EXPLANATION OF SIGNIFICANT DIFFERENCES

RECORD OF DECISION (ROD) - CAUFORNIA GULCH/YAK TUNNEL

DECLARATION S

Considering the new Information that has been developed and the changes that have been made to the

selected remedy chosen in the original ROD of March 29, 1988 as modified on March 23, 1989, EPA has

determined that the remedy remains protective of human health and the environment, complies with Federal

and State requirements that are applicable or relevant and appropriate for this initial remedial action, except

those for which an interim action waiver was invoked in the original ROD, and is cost-effective. In addition,

the revised remedy utilizes permanent solutions and alternative treatment technologies to the maximum extent

practicable for this Site. ADMINISTRATIV E RECOR D

FIL E PLAN

ames J. Scherer

Regional Administrator

0.433.41

Date /
FINA L

CLEANUP OF DISCHARGE FROM THE

Y AK TUNNEL OPERABLE UNIT

CALIFORNIA GULCH SIT E

LEADVILLE, LAKE COUNTY, COLORADO

EXPLANATION OF SIGNIFICANT DIFFERENCES

OCTOBER 1991
OVERVIEW

Thi s is the final Explanation of Significan t Differences (ESD) which explains the U.S.

Environmental Protection Agency's proposed changes in the clean-up plan for the Yak Tunnel Operable

Unit of the California Gulch Superfun d site. Thi s document explains the significant differences between

the remedy outlined in the 1988 Record of Decision (ROD), which was modified in March of 1989, and

the remedy proposed herein.

Public Involvement Opportunities

T he Administrative Record, which contains the complete documentation for the site and additional

copies of the 1988 ROD, as well as the 1989 and 1991 ESDs, is available for public review at the followin g

location:

Lake County Library

115 Harrison Avenue

Leadville, Colorado 80461

(719) 486-0569

Colorado Mountain College Library

Colorado Mountain College

Timberline Campus

Leadville, Colorado 80461

(719) 486-2015

EPA Superfun d Records Room

999 18th Street, 5th Floor

Denver, Colorado 80202-2405

(303) 293-1807

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Revision 2 October 1991 Pagel
INTRODUCTION

T he California Gulch Superfun d site is located in Leadville, Lake County, Colorado (see Figure 1). The

study area for the site includes the California Gulch watershed and the area drained by the Yak Tunnel. In

order to make the site more manageable it has been divided Into two operable units. The first Operable Unit

addressed by ERA, and the subject of this ESD, is the Yak Tunnel. The remainder of the site is currently under

investigation and may result in the definition of additional operable units.

The Yak Tunnel extends approximately 3.5 to 4 miles beneath the mountains southeast of Leadville,

Lake County, Colorado, and dewaters numerous mines. The discharge from the Yak Tunnel is contaminated

with acid mine drainage containing high concentrations of heavy metals, including cadmium, copper, lead and

zinc. These concentrations are affectin g aquatic life in the Arkansas River, and pose a potential risk to human

health and the environment. Consequently, the site was placed on the Environmental Protection Agency's

(EPA) National Priorities List (NPL) for cleanup under the Comprehensive Environmental Response,

Compensation, and Liability Act (better known as CERCLA or Superfund) . Under the Superfun d law, EPA is

charged with the responsibility of developing and implementing dean-up remedies that protect human health

and the environment.

After study and evaluation of the first operable unit, EPA issued a Record of Decision (ROD) in March

1988 describing the remedy chosen to dean up the Yak Tunnel discharge. The remedy selected was

subsequently modified because additional Information caused EPAto question the initial clean-up plan. These

changes were contained in EPA's March 1989 Record of Decision Modification, which described the differences

between the remedy proposed In the ROD and the remedy to be implemented at the site.

The ROD Modification provided greater protection of human health and the environment through

collection of contaminated water between the portal plug and second plug. Data indicated that contaminated

water from the Yak was seeping into the ground water system near the mouth of the tunnel. The interim

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SCALE OF MILES

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treatment system was changed to a permanent treatment system to treat flow collected from the tunnel

(between the flow through portal and second plugs).

In August 1988, EPA issued an Administrative Order to several Potentially Responsible Parties (PRPs)

requiring these parties to implement an interim remedy, including construction of the Surge Pond and a

filtration plant. The PRPs began construction in 1988 and completed most of the construction associated with

this interim remedy during the 1989 construction season. The filtration plant was commissioned during

December 1989 and is currently treating the discharge from the Yak Tunnel.

In March 1989, EPA issued an Administrative Order (AO) to several PRPs requiring these parties to

implement the modified remedy. During preparation of plans to implement the remedy, the PRPs expressed

concern with the modified remedy, primarily with the risk of uncontrolled and undetected migration of

contaminants in ground water due to tunnel flooding. Thereafter, the PRPs submitted to EPA an alternative

proposal for site cleanup, entitled Selected Remedy Component Change Proposal (Res-ASARCO Joint Venture, March 1990). After review of these additional considerations, EPA proposes to make further revisions to the

remedy. Followin g are the basic components to the proposed revision: (1) construction of a surge pond; (2)

installation of a water treatment plant; (3) construction of a flow control bulkhead within the tunnel; (4) sealing

of inflow into the tunnel; (5) installation of weirs In the tunnel; (6) periodic inspection of the Yak Tunnel;

(7) installation of a surface and ground water monitoring network; (8) development of a contingency plan; and

(9) operation and maintenance of the remedy. The proposed changes to the remedy will better control groundwater flow and minimize movement of contaminated ground water away from the Yak Tunnel through continued

use of the Yak Tunnel as a drain for the mine workings and surrounding area.

