Multi-isotope tracer for identifying nitrate sources in shallow groundwater in a large irrigation area, China

Nitrate pollution of groundwater associated with anthropogenic activities and social development is a global issue, especially in agricultural regions. This study investigated the source and fate of nitrate in groundwater of the Jiaokou Irrigation Area using chemical and isotopic fingerprints (isotope tracing, graphical analysis, and d-excess) in combination with end-member mixing analysis (EMMA). The hydrochemical analysis revealed that the NO₃^- concentration exceeds 20 mg/L (calculated as N) in approximately 66.67% of groundwater samples. The highest NO₃^- concentration was observed in Cl·SO₄ type water, followed by the HCO₃ and SO₄ types. Areas with serious nitrate pollution are mainly distributed in cultivated land. Groundwater mineralization is driven by silicate weathering and evaporite dissolution. Ion exchange and mixing processes are also major factors affecting the chemical composition of groundwater in the study area. The largest contributors to NO₃-N in groundwater were found to be manure and sewage input, followed by soil N (SON) and NH₄-N fertilizer (NF) application. The high nitrogen content in groundwater is mainly affected by mixing and nitrification processes. Interestingly, although evaporation is the main factor affecting groundwater mineralization in the study area, it has little effect on the high nitrate concentration in groundwater. Based on the results, a conceptual model was constructed to describe the nitrogen cycle in the irrigation area. The findings help clarify the mechanism of nitrate pollution and deepen our understanding of the role of nitrogen cycling in the soil-water-gas system.