Priority polluants in urban stormwater: part 2 – Case of combined sewers

This study has evaluated the quality of combined sewer overflows (CSOs) in an urban watershed, such as Paris, by providing accurate data on the occurrence of priority pollutants (PPs) and additional substances, as well as on the significance on their concentrations in comparison with wastewater and stormwater. Of the 88 substances monitored, 49 PPs were detected, with most of these also being frequently encountered in wastewater and stormwater, thus confirming their ubiquity in urban settings. For the mojority of organic substances, concnetrations range between 0.01 and 1 µg. l^-1, while metals tend to display concentrations above 10 µg. l^-1. Despite this ubiquity, CSO, wastewater and stormwater feature a number of difference in both their concentration ranges and pollutant patterns. For most hydrophobic organic pollutants and some particulate-bound metals, CSOs exhibit higher concentrations than those found in stormwater and wastewater, due to the contribution of in-sewer deposit erosion. For pesticides and Zn, CSOs have shown concnetrations close to those of stormwater, suggesting runoff as the major contributor, while wastewater appears to be the main source of volatile organic compounds. Surprisingly, similar concentration ranges have been found for DEHP and tributyltin coumpounds in CSOs, wastewater and stormwater. The last section of this article identifies substances for which CSO discharges might constitute a major risk of excceding Environmental Quality Standards in receiving waters and moreover indicates a significantrisk for PAHs, tributylin compounds and chloroalkanes. The data generated during this survey can subsequently be used to identify PPs of potential significance that merit further investigation. 

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