THE IMPORTANCE OF WETLAND PROCESSES IN THE REMOVAL OF SE FROM MINE DRAINAGE

High concentrations of Se are observed in surface seepage emanating from 2 separate mine dumps, which resulted from phosphate mining in the Phospohoria Fm., southeast Idaho. At Wooley Valley, a wetland has formed at the base of the waste dump, whereas seepage from the Maybe Canyon dump drains directly into a rapidly flowing stream with only a minor adjacent wetland. At Wooley Valley samples of sediment (collected to 15 cm depth), surface water, and pore water (collected to 20 cm depth) were collected and analyzed for total bulk chemistry, Fe(II), Se(IV), and Se (VI). Selective extractions were also performed on the sediment samples for exchangeable, carbonate, Mn oxide, Fe oxide, and organic matter + sulfide fractions. Se concentrations in the seepage peak at 520 ppb, although they vary spatially and are greatest during high flow periods in the spring and early summer. In surface waters of the wetland, Se concentrations decrease exponentially with distance from the seep site, decreasing to 5-10 ppb over 250 m. Pore waters average 5 ppb Se. Wetland sediments contain from <10 to >1000 ppm Se, and concentrations also decrease with distance from the seep. Selective extraction data show that Se is found within the ferrihydrite fraction of the sediment and as elemental Se. In contrast, stream waters at Maybe Canyon contain 1800 ppb Se at the seepage site. These concentrations decrease to 500 ppb over a distance of 2000 m downstream. We argue that the wetland environment at Wooley Valley creates the redox conditions necessary for Se to adsorb/co-precipitate with ferrihydrite, and to become reduced to elemental Se. Also, the slow water flow through the wetland allows sufficient time for these reactions to occur.