Under Section 117 of CERCLA, as amended by the Superfun d Amendments and Reauthorization Act

of 1986 (SARA), EPA is required to develop and publish a ROD modification or an explanation of Significan t

Difference when changes to the ROD are proposed. A ROD modification Is required when fundamental

changes are proposed to the selected remedy. EPA Is required to publish an ESD when significant, but not

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fundamental , changes are proposed to the previously selected remedy. The proposed changes reflect the

rationale presented in the original proposed plan developed prior to the ROD, and therefore, do not constitute

a fundamental change to the remedy. In addition, significant public participation in the past has fully supported

the proposed changes to the remedy.

The proposed remedy is presented as an ESD because the scope of the remedy is not significantly

changed: the remediation goals will remain the same as will the physical area of the response; the treatment

methods and the contingency remains the same. The proposed remedy would Increase protectiveness of

human health and the environment through reducing the potential for migration of contaminated ground water

away from the Yak Tunnel. This protectiveness is obtained by allowing free flow through the Yak Tunnel

(removal of the plugs except for the flow control bulkhead at the portal), and permanent treatment of the Yak

Tunnel flow Into perpetuity.

The purpose of this document Is to describe the remedial action to be undertaken for this operable unit

and explain the significant ways In which this proposal differs from the remedy selected by EPA in 1988 and

subsequently modified In 1989. This document also provides a brief history of the site.

This ESD presents only a synopsis of Information on the site. For more Information on the site or the

previous ROD Modification issued in 1989, the reader may wish to refer to the Information repositories at the

Lake County Library, Colorado Mountain College Library, Timberiine Campus, and the EPA offices in Denver.

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SUMMAR Y OF SIT E HISTOR Y AND CONTAMINATION PROBLEMS

Site History

T he California Gulch site is located in Lake County, Colorado. The study area includes the California

Gulch watershed and the area drained by the Yak Tunnel. The site Is within the historic Leadville Minin g District

where significant quantities of gold, silver, lead, zinc, manganese, and copper have been mined since the

1860s. Hundreds of mines, many mills, more than 40 smelters and several placer operations nave been active

in the area. As a result, numerous slag piles, tailings ponds and abandoned mine, mill, and smelter sites are

found within the study area.

Mining activities in the Leadville area began with placer mining of gold-bearing deposits along California

Gulch In 1859. The placer deposits were quickly exhausted and underground mining of lode veins in the upper

California Gulch area began. Silver-laden lead carbonate ore was discovered In the area, and silver mining

began in 1875. A number of mills and smelters were built to process the mined ores and the town expanded

considerably. Underground mining was extensive and mine drainage tunnels were built as a way to dewater

these workings. Construction of the Yak Tunnel began in 1889 and It was extended a number of times to the

present length of between 3.5 and 4 miles.

Near the turn of the century lead and zinc mining and ore processing also began in the area. Minin g

of all of these metals has followed a cyclic pattern since, with extensive activity when metals prices were high

and very little mining when the metals prices did not Justify the mining costs. Current mining activities in the

area are limited to the Black Cloud Mine In Iowa Gulch and several other small mining ventures. Total metal

production from the Leadville District from 1859 through 1966 totaled about 24 million tons. The principal

metals were silver, zinc, lead, gold, and copper.

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In 1982 and 1983, EPA conducted a preliminary evaluation of the site, consisting of a site inspection

and review of existing data. The site was placed on the NPL in 1983 for cleanup under the Superfun d program.

The Superfun d law requires EPA to identify those who have either contributed to the contamination at the site

or who own contaminated property. These persons or companies, called Potentially Responsible Parties

(PRPs) may be responsible for conducting dean-up activities and paying clean-up costs. Seven parties were

identified as PRPs for the site contamination based on ownership or operation of mining or mineral processing

operations in the area. These companies were notified in the spring and summer of 1983 that they were

considered potentially responsible for the release of hazardous substances, pollutants, and contaminants at

the site. They were all offered opportunity to participate voluntarily in a response action.

T he Phase I Remedial Investigation (Rl) of the site, including California Gulch and the Yak Tunnel, began

in 1984. The Phase i Rl report was released in May 1987 (EPA, 1987a). In the spring of 1986, while the Rl was

In progress, EPA Identified and notified six additional parties that they were considered PRPs.

A Feasibility Study (FS) report was released by EPA in Jun e 1987, and in August 1987, a proposed

Remedial Action Plan for the Yak Tunnel Operable Unit was released. EPA held a 90-day public comment

period and a public meeting to provide an opportunity for the public to review and comment on these plans. A ROD, dated March 1988, was Issued detailing the selected remedy for the Yak Tunnel Operable Unit. The

ROD specified the components, procedures and goals for cleanup of the Yak Tunnel discharge in this operable

unit. Future RODs will specify components, procedures and goats for cleanup of other operable units within

the site, possibly Includin g soils, ground water, and surface water.

