Polymer-specific transfer and retention of microplastics at the river–sediment–groundwater interface

Microplastic particles (MPs) are ubiquitous contaminants in fluvial systems, yet the processes governing their transport and retention in surface water–groundwater systems remain insufficiently understood. This study emphasizes the combined role of removal along subsurface flow paths from rivers to groundwater, and of lateral river hydrodynamics, in shaping the distribution of MPs in riverbeds.The spatial distribution of MPs in surface waters, sediment cores, and adjacent groundwater was investigated at two bank filtration sites in north-eastern Germany. The investigations, which took place between October 2022 and March 2024, demonstrate that the accumulation or mobilization of different polymers in riverbed sediments is controlled by a combination of hydrological exchange processes and in-channel hydrodynamics. For instance, ship-induced currents can resuspend particles, thereby preventing their deposition in navigation canals and enhancing their accumulation in riverbanks. While PP and PE dominated surface waters, negatively buoyant polymers such as PET and PVC were enriched in riverbeds and groundwater. PA, although present in surface and groundwater samples, was absent from riverbed sediments, suggesting high subsurface mobility.This work identifies critical mechanisms controlling the fate of MPs in fluvial systems. It demonstrates that the interface between surface water and groundwater can act as a sink or a conduit, depending on the polymer type. The implications of this phenomenon are significant for the protection of drinking water and the health of freshwater ecosystems, as retention hotspots and transport pathways influence the exposure risk to biota and water resources.