Temporal variations in net plastic transport from the Rhine and Saigon rivers into the sea

Estuaries are the transition zones between rivers and the ocean, and act as final buffer zones for plastic pollution before entering the sea. Plastic transport and retention in estuaries are the result of a complex interplay between tidal dynamics, freshwater discharge, and estuary characteristics. Despite its importance, net plastic transport between rivers and the sea is poorly understood and quantified. Here, we show that the net plastic transport in estuaries is highly variable over time and between rivers. We combined plastic concentration data and simulated discharge data to estimate plastic transport for a one-year period in the Saigon, Vietnam, and a five-month period in the Rhine, Netherlands. We estimated the tidal-cycle averaged net plastic transport. We defined the delivery ratio as the relative net transport over a full tidal cycle (ebb and flood phases), ranging from 1 (all plastic transport to sea) to -1 (all plastic transported upstream). We found that the delivery ratio varied between -0.87 and 1.0 for the Saigon and -0.05 and 0.84 for the Rhine. Negative delivery ratios indicate that estuaries can have a net upstream transport of plastics during certain periods. We quantified the uncertainty stemming from incomplete monitoring of full tidal cycles. We demonstrate that the uncertainty for individual tidal cycles remains large, even at 90% coverage of full tidal cycles. Our results emphasize the importance of reliable observations for full tidal cycles. We anticipate our study to provide guidance on improved monitoring, understanding, and accounting of plastic transport and retention in estuaries.