MERCURY TMDL DEVELOPMENT FOR SOUTH DAKOTA

Currently nine lakes and impoundments within South Dakota are EPA 303(d) listed as mercury impaired waterbodies due to occurrence of fish tissue concentrations exceeding 1.0 mg/kg total mercury. In cooperation with the South Dakota Department of Environment and Natural Resources (SD-DENR), a series of interdisciplinary monitoring and assessment studies commenced in 2008 to understand mercury loading and cycling behavior within South Dakota air and watersheds, with the goal of collecting data to complete a total maximum daily loading (TMDL) criterion for mercury impaired waterbodies in South Dakota. Comparisons between historical water quality and mercury fish tissue concentrations were screened and analyzed using binary logistic regression, Akaike’s Information Criterion, and stepwise logistic regression methodology. Correlations were strongest between northern pike and walleye mercury tissue concentrations and water column alkalinity, total solids, and phosphorous concentrations, and suggest productivity and consequential eutrophication are the predominant indicators of fish tissue mercury susceptibility for aquatic conditions common in South Dakota. Eleven sediment cores collected at both advisory and non-advisory lakes and impoundments were analyzed for total mercury, iron, phosphorous, sulfur, sulfate, carbon, and water content, and quantitatively dated using Pb²¹⁰ and Cs¹³⁷ radiodating to estimate historical mercury fluxes. Strong correlations exist between sulfate reduction and sediment mercury, suggesting predominant pathway of methyl mercury formation and aquatic uptake is due to sulfate reducing bacteria activity within mercury advisory lakes. Advisory lakes sediment mercury concentrations range from 13 to 310 mg/kg, with the highest concentrations generally found within the top 10cm sediment depth. Nine bulk atmospheric mercury deposition monitors were deployed throughout South Dakota, Nebraska, North Dakota, and Wyoming, and a strong positive correlation exists between local atmospheric mercury deposition and mercury fish tissue occurrences. Results from these studies provide the basis of future mercury TMDL development for South Dakota lakes and impoundments.