In August 1988, EPA issued an AO to ASARCO Inc., Res-ASARCO Joint Venture, and Resurrection

Mining Company requiring these parties to construct the surge pond portion of the Yak Tunnel remedy. The

surge pond construction is essentially complete at the time of the writing of this draft ESD. In March 1989, EPA

modified the ROD and issued a second AO to ASARCO Inc., Newmorrt Minin g Corporation, Res-ASARCO Joint

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Venture, and Resurrection Minin g Company for implementation of the remedy set forth in the ROD Modification. This remedy is under way, and Is the subject of this ESD.

Summary of Contamination Problems

ERA studies indicate that the California Gulch site is contaminated with heavy metals, including

cadmium, copper, lead, and zinc, coming from both active and abandoned mining and minerals processing

facilities .

The Yak Tunnel is the major contributor to site contamination. The tunnel was designed to provide

drainage, facilitate exploration, and provide transportation routes for removing ore from mines in the Leadville

Mining District It collects ground water from the mines and then discharges this water into California Gulch.

In an average year the tunnel discharges a combined total of approximately 210 tons of cadmium, copper, lead,

zinc and other metals into Califomia Gulch. California Gulch, in turn, flows into the Arkansas River. The tunnel

contributes 75 to 80 percent of the metal contaminants released into the Arkansas River by Califomia Gulch. Metals from the Yak Tunnel have been shown to move through soils, surface water, ground water, and air at

the site.

Cadmium, copper, lead, and zinc are key metals of concern in the Yak Tunnel discharge. The heavy

metals can be harmful to aquatic and terrestrial plants and animals, including humans. Aquatic organisms can

absorb metals. Plants can take up metals from water and soils through their root systems. Domestic animals

and wildlife can drink contaminated water or eat plants (or other animals) that have taken up metals. In turn,

humans can be exposed through Inhalation of airborne contaminants, or ingestion of contaminated water,

sediments, or food.

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The metals of concern can be harmful to animals, aquatic life and humans.

• Chronic exposure to cadmium may result in hypertension and kidney and liver damage in

humans. In low concentrations, it is toxic to freshwater fish. Terrestrial plants can accumulate

cadmium in leafy tissue to concentrations which are dangerous to animals, including humans,

without inhibiting the plant growth.

• Copper is not acutely toxic to humans, but Is one of the most harmful metals to fish and other

aquatic life.

• Long-term human exposures to lead can cause anemia, Intestinal cramps, fatigue, and

neurological damage. Even at very low levels, lead exposure can cause harmful effect s to the

nervous systems of children. Studies on laboratory animals have shown that exposure to lead

can cause cancer and an increase In reproductive complications. Lead is also toxic to aquatic

life.

• Fish, such as rainbow and brook trout, are adversely affected by exposure to zinc. Zinc is

rarely damaging to human health or terrestrial animal health; however, when humans are

exposed to zinc in combination with other metals, the results may be harmful.

Arsenic, iron, manganese, and other heavy metals are also present in the Yak Tunnel discharge;

however, for this operable unit in particular, zinc is the metal of greatest concern at the concentrations of

metals known to be in the Yak Tunnel discharge.

Studies of the Arkansas River below the confluence with California Gulch show that metals

contamination has reduced the capacity of the river to support well balanced aquatic populations. Both the

quantity and variety of fish are reduced in this portion of the river due to contamination from California Gulch.

DESCRIPTION OF THE REMEDIES

Summary of the 1988 Record of Decision

In March 1988, EPA signed a ROD outlining a dean-up plan for the Yak Tunnel portion of the California

Gulch site. The remedy was designed to minimize the flow of water out of the Yak Tunnel and to prevent the

uncontrolled release of tunnel drainage to the environment. Row from the tunnel would be minimized through

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pluggin g the tunnel and treating the water which may escape from behind the plugs. The remedy consisted

of several elements:

Construction of surge ponds to capture drainage from the tunnel and to minimize the impact

of surges on California Gulch and the Arkansas River.

Installation of an interim water treatment system to treat water from the Yak Tunnel before

discharge In California Gulch. Hoodin g the tunnel would have raised the water level depletin g

the available oxygen, thus improving the ground water quality. Treatment would only be

necessary until water quality improved.

Construction of plugs at three locations within the tunnel to stop uncontrolled discharge of

tunnel drainage into California Gulch and to prevent surges.

Sealing of shafts, drill holes and fractured rock and diversion of surface water from tunnel

recharge areas to reduce the amount of water entering the Yak Tunnel system.

Grouting of fractured rock, caved-in areas, and drill holes to prevent seepage of contaminated

water to the land surface.

Establishment of a surface and ground-water monitoring system to detect any leakage, seeps

or migration of contaminated ground water which may result from installation of the tunnel

plugs.

Installation of a pumping system to control water levels behind the portal plug, if necessary, to

prevent uncontrolled seepage. The pumped water would be routed to the interim treatment

system to remove contaminants before discharge into California Gulch. Development and implementation, as necessary, of a contingency plan to address any adverse

effect s on surface or ground water resulting from tunnel plugging.

Operation and maintenance of the remedy.

