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 sewe r overflows (CSOs) can contribute significantly to the contamination of rec eiving waters, particularly in sediment depositional areas near outfalls. A nalyses of sediment chemistry alone are not sufficient to fully assess the effects of these discharges. Toxicity testing and evaluations of benthic in vertebrate communities, in conjunction with chemical analyses, provide a mo re complete characterization This study assessed relationships among three separate aspects of the benthic environment: sediment chemistry (metals, PA Hs and nutrients) and particle size, sediment toxicity (ten endpoints with four benthic taxa), and benthic invertebrate community structure. In this i nitial survey, ten sites in five different study areas, representing a rang e of receiving water environments exposed to stormwater and CSO discharges, were sampled in October 1998. Results of analyses indicated that while con taminant (metals and PAHs) concentrations were relatively high in sediments , biological effects were not evident. Toxicity of sediments was low and al tered 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 stormw ater and CSO discharge impacts should use "upstream/downstream sampling des igns and study sites with minimal variability of habitat conditions.