Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Sulphur Bank Mercury Mine
EPA #: CAD980893275
City: Clear Lake
Congressional District: 01
Test caps installed in Clear Lake
EPA contractors installed a test cap at the southern area of Oaks Arm to cover contaminated sediments in Clear Lake. This activity was completed in February 2013 and monitoring will continue for two years. For more information please click here.
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 06/24/88
Final Date: 08/30/90
The Sulphur Bank Mercury Mine site is located at the southeastern end of the Oaks Arm of Clear Lake. The Superfund site also includes the Elem Tribal Colony which is located directly adjacent to the mine property. Approximately 70 houses are located within 3 miles of the site. A freshwater wetland is located 900 feet to the north of the mine. A critical habitat for three endangered wildlife species, the Peregrine Falcon, Southern Bald Eagle, and Yellow-Billed Cuckoo, is located less than 1/4 mile from the site.
The 150-acre Sulphur Bank Mercury Mine site initially was mined for sulfur from 1865 to 1871. Mercury ore was mined intermittently by underground methods from 1873 to 1905. The site was intermittently open pit mined from 1915 to 1957. The mine, once one of the largest producers of mercury in California, has been inactive since 1957. Approximately 150 acres of mine tailings and waste rock and a flooded open pit mine (called the Herman Impoundment) are located on the property. The mine tailings extend into the Oaks Arm of Clear Lake along 1,300 feet of shoreline. Approximately two million cubic yards of mine wastes and tailings remain on the mine site. The Herman Impoundment, which is filled with acidic water, covers 23 acres to a depth of 90 feet and is located 750 feet up gradient of the lake. EPA's remedial investigation studies have found mercury and arsenic in the mine wastes and tailings. Mercury is present in the bottom sediments in Clear Lake, and mercury has bio-concentrated in the food web of Clear Lake. The levels of mercury in fish from the lake led the State to issue an advisory to limit consumption of fish. Clear Lake is the source of water that the Clear Lake Oaks Water District provides for municipal drinking water for 4,700 people.
Contaminants and Risks
- Surface Water
- Soil and Sludges
- Environmentally Sensitive Area
The soils at the mine property and at the adjacent Elem Tribal Colony are contaminated with high levels of mercury and arsenic. Surface water and groundwater that discharge from the site contain high levels of mercury and arsenic and contaminate the natural wetlands to the north of the mine property and the sediments of Clear Lake. Mercury bio-concentrates in the Clear Lake food web. Fish and other biota in Clear Lake are contaminated with high levels of mercury.
One of the major health threats associated with the site is human exposure to levels of mercury that are in excess of the Federally recommended limit for human consumption by eating fish from Clear Lake. In addition, people who come into direct contact with contaminated soils, surface water and sediments may incidentally ingest mercury and arsenic at levels that may cause adverse health effects. Contaminants that leach into the nearby wetland expose biota to harmful discharges of mercury and arsenic.
Who is Involved
This site is being addressed through short-term and long-term Federal cleanup actions.
Investigation and Cleanup Activities
EPA is addressing the site in three phases: emergency actions to prevent short-term harm to the lake and nearby residents, and two long-term remedial phases, called "operable units". In Operable Unit-1 EPA will develop and implement a cleanup plan to address the Herman Impoundment and the mine waste piles located on the mine property. In Operable Unit-2 EPA will develop and implement a cleanup plan to address the contaminated sediments in Clear Lake and the resulting mercury accumulation in the biota.
Erosion control emergency response. In 1992, EPA cut back the slope of the mine waste pile located along the shore of Clear Lake and then reseeded this area. These measures were taken to eliminate the erosion of mine wastes containing mercury and arsenic into Clear Lake. In 1996, EPA raised a small earthen dam at the Herman Impoundment as as a temporary flood control measure.
Soil removal from residential yards. In 1997, EPA removed up to 18 inches of the mercury and arsenic-contaminated mine wastes from 17 residential yards at the Elem Tribal Colony, which is located adjacent to the mine property. EPA placed clean fill material and top soil in the residential yards. Varying amounts of contaminated mine waste remained at depth in the residential yards.
Surface Water Diversion. In 1999-2000, EPA constructed surface water diversions on the mine site to prevent contaminated sediments and water from flowing into Clear Lake. The diversion work included the construction of a 4,000' pipeline which allows clean storm water from up gradient surrounding hills to flow around the site without being contaminated by mine wastes or causing an overflow of the Herman Impoundment.
Well closure emergency response. During the 1970's owners of the mine had allowed exploration companies to drill some exploratory geo-thermal wells near the Herman Impoundment. EPA was concerned that the wells were not properly abandoned and that they might be subject to a violent release of geo-thermal gases. EPA conducted an emergency action in late 2000 and early 2001 to investigate the closure of all of the historic wells, and took action to properly close three wells that had not been properly abandoned.
