Pinto Valley Mine Draft Environmental Impact Statement Vol 2

U.S. Department of Agriculture; Forest Service
2019-12-01
Pinto Valley Mine; Tonto National Forest

United States Department of Agriculture

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Pinto Valley Mine 

Draft Environmental Impact Statement

Volume 2

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In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident.

Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through the Federal Relay Service at (800) 877-8339. Additionally, program information may be made available in languages other than English.

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov.

USDA is an equal opportunity provider, employer and lender.

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Appendix A. Maps

Map 1-1. Existing Mine Features 

Map 2-1. No Action Alternative

Map 2-2. Proposed Action 

Map 3-1. Air Quality – National Ambient Air Quality Standards Non-Attainment Areas

Map 3-2. Biological Resources – Overview 

Map 3-3. Biological Resources – Southwestern Willow Flycatcher

Map 3-4.  Biological Resources – Yellow-Billed Cuckoo

Map 3-5.  Geology – Regional Geologic Setting 

Map 3-6. Geology – Geologic Cross Sections 

Map 3-7.  Paleontology – Geologic Units Associated with Paleontological Resource Potential

Map 3-8. Hazardous Materials – Storage and Use Locations at Major Mine Facilities

Map 3-9.  Hazardous Materials – Storage and Use Locations at Mill, Concentrator, and Miscellaneous Facilities

Map 3-10. Lands and Realty – Land Ownership

Map 3-11. Livestock Grazing – Grazing Allotments and Constructed Features

Map 3-12. Recreation and Wilderness – Recreation Locations and Wilderness Areas

Map 3-13.  Soils – Soil Types 

Map 3-14. Transportation – Transportation Routes and Forest Roads

Map 3-15. Visual Resources –Viewshed and Sensitive Viewing Areas

Map 3-16.  Visual Resources – Visual Quality Objectives

Map 3-17. Visual Resources – USFS Variety Classes and BLM Scenic Quality 

Map 3-18. Visual Resources – USFS User Sensitivity / BLM Sensitivity Levels 

Map 3-19.  Visual Resources – USFS and BLM Distance Zones

Map 3-20. Visual Resources – BLM Visual Resource Inventory Classes 

Map 3-21. Visual Resources – No Action Viewshed

Map 3-22. Visual Resources – Proposed Action Viewshed

Map 3-23. Water Resources – Streams in the Hydrologic Study Area

Map 3-24. Water Resources – Springs and Seeps

Map 3-25. Water Resources – Seep and Spring Flow Rates

Map 3-26. Water Resources – Waters of the U.S.

Map 3-27. Water Resources – Water Resources Sampling Locations

Map 3-28. Water Resources – Location of Existing Pumping Wells

Map 3-29. Water Resources – Groundwater Elevation Contours – Hydrologic Study Area (June 2018)

Map 3-30. Water Resources – Groundwater Quality – Average Sulfate Concentrations (2014–2016)

Map 3-31. Water Resources – Water Rights

Map 3-32. Water Resources – Predicted Change in Groundwater Levels – Transient Period (2013–2018)

Map 3-33. Water Resources – Predicted Change in Groundwater Levels – No-Action Alternative (End Of Mining)

Map 3-34. Water Resources – Predicted Change in Groundwater Levels – No-Action Alternative (100 Years Post-Mining)

Map 3-35. Water Resources – Surface Water Resources within the Maximum Extent of the 5-Foot Drawdown Contour

Map 3-36. Water Resources – Surface Water Rights within the Maximum Extent of the 5-Foot Drawdown Contour

Map 3-37. Water Resources – Predicted Change in Groundwater Levels – Proposed Action (End of Mining)

Map 3-38. Water Resources – Predicted Change in Groundwater Levels – Proposed Action (100 Years Post-Mining)

Map 1-1. Existing Mine Features

Map 2-1. No Action Alternative

Map 2-2. Proposed Action

Map 3-1. Air Quality – National Ambient Air Quality Standards Non-Attainment Areas

Map 3-2. Biological Resources – Overview

Map 3-3. Biological Resources – Southwestern Willow Flycatcher

Map 3-4. Biological Resources – Yellow-Billed Cuckoo

Map 3-5. Geology – Regional Geologic Setting

Map 3-6. Geology – Geologic Cross Sections

Map 3-7. Paleontology – Geologic Units Associated with Paleontological Resource Potential

Map 3-8. Hazardous Materials – Storage and Use Locations at Major Mine Facilities

Map 3-9. Hazardous Materials – Storage and Use Locations at Mill, Concentrator, and Miscellaneous Facilities

Map 3-10. Lands and Realty – Land Ownership

Map 3-11. Livestock Grazing – Grazing Allotments and Constructed Features

Map 3-12. Recreation and Wilderness – Recreation Locations and Wilderness Areas

Map 3-13. Soils – Soil Types

Map 3-14. Transportation – Transportation Routes and Forest Roads

Map 3-15. Visual Resources –Viewshed and Sensitive Viewing Areas

Map 3-16. Visual Resources – Visual Quality Objectives

Map 3-17. Visual Resources – USFS Variety Classes and BLM Scenic Quality

Map 3-18. Visual Resources – USFS User Sensitivity / BLM Sensitivity Levels

Map 3-19. Visual Resources – USFS and BLM Distance Zones

Map 3-20. Visual Resources – BLM Visual Resource Inventory Classes

Map 3-21. Visual Resources – No Action Viewshed

Map 3-22. Visual Resources – Proposed Action Viewshed

Map 3-23. Water Resources – Streams in the Hydrologic Study Area

Map 3-24. Water Resources – Springs and Seeps

Map 3-25. Water Resources – Seep and Spring Flow Rates

Map 3-26. Water Resources – Waters of the U.S.

Map 3-27. Water Resources – Water Resources Sampling Locations

Map 3-28. Water Resources – Location of Existing Pumping Wells

Map 3-29. Water Resources – Groundwater Elevation Contours – Hydrologic Study Area (June 2018)

Map 3-30. Water Resources – Groundwater Quality – Average Sulfate Concentrations (2014–2016)

Map 3-31. Water Resources – Water Rights

Map 3-32. Water Resources – Predicted Change in Groundwater Levels – Transient Period (2013–2018)

Map 3-33. Water Resources – Predicted Change in Groundwater Levels – No-Action Alternative (End Of Mining)

Map 3-34. Water Resources – Predicted Change in Groundwater Levels – No-Action Alternative (100 Years Post-Mining)

Map 3-35. Water Resources – Surface Water Resources within the Maximum Extent of the 5-Foot Drawdown Contour

Map 3-36. Water Resources – Surface Water Rights within the Maximum Extent of the 5-Foot Drawdown Contour

Map 3-37. Water Resources – Predicted Change in Groundwater Levels – Proposed Action (End of Mining)

Map 3-38. Water Resources – Predicted Change in Groundwater Levels – Proposed Action (100 Years Post-Mining)

 

Appendix B. Air Quality Technical Support Document

 

United States Department of Agriculture

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Pinto Valley Mine 

Environmental Impact Statement

Air Quality Technical Support Document 

 

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In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through the Federal Relay Service at (800) 877-8339. Additionally, program information may be made available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov.

USDA is an equal opportunity provider, employer and lender.

Table of Contents

1.0 Introduction...................................................................................................... B-1

1.1 Project Description ......................................................................................... B-1

2.0 Air Quality Status ............................................................................................ B-2

2.1 Pinto Valley Area Air Quality Classification .................................................... B-2

3.0 Dispersion Modeling Input Data and Defaults ................................................ B-4

3.1 Modeling Approach ........................................................................................ B-4

3.1.1 Modeling Options ......................................................................................... B-4

3.1.2 Pollutants and Averaging Period ................................................................. B-5

3.1.3 Input Preparation ......................................................................................... B-6

3.1.4 Study Area and Receptor Network .............................................................. B-9

3.2 Background Air Quality Data ........................................................................... B-12

4.0 Emissions and Modeling Scenarios ................................................................ B-14

4.1 Modeling Scenarios: Pinto Valley Mine ........................................................... B-14

4.1.1 Annual Criteria Pollutant Emissions Modeling ............................................. B-15

4.1.2 Short-Term Criteria Pollutant Emissions Modeling ....................................... B-15

4.1.3 Source Characterization ............................................................................... B-16

4.2 Cumulative Modeling Scenario ........................................................................ B-17

5.0 Dispersion Modeling Results ........................................................................... B-19

5.1 Pinto Valley Mine ............................................................................................ B-19

5.2 Modeled Emission Rates for Precursors Analysis Tier 1 Assessment for Ozone ... B-20

5.3 Cumulative Modeling ....................................................................................... B-21

5.3.1 Deposition Impact Assessment .................................................................... B-27

6.0 Visibility Impact Assessment ........................................................................... B-28

6.1 VISCREEN Modeling ...................................................................................... B-28

6.2 PLUVUE II Modeling ....................................................................................... B-32

7.0 Conclusions .................................................................................................... B-36

8.0 References .................................................................................................... B-38

 

 

List of Attachments

Attachment A – Emissions Inventory

 

List of Tables

Table 1. Monthly seasonal profile at the Pinto Valley Mine ....................................... B-7

Table 2. Moisture classification for the Pinto Valley Mine .......................................... B-7

Table 3. Background pollutant concentrations and source locations ......................... B-14

Table 4. Summary of Pinto Valley Mine onsite emissions for the proposed action and no-action alternative (tons per year) ........................................................................ B-15

Table 5. Maximum hourly emission rates from blasting at the Pinto Valley Mine ........................ B-16

Table 6. Nearby mining and smelter facilities considered for the cumulative emissions and analysis ................................................................................................. B-18

 

Table 7. Summary of regional emissions used for the cumulative analysis (tons per year) .......... B-19

Table 8. Maximum air quality impacts for the proposed action and no-action alternative at the Pinto Valley Mine, compared to the applicable National Ambient Air Quality Standards, maximum emission year 2024 .............................................................. B-20

Table 9. Maximum cumulative air quality impacts for the proposed action compared to the applicable National Ambient Air Quality Standards, maximum emission year 2024 ....... B-22

Table 10. Class I areas and sensitive class II areas within 50 kilometers of the Pinto Valley Mine .. B-28

Table 11. Maximum emission rates during routine events at the Open Pit and during blasting ..... B-30

Table 12. VISCREEN level-1 screening modeling results of plume visibility inside class I and sensitive class II areas from blasting activities ................................................................................. B-30

Table 13. VISCREEN level-2 screening modeling results of plume visibility inside class I and sensitive class II areas from blasting activities ................................................................................. B-31

Table 14. VISCREEN level-1 screening modeling results of plume visibility inside class I and sensitive class II areas from drilling and hauling activities ............................................................... B-31

Table 15. VISCREEN level-2 screening modeling results of plume visibility inside class I and sensitive class II areas from drilling and hauling activities ............................................................... B-32

Table 16. PLUVUE II modeling results of plume visibility inside the Superstition Wilderness area from blasting activities ............................................................................................................... B-34

 

List of Figures

Figure 1. Nonattainment areas in the vicinity of the Pinto Valley Mine ............................................ B-3

Figure 2. Wind rose from the Pinto Valley Mine meteorological monitoring station ....................... B-8

Figure 3. Pinto Valley Mine proposed action mine expansion ........................................................ B-10

Figure 4. AERMOD receptor grid for the near-field modeling of the Pinto Valley Mine expansion B-11

Figure 5. AERMOD receptor grid for the far-field modeling of the Pinto Valley Mine expansion ... B-12

Figure 6. Receptors (green circles) with the maximum cumulative 1-hour nitrogen dioxide concentrations (with background levels) modeled to potentially exceed National Ambient Air Quality Standards, maximum emission year 2024 ...................................................... B-23

Figure 7. Locations and concentrations for the three highest Pinto Valley Mine contribution receptors when the total concentration exceeded the 1-hour sulfur dioxide National Ambient Air Quality Standard for the cumulative modeling analysis (includes background concentrations), maximum emission year 2024 ............................................................... B-24

Figure 8. Receptors (green circles) with the high second high cumulative 24-hour PM10 concentrations (with background concentrations) greater than 110 micrograms per cubic meter, maximum emission year 2024 .............................................................................. B-25

Figure 9. Receptors (green circles) with the high eighth high cumulative 24-hour PM2.5 concentrations (with background concentrations) greater than 25 micrograms per cubic meter, maximum emission year 2024 .............................................................................. B-26

Figure 10. Receptors (green circles) with the maximum cumulative annual PM2.5 concentrations (with background concentrations) modeled to potentially exceed National Ambient Air Quality Standards, maximum emission year 2024 ............................................................ B-27

 

 

 

 

ACRONYMS AND ABBREVIATIONS

AERMAP American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee terrain preprocessor

AERMET American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee Model meteorological data preprocessor

AERMOD American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee Model

AERSURFACE American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee surface characteristics tool

CFR Code of Federal Regulations

EIS environmental impact statement

Forest Service U.S. Forest Service

PLUVUE Plume Visibility Model

PM10 particulate matter less than 10 microns mean mass diameter 

PM2.5 particulate matter less than 2.5 microns mean mass diameter

VISCREEN plume visual impact screening model

 

 

1.0 Introduction

This air quality technical support document provides a description of the methods and results of the air quality assessment and modeling prepared for the Pinto Valley Mine environmental impact statement (EIS). The air impact analysis is based on the modeling protocol titled “Pinto Valley Mine Expansion Project EIS – Air Quality Impact Assessment Protocol” (ICF 2018) with a final version submitted to the U.S. Forest Service (Forest Service) on May 24, 2018. The air assessment protocol was developed following applicable portions of the Arizona Department of Environmental Quality’s “Air Dispersion Modeling Guidelines for Arizona Air Quality Permits” (Arizona Department of Environmental Quality 2015), U.S. Environmental Protection Agency Guidelines on Air Quality Models (Title 40, Part 51 of the Code of Federal Regulations [CFR], appendix W, May 22, 2017), and Forest Service guidance “Climate Change Considerations in Project Level NEPA Analysis” (Forest Service 2009). The air assessment examines and quantifies potential impacts on air quality and air quality–related values from emissions associated with the proposed expansion of the existing Pinto Valley Mine in Gila County, Arizona owned by Pinto Valley Mining Corp., whose parent corporation is Capstone Mining Corporation. 

1.1 Project Description

The Pinto Valley Mine is an existing open pit copper and molybdenum mine located in Gila County, Arizona, approximately 8 miles west of the Town of Miami on private and National Forest System lands in the Globe Ranger District. Pinto Valley Mine Corp.’s mining plan of operations proposes to expand existing mining operations from private lands onto National Forest System lands, extend the mine life, and consolidate prior authorizations that are reasonably incident to extraction, transportation, and processing of mineral deposits on its mining claims.

Existing facilities at Pinto Valley Mine include the Open Pit and adjacent milling and processing operations, tailings storage facilities, and waste rock disposal areas. Existing surface disturbance associated with Pinto Valley Mine facilities encompasses an estimated 3,915 acres, of which 3,349 acres are on private lands and 556 acres are on National Forest System lands. The mining plan of operations proposes an additional 1,316 acres of surface disturbance (1,087 acres on private land and 229 acres on National Forest System lands), primarily for pit expansion and tailings disposal, for a total estimated surface disturbance of 5,231 acres (4,436 acres on private land and 795 acres on National Forest System lands). Expansion of the pit and tailings area would extend the operating life of the mine by several years and enable Pinto Valley Mine Corp. to recover proven and probable mineral reserves adjacent to the existing Open Pit. 

Pinto Valley Mine Corp. proposes the following modifications and additions as part of the proposed Pinto Valley Mine Expansion Project:

. Continuation of existing authorized activities and permitted disturbances at the Pinto Valley Mine (including all State and Federal authorizations on public or private lands)

 

. Expansion of the Open Pit and Tailings Storage Facility Nos. 3 and 4 in Pinto Valley Mining Corp.’s unpatented claims on National Forest System lands

 

. Pinto Valley Mining Corp.’s actions on private lands to support ongoing and expanded operations

 

. Construction of linear features (access roads, electrical power transmission lines) to support the expansion of the Open Pit and Tailings Storage Facility No. 3 and No. 4 

 

. Extension of the mine life to 2039

 

 

Under the no-action alternative, the mine life is anticipated to continue until 2027 at current production rates. The proposed action would allow extension of the mine life until 2039. 

Pinto Valley Mine Corp. utilizes conventional open-pit hard rock mining methods employing drilling, blasting, loading, and hauling to extract copper-bearing sulfide ore. Waste rock is transported by haul truck to the waste rock storage areas. Ores bearing copper and molybdenum are extracted from an existing Open Pit, all located on private property. The extract ore processing activities are composed of milling, copper concentrating, copper leaching, and solvent extraction and electrowinning. No copper smelting takes place at the Pinto Valley Mine site. The copper and molybdenum concentrates from the milling and flotation operations are shipped off site for further processing. Under the proposed action, these same activities would continue until 2039. 

The Open Pit does not currently extend onto National Forest System lands. The approximate current size of the Open Pit is 754 acres and an approximate base elevation of 2,645 feet above mean sea level. The total depth of the pit from the bottom to its highest rim at 4,760 feet is 2,115 feet. The approximate east-west and north-south dimensions of the pit are 8,300 feet and 5,300 feet, respectively. Under the no-action alternative, Pinto Valley Mine Corp. would continue mining the Open Pit on private property and would extend the pit on private lands to the north and west to access ore deposits. For the no-action alternative, the pit would not be extended onto National Forest System lands.

 

2.0 Air Quality Status

2.1 Pinto Valley Area Air Quality Classification

The U.S. Environmental Protection Agency classifies air quality regions as “nonattainment” for a given pollutant if ambient air concentrations exceed the National Ambient Air Quality Standards, which are established separately for each of the “criteria” pollutants. These National Ambient Air Quality Standards have been promulgated under 40 CFR 50 (see https://www.epa.gov/criteria-air-pollutants for more information). Areas that are not nonattainment are either “attainment” if the National Ambient Air Quality Standards have not been exceeded, or the area is deemed unclassifiable or attainment if insufficient data exist to make a determination. Attainment status is based on the results of ambient air quality monitoring, typically performed over a 3-year period. 

The Pinto Valley Mine area is classified as attainment or unclassifiable or attainment for particulate matter less than 2.5 microns mean mass diameter (PM2.5), as well as lead, carbon monoxide, sulfur dioxide, nitrogen dioxide, and ozone. The Pinto Valley Mine area is classified as nonattainment for PM10, which is particulate matter less than 10 microns mean mass diameter and includes dust from mining operations, wind erosion, and traffic on unpaved roads. The Pinto Valley Mine area is also in a nonattainment area for the 1-hour sulfur dioxide National Ambient Air Quality Standard and is close to a major source of sulfur dioxide from the Freeport-McMoRan Smelter. Refer to Figure 1 below for the nonattainment area boundaries in proximity to the Pinto Valley Mine.  

 

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Figure 1. Non-attainment areas in the vicinity of the Pinto Valley Mine

 

3.0 Dispersion Modeling Input Data and Defaults

 

The primary air quality modeling tool used in this analysis is the American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee Model (AERMOD). AERMOD is used to assess the near-field air quality impacts for comparison with applicable air quality standards. The AERMOD analysis focuses on emissions associated with the proposed action for the expansion onto National Forest System lands and potential impacts on air quality. AERMOD is also used to conduct a modeling assessment of cumulative impacts caused by emissions from the Pinto Valley Mine and other primary emissions sources in the analysis area. 

AERMOD is a steady-state Gaussian dispersion model designed to simulate the local-scale dispersion of pollutants from low-level or elevated sources in simple or complex terrain. AERMOD is a U.S. Environmental Protection Agency “preferred” model (40 CFR 51, appendix W, Guideline on Air Quality Models) and is particularly suited to evaluating the impacts for PM10 and 1-hour sulfur dioxide (the pollutants associated with the nonattainment areas in the vicinity of the Pinto Valley Mine). Recent versions of AERMOD include algorithms for simulating deposition of gaseous and particulate pollutants. 

The AERMOD modeling system consists of four main components: 

1. The AERMOD dispersion model; 

 

2. The American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee meteorological data preprocessor (AERMET); 

 

3. The American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee terrain preprocessor (AERMAP) and American Meteorological Society and U.S. Environmental Protection Agency Regulatory Model Improvement Committee surface characteristics tool (AERSURFACE); and 

 

4. A land cover data preprocessor for surface characteristics for use in AERMET. 

 

 

The latest versions of these tools are used for this analysis and include version 18081 of AERMOD, version 16181 of AERMET, version 13016 of AERSURFACE, and version 18081 of AERMAP. The dispersion algorithms are based on the similarity theory of planetary boundary layer meteorology. The airflow and stability characteristics (such as convective versus stable) as well as the vertical structure of the boundary layer are included in simulating dispersion. Numerous features and options accommodate a variety of source types, pollutants, and land-use and topographical features. 

3.1 Modeling Approach

AERMOD was applied using 2 years of the most recently available onsite meteorological data collected at Pinto Valley Mine (January 2016 to January 2018), incorporates emissions from the proposed expansion of the mine for a near-field analysis, and includes other nearby sources from smelting and mining in the regional analysis. The emissions sources (such as engines and generators) are located throughout the Pinto Valley Mine project area and the location of the sources are specific as part of the model inputs. 

3.1.1 Modeling Options 

AERMOD was run using regulatory default options for the simulation parameters. A Tier 3 method was applied using the ozone-limiting method for nitrogen dioxide modeling. The default equilibrium nitrogen dioxide/oxides of nitrogen ratio of 0.9 was used in the analysis. In-stack nitrogen dioxide to oxides of nitrogen ratios in most cases were based on similar engines sizes available from the U.S. Environmental Protection Agency’s “NO2_ISR_database.xls” file. Where these were not available an in stack ratio of 0.1

was used for mobile source equipment as this equipment pre-dates the use of selective catalytic reduction to meet Tier 4 oxides of nitrogen emission standards. The use of selective catalytic reduction reduces oxides of nitrogen emissions but increases the initial fraction as nitrogen dioxide. Resolution Copper Mine’s East Plant hourly ozone data were used to provide background ozone concentrations needed for the ozone-limiting method.  

Dry plume depletion can be an important effect for the more distant source-receptor distances. However, the computational burden required to run AERMOD with plume depletion is large. We therefore first modeled particulate matter sources without plume depletion. We then modeled only those receptors with plume depletion that when combined with background concentrations showed a potential to exceed the particulate matter air quality standards. The combined results are reported in section 5. 

Within AERMOD, sources can be treated as point, volume, or area sources. No building downwash was used in the analysis. For this analysis, stacks associated with the generators and solvent extraction and electrowinning are treated as point sources. Emission sources such as the waste rock dump, National Forest System roads, and mobile equipment operating in specific areas (such as excavators and hydraulic shovels) were modeled as area polygons. Exhaust emissions from the mining haul trucks and blasting were modeled as line volume sources. Other default options used in the AERMOD modeling included stack-tip downwash and the incorporation of the effects of elevated terrain.

Refer to section 4.0 (“Emissions and Modeling Scenarios”) for additional information on emissions and modeling. 

 

3.1.2 Pollutants and Averaging Period 

 

AERMOD is used to examine the impacts of emissions of the following criteria pollutants: PM10, PM2.5, nitrogen oxides, sulfur dioxide, and carbon monoxide.1 For each criteria pollutant, the averaging period(s) is based on the relevant National Ambient Air Quality Standards. The averaging periods are as follows:

1 The facility has no lead emissions and was not therefore not modeled. This report does identify lead background concentrations for completeness. 

. PM10: 24-hour averaging periods

 

. PM2.5: 24-hour and annual averaging periods

 

. Nitrogen dioxide: 1-hour and annual averaging periods

 

. Sulfur dioxide: 1-hour averaging period

 

. Carbon monoxide: 1-hour and 8-hour averaging periods

 

 

While nitrogen oxides and volatile organic compounds contribute to the formation of ozone in the atmosphere, the formation of this criteria air pollutant is a result of highly nonlinear atmospheric photochemistry and is considered a regional air pollutant most often addressed in State Implementation Plans. The U.S. Environmental Protection Agency has only recently released guidance on assessing ozone formation from a single source (U.S. Environmental Protection Agency 2018a). However, this is primarily intended for major sources of emissions that require a Prevention of Significant Deterioration permit. Emissions of nitrogen oxides and volatile organic compounds associated with the Pinto Valley Mine fall well below this requirement. As a result, ozone was not assessed in detail in this study. 

 

3.1.3 Input Preparation

 

3.1.3.1 Topographical Data

The general terrain in the area of Pinto Valley Mine consists of rolling hills and interspersed mesas. Digital topographical data (in the form of 7.5-minute Digital Elevation Model files) with a resolution of . arc second ~10-meter horizontal resolution was used in the analysis for receptors located outside the high-resolution (3-foot) terrain data provided by Pinto Valley Mine Corp. These data were processed for use in AERMOD using the AERMAP preprocessor program (U.S. Environmental Protection Agency 2016a). 

 

3.1.3.2 Meteorological and Surface Characteristics

Onsite meteorological monitoring data were provided by Pinto Valley Mine Corp. for data collected at the Pinto Valley Mine for 2 complete years from 2016 and 2017. In addition, precipitation data were provided by Pinto Valley Mine Corp. for the period of 1987 through 2017 as collected at the Pinto Valley Administration rain gauge used in the climate portion of the analysis. 

 

The air quality dispersion modeling uses the onsite weather observations from the Pinto Valley Mine monitoring site (33.407 degrees north, 110.985 degrees west). Parameters measured at the site include solar radiation, precipitation, barometric pressure, air temperature at 2 and 10 meters, differential temperature between 2 and 10 meters, dew point temperature at 2 and 10 meters, relative humidity at 2 and 10 meters, and wind speed and wind direction at 10 meters. Because the Pinto Valley Mine meteorological station measures solar radiation and temperature difference, the bulk Richardson method was used in AERMET to determine turbulence for the AERMOD modeling. Hence, no adjustment was needed for the friction velocity per U.S. Environmental Protection Agency recommendations when not using cloud cover and wind speed to characterize turbulence. 

 

There were two extended time periods in 2016 with missing data: January 1 (0000 hours) through January 5 (1200 hours) and March 2 (1300 hours) through March 9 (0900 hours). Meteorological data from the Resolution Mine East Plant meteorological monitoring site (latitude 33.3037 degrees north and longitude 111.0674 degrees west, elevation 4,176 feet) were used to fill in these missing time periods. Two hours over the 2-year period did not have solar radiation or differential temperature. Sky cloud cover data from the nearest National Weather Service site were used for these 2 hours to determine boundary layer conditions. The nearest National Weather Service site is the Safford Municipal Airport (WBAN 93084). The only other data used from the Safford site were the dew point temperature2 when not available from Pinto Valley or East Plant. Upper air data are also required by AERMOD. The nearest upper air data were obtained from the National Weather Service Tucson Airport station (WBAN 23160).

2 Dew point temperature is not used in AERMOD analysis in this study. 

Meteorological data were processed using the AERMOD-ready surface and upper air input files using the AERMET program. AERMET, a component of the AERMOD modeling system, combines onsite, National Weather Service surface, and upper air data for use in AERMOD (U.S. Environmental Protection Agency 2016b). The model also calculates a number of surface boundary layer parameters (such as Monin-Obkhov length and convective scale velocity) used in AERMOD. 

 

Surface Characteristics

 

Boundary layer parameter estimates produced by AERMOD are influenced by the surface conditions or characteristics at the measurement site. Obstacles to the wind flow, the amount of moisture at the surface, and reflectivity of the surface all affect the boundary layer estimates. These influences are quantified through the surface albedo, Bowen ratio, and roughness length. To obtain reasonable

estimates of albedo, Bowen ratio, and surface roughness, the AERSURFACE pre-processor of the AERMOD modeling system was used in the analysis. 

The U.S. Geological Survey National Land Cover Dataset 1992 was downloaded from http://landcover.usgs.gov and used with version 13016 of AERSURFACE to provide the surface land-use information needed for use by the AERMET pre-processor. The Pinto Valley Mine meteorological site was used in the determination of surface characteristics in AERSURFACE. AERSURFACE was run with the specifications that the area was arid, and the site was not at an airport. Twelve (30-degree) sectors were used for processing to account for variations in land cover near the measurement site. The study radius for surface roughness was set at 1 kilometer. Table 1 below depicts the monthly seasonal profile used in the modeling. 

 

Table 1. Monthly seasonal profile at the Pinto Valley Mine

Months

Season

January, February, March, December

Late autumn after frost and harvest, or winter with no snow

April, May, June

Transitional spring with partial green coverage or short annuals

July, August, September

Midsummer 

October, November

Autumn with unharvested cropland

 

AERSURFACE was run specifying dry, average, and wet surface moisture and the results were later used to create composite surface characteristics for the third stage of AERMET.

 

Determination of Dry, Average, and Wet Months for the Analysis Period

 

Based on information provided in the AERSURFACE user’s guide (U.S. Environmental Protection Agency 2013), each month in the modeling period was classified as either dry, average, or wet, and this information was later used in stage 3 of AERMET.

 

The rainfall data from the Pinto Valley Administration rain gauge for the 30-year period ending 2017 were analyzed and a 30-year monthly average was determined for each month. The statistics for a given month were not used in the average if data from more than 5 days were missing in a given month. The precipitation total for a given month during the modeling period was computed and the corresponding 30-year monthly average was applied. If the ratio was less than 0.5, the month was designated as dry. If the ratio was greater than or equal to 0.5 but less than 2, then the month was designated as average. If the ratio was greater than or equal to 2, then the month was designated as wet. Table 2 provides the information for the moisture classification of the region.

 

Table 2. Moisture classification for the Pinto Valley Mine 

Year

Jan.

Feb.

Mar.

Apr.

May.

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

2016

average

dry

dry

average

average

wet

average

dry

average

average

average

Wet

2017

wet

average

dry

dry

average

dry

wet

dry

average

dry

dry

dry

 

 

 

Note: For January 2017, there were 6 days with missing data. Averages were computed and then compared to the 30-year monthly averages for both 2016 and 2017. Based on the ratios, the month was still deemed to be “wet.” 

 

Wind Direction

 

Figure 2 depicts a 2-year averaged (2016–2017) wind rose from the Pinto Valley Mine meteorological station. The wind rose indicates a predominant wind direction from the south-southeast with frequent low wind-speed conditions.  

 

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Figure 2. Wind rose from the Pinto Valley Mine meteorological monitoring station

 

3.1.4 Study Area and Receptor Network 

Figure 3 depicts the overall layout of the Pinto Valley Mine under the proposed action. Major project features include the tailings storage facilities, ponds, stockpiles, mill and concentrator, the Open Pit, and the general mine boundary line. The near-field modeling includes emissions from all of the Pinto Valley Mine emission sources.

 

Figure 4 depicts the near-field receptor grid consisting of 200- by 200-meter receptor cells starting at locations where the general public might be located and with additional receptors placed along the National Forest System roads (modeled as line area sources) that are publicly accessible and run through the Pinto Valley Mine. The haul roads were modeled as line-volume sources, which required that any receptors near haul roads be placed outside the exclusion zone at a distance of 8 meters from the roadway edge. These receptors extend 1,000 meters or more from the boundary of the source area and then increase to 500- by 500-meter spacing to approximately 5,000 meters (5 kilometers) from the mining area. Due to the irregular shape of the project area, the number of 200-meter spaced receptors between the boundary and the beginning of the 500-meter spaced receptors varies with location. As depicted on figure 4, the 200-meter spaced receptors comprise the denser (inner) array, and the 500-meter spaced receptors (less dense) extend outward. 

 

In addition to the receptor grid shown on figure 4, receptors were placed along the boundary of the two class I areas within 50 kilometers of the Pinto Valley Mine (figure 5). For the cumulative assessment, the receptor grid was extended to encompass the nearby Freeport-McMoRan smelter using 1,000-meter spacing but only for receptors not within the Miami Mine property where the public does not have access. 

 

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Figure 3. Pinto Valley Mine proposed action mine expansion 

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Figure 4. AEMOD receptor grid for the near-field modeling of the Pinto Valley mine expansion

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Figure 5. AERMOD receptor grid for the far-field modeling of the Pinto Valley Mine expansion

 

3.2 Background Air Quality Data

In this analysis, the total pollutant concentration used for comparison with the National Ambient Air Quality Standards is the sum of the AERMOD-derived impacts from the Pinto Valley Mine plus background pollutant concentrations. The background concentrations are intended to account for sources not included in the modeling. Background concentrations are representative of the regional air quality in the vicinity of the Pinto Valley Mine but do not include offsite sources considered in the cumulative analysis. The background monitored data to use in the analysis followed the guidance specified in the Arizona Department of Environmental Quality guidance (Arizona Department of Environmental Quality 2015), which recommends the selection of a representative site be based on three key factors: (1) monitor location; (2) data quality (90 percent completeness criteria each quarter); and (3) how current the data are. Not all pollutants of interest are measured at all air quality monitoring sites. This section summarizes the underlying rationale for the selection of the representative background monitor to use in this analysis. The background air quality values and selected monitoring stations are summarized in table 3.

Resolution Copper has conducted background hourly air quality monitoring at the proposed East Plant location for at least the past 3 years for ozone, nitrogen dioxide, PM10, PM2.5, and sulfur dioxide. The East Plant is approximately 14 kilometers (8.6 miles) from the Pinto Valley Mine site. This East Plant has had limited exploratory drilling during this time period and the monitored values are considered representative of the regional background concentration while having minimal impact from the Miami Mine, the Miami Mine Smelter, or Pinto Valley Mine (these emission sources are included in the project-specific and cumulative emissions). Resolution Copper provided these data; upon review, the data exceeded the U.S. Environmental Protection Agency’s 75 percent completeness criteria (40 CFR) for each quarter. Therefore, the East Plant air quality data were used to determine background concentrations for the monitored air pollutants. However, for particulate matter, the Resolution Mine also monitored PM10 and PM2.5 at the West Plant location. A total of 86 days over the 3-year period were incomplete for particulate matter at the East Plant. To complete the picture of the particulate matter background concentrations, a gap-filling procedure developed by Pinal County Air Quality Control District was used to complete the record. The approach using a two-step procedure:

 

. Step 1: Any missing PM10 or PM2.5 data are filled using the measured PM10 and PM2.5 collected data at the closest monitoring site if available. For Resolution Mine, this was West Plant. 

 

. Step 2: When the daily monitoring data are missing at both sites, a monthly gap-fill procedure is used based on the highest monitored concentration for the month averaged over the 3 years.

 

Following completion of this procedure, a quarterly background concentration was derived following the “Second tier 24-hour modeling analysis” methodology as described in the U.S. Environmental Protection Agency’s memorandum dated May 20, 2014 (U.S. Environmental Protection Agency 2014). We applied this methodology to determine the 24-hour quarterly background concentration for PM2.5 and followed the same methodology for PM10, with the exception of using the daily second high value rather than the 98th percentile used for PM2.5. This is consistent with the form of the PM10 standard. 

 

Resolution Copper does not monitor for carbon monoxide. The air quality team reviewed the available carbon monoxide monitoring stations within the state to determine an appropriate background monitor. The Arizona Department of Environmental Quality has only two sites, the JLG supersite in downtown Phoenix and Alamo Lake, neither representative of the Pinto Valley Mine region. Maricopa County Air Quality Department has seven sites that monitor carbon monoxide in Maricopa County and it has identified the Buckeye monitor as the “upwind background” site representing background air concentration levels. The Pinto Valley Mine site is located sufficiently far from the Phoenix metropolitan area for carbon monoxide levels to return to near-background levels. As a result, the Buckeye monitoring station serves as the carbon monoxide background monitor. 

 

The Miami Golf Course is the only lead monitor in the vicinity of the Pinto Valley Mine. It is a source-oriented monitor for lead emissions associated with the operation of the Freeport-McMoRan smelter. This monitor was used to conservatively represent background lead concentrations. 

 

Table 3. Background pollutant concentrations and source locations 

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Sources: Carbon monoxide and lead retrieved from https://www.epa.gov/outdoor-air-quality-data/monitor-values-report; all other air pollutants retrieved from Resolution Copper 2018. 

 

4.0 Emissions and Modeling Scenarios

4.1 Modeling Scenarios: Pinto Valley Mine 

The modeling scenarios were designed to capture the maximum emissions year impacts for each pollutant for each of the major activities associated with the proposed action and the no-action alternative. The modeling scenario under the proposed action includes all ongoing activities at the Pinto Valley Mine plus the expansion of the Open Pit and Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 onto National Forest System land. The changes compared to the no-action alternative are mostly spatial with expansion onto National Forest System lands for both the Open Pit and the tailings storage facilities. In addition, compared to the no-action alternative, an increase in haul truck activity would occur under the proposed action due to the extended life of the mine. To assess the maximum impact, the proposed action was modeled for the year with the highest total haul truck activity, which would be 2024 with total vehicle miles traveled of 1,063,691. Under the no-action alternative, the 2024 total vehicle miles traveled are estimated at 385,900. The major changes in spatial allocation between

the no-action alternative and proposed action included in the modeling were associated with the waste haul routes and the difference in expansion of the Open Pit and Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 onto National Forest System lands under the proposed action.5 

5 Other construction-related activities such as new access roads (such as the Tailings Storage Facility No. 4 perimeter road) and power lines (such as the Tailings Storage Facility No. 4 barges) are not scheduled to occur in 2024. The emissions associated with these activities are small compared to the increased haul truck activity under the proposed action relative to the no-action alternative in 2024. 

Because the mining operation occurs continuously 24 hours per day, 7 days per week, 365 days per year, the same daily emission rates are used throughout the year, with the exception of blasting, which is discussed under short-term emissions. 

All sources of emissions are modeled in both the no-action alternative and proposed action modeling scenarios. Differences between the two scenarios represent the incremental impact from the mine expansion onto National Forest System lands. 

4.1.1 Annual Criteria Pollutant Emissions Modeling

Table 4 provides a summary of the criteria pollutant annual average emission rates in 2024 for the proposed action and no-action alternative. Compared to the no-action alternative, the proposed action shows higher emission rates for PM10, PM2.5, nitrogen oxides, and carbon monoxide. As previously indicated, the increase in these emissions is mostly due to increases in haul truck activity under the proposed action. The largest increase for any single pollutant was for PM10 due to increases in fuel combustion and fugitive dust emissions under the proposed action. Attachment A (“Emissions Inventory”) provides additional detail on emission rates for the three major source types used in this analysis. Under the proposed action and no-action alternative, the point source emissions remain the same and only the spatial allocation of the emissions change. 

Table 4. Summary of Pinto Valley Mine onsite emissions for the proposed action and no-action alternative (tons per year) 

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Source: Pinto Valley Mine Corp. 2018. Refer to attachment A (“Emissions Inventory”).

CO = carbon monoxide; NOX = nitrogen oxides; SO2 = sulfur dioxide; VOC = volatile organic compounds

 

4.1.2 Short-Term Criteria Pollutant Emissions Modeling

To uncover the rock and ore-bearing minerals in the Open Pit, blasting of the waste rock and sulfide ore is performed about 124 times per year for waste rock and slightly more frequently at 136 times per year for the sulfide ore. These are typically separate events and were modeled separately in this analysis. The

blasting produces sizeable short-term emission events. Emissions were averaged over 1-hour then modeled with AERMOD to enable comparison to the short-term standards. Emissions from both the blasting disturbance of the rock and minerals along with blasting agent are included in the emissions. Table 5 depicts the maximum short-term emission rates associated with blasting at the Pinto Valley Mine. 

 

Table 5. Maximum hourly emission rates from blasting at the Pinto Valley Mine 

Activity

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Source: Pinto Valley Mine Corp. 2018. Refer to attachment A (“Emissions Inventory”).

CO = carbon monoxide; NOX = nitrogen oxides; SO2 = sulfur dioxide

 

These short-term emissions were modeled to determine the maximum short-term impacts in combination with the routine operational emissions. Blasting activities were modeled separately but were added together to determine the maximum impacts, as both model runs used the same set of receptors. Blasting emissions were also modeled separately for long-term averages assuming blasting occurred only between 11 a.m. and 1 p.m. and were added to routine operational annual concentrations. 

 

4.1.3 Source Characterization 

Within AERMOD, emission sources can be treated as point sources, volume sources, area sources, and Open Pit. Point sources are emissions released into the atmosphere from a stack or vented source. Volume sources are used to simulate the effects from release building rooftops and line sources (such as conveyor belts). Area sources are used to characterize fugitive emissions such as those from storage piles and waste dumps. The Open Pit algorithm is specifically used to model sources within the Open Pit for particulate and gaseous emissions. 

The sections below describe the various ways all of the sources at Pinto Valley Mine were characterized within AERMOD for the air dispersion modeling. 

4.1.3.1 Point Sources

Point sources at Pinto Valley Mine include moly thickeners, boilers, solvent extraction and electrowinning, and emergency generator(s). Emissions from these sources were modeled as individual point sources. Stack parameters (release height, temperature, exit velocity, stack diameter) and locations for these point sources were provided by Pinto Valley Mine Corp. 

4.1.3.2 Roads – Line Volume Sources 

An unpaved haul road network was provided by Pinto Valley Mine Corp. showing the anticipated haul truck routes, waste rock dumping locations, and ore unloading locations for 2024 for both the proposed action and no-action alternative. Emissions due to haul and dump trucks on the unpaved road network were modeled as volume sources and the modeling parameters were based on guidance from the Arizona Department of Environmental Quality (2015), the AERMOD user’s guide (U.S. Environmental Protection Agency 2016c), and the haul road working group (U.S. Environmental Protection Agency 2012). The height of the typical haul truck is 6 meters (20 feet). Therefore, for each road source, the volume height was set to 10.2 meters (1.7 times the height of the vehicles generating the emissions), the

initial vertical dimension was set to 4.74 meters (volume height divided by 2.15), and the release height was set to 5.1 meters (half of the volume height). The road width was estimated to be 13.7 meters (45 feet) so that the initial lateral dimension for each volume was set to 6.37 meters (width of 13.7 meters divided by 2.15). 

4.1.3.3 Roads – Line Area Sources 

Roads on which service trucks travel were identified by Pinto Valley Mine Corp., along with anticipated routes on these roadways. These roads were modeled as line area sources. The height of the typical service truck is 3.4 meters (11 feet). As a result, the initial vertical dimension was set to 1.58 meters (height divided by 2.15), and the release height was set to 1.7 meters (half of the vehicle height). The width of the disturbance area was estimated to be 10 meters (3 meters on both sides plus a 2.4-meter vehicle width) so that the initial lateral dimension for each volume was set to 3.91 meters (width of 8.4 meters divided by 2.15). 

4.1.3.4 Blasting – Line Volume Sources 

Blasting emissions were modeled as line volume sources for both waste rock and sulfide ore and for both particulate matter and gas-phase pollutants. This line source characterization best describes the lateral nature of these emissions. Each blast covers an area of approximately 84,000 square feet. The side length of each volume source was set at 70 feet (21.0 meters) (representing the average width of a blast). A typical blast sends emissions up to 70 feet (21.34 meters) in the air, so a vertical dimension of that height was assigned to the volume sources. As a result, the initial vertical dimension of each source was set to 9.93 meters (21.34 divided by 2.15) and the release height was set to 10.67 meters (half of the vertical dimension of 21.34 meters). 

Blasting typically occurs at noon but always between 11 a.m. and 1 p.m. The short-term impacts were modeled so that these emissions only occur during those hours. Therefore, for evaluating the 1-hour averaged impacts from nitrogen dioxide, sulfur dioxide, and carbon monoxide, blasting emissions were set to occur every hour between 11 a.m. and 1 p.m. 

4.1.3.5 Area Sources – Areapoly

Fugitive and low-level emissions from sources at or near ground level at Pinto Valley Mine include waste dumps, truck wash, National Forest System roads, articulator trucks, water trucks, excavators, fork lifts, hydraulic shovels, and sweepers. These were modeled as areapoly sources with multiple vertices with release heights ranging from 2.55 to 5.1 meters depending on the equipment. 

4.1.3.6 Open Pit 

Fugitive emissions (bulldozing, grading) within the Open Pit and drilling emissions were modeled using the Open Pit source algorithm within AERMOD. The Open Pit source parameters, easterly length, northerly length, and volume were based on the length and width dimensions of the rectangle drawn to best represent the Open Pit shape and the depth of the Open Pit. 

 

4.2 Cumulative Modeling Scenario 

A cumulative modeling scenario was conducted to assess the cumulative air quality impacts associated with emissions from the Pinto Valley Mine in combination with other major emission sources in the analysis area. Nearby activities include other mines and smelters and offsite activity includes the transport of concentrate and waste hauling from Pinto Valley Mine to their respective destinations. The

mines and smelters and their leaching operation that are included in the cumulative analysis are listed in table 6. In addition to these existing facilities, the emission inventory from the proposed Resolution Mine, located approximately 14 kilometers (8.6 miles) away from Pinto Valley Mine, was reviewed. Based on review of the Resolution Mine emissions inventory and due to the nature of the underground mining for the Resolution Mine, it was determined that the majority of emissions would occur at or near ground level. 

 

Table 6. Nearby mining and smelter facilities considered for the cumulative emissions and analysis 

Facility

Distance from Pinto Valley Mine (kilometers)

Freeport-McMoRan Smelter6

11

Carlota Mine7

3.2

Miami Mine8

8

ASARCO Hayden Smelter

48

 

6 The potential to emit emissions is based on the most recent permit issued by Arizona Department of Environmental Quality permit #6609, which includes substantial emission reductions particularly for sulfur dioxide as referenced under Arizona Administrative Code R 18-2-C1302. The emission reductions were done as part of an expansion in production levels at the smelter.

7 The Carlota Mine is currently using surface and sub-surface leaching to extract economic copper from the leach pads. This operation is expected to be completed before 2024, which is the maximum emission modeling year used in this analysis, and therefore emissions from the Carlota Mine are not included in the analysis.

8 The Miami Mine is currently in care and maintenance mode with only limited leaching operations from stockpiles.

 

The emissions for the mines and smelters are based on their most recent permitted emission levels obtained from the Arizona Department of Environmental Quality. The Freeport-McMoRan Smelter recently upgraded the emission control technology at the smelter and the emissions presented in table 7 reflect that change. The Freeport-McMoRan Smelter facility-wide potential to emit emissions were obtained from the Arizona Department of Environmental Quality (Arizona Department of Environmental Quality 2017a). This includes emissions from the smelter building, tail stack, fume stack, and bypass stack as well as other plant-wide activity associated with the smelting operation. In the sulfur dioxide State Implementation Plan done by the Arizona Department of Environmental Quality, the use of AERMOD or buoyant line plume models alone to model the smelter’s roofline vent emissions over-predicted the sulfur dioxide impacts from the roofline vents. The State Implementation Plan analysis used a hybrid modeling approach with AERMOD (version 14134) and buoyant line plume modeling to properly model the smelter roofline vents and the other sulfur dioxide emission sources. Since the time of the State Implementation Plan modeling, a new version of AERMOD (version 15181 and later) has been released that incorporated the specialized algorithm from the buoyant line and point source dispersion model, making a hybrid approach no longer necessary. This assessment used this buoyant line source type option in AERMOD to model the rooftop vent emissions, which included information on the roof vents as reported in the sulfur dioxide State Implementation Plan on the exit velocity, stack temperature, building length of roof top vents, height and width, and separation width between buildings. 

The ASARCO Hayden smelter potential-to-emit emission rates were obtained from the Arizona Department of Environmental Quality in December 2017 (Arizona Department of Environmental Quality 2017b). This includes emissions from the main stack, concentrator, baghouse, and acid plant, as well as other plant-wide activity associated with the smelting operation. These reflect the recently re-permitted emission rates as reported by the Arizona Department of Environmental Quality (2017b). 

Table 7. Summary of regional emissions used for the cumulative analysis (tons per year)

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Source: Arizona Department of Environmental Quality 2017a, 2017b 

 

5.0 Dispersion Modeling Results

 

5.1 Pinto Valley Mine 

Modeling results are evaluated by comparing the modeled results with the National Ambient Air Quality Standards for the applicable air quality standard. For each pollutant, the maximum ambient air quality impacts are added to the maximum background air quality concentration. The conservative modeling results show that under the no-action alternative and proposed action, the only potential for National Ambient Air Quality Standard exceedances for the maximum activity level of 2024 is the 1- hour nitrogen dioxide standard. The results are summarized in table 8 for both the proposed action and no-action alternative. The sections below present the source contribution for the 1-hour nitrogen dioxide high concentrations and for the elevated 24-hour PM10 concentrations. 

1-hour nitrogen dioxide: The maximum 1-hour nitrogen dioxide concentration is above the National Ambient Air Quality Standards for both the no-action alternative and proposed action, by 4 percent and 9 percent, respectively. The main cause is the nitrogen oxides emissions from the onsite haul trucks, as they account for approximately half of the maximum concentration. By 2024, the 2012 and 2013 haul trucks are near the end of their lifespan, and replacing the haul trucks or engines with lower nitrogen oxides–emitting engines could help to reduce concentrations. Also, the hydraulic shovels and track dozers contribute about 20 percent of the emissions, and replacing this off-road equipment with lower-emitting Tier 4 equipment earlier than 2024 would likely substantially lower their nitrogen dioxide contribution. 

24-hour PM10: The maximum 24-hour PM10 concentration is below the National Ambient Air Quality Standards, by more than 25 percent under the proposed action. At the highest concentration location, a little more than a third of the emissions are due to fugitive dust emissions mostly from truck traffic operating near Tailings Storage Facility No. 3. Fugitive dust is difficult to model accurately because of the large uncertainty associated with the silt content. The silt content used in the analysis is based on the Arizona state average from the Environmental Protection Agency’s National Emission Inventory, which is the suggested default value in the absence of locally derived surface silt content (U.S. Environmental Protection Agency 2017). 

 

Table 8. Maximum air quality impacts for the proposed action and no-action alternative at the Pinto Valley Mine, compared to the applicable National Ambient Air Quality Standards, maximum emission year 2024

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Note: Bold text identifies modeled exceedances of National Ambient Air Quality Standards. 

µg/m3 = micrograms per cubic meter

 

5.2 Modeled Emission Rates for Precursors Analysis Tier 1 Assessment for Ozone 

 

To demonstrate that the proposed action would not have a significant impact on ozone formation, a Tier 1 modeled emission rates for precursors analysis was conducted following the U.S. Environmental Protection Agency’s guidance on development of modeled emission rates for precursors for ozone (U.S. Environmental Protection Agency 2016d) as developed to satisfy compliance under the Prevention of Significant Deterioration Program. The guidance estimates single industrial source impacts on secondary pollutants for a first-tier analysis that involves the use of relationships between emissions and ambient air quality impacts developed from existing photochemical modeling studies conducted by the U.S. Environmental Protection Agency, which are sufficiently conservative for evaluating a project potential source’s single impacts. In accordance with the modeled emission rates for precursors guidance, photochemical modeling was performed to assess the maximum potential impacts of emission precursors on ozone and secondary particulate matter impacts for some 70 locations scattered across the continental United States, for three source strengths and two release characteristics (low-level and high). 

