Ecological risk assessment in a large river-reservoir: 3. Benthic invertebrates

The sediments of Poplar Creek and the Clinch River are contaminated with a wide variety of chemicals, including heavy metals, polycyclic aromatic hydrocarbons, and PCBs. Sources include the U.S. Department of Energy's Oak Ridge Reservation as well as both known and unidentified upstream activities. We investigated the risks to benthic invertebrates posed by chemicals in these sediments as part of a comprehensive ecological risk assessment performed to support Superfund clean-up decisions. Poplar Creek was the only river reach for which significant risks were determined. This conclusion was based on several lines of reasoning: sediment-associated organisms at most sites were exposed to levels of several contaminants that have been observed to be toxic; the biosurvey results show a greater than 20% reduction relative to reference sites in taxa richness and abundance; the statistical analysis of the physical, contaminant, and biosurvey data did not exclude contaminants as possible causal factors; and the sediment toxicity tests were too ambiguous to definitively exclude impacts in this reach. This assessment demonstrates the importance of collecting biological data, including sediment toxicity tests and biological surveys; statistically analyzing the relationships of chemicals, physical variables, and measured effects (e.g., toxicity or benthic invertebrate densities); and using sediment chemical and effects distributions in addition to point estimates of exposure and screening benchmarks.