Acute aquatic life criteria for selenium

The U.S. Environmental Protection Agency's (U.S. EPA's) current acute aquatic life criterion for selenium (Se), 20 micrograms/L, was based on a back-calculation from the chronic criterion of 5 micrograms/L, using the acute-to-chronic ratio in reverse. While bioaccumulation and subsequent toxicity through dietary uptake are important factors in chronic toxicity, acute toxicity is a short-term phenomenon, which should be promulgated with an acute value derived from direct toxicity studies, similar to other metals and metalloids. Acute toxicity values from the U.S. EPA Se criteria document were updated with more recent toxicity data for the two primary forms of Se, selenite (Se IV) and selenate (Se VI). Using this updated toxicity database, genus mean acute values (were calculated, and the database was ranked based on sensitivity to both forms. Following recalculation (using U.S. EPA criteria derivation guidelines), the updated final acute value (FAV) for selenite was 437 micrograms/L, resulting in a criterion maximum concentration (CMC), or acute criterion, of 219 micrograms/L. For selenate, the FAV becomes 826 micrograms/L, with a CMC of 413 micrograms/L. Unless Se speciation studies demonstrate that the primary form of Se is selenate, the conservative assumption should be that any Se present is in the more toxic selenite form, resulting in an acute criterion for Se of 220 micrograms/L. These acute criteria are scientifically sound and necessary, under U.S. EPA's regulatory structure for deriving National Pollutant Discharge Elimination System discharge permit limits.