Hydrology as a Driver of Floating River Plastic Transport.

Plastic pollution in aquatic ecosystems is a growing threat to ecosystem health and human livelihood. Recent studies show that the majority of environmental plastics accumulate within river systems for years, decades and potentially even longer. Long‐term and system‐scale observations are key to improve the understanding of transport and retention dynamics, to identify sources and sinks, and to assess potential risks. The goal of this study was to quantify and explain the variation in floating plastic transport in the Rhine‐Meuse delta, using a novel 1‐year observational data set. We found a strong positive correlations between floating plastic transport and discharge. During peak discharge events, plastic transport was found up to six times higher than under normal conditions. Plastic transport varied up to a factor four along the Rhine and Meuse rivers, which is hypothesized to be related to the complex river network, locations of urban areas, and tidal dynamics. Altogether, our findings demonstrate the important role of hydrology as driving force of plastic transport dynamics. Our study emphasizes the need for exploring other factors that may explain the spatiotemporal variation in floating plastic transport. The world's most polluted rivers are connected to the ocean through complex deltas. Providing reliable observations and data‐driven insights in the transport and dynamics are key to optimize plastic pollution prevention and reduction strategies. With our paper we aim to contribute to both advancing the fundamental understanding of plastic transport dynamics, and the establishment of long‐term and harmonized data collection at the river basin scale. Plain Language Summary: Plastic pollution in rivers and oceans harms ecosystems and human livelihoods. Especially large plastic items (>0.5 cm) can be mistaken for food by animals, damage ships, and block waterways. Knowing how much plastic is floating through rivers is important for policy‐makers to reduce plastic pollution in the environment. In our study, we measured floating plastic pollution in the Rhine and the Meuse, two large European rivers that flow into the ocean in the Netherlands. From January to December 2021, a team of students and volunteers counted plastic items floating in the rivers from bridges. We found that more plastic was counted when the river flow was higher. The highest amount of plastic was measured during two flood events, when parts of the land next to the rivers were flooded. We think that more plastic leaks into the river when streets, riverbanks, and floodplains are under water. We hope that our study can help to better predict how much plastic flows through other big rivers around the world. Only when we know how big the plastic problem is, we can successfully solve it. Key Points: Plastic pollution is a global environmental challenge, but poorly understood and quantified due to a lack of reliable observationsRiver plastic transport increases significantly during discharge during peak eventsHydrology plays a crucial role in the transport and retention dynamics, and the spatiotemporal variation of floating plastic transport [ABSTRACT FROM AUTHOR]