To assess the potential impacts of the proposed action, we started with the increase in precursor emissions relative to the no-action alternative. These are reported in table 4 and are 45 tons per year for primary PM2.5, less than 0.1 ton per year for volatile organic compounds, 345 tons per year for nitrogen oxides, and less than 0.1 ton per year for sulfur dioxide. The U.S. Environmental Protection Agency has compiled a set of the most conservative modeled emission rates for each precursor across all sources,

areas, and studies (table 7.1 in the modeled emission rates for precursors guidance). Only nitrogen oxides emission increases are above the level of significant emission rates based on the values reported in table 7.1 in the modeled emission rates for precursors guidance. The nitrogen oxides emission rate is only exceeded for 8-hour ozone impacts. Further analysis is required using a site-specific modeled emission rates for precursors analysis to determine if significant impacts would occur.  

The location, type, and source strength characteristics for Pinto Valley Mine that are closest to the ozone impact assessment done by the U.S. Environmental Protection Agency is a modeled nitrogen oxides emission release from Gila County, Arizona (source type 14, western U.S.), low-level release, with an emission release rates of 500 tons per year. 

To derive a modeled emission rates for precursors value, the model predicted relationship between precursor emissions from hypothetical sources (in this case source type 14, western U.S.) and their downwind maximum impacts can be combined with a critical air quality threshold using the following equation:

Modeled emission rates for precursors for the western U.S. region (tons per year) equals critical air quality threshold multiplied by (modeled emission rate from the western U.S. low-level release (tons per year)) divided by (modeled air quality impact from the western U.S. low-level release)

Therefore, for Pinto Valley Mine, the ozone modeled emission rates for precursors value based on a 1.0 part per billion increase threshed is:

Modeled emission rates for precursors for the western U.S. region (tons per year) equals 1.0 part per billion multiplied by (500 tons per year) divided by (1.23 parts per billion) equals 407 tons per year 

In this case, the proposed action’s emission increase is less than the calculated nitrogen oxides to 8-hour ozone modeled emission rates for precursors threshold, such that ozone impacts from Pinto Valley Mine would be expected to be less than the critical air quality threshold of 1 part per billion. Therefore, Pinto Valley Mine’s impact on ozone would be less than significant.

 

5.3 Cumulative Modeling 

Modeling results for the cumulative modeling scenario were evaluated for the proposed action by comparing the modeled results with the National Ambient Air Quality Standards for the applicable air quality standard. For each pollutant, the maximum ambient impacts are added to the maximum background concentration. The conservative modeling results show the potential for National Ambient Air Quality Standards exceedances for the maximum activity level of 2024 for 1-hour nitrogen dioxide and annual PM2.5. The results are summarized in table 9. For the two cases showing a potential National Ambient Air Quality Standards exceedance and for the 24-hour PM10 and PM2.5, the sections below present the primary reason for the highest concentrations along with a figure showing the location and spatial extent of the areas of concern.

 

Table 9. Maximum cumulative air quality impacts for the proposed action compared to the applicable National Ambient Air Quality Standards, maximum emission year 2024 

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1  Based on quarterly values of Quarter 1 46.5, Quarter 2 70.4, Quarter 3 49.5, and Quarter 4 65.7 µg/m3. 

2 Based on quarterly values of Quarter 9.1, Quarter 10.0, Quarter 10.5, and Quarter 8.0 µg/m3.

Note: Bold text identifies modeled exceedances of National Ambient Air Quality Standards. 

µg/m3 = micrograms per cubic meter

 

1-hour nitrogen dioxide: The maximum 1-hour nitrogen dioxide concentration is above the National Ambient Air Quality Standards by 9 percent. The exceedance is nearly the same as for Pinto Valley Mine only. The main cause is the nitrogen oxides emissions from the onsite haul trucks, as they account for approximately half of the maximum concentration. Other mobile equipment such as the watering trucks, dozers, hydraulic shovels, and articulator trucks also contribute. Figure 6 shows the spatial extent of two areas near the Pinto Valley Mine that could potentially have an exceedance of the 1-hour nitrogen dioxide National Ambient Air Quality Standards. The highest concentration is found along National Forest System Road 287 running through the mine near Tailings Storage Facility No. 3. The other, more distant location north of Tailings Storage Facility No. 4 is near the boundary of the unpatented claim, at a location of higher terrain, where nitrogen oxides emissions associated with operations at Tailings Storage Facility No. 4, the Main Dump, and the Open Pit all contribute to this potential 1-hour National Ambient Air Quality Standards exceedance (166 plus 23 equals 189 micrograms per cubic meter). 

 

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Figure 6. Receptors with the maximum cumulative 1-hour NO2 concentrations modeled to potentially exceed National Ambient Air Quality Standards, maximum emission year 2024

 

1-hour sulfur dioxide: A number of areas to the north and east of Pinto Valley Mine show a potential violation of the 1-hour sulfur dioxide standard. However, in all cases the 1-hour exceedance is more than 98.5 percent attributable to emissions from the Miami smelter.11 Under the U.S. Environmental Protection Agency’s Prevention of Significant Deterioration Program, it has established an interim 1-hour sulfur dioxide significant impact level (U.S. Environmental Protection Agency 2010). The significant impact level is a useful screening technique that can be used to determine whether the predicted ambient impacts as a result of the proposed action emissions increase will be considered to cause or contribute to a modeled violation of the 1-hour sulfur dioxide National Ambient Air Quality Standard. To demonstrate that Pinto Valley Mine does not exceed the 1-hour sulfur dioxide significant impact level of 3 parts per billion (7.8 micrograms per cubic meter), we identified all hours in the cumulative modeling

11 The main cause for the high sulfur dioxide concentration is the emissions from the Freeport-McMoRan smelter building and the 1,000-foot tailstack. The Arizona Department of Environmental Quality’s sulfur dioxide State Implementation Plan modeling demonstrated compliance with the 1-hour sulfur dioxide National Ambient Air Quality Standard with these same emissions; this is likely due to the Arizona Department of Environmental Quality’s use of the 30.5-meter Miami onsite meteorological tower data. Wind speeds at these heights are generally greater than at meteorological monitors at lower heights, which results in lower maximum concentrations. AERMOD only allows the use of a single meteorological station and the emphasis in this assessment was on the meteorology associated with the Pinto Valley Mine onsite operations. 

(including background) during which the 1-hour sulfur dioxide concentration exceeded the 1-hour sulfur dioxide National Ambient Air Quality Standard of 196 micrograms per cubic meter while retaining information on which sources were from Pinto Valley Mine and which were from other sulfur dioxide emission sources. We then determined the concentration contribution from Pinto Valley Mine sources for all hours and all receptors that exceeded the 1-hour sulfur dioxide National Ambient Air Quality Standard. In no case was the contribution from Pinto Valley Mine greater than 7.8 micrograms per cubic meter. As shown on figure 7, the maximum contribution from Pinto Valley Mine from any hour during a modeled exceedance of the 1-hour sulfur dioxide National Ambient Air Quality Standard was 2.46 micrograms per cubic meter. Therefore, the Pinto Valley Mine contribution would be less than significant and can be considered not to cause or contribute to a violation of the 1-hour sulfur dioxide National Ambient Air Quality Standard. 

 

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Figure 7. Locations and concentrations for the three highest Pinto Valley Mine contribution receptors when the total concentration exceeded the 1-hour SO2 National Ambient Air Quality Standards for the cumulative modeling analysis (includes background concentrations), Maximum Emission Year 2024

24-hour PM10: The maximum high second high 24-hour cumulative PM10 modeling results are shown on figure 8. The figure shows receptor locations that exceed 110 micrograms per cubic meter (all receptors are below the National Ambient Air Quality Standards). The highest concentrations are seen near Tailings Storage Facility No. 3 and are a result of the emissions associated with vehicle movements and the

activity at Tailings Storage Facility 3. None of the high concentration locations are where the general public might be reasonably anticipated to be found over a 24-hour period.  

 

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Figure 8. Receptors (green circles) with the high second high cumulative 24-hour PM10 concentrations (with background concentrations) greater than 110 micrograms per cubic meter, maximum emission year 2024

 

24-hour PM2.5: The high eighth high 24-hour cumulative PM2.5 modeling results for receptors greater than 25 micrograms per cubic meter are shown on figure 9; only a few exceed 30 micrograms per cubic meter and no receptors exceed the National Ambient Air Quality Standards. The locations for these potential exceedances are along the Forest Service roadways, with the highest found nearest the primary crusher. None of the exceedance locations are where the general public might be reasonably anticipated to be found over a 24-hour period. 

 

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Figure 9. Receptors (green circles) with the high eighth high cumulative 24-hour PM2.5 concentrations (with background concentrations) greater than 25 micrograms per cubic meter, maximum emission year 2024

 

Annual PM2.5: The maximum annual PM2.5 modeling results are shown on figure 10. The locations for potential exceedances of the National Ambient Air Quality Standards are confined to six receptors along National Forest System Road 287 just to the east of Tailings Storage Facility No. 3. This is about half due to the background and non-Pinto Valley Mine sources, with the other half due to concentrate transport, other vehicle movements along the roadway, and operations at the nearby Tailings Storage Facility No. 3. None of the exceedance locations are where the general public might be reasonably anticipated to be found for any long-term period.

 

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Figure 10. Receptors (green circles) with the maximum cumulative annual PM2.5 concentrations (with background concentrations) modeled to potentially exceed National Ambient Air Quality Standards, maximum emission year 2024

 

5.3.1 Deposition Impact Assessment 

A level-2 deposition analysis was performed using AERMOD to evaluate the deposition for gas-phase pollutants (sulfur dioxide and nitrogen oxides deposition for class I areas within 50 kilometers of Pinto Valley Mine). The deposition modeling is a non-default option in AERMOD, which required additional specifications defining seasonal categories of land use for each 10-degree sector. We used recommendations from the Federal Land Managers’ Interagency Guidance for Near Field Deposition Modeling (Forest Service et al. 2014) for the deposition velocities for nitrogen dioxide (0.05 meter per second) and sulfur dioxide (0.005 meter per second). Similarly, we used the Federal Land Managers’ recommended other deposition parameters for diffusivity in air, diffusivity in water, cuticular resistance, and henry’s law coefficient for sulfur dioxide and nitric acid for the nitrogen and sulfur deposition modeling. A receptor grid using 100-meter spacing was used within the class I areas for sulfur dioxide. However, because we have many more sources of nitrogen oxides emissions relative to the sulfur deposition, analysis model run times for the nitrogen deposition analysis are excessive. Class I areas were initially modeled with a coarse 500-meter grid spacing. The results from that modeling showed that the highest deposition occurred at the class I boundary closest to Pinto Valley Mine, so more refined grid-spacing was not needed to identify the maximum deposition. 

 

The potential acid deposition impacts are reported as total deposition of total sulfur (as sulfur dioxide) and total nitrogen (as nitric acid) in kilograms per hectare per year. The results are compared against the Federal Land Managers’ recommended nitrogen and sulfur deposition analysis thresholds of 0.010 kilogram per hectare per year and 0.005 kilogram per hectare per year, respectively (Forest Service et al. 2011). The deposition analysis threshold is the additional amount of nitrogen or sulfur deposition within the class I area, below which impacts from the proposed action are considered negligible. 

The modeling results are discussed as the change between the proposed action and no-action alternative. For sulfur dioxide, the largest increase in sulfur deposition in any class I area was 0.00013 kilogram per hectare per year, well below the threshold value of 0.005 kilogram per hectare per year. The largest decrease in sulfur deposition as a result of the proposed action was 0.0060 kilogram per hectare per year. For nitrogen, the largest increase in any class I area was 0.028 kilogram per hectare per year, which exceeds the deposition analysis threshold value of 0.010 kilogram per hectare per year. Only the Superstition Wilderness area had deposition values exceeding the deposition analysis threshold, with about half of the area of the Superstition Wilderness above the deposition analysis threshold. A more refined level-3 analysis (Forest Service et al. 2014), which more accurately accounts for the conversion of nitrogen oxide to various nitrogen species using the CALPUFF modeling system, may demonstrate nitrogen deposition below the deposition analysis threshold. However, this effort is beyond the current scope of the project. 

 

6.0 Visibility Impact Assessment

 

The Federal Land Managers’ Air Quality Related Values Work Group Phase I Report (Forest Service et al. 2010) has developed a screening methodology to evaluate the potential for plume visibility impacts for class I areas more than 50 kilometers from the source. This screening approach has been applied in this analysis to determine if emissions from the project would cause or contribute to visibility impairment of class I areas up to 50 kilometers from the Pinto Valley Mine site. The approach is based on a review of the annual emission source strength and distance from a class I area. In general, a source will not cause or contribute to visibility impairments of class I areas if the source is more than 50 kilometers from any class I area and if the source emits fewer than 500 tons per year of nitrogen oxides or sulfur dioxide (or combined nitrogen oxides and sulfur dioxide). Under the proposed action, the increase in nitrogen oxides and sulfur dioxide emissions would be fewer than 500 tons per year. 

 

6.1 VISCREEN Modeling

 

For class I areas within 50 kilometers of Pinto Valley Mine, the U.S. Environmental Protection Agency’s plume visual impact screening model (VISCREEN) (U.S. Environmental Protection Agency 1992a) was used to assess the potential for observers to see visible plumes. Two wilderness areas and one national monument were identified within 50 kilometers of the Pinto Valley Mine site, as shown in table 10. The visual range is the average annual natural visibility, which represents the pristine atmospheric state not affected by human activities. 

Table 10. Class I areas and sensitive class II areas within 50 kilometers of the Pinto Valley Mine

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Source for Superstition Wilderness and Sierra Ancha Wilderness: Forest Service Class I Wilderness Areas List (Forest Service n.d.)

 

Potential visibility impacts within the class I areas and sensitive class II areas were evaluated using the single-source VISCREEN model, in accordance with the procedures discussed in the U.S. Environmental Protection Agency’s “Workbook for Plume Visual Impact Screening and Analysis (Revised)” (U.S. Environmental Protection Agency 1992a). A level-1 assessment was performed for the project, with the visibility effects assessed for all three class I or sensitive class II areas. 

 

VISCREEN is designed to assess the impact from a single-source location, which makes application to Pinto Valley Mine somewhat problematic. However, most of the proposed action changes in emissions are associated with the movement of the haul truck traffic. To evaluate the potential visibility impacts using VISCREEN, all of the fugitive and exhaust routine emissions were considered where they are most concentrated within the Open Pit area. These routine emissions include grading, bulldozing, drilling, haul truck activity, service truck activity, and loading of waste rock. However, under both the no-action alternative and proposed action, the routine operations of grading, bulldozing, service trucks, and loading of waste rock will continue. The only additional emissions increase under the proposed action relative to the no-action alternative is for drilling and an increase in the truck hauling of waste rock. As discussed in section 4.1, an increase in truck haul miles of 677,791 would occur under the proposed action relative to the no-action alternative. Of this amount, about 35 percent of the activity would occur within the Open Pit (Pinto Valley Mine Corp. 2019). 

 

Emissions from blasting operations were evaluated separately, as all other activity is at a minimum. The blasting visibility assessment was limited to the hours during which blasting typically occurs, between 11:00 a.m. and 1:00 p.m. local time, which limits the types of meteorological conditions possible. 

 

VISCREEN evaluates the plume visual effects inside class I or class II areas for both visual contrast and human perceptibility against the sky background and terrain background for different sun angles. The model accounts for spatial geometry and sun angles that affect the visibility of a plume. We used VISCREEN in both level-1 and level-2 mode. Level-2 is a refinement in determining the 1 percent frequency threshold with which a plume is visible based on an assessment of the actual number of meteorological conditions (wind speed, direction, and stability), plus travel time that result in a threshold criteria exceedance. For this assessment, the level-1 screening was first applied, followed by level-2 screening, as described below. 

 

Level-1 screening assumes worst-case meteorology including extremely stable atmospheric conditions and low wind speeds (1 meter per second) and worst-case scenarios for the direction of emissions traveling to the class I and class II areas. Inputs for the visibility assessment included: 

. The region’s background visual range and background ozone concentration

 

. Maximum hourly emission rates from the proposed action of: 

 

o Particulate matter (PM10)

 

o Nitrogen oxides

 

o Soot or elemental carbon

 

o Primary sulfate 

 

 

 

Table 11 shows the highest emission rates within the Open Pit for drilling, for increased truck haul activity, and during blasting events. No new primary sulfate emissions would occur at Pinto Valley Mine. 

Table 11. Maximum emission rates during routine events at the Open Pit and during blasting 

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VISCREEN uses two threshold criteria to screen for potential impacts: 

. Delta-E (L*A*B*) values greater than 2.0; and 

 

. Plume contrast values of absolute magnitude greater than 0.05 

 

Delta-E (L*A*B*) is a plume perceptibility measure that is a combined parameter of brightness, hue, and saturation. The plume contrast is a criterion of the perceptibility of green light. 

 

Potential visibility impacts, or the maximum degree of plume visibility, from the proposed action for routine operations and blasting activities were evaluated against the Delta-E criterion of 2.0 and the contrast criterion of 0.05.12 The VISCREEN level-1 screening results show exceedances of Delta-E and plume contrast criteria within all three class I and sensitive class II areas, as shown in table 12 for blasting activities. 

12 All VISCREEN modeling for blasting was limited to the hours between 11 a.m. and 1 p.m. 

 

Table 12. VISCREEN level-1 screening modeling results of plume visibility inside class I and sensitive class II areas from blasting activities 

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Note: Bold text identifies an exceedance of the criterion. 

 

Consequently, a level-2 screening analysis was performed for blasting activities to determine if visibility impacts would occur for 1-percentile meteorological conditions. This also included an adjustment for the lower elevation for blasting relative to the class I area per guidance of applying VISCREEN (U.S. Environmental Protection Agency 1992a). As shown in table 13, one exceedance for terrain 1 (a viewing

angle in the direction of the sun for a dark terrain object) for Delta-E within Superstition Wilderness is indicated during blasting activities for the level-2 screening analysis. 

 

Table 13. VISCREEN level-2 screening modeling results of plume visibility inside class I and sensitive class II areas from blasting activities 

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Note: Bold text identifies an exceedance of the criterion. 

 

Table 14 shows the level-1 screening results for drilling and hauling activities for all three class I or sensitive class II areas. The VISCREEN level-1 screening results show exceedances of Delta-E and plume contrast criteria within all three class I and sensitive class II areas. 

Table 14. VISCREEN level-1 screening modeling results of plume visibility inside class I and sensitive class II areas from drilling and hauling activities 

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Note: Bold text identifies an exceedance of the criterion. 

 

Consequently, a level-2 screening analysis was performed for drilling and hauling activities to determine if visibility impacts would occur for 1-percentile meteorological conditions. This also included an adjustment for the lower elevation for blasting relative to the class I area per guidance of applying VISCREEN (U.S. Environmental Protection Agency 1992a). As shown in table 15, one exceedance for terrain 1 (a viewing angle in the direction of the sun for a dark terrain object) for Delta-E within Superstition Wilderness is indicated during drilling and hauling activities for the level-2 screening analysis.

As described below in section 6.2, the highest difference in color contrast value from the less conservative Plume Visibility Model (PLUVUE) II modeling (5.397) is lower than the highest difference in color contrast value found in the level-2 VISCREEN modeling (7.44) for blasting activities. This is a 27.5 percent reduction in the highest difference in color contrast value for blasting activities; it is reasonable to assume that a similar reduction would be found for the highest difference in color contrast value via PLUVUE II modeling for drilling and hauling activities. Therefore, we would expect that the PLUVUE II modeling for drilling and hauling activities would have the highest difference in color contrast value of approximately 1.55, which would be below the difference in color contrast threshold criterion.

 

Table 15. VISCREEN level-2 screening modeling results of plume visibility inside class I and sensitive class II areas from drilling and hauling activities 

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Note: Bold text identifies an exceedance of the criterion. 

 

6.2 PLUVUE II Modeling

 

Due to the difference in color contrast exceedance modeled within Superstition Wilderness for the level-2 VISCREEN analysis for blasting activities, a level-3 near-field refined analysis using PLUVUE II (U.S. Environmental Protection Agency 1992b, 1996) was used to estimate potential plume blight in Superstition Wilderness as a result of blasting activities. A level-3 analysis is considered to be a comprehensive analysis of the magnitude and frequency of occurrence of plume visual impacts as observed at class I or sensitive class II area vistas. PLUVUE II is a straight-line, simple terrain, Gaussian plume model designed to calculate the visual impairment from pollutants of a single point or area

source. PLUVUE II uses the actual source location, receptor locations, meteorological conditions, and time of day to determine the geometries of the sun, plume, and observer for the optical calculations.

PLUVUE II was run in observer mode to evaluate the view for an observer at the nearest point to the project site within the Superstition Wilderness class I area. Each modeling scenario is composed of emissions from blasting activities. Plume trajectories were binned into representative directions that passed through the Superstition Wilderness with 10-degree spacing from 70 degrees to 140 degrees. In PLUVUE II, the user must specify the downwind locations along these trajectories where the chemical transformation and visibility calculations are made. As per the PLUVUE guidance, the first four downwind distance were set to 1, 2.5, 5, and 8.4 kilometers (the leading edge of the boundary of Superstition Wilderness) in order to provide an accurate prediction of the oxidation of nitrogen oxides to nitrogen dioxide. Beyond 8.4 kilometers, an evaluation point was placed every 5 kilometers until the class I area was fully spanned. Although PLUVUE makes the calculation at each evaluation point, only points within the class I area were considered in the impact analysis. 

Elevated terrain can block and channel airflow, especially during stable conditions, and it can also increase mechanical mixing and enhance diffusion. To account for this, the stability class was lowered by one category (such as from D to C), as the observer is at least 500 meters above the source. Also, views with a plume offset angle of less than 11.25 degrees were eliminated. The PLUVUE II model requires background visual range and meteorological conditions as inputs. The modeling uses the same background visual ranges and meteorological conditions as used in the VISCREEN level-2 analysis. Only the hours during which blasting activities might occur were evaluated: 11 a.m., 12 p.m., and 1 p.m.

Plume blight is evaluated using absolute contrast and the difference in color contrast. Absolute contrast is the contrast parameter that accounts for the relative difference in intensity between a viewed object and its background. In PLUVUE II, contrast calculations are done at 39 wavelengths. Difference in color contrast is a color contrast parameter that is calculated for the entire visible spectrum and indicates how different the brightness and color of the plume and background are relative to each other. Difference in color contrast is probably the best single indicator of the perceptibility of a plume to both its contrast and its color with respect to a viewing background. A difference in color contrast of 1.0 represents a condition where the viewer is actively looking for a sharp-edged plume under ideal viewing conditions with a uniform viewing background. Under more diffuse plume conditions, a plume with a difference in color contrast of about 2 would be perceivable to many people. Therefore, a difference in color contrast threshold of 2 was used in this analysis and is the same threshold used in the level-2 VISCREEN analysis (U.S. Environmental Protection Agency 1992a).

The model was run for each hour of blasting activities (11 a.m., 12 p.m., and 1 p.m.) that the wind blew toward the Superstition Wilderness area to determine if the difference in color contrast was exceeded in that hour. Dates associated with equinoxes (March 20 and September 20) and solstices (June 20 and December 20) were chosen to provide a full range of solar geometry inputs. The model considered view with sky, white, gray, and black backgrounds. A total of 96 runs were completed for all combinations of wind direction, time of year, and downwind distances. The results were then reviewed to determine which hours the threshold was exceeded for Superstition Wilderness, as shown in table 16. Results for wind directions of 70 degrees, 80 degrees, 90 degrees, and 100 degrees are presented because exceedances of the difference in color contrast threshold only occur for those wind directions.

As shown in table 16, with a 70-degree wind direction, exceedances are indicated for difference in color contrast for all modeled hours and calendar dates associated with blasting activities against a black background. However, no exceedances are shown for any other modeling scenario with a 70-degree wind direction. For an 80-degree wind direction, exceedances are indicated for difference in color contrast for all modeled hours and calendar dates associated with blasting activities against all

backgrounds. Difference in color contrast values in general are highest associated with this wind direction and the highest difference in color contrast value from all PLUVUE II modeling runs is shown on September 20 at 1 p.m. against a black background (5.397). For a 90-degree wind direction, exceedances are indicated for difference in color contrast for all modeled hours and calendar dates associated with blasting activities against a black background, which is very similar to the 70-degree wind direction. However, no exceedances are shown for any other modeling scenario with a 90-degree wind direction. For a 100-degree wind direction, exceedances are indicated for difference in color contrast for the modeled hours of 12 p.m. and 1 p.m. for all modeled calendar dates associated with blasting activities against a black background. However, no exceedances are shown for any other modeling scenario with a 100-degree wind direction.

In all cases, the highest difference in color contrast values are for black backgrounds, which indicates that the plumes will be lighter in color. Note that the highest difference in color contrast value from the less conservative PLUVUE II modeling (5.397) is lower than the highest difference in color contrast value found in the level-2 VISCREEN modeling (7.44). While the potential for a visibility impairment is possible based on this visibility analysis, the frequency in which the wind direction is between 70 and 100 degrees and between the hours of 11 a.m. and 1 p.m. is less than 15 percent based on the 2 years of Pinto Valley Mine meteorological measurements. Therefore, the probability of this event occurring is relatively low. Additionally, the plume will only be visible when viewed against a black background with the maximum difference in color contrast for other backgrounds of 2.65. For these other backgrounds, the maximum difference in color contrast of 2.65 corresponds to about 60 percent of observers able to detect the color difference of a plume against a background (Malm et al. 1980). 

 

Table 16. PLUVUE II modeling results of plume visibility inside the Superstition Wilderness area from blasting activities 

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Note: Bold and underline text identifies an exceedance of the criterion. 

 

7.0 Conclusions 

 

Air quality dispersion modeling (AERMOD) for the no-action alternative, proposed action, and cumulative analysis was performed for the Pinto Valley Mine. The air quality modeling examined the impacts from the criteria pollutants of sulfur dioxide, carbon monoxide, nitrogen dioxide, PM2.5, and PM10. An ozone impact assessment was performed using the modeling emissions rates for precursor analysis. In addition, air quality–related values for class I areas and sensitive class II areas were evaluated for potential nitrogen and sulfur deposition along with a plume visibility impact assessment. 

The air quality modeling analysis for the criteria pollutants showed the following:

1-hour nitrogen dioxide maximum potential impact was only 9 percent over 1-hour National Ambient Air Quality Standards but occurred under the no-action alternative, proposed action, and cumulative assessment. The location of this impact was limited to the National Forest System road near Tailings Storage Facility No. 3. 

Annual PM2.5 maximum potential impact exceeded the National Ambient Air Quality Standards for the cumulative analysis. The locations for potential exceedances of the National Ambient Air Quality Standards are confined to six receptors along National Forest System Road 287 just to the east of Tailings Storage Facility No. 3. It is highly unlikely the general public would be found at these locations for extended durations. 

No air quality impacts were predicted for the 24-hour PM10, 24-hour PM2.5, 1-hour carbon monoxide, 8-hour carbon monoxide, 8-hour ozone, or annual nitrogen dioxide. Sulfur dioxide impact contributions from Pinto Valley Mine were shown to be less than significant. 

Deposition and Visibility 

Sulfur deposition was less than significant for all class I and sensitive class II wilderness areas. Nitrogen deposition showed the potential for adverse impacts in about half of the Superstition Wilderness area. More refined modeling or an assessment of the sensitivity of the affected area would further determine the intensity and scale of impacts.  

Potential impacts for seeing a perceptible plume from Pinto Valley Mine from the class I and sensitive class II areas were evaluated during routine operations and for blasting events. The modeling assessment for routine operations and blasting used a level-1 and level-2 screening assessment using the VISCREEN model. A level-3 assessment was done for the blasting using the plume visibility model PLUVUE-II. 

In general, potential visibility impacts were limited to the Superstition Wilderness area and included the following.

Routine Operations: Visibility impacts and visible plumes could occur within Superstition Wilderness into the direction of the sun against a dark terrain background. However, a more refined level-3 analysis would likely show that this impact is less than perceptible. 

Blasting: The level-3 analysis showed a perceptible plume was only possible with a wind direction from 70–100 degrees during a blasting event and that this wind direction only occurs 15 percent of the time.

 

8.0 References 

 

Arizona Department of Environmental Quality. 2015. “Air Dispersion Modeling Guidelines for Arizona Air Quality Permits.” Prepared by Air Quality Permit Section, Air Quality Division, December. 

Arizona Department of Environmental Quality. 2017a. Personal communication with Mr. Pervinder Tandon in December 2017.

Arizona Department of Environmental Quality. 2017b. Personal communication with Mr. Vivek Kapur in December 2017. 

ICF. 2018. “Pinto Valley Mine Expansion Project EIS – Air Quality Impact Assessment Protocol.” Draft Final, May 24, 2018. 

Malm, W. C, K. K. Leiker, and J. V. Molenar. 1980. Human perception of visual air quality. J. Air Pollution Control Association, 30 (2), 122–131.

Pinto Valley Mining Corporation. 2018. Emissions inventory information for project-related stationary and mobile sources. 

Pinto Valley Mining Corporation. 2019. Action Item Record #249 email exchange from Tim Ralston, Pinto Valley Mine Corp., May 2, 2019. 

Resolution Copper. 2018. Kami Ballard, Environmental & Permitting Advisor – Resolution Copper Mine, January 25, 2018.

U.S. Environmental Protection Agency. 1992a. “Workbook for Plume Visual Impact Screening and Analysis (Revised).” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, EPA-454/R-92-023. October. Available: https://www3.epa.gov/ttn/scram/userg/screen/WB4PlumeVisualOCR.pdf.

U.S. Environmental Protection Agency. 1992b. “User’s Manual for the Plume Visibility Model, PLUVUE-II (Revised).” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, EPA-454/B-92-008. October.

U.S. Environmental Protection Agency. 1996. “Addendum to the User’s Manual for the Plume Visibility Model, PLUVUE-II (Revised).” Final Report. June. 

U.S. Environmental Protection Agency. 2010. “Guidance Concerning the Implementation of the 1-hour SO2 NAAQS for the Prevention of Significant Deterioration Program.” Memorandum from Stephen Page, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina. August. 

U.S. Environmental Protection Agency. 2012a. “Nonattainment Areas for Criteria Pollutants (Green Book) Ozone 8-Hour Standard Non-attainment Areas (2008 Standard).” Available: https://www.epa.gov/green-book/green-book-gis-download. Accessed August 29, 2017.

U.S. Environmental Protection Agency. 2012b. “Haul Road Work Group Final Report Submission to EPA-OAQPS.” Attachment to Memorandum from Tyler Fox, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina.

U.S. Environmental Protection Agency. 2013. “AERSURFACE User’s Guide.” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (EPA-454/B-08-001).

U.S. Environmental Protection Agency. 2014. “Guidance for PM2.5 Permit Modeling.” Memorandum dated May 20, 2014, from Stephen Page, Director, Office of Air Quality Planning & Standards, page 55. Available: https://www3.epa.gov/scram001/guidance/guide/Guidance_for_PM25_Permit_Modeling.pdf. 

U.S. Environmental Protection Agency. 2016a. “User’s Guide for the AERMOD Terrain Preprocessor (AERMAP).” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (EPA-454/B-16-012).

U.S. Environmental Protection Agency. 2016b. “User’s Guide for the AERMOD Meteorological Preprocessor (AERMET).” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (EPA-454/B-16-010).

U.S. Environmental Protection Agency. 2016c. “User’s Guide for the AMS/EPA Regulatory Model—AERMOD.” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (EPA-454/B-16-0t11).

U.S. Environmental Protection Agency. 2016d. “Guidance on the Development of Modeled Emission Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for Ozone and PM2.5 under the PSD Permitting Program.” Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina (EPA-454/R-16-006). December 2, 2016. Available: https://www3.epa.gov/ttn/scram/guidance/guide/EPA454_R_16_006.pdf.

U.S. Environmental Protection Agency. 2017. “Nonattainment Areas for Criteria Pollutants (Green Book) SO2 Nonattainment Areas (2010 Standard).” Available: https://www.epa.gov/green-book/green-book-gis-download. Accessed August 29, 2017.

U.S. Environmental Protection Agency. 2018a. “Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program.” Memorandum from Peter Tsirigotis, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina. April 17, 2018. 

U.S. Environmental Protection Agency. 2018b. Air Emissions Factors and Quantification, AP-42: Compilation of Air Emissions Factors. Last updated August 1, 2018. Available: http://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emission-factors.

U.S. Forest Service. No date. Class I Wilderness Area List. Available: https://www.fs.fed.us/air/technical/class_1/alpha.php.

U.S. Forest Service. 2009. “Climate Change Considerations in Project Level NEPA Analysis.” January 13, 2009. Available: https://www.fs.fed.us/climatechange/documents/nepa-guidance.pdf.

U.S. Forest Service, National Park Service, and U.S. Fish and Wildlife Service. 2010. “Federal Land Managers’ Air Quality Related Values Work Group (FLAG) Phase I Report – Revised (2010).” Natural Resource Report NPS/NRSS/ARD/NRR—2010/232. October. 

U.S. Forest Service, National Park Service, and U.S. Fish and Wildlife Service. 2011. “Federal Land Managers’ Interagency Guidance for Nitrogen and Sulfur Deposition Analyses.” Natural Resource Report NPS/NRSS/ARD/NRR—2011/465. November. 

U.S. Forest Service, U.S. Fish and Wildlife Service, and National Park Service. 2014. Draft “Federal Land Managers’ Interagency Guidance for Near Field Deposition Modeling.” January. 

Western Region Air Partnership. 2006. “WRAP Fugitive Dust Handbook.” Prepared by Countess Environmental for the Western Governors Association. September. 

 

ATTACHMENT A: EMISSIONS INVENTORY

 

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Appendix C. Federally Listed and Forest Service Sensitive Species Screening Analysis

 

The tables below list species protected under the federal Endangered Species Act (table c-1) and the Forest Service sensitive species (table c-2) that have the potential to occur in the vicinity of the analysis area, but that will not be affected by project activities. The tables provide justifications for excluding these species from further evaluation.

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References

Arizona Department of Water Resources. 2009. Arizona Water Atlas, Volume 5, Central Highlands Planning Area. 368 p.

Arizona Game and Fish Department. 1997. Ripley Wild Buckwheat (Eriogonum ripleyi). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2000a. Cochise Sedge (Carex ultra). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2000b. Pima Indian Mallow (Abutilon parishii). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2001a. Mexican Gray Wolf, Lobo (Canis lupus baileyi). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 7 p.

Arizona Game and Fish Department. 2001b. Fish Creek Fleabane (Erigeron piscaticus). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 5 p.

Arizona Game and Fish Department. 2002a. Gila Chub (Gila intermedia). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 9 p.

Arizona Game and Fish Department. 2002b. Razorback Sucker (Xyrauchen texanus). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2002c. Northern Leopard Frog (Lithobates pipiens [Rana pipiens]). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2002d. Sonora Sucker, Gila Sucker (Catostomus insignis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 5 p.

Arizona Game and Fish Department. 2002e. Blumer’s Dock (Rumex orthoneurus). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2002f. Galiuro Sage (Salvia amissa). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2003a. Spotted Bat (Euderma maculatum). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 9 p.

Arizona Game and Fish Department. 2003b. Fossil Springsnail (Pyrgulopsis simplex). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2003c. Netwing Midge (Agathon arizonicus). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2003d. Parker’s Cylloepus Riffle Beetle (Cylloepus parkeri). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 5 p.

Arizona Game and Fish Department. 2003e. Hohokam Agave (Agave murpheyi). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 7 p.

Arizona Game and Fish Department. 2003f. Tonto Basin Agave (Agave delamateri). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2004a. Aravaipa Woodfern (Thelypteris puberula var. sonorensis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2004b. Chihuahuan Sedge (Carex chihuahuensis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2004c. Fish Creek Rockdaisy (Perityle saxicola). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2004d. Mt. Dellenbaugh Sandwort (Arenaria aberrans). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 3 p.

Arizona Game and Fish Department. 2004e. Toumey Groundsel (Senecio neomexicanus var. toumeyi). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2005a. Mexican Spotted Owl (Strix occidentalis lucida). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 12 p.

Arizona Game and Fish Department. 2005b. Arizona Phlox (Phlox amabilis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2005c. Eastwood Alum Root (Heuchera eastwoodiae). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2005d. Mapleleaf False Snapdragon (Mabrya acerifolia). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2006. Yuma Clapper Rail (Rallus longirostris yumanensis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 11 p.

Arizona Game and Fish Department. 2009. Western Barking Frog (Craugastor augusti cactorum). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 8 p.

Arizona Game and Fish Department. 2012a. Northern Mexican gartersnake, Mexican gartersnake, Northern Mexican garter snake (Thamnophis eques megalops). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 8 p.

Arizona Game and Fish Department. 2012b. Arizona Bugbane (Actaea arizonica). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 5 p.

Arizona Game and Fish Department. 2013. Northern Goshawk (Accipiter gentilis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 8 p.

Arizona Game and Fish Department. 2015a. Sonoran Desert Tortoise (Gopherus morafkai). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 10 p.

Arizona Game and Fish Department. 2015b. Headwater Chub (Gila nigra). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 6 p.

Arizona Game and Fish Department. 2015c. Roundtail Chub (Gila robusta). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 7 p.

Arizona Game and Fish Department. 2015d. Verde Breadroot (Pediomelum verdiensis). [Unpublished abstract]. Arizona Game and Fish Department, Heritage Data Management System, Phoenix, Arizona. 4 p.

Arizona Game and Fish Department. 2018a. Element Occurrence Data, Point Observation Database, and SGCN models. [Unpublished GIS shapefiles and supporting documents]. Arizona Game and Fish Department, Heritage Database Management System, Phoenix, Arizona.

Arizona Game and Fish Department. 2018b. Point Observations, Element Occurrences, and Predicted Range, AGFD Heritage Database Management System. https://www.azgfd.com/Wildlife/HeritageFund/. Accessed February 8, 2018.

Corman, T. E. and C. Wise-Gervais. 2005. Arizona Breeding Bird Atlas. UNM Press. 636 p. 

eBird. 2017. eBird: An online database of bird distribution and abundance. http://www.ebird.org. Accessed December 12 and 27, 2017. 

Jerla, C. 2010. A Class III Cultural Resources Survey of 5,324.86 Acres in Pinto Valley, Gila County, Arizona. Tucson, AZ: WestLand Resources, Inc.

Muñoz-Quesada, F. J. and R. W. Holzenthal. 2008. Revision of the Nearctic species of the caddisfly genus Wormaldia McLachlan (Trichoptera: Philopotamidae). Zootaxa 1838: 1-75.

SEINet. 2017. SEINet - Arizona Chapter. http://swbiodiversity.org/seinet/. Accessed December 29, 2017.

SEINet. 2018. SEINet - Arizona Chapter. http://swbiodiversity.org/seinet/. Accessed January 3, 2018, January 23–24, 2018, and March 7, 2018.

U.S. Fish and Wildlife Service. 2002. Colorado Pikeminnow (Ptychocheilus lucius) Recovery Goals. Denver, CO: U.S. Fish and Wildlife Service, Mountain-Prairie Region.

U.S. Fish and Wildlife Service. 2004. Endangered and Threatened Wildlife and Plants; Final Designation of Critical Habitat for the Mexican Spotted Owl. Federal Register Vol. 69 No. 168, August 31, 2004.

U.S. Fish and Wildlife Service. 2005. Endangered and Threatened Wildlife and Plants; Listing Gila Chub as Endangered with Critical Habitat. Federal Register Vol. 70 No. 211, November 2, 2005.

U.S. Fish and Wildlife Service. 2006. California least tern (Sternula antillarum browni) 5-Year Review Summary and Evaluation. Carlsbad, CA: U.S. Fish and Wildlife Service, Carlsbad Fish and Wildlife Office.

U.S. Fish and Wildlife Service. 2009a. California Least Tern (Sterna antillarum browni) Species Description. https://www.fws.gov/southwest/es/arizona/.

U.S. Fish and Wildlife Service. 2009b. Colorado Pikeminnow (Ptychocheilus lucius) Species Description. https://www.fws.gov/southwest/es/arizona/.

U.S. Fish and Wildlife Service. 2009c. Razorback Sucker (Xyrauchen texanus) Species Description. https://www.fws.gov/southwest/es/arizona/.

U.S. Fish and Wildlife Service. 2010. Endangered and Threatened Wildlife and Plants; 90-Day Finding on a Petition to List a Stonefly (Isoperla jewetti) and a Mayfly (Fallceon eatoni) as Threatened or Endangered with Critical Habitat. Federal Register Vol. 75 No. 65, April 6, 2010.

U.S. Fish and Wildlife Service. 2013. Endangered and Threatened Wildlife and Plants; Designation of Critical Habitat for the Northern Mexican Gartersnake and Narrow-Headed Gartersnake; Proposed Rule. Federal Register Vol. 78 No. 132, July 10, 2013.

U.S. Fish and Wildlife Service. 2015a. Endangered and Threatened Wildlife and Plants; Revision to the Regulations for the Nonessential Experimental Population of the Mexican Wolf. Federal Register Vol. 80 No. 11, January 16, 2015.

U.S. Fish and Wildlife Service. 2015b. Species Status Assessment for the Sonoran Desert Tortoise. Albuquerque, NM: U.S. Fish and Wildlife Service, Southwest Region.

U.S. Fish and Wildlife Service. 2015c. Candidate Conservation Agreement for the Sonoran Desert Tortoise (Gopherus morafkai) in Arizona Between the U.S. Fish and Wildlife Service and Cooperating Agencies comprising the Arizona Interagency Desert Tortoise Team. May 27, 2015.

U.S. Fish and Wildlife Service. 2015d. Endangered and Threatened Wildlife and Plants; Threatened Species Status for the Headwater Chub and a Distinct Population Segment of the Roundtail Chub. Federal Register Vol. 80 No. 194, October 7, 2015.

U.S. Fish and Wildlife Service. 2017. Mexican Wolf Recovery Program: Progress Report #19, Reporting Period: January 1 – December 31, 2016. 61 p.

U.S. Fish and Wildlife Service and U.S. Department of Agriculture, Forest Service (Forest Service). 1998. Arizona Bugbane (Cimicifuga arizonica) Conservation Agreement. U.S. Fish and Wildlife Service, Arizona Ecological Services Field Office and U.S. Department of Agriculture, Forest Service, Coconino National Forest, Kaibab National Forest, and Tonto National Forest.

WestLand Resources, Inc. (WestLand). 2016a. Cultural Resources Evaluation of AZPDES Outfall No. 005 and Powder Magazines at Pinto Valley Mine, Gila County, Arizona. [Unpublished report]. Pinto Valley Mining Corp. 16 p.

 

Appendix D. Pinto Valley Mine Hazardous and Nonhazardous Materials Inventory

 

This appendix provides an inventory and description of the hazardous and nonhazardous materials used and stored at the Pinto Valley Mine. The locations at the Pinto Valley Mine where these materials are stored and used are described in the table below and depicted on maps in appendix A, map 3-8 and map 3-9.

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References

Pinto Valley Mining Corporation. 2017a. Emergency Planning and Community Right-to-Know Act Section 311-312 Tier I and II Report.

Pinto Valley Mining Corporation. 2017b. Pinto Valley Mine Hazardous Materials Inventory – Pinto Valley Mine 2016.

Pinto Valley Mining Corporation. 2017c. Pinto Valley Mine Hazardous Materials and Nonhazardous Materials Inventory Worksheet Report.

 

 

Appendix E. Water Resources and Geochemistry Technical Report 

 

United States Department of Agriculture

U.S. Department of Agriculture logo

Pinto Valley Mine 

Environmental Impact Statement

Water Resources and Geochemistry Technical Report

 

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Forest Service Tonto National Forest MB-R3-12-08 December 2019

Forest Service Logo

 

 

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EXECUTIVE SUMMARY

AFFECTED ENVIRONMENT

The Pinto Valley Mine is situated within the Pinto Creek watershed (Hydrologic Unit Code 1506010307), which is part of the Upper Salt River drainage basin (HUC 15060103). The Pinto Creek watershed is drained by Pinto Creek, a north-flowing perennial and intermittent stream that is tributary to Roosevelt Lake. The cumulative effects analysis area for water resources and geochemistry (herein referred to as the hydrologic study area) encompasses approximately 200 square miles that includes the entire Pinto Creek watershed area, as well as a small portion of the Miami Wash and Middle Pinal Creek watersheds directly east of the Pinto Valley Mine. 

The Pinto Valley Mine is situated in a semi-arid region with an average annual precipitation ranging from 16.65 inches to 17.28 inches as recorded at the two Pinto Valley Mine meteorological stations for the period from 2006 through 2015. The estimated average open-pan evaporation rate for the mine area is 72.1 inches per year.

Surface Water Resources

Major tributaries to Pinto Creek include the West Fork of Pinto Creek, Horrell Creek, and Campaign Creek in the Lower Pinto Creek drainage area. Other tributaries in the vicinity of the Pinto Valley Mine and adjacent Carlota Mine include Gold Gulch, Miller Gulch, Eastwater Canyon, Powers Gulch, and Haunted Canyon. Pinto Creek is predominantly intermittent, although there are several perennial reaches where groundwater surfaces due to bedrock constrictions. Surface water flows in the Pinto Creek watershed are monitored by three U.S. Geological Survey gaging stations located along Pinto Creek. Pinto Creek near Miami, Arizona (also known as the Magma Weir) located downstream from Pinto Valley Mine has the longest period of record (October 1994 through present). The U.S. Department of Agriculture, Forest Service (Forest Service) has an instream flow right to use waters flowing in Pinto Creek for recreation and wildlife, including fish. An annual amount of 1,794.2 acre-feet per year is required at the Pinto Valley weir to maintain sufficient flows for riparian vegetation, wildlife, aquatic habitat maintenance, and maintenance of the reach’s recreation amenities (Forest Service 1991). 

A hydrologic modeling study of the Pinto Creek watershed was conducted to evaluate the surface flows and baseflow at the Magma Weir gaging station. The Australian Water Balance Model used Parameter-elevation Regressions on Independent Slopes Model precipitation and evapotranspiration as inputs for the watershed and was calibrated to flow conditions measured at Magma Weir for the 1996–2006 period. This calibration period was selected because mining was shut down, and there was limited pumping at Pinto Valley Mine and no pumping activity at the Carlota Mine. The calibrated model was then applied to simulate flows at the Magma Weir for the 2007 to 2016 period. Statistical analysis indicated a deterioration in model fit for the simulated periods (the predicted flows from the calibrated model no longer reflected the actual flows on the stream as they did during the calibration period). The period of the deterioration in model fit correlates with the initiation of high rates of pumping from the Peak Well System beginning in 2013 and continuing through 2016. Pumping rates during this period ranged from 2,000 to 4,000 gallons per minute (4.46 to 8.91 cubic feet per second). 

A seep and spring survey was conducted in May and June of 2018 to inventory springs and seeps in the region around the mine that may be affected by future mine-related groundwater pumping activities.

Results from the survey identified 78 seep and spring sites. Thirty-eight of the sites had reported wet or flowing conditions and the remainder were reported dry. 

A waters of the United States survey was conducted for the proposed expansion of the Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. A large portion of the study area is located within an area where surface water features and natural topography have been significantly manipulated in support of the storm water and seepage management system at the mine. Features identified within the non-discharge boundary are not considered jurisdictional. There were 14 ephemeral and intermittent drainages and 2 wetland areas identified within the discharging portion of the study area. Five of the drainage features identified as surface water features 1, 2, 3, 20, and 21 were considered jurisdictional waters. Features 1, 2, and 3 are downgradient extensions of drainage considered previously in Tailings Storage Facility No. 3 and are directly tributary to Pinto Creek. Feature 20 is also a direct tributary to Pinto Creek and shares a similar downgradient flowpath and distance to Roosevelt Lake as features 1, 2, and 3. Feature 21 is a portion of the mainstem of Pinto Creek. This reach of Pinto Creek is intermittent and flows for more than 3 months of the year. The five jurisdictional drainage features identified are not affected by proposed expansion of Tailings Storage Facility No. 3. 

The Pinto Valley Mine complex and the Pinto Creek watershed are in an area described as zone D or an undetermined risk area. These are areas where flood hazards are undetermined, but flooding is possible (Federal Emergency Management Agency 2017). At the Pinto Valley Mine site, the topography has been altered in support of the storm water and seepage management system. Most of the site is maintained as a non-discharging area where storm water runoff is contained within impoundments and water containment systems. These systems are designed and managed to contain the 100-year, 24-hour storm event. Discharge from the Pinto Valley Mine facilities occurs only at monitored discharge points. 

Groundwater Resources

Groundwater occurs both within the bedrock units that underlie the hydrologic study area and in alluvium that veneers bedrock in the valley bottom areas along the major drainage courses. The geology and structural conditions within the project area and across the hydrologic study area are complex. The geologic map and interpreted cross-sections constructed across the area identify 18 bedrock units and numerous faults. The movement and storage of groundwater in the bedrock units throughout the project area and hydrologic study area are generally controlled by the interconnection of fractures within the rock mass. The density (spacing), aperture, and interconnection of these fractures are highly variable within individual or between rock units. This variability is reflected in the results of aquifer testing conducted in the area around the Pinto Valley Mine and Carlota Mine that indicate that the bedrock complex is highly heterogeneous with respect to hydraulic conductivity.

The Gila conglomerate is the principal aquifer in the Globe-Miami Mining District, the Pinto Valley watershed, and for the Pinto Valley Mine. Pinto Valley Mine’s main water source is provided by extracting groundwater from the Peak Well System northwest of Tailings Storage Facility No. 4 on the east and west sides of Pinto Creek. The primary target for completion of these wells is the Gila conglomerate and Dacite unit. Available information from the Peak Well System indicates that Gila conglomerate and underlying Dacite unit are highly permeable and transmissivity in the well field area with yields of up to 1,100 gallons per minute is reported for individual wells (SRK Consulting, Inc. 2019a). 

