Predicted sediment toxicity downstream of combined sewer overflows corresponds with effects measured in two sediment contact bioassays

The present study investigated the sediment toxicity along 100 m transects downstream from three different combined sewer overflows (CSOs) using a coupled substance risk assessment and bioassay approach. As a first step, a risk assessment according to the consensus based sediment quality guideline was performed to estimate the probability for an incidence of toxicity to benthic organisms in freshwater sediments using their pollutant concentrations of metals, PAHs and PCBs. Secondly, two sediment contact assays (SCAs) using the nematode Caenorhabditis elegans and embryos of the fish Danio rerio were conducted. Sediments downstream from CSOs were strongly influenced by discharged particles in terms of pollution, due to associated contaminants as well as oxygen depletion caused by oxygen consuming matter. In general, the calculated probability for sediment toxicity corresponded with results of the SCAs. With increasing distance from the CSOs both the toxic potential of the sediments as well as their effects detected in the SCAs decreased. However, clear correlations between concentrations of metals as well as PCBs and toxic effects were found for the nematode SCA. The toxicity assay with embryos of D. rerio was strongly influenced by oxygen depletion in the sediments. Therefore, it was not possible to differentiate between responses mediated by pollution or oxygen depletion using the D. rerio embryo test. The results of the present study demonstrated a clear effect of CSOs on the toxicity of downstream located sediments. As particles were identified as vector for pollutants and as source for oxygen depletion, the retention of particles within the sewer network is of high importance to minimize both stressors. The present study shows that a retention zone located between the CSO and the receiving creek is an efficient measure to enhance sediment and water quality downstream of CSOs. (C) 2019 Elsevier Ltd. All rights reserved.