Mercury contamination from gold mining remains a problem in California waters

In many gold-mining areas where mercury was used, it is still relatively easy to find quantities of liquid elemental mercury in sediments and stream channels. Of even greater environmental concern is the presence of methylmercury, an organic form of mercury that is a potent neurotoxin and is especially detrimental to developing fetuses and children. Methylmercury accumulates and biomagnifies in the food chain, reaching highest concentrations in predatory fish such as bass and other species which are prized by anglers. Numerous water bodies in California have fish-consumption advisories because of mercury contamination from historical mining. Several of these advisories are based on data collected by the USGS, including those in Trinity County, and in the Bear, Yuba, and American River watersheds in the Sierra Nevada. For information on these advisories, see the web site of the California Office of Environmental Health Hazard Assessment.

Mercury from hydraulic mining has been transported with sediments downstream into the San Francisco Bay/Sacramento-San Joaquin Delta estuary, where it has contributed to elevated mercury concentrations in fish, resulting in additional consumption advisories and regulatory action through the Total Maximum Daily Load (TMDL) process. For information on the TMDL process see the web sites of the California Regional Water Quality Control Boards:

San Francisco Bay Mercury TMDL

Sacramento - San Joaquin Delta Methylmercury TMDL

The Cache Creek watershed drains eastward from the north-central part of the California Coast Range. It includes Clear Lake, the largest and oldest natural lake located entirely in California. There are several historical gold and mercury mines in the Cache Creek watershed, including the Sulphur Bank Mercury Mine, which is a USEPA Superfund site, and the Abbott and Turkey Run mines which were recently remediated by the private sector. There is a mercury TMDL in place for Cache Creek and some of its tributaries, developed by the California Regional Water Quality Control Board - Central Valley Region.

Ongoing studies by USGS are focused on characterizing and quantifying sources of mercury and methylmercury to the Bay-Delta, including continuing runoff from mercury mining areas in the Coast Ranges and from gold-mining areas in the Sierra Nevada, and from resuspension and diffusive transport from mercury-contaminated sediments already in the rivers and reservoirs in the Bay-Delta watershed. For more information, check out this USGS web page: