The role of reactive air–water interfaces in contaminant transport in the vadose zone

Under unsaturated conditions, the coexistence of air and water generates complex, dynamically evolving interfacial structures, whose impact on solute mixing, residence times, and reactivity remains poorly understood at the pore scale. Substances transported in the water phase can interact with the air phase at the fluid-fluid interface. In particular, per- and polyfluoroalkyl substances (PFAS) are emerging contaminants of concern that are known to preferentially accumulate at air–water interfaces, where interfacial processes control their retention and mobility in the vadose zone. Darcy-scale models and experimental observations suggest that transient hydrological conditions and interfacial area dynamics can strongly influence PFAS fate. However, the pore-scale mechanisms governing transport toward air–water interfaces and the resulting mixing-limited reactivity remain largely unexplored even under steady flow. This gap limits the development of models capable of upscaling pore-scale interfacial mixing processes and predicting solute fate at larger spatial and temporal scales. We investigate these mechanisms using a Lagrangian particle-tracking approach to resolve solute transport in steady two-dimensional pore-scale flow fields under partial saturation. Solute trajectories are governed by advection, diffusion, and interactions with both fluid–fluid (air–water) and fluid–solid interfaces, enabling direct quantification of interfacial encounter statistics and residence-time distributions. These metrics provide natural descriptors of mixing-limited regimes, in which effective reaction rates are controlled by transport toward interfacial zones rather than intrinsic kinetics, and allow identification of pore-scale features that control the large-scale evolution of solute transport. This study contributes to ongoing efforts to connect pore-scale physical processes with effective models of solute transport in the vadose zone, with direct implications for predicting the fate of reactive contaminants under transient unsaturated conditions.