A Multimodal Approach to Develop a TMDL for Mercury Impaired Lakes and Impoundments in 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 total maximum daily loading (TMDL) criterion for mercury impaired South Dakota waterbodies.  Comparisons between historical water quality and mercury fish tissue concentrations were analyzed using regression analysis, box-plots, and Akaike’s information criterion (AIC).  Correlations were strongest between Northern Pike and Walleye mercury tissue concentrations and water column alkalinity, total solids, dissolved oxygen, and phosphorous concentrations. Eleven sediment cores collected at both advisory and non-advisory lakes and impoundments were analyzed for total mercury, iron, phosphorous, sulfur, and water content, and quantitatively dated using Pb²¹⁰ and Cs¹³⁷ radiodating to estimate historical mercury fluxes.  Advisory lake and impoundment mercury sediment concentrations ranged 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 deposition results suggest positive correlation between monthly total mercury deposition (wet and dry) and precipitation.