Plastics Gone Missing: Resolving the Mass Balance at River Bifurcations

Recent studies suggest that more land-based plastics are accumulated and remobilized in riverine environments than exported to the ocean. Hydrodynamics and other factors like wind drag and navigation can drive plastics to riverbanks, where they can be retained in plants, on floodplains, or in stagnant water bodies. With this study, we provide additional observational evidence that a substantial share of floating plastics may not flow downstream. Every week from May to December 2021, we measured floating plastics at three bridges located in the Hong-Duong bifurcation of the Red River, Vietnam. These locations were chosen to monitor both the input plastics entering the bifurcation and the output plastics exiting the bifurcation in both branches. The upstream location is Nhat Tan, which is located on the Red River in northern Hanoi. The downstream location on the main stream is Long Bien, approximately 8 kilometers south of Nhat Tan on the Red River, while the third location is in Dong Tru, on the distributary Duong River, about 7 kilometers from Nhat Tan. We collected data on the plastic mass balance of various plastic categories in the bifurcation, including PET, PO-soft, PO-hard, multilayers, PS, and PS-E. The results indicated that the plastic mass balance does not close in general; there is more plastic upstream than in the two downstream locations combined. The dry season, from October to December, was more balanced, with a 4% difference. Meanwhile, between May and September, approximately 16% of floating plastics were discovered to be missing. Additionally, the majority of floating plastics remained in the main stream, with only 8% entering the distributary, and the division rate kept constant throughout the study period. However, the balance differed for specific categories. Five categories had missing downstream records compared to upstream, and PO-soft featured a more intricate balance mechanism with alternating changes between missing and abundant records from month to month. In the meantime, PS was seen upstream but was never detected downstream. For PS-E, most of the items found in the upstream were either detected in the distributary, or disappeared in the bifurcation; less than 1% was identified downstream. The variation in transport between PS and PS-E may be caused by deposition, extraction, accumulation, or sub-surface transport. Finally, except for PO-hard, the rate of plastic transport was found to be higher near the river banks than in the thalweg. These findings suggest that the transport mechanism of macroplastics in rivers may be more complex than previously assumed, and prompt further studies. Our data allow modelling plastic transport for different plastic categories, designing suitable monitoring or clean-up methods, and understanding the roles of hydrological components such as discharge and flow velocity in transporting plastics.