Groundwater occurs in relatively thin alluvial deposits that veneer bedrock along stream channels and associated floodplain along sections of Pinto Creek and tributaries to Pinto Creek. Groundwater in alluvium is stored and transmitted through interconnected pores in the granular deposits. The thickness of the deposits is estimated to range from 0 to 10 feet where the stream channel is narrow and

controlled by crystalline bedrock to up to an estimated 80 to 100 feet where the channel widens at the confluence of major tributaries. The shallow alluvial groundwater system is assumed to be hydraulically connected to the deeper bedrock groundwater flow system, although the hydraulic connection to the underlying bedrock aquifer has not been specifically defined (SRK Consulting, Inc. 2019a).

Groundwater extraction for the Pinto Valley Mine occurs through pumping from selected production wells within the Peak Well System, and pumping of water collected in sumps excavated into the floor of the Open Pit. The Peak Well System extends northwest of Tailings Storage Facility No. 4 and includes production wells on the east and west sides of Pinto Creek, and two additional wells (Peak Wells 26 and 29) northwest of Tailings Storage Facility No. 3 located east of Pinto Creek. Pumping rates for the well field range from 2,000 to 4,000 gallons per minute and averaged 2,980 gallons per minute for the 2013–2018 period (SRK Consulting, Inc. 2019a). Groundwater inflows into the Pinto Valley Mine Open Pit historically ranged from 200 to 300 gallons per minute during mining. Two water supply wells (TW-2, BMW 32) associated with the Carlota Mine are adjacent to Pinto Creek west of the Pinto Valley Mine. The estimated total pumping rate for the two Carlota Mine water wells is 343 gallons per minute for the 2013–2017 period. The Carlota Mine has one open pit (Cactus-Carlota Open Pit) that extends below the regional groundwater table. The estimated average groundwater inflow into the Cactus-Carlota Open Pit is less than 50 gallons per minute.

A groundwater elevation contour map for the Pinto Creek watershed was developed based on measured water levels in available wells and the surface elevation of spring and stream sites with observed surface water recorded in June 2018. The groundwater elevation contours indicate that the groundwater flow generally mimics the topography with flow from higher to lower elevations and from the southern to northern perimeter of the watershed. The groundwater contours also indicate that the predominant flow pattern in the area adjacent to and north of the mine in the watershed is toward Pinto Creek and Campaign Creek. The groundwater contours indicate that the groundwater flow pattern is disrupted by the pit dewatering because all groundwater flow in the area surrounding the pit is toward the pit.

Average inflow and outflow from the groundwater system were estimated to establish a baseline, quasi-steady-state water balance for the hydrologic study area. Existing groundwater inflow components include precipitation recharge, artificial recharge (draindown from mine facilities), and surface and subsurface flow (stream recharge to the groundwater system). Groundwater outflow components include evapotranspiration from phreatophytes, evaporation from pit lakes, groundwater pumping from existing Pinto Valley Mine and Carlota Mine operations, discharge to streams and springs, and subsurface outflow to adjacent areas outside the hydrologic study area (deep groundwater leaving the system along the northern boundary of the Pinto Valley watershed). 

Water Quality

The Arizona Department of Environmental Quality has classified segments of Pinto Creek as impaired for copper, or for selenium and copper. The Gibson Mine has been the largest source of copper to Pinto Creek, and remediation projects completed at the mine since 2007 have reduced the dissolved copper concentrations by 85 percent. 

Three permits set compliance monitoring requirements for storm water discharge and groundwater quality at points of compliance at the Pinto Valley Mine. Baseline water quality for the mine was reviewed for the 2014–2016 period. Parameters that have exceeded permit requirements (once or more for the period) for surface water quality include dissolved selenium and total selenium. The parameters of concern at Pinto Valley Mine that were reported as exceeding the Aquifer Protection Permit requirements were total dissolved solids (alert level), sulfate (alert level), and gross alpha (aquifer

quality limit). Monitoring wells immediately downgradient from Tailings Storage Facility Nos. 1 and 2, Tailings Storage Facility No. 3, and Tailings Storage Facility No. 4 have high concentrations of total dissolved solids and sulfate that exceed the non-enforceable National secondary maximum contaminant levels.

Water Rights

An inventory of active surface water rights that occur within the region surrounding the Pinto Valley Mine was prepared for the project (WestLand Resources, Inc. 2018b). The surface water rights inventory identified 24 water rights that include 18 owned by the Tonto National Forest, 3 owned by Havens Ranch, and 3 owned by Cyprus Miami Mining Corp. All of the water rights except one (described below) were established as a source of water for stock or stock and wildlife). Water rights in Arizona are managed by the Arizona Department of Water Resources. 

The Tonto National Forest has an instream flow right to use waters flowing in Pinto Creek for recreation and wildlife, including fish, under permit No. 33-89109. The priority date for this water right is December 14, 1983. An annual amount of 1,794.2 acre-feet per year is required at the Pinto Valley weir to maintain sufficient flows for riparian vegetation, wildlife, aquatic habitat maintenance, and maintenance of the reach’s recreation amenities. Instream flow requirements vary by month as specified in the permit. The water rights section of this report compares actual flow volume at the Pinto Creek near Miami, AZ (09498502) gauge to the flow volume required to maintain sufficient flows in Pinto Creek per the instream flow right. In recent years the number of consecutive months recording zero flow has increased. There were no months recording zero flow in the period of record until 2014, and 26 months between January 2014 and December 2018.

Rock Geochemistry

Sampling and Testing. Mine rock and tailings have been collected and geochemically analyzed at the Pinto Valley Mine beginning in 1995 and continuing through 2016. Sampling has been conducted at several major facilities associated with the Pinto Valley Mine site, as well as some locations not specifically identified as discrete facilities. The results of the sampling and geochemical testing were used to define the rock geochemistry of the existing site conditions. 

Historical waste rock, tailings, and heap leach rock have been submitted for various geochemical bulk characterization tests. Standard geochemical analyses of mine rock and tailings have included acid-base accounting, meteoric water mobility procedure and synthetic precipitation leaching procedure, whole rock analysis, net acid-generating leachate, and kinetic humidity cell testing. 

Geochemical Characterization. Using acid-base accounting test results, mine materials may be classified as potentially acid generating or not potentially acid generating. Using the Arizona Best Available Demonstrated Control Technology criteria, 65 percent of waste rock is not potentially acid generating, 29 percent is likely to generate acid rock drainage, and 6 percent is uncertain. For tailings, 17 percent is not potentially acid generating, 69 percent is likely to generate acid rock drainage, and 14 percent is uncertain. Materials classified as likely to produce acid rock drainage are dominated by most tailings samples and quartz monzonite (Ruin granite), with some diabase. Quartz monzonite is the dominant waste rock material for the proposed action. Not potentially acid generating material samples are dominated by altered limestone and Gila conglomerate. These materials have abundant calcium carbonate and very little, or no, sulfide mineralization. 

Humidity cell testing work on historical Pinto Valley Mine rock indicates that quartz monzonite and diabase have a potential to produce acid rock drainage. Two samples of historical tailings have been characterized using humidity cell testing. Although neither sample produced acidic leachate during the test, acid-base accounting characterization of the samples and depletion rates of neutralization potential and acid potential indicate they are likely to ultimately produce acid rock drainage. 

Regardless of the results of acid-base accounting and humidity cell testing analyses, the discharge of acid rock drainage in the field is closely linked to the water balance of mine materials. Even if sulfide mineral oxidation reactions occur to produce the chemical ingredients of acid rock drainage, there must be sufficient water to dissolve and mobilize these constituents.

Existing Facilities. The water entrained in tailings storage facilities is a neutral pH solution with high concentrations of sulfate and other chemical constituents such as calcium and magnesium. The high pH inhibits any elevated concentrations of metals. Monitoring wells immediately downgradient from the tailings storage facilities reflect the high-sulfate water quality within the tailings storage facilities. 

The leach pile currently discharges acidic leachate as part of the engineered recovery of copper. The solutions did not form their acidic character through the natural weathering of the rock but from the addition of sulfuric acid. Material in the leach dump facilities is classified as likely to produce acid rock drainage.

Most of the leachate generated from the Leach Pile is recovered and processed for copper. However, the facility is unlined and a small percentage (less than 5 percent) of the current total pregnant leach solution is assumed to seep into groundwater directly beneath the footprint of the facility (SRK Consulting, Inc. 2019a). Most of this seepage to groundwater is captured within the drawdown cone associated with active pit dewatering that is known as the Active Containment Capture Zone (SRK Consulting, Inc. 2019a). The seepage from the leach pile contained within the capture zone discharges into the pit through fracture flow and as acid in the pit wall (SRK Consulting, Inc. 2019a). The seepage from leach pile that is outside this capture zone enters into the groundwater system and flows toward Pinto Creek (SRK Consulting, Inc. 2016f, 2019a). Water quality sampling and analyses from the groundwater monitoring well APP-7 downgradient and west of the facility in Gold Gulch APP-7 (between the Leach Piles and Pinto Creek) has not indicated the presence of low pH or elevated metal concentration (attachment C, Table C-4). SRK Consulting, Inc. (2016f) estimates that seepage that infiltrates and mixes with groundwater and flows toward Pinto Creek will have an estimated travel time ranging from greater than 100 years to 500 years based on the low permeability of the bedrock and distance to Pinto Creek. SRK Consulting, Inc. (2016f) also notes that “Facility-specific closure plans will address the post-closure mine water management and potential draindown towards Pinto Creek.”

Discharges of acid rock drainage or metal-bearing solutions from waste rock have not been detected in surface water or groundwater monitoring proximal to waste rock facilities. 

ENVIRONMENTAL CONSEQUENCES 

Key issues for detailed analysis include (1) potential impacts on Pinto Creek and other streams, seeps, and springs in the area resulting from groundwater pumping; (2) potential impacts on surface and groundwater quality resulting from construction, operations, reclamation, closure, and post-closure activities at mine facilities; and (3) impacts on surface water resources associated with mine disturbance and storm water management at the Pinto Valley Mine.

Methodology

Groundwater Modeling. A calibrated three-dimensional numerical groundwater flow model was developed for the environmental impact statement (EIS) to estimate effects on groundwater and surface water resources resulting from groundwater extraction and water management activities that would occur under the no-action alternative, proposed action, and cumulative scenarios (SRK Consulting, Inc. 2019a). The groundwater model domain encompasses the entire hydrologic study area (approximately 200 square miles). The model was calibrated to both a quasi-steady state and transient conditions. The quasi-steady state period extends from January 2011 through December 2012 and corresponds to a “care and maintenance period” for the Pinto Valley Mine. The transient period for model calibration was defined as the period extending from January 2013 through December 2018. The transient period corresponds to the period when full mine operations were resumed and continued without interruption. The calibrated numerical model was used to evaluate or estimate: (1) areal extent, magnitude, and timing of drawdown and recovery of groundwater levels through the mining and post-mining periods; (2) changes in baseflow to Pinto Creek; and (3) development of the post-mining pit lake including groundwater inflow and outflow rates through the pit, and final surface water elevations of the pit lakes.

Evaluation of Impacts on Groundwater Levels. The calibrated groundwater model was used to simulate the groundwater levels and change in groundwater levels and flow rates that have occurred (through 2018) and are projected to occur in the future as a result of mine development and groundwater extraction activities associated with the no-action alternative and proposed action mining scenarios. The projected changes in groundwater levels represent the difference between the model-simulated groundwater elevations and simulated steady-state baseline groundwater elevations that existed at the end of 2012 (prior to the reinitiating of mining and groundwater extraction activities at Pinto Valley Mine).

Evaluation of Impacts on Surface Water Resources. The impact assessment identifies and evaluates potential impacts on streams, springs, and seeps located within the maximum extent of the projected 5-foot groundwater drawdown contour. The 5-foot drawdown contour defines the area where the water table would be lowered by 5 feet or more at some point in time during the mining or post-mining period. Potential impacts on riparian vegetation resulting from drawdown and baseflow reductions are discussed in section 3.3.2, “Vegetation,” of the EIS. 

Predicted Drawdown and Baseflow Reduction (2013–2018). The groundwater model was used to simulate the changes in groundwater elevations and groundwater discharge rates that occurred after restarting active mining at the Pinto Valley Mine at the beginning of 2013 and the end of 2018. At the end of 2018, the simulated drawdown associated with the Pinto Valley Mine consists of two separate drawdown cones: one associated with the portion of the Peak Well System that extends north-northwest of Tailings Storage Facility No. 4, and the other that consists of drawdown area around the pit and adjacent areas located west of the pit. The model results predict that over the 2013–2018 period, the baseflow to Pinto Creek was substantially reduced from an initial rate of 1,070 gallons per minute (start of 2013) to 188 gallons per minute (end of 2018). This represents an 82 percent reduction in baseflow compared to the estimated average baseflow conditions at the Magma Weir at the end of 2012.

Pit Lake Geochemistry Modeling. A hydrochemical evaluation of pit lake water quality was performed for the proposed project (SRK Consulting, Inc. 2019b). In this evaluation, water quality in the pit was estimated from modeling that included the following inputs and reactions: (1) the quality and quantity of groundwater inflow; (2) chemical releases from oxidized wall rock; (3) aqueous geochemical reactions

in the pit lakes; (4) evaporation from the pit lake surfaces; (5) direct precipitation into the pit lakes; (6) runoff from pit walls; and (7) exchange of carbon dioxide between the pit lakes and the atmosphere. Details regarding pit lake modeling assumptions and methodology are provided in the pit lake geochemistry modeling report for the proposed project (SRK Consulting, Inc. 2019b). 

No-action Alternative

The no-action alternative is a continuation of existing authorized activities and permitted disturbances from mining activities until 2027. 

Groundwater Drawdown. The predicted changes in groundwater levels under the no-action alternative indicate that the drawdown area (defined as the area predicted to experience a reduction in groundwater levels of 5 feet or more at any point in time over the mining or post-mining period) would expand drawdown as compared to the 2018 conditions. The drawdown results also indicate the drawdown area is predicted to continue to expand during the post-mining period from 18.9 square miles at the end of mining to 36.2 square miles at 100 years post-closure.

Drawdown Impacts on Perennial Streams. Potential impacts on streams were evaluated by (1) using available baseline data to identify perennial stream reaches located within the drawdown area; and (2) model simulations of baseflow reduction at Pinto Creek at the Magma Weir gage. The drawdown is projected to encompass a total of 5.68 miles of perennial stream length that includes portions of Pinto Creek (4.62 miles), Miller Springs Gulch (0.81 mile), and an unnamed tributary to Pinto Creek (0.25 mile). The EIS analysis assumes that baseflow in the specific stream reaches identified above are largely controlled by discharge from the regional (bedrock) aquifer system. The predicted expansion of drawdown resulting from mine-induced drawdown could result in a reduction in baseflow in these stream reaches. A reduction in baseflows would likely affect the perennial stream reach (reduce the length of or eliminate the perennial stream reach affected by drawdown). These types of impacts would likely persist until pumping ceases and groundwater levels recover to pre-pumping conditions. 

The model results for the no-action alternative predict that impacts on baseflow occurred as a result of pumping from the Peak Well System during the 2013–2018 period. Continued pumping from the Peak Well System under the no-action alternative is predicted to sustain those impacts (and result in a 10 percent increase in baseflow reduction) through the end of mining. The model simulations predict that the baseflows would recover rapidly after pumping ceases, and fully recover within 10 years. The continuation of substantial reductions in baseflow over the mine life included under the no-action alternative is anticipated to increase the duration of the impacts on baseflow (that occurred between 2013–2018) an additional 8 years. A substantial long-term reduction in baseflow resulting from groundwater pumping would likely result in a measurable reduction in flow (or elimination of surface flow) during the low-flow periods, and a corresponding reduction in the length of perennial stream reaches that existed prior to being affected by groundwater pumping. As a consequence, segments of Pinto Creek that were previously characterized as perennial prior to 2013, and that may have been affected by pumping that occurred between 2013–2018 (and no longer sustain perennial flow conditions), would be expected to continue to be affected at a similar magnitude but for a longer duration (approximately 8 years). 

Recovery of groundwater discharge (baseflow) to Pinto Creek is predicted to vary over the closure and post-closure period (Figure 56, SRK Consulting, Inc. 2019a). During the first 10 years after groundwater pumping ceases, the baseflow is predicted to fully recover to approximately 1,070 gallons per minute (the pre-pumping 2013 flow rate) as a result of continued high infiltration from Tailings Storage Facility No. 4. From 10 years to 100 years post-closure, the baseflow is predicted to gradually decrease from

approximately 1,070 gallons per minute to approximately 430 gallons per minute due to combined effects of residual drawdown and progressively reduced infiltration from draindown of Tailings Storage Facility No. 4. The predicted baseflow rate of 430 gallons per minute at 100-years post mining represents an approximate 60 percent reduction in baseflow as compared to the simulated pre-pumping conditions (1,070 gallons per minute) at the start of 2013. After 100 years post-mining, the baseflow is predicted to eventually reach a steady-state condition of approximately 407 gallons per minute, which represents a 62 percent reduction in flow compared to conditions at the start of 2013. The predicted long-term reduction of baseflow in Pinto Creek during the post-mining period is attributed to residual drawdown from groundwater pumping and Open Pit mining activities. 

Drawdown Impacts on Perennial Springs. There are 17 inventoried spring sites (6 wet and 11 dry) within the drawdown area. One of these perennial springs also occurs within the predicted drawdown area that resulted from pumping during the 2013–2018 period. The six wet (flowing) springs are considered perennial springs with baseflow controlled by discharge from the groundwater system. The actual impacts on individual seeps or springs would depend on the source of groundwater that sustains the perennial flow (perched or hydraulically isolated aquifer versus regional groundwater system) and the actual extent of mine-induced groundwater drawdown that would occur in the area. The interconnection (or lack of interconnection) between the perennial surface waters and deeper groundwater sources is largely controlled by the specific hydrogeologic conditions that occur at each site. Considering the uncertainty between the actual groundwater elevations and model-simulated groundwater elevations in this area, and the absence of data to define if these springs are perched or connected to the deeper groundwater aquifer system, the EIS analysis conservatively assumed that there is a potential risk that drawdown associated with groundwater pumping for the mine could reduce baseflow to the six perennial springs identified within the drawdown area. Depending on the severity of the reductions in flow, this could result in the drying up of springs and a reduction in the size of any associated wet soil or groundwater-dependent vegetation areas. The groundwater modeling results indicate that residual drawdown would persist for at least 100 years after mine closure. Therefore, any reduction in flow at these spring sites resulting from mine-induced drawdown would persist for the foreseeable future.

Drawdown Impacts on Surface Water Rights. There are 26 surface water rights in the drawdown area that include 17 used for livestock, 7 used for stock and wildlife, and 1 (the Tonto National Forest Pinto Creek in-stream flow water right) used for recreation and wildlife. Nine of these 26 surface water rights also occurred within the projected drawdown area resulting from pumping between 2013 and 2018 (transient period). The actual impacts on individual surface water rights would depend on the site-specific hydrologic conditions that control surface water discharge. Only those waters sustained by discharge from the regional groundwater system would be likely to be affected. For surface water rights that are dependent on groundwater discharge, a potential reduction in groundwater levels could reduce or eliminate the flow available at the point of diversion for the surface water right.

Watershed Impacts. Pinto Valley Mine operations have altered the natural watersheds on the private mine property. Under the no-action alternative, there would be 4,820.3 acres of disturbed land at the Pinto Valley Mine. The percentage of surface disturbance within the Upper Pinto Creek subwatershed is approximately 21 percent, an increase of about 3.4 percent from the existing conditions. 

Most of the disturbed areas at the Pinto Valley Mine currently fall within the non-discharging boundary located within the Upper Pinto Creek subwatershed. Of the 4,553.0 acres of surface disturbance in the Upper Pinto Creek subwatershed as a result of the no-action alternative, 3,859.0 acres of that disturbance would be within the non-discharging boundary. Runoff from the remaining 691.1-acre disturbance area in the Upper Pinto Creek subwatershed would discharge to Pinto Creek. Under existing

conditions, there are 647.8 acres of disturbance within the discharging area. There would be an increase of surface disturbance within the discharging portion of the Pinto Valley Mine of 43.3 acres under the no-action alternative. Based on the relatively small increase in surface disturbance compared to the existing conditions, the impacts of surface disturbance under the no-action alternative from the discharging areas at the Pinto Valley Mine on flow quantities monitored at Magma Weir gaging station would likely not be measurable.

Pit Lake Development. The groundwater model simulations predict that a pit lake would start to develop in 2027 and reach 95 percent of full recovery within approximately 182 years after closure. At full recovery, the pit lake is predicted to have a groundwater inflow rate of 99 gallons per minute and evaporation rate of 310 gallons per minute. The numerical modeling results indicate that the pit lake would behave as a strong hydraulic sink (hydrologic capture zone where there is groundwater inflow that is lost to evaporation but no outflow to the groundwater system).

Pit Lake Water Quality. For the no-action alternative, the water quality of the pit lake is predicted to have low pH (2.02–2.22 standard units), high sulfate concentrations (20,336–32,814 milligrams per liter), and high metal concentrations over the simulated post-mining period. Although chemical weathering of pit wall rock is anticipated to provide a source of chemical loading to the lake, the chemical composition of the lake is fundamentally dictated by the addition of pregnant leach solution. Because the pit lake is expected to behave as a strong hydraulic sink, the pit lake water would be fully contained within the pit and would not discharge to groundwater or surface water resources outside the pit boundaries. An ecological risk assessment was used to evaluate risk to terrestrial and avian life from potable consumption and interaction with the pit lake water quality. The results of the ecological risk assessment and the evaluation of potential impacts on terrestrial and avian life are provided in section 3.3, “Biological Resources,” of chapter 3 in the Pinto Valley Mine EIS. 

Tailings Facilities Water Quality Impacts. Currently, tailings storage facilities discharge alkaline, high total dissolved solids, high sulfate concentration water with variable but low metal concentrations. These conditions would persist under the no-action alternative. During mining, the high total dissolved solids, high sulfate water leachate would continue to drain through the tailings and seep out of the base of the facility and enter the groundwater flow system. A series of existing groundwater production wells that are part of the Peak Well System are used to capture and pump back the high sulfate water immediately downgradient from Tailings Storage Facility No. 4 and Tailings Storage Facility No. 3. This pump-back system serves to prevent high total dissolved solids, high sulfate process water in the tailings facilities from migrating outside of the project area or discharging into Pinto Creek. 

After closure, seepage resulting from draindown from Tailings Storage Facility No. 4 is predicted to continue at progressively reduced rates until approximately year 2080. However, the current mine closure and reclamation plans do not include a commitment for the construction, long-term operation, and maintenance of a pump-back system (seepage capture system) and treatment system to manage the predicted seepage. Without long-term capture and treatment, seepage from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would likely enter the groundwater system and migrate downgradient (outside of the Pinto Valley Mine project boundary) and potentially discharge as baseflow (and degrade water quality) in Pinto Creek. Therefore, under the no-action alternative, there is a potential for long-term impacts on groundwater and surface water quality from draindown of entrained process water during the post-closure period. Because the tailings solids are considered to be likely to generate acid rock drainage, discharge of low pH metal-laden water could potentially occur. However, the combination of reclamation and the relatively arid climate is expected to limit the flux of water through tailings after initial draindown. Therefore, problematic discharge of acid rock drainage to negatively affect water resources is not regarded as a significant threat.

Heap Leach Water Quality Impacts. The heap leach facility is predicted to discharge low pH, high total dissolved solids, metal-laden water for hundreds of years. Most of this discharge would be captured in a Pregnant Leach Solution Pond and pumped to the Open Pit lake, but some would continue to infiltrate to groundwater. A large portion of the seepage into groundwater would flow into the Open Pit. However, a small part of the seepage into the groundwater system would flow toward Pinto Creek. The portion of the seepage that is predicted to flow toward Pinto Creek has the potential to degrade groundwater quality west of the facility and eventually affect the water quality in Pinto Creek.

Waste Rock Facilities Water Quality Impacts. The basic observations for historical, existing waste rock is assumed to represent anticipated conditions during the mining and post-mining periods. Acid rock drainage products can be expected to accumulate as highly water-soluble salts on waste rock due to exposure to air, occasional rain, and humidity. In response to more pronounced precipitation events, acid rock drainage–like solutions with low pH and elevated sulfate and metals are likely to be temporarily released in storm water. For waste rock dumps, any production of acid rock drainage in response to rain events would tend to react with other rock materials, which may act to dilute and neutralize to an unknown extent. Currently, groundwater wells proximal to waste rock dumps do not show impacts from acid rock drainage. This and the absence of reported routine or sporadic acid rock drainage seeps or runoff suggest that infiltration of meteoric precipitation into the waste rock dumps is insufficient to produce a discharge of acid rock drainage in the long term. 

Proposed Action

Groundwater Drawdown. The predicted changes in groundwater levels under the proposed action indicate the drawdown area is predicted to continue to expand during the post-mining period from 23.8 square miles at the end of mining to 37.9 square miles at 100 years post-closure. Compared to the drawdown area predicted under the no-action alternative, the predicted drawdown area for the proposed action represents an increase of 26 percent and 5 percent at the end of mining and 100-year post-mining timeframes, respectively.

Drawdown Impacts on Perennial Streams. Potential impacts on streams were evaluated by (1) using available baseline data to identify perennial stream reaches located within the drawdown area; and (2) model simulations of baseflow reduction at Pinto Creek at the Magma Weir monitoring gage. The drawdown is projected to encompass a total of 5.87 miles of perennial stream length that includes portions of Pinto Creek (4.80 miles), Miller Springs Gulch (0.82 mile), and an unnamed tributary to Pinto Creek (0.25 mile). The EIS analysis assumes that baseflow in the specific stream reaches identified above are largely controlled by discharge from the regional (bedrock) aquifer system. (These results are similar to but slightly larger than those defined under the no-action alternative.) The predicted expansion of drawdown resulting from mine-induced drawdown could result in a reduction in baseflow in these stream reaches. A reduction in baseflows would likely affect the perennial stream reach (reduce the length of or eliminate the perennial stream reach affected by drawdown). These types of impacts would likely persist until pumping ceases and groundwater levels recover to pre-pumping conditions. 

Model simulations predict that impacts on baseflow occurred as a result of pumping from the Peak Well System during the 2013–2018 period, and then would continue at slightly greater magnitude under the no-action alternative (2019–2027), and would continue at a similar magnitude until pumping ceases under the proposed action. Recovery of groundwater discharge (baseflow) to Pinto Creek is predicted to follow the same pattern as described under the no-action alternative scenario over the closure and post-closure period (SRK Consulting, Inc. 2019a). During the first 10 years after groundwater pumping ceases, the baseflow is predicted to fully recover to approximately 1,070 gallons per minute (the pre-pumping 2013 flow rate) as a result of continued high infiltration from Tailings Storage Facility No. 4.

From 10 years to 100 years post-closure, the baseflow is predicted to gradually decrease from approximately 1,070 gallons per minute to approximately 430 gallons per minute due to combined effects of residual drawdown and progressively reduced infiltration from draindown of Tailings Storage Facility No. 4. The predicted baseflow rate of 430 gallons per minute at 100-years post mining represents an approximate 60 percent reduction in baseflow as compared to the simulated pre-pumping conditions (1,070 gallons per minute) at the start of 2013. After 100 years post-mining, the baseflow is predicted to eventually reach a steady-state condition of approximately 407 gallons per minute, which represents a 62 percent reduction in flow compared to conditions at the start of 2013. The predicted long-term reduction of baseflow in Pinto Creek during the post-mining period is attributed to residual drawdown from groundwater pumping and Open Pit mining activities.

Continued pumping from the Peak Well System under the proposed action scenario is predicted to extend the duration of the predicted impacts on baseflow for an additional 12 years (compared to the no-action alternative). The potential impacts on baseflow and perennial stream flow in Pinto Creek in the affected area would be the same as those described under the no-action alternative in that long-term reduction in baseflow would likely result in a measurable reduction in flow (or elimination of surface flow) that would be particularly noticeable during the low-flow periods. As a consequence, segments of Pinto Creek that were previously characterized as perennial prior to 2013, and that may have been affected by pumping that occurred between 2013–2018 (and no longer sustain perennial flow conditions), would be expected to continue to be affected at a similar magnitude but longer duration (an additional 12 years). The reduction of stream flow in segments of Pinto Creek could adversely affect the surface water quality in those reaches, particularly during low-flow conditions. 

Drawdown Impacts on Perennial Springs. There are 19 inventoried spring sites (7 wet and 12 dry) within the drawdown area. The seven wet (flowing) springs include six springs that also occur within the drawdown area defined under the no-action alternative. As described for the no-action alternative, one of these perennial springs also occurs within the predicted drawdown area that resulted from pumping during the 2013–2018 period. Potential impacts on perennial springs would be the same as those described for the no-action alternative. 

Drawdown Impacts on Surface Water Rights. There are 26 surface water rights in the drawdown area that include 17 used for livestock, 7 used for stock and wildlife, and 1 (the Tonto National Forest Pinto Creek in-stream flow) used for recreation and wildlife. Nine of these 26 surface water rights also occurred within the projected drawdown area resulting from pumping between 2013–2018 (transient period). As shown in table 3-5, the same 26 water rights identified within the drawdown area under the proposed action were identified within the projected drawdown area under the no-action alternative. Potential impacts on surface water rights would be similar to those described under the no-action alternative. However, the results of the groundwater modeling indicate that the impacts on baseflow in Pinto Creek would persist for a longer period under the proposed action compared to the no-action alternative. Therefore, potential impacts on the Forest Service instream flow right could persist for a longer period (an additional 12 years) compared to the no-action alternative. 

Watershed Impacts. Under the proposed action, there would be 5,231.8 acres of disturbed land at the Pinto Valley Mine that include an additional 1,454.7 acres of surface disturbance (1,301.1 on private land and 153.7 acres on National Forest System lands). The percentage of surface disturbance within the Upper Pinto Creek subwatershed is approximately 22 percent, an increase of about 5 percent from existing conditions. 

Most of the disturbed areas at the Pinto Valley Mine currently fall within the non-discharging boundary. Of the 4,961.3 acres of surface disturbance in the Upper Pinto Creek subwatershed that would occur under the proposed action, 4,214.9 acres of that disturbance would be within the non-discharging

boundary. Runoff from the remaining 746.4-acre disturbance area in the Upper Pinto Creek subwatershed would discharge to Pinto Creek. Under existing conditions, there are 647.8 acres of disturbance within the discharging area. There would be an increase of surface disturbance within the discharging portion of the Pinto Valley Mine of 98.6 acres under the proposed action. The impacts of the surface disturbance under the proposed action would be similar to those under the no-action alternative and would likely not be measurable. 

Pit Lake Development. The groundwater model simulations predict a pit lake would start to develop in 2038 during the first year after Open Pit mining ceases as a result of passive inflow of groundwater. The pit lake is projected to reach 95 percent of full recovery within approximately 220 years after closure. In comparison to the predicted pit lake that would develop under the no-action alternative, the proposed action pit lake would be deeper and have a larger surface area and volume and proportionally larger rate of groundwater inflow and net evaporation. At full recovery, the pit lake is predicted to have a groundwater inflow rate of 121 gallons per minute and evaporation rate of 392 gallons per minute. The pit lake is expected to behave as a strong hydraulic sink (hydrologic capture zone where there is groundwater inflow that is lost to evaporation but no outflow to the groundwater system).

Pit Lake Water Quality Impacts. The water quality of the pit lake is predicted to have a low pH, high sulfate concentrations, and high metal concentrations over the entire post-mining period. Compared to the no-action alternative, the proposed action would result in a larger pit with a larger pit lake and a larger pit lake volume; this larger volume would act to dilute the effect of pregnant leach solution. As a result, the proposed action pit lake water quality would be better than predicted for the no-action alternative pit lake. Over the first 100 years following closure, the no-action alternative pit lake is predicted to have a pH ranging from 2.0 to 2.2, sulfate from 19,574 to 32,814 milligrams per liter, with a range of elevated metal concentrations. In comparison, the proposed action pit lake is predicted to have a pH range from 2.3 to 2.4, sulfate range of 12,688 to 15,459 milligrams per liter, and elevated metals. As with the no-action alternative pit lake, the proposed action pit lake water would be fully contained within the pit and would not discharge to groundwater or surface water resources outside the pit boundaries. An ecological risk assessment was used to evaluate risk to terrestrial and avian life from potable consumption and interaction with the pit lake water quality (section 3.3, “Biological Resources” of chapter 3 in the Pinto Valley Mine EIS). 

Tailings Facilities Water Quality Impacts. The potential impact on water resources from the tailings storage facilities under the proposed action would be similar to but greater than that of the no-action alternative. Owing to the expanded volume of tailings storage under the proposed action, a greater volume of neutral pH, high total dissolved solids, high sulfate leachate would be generated and stored in the larger facility. During mining, the high total dissolved solids, high sulfate leachate would continue to drain through the tailings and seep out of the base of the facility and enter the groundwater flow system. A series of existing groundwater production wells that are part of the Peak Well System are used to capture and pump back the high-sulfate water immediately downgradient from Tailings Storage Facility No. 4 and Tailings Storage Facility No. 3. This pump-back system serves to prevent high-sulfate process water in the tailings facilities from migrating outside of the project area or discharging into Pinto Creek. 

As part of closure and reclamation, the top surface for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would be covered with a minimum of 1 foot of cover material and revegetated, and the side slopes would be covered with a minimum of 2 feet of cover material followed by 6 inches of rock armor. Seepage from Tailings Storage Facility No. 3 would continue at progressively reduced rates during the closure and post-closure period until about 2050 (under both the no-action alternative and proposed action). Seepage resulting from draindown from Tailings Storage Facility No. 4 is predicted to

continue at progressively reduced rates until approximately year 2100 (SRK Consulting, Inc. 2019a). The length of time estimated to reach a steady-state condition under the proposed action is approximately 20 years longer than predicted under the no-action alternative. The current mine closure and reclamation plans do not include a commitment for the construction, long-term operation, and maintenance of a pump-back (seepage capture system) and treatment system to manage the predicted seepage over the post-closure period for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. Without long-term capture and treatment, seepage from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would likely migrate downgradient (outside of the Pinto Valley Mine project boundary) and potentially discharge as baseflow (and degrade water quality) in Pinto Creek. High total dissolved solids, high sulfate concentrations in the seepage from the facilities would likely degrade water quality in the groundwater system and in Pinto Creek downgradient of these facilities. Therefore, under the proposed action there is a potential for long-term impacts on groundwater and surface water quality during the post-closure period. 

Heap Leach Facility Water Quality Impacts. There would be no change to the currently authorized operation and planned closure of the heap leach facility under the proposed action. Potential impacts associated with the closure of the heap leach facility would be similar to those described under the no-action alternative. The footprint of the heap leach facility is the same under both the proposed action and no-action alternative. However, because the long-term residual drawdown area is larger under the proposed action compared to the no-action alternative, the percentage of the facility area that would be within the groundwater capture zone is larger under the proposed action (94 percent) compared to the no-action alternative (72 percent). Consequently, the facility area located outside of the groundwater capture zone is smaller (6 percent) compared to the no-action alternative (28 percent). Therefore, the rate of seepage that would enter and mix with the groundwater system and flow west toward Pinto Creek would be less than projected under the no-action alternative. As with the no-action alternative, the small portion of the seepage that is predicted to flow toward Pinto Creek has the potential to degrade groundwater quality west of the facility and could potentially affect the water quality in Pinto Creek. 

Waste Rock Facilities Water Quality Impacts. Impacts on surface and groundwater quality is expected to be the same as described under the no-action alternative. 

 

CUMULATIVE EFFECTS

The cumulative effects analysis area boundary for water resources and geochemistry includes the entire Pinto Creek watershed area, as well as a small portion of the Miami Wash and Middle Pinal Creek watersheds directly east of the Pinto Valley Mine (figure 1-1). The cumulative effects analysis area was defined to include the maximum geographic extent of effects from surface disturbances and water management activities associated with the proposed project (and interrelated actions) and past, present, and reasonably foreseeable future actions. The total area of the cumulative effects analysis area encompasses approximately 200 square miles (127,000 acres). 

Within this cumulative effects analysis area, past and present disturbance has resulted from the following activities: mineral development and exploration projects; utilities, infrastructure, and roads; and livestock grazing and dispersed recreation. Wildland fires are another major disturbance. These can cumulatively affect surface water quality by removing the vegetation layer, increasing erosion and downstream turbidity. Storms can cause mass losses of sediment along eroded embankments, altering the course of hydrological systems. Wildland fires also can change the ecosystem, replacing shrub

habitat with grasslands. Shrubs are more resistant to erosion, but grasslands are more adaptable to changing environmental conditions. 

Rangeland management also is an important disturbance to, and utilizer of, water resources in the cumulative effects analysis area. Rangeland management relies on predictable subsurface and surface water quantity and quality to sustain activities. This source can contribute to changes in water quality through the additions of nitrogen and other constituents. Livestock also can trample vegetation around water sources, degrading surface water quality through the subsequent erosion.

Mining also has the potential for cumulative impacts on water quality and quantity. Individually insignificant dewatering of numerous mine pits can cause cumulative effects analysis area–wide changes in both groundwater and surface water quantity. Exposure of naturally occurring geochemical conditions can cause harmful constituents to enter the watershed through inadvertent release. Waste rock poses a threat for erosion and sedimentation to the watershed. Individual mine impacts may be minor to negligible, while cumulative mining activity can pose the potential for significant impacts on water quality in the cumulative effects analysis area. 

Previous construction associated with utilities, infrastructure projects, and roads may have used water during construction, and the largest potential post-construction effect likely is related to erosion and sedimentation associated with access roads or reclaimed disturbances. All roads can present water quality impacts due to inadvertent spills or releases during vehicular accidents. Unpaved roads, such as those crossing public lands and those within recreation sites in the cumulative effects analysis area, also can be a source of increased erosion and sedimentation. Paved roads may cause water quality issues resulting from increased storm water runoff.

No-action Alternative

There would be an increase in surface disturbance of 949.8 acres on private land and no increase in disturbance on National Forest System lands under the no-action alternative. Most of the land disturbance occurs in the Upper Pinto Creek subwatershed. Other mine-related disturbance in the cumulative effects analysis area includes the Carlota Copper Mine (3,054 acres) located in the Upper Pinto Creek and Haunted Canyon subwatershed areas, and an abandoned historical mine (less than 100 acres) located within the Upper Pinto Creek subwatershed. 

The calibrated groundwater model was used to simulate the cumulative change in groundwater levels and flow rates that are projected to occur in the future as a result of mine development and groundwater extraction activities associated with the no-action alternative (and proposed action). All of the model drawdown predictive simulations represent cumulative effects in that they incorporate other water pumping stresses in the model area, including water supply pumping and mitigation activities at the adjacent Carlota Mine and pumping by other private parties in the cumulative effects analysis area. No other past, present, or reasonably foreseeable future actions are known that would affect the groundwater modeling (SRK Consulting, Inc. 2019a). 

The drawdown results also indicate the cumulative drawdown area is predicted to continue to expand during the post-mining period. For example, the area predicted to experience a reduction in groundwater levels of 5 feet or more is predicted to encompass 18.9 square miles at the end of mining and expand to 36.2 square miles at 100 years post-closure. The maximum area of drawdown (defined by the 5-foot contour) is a southeast to northwest–oriented elongated area that extends from the southeast margin to the center of the hydrologic study area. 

The cumulative impacts on perennial streams (including Pinto Creek), seeps and springs, and water rights associated with drawdown are the same as those described for the no-action alternative (section 3.2.1).

Cumulative impacts on water quality under the no-action alternative are most strongly related to the high volume of neutral pH, high total dissolved solids, high sulfate water that will drain from tailings storage facilities during the closure and post-closure periods. Unless recovered by pump-back wells, this solution would report to the Pinto Creek drainage. Discharge from tailings storage facilities could develop acid rock drainage characteristics over time depending on the degree and success of reclamation that should limit oxygen and water ingress.

Temporal occurrences of acid rock drainage are likely from localized positions on tailings storage facility embankments and waste rock facilities. These discharges are anticipated to be in response to storm water runoff and are unlikely to be sustained discharges. The limited production of acid rock drainage from storm water will interact with other, acid-consuming materials (such as Gila conglomerate), which would limit the extent of water resource impacts from acid rock drainage in the short and long terms.

Proposed Action

Under the proposed action, the surface disturbance would increase 39 percent from the disturbance that would occur under the no-action alternative. There would be an increase in surface land disturbance of 1,301.1 acres on private land and an increase of 153.7 acres in land disturbance on National Forest System lands. Most of the land disturbance occurs in the Upper Pinto Creek subwatershed. 

Carlota Mine and several smaller historical mines are also located within the Upper Pinto Creek subwatershed. Surface disturbance from these mines would be the same as for the no-action alternative. 

As described under the no-action alternative, all of the model drawdown predictive simulations presented in section 3.3 represent cumulative effects in that they incorporate other water pumping stresses in the model area, including water supply pumping and mitigation activities at the adjacent Carlota Mine and pumping by other private parties in the cumulative effects analysis area. No other past, present, or reasonably foreseeable future actions are known that would affect the groundwater modeling (SRK Consulting, Inc. 2019a). 

The maximum areal extent of the 5-foot drawdown contour under the proposed action scenario is presented on figure 3-8. This figure shows the predicted outer limit of the 5-foot drawdown contour as determined by overlaying a series of 10-foot drawdown contours for representative points in time over the 100-year post-mining period (SRK Consulting, Inc. 2019a). The maximum area of drawdown (defined by the 5-foot contour) is very similar to and slightly larger than the area predicted under the no-action alternative. 

The cumulative impacts on perennial streams (including Pinto Creek), seeps and springs, and water rights under the proposed action cumulative scenario are the same as those described in section 3.3 for the proposed action.  

Cumulative impacts on water quality under the proposed action are essentially the same as those described under the no-action alternative except for a greater amount of initial draindown of neutral pH, high total dissolved solids, high sulfate water. This greater amount of discharge has the potential to affect water resources at greater distances from the project site, for longer periods of time. The net

total chemical mass discharged from the site is somewhat greater under the proposed action than under the no-action alternative.

 

PROPOSED MONITORING AND MITIGATION 

Based on the potential impacts presented in this report, five mitigation measures are proposed that would avoid, minimize, reduce, rectify, or restore identified key resource impacts. The proposed mitigation measures are summarized below: 

. Mitigation Measure WR-1a: Comprehensive Water Resources Monitoring Plan Development: The mine owner/operator would be responsible for the development of a comprehensive water resources monitoring plan for the project. The monitoring plan would be designed to monitor both groundwater and surface water resources within and near (within 1 mile of) the projected maximum extent of the drawdown area that could occur as a result of the proposed action or cumulative scenarios.

 

. Mitigation Measure WR-1b: Groundwater Modeling Recalibration. There is uncertainty regarding the groundwater modeling predictions. The numerical groundwater model developed for this EIS would be updated and recalibrated throughout the life of the project based on the actual observed changes in groundwater elevation and additional hydrogeologic, surface water, and groundwater-related data collected during operation. Geochemical modeling would be updated as necessary (if requested by the Forest Service or Arizona Department of Environmental Quality) if different results are predicted from the updated groundwater modeling or different results are obtained through the ongoing geochemical characterization during mining. 

 

. Mitigation Measure WR-1c: Surface Water Impacts Workshop. The Forest Service and Pinto Valley Mining Corp. (and other Pinto Creek stakeholders including basin surface water rights holders and Arizona regulatory agencies as appropriate) will convene for an annual meeting to discuss the water budget for the Pinto Valley watershed.

 

. Mitigation Measure WR-2: Water Rights Mitigation. The mine operator would be responsible for monitoring groundwater levels between the mine and surface water rights within the projected mine-related and well field–related drawdown area as part of the water resources monitoring program (WR-1). Adverse impacts on water wells and surface and groundwater rights would be identified and mitigated, as required under Arizona state law.

 

. Mitigation Measure WR-3: Post-Closure Tailings Seepage Management Plan: The mine operator would be responsible for development of a detailed, long-term management plan to capture and treat groundwater flowing out of the Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 facilities if monitoring indicated contaminants in excess of regulatory thresholds would be introduced into groundwater flowing onto National Forest System lands. The plan would specify the construction, operation, and maintenance of capture wells and water quality treatment and management system (or other approved demonstrated technologies) designed to prevent downgradient groundwater and surface water degradation. 

 

 

 

 

TABLE OF CONTENTS

Executive Summary ........................................................................................................................... E-i

Affected Environment ................................................................................................................... E-i

Surface Water Resources .................................................................................................... E-i

Groundwater Resources ..................................................................................................... E-ii

Water Quality .................................................................................................................... E-iii

Water Rights ...................................................................................................................... E-iv

Rock Geochemistry ............................................................................................................ E-iv

Environmental Consequences ...................................................................................................... E-v

Methodology ..................................................................................................................... E-vi

No-action Alternative ....................................................................................................... E-vii

Proposed Action ................................................................................................................. E-x

Cumulative Effects ..................................................................................................................... E-xiii

No-action Alternative ...................................................................................................... E-xiv

Proposed Action ............................................................................................................... E-xv

Proposed Monitoring and Mitigation ........................................................................................ E-xvi

1.0 Introduction ......................................................................................................................... E-1

1.1 Area of Analysis ................................................................................................................... E-1

1.2 Regulatory Framework ........................................................................................................ E-1

1.2.1 Lands and Resource Management Plan ................................................................. E-1

1.2.2 Federal Law ............................................................................................................ E-4

1.2.3 State and Local Law................................................................................................ E-4

2.0 Affected Environment .......................................................................................................... E-5

2.1 Hydrologic Setting ............................................................................................................... E-5

2.2 Climatic Setting ................................................................................................................... E-5

2.3 Surface Water Resources .................................................................................................... E-8

2.3.1 Streams .................................................................................................................. E-8

2.3.2 Springs and Seeps ................................................................................................ E-11

2.3.3 Waters of the U.S. ................................................................................................ E-18

2.3.4 Flood Hydrology ................................................................................................... E-20

2.3.5 Existing Mine Facilities and Mine Water Management Facilities ........................ E-20

2.4 Groundwater Resources ................................................................................................... E-25

2.4.1 Hydrogeologic Unit .............................................................................................. E-25

2.4.2 Groundwater Extraction ...................................................................................... E-27

2.4.3 Groundwater Levels, Gradients, and Flow Patterns ............................................ E-27

2.4.4 Regional Groundwater Budget ............................................................................ E-30

2.5 Water Quality .................................................................................................................... E-33

2.5.1 Water Quality Standards ...................................................................................... E-33

2.5.2 Total Dissolved Solids and Sulfate Concentrations .............................................. E-38

2.5.3 Surface Water Quality .......................................................................................... E-39

2.5.4 Groundwater Quality ........................................................................................... E-41

2.6 Rock Geochemistry ........................................................................................................... E-45

2.6.1 Material Sampling and Testing ............................................................................ E-45

2.6.2 Geochemical Testing Results ............................................................................... E-50

2.6.3 Facility Characterization ....................................................................................... E-65

 

2.7 Water Rights ...................................................................................................................... E-69

3.0 Environmental Consequences ............................................................................................ E-74

3.1 Introduction ...................................................................................................................... E-74

3.1.1 Key Issues for Detailed Analysis ........................................................................... E-74

3.1.2 Resource Indicators ............................................................................................. E-74

3.1.3 Methodology ........................................................................................................ E-75

3.2 No-action Alternative ........................................................................................................ E-86

3.2.1 Water Quantity Impacts ....................................................................................... E-86

3.2.2 Water Quality Impacts ....................................................................................... E-104

3.3 Proposed Action .............................................................................................................. E-108

3.3.1 Water Quantity Impacts ..................................................................................... E-108

3.3.2 Water Quality Impacts ....................................................................................... E-117

3.4 Cumulative Effects .......................................................................................................... E-120

3.4.1 Introduction ....................................................................................................... E-120

3.4.2 Past, Present, and Reasonably Foreseeable Future Actions .............................. E-121

3.4.3 Cumulative Effects ............................................................................................. E-121

4.0 Proposed Monitoring and Mitigation ............................................................................... E-124

4.1 Residual Impacts ............................................................................................................. E-127

5.0 References ....................................................................................................................... E-128

 

 

 

LIST OF ATTACHMENTS

Attachment A Seep and Spring Locations Visited between May 30 and June 22, 2018

Attachment B Pinto Valley Mining Corp. Arizona Pollutant Discharge Elimination System Compliance Summary Tables 2014–2016

Attachment C Pinto Valley Mining Corp. Aquifer Protection Permit Compliance Summary Tables 2014–2016

 

LIST OF TABLES

Table 2-1. Sub-watershed area characteristics .................................................................................... E-5

Table 2-2. Mining activity from Pinto Valley and Carlota Mines in the Pinto Creek watershed ......... E-9

Table 2-3. Comparison of zero-flow days at Magma Weir to average precipitation from 1996–2017................................................................................................................................... E-11

Table 2-4. Summary of jurisdictional drainage features within the National Forest System lands analysis area ...................................................................................................................... E-20

Table 2-5. Storm water basins and surface water monitoring at Pinto Valley Mine ......................... E-22

Table 2-6. Estimated annual groundwater budget for the hydrologic study area (quasi-steady state) ................................................................................................................................. E-30

Table 2-7. Surface water quality standards ....................................................................................... E-34

Table 2-8. Groundwater quality standards ........................................................................................ E-37

Table 2-9. Water classification based on total dissolved solids ......................................................... E-38

Table 2-10. Average Arizona Pollutant Discharge Elimination System data for parameters that exceeded Arizona Pollutant Discharge Elimination System or other standards 2014–2016 at Pinto Valley Mine ................................................................................................. E-40

Table 2-11. Average Aquifer Protection Permit data for parameters that exceeded Arizona Pollutant Discharge Elimination System or other standards 2014–2016 at Pinto Valley Mine........................................................................................................................ E-41

Table 2-12. Static test samples by facility ............................................................................................ E-46

Table 2-13. Static test samples by lithology ......................................................................................... E-47

Table 2-14. Completed humidity cell tests .......................................................................................... E-49

Table 2-15. Results of humidity cell testing ......................................................................................... E-55

Table 2-16. Acid-base accounting of waste rock dumps ...................................................................... E-65

Table 2-17. Summary of tailings acid-base accounting ........................................................................ E-67

Table 2-18. Surface water rights in the Pinto Creek watershed .......................................................... E-70

Table 2-19. Recorded flow volume at Pinto Creek near Miami, AZ (U.S. Geological Survey 09498502) compared to the flow volumes specified in the Tonto National Forest instream flow right (permit 33-89109) (acre-feet) ........................................................... E-72

Table 3-1. Resource indicators and measures for assessing effects .................................................. E-74

Table 3-2. Summary of facilities under the no-action alternative ..................................................... E-86

Table 3-3. Perennial stream miles within the projected drawdown areas (no-action alternative and proposed action) ........................................................................................................ E-92

Table 3-4. Summary of seeps and springs within the drawdown area (no-action alternative and proposed action) ............................................................................................................... E-94

Table 3-5. Surface water rights within the predicted drawdown area (no-action alternative and proposed action) ............................................................................................................... E-97

Table 3-6. Predicted pit lake development summary (no-action alternative and proposed action) ............................................................................................................................. E-101

Table 3-7. Surface disturbance under the no-action alternative by watershed .............................. E-103

Table 3-8. Predicted pit lake water quality (no-action alternative) ................................................. E-104

Table 3-9. Summary of facilities under the proposed action ........................................................... E-108

Table 3-10. Surface disturbance under the proposed action by watershed ..................................... E-116

Table 3-11. Predicted pit lake water quality (proposed action) ........................................................ E-117

 

LIST OF FIGURES

Figure 1-1. Hydrologic study area ......................................................................................................... E-2

Figure 1-2. Existing and authorized facilities and proposed expansion areas ...................................... E-3

Figure 2-1. Topographic influence on precipitation .............................................................................. E-7

Figure 2-2. Streams in the hydrologic study area ............................................................................... E-12

Figure 2-3. Analysis of Pinto Creek near Miami hydrograph 1998–2006 ........................................... E-13

Figure 2-4. Observed versus simulated monthly streamflow at Pinto Creek near Miami for the period 1996–2006 (model including 1 cubic foot per second baseflow) .......................... E-14

Figure 2-5. Observed versus simulated monthly streamflow at Pinto Creek near Miami for the period 2007–2016 ............................................................................................................. E-15

Figure 2-6. Springs and seeps .............................................................................................................. E-16

Figure 2-7. Seep and spring flow rates ................................................................................................ E-17

Figure 2-8. Waters of the U.S. ............................................................................................................. E-19

Figure 2-9. Water resources sampling locations ................................................................................. E-24

Figure 2-10. Location of existing pumping wells ................................................................................... E-28

Figure 2-11. Groundwater elevation contours – hydrologic study area (June 2018) ........................... E-29

Figure 2-12. Conceptual model (2011–2012) ........................................................................................ E-32

Figure 2-13. Groundwater quality – average sulfate concentrations (2014–2016) .............................. E-44

Figure 2-14. Acid-base accounting data facility .................................................................................... E-52

Figure 2-15. Acid-base accounting data by lithology ............................................................................ E-53

Figure 2-16. Meteoric water mobility procedure and synthetic precipitation leaching procedure copper results by lithology ................................................................................................ E-59

Figure 2-17. Meteoric water mobility procedure and synthetic precipitation leaching procedure iron results by lithology ..................................................................................................... E-60

Figure 2-18. Meteoric water mobility procedure and synthetic precipitation leaching procedure zinc results by lithology ..................................................................................................... E-61

Figure 2-19. Net acid-generating copper results by lithology ............................................................... E-62

Figure 2-20. Net acid-generating iron results by lithology .................................................................... E-63

Figure 2-21. Net acid-generating zinc results by lithology .................................................................... E-64

Figure 2-22. Water rights ...................................................................................................................... E-73

Figure 3-1. Groundwater model domain ............................................................................................ E-77

Figure 3-2. Water level monitoring wells used for model calibration ................................................ E-78

Figure 3-3. Conceptual model (2013–2039) ........................................................................................ E-79

Figure 3-4. Predicted change in groundwater levels – transient period (2013–2018) ....................... E-84

Figure 3-5. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir (2013–2019) ................................................................................................ E-85

Figure 3-6. Predicted change in groundwater levels – no-action alternative (end of mining) ........... E-87

Figure 3-7. Predicted change in groundwater levels – no-action alternative (100 years post-mining) .............................................................................................................................. E-88

Figure 3-8. Surface water resources within the maximum extent of the 5-foot drawdown contour .............................................................................................................................. E-90

Figure 3-9. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir – no-action alternative (2013–2027) .......................................................... E-93

Figure 3-10. Surface water rights within the maximum extent of the 5-foot drawdown contour ....... E-96

Figure 3-11. Predicted post-mining pit lake development (no-action alternative and proposed action) ............................................................................................................................. E-100

 

Figure 3-12. Predicted change in groundwater levels – proposed action (end of mining) ................. E-110

Figure 3-13. Predicted change in groundwater levels – proposed action (100 years post-mining) ... E-111

Figure 3-14. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir – proposed action (2013–2038) ................................................................ E-112

 

 

ACRONYMS AND ABBREVIATIONS

EIS environmental impact statement

Forest Service U.S. Department of Agriculture, Forest Service

 

1.0 Introduction

1.1 Area of Analysis

The cumulative effects analysis area for water resources and geochemistry (herein referred to as the hydrologic study area) encompasses approximately 200 square miles that includes the entire Pinto Creek watershed area, as well as a small portion of the Miami Wash and Middle Pinal Creek watersheds directly east of the Pinto Valley Mine (figure 1-1). The project analysis area for direct and indirect impacts for water resources is defined as those areas within the hydrologic study area where groundwater or surface water could be affected by the proposed project. 

