epa superfund record of decision• collection and treatment of acid rock drainage (ard) from mine...

EPA/ROD/R08-03/0152003

EPA Superfund

Record of Decision:

CALIFORNIA GULCHEPA ID: COD980717938OU 06LEADVILLE, CO09/25/2003

FINAL RECORD OF DECISION

OPERABLE UNIT 6 CALIFORNIA GULCH SUPERFUND SITE

LEADVILLE, COLORADO

September 2003

U. S. Environmental Protection Agency 999 18th Street, Suite 500

Denver, CO 80202

RECORD OF DECISION

OPERABLE UNIT 6 CALIFORNIA GULCH SUPERFUND SITE

LEADVILLE, COLORADO

The U. S. Environmental Protection Agency (EPA), with the concurrence of the Colorado Department of PublicHealth and Environment (CDPHE), presents this Record of Decision (ROD) for Operable Unit 6 (OU6) of theCalifornia Gulch Superfund Site (Site) in Leadville, Colorado. The ROD is based on the Administrative Record forOU6, including the Remedial Investigation/Feasibility Study (RI/FS), the Proposed Plan, the public commentsreceived, and EPA responses. The ROD presents a brief summary of the RI/FS, past Response Actions, actual andpotential risks to human health and the environment, and the Selected Remedy. EPA followed the ComprehensiveEnvironmental Response, Compensation, and Liability Act, as amended, the National Contingency Plan (NCP), andEPA guidance (EPA, 1999a) in preparation of the ROD. The three purposes of the ROD are to:

1. Certify that the remedy selection process was carried out in accordance with the requirements of theComprehensive Environmental Response, Compensation, and Liability Act, 42 U. S. C. 9601 et seq.. asamended by the Superfund Amendments and Reauthorization Act (collectively, CERCLA), and, to theextent practicable, the NCP;

2. Outline the engineering components and remediation requirements of the Selected Remedy; and 3. Provide the public with a consolidated source of information about the history, characteristics, and risk

posed by the conditions at OU6, as well as a summary of the cleanup alternatives considered, theirevaluation, the rationale behind the Selected Remedy, and the agencies' consideration of, and responses to,the comments received.

The ROD is organized into three distinct sections:

1. The Declaration Section functions as an abstract and data certification sheet for the key information in theROD and includes the formal authorizing signature page for the ROD.

2. The Decision Summary provides an overview of the characteristics of OU6, alternatives evaluated, and theanalysis of those options. It also identifies the Selected Remedy and explains how the remedy fulfillsstatutory and regulatory requirements.

3. The Responsiveness Summary serves the dual purposes of: (1) presenting stakeholder concerns about OU6and preferences regarding the remedial alternatives; and (2) explaining how those concerns were addressedand the preferences were factored into the remedy selection process.

DECLARATION

DECLARATION

SITE NAME AND LOCATION

Operable Unit 6 California Gulch Superfund Site Leadville, Colorado CERCLIS # COD980717938

STATEMENT OF BASIS AND PURPOSE

This decision document presents the Selected Remedy for OU6 of the Site in Leadville, Colorado. The EnvironmentalProtection Agency selected the remedy in accordance with the Comprehensive Environmental Response,Compensation, and Liability Act (CERCLA), as amended by the Superfund Amendments and Reauthorization Act(SARA), and to the extent practicable, the NCP. This decision is based on the Administrative Record file for OU6.The State of Colorado concurs with the Selected Remedy.

ASSESSMENT OF SITE

The Response Action selected in this Record of Decision is necessary to protect public health or welfare or theenvironment from actual or threatened releases of hazardous substances and pollutants or contaminants from OU6which may present an imminent and substantial endangerment to public health or welfare.

DESCRIPTION OF SELECTED REMEDY

The Site was added to the National Priority List in 1983. In 1994 the Site was divided into 10 geographically-basedareas, also called operable units. This was accomplished through a Consent Decree with ASARCO and ResurrectionMining Company (the mining companies). OU6 is one of these 12 operable units and covers approximately 3.4 squaremiles in the northeastern portion of the Site. OU6 includes the Stray Horse Gulch watershed and the upper and lowerportions of the Evans Gulch watershed. Investigation of the Site began in the mid-1980s and continues today. Mostof the investigation in OU6 was performed as a part of broader studies of the entire Site. A complete list ofinvestigative reports on OU6 can be found in the OU6 Focused Feasibility Study (EPA, 2002a) in the AdministrativeRecord. Beginning in 1990, EPA and the mining companies began a series of Response Actions to systematicallycleanup the majority of the mine wastes causing contamination in OU6. A Response Action is a cleanup project thatby itself may not achieve total site cleanup. However, taken together, all of the Response Actions performed to datein OU6 have addressed most of the mine wastes considered to be sources of contamination.

The major OU6 Response Actions have included:

• Consolidation and capping of selected mine waste piles.

• Collection and treatment of acid rock drainage (ARD) from mine waste piles. Treatment occurs at a facilityoperated by the US Bureau of Reclamation (BOR) at the portal of the Leadville Mine Drainage Tunnel. BORhas cooperated with EPA to treat OU6 water on an interim basis to determine the feasibility of using itsfacility to treat OU6 water for the long- term.

• Diversion of clean surface water around mine wastes.

• Rehabilitation of Stray Horse Gulch and Starr Ditch.

Although these Response Actions have cleaned-up most of the mine wastes in OU6, it is not certain thatmaintaining these remedies into the future is the most cost effective and reliable way to maintain the improvementsachieved to date. Therefore, EPA will augment portions of the work performed to date and/or use new kinds ofcleanup methods as presented in this ROD.

The cleanup plan for OU6 is one of the many remedies either completed or being considered for each of the operableunits in the Site. Pursuant to the agreement reached between EPA, the State, ASARCO and Resurrection in the 1994Consent Decree, the remedies for OUs 2 through 11 typically target the source materials such as mine waste rock andmill tailing. The remedies for all but OU6, OUS (AV/ CZL Smelter), OU11 (Arkansas River Floodplain), and OU12(Site-Wide Surface and Groundwater) have been constructed.

EPA will monitor the improvements in surface and groundwater quality as the source control remedies in each of theoperable units are completed. If the remedies don't sufficiently improve the quality of surface and groundwater, EPAand the State of Colorado may require further source control measures or other cleanup actions. Operable Unit 12(OU12) will assess and address site- wide surface and ground water quality, if necessary. Unlike the other operableunits, OU12 includes the entire Site.

The OU6 Focused Feasibility Study (FFS), Waste Rock Remedial Investigation Report and other investigativereports describe those waste rock piles which generate acid rock drainage (ARD) in quantities above a level ofconcern. Waste rock piles in OU6 have been addressed through prior Response Actions or will be addressed underthe Selected Remedy for OU6.

These prior Response Actions will be maintained under the Selected Remedy. In addition to maintaining priorResponse Actions, the Selected Remedy will result in the removal of additional mine waste rock to an on- Siterepository planned for the Site, thereby isolating the waste from the environment. Other components of the SelectedRemedy address ARD derived from mine waste rock.

The major components of the Selected Remedy include:

• Continued collection and management of ARD with management by:

• Treatment to remove contaminants at a water treatment facility operated by the US Bureau ofReclamation (BOR).

• Discharge to surface water at a controlled rate to minimize impacts to the Parkville Water District. • Evaporation.

• Continued maintenance of consolidated and capped waste piles and surface water management featuresconstructed during prior Response Actions. Maintenance will include inspections and repairs to caps andperiodic cleaning/ repairing of surface water management features. Wastes generated during maintenanceactivities will be disposed in an on- Site repository planned for the Site.

• Construction of a plug (bulkhead) in the Leadville Mine Drainage Tunnel (LMDT) to allow bettermanagement of groundwater in flooded underground mine workings connected to the LMDT (mine pool).

• Dewatering of the mine pool from a location upstream of the bulkhead and delivery of pumped groundwaterto the BOR's treatment plant via a buried gravity pipeline.

• Establishing a groundwater monitoring network to observe water level and quality conditions in and aroundthe mine pool.

• Relocation of additional mine waste rock (e.g., Ponsardine mine waste pile) to the on-Site repositoryplanned for the Site.

• Stabilization of a retaining wall (crib wall) associated with the Emmet waste rock pile.

• Establishment of land use controls under an Institutional Control Overlay (ICO) District to ensure that anyfuture changes in land use are consistent with the final remedy.

STATUTORY DETERMINATIONS

The Selected Remedy is protective of human health and the environment, complies with federal and Staterequirements that are applicable or relevant and appropriate to the remedial action, is cost effective, and utilizespermanent solutions and alternative treatment technologies to the maximum extent practicable. This remedy alsosatisfies the statutory preference for treatment as a principal element of the remedy. Because this remedy will resultin hazardous substances, or pollutants or contaminants remaining in OU6 above levels that allow for unlimited useand unrestricted exposure, a statutory review will be conducted within five years after initiation of remedial action toensure that the remedy is, or will be, protective of human health and the environment. This remedy is acceptable toboth the State of Colorado and the community of Leadville.

DATA CERTIFICATION CHECKLIST

The following information is included in the Decision Summary section of this Record of Decision. Additionalinformation can be found in the Administrative Record file for this site.

• Contaminants of concern (COCs) and their respective concentrations.• Baseline risk represented by the COCs. • Cleanup levels established for COCs and the basis for these levels. • Current and reasonably anticipated future land use assumptions used in the baseline risk assessment and

ROD. • Potential land use that will be available at OU6 as a result of the Selected Remedy. • Estimated capital, annual operation and maintenance (O& M), and total present worth costs, discount rate,

and the number of years over which the remedy cost estimates are projected. • Key factor( s) that led to selecting the remedy.

that the remedy is, or will be, protective of human health and the environment. Thisremedy is acceptable to both the State of Colorado and the community of Leadville.

DATA CERTIFICATION CHECKLIST

The following information is included in the Decision Summary section of this Record ofDecision. Additional information can be found in the Administrative Record file for thissite.

• Contaminants of concern (COCs) and their respective concentrations.• Baseline risk represented by the COCs.• Cleanup levels established for COCs and the basis for these levels.• Current and reasonably anticipated future land use assumptions used in the

baseline risk assessment and ROD.• Potential land use that will be available at OU6 as a result of the Selected

Remedy.• Estimated capital, annual operation and maintenance (O&M), and total present

worth costs, discount rate, and the number of years over which the remedy costestimates are projected.

• Key factor(s) that led to selecting the remedy.

AUTHORIZING SIGNATURE

Max H. Dodson DateAssistant Regional AdministratorEcosystems Protection and RemediationU.S. Environmental Protection Agency, Region VIII

Final Record of Decision D-4OU6 California Gulch NPL Site

DECISION SUMMARY

TABLE OF CONTENTS

1.0 SITE NAME, LOCATION AND DESCRIPTION.......................................................................... DS-1 2.0 SITE HISTORY AND ENFORCEMENT ACTIVITIES................................................................. DS-2 3.0 COMMUNITY PARTICIPATION.................................................................................................. DS-7 4.0 SCOPE AND ROLE OF OPERABLE UNIT OR RESPONSE ACTION....................................... DS-8 5.0 SITE CHARACTERISTICS............................................................................................................. DS-10

5.1 PHYSICAL SETTING....................................................................................................... DS-10 5.1.1 Site Physiology................................................................................................. DS-10 5.1.2 Regional Geology............................................................................................. DS-10 5.1.3 Climate................................................................................................................ DS-11

5.2 SURFACE WATER AND RELATED MEDIA............................................................. DS-115.2.1 Surface Water Hydrology................................................................................ DS-11 5.2.2 Surface Water Chemistry................................................................................. DS-12 5.2.3 Fluvial Tailing.................................................................................................... DS-13 5.2.4 Stream Sediments.............................................................................................. DS-13

5.3 GROUNDWATER............................................................................................................ DS-13 5.3.1 Hydrogeology................................................................................................... DS-13 5.3.2 Groundwater Chemistry................................................................................... DS-15

5.4 MINE WASTE.................................................................................................................. DS-15 5.4.1 Mine Waste Types........................................................................................... DS-15 5.4.2 Mine Waste Locations..................................................................................... DS-16 5.4.3 Mine Waste Quantities.................................................................................... DS-19

6.0 SUMMARY OF SITE RISKS.......................................................................................................... DS-20 6.1 HUMAN HEALTH RISKS.............................................................................................. DS-20

6.1.1 Contaminants of Concern................................................................................ DS-20 6.1.2 Exposure Assessment...................................................................................... DS-21 6.1.3 Toxicity Assessment........................................................................................ DS-21 6.1.4 Risk Characterization........................................................................................ DS-22

6.2 ECOLOGICAL RISKS................................................................................................................. DS-23 6.2.1 Contaminant Identification.............................................................................. DS-23 6.2.2 Exposure Assessment....................................................................................... DS-24 6.2.3 Toxicity Assessment......................................................................................... DS-24 6.2.4 Risk Characterization........................................................................................ DS-25

7.0 REMEDIAL ACTION OBJECTIVES.............................................................................................. DS-26 7.1 INTRODUCTION.............................................................................................................. DS-26 7.2 MEDIA OF CONCERN.................................................................................................... DS-27 7.3 MIGRATION PATHWAYS OF CONCERN.................................................................. DS-28 7.4 REMEDIAL ACTION OBJECTIVES.............................................................................. DS-28

8.0 DESCRIPTION OF ALTERNATIVES............................................................................................ DS-29 8.1 ALTERNATIVE 1: NO ACTION..................................................................................... DS-29 8.2 ALTERNATIVE 2: MAINTAIN CURRENT REMEDIES W/LAND

USE CONTROLS............................................................................................................... DS-29

8.3 ALTERNATIVE 4: IN- SITU CHEMICAL STABILIZATION OR RELOCATION (IN COMBINATION WITH ALTERNATIVE OPTIONS 2A THROUGH 2H)........................................................................................................... DS-31

8.4 ALTERNATIVE 5: CONSOLIDATE AND CAP W/LAND USE CONTROLS....................................................................................................................... DS-32

8.5 ALTERNATIVE 6: EXCAVATE, TRANSPORT AND ON-SITE DISPOSAL W/LAND USE CONTROLS........................................................................ DS-32 9.0 SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES...................................... DS-33

9.1 NCP CRITERIA................................................................................................................. DS-33 9.1.1 Overall Protection of Human Health and the Environment......................... DS-33 9.1.2 Compliance with Applicable or Relevant and

Appropriate Requirements.............................................................................. DS-33 9.1.3 Long-Term Effectiveness and Permanence................................................... DS-33 9.1.4 Reduction of Toxicity, Mobility, or Volume Through

Treatment........................................................................................................... DS-34 9.1.5 Short- Term Effectiveness............................................................................... DS-35 9.1.6 Implementability................................................................................................ DS-35 9.1.7 Cost..................................................................................................................... DS-35 9.1.8 State Acceptance.............................................................................................. DS-36 9.1.9 Community Acceptance................................................................................... DS-36

9.2 WAMP CRITERIA........................................................................................................... DS-36 9.2.1 Surface Erosion Stability.................................................................................. DS-36 9.2.2 Slope Stability................................................................................................... DS-37 9.2.3 Flow Capacity and Stability............................................................................ DS-37 9.2.4 Surface Water and Groundwater Loading Reduction.................................. DS-38 9.2.5 Terrestrial Ecosystem Exposure...................................................................... DS-39 9.2.6 Non- Residential Soils...................................................................................... DS-40

10.0 PRINCIPAL THREAT WASTES.................................................................................................... DS-41 11.0 SELECTED REMEDY....................................................................................................................... DS-42

11.1 DESCRIPTION OF THE SELECTED REMEDY............................................................ DS-42 11.2 SUMMARY OF ESTIMATED REMEDY COSTS........................................................ DS-44 11.3 CONTINGENCY MEASURES AND LONG-TERM MONITORING.......................... DS-45

12.0 STATUTORY DETERMINATIONS.............................................................................................. DS-47 12.1 PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT........................ DS-47 12.2 COMPLIANCE WITH APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS................................................................................ DS-47 12.3 COST-EFFECTIVENESS.................................................................................................. DS-48 12.4 UTILIZATION OF PERMANENT SOLUTIONS AND ALTERNATIVE

TREATMENT (OR RESOURCE RECOVERY) TECHNOLOGIES TO THE MAXIMUM EXTENT PRACTICABLE (MEP).................................................. DS-49

12.5 PREFERENCE FOR TREATMENT AS A PRINCIPAL ELEMENT............................ DS-49 12.6 FIVE-YEAR REVIEW REQUIREMENTS........................................................................ DS-49

13.0 DOCUMENTATION OF SIGNIFICANT CHANGES FROM PREFERRED ALTERNATIVE OF PROPOSED PLAN........................................................................................ DS-50

14.0 REFERENCES.................................................................................................................................... R- l

LIST OF TABLES

TABLE

1 1995 CONTAMINANT CONCENTRATIONS IN STREAM BED SEDIMENTS2 SUMMARY OF PILES IDENTIFIED AS CANDIDATES FOR REMEDIAL ACTION 3 ZINC AND CADMIUM CONCENTRATIONS AT SHG-08 AND SHG-09 4 ARD-GENERATING MINE WASTE VOLUMES 5 HAZARD INDICES FOR SOLID SURFICIAL MEDIA BY RECEPTOR FOR OU6 6 COMPARATIVE ANALYSIS USING NCP CRITERIA 7 COMPARATIVE ANALYSIS USING ADDITIONAL CRITERIA 8 SELECTED REMEDY COST ESTIMATE FOR ALTERNATIVE 2G AND 4B 9 CHEMICAL-SPECIFIC ARARs 10 LOCATION-SPECIFIC ARARs 11 ACTION- SPECIFIC ARARs

LIST OF FIGURES

FIGURE

1 LOCATION MAP CALIFORNIA GULCH SUPERFUND SITE 2 SITE LOCATION MAP CALIFORNIA GULCH OU6 3 PREVIOUS REMOVAL ACTION AREAS 4 STRAY HORSE GULCH SURFACE WATER MANAGEMENT REMEDIES 5 SURFACE WATER FEATURES AND WETLAND AREAS 6 PRE-REMEDIAL 1995 SURFACE WATER CONTAMINANT LOADING7 FLUVIAL TAILINGS LOCATIONS 8 ALLUVIAL AQUIFER WATER TABLE CONTOURS 9 POTENTIOMETRIC SURFACE CONTOURS OF BEDROCK AQUIFER 10 BEDROCK AQUIFER GROUNDWATER CHEMISTRY NOV 1991- JAN 1992 11 WASTE ROCK PILE LOCATIONS 12 ARD-GENERATING SOURCE AREAS OF CONCERN 13 AVIRIS MINERAL ASSEMBLAGES 14 TOTAL CADMIUM LOADING TRENDS 15 TOTAL ZINC LOADING TRENDS 16 ALTERNATIVE 2G

1.0 SITE NAME, LOCATION AND DESCRIPTION

Site Name: Operable Unit 6 California Gulch Superfund Site Site Location: Leadville, Colorado National Superfund electronic database identification number: CERCLIS No.

COD980717938 Lead Agency: US Environmental Protection Agency Support Agency: Colorado Department of Public Health and Environment (CDPHE)

Cleanup funding: Superfund trust fund and matching funds provided by CDPHE.

Site Description:

The California Gulch Superfund Site (Site) is comprised of approximately 16.5 square miles of mountainous terrain inLake County, Colorado, approximately 100 miles southwest of Denver (Figure 1). The Site is divided into 12 OperableUnits (Figure 2). Operable Unit No. 6 (OU6) covers approximately 3.4 square miles in the northeastern quadrant of theSite and includes the Stray Horse Gulch watershed and the upper and lower portions of the Evans Gulch watershed.Historic mining activities resulted in placement of mine wastes on the land surface. Some of these mine wastes maycontain chemicals at concentrations posing an unacceptable human health risk as determined by EPA human healthrisk assessments. In addition, some of these mine wastes generate acid rock drainage (ARD) in concentrations toxicto aquatic organisms.

2.0 SITE HISTORY AND ENFORCEMENT ACTIVITIES

The Site is located in a highly mineralized area of the Colorado Rocky Mountains. Mining, mineral processing, andsmelting activities have produced gold, silver, lead and zinc for more than 140 years. Mining began in the Leadvillearea in 1859 when prospectors working the channels of Arkansas River tributaries discovered gold at the mouth ofCalifornia Gulch. Initial activities consisted only of small-scale placer mining until 1868, when the first gold ore veinswere discovered along California Gulch. By 1872, however, problems with water, transportation and labor made oreremoval so difficult that most miners had left the area. In 1874, silver-bearing lead carbonate was discovered, andmining in the Leadville district boomed.