Summary of the 1989 ROD Modification

The ROD was modified In 1989 because of new information, specifically seepage entering the groundwater system near the portal plug. Therefore, the portal plug was changed to a flow through plug. Also, the

interim water treatment system was changed to a permanent system to treat the water from the flow through

portal plug in perpetuity.

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T he elements of the remedy as modified in 1989 are listed below. The changes from the initially

selected remedy are noted by highlighting, and are discussed within brackets ([ ]).

Construction of a surge pond or ponds to capture drainage from the tunnel and to minimize

the impact of surges on California Gulch and the Arkansas River. [The multiple surge ponds

initially selected were replaced with the possibility of a single surge pond.]

Installation of a permanent water treatment system to treat water from the Yak Tunnel before

discharge in California Gulch. [The interim treatment facility from the Initially selected

remedy was replaced with a permanent treatment system.]

Construction of plugs at three locations within the tunnel to stop uncontrolled discharge of mine

drainage into California Gulch and to prevent surges.

Implementation of water control measures to reduce surface water inflow into the tunnel

system and to minimize uncontrolled ground-water outflow from the tunnel system

followin g plugging, as necessary, for proper performance of the remedy. [Reduction of

seepage and recharge was made optional in the modified remedy, contingent upon the

ability of the water treatment system to successfully treat seepage and recharge. Grouting

of fractured rock, caved-in areas, and drill holes to prevent seepage of contaminated water

to the land surface was deleted from the modified remedy.]

Establishment of a surface and ground-water monitoring system to detect any leakage, seeps

or migration of contaminated ground water which may result from Installation of the tunnel

plugs.

Installation of a pumpin g or drainage mechanism to control water levels behind the portal plug. The pumped water would be routed to the treatment system to remove contaminants before

discharge Into California Gulch. [The portal plug became a flow through plug, rather than

a solid plug. This flow through plug was intended to prevent surges from affectin g the

treatment system, not to flood the tunnel.]

Development and Implementation as necessary of a contingency plan to address any adverse

effect s on surface or ground water resulting from tunnel plugging.

Operation and maintenance of the remedy.

These changes were discussed in detail in the 1989 ROD Modification. The reader may wish to refer

to the 1988 ROD and the 1989 ROD Modification for a more detailed discussion of the remedy selected in 1988

and modified in 1989. These documents are available at the Lake County Library, Colorado Mountain College

Library, Timberline Campus, and the EPA offices in Denver.

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Summary of the Proposed 1991 Remedy

Both the 1988 ROD and the 1989 ROD Modification will be further altered by the remedy proposed In

this Draft ESD upon approval by EPA. Many of the elements have not changed, or have changed in number

or location, but remain fundamentally the same as the elements described in the ROD and ROD Modification. These elements are discussed below in greater detail than the foregoing summaries of the previous remedies

to help the reader understand what the proposed remedy includes. Refer to the section "Basis for Modifying

the Remedy" in this ESD for an explanation of the basis for the changes. In the discussion below, the

components which have changed from previous remedies will be noted by highlighting, and discussed at the

end of the paragraph within brackets ([ ]). The proposed remedy, designed to allow free flow from the tunnel,

through elimination of two of the plugs and replacement of the portal plug with a flow control bulkhead. The

proposed remedy consists of:

• Construction of a surge pond to capture drainage from the Yak Tunnel and to minimize the Impact of surges from the tunnel on California Gulch and the Arkansas River. The surge pond Is proposed to be a permanent part of the remedy. (Construction of this surge pond is

essentially complete at the time of the writing of this draft ESD.)

• Installation of a permanent water treatment system to treat water from the Yak Tunnel before

discharge into California Gulch.

• Construction of a flow control bulkhead within the tunnel to prevent surges. [Two of the three

plugs are proposed to be eliminated from the remedy, leaving the portal plug as a flow

control bulkhead to prevent surges (redundant to the surge pond), and to allow for

operational flexibility within the water treatment facility . The flow control bulkhead is

proposed to be constructed in competent rock, approximately 2,000 feet into the tunnel. The tunnel will be rehabilitated to the extent required for bulkhead placement

(construction) and long-term stability of the tunnel discharge outlet.]

• Sealing of shafts and diversion of surface water from tunnel recharge areas to reduce the

amount of water entering the Yak Tunnel system, as necessary, for proper performance of the

remedy.

• Establishment of a surface and ground-water monitoring system to detect seeps, ground-water

quality and flow direction, and potential gradient reversal resulting in migration of

contaminated ground water away from the tunnel. The gradient reversal may result from a rockfall or cave-in within the tunnel. Thi s monitoring system is proposed to include a

minimum of seven ground-water monitoring wells. [Both the ROD and the 1989 ROD Modification required establishment of a minimum of 23 monitoring wells. Since the

remedy proposed in this ESD eliminates floodin g of the mine workings, the potential for

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migration of contaminants is greatly reduced. This reduces the number and locations of

the monitoring wells needed to detect movement of contaminants.]

• Placement of six or more weirs, or other flow measuring devices, at key locations in the Y ak Tunnel. The weir locations will be selected during an initial Inspection of the tunnel.

• Periodic inspection of the Yak Tunnel. Qualified mining crews will enter the tunnel

annually to inspect and maintain weirs and other structures in the tunnel. Crews will also

enter the tunnel to determine the cause of unexpected increases or decreases in flow

within the Yak Tunnel.