The locations of major existing and proposed mine facilities at the Pinto Valley Mine are shown on figure 1-2. Details regarding the existing and authorized and proposed facility expansions included under the proposed action are provided in chapter 2 (“Alternatives, Including the Proposed Action”) of the Pinto Valley Mine EIS and in the mining plan of operations for the Pinto Valley Mine (WestLand Resources, Inc. 2016a). 

The temporal boundary for analyzing the direct and indirect effects on water resources includes the time frames associated with construction, operation, mine closure, and final reclamation. The analysis also evaluates potential long-term, post-closure impacts on water quantity and water quality that could persist into the post-closure period.

1.2 Regulatory Framework

The regulation, appropriation, and preservation of water in Arizona falls under both Federal and State jurisdiction. Key water-related components of the Tonto National Forest Land and Resource Management Plan and Federal and State law are outlined below. 

1.2.1 Lands and Resource Management Plan

The Tonto National Forest Land and Resource Management Plan provides the following management directions and prescriptions for water resources (Forest Service 1985).

Provide direction and support to all resource management activities to: 

. Meet minimum water quality standards;

 

. Emphasize improvement of water quality; 

 

. Augment water supplies when compatible with other resources; 

 

. Enhance riparian ecosystems, by improved management; and

 

. Obtain water rights necessary to ensure orderly resource development.

 

 

In September, 2001, a supplement to the Forest Service Manual, Southwestern Region (Region 3), chapter 2540 was issued concerning water uses and development (Forest Service Manual 2500, chapter 2540) (Forest Service 2001). This guidance clarifies responsibilities of the Forest Service to document and manage water uses of the Forest Service, and the special use authorization of water developments on National Forest System lands.

 

 

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Figure 1-1. Hydrologic study area 

 

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Figure 1-2. Existing and authorized facilities and proposed expansion areas 

1.2.2 Federal Law

1.2.2.1 Clean Water Act Section 303(d)

The Clean Water Act section 303(d) authorizes listing impaired waters and developing total maximum daily loads for these waterbodies.

1.2.2.2 Clean Water Act Section 402

The Clean Water Act section 402 establishes the National Pollutant Discharge Elimination System. Arizona Department of Environmental Quality is the permitting authority in the State of Arizona for the issuance of point-source Arizona Pollutant Discharge Elimination System permits and the storm water permitting program (multisector general permit) pursuant to section 402 of the Clean Water Act. 

1.2.2.3 Clean Water Act Section 404

The Clean Water Act section 404 regulates the discharge of dredged or fill material into waters of the U.S.

1.2.2.4 Executive Orders

. Executive Order 11990 (as amended by Executive Order 12608) – Protection of Wetlands. Provides for “no net loss” of wetlands policy outlined in an agreement between the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency.

 

. Executive Order 11988 (revised Executive Order 13690 was revoked by Executive Order 13807, reverted to Executive Order 11988) – Floodplain Management. Requires Federal agencies to avoid the long- and short-term adverse impacts associated with occupancy and modification of floodplains. 

 

 

1.2.3 State and Local Law

. Arizona Department of Environmental Quality, Aquifer Protection Permit – requires the operator of a mine tailings storage facility to obtain an Aquifer Protection Permit. Requires demonstration of Best Available Demonstrated Control Technology to prevent or reduce the discharge of pollutants to the aquifer and that aquifer quality standards will not be exceeded at the point of compliance at the time of permit issuance. 

 

. Arizona Department of Water Resources, Dam Safety Permit

 

. Arizona Department of Water Resources, Water Rights

 

. Gila County Floodplain Management Ordinance – similar to Executive Order 11988

 

 

 

2.0 Affected Environment

2.1 Hydrologic Setting

The Pinto Valley Mine is situated within the Pinto Creek watershed, which is part of the Upper Salt River drainage basin (Hydrologic Unit Code 15060103). The Pinto Creek watershed (HUC 1506010307) is drained by Pinto Creek, a north-flowing perennial and intermittent stream that is tributary to Theodore Roosevelt Lake (also called Roosevelt Lake) (figure 1-1). Roosevelt Lake is a large reservoir constructed by damming the Salt River, and is one of seven reservoirs included in the water storage and delivery system of the Salt River Project. The Salt River Project is composed of the Salt River Valley Water Users’ Association and the Salt River Project Agricultural Improvement and Power District. Collectively, the Salt River Project delivers approximately 8,700,000 acre-feet of water annually to a 248,000-acre service area in central Arizona and supplies water to a large portion of the Phoenix metropolitan area (Salt River Project 2019). 

The topography across the hydrologic study area is characterized by high relief dominated by steeply sloping mountains and ridges and incised drainages. Elevations ranges from approximately 6,500 feet above mean sea level in the headwaters of the Pinal Mountains near the southern boundary to approximately 2,200 feet above mean sea level along the northern boundary near where Pinto Creek flows into Roosevelt Lake. The elevation across the Pinto Valley Mine Project area ranges from approximately 4,900 feet above mean sea level on a ridge east of the mine site to approximately 3,000 feet above mean sea level where Pinto Creek exits the northern boundary (WestLand Resources, Inc. 2016a). 

The Pinto Creek watershed encompasses approximately 186 square miles and includes the six sub-watersheds (or drainage areas) listed in table 2-1 and depicted on figure 1-1. Mining facilities at Pinto Valley Mine are predominantly within the Upper Pinto Creek drainage area; the Peak Well System is located primarily within the Middle Pinto Creek drainage area. The Carlota Mine, an open pit copper mine that started mining in 2008 (and has experienced intermittent mining activity through mid-2019), is located in the Haunted Canyon and Upper Pinto Creek drainage basins. In addition to these two mines, a number of other inactive or abandoned mines occur in the Upper Pinto Creek drainage area. 

Table 2-1. Sub-watershed area characteristics

TABLE

 

 

2.2 Climatic Setting

The Pinto Valley Mine is in a semi-arid region with an average annual temperature of 65 degrees Fahrenheit. Precipitation patterns show two distinct precipitation periods on an annual basis. The winter

precipitation period (November through March) is characterized by longer-duration, low-intensity storms. The summer monsoon period (June through October) is characterized by short-duration, high-intensity events. April and May typically experience little to no precipitation (SRK Consulting, Inc. 2019a). 

Precipitation data are recorded at four locations (two meteorological stations, #1 and #2, and two rain gauges) within the Pinto Valley Mine project boundary. The average annual precipitation recorded at the two meteorological stations for the 2006 through 2015 period was 17.28 inches at station #1 and 16.65 inches at station #2. 

Available precipitation data from 15 National Oceanic and Atmospheric Administration stations within and adjacent to the region were used to establish a correlation between elevation and mean annual precipitation (SRK Consulting, Inc. 2019a). Parameter-elevation Regressions on Independent Slopes Model data (figure 2-1) were then used to estimate mean annual precipitation rates within elevation zones defined across the hydrologic study area. Parameter-elevation Regressions on Independent Slopes Model data were also used to estimate the open-pan evaporation rate for the mine area of 72.1 inches per year. Additional details regarding the analysis to define precipitation zones and evaporation for the hydrologic study area are provided in the groundwater model report (SRK Consulting, Inc. 2019a).

 

 

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Figure 2-1. Topographic influence on precipitation 

 

2.3 Surface Water Resources

Surface water resources include streams, springs, and seeps in the vicinity of the project area (cumulative effects analysis area) as well as within the Pinto Valley Mine area. 

 

2.3.1 Streams

Streams within the hydrologic study area are presented on figure 2-2. Major tributaries to Pinto Creek include the West Fork of Pinto Creek and Horrell Creek that enter Pinto Creek north of Tailings Storage Facility No. 4, and Campaign Creek in the Lower Pinto Creek drainage area. Other tributaries in the vicinity of the Pinto Valley Mine and adjacent Carlota Mine include Gold Gulch, Miller Gulch, Eastwater Canyon, Powers Gulch, and Haunted Canyon. 

Perennial, intermittent, and ephemeral stream reaches were defined based on the Tonto National Forest geographic information systems layer combined with the U.S. Geological Survey National Hydrography Database to represent historical conditions predating the 2013 resumption of mining activity at the Pinto Valley Mine (Forest Service 2019).

Pinto Creek is predominantly intermittent, although there are several perennial reaches where groundwater surfaces due to bedrock constrictions. Perennial reaches within Pinto Creek have historically been found in three areas: 

. From the confluence of Miller Gulch to a point downstream of the confluence with Haunted Canyon;

 

. From below Iron Bridge to a point above the confluence with West Fork Pinto Creek; and

 

. From U.S. Geological Survey Station 09498502, Pinto Creek near Miami, Arizona to a point upstream from the Blevens Wash confluence (Arizona Department of Environmental Quality 2016a).

 

 

In its upper reaches, Pinto Creek flows through a deeply incised, V-shaped channel with little alluvium. The middle reach of the stream begins at the confluence of West Fork Pinto Creek with Pinto Creek, where the stream widens into an alluvial basin about 0.5 mile wide and over a mile long. Three tributaries drain into Pinto Creek along the western side of the alluvial basin: West Pinto Creek, Horrell Creek (located within the Middle Pinto Creek basin), and an unnamed tributary that enters from the northwest within the Upper Pinto Creek Basin. The alluvial basin is covered with stream alluvium of unknown thickness composed of granular materials ranging in size from sand to boulders. The Pinto Creek Valley narrows toward the north end of the alluvial basin. North of the alluvial basin, the valley varies in width from a few feet in the Box Canyon below the Henderson Ranch to a maximum width of about 500 feet. Near the confluence with Roosevelt Lake, Pinto Creek is characterized as braded (Forest Service 1991). 

Surface water flows in the Pinto Creek watershed are monitored by three U.S. Geological Survey gaging stations located along Pinto Creek (figure 2-2): 

. Pinto Creek above Haunted Canyon (U.S. Geological Survey Station 094985005);

 

. Pinto Creek below Haunted Canyon (U.S. Geological Survey Station 09498501); and

 

. Pinto Creek near Miami, Arizona (Magma Weir) (U.S. Geological Survey Station 09498502).

 

 

Pinto Creek near Miami, Arizona (also known as the Magma Weir) has the longest monitoring period of record from October 1994 through present. The U.S. Geological Survey began monitoring flows at Pinto

Creek below Haunted Canyon in October of 1995, and above Haunted Canyon in October of 2007. The quality of automated stream flow records at the U.S. Geological Survey stations along Pinto Creek has varied. Monitoring data from the Magma Weir gage are generally fair, although daily estimates are poor during the 2003–2018 period of record (WestLand Resources, Inc. 2019). Earlier data quality information for automated stream flow records for the period from 1996 through 2002 were not available. Although daily estimates are also poor at Pinto Creek above Haunted Canyon and below Haunted Canyon, the automated gage records are generally good. 

Surface water flow in Pinto Creek is influenced by several components. Rainfall runoff is the overland flow from precipitation. Flow in Pinto Creek from precipitation events has a typical response time in hours. Interflow is shallow groundwater and infiltration within the alluvium that has a typical response time in days. Base flow from groundwater usually has a response time in months or years. 

SRK Consulting, Inc. analyzed the flow data from the three U.S. Geological Survey gaging stations on Pinto Creek to estimate the baseflow (groundwater contribution component to total stream flow) (SRK Consulting, Inc. 2018). The results of the study concluded that dry conditions were recorded 44.8 percent, 14.3 percent, and 13.7 percent of the time at stations 94985005, 9498501, and 9498502, respectively. The study also noted that the baseflow at the Magma Weir decreased after the last quarter of 2013 compared to the 2010 to 2013 period (SRK Consulting, Inc. 2018). 

2.3.1.1 Pinto Creek Watershed Water Balance Model

BGC Engineering USA, Inc. conducted a hydrologic modeling study of the Pinto Creek watershed to evaluate the surface flows and baseflow at the Magma Weir monitoring gage (BGC Engineering USA, Inc. 2019). The hydrologic model was developed using the Australian Water Balance Model. The Australian Water Balance Model used Parameter-elevation Regressions on Independent Slopes Model precipitation and evapotranspiration as inputs for the watershed and was calibrated to flow conditions measured at Magma Weir. The Australian Water Balance Model outputs were direct runoff and interflow, from which baseflow was estimated by subtracting the flow recorded at the Magma Weir monitoring gage. 

The calibration period was selected after evaluating the mining activities at the Pinto Valley and Carlota Mines in the Pinto Creek basin. Both the Pinto Valley Mine and Carlota Mine extract groundwater from production wells for processing and other operations. The periods of mining and estimated groundwater pumping rates are summarized in table 2-2. Two care and maintenance periods for the Pinto Valley Mine are identified in table 2-2. As indicated in table 2-2, during these periods, pumping from groundwater was substantially reduced. The care and maintenance period from 1988 through 2007 included a period of limited activity for the Pinto Valley Mine, and Carlota mine operations had not begun. The second care and maintenance period for the Pinto Valley Mine was from 2009–2012. Carlota Mine operations were active from 2007 through 2014. 

Table 2-2. Mining activity from Pinto Valley and Carlota Mines in the Pinto Creek watershed

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Source: SRK Consulting, Inc. 2019a

 

The Australian Water Balance Model was calibrated to stream flow from the U.S. Geological Survey Magma Weir gaging station during a calibration period of 1996–2006 for Pinto Creek near Miami. This represents a care and maintenance period at the Pinto Valley Mine when there were limited pumping stresses from the Pinto Valley Mine and when there was no pumping activity at the Carlota Mine. Results of the initial calibration showed the model was underestimating streamflow during periods of very low streamflow. Analysis of the hydrograph for the Magma Weir indicates that the creek had a baseflow of approximately 1 cubic foot per second during the 1998–2006 period conceptualized as discharge to the stream from deep groundwater flow within the bedrock (figure 2-3). The results of the baseflow analysis were used as the rationale to adjust the water balance model to account for steady-state inflow of 1 cubic foot per second to replicate the baseflow input. After the baseflow component was included in the model (figure 2-4), the results of the calibration for the 1998–2006 period improved substantially (BGC Engineering USA, Inc. 2019).

The calibrated model was then applied to simulate flows at the Magma Weir for the 2007 to 2016 period (figure 2-5). The results indicate a substantial reduction in model fit statistics between the 1998–2006 calibration period and the 2013–2016 simulation period. The period of the deterioration in model fit correlates with the initiation of high rates of pumping from the Peak Well System beginning in 2013 and continuing through 2016 (BGC Engineering USA, Inc. 2019). Pumping rates during this period range from 2,000 to 4,000 gallons per minute (4.46 to 8.91 cubic feet per second). 

The average streamflow for the Pinto Creek near Miami gage was approximately 10 cubic feet per second during the calibration period of 1998–2006. During that period there were no days with zero flow reported at the gage. In contrast, the average streamflow at the Pinto Creek near Miami gage for the period from 2013 to 2016 was 4.3 cubic feet per second. Reported days with zero flow at this gage during this period were compared to average precipitation in inches used in the model from the period of 1996 to 2017. There was an increase of zero flow days reported at the Magma Weir from 2013 through 2016. Variation in corresponding precipitation during the same time period does not explain the increase in zero-flow days because two of the years, 2014 and 2015, showed annual precipitation above average (table 2-3). 

 

Table 2-3. Comparison of zero-flow days at Magma Weir to average precipitation from 1996–2017

Formula Placeholder

 

2.3.2 Springs and Seeps

A total of 78 springs and seeps have been identified within the area. Ajax, Ltd. completed a survey of seeps and springs within the hydrologic study area to define baseline conditions in areas that may be affected by future groundwater pumping activities (Ajax, Ltd. 2018). In support of the survey, WestLand Resources, Inc. performed a remote-sensing image analysis to assist in the identification of potential seep and spring locations. The analysis used infrared imagery to identify actively growing vegetation by the Normalized Difference Vegetation Index (WestLand Resources, Inc. 2018a). 

Results from the WestLand Resources, Inc. study were used by Ajax, Ltd. to direct a field surveys that identified 78 seep and spring sites (figure 2-6) (Ajax, Ltd. 2018). The field surveys were conducted in May and June, 2018, prior to the onset of the summer monsoon. The survey occurred during an extreme drought period (National Weather Service 2018) and rainfall was between 25 percent and 50 percent of normal for the 2018 water year to date. Wet or flowing seeps and springs identified in the survey are considered perennial water sources because the wet or flowing conditions were observed during a period of extreme drought. The locations of springs and seeps identified within the inventoried area are shown on figure 2-6; available information on these springs and seep sites is summarized in table A-1 in attachment A. The locations and flow rates for springs and seeps measured during the field surveys are shown on figure 2-7. 

 

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Figure 2-2. Streams in the hydrologic study area

 

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Figure 2-3. Analysis of Pinto Creek near Miami hydrograph 1998–2006

 

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Figure 2-4. Observed versus simulated monthly streamflow at Pinto Creek near Miami for the period 1996–2006 (model including 1 cubic foot per second baseflow)

 

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Figure 2-5. Observed versus simulated monthly streamflow at Pinto Creek near Miami for the period 2007–2016

 

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Figure 2-6. Springs and seeps

 

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Figure 2-7. Seep and spring flow rates

 

2.3.3 Waters of the U.S.

 

Waters of the United States were investigated by WestLand Resources, Inc. in 2015 (WestLand Resources, Inc. 2015a, 2015b) and 2017 (WestLand Resources, Inc. 2017). Figure 2-8 shows the analysis areas for the three Requests for Approved Jurisdictional Waters Determination submitted to the U.S. Army Corps of Engineers. The U.S. Army Corps of Engineers found the WestLand Resources, Inc. reports to be accurate and complete and approved the jurisdictional determinations.

The initial jurisdictional request was focused on an area that comprises the proposed footprint for the extension of Tailings Storage Facility No. 3. Surface water features and natural topography within the initial analysis area #1 have been significantly manipulated in support of the storm water and seepage management system at the mine. Approximately 1,065 acres of the 1,312-acre analysis area #1 are managed by Pinto Valley Mining Corp. as a non-discharging area, where storm water runoff is contained within impoundments and water containment systems for precipitation events. Analysis focused on the remaining 247 acres of analysis area #1 considered the discharging area. All of the surface water features within the discharging portion of analysis area #1 were ephemeral drainage features, spatially intermittent drainage features, or wetlands. The drainage features within analysis area #1 do not qualify as traditional navigable waters or relatively permanent waters. One of the drainage features is classified as spatially intermittent; however, this feature would not be considered a relatively permanent water for the purpose of a significant nexus analysis. There were no features considered jurisdictional waters in analysis area #1 (WestLand Resources, Inc. 2015a).

A supplement to the initial Approved Jurisdictional Waters Determination request was submitted on June 19, 2015. The analysis area #2 supplement includes the original area and a supplemental area that includes an additional 1,640 acres of private lands. All of the features identified in analysis area #2 were ephemeral washes isolated from downstream waters by Tailings Storage Facility No. 4. These ephemeral features are isolated and do not discharge to downstream receiving waters and were not considered jurisdictional waters (WestLand Resources, Inc. 2015b). 

A final Approved Jurisdictional Waters Determination request was submitted on January 5, 2017. Mine operation alternatives for the expansion of Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would include use of National Forest System lands. Features in these areas were designated the National Forest System lands analysis area #3. The National Forest System lands analysis area #3 consists of approximately 687 acres. The Forest Service manages 667 acres and Pinto Valley Mining Corp. owns approximately 20 acres. Features identified in analysis area #3 include 14 ephemeral and intermittent drainages and 2 wetland areas. Five of the drainage features are west and downgradient of the Pinto Valley Mine within the discharging portion of analysis area #3 that is tributary to Pinto Creek. Features 1, 2, 3, 20, and 21 shown on figure 2-8 and summarized in table 2-4 were considered jurisdictional waters. Features 1, 2, and 3 are downgradient extensions of drainage considered previously in Tailings Storage Facility No. 3 and are directly tributary to Pinto Creek. These features are ephemeral drainages that were found to have a significant nexus to the nearest downgradient traditional navigable water, Roosevelt Lake. Feature 20 is a newly delineated feature that is also a direct tributary to Pinto Creek and shares a similar downgradient flowpath and distance to Roosevelt Lake as features 1, 2, and 3. Feature 21 is a portion of the mainstem of Pinto Creek. This reach of Pinto Creek is intermittent, and flows for more than 3 months of the year. Pinto Creek meets the definition of a relatively permanent water and is, therefore, a jurisdictional water. The remaining 11 features are all within the zero-discharge boundary of the Pinto Valley Mine and are not considered jurisdictional waters. The five jurisdictional drainage features identified are not affected by proposed expansion of Tailings Storage Facility No. 3. 

 

 

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Figure 2-8. Waters of the U.S.

 

Table 2-4. Summary of jurisdictional drainage features within the National Forest System lands analysis area

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Source: WestLand Resources, Inc. 2015a, 2015b, 2017

 

2.3.4 Flood Hydrology

 

The Pinto Valley Mine complex and the Pinto Creek watershed are in an area described as zone D or an undetermined risk area. These are areas where flood hazards are undetermined, but flooding is possible (Federal Emergency Management Agency 2017). The Pinto Valley Mine is east of Pinto Creek in the hills above the valley floor. At the Pinto Valley Mine site, the topography has been altered in support of the storm water and seepage management system. Most of the site is maintained as a non-discharging area where storm water runoff is contained within impoundments and water containment systems. These systems are designed and managed to contain the 100-year, 24-hour storm event. Discharge from the Pinto Valley Mine facilities occurs only at monitored discharge points (WestLand Resources, Inc. 2016b). Key components of the mine water management system are summarized in the following section. 

 

2.3.5 Existing Mine Facilities and Mine Water Management Facilities

 

2.3.5.1 Mine Facilities 

Two active water supply reservoirs have been authorized on National Forest System lands: Cottonwood Reservoir and Mine Reservoir. Cottonwood Reservoir is the primary water storage facility for the Pinto Valley Mine. The Mine Reservoir stores water from the Burch pipeline. Water is temporarily stored in the reservoir and used for dust control on haul roads in the Open Pit or waste rock dumps reporting to the Open Pit. 

Cottonwood Tailings Impoundment is immediately downstream of Cottonwood Reservoir. The tailings storage facility encompasses 322.0 acres: 278.2 acres on National Forest System lands and 43.8 acres on private land. The tailings storage facility was approved in 1944 by the Bureau of Land Management and the land was transferred to the Forest Service in 1989. Tailings were deposited in the Cottonwood Tailings Impoundment from 1944 through 1954 and again in 1974 through 1984. The surface of the facility was capped with 6-inch inert material (Gila conglomerate) in 1988 and remains inactive. 

Infrastructure relating to the impoundment that are still active include:

. Seepage collection system at the embankment. PV-004 is located within this seepage collection system. Water from this system is directed to the service water circuit. 

 

. Reclaimed water system within the impoundment that provides water to the service water circuit. 

 

 

. PV-005, an outfall at the southeastern corner of the impoundment, discharging to an unnamed drainage that reports to Pinto Creek. 

 

. Evaporation and settling ponds that manage storm water that falls on the impoundment surface.

 

. National Forest System Road 287, located along the west side of the impoundment. 

 

 

The inactive 19 Dump is located entirely on National Forest System lands (75.7 acres). The waste rock dump was authorized by the Forest Service in 1985. The toe of the dump near the Cottonwood Reservoir was authorized to extend onto the right-of-way for the Cottonwood Tailings Impoundment and Reservoir. 

Tailings Storage Facility Nos. 1 and 2 merged over time into a single impoundment area. This facility is an inactive, unlined tailings storage facility west of the Open Pit on 403.8 acres of private lands. As described in section 3.8 of the EIS (“Geology, Minerals, Geotechnical Stability, and Paleontology”), a slope failure occurred at this facility in 1997, which resulted in 370,000 cubic yards of tailings and non-mineralized waste rock being released to Pinto Creek. A rapid-response program of abatement and restoration was established by the Forest Service to remedy the consequences of the incident. The removal of materials from National Forest System lands was completed in July 1998. An assessment of the potential effects on human health, water quality, and riparian and upland flora and fauna was made and a monitoring program was developed. 

Tailings Storage Facility No. 3 is an unlined tailings impoundment currently used as a backup to Tailings Storage Facility No. 4 for disposing tailings from the mill. The current footprint of Tailings Storage Facility No. 3 includes 263.8 acres of private land and 5.7 acres of National Forest System lands. An encroachment of tailings occurred onto unpatented claim on National Forest System lands in 2013. A boundary dam was constructed to isolate the encroachment area and the tailings were removed in 2014. A second encroachment onto unpatented claims on National Forest System lands is associated with a 0.53-acre sediment trap and adjoining 2.2-acre area. This sediment trap collects runoff from a portion of the embankment and acts as a small detention basin. Water from the trap flows to the Slack Pond and is pumped back to the Pinto Valley Mining Corp. water management system. The reclaimed water system at Tailings Storage Facility No. 3 conveys water from the southern end of the Tailings Storage Facility No. 3 impoundment. 

Tailings Storage Facility No. 4 is an active, unlined tailings impoundment with a current footprint of 703.9 acres on private land. Boundary dams constructed since 2016 prevent tailings from extending onto National Forest System lands. The existing reclaimed water system conveys water from the southern end of the Tailings Storage Facility No. 4 supernatant pool to the mill water supply tank. 

2.3.5.2 Mine Water Management Facilities

Drainage areas for the Pinto Valley Mine are shown on figure 2-9. Pinto Valley Mining Corp. operates under a storm water pollution prevention plan that includes Pinto Valley Mine facility operations on private and public lands. The storm water pollution prevention plan follows requirements of the storm water multisector general permit (No. AZMSG2010-003) (Arizona Department of Environmental Quality 2010). Process and storm water discharges that do not qualify for coverage under the multisector general permit are covered under an Arizona Pollutant Discharge Elimination System permit (Individual Permit No. AZ0020401) (Pinto Valley Mining Corp. 2014). Topography and natural drainage at the Pinto Valley Mine have been altered so that natural flow does not enter Pinto Creek. Storm water and seepage management systems have been constructed to contain storm water runoff on site through a

system of impoundments and water-containment structures designed and managed to contain the 100-year or 24-hour storm event. There are seven discharge points draining to Pinto Creek from the Pinto Valley Mine property. These discharge points are identified in table 2-5, in the column labeled “Arizona Pollutant Discharge Elimination System Compliance Monitoring Locations” (WestLand Resources, Inc. 2016b). The locations of the compliance monitoring points for the Arizona Pollutant Discharge Elimination System permit and the Aquifer Protection Permit are shown on figure 2-9.

Non-discharging basins are areas where runoff is contained and does not discharge to downstream receiving waters. These waters are contained in storm water and seepage ponds that collect flows that are either used in the service water circuit or left in the collection area to evaporate. 

In addition to the Arizona Pollutant Discharge Elimination System compliance monitoring locations, there are three locations in West Basin and Lower Cottonwood Canyon Basin that are regulated by the multisector general permit. SW-WB3 is an outfall in West Basin that contains a sampler for storm water monitoring. Seep MG-5b is a French drain used to relieve saturation (storm water) for the Nos. 1 and 2 Tailings basin. This drain, also in West Basin, has been dry for many years. SW-LCC1 is an outfall that consolidates and discharges storm water runoff from the Lower Cottonwood Canyon Basin. 

A Best Management Practices Plan is part of the Arizona Pollutant Discharge Elimination System Permit (part IVB) and is reviewed annually and disclosed in the Arizona Pollutant Discharge Elimination System report each year. Two categories of best management practices are required in the storm water pollution prevention plan. 

Structural best management practices are installed to provide appropriate storm water runoff control and adequate sediment and erosion protection to reduce sediment and associated pollutant loading the receiving waters. These include diversions, culverts, riprap, and sediment control ponds; soil covers over tailings impoundments or embankments; and rock armor placed on the downstream face of the No. 4 Tailings embankment (Pinto Valley Mining Corp. 2015a).

Non-structural best management practices incorporate good housekeeping measures and preventative maintenance. Good housekeeping measures include proper storage of materials and removal of nonessential or waste materials from the site. Preventative maintenance includes keepings pumps, motors, and mobile equipment in good operating order and documenting records of maintenance (Pinto Valley Mining Corp. 2015a).

Table 2-5. Storm water basins and surface water monitoring at Pinto Valley Mine

Basin Name

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Sources: WestLand Resources, Inc. 2016b; Pinto Valley Mining Corp. 2015a

 

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Figure 2-9. Water resources sampling locations

 

2.4 Groundwater Resources

 

Recharge, storage, and movement of groundwater depend on bedrock and alluvial geologic conditions, climate, and topography of a site. The general stratigraphic and structural framework throughout the hydrologic study area and the proposed project area are described in section 3.8, “Geology, Minerals, Geotechnical Stability, and Paleontology,” in the Pinto Valley Mine EIS. Interpreted subsurface geology across the hydrologic study area is based on information compiled from data and maps published by the U.S. Geological Survey and Arizona Geological Survey and data provided by Pinto Valley Mining Corp. in the “Pinto Valley Mine Groundwater Modeling Report for Mine Extension (Revised)” (SRK Consulting, Inc. 2019a). 

 

2.4.1 Hydrogeologic Unit

Groundwater occurs both within the bedrock units that underlie the hydrologic study area and in alluvium that veneers bedrock in the valley bottom areas along the major drainage courses. These units are described below and additional information can be found in the “Pinto Valley Mine Groundwater Modeling Report for Mine Extension (Revised)” (SRK Consulting, Inc. 2019a).

 

2.4.1.1 Bedrock Units

Interpreted surface and subsurface geology and hydrogeology across the hydrologic study area are based on information compiled from data and maps published by the U.S. Geological Survey and Arizona Geological Survey and unpublished data provided by Pinto Valley Mining Corp. This information was used to construct a geologic map and series of interpretive geologic and hydrogeologic cross-sections across the hydrologic study area as presented in appendix A in the “Pinto Valley Mine Groundwater Modeling Report for Mine Extension (Revised)” (SRK Consulting, Inc. 2019a). The results of borehole aquifer tests conducted both at the Pinto Valley Mine and the adjacent Carlota Mine to estimate aquifer parameters are also summarized in the groundwater model report (SRK Consulting, Inc. 2019a). 

The geology and structural conditions within the project area and across the hydrologic study area are complex. The geologic map and interpreted cross-sections constructed across the area identify 18 bedrock units that range from Precambrian (greater than 600 million years before present) to late Tertiary (5 million years before present) and numerous faults. With the exception of the Gila conglomerate aquifer described below, the movement and storage of groundwater in the bedrock units throughout the project area and hydrologic study area are generally controlled by the interconnection of fractures within the rock mass. Typically, in these types of rocks, the density (spacing), aperture, and interconnection of these fractures are highly variable within individual or between rock units. This variability is reflected in the results of aquifer testing conducted in the area around the Pinto Valley Mine and Carlota Mine that indicate that the bedrock complex is highly heterogeneous with respect to hydraulic conductivity, which range over six orders of magnitude overall and up to four orders of magnitude within a specific rock unit (SRK Consulting, Inc. 2019a).

The Gila conglomerate is the principal aquifer in Globe-Miami Mining District, the Pinto Valley watershed, and for the Pinto Valley Mine. Historically, the Gila conglomerate has served as the primary water source of water for several mining projects in the Globe-Miami Mining District. However, as early as 1962, the U.S. Geological Survey recognized that overdraft was depleting the quantity of water available in the Gila conglomerate in the Globe Valley (located in the adjacent area east of the hydrologic study area) (Peterson 1962). Pinto Valley Mine’s main water source is provided by extracting groundwater from the Peak Well System northwest of Tailings Storage Facility No. 4 on the east and

west sides of Pinto Creek. The primary target for completion of these wells is the Gila conglomerate and Dacite unit. Available information from the Peak Well System indicates that Gila conglomerate and underlying Dacite unit are highly permeable, and transmissivity in the well field area with yields of up to 1,100 gallons per minute is reported for individual wells (SRK Consulting, Inc. 2019a). Based on review of available data from the Peak Well System, SRK Consulting, Inc. (2019a) concluded the high permeability and transmissivity of the Gila conglomerate and Dacite unit resulted from “significant fracturing throughout the Peak wellfield.” 

 

2.4.1.2 Alluvial Unit

Groundwater occurs in relatively thin alluvial deposits that veneer bedrock along stream channels and associated floodplain along sections of Pinto Creek and tributaries to Pinto Creek. Alluvial deposits consist of unconsolidated gravel, sand, and silt, with the coarser material deposited along the stream channel and finer-grained material deposited in the floodplain. Groundwater in alluvium is stored and transmitted through interconnected pores in the granular deposits. The width and thickness of the alluvial deposits is highly variable along Pinto Creek (SRK Consulting, Inc. 2019a). In the upper portions of the watershed, the width of the alluvium along Pinto Creek is typically on the order of a few tens of feet, whereas in the lower portion of the watershed, the width of alluvium increases to up to approximately 1,500 feet. The thickness of the deposits is estimated to range from 0–10 feet where the stream channel is narrow and controlled by crystalline bedrock up to an estimated 80 to 100 feet where the channel widens at the confluence of major tributaries (SRK Consulting, Inc. 2019a). The shallow alluvial groundwater system is assumed to be hydraulically connected to the deeper bedrock groundwater flow system, although the hydraulic connection to the underlying bedrock aquifer has not been specifically defined (SRK Consulting, Inc. 2019a).

 

2.4.1.3 Hydrostructural Units

Groundwater flow pathways are influenced by major faults that offset and displace rock units. Depending on the physical properties of the rocks involved, faulting may create either barriers or conduits for groundwater flow. For example, faulting of softer, less-competent rocks typically forms zones of crushed and pulverized rock material that behave as a barrier to groundwater movement. Faulting of hard, competent rocks often creates conduits along the fault trace, resulting in zones of higher groundwater flow and storage capacity along the fault zone compared to the un-faulted surrounding rock.

Major regional faults identified in the hydrologic study area are shown on the geologic maps of the Pinto Valley watershed area (map 3-5) and Pinto Valley Mine area (map 3-6) and in selected cross-sections shown in figures 3-11, 3-12, 3-13a, and 3-13b in section 3.8, “Geology, Minerals, Geotechnical Stability, and Paleontology,” in the EIS. Major regional faults identified in the hydrologic study area include the Kennedy fault (a northeast-trending fault located west of the Peak Well System), and the West Branch of the Gold Gulch fault (a north- to northwest-trending fault zone) that traverses the western portion of the Open Pit and east side of Tailings Storage Facility No. 4. Other important fault zones that occur in the mine area include the Gold Gulch fault system, Jewel Hill fault system, South Hill fault, and Dome Hill fault.

A hydrogeologic investigation was conducted in late 2015 and early 2016 to evaluate the hydraulic conductivity groundwater elevations in the Gold Gulch, Jewell Hill, and South Hill fault zones that flank the Open Pit (SRK Consulting, Inc. 2016a). The investigation included drilling core holes, conducting packer testing, and installing vibrating wire piezometers in the hanging wall central portion and footwall

of each fault zone. The results of the investigation combined with the water level monitoring indicated that the faults were not barriers to groundwater flow (SRK Consulting, Inc. 2016a). Information regarding the permeability of the other faults in the hydrologic study area is not available.

 

2.4.2 Groundwater Extraction

The location of currently used water supply wells for the Pinto Valley Mine and Carlota Mine are shown on figure 2-10. Groundwater extraction for the Pinto Valley Mine occurs through pumping from selected production wells within the Peak Well System, and pumping of water collected in sumps excavated into the floor of the Open Pit. The Peak Well System extends northwest of Tailings Storage Facility No. 4 and includes production wells on the east and west sides of Pinto Creek, and two additional wells (Peak Wells 26 and 29) northwest of Tailings Storage Facility No. 3 east of Pinto Creek. Pumping rates for the well field range from 2,000 to 4,000 gallons per minute and averaged 2,980 gallons per minute for the 2013–2018 period (SRK Consulting, Inc. 2019a). Groundwater inflows into the Pinto Valley Mine Open Pit historically ranged from 200 to 300 gallons per minute during mining and were estimated to be approximately 100 gallons per minute at the end of the steady-state period (end of 2013). 

Two water supply wells (TW-2 and BMW 32) associated with the Carlota Mine are adjacent to Pinto Creek west of the Pinto Valley Mine (figure 2-10). The estimated total pumping rate for the two Carlota Mine water wells is 343 gallons per minute for the 2013–2017 period. The Carlota Mine has one open pit (Cactus-Carlota Open Pit) that extends below the regional groundwater table. The estimated average groundwater inflow into the Cactus-Carlota Open Pit is fewer than 50 gallons per minute (SRK Consulting, Inc. 2019a).

 

2.4.3 Groundwater Levels, Gradients, and Flow Patterns 

A groundwater elevation contour map for the Pinto Creek watershed (figure 2-11) was developed based on measured water levels in available wells and the surface elevation of spring and stream sites with observed surface water recorded in June 2018 (Ajax, Ltd. 2018). The water level data used to construct the contour map include measurements from inactive Peak Wells and shallow alluvial wells within the Pinto Valley Mine property, bedrock and alluvial monitoring wells associated with the Carlota Mine, and private ranch wells identified within the northern portion of the watershed. As summarized in section 2.3, the surface water inventory identified surface water or surface water flow at 39 sites. The occurrence of surface water flow identified during the field survey is assumed to reflect discharge from the groundwater flow system. 

The groundwater elevation contours indicate that the groundwater flow generally mimics the topography with flow from higher to lower elevations and from the southern to northern perimeters of the watershed. The groundwater contours also indicate that the predominant flow pattern in the area adjacent to and north of the mine in the watershed is toward Pinto Creek and Campaign Creek.

 

 

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Figure 2-10. Location of existing pumping wells

 

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Figure 2-11. Groundwater elevation contours – hydrologic study area (June 2018)

 

Groundwater elevations range from approximately 4,300 feet above mean sea level south of Pinto Valley Mine to approximately 2,270 feet above mean sea level near the confluence of Pinto Creek and Campaign Creek near the northern boundary of the Pinto Creek watershed (SRK Consulting, Inc. 2019a). The general direction of groundwater flow in the vicinity of the Pinto Valley Mine is from higher elevations in the south-southeast of the mine to lower elevations north-northwest of the mine. However, the groundwater contours indicate that the groundwater flow pattern is disrupted by the pit dewatering, as illustrated by the fact that all groundwater flow in the area surrounding the pit is toward the pit.

 

2.4.4 Regional Groundwater Budget 

Average Inflow and outflow from the groundwater system were estimated by SRK Consulting, Inc. (2019a) to establish a baseline quasi-steady-state water balance for the hydrologic study area. The quasi-steady-state period extends from January 2011 through December 2012 and corresponds to a “care and maintenance period” for the Pinto Valley Mine. A conceptual model of the groundwater flow system and description of the water balance components are provided on figure 2-12. The estimated average annual steady-state (2011–2012 period) groundwater budget is presented in table 2-6. Existing groundwater inflow components include precipitation recharge, artificial recharge (draindown from mine facilities), and surface and subsurface flow (stream recharge to the groundwater system). The estimated water balance assumes that that no groundwater inflow occurs from areas outside the hydrologic study area, and all of the water discharged to Haunted Canyon for mitigation of riparian vegetation is consumed by evapotranspiration and does not recharge the groundwater system (SRK Consulting, Inc. 2019a). 

Groundwater outflow components include evapotranspiration from phreatophyte, evaporation from pit lakes, groundwater pumping from existing Pinto Valley Mine and Carlota Mine operations, discharge to streams and springs, and subsurface outflow to adjacent areas outside the hydrologic study area (deep groundwater leaving the system along the northern boundary of the Pinto Valley watershed) (SRK Consulting, Inc. 2019a).

 

Table 2-6. Estimated annual groundwater budget for the hydrologic study area (quasi-steady state1)

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Source: SRK Consulting, Inc. 2019a

 

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Figure 2-12. Conceptual model (2011–2012)

 

2.5 Water Quality

 

2.5.1 Water Quality Standards

Federal regulations ensure the protection of water resources under the Safe Drinking Water Act and the Clean Water Act. The Arizona Department of Environmental Quality is responsible for implementing these regulations in Arizona. 

The Arizona Administrative Code, title 18, Environmental Quality, chapter 11, Water Quality Standards for the State of Arizona, article 1 defines water quality standards for surface waters, article 4 defines aquifer water quality standards, and article 6 describes impaired water identification (Arizona Department of Environmental Quality 2009a, 2009b). Water quality standards for surface water and groundwater, as well as compliance standards, are listed in table 2-7 and table 2-8, respectively.

 

2.5.1.1 Impaired Waters

Section 303(d) of the Clean Water Act authorizes the U.S. Environmental Protection Agency to assist States in listing impaired waters and developing total maximum daily loads for these waterbodies. A total maximum daily load establishes the maximum amount of a pollutant allowed in a waterbody and serves as the starting point or planning tool for restoring water quality. The Arizona 2016 303(d) list of impaired waters identifies three segments in the Pinto Creek watershed that are impaired (Arizona Department of Environmental Quality 2016b) (shown on figure 2-2):

. Pinto Creek from the headwaters to the confluence of West Fork Pinto Creek with Pinto Creek: impaired for copper

 

. Five Point Tributary from the headwaters of the tributary to the confluence with Pinto Creek: impaired for copper (dissolved)

 

. Pinto Creek from the confluence of West Fork Pinto Creek to Roosevelt Lake: impaired for selenium and copper (Arizona Department of Environmental Quality 2016a)

 

 

The Gibson Mine has been the largest source of copper to Pinto Creek. Several remediation projects have been completed at the mine since 2007, reducing the dissolved copper concentrations to the Gibson Mine tributary by 85 percent. Additional remediation is ongoing (Arizona Department of Environmental Quality 2017a).

 

2.5.1.2 Monitoring Requirements 

Three permits set compliance monitoring requirements for storm water discharge and groundwater quality at points of compliance at the Pinto Valley Mine. Arizona Pollutant Discharge Elimination System permit No. AZ0020401 authorizes storm water discharge mixed with mine process water and mine drainage from the Pinto Valley Mine (Arizona Department of Environmental Quality 2014). The Arizona Pollutant Discharge Elimination System multisector general permit No. AZMSG2010-003 authorizes discharge under the Arizona Pollutant Discharge Elimination System program (Arizona Department of Environmental Quality 2010). The Aquifer Protection Permit No. P-100329 sets the aquifer water quality standards for the point of compliance wells for the Pinto Valley Mine (Arizona Department of Environmental Quality 2017b). The Arizona Pollutant Discharge Elimination System and Aquifer Protection Permit monitoring locations are shown on figure 2-9. 

 

Table 2-7. Surface water quality standards

Formula Placeholder

Sources: Title 40, Part 141.51 of the Code of Federal Regulations; Title 40, Part 143.3 of the Code of Federal Regulations; Arizona Administrative Code R 18-11, Article 4; Arizona Administrative Code R18-11, Article 1; Arizona Department of Environmental Quality Permit No. AZ0020401

All units in milligrams per liter except where noted in constituent name.

 

Table 2-8. Groundwater quality standards

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Sources: Title 40, Part 141.51 of the Code of Federal Regulations; Title 40, Part 143.3 of the Code of Federal Regulations; Arizona Administrative Code R 18-11, Article 4; Arizona Aquifer Protection Permit No. P-100329

 

2.5.2 Total Dissolved Solids and Sulfate Concentrations

 

The total dissolved solids concentrations is commonly used as a general indicator as to a water’s suitability for a particular use (Driscoll 1986). A simple, widely used classification for water based on the total dissolved solids concentrations (Freeze and Cherry 1979) is presented in table 2-9. In accordance with this classification waters with total dissolved solids concentrations of 1–1,000 milligrams per liter are considered “fresh water”; and, waters with total dissolved solids concentrations of 1,000 to 10,000 milligrams per liter are considered “brackish.” Brackish waters are characterized as distastefully salty (U.S. Geological Survey 2019a). In addition, brackish waters are considered unfavorable to the growth of most plant species and “without appropriate management it is damaging to the environment” (U.S. Geological Survey 2019a). 

Beneficial uses of water include domestic and municipal drinking water, livestock and wildlife water sources, irrigation and industrial (mining, manufacturing). Water that contains high total dissolved solids concentrations may not be suitable for certain beneficial uses. Water that contains less than 500 milligrams per liter total dissolved solids concentrations is acceptable for domestic use and most other beneficial uses. Water with greater than 1,000 milligrams per liter contains minerals that give water an unacceptable taste as a drinking water supply and may be unsuitable for other uses in other ways. For example, wells with high total dissolved solids are potentially corrosive to metal well screens and other parts of well structure regardless of other chemical characteristics (Driscoll 1986). 

 

Table 2-9. Water classification based on total dissolved solids

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Source: Freeze and Cherry 1979

 

The National Secondary Drinking Water Regulation uses 500 milligrams per liter for total dissolved solids and 250 milligrams per liter for sulfate. These secondary standards defined as part of the National

Secondary Drinking Water Regulations (U.S. Environmental Protection Agency 2016) are non-enforceable guidelines regulating contaminants in drinking water that may cause cosmetic effects (such as skin or tooth coloration) or aesthetic effects (such as taste, odor, or color). U.S. Environmental Protection Agency recommends that systems for drinking water comply with these standards (although compliance is not required). Some States (including California, Nevada, and Utah) have adopted secondary standards for total dissolved solids and sulfate as enforceable standards for the protection of groundwater quality. However, Arizona has not adopted secondary standards or enforceable secondary standards to regulate total dissolved solids or sulfate concentrations.