Extensive replacement deposits of lead, silver and gold ores associated with fissure veins were discovered andmined. Zinc and manganese, which were of little value in the early days, were later mined extensively. As surfaceveins diminished, miners tunneled deeper into the mountains. Underground mines were developed east andsoutheast of Leadville. As mines were developed, waste rock was excavated along with the ore. The waste rock wasplaced near the mine entrance, and the ore was transported to the mill. At the mill, ores were crushed and separatedinto metallic concentrates and waste products by physical processes. The metallic concentrates were then shippedelsewhere or further processed at a smelter in the area. The waste products (mill tailing) were generally placed nearthe mill in a tailing pond. In the smelters, the high- grade ores were refined and concentrated into higher- gradeproducts. Waste products from the smelters included slag and dust, and off-gases. Forty-four known smelters werein the district (Woodward-Clyde, 1994a).

The EPA proposed adding the Site to the National Priorities List (NPL) on December 30, 1982. The site was formallylisted on September 8,1983. The following is a brief chronological summary of the major regulatory actions taken atthe Site.

• 1982 - The Site is proposed for the NPL.

• 1983 - The Site is formally added to the NPL.

• 1986 - EPA emergency response workers extended public water supply system lines to residencesusing private wells.

• 1987 - EPA began an investigation of mine wastes. Approximately 2,000 mine waste piles within theSite were screened to identify those larger than 100,000 cubic yards. Further screening was basedon proximity to populated areas, roadways, and surface water, and potential pile instability.Forty-five waste deposits were selected for field inspection and sampling based on access, size,waste type, stability, and proximity to residential areas and/or watercourses. Eleven of these siteswere mine waste piles, with the remainder being slag piles and tailing impoundments(Woodward-Clyde, 1994b).

• 1994 - The United States, the State of Colorado, and the Potentially Responsible Parties (PRPs)entered into a Consent Decree (CD). The CD divided the Site into 12 Operable Units (OUs) for thecleanup of geographically based areas within the site (USDC, 1994). OU6 is one of these OU's.

The EPA and ASARCO have implemented a number of Time Critical and Non-Time Critical Response Actions withinOU6. These Response Actions were conducted primarily to prevent exposure of human populations to contaminantsfrom mine wastes and to reduce leaching and migration of metals from the wastes into surface waters. This sectionbriefly summarizes the Response Actions implemented to date in chronological order (See Figure 3). Detaileddescriptions of the individual Actions are available within the documents referenced.

1. In 1990, ASARCO performed improvements along 5th Street and Starr Ditch between East 5th Street and theHarrison Street slag pile. The improvements involved converting existing open ditches to culverts along

both sides of East 5th Street, including the construction of the 5th Street headwall. Starr Ditch was alsofenced to restrict public access from just north of 5th Street to Monroe Street, just east of the HarrisonStreet slag pile (USEPA, 1995a).

2. During the summer of 1994, the BOR, on behalf of EPA, implemented sediment control measures on Hamm'sTailing Impoundment as a Time Critical Response Action (USEPA, 1995a).

3. In 1995 and 1996 EPA implemented a Time Critical Response Action at Hamm's Tailing Impoundment. TheAction involved removal of sediment from sedimentation ponds, enlargement of selected sedimentationponds, rehabilitation of straw dams and ponds, removal of sediment from culverts and associated drainagestructures, construction of an upgradient run- on control ditch, and reestablishment of Starr Ditch at thebase of Harrison Street slag pile (USEPA, 1995a). The removed sediment was transported to the Hamm'sTailing Impoundment for disposal (USEPA, 1996a).

4. Also in 1996, EPA conducted a Time Critical Response Action for the Hamm's Tailing Impoundment and thePenrose Mine Waste Pile. The majority of mine waste in the Penrose Pile was transported to the Hamm'sTailing Impoundment. The remaining waste at the Penrose Pile was graded to a 3:1 configuration, coveredwith a soil cap, and revegetated. The Hamm's Impoundment was then consolidated, compacted and gradedto a stable 3:1 configuration, covered with a soil cap, and revegetated (USEPA, 1996b).

5. In 1997, EPA implemented Phase I of a five- phase OU6 Response Action. The purpose of the Non-TimeCritical Response Action was to mitigate the majority of the source areas impacting water quality in theStray Horse Gulch drainage (USEPA, 1997). The Phase I Action included (COM, 2000a):

• Capping of three, double- compartment mine shafts. • Construction of five crib walls. • Consolidation and capping of three waste rock piles.

• Wolftone • Maid of Erin • Mahala

Work on the waste rock piles consisted of excavating, transporting, placing, reshaping, and compactingcontaminated mine waste rock from adjacent areas including approximately 5,900 cubic yards (CY) of acid generatingwaste rock from Stray Horse Gulch Road. A polyvinyl chloride (PVC) geomembrane in combination with a Mirafigeofabric was placed on top of each consolidated pile. A minimum of 8 ft of dolomite waste rock was then placed andcompacted above the Mirafi geofabric at Wolftone and Maid of Erin at a 1.5:1 side slope. White porphyry mixed withdolomite waste rock was used as a veneer material to provide a more aesthetic appearance on the Wolftone Pile. AtMahala, the rock cap was constructed as described above with the exception of the top. The top of the pile wascapped with 1-2 ft of white porphyry (Pacific Western, 2001).

The Ponsardine Pile and the source areas addressed in the 1998 Phase II Response Action (Ram, Mikado, HighlandMary, Adelaid and Pyrenees waste piles) were initially scheduled for Response Action during the Phase I work. ThePonsardine Pile, however, was eliminated due to state and local concerns about potential adverse impacts to historicand cultural resources.

6. In 1998, EPA conducted Phase II of a Non-Time Critical Response Action to mitigate additional sourceareas impacting water quality in the Stray Horse Gulch drainage (see Figure 4). The Phase II work involvedsurface water management rather than consolidation and capping to address state and local concerns aboutpotential impacts to historic and cultural resources. The following major components were completedduring Phase II ( CDM, 2000b):

• Capping of one double-compartment mineshaft. • Construction of water run-on diversions at Highland Mary, Mikados, RAM, Greenback, Pyrenees

and Adelaide-Ward. • Construction of water runoff channels at Highland Mary, Mikados, Pyrenees and Adelaide-Ward. • Construction of ARD retention basins at Highland Mary, Mikados, Pyrenees and Adelaide-Ward. • Construction of detention basins in lower Stray Horse Gulch at Adelaide Park and the Emmet

waste rock pile. • Construction of an interim water runoff collection channel and sediment basin at Fortune/

Resurrection No.1. • Rehabilitation of Stray Horse Gulch and Starr Ditch. • Sediment removal at the 5th Street Headwall.

During Phase II, the Ponsardine Pile was considered for run-on/runoff control, but was ultimately not included basedon the small size of the pile and the insignificant ARD it appeared to contribute based on available information. Inaddition, according to local observations runoff was believed to infiltrate before reaching the east side of Leadville.Subsequent opportunistic data collected during the 1998 construction season showed that the Ponsardine pile canbe a source of ARD and associated metals particularly during storm events (EPA, 1999b).

7. Phase III was implemented in 1999 as a continuation of the Phase II work. The following briefly describesthe major work conducted (CDM, 2000b):

• Construction of water runoff collection channels at RAM and Greenback. • Construction of water runoff retention basins at RAM and Greenback. • Completion of water runoff collection channel and sediment basin at Fortune/ Resurrection No. 1.

8. In 2000, EPA conducted Phase IVof the OU6 Response Action. This phase of the Non-Time CriticalResponse Action involved removal and disposal of sediments along Starr Ditch, rehabilitation andrealignment of Starr Ditch, revegetation of disturbed areas, and slope stabilization at the RAM andGreenback areas (CDM, 2000c).

During the spring runoff in 2000, the water collected in the retention basins at RAM, Greenback and Pyrenees nearlyovertopped due to under design of the retention basins. In order to prevent overtopping, the water was collected atthe Greenback Pond and siphoned to Stray Horse Gulch.

9. In the fall of 2000, as part of a Non-Time Critical Response Action, EPA constructed a discharge to a lateralconnecting the Marion and Emmet Mine Shafts to prevent the release of water into Stray Horse Gulch fromthe Greenback Retention Pond during future spring runoff events. The current capacity of several retentionponds does not accommodate a typical snowmelt event. The Marion Shaft is tributary to the Leadville MineDrainage Tunnel (LMDT) via the Robert Emmet Shaft. The LMDT then carries the collected runoff to theBOR's treatment plant at the portal of the LMDT.

The connection between the Marion Shaft and the LMDT has been under analysis by EPA. A number of tracerstudies were performed to determine the effectiveness of the LMDT as a conveyance structure for the GreenbackPond discharge. These studies confirm a hydraulic connection between the Marion Shaft and the LMDT portal.

This conclusion is based on information provided in Ground Water Hydrology Report on conditions near the LMDT(EPA, 2002b):

• Approximately 63% of the tracer injected in the Marion Shaft was recovered at the LMDT portaland additional dye was still being recovered at the portal.

• A portion of the tracer remains in the Emmet Shaft.

• Tracer has not been detected at any other point where groundwater discharges to surface water.

10. During the 2001 construction season, EPA implemented Phase V of a Non-Time Critical Response Actionfor surface water management and sediment control in the Ibex/Irene area (USEPA, 2001). This phaseinvolved constructing water run on diversion ditches on the south side of Lake County Road 1A followingthe path of the diverted Lincoln Gulch as well as additional ditches that direct surface water towards theEclipse Mine and existing drainages, eventually leading to South Evans Gulch. Surface water runoff fromARD-generating sources is redirected to a detention basin, which ultimately discharges to Lincoln Gulch. Asecond detention basin was constructed in the Old Lincoln Gulch channel near Lake County Road 3B tocollect sediment from erosion in Lincoln Gulch (CDM, 200 la).

11. During the 2002 construction season, EPA completed the rehabilitation of Stan-Ditch as a Non-Time CriticalResponse Action. The project included the installation of 550 feet of culvert in the vicinity of MonroeStreet.

3.0 COMMUNITY PARTICIPATION

The Focused Feasibility Study (FFS), Proposed Plan and numerous other documents for OU6 were made available tothe public over the course of investigative and remedial work conducted in the Site since 1983. The FFS andProposed Plan were made available in September 2002 and April 2003, respectively. They can be found in theAdministrative Record file and the information repository maintained at the EPA Docket Room in Region 8 and at theLake County Public Library. The notice of the availability of these two documents was published in the Lake CountyHerald on April 3, 2003. A public comment period was held from April 3, 2003 to May 3, 2003. In addition, a publicmeeting was held on May 1, 2003 to present the Proposed Plan to a broader community audience than those that hadalready been involved at the Site. At this meeting, representatives from EPA, Bureau of Reclamation (BOR), andCDPHE answered questions about problems at the OU6 and the remedial alternatives. EPA's response to thecomments received during this period is included in the Responsiveness Summary, which is part of this Record ofDecision.

As part of past Response Actions, EPA conducted numerous public meetings and public comment periods to allowthe public to comment on the Engineering Evaluations/Cost Analyses. EPA prepared responsiveness summariesrelated to various phases of Non-Time Critical Removal Actions.

4.0 SCOPE AND ROLE OF OPERABLE UNIT OR RESPONSE ACTION

EPA has organized work at the Site into 12 operable units. The operable units are listed below and groupedaccording to the status of remedial action:

Implemented Response Actions:

• OU1 - Yak Tunnel/Water Treatment Plant • OU2 - Malta Gulch Fluvial Tailing/Leadville Corporation Mill/Malta Gulch Tailing Impoundment • OU3 - D& RGW Slag Piles/ Railroad Easement/ Railroad Yard and Stockpiled Fine Slag • OU4 - Upper California Gulch • OU7 - Apache Tailing Impoundments • OU8 - Lower California Gulch • OU9 - Residential Populated Areas • OU10- Oregon Gulch

Activities Proposed Under this ROD:

• OU6 - Starr Ditch/Penrose Dump/Stray Horse Gulch/Evans Gulch

Future Response Plans:

• OU5 - ASARCO Smelter/ Slag/Mill Sites and AV/CZL Smelters• OU11 - Arkansas River Valley Floodplain • OU12 - Site- Wide Water Quality

Pursuant to the August 26, 1994 Consent Decree (CD) at this Site (USDC, 1994), it was agreed that the decision onremediation of Site-wide Surface Water and Groundwater (OU12) would be made only after remedies for sourceremediation were selected and implemented at each OU. Therefore, prior and proposed Response Actions in OU1through OU11 are intended to address mine wastes and features that are considered source materials. Remediesexcluded Site- wide surface and groundwater as specific targets for remediation in these OU's.

Source remedies were designed to minimize human exposure to mine wastes where the human health risk was outsidethe acceptable risk range and/ or minimize the discharge of ARD to surface or groundwater within a given operableunit. The intent of this approach to remediation of the Site is to lower human health risks resulting from directcontact with mine wastes to below a level of concern through remedial action within individual operable units. Inaddition, the overall improvements in Site- wide surface and groundwater (OU12) achieved through sourceremediation work in individual operable units will be assessed against the remedial action objectives for OU12.Additional work within one or more operable units may be required if remedial action objectives for OU12 are not metafter source remediation is completed in OU1 through OU11.

Therefore, through the phases of work that utilize a water management approach, the Selected Remedy for OU6minimizes the discharge of ARD to surface and groundwater. It also addresses human health risks outside of theacceptable risk range resulting from direct contact with mine wastes through removal or capping of mine waste, orthrough the ICO District.

The incremental improvement to Site- wide surface and groundwater quality (OU12) resulting from theimplementation of the Selected Remedy in OU6 will be assessed against remedial action objectives for OU12. EPAand CDPHE will establish specific surface and groundwater requirements at a later date under a ROD for OU-12. Iffurther improvement in water quality is required, consideration will be given to additional source controls or Site-wide water treatment.

5.0 SITE CHARACTERISTICS

5.1 PHYSICAL SETTING

5.1.1 Site Physiology

Operable Unit 6 lies in the Southern Rocky Mountain Physiographic Province of the United States, which ischaracterized by fault- block mountain ranges separated by intermontane valleys. Leadville is located on the eastside of the Arkansas River Valley at the base of Mount Evans near the confluence of Evans Gulch with the ArkansasRiver (see Figure 2). Evans Gulch is a large glacial valley that extends down the west slope of Mount Evans fromelevations of 13,200 ft above mean sea level (MSL) to the Arkansas Valley at approximately 9,900 ft above MSL.Evans Gulch is bordered on the north by Prospect Mountain and on the south by Iron Hill, Breece Hill, and BallMountain. Stray Horse Gulch is a small ephemeral stream that lies in the south portion of the Evans Gulch Valley,separated by Yankee Hill and a lateral moraine from the former Evans glacier.

5.1.2 Regional Geology

The bedrock formations underlying OU6 are a series of sedimentary strata that range in age from Cambrian toPennsylvanian and consist of quartzite, limestone, dolomite and shale. These Paleozoic sedimentary formations wereintruded during the late Cretaceous or early Tertiary periods in several episodes by porphory in "blanket" sills anddikes. These porphyry intrusions created the major portion of the mineralized zones and ore deposits (BOR, 1997).

The entire sequence of intruded sedimentary formations and pre-Cambrian granitic bedrock was uplifted and faultedinto a series of discrete bedrock blocks by north- south trending normal faults that step downward in elevation fromMosquito Pass on the east to the Arkansas Valley on the west. This series of faults largely controlled thedistribution and depth of the ore bodies, as well as groundwater which entered the mines in large quantities prior tothe construction of the drainage tunnels. Prior to the construction of the Yak Tunnel and the LMDT (see section onSurface Hydrology), pumping was required to dewater the lower ore body levels throughout the mining district(BOR, 1997).

Since the start of placer mining in 1859, the sedimentary bedrock units and intrusive ore deposits were mined, andwastes were deposited on the surface. These waste materials become subject to weathering which oxidize, breakdown, and release remaining contaminant metals into surface and ground water. Wastes containing significantamounts of metal sulfides generate acidic drainage further mobilizing soluble metals.

Throughout much of OU6, the bedrock is overlain by unconsolidated glacial deposits associated with the EvansGulch Glacier.

5.1.3 Climate

The topographic features of Lake County strongly influence the climatic variations in the Leadville area. Theelevation of the City of Leadville is approximately 10,000 ft above MSL. Normal temperature extremes range from - 30/ F to 86 / F, with an average minimum temperature of 21.9 / F. Average annual precipitation is 18 inches with thewettest months being July and August and the driest months being December and January. Summer precipitation isusually associated with convective showers. The annual peak snowmelt usually occurs in June. The average frost-free season is 79 days. The wind is predominantly from the northwest and ranges from calm to 30 miles per hour(Golder, 1996a).

The National Weather Service operates a meteorological station at the Leadville airport two miles southwest ofLeadville. Additional weather observations were measured at the Yak Tunnel meteorological station near the YakTunnel Water Treatment Plant. The Final Air Monitoring Report (Woodward-Clyde, 1992a) provides an evaluation oflocal meteorological data.

5.2 SURFACE WATER AND RELATED MEDIA

5.2.1 Surface Water Hydrology

Operable Unit 6 is made up of two main drainages that contribute surface water to the Arkansas River: Stray HorseGulch and Evans Gulch (see Figure 5). Stray Horse Gulch has one main tributary, Little Stray Horse Gulch, and isrouted through the eastern portion of Leadville to its confluence with California Gulch via Starr Ditch. CaliforniaGulch is a tributary to the Arkansas River. Evans Gulch contains two sub-drainage basins, Lincoln Gulch and SouthEvans Gulch. The majority of Evans Gulch lies outside of OU6 (see Figure 5).

The BOR performed a three- phase water and sediment sampling and hydrologic measurement program during 1995and 1996. Phase 1 consisted of water and sediment sampling during the 1995 spring runoff season. Phase 2 involvedsampling and analysis of mine wastes conducted in 1996. Phase 3 consisted of water and sediment sampling duringthe 1996 spring runoff.

Stray Horse Gulch is an intermittent stream that flows only during spring runoff and intense and/ or extendedprecipitation events. The drainage is approximately 12,300 feet long with an elevation loss of 675 feet. The BORcollected flow rate data during the 1995 (Phase 1) and 1996 (Phase 3) spring runoff periods. Four stations weremonitored within the Stray Horse Gulch Drainage during Phase 1: SHG-07, SHG-08, SHG-09 and SHG-10 (see Figure5). An additional station, SHG-07A, was added during the Phase 3 investigation. The flow measurements collected in1995 and 1996 show that significant surface water loss to the subsurface occurs in Stray Horse Gulch. The majorityof the loss occurs between SHG07 and SHG09. There was no net effective gain to stream flow in this reach of thedrainage (BOR, 1997). However, over the years of monitoring, Stray Horse Gulch has been both a losing and gainingstream, therefore reversals are both possible and occurring. Flows in Stray Horse Gulch ranged from 0 to 3.76 cubicfeet per second (cfs) during the 1995 runoff event. Using United States Geological Survey (USGS) peak flow datafrom Station 07081200 (Arkansas River near Leadville, CO), a flood frequency analysis was performed using the HEC-FFA ( Flood Frequency Analysis) program created by the US Army Corps of Engineers Hydrologic EngineeringCenter. Based on data from years 1968 - 2000,1995 was determined to be between a 10-and 20-year runoff event.

Evans Gulch is the longest continuous drainage within OU6 and serves as the municipal water supply for the city ofLeadville via the Parkville Water District. During Phase 1, BOR monitored eleven sampling stations within the EvansGulch drainage: EG-01, EG-02, EG-03, WE-01, WE-02, SEG-01, SEG-02, SEG-03, SEG-04, SEG-05, and LG-01 as shownon Figure 5. The Evans Gulch Stations exhibited the highest runoff stream flow volumes in OU6, up to 65.5 cfs (BOR,1996a). The peak flow at the South Evans Gulch stations was 26.6 cfs. Lincoln Gulch exhibited relatively low flows foronly a short period of time during the sampling period and carries surface flows only during spring runoff orsignificant rain events. The Phase 3 sampling program showed similar characteristics to the Phase 1 results (BOR,1997). Generally, surface flows from Evans Gulch do not reach the confluence with the Arkansas River from latesummer until the start of spring runoff and are not considered a significant source of metals to the Arkansas River.

