Superfund Information Systems: Site Profile

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The 150-acre Sulphur Bank Mercury Mine site is an abandoned open pit mercury mine located on the shoreline of Clear Lake in Lake County, California. Former mining activities at the site contaminated soils, sediments and surface water with mercury and arsenic. The mine introduced mercury to 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.

The Sulphur Bank Mercury Mine was mined intermittently for sulphur and mercury between 1865 and 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 from the lake. Clear Lake is the source of water that the Clear Lake Oaks County Water District treats and provides for municipal drinking water for 4,700 people..

Following immediate actions to protect human health and the environment, site investigations and long-term cleanup planning are ongoing.

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What Has Been Done to Clean Up the Site?


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.

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 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 eliminated potential human exposure to mine waste by containing the waste used to construct the original road.

Lake Sediment Capping Study: In 2013, EPA constructed two test “caps” or sediment covers, placing a layer of sand and gravel over mercury-containing sediments in clear lake near the mine. This test demonstrated that sediment capping is a viable technology for the site-specific conditions at Clear Lake and worthy of further consideration in the Feasibility Study.


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What Is the Current Site Status?

EPA is currently completing a Focused Feasibility Study for the terrestrial (on-land) portion of the mine site. This study will update the 2006 Feasibility Study with new information gathered from recent site investigation, and compare potential alternatives for cleanup. This report is planned for finalization in early 2018. The information from this study will be used to plan the next cleanup actions at the site.

EPA is also conducting a new Human Health Risk Assessment to evaluate the ways that contamination from the Superfund site may pose a risk to the health of community members and users of Clear Lake. EPA plans to complete this assessment in 2018.






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Activity and Use Limitations

At this site, activity and use limitations that EPA calls institutional controls are in place. Institutional controls play an important role in site remedies because they reduce exposure to contamination by limiting land or resource use. They also guide human behavior. For instance, zoning restrictions prevent land uses – such as residential uses – that are not consistent with the level of cleanup.

For more background, see Institutional Controls.

EPA installed fencing and signs around the mine site to prevent public access to the site.

The State of California issued a fish advisory recommending that people limit their consumption of certain fish from Clear Lake. Click here for more information.

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Emergency Response and Removal

EPA monitors the water level in Herman Impoundment to determine the risk of overflow into Clear Lake. Overflow has not occured since EPA's construction of a stormwater diversion system in 1999. Contingency planning for exceptionally wet winters includes deployment of emergency water treatment should overflow appear imminent.

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Enforcement Information

On April 18, 2012, the United States of America, on behalf of the Administrator of the US Environmental Protection Agency, finalized entry into a Consent Decree with Bradley Mining Company, Frederick Bradley - Trustee for the Worethen Bradley Family Trust, and the Elem Indian Colony of Pomo Indians. The Consent Decree, available here, describes the enforcement history at the site and the roles, liabilities and obligations of the involved parties.

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