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River plastic transport and storage budget
Summary
This study provides the first systematic budget of how plastic moves through rivers and where it gets stored, finding that riverbanks and floodplains trap far more plastic than the surface water layer that is typically monitored. Rivers act not just as pipelines delivering plastic to the ocean but as large reservoirs that accumulate and slowly release plastic over time. Understanding this full storage picture is essential for estimating how much microplastic will eventually reach the ocean and for designing effective river cleanup strategies.
Abstract Rivers are one of the main conduits that deliver plastic from land into the sea, and also act as reservoirs for plastic retention. Yet, our understanding of the extent of river exposure to plastic pollution remains limited. In particular, there has been no comprehensive quantification of the contributions from different river compartments, such as the surface, water column, riverbank and floodplain, to the overall river plastic transport and storage. This study aims to provide an initial quantification of these contributions. We first identified the main relevant transport processes for each river compartments considered. We then estimated the transport and storage terms, by harmonizing available observations on surface, suspended and floodplain plastic. We applied our approach to two river sections in the Netherlands, with a focus on macroplastics (>= 2.5 cm). Our analysis revealed that for the studied river sections, suspended plastics account for over 96% of item transport within the river channel, while their relative contribution to mass transport is only 30-37% (depending on the river section considered). Surface plastics predominantly consisted of heavier items (mean mass: 7.1 g/#), whereas suspended plastics were dominated by lighter fragments (mean mass: 0.1 g/#). Additionally, the majority (98%) of plastic mass was stored within the floodplains, with the river channel accounting for only 2% of the total storage. Our study developed a harmonized approach for quantifying plastic transport and storage across different river compartments, providing a replicable methodology applicable to different regions. Our findings emphasize the importance of systematic monitoring programs across river compartments for comprehensive insights into riverine plastic pollution.