Ecotoxicological risk assessment of micropollutants from treated urban wastewater effluents for watercourses at a territorial scale: Application and comparison of two approaches.

In most cases, urban Wastewater Treatment Plants (WWTP) only partially abate pollutants occurring in the influent. Treated effluents can thus contain a complex mixture of ecotoxic pollutants, such as heavy metals, detergents, disinfectants, plasticizers, pharmaceuticals residues or pesticides. In this context, Ecotoxicological Risk Assessment (ERA) provide essential decision-making tools to public authorities for establishing environmental policies and conducting territorial planning. The present work aims to develop a territorial-scale ERA methodology using two complementary approaches based on a Risk Quotient (RQ) calculation: (1) the first, based on the risk linked to each individual pollutant (single substances ERA); (2) the second, considering all pollutants present, and the "cocktail effect" (mixture ERA). This research was performed at 33 urban WWTPs of in a highly urbanized part of France (Lyon area). Initial minimum, median and maximum pollutant concentrations in treated effluents were obtained from a literature review of physico-chemical analysis studies, to reconstitute "typical" effluents. The classical approach (single substances ERA) identified the riskiest substances (e.g. endocrine disruptors, as the Estrone with RQ up to 593.75), and showed the risks for each WWTP. The mixture ERA approach revealed new risks, which were not highlighted in the classical ERA approach, thus increasing the number of WWTPs identified as at risk. This study shows the importance of accounting for the cocktail effect, which is not considered in current regulatory decisions. Finally, this methodology allowed us to identify the riskiest situations (often medium sized WWTPs, releasing into small streams), that could worsen in the context of climate change.