Using nitrogen and oxygen isotopes to access sources and transformations of nitrogen in the Qinhe Basin, North China

Nitrate pollution in water is a common environmental problem worldwide. The Qinhe Basin (QHB) faces with the risk of eutrophication. To clarify nitrate pollution of river water, water chemical data, water isotope values (δD and δ¹⁸O-H₂O), and dual nitrate isotope values (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were used to discern sources and transformation mechanisms of nitrogen in the QHB. The nitrate concentrations of river water ranged from 0.71 to 20.81 mg L^-1. The δD and δ¹⁸O-H₂O values of river water varied from - 74 to -52‰ and from - 10.8 to - 7.2‰, with an average value of - 60‰ and - 8.2‰, respectively. The δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values of nitrate ranged from - 6.7 to + 14.8‰ and from - 6.0 to + 5.6‰, with a mean value of + 4.6‰ and - 0.6‰, respectively. Assimilation by algae and the mixing of soil nitrogen, chemical fertilizer, sewage, and industrial wastewater could account for increasing δ¹⁵N-NO₃^- values. There was neither significantly positive nor negative correlation between δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in river water, indicating that no obvious denitrification shifted isotopic values of nitrate in the QHB. Based on the dual isotopic values of nitrate and land use change in the watershed, it could be concluded that intensive nitrification dominated in the QHB, and dissolved nitrate was mainly derived from nitrification of ammonium in fertilizer, soil nitrogen, and domestic sewage. As the primary nitrate sources identified in the QHB, effective fertilization and afforestation can be taken to protect water resource from nitrate pollution.