• Development and implementation, as necessary, of a contingency plan to address any adverse

effect s on surface or ground water resulting from tunnel blockage. [Implementation of the

contingency plan will be based upon: 1) decrease In flow from the Yak Tunnel; 2) rise in

water levels in monitoring wells located outside the mine workings; 3) indication of

gradient reversal (flow away from the tunnel); and 4) degradation of water quality as

detected by the monitoring network. A baseline for Yak Tunnel flow, water levels,

hydraulic gradient, and water quality will be established for comparison with future data. T he remedy will also be monitored for seeps by periodic site reconnaissance looking for

surface seeps.]

• Operation and maintenance of the remedy.

Explanation of Significant Differences

The 1988 remedy, as modified by the 1989 remedy, and the proposed 1991 remedy remain

fundamentally the same. The same water treatment processes will be employed. In addition, a flow control

bulkhead and a monitoring network will be constructed. The differences between the 1989 ROD Modification

and the 1991 proposed remedy are:

1. Removal of two of the three plugs to allow free flow from the tunnel, and placement of a flow

through bulkhead at the portal for surge protection;

2. Reduction in the proposed ground-water monitoring system due to the greatly reduced chance

of contaminated water movement into the ground water system;

3. Continued use of the Yak Tunnel as a drain for contaminated water; and

4. Treatment of all Yak Tunnel water, not just water between the portal and second plug.

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The major differences between the 1988 remedy, which was modified by the 1989 ROD Modification ,

and the proposed 1991 remedy include:

1. The nature and design of the plugs to be installed in the Yak Tunnel. A flow control bulkhead

will be installed approximately 2,000 feet Into the tunnel. The bulkhead will control flow from

the tunnel to optimize water treatment facility operation and provide surge protection. The two

remaining plugs will not be installed allowing for free flow from the tunnel.

The decision to not install the two plugs was based upon the reduction of exposure to industrial (miner) safety hazards during rehabilitation of the tunnel and the unknown water quality impacts

that pluggin g the tunnel may have caused.

2. The installation of weirs, or other flow measuring devices, In the Yak Tunnel. These weirs will allow for monitoring of the flow in various reaches of the tunnel.

The exact locations and numbers of weirs will be based upon an investigation of the condition

of the Yak Tunnel and its main laterals. Thi s investigation will determine the relative water

contribution to the tunnel from each main lateral. The investigation will also determine the

condition of fault zones which Intersect the Yak Tunnel and its main laterals. Weirs will be

located to determine baseline flow and to provide immediate warning of sudden Increases or

decreases in flow within the Yak Tunnel or its laterals.

The weirs will serve two purposes:

• The weirs will measure baseline flow in the tunnel. The seasonal variation of flow

from the main laterals may assist in determining sources of surface water Inflow

into the tunnel system. Significan t sources of inflow may then require remedial

actions to reduce the volume of water flowin g to the treatment plant.

• The weirs will provide information on rapid changes In flow in the tunnel. In the

case of rapid increase in flow, flow measurement will assist the treatment plant

operator in protecting the plant from surges. In the case of rapid decrease in

flow, flow measurement will trigger implementation of contingency plans,

including relnspection of the Yak Tunnel.

I f, during the investigation of the Yak Tunnel, EPA determines that conditions in the tunnel will

not allow Installation and maintenance of the weirs, EPA will develop an alternate method for

monitoring the tunnel. This alternate method will be presented to the public prior to

implementation, either In a Fact Sheet or another ESD.

3. The modification of the monitoring network reflects the use of weirs for directly monitoring the

changes In water flow rate within the tunnel. In addition, monitoring wells constructed within

selected faul t zones outside of known mine workings will be used to monitor water level, water

quality and flow direction to or from the tunnel. This monitoring system will determine potential

contaminant movement away from the tunnel. Thi s change also reflects the change In the

floodin g component of the remedy, with a greatly reduced chance of movement of

contaminated water into the ground water system.

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Basis for Modifyin g the Remedy

Previous investigations at the site indicated that the geological setting of the Leadville Minin g District

was complex. However, these investigations were not able to fully consider the difficulties associated with

monitoring and detecting migration of contaminated ground water in the mining district, including the Yak

Tunnel.

New Information has become available to EPA followin g remedial actions at the Eagle Mine near

Mlnturn, Eagle County, Colorado. At that site, after pluggin g several adits, mine water began seeping from

adjacent bedrock fracture zones (very little water seeps from the adits themselves). This surface, and

subsurface, seepage has Increased the concentrations of heavy metals In the Eagle River below the mine

workings and In Rock Creek. EPA has focused on the observed short-term effect s of mine pluggin g at the

Eagle site in forming its decision regarding the Yak Tunnel. Although it is not known whether these effect s will

continue over the long term, EPA recognizes that source control remediation measures which have been

implemented may well prove to be effective at Eagle.

