Groundwater Quality Characteristics And Their Formation Mechanism In The Plain Area Of The Yili River Basin

Groundwater is considered as one of the most important water resource which controls social and economic development,constitutes and affects ecological environment,and maintains regional environmental stability.In arid and semi-arid regions,groundwater is often the main source of water supply due to the lack of surface water.In recent years,the groundwater in many areas has been polluted due to the acceleration of urbanization.The groundwater in arid and semi-arid areas in northwestern China also shown sign of deterioration.The impact of groundwater pollution has had a greater impact on the sustainable development of the regional economy.Based on the systematic groundwater hydrochemical characteristics in the plain area of the Yi Li River Basin,combined with regional geographic characteristics,geological conditions,and hydrogeological characteristics,this study carried out groundwater hydrochemical evolution and water quality evaluation,and explored the causes of groundwater hydrochemical characteristics.Based on groundwater data collected in 2011 and 2018 from the plain area of the Yili River Basin,Xinjiang.The MAPGIS 6.7 software and SURFER 11 software were used to map the location of the groundwater sampling points,and the chemical composition of groundwater and mineral sources were analyzed.Tri-linear diagram of Piper and SPSS 19.0 software with multivariate statistical analysis were used to analyze the hydrochemical characteristics of groundwater in different years.The hydrogeochemical analysis and inverse geochemical modeling were carried out to study the chemical evolution characteristics of groundwater in different hydrogeological sections in different years.The PHREEQC hydrogeochemical simulation software was used to analyze the dissolution and precipitation pattern,migration law of groundwater chemicals in different groundwater paths.The Matlab software was used to establish an optimized fuzzy comprehensive evaluation model to evaluate the groundwater quality in 2011 and 2018 in the study.The main conclusions are as follow:（1）The groundwater is mostly neutral-weak alkaline groundwater with a high total hardness level in the study area,the chemical components shows small variation throughout the study area,with the main cation of Ca²⁺and the main anion of HCO₃^-.The main groundwater hydrochemical types can be divided into five types which are HCO₃-Ca,HCO₃·SO₄-Ca,HCO₃·SO₄-Ca·Mg,HCO₃·SO₄-Ca·Na,and HCO₃-Ca·Mg.Although the groundwater chemical composition is mainly controlled by rock weathering and evaporation concentration,the cation exchange process also effects the groundwater chemical composition in the study area.Further study revealed that water quality of certain groundwater samples deteriorates due to might human activities.The groundwater Na⁺,Cl^-and SO₄^2-were mainly came from the dissolution of rock salt,gypsum and other evaporates,and the dissolution of calcite,dolomite and gypsum provide groundwater with Ca²⁺and Mg²⁺.（2）The main groundwater hydrochemical types include HCO₃-Ca·Mg,HCO₃·SO₄-Ca·Mg,HCO₃·SO₄-Ca·Na and HCO₃·SO₄-Ca·Na·Mg in 2011,and the main groundwater hydrochemical types are HCO₃-Ca·Mg,SO₄·HCO₃-Ca,HCO₃-Ca,HCO₃·SO₄-Ca,HCO₃·SO₄-Ca·Mg and HCO₃·SO₄-Ca·Mg·Na in2018.In comparison,the groundwater hydrochemical types clearly variate in 2018,the main hydrochemical types increases and are more complex.Anhydrite,calcite,dolomite,gypsum and rock salt were selected as"possible mineral phases"for the inverse geochemical modeling.in simulated paths 1and 2,the dissolution of dolomite and gypsum supplement groundwater with additional Ca²⁺,Mg²⁺and SO₄^2-causing a increase in Ca²⁺,Mg²⁺and SO₄^2-concentration,with the precipitation of rock salt the concentration of groundwater Na⁺decreases along the simulated paths.In path 3,the Ca²⁺and SO₄^2-content in groundwater mainly comes from the dissolution of gypsum,while Mg²⁺mainly comes from the dissolution of dolomite.In path 4,despite anhydrite and gypsum exhibits opposite dissolution and precipitation pattern,the concentration of groundwater SO₄^2-increases along the simulated path in both years.Confirming that the dissolution of silicate rock salt provides additional SO₄^2-.（3）The overall groundwater quality in the study area in 2011 and 2018 ranged from Grade Ⅰ to Ⅲ.In2011 and 2018,the groundwater quality of Gongliu County in the eastern part of the study area was mainly classified as Grade Ⅰ to Ⅲ.Compared with 2011,the quality of some groundwater sample sights in 2018have changed.Namely,the groundwater in Huocheng County and Horgos city in the northwestern part of the study area have improved.In 2011,the groundwater quality was mainly Grade Ⅳ while in 2018,the groundwater quality was mainly Grade Ⅰ～Ⅲ;the groundwater quality of Chabuchaer County in the southwestern part of the study area was partially degraded.In 2011,the groundwater quality was mainly Grade I,and the water quality of one sample sight was V.In2018,the water sample points were mainly Grade Ⅰ～Ⅲ water,and some of the water sample were Grade Ⅳ water.The water quality of some water samples deteriorated in Yining City and Yining County in the central part of the study area.In 2011,the groundwater quality were all Grade Ⅰ～Ⅲ.In 2018,the groundwater was mainly Grade Ⅰ～Ⅲ water,but water quality of few water samples was Grade Ⅳ and Ⅴ.（4）Among the chemical components that affect the groundwater quality in 2011 and 2018,the groundwater total hardness,TDS,Na⁺,Cl^-and SO₄^2-,gradually increase under rock weathering,evaporation and concentration process which are also the main controlling factor of groundwater quality in the study area.NH₄⁺and NO₃^-in groundwater mainly come from agricultural fertilizers and domestic sewage,and their concentration gradually increases in weakly alkaline groundwater environment.Groundwater F^-comes from minerals that contain fluorine in the aquifer,and the weakly alkaline nature of the groundwater environment contributes to the enrichment of F^-,indicating that the groundwater environment also contributes to the deterioration of groundwater quality.In comparison,The impact of human activities such as agricultural fertilizers and domestic sewage on groundwater quality has increased in 2018.