Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Lava Cap Mine
EPA #: CAD983618893
City: 5 miles SE of Nevada City
Congressional District: 02
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 09/29/98
Final Date: 01/19/99
The Lava Cap Mine site occupies approximately 33 acres in a semi-rural residential area of the Sierra Nevada foothills in western Nevada County, California. The site is approximately 5 miles southeast of Nevada City and 6 miles east of Grass Valley at an elevation of about 2700 feet. The site includes the mining area where ore was processed to recover gold, and areas where tailings which originated at the mine have been washed downstream and deposited over time. The downstream areas of the site include Lost Lake, a private lake surrounded by homes, located approximately 1-1/4 miles downstream of the Lava Cap mine site.
In 1994, an estimated 1,776 people lived within one mile of the site, and 24,091 lived within four miles of the site. The immediate watershed basin ecosystem contains two California Species-of-Special-Interest: foothill yellow-legged frog and western pond turtle, in addition to more common species of reptiles, amphibians, fish, birds, and mammals.
Gold and silver mining activities were initiated at Lava Cap Mine in 1861. From 1861 to 1918, processing of the ore and disposal of the waste rock, overburden, and tailings occurred off-site at the Banner Mine, which is located approximately 1.5 miles north of the Lava Cap Mine.
The Lava Cap Mine was inactive from 1918 to 1934, at which time mining activities were resumed and a flotation plant was built to process the ore at the site. The gold and silver concentrates from the flotation plant were shipped to two smelters, one in California and the other in Washington. In 1940, a cyanide plant was built to recover the concentrates on site. However, this operation proved to be relatively ineffective. From 1941 to 1943, the cyanide plant only handled the middlings and tailings from the flotation plant. The middlings and tailings were ground to a very fine size (i.e., able to pass through a 400-mesh screen), then vat leached with cyanide to remove the residual gold and silver. Slurries from the flotation and cyanide processes were deposited in a ravine on the site. Where the ravine steepened and narrowed, a log dam approximately 60 feet high was built to hold the tailings in place. The waste rock and overburden were also deposited in two piles located at the site between the mineshaft and the tailings pond. In 1943, Lava Cap Mine was closed due to World War II. An attempt was made to re-open the mine in the mid-1980s. However, community opposition resulted in the defeat of a proposed re-zoning of the property which would have allowed mining activities to resume at the site.
In 1979, complaints from local residents initiated an action from California’s Central Valley Regional Water Quality Control Board (RWQCB) that led to issuance of a Cleanup and Abatement Order (CAO). The CAO called for the property owners at that time to take measures to limit tailings discharges to Little Clipper Creek, to divert surface water runoff from the mine and mill waste fill deposits, and to obtain an evaluation of the dam. This Order resulted in the construction of three small settling basins below the dam and some flow diversions around the tailings. No improvements were made to the dam.
During a major winter storm in January 1997, the upper half of the log dam collapsed, releasing over 10,000 cubic yards of tailings into Little Clipper Creek. In early 1997 staff from the California Department of Fish and Game and the Nevada County Department of Environmental Health inspected the site. Extensive deposits of tailings were observed in and on the shoreline of Little Clipper Creek, at the confluence of Little Clipper and Clipper Creeks, and in and on the shoreline of Lost Lake. The tailings were also observed in wetland areas contiguous with these water bodies, in some cases completely covering the vegetation.
Several times during 1997, DTSC has conducted sampling at the mine site and off-site at Lava Cap Mine to determine the locations and concentrations of arsenic contamination. Following the dam collapse in January 1997, the current property owner constructed a drainage ditch upstream of the mill tailings which partially diverted surface away around the tailings.
The EPA formally listed the Lava Cap Mine site on the National Priorities List (NPL) in February 1999, allowing Superfund funding to be spent on investigation and cleanup of the site. In 2004, as a result of initial studies of the site, EPA divided site into four project areas, or Operable Units. The four are:
- Operable Unit 1, the Mine Area
- Operable Unit 2, Groundwater
- Operable Unit 3, the Lost Lake area
- Operable Unit 4, Mine Area Residences
Contaminants and Risks
- Surface Water
- Soil and Sludges
The primary contaminant from the Lava Cap mine is arsenic in the mine tailings both on the mine property and in the Little Clipper Creek watershed from the mine down to Lost Lake. Arsenic is also found at elevated levels in the water of the creek and the lake and in shallow groundwater below the tailings. Samples of drinking water wells near the mine site have shown elevated levels of mine-related arsenic, in concentrations varying from two to ninety times the drinking water standard of 10 parts per billion (ppb). Each of these residences has a treatment unit for their drinking water supply, so EPA believes noone is currently ingesting arsenic-contaminated drinking water above the drinking water standard.
