Chemical composition of bottom sediments within black hills region reservoirs of South Dakota and Wyoming

The objectives of this study were to determine the effect of heavy metals transported from mining areas due to historical mining activities within the Black Hills region of South Dakota and Wyoming US, and to evaluate the presence of major and trace elements resulting from influence of provenance, weathering, erosion, and sediment sorting within reservoir sediments. Soil core samples were collected from both inflow and outflow of the reservoirs. X-ray florescence analyses were performed to determine the presence of major and the trace elements concentration. Pactola and Sheridan Lakes had the highest concentration of As, Fe, Mn, and Rb/Sr ratios, while the Belle Fourche Reservoir had the highest U concentration. Spatial and temporal patterns of Rb/Sr ratio of the reservoir sediments were used to determine the weathering intensity of the catchment areas. With the exception of Angostura and LAK reservoirs, all Black Hills reservoirs were dominated by physical weathering processes within their respected watersheds. Immobile high field strength elements from within the reservoir sediments were higher compared to upper continental crust (UCC) concentrations, and further suggest the dominance of physical weathering processes for all reservoirs except LAK. Spider diagrams normalized to UCC concentrations indicate that Black Hills regional reservoirs were elevated in Zr, Hf, and U, further supporting the dominance of physical or mechanical weathering influences from their watersheds. Watersheds dominated by physical weathering processes are commonly subjected to high erosion, anthropogenic activities, and/or high precipitation processes.