 

2.5.3 Surface Water Quality

Surface water quality data collected by Pinto Valley Mining Corp. during the baseline period of 2014 through 2016 were reported in the Pinto Valley Mining Corp. Arizona Pollutant Discharge Elimination System Annual Reports (Pinto Valley Mining Corp. 2015a, 2016a, 2017a). These data were compared to standards found in table 2-7 (surface water quality standards). Surface water quality summaries are provided in attachment B (“Pinto Valley Mining Corp. Arizona Pollutant Discharge Elimination System Compliance Summary Tables”). Table 2-10 shows the average annual result for parameters that exceeded standards organized by monitoring location and year. 

The parameters of concern for the Arizona Pollutant Discharge Elimination System permit at Pinto Valley Mine that were reported as exceeding the Arizona Pollutant Discharge Elimination System permit requirements are dissolved selenium and total selenium. The standard was exceeded at MG1-12b, MG2-8b, and MPO-1b during the 2014–2016 period. Average annual results for total chromium for PV-005 were included in table 2-10 because there is a note in the permit that requires additional analysis if total chromium is detected above 0.008 milligram per liter. Total chromium results were reported as 0.0081 milligram per liter on February 4, 2014. If total chromium exceeds 0.008 milligram per liter, the permit states that Pinto Valley Mining Corp. must sample for chromium VI for the remainder of the permit. 

The Arizona Pollutant Discharge Elimination System permit also requires that total dissolved solids, dissolved sulfate, and total sulfate be reported. However, there are no surface water quality standards (or exceedance limits) assigned to these parameters in the Arizona Pollutant Discharge Elimination System permit. 

 

Table 2-10. Average Arizona Pollutant Discharge Elimination System data for parameters that exceeded Arizona Pollutant Discharge Elimination System or other standards 2014–2016 at Pinto Valley Mine

Formula Placeholder

Sources: Pinto Valley Mining Corp. 2015a, 2016a, 2017a

PV005 reported chromium (T) result higher than limit of 0.008 milligram per liter in quarter 1 of 2014 (0.0081 milligram per liter on 2-4-14). According to the Arizona Pollutant Discharge Elimination System permit, if total chromium exceeds 0.008 milligram per liter, Pinto Valley Mining Corp. must sample for chromium VI for the remainder of the permit. 

Averages were calculated using 1/2 detection limit for non-detected parameters. 

 

2.5.4 Groundwater Quality

Groundwater quality data collected by Pinto Valley Mining Corp. during the baseline period of 2014 through 2016 were reported in the Pinto Valley Mining Corp. Aquifer Protection Permit Annual Reports (Pinto Valley Mining Corp. 2015b, 2016b, 2017b). These data are summarized in tables C-1, C-2, C-3, and C-4 provided in attachment C. The groundwater quality data were compared to standards listed in table 2-8.

The parameters of concern for the Aquifer Protection Permit at Pinto Valley Mine that were reported as exceeding the Aquifer Protection Permit requirements are total dissolved solids (alert level), sulfate (alert level), and gross alpha (aquifer quality limit). Table 2-11 summarizes the alert level and average annual result for total dissolved solids and sulfate for each compliance monitoring location. Total dissolved solids and sulfate have varying alert levels that are shown for each compliance location (table 2-11). The alert levels for total dissolved solids and sulfate were removed in the fourth quarter of 2015. Total dissolved solids and sulfate are currently designated in the compliance permit as “monitor only.” Figure 2-13 illustrates the relative magnitude of average annual sulfate concentrations for the 2014–2016 period by location. 

The national secondary standards (secondary maximum contaminant levels) for drinking water for total dissolved solids (500 milligrams per liter) and sulfate (250 milligrams per liter) are provided in table 2-11 for comparative purposes. As listed in table 2-11 and illustrated on figure 2-13, monitoring wells immediately downgradient from Tailings Storage Facility Nos. 1 and 2, Tailings Storage Facility No. 3, and Tailings Storage Facility No. 4 have relatively high concentrations of total dissolved solids and sulfate that exceed their respective secondary maximum contaminant levels.

Results from Aquifer Protection Permit-3A and Aquifer Protection Permit-3B for gross alpha (total and dissolved) exceeded the aquifer quality limit of 15 picocuries per liter consistently from the first quarter of 2014 through the third quarter of 2015. In the fourth quarter of 2015, the aquifer quality limit was removed from the permit. The requirement for gross alpha (total and dissolved) from the fourth quarter of 2015 through present is “monitor only.” It should be noted that gross adjusted alpha 6 for Aquifer Protection Permit-3A and Aquifer Protection Permit-3B was found to be below the 15 picocuries per liter aquifer quality limit from first quarter 2014 through fourth quarter 2016. 

 

Table 2-11. Average Aquifer Protection Permit data for parameters that exceeded Arizona Pollutant Discharge Elimination System or other standards 2014–2016 at Pinto Valley Mine

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Sources: Pinto Valley Mining Corp. 2015b, 2016b, 2017b

Alert level for total dissolved solids and sulfate (D, T) are specific to compliance point.

Averages calculated using 1/2 detection limit for non-detected parameters.

 

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Figure 2-13. Groundwater quality – average sulfate concentrations (2014–2016)

 

2.6 Rock Geochemistry 

 

2.6.1 Material Sampling and Testing

Mine rock and tailings have been collected and geochemically analyzed at the Pinto Valley Mine beginning in 1995 and continuing through 2016. The purpose for sampling has included compliance with the Arizona Department of Environmental Quality Aquifer Protection Permit program, site understanding to support potential reclamation activities, and due diligence related to acquisition.

 

2.6.1.1 Geochemical Sampling

Sampling has been conducted at several major facilities associated with the Pinto Valley Mine site, as well as some locations not specifically identified as discrete facilities. The results of the sampling and geochemical testing were used to define the rock geochemistry of the existing site conditions. Table 2-12 and table 2-13 report the number of samples collected in terms of mine facility (table 2-12) and rock type (table 2-13). These tables are taken from Schlumberger Water Services (2008a), which compiles work from Schafer & Associates (1995) and MWH Americas, Inc. (2005). Schlumberger Water Services (2008a) also reports samples collected broken down by rock type (lithology). This information is summarized in table 2-13 by lithology. 

Sampling of waste rock dumps did not include drilling of the dumps to depth, but instead focused on more easily accessible surficial positions. Schafer & Associates (1995) collected samples from 6–18 inches in depth from the north and south waste dumps, as well as the leach dump. The study describes that the waste rock dumps were constructed by end-dumping. This provided the opportunity to sample across the tops of the dumps and sample rocks deposited over the time of dump construction and estimate the volumes of different rock types. MWH Americas, Inc. (2005) collected samples of waste rock dumps using test pits that ranged in depth from 2–20 feet. The study provides no description of sampling transects or other position information. Specific non-tailings locations sampled are listed in table 2-12.

Sampling of tailings conducted by Schafer & Associates (1995) were, like waste rock, from very surficial depths. MWH Americas, Inc. (2005) collected samples of tailings from much greater depths (as deep as 200 feet). Both of these investigations collected samples from the tailings storage facilities shown in table 2-12. 

A single composite sampled of Ruin granite (quartz monzonite) was prepared from exploration cores (SRK Consulting, Inc. 2016a). Four exploration drill cores were sampled from depths ranging from 330 to 810 feet. This composite was prepared to represent the average geochemical character of the rock anticipated to be excavated as part of the proposed action. SRK Consulting, Inc. (2016b) describes the composite as prepared by Capstone resource geologists as “representative,” although the basis for this description is not documented. 

Other tested samples include samples of metallurgically prepared tailings that are the expected geochemical character of tailings to be produced under the proposed action (SRK Consulting, Inc. 2016c). 

 

Table 2-12. Static test samples by facility

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Source: Schlumberger Water Services 2008a

 

Table 2-13. Static test samples by lithology

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Source: Schlumberger Water Services 2008a

 

2.6.1.2 Geochemical Characterization Testing

To date, 648 samples of historical waste rock, tailings, and heap leach rock have been submitted for various geochemical bulk characterization tests. As summarized by Schlumberger Water Services (2008a) and SRK Consulting, Inc. (2008), 647 were submitted between 1995 and 2008. One composite sample of Ruin granite has been prepared (SRK Consulting, Inc. 2016b) that is intended to represent waste rock produced under the proposed action. 

Standard geochemical analyses of mine rock and tailings have been conducted at Pinto Valley Mine and include:

. Acid-base accounting

 

. Meteoric water mobility procedure and synthetic precipitation leaching procedure

 

. Whole rock analysis

 

. Net acid-generating leachate

 

. Kinetic humidity cell testing

 

 

Acid-base accounting analysis determines the acid potential and neutralization potential of mine materials. Using acid potential and neutralization potential, mine materials may be classified as potentially acid generating or not potentially acid generating, but acid-base accounting analysis does not provide any indication of concentration of chemical constituents in contact water. To date, 384 samples have been submitted for acid-base accounting testing (table 2-12 and table 2-13). This includes the 383 shown in table 2-12 (Schlumberger Water Services 2008a) plus one from SRK Consulting, Inc. (2016b). These samples also include paste pH measurements. Paste pH measures the pH of a solid: water slurry. If products of sulfide mineral oxidation are present on the surface of the material, an acidic, low pH is observed for the slurry.

In addition to sampling for acid-base accounting, 88 samples have been collected for meteoric water mobility procedure and synthetic precipitation leaching procedure (table 2-12 and table 2-13). These tests are best described as simple rinse tests to gauge the release of water-soluble chemical constituents from the solid sample. The results may be used to describe the chemical quality of rain runoff from waste rock dumps, tailings embankments, and pit walls.

Whole rock analysis (126 samples; table 2-12 and table 2-13) (Schlumberger Water Services 2008b) provides no information regarding the potential to produce good or poor contact water quality. It does provide insight into which chemical constituents are present in the rock and offers a basis for not being concerned about constituents that are not present. For example, if arsenic is not detected from whole rock analysis, arsenic can be disregarded as a constituent of concern.

Net acid-generating leachate testing (35 samples; table 2-12 and table 2-13) combines a solid sample with hydrogen peroxide to rapidly oxidize reactive sulfide minerals. The sample produces all acidity, releases all metals, and allows any possible acid neutralization to occur. The pH of the resulting solution provides an indication of the likelihood of the material to eventually produce acid rock drainage. Analysis of the solution for metals gives insight into which metals might be released during weathering under field conditions. The concentrations are not, however, particularly helpful for understanding their concentration in field contact water.

Humidity cell testing is a kinetic test (28 samples; table 2-14) that exposes a solid sample to abundant water and air to oxidize reactive sulfide minerals. The sample is rinsed weekly to measure the release of

chemical constituents. The test provides two useful types of data. First, the test gauges the relative rate of acid production through the oxidation of sulfide minerals (such as pyrite and iron disulfide) and the consumption of acid by neutralizing minerals (such as calcite and calcium carbonate). These relative rates of acid production are routinely used to gauge the timing for any potential onset of acid rock drainage; acid production may occur in the near term, the long term, or not at all. The second type of data provided by humidity cell testing analysis is an assessment of the rate of release of chemical constituents that may dissolve into contact water. These data provide a basis for estimating the quality of water contacting mine materials over extended times. The first rinse of the humidity cell testing is similar to a meteoric water mobility procedure or synthetic precipitation leaching procedure in that it gauges the release of water-soluble chemical constituents initially present on a solid sample. 

 

Table 2-14. Completed humidity cell tests

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Sources: Schafer & Associates 1995; MWH Americas, Inc. 2005; Schlumberger Water Services 2010; SRK Consulting, Inc. 2016b

 

2.6.2 Geochemical Testing Results

 

2.6.2.1 Potential to Produce Acid Rock Drainage

Acid-base accounting data (Schlumberger Water Services 2008c) are interpreted in several ways to characterize the potential for a mine material to develop acid rock drainage. Typical parameters to consider include:

. The neutralization potential ratio, which is the ratio of neutralization potential to acid potential

 

. The net neutralization potential, which is the difference between neutralization potential and acid potential

A standard graphical assessment of acid-base accounting data is to plot the neutralization potential versus the acid potential. Data points that plot below a one-to-one (1:1) line (closer to the acid potential axis) are considered likely to produce acid rock drainage. Data points above a one-to-three (1:3) line (closer to the neutralization potential axis) are considered not potentially acid generating. Data points that fall between these lines are considered uncertain with respect to their potential to produce acid rock drainage. Likely and uncertain samples are together characterized as potentially acid generating.

Figure 2-14 (Schlumberger Water Services 2008c) is a graph of neutralization potential versus acid potential for Pinto Valley Mine acid-base accounting data (except for data from SRK Consulting, Inc. 2016b). The figure shows acid-base accounting by facility at the mine site. Figure 2-15 is an identical graph except that data are illustrated by lithology. For either graph, samples tend to be either not potentially acid generating or likely to produce acid rock drainage. Very few samples are considered uncertain. 

In terms of lithology, not potentially acid generating material samples are dominated by altered limestone and Gila conglomerate. These materials have abundant calcium carbonate and very little or no sulfide mineralization. Materials classified as likely to produce acid rock drainage are dominated by most tailings samples and quartz monzonite (Ruin granite), with some diabase. Quartz monzonite is the dominant waste rock material for the proposed action.

In Arizona, draft guidance exists (Arizona Department of Environmental Quality 1998) to classify mine material as chemically inert and not potentially acid generating. This guidance indicates that mine materials are not potentially acid generating if either of the following conditions is true:

. Net neutralization potential is greater than zero AND total sulfur is less than or equal to 0.3 percent, or  

. Neutralization potential ratio is greater than three AND total sulfur is less than or equal to 0.3 percent.

 

There are also current prescriptive guidelines (Arizona Department of Environmental Quality 2004) for classifying mine materials. Arizona Best Available Demonstrated Control Technology guidance specifies the interpretation of neutralization potential and acid potential values as follows: 

. If net neutralization potential is less than -20, the rock can be considered acid generating; 

. If net neutralization potential is greater than +20, the rock can generally be considered not potentially acid generating; and 

. Samples that fall between -20 and +20 are considered uncertain, and may be tested further using kinetic testing methods that would be expected to assist in the design of the facility, specifically design of acid rock drainage controls. 

In terms of neutralization potential ratio, Best Available Demonstrated Control Technology specifies:

. Neutralization potential ratio greater than 3 indicates low risk for acid drainage to develop; 

. Neutralization potential ratio less than 1 indicates that the rock is “more likely to generate acid”; and

 

. Neutralization potential ratio between 1 and 3 indicates uncertainty and additional testing is usually necessary using kinetic test methods. 

 

For the 384 Pinto Valley waste rock samples tested, 65 percent are considered not potentially acid generating, or chemically inert, by the Arizona draft guidance (Arizona Department of Environmental Quality 1998). For tailings, using the draft Arizona guidance, only about 17 percent of samples can be considered inert. The median (average) neutralization potential ratio for tailings is 0.65 and the median net neutralization potential is -1.8. On the whole, tailings are characterized by this guidance as likely to generate acid rock drainage.

Using the Best Available Demonstrated Control Technology criteria (Arizona Department of Environmental Quality 2004), 65 percent of waste rock is not potentially acid generating, 29 percent is likely to generate acid rock drainage, and 6 percent is uncertain. For tailings, very consistent with Arizona Department of Environmental Quality criteria (1998), 17 percent is not potentially acid generating, 69 percent is likely to generate acid rock drainage, and 14 percent is uncertain.

Paste pH measurements of waste rock and tailings can provide an indication of past oxidation of sulfide minerals under field conditions. Values for paste pH vary widely across the site (Schlumberger Water Services 2008d), from very acidic, low pH, to alkaline (high pH). The range in values is reflective of not only the acid-base accounting, net capacity to produce acid rock drainage, but also the time that the material may have been in the field exposed to weathering. Acidic paste pH measurements are dominated by tailings, leach dump samples, and quartz monzonite.

In contrast to acid-base accounting testing to gauge the net capacity to produce acid rock drainage, humidity cell testing characterization describes the rate at which acid-generating reactions occur relative to the rate of acid-neutralizing reactions. For mine materials that are considered likely, or uncertain, to produce acid rock drainage, the results of humidity cell testing provide an indication of how rapidly neutralization potential may be consumed for acid rock drainage to develop, if at all. In some cases, reactive sulfides in mine material are bound up in chemically inert silicate minerals and do not oxidize rapidly, or at all.

 

 

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Figure 2-14. Acid-base accounting data facility

Source: Schlumberger Water Services 2008c 

 

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Figure 2-15. Acid-base accounting data by lithology

Source: Schlumberger Water Services 2008c

 

Humidity cell testing work on historical Pinto Valley Mine rock indicates that quartz monzonite and diabase have a potential to produce acid rock drainage (table 2-15). In total, seven samples of quartz monzonite have been characterized using humidity cell testing. Six samples are reported by Schafer & Associates (1995) that had weathered under field conditions and one, a composite sample produced from exploration core, is reported by SRK Consulting, Inc. (2016b). Six of the seven samples developed low pH leachate (acid rock drainage) during the test. These tests also produced very high sulfate concentrations on the first flush of the humidity cell testing procedure, although analyzed metals were below detection limits. The most recent sample (SRK Consulting, Inc. 2016b), most representative of waste rock to be produced under the proposed action, reacted more slowly during testing than any other sample of quartz monzonite tested. This is attributed to reactive sulfide minerals being either relatively large or encased to some degree by unreactive minerals in the rock matrix. Either of these conditions reduces the exposed reactive surface area of the sulfide minerals compared with their total mass. Ultimately however, this sample is projected to have the potential to produce acid rock drainage in the long term. There is no description of the sulfide mineral occurrences in historical samples to document the extent that these materials are different from the SRK Consulting, Inc. (2016b) sample.

Only two samples of historical tailings have been characterized using humidity cell testing. Although neither sample produced acidic leachate during the test, acid-base accounting characterization of the samples and depletion rates of neutralization potential and acid potential indicate they are likely to ultimately produce acid rock drainage (MWH Americas, Inc. 2006). The 26 weeks of humidity cell testing was not long enough to deplete the tailings of neutralization potential to allow the onset of acidic leachate. 

 

Table 2-15. Results of humidity cell testing

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Sources: Schafer & Associates 1995; MWH Americas, Inc. 2006; Schlumberger Water Services 2010; SRK Consulting, Inc. 2016d, 2016b

 

2.6.2.2 Potential to Leach Metals

Meteoric water mobility procedure and synthetic precipitation leaching procedure are used to gauge the potential for contact water from rainfall to mobilize metals (and other chemical constituents). Similar to the wide range of paste pH for sampled mine materials, meteoric water mobility procedure and synthetic precipitation leaching procedure results display a wide range of concentrations of metals. Schlumberger Water Services (2008e) tabulates measured concentrations of metals for waste rock and tailings samples. Figure 2-16, figure 2-17, and figure 2-18 illustrate these data for copper, iron, and zinc to illustrate the wide range in concentrations measured. All rock types can release metals to contact water.

Meteoric water mobility procedure and synthetic precipitation leaching procedure results are commonly compared to different regulatory standards for perspective. Metals concentrations may be compared with aquifer water quality standards to identify runoff water quality that will not degrade groundwater quality upon infiltration. These concentrations may also be compared with surface water quality standards if the runoff is anticipated to be discharged to a stream. The highly variable meteoric water mobility procedure and synthetic precipitation leaching procedure metal concentrations result in instances when regulatory thresholds may be exceeded and many instances where they are not.

Net acid-generating leachate testing can also be used to gauge potential leaching of metals. This test uses peroxide on the sample and is not truly representative of field conditions. Therefore, it is used to characterize which metals may be mobilized upon oxidation of sulfide minerals. Like meteoric water mobility procedure and synthetic precipitation leaching procedure testing, net acid-generating leachate results indicate that a wide range of several metals could be released from mine materials upon oxidation in the field. Schlumberger Water Services (2008f) tabulates results for net acid-generating leachate metals. Figure 2-19, figure 2-20, and figure 2-21 illustrate typical data for copper, iron, and zinc. All rock types release metals to contact water upon exposure to oxidizing conditions.

 

 

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Figure 2-16. Meteoric water mobility procedure and synthetic precipitation leaching procedure copper results by lithology 

Source: Schlumberger Water Services 2008e 

 

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Figure 2-17. Meteoric water mobility procedure and synthetic precipitation leaching procedure iron results by lithology 

Source: Schlumberger Water Services 2008e

 

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Figure 2-18. Meteoric water mobility procedure and synthetic precipitation leaching procedure zinc results by lithology

Source: Schlumberger Water Services 2008e

 

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Figure 2-19. Net acid-generating copper results by lithology

Source: Schlumberger Water Services 2008e

 

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Figure 2-20. Net acid-generating iron results by lithology

Source: Schlumberger Water Services 2008e

 

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Figure 2-21. Net acid-generating zinc results by lithology

Source: Schlumberger Water Services 2008e

 

2.6.3 Facility Characterization

 

Mine facilities of geochemical concern include waste rock dumps, leach dumps, and tailings storage facilities.

 

2.6.3.1 Waste Rock

Geochemical characterization data are available primarily for the north dump, south dump, and 19 Dump waste rock storage facilities. There are very limited data for the northwest dump and West Dump.

On a facility basis, it is generally difficult to characterize the overall potential to produce acid rock drainage. However, Schafer & Associates (1995) reports that Magma Pinto Valley (an earlier mine operator) and Hargis & Associates compiled volume estimates for the amount that each principal rock type composes of the two major waste rock dump complexes at that time, the north and south waste rock dumps. Tables of the estimated volumes of various lithologies in these dumps are presented in Schafer & Associates (1995). These volume estimates can be used in combination with the average acid potential and neutralization potential of rock types to estimate an overall net neutralization potential and neutralization potential ratio. This then gives an indication of the overall likelihood of a waste rock dump to produce acid rock drainage. Table 2-16 presents the volume percentages of rock types in the north and south waste rock dumps (Schafer & Associates 1995) and the average acid potential and neutralization potential values for rock types (Schlumberger Water Services 2008d). Calculated overall net neutralization potential and neutralization potential ratio values are also shown.

 

Table 2-16. Acid-base accounting of waste rock dumps

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Sources: Schafer & Associates 1995; Schlumberger Water Services 2008d

 

The average overall net neutralization potential and neutralization potential ratio values calculated for the north and south waste rock dumps indicate that the facilities are not likely to produce acid rock drainage. Note, however, that given the range of acid-base accounting data for any given rock type, localized hotspots for acid rock drainage generation are quite possible. 

The 19 Dump has been well characterized to support its use as a source as a reclamation stockpile (SRK Consulting, Inc. 2016e). Data used to characterize this facility include data reported by Schafer & Associates (1995) as “south dump” and it is unclear if 19 Dump is a subset of south dump or a separate facility. There are 29 samples reported by SRK Consulting, Inc. (2016a) for 19 Dump. On a net neutralization potential and neutralization potential ratio basis, no sample is identified as likely to produce acidic contact water. On a net neutralization potential basis, six samples are net acid consumers (net neutralization potential greater than 20 and neutralization potential ratio greater than 3). On a neutralization potential ratio basis, all but one sample are net acid consumers (neutralization potential ratio greater than 3). Only one sample can be clearly classified as uncertain to produce acid rock drainage. It has a net neutralization potential less than 20, but greater than -20, and a neutralization potential ratio less than 3 but greater than 1. Pinal schist and Gila conglomerate are the only rock types reported to occur in this facility (SRK Consulting, Inc. 2016a), and with their average net neutralizing characteristics (table 2-16) it is likely that this facility represents overall net acid-consuming material.

Currently, there are no documented instances of persistent discharges of acid rock drainage or metal-baring solutions from waste rock. Similarly, there are no reported data that describe the chemical quality of storm water runoff. Wells proximal to waste rock facilities do not show the effects of acid rock drainage.

2.6.3.2 Leach Dumps

Leach dumps are more problematic than waste rock dumps. These facilities currently discharge acidic leachate as part of the engineered recovery of copper. The solutions did not form their acidic character through the natural weathering of the rock but from the addition of sulfuric acid. Leach dump rock itself has a median net neutralization potential of -14.1 and median neutralization potential ratio of 0.1, which is strongly affected by the removal of neutralization potential in the rock by the acidic leach process solutions. Material in the leach dump facilities is classified as likely to produce acid rock drainage.

The Leach Pile currently discharges acidic leachate as part of the engineered recovery of copper. The solutions did not form their acidic character through the natural weathering of the rock but from the

addition of sulfuric acid. Material in the leach dump facilities is classified as likely to produce acid rock drainage. 

Most of the leachate generated from the Leach Pile is recovered and processed for copper. However, the facility is unlined and a small percentage (less than 5 percent) of the current total pregnant leach solution is assumed to seep into groundwater directly beneath the footprint of the facility (SRK Consulting, Inc. 2019a). Most of this seepage to groundwater is captured within the drawdown cone associated with active pit dewatering that is known as the Active Containment Capture Zone (SRK Consulting, Inc. 2019a). The seepage from the leach pile contained within the capture zone discharges into the pit through fracture flow and as acid in the pit wall (SRK Consulting, Inc. 2019a). The seepage from leach pile that is outside this capture zone enters into the groundwater system and flows toward Pinto Creek (SRK Consulting, Inc. 2016f, 2019a). Water quality sampling and analyses from the groundwater monitoring well APP-7 located downgradient and west of the facility in Gold Gulch APP-7 (between the Leach Piles and Pinto Creek) has not indicated the presence of low pH or elevated metal concentration (attachment C, Table C-4). SRK Consulting, Inc. (2016f) estimates that seepage infiltrates and mixes with groundwater and flows toward Pinto Creek with an estimated travel time ranging from greater than 100 years to 500 years based on the low permeability of the bedrock and distance to Pinto Creek. SRK Consulting, Inc. (2016f) also notes that “Facility-specific closure plans will address the post-closure mine water management and potential draindown towards Pinto Creek.”

 

2.6.3.3 Tailings Storage Facilities

In contrast to waste rock, tailings are a processed, homogenized material and characterization of surficial samples is often thought to be more representative of an overall tailings storage facility than would be true for a waste rock dump. However, acid-base accounting data for Pinto Valley tailings show a wide range of acid-base accounting character. General statistics for tailings are presented in table 2-17. As shown in table 2-17, the acid-base accounting characteristics and paste pH of tailings in different facilities vary widely. As with waste rock, it is possible for tailings to develop acid rock drainage hotspots. For material within the tailings storage facility impoundment, any acidic solutions produced comingle with the bulk of the entrained water, which is alkaline, which neutralizes it. On the other hand, embankments can become unsaturated and exposed to the atmosphere (oxygen) and locally produce acid rock drainage products. These hotspots may, like waste rock dumps, result in acid rock drainage quality storm water in response to precipitation events.

 

Table 2-17. Summary of tailings acid-base accounting

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Source: Schlumberger Water Services 2008c

 

 

The potential for tailings to leach metals is, like other mine materials, highly variable. Examples of metal-leaching potential from tailings using the synthetic precipitation leaching procedure and meteoric water mobility procedure tests are shown on figure 2-16, figure 2-17, and figure 2-18.

The oldest facility is the Cottonwood Tailings Impoundment. Acid-base accounting data for this facility (SRK Consulting, Inc. 2016e) for data collected from near the surface (0–12 inches) through 40 feet deep are consistent with all other tailings storage facilities (see table 2-17). Also consistent with other facilities, the measured paste pH values ranged from acidic to alkaline. Paste pH values increased with depth and greater isolation from oxygen, ranging from 3.3 to 4.9 at the surface and up to 6.6 to 7.7 at depth. Synthetic precipitation leaching procedure and meteoric water mobility procedure tests yielded results that were all above Arizona Water Quality Standards.

The water entrained in tailings storage facilities is a neutral pH solution with high concentrations of sulfate and other chemical constituents such as calcium and magnesium. The high pH inhibits any elevated concentrations of metals. Monitoring wells immediately downgradient from the tailings storage facilities reflect the water quality within the tailings storage facilities. SRK Consulting, Inc. (2016c) reports water quality data for tailings facilities, and MWH Americas, Inc. (2005) provides water quality data for monitoring wells at the toe.

Currently, tailings storage facilities discharge alkaline, high sulfate concentration water with variable but low metal concentrations. Some acid rock drainage storm water runoff may occur from embankments.

 

2.6.3.4 For All Facilities

It is important to note that regardless of the acid-base accounting (and paste pH) characterization of potential to produce acid rock drainage, the actual production of acid rock drainage in field settings can be limited. Oxidation of reactive sulfide minerals requires the simultaneous presence of water and oxygen. Some facilities, like newer tailings storage facilities, contain water, but the bulk of the material is saturated with water and is mostly isolated from oxygen. As such, in active tailings storage facilities, acid rock drainage can only be produced from unsaturated surface material. Most characterized tailings samples were from near-surface depths (see above) and have an average paste pH of 5.9, which is only slightly acidic. However, the minimum tailings paste pH is 2.3 and indicates that greater amounts of oxidation can occur and lead to acidic contact water. As decommissioned tailings storage facilities desaturate to field capacity, oxygen can penetrate further, deepening any oxidation. 

In contrast to tailings, waste rock dumps are more freely oxygenated and may contain some moisture. Hence, sulfide minerals can more easily oxidize. However, any potential discharge of acid rock drainage from waste rock dumps requires sufficient precipitation (rain or snow melt) to produce flow through the material. No acid seeps are reported for waste rock or tailings storage facilities. Groundwater monitoring close to these facilities shows the effects of sulfide mineral oxidation (elevated sulfate concentrations), but not low pH. 

In summary, regardless of the results of acid-base accounting and humidity cell testing analyses, the discharge of acid rock drainage in the field is closely linked to the water balance of mine materials. Even if sulfide mineral oxidation reactions occur to produce the chemical ingredients of acid rock drainage, there must be sufficient water to dissolve and mobilize these constituents.

 

2.7 Water Rights

 

WestLand Resources, Inc. prepared an inventory of surface water rights that occur within the region surrounding the Pinto Valley Mine (WestLand Resources, Inc. 2018b). The inventory included all active surface water rights as shown on figure 2-22. The Arizona Department of Water Resources database was searched for all active surface water right applications, claims, permits, and certificates within the analysis area.

Table 2-18 provides a summary of inventoried water rights and includes the map ID corresponding to figure 2-22, location and permit information, priority dates, owner, type of use, and quantity of the right. 

The surface water right inventory identified 24 water rights that include 18 owned by the Tonto National Forest, 3 owned by Havens Ranch, and 3 owned by Cyprus Miami Mining Corp. All of the water rights except one (described below) were established as a source of water for stock or stock and wildlife.

The Tonto National Forest has an instream flow right to use waters flowing in Pinto Creek for recreation and wildlife, including fish, under permit No. 33-89109. The priority date for this water right is December 14, 1983 (Arizona Department of Water Resources 1999). An annual amount of 1,794.2 acre-feet per year (approximately 2.48 cubic feet per second on an annual basis) is required at the Pinto Valley weir to maintain sufficient flows for riparian vegetation, wildlife, aquatic habitat maintenance, and maintenance of the reach’s recreation amenities (Forest Service 1991). Instream flow requirements vary by month as specified in permit 33-89109 (Arizona Department of Water Resources 1999), also found in WestLand Resources, Inc. 2018b. 

Table 2-19 shows a comparison of actual flow volume at the Pinto Creek near Miami, AZ (09498502) gauge to the flow volume required to maintain sufficient flows in Pinto Creek per the instream flow right. In recent years the number of consecutive months recording zero flow has increased. There were no months recording zero flow in the period of record until 2014, and 26 months with no flow between January 2014 and December 2018.

The Arizona Groundwater Management Act was adopted in 1980 and is administered by Arizona Department of Water Resources. The goals of the Groundwater Management Act were to control overdraft occurring at the time in several areas within the State, provide a means to allocate the State’s limited groundwater resources, and augment the State’s groundwater through water development. In response to the Groundwater Management Act, the Arizona Department of Water Resources established a comprehensive management framework that established three levels of water management applicable to different areas with different groundwater conditions. The lowest level of management applies statewide. The second level applies to irrigation non-expansion areas. The highest level of management with the most restrictive provisions applies to active management areas that were designated to control severe overdraft. 

The entire hydrologic study area is situated outside of active management areas and irrigation non-expansion areas. Within areas outside of active management areas and irrigation non-expansion areas, groundwater may be withdrawn for beneficial use without a groundwater withdrawal permit. In other words, water rights or permits for the withdrawal of groundwater are not required to withdraw groundwater for beneficial use within the hydrologic study area. Therefore, the water rights inventory did not identify groundwater users (or groundwater withdrawal permits) because groundwater withdrawal permits are not required by the State.

 

Table 2-18. Surface water rights in the Pinto Creek watershed

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Source: WestLand Resources, Inc. 2018b, Inventory of Surface Water Rights near the Pinto Valley Mine.

 

 

Table 2-19. Recorded flow volume at Pinto Creek near Miami, AZ (U.S. Geological Survey 09498502) compared to the flow volumes specified in the Tonto National Forest instream flow right (permit 33-89109) (acre-feet)

TABLE

Source: U.S. Geological Survey 2019b, 2019c.

Note: Values in highlighted cells are less than the monthly or annual volume in acre-feet specified in Arizona Water Rights Permit 33-89109

 

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Figure 2-22. Water rights 

 

3.0 Environmental Consequences

 

3.1 Introduction

 

3.1.1 Key Issues for Detailed Analysis

Key issues for detailed analysis identified in the “Pinto Valley Mine Environmental Impact Statement Draft-Final Scoping and Issues Report” for water resources include:

. Issue 9A: Impacts on Groundwater Quantity – Effects from water use by the Pinto Valley Mine Project, which includes groundwater pumping in the Pinto Creek watershed, would include potential long-term impacts for the life of the project and beyond depending on length of time for recovery of groundwater levels after groundwater pumping ceases. 

 

. Issue 9B: Impacts on Groundwater Quality – The Pinto Valley Mine Project would result in potential impacts on groundwater quality from mine facilities during construction, operations, reclamation, closure, and post-closure activities. 

 

. Issue 9C: Impacts on Surface Water Quantity in the Pinto Creek Watershed – Reduction in groundwater quantity associated with Pinto Valley Mine water use could result in decreased flow and supply to the surface waters, which are fed by groundwater, including Pinto Creek and other streams, seeps, and springs in the area. Storm water management at Pinto Valley Mine facilities could change the amount of surface water moving downstream in the Pinto Creek drainage. 

 

. Issue 9D: Impacts on Surface Water Quality in the Pinto Creek Watershed – Storm water runoff could interact with hazardous materials, tailings, waste rock, spent heap leach materials, and ore stockpiles, which could result in contaminants moving downstream. Surface disturbance could result in increased sediment transport to downstream waters and cause aggradation or erosion in the downstream channels, leading to degradation of riparian habitat or impacts on surface water uses. 

 

 

3.1.2 Resource Indicators

Table 3-1 provides the resource indicators and measures for assessing potential effects on water resources.

 

Table 3-1. Resource indicators and measures for assessing effects

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3.1.3 Methodology

 

3.1.3.1 Groundwater Modeling

 

A calibrated three-dimensional numerical groundwater flow model was developed for the EIS to estimate effects on groundwater and surface water resources resulting from groundwater extraction and water management activities that would occur under the no-action alternative, proposed action, and cumulative scenarios (SRK Consulting, Inc. 2019a). Specifically, the numerical model was used to evaluate or estimate: (1) areal extent, magnitude, and timing of drawdown and recovery of groundwater levels through the mining and post-mining periods; (2) changes in baseflow to Pinto Creek; (3) development of the post-mining pit lake including groundwater inflow and outflow rates through the pit and final surface water elevations of the pit lakes; and (4) potential changes in the water balance in the hydrologic study area. 

The groundwater flow model was developed by SRK Consulting, Inc. (SRK Consulting, Inc. 2019a) using an enhanced version of the U.S. Geological Survey groundwater flow program MODFLOW (McDonald and Harbaugh 1988) known as MODFLOW-SURFACT (HydrogeoLogic 2011). MODFLOW-SURFACT contains many improvements over MODFLOW, including enhanced simulation capabilities for handling complex field conditions (including simulating large groundwater elevation fluctuations resulting in drying and wetting of grid cells). MODFLOW originally was designed to simulate flow through porous

media. MODFLOW models have been used to simulate groundwater flow in bedrock aquifers where flow through the bedrock system is controlled by interconnected fracture networks that behave similarly to porous media flow.

The groundwater model domain encompasses the entire hydrologic study area (approximately 200 square miles) as shown on figure 3-1. The numerical groundwater model consists of 21 horizontal layers to simulate the vertical range extending from 6,549 feet above mean sea level (the highest elevation in the hydrologic study area near the southern model boundary) to 1,500 feet below mean sea level (the uniform bottom elevation of the model). To provide more detailed flow information in the project area, the grid cell dimensions vary horizontally from 250 feet by 250 feet in the mine area and gradually coarsen out to 2,000 feet by 2,000 feet at the outer margins of the model. The more detailed grid cells in the mining area allow the model to more accurately match known or inferred conditions in the vicinity of the Open Pit, Peak Well System, and major mine facilities (such as pit geometry, well locations, groundwater levels, and hydrogeologic units). 

The model was calibrated to both quasi-steady-state and transient conditions. The quasi-steady-state period extends from January 2011 through December 2012 and corresponds to a “care and maintenance period” for the Pinto Valley Mine. During the care and maintenance period, leaching operations continued but there was no mining, processing of sulfide ore, or deposition of tailings. Make-up water for the leaching operation was predominantly sourced from pumping the sump-like pit lake depression in the floor of the Open Pit. When additional water was required, it was pumped from wells in the Peak Well System at a rate of 100 to 200 gallons per minute (SRK Consulting, Inc. 2019a).

The transient period for model calibration was defined as the period extending from January 2013 through December 2018. The transient period corresponds to the period when full mine operations were resumed and continued without interruption. Full mine operations included Open Pit mining, sulfide ore processing, and tailings deposition. The primary source of water for this period was from groundwater extraction from the 21 production wells (figure 2-10) in the Peak Well System and pit dewatering system located on Pinto Valley Mine property. During the 2013–2018 (transient) period, the Peak Well System was pumped at an estimated average production rate of 2,980 gallons per minute (SRK Consulting, Inc. 2019a). Other sources of groundwater extraction simulated during the transient period include inflow to the Pinto Valley Mine Open Pit (100 gallons per minute), inflow into the Cactus-Carlota Open Pit (50 gallons per minute), and pumping of production at the adjacent Carlota Mine (343 gallons per minute) (SRK Consulting, Inc. 2019a). 

The model was calibrated to measure groundwater levels, water supply pumping rates, estimates of groundwater baseflow discharge rates to Pinto Creek, and pit inflow estimates. The locations of wells with measured water levels that were used for steady-state and transient calibration are shown on figure 3-2. Key components of the conceptual model relative to the Pinto Valley Mine used for development of the numerical model are illustrated in the block diagram presented on figure 3-3. Details regarding the conceptual hydrogeologic model, modeling approach and setup, steady-state and transient calibrations, and simulations are presented in the “Pinto Valley Mine Groundwater Modeling for Mine Extension (Revised)” (SRK Consulting, Inc. 2019a).

 

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Figure 3-1. Groundwater model domain

 

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Figure 3-2. Water level monitoring wells used for model calibration

 

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Figure 3-3. Conceptual model (2013–2039)

 

3.1.3.2 Evaluation of Impacts on Groundwater Levels

The calibrated groundwater model was used to simulate the groundwater levels and change in groundwater levels and flow rates that have occurred (through 2018), and are projected to occur in the future as a result of mine development and groundwater extraction activities associated with the no-action alternative and proposed action mining scenarios. For model simulation purposes, the no-action alternative scenario assumes that mining would continue at the current mining rate through the end of 2026 with ore processing and tailings deposition continuing through the end of 2027. Under the proposed action, the model scenario assumes that mining would continue at the current mining rate through June 2038 with ore processing and tailings deposition continuing through June 2039.

For the evaluation of changes in groundwater levels through the end of 2018, and for the no-action alternative and proposed action, the projected changes in groundwater levels represent the difference between the model-simulated groundwater elevations and simulated steady-state baseline groundwater elevations that existed at the end of 2012 (prior to the reinitiating of mining and groundwater extraction activities at Pinto Valley Mine). In other words, drawdown maps presented in this report represent the predicted total changes to groundwater elevations resulting from the combined effects of recent historical (2013–2018) and projected future pumping and mining activities that would have occurred since the end of 2012. All of the predictive scenarios incorporate other water pumping stresses in the model area, including water supply pumping and mitigation activities at the adjacent Carlota Mine and pumping by other private parties in the hydrologic study area. No other past, present, or reasonably foreseeable future actions are known that would affect the groundwater modeling (SRK Consulting, Inc. 2019a).

3.1.3.3 Evaluation of Impacts on Surface Water Resources

The impact assessment identifies and evaluates potential impacts on streams, springs, and seeps located within the maximum extent of the projected 5-foot groundwater drawdown contour. The 5-foot drawdown contour defines the area where the water table would be lowered by 5 feet or more at some point in time during the mining or post-mining period. Changes in groundwater levels of fewer than 5 feet often are difficult to distinguish from natural seasonal and annual fluctuations in groundwater levels. For discussion purposes, the area contained within the maximum extent of the 5-foot drawdown contour is referred to in the remainder of the analysis for surface water resources as the “drawdown area.”

The springs and streams in the region can be characterized as either ephemeral, intermittent, or perennial. Ephemeral and intermittent springs and stream reaches flow only during or after wet periods in response to runoff events. By definition, these surface waters are not controlled by discharge from the groundwater flow systems. During the low-flow period of the year, ephemeral and intermittent springs and stream reaches typically are dry. In contrast, perennial springs and stream reaches generally flow throughout the year. Flows observed during the wet periods in perennial springs and streams include a combination of surface runoff and groundwater discharge, whereas flows observed during the low-flow period are sustained entirely by discharge from the groundwater system. If the flow from the perennial spring or stream is controlled by discharge from the aquifer affected by mine-induced drawdown, a reduction of groundwater levels could reduce the groundwater discharge to perennial springs or streams with a corresponding reduction in spring flows, lengths of perennial stream reaches, and their associated riparian or wetland areas. 

Potential impacts on riparian vegetation resulting from drawdown and baseflow reductions are discussed in section 3.3.2, “Vegetation,” of the EIS. 

3.1.3.4 Pit Lake Geochemistry Modeling

A hydrochemical evaluation of pit lake water quality was performed for the proposed project (SRK Consulting, Inc. 2019b). In this evaluation, water quality in the pit was estimated from modeling that included the following inputs and reactions: (1) the quality and quantity of groundwater inflow; (2) chemical releases from oxidized wall rock; (3) aqueous geochemical reactions in the pit lakes; (4) evaporation from the pit lake surfaces; (5) direct precipitation into the pit lakes; (6) runoff from pit walls; and (7) exchange of carbon dioxide between the pit lakes and the atmosphere. Details regarding pit lake modeling assumptions and methodology are provided in the pit lake geochemistry modeling report for the proposed project (SRK Consulting, Inc. 2019b). 

Data Sources and Development of Model Inputs

The data sources for the pit lake geochemistry model are relatively common for pit lake models and capture the pertinent components. Some of the geochemical data were scaled from the fundamental laboratory measurements to produce data suitable for use in the overall pit lake model. Specifically, humidity cell testing data from a test of the composite quartz monzonite sample (SRK Consulting, Inc. 2016b) is reported as scaled to adjust for the difference in rock-to-water ratio between the humidity cell testing test and the field (SRK Consulting, Inc. 2019b).

Five solution inputs are identified as reporting to the ultimate pit lake:

. Direct precipitation, 

 

. Hydrologic catchment runoff, 

 

. Groundwater,

 

. Pit wall runoff, and 

 

. Discharge of pregnant leach solution either from heap leach leakage or direct discharge. 

 

 

The first two are assigned chemical compositions that are direct measurements of site conditions. They appear appropriate and need no further consideration. The latter three are derived from models applied to laboratory and field data. 

Of the five solution inputs to the pit lake listed above, pregnant leach solution is the largest contributor of chemical load to the pit lake. Pregnant leach solution is, simply stated, a very concentrated, high total dissolved solids solution with a wide range of chemical constituents of concern. The chemical composition of this discharge is described as having been derived from a leach pad draindown model (SRK Consulting, Inc. 2015). Of the remaining solution inputs, the chemical load from groundwater is most significant. This term combines the chemical load of ambient groundwater with weathering products of sulfide mineral oxidation that are released from the fractured pit shell. 

Based on laboratory testing, the bulk of the pit wall rock (quartz monzonite) is mineralized and will oxidize to release water-soluble chemical constituents. These constituents will report to the pit lake either as pit wall runoff of meteoric precipitation, or by rinsing with groundwater that flows through the pit wall face, or both. The model assumes the pit wall runoff chemistry will be consistent with short-term leach test results (meteoric water mobility procedure). For quartz monzonite, this term was developed by laboratory testing of rock (Ruin granite) that had been composited from exploration core. These data were augmented by meteoric water mobility procedure or synthetic precipitation leaching

procedure data for other rock units composing the pit wall when available, or from later-term (week 10) humidity cell testing data if short-term leach test data were unavailable. For groundwater that flows through the fractured pit wall (seepage face), humidity cell testing is used as the basis and is scaled to account for differences between laboratory test and field conditions, such as the ratio of the mass of rock to mass of contacting water.

The chemical concentration of discharge from the seepage face is appropriately assigned to a disturbed, fractured pit wall depth. However, scaling results from humidity cell testing to account for water to rock ratio and contact time differences between the humidity cell testing and the field conditions appear to be problematic. The humidity cell testing of core composites (SRK Consulting, Inc. 2016b) does not well represent quartz monzonite that has weathered under field conditions for an appreciable time. As described in section 2.6.1, the composite was made from core at least 310 feet deep into the rock mass. Although the cores may have oxidized to some extent during storage, initial sulfate concentrations in humidity cell testing suggest limited oxidation compared with historical testing of quartz monzonite. Of the seven humidity cell testing evaluations conducted on quartz monzonite, the composited sample is the only one that did not produce low pH leach solutions. It also showed very diminished rates of release of sulfate. Other tested samples came from waste rock or leach dumps that had experienced weathering under field conditions. These samples not only achieved greater weathering rates over the entire humidity cell testing procedure, but exhibited a low pH, high sulfate first flush. This first flush represents the release of accumulated weathering (acid rock drainage) products. There is no term in the pit lake model to address an anticipated, and field-documented, first flush of acid rock drainage products. Note that the generally un-weathered character of the composite sample also affects the source term for pit wall runoff (meteoric water mobility procedure).

Table 6 of the Pit Lake geochemistry modeling memorandum (SRK Consulting, Inc. 2019b) presents the concentration of solutes discharged to the pit lake that originate from rinsing of the seepage face. While historical testing of quartz monzonite (via humidity cell testing) reports first-flush concentrations on the order of 3,000 milligrams per liter, the model term is on the order of 0.1 milligram per liter. This seems incongruous with concentrations in contact water for historical samples, regardless of scaling effects. Therefore, as reported, the pit lake model does not account for the accumulation of oxidation products on the pit shell that are rinsed into the lake by the inflow of groundwater. The specific details of the scaling are not presented. 

As reported (SRK Consulting, Inc. 2019b), water flows, other than pregnant leach solution, reporting to the pit with the highest concentration of sulfate have on the order of 600 milligrams per liter of sulfate (SRK Consulting, Inc. 2019b, 2019ff). Groundwater is the largest single flow reporting to the pit lake and has a sulfate concentration of 540 milligrams per liter. These source terms do not provide a basis to account for the concentration of sulfate (2,270 milligrams per liter) reported for the short-term pit lake that formed in 2015. Therefore, unless there is some unaccounted-for source of sulfate (such as accumulated weathering products or lost heap leach seepage), the model source terms may under-predict solute loading to the lake. SRK Consulting, Inc. (2019b) indicates that pregnant leach solution seepage from the heap leach to the pit (small) is accounted for, but its contribution to the modeled lake chemistry is not described.

Regardless, with the addition of pregnant leach solution during draining of the heap leach facility, the modeled ultimate pit lake water quality is, in practical terms, appropriate. It is unclear if alternate scaling of humidity cell testing for calculating seepage face chemical loading would result in substantial changes to metal concentration calculated for the pit lake that might affect ecotoxicological assessments. As a result, the chemical load to the pit lake associated with groundwater passing through

the seepage face, and pregnant leach solution leakage through pit walls, should be regarded as representing a component of uncertainty in the model. 

Geochemical Modeling

Following the mass balance calculation of water and dissolved chemical constituents reporting to the pit lake at each time step in the model, geochemical equilibrium conditions were imposed. This geochemical modeling is accomplished using the computer program PHREEQC, which is a standard program for the purpose and appropriate. PHREEQC requires a range of assumptions for the selection of input parameters concerning expected site conditions.

After assuming the water and solutes mix thoroughly, a range of mineral phases that might possibly form if their saturation limits are reached is specified. The range of minerals assumed to be active is fundamentally sound and supported by peer-reviewed literature study (Eary 1999). Any precipitated minerals are appropriately assumed to remove chemical mass from the pit lake water. The model appropriately specifies gas phase equilibrium with atmospheric carbon dioxide and oxygen (being as the pit lake sits in the open). The calculations assume that the pit lake water is well mixed and does not develop any chemical or thermal stratification. This assumption seems reasonable, as it may bias chemical concentrations at the pit lake surface (tendency for higher concentrations at the pit lake surface where biological receptors are most prevalent).

3.1.3.5 Predicted Drawdown and Baseflow Reduction (2013–2018)

The groundwater model was used to simulate the changes in groundwater elevations and groundwater discharge rates that occurred after restarting active mining at the Pinto Valley Mine at the beginning of 2013 and the end of 2018. The predicted drawdown and associated predicted reductions in baseflow to Pinto Creek at the Magma Weir gage that occurred up through the end of 2018 were used as a point of reference for comparison and discussion of impacts associated with the no-action alternative and proposed action. The model predicted changes in groundwater levels at the end of 2018, and reductions in baseflow at the Magma Weir (Pinto Creek) are presented in figure 3-4 and figure 3-5, respectively. At the end of 2018, the simulated drawdown associated with the Pinto Valley Mine consists of two separate drawdown cones: one associated with the portion of the Peak Well System that extends north-northwest of Tailings Storage Facility No. 4, and the other that consists of the drawdown area around the pit and adjacent areas west of the pit. 

Figure 3-5 presents the model-simulated pumping rates from the Peak Well System, seepage rates at Tailings Storage Facility No. 4, and baseflow rates at the Magma Weir (Pinto Creek gaging station) over the 2013–2018 period. The model results predict that over the 2013–2018 period, the baseflow to Pinto Creek was substantially reduced from an initial rate of 1,070 gallons per minute (start of 2013) to 188 gallons per minute (end of 2018). This represents an 82 percent reduction in baseflow compared to the estimated average baseflow conditions at the Magma Weir at the end of 2012. As illustrated on figure 3-5, the simulations indicate that most of the reduction occurs during the 2013–2016 period, followed by a smaller rate of reduction over the 2016–2018 period. The model simulated reductions in baseflow to Pinto Creek at the Magma Weir attributed to drawdown effects resulting from pumping from the Peak Well System over the 2013–2018 period. 