There are a number of possible routes of exposure to arsenic at the site. Dust-like tailings which can become airborne in deposition areas, or which may be tracked into homes, could cause health problems if the tailings were inhaled over a period of time. To minimize this risk, EPA recommends that people limit their contact, and their pets' contact, with the mine tailings, and that those who walk along the tailings deposition areas of Little Clipper Creek and Lost Lake rinse their shoes and pets afterward. The levels of arsenic in water in Little Clipper Creek and in Lost Lake have exceeded the drinking water standard in some samples. EPA recommends that people limit the ingestion of water from these sources; however, the risk to people during boating or even swimming is considered relatively low. Fish in Lost Lake have been found to contain arsenic at low levels and consumption of these fish ordinarily would not be considered to present a high risk to people; however, EPA recommends limiting fishing recreation to catch-and-release at this time. Some data from wells on and near the site indicates that arsenic may be affecting groundwater underlying the area, and EPA has begun a groundwater study. The human health risk assessment for the site describes potential health effects in detail.
Investigation and Cleanup Activities
EPA has taken an initial action to stabilize the mine site and is planning further stabilization work. EPA is also conducting a study to determine the best approach to long-term cleanup of the affected area.
Interim Record of Decision for Groundwater/Drinking Water. In September, 2008, EPA signed a Record of Decision that specified a remedy to address mine-related arsenic contamination in drinking water wells near the Lava Cap Mine site. The selected remedy consists of constructing a public water supply line to affected residences. This is an interim remedy for the overall Groundwater Operable Unit, since further study is required, due to the complexities and interactions between the fractured bedrock aquifer and surface water. A final remedy decision will be made once additional data are gathered and analyzed. The Interim Record of Decision can be found in the Technical Documents section, dated September 30, 2008.
Initial response. In October 1997, the EPA Region 9 Emergency Response Office determined that conditions associated with the tailings release from the Lava Cap Mine site met the National Contingency Plan (NCP) section 300.415(b)(2) criteria for a removal action. During October and November 1997, 4,000 cubic yards of tailings were removed from the damaged dam area and stockpiled on the waste rock pile immediately to the north of the tailings pile. The lower half of the dam (i.e., approximately 30 feet in height) was found to be in relatively good condition. The oversteepened slopes of the tailings pile immediately behind the dam were graded and the entire tailings pile was covered with waste rock. Stream diversions were also created around the tailings pile. In February 1998 a second response was constructed at the site to stabilize another tailings release and to further improve the drainage. The removal action has been completed and included covering approximately 4,000 cubic yards of stockpiled tailings with a clay cap.
Remedial investigation. As part of its longer term study of the site, EPA conducted several rounds of sampling upgradient from the mine, on the mine property, along Little Clipper and Clipper Creeks in the stretch from approximately 2,000 feet upstream of the mine to approximately 1,000 feet downstream of Lost Lake, and from in and around Lost Lake. EPA sampled several media, including surface soil, subsurface soil, air, groundwater, mine discharge, surface water, and sediment. To collect subsurface soil and groundwater samples, EPA installed borings and constructed groundwater monitoring wells using a truck-mounted drilling rig. The resulting remedial investigation report was released in November 2001. It concludes that arsenic in mine tailings is the primary threat to human health, while both arsenic and metals threaten the ecology of the area.
A Remedial Investigation for the Groundwater Operable Unit was completed in July 2008 and is available in the information repositories for review. A supplement to this document may be developed once further studies of the groundwater/surface water interactions have been completed.
EPA distributed the public release draft of the Feasibility Study for the Mine Area in OU1 where mining took place in 2004. The feasibility study defined the goals of EPA's remedial action, evaluated different technologies for reaching those goals, and combined those available technologies into several alternative cleanup plans. Alternatives considered include:
- taking no action
- installing physical access and legal land use controls
- decontaminating the mine buildings
- demolishing the mine buildings
- capping the tailings and waste rock piles and constructing surface water diversion channels
- excavating and disposing of the tailings and waste rock pile in an on-site landfill
- excavating and shipping the tailings and waste rock pile to an off-site disposal facility
- capping tailings and contaminated sediment in Little Clipper Creek down to Greenhorn Road
- excavating the contaminated sedimet in Little Clipper Creek down to Greenhorn Road
The Feasibility Study was completed for the drinking water component for Groundwater (OU2) in July 2008. Alternatives considered in this study include:
- taking no action
- installing point-of-use undersink treatment units, land use notifications, and monitoring of groundwater
- installing wellhead treatment units, land use notifications, and monitoring of groundwater
- providing an alternative water supply via pipeline from Nevada Irrigation District, land use notifications, and monitoring of groundwater
The proposed plan is available now for the drinking water component of the Groundwater Operable Unit.