EPA has also gained additional information about the difficulties of installing an "accurate" monitoring

network in highly faulted areas, such as the Leadville Mining District. During the construction of the Surge

Pond in 1989, the presence of a geologic fault Immediately downgradient of the Surge Pond made installation

of a ground-water monitoring network extremely difficult. The fault had originally been thought to be located

several hundred feet down gradient of the surge pond, not Immediately down gradient of the surge pond. Due

to the fault, the bedrock and the unconsolidated sediments above the bedrock are differen t upgradient and

downgradient of the Surge Pond. The bedrock and unconsolldated sediments upgradient of the pond consist

of porphyry or weathered porphyry; the bedrock and unconsolidated sediments downgradient of the pond

consist of dolomite or weathered dolomite. Since the chemistry of the geological deposits is different,

interpretation of chemical analyses on ground water would be extremely difficult. In addition, the complex step

faul t appears to be acting as either a dam or a sink, since alluvial ground water is only present on one side

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of the fault . These conditions are not expected to be unique to the vicinity of the Surge Pond; there may be

other complex fault s which Intersect the Yak Tunnel. This additional information Is presented In Hydrologic

Investigation of Surge Pond Area (Res-ASARCO Joint Venture, March 1990); supplemental Information is

presented in Appendix A. Selected Remedy Component Change Proposal (Res-ASARCO Join t Venture, March

1990). These technical documents are available at the information repositories at the Lake County Library,

Colorado Mountain College Library, Timberlln e Campus, and the EPA offices in Denver.

E PA has worked extensively with the PRPs, Includin g ASARCO, Newmont Mining, Resurrection Mining,

and the Res-ASARCO Joint Venture, In developing this ESD. Many components of this remedy have been

proposed by the PRPs.

T he National Contingency Plan (NCP), the regulations developed by EPA to implement the Superfun d

law, requires EPA to use nine criteria to evaluate proposed remedies at Superfun d sites. These nine criteria

were established to provide for consistent evaluation of proposed remedies throughout the country. The

following paragraphs present a brief evaluation of the proposed remedy using the nine criteria.

1. Overall Protection of Human Health and the Environment:

Partial tunnel rehabilitation, collection, and treatment of the mine drainage will adequately

protect human health and the environment from unacceptable risks posed by the hazardous

substances In both the short term and the long term. The proposed remedy reduces or

eliminates impacts on surface and ground water, through collection and treatment of the Yak Tunnel discharge.

2. Compliance with Applicable or Relevant and Appropriate Requirements (ARARs):

The treatment facility will attain compliance with all applicable or relevant and appropriate Federal and State requirements, except for requirements pertaining to instream water quality. Most of the area from which the contaminated discharges to the Arkansas originate has not yet

been adequately characterized. Thus, it is not yet possible to perform a wasteload allocation

to determine the extent to which the Yak Tunnel discharge and each of the other discharges

must be treated In order for the Arkansas River to meet instream water quality requirements. Water quality-based effluen t limitations and standards for the upper Arkansas River have

therefore been interim waved. Technology-based treatment requirements, however, represent

the minimum level of control that has not been waived. Initially upon start-up of the water

treatment plant, New Source Performance Standards (NSPS) must be met. By the end of the

second year of operation, the water treatment plant effluent must meet best available technology

requirements based upon acute toxicity testing. These requirements can be translated into

Yak Tunnel Operable Unit Revision 2 October 1991

Explanation of Significant Differences Page 16 Document Control Number 4800-01-0600

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interim numeric standards as a design basis and compliance threshold. When the area has

been studied, the wasteload allocation will be performed, water quality-based treatment

requirements for the Yak Tunnel and other discharges will be determined, and the Arkansas River will meet Instream water quality standards.

3. Long-Term Effectiveness and Permanence:

The proposed remedy provides for long term maintenance of the collection system and the

treatment facility thus reducing or eliminating residual risk remaining from untreated waste. Operation and maintenance procedures and monitoring will be required to ensure the

effectiveness of the proposed remedy in perpetuity. Future treatment technologies will be

evaluated when the treatment facility requires updating. Tunnel blockage In the form of natural

rockfalls may eventually occur, causing a reversal in ground-water gradient which may adversely

impact the local or regional ground-water system. However, the contingency plan will mitigate

this impact and will allow response time to assess the impact and develop alternatives for

effectively collecting contaminated ground water and containing sources of acid mine drainage.

4. Reduction of Toxicity, Mobility, or Volume through Treatment:

The principal threats posed by the site include mine drainage which is contaminated with high

concentrations of metals and low pH. The treatment facility will remove the metals from the

tunnel discharge and increase the pH to acceptable levels. The sludge produced by the

treatment process will contain metal hydroxides which have low mobility under nonacidlc

conditions. In addition, the sludge will recover sufficien t zinc to allow recycling.

5. Short-Term Effectiveness:

No short-term risks posed to the community during implementation are anticipated. Potential Impacts on workers during implementation include manageable underground construction safety

hazards during tunnel rehabilitation which will be conducted by personnel familiar with the safety

hazards sometimes encountered in underground work. The probability of occurrence of safety

hazards has been reduced from the original proposed plan because of the reduced duration

of exposure in that shorter length of tunnel requiring rehabilitation. No potential environmental

impacts are anticipated during Implementation.

6. Implementability :

The tunnel rehabilitation and the treatment facility both utilize known technologies which are

technically feasible, reliable, and implementable. Monitoring of the collection system by way

of monitoring wells and periodic tunnel inspections will be conducted. The effectiveness of the

treatment facility wOl be verified by way of effluent testing. The treatment process may be upset

by changing conditions of influent resulting in less than desired effluent quality for short periods

of time. Good design, maintenance, and operational practices are required to meet

performance standards. The remedy includes a contingency plan to relieve water

accumulations occurring behind tunnel blockages.