A Wetlands Study was performed by Woodward- Clyde (1992b) to delineate the existing wetlands within the Site.The wetlands within OU6 are shown on Figure 5, and consist primarily of the upper reach of Stray Horse Gulch atAdelaide Park and most of the floodplain of the Evans Gulch drainage.

5.2.2 Surface Water Chemistry

During the Phase 1 and Phase 3 surface water sampling events, Stray Horse Gulch exhibited acidic, sulfate- rich waterindicative of ARD. The exception was at the upstream station SHG- 07, directly downgradient of the Adelaidewetlands, where the water had a near- neutral pH (see Figure 6). The pH decreased at each downstream station whilezinc and cadmium loadings increased (BOR, 1996a).

Figure 6 provides the 1995 zinc and cadmium loading values for OU6 during spring runoff as calculated by the BORin their Phase 1 report (BOR, 1996a). Zinc and cadmium are shown, as they are generally good indicators of waterquality and are contaminants of concern (Weston, 1995a). The BOR calculated flow- weighted metal loading usingthe following equation: [(flow,cfs) x (28.3168 L/cf) x (6.048xl05 s/wk) x.(metal cone, µg/L)] /(l. OxlO9 µg/kg)

The reported value was then converted to pounds per day for use in this document.

5.2.3 Fluvial Tailing

The Mine Waste Piles and Tailing Disposal Area Remedial Investigations (Woodward-Clyde, 1994a and b) identifiedthe fluvial tailing within the Site and evaluated whether surface and/ or groundwater had been affected. Fluvialtailings were categorized as suspected source areas and are shown on Figure 7. Note, however, that no fluvial tailingexist within OU6.

5.2.4 Stream Sediments

Stream sediments are naturally occurring throughout drainages of OU6 and have historically been disturbed due toplacer mining. Table 1 presents a summary of the median concentration of various analytes at multiple stationswithin each watershed for the 1995 spring runoff.

Median stream bed sediment contaminant metal concentrations were highest in Stray Horse Gulch, followed byLincoln Gulch, Evans Gulch, and then finally by South Evans Gulch where median concentrations were the lowest.Contaminant metal concentrations typically increased with decreasing station elevation. This is consistent with thegreater contributing watershed at lower surface water monitoring stations ( BOR, 1996a).

5.3 GROUNDWATER

5.3.1 Hydrogeology

No hydrogeologic study existed at the time the OU6 Focused Feasibility Study was prepared. Therefore, relevantinformation from the California Gulch Site Hydrogeologic RI (Golder, 1996a) is used in this document to describe thehydrogeology of the area. This information is supplemented with the results of a recent study of the hydrogeologyin the vicinity of the LMDT (EPA, 2002b).

The Site contains two hydrogeologic units: one consists of unconsolidated sediments and the other is a series ofigneous and sedimentary bedrock formations. The unconsolidated sediment unit includes a saturated section(alluvial aquifer) and several perched groundwater zones. The bedrock aquifer is the saturated portion of the bedrockunit. Groundwater recharge to the hydrologic units is from infiltration of precipitation, including snowmelt andsurface water (Golder, 1996a).

The alluvial aquifer is largely contiguous and primarily under unconfined conditions although perched groundwatercan occur locally. Depth to groundwater varies from less than one foot near California Gulch to approximately 250feet at higher elevations, and saturated thickness ranges from 0 to over 1,000 feet in the alluvial aquifer above thebedrock contact. The average groundwater flow direction of the alluvial aquifer is east to west (S88 / W) with a 0.03foot/foot (ft/ ft) hydraulic gradient. Lithologic variability, variable recharge rates, and interactions with surface waterand/ or groundwater cause local variation in the groundwater flow direction and hydraulic gradient (Golder, 1996a).Alluvial aquifer water table contours from 1992 are provided in Figure 8.

The bedrock aquifer refers to the areas of granitic, metamorphic and sedimentary bedrock through whichgroundwater flows. This groundwater flow is primarily controlled by fracture zones associated with faults, solutionfeatures associated with calcareous sedimentary rocks, and mine workings. The bedrock aquifer is primarily underunconfined conditions in the eastern third of the Site and confined or partially confined conditions in the western

two-thirds of the Site. Depth to groundwater in the bedrock aquifer ranges from approximately 28 feet to 796 feetbelow ground surface (Golder, 1996a). Groundwater levels and flow directions indicate that the bedrock aquifer isflowing into- both the Yak Tunnel and the LMDT. The hydraulic gradient near the Yak Tunnel ranges from 0.54 ft/ftto 0.13 ft/ ft (Golder, 1996a). Near the LMDT, the hydraulic gradient is estimated to be 0.04 ft/ ft. Outside of theinfluence of these drainage tunnels, the groundwater flow direction is west- northwest with a hydraulic gradient of0.02 ft/ ft (Golder, 1996a). Potentiometric surface contours for the bedrock aquifer, based on water levelmeasurements taken in 1992, are provided in Figure 9.

Limited information concerning the alluvial/ bedrock aquifer interaction is available from the Hydrogeologic RI.Groundwater level data was collected from alluvial aquifer piezometer/ bedrock monitoring well pairs PZ-4/BMW-1,PZ-6/BMW-2, PZ-10/BMW-3 (see Figure 9). At PZ-4/BMW-1 AND PZ-6/BMW-2, an upward gradient of 0.26 and0.07 ft/ft was observed, respectively. The well pair PZ-10/BMW-3 exhibited a downward gradient of 0.39 ft/ftbetween the aquifers (Golder, 1996a). Subsurface drainage of bedrock aquifer groundwater by the LMDT may causethe relatively low hydraulic head exhibited in the third well pair. There is no direct evidence of flow between bedrockand alluvial aquifer groundwater at this location.

The Yak Tunnel was developed to reduce mine-flooding problems experienced in the Iron Hill region during hard-rock mining for lead ores in the late 1800' s. Yak Tunnel construction began in 1895 at a bottom elevation of 10,330 ft.The tunnel proved so effective at draining the Iron Hill area that it was extended to connect with the Ibex andResurrection mines (CDM, 1997). Construction began on a second tunnel, the LMDT, in 1943 to drain mine workingsbelow 10,063 ft including the lower parts of the Iron Hill basin and the Downtown, Fryer Hill, and Carbonate Hillbasins. The LMDT was completed in 1952. Recent studies of the LMDT and vicinity (EPA, 2002b) suggest thatcollapses in the LMDT have impounded substantial amounts of water in the LMDT and hydraulically connectedunderground workings (mine pool). The specific locations of the collapses are not known, and warrant furtherinvestigation. These collapses have resulted in changes in the groundwater levels or conditions in the vicinity of theLMDT, OU6, California Gulch, and possibly other locations throughout the Leadville area. Collapses are suspectednear the LMDT portal and near the Robert Emmet Shaft, approximately 10,000 feet from the tunnel portal (EPA,2002b).

The increased hydraulic head associated with flooded mine workings (mine pool) in the vicinity of the Robert EmmetShaft has caused recharge from the bedrock aquifer to the alluvial aquifer via local fault systems (Pendry Fault)resulting in rising groundwater levels in the alluvium of Lower California Gulch.

Dye tracer testing in the Marion and Robert Emmet Shafts has not substantiated concerns over migration of minepool water away from the LMDT. Dye tracers introduced at the Marion and Robert Emmet Shafts have beenrecovered only at the LMDT portal. Other than this detection, dye has not been detected at any other surface wateror groundwater monitoring location (EPA, 2002b).

5.3.2 Groundwater Chemistry

Only limited information is available regarding groundwater quality in OU6. Figure 10 illustrates the groundwater pHvalues as well as dissolved zinc and cadmium concentrations for November 1991 through January 1992 as reported inthe Hydrogeologic RI (Golder, 1996a). Background water quality was also investigated in the Hydrogeologic RI. Theinvestigation showed that both the alluvial and bedrock aquifers are of the calcium- magnesium- carbonate/ sulfatetype with a pH range from 7.3 to 8.0. Dissolved metals were rarely above the detection limits.

The wells within OU6 (Figure 10) have been monitored periodically by CDPHE for chemicals other than zinc andcadmium. A comprehensive summary of these groundwater quality data for the Site can be found in RMC, 200la.Concentrations of cadmium, iron, magnesium, zinc and sulfate are elevated. Low pH values are also common.

Recent unpublished data (Tetra Tech- RMC, 2003) suggests that shallow alluvial groundwater is being impacted byinfiltration of ARD stored in some of the detention ponds constructed during prior Response Actions (Section 2.0).Elevated levels of arsenic and cadmium have been observed in monitoring wells proximal to the detention ponds.

Access to the flooded mine workings under OU6 is very limited and water quality sampling has been limited to asingle sample collected from the Robert Emmet Shaft located just north of the Marion Mineshaft illustrated on Figure3. The sample contained 2.66 µg/1 of cadmium, 18.5 µg/1 of copper, 22.5 µg/1 of lead, 185,150 µg/1 of manganese and164,850 µg/1 of zinc. The sample was collected in 1999 and represents conditions prior to the introduction of ARDinto the Marion mineshaft.

5.4 MINE WASTE

5.4.1 Mine Waste Types

Two types of mine wastes are present within OU6: waste rock and tailing. Mine waste rock piles are usually locatednear adit and shaft entrances and are comprised of rock excavated during mine development, gangue (un-mineralizedrock), and low-grade ores (Water, Waste and Land, 1990). Gangue consists of material such as chert, limestone,quartzite, and minor quantities of rock with metal sulfide mineralization including pyrite, chalcopyrite, sphalerite, andgalena. Surface exposure and weathering of these minerals may lead to pyrite oxidation resulting in the production ofARD with elevated metals concentrations.

Mill tailing is the waste generated during the processing of ore. When ores were taken to the mill they were firstcrushed and separated into metallic concentrates and waste products. These waste products were generally placednear the mill in a tailing pond. Tailing occurrence in OU6 is limited to the Hamms Tailing. Mine wastes in the OU6area are illustrated on Figure 11.

5.4.2 Mine Waste Locations

As part of the FFS, a number of mine waste areas were identified as candidates for possible remedial action (seeFigure 12 and Table-2). These areas were identified from available surface water quality data, Airborne Visible andInfra- Red Imaging Spectroscopy (AVIRIS) data (Figure 13), surface soil chemical data, and field observations ofmineralogy. In addition, any mine waste area that was subjected to a prior Response Action resulting in mine wasteexposed at the surface (i. e. that were not capped) was considered to be a candidate for remedial action withoutfurther analyses.

Surface water quality data was obtained from investigations conducted by the BOR as well as 2000 and 2001synoptic sampling events by the Colorado Mountain College (CMC) and Rocky Mountain Consultants (RMC).Synoptic sampling is the sampling of a slug of water as it moves through a hydrologic system (RMC, 2001b). Thesurface water data was first analyzed to identify drainages that contribute significant loading downstream. When apotential source drainage was identified, the contaminant loading trends between individual surface watermonitoring stations were evaluated to isolate sub-drainages of concern.

The sub-drainages were then analyzed with respect to surface mineralogy obtained from remote sensing analysis ofOU6 to map the distribution of surface minerals (AVIRIS). The AVIRIS instrument collects data from a NASA ER-2aircraft at an altitude of 65,000 ft with resolution of approximately 17 meters (BOR, 1997). The data is presented byshowing the predominant mineral in each 17 x 17 meter pixel as illustrated in Figure 13.

These data were used extensively to help prioritize ARD-generating source areas for Response Actions implementedin OU6. The key indicators of ARD, as determined by the BOR investigation are pyrite and its secondary minerals aslisted below.

Mineral Chemical Formula ARD-Generation • Pyrite FeS2 High • Copiapite FeFe4(S04) 60(OH)- 20H20 • Jarosite (Na,K) Fe3(SO4) 2(OH) 6 • Goethite alpha-FeO(OH) • Hematite alpha-Fe2O3 Low

The last set of screening tools for identification of ARD-generating mine waste areas is surface soil chemical dataand field observations of mineralogy. This information was primarily obtained from the Mine Waste Piles RI(Woodward- Clyde, 1994a). Based on field reconnaissance, the Mine Waste Piles RI categorized the inventoried minewaste piles into five groups:

ARD-Generation • Group 1: No observed minerals of concern. Low • Group 3: Minor amounts of sulfide minerals such as pyrite, sphalerite and chalcopyrite, but no galena. • Group 2: Predominantly manganese and carbonate minerals

with occasional traces of pyrite. • Group 4: Abundant amounts of sulfide minerals

(except galena) and occasional manganese and carbonate minerals. • Group 5: Similar to Group 4 piles, but contain visible galena. High

The results of this screening identified mine waste areas in Stray Horse, Little Stray Horse and Upper Lincoln Gulch(Ibex/Irene Area). The general areas containing ARD-generating mine wastes-are illustrated on Figure 12.

The individual mine waste piles identified as candidates for remedial action are listed below. Those that have alreadybeen addressed by prior Response Actions are boldfaced.

• Greenback • RAM • Old Mikado • New Mikado • Highland Mary • Adelaide/Ward • Pyrenees • Fortune/Resurrection • Ponsardine • Emmet • Evans F& G

After further analysis, several portions of the Emmet mine waste area and the Evans F & G waste piles were droppedfrom consideration for additional remedial action as discussed below.

The Emmet mine area consists of five piles as identified in the Final Mine Waste Piles RI (Woodward-Clyde, 1994a).Two of these piles were directly associated with the Emmet Mine, while the other three are located in the vicinity.The 2001 synoptic sampling data collected by the Colorado Mountain College demonstrates a significant increase incontaminant concentrations between stations SHG-08 and SHG-09, as shown in Table 3, which lie just up- anddown-stream of the Emmet Pile (see Figure 5). While this increase may be attributable in part to the loadingcontribution from the Emmet Pile, it should be noted that there is uncertainty as to what portion of the loading isfrom the Emmet Pile and what portion may be from remediated source areas on the south side of Stray Horse Gulch orin-channel seeps/springs. These areas (including Maid of Erin, Wolftone and Mahala) have been consolidated andcapped. However, the soils underlying the waste piles may continue to discharge ARD as the remedies stabilize.

Based on a Group 5 categorization assigned by the Mine Waste Piles RI as well as visual observations, two of thefive piles in the Emmet Mine area were identified as ARD-generating (see Figure 12).

The final potential source area of concern identified during this process is referred to as Evans F& G. This area wasdelineated as two areas (Evans F & Evans G) in the Draft BOR Value Analysis Report (BOR, 1996b). The labeling

scheme for these areas was retained from the report for continuity. The Evans F& G area was initially identified dueto significant contaminant concentration increases between stations SEG-03, SEG-04 and SEG-05 during the 2000spring run-off sampling event. In addition, AVIRIS data showed ARD-generating mineralogies present in Evansareas F&G.

However, 1995 BOR water chemistry data showed that South Evans Gulch had the lowest median flow-weightedmetal loading in OU6 (BOR, 1996a). Subsequent work identified Lincoln Gulch as the largest source for loading to theEvans Gulch watershed. This was the basis for the Ibex/ Irene Response Action documented in an ActionMemorandum (EPA, 2001). Based on this information, and the fact that Evans Gulch is not considered to be a majorloading source to the Arkansas River, the Evans F& G area was not retained as a candidate for remedial action.However, long- term water quality monitoring will be performed as part of the remedy to observe any changes thatmay affect the Parkville Water Supply. As data are collected, and during the five-year remedy review, changes inwater quality will be assessed to determine if further work is necessary

Table 2 provides a summary of the screening process used to define each of the candidate piles identified in thisdocument.

5.4.3 Mine Waste Quantities

The volume of each mine waste area identified in Figure 12 is listed in Table 4. The volumes were obtained from theDraft Value Analysis Report (BOR, 1996b) except where otherwise noted.

Since the Emmet mine area was not identified during the Value Analysis, an estimated volume for the two Emmetpiles of concern was obtained using GIS topographic and pile location data obtained from the Leadville area GISdatabase provided by EPA Region 8. Initially, a Triangulated Irregular Network (TIN) model was created usingcurrent topographic information. A pre-mining TIN model was then produced by estimating contour locations beforemine waste piles began to influence topographic features. These two TIN models were subtracted from one anotherto obtain approximate pile volumes. This volume is given in Table 4.

Recent studies of the LMDT and vicinity (EPA, 2002b) suggest that collapses in the LMDT have impoundedsubstantial amounts of contaminated groundwater in the LMDT and hydraulically connected underground workings(mine pool). Although not supported by recent dye tracer studies (EPA, 2002b) the potential or this impoundedwater to migrate in the subsurface and/or to discharge to surface water remains. Therefore, the mine pool will beaddressed under the Selected Remedy. The volume of water impounded in the mine pool was estimated to range from539,000,000 gallons to 1,465,000,000 gallons (EPA, 2002b).

6.0 SUMMARY OF SITE RISKS

The Baseline Risk Assessment (BRA) estimates what risks site contaminants pose if no action is taken. It providesthe basis for taking action and identifies the contaminants and exposure pathways that need to be addressed by theremedial action. This section of the ROD summarizes the results of the BRA for the Site as they apply to OU6.

6.1 HUMAN HEALTH RISKS

Although a BRA was not prepared specifically for OU6, a number of BRAs were completed for the Site. These arelisted and briefly described below:

• Preliminary Human Health Baseline Risk Assessment for the California Gulch NPL Site (Weston,1991).

• Baseline Human Health Risk Assessment for the California Gulch Superfund Site. Part C:Screening Level Soil Concentrations for Workers and Recreational Site Visitors Exposed toLead and Arsenic (Weston, 1995b).

• Baseline Human Health Risk Assessment for the California Gulch Superfund Site. PartC: Evaluation of Worker Scenario.

• Baseline Human Health Risk Assessment for the California Gulch Superfund Site. PartC: Evaluation of Recreational Scenarios.

• Baseline Human Health Risk Assessment for the California Gulch Superfund Site. Part A - Risksto Residents from Lead (Weston, 1996a).

• Baseline Human Health Risk Assessment for the California Gulch Superfund Site. Part B - Risksto Residents from Contaminants other than Lead (Weston, 1996b).

The findings of the BRAs regarding contaminants of concern, completed and significant exposure pathways andnumerical cleanup goals are relevant to OU6.

6.1.1 Contaminants of Concern

Historic mining, milling and smelting operations typically contaminated the environment with a number of metals.This includes many of the metals which were the main objective of historic mining and refining activities (copper,lead, silver, zinc), as well as a variety of other metals that exist in the ore body (arsenic, antimony, barium, beryllium,cadmium, chromium, nickel, manganese, mercury, thallium). Essentially, all of these chemicals occur at elevatedconcentrations (compared to background) in on-Site media (including soil, mine wastes, surface water andgroundwater), and all of these metals are capable of causing adverse effects in humans if exposure is high enough(Weston 1996b).

Even though many metals occur at elevated concentrations in the environment around Leadville, experience at othersites, as well as the results of preliminary calculations at this site indicate that lead and arsenic are the "risk drivers".Thus, the Baseline Human Health Risk Assessment for the California Gulch Superfund Site. Part C: Evaluation ofWorker and Recreational Scenarios focuses on these two chemicals (Weston, 1995b).

6.1.2 Exposure Assessment

The non-residential areas of OU6 are currently zoned Industrial Mining. However, the current land use is primarilyrecreational. Risks associated with direct contact with mine waste and contaminated soils by the recreational adultand adult worker were evaluated in the BRA. .

Human exposure to contaminants in surface water through direct contact with water or sediment is considered to besufficiently minor across the Site that quantitative evaluation was not warranted (Weston, 1995b).

Two small portions of OU6 are either zoned residential or adjoin residential areas. These include the Penrose MineWaste Pile and Starr Ditch. The Penrose Mine Waste Pile is zoned residential and was capped with a soil coverduring a prior Response Action. Therefore, the human exposure pathway has been interrupted at this location. StarrDitch was remediated under a prior Response Action by removing contaminated sediments. In addition, an extensiveeducation and intervention program to manage lead exposure at the Site is included in the Lake County CommunityHealth Program (LCCHP) implemented under the Record of Decision for OU9. The LCCHP combines blood leadmonitoring, education, community awareness, and residence- specific Response Actions to reduce the risk of leadexposure to children in Leadville and surrounding residential areas. The program addresses lead from soil and dust,interior and exterior paint, leaded plumbing fixtures, and other potential sources beyond an individual residence. Anextensive education and intervention program to manage exposure at the site is an integral part of the program. Forthese reasons, it is believed that there are no known residential exposures to unacceptable levels of contamination.

6.1.3 Toxicity Assessment

EPA and CDPHE have identified many chemicals at the Site that pose unacceptable risk to human health and theenvironment. Of these chemicals of concern, the two chemicals described below are of primary concern at the Site.

