Numerical modelling of the effects of sub-irrigation using treated wastewater on nitrogen species and groundwater levels using HYDRUS-2D

In this study, a reactive transport model using HYDRUS-2D is built for an agricultural field in Kinrooi (Belgium), irrigated with treated wastewater through a sub-irrigation system. The model evaluates groundwater level fluctuations and transformations of NH₄⁺, NO₂⁻, and NO₃⁻ within the unsaturated zone and groundwater over a three-year period. Hydrodynamic and reactive transport components were calibrated against high-resolution groundwater level time series and nitrogen compound concentrations. Model shows that sub-irrigation raised groundwater levels by up to 0.4 m during dry periods, while after irrigation ceased, groundwater levels declined nonlinearly, with roughly half of the increase dissipating within the first month and the remainder gradually returning to baseline over the following three months. Although sub-irrigation slightly affected nitrogen transport and leaching, these impacts were spatially limited to irrigated zone and diminished one month after irrigation ended where concentrations within unsaturated zone for NH₄⁺ and NO2⁻ decreases by 10 %. Groundwater concentrations remained below permitted threshold values in both irrigated and non-irrigated zones. Elevated background nitrate from long-term fertilizer use reduced the relative effect of sub-irrigation on groundwater nitrate. Scenario analyses further revealed that natural processes, including plant uptake, sorption, and particularly nitrification and denitrification play a key role in regulating NH₄⁺ and NO₃⁻ concentrations, while their absence leads to higher concentrations due to reduced transformation and retention.