Long-term nitrate legacy in the vadose zone–groundwater system: Integrated modeling of intensive agriculture impacts

Agricultural intensification has led to persistent nitrate (NO₃^⁻) accumulation in the vadose zone (VZ)–groundwater system, yet the long-term fate of this NO₃^⁻ remains insufficiently understood. This study develops a high-resolution coupled modeling framework that integrates a multiple-column VZ model (MCM) with a MODFLOW-MT3D groundwater model. The framework reconstructs NO₃^⁻ dynamics over four decades (1982–2022) in China’s Guanzhong Plain, captures the complete transport chain from agricultural inputs and VZ processes to riverine discharge, and has been validated against multi-source observations. The results show that the mean NO₃^⁻ leaching fluxes increased sharply after 1987. Correspondingly, the VZ NO₃^⁻ storage increased nearly tenfold, from 670 kt in 1982 to 7,631 kt in 2022. Pronounced spatial heterogeneity was observed, with VZ NO₃^⁻ residence times exceeding 100 years in peripheral thick-loess areas, but only 10–27 years in central thin-loess zones. Groundwater NO₃^⁻ concentrations exhibited a spatial pattern closely consistent with leaching distributions, while groundwater NO₃^⁻ storage showed a turning point around 1988, decreasing to 3,190 kt before increasing to 3,328 kt by 2022. This pattern reflects the delayed but direct transmission of NO₃^⁻ from the VZ to groundwater. Over the study period, the average groundwater discharge to the Yellow River was 4.2 × 10⁸ m³ yr⁻¹, while the NO₃^⁻ export via this discharge decreased from 77 kt to 12.7 kt. When combined with irrigation return flows, this reduction forms a “closed N cycle” that enhances subsurface NO₃^⁻ accumulation. This coupled framework provides a transferable approach for quantifying NO₃^⁻ storage, residence times, and release dynamics in intensively cultivated regions. It provides critical insights into legacy N risks and facilitates the development of long-term groundwater protection strategies.