Unraveling microplastic retention distribution in porous media: A unified framework coupling flow conditions and particle properties.

The vertical transport of microplastics (MPs) in porous media poses a significant threat of deep aquifer contamination, yet predicting their spatial distribution remains challenging due to complex particle-media interactions. This study integrates column experiments with numerical modeling to elucidate the vertical retention dynamics of MPs in saturated porous media. The results demonstrate that MP retention profiles are shaped by a dynamic competition between physicochemical attachment and mechanical straining. Contrary to the exponential decay typically observed in clean-bed filtration, higher influent particle concentrations shifted the retention profile toward deeper layers, driven by the blocking effect following the rapid saturation of surface deposition sites. Elevated flow rates diminished surface accumulation and facilitated deeper infiltration by reducing residence time and generating strong hydrodynamic shear forces that continuously opposed stable deposition, driving ongoing particle re-migration. Regarding physical properties, particle size dictated the vertical distribution mode: nano-sized MPs exhibited high mobility with uniform retention profiles, whereas micron-sized MPs were confined to shallow layers via mechanical straining. While polymer density acted as a gravitational filter, the enhanced settling of denser PVC-MPs subtly shifted their center of mass toward shallower layers. Crucially, a unified predictive framework based on the Damköhler number (Da) was established, which successfully categorizes MP transport into retention-limited or transport-limited regimes. This study provides a scalable theoretical tool for assessing the depth-dependent vulnerability of groundwater systems to MP contamination.