Seasonal and spatial variation, and land-use influences on riverine microplastics and their ecological risks in the Yanshui River Basin

This study presents a basin‑wide with year-round monitoring of microplastics (MPs) sampling both surface water and sediment at 14 sites across the Yanshui River Basin, covering agricultural, industrial, and urban land uses to capture seasonal and spatial variability. MPs were detected at all sites, with average abundance of 235.1 ± 219.9 items/L in surface water and 20370 ± 20550 items/kg in sediment. MPs abundance peaked at upstream agricultural and downstream drainage sites, with sediment accumulation inversely related to water depth. MPs also tended to accumulate in sediment, which functions as a major sink. Over 93% of detected MPs were <500 µm, predominantly consisting of particles (>67%) made of PE, PP, SBR, and PET. Redundancy analysis revealed that surface water MPs were strongly linked to agricultural land use; sediment MPs showed no such correlation. MPs abundance increased in the wet season, but sediment polymer weight was higher in the dry season, suggesting heavier polymers settled under low-flow conditions. Ecological risk assessments showed contrasting results, with all sites being low-risk by Pollution Load Index (<10) but exceeding the Polymer Risk Index danger threshold (>100, max 5400). PVC and PU drove up to 99.6% of Potential Ecological Risk Index, underscoring the need for polymer-aware risk assessments. Notably, surface water samples contained elevated proportions of these high-hazard polymers, which disproportionately drove the overall ecological risk. These findings underscore the ecological threat posed by toxic polymers and emphasize the need for source-specific, polymer-aware risk assessments in riverine environments. • Year-round monitoring confirmed that sediments act as persistent sinks for MPs • Sediment MPs showed no clear land use pattern but negatively related to water depth • Surface water MPs were significantly influenced by agricultural land use ( p =0.026) • High-risk polymers (PVC, PU) drove elevated PRI and PERI values in surface waters