Microplastic Emission Dynamics in an Urban River under Rainfall Events

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.