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(202) Transport and Retention of Selenium in Drainage Emanating from Mine Waste, Southeast Idaho.
Stillings, Lisa*,1, Amacher, Michael2, 1 U.S. Geological Survey, Reno, NV2 USDA Forest Service, Logan, UT
ABSTRACT- The Phosphoria Fm in southeast ID is a black shale that has been mined for phosphate during the past several decades. Trace elements can be found in high concentrations throughout the section, leading to concerns over the disposal of mine waste and the mobility of these trace elements during weathering. Our studies focus on the release and mobility of Se from mine waste, generated during phosphate mining. Samples of sediment, surface water, and porewater were collected from a wetland that formed from seepage at the base of a waste-rock pile. Surface water and porewater (collected to a depth of 20 cm) were analyzed for total bulk chemistry, Fe(II), Se(IV), and Se(VI). Sediment samples (collected to a depth of 15 cm) were analyzed for total bulk chemistry, and selective extractions were performed for exchangeable, carbonate, Mn oxide, Fe oxide, and organic matter + sulfide fractions. Se concentrations in seep water can be as high as 520 ppb, although they vary spatially and temporally and are greatest during high water flow. Se concentrations in wetland surface water decrease exponentially with distance from the seep, dropping to 5-10 ppb over 250 m. Sediment pore water contains low Se concentrations, with an average of 5 ppb. Concentrations in both surface and porewater vary inversely with Fe(II). Wetland sediment contains from <10 to >1000 ppm Se, and concentrations decrease with distance from the seep. Selenium is found within the ferrihydrite fraction of the sediment, and also as elemental Se. We hypothesize that Se is removed from surface and porewater by adsorption and coprecipitation with ferrihydrite, and by reduction to elemental Se. The presence of a wetland environment is critical for creation of the redox conditions responsible for the observed Se partitioning. In contrast, water draining from mine waste without surrounding wetlands retains high concentrations of Se in solution.
Key words: selenium, ferrihydrite, Phosphoria Formation, wetland
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