MERCURY SOURCES, TRANSFORMATIONS, AND TRANSFER THROUGH THE CLEAR LAKE ECOSYSTEM, CALIFORNIA, USA

Clear Lake is a 180 km² freshwater lake located in California’s northern Coast Ranges, approximately 120 km northwest of Sacramento. The lake supports a wide variety of fish and bird species and is a popular sport-fishing destination; however, mercury (Hg) concentrations in the food web pose a threat to both human and ecosystem health. The Sulfur Bank Mercury Mine (SBMM), on the eastern shore of the Oaks Arm of Clear Lake, was mined using both open-pit and underground methods (active from the 1870’s until 1957) and is now a U.S. Environmental Protection Agency (EPA) Superfund site. This former Hg mining area is likely the dominant contributor of Hg to Clear Lake and is the focus of continued clean-up efforts to minimize Hg impacts at the mine site and to Clear Lake. The SBMM area is also part of the Geysers-Clear Lake geothermal area, with gas fumaroles containing CO₂, CH₄, and H₂S, with trace amounts of gaseous Hg(0). Studies by M. Gustin and others revealed that Hg(0) can evade from mine waste, potentially re-entering the local cycle. Successful remediation of the SBMM requires a comprehensive understanding of the sources and forms of mercury that are transported away from the mine site, and that are transformed to methylmercury (MeHg), a more toxic form of mercury that is bioaccumulated in the food web. In collaboration with the EPA, the U.S. Geological Survey (USGS) is analyzing naturally occurring Hg isotopes in lake water and lake sediment, pore water and ground water flowing toward the lake from a mine waste area, air, lichen, and several trophic levels of biota within the lake to determine sources and pathways for Hg and MeHg in the Clear Lake food web. Hg isotope analyses should provide information on Hg sources at SBMM, and allow quantification of long-range atmospheric transport (e.g., from Asia). Elevated concentrations of MeHg in sediment, water, and biota of Clear Lake are associated with prolonged low-oxygen events at depth in the Oaks Arm. A pilot test is being planned to evaluate hypolimnetic oxygenation as a strategy to mitigate MeHg formation. Adding Hg cycling to an existing 3-D hydrodynamic model of Clear Lake will facilitate hypothesis testing. New insights on vectors for Hg flow through the Clear Lake ecosystem from these efforts will support EPA’s decision making regarding potential remedial actions to aid in the lake’s recovery from elevated Hg concentrations in the food web.