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

Microplastic particles (MPs) are ubiquitous across aquatic compartments, yet their transport and retention dynamics in surface water-groundwater systems remain poorly understood. This study investigates polymer-specific microplastic fate across the sediment-water interface at two bank filtration sites in northeastern Germany, both characterized by sandy riverbeds and permanently losing hydraulic conditions. Using harmonized sampling and analytical methods, MPs ≥32 µm were quantified in surface waters, sediment cores, and adjacent groundwater from October 2022 to March 2024. MP concentrations in surface waters were significantly reduced by over 84 % in adjacent groundwater, indicating high MP removal efficiency during bank filtration. Spatial heterogeneity in riverbed MP concentrations reflects hydraulic stress regimes, with ship-induced currents resuspending particles, preventing deposition in navigation canals, and enhancing accumulation in river banks. Non-metric multidimensional scaling confirmed significantly distinct polymer composition patterns across environmental compartments. While polyethylene and polypropylene dominated surface waters, negatively buoyant polymers such as polyethylene terephthalate and polyvinyl chloride were enriched in riverbeds and groundwater. Polyamide, although present in surface and groundwater samples, was absent from sediments, suggesting high subsurface mobility. Median MP sizes remained relatively constant across compartments, indicating that local hydrodynamics and polymer properties govern transport more than size alone. These findings emphasize the role of both removal along the subsurface flow path and lateral river hydrodynamics in MP distribution. Overall, this study provides robust field-based evidence of polymer-specific MP retention and transport at the surface water-groundwater interface, with implications for drinking water protection and freshwater ecosystem health.