A critical view on determination of annual microplastic loads in the Rhine River

Microplastics (MP) have become a major focus in environmental research, particularly in rivers. Despite numerous studies, there are no standardized methods available, neither for sampling nor for quantification, leading to significant differences in reported loads of microplastic. This study focused on assessing MP loads for polypropylene (PP), polyethylene (PE), polystyrene and poly(vinyl chloride) in the Rhine River at Koblenz using two extensive datasets from 2019 to 2023 based on sampling with continuous flow centrifuge (CFC) and sedimentation boxes (SB). For the load calculations, well-established discrete and integrated calculation approaches were applied, originating from hydro-geomorphology. Annual MP loads of up to 280±77 t (CFC) and 429±125 t (SB) were determined. The sampling technique strongly influenced the calculated MP loads based on its sampling advantages and disadvantages. For polymers with densities <1 g/cm³ separation techniques seem to be technically limited. CFCs are often used as the standard sampling device in suspended sediment concentration monitoring strategies, due to its high sampling efficiency for mineral particles, but have limited abilities to gather lighter polymers such as PE and PP. Recovery experiments indicated concentration shifts in the SB (up to twice the polymer concentration), and incomplete recoveries of PE (23±9%) and PP (48±12%) with the CFC. Therefore, when using a CFC it is recommended to additionally sample and analyze the residual water, to avoid the loss of low-density polymers (<1 g/cm³). Our results suggest that the suitability of both, discrete and time-integrated sampling approaches for microplastic load estimation strongly depends on the considered particles.