MERCURY CONTAMINATION FROM HISTORICAL GOLD MINING IN THE SIERRA NEVADA, CALIFORNIA: EFFECTS ON WATER QUALITY, SEDIMENT, AND BIOACCUMULATION

Following the discovery of gold at Sutter’s Mill in 1848, the California Gold Rush of 1849 evolved into more organized mining activities in the 1850s and 1860s, including the development of hydraulic mining methods for unconsolidated gold placer deposits. Mercury (Hg) amalgamation was widely used, enhancing gold recovery especially for fine-grained particles. Cyanide recovery replaced Hg amalgamation at many gold mines starting in the 1910s and 1920s. About 4.5 million kg of Hg was lost to the environment at placer gold mining operations in California, of which 80–90% was lost in the Sierra Nevada. An additional 1.4 million kg of Hg is estimated to have been lost at stamp mills at hardrock gold mines in the Sierra Nevada. Studies by D. Slotton and others at the University of California, Davis beginning in the early 1990s and by the U.S. Geological Survey (USGS) starting in 1999 have documented widespread Hg contamination of water, sediment, and biota in the Sierra Nevada downstream of historical gold mining operations. The USGS collected samples from 208 locations in the Sierra Nevada during 1999–2004 and from 22 locations during 2011–12, in cooperation with several other federal, state, and local agencies (Alpers and others 2024, https://doi.org/10.5066/P9EL0UN9). Areas were identified with elevated Hg and (or) methylmercury (MeHg) in water, sediment, and biota (invertebrates, frogs, and fish) samples, leading to remediation efforts by federal land-management agencies and fish consumption advisories by state and local public-health agencies. Field experiments by USGS in cooperation with the Bureau of Land Management and the California State Water Resources Control Board in 2007–08 demonstrated that fine-grained particles of elemental Hg(0) and Au-Hg amalgam occurring in stream sediments downstream of hydraulic gold mines are subject to resuspension by suction dredging. Laboratory experiments demonstrated that very small amounts of fine-grained sediment enriched in inorganic Hg(II) will stimulate MeHg production when mixed with sediment from reservoirs and wetlands typical of areas downstream from historic hydraulic mining sites. This presentation will discuss correlations between Hg and MeHg concentrations among sites with jointly collected water, sediment, and (or) biota, as well as correlations with land use, including the distribution of historical mining.