Microplastic Pollution in a Small River with an Agriculture-dominated Catchment under Different Rainfall Conditions: Transport Scaling, Characteristics and Ecological Risk Assessment

The agriculture cluster is recognized as a cryptic hotspot of microplastic pollution. Especially, rainfall-driven microplastic (MP) dynamics in small agricultural rivers have not been adequately quantified under various hydrological conditions. This study quantified MP concentrations, loads, characteristics, and ecological risks under baseline dry conditions and during four rainfall events of varying intensities in a small agricultural river. During baseline conditions, MP concentrations were low (10–53 particles/m 3 ), whereas during heavy rainfall events, concentrations peaked at 2,187–3,253 particles/m 3 , representing up to 1,100-fold increases compared to dry conditions. The power-law equation and L–Q analysis showed shallower MP scaling ( n = 1.38) than total suspended solids (TSS) ( n = 2.22), reflecting source-limited and neutrally buoyant transport. Polyethylene was the dominant polymer (76–95%), and film-shaped particles comprised more than 50% of the morphologies, which is consistent with agricultural plastic use. The potential risk index (PRI) increased sharply from moderate under baseline conditions (151) to extremely high during heavy rainfall (5,400), driven by the episodic mobilization of highly toxic polymers such as polyvinyl chloride and phenol–formaldehyde. These results demonstrate the episodic nature of the MP ecological risk in agricultural rivers and underscore the need for targeted management during rainfall periods.