Tidal-driven transport and retention of polydisperse cylindrical microplastics in porous media: Insights from column and visualized chamber experiments

Coastal aquifers act as important sinks and transport pathways for microplastics (MPs) originating from both terrestrial and marine sources. However, the mechanisms governing their vertical transport and retention under tidal forcing in groundwater systems remain poorly understood. This study combined column experiments with real-time visualization chambers to investigate the transport of polydisperse cylindrical MPs in porous media under tidal cycling conditions. Results showed that tidal cycles promoted vertical redistribution of MPs, with cumulative upward displacement increasing with cycle number. Flow velocity, glass bead (GB) size and salinity synergistically affected MP migration. Higher flow velocity intensified upward transport and altered spatial distribution patterns in 6.0 mm GB media. Decreasing GB size from 6.0 mm to 2.5-3.0 mm significantly inhibited MP upward migration under all tested salinity conditions. The inhibitory effect of increased salinity on upward MP migration was more pronounced under lower flow velocity and in finer porous media. Mechanical processes including bridging and straining governed by particle-to-grain size ratio played a dominant role in MP retention, particularly in finer porous media. In polydisperse systems, size-selective transport emerged primarily in finer porous media, with smaller MPs preferentially migrating upward while larger particles were retained in deeper layers. MP migration was mainly controlled by the coupled effects of tidal-driven hydrodynamics, capillary-driven entrainment and interfacial trapping, as well as pore-scale straining and bridging. These findings provide new insights into MP transport in coastal porous media and improve understanding of land-to-sea MP fluxes and associated environmental risks.