555707 _———•————— SDMS Document ID Illlllllllllllllllllllllllllll200430l2 RECORD OF DECISION FOR SUMMITVILLE MINE FINAL SITE-WIDE REMEDY OPERABLE UNIT 5 SUMMITVILLE MINE SUPERFUND SITE RIO GRANDE COUNTY, COLORADO RECORD OF DECISION FOR SUMMITVILLE MINE FINAL SITE-WIDE REMEDY OPERABLE UNIT 5 SUMMITVILLE MINE SUPERFUND SITE RIO GRANDE COUNTY, COLORADO DECLARATION FOR THE SITE-WIDE RECORD OF DECISION Site Name and Location Summitville Mine Superfund Site, Summitville, Rio Grande County, Colorado. Statement of Basis and Purpose This document represents the final Record of Decision for the Summitville Mine Superfund site (site) final site-wide remedial action, designated as Operable Unit (OU) 5. The site is defined as the permitted 1,231- acre mine site that is located in the southeastern portion of the San Juan Mountains, in the southwest comer of Rio Grande County, Colorado. This decision document presents the Selected Remedy for the site, which was chosen in accordance with Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1986, as amended by Superfund Amendments and Reauthorization Act of 1986, and the National Oil and Hazardous Substance Pollution Contingency Plan (NCP). This decision is based on the Administrative Record file for this site. The U.S. Environmental Protection Agency's (U.S. EPA) CERCLA identification number for the site is COD983778432. This document is issued by the Colorado Department of Public Health and Environment (CDPHE), the lead agency for the site-wide Remedial Investigation and Feasibility Study, and the U.S. EPA Region VIII. Both U.S. EPA and the State of Colorado concur with the Selected Remedy presented herein. The remedial action selected in this Record of Decision is necessary to protect the public health or welfare or the environment from actual or threatened releases of hazardous substances from the site into the environment. Assessment of the Site Past mining operations at the site resulted in contamination of surface water, groundwater, and sediments. The principal threat waste includes mobile source materials, such as acid mine drainage originating at the site, that ultimately impacts downstream waters. The acid mine drainage is also accompanied by naturally occurring acid rock drainage from mineralized terrains at the site and other areas within the Alamosa River watershed. The acidic drainage is characterized by high metals concentrations and low pH, typically below four standard units. Metal contaminants at the site include copper, iron, manganese, zinc, aluminum, and cadmium, among others. In addition, heap leach operations used sodium cyanide to extract precious metals for crushed ore. Minor amounts of residual cyanide and cyanide degradation products remain within Heap Leach Pad waste materials, and are therefore included as site contaminants. Summitville Mine - Declaration Page for Operable Unit 5 On December 3, 1992, Summitville Consolidated Mining Company, Inc., the operator of the mine, announced pending bankruptcy and informed the State of Colorado that financial support for site operations would not continue beyond December 15, 1992. On December 4, 1992, the State of Colorado requested emergency response assistance from the U.S. EPA. On December 16, 1992, the U.S. EPA Region VIII Emergency Response Branch assumed control of the site as part of an Emergency Response Removal Action. The U.S. EPA immediately began water treatment plant modifications to treat cyanide- contaminated leachate and acid mine drainage from numerous sources at the site. Preliminary Remedial Action Objectives were developed, to the extent practicable, in compliance with applicable or relevant and appropriate requirements to the early site cleanup actions. The site cleanup targeted five areas of primary concern for emergency response actions or interim remedial actions. Emergency response actions included plugging of the Reynolds and Chandler Adits to reduce a major source of acid mine drainage. The other areas of concern were addressed through Interim Records of Decision as described below: > Water Treatment, designated QUO. > Heap Leach Pad Detoxification/Closure, designated OU1. > Excavation of mine wastes from the Cropsy Waste Pile, Beaver Mud Dump and the Cleveland Cliffs Tailings Impoundment, placement of this material in the mine pits, and mine pit closure. designated OU2. > Site-wide reclamation activities, designated OU4. Groundwater contamination within South Mountain was also an area of concern and originally designated OU3. An Interim Record of Decision for South Mountain Groundwater (OU3) was never drafted. Instead, groundwater concerns were addressed through the site-wide Remedial Investigation and Feasibility Study and incorporated into the final remedy (OU5). The