Characteristics And Key Influencing Factors Of Groundwater Nitrogen Pollution In Typical Irrigation Areas Of North China Plain

In recent years,with the acceleration of agricultural development and urbanization,excessive application of chemical fertilizers and the random discharge of domestic sewage make the nitrogen pollution of groundwater increasingly serious.In order to study the characteristics and key influencing factors of groundwater nitrogen pollution,this paper selects People’s Victory Canal Irrigation Area in Xinxiang City,Henan Province as the research area,and collects the data from May 2017 to April 2019through field monitoring and laboratory test once a month.Based on multivariate statistical analysis and isotope tracer technology,the pollution characteristics of groundwater inorganic chemistry nitrogen in the study area,the key factors affecting groundwater nitrate nitrogen content and its migration and transformation law were analyzed.The pollution sources of groundwater nitrate nitrogen were identified and the contribution rate was estimated.The research results are of great significance to the prevention and control of groundwater nitrogen pollution,the safety of water use in agricultural irrigation areas and the sustainable development of agriculture.The main research contents and conclusions are as follows:（1）The physical and chemical indicators of hydrology,soil and groundwater from May 2017 to April 2019 were obtained through field monitoring,laboratory experiments and surveys.And the pollution status and temporal and spatial distribution characteristics of groundwater inorganic chemistry nitrogen were analyzed.The average concentrations of NO₃^--N,NH₄⁺-N and NO₂^--N in groundwater were 8.31 mg/L,1.52mg/L and 0.08 mg/L,respectively.The proportions of water samples below the standard of groundwater type III were 8.63%,95.54%and 63.69%.The exceeding standard of NH₄⁺-N was the most serious,followed by NO₃^--N and NO₂^--N.The concentration of groundwater inorganic chemistry nitrogen has an increasing trend in time scale,and the concentration is lower in the east,higher in the north-central and northwest in space.The concentration of groundwater inorganic chemistry nitrogen in maize growth period（May-September）was generally higher than that in wheat growth period（the concentration of groundwater ammonium nitrogen was low from May to September2017）.NNM,LDZ and QLC had higher nitrate concentration,NNM and LDZ had higher nitrate concentration,FZ,LDZ and WGY had higher nitrite concentration.The concentrations of ammonium nitrogen and nitrate nitrogen in SXC were the highest,and the concentration of nitrite nitrogen was higher than most observation points,and the pollution was the most serious.（2）Multivariate statistical method was used to analyze the influencing factors of nitrate nitrogen content and its migration and transformation in groundwater,and the pollution sources and key influencing factors were identified.According to the correlation analysis results,groundwater depth,precipitation infiltration,temperature,ORP,DO and soil nitrogen all affected the migration and transformation of nitrate nitrogen in groundwater.Six principal components were extracted by principal component analysis（PCA）,and the results showed that groundwater environment was mainly affected by precipitation leaching and migration,mineral salt leaching and human activities,indicating that groundwater nitrate nitrogen was affected by external nitrogen leaching,soil nitrogen transformation,human activities and groundwater environment.From the structural equation model,it can be seen that soil surface nitrogen is composed of NH₄⁺-N、NO₃^--N、NO₂^--N and SON,and the path coefficients are 0.44、0.22、0.51 and-0.53,respectively.The path coefficient of direct impact on groundwater nitrate nitrogen is 0.45.At the same time,NO₃^--N in groundwater was indirectly affected by surface nitrogen through groundwater index and directly affected by groundwater environment,and the path coefficient was 0.94.Groundwater depth,soil sand content and water content affect the rate of nitrogen leaching into groundwater.（3）The nitrogen and oxygen isotope characteristics of groundwater in the study area were systematically analyzed,and the migration and transformation rules were identified.In the study area,δ¹⁵N-NO₃^-was between-20.16‰～107.88‰,with an average of 11.16‰,andδ¹⁸O-NO₃^-was between 15.69‰～56.56‰,with an average of8.39‰.The concentration of DO in groundwater was 2.65～15.25 mg/L.There was no significant negative correlation betweenδ¹⁵N-NO₃^-and ln[NO₃^-].The groundwater environment was not conducive to the occurrence of denitrification during the whole study period,and the denitrification was not obvious.However,the slopes of the linear relationship betweenδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-of QLC and GDM were 0.50（R²=0.05）and 0.59（R²=0.90）,and there was certain denitrification.More than half of the samples haveδ¹⁸O-NO₃^-values ranging from-10‰～10‰,and the nitrification is the main process of nitrate formation in groundwater in the study area.（4）The sources of nitrate nitrogen pollution in groundwater were qualitatively analyzed by isotope tracer method and MixSIAR model,and the contribution rates of each source were estimated.According to the distribution relationship of groundwater abundances ofδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-and the range ofδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-of nitrate nitrogen pollution endmembers,the pollution sources of groundwater nitrate nitrogen are mainly precipitation and ammonium nitrogen fertilizer,soil nitrogen,atmospheric deposition,manure and sewage.The contribution rates of groundwater nitrate nitrogen by chemical fertilizer,soil nitrogen,atmospheric deposition,manure and sewage pollution were 19.78%,28.88%,9.04%and 42.69%,respectively.The contribution rates of nitrate nitrogen to soil nitrogen,manure and sewage,chemical fertilizer and atmospheric deposition pollution in silt farmland groundwater were49.76%,30.07%,13.68%and 6.51%,respectively.The contribution rates of soil nitrogen,manure and sewage,chemical fertilizer and atmospheric deposition pollution to nitrate nitrogen in farmland groundwater in silty loam soil were 23.51%,50.61%,18.66%and 7.79%,respectively.The contribution rates of nitrate nitrogen in sandy loam farmland groundwater from manure and sewage,chemical fertilizer,soil nitrogen and atmospheric deposition were 27.31%,27.18%,31.06%and 14.46%.