STREAM SEDIMENT TRANSPORT AND WETLAND SEQUESTRATION OF TRACE METALS IN A NORTHERN CALIFORNIA COAST RANGE WATERSHED IMPACTED BY HISTORIC MERCURY MINING

The transport and biogeochemical interactions of Cr, Ni and Hg in stream sediment and wetland soils forming from these sediments was studied in a small watershed in northern California that has been impacted by historic mercury mining. Bedrock in the Davis Creek watershed includes silicic volcanic rocks, marine siltstone, serpentinite, and hydrothermally-altered serpentinite. Elevated concentrations of Cr and Ni result from the weathering of serpentinite, with elevated Hg concentrations associated with hydrothermally altered serpentinites. Stream sediment derived from the erosion of soils forming on the upland parent material is transported downstream and is deposited along overbank deposits and along a delta entering Davis Creek Reservoir. The sediments are parent material for wetland soils in these depositional environments. The relationship between phospholipid fatty acid (PLFA)-defined soil microbial communities, hydrology, and soil chemistry was used to evaluate the role of wetland soils in sequestering trace metal downstream from the historic mercury mines and serpentinite soils. Stream sediments (approximately 70 percent derived from readily weathering siltstones) have elevated chromium (610 to 1430 ppm Cr) and nickel (410 to 550 ppm Ni) concentrations that reflect the input of these metals from weathering serpentinite. Mercury derived from weathering waste rock and altered serpentinite is also elevated in stream sediments (3 to 11 ppm Hg). Wetland soils have elevated trace metal concentrations (820 to 2540 ppm Cr, 550 to 840 ppm Ni, and 6 to 55 ppm Hg) relative to the stream sediment parent, suggesting complexation by soil organic matter. Statistical analyses of soil chemistry and PLFA profiles indicate distinct microbial communities associated with serpentinite soils, consistent with the elevated Cr and Ni concentrations. Methyl-Hg assays and PLFA biomarkers for sulfur reducing bacteria suggested that Hg is, in part, released through microbially-mediated methylation. These data indicate that wetland soils are important in the retention of trace metals in this watershed through processes including sorption and microbial cycling.