Operable Unit-1: Herman Impoundment and Waste Piles Investigation. In 1990, EPA began an investigation into the nature and extent of contamination associated with the Herman Impoundment and mine waste piles. EPA initially completed a remedial investigation and feasibility study (RI/FS) and issued a proposed cleanup plan for Operable Unit-1 in 1994. Following the plan's release, the seven-year drought ended with severe flooding, revealing a major acid mine drainage problem affecting Clear Lake. EPA decided to re-open the RI/FS to identify sources of the discharge of mercury-rich acid mine drainage into Clear Lake and to identify appropriate control strategies. In late 2000, the Department of Energy completed an aerial survey of geological resistivity at the site to help define the pathways of contaminated groundwater movement from the mine property to Clear Lake. In early 2001, EPA's contractors completed a hydrogeological study of groundwater movement through the site. EPA has completed the remedial investigation and is currently performing a feasibility study to develop and evaluate cleanup alternatives for the Herman Impoundment and mine waste piles in Operable Unit-1.
Operable Unit-2: Lake Sediments and North Wetland Investigation. In 1991, EPA began an investigation into the nature and extent of contamination in the lake sediments, but put this study on hold in order to focus on the sources of the contaminant discharges at the mine property. Meanwhile, other entities, notably the University of California - Davis, have continued to study the processes affecting bioavailability of mercury in Clear Lake. In 2000, EPA convened a conference of national experts on mercury contamination. EPA is currently conducting additional studies to characterize the nature and extent of mine waste contaminants in the lake sediments and in the North Wetland. EPA is also conducting studies to define the baseline conditions of water quality and the biota in the North Wetland and Clear Lake prior to the performance of the cleanup to provide data to assess the effectiveness of the cleanup.
Soil removal from Elem Indian Colony. In June 2006, EPA began work to remove 28,000 cubic yards of mercury and arsenic contaminated mine waste from the Elem Indian Colony residential area. EPA removed mine wastes from 14 residential yards, several inactive gravel roadways, and from beneath the entire paved roadway system. Clean fill was imported and used to prepare foundations and clean topsoil was brought in for all yards and open areas. EPA installed five new homes, refurbished seven existing homes, repaved the roadway, installed new curbs, sidewalks, water system and storm drains, and improved the sewer system. All Elem residents participated in the Temporary Transitional Housing Program during the mine waste removal action. In December 2006, 15 of 17 households returned to their permanent homes at the Elem Indian Colony. EPA completed the installation of two additional trailers and completed several roadway improvements in June 2007. The remaining two households have returned to their permanent homes.
Mine Waste Removal from Sulphur Bank Mine Road. In January 2008, EPA began work to remove mercury and arsenic contaminated mine waste from the gravel roadways and adjacent areas in a residential area located to the south of Sulphur Bank Mercury Mine along Sulphur Bank Mine Road. EPA completed the mine waste removal action in April 2008.
Mine Waste Capping on BIA Route 120. In late 2010, EPA completed a removal action to re-build Route 120, the main access road to the residential area of the Elem Colony. The re-construction eliminates potential human exposure to mine waste by containing the waste used to construct the original road.
Cleanup Results to Date
EPA has completed erosion control measures to stabilize the shoreline waste pile. EPA has removed all of the contaminated soil from residential yards in the Elem Tribal Colony which is located adjacent to the mine property. EPA has constructed surface water controls to divert storm water away from the Herman Impoundment to prevent overflows of contaminated water to Clear Lake. EPA has completed the proper closure and abandonment of three geothermal wells at the mine property. EPA has removed contaminated mine waste from the gravel roadway and adjacent areas along Sulphur Bank Mine Road.
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.
EPA has identified the Bradley Mining Company and the Worthen Bradley Trust as potentially responsible parties for the Sulphur Bank Mercury Mine site.
Additionally, EPA has identified NEC Acquisition Company as a potentially responsible party with respect to the improper abandonment of three geothermal wells at the mine property. EPA has entered into a settlement with NEC Acquisition Company to recover a substantial portion of the costs that EPA incurred to complete the proper closure and abandonment of the three geothermal wells.
Documents and Reports
Public Information Repositories
The public information repositories for the site are at the following locations:
Lake County Library
1425 North High Street
Lakeport, CA 95453
4700 Golf Avenue
Clearlake, CA 95422
EPA Site Manager
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
8800 Cal Center Drive
Sacramento, CA 95826-3200
PO Box 757
Lower Lake, CA 95457
After Hours (Emergency Response)