Identifying nitrate sources and transformation in groundwater in a large subtropical basin under a framework of groundwater flow systems

Abstract

Land use patterns and regional hydrogeological conditions have a great impact on nitrate sources and transformation in groundwater at the large-basin scale. In this study, hydrochemistry, multiple isotopes (δD-H2O, δ18O-H2O, δ15N-NH4+, δ15N-NO3- and δ18O-NO3-), and a Bayesian model were integrated to identify nitrate sources and transformation in groundwater in the Qingyi River basin (QRB), a large subtropical basin in East China. Overall, increases in total dissolved solids, NO3-, and decreases in d-excess and dissolved oxygen concentrations were observed along the regional groundwater flow direction, suggesting groundwater in recharge and transition areas had fewer anthropogenic N inputs, shorter residence times, and more oxidizing environments. As demonstrated by the negative correlation of δ15N-NH4+ and δ15N-NO3-, nitrification was dominant in recharge and transition areas, and δ18O-NO3- values above theoretical nitrification line indicated a mixing of atmospheric NO3- and nitrified NO3-. By contrast, denitrification was a major transformation in discharge areas. In recharge areas (with an area of 95.9% forest), soil organic nitrogen and atmospheric precipitation contributed 82.7% and 17.3%, respectively. In transition areas (54.3% forest, 42.5% farmland), the major nitrate sources were soil organic nitrogen (38.7%) and chemical fertilizer (27.7%). In discharge areas (52.4% farmland, 15.0% residential area), manure/sewage and soil organic nitrogen accounted for 34.6% and 28.4% of total nitrate. High soil organic nitrogen contribution in the QRB suggested land use in recharge areas is essential for controlling the groundwater nitrate contamination in basins. Finally, we proposed a conceptual model of the nitrate sources and transformation in groundwater in the QRB, revealing the nitrate behaviors in groundwater in a large basin within a framework of groundwater flow systems. This study proves that groundwater flow system is effective to fuse multi-information to identify nitrate sources and fate in groundwater in a large basin, which is helpful to regional groundwater management and protection in similar subtropical basins.