Study On Formation Mechanism Of Poor Quality Groundwater In Western Kashgar Prefecture Of Xinjiang

Western Kashgar Prefecture is located in the Kashgar River Delta of Xinjiang,which belongs to the Kashgar River Basin.The groundwater system is divided into the Kezilesu River groundwater system in northern part and the Gaizi River groundwater system in southern part in terms of the Kumutage anticline in the central part in the study area.Constrained by factors such as natural conditions and social-economic development,water resources in the study area are scarce.Therefore,groundwater is the main water supply source.However,the groundwater quality significantly exceeded the limits in the study area,which will seriously affect the local water demand.Research on chemical characteristics and formation mechanism of poor quality groundwater and major influence factors of groundwater quality degradation can provide a theoretical basis for the reasonable exploitation,utilization and treatment of groundwater.Besides,for areas have similar geological and climatic conditions,it has reference and guiding significance to the researches on poor quality groundwater.Taking poor quality groundwater in Western Kashgar Prefecture of Xinjiang as research object,this paper used mathematical statistics analysis,hydrogeochemical analysis,isotope analysis and hydrogeochemical simulation to study groundwater chemical characteristics,classify the poor quality groundwater types,discuss the major factors influencing groundwater degradation,reveal groundwater chemical formation and evolution in the study area.The main research results were as follows:(1)Unconfined groundwater was mainly SO₄ type and confined water were SO₄and SO₄·Cl type in the Kezilesu River groundwater system in northern part.While in the Gaizi River groundwater system in southern part,unconfined groundwater were dominantly SO₄·Cl and Cl·SO₄ type and confined water was SO₄·Cl type.Hydrochemistry type zoning varied widely in the study area from 1985 to 2014.The area of SO₄ type groundwater generally decreased,in contrast,the area of Cl and HCO₃ type groundwater showed an overall increase.(2)The groundwater quality was very poor in the study area.The point exceeding ratio of TDS,TH,Na⁺,Cl^-,SO₄^2-,F^-,As,Fe and Mn in groundwater were very high,which were the main indicators of groundwater quality degradation.(3)Based on the results of cluster analysis coupled with groundwater flow system,we classified the poor quality groundwater into 10 types.In the Kezilesu River groundwater system in northern part,poor quality unconfined groundwater varied from A_?type(high TDS,high TH,high F^-,high Fe,high Mn)in the west to B_?type(high TDS,high TH)in the east;shallow confined groundwater was mainly C_?type(high TDS,high TH,high F^-,high As,high Fe,high Mn),while deep confined groundwater was mainly E_?type(high TDS,high TH,high F^-,high Fe,high Mn).In the Gaizi River groundwater system in southern part,all the poor quality unconfined groundwater was A_?type(high TDS,high As,high Fe,high Mn),while shallow confined groundwater was mainly C_?type(high TDS,high TH,high F^-,high Fe,high Mn)and deep confined groundwater was all of E_?type(high TDS,high TH,high Fe,high Mn).(4)The formation of poor quality groundwater in the study area was commonly influenced by primary geological environment,climatic conditions,hydrogeological conditions,surface water quality,groundwater chemical environment(p H value)and human activities(groundwater exploitation and domestic,industrial and agricultural pollution).Among these,climatic conditions were the main factors that affect the degradation of shallow groundwater.Hydrogeological conditions and groundwater exploitation were the main factors that affect the degradation of deep confined groundwater in the Kezilesu River groundwater system in northern part.Groundwater degradation in the study area was greatly affected by domestic and industrial pollution.Agricultural pollution affected groundwater quality only in partial areas.(5)The formation and evolution of groundwater chemical components were mainly affected by leaching,evaporation-concentration,cation exchange,desulphidation and mixing action.The dissolution of gypsum,halite and fluorite minerals provided important sources of SO₄^2-,Na⁺,Cl^-and F^-in groundwater.Ca²⁺and Mg²⁺were mainly derived from dissolution of minerals such as gypsum,feldspar,calcite and dolomite.Gibbs diagram showed that the Kezilesu River groundwater system in northern part was mainly affected by rock weathering and evaporation-concentration.Chemical components of recharge sources in unconfined and confined groundwater in the Gaizi River groundwater system in southern part were mainly affected by evaporation-concentration.Besides,several shallow confined groundwater sampling points were affected by rock weathering and evaporation-concentration.Cation exchange in the Kezilesu River groundwater system in northern part was stronger than that in the Gaizi River groundwater system in southern part.In general,cation exchange gradually increased with aquifer depth downward.The desulphidation occurred in partial section of the Southern Gaizi River groundwater system.Mixed action occurred not only between surface and groundwater,but also among different aquifers.(6)The?D and?¹⁸O values of surface water and groundwater were both distributed below the local meteoric water line(LMWL),indicating a certain degree of evaporation during precipitation recharge.Vertically,?D and?¹⁸O values of unconfined groundwater,shallow confined groundwater and deep confined groundwater were similar,demonstrated a closer hydraulic connection among different aquifers.(7)Inverse hydrogeochemical simulation results showed that simulated pathways of typical section all experienced mineral dissolution-precipitation and cation exchange during the hydrogeochemical evolution of groundwater in the study area.The effect of mineral dissolution-precipitation was slightly greater than cation exchange.