ENVIRONMENTAL GEOCHEMISTRY AND REMEDIATION OF ABANDONED, MERCURY-CONTAMINATED, PLACER GOLD MINES IN THE SIERRA NEVADA, CALIFORNIA

The extensive use of elemental mercury (Hg) to enhance gold recovery during the late 19^th and early 20^th centuries resulted in widespread Hg contamination along the western slope of California's Sierra Nevada. Although Hg was used in conjunction with both lode and placer gold mining operations, historical records indicate that significantly more Hg was used and lost to the environment from placer gold mining activities, including hydraulic mining, drift mining, and dredging, compared with Hg use and loss from hard-rock stamp mills and associated retorts. To identify Hg “hot spots” as remediation targets on public and private lands in the Bear River and Yuba River watersheds (NW Sierra Nevada), the U.S. Geological Survey worked in cooperation with other federal, state, and local agencies on detailed studies during 1999–2004. Samples of water, sediment, fish, frogs, and insects from abandoned placer gold mine sites and receiving waters were analyzed for total Hg and methylmercury (MeHg), a toxic, organic form of Hg that readily bioaccumulates. Based on results of this effort, the State of California issued a local fish-consumption advisory and several remediation projects at abandoned placer gold mines were planned, involving removal and (or) isolation of Hg-contaminated sediments. Three such remediation projects have been completed by federal agencies since 2000 in the Bear River watershed. In 2000, the Polar Star mine tunnel, a 150-meter-long sluice tunnel in the Dutch Flat mining district, was remediated by the U.S. EPA. In 2003, the U.S. Forest Service remediated a Hg-contaminated tunnel at the Sailor Flat mine, and in 2006, the Bureau of Land Management completed remediation of the Boston mine tunnel, a 130-meter-long sluice tunnel in the Red Dog mining district. These pilot efforts to remove Hg from flowing sluice tunnels has provided environmental benefits by reducing Hg and MeHg loads to receiving waters. However, achieving the ultimate goal of reducing environmental risk by lowering MeHg concentrations in biota will require a more comprehensive understanding of environmental processes affecting Hg methylation and transport, such as the interaction of Hg speciation, organic carbon, sulfate, iron, and microbial activity, and the seasonality of nutrient fluxes affecting food web bioaccumulation dynamics.