Identification Of Nitrate Pollution Sources Of Groundwater In Agricultural Areas Based On Stable Isotopes

With the development of modern agriculture,a large number of nitrogen-containing compounds,discharged by human activities such as production and living in rural areas,have led to the prominent problem of groundwater nitrogen pollution,in which nitrate nitrogen is the main form.In view of the importance and fragility of groundwater resources,it is urgent to effectively prevent nitrate nitrogen pollution in groundwater.Once groundwater is contaminated,repair time and economic costs are relatively high.Therefore,taking measures to protect groundwater and prevent pollution should be given more priority than taking measures treat groundwater pollution.So,accurate identification of groundwater pollution sources and their contributions are very important for groundwater pollution prevention and control.At present,most of the studies on traceability of groundwater pollution sources are always limited to single pollution sources.However,the change of pollution concentration caused by the migration and transformation of pollutants in the environment will also affect the final traceability results,which may lead to great uncertainty in the traceability results of composite pollution sources.In addition,the quantitative identification of groundwater pollution sources will help to accurately formulate regional groundwater pollution prevention measures,and provide a scientific basis for groundwater nitrate nitrogen pollution prevention and control.Therefore,considering the uncertainty and composite characteristics of the groundwater pollution sources,Jingyang District of Deyang City are taken as the research objects in this paper.Combining with land use types,isotope traceability technology,migration and transformation mechanism and hadrochemical analysis methods,the environmental factors that affect the content of nitrate and nitrogen in groundwater were analyzed,and IsoSource model and SIAR model are used to quantitatively study the pollution composition and contribution of nitrate nitrogen in groundwater.The main research results are as follows:1.The analysis results of water chemistry characteristics in the study area show that the groundwater in the area is neutral or weakly alkaline,and 90%of the water samples have oxidizing ability and greatly fluctuating dissolved oxygen.Denitrification reaction were showed in the results,but the reaction rate is low.At the same time,industrial land type dissolved oxygen is significantly lower than arable land,vegetable garden,farm land for the other three types.The main hydrochemical type of groundwater in the study area is Ca·Mg-HCO₃ type,followed by Na·K-HCO₃ and Ca·Mg-SO₄·Cl type.2.More than 70%of the water samples in the area have nitrate nitrogen exceeding or near exceeding the standard in this study.Based on Arc GIS for spatial analysis,the inverse distance interpolation method was used to draw the spatial distribution map of nitrate and its nitrogen and oxygen isotopes.Comparing it with the local land type map,it is found that there is a certain correlation between the spatial changes of groundwater nitrate and its isotopes and the Land application type.The concentration of nitrate in the agricultural area is higher and itsδ¹⁵N value is lower,while theδ¹⁵N andδ¹⁸O value in the industrial area are higher.3.Qualitative analysis of the sources of groundwater pollution in the study area was based on the distribution ofδ¹⁵N andδ¹⁸O characteristic values of the water samples in the study area in the dual isotope map.Preliminarily results determined that the sources of groundwater nitrate pollution in the study area are mainly rural domestic sewage,livestock and poultry manure and soil Organic nitrogen.Subsequently,the influence of the migration and transformation process on the nitrogen and oxygen isotopes of groundwater nitrate was demonstrated,and the nitrateδ¹⁸O value estimated by the formula was consistent with the content of each type of land and was within the theoretical abundance range of nitrate,indicating that there is a weak nitrification reaction in the study area.At the same time,a negative correlation showed between the concentration of nitrate and its isotopic abundance,with correlation coefficients of-0.11 and-0.24,respectively,and the effect of denitrification was poor.In order to further verify the results,the relationship between chloride ion and nitrate nitrogen and oxygen isotope was studied,and it was found that their correlation coefficients were extremely low,0.04 and 0.17 respectively.Combined with the situation that the dissolved oxygen concentration of the water samples in the study area is higher than 2 mg/L,it is determined that the denitrification effect of groundwater is not obvious,and the fractionation effect is not significant.4.The two models of IsoSource and SIAR are used to quantitatively analyze the contribution rates of different pollution sources in the study area.The results of the two are basically consistent.However,the data of the IsoSource model has a large degree of dispersion and the SIAR model data shall prevail in the following.Among arable land,vegetable plots and farms,chemical fertilizers have the highest contribution rate,with a contribution rate of more than 30%,followed by soil organic nitrogen,livestock and poultry manure,atmospheric rainfall and sewage.The contribution rates are 24%,19%,18%,9%.The composition of the nitrate contribution of groundwater in industrial areas is different,mainly affected by industrial,domestic sewage,and atmospheric rainfall.The contribution rates are 28%and 27%.The proportion of soil organic nitrogen is slightly lower at 19%,and the proportion of chemical fertilizer and livestock manure is the lowest,at 16%and 10%.In summary,this simulation results were in line with the actual situation.The results of the identification of pollution sources based on stable isotopes showed that the long-term irrational fertilization management and irregular discharge of sewage and wastewater from domestic production in the study area were the main factors that caused the groundwater nitrate nitrogen pollution in the area.In addition,various gaseous nitrides produced by combustion in industrial parks also have a certain impact on groundwater nitrate nitrogen pollution.The quantitative identification of pollution sources and their transformation laws in this study will help local governments formulate targeted groundwater nitrate mitigation policies and provide a scientific basis for groundwater nitrate nitrogen pollution prevention and control.