Prediction of non-equilibrium transport of nitrate nitrogen from unsaturated soil to saturated aquifer in a watershed: Insights for groundwater quality and pollution risk assessment

This study introduces an innovative integrated modeling framework to elucidate nitrate nitrogen migration within heterogeneous vadose zones, addressing key challenges in simulating non-equilibrium pollutant transport at the watershed scale. A novel in-situ device was developed for efficient, large-scale soil solute breakthrough curve (BTC) collection, critical for field-scale simulations. Fitting these BTCs, the Mobile-Immobile Model (MIM) (mean R² = 0.94) outperformed the Convection-Dispersion Equation (CDE) (mean R² = 0.88), underscoring the prevalence of non-equilibrium transport in the study area. Bootstrap resampling validated sample adequacy for estimating transport parameters v (velocity) and D (dispersion), with confidence intervals stabilizing below 10 %. The Ensemble improved Stream Tube Model (ESTM), incorporating Pearson Type III (P-III) distributions for transport variables (validated against lognormal; R² up to 0.92) and nitrate degradation/adsorption, significantly enhanced predictive precision. This model accurately predicted high pollutant concentrations during rainy seasons (NSE = 0.97) and reasonably estimated dry season low concentrations, despite slight overestimations attributed to sensor limitations at low moisture. When integrated with a groundwater solute transport model, the framework effectively simulated long-term nitrate dynamics in both vadose and saturated zones under fertilizer application, closely matching observations. Sensitivity analysis highlighted mean dispersion, its skewness, mean velocity, and adsorption as critical for non-equilibrium transport. Critically, by explicitly modeling v and D joint heterogeneity, our ESTM markedly outperformed traditional models in simulating early breakthrough and tailing (R² = 0.875 vs. 0.623). This research provides a robust, adaptable approach for understanding groundwater nitrate dynamics and pollutant fate across diverse environmental conditions.