Multi-contaminants in road runoff of a compact city: Characteristics, interactions, and ecological risks

As urban road runoff is a critical source of water contamination, it poses a persistent challenge to water quality management, particularly in compact cities. Yet the sources, transport mechanisms, interactions, and ecological risks of these co‑occurring contaminants remain insufficiently understood. Therefore, we investigated 32 stormwater quality parameters in runoff from six representative road sites across 11 rainfall events over a 17-month period in Hong Kong. Contaminant levels in initial road runoff were markedly higher than those in natural rainwater; notably, Escherichia coli concentrations were more than four orders of magnitude greater. Concentrations of organic matter, solids, nutrients, pathogens, and metals in the initial road runoff exceeded the Water Pollution Control Ordinance (WPCO) objectives in Hong Kong by several to dozens of times. A pronounced first flush effect was observed for chemical oxygen demand (COD), total suspended solids (TSS), zinc (Zn), and microplastics (MPs). MPs correlated positively with pH, E. coli, phosphate (PO₄^3-), and nitrate nitrogen (NO₃^-N), and negatively with dissolved oxygen (DO) and iron (Fe), suggesting their role as vectors for nutrients and pathogens under oxygen-poor, biologically enriched conditions. Contaminant levels varied by land use and season, with higher concentrations on residential roads and winter peaks for COD, chloride (Cl^-), Zn, Fe, and TSS. Ecological risk indices indicated very high metal risk, primarily from lead (Pb) and nickel (Ni), and moderate-to-high polymer risk from polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA). These findings highlight road runoff as a key driver of urban water quality degradation, emphasizing the importance of managing first-flush discharges through land-use-specific controls, seasonally optimized street cleaning, and targeted treatment of high-risk contaminants.