Project Description: Region Specific Time Scale Toxicity in Aquatic Ecosystems

Urban and nonpoint source runoff, particularly discharges during wet weather, present a contaminant source to receiving systems that has specific analysis needs. During a storm event, concentrations of contaminants may vary over very short time intervals requiring modification of standardized test procedures. To adequately assess the potential toxicity of short-term exposure to storm-related contaminants in wet weather flows a test system must meet several criteria: 1) fast response time with a time scale for the response matching the time scale of the exposure, 2) capacity to respond to changing conditions (e.g. track changing toxicity), and 3) respond to the effects of a single stressor, as well as integrate the effect of multiple physical, chemical, and biological stressors (Herricks et al. 1994). Although these criteria may be contradictory for a single test system, all criteria can be met in a test battery involving multiple test systems (Milne et al. 1996). Where wet weather flows produce short exposure times with large concentration transients, rapid test procedures such as Microtox and an enzyme inhibition assay (IQ testing) are appropriate. Lethality tests with commonly used species such as Ceriodaphnia dubia and Hyalella azteca have also been developed to screen toxicity and provide a time scale of exposure that is matched to that of the runoff event. For discharges modified by best management practices, or where sampling provided time-based composites, more standard whole effluent-based test designs with C. dubia and H. azteca are appropriate. Toxicity testing conducted in the absence of field validation may lack a connection to observed receiving system effects. To establish a foundation for interpretation laboratory-based toxicity testing with field-based studies, the laboratory-based testing is compared with in situ testing with caged organisms or field bioassessments. Test application results indicate different degrees of concordance between tests in measured degree of toxicity; some tests give consistently similar results whereas others show more complex and variable patterns. Toxicity testing in single events may not be predictive of long-term effects in receiving waters, but multiple event analysis provides information on sources and variability of toxicity that is useful in watershed management.

Following a review of test systems available to assess time scale toxicity, a field and laboratory evaluation program was undertaken in 1994 to identify a test battery that responds effectively to regional or site-specific episodic event toxicity. Test systems were selected from two primary sources. The first soruce included tests systems and/or testing procedures previously used in assessing wet weather/episodic event toxicity. The second source was found in standard toxicity testing methods and species, where the test systems selected included both standard procedures that support the comparison of test system performance with the extensive literature on toxic effect, and modified testing procedures that are designed to meet the specific requirements of time scale toxicity testing. Of particular importance in the test system selection process is practical implementation, reflected in the weighting of criteria associated with the costs, in terms of equipment, personnel and unit costs, associated with test system implementation.