Lead is present mostly in the waste rock, slag, and soil. Lead can accumulate in the body over time if exposure isfrequent or continuous. It can cause harm if present above certain levels in the human body. Lead can affect thedevelopment of the nervous system, including impaired learning ability and hearing, and the reproductive system.Children are especially vulnerable to lead contamination for the following reasons: Their bodies and brains are stilldeveloping and they absorb more lead than adults, and children often play outside where they are more likely to beexposed to lead in the soil. They are more likely to put dirty fingers and toys in their mouths.

Arsenic is also present in mine waste rock, slag, soil and groundwater at the Site. Arsenic does not easily accumulatein the body. Most arsenic that is absorbed into the body is efficiently passed in the urine. Harmful health effectsrelated to long- term exposure to too much arsenic include lung and skin cancer and digestive tract problems. Oralexposure to high doses of arsenic produces marked acute irritation of the gastrointestinal track, leading to nauseaand vomiting.

6.1.4 Risk Characterization

Weston (1995b) developed risk- based action levels for lead and arsenic rather than calculating risks for all areas ofthe Site. Action levels were developed for recreational and worker scenarios. The action levels represent risk-basedconcentrations protective of human health and may be used to identify soils (or mine waste) of potential concern torecreational visitors or workers.

For the recreational scenario, lead action levels ranged from as low as 5,000 mg/kg to 85,000 mg/kg, depending uponwhich input parameters were used (Weston, 1995b). A lead concentration of 16,000 mg/kg was selected forcomparison to soil concentrations for lead (Weston 1995b). For arsenic, action levels ranged from 1,400 to 3,200 mg/kg based on carcinogenic and systemic effects, respectively (Weston, 1995b). An arsenic concentration of 1,400 mg/kg was selected for comparison to soil arsenic concentrations, based on the potential carcinogenic health effects(Weston, 1995b). The recreational scenario considered all appropriate age groups.

For the worker scenario, plausible action levels for lead ranged from as low as 2,200 mg/kg to as high as 19,100mg/kg. The central tendency values include 6,100 and 7,700 mg/ kg (geometric and arithmetic mean, respectively;(Weston, 1995b). Lead concentrations in this range are likely to be protective with a reasonable degree of confidence(Weston, 1995b). For arsenic the plausible action levels range from 330 mg/kg to 1,300 mg/kg. The central tendencyvalues include 610 and 690 mg/ kg (geometric and arithmetic mean, respectively, Weston, 1995b).

For the residential scenario, the lead action level, as identified in the OU9 ROD, is 3,500 mg/kg USEPA, 1999c). Forarsenic, PRGs ranged from 120 mg/kg to 340 mg/kg (Weston, 1996b). Action levels for arsenic and lead are likelyexceeded in some locations within OU6 and will be addressed through implementation of the ICO District.

In order to identify areas where action levels might be exceeded, the action levels were compared to soilconcentration values presented in previous Remedial Investigations (RIs). Note that it is the average lead level and95% upper confidence limit of the mean arsenic level over an area that should be compared to the soil action level.Occasional measurements of concentrations above the action level do not necessarily constitute evidence that anarea is unsafe (Weston, 1995b).

Inspection of prior Site-wide RIs (containing data from OU6) shows that average lead levels are generally well belowthe action level of 16,000 parts per million (ppm) for areas where recreational scenarios are considered likely (Weston,1995b). For the worker scenario, the average lead levels are mostly below the central-tendency range of plausibleaction levels (6100-7700 ppm) for most areas zoned for commercial land use, with the possible exception of someareas in the historic mining area east of town (including portions of OU6) (Weston, 1995b). Therefore, the potentialexists for action levels to be exceeded under the worker scenario in portions of OU6.

In accordance with the 1994 Consent Decree (USDC, 1994), remedial action objectives (RAO's) for OU6 do notinclude achieving numerical water quality goals. Achievement of quantitative water quality goals will be addressedunder the RAO's for OU12.

6.2 ECOLOGICAL RISKS

Although an assessment of ecological risks was not prepared specifically for OU6, a number of ecological riskassessments (ERA's) were completed for the Site. These are listed below:

• Final Baseline Aquatic Ecological Risk Assessment for the California Gulch NPL Site (Weston,1995a).

• Ecological Risk Assessment for the Terrestrial Ecosystem, California Gulch NPL Site (Weston,1997).

6.2.1 Contaminant Identification

The Aquatic ERA (Weston, 1995a) identifies the impact of mine waste contamination on the aquatic ecosystem atthe Site. The media of concern were surface water and sediments. The contaminants evaluated included aluminum,arsenic, barium, cadmium, copper, iron, lead, manganese, nickel, selenium, and zinc. Further assessment has lead to arefinement of this initial list to the following chemicals of potential concern for surface water:

• Aluminum • Cadmium • Copper• Lead • Zinc

Media evaluated in the Terrestrial ERA (Weston, 1997) included soil, slag, waste rock, and tailing in upland areas,and fluvial tailing and sediments in riparian areas. Only data from the top two inches of these media were evaluated.Adverse impacts on the terrestrial ecosystem from exposure to contaminants in surface water were also evaluated.Contaminants evaluated included arsenic, antimony, barium, beryllium, cadmium, chromium, copper, lead, nickel,manganese, mercury, silver, thallium, and zinc.

6.2.2 Exposure Assessment

The Aquatic ERA evaluated ecological receptors typical of those present or historically present at the Site,consisting of aquatic plants, benthic macroinvertebrates, and fish (primarily trout species). The potential exposurepathways for aquatic receptors were ingestion of surface water, sediments, and dietary items, and direct contact withsurface water, sediments, and modeled concentrations of dissolved contaminants in sediment pore water.

Aquatic risks were assessed by sampling station rather than by OU. Sampling stations of concern in OU6 were in theStray Horse Gulch and Starr Ditch drainages. However, the physical limitations of these and other OU6 tributariespreclude the support of aquatic life. Therefore, risk evaluations were focused on California Gulch and the ArkansasRiver (CDM, 1997).

Receptors evaluated in the Terrestrial ERA were representative of those found at OU6; upland and riparianvegetation communities, birds, and herbivorous and predatory mammals. Contaminant intakes were estimated forthese receptors based on assumptions regarding exposure, such as food ingestion rates and body weight. Exposurepathways evaluated included direct exposure to contaminated media, ingestion of contaminated ponded water orsurface runoff, incidental ingestion of contaminated media, and indirect exposure through the food chain (Weston,1995b).

The Aquatic ERA used the 95% upper confidence limit (UCL95) as the exposure point concentration (EPC) forchronic exposure. If the UCL95 was greater than the maximum contaminant concentration, the maximum was used asthe chronic EPC. The maximum contaminant concentration was used to represent the acute exposure (Weston,1995a).

The Terrestrial ERA used the UCL95 as the EPC to evaluate risk by OU. If the maximum contaminant concentrationwas less than the UCL95, the maximum was used as the chronic EPC. Risks were also characterized by samplingstation. The maximum contaminant concentrations were used to calculate risks at individual sampling stations due tolimited data quantities per station.

6.2.3 Toxicity Assessment

The toxicity assessment in the Aquatic ERA discusses the toxicity of inorganic elements to aquatic species.However, the toxicity varies so widely that it is not practical to list specific concentrations that cause effects underconditions of acute or chronic exposure for all species. The Aquatic ERA reaches the general conclusion thatconcentrations below the Ambient Water Quality Criteria (AWQC) or state standards are unlikely to adversely affectpopulations of fish or macro invertebrates.

The toxicological, literature was reviewed in the Terrestrial ERA to obtain acceptable chemical concentrations in theenvironment for plants and soil fauna, as well as acceptable intakes for birds or mammals. The goal of the literaturereview was to document No Adverse Effect Levels (NOAELs) for each receptor and chemical. Because literaturevalues for any given wildlife species are often lacking, there was typically only one good study for birds, and one formammals, which was used to represent the avian and mammalian receptors, respectively. One study was selected torepresent the plants, and one study selected to represent soil fauna. Even this distillation of available literatureinformation is too large for inclusion in this ROD. The relevant toxicological information is provided in the TerrestrialERA (Tables 4- 4 and 4- 5, Weston 1977).

6.2.4 Risk Characterization

The Aquatic ERA used EPA AWQC as well as standards developed by the State of Colorado to evaluate the toxicityof contaminants in surface water to aquatic receptors. Sediment toxicity values were derived from the toxicologicalliterature. Sediment and surface water toxicity criteria were compared to contaminant EPCs to determine risk toaquatic receptors. The resulting value is termed a hazard quotient (HQ). An HQ less than one indicates there is littlepotential for adverse effects to occur. An HQ greater than one indicates a potential for risk but does not necessarily

mean that adverse effects will occur. The sum of the HQ's is the hazard index (HI).

HQs and HI's specific to OU6 were not presented in the ERA's. Therefore, this summary does not providequantitative risks associated with surface water in OU6. Results of the Aquatic RA indicate that mine waste posespotential risk to all aquatic species.

In accordance with the 1994 Consent Decree (USDC, 1994), remedial action objectives ( RAO's) for OU6 do notinclude achieving numerical water quality goals. Achievement of quantitative water quality goals will be consideredunder the RAO's for OU12.

To quantify terrestrial risks, exposure intakes were estimated for upland and wetland receptors and hazard quotients(HQs) were calculated by analyte for each receptor. Due to the large number of analytes, receptors and mediaevaluated, Hazard Indices (HIs) were estimated by summing the HQs for each exposure pathway for all analytes. TheOU6 His are given in Table 5. A review of this table reveals ecological risks above a level of concern for several birdsand mammal. Species with Hi's above 20 include Blue grouse, Mountain Bluebird and Least Chipmunk.

7.0 REMEDIAL ACTION OBJECTIVES

7.1 INTRODUCTION

The objectives of the Response Actions taken to date in OU6 and documented in this ROD are very specific andoriginate in a Consent Decree (CD; USDC, 1994). The United States, the State of Colorado, and the PotentiallyResponsible Parties entered into the CD in 1994. The objectives set forth in the CD were to:

1. Protect public health, welfare, and the environment from releases or threatened releases of wastematerial at or from the Site;

2. To divide the Site into areas of responsibility among the parties;

3. To improve the quality of Site- wide Surface and Ground Waters through Source Remediation;

4. To reimburse the Past and Future Response Costs of the Plaintiffs;

5. To resolve the liabilities of the Settling Defendants at the Site

6. To resolve the claims of the Settling Defendants against the United States and the State.

The CD defines Source Remediation (mentioned in Item 3, above) as:

" Response Actions designed to prevent or control the release or threatened release of waste materialfrom sources of contamination such as tailings impoundments, fluvial tailings, waste rock piles and soilsinto all pathways of migration, but shall not include any treatment of Site-wide Surface or GroundWaters."

Item 3, above, provides the most specific direction for remedial measures in OU6. The requirement to improve waterquality through source remediation presents an inherent limitation in the scope of the remedial alternatives andremedial objectives. Restricting remedial action to source remediation may preclude the achievement of any specificnumerical water quality objectives for surface or groundwater in OU6. This is due to:

• The presence of non- point source pollution. • The discharge of contaminated groundwater to surface water. • Natural background contaminant levels.

These limitations coupled with the specific exclusion of Site-wide surface or ground water treatment leads to theconclusion that remedial action objectives should not, at this time, include chemical-specific numerical water qualitystandards applied within OU6 boundaries. Rather, the objective is to reduce metal loading to the watershed to the extent practicable. Although this is a subjective standard, the effectiveness of existing and proposed remedialactions can be measured or estimated as a relative improvement in water quality from pre- remedial conditions.

Chemical-specific water quality standards will be formally addressed under OU12. The Parties to the 1994 ConsentDecree agreed the OU was established to determine final Site-wide surface and groundwater quality standards and aprocess for achieving those standards after source remediation is completed.

7.2 MEDIA OF CONCERN

Before setting the Remedial Action Objective (RAO's) for OU6, consideration was given to the specificenvironmental media that constitute a source for metals in the watershed and therefore should be targeted forremedial action.

The CD suggests materials that may be considered sources:

" Response Actions designed to prevent or control the release or threatened release of waste materialfrom sources of contamination such as tailings impoundments, fluvial tailings, waste rock piles and soilsinto all pathways of migration, but shall not include any treatment of Site-wide Surface or GroundWaters."

As discussed in Section 2.0 the only fluvial or impounded tailing in OU6 is the former Hamm's Tailing Impoundment.These mine wastes were consolidated and capped in 1996. No tailing remain exposed at the surface in OU6. Nativesoils are not considered to be a significant source of ARD.

Although not specifically identified in the CD's definition of Source Remediation, stream sediments were alsoconsidered as a medium of potential concern. Most of the prior Response Actions included the removal of streamsediments, construction of sediment basins and the re-routing of stream channels around ARD-generating minewastes. In areas of OU6 where stream sediments have not been physically removed (Evans Gulch), metal loading atthe OU6 boundary has been relatively low and is not believed to significantly contribute to human or ecological risk.Therefore, stream sediments are not considered a medium of concern and are not targeted for remedial actionthrough physical removal.

Mine waste rock remains the primary source for metal loading to surface and ground water in OU6.

7.3 MIGRATION PATHWAYS OF CONCERN

Before setting the RAO's for OU6, consideration was given to the potential contaminant pathways of concern. As astarting point, the migration pathways of concern listed in the Screening Feasibility Study (SFS) (EPA, 1993) wereconsidered including:

• Control wind erosion of waste rock materials from the source locations. • Control water erosion of waste rock materials from the source locations. • Control leaching and migration of metals from waste rock into surface water. • Control leaching of metals from waste rock into groundwater.

The first migration pathway (airborne transport) was determined to present a human health risk below a level ofconcern. The Baseline Human Health Risk Assessment - Part A: Risks to Residents from Lead (Weston, 1996a)concluded that " inhalation exposure to lead is minimal at this site, and the inhalation exposure was not consideredfurther in this assessment. " Based on this conclusion, airborne transport of contaminants was not consideredfurther.

The second, third and fourth migration pathways relate to the release of metals from source material ( mine wasterock, in the case of OU6) to surface and ground water. These migration pathways were considered relevant.

7.4 REMEDIAL ACTION OBJECTIVES

Based on the forgoing, the RAO's for OU6 include:

1. Control erosion of mine waste rock and deposition into local water courses. 2. Control leaching and migration of metals from mine waste rock into surface water. 3. Control leaching of metals from mine waste rock into groundwater. 4. Prevent direct unacceptable exposures to elevated concentrations of contaminants in the soil and waste

rock.

8.0 DESCRIPTION OF ALTERNATIVES

This section provides a brief explanation of the remedial alternatives developed for OU6. Remedial alternativesdescribed below were retained after preliminary screening and were evaluated using the nine criteria required by theNCP and six additional criteria required by the Work Area Management Plan (WAMP) as part of the CD. Alternative3 (Expand Surface Water Management) was not retained and is not discussed further.

8.1 ALTERNATIVE 1: NO ACTION

Estimated capital and operating cost: $ 0 Implementation time: Immediate

This alternative leaves waste rock in OU6 in its current condition. Remedies previously implemented as ResponseActions would be abandoned as- is with no monitoring or maintenance. The current discharge of contaminatedsurface water to the LMDT (introduction into the Marion Shaft) would be abandoned and the flows allowed to enterthe Stray Horse Gulch channel when the collection ponds overtop.

8.2 ALTERNATIVE 2: MAINTAIN CURRENT REMEDIES W/LAND USE CONTROLS

This alternative involves maintaining the prior Response Actions and the addition of land use controls through anICO District. Maintenance of capped mine waste materials would require periodic inspection, repair of erosionalfeatures and other minor repairs. Maintenance of the water management remedies would require sediment removalfrom retention ponds and other catch basins, and the conveyance, storage, and treatment of collected ARD.

Alternative 2 includes seven ARD conveyance, storage and treatment options, detailed below. ARD consideredunder the conveyance/ treatment options includes only the current discharge to the Marion Shaft. Treatmentoptions include the Yak and BOR treatment plants or a new, dedicated treatment facility. Cost estimates for each ofthe options under Alternative 2 assume that an on- Site waste repository would be developed within the Site andwould accept wastes generated under this alternative.

Land use controls would be implemented as part of this remedy and would limit access to or use of the areasremediated through prior Response Actions. EPA will work with Lake County on the ICO District for theunremediated portions of OU6, including exposed mine wastes and contaminated soils remaining in- place. Thesecontrols will protect existing remedies including caps and diversions and would ensure that future changes in landuse are protective of human health and the environment.

• Alternative 2a - Pressurized Pipeline to Yak Tunnel

Estimated capital and operating cost: $ 12,589,648 Implementation time: 2-years

This alternative involves the construction of a lift station at the Greenback Pond to deliver ARD via apressure line to a vertical bore advanced into the Yak Tunnel at a location up-stream of the existingbulkhead. The subsurface mine workings would be used to store contaminated water that would be meteredout and treated at the Yak Treatment Plant.

• Alternative 2b - Gravity Pipeline to Yak Water Treatment Plant w/Storage


This alternative involves delivering ARD to the Yak surge pond via a gravity pipeline. A 10 million gallonlined impoundment would be constructed along the pipeline alignment to permit water to metered out to the

Yak surge pond.

• Alternative 2e -Gravity Pipeline to Existing Pumping Well along LMDT w/Storage


This alternative-involves delivering ARD to the existing extraction well along the lower portion of theLMDT. The existing pipeline between the extraction well and the BOR treatment plant would be used toconvey OU6 ARD to the BOR's plant. A 10-million gallon lined impoundment would be constructed alongthe pipeline alignment to permit water to be metered out to the BOR's plant.

• Alternative 2g - Install Bulkhead in LMDT and Dewater Mine Pool w/Gravity Pipeline to BOR TreatmentPlant (Selected Alternative)


This alternative involves continuing the introduction of ARD into the Marion Shaft. A bulkhead would beconstructed in the LMDT where it passes through competent rock. The ground water upstream of thebulkhead (mine pool) would be pumped and delivered to the BOR treatment plant via a gravity pipeline.Significant plant upgrades would be needed in order for this alternative to be selected. This alternativewould result in the treatment of collected ARD as well as the accelerated treatment of contaminatedgroundwater filling the mine workings in OU6.

• Alternative 2h - Gravity Pipeline to Dedicated Water Treatment Plant


This alternative involves the construction of an independent water treatment facility to treat ARD collectedin OU6. The water would be conveyed to the treatment facility from the Greenback Pond via a gravitypipeline and returned to Stray Horse Gulch after treatment. The alternative would result in the long- termtreatment of collected ARD.

8.3 ALTERNATIVE 4: IN-SITU CHEMICAL STABILIZATION OR RELOCATION (IN COMBINATIONWITH ALTERNATIVES OPTIONS 2A THROUGH 2H)

This alternative involves maintaining the existing remedies (identical to Alternative 2) in addition to selecting one oftwo options for addressing waste rock piles that are considered to be sources for ARD that have not beenpreviously addressed through Response Actions. These include the Ponsardine and Emmet waste rock piles.

• Alternative 4a - In- Situ Chemical Stabilization

Estimated capital and operating cost: $ 400,000 Implementation time: 1-year

This alternative would involve the injection and dispersion of buffering agents into the Ponsardine wastepile so that a final equilibrium is reached that inhibits acid generation. Chemically stabilized systems aresusceptible to weathering and chemical decomposition therefore the potential for contact by surface waterrun on or runoff during storm events should be minimized. This process would maintain the generalintegrity of the waste piles for cultural and historical aesthetics. However, some disturbance of the pilewould be expected, as equipment will need to access all portions of the pile.

• Alternative 4b - Relocation (Selected Alternative)

Estimated capital and operating cost: $ 292,236 Implementation time: 1-year

This alternative involves the relocation of the Ponsardine waste pile to the on-Site repository planned forthe Site as discussed under Alternative 2. A four-mile haul distance is assumed for costing purposes. Thisalternative includes repairs (physical stabilization) of a retaining crib wall on the south side of the Emmetwaste rock pile.

8.4 ALTERNATIVE 5: CONSOLIDATE AND CAP W/ LAND USE CONTROLS


This alternative involves consolidating and capping of waste rock piles that are considered to be sources for ARD.This alternative excludes those mine wastes that have already been consolidated and capped. The cap design wouldfollow that implemented during prior Response Actions including a geomembrane and an 8-foot thick dolomitewaste rock cap. Cap material other than dolomite may be considered equally effective.