 

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Figure 3-4. Predicted change in groundwater levels – transient period (2013–2018)

 

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Figure 3-5. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir (2013–2019) 

Source: SRK Consulting, Inc. 2019a

 

3.2 No-action Alternative

 

The no-action alternative is a continuation of existing authorized activities and permitted disturbances from mining activities until 2027. Major mining activities included under the no-action alternative are summarized in table 3-2. Details regarding the no-action alternative are provided in chapter 2 of the Pinto Valley Mine EIS. 

Use of permitted facilities, such as Cottonwood Reservoir and Mine Reservoir, will continue to be utilized under the no-action alternative. Some unpermitted facilities that encroach on National Forest System lands such as roads, storm water ponds, and the Burch pipeline will be addressed in separate processes, but continue to operate under the no-action alternative. Any expansion of facilities including the Open Pit, Inert Limestone Stockpile, Main Dump, Castle Dome Marginal Dump, Waste Dump, borrow and riprap sources, and Tailings Storage Facility No. 4 will occur on private lands. 

 

Table 3-2. Summary of facilities under the no-action alternative

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3.2.1 Water Quantity Impacts

 

3.2.1.1 Drawdown Impacts on Groundwater Levels

The groundwater modeling (SRK Consulting, Inc. 2019a) predicted changes in groundwater levels under the no-action alternative at the end of mining (2027) and 100 years post-mining as shown on figure 3-6 and figure 3-7, respectively. These predictions indicate an increase in the area affected by drawdown as compared to simulated end of 2018 conditions (figure 3-4). The drawdown results also indicate the drawdown area is predicted to continue to expand during the post-mining period. For example, the area predicted to experience a reduction in groundwater levels of 5 feet or more is predicted to encompass 18.9 square miles at the end of mining and expand to 36.2 square miles at 100 years post-closure.

 

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Figure 3-6. Predicted change in groundwater levels – no-action alternative (end of mining)

 

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Figure 3-7. Predicted change in groundwater levels – no-action alternative (100 years post-mining) 

 

The maximum areal extent of the 5-foot drawdown contour under the no-action alternative scenario is presented on figure 3-8. This figure shows the predicted outer limit of the 5-foot drawdown contour as determined by overlaying a series of 10-foot drawdown contours for representative points in time over the 100-year post-mining period (SRK Consulting, Inc. 2019a). The maximum area of drawdown (defined by the 5-foot contour) is an southeast to northwest–oriented elongated area that extends from the southeastern margin to the center of the hydrologic study area (figure 3-8).

 

3.2.1.2 Drawdown Impacts on Surface Water Resources

 

Perennial Streams

Potential impacts on streams were evaluated by (1) using available baseline data to identify perennial stream reaches within the drawdown area; and (2) model simulations of baseflow reduction at Pinto Creek at the Magma Weir monitor. Perennial stream reaches identified in the hydrologic study area are shown on figure 2-2. Perennial stream reaches within the no-action alternative drawdown area are shown on figure 3-8 and listed in table 3-3. The drawdown is projected to encompass a total of 5.68 miles of perennial stream length that includes portions of Pinto Creek (4.62 miles), Miller Springs Gulch (0.81 mile), and an unnamed tributary to Pinto Creek (0.25 mile). The EIS analysis assumes that baseflow in the specific stream reaches identified above are largely controlled by discharge from the regional (bedrock) aquifer system. The predicted expansion of drawdown resulting from mine-induced drawdown could result in a reduction in baseflow in these stream reaches. A reduction in baseflows would likely affect the perennial stream reach (reduce the length of or eliminate the perennial stream reach affected by drawdown). These types of impacts would likely persist until pumping ceases and groundwater levels recover to pre-pumping conditions.

Figure 3-9 presents the model-simulated pumping rates from the Peak Well System, seepage rates at Tailings Storage Facility No. 4, and baseflow rates at the Magma Weir (Pinto Creek gaging station) over the 2013–2027 period. The model results for the no-action alternative scenario include the predictions for the 2013–2018 period discussed previously that indicate a simulated reduction from an initial 1,070 gallons per minute (start of 2013) to 188 gallons per minute (end of 2018)—an 82 percent reduction. Between the end of 2018 and the beginning of 2027, the simulations predict that baseflow would be further reduced from 188 gallons per minute (2018) to 76 gallons per minute (2027). Compared to the assumed steady-state conditions at the end of 2012, the model simulations predict that pumping from the Peak Well System over the 2013 to 2027 period would result in an estimated total reduction in baseflow of 92 percent (an increase of approximately 10 percent over the predictions for the 2013–2018 period) and that the pumping conducted as part of the no-action alternative would extend the duration of maximum impacts on baseflow for an additional 8 years. 

 

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Figure 3-8. Surface water resources within the maximum extent of the 5-foot drawdown contour

 

Recovery of groundwater discharge (baseflow) to Pinto Creek is predicted to vary over the closure and post-closure period (SRK Consulting, Inc. 2019a). During the first 10 years after groundwater pumping ceases, the baseflow is predicted to fully recover to approximately 1,070 gallons per minute (the pre-pumping 2013 flow rate) as a result of continued high infiltration from Tailings Storage Facility No. 4. From 10 years to 100 years post-closure, the baseflow is predicted to gradually decrease from approximately 1,070 gallons per minute to approximately 430 gallons per minute due to combined effects of residual drawdown and progressively reduced infiltration from draindown of Tailings Storage Facility No. 4. The predicted baseflow rate of 430 gallons per minute at 100-years post mining represents an approximate 60 percent reduction in baseflow as compared to the simulated pre-pumping conditions (1,070 gallons per minute) at the start of 2013. After 100 years post-mining, the baseflow is predicted to eventually reach a steady-state condition of approximately 407 gallons per minute, which represents a 62 percent reduction in flow compared to conditions at the start of 2013. The predicted long-term reduction of baseflow in Pinto Creek during the post-mining period is attributed to residual drawdown from groundwater pumping and Open Pit mining activities. 

For the purposes of this analysis, baseflow is defined and simulated as groundwater discharge to the alluvium at the alluvial and bedrock interface. In a natural stream setting in the arid Southwest, the stream flow component that sustains the perennial characteristic of a perennial stream reach is primarily this baseflow. In other words, during extended seasonal dry periods, the perennial flow that persists through these periods is sustained by groundwater discharge (baseflow). A substantial long-term reduction in baseflow resulting from groundwater pumping would likely result in a measurable reduction in flow (or elimination of surface flow) during the low-flow periods, and a corresponding reduction in the length of perennial stream reaches that existed prior to being affected by groundwater pumping.

In summary, model simulations predict that impacts on baseflow occurred as a result of pumping from the Peak Well System during the 2013–2018 period. Continued pumping from the Peak Well System under the no-action alternative scenario is predicted to sustain those impacts (and result in a 10 percent increase in baseflow reduction) through the end of mining. The numerical model simulation predicts that the baseflow in Pinto Creek at the Magma Weir would rapidly recover after pumping ceases and fully recover within 10 years. The continuation of substantial reductions in baseflow over the mine life included under the no-action alternative is anticipated to increase the duration of the impacts on baseflow to Pinto Creek (that occurred between 2013–2018) an additional 8 years. As a consequence, segments of Pinto Creek that were previously characterized as perennial prior to 2013, and that may have been affected by pumping that occurred between 2013–2018 (and no longer sustain perennial flow conditions), would be expected to continue to be affected at a similar magnitude but for a longer duration (approximately 8 years). 

 

Table 3-3. Perennial stream miles within the projected drawdown areas (no-action alternative and proposed action)

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28 Perennial stream reaches within the maximum extent of predicted 5-foot drawdown contour at the end of the 2013–2018 period.

29 Perennial stream reaches within the predicted maximum extent of 5-foot drawdown contour over the mining and post-mining periods.

30 Ibid.

 

 

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Figure 3-9. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir – no-action alternative (2013–2027)

Source: SRK Consulting, Inc. 2019a

 

Perennial Springs

The locations of springs and seeps within the drawdown areas under the no-action alternative are shown on figure 3-8. There are 17 inventoried spring sites (6 wet and 11 dry) within the drawdown area (table 3-4). One of these perennial springs (map ID SP77) that also occurs within the predicted drawdown area resulted from pumping during the transient period (2013–2018). The six wet (flowing) springs are considered perennial springs with baseflow controlled by discharge from the groundwater system.

The actual impacts on individual seeps, springs, or stream reaches would depend on the source of groundwater that sustains the perennial flow (perched or hydraulically isolated aquifer versus regional groundwater system) and the actual extent of mine-induced groundwater drawdown that would occur in the area. The interconnection (or lack of interconnection) between the perennial surface waters and deeper groundwater sources is largely controlled by the specific hydrogeologic conditions that occur at each site. Considering the uncertainty between the actual groundwater elevations and model-simulated groundwater elevations in this area, and the absence of data to define if these springs are perched or connected to the deeper groundwater aquifer system, the EIS analysis conservatively assumed that there is a potential risk that drawdown associated with groundwater pumping for the mine could reduce baseflow to the six perennial springs identified within the drawdown area. Depending on the severity of the reductions in flow, this could result in the drying up of springs and a reduction in the size of any associated wet soil or wetland vegetation areas. The groundwater modeling results indicate that residual drawdown would persist for at least 100 years after mine closure (see figure 3-7). Therefore, any reduction in flow at these spring sites resulting from mine-induced drawdown could persist for the foreseeable future. 

 

Table 3-4. Summary of seeps and springs within the drawdown area (no-action alternative and proposed action)

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31 Values with”-“indicates sites with no flow rate recorded.

 

Waters of the U.S.

The no-action alternative would not result in any direct disturbance including filling, excavation, or any other surface disturbance to the 0.841 acre of jurisdictional drainages identified as features 1, 2, 3, 20, and 22 in section 2.3.3. 

3.2.1.3 Drawdown Impacts on Surface Water Rights

Surface water rights within the predicted maximum extent of the drawdown area are shown on figure 3-10 and listed in table 3-5. There are 26 surface water rights in the drawdown area that include 17 used for livestock, 7 used for stock and wildlife, and 1 (the Tonto National Forest Pinto Creek in-stream flow) used for recreation and wildlife. Nine of these 26 surface water rights also occurred within the projected drawdown area resulting from pumping between 2013 and 2018 (transient period). 

The actual impacts on individual surface water rights would depend on the site-specific hydrologic conditions that control surface water discharge. Only those waters sustained by discharge from the regional groundwater system would be likely to be affected. For surface water rights that are dependent on groundwater discharge, a potential reduction in groundwater levels could reduce or eliminate the flow available at the point of diversion for the surface water right.

 

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Figure 3-10. Surface water rights within the maximum extent of the 5-foot drawdown contour

Table 3-5. Surface water rights within the predicted drawdown area (no-action alternative and proposed action)

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3.2.1.4 Predicted Pit Lake Development

The numerical groundwater flow model developed for the project was used to predict the rate of recovery and pit lake development for the final Open-Pit configuration under the no-action alternative. The groundwater model simulations predict that a pit lake would start to develop in 2027 and reach 95 percent of full recovery within approximately 182 years after closure. The predicted final pit lake area is illustrated on figure 3-11; predicted surface area, volume, groundwater inflow, and evaporation rates at full recovery are summarized in table 3-6. 

At full recovery, the pit lake is predicted to have a groundwater inflow rate of 99 gallons per minute and evaporation rate of 310 gallons per minute. The pit lake is expected to behave as a strong hydraulic sink (hydrologic capture zone where there is groundwater inflow that is lost to evaporation but no outflow to the groundwater system) (SRK Consulting, Inc. 2019a). The predicted water quality of the pit lakes is discussed in section 3.2.2, “Water Quality Impacts.”

 

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Figure 3-11. Predicted post-mining pit lake development (no-action alternative and proposed action)

 

Table 3-6. Predicted pit lake development summary (no-action alternative and proposed action)

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Source: SRK Consulting, Inc. 2019a

 

3.2.1.5 Watershed and Drainage Area Impacts 

Pinto Valley Mine operations have altered the natural watersheds on the private mine property. Non-discharging basins are areas where runoff is contained and does not discharge to downstream receiving waters. These waters are contained in storm water and seepage ponds that are either used in the service water circuit or left in the collection area to evaporate. 

Under the no-action alternative, there would be 4,823.1 acres of disturbed land at the Pinto Valley Mine. The majority of the disturbance would occur as follows: 

. Upper Pinto Creek (34.6 square miles): 4,553 acres (increase from 3,780 acres under existing conditions) of mining, milling and processing, roads, electrical power line, and water supply, storage, and distribution.

 

. Middle Pinto Creek (36.1 square miles): 225.5 acres (increase from 96.8 acres under existing conditions) of milling and processing, roads, electrical power lines, and water supply, storage, and distribution. 

 

. West Fork Pinto Creek (28.3 square miles): 4.4 acres of roads currently existing. There are no plans under the no-action alternative to increase disturbance in this watershed.

 

. Miami Wash (Pinal Creek watershed) (21.8 square miles): 39.8 acres (increase from 33.5 acres under existing conditions) of mining, roads, electrical power lines, and water supply, storage, and distribution. There would be an increase of acreage required for expansion of the Open Pit under the no-action alternative (from 3.5 acres to 9.8 acres).

 

. Bloody Tanks Wash (Pinal Creek watershed) (20.6 square miles): 0.5 acre of roads currently exists in this watershed. There would be no further disturbance in this watershed under the no-action alternative.  

The percentage of surface disturbance within the Upper Pinto Creek subwatershed is approximately 21 percent, an increase of about 3.4 percent from the existing conditions. The percentage of surface disturbance within the Middle Pinto Creek subwatershed is about 1 percent, an increase of approximately 0.6 percent from existing conditions. Total disturbance under the no-action alternative as a percentage of the entire Pinto Creek watershed is approximately 4 percent, which is about a 0.8 percent increase from existing conditions. There would be a surface disturbance increase from existing conditions of 0.05 percent, for a total of 0.29 percent disturbance in Miami Wash. Disturbance from Pinto Valley Mine activities under the no-action alternative amount to 0.03 percent of the total Pinal Creek watershed. 

Table 3-7 shows the facility type and disturbance in the Pinto Creek watershed as well as limited disturbance in Miami Wash, which is in the Pinal Creek watershed east of the Open Pit. 

Most of the disturbed areas at the Pinto Valley Mine currently fall within the non-discharging boundary located within the Upper Pinto Creek subwatershed as shown on figure 2-9. Of the 4,553.0 acres of surface disturbance in the Upper Pinto Creek subwatershed as a result of the no-action alternative, approximately 3,859.0 acres of that disturbance would be within the non-discharging boundary. Runoff from the remaining 691.1-acre disturbance in the Upper Pinto Creek subwatershed would discharge to Pinto Creek. Under existing conditions, there are 647.8 acres of disturbance within the discharging area. There would be an increase of surface disturbance of within the discharging portion of the Pinto Valley Mine of 43.3 acres under the no-action alternative.

 

Table 3-7. Surface disturbance under the no-action alternative by watershed 

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Notes: 

To avoid double counting disturbance acreages, overlapping areas of disturbance between footprints of different facilities were assigned to one facility type.

Pinto Valley Mining Corp. facilities occur in the Bloody Wash south of Miami Wash (0.5 acre of roads).

Based on the relatively low increase in surface disturbance compared to the existing conditions, the impacts of surface disturbance under the no-action alternative from the discharging areas at the Pinto Valley Mine on flow quantities monitored at Pinto Creek near Miami would likely not be measurable.

 

3.2.2 Water Quality Impacts

 

3.2.2.1 Pit Lake Water Quality

Predicted pit lake water quality under the no-action alternative at selected time intervals over the 500-year post-mining period are provided in table 3-8 (SRK Consulting, Inc. 2019b). For the no-action alternative, the water quality of the pit lake is predicted to have low pH (2.02–2.22 standard unit), high sulfate concentrations (20,336–32,814 milligrams per liter), and high metal concentrations over the simulated 500-year post-mining period. Although chemical weathering of pit wall rock is anticipated to provide a source of chemical loading to the lake, the chemical composition of the lake is fundamentally dictated by the addition of pregnant leach solution. Table 8 of SRK Consulting, Inc. (2019b) presents the modeled chemical composition of heap leach draindown solutions. Over the first 100 years following closure, the leach draindown is predicted to have a low pH (ranging from 2.2 to 3.1), high sulfate concentrations (ranging from 71,500 to 86,207 milligrams per liter), and high metals concentrations.

The pit lake is expected to behave as a strong hydraulic sink (no outflow to groundwater) (SRK Consulting, Inc. 2019a). Therefore, the pit lake water would be fully contained within the pit and would not discharge to groundwater or surface water resources outside the pit boundaries. An ecological risk assessment was used to evaluate risk to terrestrial and avian life from potable consumption of and interaction with the pit lake water quality. The results of the ecological risk assessment and the evaluation of potential impacts on terrestrial and avian life are provided in section 3.3, “Biological Resources,” of chapter 3 in the Pinto Valley Mine EIS. 

 

Table 3-8. Predicted pit lake water quality (no-action alternative)

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Arizona Aquifer Water Quality Standards included for comparative purposes.

Source: SRK Consulting, Inc. 2019b

 

3.2.2.2 Tailings Storage Facilities 

Impacts on water quality from tailings storage facilities are associated with two chemical loading terms. Initially, the impact on water resources from tailings storage facilities is due to the draindown and discharge of entrained process solutions. Later in time, after initial draindown of entrained process solutions, acid rock drainage is possible. Future tailings are anticipated to have a net neutralization potential higher than, and a neutralization potential ratio similar to, tailings within Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 (SRK Consulting, Inc. 2016c). Consistent with humidity cell testing characterization of material from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 (MWH Americas, Inc. 2006), future tailings (SRK Consulting, Inc. 2016c) display a sluggish weathering rate (release of sulfate), but observed depletion rates for neutralization potential and acid potential indicate a high potential for tailings to eventually produce acid rock drainage constituents. However, as the water-saturated conditions at depth within an operational tailings storage facility nearly eliminate sulfide mineral oxidation due to exclusion of oxygen, any long-term weathering of tailings must be preceded by draindown of the tailings storage facility. Furthermore, tailings will be reclaimed by capping, which will act as an additional long-term barrier to oxygen ingress. Overall, the initial impact on water resources would be due to the discharge of large volumes of entrained process water. Later, because tailings solids are considered to be likely to generate acid rock drainage, discharge of low pH metal-laden water could potentially occur. However, the combination of reclamation and the relatively arid climate is expected to limit the influx of water through tailings after initial draindown. Therefore, problematic discharge of acid rock drainage to negatively affect water resources is not regarded as a significant threat. Additional description of these potential impacts on water resources is provided in the following paragraphs. 

As described in section 2.6.3.3, localized hotspots of acid rock drainage production on tailings storage facility embankments should be expected (Schlumberger Water Services 2010), but may or may not produce any measurable impacts, pending reclamation. Impacts from these hotspots would most likely be localized and associated with storm water runoff and not persistent discharges. 

Monitoring wells immediately downgradient from the tailings storage facilities are used to detect the water quality seeping from the tailings storage facilities. SRK Consulting, Inc. (2016c:Table 17) reports water quality data for tailings storage facilities, and MWH Americas, Inc. (2005) provides water quality data for monitoring wells at the toe. More recent (2014–2016 period) water quality data for these wells

are provided in attachment C and summarized in table 2-11. In the short term, discharge from the tailings storage facilities can be expected to have a chemical composition as has been observed in entrained water and monitoring wells (SRK Consulting, Inc. 2016c). The water entrained in tailings storage facilities is a neutral pH solution with high total dissolved solids concentrations and high concentrations of sulfate and other chemical constituents such as calcium and magnesium. The high pH inhibits any elevated concentrations of metals. 

Currently, tailings storage facilities discharge alkaline, high total dissolved solids and sulfate concentration water with variable but low metal concentrations. Some acid rock drainage storm water runoff may occur from embankments. These conditions will persist under the no-action alternative. During mining, the high sulfate water leachate would continue to drain through the tailings and seep out of the base of the facility and enter the groundwater flow system. A series of existing groundwater production wells that are part of the Peak Well System is used to capture and pump back the high total dissolved solids and sulfate water immediately downgradient from Tailings Storage Facility No. 4 and Tailings Storage Facility No. 3. This pump-back system serves to prevent high total dissolved solids and sulfate process water in the tailings storage facilities from migrating outside of the project area or discharging into Pinto Creek. 

After closure, seepage resulting from draindown from Tailings Storage Facility No. 4 is predicted to continue at progressively reduced rates until approximately year 2080 (SRK Consulting, Inc. 2019a). However, the current mine closure and reclamation plans do not include a commitment for the construction, long-term operation, and maintenance of a pump-back system (seepage capture system) and treatment system to manage the predicted seepage. Without long-term capture and treatment, seepage from Tailings Storage Facility No. 4 would migrate downgradient (outside of the Pinto Valley Mine project boundary) and potentially discharge as baseflow (and degrade water quality) in Pinto Creek. Therefore, under the no-action alternative, there is a potential for long-term impacts on groundwater and surface water quality from draindown of entrained process water during the post-closure period. Furthermore, as described above, long-term impacts from formation and discharge of acid rock drainage is possible, but not considered likely to result in significant impacts. 

 

3.2.2.3 Heap Leach Facility

Under the no-action alternative, seepage of residual acidic leachate generated from the heap leach facility would continue to drain at progressively reduced rates over the closure and post-closure periods compared to the active operation period. At closure (2028), the seepage rate from the facility is estimated to be approximately 250 gallons per minute. From 2028 to 2050, the seepage rate is predicted to gradually reduce to approximately 25 gallons per minute, which is projected to be the average steady-state seepage rate for the post closure period after 2050 (SRK Consulting, Inc. 2019a). 

At the end of draindown, the remaining rock in the facility would continue to chemically weather any remaining sulfide minerals. Acid-base accounting and humidity cell testing characteristics of the leach dump materials suggest that the formation of acid rock drainage is likely (Schafer & Associates 1995) given sufficient infiltration to produce discharge. Over the long term, on closure, the heap leach facility is modeled to discharge low pH, high total dissolved solids, metal-laden water for hundreds of years (SRK Consulting, Inc. 2015, 2019b).

The heap leach facility is situated on low-permeability Ruin granite, and during the closure and post-closure periods most of the seepage out of the facility would be captured in a lined pond located at the toe of the facility and discharged by pumping into the Open Pit (pit lake). A small portion of the seepage

(less than 5 percent) is assumed to infiltrate directly into the low-permeability substrate beneath the heap leach facility footprint (SRK Consulting, Inc. 2019a). 

A groundwater capture zone currently exists around the pit where the drawdown of groundwater levels around the pit results in steep groundwater flow gradients toward the Open Pit such that all groundwater flow within this zone is captured and flows (and eventually discharges into) the Open Pit. Residual drawdown is predicted to persist around the pit during the closure and post-closure periods. As a result, the groundwater capture zone is predicted to persist in the area around the pit over the closure and post-closure period. Most of the seepage that infiltrates to groundwater beneath the facility during the operation, closure, and post-closure periods would flow into the Open Pit. At 500 years post-closure, an estimated 72 percent of the facility would be within the groundwater capture zone and would flow toward and ultimately discharge into the pit lake. The remaining 28 percent of the seepage that infiltrates to groundwater would be located outside of the groundwater capture zone. For this area, the seepage would enter and mix with the groundwater system and flow west toward Pinto Creek. The portion of the seepage that is predicted to flow toward Pinto Creek has the potential to degrade groundwater quality west of the facility and could potentially affect the water quality in Pinto Creek. 

 

3.2.2.4 Waste Rock Facilities

The basic observations for historical, existing waste rock (see section 2.6.1.2) apply under the no-action alternative. As noted above, the actual release of acid rock drainage products from waste rock is dependent upon the presence of sufficient water to result in discharge from the facility. Acid rock drainage products can be expected to accumulate as highly water-soluble salts on waste rock due to exposure to air, occasional rain, and humidity. In response to more pronounced precipitation events, acid rock drainage–like solutions with low pH and elevated sulfate and metals are likely to be temporarily released in storm water, and the first flush from pit walls upon formation of the post-mining pit lake. For waste rock dumps, any production of acid rock drainage in response to rain events will interact with other rock materials, which may act to dilute and neutralize to an unknown extent.

Currently, groundwater wells proximal to waste rock dumps do not show impacts from acid rock drainage. This and the absence of reported routine or sporadic acid rock drainage seeps or runoff suggest that infiltration of meteoric precipitation into the waste rock dumps is insufficient to produce a discharge of acid rock drainage in the long term. 

3.2.2.5 Storm Water Management

Storm water management under the no-action alternative would be a continuation of the existing storm water management plan as described in section 2.3.5.2. Storm water and seepage management systems have been constructed to contain storm water runoff on site through a system of impoundments and water containment structures designed and managed to contain the 100-year/24-hour storm event. Some storm water management structures would be reconfigured to accommodate expansions of the Open Pit, Tailings Storage Facility No. 3, and Tailings Storage Facility No. 4. 

Sampling at all compliance monitoring sites will continue and would be expected to show similar water quality results as the water quality baseline period of 2014–2016. 

Selenium, dissolved was found to exceed the Arizona Surface Water Quality Standards set in the Arizona Pollutant Discharge Elimination System permit at MG1-12b for 5 of 12 quarters between 2014 and 2016, at MG2-8b for 1 of 12 quarters, and at MPO-1b for 4 of 12 quarters. 

Although the permit requires sulfate (dissolved and total) and total dissolved solids to be reported only, the results of sampling show high concentrations at all stations reporting sampling results (attachment B, “Arizona Pollutant Discharge Elimination System Pinto Valley Mining Corp. Arizona Pollutant Discharge Elimination System Compliance Summary Tables 2014–2016”). 

The closure period under the no-action alternative would begin in 2027. Closure is expected to last 3 years. Closure and post-closure plans will be completed near the end of mining in 2027 that would define requirements for storm water management after mining concludes. Maintenance and monitoring of the storm water management system would be defined at that time.

 

3.3 Proposed Action

The proposed action would extend the life of the mine 12 years, until 2039. Activities proposed under the proposed action are described in table 3-9. 

 

Table 3-9. Summary of facilities under the proposed action 

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3.3.1 Water Quantity Impacts

 

3.3.1.1 Drawdown Impacts on Groundwater Levels

The groundwater model (SRK Consulting, Inc. 2019a) predicted changes in groundwater levels under the proposed action at the end of mining (2039) and 100 years post-mining as shown on figure 3-12 and figure 3-13, respectively. The drawdown results also indicate the drawdown area is predicted to continue to expand during the post-mining period. For example, the area predicted to experience a

reduction in groundwater levels of 5 feet or more is predicted to expand from 23.8 square miles at the end of mining to 37.9 square miles at 100 years post-closure. 

The maximum areal extent of the 5-foot drawdown contour under the proposed action scenario is presented on figure 3-8. This figure shows the predicted outer limit of the 5-foot drawdown contour as determined by overlaying a series of 10-foot drawdown contours for representative points in time over the 100-year post-mining period (SRK Consulting, Inc. 2019a). Compared to the drawdown area predicted under the no-action alternative scenario, the predicted drawdown area for the proposed action represents an increase of 26 percent and 5 percent at the end of mining and 100-year post-mining timeframes, respectively.

 

3.3.1.2 Drawdown Impacts on Surface Water Resource

Perennial Streams

Potential impacts on streams were evaluated by (1) using available baseline data to identify perennial stream reaches within the drawdown area; and (2) model simulations of baseflow reduction at Pinto Creek at the Magma Weir monitoring gage. Perennial stream reaches identified in the hydrologic study area are shown on figure 2-2. Perennial stream reaches within the proposed action drawdown area are shown on figure 3-8 and listed in table 3-3. The drawdown is projected to encompass a total of 5.87 miles of perennial stream length that includes portions of Pinto Creek (4.80 miles), Miller Springs Gulch (0.82 mile), and an unnamed tributary to Pinto Creek (0.25 mile). The EIS analysis assumes that baseflow in the specific stream reaches identified above are largely controlled by discharge from the regional (bedrock) aquifer system. The predicted expansion of drawdown resulting from mine-induced drawdown could result in a reduction in baseflow in these stream reaches. A reduction in baseflows would likely affect the perennial stream reach (reduce the length of or eliminate the perennial stream reach affected by drawdown). These types of impacts would likely persist until pumping ceases and groundwater levels recover to pre-pumping conditions. The reduction of stream flow in segments of Pinto Creek could adversely affect the surface water quality in those reaches, particularly during low-flow conditions. 

Figure 3-14 presents the model-simulated pumping rates from the Peak Well System, seepage rates at Tailings Storage Facility No. 4, and baseflow rates at the Magma Weir (Pinto Creek gaging station) over the 2013–2038 period. The model results for the proposed action scenario include the predictions discussed previously for the transient (2013–2018) and no-action alternative (2019–2027) scenarios. Under the transient scenario (2013–2018), the model simulations predict reductions in baseflow from an initial 1,075 gallons per minute (2013) to 188 gallons per minute (2018): an 82 percent reduction. Under the no-action alternative scenario (2019–2027), the simulations predict additional reduction from 188 gallons per minute (2019) to 76 gallons per minute (2027) that would increase the total reduction in baseflow to 92 percent (compared to the initial 2013 flow rate).

Under the proposed action model scenario, pumping from the Peak Well System would continue for an additional 12 years (2027–2039). Over this period, the baseflow rate is predicted to range from a minimum of 76 gallons per minute to 165 gallons per minute, which represents a total flow reduction ranging from 92 percent to 84 percent, respectively (as compared to the simulated baseflow rates at the start of 2013). The potential impacts on baseflow and perennial stream flow in Pinto Creek in the affected area would be similar to those described under the no-action alternative. 

 

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Figure 3-12. Predicted change in groundwater levels – proposed action (end of mining)

 

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Figure 3-13. Predicted change in groundwater levels – proposed action (100 years post-mining) 

 

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Figure 3-14. Simulated pumping, Tailings Storage Facility No. 4 infiltration, and baseflow at the Magma Weir – proposed action (2013–2038)

Source: SRK Consulting, Inc. 2019a

 

Recovery of groundwater discharge (baseflow) to Pinto Creek is predicted to follow the same pattern as described under the no-action alternative scenario over the closure and post-closure period (Figure 67, SRK Consulting, Inc. 2019a). During the first 10 years after groundwater pumping ceases, the baseflow is predicted to fully recover to approximately 1,070 gallons per minute (the pre-pumping 2013 flow rate) as a result of continued high infiltration from Tailings Storage Facility No. 4. From 10 years to 100 years post-closure, the baseflow is predicted to gradually decrease from approximately 1,070 gallons per minute to approximately 430 gallons per minute due to combined effects of residual drawdown and progressively reduced infiltration from draindown of Tailings Storage Facility No. 4. The predicted baseflow rate of 430 gallons per minute at 100-years post mining represents an approximate 60 percent reduction in baseflow as compared to the simulated pre-pumping conditions (1,070 gallons per minute) at the start of 2013. After 100 years post-mining, the baseflow is predicted to eventually reach a steady-state condition of approximately 407 gallons per minute, which represents a 62 percent reduction in flow compared to conditions at the start of 2013. The predicted long-term reduction of baseflow in Pinto Creek during the post-mining period is attributed to residual drawdown from groundwater pumping and Open Pit mining activities. 

In summary, model simulations predict that impacts on baseflow occurred as a result of pumping from the Peak Well System during the 2013–2018 period, would continue at slightly greater magnitude under the no-action alternative scenario (2019–2027), and would continue at a similar magnitude until pumping ceases under the proposed action. Continued pumping from the Peak Well System under the proposed action scenario is predicted to extend the duration of the predicted impacts on baseflow for an additional 12 years (compared to the no-action alternative). The potential impacts on baseflow and perennial stream flow in Pinto Creek in the affected area would be the same as those described under the no-action alternative in that long-term reduction in baseflow would likely result in a measurable reduction in flow (or elimination of surface flow) that would be particularly noticeable during the low-flow periods. As a consequence, segments of Pinto Creek that were previously characterized as perennial prior to 2013, and that may have been affected by pumping that occurred between 2013–2018 (and no longer sustain perennial flow conditions), would be expected to continue to be affected at a similar magnitude but for longer duration (an additional 12 years).

Perennial Springs

The locations of springs and seeps within the drawdown areas under the proposed action are shown on figure 3-8. There are 19 inventoried spring sites (7 wet and 12 dry) within the drawdown area (table 3-4). The seven wet (flowing) springs include six springs that also occur within the drawdown area defined under the no-action alternative. As described for the no-action alternative, one of these perennial springs (map ID SP77) that also occurs within the predicted drawdown area resulted from pumping during the transient period (2013–2018). 

Potential impacts on perennial springs would be the same as those described for the no-action alternative. In summary, there is a potential risk that drawdown associated with groundwater pumping for the mine could reduce baseflow to perennial springs identified within the drawdown area. Depending on the severity of the reductions in flow, this could result in the drying up of springs and a reduction in the size of any associated wet soil or wetland vegetation areas. The groundwater modeling results for the proposed action indicate that residual drawdown would persist for at least 100 years after mine closure. Therefore, any reduction in flow at these spring sites resulting from mine-induced drawdown would likely persist for the foreseeable future.

Waters of the U.S. 

Similar to the no-action alternative, the proposed action would not result in any direct disturbance of the jurisdictional drainages. 

3.3.1.3 Drawdown Impacts on Surface Water Rights

Surface water rights within the predicted maximum extent of the drawdown area are shown on figure 3-10 and listed in table 3-5. There are 26 surface water rights in the drawdown area that include 17 used for livestock, 7 used for stock and wildlife, and 1 (the Tonto National Forest Pinto Creek in-stream flow) used for recreation and wildlife. Nine of these 26 surface water rights also occurred within the projected drawdown area resulting from pumping between 2013 and 2018 (transient period). As shown in table 3-5, the same 26 water rights identified within the drawdown area under the proposed action were identified within the projected drawdown area under the no-action alternative. 

The actual impacts on individual surface water rights would depend on the site-specific hydrologic conditions that control surface water discharge. Only those waters sustained by discharge from the regional groundwater system would be likely to be affected. For surface water rights that are dependent on groundwater discharge, a potential reduction in groundwater levels could reduce or eliminate the flow available at the point of diversion for the surface water right. 

The results of the groundwater modeling indicate that the impacts on baseflow in Pinto Creek would persist for a longer period under the proposed action compared to the no-action alternative. Therefore, potential impacts on the Forest Service in-stream flow right could persist for a longer period (an additional 12 years) compared to the no-action alternative. 


3.3.1.4 Predicted Pit Lake Development

The numerical groundwater flow model developed for the project was used to predict the rate of recovery and pit lake development for the final Open-Pit configuration under the proposed action. The groundwater model simulations predict a pit lake would start to develop in 2038 during the first year after Open Pit mining ceases as a result of passive inflow of groundwater. The pit lake is projected to reach 95 percent of full recovery within approximately 220 years after closure. 

The final pit lake area is illustrated on figure 3-11; predicted surface area, volume, groundwater inflow, and net evaporation rates at full recovery are summarized in table 3-6. The summary provided in table 3-6 also provides a direct comparison of the depth, surface area, volume, groundwater inflow rate, and net evaporation rate for the pit lake predicted to occur under the no-action alternative and proposed action scenarios. In comparison to the predicted pit lake that would develop under the no-action alternative, the proposed action pit lake would be deeper, have a larger surface area and volume, and have a proportionally larger rate of groundwater inflow and net evaporation.

At full recovery, the pit lake is predicted to have a groundwater inflow rate of 121 gallons per minute and evaporation rate of 392 gallons per minute. The pit lake is expected to behave as a strong hydraulic sink (hydrologic capture zone where there is groundwater inflow that is lost to evaporation but no outflow to the groundwater system) (SRK Consulting, Inc. 2019a). The predicted water quality of the pit lakes is discussed in section 3.2.2, “Water Quality Impacts.”

 

3.3.1.5 Watershed and Drainage Area Impacts 

Under the proposed action, there would be an additional 1,316.5 acres of surface disturbance (1,087.1 acres on private land and 229.4 acres on National Forest System lands) for a total of 5,231.7 acres of disturbed land at the Pinto Valley Mine. The majority of the disturbance would occur as follows: 

. Upper Pinto Creek (34.6 square miles): 4,931.3 acres (increase from 3,569.10 acres under existing conditions) of mining, milling and processing, roads, electrical power line, and water supply, storage, and distribution.

 

. Middle Pinto Creek (36.1 square miles): 225.8 acres (increase from 96.8 acres under existing conditions) of milling and processing, roads, electrical power lines, and water supply, storage, and distribution. 

 

. West Fork Pinto Creek (28.3 square miles): 4.4 acres of roads currently existing. There are no plans under the proposed action to increase disturbance in this watershed.

 

. Miami Wash (Pinal Creek watershed) (21.8 square miles): 39.8 acres, which is the same as under the no-action alternative.

 

. Bloody Tanks Wash (Pinal Creek watershed) (20.6 square miles): 0.5 acre of roads currently exists in this watershed. There would be no further disturbance in this watershed under the proposed action. 

 

The percentage of surface disturbance within the Upper Pinto Creek subwatershed is approximately 22 percent, an increase of about 5 percent from the existing conditions. The percentage of surface disturbance within the Middle Pinto Creek watershed is the same as under the no-action alternative. Total disturbance under the proposed action as a percentage of the entire Pinto Creek watershed at Roosevelt Lake is approximately 4.4 percent, which is about a 1.1 percent increase from existing conditions. There would be no change from the no-action alternative within the Miami Wash watershed. 

Table 3-10 shows the facility type and disturbance in the Pinto Creek watershed as well as limited disturbance in Miami Wash, which is located in the Pinal Creek watershed east of the Open Pit. 

Most of the disturbed areas at the Pinto Valley Mine currently fall within the non-discharging boundary shown on figure 2-9. Of the 4,961.3 acres of surface disturbance in the Upper Pinto Creek subwatershed as a result of the proposed action, 4,214.9 acres of that disturbance would be within the non-discharging boundary. Runoff from the remaining 746.4-acre disturbance in the Upper Pinto Creek subwatershed would discharge to Pinto Creek. Under existing conditions, there are 647.8 acres of disturbance within the discharging area. There would be an increase of surface disturbance within the discharging portion of the Pinto Valley Mine of 98.6 acres under the proposed action. The impacts of the surface disturbance under the proposed action would be similar to those of the no-action alternative and would likely not be measurable. 

 

Table 3-10. Surface disturbance under the proposed action by watershed 

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Notes:

To avoid double counting disturbance acreages, overlapping areas of disturbance between footprints of different facilities were assigned to one facility type.

Pinto Valley Mining Corp. facilities occur in the Blood Tanks Wash south of Miami Wash (0.5 acre of roads).

 

3.3.2 Water Quality Impacts

 

3.3.2.1 Pit Lake Water Quality 

Table 3-11 presents the predicted water quality at selected time intervals in the post-closure period for the proposed action pit lake. The water quality of the pit lake is predicted to be poor and characterized by low pH, high sulfate concentrations, and high metal concentrations over the entire 500-year post-mining period. Compared to the no-action alternative, the proposed action would result in a larger pit with a larger pit lake and a larger pit lake volume (table 3-6); this larger volume would act to dilute the effect of pregnant leach solution. Although the proposed action pit lake water quality is predicted to be poor, the water quality would be better than predicted for the no-action alternative pit lake (table 3-8). Over the first 100 years following closure, the no-action alternative pit lake is predicted to have a pH ranging from 2.0 to 2.2 and sulfate ranging from 19,574 to 32,814 milligrams per liter, with a range of elevated metal concentrations. In comparison, the proposed action pit lake is predicted to have a pH range from 2.3 to 2.4, sulfate range of 12,688 to 15,459 milligrams per liter, and elevated metals. 

Even though the pit lake water quality is predicted to be poor, the pit lake is expected to behave as a hydraulic sink (no outflow to groundwater) (SRK Consulting, Inc. 2019a). Therefore, the pit lake water would be fully contained within the pit and would not discharge to groundwater or surface water resources outside the pit boundaries. An ecological risk assessment was used to evaluate risk to terrestrial and avian life from potable consumption and interaction with the pit lake water quality. The results of the ecological risk assessment and the evaluation of potential impacts on terrestrial and avian life are provided in section 3.3, “Biological Resources,” of chapter 3 in the Pinto Valley Mine EIS. 

 

Table 3-11. Predicted pit lake water quality (proposed action)

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Arizona Aquifer Water Quality Standards included for comparative purposes.

Source: SRK Consulting, Inc. 2019b

 

3.3.2.2 Tailings Storage Facilities

The potential impact on water resources from the tailings storage facilities under the proposed action would be similar to but greater than those of the no-action alternative. Owing to the expanded volume of tailings storage under the proposed action, a greater volume of neutral pH, high total dissolved solids, and sulfate leachate would be generated and stored in the larger facility. During mining, the high total dissolved solids and sulfate leachate would continue to drain through the tailings, seep out of the base of the facility, and enter the groundwater flow system. A series of existing groundwater production wells that are part of the Peak Well System is used to capture and pump back the high-sulfate water immediately downgradient from Tailings Storage Facility No. 4 and Tailings Storage Facility No. 3. This pump-back system serves to prevent high-sulfate process water in the tailings storage facilities from migrating outside of the project area or discharging into Pinto Creek. 

As part of closure and reclamation, the top surface for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would be covered with a minimum of 1 foot of cover material and revegetated, and the side slopes would be covered with a minimum of 2 feet of cover material followed by 6 inches of rock armor. Seepage from Tailings Storage Facility No. 3 would continue at progressively reduced rates during the closure and post-closure period until about 2050 (under both the no-action alternative and proposed action). Seepage resulting from draindown from Tailings Storage Facility No. 4 is predicted to continue at progressively reduced rates until approximately year 2100 when the flow is projected to reach a long-term steady-state condition (SRK Consulting, Inc. 2019a). The length of time estimated to reach a steady-state condition under the proposed action is approximately 20 years longer than predicted under the no-action alternative. The current mine closure and reclamation plans (SRK Consulting, Inc. 2016b) do not include a commitment for the construction and long-term operation and maintenance of a pump-back (seepage capture system) and treatment system to manage the predicted seepage from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. Without long-term capture and treatment, seepage from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 would likely migrate downgradient (outside of the Pinto Valley Mine project boundary) and potentially discharge as baseflow (and degrade water quality) in Pinto Creek. The high total dissolved solids and sulfate concentrations in the seepage from the facilities would likely degrade water quality in the groundwater system and in Pinto Creek downgradient of these facilities. Therefore, under the proposed action there is a potential for long-term impacts on groundwater and surface water quality during the post-closure period. 

The likely acid rock drainage potential from existing tailings, which can lead to acid rock drainage production in the long term, would also apply to future tailings stored in the expanded tailings storage facilities. Future tailings are anticipated to have a net neutralization potential higher than, and a neutralization potential ratio similar to, tailings within the existing Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 (SRK Consulting, Inc. 2016c). Consistent with humidity cell testing characterization of material from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 (MWH Americas, Inc. 2006), future tailings (SRK Consulting, Inc. 2016c) display a sluggish weathering rate (release of sulfate), but observed depletion rates for neutralization potential and acid potential indicate a high potential for tailings to eventually produce acid rock drainage constituents. However, as the water-saturated conditions at depth within an operational tailings storage facility nearly eliminate sulfide mineral oxidation due to exclusion of oxygen, any long-term weathering of tailings must be preceded by draindown of the tailings storage facility. Furthermore, the tailings storage facilities would be reclaimed by capping, which will act as an additional long-term inhibitor to oxygen and water ingress. The combination of reclamation and the relatively arid climate is expected to limit the influx of water through tailings after initial draindown. Therefore, problematic discharge of acid rock drainage to negatively affect water resources is not regarded as a significant threat.

In summary, under the proposed action there is a potential for long-term impacts on groundwater and surface water quality from seepage of entrained process water during the post-closure period. Furthermore, as described above, long-term impacts from formation and discharge of acid rock drainage are possible, but are not considered likely to result in significant impacts. 

3.3.2.3 Heap Leach Facility

There would be no change to the currently authorized operation and planned closure of the heap leach facility under the proposed action. Potential impacts associated with the closure of the heap leach facility would be similar to those described as part of the no-action alternative (section 3.2.2). 

As described under the no-action alternative, a groundwater capture zone currently exists around the pit where the drawdown of groundwater levels around the pit results in steep groundwater flow gradients toward the Open Pit such that all groundwater flow within this zone is captured and flows (and eventually discharges into) the Open Pit. Residual drawdown is predicted to persist around the pit during the closure and post-closure periods. Most of the seepage that infiltrates to groundwater beneath the facility during the operation, closure, and post-closure periods would flow into the Open Pit. At 500 years post-closure, an estimated 94 percent of the facility would be within the groundwater capture zone and seepage would flow toward and ultimately discharge into the pit lake. The remaining 6 percent of the seepage that infiltrates to groundwater would be outside of the groundwater capture zone. 

The footprint of the heap leach facility is the same under both the proposed action and no-action alternative. However, because the long-term residual drawdown area is larger under the proposed action compared to the no-action alternative, the percentage of the facility area that would be within the groundwater capture zone is larger under the proposed action (94 percent) compared to the no-action alternative (72 percent). Consequently, the facility area outside of the groundwater capture zone is smaller (6 percent) compared to the no-action alternative (28 percent). Therefore, the rate of seepage that would enter and mix with the groundwater system and flow west toward Pinto Creek would be less than projected under the no-action alternative. As with the no-action alternative, the small portion of the seepage that is predicted to flow toward Pinto Creek has the potential to degrade groundwater quality west of the facility and could potentially affect the water quality in Pinto Creek. 

 

3.3.2.4 Waste Rock Facilities

The basic observations for historical waste rock (see section 2.6.1.2) extend to future dump construction under the proposed action. Only one composite sample of quartz monzonite (Ruin granite) has been chemically analyzed to represent the dominant waste rock to be generated under the proposed action. Being a composite of 40 feet of drill core, this sample presents an overall average characterization. As reported by SRK Consulting, Inc. (2016b) this sample, and future waste rock, will have a net neutralization potential of -47 and a neutralization potential ratio of 0.2. This classifies waste rock to be produced under the proposed action as likely to produce acid rock drainage. Because this material historically shows a relatively wide range of acid rock drainage formation potential, zones of greater and lesser acid rock drainage potential can be expected in future waste rock. The single composite sample returned humidity cell testing neutral pH and low sulfate concentrations that are at odds with humidity cell testing analysis of six other samples of quartz monzonite that yielded acidic pH values and generally much greater rates of sulfate production. For all historical samples of quartz monzonite that predate the SRK Consulting, Inc. (2016b) composite sample, the median net neutralization potential and neutralization potential ratio is -14.7, and 0.11, respectively. Unlike testing of the composite core sample, all historical samples of quartz monzonite produced a median paste pH of 3.3 from samples that had weathered under field conditions. The composite sample had a paste pH of 7.7. Similarly, humidity cell testing characterization of historical quartz monzonite samples yielded acidic pH, higher rates of release of sulfate, and a very high first flush that rinsed off accumulated weathering products (see table 2-15). Despite the humidity cell testing results for the recent composite core sample, waste rock produced under the proposed action should be anticipated to produce acid rock drainage products. However, the potential for discharge of acid rock drainage would require sufficient infiltration to produce discharge. To date, no surface or groundwater discharge of acid rock drainage from waste rock has been noted. This and the absence of reported routine or sporadic acid rock drainage seeps or runoff suggest that infiltration of meteoric precipitation into the waste rock dumps is insufficient to produce a discharge of acid rock drainage in the long term. 

 

3.3.2.5 Storm Water Management

Storm water management under the proposed action would be the same as described in the no-action alternative, but would be extended for 12 years. The closure and post-closure periods would begin in 2039. 

 

3.4 Cumulative Effects

 

3.4.1 Introduction

The cumulative effects analysis area boundary for water resources and geochemistry includes the entire Pinto Creek watershed area, as well as a small portion of the Miami Wash and Middle Pinal Creek watersheds directly east of the Pinto Valley Mine (figure 1-1). The cumulative effects analysis area was defined to include the maximum geographic extent of effects from surface disturbances and water management activities associated with the proposed project (and interrelated actions) and past, present, and reasonably foreseeable future actions. The total area of the cumulative effects analysis area encompasses approximately 200 square miles (127,000 acres). The baseline surface water, groundwater, and geochemistry for the cumulative effects analysis area is described in chapter 2, “Affected Environment.” 

 

3.4.2 Past, Present, and Reasonably Foreseeable Future Actions

Past, present, and reasonably foreseeable future actions are discussed in section

 

3.23.1 and summarized in table 3-125 in the draft EIS.

Within the cumulative effects analysis area, past and present disturbance has resulted from the following activities: mineral development and exploration projects; utilities, infrastructure, and roads; and livestock grazing and dispersed recreation. Wildland fires are another major disturbance. These can cumulatively affect surface water quality by removing the vegetation layer, increasing erosion and downstream turbidity. Storms can cause mass losses of sediment along eroded embankments, altering the course of hydrological systems. Wildland fires also can change the ecosystem, replacing shrub habitat with grasslands. Shrubs are more resistant to erosion, but grasslands are more adaptable to changing environmental conditions. 

Rangeland management also is an important disturbance to, and utilizer of, water resources in the cumulative effects analysis area. Rangeland management relies on predictable subsurface and surface water quantity and quality to sustain activities. This source can contribute to changes in water quality through the additions of nitrogen and other constituents. Livestock also can trample vegetation around water sources, degrading surface water quality through the subsequent erosion.

Mining also has the potential for cumulative impacts on water quality and quantity. Individually insignificant dewatering of numerous mine pits can cause cumulative effects analysis area-wide changes in both groundwater and surface water quantity. Exposure of naturally occurring geochemical conditions can cause harmful constituents to enter the watershed through inadvertent release. Waste rock poses a threat for erosion and sedimentation to the watershed. Individual mine impacts may be minor to negligible, while cumulative mining activity can pose potential for significant impacts on water quality in the cumulative effects analysis area. 

Previous construction associated with utilities, infrastructure projects, and roads may have used water during construction, and the largest potential post-construction effect likely is related to erosion and sedimentation associated with access roads or reclaimed disturbances. All roads can present water quality impacts due to inadvertent spills or releases during vehicular accidents. Unpaved roads, such as those crossing public lands and those within recreation sites in the cumulative effects analysis area, also can be a source of increased erosion and sedimentation. Paved roads may cause water quality issues resulting from increased storm water runoff.

