Microplastic transport regulated by land use, point source and tidal forces in a coastal river-estuary system

Microplastic (MP) pollution in coastal river-estuary systems is governed by the combined effects of terrestrial inputs, in-channel deposition, and tidal resuspension; however, the coupled influence of diffuse land-use sources, localized point discharges, and marine forcing on land-sea MP transport remains insufficiently resolved. This study develops an integrated land-sediment-water analytical framework to quantify how land-use patterns and engineered point sources jointly regulate sedimentary MP accumulation and how tidal fluctuations remobilize benthic MPs into the water column. Land-use spatial analysis was combined with principal component analysis (PCA) and multiple linear regression (MLR) to distinguish diffuse non-point inputs from localized point-source contributions. A distance-decay-based composite exposure index was constructed to integrate multiple upstream discharges into a spatially weighted metric. The final combined PCA-MLR model incorporating land-use gradients and the point-source exposure index explained 71.9% of the spatial variation in sedimentary MPs (R² = 0.719, adjusted R² = 0.649, p = 0.0013). Sedimentary MP concentrations were positively associated with both anthropogenic land-use gradients and point-source intensity. In contrast, riverine MPs showed negligible correlation with surrounding land-use variables but were significantly predicted by sedimentary MP levels (R² = 0.503, p < 0.005), indicating that estuarine sediments act as long-term sinks for terrestrial MPs while simultaneously serving as secondary sources through tide-driven resuspension. These findings identify sediment-water coupling as a critical exposure pathway and provide quantitative support for risk-informed MP management in transitional coastal environments.