Characteristics And Formation Mechanism Of Groundwater Quality In Plain Area Of Kashgar River Basin

The plain area of Kashgar River Basin is located in the west of Tarim Basin in Xinjiang and is one of the production bases for grain and cotton planting in Xinjiang.Groundwater is the main available water resources in the plain of Kashgar River Basin,but the rapid development of social economy has had a negative impact on groundwater quality.The groundwater quality requires necessary monitoring and comprehensive evaluation.The hydrochemical components and isotopic characteristics of groundwater contain information on the evolution of groundwater quality.Therefore,the characteristics of groundwater hydrochemical components and isotope in the plain area of Kashgar River Basin were studiedin order to provide a scientific basis for the exploitation and utilization of groundwater in the study area.In this paper,the characteristics and formation mechanism of groundwater quality in plain area of Kashgar River basin were studied by means of regional hydrogeological survey,water sample collection,graphic method,statistical analysis method and environmental isotope method.The main research results were as follows:(1)Hydrochemical characteristics of groundwater in the study areaBy means of mathematical statistics,Schucarev classification and Schoeller diagram,the characteristics of groundwater chemistry in the study area were analyzed.Unconfined groundwater was more affected by external factors than confined groundwater,proned to ion reaction.The hydrochemical types of groundwater had changed obviously,the distribution area of SO₄·Cl type groundwater and SO₄·HCO₃ type groundwater had increased obviously,and the salinization and hardening of groundwater were obvious.Using Map GIS and Surfer to draw the distribution map of groundwater TDS,Cl^-,SO₄^2-and salinization area,the results showed that the Cl^-content in unconfined groundwater increased gradually from south to north,and the Cl^-content in confined groundwater showed the characteristics of low in west and high in east.The SO₄^2-content in unconfined groundwater increased from south to north,the SO₄^2-content in shallow confined groundwater increased gradually from west to east,and the SO₄^2-content in deep confined groundwater alternated between high and low values.The unconfined groundwater TDS increased from south to north.The TDS distribution characteristics of shallow confined groundwater were high in east and low in west.The TDS distribution characteristics of deep confined groundwater were high in east and low in west,high in north and low in south.The salinization area of groundwater in other aquifers was increasing except the shallow confined groundwater.The recharge source,groundwater age and sulfate source of groundwater in the study area were discussed by isotope analysis.The stable isotopic characteristics of hydrogen and oxygen in the study area showed that the recharge source of unconfined groundwater and confined groundwater was atmospheric precipitation.³H isotopic characteristics showed that unconfined groundwater mainly received precipitation recharge.The groundwater age of shallow confined groundwater was less than 60 years.Deep confined groundwater was mainly recharged by early precipitation.³⁴S isotopic characteristics showed that SO₄^2-of groundwater in the study area were mainly derived from the dissolution of terrestrial deposit evaporate filtration.⁸⁷Sr/⁸⁶Sr ratio characteristics showed that the main hydrochemical components of surface water and groundwater in the study area were mainly derived from limestone and dolomite weathering dissolution.(2)Evaluation of groundwater quality and suitability of irrigation water in the study areaThe groundwater quality in the study area was evaluated by fuzzy comprehensive evaluation method based on entropy weight method.The results showed that the proportion of groundwater(I?III)available for drinking in each aquifer in the study area was 64.1%,46.2%and 37.6%,respectively.The proportion of IV and V groundwater in unconfined groundwater,shallow confined groundwater and deep confined groundwater were 35.9%,53.8%and 62.4%,respectively.The overall performance of water quality in the study area was that unconfined groundwater was better than shallow confined groundwater,shallow confined groundwater was better than deep confined groundwater.A fuzzy matter element model based on entropy weight was established to evaluate the irrigation adaptability of groundwater in the study area.42.7%unconfined groundwater,24.7%shallow confined groundwater and 26.2%deep confined groundwater belonged to class I groundwater,which was suitable for irrigation water source.(3)Formation mechanism of groundwater qualityBy using the methods of Gibbs diagram,ion ratio diagram,hydrogeochemical simulation and factor analysis,the interaction relationship between groundwater and external environment was studied.The results showed that the main mechanisms of groundwater quality formation in this area were effects of precipitation and surface water,evaporation and concentration,cation alternate adsorption,dissolution of minerals,human activities.