Nitrate sources and transformation in surface water and groundwater in Huazhou District, Shaanxi, China: Integrated research using hydrochemistry, isotopes and MixSIAR model

Global water resources affected by excessive nitrate (NO₃^-) have caused a series of human health and ecological problems. Therefore, identification of NO₃^- sources and transformations is of pivotal significance in the strategic governance of widespread NO₃^- contaminant. In this investigation, a combination of statistical analysis, chemical indicators, isotopes, and MixSIAR model approaches was adopted to reveal the hydrochemical factors affecting NO₃^- concentrations and quantify the contribution of each source to NO₃^- concentrations in surface water and groundwater. The findings revealed that high groundwater NO₃^- concentration is concentrated in the southwestern region, peaking at 271 mg/L. NO₃^- concentration in the Wei River and Yuxian River exhibited an increase from upstream to downstream, but in the Shidi River and Luowen River, its concentration was highest in the upstream. Groundwater NO₃^- has noticeable correlation with Na⁺, Ca²⁺, Mg²⁺, Cl^-, HCO₃^-, TDS, EC, and ORP. In surface water, NO₃^- level is significantly correlated with NH₄⁺ and ORP. Major sources of NO₃^- in surface and groundwater comprise manure & sewage and soil nitrogen. Source contribution for surface water was calculated by MixSIAR model to obtain soil nitrogen (57.7%), manure & sewage (23.8%), chemical fertilizer (12%), and atmospheric deposition (6.4%). In groundwater, soil nitrogen and manure & sewage accounted for 19% and 63.8% of nitrate sources, respectively. Both surface water and groundwater exhibited strong oxidation, with nitrification the primary process. It is expected that this study will provide insights into the dynamics of NO₃^- and contribute to the development of effective strategies for mitigating NO₃^- contaminant, leading to sustainable management of water resources.