A revised Feasibility Study and proposed cleanup alternatives for the Lost Lake Area (OU3) are expected in 2013.
Mine Area Cap
Construction of the cap on the mine area in OU1 has been essentially completed. Excavated tailings and contaminated soils from Little Clipper Creek as well as surrounding the residences have been consolidated under the multi-layered cap. Surface water is routed around the cap and over the rock-wall buttress, preventing water from infiltrating the mine tailings. Revegetation along Little Clipper Creek was in November 2007.
Surface Water Treatment
Water that is discharging from the mine will be treated to ensure water quality downstream. The design is anticipated by 2013.
EPA waterline OU2
The project is part of the Operable Unit 2 (OU2) groundwater remedy at the Lava Cap Mine Superfund Project to provide drinking water to residents whose drinking water wells are impacted with arsenic contamination from the Lava Cap Mine Superfund Site. EPA will build an approximately 1.5 mile 8-inch distribution line and service lines, to connect only impacted residences and non-impacted residences where the pipeline will go through their properties.
Cleanup Results to Date
This site has four operable units (OUs) and two signed Records of Decision (RODs). The OU1 ROD includes the tailings and adit water in the mine area and the mine residences, which was later separated into OU1 and OU4. The OU2 Interim ROD for groundwater was signed in 2008. The ROD for OU1 (the Lava Cap Mine Area) is being implemented as two distinct remedies. The first remedy included excavation of tailings and tailings consolidation, vegetative covers, a tailings pile cap, a rock buttress, and drainage channels. The second remedy for OU1, which is still in the remedial design phase, is treatment of adit water emanating from the mine area. The remedy also includes institutional controls to minimize potential future exposure to remaining contaminated materials. The remedy for OU4 (mine residences) consisted of demolition of mine residences followed by removal of contaminated debris and soils. Construction activities for OU4 were completed in December 2005.
Potentially Responsible Parties
Potentially responsible parties (PRPs) refers to companies that are potentially responsible for generating, transporting, or disposing of the hazardous waste found at the site.
Online information about the PRPs for the site is not yet available.
Documents and Reports
Public Meetings: EPA is committed to involving the public in the cleanup decision-making process. Its Community Involvement Program focuses on answering the community’s questions about the cleanup effort, providing information to the community about site activities, and incorporating community issues and concerns into Agency decisions, particularly when a cleanup remedy is proposed.
Community Involvement History:
In November 2000, EPA held public meetings in Grass Valley and Nevada City on our preliminary results from initial rounds of sampling.
In November 2001, EPA again held public meetings in the same locations, in cooperation with the Lava Cap Mine Superfund Coalition, recipient of an EPA grant to provide technical expertise to the community in understanding the Lava Cap cleanup. At these meetings, EPA presented the results of its human health and ecological risk assessments. EPA also began a discussion with community members on their preferences regarding possible actions EPA might take to clean up the site.
In December 2002, EPA conducted public workshops on cleanup alternatives for the Lost Lake area of the site and also met with area residents at home. EPA:
- described the process used to develop alternatives,
- presented sample alternatives,
- and conducted an exercises that allowed the community members to discuss the attributes of the site that they valued and wanted to retain or restore and to provide specific ideas about their preferred cleanup alternatives.
In February 2004, EPA held a formal public hearing on the Proposed Plan for the Mine Area of the site. See U.S. EPA Proposes Cleanup Plan for Mine Area Operable Unit Fact Sheet in Documents and Reports section above.
In August 2008, EPA invited the public to attend a public meeting to hear a presentation on the Proposed Plan.
- discussed the results of the investigations into mine-related arsenic contamination in groundwater
- described the cleanup options EPA evaluated for addressing this contamination
- explained EPA's preferred alternative
- encouraged the public to comment on any or all of the alternatives
Public Information Repositories
The public information repositories for the site are at the following locations:
Grass Valley Public Library
207 Mill Street
Grass Valley, CA 95945
Nevada County Library
980 Helling Way
Nevada City, CA 95959
EPA Site Manager
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
Department of Toxic Substances Control,
8800 Cal Center Drive
Sacramento, CA 95826
After Hours (Emergency Response)