Development of a bioaccumulation model for selenium oxyanions and organoselenium in stream biota

Selenium (Se) occurs in natural surface waters as a variety of inorganic and organic chemical species, typically dominated by the oxyanions selenate and selenite. Organoselenium species, although hypothesized to be more bioavailable than oxyanions, have rarely been identified or quantified in natural waters and little is known about their fate or bioaccumulative potential. We studied spatial patterns of bioaccumulation in relation to aqueous Se speciation over 5 years at more than 100 locations near coal mine operations in southeast British Columbia, Canada. We used a sequential approach to fitting bioaccumulation model coefficients, first using sites with no detectable organic Se species (< 0.01 µg L-1) to describe the bioaccumulation of selenate and selenite, then applying those relationships to the remaining sites to infer the bioavailability of detectable organoselenium species. Our analysis indicated that the methylated species methylseleninic acid was the most bioaccumulative form, followed by dimethylselenoxide. Organoselenium species were associated primarily with mine sedimentation ponds and are presumed to be products of Se metabolism by algae and bacteria. Highly bioavailable organoselenium species exported from the ponds appear to be responsible for enhanced Se bioaccumulation in biota in downstream lotic reaches, with this influence diminishing with distance from ponds as concentrations decline. Our findings indicate that managing biological productivity in mine sedimentation ponds could help manage Se risk in the receiving environment.