Toxicity testing for controlling urban wet-weather pollution: advantages and limitations

Toxicity of Combined Sewer Overflows (CSOs) and stormwater was studied at 15 sites in Southern Ontario, using a battery of seven bioassays. The highest frequencies of severe and moderate toxicity (19% and 24%, respectively) were found at highway runoff sites; frequencies of CSO toxicity were much lower (from 6.6% to 13%, for moderate and severe toxicity combined). Stormwater ponds contributed to toxicity reduction, with respect to both water and sediment downstream of ponds. Conventional toxicity testing was found useful for screening and assessing potential receiving water impacts, but was somewhat limited by the dynamic nature and large variety of wet-weather pollution sources.