8.5 ALTERNATIVE 6: EXCAVATE, TRANSPORT AND ON-SITE DISPOSAL W/LAND USE CONTROLS


This alternative would involve the excavation, transport, and disposal in an waste repository (constructed within theOU6 boundaries) of waste rock piles (and underlying soils to a depth of 1- foot) that are considered to be sources forARD. This alternative excludes those mine wastes that have already been consolidated and capped. The pilefootprint would be vegetated after removal. The repository is not associated with the on-Site waste repositorydiscussed in Alternative 2. Rather, it would be located within OU6 and would meet most of the requirements for anindustrial solid waste landfill cell including a geomembrane bottom liner and cover, or appropriate alternatives.

9.0 SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES

Section 300.430(e)(9) of the NCP requires that the EPA evaluate and compare the remedial cleanup alternatives usingthe nine criteria listed below. The first two criteria, (1) overall protection of human health and the environment and (2)compliance with applicable or relevant and appropriate requirements (ARAR), are threshold criteria that must be metfor the Selected Remedies. The Selected Remedies must then consider the remaining 5 balancing and 2 acceptancecriteria. In addition, the cleanup alternatives were evaluated using six performance criteria specified in the WAMP(USDC, 1994) to assist in evaluating the effectiveness of each alternative. A summary of the comparative analysis ispresented on Tables 6 and 7.

9.1 NCP CRITERIA

9.1.1 Overall Protection of Human Health and the Environment

Overall protection of human health and the environment addresses whether each alternative can adequately protecthuman health and the environment, in both the short and long-term, from unacceptable risks posed by hazardoussubstances, pollutants, or contaminants present at the site by eliminating, reducing, or controlling human andenvironmental exposures.

The range of alternatives provides the full spectrum of protectiveness from No Action (Alternative 1) throughalternatives that result in the isolation of source material from humans and the environment (Alternative 5 and 6).The intermediate alternative (Alternative 2) offers protectiveness equal or somewhat greater than that alreadyachieved through prior Response Actions.

All of the alternatives (except Alternative 1) require institutional controls under the ICO District to minimize thelikelihood of human health risks above a level of concern.

9.1.2 Compliance with Applicable or Relevant and Appropriate Requirements

Compliance with ARARs addresses whether a remedy will meet all of the applicable or relevant and appropriaterequirements under federal environmental laws and state environmental or facility siting laws or provide grounds forinvoking waivers.

There are no chemical-specific ARARs for OU6. All of the alternatives are expected to comply with action- andlocation- specific ARARs.

9.1.3 Long-Term Effectiveness and Permanence

Long- term effectiveness and permanence refers to expected residual risk and the ability of a remedy to maintainreliable protection of human health and the environment over time, once cleanup levels have been met. This criterionincludes the consideration of residual risk that will remain on-Site following remediation and the adequacy andreliability of controls.

The variations on Alternative 2 require perpetual operation of ARD collection, conveyance and treatment facilities.Therefore, while these alternatives are effective in the long- term they are less permanent. Alternatives 4b, 5 and 6offer equal or greater long- term effectiveness when compared with the other alternatives. Alternatives 4b, 5 and 6also offer greater permanence than the other alternatives. The isolation of ARD-generating source material eitherunder engineered caps or through placement in a landfill cell provides a greater level of permanence with lowmaintenance.

Within the variations on Alternative 2, the Selected Remedy (Alternative 2g - construct bulkhead in LMDT) offerssome additional long- term effectiveness when compared with the other Alternative 2 options. Under option 2g, the

likelihood of untreated ARD being discharged to surface water is minimized through the use of the LMDT mine poolto store ARD even during high runoff years. Groundwater entering the LMDT downstream of the bulkhead isexpected to eventually be relatively clean. This may permit discharge of LMDT portal flows without treatmentthereby improving the efficiency of the BOR's water treatment program. In addition, dewatering of the mine pool overtime will reduce the potential for contaminated groundwater to discharge to surface water through seeps, springsand flowing mine shafts.

The degree of effectiveness and permanence realized through implementation of Alternative 2g depends, in part, onthe competency of the LMDT and flooded mine workings connected to the LMDT. The area of the LMDT targetedfor bulkhead construction is considered to be competent bedrock. However, the current and potential futureconditions in the LMDT are not well understood.

In order to help assess long-term effectiveness, a monitoring program will be implemented to observe groundwaterquality, flows and hydraulic head levels. This is discussed further in section 11.3 (Contingency Measures andLong-Term Monitoring)

9.1.4 Reduction of Toxicity, Mobility, or Volume Through Treatment

Reduction of toxicity, mobility, or volume through treatment refers to the anticipated performance of the treatmenttechnologies that may be included as part of a remedy. Alternatives are assessed for the degree to which theyemploy recycling or treatment that reduces toxicity, mobility, or volume, including how treatment is used to addressthe principal threats posed by the site.

Treatment of source materials is provided only under Alternative 4, In-situ Chemical Stabilization. This technology, ifeffective, will reduce the mobility of the contaminants through chemical and possibly physical fixation. Alternative 2will treat collected ARD and convert the dissolved metals to solid metal complexes thereby reducing the mobility ofthe contaminants. Alternatives 5 and 6 will minimize the generation of ARD, thereby reducing the mobility of thecontaminants. None of the alternatives affect contaminant volume or toxicity.

9.1.5 Short-Term Effectiveness

Short-term effectiveness addresses the period of time needed to implement the remedy and any adverse impacts thatmay be posed to workers, the community and the environment during construction and operation of the remedy untilcleanup levels are achieved.

Alternatives 1, 2, and 4a do not require significant disturbance of mine wastes. Alternatives-4b, 5 and 6 involvemoderate to large-scale disturbance of mine wastes. Therefore, relatively greater short-term impacts would beexpected.

9.1.6 Implementability

Implementability addresses the technical and administrative feasibility of a remedy from design through constructionand operation. Factors such as availability of services and materials, administrative feasibility, and coordination withother governmental entities are also considered.

All of the alternatives are technically and administratively implementable. However, past community objection toremedies that result in significant disturbance to mine wastes may render Alternatives 5 and 6 not implementable.

The variations on Alternatives 2 require a range of operation and maintenance activities including long- term watertreatment. The implementation of Alternative 2, including the Selected Remedy (2g) will require the development oflong- term operating agreements between the State of Colorado, USEPA and the Bureau of Reclamation. Therefore,the implementability of Alternative 2 is considered to be moderate.

Land use controls would be implemented under an overlay district planned for the Site. Implementability of theoverlay district is considered to be moderate.

9.1.7 Cost

The costs of Alternatives 2g and 4b (the Selected Remedy) are difficult to quantify at this time given uncertaintiesregarding water treatment plant upgrades that may be needed. In addition, the BOR plant replacement costs are alsouncertain and impact Alternatives 2e and2g.

Despite these uncertainties, it is possible to rank the alternatives by cost. Alternative 1 has no costs associated withit. The options under Alternative 2 are all expected have a lower present worth cost than Alternatives 5 and 6.Alternative 6 is less costly than Alternative 5. All costs are summarized on Table 8.

9.1.8 State Acceptance

The State has been consulted throughout this process and concurs with the Selected Remedy.

9.1.9 Community Acceptance

Public comment on the FFS and Proposed Plan was solicited during a formal public comment period extending fromApril 3 through May 3, 2003. The community is generally supportive of the selected remedial alternatives. Writtencomments and comments received during the public meeting pertained to clarification of specific issues associatedwith the selected remedial alternatives. One comment objecting to the relocation of the Ponsardine mine waste pilewas received from a private citizen who resided near OU6. A private citizen who owns property within OU6submitted one comment expressing concern over the proposed use of the Black Cloud Tailing Impoundment as thelocation for an on- Site repository. Otherwise, there were no objections to the selected remedial alternatives andquestions posed during the public meeting appeared to be satisfactorily addressed during the meeting. TheResponsiveness Summary addresses all comments received during the public comment period.

9.2 WAMP CRITERIA

WAMP Criteria are based on a Draft Work Area Management Plan prepared by USEPA in 1995. The plan identifiescriteria to be used to evaluate the effectiveness of remedial alternatives developed in an FS. These criteria wereincluded in the OU6 FFS to ensure consistency in remedial design and construction at all operable units as describedin the 1994 Consent Decree. The description of the criteria follows EPA, 1995b.

9.2.1 Surface Erosion Stability

Remedial alternatives for source material were assessed for surface erosion stability. Erosion stability would beachieved through the development of surface configurations and implementation of erosion protection measures.Predictions of erosion stability and erosion protective measures include:

a) Erosional releases of waste material will be predicted by use of all or some of the following procedures:the Revised Universal Soils Loss Equation (RUSLE), wind erosion soil loss equation and the procedures setforth in the U. S. Nuclear Regulatory Commission's Staff Technical Position on the Design of ErosionProtection Cover For Stabilization of Uranium Mill Tailing Sites for site specific storm flow condition setforth in (b), below or other standard recognized engineering methods.

b) Remediated surfaces located within the 500-year floodplain shall be stable under 500-year, 24-hour and 2-hour storm events. Remediated surfaces located outside the 500-year floodplain shall be stable under100-year, 24-hour and 2-hour storm events. On source area embankments or where the slope of thereconstructed source is steeper that 5: 1, surface flow shall be concentrated by a factor of 3 for purposes ofevaluating erosion stability.

Alternatives 1, 2, and 4A do not require significant disturbance of mine wastes. Waste piles consolidated andcapped under Alternative 5 and 6 will comply with this WAMP criterion. Alternative 4B will result in the removal ofthe Ponsardine waste pile and Alternative 6 leave none of the ARD- generating mine waste targeted for remediationexposed at the surface.

9.2.2 Slope Stability

Source remediation alternatives were assessed for geotechnical stability. Geotechnical stability would be ensuredthrough the development of embankments or slope contours. The remedial design would meet the following:

a) Impounding embankments shall be designed with a Factor of Safety (Safety Factor) of 1.5 for staticconditions and 1.0 for pseudo-static conditions.

b) Recontoured slopes shall be designed with a Safety Factor of 1.5 for static conditions and 1.0 forpseudo- static conditions.

c) Analysis of geotechnical stability shall be performed using an acceptable model. Material and geometryinput parameters would be obtained from available data.

Alternatives 1, 2, and 4A do not require significant disturbance of mine wastes. Waste piles consolidated andcapped under Alternative 5 and 6 will comply with this WAMP criterion. Alternative 4B will result in the removal ofthe Ponsardine waste pile and Alternative 6 leave none of the ARD-generating mine waste targeted for remediationexposed at the surface.

9.2.3 Flow Capacity and Stability

Remedial alternatives were assessed for conformance with flow capacity and stability requirements. The remedialdesign would meet the following criteria:

a) Capacity: Diversion ditches shall be sized to convey the 100-year, 24-hour and 2-hour storm events.Reconstructed stream channels shall be sized to convey flow equal to or greater than the flow capacityimmediately upstream of the reconstruction.

b) Stability: Erosional releases of Waste Material from ditches, stream channels, or retaining structures asdetermined by all or some of the following models and engineering methods: U. S. Army Corps of EngineersHydrologic Engineering Center HEC-1 and HEC-2 models or by other recognized engineering erosionalmodels.

1. Diversion Ditches and Reconstructed Stream Channels: Remedial construction located within the 500-year floodplain shall be designed to be stable under flows resulting from a 500-year, 24-hour and 2-hourstorm events. Remedial construction outside of the 500-year floodplain shall be designed to withstandflows resulting from 100-year, 24-hour and 2-hour storm events. Reconstructed stream channels shall beconfigured to the extent practicable to replicate naturally occurring channel patterns.

2. Retaining structures: Structures such as gabions, earth dikes, or rip rap shall be designed to be stableunder the conditions stated above in item (a) for the diversion ditch or stream channel with which thestructure is associated. If rip rap is to be placed in stream channels or ditches, the rip rap will be sizedutilizing one of the following methods:

• U. S. Army Corps of Engineers • Safety Factor Method • Stephenson Method • Abt/ CSU Method

Selection of one of these methods will be based on the site- specific flow and slope conditionsencountered.

None of the alternatives involve the construction of channels with the possible exception of Alternative 6, whichmay involve the diversion of storm water around and off of the on-Site repository.

9.2.4 Surface Water and Groundwater Loading Reduction

Remedial alternatives were assessed for reduction of mass loading of Contaminants of Concern (COCs), includingTotal Suspended Solids (TSS) and sulfate, as defined in the Aquatic Ecosystem Risk Assessment, and change in pH,resulting from run-on, runoff and infiltration from source areas. This criterion incorporates the following:

• For each source of contamination evaluated, the present mass loading of COCs (including TSS andsulfate) and present pH measurements should be calculated for both surface water and groundwater using scientifically accepted methods.

• For each source of contamination evaluated, the net loading reduction of COCs (including TSSand sulfate) and change in pH resulting from implementation of each remedial alternative should becalculated for surface water and groundwater. Scientifically accepted methods for calculating massloading shall be used.

Alternative 1 is expected to achieve little reduction in surface and groundwater loading. Alternative 2 maintainsexisting improvements in metal loading. Therefore, actual rather than modeled reductions in mass loading of cadmiumand zinc to surface water are provided on Figures 14 and 15. It is possible that ARD detention ponds included underAlternative 2 will result in an increase in loading to groundwater. Section 11.3 discusses contingencies should long-term monitoring demonstrate an increase in loading to groundwater under Alternative 2.

Significant reduction in mass loading is expected under Alternative 4a. However, the magnitude of the reduction is afunction of the effectiveness of the chemical/physical stabilization process. Reduction in mass loading underAlternative 4b is expected to be complete for the Ponsardine waste pile. Stabilization of the Emmet crib wall isexpected to achieve some reduction in mass loading to surface water. Water quality monitoring of this portion ofStray Horse Gulch will be needed to assess the degree of loading reduction. Reduction in mass loading underAlternatives 5 and 6 is expected to be nearly complete as these alternatives result in the isolation of the waste rockfrom the environment..

9.2.5 Terrestrial Ecosystem Exposure

Remedial alternatives were assessed with respect to reduction of risk to the terrestrial ecosystem within OU6. Thisassessment was based on area-wide estimations of risk to receptor populations. Exposure estimations for assessingthis risk considered factors that affect frequency and duration of contact with contaminated media, such as (1) theconcentrations and horizontal extent of contamination; and, (2) the effect of home range on the amount of time agiven species will spend in contact with contaminated media. For each source of contamination, the reduction of thepotential exposure predicted to result from the implementations of each remedial action alternative should becompared to the present potential exposure predicted by the Terrestrial Risk Assessment as follows:

a) For each source of contamination evaluated, the present risk due to exposure as defined in the TerrestrialEcosystem Risk Assessment should be calculated for soil, each source of contamination, and pondedsurface water associated with each source of contamination.

b) For each source of contamination evaluated, reduction of exposure and ecological risk resulting fromimplementation of each RA alternative should be calculated for soil and other media described above. The

potential exposure predicted to result from implementation of each RA alternative should be compared tothe present potential baseline exposure predicted by the Terrestrial Ecosystem Risk Assessment.

The options under Alternative 2 through 6 involve the maintenance of existing consolidated and capped minewastes. The caps break the exposure pathway for terrestrial receptors thereby reducing risks to the terrestrialecology. Mine waste excavation and disposal under Alternative 4b further reduces exposure of terrestrial receptorsto mine waste. In addition, unremediated mine waste piles typically are devoid of vegetation rendering them poorhabitat for terrestrial receptors resulting in limited exposure to Site wastes.

Some variations on Alternative 2 involve the construction of large lined impoundments to store ARD prior totreatment. These impoundments would have to be monitored to assure they are not attractive to terrestrial receptors(the Selected Remedy does not include such impoundments). All variations under Alternative 2 involve thecontinued use of small detention and retention ponds constructed at the base of several mine waste piles as part ofprevious Response Actions (Figure 4). These ponds collect and store ARD. Although it is possible that these pondsattract terrestrial receptors, their combined area is limited. In addition, these ponds have existed since 1998 and EPAhas received no reports of terrestrial receptor contact with ponded ARD or acute impacts to wildlife such as animalcarcasses in or proximal to the ponds.

Alternatives- 5 and 6 result in the isolation of ARD-generating mine wastes from the environment eliminating theterrestrial exposure pathway. Non-ARD generating mine wastes will not be addressed under any of the alternatives.

9.2.6 Non- Residential Soils

Non-residential areas were addressed in the FFS. These nonresidential soils are in areas zoned agricultural/forest,and industrial mining. The nonresidential areas within OU6 were evaluated in the ecological risk assessments andFFS consistent with current and likely future land use. Given existing zoning and land use development patterns,EPA expects that current agricultural/forest, and industrial/mining land uses will not change substantially.

All of the alternatives (except Alternative 1) include institutional controls under the ICO District to minimize thelikelihood of unacceptable human health risks from nonresidential soils. Alternatives 5 and 6 will result in theisolation of ARD-generating mine wastes from humans.

10.0 PRINCIPAL THREAT WASTES

The NCP establishes an expectation that EPA will use treatment to address the principal threats posed by a sitewherever practicable (NCP 300.430(a)(l)(iii)(A)). Identifying principal threat wastes combines concepts of both hazardand risk. In general, principal threat wastes are those source materials considered to be highly toxic or highly mobilewhich generally cannot be contained in a reliable manner or would present a significant risk to human health or theenvironment should exposure occur. Conversely, nonprincipal threat wastes are those source materials thatgenerally can be reliably contained and that would present only a low risk in the event of exposure.

Mine waste rock is not considered to be principal threat waste.

11.0 SELECTED REMEDY

11.1 DESCRIPTION OF THE SELECTED REMEDY

The Selected Remedy for OU6 includes Alternatives 2g and 4b.

Alternative 2g involves continued maintenance of consolidated and capped waste piles including the Wolftone,Maid of Erin and Mahala. This alternative also involves maintenance of surface water management featuresconstructed during prior Response Actions at Ram, Mikado, Highland Mary, Adelaide, Pyrenees and Ibex/Irene mine

waste piles/ areas as well as portions of the Stray Horse Gulch channel. Maintenance will include inspections andrepairs to caps and periodic cleaning/ repairing of surface water management features. Wastes generated duringmaintenance activities will be disposed in an on- Site repository planned for the Site.

Alternative 2g also includes continued collection of ARD with management by:

• Discharge to surface water at a controlled rate to minimize impacts to the Parkville Water District(detention pond constructed at Ibex/irene area (Figure 3) under prior Response Action).

• Evaporation (detention ponds constructed at New and Old Mikado, Highland Mary andAdelaide/Ward area (Figure 3) under prior Response Actions).

• Treatment to remove contaminants at a water treatment facility operated by the BOR. This portionof the remedy includes most new components to be constructed under this ROD as discussedbelow.

Alternative 2g continues the introduction of ARD into the Marion Shaft. A plug would be constructed in the LMDTwhere it passes through competent rock (See Figure 16). The resulting impounded groundwater (mine pool) would bepumped from a location up stream of the concrete plug and delivered to the BOR treatment plant via a gravitypipeline. Groundwater entering the LMDT below the plug would ultimately be allowed to exit the tunnel portal andflow downstream without treatment. EPA anticipates that the water quality would improve over time to a levelcomplying with discharge permits currently held by the BOR. Water treatment would be required until the waterquality reached the level required by discharge permits.

The total estimated ARD discharged to the Marion Shaft in 2001 is 9,600,000 gallons with an estimated peak flow of550 gpm. The anticipated available capacity of the BOR Treatment Plant is not known with certainty. However, theplant is believed to have current excess capacity of at least 50 gpm of typical OU6 ARD. This quantity is the basisfor sizing remedial designs involving the BOR facilities.

The estimated volume of water in the mine pool above the elevation of the LMDT is 750 million gallons. A pumpingrate of 3,000 gpm was estimated in order to dewater the mine pool in less than two years to the elevation of theLMDT. After dewatering the mine pool, an equilibrium- pumping rate would be maintained in perpetuity. The actualpumping rate will be determined during remedial design. However, for the purposes of developing a cost for thisalternative a pump system and pipeline capable of delivering 3,000 gpm is assumed.

The BOR plant does not have the capacity to treat water at an initial pumping rate of 3,000 gpm. As discussed above,the actual pumping rate and treatment capacity will be determined during remedial design. If the selected pumpingrate exceeds the existing BOR plant capacity, plant upgrades will be necessary. The type of plant upgrades dependsnot only on the pumping rate but also on the quality of the water in the mine pool. At the present time the mine poolwater chemistry is not fully characterized. As an alternative, the mine pool may be dewatered at a much lowerpumping rate for a longer period of time. In either case, an equilibrium-pumping rate will have to be maintained inperpetuity. The pipeline alignment and locations of the proposed bulkhead in the LMDT is shown on Figure 16.

