Transport and retention of microplastics during riverbank filtration

Introduction Riverbank filtration (RBF) is a widely used natural water treatment process; however, the fate and attenuation of microplastics (MPs), within river–aquifer systems is not well understood. This study investigated the occurrence and characteristics of MPs within an RBF system to comprehend the MPs transport behaviour. Methods Five sampling campaigns were conducted over the course of a year. These covered rivers, groundwater wells and an abstraction well across six locations within the RBF site, which extracts water from the Leine River in Germany. MPs were collected and characterized through stereomicroscopy and chemically identified using ATR-FTIR. In this study, MPs were detected throughout the RBF system but also in groundwater without river influence during all sampling campaigns, demonstrating their continuous presence and persistence within the hydrological continuum. Results and discussion River water consistently exhibited the highest MP concentration (74.4 ± 23.5 MPs/100 L), whereas the abstraction well showed low concentration (9.4 ± 7.8 MPs/100 L), significant differences in MP concentrations were observed ( p = 0.002). MP transport and retention were strongly influenced by MP characteristics such as shape and type. Overall, fiber shapes (88.5% of all particles) and polyethylene terephthalate (PET) (73.8% of total identified polymers) dominated. The prevalence of fibers suggests that their elongated morphology may enhance mobility through interconnected pore spaces. Within the collected MP size range of 20 to 5,000 μm, the most abundant class was 100–200 μm, indicating that particle size may regulate pore-scale straining. These results indicate that, despite their high density, PET fibers can pass through pore spaces and potentially bypass natural filtration barriers due to their small size. The study emphasizes that RBF is an effective spatial concentration attenuation system for reducing MP contamination in drinking water sources and highlight the importance of natural attenuation process.