HR: 1340h
AN: H43D-0391    [Abstracts]
TI: Biogeochemistry of mercury in soils and sediments in a mining-impacted watershed, California
AU: * Holloway, J M
EM: jholloway@usgs.gov
AF: U.S. Geological Survey, Bldg 20, DFC, MS 973, Denver, CO 80225 United States
AU: Goldhaber, M B
EM: mgold@usgs.gov
AF: U.S. Geological Survey, Bldg 20, DFC, MS 973, Denver, CO 80225 United States
AB: The East Davis Creek watershed, located in the California Coast Ranges, is host to historic mines that provided mercury for recovery of gold in the Sierra Nevada goldfields in the mid-to-late 1800s. Bedrock in this watershed includes marine sedimentary rock, serpentinite, and hydrothermally altered serpentinite. Cinnabar (HgS) found in the altered serpentinite is the primary ore mineral for mercury. We evaluated the hypothesis that mercury is sequestered in soil organic matter downstream from source areas, releasing a fraction as water-soluble methylmercury. Microbial biomass and the presence of sulfur-reducing bacteria implicated in mercury methylation were quantified using phospholipid fatty acid (PLFA) data. Methylation incubations were performed on soil and sediment inoculated with water from Davis Creek Reservoir and sealed in glass containers under an anoxic headspace for 21 days. Methylmercury was measured on extracts of the soils at the start and at the end of the incubation period. Two sources of mercury to stream sediments, a soil with an altered serpentinite parent and mine tailings, were incubated. Stream sediment, an overbank deposit soil and a wetland soil forming from these sediments were also incubated. The overbank deposit soil is periodically flooded. The wetland soil around the edge of Davis Creek Reservoir is perennially saturated with water. The altered serpentinite soil and mine tailings had the highest total mercury concentrations (170 and 150 ng Hg /g, respectively). Total mercury concentrations in stream sediments are low (­š1 ng Hg/g), with higher mercury concentrations in the overbank (3 ng/g) and wetland soils (18 ng Hg/g). Mercury leached from altered serpentinite soils and mine tailings may be transported downstream and sequestered through sorption to organic matter in the overbank and wetland soils. PLFA biomarkers for {\it Desulfobacter} (10Me16:0) and {\it Desulfovibrio} (i17:1) were present in all incubated materials, with lower concentrations in mine tailings and stream sediment relative to the three soils examined. Methylmercury was initially present in greater concentrations in the overbank deposit (23 ng HgMe/g) soils. The elevated methyl mercury in the overbank deposit soil may be due to the greater biomass of sulfur reducing bacteria indicated by the 10Me16:0 and i17:1 biomarkers. During the 21-day incubation, methylmercury increased from 0.6 to 15 ng HgMe/g in the wetland soil concomitantly with sulfate decreasing from 130 to 7.0 mg SO$_{4}$$^{=}$/g. Methylmercury concentrations did not change appreciably in the other soils, although sulfate decreased from 19 to 2.0 mg SO$_{4}$$^{=}$/g in the overbank deposit soil. These data suggest that overbank deposits and wetland soils sequester mercury leached from upstream sources, with a fraction of this mercury released through microbial methylation.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting