An Urban Stormwater Contaminant Signature: Defining Priority Contaminants for Urban Stormwater Research.

Stormwater ponds (SWPs) are a common feature of urban landscapes, designed to manage runoff and reduce flooding. Increasingly, they are also recognized as seminatural habitats supporting aquatic biodiversity. However, SWPs receive complex mixtures of contaminants from surrounding urban areas, and the extent of contamination within the ponds themselves remains underexplored. Most studies have focused on outflows or a narrow set of targeted analytes, limiting our understanding of the exposure risks for organisms residing within these systems. To address this gap, we assessed contaminant profiles in 21 SWPs across a highly urbanized city in Ontario, Canada, using three complementary sampling approaches: time-integrated water samples, biofilm on artificial substrates, and organic diffusive gradients in thin films (o-DGTs). Across all sites, we detected 200 organic compounds, including pesticides, pharmaceuticals, industrial chemicals, and compounds, linked to vehicles and infrastructure. Additionally, we documented widespread chloride and fecal contamination and elevated levels of traffic-related metals in biofilms. From these data, we identified a set of frequently detected and environmentally relevant contaminants, which we term the urban stormwater contaminant signature (USCS). This proposed list may support the development of targeted monitoring strategies and help focus future research on mixture toxicity and risk to aquatic biota. Given the apparent ecological role of SWPs and the range of stressors they contain, assessing cumulative exposures is critical for understanding the potential impacts of urban runoff on resident organisms.