Spatiotemporal dynamics of microplastics in urban stormwater runoff: Functional area effects and transport pathways (Shanghai, China)

Urban stormwater runoff is a critical pathway for microplastics pollution, yet its detailed transport dynamics remain poorly characterized. This study employed intra-event time-series sampling (at intervals of 0, 5, 15, 30, 60, 120, and 240 min after runoff initiation) during a heavy rainfall event in Shanghai (China) to investigate microplastics concentrations and characteristics across three urban functional areas. Our results revealed that microplastic pollution levels were strongly land-use-dependent: the dining area was a severe hotspot, with a time-weighted average concentration of 689.7 ± 214.1 items/L, which was significantly higher than the residential area (215.6 ± 38.9 items/L) and the parking area (172.8 ± 18.8 items/L), and all concentrations far exceeded local aquatic background values. A pronounced first flush effect was observed, particularly in the dining area, where the peak concentration was reached within just 5 min. The runoff was dominated by small-sized (<1.0 mm) and fibrous microplastics composed of PET and PP. These small fibers were preferentially exported in the early phase of runoff (within the first 30 min), whereas granules and larger-sized microplastics accumulated in the later phase. By elucidating the land-use-dependent transport dynamics and fate of microplastics, this study provides a scientific basis for targeted source control, including prioritizing initial flush interception, and stormwater management in global megacities.