emergency response and interim remedial actions (OU1 and OU2) have been completed, with only minor reclamation work (OU4) remaining after the 2001 construction season. Water treatment (QUO), which is on going, has been successful at achieving Interim Action Levels established in the OU4 Interim Record of Decision for several site contaminants. However, achievement of Interim Action Levels for copper (the ecological risk driver) and aluminum has been infrequent, typically less than 10 percent of the time. In addition, State of Colorado water quality standards for the Alamosa River have been frequently exceeded over the past several years. These exceedances have been due, in part, to release of contaminated water from the site impoundment, Summitville Dam Impoundment, during years of normal or above normal precipitation.1 Description of the Selected Remedy The site-wide remedial action (OU5) selected in this Record of Decision is a final action that will address the threats to the environment that remain at the site after completion of emergency and interim remedial actions. The goal of the Selected Remedy is to capture the mobile source material, (i.e., acid mine drainage), contain it in an on-site impoundment, and remove metals to achieve water quality standards in the Alamosa River. The Selected Remedy continues the benefits achieved through the emergency actions and interim remedial actions and further reduces and controls threats to the environment. The Selected Several sources of acid mine drainage present at the site are not addressed by the Interim Records of Decision. The combination of inadequate storage and treatment capacity with these acid mine drainage sources, necessitate additional remedial action to further stabilize the site and to meet water quality goals in the Alamosa River and Terrace Reservoir. Summitville Mine - Declaration Page 2 for Operable Unit 5 Remedy will maintain interim remedial actions for OU1, OU2, and OU4. The major components of the Selected Remedy include the following: > On-site contaminated water impoundment upstream of the Wightman Fork-Cropsy Creek confluence; > Construction of a new gravity-fed water treatment plant downstream of the contaminated water impoundment; ^ Possible breach and removal of the existing Summitville Dam Impoundment; > Construction of a sludge disposal repository; > Upgrade of Wightman Fork Diversion; > Upgrade of select site ditches; > Construction of groundwater interceptor drains; > Construction of a Highwall ditch; > Rehabilitation of Reynolds Adit; > Management of mine pool water; > Continued site maintenance, and groundwater/surface water and geotechnical monitoring on-site; and > Surface water, sediment, and aquatic life monitoring in Alamosa River and Terrace Reservoir. Determination of impoundment size, and exact location and capacity of the water treatment plant are deferred to the Remedial Design phase. The data to support design of these two components will be collected and evaluated during 2002 and 2003. Additional data collected during the Remedial Design phase will be used to assess the success of OU4 reclamation in neutralizing acid mine drainage at the site. Design of hydraulic structures will conform to the design event (100-year snow melt and 500-year, 24-hour duration precipitation). The volume of water from the 100-year snow melt drives the sizing of the impoundment, whereas design of ditches is driven by the 500-year precipitation. Institutional controls, other than continued restricted access to the site, are not components of the remedy. Release of contaminated water from the Summitville Dam Impoundment during springtime snow melt runoff has been necessary during years of average or greater snow pack because the combined storage and treatment capacity of the existing impoundment and water treatment plant is exceeded. The releases have immediate, detrimental impacts to the downstream environment by lowering the pH of Wightman Fork for weeks and adding a considerable metal load to the Alamosa River system. The Selected Remedy includes a new water treatment plant that employs a proven and effective active water treatment technology. A more reliable influent delivery system will be constructed that requires low levels of operation and maintenance, and could be operated year-round if necessary. The combination of a new water treatment plant, reliable influent delivery system, and storage impoundment should eliminate releases of contaminated water. The remedy also includes water diversions designed to route clean water from reclaimed areas around the treatment system, and measures to control or mitigate contaminated water from source areas that remain after site-wide reclamation (OU4) is completed. It is expected that