Spatial heterogeneity and vertical redistribution of microplastics in floodplain soils

Floodplains are increasingly recognised as important sinks for microplastics (MPs) at the interface between terrestrial and aquatic systems, yet their role in the long-term retention, redistribution, and vertical transport of MPs in soils remains poorly understood. While rivers are known pathways for microplastics to marine environments, floodplain soils may act as intermediate storage compartments where hydrological dynamics, sediment properties, and biological activity jointly control plastic fate.Here, we investigate the spatial and vertical distribution of microplastics in floodplain soils along the Rhine River, combining field observations, depth-resolved soil analyses, and hydrodynamic flood modelling. Soil profiles were sampled across multiple transects spanning contrasting floodplain topographies and flooding frequencies. Microplastic abundance and mass concentrations were quantified using FTIR spectroscopy and pyrolysis GC/MS. To assess controls on vertical redistribution, top soils and one selected soil profile were studied, supported by physico-chemical analyses and dating. In parallel, a hydrodynamic flood model was used to relate observed microplastic patterns to flood frequency and inundation dynamics.Our results reveal pronounced spatial heterogeneity in MP distribution across floodplains. Highest concentrations consistently occur in topographic depressions characterised by frequent inundation and enhanced sediment deposition. Vertically, microplastics are predominantly enriched in upper soil horizons but are also detected at depth, indicating downward transport beyond simple surface accumulation. Associations with finer-grained horizons suggest a role of soil physical structure in regulating retention, while deviations from this pattern point to the modifying influence of biological activity and soil mixing processes.These findings highlight floodplains as dynamic and heterogeneous microplastic sinks, where hydrological connectivity, local topography, and soil properties interact to control transport and long-term fate.