Mercury Contamination of Water, Sediment, and Biota in Watersheds Affected by Historic Hydraulic Gold Mining in California

Abstract for 2001 DOI Conference on the Environment
Conference held March 13-15,2001, Albuquerque, NM

Charles N. Alpers(1), Michael P. Hunerlach(1), Roger L. Hothem(2), Jason T. May(3), James J. Rytuba (4), and David Lawler(5)

(1) U.S. Geological Survey, Water Resources Division, 6000 J St., Placer Hall, Sacramento, CA 95819 Tel.: 916-278-3134; Fax: 916-278-3013; e-mail: cnalpers@usgs.gov
(2) U.S. Geological Survey, Biological Resources Division, 1 Shields Ave., Davis, CA 95616
(3) California State University Sacramento Foundation, 6000 J St., Sacramento, CA 95819
(4) U.S. Geological Survey, Geologic Division, 345 Middlefield Rd., Mailstop 901, Menlo Park, CA 94025
(5) Bureau of Land Management, 2800 Cottage Way, Sacramento, CA 95825

Mercury, which was used extensively in gold processing in the Sierra Nevada and Trinity-Klamath regions of California during the late 19th and early 20th centuries, has contaminated water, sediment, and biota on a watershed scale. Hydraulic mining has severely modified the geomorphology and hydrology of these regions, leading to increased turbidity of the natural waters, siltation of riverbeds, and contamination of the bottom sediments of reservoirs downstream of the mines. At hundreds of individual mines, thousands of pounds of mercury were used to facilitate gold recovery by amalgamation within sluice boxes during hydraulic mining. Visible concentrations of elemental mercury remain in the sediment of some sluices and tunnels that drain abandoned mine pits. The continued interaction between the mercury-laden sediment and flowing water is responsible for ongoing mercury and methylmercury contamination of surface waters.

The Bear River and South Yuba River watersheds of the northern Sierra Nevada are among the most highly impacted areas. During 1999Ð2000, an interagency team, including scientists from the U.S. Geological Survey, the Bureau of Land Management, the U.S. Forest Service, as well as state and local agencies, sampled water, sediment, and biota from several abandoned mine land (AML) sites and downstream reservoirs in these watersheds. In 17 of 53 unfiltered water samples, total recoverable mercury (TR-Hg) was above the 50 ng/L (nanograms per liter) EPA aquatic life criterion. Water flowing from two tunnels in the Bear River watershed had TR-Hg concentrations greater than 100,000 ng/L, well above EPA's drinking water standard of 2,000 ng/L. Downstream of the mine sites, the Bear River had unfiltered concentrations of methylmercury (MeHg), a toxic form that readily bioaccumulates, greater than 0.4 ng/L during JulyÐAugust 1999 and January 2000. Elevated Hg and MeHg in several taxa of predaceous invertebrates and elevated Hg in amphibians generally occur at, and downstream of, the hydraulic mine sites with the most elevated TR-Hg and MeHg concentrations in water and sediment. Fourteen percent (8 of 57) of bass samples had Hg concentrations (in fillet) greater than or equal to 1 ppm (wet weight), and 52 percent (74 of 141) of all fish analyzed had concentrations of Hg greater than or equal to 0.3 ppm, the new (December 2000) EPA water-quality criterion. Bioaccumulation of methylmercury in sport fish downstream of the hydraulic mine sites may be of concern to human health, especially in reservoirs with shallow, relatively warm-water conditions. On the basis of the results of this interagency study, three counties have issued public notification recommending limited fish consumption-the first such advice from a public agency with regard to reservoirs and streams in the Sierra Nevada.

During 2000, the interagency AML team studied mercury contamination in areas affected by historic gold mining in both the Bear-Yuba and the Trinity River watersheds. Concentration data for water, sediment, and biota (invertebrates, amphibians, and fish) in both watersheds will be used to prioritize sites for eventual remediation. Monitoring of key sites before and after remediation will allow documentation of the benefits in terms of reduced mercury bioaccumulation and mercury loading to downstream waterbodies.