Alternative 4b involves the relocation of the Ponsardine Waste Pile to an on- Site repository planned for the Site inaddition to stabilization of the Emmet Waste Pile crib wall. Relocation of the Ponsardine Waste Pile will minimizeARD runoff and infiltration. Stabilization of the Emmet Waste Pile crib wall will reduce the amount of contaminatedseepage originating from the current crib wall and minimize the likelihood of a structural collapse of the portion of thewaste pile retained by the existing crib wall.

Controls on land use would be implemented as part of this remedy. Land Use Controls would limit access to or use ofthe areas remediated through prior Response Actions. These include capped and consolidated waste piles, areaswith clean water diversion and ARD collection structures. Permanent measures to be considered would include legalor institutional mechanisms to provide notification that a Superfund remedy is in place and establish restrictions/requirements for future activities to maintain the integrity and effectiveness of the remedies. Modifications to county

and/ or city zoning ordinances would involve the creation of an "overlay district" to provide a screening process toidentify properties where special precautions or requirements may be needed. Land use and plans/ proposals forfuture land use would be monitored and evaluated as part of the five- year review process.

EPA will work with Lake County on an ICO District that would be implemented as part of this remedy and would limitaccess to or use of the areas remediated through prior Response Actions. These controls will protect existingremedies including caps and diversions and would also ensure that future changes in land use are protective ofhuman health and the environment.

Groundwater monitoring will include but not be limited to water quality and elevation monitoring of the mine poolimpounded behind the bulkhead to be installed in the LMDT. The purpose of the mine pool monitoring is todetermine to what degree pumping of groundwater from the LMDT has arrested or retarded the migration of minepool water away from OU6.

The remedy would consist of the following elements:

• 375- feet of 2.4 meter diameter vertical shaft. • Concrete bulkhead in LMDT. • Pump system capable of delivering 3,000 gpm. • 7,287- feet of 18- inch I. D., PVC pipe buried below frost depth (including a pipe "pig" and launcher for

pipeline maintenance) • Treatment at the BOR's plant of 50 gpm of piped mine pool water in addition to existing waters collected at

the portal and dewatering wells, assuming no plant upgrades. Periodic cleaning of retention ponds andsediment basins.

• Transport of water treatment residuals, and pond and sediment basin deposits to an on- Site repository. Operation and maintenance of new and existing remedy components.

• Groundwater monitoring to observe water level and quality conditions in and around the mine pool, theEmmet waste piles, and retention ponds.

• Removal of Ponsardine and disposal to an on- Site Repository. • Construction of Emmet Waste Pile crib wall. • Institutional controls under an ICO District.

11.2 SUMMARY OF ESTIMATED REMEDY COSTS

The detailed cost estimate and present worth analysis for the Selected Remedy is presented in Table 8. The presentworth value of the estimated capital and operating cost for a 100-year period is approximately $ 13,605,227. Thepresent worth of implementing this remedy over 100-years is based on a 7 percent discount rate. The constructioncost of the BOR's plant is not available for use in calculating the periodic replacement costs for this facility.Therefore, the actual construction cost of the Yak water treatment plant is used for developing a cost estimate forthis alternative.

For costing purposes, the quantity of mine pool water treated annually is based on the current estimated minimumexcess capacity of 50 gpm. It is assumed that a minimum of 50 gpm of mine pool water would be delivered to the BORtreatment plant continuously throughout the year. However, pumping and conveyance equipment proposed underthis alternative is sized for a maximum pumping rate of 3,000 gpm.

Costing of operation and maintenance wastes assumes that an on- Site repository will be developed within the Siteand will be able to accept wastes generated under the Selected Remedy. The wastes include water treatmentresiduals and sediment periodically removed from detention ponds.

For costing the relocation of the Ponsardine Waste Pile, a four-mile haul distance is assumed.

11.3 CONTINGENCY MEASURES AND LONG-TERM MONITORING

Specific water quality goals for surface streams and heavy metals contamination have not been established at thistime. EPA and CDPHE will establish specific surface and groundwater requirements at a later date under a Record ofDecision for OU12.

Pre- remedial data will be compared to water quality data collected after the Selected Remedy has been implemented.EPA and CDPHE will make an evaluation of the degree of surface water quality improvement at that time. If theimprovement in OU6 surface water is not considered sufficient to meet OU12 water quality standards, additionalResponse Actions may be required under an OU12 record of decision.

Additional Response Actions will be considered in the event that groundwater under ARD detention pondsbecomes adversely affected by infiltration from the detention ponds. The additional Response Actions may include:

• Construct liners or abandon existing ARD detention ponds associated with the Greenback, RAMand Pyrenees mine waste piles (Figure 3) and allow ARD to drain to the Marion Mineshaft withoutdetention.

• Construct liners in existing ARD detention ponds associated with the New and Old Mikado,Highland Mary and Adelaide/Ward mine waste piles (Figure 3).

• Construct a gravity pipeline to convey ARD from the New and Old Mikado, Highland Mary andAdelaide/Ward mine waste pile detention ponds (Figure 3) to the Marion Mineshaft.

If monitoring indicates that significant surface loading between SHG-08 and SHG-09 continues, and is notattributable to infiltration from impoundments, remedial alternatives for addressing ARD from the Emmet piles will beevaluated.

Because most mine wastes will remain in- place, the Selected Remedy will require a five-year review under Section 12(c) CERCLA and Section 300.430(f)(4) ii) of the NCP. The five-year review includes a review of groundwater andsurface water monitoring data, inspection of the integrity of the covers, diversion channels and impoundments andhow well the Selected Remedy is achieving the remedial action objectives and ARAR's that it was designed to meet.Groundwater monitoring will include but not be limited to water quality and elevation monitoring of the mine poolimpounded behind the bulkhead to be installed in the LMDT. The purpose of the mine pool monitoring is todetermine to what degree pumping of groundwater from the LMDT has arrested or retarded the migration of minepool water away from OU6. A monitoring plan specific to the mine pool will be developed during the remedial designphase of remedy implementation. The monitoring plan will define ambient conditions to ensure that if conditionschange in the LMDT or attendant mine workings they will not go undetected.

In the event that conditions in the mine pool change beyond certain thresholds, as defined by the monitoring plan,EPA will consider such changes as constituting a "Remedy Failure". At such time EPA will implement all monitoringand remedial Response Actions required to address the failure in a timely manner. Allocation of the USEPA, USBOR,and State of Colorado costs and responsibilities will be subject to agreements that will be defined in Long-TermOperation & Maintenance Agreements between the parties and in the EPA- CDPHE State Superfund Contract (SSC).

Portions of the Phase II Response Action (the consolidation and capping of waste rock piles) may not haveachieved the 1.5 slope stability factor of safety for static conditions and 1.0 for psuedo-static conditions specified inthe Work Area Management Plan (WAMP). If slope failure should occur, EPA will consider the event a " RemedyFailure", and at such time implement all remedial Response Action required (and monitoring if needed) to address thefailure in a timely manner.

12.0 STATUTORY DETERMINATIONS

Under CERCLA Section 121 and the NCP, EPA must select a remedy that is protective of human health and theenvironment; that complies with ARARs; is cost- effective; and utilizes permanent solutions, alternative treatmenttechnologies, or resource recovery technologies to the maximum extent practicable. In addition, CERCLA includes apreference for remedies that employ treatment that permanently and significantly reduces the volume, toxicity, ormobility of hazardous wastes as a principal element and a bias against off- site disposal of untreated wastes. Thefollowing sections discuss how the Selected Remedy meets these statutory requirements.

12.1 PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT

The Selected Remedy will protect human health and the environment through the maintenance of prior ResponseActions and the perpetual treatment of ARD. Dewatering of the mine pool will result in a reduction in bedrock watertable potentially reducing the number of springs in the area discharging contaminated groundwater. Long- termsurface and groundwater monitoring will be used to assess the effects of ARD discharge to the Marion Shaft as wellas dewatering of the mine pool. The details of the long- term monitoring will be developed during remedial design.Land use controls under an ICO District will minimize the likelihood of human health risk above a level of concernfrom exposure to nonresidential soils and mine wastes.

The Selected Remedy is protective of terrestrial ecology through the maintenance of existing consolidated andcapped mine wastes and the removal of the Ponsardine mine waste pile. The caps break the exposure pathway forterrestrial receptors thereby reducing risks to the terrestrial ecology. In addition, unremediated mine waste pilestypically are devoid of vegetation rendering them poor habitat for terrestrial receptors resulting in limited exposure toSite wastes.

The Selected Remedy includes the continued use of small detention and retention ponds constructed at the base ofseveral mine waste piles as part of previous Response Actions. These ponds collect and store ARD. Although it ispossible that these ponds attract terrestrial receptors, their combined area is limited. In addition, these ponds haveexisted since 1998 and EPA has received no reports of terrestrial receptor contact with ponded ARD or acute impactsto wildlife such as animal carcasses in or proximal to the ponds.

12.2 COMPLIANCE WITH APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS

The Selected Remedy will comply with all ARARs identified in Tables 9 through 11. No waiver of ARARs will benecessary. Final performance standards will not include ARARs for Site- Wide Surface and Ground Water or requirea specified decrease in concentrations of COCs to Site- Wide Surface and Ground Water (USCD, 1994). It was agreedthat the decision on remediation of Site- Wide Water Quality (OU12) would be made only after remedies for sourceremediation were selected and implemented at each OU (USDC, 1994). As a result, specific water quality goals forsurface streams and groundwater have not been established at this time.

Chemical Specific ARAR's. There are no chemical specific ARAR's associated with the Selected Remedy.

Location Specific ARAR's The selected remedy will comply with all location specific ARAR's. Several regulationspertaining to the preservation of historic features have been identified as ARARs. Compliance will be achievedthrough implementation of procedures to preserve historical and archeological data should qualifying historicalfeatures be affected by the remedy.

Action Specific ARAR's. The selected remedy will comply with all action specific ARAR's. Environmental covenantsare. required whenever contamination is left in place, such as mine waste. Compliance with this ARAR will beachieved through the creation of an institutional control overlay (ICO) district. Underground injection control (UIC)regulations are triggered by the introduction of collected ARD into the Marion mineshaft via a class 5 well. The UICregulation requires that injected waters not degrade drinking water sources. Current data indicate degradation of

groundwater outside of the LMDT and Marion mineshaft resulting from the injection of ARD into the Marion Shafthas not occurred (Section 2.0). Similarly the State PDES and CBGS are applicable and require that there be nodegradation of ground water from the introduction of the collected ARD water in the Marion mine shaft. Points ofCompliance for this ARAR will be established and monitored through the implementation of long- term groundwaterquality monitoring program.

12.3 COST- EFFECTIVENESS

In EPA's judgment, the Selected Remedy is cost-effective and represents a reasonable value for the money to bespent. In making this determination, the following definition was used: " A remedy shall be cost-effective if its costsare proportional to its overall effectiveness (NCP 300.430(f)(l)(ii)(D))". This was accomplished by evaluating the "overall effectiveness" of those alternatives that satisfied the threshold criteria (i. e., were both protective of humanhealth and the environment and ARAR-compliant). Overall effectiveness was evaluated by assessing three of thefive balancing criteria in combination (long-term effectiveness and permanence; reduction in toxicity, mobility, andvolume through treatment; and short-term effectiveness). Overall effectiveness was then compared to costs todetermine cost- effectiveness. The relationship of the overall effectiveness of this remedial alternative wasdetermined to be proportional to its costs and hence this alternative represents a reasonable value for he money tobe spent.

The estimated present worth cost of the Selected Remedy is $ 13,998,431.

12.4 UTILIZATION OF PERMANENT SOLUTIONS AND ALTERNATIVE TREATMENT ( OR RESOURCERECOVERY) TECHNOLOGIES TO THE MAXIMUM EXTENT PRACTICABLE (MEP)

EPA has determined that the Selected Remedy represents the maximum extent to which permanent solutions andtreatment technologies can be utilized in a practicable manner at OU6. Of those alternatives that are protective ofhuman health and the environment and comply with ARARs, EPA has determined that the Selected Remedyprovides the best balance of trade- offs in terms of the five balancing criteria, while also considering the statutorypreference for treatment as a principal element and bias against off-site disposal and considering State andcommunity acceptance.

The large volume of mine wastes precludes treatment or off- Site disposal as a viable option. In addition, the minewaste and related ARD do not contain resources that may be recovered economically at the present time. However,the potential remains for reprocessing of mine waste in the future should it become economically viable.

12.5 PREFERENCE FOR TREATMENT AS A PRINCIPAL ELEMENT

The Selected Remedy includes treatment of ARD as a principal element.

12.6 FIVE-YEAR REVIEW REQUIREMENTS

Because this remedy will result in hazardous substances, pollutants, or contaminants remaining in OU6 above levelsthat allow for unlimited use and unrestricted exposure, a statutory review will be conducted within five- years afterinitiation of remedial action to ensure that the remedy remains protective of human health and the environment.

13.0 DOCUMENTATION OF SIGNIFICANT CHANGES FROM PREFERRED ALTERNATIVE OF PROPOSEDPLAN

The Proposed Plan for OU6 was released for public comment in April 2003. The Proposed Plan identified Alternative2g and 4b as the Preferred Alternative for waste rock. EPA reviewed all written and verbal comments submittedduring the public comment period. It was determined that no significant changes to the remedy, as originallyidentified in the Proposed Plan, were necessary or appropriate.

14.0 REFERENCES

CDM, 1997. Engineering Evaluation/Cost Analysis for Stray Horse Gulch, Operable Unit 6, California Gulch NPLSite, Leadville, Colorado, June 1997.

CDM, 2000a. Final Phase I Removal Action Completion Report for the California Gulch Superfund Site, LakeCounty, Colorado, Operable Unit 6, December 11, 2000.

CDM, 2000b. Final Phase II/ IH Removal Action Completion Report for the California Gulch Superfund Site, LakeCounty, Colorado, Operable Unit 6, December 2000.

CDM, 2000e. Final Phase IV Removal Action Completion Report for the California Gulch Superfund Site, LakeCounty, Colorado, Operable Unit 6, December 2000.

CDM, 200la. Final Technical Memorandum Response Action Alternative Evaluation of Ibex/Irene Site atOperable Unit 6, California Gulch Superfund Site, Lake County, Colorado, July 2001.

Golder Associates, Inc., 1996a. Hydrogeologic Remedial Investigation Report, California Gulch Site, Leadville,Colorado, Volume I, May 1996.

Pacific Western Technologies, Ltd., 2001. Evaluation of Mahala Waste Pile, Stray Horse Gulch, Removal Actionfor the California Gulch Superfund Site, OU6, June 27, 2001.

Rocky Mountain Consultants, Inc., 2001 a. Summary of Groundwater Quality Data, California Gulch NPL Site,Lake County, Colorado, March 26, 2001.

Rocky Mountain Consultants, Inc., 2001 b. California Gulch Superfund Site, Synoptic Sampling of Stray HorseGulch, Starr Ditch, and Lower California Gulch (OU6), Spring 2000, January 2001.

Roy F. Weston, Inc., 1991. Preliminary Human Health Baseline Risk Assessment for the California Gulch NPL site,Leadville, Colorado, December 1991.

Roy F. Weston, Inc., 1995a. Final Baseline Aquatic Ecological Risk Assessment for the California Gulch NPL Site,September 1995.

Roy F. Weston, Inc., 1995b. Baseline Human Health risk Assessment for the California Gulch Superfund Site, PartC — Evaluation of Worker and Recreational Scenarios, April 1995.

Roy F. Weston, Inc., 1996a. Baseline Human Health Risk Assessment, California Gulch Superfund Site, Leadville,Colorado, Part A - Risks to Residents from Lead, January 1996.

Roy F. Weston, Inc., 1996b. Baseline Human Health Risk Assessment, California Gulch Superfund Site, Leadville,Colorado, Part B - Risks to Residents from Contaminants other than Lead, January 1996.

Roy F. Weston, Inc., 1997. Ecological Risk Assessment for the Terrestrial Ecosystem, California Gulch NPL Site,Leadville, Colorado, January 1997.

United States Department of the Interior, Bureau of Reclamation, 1996a. Phase I: Feasibility Study, Water andSediment Sampling and Hydrologic Measurement Program, Results and Findings, 1995 Spring Runoff forOperable Unit 6, California Gulch NPL Site, Leadville, Colorado, November 1996.

United States Department of the Interior, Bureau of Reclamation, 1996b. Value Analysis, Draft-Presentation

Report, Project: California Gulch OU6 Removal Action Evaluation and Decision Phase, Leadville, Colorado,April 1996.

United States Department of the Interior, Bureau of Reclamation, 1997. Draft Environmental Geology of OperableUnit 6, Removal Action Design Data, California Gulch Superfund Site, Leadville, Colorado, February 1997.

United States District Court for the District of Colorado. Consent Decree with Asarco Incorporated, ResurrectionMining Company, Newmont Mining Corporation, and the Res-Asarco Joint Venture, Civil Action No. 83-C-2388,August 25, 1994.

USEPA, 1993. Final Screening Feasibility Study for Remediation Alternatives at the California Gulch NPL Site,Leadville, Colorado, September 1993.

USEPA, 1995a. Action Memorandum. Subject: Request for Removal (Response) Action at the California GulchNational Priorities List Site, Leadville, Colorado: ACTION MEMORANDUM for an Emergency Removal Action forRehabilitation and Construction of Drainage and Sediment Control Features, Hamm's Tailing Impoundment. Ref:8HWM- SR, November 6,1995.

USEPA, 1995b. Work Area Management Plan for the California Gulch Superfund Site, Implementation by the U. S.Environmental Protection Agency (draft), June 14, 1995.

USEPA, 1996a. Action Memorandum. Subject: Request for Removal (Response) Action at the California GulchNational Priorities List Site, Leadville, Colorado: ACTION MEMORANDUM for an Emergency Response Removalfor the removal of sediments from the 5th Street Drainage Ditch and Starr Ditch, Operable Unit 6, (OU6). Ref:EPRSR, May 1,1996.

USEPA, 1996b. Action Memorandum. Subject: Request for Removal (Response) Action at the California GulchNational Priorities List Site, Leadville, Colorado: ACTION MEMORANDUM for Time Critical Removal Actions forHamm's Tailings Impoundment and the Penrose Mine Waste Pile. Ref: 8EPR-SR, July 26, 1996.

USEPA, 1997. Work Area Management Plan for the California Gulch Superfund Site, Implementation by the U. S.Environmental Protection Agency: ACTION MEMORANDUM for Non-Time Critical Removal Actions for SourceControl Activities at Designated Mine Waste Piles (Operable Unit VI). Ref: 8EPR- SR, June 24,1997.

USEPA, 1999a A Guide to Preparing Superfund Proposed Plans, Records of decision, and Other Remedy SelectionDecision Documents, OSWER 9200.1- 23P, EPA 540-R-98-031, July 30, 1999.

USEPA, 1999b Action Memorandum. Subject: Request for Removal (Response) Action at the California GulchNational Priorities List Site, Leadville, Colorado: ACTION MEMORANDUM AMENDMENT for subsequentNon-Time Critical Removal Actions for Surface Water Management at Designated Mine Waste Piles. Ref: 8EPR-SR, June 2, 1999.

USEPA, 1999c Record of Decision, Residential Populated Areas, Operable Unit 9, September 1999.

USEPA, 2001. Action Memorandum. Subject: Request for Removal (Response) Action at the California GulchNational Priorities List Site, Leadville, Colorado: ACTION MEMORANDUM for a Non- Time Critical RemovalAction for Water Management Activities at the Ibex/ Irene Mine Waste Pile at the upper end of Lincoln Gulch. (Aportion of Operable Unit 6)(Phase V) Ref: 8EPR- SR, June 25, 2001.

USEPA, 2002a, Final Feasibility Study, Operable Unit 6, California Gulch NPL Site, September 11, 2002.

USEPA, 2002b. Draft Groundwater Hydrology in the Vicinity of the Leadville Mine Drainage Tunnel, OperableUnit 6 and Affected Areas, August 2002.

Water, Waste and Land, Inc., 1990. California Gulch Hydrologic Investigation, Leadville, Colorado, August 1990.

Woodward-Clyde Consultants, 1992a. Final Air Monitoring Report, California Gulch Site, Leadville, Colorado,Volume I, May, 1992.

