Modeling of Nitrate Transport in the Vadose Zone by Considering the Mobile–Immobile Approach Using a Sand Tank Experiment

Precise process-based modeling of nonequilibrium water flow and solute transport continues to pose a significant challenge in vadose zone hydrogeology. Nonequilibrium water flow and solute transport in the vadose zone bring about various complexities associated with heterogeneity, variability, unsaturated flow processes, hysteresis effects, and time and scale dependencies. These challenges necessitate the utilization of advanced measurement techniques, robust modeling approaches, and a comprehensive understanding of the physical and chemical processes that take place in the vadose zone. This study focuses on the experimental and process-based modeling of nitrate transport that considers equilibrium and nonequilibrium (physical nonequilibrium) transport processes. This study was carried out by performing a laboratory-scale experimental study followed by a numerical simulation modeling study. The experimental study was conducted using a tank setup 60 cm long, 30 cm wide, and 60 cm deep, which was filled with sandy soil. The nitrate concentration was measured at different soil depths after a constant water flux, which contained 300 mg/L nitrate, was applied to the tank top surface. A numerical simulation modeling study was performed using HYDRUS-1D. Simulation runs were carried out by considering equilibrium (single porosity) and physical nonequilibrium (dual-porosity model) conditions. Various simulations used dual-porosity models, which consider different proportions of sorption sites that interact with the mobile water content and varying mass transfer coefficients. The simulation with a 75% mobile zone with a 10 day−1 mass transfer coefficient agreed well with the experimental data.