The mobilization of arsenic (As)-rich floodplain sediments resulting from gold mining legacy along the Cheyenne River and Whitewood Creek, South Dakota, were examined using aqueous speciation chemistry, X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Powder Diffraction (XRD). This study seeks to understand the release of arsenic and potential mechanism for mobility with the As-Fe-bearing solids found in the Cheyenne River. The highest concentrations of acid-extractable As measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) from solid samples in the Cheyenne River bank collected in June 2016 at Deal Ranch (DR) were in the range of 259-386 mg/kg. Closer to the Homestake Mine on Whitewood Creek (WW), As concentrations were as high 2,020 mg/kg. Similar mineralogy between the DR and WW samples was identified through XRD analyses, with the predominance of alumino-silicate and iron-bearing minerals. A batch experiment reacting these sediments (DR, WW) with phosphate (0.1 and 10 mM) and bicarbonate (0.2 mM and 20 mM) under environmentally relevant pH conditions suggested that arsenic is mostly (&gt;90%) released as As(V). The mobilization of As(V) with the bicarbonate treatment was 17 µg/L, however the highest release (6,234 µg/L) occurred with 10 mM sodium phosphate treatment. Furthermore, a decrease of 16% As(V) and 12% Fe(II) was detected from both solids using XPS after being reacted with sodium phosphate. These results indicate that arsenic release is favored after reaction of sediments by competitive ion exchange with 10 mM PO43- compared to 10 mM HCO3-. The results from this study aid in a better understanding of arsenic in surface waters affected by mining legacy.

Arsenic mobility and reactivity near abandoned mine wastes in Cheyenne River, South Dakota (CRST)

255th National Meeting (2018)

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