Woodward-Clyde Consultants, 1992b. Wetlands Map for California Gulch RI/FS Study Area, September 1992.

Woodward-Clyde Consultants, 1994a. Final Mine Waste Piles Remedial Investigation Report, California GulchSite, Leadville, Colorado, January 1994.

Woodward-Clyde Consultants, 1994b. Final Tailings Disposal Area Remedial Investigation Report, CaliforniaGulch Site, Leadville, Colorado, January 1994.

TABBED PAGE: TABLES

Table 11995 Contaminant Concentrations in Stream Bed Sediments (mg/kg)

Analyte Evans Gulch South Evans Gulch Lincoln Gulch Stray Horse Gulch

ConcentrationRange

ConcentrationRange

Concentration ConcentrationRange

Aluminum 1,540 - 3,930 1,535 - 3,730 3,930 2,740 - 5,200

Arsenic 4 - 93 9 - 50 78 7 - 106

Cadmium 0.6 - 13 0.3 - 10 41 2 - 26

Copper 9 - 212 4 - 52 485 83 - 334

Iron 6,220 - 26,100 8,050 - 12,455 39,700 9,510 - 70,300

Lead 81 - 1,080 41 - 848 2,150 188 - 4,690

Manganese 281 - 2,240 142 - 941 2,190 491 - 5,620

Zinc 163 - 6,540 43 - 1,960 1,110 320 - 4,550

(BOR, 1996a)1- Concentration shown are the minimum and maximum median contaminant concentration for sample stations within a given watershed.2- Only one sample station.

Table 2 Summary of Piles Identified as Candidates for Remedial Action

Pile/Area

PreviousResponseAction(Y/ N) AVIRIS

Mile Waste Pile RI Water Quality

Candidate forFurtherRemedialActions (Y/N)

Greenback Y NA NA NA Y

RAM Y NA NA NA Y

Old Mikado Y NA NA NA Y

New Mikado Y NA NA NA Y

Highland Mary Y NA NA NA Y

Adelaide/ Ward Y NA NA NA Y

Pyrenees Y NA NA NA Y

Fortune/Resurrection

Y NA NA NA Y

Ponsardine N NA NA NA Y*

Emmett N Y Y Y Y

Evans F& G N Y NA N N

* Although the Ponsardine Pile has not been part of a prior Response Action, it was scheduled for Response Action activities, and therefore identified as ARD-generating.

NA - Not applicable. Sources identified during prior Response Actions were not analyzed further. The MineWaste Pile RI only encompassed the populated areas of Leadville and therefore did not inventory theEvans area.

AVIRIS - airborne visible and infrared imaging spectroscopy

Table 3 Zinc and Cadmium Concentrations at SHG- 08 and SHG- 09

Station Date Analyte Units Result

SHG-08 5/15/ 01 Total Cadmium µg/l 19.8

SHG-09 5/15/ 01 Total Cadmium µg/l 155

SHG-08 5/15/ 01 Total Zinc µg/l 2,200






( RMC, 2001)

Table 4 ARD-Generating Mine Waste Volumes

Pile/Area Volume (CY)

Greenback 44,000

RAM 24,800

Old Mikado 42,000

New Mikado 44,000

Highland Mary 27,000

Adelaide/Ward 54,000

Pyrenees 60,500

Fortune/Resurrection 200,000

Ibex/Irene 223,000*

Ponsardine 11,000

Emmett 11,800**

Total: 742,100

Volumes obtained from the Value Analysis Report (BOR, 1996b) unless otherwise noted. * Used sum of volumes reported in Value Analysis for areas Ibex- H and Ibex- I ** Calculated by HDR during FFS process.

Table 5 Hazard Indices for Solid Surficial Media by Receptor for OU6

Operable Unit OU6a OU6b

Blue Grouse 25 3

Mountain Bluebird 634 463

American Kestrel 18 0

Red-tailed Hawk 12 0

Bald Eagle 14 0

Least Chipmunk 41 31

Mule Deer 3 0

Red Fox 16 0

(Weston, 1997) OU6a - Majority of OU6 OUb - Small area in western OU6 including Starr Ditch

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National ambient air quality slandafds (NAAQS) are implemented through the NewSource Review Program and Slate Implementation Plans (SIPs). The federal NewSource Review program address only major sources. Current air emissions fromundisturbed nine waste are below a level of concern per EPA's Baseline Humanhealth Risk Assessment, Part A (19%). Emissions associated with remedial action areaddressed under Action-specific ARARs.

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definition of a hazardous waste. Further, waste placement pre-dales the RCRA. A

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LDR's as ARAR for treatment residuals (water plant sludge and impoundment

sediment) is provided under Action-Specific ARAR's.

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Portions of OU6 lie wilhin a 100-year floodplai

however, implemenlalion of (he selected remed

for OU 6 will not result in any development in

the floodplain that will result in direct or indireadverse impacts.8

uircs federal agencies to evaluate the potential

els of actions (hey may lake in a floodplain lo

d. lo (he extent possible, adverse effects

ciated with direct or indirect development of a

dplain.

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Wellands are present in portions of OU6 as

defined in a 1992 study by Woodward Clyde,

however implemenlation of (he selected remed]

for OU 6 will not result in any construction or

activities in any identified wetlands.

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NKS federal agencies conducting certain

rilies lo avoid, to the extent possible, Ihe

«e impacts associated wilh the destruction or

of wedands and to avoid support of new

ilruclion in wetlands if a practicable•native exist-:.

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Wetlands are present in portions of OU6 as

defined in a 1992 study by Woodward Clyde ai

Stray Horse Gulch is considered "waters of Ihe

United Stales", implemenlation of the selected

remedy will not result in the discharge of dredgor fill material into waters of the United Slates.

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uires Federal agencies (o avoid, lo the extent

lible, adverse impacts associated wilh

niction or loss of wetlands. Regulates the

harge of dredged or fill material into waters of

. Consultation with Ihe Regional Responsem required.

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Provides protection for threatened and

endangered species and their habitats. Howevei

site-specific studies did not document the

presence of threatened or endangered species. I

threatened or endangered species are encounter

during remedial activities in OU6, then

requirements of the Act would be applicable.

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eels endangered species and threatened species

preserves their habitat. Requires coordination

i federal agencies for mitigation of impacts.

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wildlife resources, EPA will coordinate wilh be

the U.S. Fish and Wildlife Service and the

Colorado Department of Natural Resources.

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uires coordination with federal and slate

icies on activities affecling/modifying streams

vers if (he activity has a negative impact on

or wildlife. Requires coordination wilh federal

state agencies to provide protection of fish and

Ilife in water resource development programs;

ilates actions thai impound, divert, control, or

lify any body of water.

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Stray Horse Gulch, Evans Gulch and Starr Ditc

are not considered "navigable rivers."

oZ.02lion 10 permit required for structures or work

r affecting navigable waters.

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Leadville National Historic Landmark DistrictProgrammatic Agreement has been entered intbetween the EPA, Ihe Advisory Council onHistoric Preservation, and (he Colorado State

Historic Preservation Officer in accordance svi

Sections 106 and 1 10(0 of NHPA.

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Requires the preservation of historic properties

included in or eligible for the National Register oHistoric Places and to minimize harm to NationaHistoric Landmarks.

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A cultural resource survey was completed inOU6 to identify historic properlies which mayaffected by removal activity. EPA will preserv

historical and archaeological data encounteredduring implemenlation of Ihe remedy.

S3>eEstablishes procedures lo provide for preservalioi

of historical and archcological data which might

be destroyed through alteration of terrain as aresult of a federal construction project or afederally licensed activity program.

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If any remedial activity involves removal of

archeological resources; EPA will comply wiltthe substantive requirements of (he Act.

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archeological resources from public lands or

Indian lands.

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There are no features within OU6 Ihnt are eligiunder Ihe Ac(.

oZoZEnables (he National Park Service (o preservehistoric resources for public use.

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The site is not within a federally-owned areadesignated as a wilderness area or a National

Wildlife Refuse System.

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Limits activities wilhin areas designated as

wilderness areas or National Wildlife RefugeSystems.

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The Arkansas River is not listed as a Wild andScenic River.

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repository will be sited. Such a facility is notbeing selected as part of this remedy.

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Facilities where treatment, storage, or disposal ofsolid waste will be conducted must meet cerlain

location standards. These include location

restrictions on proximity to airports, floodplains,

wetlands, fault areas, seismic impact zones, andunstable areas.

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No State lands included.

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Establishes procedures and requires a permit forinvcsligalion, excavation, gathering, or removalfrom Hie natural slate of any historical,prehislorical, or archcological resources on slatelands for the benefit of recognized scientific oreducational institutions. Also requires anexcavation permit and nolificalion if humanremains are found on slate land.

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of historical significance.

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Standards for regulation of non-game wiand threatened and endangered species,specific studies did not document the prthreatened or endangered species. If (hreendangered species are encountered duriremedial activities at OU6, (hen requireiAct will be applicable.

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Protects endangered and threatened species andpreserves (heir habitats. Requires coordinationwilh Ihe Division of Wildlife if remedial aclivilitimpact on state-listed endangered/threatenedspecies or (heir habitat.

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Tabbed Page:Responsiveness

Summary

Responses to Comments Received on the Proposed Plan Operable Unit No. 6

California Gulch NPL Site

Comments are presented in standard type. Responses are provided in italics.

Comments received during May 1, 2003 Public Meeting

1. Brian Pearson of the US Bureau of Reclamation (BOR) expressed support for the preferred alternative butrecognized the need to confirm that BOR has the authority to treat OU6 water over the long-term.

EPA appreciates the BOR's support for the preferred alternative and recognizes its need to formalizewithin its own organization the decision to treat OU6 water over the long-term.

2. Russ Allen of the Colorado Department of Public Health and Environment (CDPHE) expressed support forthe preferred alternative but recognized the need for EPA and CDPHE to reach agreement on the CDPHE'soperation and maintenance responsibilities. CDPHE also requested more technical assurance regarding thetechnical implementability of the remedy.

EPA has identified in Section 11.3 (Contingencies), those remedy elements that would be theresponsibility of EPA. Remaining operation and maintenance activities will be defined throughdiscussions with CDPHE during remedial design. Technical implementability will also be furtherevaluated during remedial design.

3. What happens to the water that is collected at the Greenback settlement pond?

This collection pond discharges through a culvert and the water flows in a constructed channel to theMarion collection pond. The water collected in the Marion pond is discharged through a borehole to anunderground mine working connecting the Marion and Emmet mine shafts and the Leadville MineDrainage Tunnel (LMDT). Water flowing through the LMDT is treated in a facility operated by the U. S.Bureau of Reclamation.

The connection between the Marion Shaft and the LMDT has been under analysis by EPA. A number oftracer studies were performed to determine the effectiveness of the LMDT as a conveyance structure forthe Greenback Pond discharge. These studies confirm a hydraulic connection between the Marion Shaftand the LMDT portal. This conclusion is based on information contained in the August 2002 GroundWater Hydrology in the Vicinity of the Leadville Mine Drainage Tunnel, Operable Unit 6 and AffectedAreas including:

C Approximately 60% of the tracer injected in the Marion Shaft was recovered at the LMDT portalas of the summer of 2003 and additional dye was still being recovered at the portal.

C A portion of the tracer remains in the Marion Shaft.

C Tracer has not been detected at any other point where groundwater discharges to surface water.

4. Are the collection ponds lined?

The collection ponds are underlain with interbedded " Lake Bed" deposits to varying degrees. TheseLake Bed deposits serve somewhat as a natural (clay) barrier. Additional Response Actions will be

considered in the event that groundwater under ARD detention ponds becomes adversely affected byinfiltration from the detention ponds. The additional response actions may include:

C Abandon existing ARD detention ponds associated with the Greenback, RAM and Pyrenees minewaste piles and allow ARD to drain to the Marion Mineshaft without detention.

C Construct liners in existing ARD detention ponds associated with the New and Old Mikado,Highland Mary and Adelaide/Ward mine waste piles.

C Construct a gravity pipeline to convey ARD from the New and Old Mikado, Highland Mary andAdelaide/ Ward mine waste pile detention ponds to the Marion Mineshaft.

5. What happens to water coming off of the Emmet?

Surface water coming into contact with rock in back of the Emmet crib wall is producing acid rockdrainage and will be mitigated under the OU6 Response Actions.

6. What is the plan for relocation of mine waste?

We are considering development of an on-Site waste repository at the Black Cloud tailing pond area inIowa Gulch.

7. How will EPA develop the bulkhead?

We are currently surveying the tunnel and analyzing water levels. Plans will likely involve drilling down380 feet offset of the shaft to develop a temporary plug, dewatering the area, then placing a permanentplug. Specific plans for the plug will be finalized during the Remedial Design phase.

8. What is the maximum capacity of the Leadville Mine Drainage Tunnel?

The plant is designed to handle about 2200 gallons of water per minute, although in reality it isprobably more like 1,900 gallons per minute.

9. What happens to the sludge derived from treated water?

Hazardous waste is transported to hazardous waste sites; non hazardous waste is placed in a solid wastelandfill.

10. How will institutional controls affect companies that are interested in conducting future mining activitieshere?

Institutional controls are not designed to prevent mining, which would be regulated by the State ofColorado Division of Minerals and Geology. Any activities that might compromise the effectiveness ofcurrent remedies will need EPA approval. Excavation activities may require county approval.

Comments received from Resurrection Mining Company

11. The Proposed Plan issued by EPA describes and summarizes the evaluation of six remedial alternatives forcleanup of Operable Unit No. 6 (OU6). The EPA intends to select a final remedy for OU6 following reviewand consideration of information received during the public comment period. As described in the ProposedPlan, the preferred alternative consists of Alternatives 2g and 4b, which includes the following principalcomponents:

C continued operation and maintenance of remedial actions previously completed by the EPA; C continued collection of contaminated runoff and seepage from mine waste rock piles within a

portion of the OU6 area and delivery of the water to the LMDT via the Marion Shaft for eventualtreatment at the BOR's water treatment plant;

C construction of a bulkhead in the LMDT for hydraulic head control of the mine water pool in theunderground mine workings connected to the LMDT;

C pumping of mine water stored behind the bulkhead in a pipeline to the BOR's water treatmentplant;

C removal of the Ponsardine waste rock pile to a yet to be determined on-Site repository; andC physical stabilization of the existing crib wall at the Robert Emmet waste rock pile.

Resurrection Mining Company (Resurrection) has reviewed the remedial alternatives described in the OU6Proposed Plan from the perspective of the expected reduction that will be achieved in the metal loadingcurrently being contributed by Stray Horse Gulch to lower California Gulch and ultimately to the ArkansasRiver. EPA previously completed considerable response actions in OU6 for mill tailing and selected minewaste rock piles. It is unlikely, however, that the preferred alternative described in the OU6 Proposed Planwill achieve any further measurable reduction in metal loading from Stray Horse Gulch, as the alternativeessentially maintains existing conditions. Resurrection suggests that the EPA consider remedial actionsthat provide further reduction in metal loading being contributed by Stray Horse Gulch.

The Focused Feasibility Study for OU6 estimated a 90% reduction in zinc and cadmium loading at thedownstream boundary of OU6 (Stations SHG-09A and SD-3) between 1995 and 2001 (OU6 FFS, Figure4-5 and 4-6). Most of the ARD-generating mine wastes in OU6 have already been addressed throughsource control measures implemented during eleven time-critical and non-time critical removal actionsperformed between 1990 and 2001. Additional reductions in metal loading through source controlmeasures are expected to be small. Nevertheless, the selected remedy includes the physical removal ofover 10,000 cubic yards of ARD-generating mine waste (Ponsardine waste rock pile).

Dewatering of the mine pool is a major component of the selected remedy. This portion of the remedy isintended to further reduce metal loading to surface water through reducing discharge of contaminatedwater at springs and seeps and from bedrock to alluvium and ultimately to surface water. Theseimprovements are expected not only in OU6 but also in other OU's.

Further remedial action in OU6 would be implemented under OU12, Site-Wide Surface and Groundwater.

12. Metal Loading in Lower Stray Horse Gulch - Based on past evaluation of surface water quality data forOU6, a significant increase in metal loading and decrease in pH was observed in the reach of Stray HorseGulch from approximately the Robert Emmet mine downstream to below Hamm's Mill Tailing Impoundment,as represented by the water quality data collected at sampling station SHG-09A. Previous response actionscompleted by EPA in the drainage area contributing to this reach were beneficial in reducing metal loading.However, based on initial review of recent water quality data collected at SHG-09 A, this reach continues tobe a source of metal loading. Specifically, as identified during the public meeting of May 1, 2003, evidencesuggests the Robert Emmet waste rock pile is contributing metals to Stray Horse Gulch. The proposed plandoes not propose any response actions that would result in any further reduction in metal loading in thisreach. The previously completed surface water management activities, which intercept and transfer waterinto the Marion Shaft, are in areas upstream of this reach. Resurrection suggests that EPA evaluate theRobert Emmet waste rock pile and other potential sources of surface water metal loading and consideradditional remedial actions to mitigate the loading sources in this lower reach of Stray Horse Gulch.

Available data are insufficient to determine the magnitude of the metal load contribution from the RobertEmmet waste pile, if any. A large volume of ARD-generating mine waste exists between surface watermonitoring stations SHG-08 and SHG-09A. The vast majority of these wastes have been remediatedthrough in- place capping and surface water management including collection and treatment of ARD.

The Emmet waste rock pile may be contributing to metal loading in this teach of Stray Horse Gulch.However, grouting and stabilization of the Emmet crib wall is expected to achieve some reduction in massloading to surface water. Water quality monitoring of this portion of Stray Horse Gulch will be needed toassess the degree of loading reduction.

13. Location of Proposed LMDT Bulkhead in Relation to Pendrv Fault - The specific location of the proposedbulkhead in the LMDT is not identified in the Proposed Plan or in the Focused Feasibility Study. Severalfaults intersect the LMDT including the Pendry Fault, which has been shown to provide a groundwaterflow pathway to California Gulch alluvium under high hydraulic head conditions in the LMDT. Risinggroundwater levels in California Gulch alluvium have been attributed to the migration of groundwater flowalong the Pendry Fault upon increased head in the LMDT. Based on mapping provided in DraftGroundwater Hydrology in the Vicinity of the Leadville Mine Drainage Tunnel, Operable Unit 6 andAffected Areas ( EPA, August, 2002), the Pendry Fault crosses the LMDT approximately 5,000 feet from theportal. Information provided in the Focused Feasibility Study suggests that the bulkhead be locatedbetween 4,000 and 6,000 feet from the portal. Resurrection recommends that the bulkhead be located incompetent rock upgradient of the Pendry fault to minimize the potential for groundwater flow to migratefrom the LMDT mine pool via the Pendry Fault to the California Gulch Alluvium.

Information should also be presented by the EPA regarding management of the. mine pool behind theproposed LMDT bulkhead. The management plan would define the normal operation parameters, includingthe proposed operating head behind the bulkhead. As part of the remedial design for the selected OU6remedy, EPA should develop a management and contingency plan for conditions that may potentiallyexceed normal operating parameters.

The location, design, normal and contingency operation and method of construction for the proposedbulkhead will be determined during remedial design. The placement of the bulkhead upstream of thePendry Fault is one of several considerations when selecting a suitable location. EPA agrees that it isdesirable to place the bulkhead upstream of the Pendry Fault.

14. Existing Surface Water Management Activities

a) As described in the Proposed Plan, the preferred alternative includes the continuation of the collection of runoff and seepage from mine waste rock piles within a portion of the OU6 area and delivery of the water to the LMDT via the Marion Shaft for eventual treatment at the BOR's water treatment plant. The details of the collection and delivery systems are not described in the Proposed Plan.

Details of the collection and deliver system is provided the Final Phase II/ III Removal ActionCompletion Report (CDM, 20001) and summarized on Figure 4-3 of the OU6 FFS.

b) The long- term effectiveness of the existing retention and overflow system is obviously dependent onthe criteria used in the design of the storage capacities of the basins and the sizing of the conveyance features. Presumably, flow in excess of the design criteria will return untreated into Stray Horse Gulch.

The design of collection and delivery systems to Marion Pond are consistent with Work AreaManagement Plan criteria. Construction was designed to withstand flows resulting from 100-year, 24-hour and 2-hour storm events.

c) The Focused Feasibility Study states that EPA was forced to siphon water from the retention basinsduring the Spring 2000 due to inadequate sizing of the basins. In the Focused Feasibility Study, an estimateof the water volume and peak flow discharged during 2001 from the retention basin into the Marion Shaft isused for conceptual sizing of remedial alternatives. The data collected in 2001 was only for part of the yearand runoff conditions in 2001 were below average. Sizing of facilities based on partial data from one belowaverage year is highly uncertain.

