Development of aquatic life criteria for selenium: a regulatory perspective on critical issues and research needs

The US is currently in the process of revising its freshwater, chronic aquatic life criterion for selenium. The fundamental issues being addressed include which environmental compartment(s) support the most reliable expression of the criterion, which form(s) of selenium should be measured in the medium (media) of choice, and which site-specific water quality (or other factors) should be linked to the expression of the criterion. Literature reviews and a recent workshop were conducted to assess the state of the science on various issues related to water-, tissue- and sediment-based criteria for selenium. Evaluation of many of these issues is ongoing. In terms of water column criteria issues, data limitations will likely restrict the expression of a criterion to operationally defined forms (e.g. total recoverable, dissolved). The specific identity of organoselenium in natural systems is lacking and may not be appropriately represented by free seleno-amino acids (e.g. selenomethionine). The available data do not appear to support quantitative relationships between chronic toxicity and water quality characteristics. In terms of a tissue-based criterion, reproductive tissue (ovary, egg) has been recommended as the tissue of choice, but practical concerns and data availability require consideration of other tissues (e.g. whole-body). Organoselenium (bound to peptides or proteins) is thought to be the form of greatest toxicological importance in fish, however, direct measurements of organoselenium compounds in tissues are very limited. Route of exposure (food vs. water uptake) may prove important for establishing diagnostic tissue residues for selenium based on laboratory data. Data on toxicological aspects of selenium in sediments appear sparse, particularly in relation to different sedimentary forms. Reliable assessments of bioaccumulation will likely be critical for making site-specific modifications to chronic selenium criteria, however, many technical issues for assessing bioaccumulation remain. The need for improved analytical methods for directly speciating organoselenium in various environmental media underpins many of the current data gaps. Improving analytical methodologies to enable affordable and reliable measurement of organoselenium compounds holds significant promise for advancing selenium ecotoxicological research.