Groundwater Nitrate Source Identification Based On Hydrochemical And Isotopes

As the groundwater pollution has complexity,concealment and hysteresis,and the limited self-purification capacity for nitrogen in the aquifer,nitrate nitrogen,once nitrite nitrogen and ammonia nitrogen enter the aquifer,it will seriously endanger the safety of the groundwater environment.Besides,it is difficult to remediate when the groundwater is contaminated,pollution source prevention and control is essential for groundwater environment protection.However,the subsurface uncertainty and various pollution sources mixed action bring great difficulty for groundwater nitrate source identification.The shallow groundwater of Yanghe north shore,Xuanhua district,Zhangjiakou was selected as the research object,and the hydrogeology and groundwater recharge and discharge characteristics of the study area was identified.Groundwater age was innovative coupling with groundwater nitrate source identification,to identify the time period of pollution sources.The source location,time and contribution rate of groundwater nitrogen pollution was analysised through isotopic analysis and SIAR model.The results will provide technical guidance for groundwater nitrogen pollution sources prevention and control.The main conclusions of this paper are as follows:1.The spatial and temporal variation of groundwater NO₃^-in the study area was significant.In order to identify the relationship between groundwater NO₃^-variation with human activities and hydrogeology,the hydrogeology data,surface water and groundwater physicochemical indexes was analyzed,especially the nitrogen and isotopes variation.NO₃^-pollution was the most serious in groundwater nitrogen pollution.The annual average was 16.21 mg/L for NO₃^-,which exceeding the limit value 10 mg/L in drinking water of World Health Organization.According to the ArcGIS results,different land use type had different concentration,dry land（25.13 mg/L）>urban system（13.84mg/L）>cultivated land（13.58 mg/L）.The NO₃^-concentration was more serious in summer.According to the results of multivariate statistical analysis,NO₃^-is all positive correlation with Cl^-、SO₄^2-、EC、Mg²⁺and HCO₃^-.The concentrations of the above ions were 1.1 to 5.1 times higher than the background values.The groundwater environment is obviously affected by human activities,and groundwater contamination comes from similar sources of pollution,including domestic sewage,industrial wastewater,agricultural fertilizer and manure,etc.The physical and chemical indexes of Yang river surface water were well fitted with groundwater,indicating that the surface water has obviously hydraulic connection with groundwater,and the polluted surface water was easy to affect the groundwater environment.2.According to qualitative identification results of isotope analysis and groundwater renewal age,the potential source of groundwater NO₃^-pollution was manure and sewage,soil nitrogen,and ammonia nitrogen in precipitation and fertilizers,and has little relationship with industrial pollution sources nearly 30 years.The double isotope characteristic values ofδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-in groundwater was all confirm to the values of manure and sewage,soil nitrogen,and ammonia nitrogen in precipitation and fertilizers.Groundwater age was coupling with groundwater nitrate source identification,to identify the time period of pollution sources.The groundwater age was 47 to 70 years based on ³H radioisotope dating.According to the pollution sources spatial and temporal distribution in the study area,the potential source of groundwater NO₃^-pollution was sewage discharge of domestic pollution sources,livestock and poultry industry,agriculture source,and a 70 year brewhouse,which has little relationship with industrial pollution sources nearly recent decades.3.The mainly migration and transformation process of groundwater NO₃^-in the study area was nitrification.The pollution source proportion sequence was manure and sewage,soil nitrogen,and ammonia nitrogen in precipitation and fertilizers.The specific value betweenδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-in groundwater was far away from the trend line of denitrification ratio,and denitrification was limited by high dissolved oxygen concentration in shallow groundwater（6.10 mg/L）and low soil moisture content.The results of isotopic characteristic values indicate that the mainly migration and transformation process was nitrification.Isotope fractionation of nitrification was not obvious as there is no NH₄⁺accumulation in the subsurface.Isotope mass balance and SIAR model were used to quantitatively analyze the groundwater NO₃^-pollution sources.The uncertainty of spatial and temporal variation of NO₃^-in complex hydrogeological conditions was reduced through SIAR.The results of pollution sources average contribution rate was 46.9%for manure and sewage,41.2%for soil nitrogen,and 11.9%for ammonia nitrogen in precipitation and fertilizers.The contribution rate shows interannual variation characteristics.