Tracking microplastics across urban drainage systems and highway runoff: a snapshot assessment

Conceptual diagram of microplastic transport in urban drainage systems showing traffic-derived tire wear particles and other microplastics entering stormwater and combined sewer systems, with indicated concentrations of PE, PP, and SBR. Microplastics (MP) were quantified during an initial rainfall event in size-fractionated (63–2,000 and <63 μm) total suspended solids (TSS) from urban drainage systems and highway runoff. Polyethylene (PE), polypropylene (PP), and styrene–butadiene rubber (SBR) were quantified. At combined sewer overflow (CSO) sites, PE concentrations ranged from 0.4 to 0.5 mg L−1 in the influent and 0.3–0.4 mg L−1 in the effluent, while PP ranged from 0.04 to 0.08 mg L−1 and SBR from 0.08 to 0.25 mg L−1. Highway runoff showed the highest SBR levels, reaching 2.1 mg L−1 in the <63-μm fraction and 0.5 mg L−1 in the 63- to 2,000-μm fraction. Sedimentation-based treatment preferentially removed TSS in the coarse fraction (65–78%) compared with the fine fraction (26–37%), whereas MP removal efficiencies were generally lower (14–42%). Additional removals of 76–82% were achieved by pile cloth filtration. Ln-transformed SBR–TSS relationships at highway sites showed significant correlations and consistent slopes, supporting TSS as a potential proxy for traffic-derived tire wear particle loads. The study provides a multimatrix snapshot across CSO-related and traffic-influenced systems, enabling comparison of treatment performance and highlighting the dominance of the <63 μm fraction, particularly in road runoff.