Diversion of Greenback Pond overflows to the Marion Pond and finally to the LMDT via EmmetShaft mitigated the inadequate sizing of the basins in that ARD collection area. The FFS used anestimate of 2001 flows in order to develop comparable cost estimates among the variousalternatives involving water treatment. Given Alternative 2g employs underground mineworkings to store collected ARD, the ability exists to collect, store and treat ARD in excess of the2001 flows. The actual capacity of the remediation system will be determined during remedialdesign.

d) The design criteria for the existing water management system or the preferred alternative are not specifiedin the Proposed Plan or in the Focused Feasibility Study and it is unclear whether the existing system willbe upgraded as part of the preferred plan. EPA should provide additional information with regard to thedesign and operation of the current and proposed water management systems in order to allow a thoroughevaluation of the proposed alternative.

See response to comment nos. 12b and c.

e) The long- term effectiveness of the surface water management system is also dependent on the hydraulicconnection between the Marion Shaft and the LMDT, which is used to convey the OU6 surface water tothe LMDT mine pool for eventual treatment at the existing BOR plant. EPA commissioned a study toevaluate the connection as described in the Draft Groundwater Hydrology in the Vicinity of the LeadvilleMine Drainage Tunnel, Operable Unit 6 and Affected Areas (EPA, August, 2002). The results of that studyindicate a hydraulic connection exists between the Marion and Robert Emmet Shafts and the LMDT portal,but that only limited discharge occurs from the mine pool impounded in the Robert Emmet Shaft to theLMDT portal. As such, the EPA should include a long- term comprehensive groundwater monitoring anddetection program as part of the preferred plan to ensure that the OU6 water delivery system and the LMDTremains functional.

A long-term comprehensive groundwater monitoring and detection program to ensure that theOU6 water delivery system and the LMDT remains functional will be developed during remedialdesign.

f) Under the preferred alternative, the Marion Shaft will receive water overflow from only three of the eightretention basins constructed by the EPA in OU6 according to information presented in the FocusedFeasibility Study. Any water overflow from the five additional retention basins (including Highland Mary,New and Old Mikado, Adelaid/Ward group, and Stray Horse Gulch at Robert Emmet) will presumably reportto Stray Horse Gulch. Periodic overflows from these ponds would potentially affect water quality in StrayHorse and California Gulch. Again, the design criteria or as built information for the existing watermanagement systems are not identified in the Proposed Plan or in the Focused Feasibility Study. EPAshould provide the design criteria for these ponds and address the long- term management of potentialoverflows from these retention basins as well as the potential impact to shallow groundwater resulting fromimpoundment of contaminant for extended periods.

The watershed served by the three retention basins mentioned in the comment is the largest ofthe water management areas in Stray Horse Gulch. While it is likely the remaining retentionbasins will overflow in the future, the frequency and volume of such overflows is expected to befar less than occurs on a regular basis at the Greenback pond. The decision to exclude theremaining retention basins from the Marion Pond collection system was based on:

C The low frequency and magnitude of anticipated overflows from the remainingretention ponds.

C The difficulties in plumping multiple remotely located basins to a commondischarge point.

Also, see response to comment nos. 12a, b and c.

15. Water Treatment Cost for the Preferred Remedy - In reviewing the estimated costs presented in theProposed Plan and Focused Feasibility Study for the preferred alternative, it appears that the treatmentcosts may have been underestimated. The proposed Plan indicated that continued treatment of OU6 watermay require an expansion of the BOR treatment plant. The Focused Feasibility Study states that the existingcapacity of the BOR treatment plant is insufficient to treat the volume of water estimated for this alternativean that significant plant upgrades would be required. However, costs associated with expansion of the BORtreatment plant have not been included. The cost of water treatment for the preferred alternative should bemore fully evaluated by the EPA, as the cost for this alternative may be significantly greater than thatpresented in the Proposed Plan.

Alternative 2g may be implemented using only the current excess capacity at the BOR's facility. Thevolume of collected ARD during the 2001 runoff season was estimated to be 9,600,000 gallons. Theestimated 50 gpm of existing excess capacity of the BOR's facility equates to 26,280,000 gallon per year.Therefore, it will be possible to dewater the mine pool and treat collected ARD without expansion of theBOR's facility. More rapid dewatering of the mine pool (if desirable) may require plant expansion.

16. Relocation of the Ponsardine Waste Rock Pile - As part of the preferred alternative, the Ponsardine wasterock pile in Little Stray Horse Gulch will be relocated to a proposed on- Site repository. As described in theFocused Feasibility Study, the Ponsardine waste pile was previously identified as a candidate for remedialaction due to potential ARD- generation, high lead concentrations, and potential degradation todowngradient water quality. Given the limited information presented in the Focused Feasibility Study andthe Proposed Plan, including the apparent absence of data demonstrating an impact to surface waterquality, relocation of the pile does not appear to be cost effective. The only other alternative described forthis pile in the Proposed Plan was in- situ chemical stabilization. Resurrection recommends that the EPAevaluate other alternatives for the pile, specifically in- situ stabilization by placement of a cover. It isanticipated that the pile could be regraded and covered with a relatively thin cap ( earthen, or possibly rockin conjunction with a geosynthetic liner) to achieve the remedial action objectives. Placement of an 8- footthick cover, as was completed during the previous response actions for consolidation and capping ofselected waste rock piles, does not appear to be warranted based on specific site conditions. In- situstabilization would eliminate the need to excavate and transport the waste rock, and to construct arepository.

EPA has assessed in place capping and water management for the Ponsardine, and determined removalof the pile to be the most effective due to the location of the pile adjacent to 7th Street and the lack ofadequate space for capping or water management techniques. The proposed on- Site repository isintended to receive Superfund wastes from many sources including treatment plant sludges, residentialsoils, and other wastes. Therefore, an alternative remedy for the Ponsardine waste rock pile would notpreclude the Repository.

Comments from Peter G. MoIIer

17. I have studied the " Proposed Plan for Operable Unit No. 6 of the California Gulch National Priority ListSite" and would like to indicate my support for the " Preferred Alternative" 2g and 4b. I was glad to see thatthis alternative " will ensure that all the prior source control measures are maintained and that treatment ofcontaminated runoff and seepage continues indefinitely." I would think that maintenance of existingsediment ponds and ditches diverting clean water around contaminated sites would be a concern.

Comment Acknowledged

18. I do have a couple of questions. First, where will the planned site- wide repository be located?

The Black Cloud tailing impoundment is the most likely candidate location for the on-Site repository.

19. Second, in what ways have you been able to utilize results of the revegtation plots on pyritic waste rock atthe "Denver City" mine site? I believe you have a copy of the report I prepared on the project, which waspublished in June, 1996 " Proceedings: High Altitude Revegetation Workshop No. 12". I have spoken withKarmen King of the Natural Resource Management program at CMC here in Leadville about the importanceof having the NRM program continue with the monitoring of the plots and expanding upon the project. Iwould be interested in getting your feedback.

EPA will continue to support Colorado Mountain College's monitoring of the plots in anyway possible.EPA recognizes the importance of long-term monitoring of the plots to determine the effectiveness ofestablishing vegetation in high metal soils.

Comments from James Tiffany

20. I favor alternative 4, chemical stabilization of the Ponsardine and Emmet waste piles plus stabilization of thecrib wall on the south side of the Robert Emmet. This plan maintains a bit of the " flavor of the miningcamp" and is a low cost solution. Monitoring over a relatively short time should indicate if this solution issuccessful. If not corrections could be implemented.

Chemical stabilization has several drawbacks including:

• Disturbance of the crusted surface of the waste pile by track- mounted equipment used to injectthe proprietary agent. This can lead to subsequent erosion.

• Unknown effectiveness of the stabilization process. EPA is reluctant to perform a demonstrationproject at a time when the final remedies should be completed. The prospect of having toimplement a contingency remedy at a later date is undesirable.

• The stabilization process (if effective) would not prevent mechanical erosion of the mine wasterock during storm events and snow melt.

21. It is obvious that the BOR and Yak tunnels need to be kept open and draining. Bulkheading and floodingmine workings above the tunnel levels will cause them to collapse much quicker than if they were freedraining. It also makes repairs to the tunnels difficult and costly at best. Seasonal storage would be betteraccommodated in surface ponds. Consider the problems caused by multiple caved sections with waterbacked up behind each. Alternative 2a will surely collapse the Yak at the point where water enters thetunnel.

Alternative 2g does not address the Yak tunnel (the commenter mentions the Yak Tunnel).

The LMDT is not believed to currently be open and free draining. Suspected blockages currentlyimpound water in flooded mine workings. The intent of Alternative 2g is to install the bulkhead incompetent rock and to dewater the mine pool upstream of the bulkhead. This approach would dewatermine workings that currently are flooded. Seasonal storage of collected ARD in surface ponds as severaldisadvantages including:

• Eliminates the portion of Alternative 2g intended to mitigate the adverse environmental impacts ofthe mine pool.

• Increases the likelihood of discharge of untreated ARD during high runoff years. It is impracticalto construct an impoundment to contain runoff during very high precipitation conditions.

• A large impoundment would be visually unattractive and would result in significant evaporativelosses requiring mitigation.

22. I also favor option 2g due to the bulkhead proposal. In locations where mining occurred below the tunnellevel; would it be possible to install a well or wells drilled from the surface into these areas and dewater themine pool? If it were possible to discharge the wells directly into the BOR tunnel the power required wouldbe far less than pumping to the surface. This might permit the mine pool to be used for storage below thetunnel level. Options 2b, 2e and 2h are viable. I am assuming that long term operating and maintenancecosts, including tunnel maintenance, are included in the present worth numbers. Without tunnelmaintenance all systems fail to be viable.

The LMDT was designed to drain mine workings at or above its level. It is unlikely that pumping from asingle mine working below the level of the LMDT would affect a large area. In addition, discharge ofpumped water to the LMDT would require the well to be vertically aligned with both the LMDT and thedeeper flooded mine working. This scenario is unlikely and would be difficult if not impossible toimplement.

23. My choice for the use of the LMDT is a variation of 2g. Pipe the contaminated water from the Marion Shaftarea to a new dedicated treatment plant or to the BOR plant. Keep the LMDT free draining and perhapspump ground water from below the tunnel level.

See response to comments 21 and 22.

Comments from Bob Calder:

24. My first choice would be Alt. 1, but we all know that will not happen.

Comment Acknowledged

25. Alt. 2: 1 think that the most critical point now would be to have ASARCO Calif, gulch treatment plant,working and on line, to aid the LMDT in the volume of water they are unable to handle. Any and allchances of water from Iron Hill should be sent out the Yak Tunnel. The LMDT treatment plant should beupgraded itself as opposed to a shaft, bulkhead, and miles of pipeline. When the metals from Iron Hill mightbetter be processed and treated at the Yak plant. Possibly selling reduced materials. Pipelines to the newbulkhead/ Shaft, will cause eco- damage that will take a lifetime to recover. Motorized vehicles now also goeverywhere, and they will follow these work areas and make more problems. I would please ask that allattempts to upgrade the LMDT plant be a first consideration, along with more thoughts towards drainagetowards the Yak Plant.

The selection of the BOR's Water Treatment Plant were driven by costs. Water treatment costs aresignificantly lower at the BOR's facility than at the Yak water treatment facility. In addition, treatment atthe Yak facility would require a new pipeline (either to the plant itself or to a location where water couldbe introduced into the Yak Tunnel). The installation of the bulkhead in the LMDT is to facilitate thedewatering of the flooded mine workings Treatment plant sludge does not contain economicallyrecoverable metals.

26. Alt. 4 I am absolutely opposed to the removal of the Posardine Mine pile. The contamination of water, fromthis pile, and down 7th street will not cease because of the removal of this pile. Next, it will come again fromthe east of the Pons., and then east of that etc. etc.. I also believe that the Ponsardine dump, truly givesvisitors to Lake County of the great mining history we have, as the road leads into the mining district. Otherpiles that were removed a few years ago, took away much of Leadville's unique mining character. The Pons.removal will just take away more history. I have designed an idea that I believe will keep the Pons dumpintact, and also filter the runoff from this area, as well as historically and aesthetically appeal to all. I also

have drawings that will aid in my proposal.

The idea is a drain along the toe of the dump, and 7th street, to a settle pond on the west of the pile, whichwill also catch any seepage from the old rail grade. The drain will be along the idea of a french drain, ie. toallow seepage to the pond. This pond will be excavated to the west, N. west of the main dump, and into thehillside south if necessary. Along with a small patch of existing aspen trees and a few lodgepole pines, theplan asks for a crib wall to hide the drain system and settle pond. I believe that this might look very good,and serve well the intended purpose. I have done an artist's rendition of what I think it will look like. The topof the Pons. dump will also be graded lightly to drain towards the shaft subsidence.

There could be a possibly be a core hole drain, easterly, looking towards the Hayden workings which havea connection to the LMDT. I really like this idea, and although I am not an engineer, I worked in and aroundmines for many years, and I think it can be done, and done historically appealing.

As mentioned previously, due to the lack of space between the County road and the toe of the pile in-place stabilization and surface water management features would be difficult to implement.

Comments from Bob Elder

27. Both the EPA and State of Colorado have chosen to combine Alternatives 2g and 4b as the preferred actionfrom among the several alternatives evaluated. I believe that this selection is the correct one, as it willsatisfactorily address the long-term water quality issues remaining in Operable Unit 6.

Comment acknowledged

28. Alternative 2g - Install Bulkhead in LMDT and Dewater Mine Pool, with Gravity Pipeline to BOR TreatmentPlant.

This alternative uniquely includes the important goal of lowering the level of the presently elevated waterpool in the mine workings beneath Fryer Hill, Carbonate Hill and Graham Park. Implementation of thisalternative will greatly lessen the ability of the metals laden water in the elevated pool to migrate intoadjacent ground water resources. It will also eliminate the possibility of a catastrophic release of impoundedmine water from the caved LMDT.

Alternative 2g will eventually eliminate the unavoidable mixing of contaminated mine water and cleanalluvial ground water in the discharge from the LMDT. At the present time, pumping of the mixture of minewater and alluvial water from the shallow well system near the LMDT portal draws down the level of alluvialwater in the area.

When Alternative 2g allows discontinuance of this well pumping, restoration of the local alluvial water tablemay significantly increase the domestic water flow that issues form Parkville Water District's nearbyCanterbury Tunnel portal.


29. There is a potential maintenance problem to be addressed under Alternative 2g. This is in regard to theplanned use of a dewatering well and gravity pipeline to deliver mine water to the BOR treatment Plant atLMDT portal. We recognized at the Climax mine that the continued pumping of heavy metals laden watertended to build up hard deposits in the pump columns and delivery lines involved.

The build up of hard deposits stems from the air entrained in the water during the pumping process. Thecontained oxygen tends to precipitate the iron and manganese sulfates contained in the acid mine drainage(AMD) as the basic sulfate, jarosite. The jarosite deposits reduce and constrict the cross section of the

pipelines, and can completely block water flow if neglected.

Timely inspection of the lines and removal of the hard deposits with rotary cutters will be necessary toinsure delivery of water through the planned system. Providing sufficient manholes along the gravity line toallow equipment access to pipe runs of workable lengths will be important for long- term maintenance of thesystem.

Alternative 2g includes the cost of a pipe pig and launcher for regular removal of scale.

30. Alternative 4b - Relocation of the Ponsardine waste dump to the on-Site repository and physicalstabilization of the retaining wall on the south side of the Robert Emmet waste dump.

Alternative 4b will certainly be a more compatible solution to the Ponsardine and Robert Emmetenvironmental conditions than the alternative actions studied. The other actions considered included eitherin- situ chemical stabilization or consolidation and capping of the waste deposits.

hi situ chemical stabilization is not only expensive to implement, but is also uncertain in its effectiveness ineach specific application. Consolidation and capping of the waste deposits requires a large capitalexpenditure. It also has the unfavorable effect of greatly altering the historic aspect of the Mining District.

The existing historic landscape is highly prized not only by the local citizens but also by Lake County'stouring visitors.

The Robert Emmet dump cribbing has not been repaired or replaced since the mine plant was lastreconstructed by ASARCO in 1951- 52. Stabilization of the deteriorated cribbing will prevent future collapseof the retained dump material down slope onto County Road 1. Possible exposure of additional mineralizedmaterial to run off water will thus be avoided.

Removal of the Ponsardine waste material to the planned on- Site repository will eliminate the sulfide-bearing waste rock presently being subjected to run off water in Little Stray Horse Gulch. The sulfidebearing material may prove to be mainly at the surface of this waste deposit, and the underlying portion ofthe dump may consist of surficial moraine and unmineralized bedrock placed on the dump in the first stagesof the shaft operation.

In any case, the Ponsardine dump will furnish a considerable volume of coarse waste rock that canbeneficially layered and mixed with the water plant sludges to be routinely placed in the on- Site repository.


31. The Proposed Plan does not specify a preferred site for the Site- Wide Repository. I understand that theexisting Black Cloud tailing pond is being considered for use as this repository.

It would be a serious mistake for the EPA and the State of Colorado to become responsible for the integrityof this tailing pond as a result of the repository use. The Black Cloud pond has severe environmentalproblems that will only become worse as time passes.

I was very familiar with the surface area now underlying the pond prior to 1970, when the pond constructionwas commenced. The area consisted entirely of a deep wetland, and the water feeding the area was notdrawn from Iowa Gulch Creek. Instead, the wetland was well fed from springs and seeps contained in thelandslide topography to the immediate south, on the steep north- facing slope of Long & Derry Hill.

The springs and seeps are associated with the Weston Fault branches exposed on Long & Derry Hill Thefault branches connect with the extensive manganese mineralization present in the Leadville dolomite. Theblack manganese oxide mineralization can be seen hi outcrop on the cliff further west along the Long &Derry ridge, and also is evident on the dumps of the old Doris and Frank mine workings down dip to theeast beneath the prominent rock glacier.

This prevalent manganese mineralization accounts for the high manganese content of the water issuingfrom the springs and seeps, and has necessitated the special effort of the Black Cloud water treatment plantto lower manganese content of the plant effluent by employment of liquid oxygen.

The wetland water must account for most of the flow that exits at the western toe of the tailing dam. Unlessan extraordinary effort to contain this flow is undertaken, such as a cofferdam, it will probably continue inperpetuity.

The landslide in contact with the south abutment of the tailing pond dam is a continuing threat to theintegrity of the pond. This landslide extends several hundred feet up the steep slope of Long & Derry Hill.It has been actively moving in recent time. This is evidenced by the pronounced slant of the still livingevergreen trees rooted in the slide. They stand at all angles from vertical to horizontal, a result of thegradual descent of the slide.

The landslide material is saturated with water, and crossed by many collapse fissures. Continued downwardmovement of the slide is exerting lateral pressure upon the south abutment of the tailing dam. At sometimein the future, this pressure will be relieved by a failure of the west face of the dam. The dam is constructedof deposited tailing sand and faced with Rock, and does not have the structural cohesion necessary toresist the lateral pressure.

The Black Cloud Mine (BCM) operator installed a toe- drain system to dewater the tailing dam. Thisdewatering system was built in part to address the seeps associated with-the Long and Derry hillside, andis a major source of the seep water historically treated by the Black Cloud facility.

The Black Cloud operator and DMG also regularly monitored the movement of the hillside south of theimpoundment during the course of tailing pond operation. The hillside monitoring did indicate smallamounts of lateral and vertical movement. The tailing dam did not show signs of movement, cracking,buckling or other adverse local conditions. The hillside monitoring was performed to predict hillsidemovement that might have caused displacement of tailing pond water into Iowa Gulch. There was noindication of tailing dam failure associated with hillside creep.

The Administrative Record indicate that the tailing dam is an earthen dam constructed of rolled earthand rock fill with centerline raises constructed as necessary. The dam was analyzed for slope stabilityunder both static and dynamic loading conditions as well as a Probable Maximum Flood Event analysis.The extensive evaluation of the tailing facility required under DMG closure requirements revealed noshortcomings in the long-term structural performance and integrity of the impoundment.

As a result, EPA will require the following items, should the BCM tailing impoundment be selected as the On-SiteRepository.

23. Evaluate failure due to rock slide during design of the on-Site repository.

24. Re-institute hillside monitoring and inspection as part of the on-Site repository operation andmaintenance.

EPA appreciate Mr. Elder's comments because they will keep the public attention focused on safe design andmonitoring systems,