Application of the WERF methodology for aquatic ecological risk assessment to a stream receiving municipal wastewater discharges

We applied our WERF Methodology for Aquatic Ecological Risk Assessment to assess the risks to aquatic life from toxic chemicals present in the Trinity River, TX, during 1987 and 1988. During that period, the river received large volumes of chlorinated municipal wastewater, as well as non-point sources of contaminants. Our objective was to evaluate how well the methodology worked and to determine how it could be improved. Methodology evaluation was done using 'hindcasting'. In other words, we applied it to an existing data-set, which included data on stream and sediment chemistry, stream and sediment toxicity, benthic macroinvertebrates, fish, and algae. First, we conducted a Screening-Level Risk Assessment to identify those chemicals presenting significant risks. Chemicals of concern include chlorine, ammonia, metals, and pesticides. Second, we quantified ecological risks and their associated uncertainties for single chemicals one-at-a-time and for multiple chemicals, in combination. Third, we compared our estimated risks with actual instream data on benthic macroinvertebrate and fish community structure and ambient toxicity to determine how precise our predictions were. Finally, we identified components of our risk assessment with the most imprecise predictions of risks and largest uncertainties. From these evaluations we determined how to reduce those uncertainties with collections of additional site-specific data. (DBO)