Soil structure strongly controls vertical microplastic transport in floodplain soils

Floodplains are highly dynamic systems and can accumulate large quantities of microplastics (MPs), yet the mechanisms controlling their vertical redistribution after deposition remain poorly constrained. We investigate MP infiltration and transport in undisturbed Rhine floodplain soils using intact 0–20 cm cores subjected to a controlled flooding scenario. A particle mix of polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) (20–75 µm), cryomilled and pre-incubated in Rhine water to allow biofilm formation, was applied to the soil surface prior to flooding. Particles were labelled with Rhodamine-B and metal oxides to enable complementary optical and elemental tracing. Water flow and tracer transport were monitored using D₂O breakthrough curves and continuous gravimetric measurements.

After freezing, soil columns were sectioned into 2 cm layers, and MPs were quantified by fluorescence microscopy, µ-XRF, and AI-assisted particle recognition. Results indicate rapid MP infiltration and vertical transport within the soil. MP breakthrough was observed in all columns, although breakthrough timing and concentrations varied among replicates.  Vertical transport was strongly governed by spatial heterogeneity and preferential flow paths, particularly biogenic macropores, whereas saturated hydraulic conductivity alone did not reliably predict MP movement. These findings highlight the dominant role of soil structural controls in floodplain MP transport and challenge the use of bulk hydraulic parameters for predicting MP redistribution during flooding events.