Urban Runoff Toxicity to Aquatic Species: Physiological and Biomarker Responses with Toxicant Characterization

Urban runoff often contains a complex mixture of contaminants that may include plastics, pesticides, personal care products, pharmaceuticals, metals, tire wear debris, and per- and polyfluoroalkyl substances. Widespread urbanization has made untreated urban runoff a major pathway for environmental contamination that can pose a significant threat to aquatic ecosystems. While existing literature reviews have focused on the sources and composition of urban runoff or the toxicity of the single contaminants, a comprehensive understanding of the ecotoxicological effects of whole complex mixtures of urban runoff contaminants is lacking, particularly regarding their short- and long-term impacts across different trophic levels. This review uniquely synthesizes the effects of urban runoff mixtures, examining both organism- and molecular-level responses in aquatic species. In addition, we developed the UrbanRunoffRisk R package to automate and improve risk assessments of urban runoff mixtures. The tool integrates predicted environmental concentration (PEC) with EC₅₀/LC₅₀ values for key aquatic species, ensuring reliable and reproducible calculations. Our analysis reveals that most existing studies focus on organismal and physiological responses to acute exposures, with limited attention paid to molecular mechanisms or identification of the toxicants driving observed effects. By combining organismal and molecular insights with a mixture of toxicity assessment and model-based risk evaluation, this review provides a novel synthesis of urban runoff ecotoxicology. Future research should also explore the molecular mechanisms underlying the effects of urban runoff mixtures and the development of practical and diverse risk assessment models. Such advancements are critical for understanding and mitigating the ecological impacts of chemical mixtures in urban runoff.