| Two pilot-scale wetland treatment system cells (nutrient amended and unamended) were designed and constructed to reduce aqueous Se concentrations in simulated energy-derived water. Specific objectives of this study were: (i) measure and correlate hydrosoil conditions with Se concentrations vertically through the hydrosoil; (ii) investigate Se-accumulating biogeochemical processes (dissimilatory Se reduction and sorption) operating in the hydrosoil; and (iii) evaluate the effect of a nutrient amendment on hydrosoil conditions, Se accumulation, and Se-sequestering biogeochemical processes in the hydrosoil. Se accumulation (i.e. total Se concentration) and hydrosoil conditions were measured with depth in the hydrosoil. Se-sequestering biogeochemical processes were investigated by counting Se-reducing microbial colony forming units (CFUs) and identifying Se geochemical fractions at various depths in the hydrosoil. The detritus (0-21 cm in nutrient amended cell and 0-14 cm in unamended cell) contained greater Se concentrations (308-830 mug/g and 138-569mug/g) and greater CFUs (2,700-22,000 CFUs/mL pore water and 9,300-15,000 CFUs/mL pore water) than the underlying sandy sediment. Correlation of organic matter content with Se concentration (r = 0.95; p<0.00001 in nutrient amended and r = 0.87; p<0.00002 in unamended) suggests organic matter influences Se-accumulating biogeochemical processes. In detritus, mean hydrosoil conditions (redox: -2 to -173 mV, pH: 6.20-6.46, and organic matter: 52-86%) were more favorable for sorption than for dissimilatory Se reduction; however, the majority of Se measured in the detritus was elemental (52.1%-58.0% in the nutrient amended cell and 21.1%-62.6% in the unamended cell) suggesting that dissimilatory Se reduction is the dominant biogeochemical process sequestering Se in the detritus. T-tests indicate significant difference in pH (t = 2.87, p = 0.0132) of the hydrosoil between the nutrient amended cell and unamended cell, but no significant differences (p<0.005) in redox potential, organic matter content, and Se concentration. Greater Se concentrations and percent of elemental Se in the nutrient amended cell than the unamended cell suggests that the nutrient amendment enhanced dissimilatory Se reduction and therefore Se accumulation in the hydrosoil. |
