Spatio-temporal characteristics of microplastics in river surface water: insights from multi-year field monitoring

Despite the well-established role of rivers as the primary pathway for microplastics from land to sea, the effect of seasonal hydrological signals on riverine microplastic loads is still a matter of considerable controversy. This study conducted four seasonal sampling campaigns (rainy and dry seasons) over two consecutive hydrological years to clarify the spatiotemporal patterns and key drivers of microplastic occurrence in rivers from an interannual perspective. Across both hydrological years, microplastic abundance was consistently higher during the dry season (49.13 and 51.45 n/m³) than during the rainy season (36.38 and 29.70 n/m³), a pattern primarily driven by the dilution effect of elevated river discharge. Urban point sources were identified as the dominant contributors to microplastic pollution, generating persistent contamination hotspots. Interestingly, we observed a pronounced spatial decoupling between abundance hotspots (W9 and W15) and flux hotspots (W11 and W15). Since microplastic flux is less sensitive to dilution, flux-based assessments more accurately reflect the in-situ pollutant loads and ecological risks than abundance-based ones. Additionally, we have established a linear relationship between microplastic flux and river discharge, offering a valuable tool for extrapolating results from short-term baseline studies to long-term flux dynamics. Collectively, these findings provide a scientific basis for developing long-term monitoring and mitigation strategies for microplastics at the catchment scale.