Mercury Methylation and Bioaccumulation in the Bear River Watershed

Science Center Objects

Extensive mercury use during historical gold processing has resulted in widespread mercury contamination and bioaccumulation of methylmercury in watersheds in the northwestern Sierra Nevada (Slotton and others 1997; Alpers and Hunerlach, 2000). A recent USGS report (May and others, 2000) documented the presence of elevated mercury in fish tissues in five reservoirs in the Bear River, Deer Creek and South Yuba River watersheds, which are among the most intensely mined.

Bear River California

Bear River in Northern California

The two reservoirs with the highest concentrations of mercury in fish (of the five tested) are Lake Combie and Camp Far West Reservoir, both in the Bear River watershed. There is presently little information on biogeochemical processes controlling rates of mercury methylation/demethylation and bioaccumulation in Sierra Nevada watersheds. An ongoing USGS study (project CA556; Hunerlach and others, 1999; 2000) has identified mercury "hot spots" at abandoned mine sites, especially certain ground and tunnel sluices used for gold recovery in conjunction with hydraulic mining. Several of the most concentrated "hot spots" are located in the Bear River watershed, including the Buckeye Flat mining district and the Polar Star mine tunnel, which was remediated during 2000 by the EPA as a Superfund emergency response removal action. Elevated concentrations of mercury and methylmercury are present in water, sediment, and biota at these "hot spots"; however, it remains unknown whether remediation of these mine sites would have a measurable effect on reducing the mercury content of fish in downstream reservoirs. The source(s) of methylmercury to the downstream reservoirs and the factors that control methylation, demethylation, and bioaccumulation of mercury remain undetermined and unquantified in Sierra Nevada watersheds.

A better understanding of hydrogeochemical and microbiological processes that control mercury cycling is critical so that a scientific basis can be established for resource management both in the Bear River and in downstream areas with impaired beneficial uses because of mercury, such as the Sacramento River and the Sacramento-San Joaquin Delta. The latter areas are currently on the State's 303(d) list as impaired for mercury, primarily because of bioaccumulation in fish, and are high priorities for the development of Total Maximum Daily Loads (TMDLs). The Bear River watershed is a likely candidate for 303(d) listing and future TMDL development based on USGS data on fish (May and others, 2000) and water quality. It would be advantageous for TMDL development in these watersheds if predictions could be made with reasonable certainty regarding the likely consequences to mercury bioaccumulation of different management scenarios, including load reductions of mercury, nutrients, sulfate, or sediment.