7. Cost:

EPA is unable to determine a significant difference In cost between the 1989 ROD Modification

and the 1991 Proposed Remedy. The 1991 Proposed Remedy provides greater reliability for

the collection of contaminated ground water.

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Explanation of Significant Difference* Page 17 Document Control Number 4800-01-0600

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8. State Acceptance:

T he State of Colorado acknowledged the validity of the additional information evaluated in

reconsidering a pluggin g remedy. The State has determined that the proposed changes are

appropriate for the Yak Tunnel situation, but at the same time, maintains that mine pluggin g

remedial alternatives may be effective in other situations.

9. Community Acceptance:

EPA has worked extensively with the community to understand the residents concerns

regarding the previously selected remedies. EPA believes that the proposed remedy will receive

better support from the community than the previous remedies. This is because the proposed

remedy will allow continued mining in areas which would have been flooded under the previous

remedies. In addition, the proposed remedy removes many of the uncertainties associated with

pluggin g and floodin g the mine workings.

Informal public meetings have been conducted and the oral and written comments received

have been generally supportive of this proposed change to the remedy. EPA will continue to

work with the community and will attempt to address the community' s comments regarding this

final ESD.

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Explanation of Significant Differences Page 18 Document Control Number 480041-0800

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SUPPORTIN G AGENC Y COMMENT S

T he Colorado Department of Health has substantively participated in the 1991 ESD development, EPA

responded to the State's concerns and incorporated their comments into this ESD. The Colorado Department

of Health concurs with EPA in the proposed modifications to the remedy.

STATUTOR Y DETERMINATION S

EPA has determined that the remedy, as modified, remains protective of human health and the

environment, and Is cost effective. In addition, the modified remedy utilizes permanent solutions and innovative

treatment or resource recovery technologies to the maximum extent practicable.

As mentioned above, when the area has been adequately studied, a wasteload allocation will be

performed, and water quality-based requirements for the Yak Tunnel and other discharges to the Arkansas

River will be determined. At that time, Instream water quality requirements for the Arkansas River will be

achieved.

SCHEDULE FOR THE PROPOSED REMEDY

The schedule for design and construction of the Yak Tunnel remedy is presented in Figure 2. The

construction of the water treatment plant is scheduled to be completed during the 1991 construction season;

site development work began during 1990 to allow for construction of the plant in 1991.

Yak Tunnel Operable Unit Revision 2 October 1991

Explanation of Significant Drfferencet Page 19 Document Control Number 4800-01-0800

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Name HATER TREATMENT PLANT

Construction

Start-up

Routine Operation

GROUND-HATER MONITORING

Hel l Installatio n

Baseline Monitoring

Routine Monitoring

TUNNEL FLOH MEASUREMENT

Y ak Tunnel Inspection

Flow Device Installatio n

Routine Flow Measurement

DurationOd

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FIGURE 2. YAK TUNNEL PROJECT SCHEDULE
PUBLI C PARTICIPATION

Persons desiring further information on this Draft Explanation of Significan t Differences and those

interested in procedures for providing comments to EPA may contact:

James Hanley Eleanor Dwight

Y ak Tunnel Remedial Project Manager Community Involvement Coordinator

Phone #: (303) 293-1649 Phone #: (303) 294-1130

Toll-free Number: 1 800-759-4372 (in Colorado)
Mailing List Additions

If you did not receive this ESD by mail, and you would like to be added to EPA's mailing list for the

California Gulch site, please send the followin g information to:

Eleanor Dwight

Community Involvement Coordinator

U.S. EPA, Region VIII, 80EA

999 18th Street, Suite 500

Denver, Colorado 80202-2405

Name:

Address:

City/State:. .Zip:.

Company, organization, or governmental entity:.

Yak Tunnel Operable Unit Explanation of Significant Difference* Document Control Number 4800014800

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Revision 2 October 1991 Page 21
GLOSSAR Y

Adit - An almost horizontal entrance to a mine.

Administrative Order - See Order below.

Cadmium - A heavy metal found in the ores in the Leadville Mining District. Exposure to cadmium may result

in hypertension and kidney and liver damage in humans. Cadmium is used in metal plating, pigments, and

chemical production.

CERCLA - The Comprehensive Environmental Response, Compensation, and Liability Act of 1980 -A federal

law passed in 1980 and modified in 1986 by SARA (see below). The Acts created a special tax that goes into

a trust fund, commonly known as Superfund, to investigate and dean up abandoned or uncontrolled hazardous

waste sites. Under the legislation, EPA can either:

• Pay for site clean-up when parties responsible for the contamination cannot be located or are

unwilling or unable to perform the work; or

• Take legal action to force responsible parties for site contamination to dean up the site or pay

back the Federal government for the cost of the dean-up.

Contaminants - Any chemical that poses a threat to public health and/or the environment. Contaminants

may be toxic, ignitable, explosive, or chemically reactive. In the Leadville Mining District, the principal

contaminants are toxic heavy metals.

