Quality Evaluation And Causes Analysis Of Phreatic Water In Plain Area Of Keriya River Basin

The plain area of the Keriya River Basin is a typical oasis area in the Tarim Basin.The economic development model is dominated by agriculture and supplemented by animal husbandry,and it is also a region where the population is mainly concentrated.Because of economic development and improvement of living standards,more and more water is used by human beings.Human impact on groundwater is also growing and groundwater environmental problems are not endless.In order to find out the current situation of groundwater pollution in the plain area of the Keriya River Basin,the statistical characteristics and spatial distribution characteristics of groundwater components in the study area were analyzed by mathematical statistics and Kriging interpolation.Combined with groundwater quality evaluation,pollution evaluation and irrigation water applicability evaluation,the groundwater pollution status and irrigation water applicability in the study area were mastered.The causes of total dissolved solids（TDS）,Cl^-、SO₄^2-,total hardness（TH）and nitrogen pollution in groundwater were analyzed by Gibbs diagram method,chlor-alkali index method,mineral saturation index method,factor analysis,cluster analysis and water quality analysis.It is of great significance to local groundwater pollution control and sustainable utilization.The main conclusions are:（1）The content characteristics of main component in groundwater in the study area in 2021 were as follows:the content of Cl^-ranged from 184.34 to 847.26 mg/L,and the content of SO₄^2-ranged from173.84 to 784.93 mg/L.NH₄⁺-N content was 0.08～1.32 mg/L,NO₃^--N content was 0.19～28.09 mg/L,TDS was 840.14～3175.01 mg/L,and TH was 376.73～1175.39 mg/L in groundwater.（2）From 1980 to 2021,the interannual variation characteristics of each component in groundwater in the study area are as follows:the annual variation of major ions,TDS,TH,Fe and Mn is not obvious,but the annual variation of NH₄⁺-N,NO₂^--N and NO₃^--N is obvious,showing a significant increase trend.（3）The spatial distribution characteristics of each index in groundwater in the study area are as follows:the distribution of Na⁺+K⁺,Ca²⁺,Mg²⁺,Cl^-,HCO₃^-,SO₄^2-in groundwater is similar.The distribution law shows that the north is high and the south is low,and the content in the middle and lower reaches of the basin is significantly higher than that in other regions.The high value area of NH₄⁺-N content was scattered,and the high value area of NO₃^--N is mainly distributed near Yutian County.the NO₂^--N content was generally low.The groundwater in the study area is neutral and weakly alkaline,and the spatial distribution of Fe,Mn,and I^-contents is quite different.The TDS and TH are higher in the north than in the south,and higher in the west than in the east.（4）The evaluation result of groundwater quality is that most of the groundwater quality in the study area is extremely poor.The results of pollution assessment show that nitrogen pollution exists in groundwater compared with the background value of groundwater environment in the study area.The evaluation results of irrigation water applicability show that most points in the study area are suitable for agricultural irrigation,and a few points are not suitable for agricultural irrigation.（5）The excessive TDS in groundwater is due to evaporation concentration and the dissolution of anhydrite,gypsum and rock salt.The reason for excessive Cl^-is the influence of human factors and the dissolution of rock salt.The excessive SO₄^2-is due to the dissolution of gypsum in aquifer.The reason of excessive TH is affected by cation alternate adsorption,salt effect and human factors.Groundwater nitrogen pollution is mainly affected by human activities,among which agricultural fertilizers,feces and domestic sewage are the main sources of NO₃^--N.