SEDIMENT MERCURY BEHAVIOR IN SOUTH DAKOTA LAKES AND IMPOUNDMENTS

Currently seven lakes and impoundments within South Dakota are EPA 303(d) listed as mercury impaired water bodies where mercury fish tissue concentrations have exceeded 1.0 mg/kg total mercury. The objectives of this study were to quantify the extent of sediment mercury within a subset of South Dakota water bodies, and to establish sediment and watershed relations responsible for biogeochemical methyl-mercury production and eventual accumulation by fish. Sediment cores were collected from eleven water bodies throughout South Dakota based upon current mercury fish advisory status and lake depth, and analyzed for total mercury, a suite of total metals, water content, total carbon and nitrogen concentration, and radiometric dating. Sediment profiles collected at four mercury advisory lakes contained low total sulfur (< 6 mg/g), moderate total iron (20-35 mg/g) and high total mercury (60-200 ng/g) compared to non mercury advisory lakes . The presence of low sulfur (< 6 mg/L) within advisory lake sediments suggest much of the available sulfur was mobilized through biological sulfate reduction-related processes, and appears responsible to increased methyl-mercury production. High sulfur sediment concentrations favor the formation of sulphides that are effective mercury complexants, thus rendering mercury unavailable for methylation. On-going radiometric dating analyses (Pb²¹⁰ and Cs¹³⁷) will provide estimates of historical watershed mercury fluxes entrapped within lake sediments. Mercury non-advisory lake watersheds generally contained higher percentage of wetlands compared to advisory lakes, suggesting that wetlands appear to be effective scavengers or sinks of mercury and effectively reduce mercury transport into the lakes. Catchment area to lake area ratios (CA: LA) correlated positively with organic solids and mercury lake loading. The extent of watershed grasslands correlated positively with sediment mercury, while cultivated land exhibited a negative correlation, suggesting that land use appears to influence mercury watershed behavior. Findings from this study provide a better understanding of mercury behavior within watersheds and sediments of South Dakota lakes and impoundments.