Pollutants in urban runoff: Scientific evidence on toxicity and impacts on freshwater ecosystems

Urban runoff effluents transport multiple pollutants collected from urban surfaces. which ultimately reach freshwater ecosystems. We here collect the existing scientific evidence on the urban runoff impacts on aquatic organisms and ecosystem functions, assessed the potential toxicity of the most common pollutants present in urban runoff, and characterized the ecotoxicological risk for freshwaters. We used the Toxic Units models to estimate the toxicity of individual chemicals to freshwater biota and observed that the highest ecotoxicological risk of urban runoff was associated to metals, polycyclic aromatic hydrocarbons (PAHs) and pesticides and, in a few cases, to phthalates. The potential risk was highest for copper and zinc, as well as for anthracene, fluoranthene, Di(2-ethylhexyl) phthlate (DEHP), imidacloprid, cadmium, mercury, and chromium. These pollutants had contrasting effects on freshwater biological groups, though the risk overall decreased from basal to upper trophic levels. Our analysis evidenced a lack of data on ecotoxicological effects of several pollutants present in urban runoff effluents, caused by lack of toxicity data and by the inadequate representation of biological groups in the ecotoxicological databases. Nevertheless, evidence indicates that urban runoff presents ecotoxicological risk for freshwater biota, which might increase if hydrological patterns become extreme, such as long dry periods and floods. Our study highlights the importance of considering both the acute and chronic toxicity of urban effluent pollutants, as well as recognizing the interplay with other environmental stressors, to design adequate environmental management strategies on urban freshwater ecosystems receiving urban runoff.