Riverine transport of microplastics from the Dutch border to the North sea

Microplastics are transported with the flow in the river. In this study we modelled the fate of microplastics released at Lobith in the Rhine and at Eijsden in the Meuse, using a depth and width averaged flow model for the Netherlands (Landelijk SOBEK model). For the transport of microplastics the processes advection, deposition and hetero-aggregation of microplastics with sediment are included. Overall, the model results suggest that the deposition is small: about 66 to 90 percent of the released microplastics are transported out of the model towards the sea, meaning that 10-34 percent are either deposited to the river bed or are stored in the water column. Resuspension of deposited microplastics was not included in the model. A sensitivity study for which resuspension was included suggests that it is not an important process in the current 1 D simulation, since the flow velocities at accumulation areas rarely exceed the critical flow velocity for resuspension. The simulated annual transport of microplastics is higher than estimates based on observations, although sources within the Netherlands are not yet included in the model. This needs to be re-evaluated in the future, after sources of microplastics from within The Netherlands have been introduced in the model.