Environmental risk ranking of 20 metal(loid)s in urban runoff: A Swedish nationwide meta-analysis

This meta-analysis integrates data from 182 runoff events across 15 Swedish sites representing diverse land uses and seasonal conditions to identify and prioritize metals of concern in urban areas. Using a data-driven risk assessment, we compared concentrations and environmental risks of 20 metal(loid)s against international databases and regulatory thresholds, evaluating both acute and chronic effects while accounting for bioavailability for seven metals. Event- and site-level analyses revealed large temporal and spatial variability in concentrations and risks, reflecting differences in emission sources, land-use characteristics, and seasonal hydrological and physicochemical conditions. Spatially, "Parking/Road", "Main Road/Highway", and "Mixed Urban" catchments were the most influential for metal concentrations and associated risks. Seasonally, snowmelt was characterized by higher total concentrations and associated total-based acute/chronic risk quotients, primarily associated with elevated TSS and pH during snowmelt. In contrast, stormwater showed higher dissolved concentrations and potentially resulting in greater dissolved- and bioavailable-based risks during stormwater events. These contrasting pathways highlight the need for dynamic, chemistry-dependent management strategies, while acknowledging variabilities related to sampling approaches and regulatory thresholds. Overall, Zn, Cu, Al, Fe, and Co ranked as the highest-risk metals, followed by Mn, V, Ba, and Ni. Effective mitigation, therefore, requires site- and season-specific control through combining targeted source reduction, solids management, and treatment adapted to metal speciation and runoff chemistry.