Using Nitrogen And Oxygen Isotopes To Resolve The Sources Of Nitrate In The Water Bodies Of The Xianyang-Xi’an Section Of The Weihe River

In recent years,nitrate pollution of water bodies has become a major environmental problem with the accelerating urbanization and rapid development of agriculture.The cities of Xianyang and Xi’an are in the more agriculturally intensive Guanzhong region of Shaanxi Province,where high intensity human activities have led to increasing nitrate pollution in the Weihe River basin.At the same time,rainfall,as the primary driver of non-point source pollution into surface water,has indirectly led to the increase of river nitrate pollution.Therefore,this study focuses on the Xianyang-Xi’an section of the Weihe River basin as a research target,the sources of nitrate in different rivers within Xianyang and Xi’an and the influence of rainfall on nitrate behavior in the river were explored by stable isotopes techniques(δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-)combined with water chemistry analysis,which can provide a reference for water environment management of surface rivers in the Guanzhong area.The primary findings were as follows:（1）The results of the analysis of total nitrogen and the content of each form of nitrogen in the water bodies of the Xianyang-Xi’an section of the Weihe River showed that the concentration of total nitrogen in the water column during the dry season was higher than the standard of surface water environmental quality category V,and nitrate nitrogen was the primary form of inorganic nitrogen.Among them,the spatial variation of nitrate content in the water bodies of the Xianyang-Xi’an section of the Weihe River was mainly influenced by a combination of agricultural pollution and sewage discharge.The results of water chemistry and stable isotopes analysis showed that nitrate in the water bodies of the Xianyang-Xi’an section of the Weihe River mainly came from ammonium nitrogen fertilizer,soil nitrogen,manure and sewage,and the nitrate in the Xianyang-Xi’an section of the Weihe River,the Bahe River and the Fenghe River was more influenced by manure and sewage.（2）The MixSIAR model was used to estimate the relative contribution of different nitrate sources,the results showed that manure and sewage contributed the most to nitrate in the water bodies of the Xianyang-Xi’an section of the Weihe River.The average proportion of contribution from different rivers were:the Xianyang-Xi’an section of the Weihe River（73.4±12.8%）>the Fenghe River（58.0±14.0%）>the Bahe River（53.9±15.0%）>the Heihe River（49.5±13.4%）>the Laohe River（42.6±14.1%）;followed by nitrogen fertilization,the average proportion of contribution from different rivers were:the Heihe River（38.3±16.7%）>the Laohe River（31.0±18.9%）>the Bahe River（28.1±16.7%）>the Fenghe River（27.1±15.7%）>the Xianyang-Xi’an section of the Weihe River（15.7±11.8%）.（3）Water chemistry and stable isotopes evidence indicated that nitrification have occurred in the Xianyang-Xi’an section of the Weihe River,the Bahe River and the Fenghe River,where it dominated the nitrogen transformation process,and denitrification was not significant.However,in the Heihe and Laohe Rivers,both denitrification and nitrification were not significant.（4）In the Xianyang section of the Weihe River,the results of nitrogen concentration and isotope analysis before and after rainfall showed that the overall nitrate nitrogen concentration,δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-values in the river increased after rainfall compared with those before rainfall.The MixSIAR model estimation results showed that the average contribution of the manure and sewage increased from 49.3±14.9%before rainfall to 52.2±17.6%after rainfall,the average contribution of nitrogen fertilizer and soil nitrogen decreased from29.6±17.4%and 18.1±13.8%before rainfall to 27.5±16.5%and 16.8±13.0%,respectively.Water chemistry and stable isotopes evidence suggested that both nitrification and denitrification were not evident in the river water before the rainfall,however,nitrification occurred after the rainfall and denitrification may have occurred in the riparian zone.