Evolution Of Groundwater Quality And Its Formation Mechanism In Plain Area Of Yarkant River Basin

Groundwater,as an important part of water resources,plays a significance role in human production and life.In recent years,various adverse ecological and environmental problems have emerged in inland arid region due to global climate change and the impact of human activities on groundwater.Therefore,carrying out groundwater investigation,evaluation and management has attracted much attention.Based on the analysis of the natural geographical conditions,socio-economic conditions and hydrogeological conditions in the plain area of Yarkant River basin,Xinjiang,this paper analyzes the surface water and groundwater hydrochemical characteristics and the evolution of groundwater quality and its relationship with land use change based on the groundwater water chemistry test data and the comprehensive use of hydrogeochemical methods,multivariate statistical analysis method,geostatistics,RS and GIS spatial analysis to reveal the evolution mechanism of water quality.It provides a scientific basis for realizing the sustainable utilization of groundwater resources in this area.The main results are as follows:(1)The pH of surface water ranged from 7.40 to 8.33,with a mean value of 7.92,which was weakly alkaline.The mean value of total dissolved solid(TDS)of river,canal and reservoir water showed an increasing trend.The TDS mean of river was 429.24 mg·L^-1,higher than the average value of world rivers(115 mg·L^-1).The hydrochemical types of river water was mainly HCO₃·SO₄-Ca·Na and SO₄·HCO₃·Cl-Ca·Na·Mg types,of canal water was mainly HCO₃·SO₄·Cl-Ca·Na type,and of reservoir water was mainly SO₄·Cl-Na·Ca type.The TDS of the river water along the Yarkant River and Tiznap River showed continuous increase and fluctuation trend,respectively,while the variation of the main ions was relatively complex.(2)From 1980 to 2018,the overall pH of groundwater was neutral or weakly alkaline.The mean value of TDS and total hardness(TH)showed an increasing trend from 1980 to 2018.The spatial distribution of major ion concentrations in unconfined groundwater and shallow confined groundwater showed a trend of low in the south and high in the north,while the pH showed a trend of low in the north and high in the south.From 1980 to 2018,the detection rates of NO₂^-and NH₄⁺were both low,and the mean concentration was small,while the detection rates of NO₃^-were high and the mean concentration was also high.Only the detection data of F^-,I^-,As and heavy metals in the groundwater of the study area in 2014and 2018 were obtained,and the detection rate of F^-,Fe and Mn concentrations was high.The detection rate of As was higher in 2014.In 2018,the detection rate of Ni was relatively high,while the detection rate of other indexes was relatively low.(3)The single factor evaluation of groundwater quality showed that the rates of groundwater quality exceeding the standard were 75.00%,88.46%,95.83%and 91.51%,respectively,in 1980,2010,2014 and2018,and the over standard rates of SO₄^2-,TH,TDS,Cl^-and Na⁺were relatively large.The comprehensive evaluation results of water quality index(WQI)based on entropy weight showed that the average WQI in 1980 was 140.02,and the proportions of excellent,good,medium,poor and extremely poor water quality were 8.33%,41.67%,29.17%,12.50%and 8.33%,respectively.The mean value of WQI in 2010 was 175.34,and the proportion of excellent,good,moderate,poor and extremely poor water quality was 3.85%,38.45%,34.62%,11.54%and 11.54%,respectively.The average WQI in 2014 was234.20,and the proportions of excellent,good,moderate,poor and extremely poor water quality were12.50%,25.00%,22.22%,8.33%and 31.95%,respectively.The mean value of WQI in 2018 was 179.48,and the proportion of excellent,good,moderate,poor and extremely poor water quality was 7.55%,34.91%,22.64%,19.81%and 15.09%,respectively.(4)The spatial correlation of WQI was analyzed based on geostatistics,and the best fitting model and accuracy were judged by the determination coefficient and the least residual sum of squares.In 2014,the Gaussian model was used to fit the best WQI for unconfined and shallow confined groundwater.In 2018,the spherical model and exponential model were used to fit the best WQI for unconfined and shallow confined groundwater,respectively.There was a strong spatial correlation between WQI of unconfined and shallow confined groundwater in 2014 and unconfined groundwater in 2018,and there was a moderate spatial correlation between WQI of shallow confined groundwater in 2018.In 2014 and 2018,the WQI of unconfined groundwater showed a trend of lower in the south and higher in the north.In 2014 and 2018,the WQI of shallow confined groundwater showed a distribution characteristic of lower in the south and higher in the north.(5)From 1980 to 2018,the mean value of WQI of groundwater showed a trend of"increasing,increasing and decreasing",and the mean value of WQI of single structure unconfined groundwater showed a trend of"increasing,decreasing and decreasing",but the change range was small.The mean value of WQI unconfined groundwater in confined groundwater area showed a trend of"decrease,increase,decrease",and the mean value of WQI in shallow confined groundwater showed a trend of"increase,decrease,decrease".(6)The analysis of groundwater chemical composition showed that groundwater hydrochemistry was mainly affected by rock leaching,evaporation and concentration.Groundwater chemical ions were mainly dissolved by evaporative salt rock and hydrolyzed by silicate mineral weathering.The dissolution of evaporite minerals such as rock salt gypsum and anhydrite were the main source of groundwater chemical ion concentration.In addition to mineral dissolution,cation exchange was also significant,and human activities have a certain influence on it.(7)Factor analysis showed that groundwater quality was mainly controlled by three main factors from1980 to 2018:hydrogeological condition(F1),hydrochemical environment(F2)and human activity factor(F3).The contribution rate of F1 was the largest with a decreasing trend;the contribution rate of F2 and F3was smaller with the increasing trend.(8)The change of cultivated land,construction land and water area had a great relationship with groundwater quality.When the area of cultivated land and construction land increased significantly and the area of water area decreased significantly,the WQI of groundwater increased and the water quality deteriorated.The area of cultivated land and construction land increased slightly and the water area increased significantly,while the WQI decreased and the trend of water quality deterioration slowed down.