 

3.4.3 Cumulative Effects

 

3.4.3.1 No-action Alternative

Watershed Disturbance

As described in section 3.2.1.5 (“Watershed and Drainage Area Impacts”), most of the existing disturbed areas at the Pinto Valley Mine fall within the non-discharging boundary located within the Upper Pinto Creek subwatershed. Other existing mine-related disturbance in the Upper Pinto Creek subwatershed includes the Carlota Copper Mine (3,054 acres) and other abandoned historical mines (fewer than 100 acres). The cumulative effects from this existing disturbance combined with new surface disturbance within the non-discharging boundary under the no-action alternative (773 additional acres) would be contained within the Upper Pinto Creek subwatershed. Any additional runoff and sediment resulting from these disturbances would be contained within impoundments and water containment systems.

There would be an increase of surface disturbance within the discharging portion of the Pinto Valley Mine of 43.3 acres under the no-action alternative. Based on the relatively small increase in surface disturbance compared to the existing conditions, the incremental effects of additional surface disturbance from the discharging areas at the Pinto Valley Mine on flow quantities monitored at Magma Weir gaging station are not anticipated to be measurable under the no-action alternative.

Water Quantity

The calibrated groundwater model was used to simulate the cumulative change in groundwater levels and flow rates that are projected to occur in the future as a result of mine development and groundwater extraction activities associated with the no-action alternative and proposed action. All of the model drawdown predictive simulations presented above in chapter 3 represent cumulative effects in that they incorporate other water pumping stresses in the model area, including water supply pumping and mitigation activities at the adjacent Carlota Mine and pumping by other private parties in the cumulative effects analysis area. No other past, present, or reasonably foreseeable future actions are known that would affect the groundwater modeling (SRK Consulting, Inc. 2019a). The projected changes in groundwater levels represent the difference between the model simulated groundwater elevations and simulated steady-state baseline groundwater elevations that existed at the end of 2012 (prior to the reinitiating of mining and groundwater extraction activities at Pinto Valley Mine). 

The drawdown results also indicate the cumulative drawdown area is predicted to continue to expand during the post-mining period. For example, the area predicted to experience a reduction in groundwater levels of 5 feet or more is predicted to encompass 18.9 square miles at the end of mining and expands to 36.2 square miles at 100 years post-closure. The maximum area of drawdown (defined by the 5-foot contour) is a southeast to northwest–oriented elongated area that extends from the southeastern margin to the center of the hydrologic study area. 

The maximum areal extent of the 5-foot drawdown contour under the no-action scenario is presented on figure 3-8. This figure shows the predicted outer limit of the 5-foot drawdown contour as determined by overlaying a series of 10-foot drawdown contours for representative points in time over the 100-year post-mining period (SRK Consulting, Inc. 2019a). 

The cumulative impacts on perennial streams (including Pinto Creek), seeps and springs, and water rights under the no-action scenario would be the same as those described in section 3.2.1. 

Water Quality

Cumulative impacts on water quality under the no-action alternative are most strongly related to the high volume of neutral pH, high total dissolved solids, high sulfate water that will drain from tailings storage facilities during the closure and post-closure periods. Unless recovered by pump-back wells, this solution would report to the Pinto Creek drainage. Discharge from tailings storage facilities could develop acid rock drainage characteristics over time depending on the degree and success of reclamation that should limit oxygen and water ingress.

Temporal occurrences of acid rock drainage are likely from localized positions on tailings storage facilities embankments and waste rock facilities. These discharges are anticipated to be in response to storm water runoff and are unlikely to be sustained discharges. The limited production of acid rock drainage from storm water would interact with other acid-consuming materials (such as Gila conglomerate), which would limit the extent of water resource impacts from acid rock drainage in the short and long terms.

 

3.4.3.2 Proposed Action

Watershed Disturbance

As described in section 3.3.1.5 (“Watershed and Drainage Area Impacts”), most of the existing disturbed areas at the Pinto Valley Mine fall within the non-discharging boundary located within the Upper Pinto Creek subwatershed. Other existing mine-related disturbance in the Upper Pinto Creek subwatershed includes the Carlota Copper Mine (3,054 acres) and other abandoned historical mines (fewer than 100 acres). The cumulative effects from this existing disturbance combined with new surface disturbance within the non-discharging boundary under the proposed action (1,362 additional acres) would be contained within the Upper Pinto Creek subwatershed. Any additional runoff and sediment resulting from these disturbances would be contained within impoundments and water containment systems.

There would be an increase of surface disturbance within the discharging portion of the Pinto Valley Mine of 98.6 acres under the proposed action. Based on the relatively small increase in surface disturbance compared to the existing conditions, the incremental effects of additional surface disturbance from the discharging areas at the Pinto Valley Mine on flow quantities monitored at Magma Weir gaging station are not anticipated to be measurable under the proposed action.

Water Quantity

As described under the no-action alternative, all of the model drawdown predictive simulations presented above in chapter 3 represent cumulative effects in that they incorporate other water pumping stresses in the model area, including water supply pumping and mitigation activities at the adjacent Carlota Mine and pumping by other private parties in the cumulative effects analysis area. No other past, present, or reasonably foreseeable future actions are known that would affect the groundwater modeling (SRK Consulting, Inc. 2019a). 

The maximum areal extent of the 5-foot drawdown contour under the proposed action scenario is presented on figure 3-8. This figure shows the predicted outer limit of the 5-foot drawdown contour as determined by overlaying a series of 10-foot drawdown contours for representative points in time over the 100-year post-mining period (SRK Consulting, Inc. 2019a). The maximum area of drawdown (defined by the 5-foot contour) is very similar to and slightly larger than the area predicted under the no-action scenario. 

The cumulative impacts on perennial streams (including Pinto Creek), seeps and springs, and water rights under the proposed action scenario are the same as those described in section 3.3.1.

Water Quality

Cumulative impacts on water quality under the proposed action are essentially the same as under the no-action alternative except for a greater amount of initial draindown of neutral pH, high total dissolved solids, high sulfate water. This greater amount of discharge has the potential to affect water resources at greater distances from the project site, for longer periods of time. The net total chemical mass discharged from the site would be somewhat greater under the proposed action than for the no-action alternative.

 

4.0 Proposed Monitoring and Mitigation

 

Based on the potential impacts presented in this report and summarized below, this section presents proposed mitigation measures that would avoid, minimize, reduce, rectify, or restore identified key resource impacts. 

Impact: The results of groundwater model simulations for Pinto Valley Mine (SRK Consulting, Inc. 2019a, 2019b) indicate the drawdown associated with the projected pumping from the Peak Well System under the proposed action would contribute to, or extend the duration of, reductions in groundwater levels over a broad area surrounding the well field and reduced baseflows in Pinto Creek adjacent to and downstream of the well field. The projected drawdown area indicates that drawdown would also likely encompass other identified surface water resources (seeps and springs) and water rights. Reductions in baseflow and seepage from mine facilities could affect surface water and groundwater quality. Mitigation Measures WR-1a (Comprehensive Water Resources Monitoring Plan), WR-1b (Groundwater Modeling Recalibration), and WR-1c (Surface Water Impact Mitigation) are intended to provide for an adaptive management approach for monitoring and mitigation of impacts on surface water and groundwater resources resulting from drawdown-related impacts resulting from the project. Potential impacts on riparian vegetation are addressed in section 3.3, “Biological Resources (Vegetation, Fish and Wildlife, Special Status Species),” of the EIS.

. Mitigation Measure WR-1a, Comprehensive Water Resources Monitoring Plan Development. Pinto Valley Mining Corp. would be responsible for the development of a comprehensive water resources monitoring plan for the project. The monitoring plan would be designed to monitor both groundwater and surface water resources within and near (within 1 mile of) the projected maximum extent of the drawdown area that could occur as a result of the proposed action or cumulative scenarios. For groundwater monitoring, the plan would include a network of monitoring wells intended to supplement the monitoring required under the Arizona Aquifer Protection Permit. Monitoring wells may include a combination of retrofitted Peak Wells that are not proposed for use as future production wells and new monitoring wells designed to monitor water levels in both the bedrock and alluvial aquifer systems. The groundwater monitoring network would include two sets of “nested wells” to be installed in the vicinity of U.S. Geological Survey monitoring station Pinto Creek near Miami, Arizona (09498502), and in the vicinity of the confluence of the West Fork and Pinto Creek. The nested wells would be designed to monitor conditions in the shallow (alluvial) and deeper (bedrock) groundwater systems as appropriate based on the actual site conditions.

 

The monitoring plan would also include surface water monitoring stations at selected representative sites to monitor changes in baseflow in Pinto Creek and at perennial spring sites within the drawdown area. At a minimum, the plan would provide for the continuation of uninterrupted surface water flow monitoring at the Magma Weir (09498502), Pinto Creek above Haunted Canyon (094985005), and Pinto Creek below Haunted Canyon (09498501). Flow monitoring at the upper two stations (094985005 and 09498501) is currently available from the U.S. Geological Survey. Pinto Valley Mining Corp. would be responsible for providing the funding mechanism for the maintenance and monitoring at the existing and new surface water monitoring stations and the two upstream stations (094985005 and 09498501) after the cessation of mining operations and closure at the Carlota Mine.

Groundwater and surface water quality would also be included to monitor potential water quality degradation between the mine and Pinto Creek, and along Pinto Creek. Water quality monitoring requirements for the selected new monitoring sites would initially follow monitoring

required by the Aquifer Protection Permit for groundwater wells and the Arizona Pollution Discharge Elimination System permit for surface water, including constituents that are listed as “monitor only.” As monitoring progresses, the constituents to be sampled may be reduced as data developed from monitoring are reviewed as part of the adaptive management.

The monitoring plan would include text, tables, and maps, as appropriate, to document (1) proposed monitoring sites; (2) design and completion of wells and surface water monitoring stations; (3) monitoring protocol and identification of water quality parameters; (4) monitoring frequency; (5) identification of mitigation thresholds to be used to trigger the development and implementation of site-specific mitigation measures as outlined in WR-1c; and (6) reporting requirements. A draft of the comprehensive water resources monitoring plan would be provided to the Forest Service for review and approval prior to implementation. Pinto Valley Mining Corp. would be responsible for constructing and maintaining the groundwater monitoring network and surface water monitoring stations during the operation and closure periods, and for a period of time during the post-closure period. The plan would be reviewed and approved by the Forest Service and implemented prior to the commencement of mining. This measure would provide for early identification of potential impacts on surface or groundwater quantity and quality impacts and trigger the development and implementation of any necessary mitigation measures (outlined in WR-1c). 

. Mitigation Measure WR-1b, Groundwater Modeling Recalibration. There is uncertainty regarding the groundwater modeling predictions. The numerical groundwater model developed for the project for this EIS would be updated and recalibrated throughout the life of the project based on the actual observed changes in groundwater elevation and additional hydrogeologic, surface water, and groundwater-related data collected during operation. The groundwater model must be recalibrated every 2 years for the first 8 years after the record of decision is signed. After 8 years, the Forest Service will reevaluate the time interval for subsequent model recalibrations. Geochemical modeling would be updated as necessary (if requested by the Forest Service or Arizona Department of Environmental Quality) if different results are predicted from the updated groundwater modeling or different results are obtained through the ongoing geochemical characterization during mining. 

 

. Mitigation Measure WR-1c, Surface Water Impacts Workshop. The Forest Service and Pinto Valley Mining Corp. (and other Pinto Creek stakeholders including basin surface water rights holders and Arizona regulatory agencies as appropriate) will convene for an annual meeting to discuss the water budget for the Pinto Valley watershed. This workshop would review observed and modeled surface water flows, collaboratively examine the causes for divergence, and propose mitigations to minimize, reduce, or eliminate impacts, which would include reviewing options for acquisition of future water supplies from elsewhere within or outside the Pinto Creek basin in order to reduce the impacts on Pinto Creek surface resources from groundwater withdrawals. The proceedings of the workshop would be published in an annual report for the project record.

 

Effectiveness: Successful implementation of the monitoring and mitigation measures would minimize or eliminate most residual adverse effects on water resources. It is anticipated that the Forest Service review of monitoring results (Mitigation Measure WR-1a), combined with the updated groundwater modeling predictions (Mitigation Measure WR-1b), would provide early warning of potential undesirable impacts on water-dependent resources. Annual workshops would be held to discuss strategies to conserve existing water resources or identify additional sources from outside Pinto Creek as outlined in Mitigation Measure WR-1c. However, an area of residual mine-related groundwater drawdown is

predicted to persist for the foreseeable future under the proposed action. Model simulations estimate that the long-term groundwater flow into the pit would be approximately 121 gallons per minute (195 acre-feet per year) under the proposed action. The long-term groundwater inflow to the pit that would be lost by evaporation is a residual impact and would permanently reduce the amount groundwater available from the hydrologic study area.  

Impact: Mine-induced drawdown and well field pumping could potentially reduce water levels and affect the active water rights within the projected drawdown areas. 

. Mitigation Measure WR-2, Water Rights Mitigation. Pinto Valley Mining Corp. would be responsible for monitoring groundwater levels between the mine and surface water rights within the projected mine-related and well field–related drawdown area as part of the water resources monitoring program (WR-1). Adverse impacts on water wells and surface and groundwater rights would be identified and mitigated as required under Arizona State law. 

 

Effectiveness: Mitigation Measure WR-2 discloses the ability to mitigate adverse impacts on water wells and water rights under Arizona State law.

Impact: Seepage resulting from draindown from Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 is predicted to continue at progressively reduced rates for several decades after mine closure and then reach a low steady-state flow rate that would persist for the foreseeable future. The seepage would likely migrate downgradient (outside of the Pinto Valley Mine project boundary) and potentially discharge as baseflow in Pinto Creek. The high total dissolved solids and sulfate concentrations in the seepage from the facilities would likely degrade water quality in the groundwater system and in Pinto Creek downgradient of these facilities during the post-closure period and affect potential beneficial uses.

. Mitigation Measure WR-3, Post-Closure Tailings Seepage Management Plan. Pinto Valley Mining Corp. would be responsible for development of a detailed, long-term management plan to capture and treat groundwater flowing out of Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 if monitoring indicated contaminants in excess of regulatory thresholds would be introduced into groundwater flowing onto National Forest System lands. The plan would specify the construction, operation, and maintenance of capture wells and water quality treatment and management system (or other approved demonstrated technologies) designed to prevent downgradient groundwater and surface water degradation. The plan development process would request involvement by affected water rights holders, state and Federal regulatory agencies, including the Forest Service, and private landowners potentially affected. The plan would provide plans for installation and operation of a compliance monitoring system that would include monitoring wells located downgradient of the capture wells. The plan also would include a cost estimate for construction and long-term operation of the system (including monitoring) during the closure and post-closure periods. This plan would be submitted for approval by the Forest Service and Arizona Department of Environmental Quality prior to final authorization to commence mining included as part of the proposed action. 

 

Effectiveness: Mitigation Measure WR-3 would ensure that a plan is developed and implemented to capture and treat groundwater flowing out of the Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 facilities during the post-closure period.

4.1 Residual Impacts 

Successful implementation of the monitoring and mitigation measures would minimize or eliminate most residual adverse effects on water resources. However, an area of residual mine-related groundwater drawdown is predicted to persist for the foreseeable future under the proposed action. Model simulations estimate that the long-term groundwater flow into the pits would be approximately 121 gallons per minute (195 acre-feet per year) under the proposed action. The long-term groundwater inflow to the pits that would be lost by evaporation is a residual impact and would permanently reduce the amount of groundwater available from the hydrologic study area.

 

5.0 References

 

Ajax, Ltd. 2018. Seep, Spring, and Well Inventory in the Pinto Creek Watershed. July 30, 2018.

Arizona Department of Environmental Quality. 1998. Policy for the Evaluation of Mining Rock Materials for Determination of Inertness. Draft guidance document by Water Permits Section, Mining Unit of the Arizona Department of Environmental Quality.

Arizona Department of Environmental Quality. 2004. Arizona Mining Guidance Manual Best Available Demonstrated Control Technology. Arizona Department of Environmental Quality Publication No. #TB04-01.

Arizona Department of Environmental Quality. 2009a. Title 18, Environmental Quality; Chapter 11, Department of Environmental Quality Water Quality Standards; Article 1, Water Quality Standards for Surface Waters. Available at: http://www.azdeq.gov/environ/water/standards/download/SWQ_Standards-1-09. Accessed 3-7-2019.

Arizona Department of Environmental Quality. 2009b. Title 18, Environmental Quality; Chapter 11, Department of Environmental Quality Water Quality Standards; Article 4, Aquifer Water Quality Standards. Available at: http://www.azdeq.gov/environ/water/standards/download/SWQ_Standards-1-09. Accessed 3-7-2019.

Arizona Department of Environmental Quality. 2010. Arizona Pollutant Discharge Elimination System Multi-Sector General Permits for Stormwater Discharges Associated with Industrial Activity – Mineral Industry. Permit No. AZMSG2010-003.

Arizona Department of Environmental Quality. 2014. Authorization to Discharge Under the Arizona Pollutant Discharge Elimination System. Permit No. AZ0020401ADEQ (2004) Arizona Mining Guidance Manual Best Available Demonstrated Control Technology. Arizona Department of Environmental Quality Publication No. #TB04-01.

Arizona Department of Environmental Quality. 2016a. Pinto Creek Site-Specific Water Quality Standard for Dissolved Copper, Salt River Watershed-HUC# 15060103-018, Gila, Maricopa and Pinal counties, Arizona. October 2016. Open File Report 16-04.

Arizona Department of Environmental Quality. 2016b. 2016 Clean Water Act Assessment (July 1, 2010 to June 30, 2015) Arizona’s Integrated 305(b) Assessment and 303 (d) Listing Report. Revised July 2016. 

Arizona Department of Environmental Quality. 2017a. Pinto Creek Dissolved Copper TMDL, Salt River Watershed. April 2017. Open File Report 16-05.

Arizona Department of Environmental Quality. 2017b. Other Amendment to Aquifer Protection Permit No. P-100329, Place ID 838, LTF 64383. File name: P-100329 Permit 3-13-2017 searchable.pdf.

Arizona Department of Water Resources. 1999. Certificate of Water Right, Certificate No. 89109.0000. June 8, 1999.

BGC Engineering USA, Inc. 2019. Pinto Valley Mine EIS, Hydrologic Modeling of Pinto Creek – FINAL. February 7, 2019. Project No. 1952-001.

BHP Copper, Inc. 1999. Final Removal Action Report BHP Copper/Pinto Valley Mine Tailings Impoundment Failure Gila County, Arizona. Miami, AZ: BHP Copper, Inc.

Capstone Mining Corporation. 2017. Pinto Valley Mine APP, AZPDES & SWPPP Site Map.

Capstone Mining Corporation. 2018. Pinto Valley Mine Boundaries, and Project Component Alternatives for Existing Disturbance, No Action Alternative and Proposed Action GIS data. 

Driscoll, F. G. 1986. Groundwater and Wells, second edition. Johnson Division, St. Paul Minnesota, 1,089 pp.

Eary, Edmond, L. 1999. Geochemical and Equilibrium Trends in Mine Pit Lakes. Allied Geochemistry 14 (1999):963–987. 

Federal Emergency Management Agency. 2017. National Flood Hazard Layer. Available at: https://hazards.fema.gov/femaportal/NFHL/. November 28, 2017

Freeze, A. R., and J. A. Cherry. 1979. Groundwater. Prentice Hall Inc. Englewood Cliffs, NJ, 604 pp. 

HydroGeoLogic, Inc. 2011. MODHMS/MODFLOW-SURFACT: A comprehensive MODFLOW-based hydrologic modeling system, version 4.0 code documentation report. Reston, VA: HydroGeoLogic Inc.

ICF. 2019. ICF per direction from Tonto National Forest. 2019. NHD Flowline FCode Type (Perennial, Intermittent, etc.) edited per Tonto National Forest guidance.  

McDonald, M. G. and A. W. Harbaugh. 1988. A modular three-dimensional finite-difference ground-water flow model. In: Techniques of Water-Resources Investigations, Book 6. Washington, D.C.: U.S. Geological Survey/U.S. Government Printing Office. Available at: http://pubs.usgs.gov/twri/index090905.html.

MWH Americas, Inc. 2005. Site investigation results – Final draft: unpublished report prepared for BHP Copper Inc., April 2005, 61 p. plus tables, figures, and appendices.

MWH Americas, Inc. 2006. Pinto Valley humidity cell testing results: unpublished technical memorandum prepared for BHP Copper Inc., February 20, 2006, 18 p.

National Weather Service. 2018. Drought Information for Arizona and Southeast California (Updated June 21, 2018), Extreme to Exceptional Drought Persists and Expands Heading into The Monsoon. Available at: https://www.weather.gov/psr/drought_June2018. June, 21, 2018.

Peterson, N. P. 1962. Geology and Ore Deposits of the Globe-Miami District, Arizona: U.S. Geological Survey Professional Paper 342, 157 p. 

Pinto Valley Mining Corporation. 2014. Authorization to Discharge Under the Arizona Pollutant Discharge Elimination System for the Pinto Valley Mine. Permit Number AZ0020401. ADEQ Inventory No. 100329. May 29, 2014.

Pinto Valley Mining Corporation. 2015a. 2014 Arizona Pollutant Discharge Elimination System Annual Report, Pinto Valley Mine – Permit No. AZ0020401. Capstone Pinto Valley Mine, 3/27/2015.

Pinto Valley Mining Corporation. 2015b. 2014 Aquifer Protection Permit Annual Report, Pinto Valley Mine – Permit No. P-100329. Capstone Pinto Valley Mine, 3/30/2015.

Pinto Valley Mining Corporation. 2016a. 2015 Arizona Pollutant Discharge Elimination System Annual Report, Pinto Valley Mine – Permit No. AZ0020401. Capstone Pinto Valley Mine, 3/31/2016.

Pinto Valley Mining Corporation. 2016b. 2015 Aquifer Protection Permit Annual Report, Pinto Valley Mine – Permit No. P-100329. Capstone Pinto Valley Mine, 3/30/2016.

Pinto Valley Mining Corporation. 2017a. 2016 Arizona Pollutant Discharge Elimination System Annual Report, Pinto Valley Mine – Permit No. AZ0020401. Capstone Pinto Valley Mine, 3/31/2017.

Pinto Valley Mining Corporation. 2017b. 2016 Aquifer Protection Permit Annual Report, Pinto Valley Mine – Permit No. P-100329. Capstone Pinto Valley Mine, 3/30/2017.

PRISM Climate Group, Oregon State University. 2010. Average 30-Year Normalized Precipitation, 1981–2010. Data available to download at: http://www.prism.oregonstate.edu/normals/.

Salt River Project. 2019. Salt River Project, Pinto Valley Mine Environmental Impact Statement. Letter provided to the Tonto National Forest. March 4, 2019.

Schafer & Associates. 1995. Geochemical Evaluation Magma Copper Pinto Valley Mine. Report by Schafer & Associates dated February 21, 1995.

Schlumberger Water Services. 2008a. Material Characteristics and Geochemistry. Appendix 4A from unknown report.

Schlumberger Water Services. 2008b. Whole rock analysis data and plots. Appendix 4F from unknown report.

Schlumberger Water Services. 2008c. Appendix 4C ABA Data and Plots. Appendix 4C from unknown report.

Schlumberger Water Services. 2008d. Appendix 4B Paste pH data and plots. Appendix 4B from unknown report.

Schlumberger Water Services. 2008e. Appendix 4D MWMP and SPLP Data and Plots. Appendix 4D from unknown report.

Schlumberger Water Services. 2008f. Appendix 4E NAG pH and leachate data and plots. Appendix 4E from unknown report.

Schlumberger Water Services. 2010. PVU Humidity cell test results summary. Memo from Jeremy Dowling and Pamela Rohal to Craig Stevens and Sergio Gonzalez. Dated May 27, 2010.

SRK Consulting, Inc. 2008. Summary of Geochemical Characteristics - Overburden Materials at BHP Copper Pinto Valley Mine. Memo from C. Hoag to BHP Copper Pinto Valley Operations dated June 6, 2008.

SRK Consulting, Inc. 2015. Draindown Flow Estimates for the Decommissioned Pinto Valley Low-Grade Ore Leaching Piles, Pinto Valley Mine: Report prepared for Pinto Valley Mining Corp. by SRK Consulting, dated June 2015. 

SRK Consulting, Inc. 2016a. Pinto Valley pit area hydrogeological field investigation and test results: unpublished technical memorandum prepared for Pinto Valley Mining Corp., April 7, 2016,15 p., 5 attachments.  

SRK Consulting, Inc. 2016b. Results of static and kinetic testing of a representative Pinto Valley PV3 waste rock composite sample. Memo from C. Hoag and D. Bird to T. Ralston, dated July 21, 2016.

SRK Consulting, Inc. 2016c. TSF3 and TSF4 Tailings Characterization and Reclamation Strategies. Memo from C. Hoag, J. Rasmussen, D. Bird, M. York, C. Kelley and R. Peroor to Tim Ralston, dated May 4, 2016.

SRK Consulting, Inc. 2016d. Geochemical characterization of Pinto Valley TSF 4 tailings samples related to construction of sand core boundary dams. Memo from C. Hoag and D. Bird to T. Ralston, dated February 7, 2016.

SRK Consulting, Inc. 2016e. 19 Dump Characterization (Support for reclassifying a Waste Rock Dump as a Reclamation Stockpile under APP No. P-100329, Pinto Valley Mine). Memo from C. Hoag, J. Rasmussen, C. Kelley and R. Peroor to Tim Ralston, dated March 26, 2016.

SRK Consulting, Inc. 2016f. Passive Containment Capture Zone – Pinto Valley Mine, APP No. P-100329. Memo from C. Hoag and K. Matos to Tim Ralston, dated July 15, 2016.

SRK Consulting, Inc. 2018. Pinto Creek Stream Gauge Data Reduction – Response to Pinto Valley Mine EIS Data Request #108, April 11, 2018.

SRK Consulting, Inc. 2019a. Pinto Valley Mine – Groundwater Modeling for Mine Extension (Revised). SRK Consulting (U.S.), Inc., SRK Project No. 219500.530. April 19, 2019.

SRK Consulting, Inc. 2019b. Pinto Valley Post-Closure Pit Lake Chemistry Predictive Models for Proposed Action and No Action Post-Closure Scenarios. Technical Memo to Timothy Ralston, Pinto Valley Mine from David Bird, P.G., SRK. April 19, 2019.

U.S. Department of Agriculture, Forest Service (Forest Service). 1985. Tonto National Forest Plan. October 1985. Available at: https://www.tontoplan.org/Documents-and-Resources. 

U.S. Department of Agriculture, Forest Service (Forest Service). 1991. Pinto Creek Instream Flow Assessment, Gila County, Arizona. Prepared for the USDA Forest Service by Rivers West, Inc. and Water and Environmental Systems Technology, Inc. November 25, 1991.

U.S. Department of Agriculture, Forest Service (Forest Service). 2001. Forest Service Manual Southwestern Region (Region 3, FMS 2500-Watershed and Air Management, Chapter 2540- Water Uses and Development. Albuquerque, New Mexico, September 5, 2001. Available at: https://www.fs.fed.us/cgi-bin/Directives/get_dirs/fsm?2500 (select Region 3, 2540.doc).

U.S. Department of Agriculture, Forest Service (Forest Service). 2019. Email correspondence from Lee Ann Atkinson, Minerals NEPA Coordinator, Tonto National Forest, Supervisor’s Office that transferred digital files: unpublished geodatabase of the Tonto National Forest perennial and intermittent stream layer; National Hydrography Dataset perennial stream segments; and Regional Riparian Mapping Project layer to the EIS water resources team. February 8, 2019. 

U.S. Environmental Protection Agency. 2016. National Primary and Secondary Drinking Water Regulations. June 2016. Available at: https://www.epa.gov/sites/production/files/2016-06/documents/npwdr_complete_table.pdf.

U.S. Geological Survey. 2017. National Hydrography Dataset. Reston, Virginia.

U.S. Geological Survey. 2019a. What is “Brackish”? Available at: https://water.usgs.gov/ogw/gwrp/brackishgw/brackish.html

U.S. Geological Survey. 2019b. Water year summaries for USGS station 09498502 for 2003-2018. Available at: https://waterdata.usgs.gov/az/nwis/wys_rpt/?site_no=09498502&agency_cd=USGS

U.S. Geological Survey. 2019c. Personal communication between Robert Fitzinger, NWIS Local Database Manager and Janet Shangraw, 9-5-2019; Water Data Reports data from 1999-2003.

WestLand Resources, Inc. 2015a. Request for Approved Jurisdictional Waters Determination for the Pinto Valley Mine TSF3 Extension, Gila County, Arizona. February 18, 2015.

WestLand Resources, Inc. 2015b. Supplement to Pinto Valley mine Approved Jurisdictional Waters Determination Request Corps File No. SPL-2015-00139-MWL. June 19, 2015.

WestLand Resources, Inc. 2016a. Plan of Operations, Pinto Valley Mine. Prepared for Pinto Valley Mining Corp. Miami, Arizona. May 13, 2016.

WestLand Resources, Inc. 2016b. Summary of Stormwater and Process Water Management on National Forest System Lands in Association with Operation of the Pinto Valley Mine. May 5, 2016.

WestLand Resources, Inc. 2017. Pinto Valley Mine Forest Service Lands Approved Jurisdictional Delineation Request. January 5, 2017. 

WestLand Resources, Inc. 2018a. Technical Memorandum Remote Sensing to Support Seep and Spring Inventory Near Pinto Valley Mine, Gila County, Arizona. July 10, 2018. 

WestLand Resources, Inc. 2018b. Inventory of Surface Water Rights near Pinto Valley Mine. August 28, 2018. 

WestLand Resources, Inc. 2019. Magma Weir History, Pinto Creek Gila County, Arizona. February 21, 2019.

 

Attachment A. Seep and Spring Locations Visited between May 30 and June 22, 2018 

 

Source:

Seep, Spring and Well Inventory in the Pinto Creek Watershed (Ajax, Ltd. 2018)

 

Table A-1. Seep and spring locations visited between May 30 and June 22, 2018

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Attachment B. Pinto Valley Mining Corp. Arizona Pollutant Discharge Elimination System Compliance Summary Tables 2014–2016

 

Table B-1. Arizona Pollutant Discharge Elimination System compliance water quality sampling for Pinto Valley Mine, 2014–2016

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Source: Data provided from Pinto Valley Mine database (Pinto Valley Mine Compliance

 

Arizona Pollutant Discharge Elimination System List)

Table B-2. Arizona Pollutant Discharge Elimination System compliance water quality sampling for Pinto Valley Mine 2014–2016

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Source: Data provided from Pinto Valley Mine database (Pinto Valley Mine Compliance Arizona Pollutant Discharge Elimination System List)

 

Attachment C. Pinto Valley Mining Corp. Aquifer Protection Permit Compliance Summary Tables 2014–2016

 

Table C-1. Aquifer Protection Permit compliance water quality sampling for Pinto Valley Mine

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Table C-2. Aquifer Protection Permit compliance water quality sampling for Pinto Valley Mine

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Table C-3. Aquifer Protection Permit compliance water quality sampling for Pinto Valley Mine

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Table C-4. Aquifer Protection Permit compliance water quality sampling for Pinto Valley Mine

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Appendix F. Geotechnical Stability Appendix 

 

F.1 Basis of Design for Tailings Storage Facilities

 

F.1.1 Regulatory Requirements and Guidance

The design, construction, and operation of the Pinto Valley Mine tailings storage facilities are subject to the Arizona Department of Environmental Quality’s Best Available Demonstrated Control Technology requirements. Best Available Demonstrated Control Technology design criteria were developed in accordance with Arizona Revised Statute 49-243.B.1 (Arizona Department of Environmental Quality 2004), which requires all permitted facilities to utilize Best Available Demonstrated Control Technology in their design, construction, and operation while considering various factors depending on whether the facility is new or existing. The Best Available Demonstrated Control Technology geotechnical criteria for static and dynamic stability of tailings dams are summarized in table 1, and are discussed in this section.

Pinto Valley Mining Corp. and its consultants also considered guidelines published by the Canadian Dam Association (Canadian Dam Association 2013, 2014) for comparison to the Arizona Department of Environmental Quality’s Best Available Demonstrated Control Technology engineering design guidance for seismic design criteria. The Canadian Dam Association guidelines for mining dam stability criteria are summarized in table 1, along with the Best Available Demonstrated Control Technology criteria. The Canadian Dam Association guidelines are based on a dam classification, as defined in table 2. 

The Arizona Department of Environmental Quality’s Best Available Demonstrated Control Technology guidance for earthquake design criteria recommends a minimum project earthquake be considered for structures having a relatively short design life and minimum potential threat to human life or environment. The minimum earthquake is the maximum probable earthquake likely to occur during a 100-year time interval. The maximum probable earthquake is defined in the Best Available Demonstrated Control Technology guidance as an event having an annual exceedance probability of 1 in 475. Where human life is potentially threatened, Best Available Demonstrated Control Technology guidance recommends using a maximum credible earthquake, which is the largest earthquake that appears capable of occurring under the presently known tectonic setting. In the Canadian Dam Association guidance, the maximum credible earthquake or an event having an annual exceedance probability of 1 in 10,000 is recommended for design of structures in the very high to extreme dam classification categories (table 2). For structures in the high category, the Canadian Dam Association recommends considering earthquakes with annual exceedance probability of at least 1 in 2,475 for operating conditions. For post-closure, the Canadian Dam Association recommends evaluating stability for earthquakes with annual exceedance probabilities between 1 in 2,475 and 1 in 10,000 for high hazard dam classification. 

The Best Available Demonstrated Control Technology guidance allows for engineering judgment to establish the design-basis earthquake between the extreme events (minimum/maximum probable earthquake and maximum/maximum credible earthquake). Best Available Demonstrated Control Technology guidance advises that the design-basis earthquake should consider site-specific conditions and the following factors: potential for threat to human life or the environment, facility life, potential future property development downstream from the facility, and seismic history in the area. For operational permitting, the Arizona Department of Environmental Quality historically has accepted a design-basis earthquake with annual exceedance probability of 1 in 975 for both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. The proposed action expansion design for both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 also considered stability for a more conservative design-basis earthquake, having an annual exceedance probability of 1 in 2,475, which is consistent with the

Canadian Dam Association recommendation for operational conditions of a high-hazard facility during the operational phase (table 2).

This environmental impact statement was triggered, in part, by the proposed expansion of Pinto Valley Mine tailings storage facilities onto Federal lands managed by the United States Forest Service (Forest Service). A tailings storage facility is a special type of embankment dam. As a Federal agency, the Forest Service subscribes to dam safety guidelines as outlined by the National Dam Safety Program, which is coordinated by the Federal Emergency Management Agency. The Federal Emergency Management Agency has published guidance documents for dam classification and design criteria (Federal Emergency Management Agency 2004a, 2004b, 2005, 2013), and for dam safety risk management (Federal Emergency Management Agency 2015). The first step is to define the dam hazard potential classification in accordance with the Federal Emergency Management Agency (2004b). Dam hazard potential classification is based on downstream impacts of failure. The Federal Emergency Management Agency (2004b) defines three categories as follows:

1. HIGH HAZARD POTENTIAL: Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.

 

2. SIGNIFICANT HAZARD POTENTIAL: Dams assigned the significant hazard potential classification are those dams for which failure or mis-operation results in no probable loss of human life but can cause economic loss, environmental damage, or disruption of lifeline facilities, or can affect other concerns. Significant hazard potential dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.

 

3. LOW HAZARD POTENTIAL: Dams assigned the low hazard potential classification are those dams for which failure or mis-operation results in no probable loss of human life and low economic or environmental losses. Losses are principally limited to the dam owner’s property.

 

The key distinction for a high hazard potential classification is the potential for life loss. Significant and low hazard classifications have no life loss anticipated. For high-hazard-potential dams, the controlling design earthquake event usually is equated with the controlling maximum credible earthquake, and the prescriptive hydrologic event is the probable maximum flood. For significant-hazard dams, the design earthquake may be determined based on faults active in Holocene time, and the design hydrologic event is a 0.1 percent annual chance exceedance flood (1,000-year flood) using available flood frequency techniques based on regional streamflow or precipitation data.  

 

Table 1. Tailings dam embankment stability design criteria based on two guidance documents 

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Table 2. Dam classification in accordance with Canadian Dam Association guidelines 

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Source: Canadian Dam Association 2013

 

Definitions for population at risk:

None: There is no identifiable population at risk, so there is no possibility of loss of life other than through unforeseeable misadventure.

Temporary: People are only temporarily in the dam-breach inundation zone (such as seasonal cottage use, passing through on transportation routes, participating in recreational activities).

Permanent: The population at risk is ordinarily located in the dam-breach inundation zone (such as permanent residents); three consequence classes (high, very high, extreme) are proposed to allow for more detailed estimates of potential loss of life (to assist in decisionmaking if the appropriate analysis is carried out).

Implications for loss of life:

Unspecified: The appropriate level of safety required at a dam where people are temporarily at risk depends on the number of people, the exposure time, the nature of their activity, and other conditions. A higher class could be appropriate, depending on the requirements. However, the design flood requirement, for example, might not be higher if the temporary population is not likely to be present during the flood season.

 

F.2 Geotechnical Design Criteria

 

The U.S. Department of Agriculture and Forest Service subscribe to the National Dam Safety Program and associated Federal Emergency Management Agency guidance documents for dam projects under their jurisdiction, including mine tailings dams. Meeting federally designated design criteria is currently the best standard of practice for facilities on National Forest System lands, and is consistent with guidance followed for new mining projects on Forest Service–administered lands. However, the Pinto Valley Mine is an existing mine, the majority of which is located on patented claims or private land (private Pinto Valley Mining Corp. property). The entirety of Tailings Storage Facility No. 4 and most of Tailings Storage Facility No. 3, as currently configured, are located on private land. The mine is currently, and has historically been, regulated by the State of Arizona under the Aquifer Protection Program. Under the jurisdiction of the State of Arizona, the guidance followed for design of tailings storage facilities is provided by Department of Environmental Quality Best Available Demonstrated Control Technology (2004) documents. 

Under the proposed action, some of the tailings stored in Tailings Storage Facilities No. 3 and No. 4 will be on National Forest System lands. This action, in part, triggered the need for this environmental

impact statement. The Forest Service has made the determination that State of Arizona geotechnical design guidelines will be considered applicable for these already existing tailings storage facilities. 

The geotechnical engineering design criteria for embankment stability for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 are summarized in table 3. These criteria are accepted by the Arizona Department of Environmental Quality as meeting the requirements for permitting under the individual Best Available Demonstrated Control Technology review process. Because this is an existing facility, the Arizona Department of Environmental Quality–approved design criteria summarized in table 3 are considered applicable for purposes of geotechnical stability assessment in this environmental impact statement. 

 

Table 3. Geotechnical stability design criteria for extension of Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4

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Sources: Wood 2019a; AMEC 2018a

 

F.3 Construction and Geotechnical Conditions at Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 

 

Physical information about the mine’s active tailings storage facilities Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 is summarized in table 4. The following paragraphs summarize relevant construction and geotechnical characteristics of the existing facilities.

 

Table 4. Physical data for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4

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Sources: Wood 2019b, 2019c; AMEC 2014

 

F.3.1 Tailings Storage Facility No. 3

The following description for Tailings Storage Facility No. 3 is generally summarized from technical memoranda provided by Pinto Valley Mining Corp. in response to environmental impact statement data requests (AMEC 2018b, 2018c), and revised data requests (Wood 2019a, 2019b). Figure 1 presents the plan layout of Tailings Storage Facility No. 3, and figure 2 and figure 3 show cross-sections A and B, for the no-action alternative configuration. Tailings Storage Facility No. 3 is formed by two embankments, one trending in the east-west direction, known as the No. 3 dam, and the other trending north-south, known as the No. 2½ dam. The facility was placed in service in 1973 and was operated until early 2009, with several non-operating periods. The No. 3 starter dam, built in 1973–1974, was a 55-foot-high embankment constructed with native soils borrowed from the general vicinity of the site. At the maximum section, the starter dam crest was 35 feet wide at an elevation of 3,344 feet. The starter dam downstream slope was approximately 2.5 horizontal to 1 vertical and equipped with a 100-foot-wide and 3-foot-thick drainage blanket extending through the dam to 175 feet upstream from the dam. Tailings materials were deposited into the Tailings Storage Facility No. 3 impoundment through cyclones or by spigotting. The original perimeter embankment for Tailings Storage Facility No. 3 ultimately reached a crest elevation of 3,742 feet, with a maximum height of 453 feet and a downstream slope ratio of 2.9 horizontal to 1 vertical. In 2011 and 2012, the configuration of the facility was altered by shifting the active deposition inward from the original dam crest. The inset Tailings Storage Facility No. 3 starter embankment was offset about 560 to 750 feet from the outer dam crest. The inset dam has been subsequently raised to a crest elevation of 3,773 feet (Wood 2019a). A boundary dam was constructed in late 2013, and tailings that had crossed into the adjacent National Forest System land in mid-2013 were removed. The boundary dam has been raised in conjunction with the main inset dam raise to an existing (2018) elevation of 3,773 feet.

The no-action alternative for Tailings Storage Facility No. 3 would raise the existing inset embankment and boundary dam to a maximum crest elevation of 3,780 feet (Wood 2019b).

The proposed action for Tailings Storage Facility No. 3 would involve a raise of a relocated inset embankment that would be placed between the existing inset embankment and the outer perimeter embankment. The proposed action configuration for Tailings Storage Facility No. 3 is depicted on figure 4 (plan) and figure 5 (cross-sections A and B). The raised embankment would be founded on the existing tailings surface at an approximate elevation of 3,725 feet, approximately 300 to 400 feet inboard from the crest of the outer perimeter embankment (AMEC 2018b). The proposed action would raise the inset embankment to a crest elevation of 3,857 feet, with an outer (downstream) slope ratio of 3 horizontal to 1 vertical. Under the proposed action, the Tailings Storage Facility No. 3 boundary dam would be buried by tailings as the land use boundary would be shifted to accommodate the enlarged tailings facility footprint.

 

F.3.2 Tailings Storage Facility No. 4

The following description for Tailings Storage Facility No. 4 is generally summarized from AMEC 2014 and 2017a and Wood 2019b. Figure 6 is a plan view and figure 7 and figure 8 are cross-sections showing the general layout of Tailings Storage Facility No. 4 for the proposed action configuration. Tailings Storage Facility No. 4 was placed in operation in 1977. The starter dam for the impoundment was constructed as a zoned earth and rock dam approximately 120 feet high, with a crest elevation of 3,355 feet. A secondary dam, referred to on the drawings as the “Slimes Dam,” was constructed about 1,500 feet upstream to store the cyclone underflow slimes during startup of the facility. Embankment raises above the starter dam crest were accomplished using both cyclone tailings sand and borrow materials.

Coarse tailings (underflow) were initially deposited behind the starter dam using a cyclone system, with the slimes overflow being deposited behind the slimes dam. The overall downstream slope ratio of the original No. 4 dam embankment was about 3 horizontal to 1 vertical. The slimes dam was subsumed by the deposited tailings in approximately 1982. Once the starter dam and subsequent raises reached 3,455 feet, the cyclone system was abandoned and whole tailings were spigotted from the dam crest until the impoundment reached an elevation of about 3,540 feet. The tailings deposition operation was switched back to cycloning in 1988. 

Deposition into Tailings Storage Facility No. 4 was halted in 1997 to facilitate placement of a new tailings pipeline. However, water continued to be stored in the tailings pond from runoff and for temporary storage of contact water from other sources. In May 1998, the dam crest elevation varied between 3,778 and 3,785 feet. In 2008, deposition was resumed for about 1 year at a relatively low mill throughput, bringing the dam crest elevation to 3,795 feet. Milling resumed in late 2012 and tailings were deposited in Tailings Storage Facility No. 4 beginning in early 2013. The east abutment of the dam was reconstructed in 2014 to realign the embankment crest and to extend cycloning operations across the full length of the dam crest. The present (2018) crest of Tailings Storage Facility No. 4 is at an elevation of 3,906 feet (Wood 2019b). The raise of Tailings Storage Facility No. 4 is currently constrained by the Pinto Valley Mine land boundary. Boundary dams are required along the eastern edge of the facility to maintain tailings deposition within this boundary. Boundary dam construction began in August 2015. The no-action alternative would raise the main dam and boundary dams to a crest elevation of 4,090 feet (Wood 2019b). The proposed action would raise the crest to elevation 4,250 feet for a final dam height of 1,045 feet.

 

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Figure 1. Tailings Storage Facility No. 3 site plan – no-action alternative

Source: AMEC 2018a, figure 1

 

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Figure 2. Tailings Storage Facility No. 3 stability analysis cross-section A – no-action alternative

Source: AMEC 2018a, figure 11

 

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Figure 3. Tailings Storage Facility No. 3 stability analysis cross-section B – no-action alternative

Source: AMEC 2018a, figure 13

 

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Figure 4. Tailings Storage Facility No. 3 site plan – proposed action

Source: AMEC 2018b, figure 1

 

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Figure 5. Tailings Storage Facility No. 3 cross-sections – proposed action

Source: AMEC 2018b, figure 2

 

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Figure 6. Tailings Storage Facility No. 4 site plan – proposed action

Source: AMEC 2017a, figure 2

 

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Figure 7. Tailings Storage Facility No. 4 cross-section P – proposed action

Source: AMEC 2017a, figure 11

 

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Figure 8. Tailings Storage Facility No. 4 cross-section R – proposed action

Source: AMEC 2017a, figure 12

 

F.3.3 Geotechnical Characteristics of Tailings Materials

When tailings materials are deposited by cyclones or by spigotting, segregation during deposition typically results in coarse tailings being deposited closer to the perimeter embankment and finer tailings deposited farther into the interior of the impoundment area, upstream from the embankment. For purposes of analyses, the tailings materials are broadly classified as coarse, fine, and intermediate tailings, based on the percentage of fines (percentage finer than a U.S. standard #200 sieve) as follows:

. Coarse tailings: less than 20 percent fines

 

. Intermediate tailings: 20 percent to 40 percent fines

 

. Fine tailings: greater than 40 percent fines

 

Tailings material geotechnical properties and in-situ pore pressure conditions within Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 have been characterized by cone penetration test soundings; installation of piezometer arrays; and drilling, sampling, and laboratory testing of samples from boreholes. 

For Tailings Storage Facility No. 3, the interpreted results from subsurface investigations are represented on two idealized cross-sections: A (figure 2) and B (figure 3). Cross-section A is representative of the No. 3 (east-west) dam, and cross-section B depicts inferred conditions for the No. 2½ (north-south) dam. These two idealized cross-sections were developed for purposes of slope stability analyses and illustrate the approximate distribution of existing coarse, intermediate, and fine tailings as well as the extent of new tailings that would be placed under the no-action alternative. Based on historical and recent piezometer measurements, pore pressures are different within the two dam segments. The north-south No. 2½ dam (cross-section B, figure 3) has a higher phreatic surface (higher pore pressures) than the east-west No. 3 dam segment (cross-section A, figure 2). These conditions were considered in the stability analyses for the existing facility and no-action alternative, which are discussed in the next section. 

For Tailings Storage Facility No. 4, interpretive cross-sections P (figure 7) and R (figure 8) depict the inferred subsurface conditions and approximate distribution of fine, intermediate, and coarse tailings. Figure 7 and figure 8 show that existing layers of fine tailings are present close to the perimeter embankment at elevations between approximately 3,480 and 3,550 feet. These elevations coincide with a time period (mid to late 1980s) when the tailings impoundment had reached elevations above the slimes dam. During that period, tailings deposition was changed to deposit whole tailings by spigotting, resulting in fines within the outer perimeter zones. Pore pressure measurements in Tailings Storage Facility No. 4 indicate that these fine tailings layers cause perched conditions to develop, in which downward drainage within the tailings deposit is impeded by the low-permeability layers. These conditions were accounted for in the slope stability analyses, which is summarized in the next section. 

 

F.4 Geotechnical Stability Results

 

Slope stability factors of safety have been computed for various configurations of Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. The results are generally summarized in table 5. Refer to the specific source reports for table 5 for details of the slope stability analyses input parameters, loading assumptions, and detailed results. This section provides a general discussion of the slope stability analyses for the active facilities.

 

F.4.1 Tailings Storage Facility No. 3

Tailings Storage Facility No. 3 slope stability modeling parameter assumptions and results are summarized in technical memoranda prepared for the environmental impact statement (AMEC 2018a, 2018b; Wood 2019a). The slope stability model parameter inputs and results for the no-action alternative are shown, for example, on figure 2 and figure 3, for cross-sections A and B, respectively. Slope stability for the existing conditions, the no-action alternative, and proposed action configurations of Tailings Storage Facility No. 3 were evaluated using three different methods of analysis to evaluate varying assumptions of drained and undrained strength behavior of the in-place tailings materials. The range of minimum factors of safety computed for the different cases analyzed in cross-sections A and B for the existing, no-action alternative, and proposed action is summarized in table 5. Slope stability factors of safety for both static and pseudo-static (seismic) loading conditions were shown to meet the minimum factors of safety recommended under the Best Available Demonstrated Control Technology guidance for “with testing” conditions.11

11 Refers to site-specific testing of material shear strengths and liner interface strengths and quality control testing (such as moisture, density, and grain size) during construction. The testing program should establish drained shear strength parameters for long-term (static) stability analyses and, where appropriate, undrained shear strength parameters for rapid loading conditions (such as earthquake or rapid drawdown).

The stability results for Tailings Storage Facility No. 3 are sensitive to pore pressures, and the position of the phreatic surface and location of excess pore pressure conditions within the impoundment are critical factors affecting stability. Reported stability analyses factors of safety (table 5) for the raised configurations of Tailings Storage Facility No. 3 under the no-action alternative and proposed action assumed a rise in the phreatic surfaces of approximately 40 percent of the raise height of tailings (Wood 2019a). The resulting factors of safety under those assumptions meet the Best Available Demonstrated Control Technology design criteria. Cross-section B on the 2½ dam is the most critical section for stability, and the minimum factors of safety for both static and pseudo-static loading conditions under deep-seated, undrained conditions are right at the minimum factors required of 1.30 and 1.00, respectively (figure 9). 

