Methodology for the evaluation of cumulative episodic exposure to chemicalstressors in aquatic risk assessment

An ecological risk assessment method was developed to evaluate the magnitud e, duration, and episodic nature of chemical stressors on aquatic communiti es. The percent of an ecosystem's species at risk from a designated chemica l exposure scenario is generated. In effects assessment, probabilistic extr apolation methods are used to generate estimated safe concentrations (ESCs) for an ecosystem using laboratory toxicity test results. Fate and transpor t modeling is employed to generate temporal stressor concentration profiles . In risk characterization, area under the curve integration is performed o n predicted exposure concentration profiles to calculate a cumulative expos ure concentration (CEC) for the exposure event. A correction is made to acc ount for the allowable exposure duration to the stressor ESC. Finally, the CEC is applied to the extrapolation model (curve) of the stressor to predic t percent species at risk to the episodic exposure. The method may be used for either prospective or retrospective risk assessments. The results of a retrospective risk assessment performed on the Leadenwah Creek, South Carol ina, USA, estuarine community are presented as a case study. The creek expe rienced periodic episodes of pesticide-contaminated agricultural runoff fro m 1986 through 1989. Although limited biological data were available for me thod validation, the risk estimates compared well with the Leadenwah Creek in situ bioassay results.