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Hydrological Seasonality Mediates Scale-Dependent Land Use Effects on Riverine Microplastic Pollution
Summary
Scientists studying a major river in China found that heavy rainfall washes way more plastic pollution (tiny plastic bits called microplastics) into waterways than dry periods, with most of it coming from urban wastewater and farming runoff. This matters because rivers like this one are a key route for plastic pollution to reach our oceans, drinking water, and food supply, meaning better management of wastewater treatment and farm runoff during rainy seasons could meaningfully cut the amount of microplastics we're exposed to.
Rivers serve as major pathways for transporting land-derived microplastics (MPs) to oceans, yet how land use shapes riverine MPs across spatial scales remains poorly understood. This study investigates MPs in the Yuan River basin, a system characterized by pronounced land use heterogeneity. By integrating multivariate statistics and Positive Matrix Factorization (PMF) with data from six field campaigns conducted between March 2023 and January 2024, we disentangled the effects of hydrological seasonality and multi-scale land use on MP distribution and sources. MP abundances exhibited marked spatiotemporal variability, where wet-season concentrations (25,800 ± 17,120 items/m³) significantly exceeded dry-season levels (12,240 ± 8320 items/m³), indicating relatively high MP contamination in the Yuan River compared with many reported freshwater systems worldwide. Notably, during high-flow periods, the explanatory power of sub-basin-scale land use (43.4%) surpassed that of the 1-km circular buffer zone (19.6%), with cropland and forest configurations, particularly the Largest Patch Index (LPI) and Shannon's Diversity Index (SHDI), acting as primary drivers. Conversely, under low-flow conditions, factors within the 1-km circular buffer (41.4%) outweighed sub-basin-scale factors (36.0%), dominated by the extent and configuration of water bodies and riparian forests. This scale-dependent shift highlights a hydrologically mediated mechanism where high flows integrate and transport MPs from broader catchment sources, whereas low flows accentuate the influence of proximal, near-channel inputs. PMF analysis identified three major MP sources: urban domestic wastewater (63.5% and 75.5% in the wet and dry seasons, respectively), agricultural activities (17.3% and 17.7%, respectively), and construction & industrial inputs (19.1% and 6.8%, respectively). Our findings emphasize the necessity of accounting for hydrological seasonality and cross-scale land use structures in risk assessments, prioritizing the fine-scale control of urban wastewater and agricultural non-point source pollution in mitigation strategies.