Distribution,Transformation Process And Source Analysis Of Nitrate In Typical Karst Basin

In this study,Huaxi river basin in Guiyang city was selected as the research object.The distribution characteristics,the source and forming process of nitrate in surface and ground water were revealed by the isotopes(?¹⁵N-NO₃^-,?¹⁸O-NO₃^-and?¹⁸O-H₂O)and hydrochemical analysis.Meanwhile,according to the SIAR?stable isotope analysis in R?model,the contribution rates of different nitrate sources in surface and groundwater were also identified quantitatively.We hope that our finding can provide a scientific basis for the effective control of nitrate pollution in the typical karst basin represented by Huaxi River Basin.The main conclusions are as follows:?1?Inorganic nitrogen pollution in Huaxi River Basin is relatively serious.In both normal and dry seasons,the concentration of NH₄⁺-N in the surface water sampling points is higher than the corresponding water standard of the water function area.In addition,the over standard rates of NO₃^--N and NH₄⁺-N in normal season are 10%and73.33%respectively,and the over standard rates of NO₃^--N and NH₄⁺-N in dry season are 21.28%and 76.60%respectively.Affected by the fertilization process,the total nitrogen concentration?the sum of the three inorganic nitrogen concentrations?of the surface and ground water in this basin is generally higher in the normal season than that in the dry season.The heterogeneity of the karst aquifer made the spatial distribution of total nitrogen concentration in the groundwater of Huaxi River Basin,NO₃^--N is the main form of dissolved inorganic nitrogen in the study area.?2?According to the method of hydrochemistry,the main sources of nitrate in this basin are analyzed.The results show that the source of nitrate in the surface water and groundwater are similar,they were both affected by the agricultural source greatly But they had obvious divisions,which indicates that the source composition of nitrate in the water body may be incomplete.According to the?¹⁵N-NO₃^-,?¹⁸O-NO₃^-values of different water in this area,the main sources of nitrate pollution in Huaxi River Basin during the normal season are chemical fertilizer,soil nitrogen and manure/sewage.?3?It can be seen that ammonia volatilization and denitrification have significantly higher isotopic fractionation effect on nitrate than nitrification from the isotopic fractionation characteristics of nitrate.In this study,nitrification plays a leading role in the formation of nitrate in different waters,and ammonia volatilization and denitrification do not occur,indicating that the initial value of nitrate isotopes is not affected by fractionation effect in the formation process.?4?As the results of SIAR model,the order of contribution rate of nitrate source in surface water was soil organic nitrogen>manure sewage>chemical fertilizer>atmospheric sedimentation,while the order of contribution rate of nitrate source in groundwater was manure sewage>soil organic nitrogen>chemical fertilizer>atmospheric sedimentation.the contribution rates of precipitation,fertilizer,soil organic nitrogen,and manure/sewage were0.97%,23.33%,43.35%and 32.35%,respectively,and the contribution to the nitrate in groundwater is 0.93%,18.37%,35.46%and 46.25%,respectively.Further,the source of nitrate in each sampling point is estimated,which was combined with the field investigation and the analysis of the land use type of the study area.It is concluded that the land use type and the local human activity had a certain impact on the migration of nitrate from different sources to the water body of the karst area.