Hydrological and biogeochemical controls governing the speciation and accumulation of selenium in a wetland influenced by mine drainage.

Controls governing the speciation and accumulation of Se in a 3.7-ha marsh influenced by mine drainage were assessed through examination of water balance, water quality, sediment, and plant tissue components. Over the 8-mo study period (April through November, 2009), mean monthly flows ranged from 1600 to 2300 m3  d-1 (hydraulic retention time of 1-3 d). Total Se concentrations in the marsh outflow were lower than the inflow by 0.4 to 6.2 μg L-1 (mean difference = 3.3 μg L-1 ), illustrating Se removal. The Se accumulation pathways are illustrated by elevated concentrations of Se in sediments (3-35 mg kg-1 dry wt) as well as in below-ground (2-41 mg kg-1 dry wt; mean = 10 mg kg-1 dry wt) and above-ground (0.8-6.3 mg kg-1 dry wt; mean = 2 mg kg-1 dry wt) emergent plant tissues. Redox stratification in the shallow water column had a marked effect on Se speciation and behavior, illustrating bottom water removal of dissolved selenate in suboxic horizons and increased mobility of dissolved organo-Se. Mass balance data yielded inflow and outflow loading rates for Se of 27 and 23 g d-1 , respectively (net accumulation rate of 4 g d-1 or 0.11 mg m2  d-1 ). The rate of accumulation as calculated from the mass balance agrees with independently measured rates of Se accumulation in sediments for the site (3.6-8.1 g d-1 or 0.10-0.22 mg m-2  d-1 ). Environ Toxicol Chem 2018;37:1824-1838.