Abstract:
Urban rivers are key pathways for land-sourced microplastics (MPs) to the oceans. However, their
real-time emission dynamics during rainfall events remain poorly understood. This study presents
high-frequency monitoring of MPs in an urban river in Japan over three rainfall events (light,
moderate, and heavy). The event mean concentrations (EMCs) of MPs were influenced by the
number of antecedent dry days, while total loads linked with the amount of rainfall. During
moderate and heavy rainfall events, microplastic (MP) loads increased by up to 100 times and event
mean concentrations (EMCs) by up to 350 times, compared to dry weather. MPs smaller than 2000
μm displayed first-flush effect during light and moderate rainfall, with over 50% of their total mass
released within the initial 20–40% of runoff flow. The smallest particles (10–40 μm) were
mobilized rapidly even at low rainfall intensities, whereas larger MPs (>2000 μm) were discharged
around or after the peak rainfall. The polymer composition of riverine MPs was significantly
impacted by rain, with polymer characteristics suggesting a likely origin from local road dust. The
levels of MPs and total suspended solids (TSS) in river water during rainfall were well-correlated,
suggesting the possibility to use TSS measurements as a useful indicator for predicting MP levels.
Finally, the findings highlighted the importance of targeting moderate to heavy rainfall events and
intercepting stormwater first flush for effective control of MP pollution in urban rivers.
