Conducting site-specific assessments of selenium bioaccumulation in aquatic systems.

Selenium (Se) is a chemical of concern at many locations across North America and elsewhere, and site-specific conditions are important when evaluating its bioaccumulation and effects in aquatic ecosystems. Most regulatory criteria and guidelines are based on waterborne Se concentrations. In contrast, the draft water quality chronic criterion of the US Environmental Protection Agency (USEPA) is based on Se concentrations in whole-body fish, and current information suggests the agency will issue a new draft criterion based in part on fish egg and/or ovary Se concentrations. However, implementation guidance is not available from the agency for either of these tissue-based criteria. Therefore, we describe a phased approach for field and laboratory assessments of Se bioaccumulation in fish and aquatic-dependent birds that can be applied in different environmental settings with the goal of developing and interpreting a tissue-based Se value. We recommend here the use of decision trees, conceptual models, and data quality objectives toward defining what should be done during the assessment, plus sampling and monitoring procedures for the assessment. First, available tissue or waterborne Se concentrations should be compared to tissue residue guidelines or adopted water quality criteria and guidelines. When needed, reproductive toxicity testing and assessment of fish populations should also be conducted in the area of interest. In addition, extensive data on the effects of Se on fish and bird species have been developed, and describing the associations between fish or bird egg tissue, aqueous Se, and potential effects is important for sites where Se may be a concern. Selenium bioaccumulation and toxicity also are of concern for amphibians and oviparous reptiles, but interpretive information is very limited for those species. Recent science indicates that effects are more strongly related to tissue concentrations of Se (especially in the eggs or ovaries of oviparous vertebrates) than to waterborne concentrations. Overall, we conclude that the approach for site-specific assessment must be flexible enough to allow what is appropriate for the situation. Furthermore, risk management and remediation decisions should be based on combined biology and chemistry data, using multiple lines of evidence in the assessment.