Copper - A heavy metal found in ores In the Leadville Mining District. Copper is not acutely toxic to humans,

but is one of the most harmful metals to fish and other aquatic life. Copper is a semiprecious metal used In

electrical and electronic equipment, coins, metal plating, and metal alloying. Dolomite - A form of limestone which is rich in magnesium.

Explanation of Significan t Differences (ESD) - A document which EPA Is required to prepare when changes

are proposed In the remedy for a Superfun d site. An ESD is issued if the change In the remedy is significant,

but not fundamental. The ROD must be modified or amended if the change is fundamental.

Factor of Safet y • Factors of safety are used to incorporate the uncertainties in the design, and to reduce the

probability of failure of the design. In engineering design, the ratio of the design property to the required

property. For example, a pump station designed to store and pump 150 gpm in a portion of the tunnel In which the average flow rate is 100 gpm would have a factor of safety of 1.5. A factor of safety of 1.5 Is

common in many engineering designs.

Feasibility Study - See Remedial Investigation/Feasibility Study below.

Gold • A precious heavy metal found In ores in the Leadville Minin g District.

Gray porphyry - See porphyry below.

Hazardous Substances - Synonymous with Contaminants. Head - The pressure exerted by liquid or gas. In the Yak Tunnel , head is equal to the depth of water standing In the tunnel, as long as the tunnel Is not flooded. Hydraulic Gradient - The potential for ground water to move in a given direction due to the difference In

pressure between two points in the aquifer.

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Lead - A heavy metal found in ores in the Leadville Mining District. Exposure to lead may result in damage

to the central nervous system in humans; in addition, exposure to lead may cause cancer. Lead is used in

batteries, foil, solder, pigments, and construction equipment, and as an additive in gasoline. Magnesium - A nonmetal found in limestone and ore bodies In the Leadville Minin g District. Magnesium is

not a contaminant in environmental media in the Leadville area.

Manganese - A heavy metal found in ores in the Leadville Minin g District. Manganese can be toxic to plants

in acidic soils. National Priorities List (NPL) - EPA's list of the most serious uncontrolled or abandoned hazardous waste

sites identified for long-term remedial response using money from the Superfund.

Operable Unit - An action taken as one part of an overall site clean-up. A number of operable units can be

used in the course of a site dean-up.

Order - A legal document which describes the actions which PRPs are required to perform at a Superfun d

site. Several types of Orders may be used, Including:

• A Unilateral Administrative Order (UAO) which is a document prepared by EPA without formal

concurrence by the PRPs;

• An Administrative Order on Consent (AOC) which Is a legal agreement between EPA and the PRPs whereby the PRPs agree to perform or pay the cost of Superfun d site clean-up; or

• A Consent Decree (CD) which is a legal agreement (essentially an AOC), approved and issued

by a judge, between EPA and the PRPs whereby the PRPs agree to perform all or part of a Superfun d site dean-up. A ll orders issued to PRPs for the Yak Tunnel Operable Unit have been Unilateral Administrative Orders.

pH - Scientific measure of the relative acidity of a water solution on a logarithmic scale from 1 to 14; low pH numbers in the 3 to 5 range are typical for strongly acidic minewater draining into the California Gulch area.

Porphyry - Rock having relatively large conspicuous crystals In a fine-grained igneous matrix. Thi s type of

rock pushed up through the surrounding sedimentary rocks and was formed after rapid cooling from the molten state.

Potentially Responsible Party (PRP) - An individuals) or company(s) (such as owners, operators,

transporters, or generators) potentially responsible for, or contributing to, the contamination problems at a Superfun d site. Whenever possible, EPA requires PRPs, through administrative or legal actions, to dean up

hazardous waste sites they have contaminated. Under SARA, some current landowners are exempt If they did

not know about the hazardous substances on their property and exercised due care in acquiring the property.

Record of Decision (ROD) - A public document that explains which clean-up alternative(s) will be used at N PL sites. The ROD Is based upon information and technical analysis generated during the RI/FS and

consideration of public comments and community concerns.

Remedial Action - The actual construction or implementation phase that follows remedial design of the

selected clean-up alternatives at a site on the NPL

Remedial Design - An engineering phase that follows the ROD, In which technical drawings and

specifications are developed for the subsequent remedial action at a site on the NPL

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Remedial Investigation/Feasibilit y Study (RI/FS) - Investigative and analytical studies usually performed

at the same time in an interactive, iterative process, and together referred to as the RI/FS. They are intended

to:

• Gather data necessary to determine the type and extent of contamination at a Superfun d site;

• Establish criteria for cleaning up a site;

• Identify and screen clean-up alternatives for remedial action; and

• Analyze in detail the technology and costs of the alternatives.

SARA • The Superfun d Amendments and Reauthorization Act of 1986. This law amended CERCLA

Silver - A precious heavy metal found in ores in the Leadville Minin g District.

Superfun d - The common name for CERCLA

Unconsolidated sediments - The geologic material above bedrock.

Weathered Dolomite - Physically altered dolomite. The term "weathered" means that the bedrock has begun

to break down into soli.

Zinc - A heavy metal found in ores in the Leadville Mining District. Zinc is rarely damaging to human health;

however, when humans are exposed to zinc In combination with other metals, the results may be harmful. At high concentrations, zinc can be harmful to fish. Zinc is used in batteries, fuses, pigments, metal platin g and

metal alloying.

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