Derivation of site-specific selenium water quality criteria: A comparison of two methods and regulatory implications.

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In 2016, the US Environmental Protection Agency (USEPA) issued revised aquatic life water quality criteria for selenium (Se). The criteria ("elements") consisted of threshold concentrations applicable to fish tissue (three tissue types, though the egg and ovary tissue takes precedence over the whole-body and muscle tissue thresholds), and water column. The agency rationalized that measured concentrations of Se in fish tissue were more predictive of potential adverse reproductive effects than those measured in external media. The agency provided two mechanisms for derivation of site-specific Se water criteria: a bioaccumulation factor (BAF) approach, and a partitioning-based bioaccumulation model approach. The use of either approach assumes that fish tissue concentrations exceed one or more of the tissue criteria. We compared the two approaches using fish tissue samples from various species in the Ohio River to evaluate resulting similarities and differences in the calculated Se water quality criteria. Fish (five species) were collected near two coal-fired power plants at sites unaffected by Se from wastewater discharges. Using results for all species and all sites combined, the resulting site-specific Se criteria for the BAF and partitioning-based model approach (median values) were 2.0 and 1.5 µg/L, respectively. Considering all species, resulting criteria differed little between the two power plant locations. Resulting criteria for both methods were strongly influenced by a small (less than detection) background Se water concentration. At least for the upper Ohio River, the BAF approach (requiring less input data) seems adequate for derivation of site-specific Se water criteria. In the current study, however, none of the tissue samples exceeded USEPA's tissue criteria. Thus, the decision to derive site-specific Se water quality criteria should be evaluated on a case-by-case basis, because the process may result in either a more stringent or less stringent value, wholly dependent on local factors. Integr Environ Assess Manag 2021;17:1255-1262. © 2021 SETAC.