Formation Mechanism Of High Sulfate Groundwater In Kashgar River Basin,Xinjiang

The Kashgar River Basin is located on the southwestern edge of the Tarim Basin,which is the core area of the“Silk Road Economic Belt”in China.Groundwater in this area is the main water resource,and the sulfate in groundwater generally exceeds the standard,which is an important factor causing regional quality-oriented water shortage.Based on the test of groundwater hydrochemical ions,hydrogen and oxygen isotopes,and sulfur isotopes in Kashgar River Basin,Xinjiang.Multivariate statistical techniques,Gibbs graph,ion ratio and inverse hydrogeochemical simulation methods were used to explore the formation mechanism of high sulfate groundwater,and comprehensively determined the effects of natural and human factors on the groundwater chemical composition.The main research results were as follows:（1）Groundwater was weakly alkaline in general.According to the division of total dissolved solids（TDS）,phreatic water in alluvial-proluvial gravel plain of the piedmont slope（PPS）was mainly composed of freshwater and brackish water,with salt water accounting for only 2.0%,while the percentages of salt water in phreatic water in alluvium plain（PA）and confined groundwater in alluvium plain（CA）were29.5%and 32.0%,repectively.According to the division of total hardness（TH）,the percentages of extremely hard water in PPS,PA and CA were 51.0%,70.5%and 62.9%,respectively.Cations concentrations showed a descending order of K⁺+Na⁺,Ca²⁺and Mg²⁺,anions concentrations showed a descending order of SO₄^2-,Cl^-and HCO₃^-.The excess rates of SO₄^2-in PPS,PA and CA were 61.2%,82.1%and 81.1%,respectively.The horizontal distribution of groundwater SO₄^2-showed obvious correlation.Along groundwater flow direction,the concentrations of SO₄^2-showed an increased trend.（2）The groundwater hydrochemical types of in PPS,PA and CA were similar.The main groundwater hydrochemical types were included SO₄·Cl-Na,SO₄·Cl-Na·Ca,SO₄·Cl-Na·Mg,SO₄·Cl-Na·Ca·Mg,SO₄·HCO₃-Na·Ca·Mg.which could be inferred that there was a certain hydraulic connection between various types of groundwater,and the hydrochemical types of anions and cations in groundwater showed obvious zonation.（3）TheδD andδ¹⁸O values of groundwater samples were distributed along the Local Meteoric Water Line（LMWL）δD=8δ¹⁸O+15,the majority of groundwater samples were distributed below the precipitation line,indicating a certain degree of evaporation during precipitation recharge.The correlation coefficients between K⁺+Na⁺,Ca²⁺,Mg²⁺,Cl^-,SO₄^2-and TDS were all greater than 0.700,indicated that the main sources of these ions were the same.The results of correlation and principal component analysis preliminarily speculated that the major ions in groundwater mainly originate from dissolution of evaporite minerals such as salt rock and gypsum,and carbonate minerals such as calcite and dolomite.（4）The results of Gibbs diagram and ion ratio showed that evaporative concentration and rock weathering were the primary factors affected and controlled the chemical composition of groundwater.The rock dissolution was dominated by the dissolution of evaporite minerals（such as gypsum,rock salt and mirabilite）and silicate minerals（such as plagioclase）,and was less affected by the dissolution of carbonate minerals and cation exchange.The results of the relationship betweenδ³⁴S and SO₄^2-,1/SO₄^2-showed that SO₄^2-in PA originated from the leaching of evaporative rock in the terrestrial sedimentation zone,the oxidation of sulfide minerals and human activities.SO₄^2-in PPS and CA originated from the leaching of evaporative rock in the sea-land interfacing sedimentation zone and terrestrial sedimentation zone,the oxidation of sulfide minerals and human activities.The reverse hydrogeochemical simulation results showed that the high sulfate in the groundwater of the Kashgar River Basin was mainly originated from the the dissolution of gypsum and other minerals.