Simulating the fate and transport of microplastics in a river corridor (Rhine River)

The extensive use of plastics has led to a widespread presence of microplastics (MPs) across various environmental compartments. Rivers and their floodplains not only play a crucial role in transporting these particles from terrestrial sources to lakes and oceans, but can also act as temporary sinks. Despite the significance of rivers as transport pathways for MPs to the ocean, our understanding of the dominant transport and retention process in river corridors is still limited. This study investigates the transport, deposition and remobilization processes of MP along a 3.5km reach of the river Rhine between Cologne and Düsseldorf, Germany. 

A three-dimensional hydrodynamic model with the morphological module (D-Morphology) was developed using the Delft3D FM software. Two types of microplastic particles with diameter of 0.1mm and different densities, 1030 kg/m³ Polystyrene and 1195 kg/m³ Polyvinyl Chloride (PVC) were used to assess their transport behavior under different flow scenarios. The model was calibrated against observed water levels on Manning’s roughness coefficient and subsequently validated against an independent data set. A continuous flux of microplastics at a concentration of 1μg/m³ was introduced into the hydrodynamic model at the upstream boundary.

First, results indicate that advection and flow turbulence are the dominant processes governing microplastic transport. Higher discharge rates enhance microplastic transport by increasing suspended concentrations, while reducing the mass of the sedimented particles. The percentage of sedimented Polystyrene was found to be about 2.5% of total input at the end of a simulated flood event in 2021. Resuspension was found to be about 40% of the sedimented mass along the river banks and floodplain during peak flood. During the recession limb of the flood event, sedimented microplastic load increased gradually whereas suspended load decreased. Additionally, the density and size of the microplastic particles along with hydrodynamic conditions significantly influence their spatial distribution and storage within the river corridor.