Deep transport potential of microplastics in estuarine riparian soils: coupled controls of pore structure, hydrodynamics, and interfacial adhesion

Estuarine riparian soils are critical interfaces connecting surface water, soils, and shallow aquifers, yet the pore-scale mechanisms governing microplastic (MP) transport and retention remain poorly understood. Here, undisturbed soil cores from the Huangpu River and Yangtze River riparian zones were investigated using stratified MP analysis, Py-GC/MS, micro-CT, and CFD-DEM simulations coupled with XDLVO calculations. MP abundance increased from 415.6 ± 20.8-960.1 ± 48.0 n/kg in Huangpu River soil and from 58.7 ± 2.9-187.1 ± 9.4 n/kg in Yangtze River soil from the 0-4-8-12 cm layers, while median particle size decreased from 118.82 to 19.89 μm and from 42.79 to 17.51 μm, respectively. These patterns were consistent with depth-dependent enrichment and size-selective redistribution. Micro-CT analysis showed that Huangpu River soil had finer pore throats and more tortuous pore networks, favoring shallow retention, whereas the Yangtze River soil contained larger, more directionally continuous pathways that facilitated deeper migration, particularly under water-scour conditions. XDLVO analysis further revealed contrasting interfacial adhesion behaviors between the two soils. Overall, MP migration and retention were jointly controlled by pore-network accessibility, hydrodynamics, and interfacial interactions, providing a mechanistic basis for assessing potential MP transfer toward deeper soils and shallow groundwater.