Selenium transport and bioaccumulation in aquatic ecosystems: a proposal for water quality criteria based on hydrological units.

Local water quality criteria for selenium should be based on an assessment of the degree of toxicological hazard to fish and wildlife, which is influenced by the spatial and temporal variation of the selenium cycle at the site under consideration. The physical area from which measurements are taken to evaluate selenium residues and biological effects, i.e., the database for setting site-specific criteria, must encompass more than an isolated segment of river, a tributary stream, etc. Because of hydrological connections between the various aquatic habitats that may be present in a watershed basin-wetlands, rivers, streams, and impoundments-the toxic threat from selenium contamination is also connected. For example, a criterion that is appropriate for a stream or river where low bioaccumulation occurs may result in seemingly harmless concentrations of selenium becoming a problem in downstream impoundments or in off-channel bays and wetlands where bioaccumulation is greater. The hydrologically connected parts of a basin downstream of a selenium discharge (natural or synthetic selenium source), extending to the point at which new sources of low-selenium water dominate the hydrology (e.g., confluence with larger tributary or river, spring or groundwater inflow), should be the area evaluated and given a specific criterion, not isolated components. Thus, a hydrological unit should be identified and used as the "site" for the purpose of setting criteria. Importantly, criteria derived in such a fashion will reflect the transport and bioaccumulation of selenium within the entire hydrological unit rather than simply focusing on a small, artificially designated segment of the system. Failure to use a hydrological unit approach can set the stage for significant biological and legal problems. Copyright 1999 Academic Press.