ROLE OF GASTROPODS IN ARSENIC AND SELENIUM BIOGEOCHEMICAL CYCLING IN A CONSTRUCTED WETLAND TREATMENT SYSTEM

Arsenic (As) and selenium (Se) are problem constituents in mine wastewaters and are toxic to aquatic life, even at low concentrations. Both As and Se are redox active and have complex biogeochemical interactions with their environment, which complicates treatment strategies. Constructed wetland treatment systems (CWTS) hold promise as a less industrially intensive approach for removing As and Se from water, transforming and transferring these elements to sediments where they may have improved geochemical stability and decreased bioavailability. Gastropods (snails) rapidly colonize wetland systems and some species are tolerant of both elevated As and Se concentrations and the varying biogeochemical conditions in CWTS environments. Snails can accumulate Se and As, and are often a food source for aquatic animals that are sensitive to As and Se; they also produce voluminous quantities of fecal pellets. We hypothesize that these fecal pellets may play an important role in microbiological community dynamics in these systems and/or function as a vector for As and/or Se transport to sediments. In this study, we characterized the distribution of As and Se in gastropod soft tissues and their fecal pellets as part of a larger study of a pilot CWTS system designed by Contango Strategies Ltd (Saskatoon, SK). XRF maps of the distribution of As and Se in thin sections of gastropod soft tissues and their fecal pellets were collected on the VESPERS (07B2-1) beamline at the Canadian Light Source. Particles containing elevated concentrations of As and Fe were identified in the fecal pellets, but As was at background levels in the gastropod tissues. Conversely, elevated concentrations of Se were observed in some of the gastropod tissues, but Se hotspots were not observed in the fecal pellets. Based on this data, it appears gastropod fecal pellets may be a vector for transport of As into the CWTS sediments. Our ongoing efforts to characterize these systems include assessment of the microbiological community in the fecal pellets and sediments and quantitative geochemical assessment of the form and concentration of As and Se in these materials.