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Microplastic transport regulated by land use, point source and tidal forces in a coastal river-estuary system
Summary
Scientists found that microplastic pollution near coastal areas comes mainly from nearby human land use (like farms, cities, and factories) and specific pollution sources like sewage outflows, which settle into river-bottom sediment over time. Even more concerning, these sediments don't just trap the plastic permanently — tides stir them back up into the water, meaning the plastic can keep re-entering the water we and marine life come into contact with long after it first settled. This matters because it shows river and estuary sediments act like a "reservoir" that keeps releasing microplastics, so cleaning up pollution sources today may not fully solve the problem without
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.