 

F.4.2 Tailings Storage Facility No. 4

Tailings Storage Facility No. 4 slope stability results for the no-action alternative are summarized in AMEC 2014 and for the proposed action in AMEC 2017a and 2017b. Minimum factors of safety for the existing conditions were provided in Wood 2019c in response to a data request from the environmental impact statement team. It should be noted that the no-action alternative stability analyses were performed for an assumed embankment crest elevation of 4,005 feet, which is 85 feet lower than the no-action alternative final crest elevation of 4,090 feet. For purposes of the environmental impact statement review, the range of factors of safety determined for the 800-foot-high dam is assumed to be reasonably close to factors of safety that would be determined for an 885-foot-high dam with similar slope inclinations, pore pressure conditions, and geotechnical material parameters. 

The stability analyses for Tailings Storage Facility No. 4 were performed for both drained and undrained pore pressure conditions to determine lower bound and upper bound factors of safety. Six cases were analyzed with different assumptions of drainage and pore pressure behavior. The various cases analyzed are described in the stability report (AMEC 2017a). The results are summarized in table 5 for shallow, intermediate, and deep failure surfaces on the two representative cross-sections P and R (figure 7 and figure 8). An example of the slope stability input parameters and critical failure surfaces computed for representative cross-section R is provided as figure 10. Slope stability factors of safety for both static and pseudo-static (seismic) loading conditions were shown to meet the minimum factors of safety

recommended under the Best Available Demonstrated Control Technology guidance for “with testing” conditions.

 

Table 5. Slope stability minimum factors of safety for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4

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Sources: AMEC 2018a (table 5.9); Wood 2019a (table 3.4, table 3.5); Wood 2019c (table B); AMEC 2014 (table 5.4); AMEC 2017a (table 5.5, table 5.6)

 

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Figure 9. Tailings Storage Facility No. 3 raised embankment stability section B – proposed action

Source: Wood 2019a, figure 5

 

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Figure 10. Tailings Storage Facility No. 4 stability cross-section R (deep) – proposed action

Source: AMEC 2017a, figure 20 

 

F.5 Liquefaction and Seismic Deformation Potential

 

In addition to static and pseudo-static slope stability analyses, the existing tailings materials in both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 were evaluated to determine their liquefaction potential (AMEC 2015b). Liquefaction is the sudden loss of strength that occurs when layers of saturated, loose soil, typically granular material such as sand or silty sand, are loaded and cannot drain. Liquefaction may be triggered by increases in pore water pressures, stress increases (such as due to dam raise by upstream construction methods), vibrations (earthquakes, blasting or equipment), and other factors. 

Liquefaction potential is a function of soil gradation, fines content and plasticity, and in-place soil density. Typically, saturated, loose silts and sands are most prone to liquefaction compared to clayey or gravelly materials, or dense sands. The tailings within Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 consist primarily of low-plasticity to non-plastic silty sand to sandy silt. Based on fines content and Atterberg limits (liquid limit and plastic limit moisture contents), and their natural moisture content, the tailings materials in Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 meet screening criteria for liquefaction-susceptible soils. 

The potential for liquefaction of the tailings contained within Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 during the occurrence of a design earthquake event was evaluated by analyzing in-situ penetration testing data (cone penetration tests). The cone penetration test results are used to compute a parameter called the cyclic resistance ratio, which is a measure of the soil’s resistance to strength loss under cyclic loading. To determine liquefaction potential, the cyclic resistance ratio is compared to the imposed cyclic stress ratio that is caused by the earthquake. 

The cyclic stress ratio for the site was calculated for ground motions equivalent to a 1 in 975 annual exceedance probability (annual exceedance probability) event. For the Pinto Valley Mine site, the 1 in 975 annual exceedance probability event has a peak ground acceleration at the bedrock surface of 0.082 gravitational acceleration. This level of shaking is lower than the design-basis earthquake event used for pseudo-static stability evaluations of Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4, which was a 1 in 2,475 annual exceedance probability event with a peak ground acceleration of 0.137 gravitation acceleration. 

The factor of safety against liquefaction under cyclic loading is determined as the ratio of the cyclic resistance ratio to the cyclic stress ratio. Factors of safety equal to or in excess of 1.0 indicate resistance to triggering liquefaction. The analysis for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 indicate that liquefaction will not occur in the tailings of either Tailings Storage Facility No. 3 or Tailings Storage Facility No. 4 under the assumed 1 in 975 annual exceedance probability event, if current conditions within the tailings do not change. A minimum factor of safety of 1.3 was obtained for these analyses. The report indicates that higher site accelerations (on the order of 0.15 gravitational acceleration) would trigger liquefaction within portions of the impounded tailing (from section 5.2.3 in AMEC 2014):

“The analysis does indicate that an earthquake generating a peak horizontal ground acceleration of 0.15 [gravitational acceleration], or greater would be capable of triggering liquefaction within portions of [Tailings Storage Facility No. 4].”

Seismic deformation analyses also were performed for the existing Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 (AMEC 2015a). Permanent deformations resulting from the previous design-basis earthquake event (1 in 975 annual exceedance probability) were estimated using the Fast Lagrangian Analysis of Continua modeling software, which is a two-dimensional, nonlinear finite-

difference program. The Fast Lagrangian Analysis of Continua program models the response of the entire dam and impounded tailings to input earthquake ground motions. The program estimates the deformed shape of the embankment during and after shaking. The model also can predict excess pore pressure generation and liquefaction. The horizontal and vertical components of spectrally matched earthquake time histories from several different historical earthquakes were used, scaled to the peak bedrock ground acceleration (peak ground acceleration) of an earthquake having a peak ground acceleration of 0.082 gravitational acceleration (the 1 in 975 annual exceedance probability event). 

The main conclusion of the original Fast Lagrangian Analysis of Continua analyses is that the Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 embankments would experience some limited deformations during and after a level of ground shaking equivalent to a 1 in 975 annual exceedance probability event for the site, which was the design-basis event previously approved by the Arizona Department of Environmental Quality for mine operational permitting. Such deformations would not result in triggering liquefaction or a flow failure of the embankments. Permanent deformation results from the Fast Lagrangian Analysis of Continua analyses are summarized in table 6.

Subsequent, updated permanent deformation analyses were completed for the proposed action raise configuration of Tailings Storage Facility No. 4 using different dynamic modeling tools (QUAKE/W, SIGMA/W, and SLOPE/W) and a revised design-basis earthquake updated to the 1 in 2,475 annual exceedance probability event (AMEC 2017b). The conclusion from the updated dynamic analyses was that the Tailings Storage Facility No. 4 structure would experience limited deformation during and after the design-level ground shaking for the site, but this deformation would not result in a flow failure of the embankment. The maximum permanent horizontal deformation was determined to be about 4 inches if the dam is subjected to the shaking represented by a 2,475-year recurrence interval earthquake event. No liquefaction zones were predicted within the Tailings Storage Facility No. 4 embankment by the updated dynamic modeling.

 

Table 6. Seismic permanent deformation analysis for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 for 1 in 975 annual exceedance probability event

Formula Placeholder

Sources: AMEC 2015a, Table 7; AMEC 2017b

Negative values are settlement.

 

F.6 Storm Water Management Considerations

Tailings Storage Facilities No. 3 and No. 4 are located within the Pinto Creek Basin on the north side of the Pinal Mountains. The basin is 152 square miles with elevations ranging from 6,700 feet to 2,200 feet, with an average elevation of 3,916 feet. The general climate of the watershed is semi-arid. During active mining, both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 are designed to store the project design storm as defined by the Arizona Mining Guidance Manual Best Available Demonstrated Control Technology (Arizona Department of Environmental Quality 2004, Section E3.2). The project design flood identified for both is the probable maximum flood, which results from a probable maximum precipitation event on the contributing watersheds. 

Storm water management during active mine operations for Tailings Storage Facility No. 3 is described in AMEC 2018e for both the no-action alternative and the proposed action. Storm water management for Tailings Storage Facility No. 3 for the post-closure period is described in a technical memorandum (Wood 2018), which is attached as appendix G-2 to the Application for Amendment to the Aquifer Protection Plan No. 100329 for Tailings Storage Facility No. 3 and Gold Gulch Repurposing (AJAX 2019). Storm water management for Tailings Storage Facility No. 4 for the proposed action is described in AMEC 2017c, which is attached as appendix F to the Application for Significant Amendment to Aquifer Protection Plan No. P-100329, Tailings Storage Facility No. 4 Extension Design (SRK 2017). 

During active mining, storm water from contributing watersheds above and including the surface area of both Tailings Storage Facility No. 3 and No. 4 is to be managed by storage within the impoundment areas upstream from the dams. As summarized in table 7, the facilities are designed with sufficient freeboard to fully contain flood water volumes up to a probable maximum precipitation event, on top of the maximum operational reclaim pool elevations.

 

Table 7. Freeboard and beach length at Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 for probable maximum precipitation flood event – proposed action

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Source: Wood 2019d, table 4.2 

 

During the last 10 years of active mining, Tailings Storage Facility No. 4 will be modified by construction of internal benches and berms on top of the facility to create three separate cells for storage of tailings, process water, and storm water. Tailings will be deposited behind the benches to create an approximately flat tailings surface. Berms will be constructed on top of the benches to provide freeboard for storing storm water. The total storage capacity within the three cells, behind the berms, is sufficient to store storm water volumes resulting from the probable maximum flood event from the contributing watersheds to each cell (AMEC 2017c). Table 8 summarizes the storm water volume and available freeboard within each cell.

 

Table 8. Probable maximum precipitation storm water volume and freeboard in Tailings Storage Facility No. 5 post-closure cells

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Source: AMEC 2017c, table 4.1 

 

After mining, storm water will be managed by constructing systems of open channels to capture and convey storm water around and across the surfaces of both tailings storage facilities. The post-closure drainage designs are based on a 100-year, 24-hour design storm event. Channels are sized and armored with riprap to convey predicted peak flows and velocities resulting from the 100-year design event, with sufficient freeboard on the channels to contain, without overtopping, the flows from a 500-year event. 

The post-closure drainage design features for Tailings Storage Facility No. 3 are shown schematically on figure 11, and are summarized as follows:

. One top surface impoundment channel will be constructed to discharge to a natural drainage via a spillway on the western side of the impoundment following full reclamation. The impoundment channel will be armored with 6-inch riprap.

 

. Two bench channels will be constructed around the base of the inset east-west and north-south legs of the upper embankment at an approximate bench elevation of 3,725 to 2,730 feet. These two channels will connect to a drop channel on the northeast side of the embankment to discharge storm water to a northern perimeter storm water run-on interceptor channel and existing ponds. This water will be discharged to a natural drainage when reclamation has been completed. The bench channels will be armored with 6-inch riprap.

 

. Post-closure design also includes providing a minimum 2 feet of cover overlain by 3-inch rock armoring of the embankment outer slope to protect against erosion rills and gullying. The top surface will be covered with a minimum of 1 foot of soil cover. 

 

The post-closure drainage design features for Tailings Storage Facility No. 4 are shown on figure 12, and are summarized as follows:

. Two impoundment channels will run northwest across the top of the reclaimed impoundment surface. The impoundment channels will receive runoff from both offsite natural drainages and flow generated on the Tailings Storage Facility No. 4 surface. The secondary channel will join the main channel in the center of the impoundment. The main channel will outfall to an existing drainage feature on the west side of the Tailings Storage Facility No. 4 embankment. The impoundment channels will require 6-inch riprap armoring for erosion protection. An apron design includes a rock size of 24 inches for the erosion protection downstream from the channel discharge.

 

. Four bench channels will collect and drain flows off the downstream face of the dam. Two bench channels are at the approximate bench elevation of 3,800 feet, and would drain the face of the dam to the east and west. Two upper channels drain east and west approximately along a bench elevation of 4,100 feet. The easterly draining channels deliver water into the east channel. The upper west-draining channel will deliver water into the impoundment channel, and the lower west-draining channel will flow into a natural wash on the west side. The bench channels will be lined with 18-inch rock riprap for erosion protection for the 100-year flow event. A riprap apron with 12-inch rock is planned for the lower (3,800-foot bench) easterly draining channel where it converges with the East Channel.

 

. The east channel acts as a spillway on the northeast side of Tailings Storage Facility No. 4. The east channel will be constructed and armored to convey offsite runoff from the east watershed and a portion of flow at the toe of the main embankment down the embankment toward a natural wash. The east channel has grades as steep as 26 percent, and will require substantial erosion protection with concrete or other means. 

 

. Post-closure design also includes providing a minimum of 2 feet of soil cover, overlain by 4- to 5-inch rock armoring of the embankment outer slope to protect against erosion rills and gullying during a 100-year runoff event. The top surface will be covered with minimum of 1 foot of soil cover.

 

 

 

 

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Figure 11. Tailings Storage Facility No. 3 post-closure drainage map

Source: Wood 2018, figure RE-001-TSF3

 

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Figure 12. Tailings Storage Facility No. 4 post-closure drainage map

Source: AMEC 2017c, exhibit 2

 

F.7 Dam Breach Analysis

 

F.7.1 Purpose

Dam breach assessments are used to prepare emergency action plans and environmental impact assessments. Dam breach studies were completed by Pinto Valley Mining Corp. in 2019 in response to requests from the Pinto Valley Mine environmental impact statement review team for supporting information related to the potential downstream consequences in the event of a failure of the Tailings Storage Facility No. 3 or Tailings Storage Facility No. 4 embankments and downstream release of tailings and tailings-impacted fluids. The purpose of dam breach analyses is not to assess the probability of a breach, or the likelihood of events or conditions that could result in a breach, but rather to define the downstream consequences should a breach occur. 

F.7.2 Methodology

Tailings dam failures are different from water dam failures in that in addition to a potential release of the pool of water stored behind the dam, there is a potential for the dam breach to also result in the release of liquefied tailings as a flow slide. Water flows much differently than liquefied tailings, and the outflow from breached tailings embankments typically does not involve release of the entire tailings volume. Whether the tailings will liquefy and how far the flow slide may travel downstream depends on the physical properties of the tailings and the topography of the downstream terrain. The dam breach models accounted for these factors.

The tailings dam breach studies were performed in a staged manner for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. 

F.7.2.1 Preliminary Analysis (AMEC 2018d)

A preliminary empirically based parametric analysis was completed to determine the possible amount and extent of outflow related to a breach. The details of these analyses were provided for the environmental impact statement in a technical memorandum (AMEC 2018d). Preliminary dam break analyses for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 were performed considering the dam geometries based on the maximum planned dam heights of the facilities for the proposed action: 560 feet for Tailings Storage Facility No. 3 and 1,008 feet for Tailings Storage Facility No. 4. 

. A flow slide calculator method described by Jeyapalan (1982) and available at the web site www.wise-uranium.org was used. This procedure computes the maximum runout distance using a Bingham plastic model (yield strength and plastic viscosity) to represent the flow behavior of the liquefied tailings. As inputs to the procedure, the calculator also requires the unit weight of the tailings and the geometric parameters of the initial height of the dam and the bed slope, or slope of the ground downstream from the tailings dam.

 

. Empirical charts and regression analyses as developed by Rico et al. (2007), and as updated by the Canadian Dam Association (Canadian Dam Association 2014), also were utilized for the preliminary analyses. The Rico empirical method utilizes regression curve relationships to estimate outflow volume, runout distance, and peak discharge based on the data collected from 29 past dam failures. The empirical model predicts tailings outflow volume based on the total pond volume; runout distance based on dam height, a dam factor (a function of dam height and contained volume), and outflow volume; and peak discharge based on dam height and the same dam factor. The study recommends that the empirical model be used with caution due to the uncertainty present in documentary evidence and the diversity of tailings dams.

 

F.7.2.2 Dam Breach Physical Models (Wood 2019d)

The preliminary analysis was followed by a more rigorous physical model approach described in a report by Wood (2019d). The procedure incorporated site hydrology, the physical and mechanical properties of the tailings materials, and the downstream topography. The physical models were used to develop maps showing the extent of downstream runout of released solids and fluids. 

The physical model considered dam breach scenarios under two initial hydrologic conditions, as follows:

. Sunny-day failure – a sudden dam failure that occurs during normal operations, which may be caused by internal erosion, piping, earthquakes, mis-operation leading to overtopping, or another event.

 

. Flood-induced or rainy-day failure – a dam failure resulting from a natural flood of a magnitude that is greater than what the dam can safely pass.

 

The breach modeling methodology was different for each of the two conditions. 

Sunny-Day Breach Model. The sunny-day scenario considered a slope-instability mode of failure involving displacement of a 3-dimensional wedge of the tailings dam and tailings from the face of the dam. The volume of material displaced lies above the slope failure surface. In a sunny-day breach scenario, the pool within the facility is assumed to be at a normal operating level, sufficiently far away from the dam face to not be intersected by any slope failure wedges. As stated in section 3.1 of the report (Wood 2019d): 

The tailings mass was considered to slump through the breach in the absence of free water because the failure scarp was not considered to intersect the pool. The slumped tailings mass would result in a conical shaped deposition downstream of the breach, similar to a debris flow slide or an alluvial fan deposit. The deposition of the tailings material that discharges through this process mainly depends on the downstream topography and stream/valley slopes, with the tailings materials stabilizing at slopes of 1 to 4 degrees (Lucia et al. 1981; Blight and Fourie 2003).

Section 5.2.1 of the report states (Wood 2019d):

The sunny-day modeling was conducted considering that the tailings would not flow, but would slump and be deposited downstream from the embankment as a debris flow.

The volume of tailings released during a sunny-day breach was determined from the assumed shape of the failed wedge based on stability analyses performed using conventional limit equilibrium methods to analyze the slope stability of the structure under static or pseudo-static loading. Deeper critical failure surfaces were considered for the breach analyses. Although deeper failure surfaces had higher factors of safety than shallower failures for both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4, deep-seated failures would release larger volumes of tailings. In addition to the initial, deep-seated failure, a secondary or progressive failure resulting in a loss of more tailings from the impoundments also was evaluated. Such a secondary failure could result in flow of fine-grained tailings due to the loss of confinement of the coarse- and intermediate-grained zones of the outermost tailings embankment. The potential impact of a secondary failure also was included in the modeling results.

The volume of the released tailings was modeled using AutoCAD as displaced along the downstream slope into the natural drainages downstream from the tailings storage facilities. The runout distance was calculated considering the total volume displaced, the natural topography downstream, and the final

(residual) slope of the displaced mass of tailings within the downstream areas. The residual slope of the released tailings in the channel was assigned values of 2.6 percent for the primary failure and 2.0 percent for the secondary failure based on studies summarized by Martin et al. (2015) and historic case studies by Lucia et al. (1981). The runout distance and deposition of tailings in areas downstream from the tailings storage facilities were first estimated for the initial or primary failure condition. The released tailings from the secondary failure were then modeled as being deposited on top of the primary deposition and farther downstream.

Flood-Induced Breach Model. A flood-induced failure is caused by an extreme rainfall event that results in overtopping of the tailings storage facility embankment. The hydraulic models HEC-RAS 2D (U.S. Army Corps of Engineers 2016) and FLO-2D (2018) were used to estimate the dam breach hydrographs and dam breach inundation areas. Hydrology for the project included determination of the inflow to the tailings storage facilities from upgradient watersheds, and ambient flow conditions prior to breach in the receiving watershed downstream. The hydrology of the receiving watershed assumed a probable maximum precipitation event occurring within the contributing watersheds of the tailings storage facilities concurrent with a 100-year flood event occurring in the receiving Pinto Creek watershed. 

For this study, during a flood-induced event, the pools within Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 were assumed to be full with the combined storm water runoff volume from a probable maximum precipitation event and the supernatant pool volume for maximum permissible operating level, prior to breach of the crest of the dam. Fontaine and Martin (2015) outline the procedure that was used to estimate the total flood outflow volume, which consists of the initial supernatant pond volume, the probable maximum precipitation storm water volume, and the volume of mobilized tailings solids and interstitial water. The mobilized tailings volume was estimated as a function of the volume of free water and the physical characteristics of the tailings materials. Conceptually, this means that a larger volume of free water will mobilize more tailings and embankment material than a smaller volume of free water. The tailings materials mobilized at the time of breach are characterized by the mass of solids and water present in the deposit, the specific gravity of the tailings, and the average dry density of the tailings. A spreadsheet was developed to estimate the released volume of the tailings material based on the procedure presented by Fontaine and Martin (2015).

Breach hydrographs were developed using HEC-RAS 2D, in consideration of predictive equations for peak discharge that are based on breach characteristics including depth, bottom width, side slopes, and time of development. The predictive equations were developed from case studies as documented by the U.S. Bureau of Reclamation (1998). Using sensitivity analysis, final breach parameters were developed for both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4. For Tailings Storage Facility No. 4, the assumed breach dimensions were 150 feet deep by 405 feet wide (at base) with 1 horizontal to 1 vertical side slopes and a time to develop of 1.5 hours. For Tailings Storage Facility No. 3, the final breach dimensions were 75 feet deep by 150 feet wide with 1 horizontal to 1 vertical side slopes and time to development of 1 hour. The same breach parameters were used for both the no-action alternative and proposed action dam heights for both structures.

HEC-RAS provided a convenient means of generating the breach hydrographs, but the model is unable to account for viscosity and yield stresses (non-Newtonian) flow that is characteristic of tailings dam breach outflows, which have a high sediment content and are more viscous than clear water. For this reason, HEC-RAS was used only to generate the breach hydrograph but not to model the flooding downstream. The breach outflow hydrographs simulated using HEC-RAS 2D were input to a different model, FLO-2D (2018), to route the floods downstream. FLO-2D is an unsteady, 2-dimensional flood routing model for channel and unconfined overland flows. The model simulates a flood over complex topography and roughness and calculates flood distributions over the computational domain. For this

modeling step, the HEC-RAS breach hydrographs were transformed to clear-water hydrographs and coupled with sediment concentration distributions within the FLO-2D model to result in an equivalent model breach volume. Rheological parameters for the tailings materials were developed and accounted for as part of the downstream flood routing using FLO-2D. 

The hydrologic dam breach scenarios assumed that during a probable maximum precipitation event affecting the tailings facilities, a 100-year flood event also is occurring in the Pinto Creek watershed. The report includes preliminary 100-year floodplain maps that were developed based on regression equations. The dam breach inflow hydrograph start times were set so that peak outflows from the breach would coincide with the peak of the 100-year flood event hydrographs. This resulted in the highest peak flow in the inundation area and simulated the most conservative results in terms of velocity and depths of flood flows.

 

F.7.3 Results

 

F.7.3.1 Preliminary Dam Breach Analysis Results

The results of the preliminary dam breach analysis were summarized in the technical memorandum (AMEC 2018d) as follows.

Results from the flow slide calculator method:

The general topographic slope from the Pinto Valley Mine to Roosevelt Lake is about 1 percent. Considering this slope, a tailings unit weight of 110 [pounds per cubic foot], and median tailings properties of a viscosity of 40 [pound-force second per square foot] and yield strength of 30 [pounds per square foot], run-out distances of 6.4, 14.2 and 26.5 miles were computed for dam heights of 300, 460 ([Tailings Storage Facility No. 3] current height) and 650 ([Tailings Storage Facility No. 4] current height) feet. If the proposed dam heights of 1008 feet for [Tailings Storage Facility No. 4] and 560 feet for [Tailings Storage Facility No. 3] are considered, the computed runout distances are 57 and 21 miles, respectively. In either case the flow would reach Roosevelt Lake.

Results from the Rico et al. (2007) empirical equations:

If the present dam height of Tailings Storage Facility No. 4 (650 feet or about 200 meters) is considered, the run-out distance estimated by the regression formula … is 98 kilometers or 58 miles. This is a meaningless estimate however, as it fails to consider the topography and features within Pinto Creek downstream of Tailings Storage Facility No. 4. … The maximum dam height considered for the empirical methods based on the historical record is about 100 meters (m). Also, the compiled data do not distinguish partial dam height breaches from full height dam breaches, and thus can only consider a full height failure. It generally is not appropriate to extrapolate any empirical model when the parameters of importance are outside the assumptions on which the empirical model was developed. The maximum design height for [Tailings Storage Facility No. 4] (307 m or 1008 feet) and [Tailings Storage Facility No. 3] (170 meters or 560 feet) both exceed 100 meters…

The conclusions from the preliminary analysis can be summarized as follows:

1. The results of the dam break analysis based on either the empirical method (Rico et al. 2007) or the flow slide calculator method (www.wise-uranium.org) indicate that a complete breach (full height dam failure) of either Tailings Storage Facility No. 3 or Tailings Storage Facility No. 4 could result in flow of supernatant water and tailings into Pinto Creek and then downstream to Roosevelt Lake, about 21 miles from the Pinto Valley Mine.

2. The available records for runout distance in the data that would correlate to the Pinto Valley Mine tailings storage facilities are very limited, if present at all. None of the available records considered in the development of the Rico et al. (2007) empirical equations contained information on dams as high as the planned Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 dams; records are limited to dams fewer than 100 meters (approximately 300 feet) high. 

3. The empirical models do not consider physical constraints provided by downstream topography, which can obstruct or change the direction of the flow and affect the runout distance.

 

F.7.3.2 Dam Breach Physical Model Results

Sunny-Day Breach Results. The embankment failure geometries of the sunny-day analyses for both Tailings Storage Facility No. 4 and Tailings Storage Facility No. 3 do not intersect water impounded on the upper surface of the facilities. The assumed failure mode was a slope failure, resulting in partially saturated embankment and tailings materials slumping out from the face of the dam and being deposited downgradient in a manner similar to a debris flow. The limits of the displaced tailings from these sunny-day models were determined based on the topography of the downstream drainage, and the physical properties of the tailing materials. The results are summarized in table 9 in terms of runout distance downgradient from the toe of each dam. Under the sunny day scenario, the maximum distance of runout of the tailings from Tailings Storage Facility No. 3 under the proposed action was estimated to be about 8,700 feet (1.65 miles), as illustrated on figure 13. The maximum distance of runout of the tailings from Tailings Storage Facility No. 4 under the sunny day scenario was determined to be approximately 14,500 feet (2.75 miles), as illustrated on figure 14. 

 

Table 9. Sunny-day breach impacts – maximum runout distance

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Source: Wood 2019d, table 5.1 

 

Flood-Induced Breach Results. The flood-induced dam breach models for Tailings Storage Facilities No. 3 and No. 4 considered release of the combined volumes of supernatant tailings fluids (with the ponds assumed at maximum permitted operating levels) and storm water runoff from a probable maximum flood event that reports to the facilities, plus an estimated volume of tailings materials that are mobilized by the release of the free water. Concurrent 100-year flooding in the Pinto Creek drainage also was included in the model. Analyses were performed for the no-action alternative and proposed action geometries for both storage facilities. The results were presented in Wood 2019d as a series of flood inundation maps for five different cases: (1) the 100-year flood with no dam breach; (2) no-action alternative and (3) proposed action dam breaches for Tailings Storage Facility No. 4; and (4) no-action alternative and (5) proposed action dam breaches for Tailings Storage Facility No. 3.

 

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Figure 13. Tailings Storage Facility No. 3 proposed action sunny day breach runout

Source: Wood 2019d, figure 5.12

 

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Figure 14. Tailings Storage Facility No. 4 proposed action sunny day breach runout

Source: Wood 2019d, figure 5.6

 

The results and flood inundation mapping indicate that flood-induced breaches at either tailings storage facility for both the no-action alternative the proposed action will affect the full length of the Pinto Creek drainage from the tailings storage facilities down to Theodore Roosevelt Lake. The following general observations about downstream impacts are summarized based on the model results and flood inundation maps:

. There are ranch buildings owned by Pinto Valley Mining Corp. within the Pinto Creek drainage in close proximity (approximately 1 mile) downstream from Tailings Storage Facility No. 4. As of June 2019, the ranch is no longer permanently inhabited. Peak flow arrival time to the structures nearest the tailings storage facilities is estimated at approximately 30 minutes, as measured from the start of the breach process. Maximum flood depths are greater than 20 feet at the ranch buildings for breach of Tailings Facility No. 4 under the flooding scenarios modeled. 

 

. There are no permanently inhabited residences between the mine site and State Route 188, which is approximately 11.3 miles downstream. 

 

o The bridge over Pinto Creek on State Route 188 would be overtopped for the modeled flood-induced breach at Tailings Storage Facility No. 4. Flows from a breach of Tailings Storage Facility No. 3 are predicted to flow beneath the bridge.

 

. Between State Route 188 and Theodore Roosevelt Lake are two adjacent communities, Roosevelt Shores and Roosevelt Estates.

 

o Residences in both communities, which lie adjacent to Pinto Creek between State Route 188 and Roosevelt Lake, are subject to significant inundation depths (5 to 15 feet) in the event of a dam breach. 

 

o Some of these residences also are within the Federal Emergency Management Agency delineated 100-year floodplain, and would be subject to flooding without a dam break. Dam break on top of a natural flood event would increase the depths, velocities, and lateral extents of flooding in these areas. 

 

o The time to arrival of peak flow in the area of the communities is estimated at 2.5 to 3 hours, as measured from the initiation of the dam breach. 

 

 

The flood-induced dam breach models for Tailings Storage Facility No. 4 resulted in the highest levels of flooding. Results were similar for both the no-action alternative and proposed action configurations. The inundation mapping results for the proposed action for breach of Tailings Storage Facility No. 4 are provided as figure 15 through figure 17. Results for peak flow, depth, and arrival time near the Roosevelt Shores community are summarized in table 10. 

 

Table 10. Flood-induced breach impacts – peak flow, depth, and time near Roosevelt Shores community

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Source: Wood 2019d, table 8.3 

 

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Figure 15. Tailings Storage Facility No. 4 proposed action flood breach inundation limits

Source: Wood 2019d, figure 8.3

 

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Figure 16. Tailings Storage Facility No. 4 proposed action flood breach inundation limits at ranch near mine

Source: Wood 2019d, figure 8.3A

 

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Figure 17. Tailings Storage Facility No. 4 proposed action flood breach inundation limits at communities near Roosevelt Lake

Source: Wood 2019d, figure 8.3B

 

F.8 Geotechnical Stability Uncertainty and Risk 

 

Tailings Storage Facilities No. 3 and No. 4 are designed to meet engineering standards and best practices in accordance with the State of Arizona guidelines for mining projects (Arizona Department of Environmental Quality 2004) referred to as Best Available Demonstrated Control Technology. These standards prescribe minimum slope stability factors of safety under static and pseudo-static (earthquake) loading for the duration of active mining and post-closure, as summarized in table 1. The slope stability design criteria summarized in table 3 have been accepted by the State regulatory agency (Arizona Department of Environmental Quality) as meeting the requirements for permitting under the Best Available Demonstrated Control Technology individual review process. Because this is an existing facility, and because the proposed expansion footprint affects a relatively small area of National Forest System land underlying Tailings Storage Facilities No. 3 and No. 4, the Forest Service has made the determination that State of Arizona guidelines should be followed for purposes of the Pinto Valley Mine environmental impact statement evaluation. 

The stability of the embankments has been evaluated using conventional, standards-based engineering methods, in which risks are controlled by established rules as to minimum factors of safety for prescribed design loads. This approach is a deterministic analysis; in other words, the outcome is “determined” by the input. Slope stability factors of safety are calculated by simulation of the slope in a computer model, with all the various soil layers and groundwater levels depicted in the simulation as accurately as possible with available information from the site. Potential sliding surfaces are examined, and the computer program solves what are called limit-equilibrium equations, in which the forces tending to drive the sliding mass downhill on the specified failure surfaces are balanced against the resisting forces. Driving forces typically include gravity and any dynamic forces caused by earthquakes. Resisting forces are primarily the available friction and shearing resistance along the sliding surface. The shear strength and anticipated pore-pressure response behavior of the embankment and tailings materials are based on geotechnical investigations. The simulated response of the embankment to loading involves assumptions about the drained or undrained response behavior of the soil. These assumptions are subject to uncertainty, and must be verified by field observations and monitoring of the actual slopes. 

In general, the factor of safety is an expression of the calculated ratio of the resisting forces to the driving forces. A factor of safety equal to 1.2, for example, implies that the calculated forces tending to resist sliding are 20 percent greater than the forces tending to drive the mass downhill. Design for stable slopes is optimized using slope-stability calculations, with the goal of achieving the minimum factors of safety as required by standards and regulations. It is important to note, however, that a computed slope-stability factor of safety is not equivalent to an actual margin of safety, as there are always uncertainties in the model input assumptions. Differences in assumed versus actual tailings properties and strength parameters, or deviations from impoundment operating assumptions that affect rate of loading and pore pressures, are examples of input variable uncertainties.

Tailings Storage Facilities No. 3 and No. 4 are being constructed using the upstream raise method, in which successive raises of the outer containment dam embankment are built upon previously placed tailings slurry. Potential loss of shear strength in saturated, slow-draining layers within the tailings pile that supports the raised dam, a phenomenon referred to as static liquefaction, is a particular concern for tailings storage facilities constructed in the upstream manner, especially for very high dams. At the end of mining under the proposed action, Tailings Storage Facility No. 4 will be over 1,000 feet high. The tailings materials on site are known to be susceptible to static liquefaction, as evidenced by a large slope failure that occurred at Pinto Valley Mine Tailings Storage Facility No. 2 in 1997 that released an estimated 370,000 cubic yards of tailings and mine waste rock. That failure was attributed to static liquefaction triggered by rapid loading of the tailings surface by placement of large lifts of waste rock. The impoundment had been considered closed at the time, and such a failure mode had not been contemplated by the mine engineers. 

The slope stability analyses for Tailings Storage Facilities No. 3 and No. 4, summarized in section F.4 of this appendix, included analyses that account for potential loss of shear strength in undrained layers due to anticipated increases in pore pressures for assumed build-up rates of the tailings piles. Minimum required static factors of safety (greater than 1.3 for the active mine operating period) are achieved based on certain assumptions about the tailings strength and pore pressure responses to assumed rates of dam raise. Coarse tailings materials appear to behave in a predicable manner, while the slow-draining, fine layers may not (AMEC 2018b). Fine tailings layers are capable of responding in an undrained manner, which is sensitive to the input assumptions. The stability results are highly dependent on the assumptions about the distribution and continuity of the undrained zones within the tailings impoundments, and the position of the phreatic surface or distribution of excess pore pressures that develop in response to the raise. It is critical that these assumptions be verified as the facilities are raised through diligent monitoring, field and laboratory testing, and engineering analyses.

Operational considerations also are important, and maintaining minimum beach distances between the outer dams and the supernatant pools is a key consideration. The minimum beach distance for Tailings Storage Facility No. 4 is 1,500 feet, although it is reported to be typically operated at a much longer beach distance. Tailings Storage Facility No. 3 is reported to be typically operated with a minimal pool, and has a minimum beach distance of 50 feet. 

The design criteria summarized in table 3 prescribe minimum static factors of safety of 1.3 for the operational period and 1.5 for the long-term, post-closure period. The lower factor of safety for the active mine operational period is allowed under the assumption that these are temporary conditions, with pore pressures elevated due to the continued addition of tailings slurry to the impoundment. Effective shear strengths of the tailings materials should theoretically improve over time after the cessation of mining because interstitial tailings pore fluids will drain down and reach a steady state equilibrium with the regional groundwater table. The lowest calculated factors of safety for static loading, summarized in table 5, were for the proposed action configurations for both Tailings Storage Facilities No. 3 and No. 4. Section B (figure 9) on Tailings Storage Facility No. 3 had the lowest result, with a factor of safety of 1.3 computed for undrained loading assumptions. This is the minimum allowed factor of safety under the Best Available Demonstrated Control Technology rules. Cross-section R (figure 10) on Tailings Storage Facility No. 4 had the lowest result for that structure, with a minimum value of 1.34 for static, undrained loading on deep-seated failure surfaces. The undrained assumptions used in the models are considered conservative for both of these results. Nonetheless, these marginally acceptable results warrant monitoring of both facilities to ensure that the pore pressure response and shear strength assumptions are valid. 

Design criteria for earthquake loading (table 3) are treated in a similar manner as the static factors of safety. Under the Best Available Demonstrated Control Technology, a lower factor of safety of 1.0 for pseudo-static stability is allowed for the operational time frame and the assumed temporary elevated construction pore pressure conditions. A minimum pseudo-static factor of safety of 1.1 is required for the long-term, steady-state pore pressure conditions. The lowest calculated factors of safety were again determined for the proposed action configurations on cross-section B (Tailings Storage Facility No. 3) and cross-section R (Tailings Storage Facility No. 4). The undrained (temporary), pseudo-static analysis for cross-section B (Tailings Storage Facility No. 3) had the lowest factor of safety, equal to 1.0, which is the minimum allowed under the Best Available Demonstrated Control Technology guidance (table 1) for materials that are not subject to liquefaction, and is the lowest value accepted by the State as the design criteria (table 3) for temporary loading conditions. The lowest pseudo-static result for cross-section R (Tailings Storage Facility No. 4) was on an intermediate-depth failure surface with a minimum factor of safety of 1.08 under undrained assumptions. 

The low factors of safety results for the pseudo-static loading analyses warranted further analyses to determine seismic liquefaction potential and deformations under seismic loading. As discussed in section F.5, liquefaction analysis for both Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 indicate that liquefaction will not occur in the tailings materials under an assumed 1 in 975 annual exceedance probability event, if current conditions within the tailings do not change. The 1 in 975 annual exceedance probability event that was evaluated was based on previous design guidance, and is a smaller event than the currently accepted design criteria (table 3). A minimum factor of safety of 1.3 against liquefaction under seismic shaking was obtained from those analyses. The liquefaction analysis report indicates that higher site accelerations (on the order of 0.15 gravitational acceleration) would trigger liquefaction within portions of the impounded tailing (AMEC 2014). The peak acceleration for the design-basis earthquake of 1 in 2,475 annual exceedance probability (table 3) is 0.137g, which is below the anticipated threshold for triggering liquefaction, but not by a large margin. 

Seismic deformation analyses for the design 1 in 2,475 annual exceedance probability earthquake were completed only for Tailings Storage Facility No. 4 (AMEC 2017b). The conclusion from the dynamic analyses was that the Tailings Storage Facility No. 4 structure would experience limited deformation, on the order of about 4 inches, during and after the design-level ground shaking for the site, but this deformation would not result in a flow failure of the embankment. No liquefaction zones were predicted for the design basis earthquake event within the Tailings Storage Facility No. 4 embankment by the updated dynamic modeling (AMEC 2017b).

The main conclusion from the dynamic stability analyses is that both active tailings storage facilities are predicted to be stable under the loading associated with the State of Arizona–accepted (table 3) design basis earthquake, which is a 1 in 2,475 annual exceedance probability event having a peak ground acceleration of 0.137g. Larger earthquakes are theoretically possible in this region, but would have lower probabilities of occurrence. Other design guidance recommendations (outside of the State of Arizona), such as the federal National Dam Safety Program, or the Canadian Dam Association guidelines for mining dams, which are summarized in table 1 and table 2, recommend using risk potential to establish design criteria. 

Risk is a function of the probability (likelihood) of failure and the consequences should a particular type of failure occur. “Risk” is defined as follows:

Risk = Likelihood (of failure) X Consequences

where: Failure Likelihood = Probability (PF)

 Consequences = magnitude of flooding, lives lost, environmental impacts, economic losses

The probability of failure is computed as:

PF = P(LOAD) X P(RESPONSE)

where: LOAD = flood, earthquake, or normal operations

 RESPONSE = dam breach, uncontrolled release

Risk is assessed as follows:

. Identify potential failure modes - screen down to risk drivers

 

 

. Estimate probability (likelihood) of load: earthquake recurrence intervals, flood frequency – hydrologic hazard curve, normal operation (probability = 1)

 

. Estimate probability of failure (response) under the load

 

. Estimate the downstream consequences

 

Under Federal agency recommendations, if the downstream consequences of dam failure indicate the potential for loss of life, the guidelines require using the most extreme prescribed design criteria for both earthquakes and flood loading. The Canadian Dam Association guidance extreme event for earthquake loading, for example, is a maximum credible earthquake or a 1 in 10,000 annual exceedance probability event (table 1) for loss of life estimates in excess of 100 people, or extreme environmental or major damage to critical infrastructure. The National Dam Safety Program and the Best Available Demonstrated Control Technology guidance (table 1) recommend using a maximum credible earthquake if any loss of life is anticipated. An earthquake larger (and less probable) than the current 1 in 2,475 annual exceedance probability design basis event for the Pinto Valley Mine facilities could cause liquefaction and loss of strength of the tailings materials, resulting in slope instability and release of tailings. Those analyses for larger earthquake events have not been done, so the actual risk is unknown for the extreme loading scenarios. 

Potential modes of dam failure other than slope instability also are possible, and these are not typically accounted for under the routine deterministic analyses and minimum calculated factors of safety that are required by regulatory agencies. Examples include failure modes such as internal erosion of embankment or foundation materials under hydraulic head gradients leading to concentrated leakage and dam breach, or overtopping due to extreme flood events. All potential failure modes, including those covered by deterministic analyses (such as slope stability calculations) and those that are not, can be evaluated in a risk analyses framework. Detailed procedures for conducting risk analyses for dams are available from several resources including the “Best Practices in Dam and Levee Safety Risk Analysis” manual developed by U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers in collaboration (U.S. Bureau of Reclamation 2018), and the “Federal Guidelines for Dam Safety Risk Management” developed by the Federal Emergency Management Agency (Federal Emergency Management Agency 2015).

A qualitative risk analysis was performed for all of the Pinto Valley Mine tailings storage facilities in 2015 (AMEC 2015). The process used followed a Failure Modes and Effects Analysis framework, as outlined by the United Kingdom Department for Environment, Flood and Rural Affairs (United Kingdom Department for Environment, Flood and Rural Affairs 2013). At the time of the 2015 risk assessment, the envisioned configurations of Tailings Storage Facilities No. 3 and No. 4 were different than the no-action alternative or proposed action configurations, and the downstream consequences had not yet been formally evaluated using dam break modeling. The 2015 qualitative risk assessment involved evaluation of the likelihood and consequences of dam failure in a workshop setting, relying on professional judgment and opinions of a group of specialists and people with first-hand knowledge of the facilities. The outcome of the process identified several credible potential failure modes, including overtopping due to precipitation events or operational problems, and slope instability due to elevated pore water pressures, among others. The consequences of dam failure were characterized qualitatively and it was recommended at the time that dam breach analyses be done to allow refinement of the risk assessment and emergency planning procedures.

As summarized in section F.7 of this appendix, dam breach and inundation analyses were conducted for the no-action alternative and proposed action for Tailings Storage Facility No. 3 and No. 4 as part of this environmental impact statement (Wood 2019). The dam breach studies indicate that consequences of a

“sunny day” (normal operational) breach of Tailings Storage Facility No. 4 could result in a runout release of tailing materials that could affect up to a 2.75-mile reach of Pinto Creek downstream from the facility. A sunny-day breach of Tailings Storage Facility No. 3 could run out over 1.6 miles downstream. Under a major flooding scenario, breach models indicate water and entrained tailings would inundate the entire reach of Pinto Creek between the mine site and Theodore Roosevelt Lake, with significant flood depths affecting a ranch on private property owned by Pinto Valley Mining Corp., State Route 188, and two communities, Roosevelt Shores and Roosevelt Estates, located near Roosevelt Lake. 

Although the consequences side of the risk equation are now fairly well understood given the Wood 2019 dam breach model results, the probability (likelihood) of all types of potential failure modes have not been evaluated or quantified for the tailings storage facilities at Pinto Valley Mine. Therefore, the probabilistic risk of dam failure is unknown, but it is not zero. There are many variables that must be evaluated to quantify probability with any level of confidence, and assigning a meaningful probability of failure to a tailings impoundment is extremely difficult. Because of the uncertainties involved, it is prudent to adopt monitoring techniques to measure and assess whether design parameters and assumptions for the deterministic slope stability analyses are being met, and to ensure that adequate monitoring is being done for all other potential failure modes that have been identified. 

The Forest Service has proposed specific monitoring measures in recognition of the uncertainties associated with design and the non-zero potential risk of tailings dam failure. In addition, the Forest Service proposes a mitigation measure to develop an emergency action plan. Having an emergency action plan is an industry standard for most high hazard water dams, and similar plans can be adapted for use with tailings impoundments. The emergency action plan mitigation measure is included in recognition of the need for timely warning of downstream persons at risk in the event of a dam breach. This mitigation measure is justified given the potential that a breach of either tailings storage facility during a significant flood event would adversely affect and inundate downstream infrastructure and communities. 

 

F.9 References

 

AJAX. 2019. Application for Amendment to APP No. P-100329, Tailings Storage Facility No. 3 and Gold Gulch Repurposing, prepared by AJAX Mountain Enterprises LLC, January.

AMEC. 2014. Design Report, Prefeasibility Study, Tailings Storage Facility Expansions, Pinto Valley Mine, Gila County, AZ, August 21.

AMEC. 2015a. Technical Memorandum, Seismic Stability Evaluation of Tailings Ponds Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4, Miami, Arizona, May 5 (provided as Appendix G in Response to Data Request, EIS Record No. 14).

AMEC. 2015b. Static and Dynamic Analysis and Piezometer Replacement Program Tailings Storage Facilities No. 3 & No. 4 Phase 2 Extension (1D Liquefaction Analysis), July 30 (provided as Appendix H in Response to Data Request, EIS Record No. 14).

AMEC. 2015c. Pinto Valley Mine, Tailings Dam Structures, Risk Assessment Summary, prepared by AMEC Foster Wheeler Environment & Infrastructure, October 13.

AMEC. 2017a. Technical Memorandum: Stability Analysis – Tailings Storage Facility No. 4, Embankment Stability Analysis, January 20.

AMEC. 2017b. Technical Memorandum, Dynamic Stability Analysis – Tailing Storage Facility No. 4 Extension, Pinto Valley Mine, March 28.

AMEC. 2017c. Hydrologic and Hydraulic Analysis, Pinto Valley Mine, Tailings Storage Facility No. 4 Extension, Drainage Report for Operations and Closure, prepared by AMEC Foster Wheeler, March 28, 2017.

AMEC. 2018a. “Summary of Design Criteria for Tailings Storage Facility No. 4 Extension” Letter Report from AMEC Foster Wheeler dated Jan 5, 2018 in response to Data Request EIS Record 13. 

AMEC. 2018b. Pinto Valley Mine Embankment Stability Analysis Tailings Storage Facility No. 3 (No Action), January 26, Response to Data Request EIS Record No. 14.

AMEC. 2018c. Pinto Valley Mine Embankment Stability Analysis Tailings Storage Facility No. 3 (Proposed Action), February 8, Response to Data Request EIS Record No. 15. 

AMEC. 2018d. Preliminary Dam Break Analyses Tailings Storage Facilities No. 3 and No. 4, Technical Memorandum, January 11, Response to Data Request EIS Record No. 18.

AMEC. 2018e. Hydrologic and Hydraulic Analysis TSF3 Drainage Report for No Action and Proposed Action Alternatives - Pinto Valley Mine, March.

Arizona Department of Environmental Quality. 2004. Arizona Mining Guidance Manual Best Available Demonstrated Control Technology, Aquifer Protection Program, Publication TB 04-01, Phoenix, Arizona.

Canadian Dam Association. 2013. “Dam Safety Guidelines 2007 (2013 Edition).” Canadian Dam Association. www.cda.ca.

Canadian Dam Association. 2014. “Technical Bulletin Application of Dam Safety Guidelines to Mining Dams.” Canadian Dam Association. www.cda.ca. 

Dames & Moore. 1985. Geotechnical Analysis and Tailings Disposal Planning, Tailings Ponds 3 and 4, Pinto Valley Operations, for Pinto Valley Copper Corporation, Newmont Mining Corporation. December 15.

Federal Emergency Management Agency. 2004a. Federal Guidelines for Dam Safety, FEMA-93, April.

Federal Emergency Management Agency. 2004b. Federal Guidelines for Dam Safety: Hazard Potential Classification System for Dams, FEMA-333, April.

Federal Emergency Management Agency. 2005. Federal Guidelines for Dam Safety: Earthquake Analysis and Design of Dams, FEMA-65 May.

Federal Emergency Management Agency. 2013. Selecting and Accommodating Inflow Design Floods for Dams, FEMA P-94, August.

Federal Emergency Management Agency. 2015. Federal Guidelines for Dam Safety Risk Management, FEMA P-1025, January.

FLO-2D Software, Inc., 2018. FLO-2D Two-Dimensional Flood Routing Model, FLO-2D QGIS Plugin User Manual, June 2018.

Jeyapalan, J. K. 1982. Dam-break studies for mine tailings impoundments. In Uranium Mill Tailings Management, Proceedings of the Fifth Symposium, Dec. 9-10, Ft. Collins, Colorado, pp 39-53.

Rico M., G. Benito, and A. Dico-Herrero. 2007. “Floods from Tailings Dam Failures.” Journal of Hazardous Materials. Vol. 154(1-3), pp 79-87.

SRK Consulting, Inc. 2017. Application for Significant Amendment to Aquifer Protection Plan (APP) No. P-100329, TSF 4 Extension Design, prepared for Capstone Pinto Valley Mine by SRK Consulting, March 31.

U.S. Department of the Interior, Bureau of Reclamation, Dam Safety Office. 1998. Prediction of Embankment Dam Breach Parameters, Dam Safety Research Report DSO-98-004, July.

U.S. Department of the Interior, Bureau of Reclamation, Dam Safety Office. 2018. Best Practices in Dam and Levee Safety Risk Analysis. https://www.usbr.gov/ssle/damsafety/risk/methodology.html. 

U.S. Army Corps of Engineers. 2016. Hydrologic Engineering Center, HEC-RAS River Analysis System User’s Manual Version 5, February.

United Kingdom Department for Environment, Flood and Rural Affairs. 2013. “Guide to risk assessment for reservoir and safety management.” March.

Wood. 2018. Technical Memorandum “Conceptual-Level Reclamation Design, Pinto Valley Mine, Tailings Storage Facility No. 3.” December 21.

Wood. 2019a. Technical Memorandum “Tailings Storage Facility No. 3 (TSF3) Embankment Stability Analysis Proposed Action,” to Pinto Valley Mining Corp. from Bibhuti B. Panda, Senior Geotechnical Engineer, Wood, July 12, 2019.

Wood. 2019b. Technical Memorandum “Revised Tailings Dam Design Criteria Request,” to Pinto Valley Mining Corp. from Bibhuti B. Panda, Senior Geotechnical Engineer, Wood, July 11, 2019.

Wood. 2019c. Letter Report “Response to TNF Data Request Record #270, Physical Aspects of TSF3 and TSF4,” to Pinto Valley Mining Corp. from Bibhuti B. Panda, Senior Geotechnical Engineer, Wood, July 12.

Wood. 2019d. Dam Breach Physical Modeling Analysis for Tailings Storage Facility No. 3 and Tailings Storage Facility No. 4 [Forthcoming].

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