Identification of groundwater nitrate sources and transformation processes under different land uses and complicated hydrological conditions in Qingyi River Basin, east China

Identification of nitrate sources and fate in basins with complex backgrounds is essential for understanding the controlling factors of regional groundwater nitrates pollution and its prevention. In this study, hydrochemistry, the concentration of dissolved nitrogenous species, multiple isotopes (δD-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NH₄⁺, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) and Bayesian model (SIMMR) were applied to identify the nitrate sources and major transformation processes under different land uses and complicated hydrological conditions in Qingyi River basin with an area of 8700km², east China. A total of 28 groundwater samples of forest-dominated areas in mountainous, forest-farmland in piedmont, and farmland-residential in plain were collected in Jul 2019. The results showed that concentrations of N species, hydrochemistry and isotopic composition had significantly differences under distinctive backgrounds generally. In mountainous area, nitrate concentrations were as low as of 1.9-6.3mg/L, and low TDS (23.6-60.8mg/L), depleted δD-H₂O(-43.3±6.7‰) and δ¹⁸O-H₂O (-7.2±1.0‰) were observed with δ¹⁵N-NO₃^- values of +1.1±0.8‰, which implies that 18.4% and 81.6% of groundwater nitrate were from soil organic nitrogen (SON) and atmosphere precipitation (AP), respectively. In piedmont areas, moderate nitrate(1.0-35.6mg/L), TDS(91.6-253.9mg/L), and relative enriched δD-H₂O(-40.1±4.1‰), δ¹⁸O-H₂O(-6.7±0.5‰) were detected with δ¹⁵N-NO₃^- values +2.8±2.2‰, and the SIMMR model suggested 37.3% nitrates were derived from SON and 31.1% from chemical fertilizers (CF) .With increasing of residential areas, higher TDS(186.5-643.8mg/L) and nitrate(5.4-58.5mg/L) as well as enriched δD-H₂O(-38.6±6.5‰) and δ¹⁸O-H₂O(-6.4±0.7‰) indicated higher anthropogenic inputs in plain areas with δ¹⁵N-NO₃^- values +6.3±2.3‰, with the origins of 31.8% SON and 30.9% manure&sewage (M&S). From the recharge and runoff areas to the discharge areas, major nitrate sources altered from SON to CF and M&S due to variation of land uses, and the denitrification became the dominant process rather than nitrification owing to gradually decreasing oxidization condition. Incomplete nitrification was proved by negative correlations of δ¹⁵N-NH₄⁺ and δ¹⁵N-NO₃^- in recharge and runoff areas. And the occurrence of obvious denitrification was deduced by low redox parameters and major ions in discharge zone. Finally, a conceptual model was proposed to reveal the pattern of groundwater nitrate sources and fate in Qingyi River Basin. This study provided a reliable and integrated approach for recognition and understanding of the nitrate sources and fate in large watershed under complicated land-uses and hydrological conditions.

Keywords: groundwater; nitrate, sources identification; δ¹⁵N-NH₄⁺; Qingyi River basin