Geometric confinement regulates shape-dependent microplastic migration across air-water interfaces in variably saturated porous media.

Microplastics (MPs) can migrate through the vadose zone, linking terrestrial plastic pollution with groundwater contamination, yet the mechanisms controlling transfer across air-water transition regions remain poorly resolved. One-dimensional column experiments were conducted to investigate the downward transport and interfacial retention of spherical, fibrous, and film-like polyethylene MPs in variably saturated porous media. Glass beads with diameters of 7 and 14 mm were used to regulate particle-to-medium size ratios and geometric confinement. Under strong confinement in the 7-mm bead columns, more than 89% of the initial MP mass was retained in the upper variably saturated section for all morphologies, limiting delivery to the operationally defined transition layer and masking shape-dependent differences. With reduced confinement, more MPs migrated downward and morphology-dependent behavior became evident. Fibrous MPs exhibited the greatest downward mobility; for G14-F1, 20.84% of the initial mass accumulated in the interfacial layer and 26.39% entered the saturated section. Spherical MPs showed the highest conditional air-water interfacial retention efficiency (EB), defined as the fraction of MP mass retained in the transition layer among the combined mass recovered from the transition and saturated sections, reaching EB = 0.94. Film-like MPs exhibited limited saturated-zone penetration because their delivery through the upper porous section was restricted. Dimensionless force analysis suggested that capillary effects contributed to interfacial retention, although pore-scale visualization is needed to verify the proposed mechanisms. These results show that geometric confinement acts as a first-order transport filter that regulates whether morphology-dependent transport behavior can emerge. They further demonstrate that groundwater transport potential depends on the sequential coupling of upstream particle delivery, interfacial retention, and saturated-zone penetration rather than on interfacial retention alone.