Estimating plastic export from estuaries into the sea using an estuarine-mass-balance model.

Estuaries act as an interface between rivers and the sea, and therefore play a key role in transporting plastic to the sea. However, plastic transport in estuaries is complex and estuary-to-sea export is still very uncertain. This study presents a macroplastic mass-balance model that can be applied to specific estuaries, and produces export estimates similar to previous studies. The daily plastic export from estuary to sea during the wet season was estimated to be 290 kg for the Rhine-Meuse Estuary (RME), and 19,431 kg for the Saigon Estuary, where the discharge into the sea is 3.5 times larger for the RME, compared to the Saigon Estuary. Due to limited river discharge measurements for the Saigon Estuary, a yearly simulation could only be performed for the RME, corresponding to an export of 13,300 kg. These estimates fall within the uncertainty range of previous studies. Deposition and remobilization from riverbanks resulted in the largest model fluxes, with magnitudes of 20 t/d for the RME and 430 t/d for the Saigon Estuary. At the same time, the model is most sensitive to the uncertainty in parameters related to deposition. Our results demonstrate that with the right measurements, making reliable estimates of estuary-to-sea export is possible. However, uncertainty in plastic transport processes, like deposition and remobilization, leads to poorly constrained parameters and increased overall model uncertainty. Automated sensors, targeted field experiments, and improved model structure can enhance export estimates and further improve understanding of plastic dynamics in estuaries.