Sulphur Bank Mercury Mine is located on the shores of Clear Lake in Lake County, California, in Clearlake Oaks. The history of mining activity at Sulphur Bank dates back to the mid 1800s, when settlers began mining sulphur for gun powder. Within a few years, they found the sulfur was contaminated with cinnabar - an ore containing mercury. By this time, miners had discovered that mercury was essential in extracting gold from ore, which spurred mercury mine development throughout California. The Sulphur Bank Mercury Mine owners dug mine shafts to facilitate removal of mercury-laden ore, which became a full-scale open pit in the 1920s when bulldozers and dump trucks were developed.

The mine was closed in 1957, leaving a hole 90 feet deep that began filling with water from creeks and geothermal springs. Dubbed the Herman Impoundment, the “lake” covers 23 acres amid 120 acres of mine tailings and waste rock.

During the 1970s and 1980s, it was discovered that catfish and bass in Clear Lake had elevated levels of mercury. A barrage of media coverage blasted the news across the State, and the local tourism industry suffered. Today, the Calif.Dept of Health Services warn people not to eat more than 1 lb of fish per month from Clear Lake, while children and pregnant women are warned not to eat any.

Despite the warnings, it took a concerted effort of scientists and residents to convince the government that problems were stemming from the mine site. It wasn’t until 1990 that the property was designated an EPA Superfund Site, and it is currently the largest mercury Superfund site in the nation. While the EPA has control over the clean-up operations, the land itself is still owned by Bradley Mine. 

In the past decade, the EPA as well as numerous scientists/researchers from U.C. Davis and world-wide, have conducted extensive tests to determine where the mercury contamination is coming from, how it is getting into Clear Lake, and most importantly, how to fix it. Despite spending millions of dollars and thousands of research hours, the answer is still not clear.

Much of the problem appears to stem from the highly acidic Herman “Pit” which has a pH level of 2.6 to 2.7, supports only bacterial life, and is filled with arsenic and other contaminants. Carbon dioxide, hydrogen sulfide and other gases fizz up from vents at the bottom of the pit. Geothermal inflows and rainwater seep into the pit and through the waste rock, become highly acidic and dissolve mercury. While there is a sizeable dam separating the pit from the lake, it is a porous barrier which allows acidic water to drain into Clear Lake, carrying mercury. Bacteria in the sediments converts this to methyl mercury, which is easily absorbed by wildlife and fish.

Today, the entire Herman Impound is fenced off from the public to ensure that contaminants are not tracked out by foot. The EPA has instituted a number of remedial projects at the Sulphur Bank Mercury Site, which have helped, but not fixed, the problems. These included emergency well closure and capping; removal of arsenic and mercury contaminated soil from the Elem Indian Colony; surface water diversion via a 4,000 foot pipeline and erosion control/regrading.

A long-term cleanup plan has been delayed by funding cutbacks as well as disputes on how to solve the problem, but the mercury continues to contaminate Clear Lake. 

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