When barriers become sources: Mechanistic insights into bidirectional diffusion under non-linear adsorption in multilayer aquitard-aquifer systems.

Predicting contaminant transport through aquitards remains a critical challenge because their apparent low permeability can mask concentration-dependent retention and release mechanisms. This study integrates non-linear Langmuir adsorption with image-based light reflection visualization (LRV) and analytical diffusion modeling to quantify bidirectional solute transport across stratified aquifer-aquitard systems. Batch adsorption experiments revealed strong non-linearity, where the retardation factor (RLangmuir) decreased sharply above 60 mg/L, indicating reduced sorption efficiency and accelerated diffusion at elevated concentrations. Incorporating this behavior into the analytical model revealed a threshold-type transition: solute migration initially slowed by strong sorption became increasingly rapid as adsorption sites approached saturation. The experimentally derived constant RLangmuir = 1.2, representing partial-capacity conditions, reproduced measured profiles with high accuracy (mean NSE = 0.96). Upward diffusion from confined aquifers breached overlying aquitards, while downward migration penetrated multiple clay layers, experimentally confirming bidirectional contaminant transport. Simulations further showed that moderate advective flow within intermediate sand layers extended both the spatial reach and persistence of contamination. These findings demonstrate that non-linear adsorption fundamentally governs the temporal evolution of aquitard behavior, transforming them from passive barriers into active secondary sources under varying boundary conditions. The proposed framework linking non-linear adsorption, quantitative visualization, and analytical modeling provides a new process-based foundation for predicting long-term contaminant persistence and developing sustainable remediation strategies in complex multilayer subsurface environments.