Stormwater impacts on the elemental and organic contaminant profiles of wastewater effluent.

Stormwater input can significantly alter the composition of wastewater effluents. However, its effect on the broad spectrum of emerging contaminants remains insufficiently understood. This study investigated stormwater impacts on treated wastewater effluent in a mid-sized city in the northeastern U.S. that has partially combined sewers and bypasses secondary treatment when flow levels exceed a certain threshold. Inorganic analysis demonstrated weather-dependent increases in copper, iron, and phosphorus during wet conditions. Targeted organic analysis identified 19 compounds, including pharmaceuticals, pesticides, and others, of which 10 showed significantly elevated concentrations during rain events. Individual and cumulative risk assessments based on predicted no effect values for Daphnia magna immobilization indicated that targeted analytes in both wet and dry weather wastewater posed chronic ecological risks to aquatic organisms. Carbendazim demonstrated the highest risk, with measured concentrations more than 100-fold greater than the no effect values. Additionally, non-targeted analysis revealed noteworthy differences in composition between wet and dry weather samples, with 1191 detected features increasing during rain events and few decreasing. This emphasizes the highly complex and variable nature of stormwater influenced effluent and has implications for ecological impacts of effluent release, as well as wastewater reuse. Overall, the current study demonstrates the importance of accounting for stormwater-driven variability in wastewater monitoring and assessment.