Plastic retention and export across Europe's rivers

Rivers play a central role in the global distribution of plastic pollution, as plastics are both retained and exported to sea. Large scale plastic transport models are used to quantify plastic export. Despite recent research showing that the majority of plastic is not transported to sea, most models have overlooked reporting a key process in plastic transport dynamics,  quantifying the retention of plastic in rivers. Moreover, previous models have carried out limited model calibrations, leading to uncertainty in export estimates. These gaps in knowledge have resulted in models potentially over or underestimating plastic export, and providing a limited assessment of the overall plastic pollution of a river system. We propose a new model to explore the state of plastic pollution accounting for both micro- and macroplastic considering both river export and retention on land and in rivers at a European scale. We find that rivers are a significant temporary sink for plastics with a large quantity of plastic being retained in Europe rivers systems on an annual scale. River basins with a large area and input of plastic to land contribute considerably to absolute plastic export and retention. When accounting for population density, we find that small coastal river basins are estimated as having a larger macroplastic export per capita compared to larger basins. Conversely large river basins are estimated as having the largest microplastic export per capita. This outcome shows the complexity of plastic export and retention and reinforces the need to model at a high spatial resolution for accurately characterizing pollution dynamics. We determine that reducing plastic generation and mismanaged inputs to land is fundamental to achieving meaningful reductions in plastic pollution. Finally, we show that using a single indicator (e.g. plastic export or retention in rivers) to determine the most polluted rivers may not be sufficient in determining the overall level of plastic pollution of a river system. By integrating estimates of plastic retention, the final assessment of plastic pollution reveals a different set of high polluting rivers, than if only export estimates were to be used. By carrying out this research we provide a holistic overview of the overall state of pollution in Europe’s rivers by evaluating plastic export and retention estimates using a well calibrated, high resolution model.