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

Debao Lu; Jian Ou; Jinglin Qian; Cundong Xu; Hui Wang

doi:10.1016/j.jconhyd.2025.104649

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

J Contam Hydrol. 2025 Jun 15:274:104649. doi: 10.1016/j.jconhyd.2025.104649. Online ahead of print.

Authors

Debao Lu¹, Jian Ou², Jinglin Qian², Cundong Xu², Hui Wang²

Affiliations

¹ Key Laboratorfigy for Technology in Rural Water Management of Zhejiang Province, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, China; College of Engineering, University of Exeter, Exeter EX4 4QF, UK. Electronic address: d.lu@exeter.ac.uk.
² Key Laboratorfigy for Technology in Rural Water Management of Zhejiang Province, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, China.

PMID: 40554323
DOI: 10.1016/j.jconhyd.2025.104649

Abstract

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.

Keywords: Agricultural pollutants；; ESTM; Integrated modeling; Nonequilibrium transport; Vadose zone.