Study On The Source And Migration Transformation Mechanism Of Surface Source Nitrogen Based On Nitrogen And Oxygen Isotope Method

Due to the complex karst geological environment in southwest China,it is difficult to determine the transformation process of nitrate from different pollution sources in karst water bodies,which leads to the lack of effective control of nitrate pollution in water bodies.In this paper,the Gaoping River Basin in Zunyi City of Guizhou Province is selected as the research object,andδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-isotope technologies are applied.The distribution characteristics of nitrate in surface water and groundwater in the study area were clarified,and its migration and transformation process was revealed.The corresponding relationship between various potential pollution sources in the environment and water quality parameters in time and space was fully considered,and the distribution characteristics of pollutants in the water body were deeply analyzed from multiple aspects.in addition,the rational application of the stable isotope analysis in R language made the accurate judgment of the sources of pollutants in each water body and the allocation of the contribution rate of each pollutant source,which was the supplement and perfection of the qualitative analysis of the sources of water pollution.It provides scientific basis for the prevention and control of nitrate pollution in typical karst watershed represented by Gaoping River Basin.The main conclusions are as follows:（1）Inorganic nitrogen pollution in the Gaoping River basin is relatively serious,and the average concentration of"three nitrogen"in the water bodies of Gaoping River basin is NO₂^--N<NH₄⁺-N<NO₃^--N.In the flat,abundant and dry periods,NH₄⁺-N concentration in surface water and groundwater sampling The concentration of NH₄⁺-N in surface water and groundwater sampling points is higher than the standard of the water body corresponding to the water function area,among which NH₄⁺-N pollution in the downstream is the most serious.The exceedance rates of NO₃^--N and NH₄⁺-N were 10%and 73.33%,respectively,in the flat water period,and 21.28%and 76.60%,respectively,in the dry water period;due to the influence of fertilizer application,the overall surface water and groundwater sampling sites in the study area showed that the total nitrogen concentration（the sum of three inorganic nitrogen concentrations）was higher in the dry water and flat water periods than in the rich water period.The non-homogeneity of karst aquifers led to the spatial distribution of total nitrogen concentration in groundwater in the study area,which was much higher than that of surface water.The main form of dissolved inorganic nitrogen in both surface water and groundwater in the study area is NO₃^--N.（2）The main sources of nitrate in the water bodies of the study area were determined based on the water chemistry method,and the results showed that most of the nitrate in the water bodies of the groundwater and surface water sampling sites originated from the manure and wastewater end-member area,half of the nitrate in the surface water bodies originated from the manure and wastewater end-member area,and half from the soil nitrogen source,but most of the nitrate in the groundwater bodies originated from the manure and wastewater end-member area,and nitrate in the surface water There are similar sources of nitrate in surface water and groundwater,but there is a significant partition between the two,indicating that there are differences in the source composition of nitrate in water bodies.Based on theδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-values in groundwater and surface water in the Gaoping River basin,it was determined that the sources of nitrate pollution in the basin water bodies were mainly soil nitrogen and manure/wastewater.（3）According to the isotopic fractionation characteristics of nitrate,the isotopic fractionation effect of nitrification on nitrate was significantly lower than that of denitrification and ammonia volatilization.However,in this study,the formation of nitrate in the waters of the Gao Ping River basin was dominated by nitrification,and neither ammonia volatilization nor denitrification occurred,indicating that the nitrogen and oxygen isotopes of nitrate in the waters of the study area were not fractionated by denitrification during the formation process.（4）The results of stable isotope analysis in R model estimation showed that the lowest contribution of nitrate source in surface water was atmospheric deposition with 9.54%;the highest contribution was soil organic nitrogen with 26.56%;followed by fertilizer contribution with26.515%and manure and sewage contribution with 25.41%.The effect of anthropogenic activities on groundwater was more obvious,with the highest percentage of soil organic nitrogen as a source of groundwater nitrate at 27.57%,the lowest percentage of atmospheric deposition contribution at10.49%,and the contribution of chemical fertilizer and manure and sewage at 25.28%and 26.57%,respectively.Atmospheric deposition has less effect on the source of NO₃^-pollution in groundwater and surface water in the study area,and soil organic nitrogen is the main source of NO₃^-pollution in groundwater and groundwater in the study area,related to the type of land use in the study area there is a typical agricultural area.Manure and dirty wastewater became the main source of nitrate in surface and groundwater in the study area.