Spatiotemporal distribution and rainfall-runoff transport characteristics of microplastics in a typical agricultural...

Rainfall runoff is one of the key drivers of microplastic transport from sources to sinks, but the dynamic changes of microplastics remain insufficiently quantified. This study systematically investigated the spatiotemporal distribution of microplastics in a typical agricultural watershed from April to December 2025 through soil sampling and monthly water monitoring, combined with high-frequency monitoring of 13 rainfall events to quantify the impact of rainfall on microplastic transport. Results showed that soil microplastic abundance was higher near residential areas. The water microplastic abundance in the downstream was higher than that in the upstream, and elevated concentrations in ponds may serve as both sinks and export sources of microplastics. The mean of event mean concentrations (EMC_MP) during rainfall events was significantly higher than that of baseflow, representing a 1.9-fold increase (p < 0.05). The predominance of counter-clockwise hysteresis (62% of events) supported the hypothesis that microplastic transport may be influenced by proximal supply limitation and distal-source delayed delivery as hydrological connectivity increases. Microplastic load was significantly correlated with most of the hydrological variables examined. Load-discharge model estimated that the 23.2% of rainfall time contributed 54.1% of the annual load, highlighting the necessity of incorporating high-frequency rainfall event monitoring into watershed microplastic load assessment frameworks.