Using benthic assessment techniques to determine combined sewer overflow and stormwater impacts in the aquatic ecosystem

Urban wet-weather sources of pollution such as stormwater and combined sewer overflows (CSOs) can contribute significantly to the contamination of receiving waters, particularly in sediment depositional areas near outfalls. Analyses of sediment chemistry alone are not sufficient to fully assess the effects of these discharges. Toxicity testing and evaluations of benthic invertebrate communities, in conjunction with chemical analyses, provide a more complete characterization This study assessed relationships among three separate aspects of the benthic environment: sediment chemistry (metals, PAHs and nutrients) and particle size, sediment toxicity (ten endpoints with four benthic taxa), and benthic invertebrate community structure. In this initial survey, ten sites in five different study areas, representing a range of receiving water environments exposed to stormwater and CSO discharges, were sampled in October 1998. Results of analyses indicated that while contaminant (metals and PAHs) concentrations were relatively high in sediments, biological effects were not evident. Toxicity of sediments was low and altered benthic communities were not detected. Neither toxicity endpoints nor benthic community descriptors were related to sediment contaminant levels. To improve the power of these assessments, future investigations of stormwater and CSO discharge impacts should use "upstream/downstream sampling designs and study sites with minimal variability of habitat conditions.