INTERIM RECORD OF DECISION ANACONDA COPPER MINE SITE Arimetco Facilities Operable Unit 8 Heap Leach Pads and Fluids Management System Lyon County, Nevada July 2017 U.S. Environmental Protection Agency U.S. Department of the Interior Co-lead Agency Bureau of Land Management Region 9 Co-lead Agency 75 Hawthorne Street Nevada State Office San Francisco, California 94105-3901 1340 Financial Boulevard Reno, Nevada 89502 Nevada Division of Environmental Protection Co-lead Agency 901 S. Stewart Street Carson City, Nevada 8970 Table of Contents ________________________________________________ List of Figures ................................................................................................................................................ iv List of Tables ................................................................................................................................................. iv List of Appendices ......................................................................................................................................... iv Acronyms and Abbreviations ......................................................................................................................... v 1.0 The Declaration............................................................................................................................... 1-1 1.1 Site Name and Location ....................................................................................................... 1-1 1.2 Statement of Basis and Purpose .......................................................................................... 1-1 1.3 Assessment of Site ............................................................................................................... 1-1 1.4 Description of Selected Remedy .......................................................................................... 1-1 1.5 Planning and Implementation of Stormwater Management Actions at OU-8 Facilities Statutory Determinations ...................................................................................................... 1-2 1.6 Data Certification Checklist .................................................................................................. 1-3 1.7 Lead Agency and Supporting Agency Signatures ................................................................ 1-4 2.0 The Decision Summary ................................................................................................................... 2-1 2.1 Project Background .............................................................................................................. 2-1 2.1.1 Site Name, Location, and Brief Description ............................................................. 2-1 2.1.2 Site History and Enforcement Activities ................................................................... 2-1 2.1.2.1 Site Mining History .................................................................................... 2-1 2.1.2.2 Arimetco Operational History .................................................................... 2-2 2.1.2.3 Agency Involvement at the Site ................................................................. 2-3 2.1.2.4 Enforcement Actions and Interim Remedial Action Summary ................... 2-5 2.1.3 Community Participation .......................................................................................... 2-6 2.1.4 Scope and Role of the Response Action ................................................................. 2-9 2.1.5 Site Characteristics................................................................................................ 2-10 2.1.5.1 Physical Setting ...................................................................................... 2-11 2.1.5.2 Geology ................................................................................................... 2-12 2.1.5.3 Hydrogeology .......................................................................................... 2-13 2.1.5.4 Surface Water ......................................................................................... 2-13 2.1.5.5 Seismicity ................................................................................................ 2-13 2.1.5.6 Ecological Setting.................................................................................... 2-14 2.1.5.7 Potential Contamination Sources ............................................................ 2-14 2.1.6 Current and Potential Future Land and Resource Uses ........................................ 2-16 2.2 Summary of Site Risks ....................................................................................................... 2-17 2.2.1 Human Health Risk Summary ............................................................................... 2-19 2.2.1.1 Identification of Chemicals of Concern .................................................... 2-19 2.2.1.2 Exposure Assessment ............................................................................ 2-19 2.2.1.3 Toxicity Assessment ............................................................................... 2-21 2.2.1.4 Risk Characterization .............................................................................. 2-21 2.2.1.5 Uncertainty Analysis ................................................................................ 2-25 2.2.2 Summary of Ecological Risk Assessment ............................................................. 2-26 2.2.3 Risk Assessment Conclusions .............................................................................. 2-27 2.3 Remedial Action Objectives ................................................................................................ 2-27 ConcDP-\\seiconfps00\TechPubs\EPA\500683 Arimetco\ROD\F\170612 Anaconda_OU8_ROD final.doc 6.12.17 i June 2017 Table of Contents (continued) ______________________________________ 2.4 Description of Alternatives .................................................................................................. 2-28 2.4.1 Remedy Components ............................................................................................ 2-29 2.4.1.1 Alternative 0 (FS Alternative 1), “No Action Alternative” .......................... 2-29 2.4.1.2 Alternative 1 (FS Alternative 2), “No Further Action Alternative” ............. 2-29 2.4.1.3 Alternative 2 (FS Alternative 6), “Passive Evaporation and Top Capping of HLPs” ...................................................................................................... 2-30 2.4.1.4 Alternative 3 (FS Alternative 8), “Passive Evaporation and Complete Capping of HLPs” .................................................................................... 2-33 2.4.1.5 Alternative 4 (FS Alternative 6a/8a), “Modified Evaporation, Complete Capping of HLPs, Upgrading Ponds, and Stormwater Management” ..... 2-35 2.4.2 Common Elements and Distinguishing Features of Each Alternative .................... 2-39 2.5 Comparative Analysis of Alternatives ................................................................................. 2-40 2.5.1 Overall Protection of Human Health and the Environment .................................... 2-41 2.5.2 Compliance with Applicable or Relevant and Appropriate Requirements .............. 2-43 2.5.3 Long-term Effectiveness and Permanence ............................................................ 2-46 2.5.4 Reduction of Toxicity, Mobility, or Volume Through Treatment ............................. 2-47 2.5.5 Short-term Effectiveness ....................................................................................... 2-47 2.5.6 Implementability .................................................................................................... 2-48 2.5.7 Cost ....................................................................................................................... 2-49 2.5.8 State Acceptance .................................................................................................. 2-49 2.5.9 Community Acceptance ......................................................................................... 2-49 2.6 Principal Threat Wastes ..................................................................................................... 2-50 2.7 Description of the Selected Remedy .................................................................................. 2-51 2.7.1 HLP Covers ........................................................................................................... 2-51 2.7.2 Upgrades to Drain-Down FMS Collection and Transfer Components ................... 2-52 2.7.3 Passive Evaporation Treatment of Drain-Down Fluids .......................................... 2-52 2.7.4 Fluids Management With Wildlife Deterrents ......................................................... 2-52 2.7.5 Upgrading of Evaporation Ponds ........................................................................... 2-52 2.7.6 Closure of the 4-Acre Pond ................................................................................... 2-52 2.7.7 Site Access and Land-Use Controls ...................................................................... 2-53 2.7.8 Stormwater Management System ......................................................................... 2-53 2.8 Summary of the Rationale for the Selected Remedy .......................................................... 2-54 2.8.1 Cost Estimate for the Selected Remedy ...............................................................