Study On Hydrochemical Characteristics And Evolution Mechanisms Of Groundwater In The Jiuquan East Basin

The Jiuquan East Basin is located in the middle of the Hexi Corridor in Gansu Province,which is the middle reaches of the Heihe River Basin.It is a typical semi desert arid climate area.Groundwater resource is an important source of people's life and production in this area.In recent years,with the construction of a large number of water conservancy projects and agricultural irrigation supporting water diversion projects and the continuous exploitation of groundwater,which has changed the relationship between the natural supply,runoff and discharge of groundwater,eventually leading to the continuous deterioration of the local ecological environment.Therefore,the study on the hydrochemical characteristics and evolution mechanisms of groundwater in the Jiuquan East Basin can provide theoretical basis for the rational development and utilization of groundwater resources and the protection of ecological environment.Based on the hydrochemical data of 111 groups of shallow groundwater and 22 groups of deep groundwater samples in the study area,combined with hydrogeochemical theory,the contents,the spatial distribution and the hydrochemical types of groundwater are studied by mathematical statistics.The results show that both shallow and deep groundwater are generally weakly alkaline freshwater,the anions are mainly SO42-and HCO3-,and the cations are mainly Mg2+ and Na+.The spatial distribution characteristics of TDS,Ca2+,Mg2+,K+,Na+,SO42-,Cl-in the shallow groundwater and TDS,Na+,SO42-,Cl-in the deep groundwater have certain similarity,which are mainly along the groundwater runoff direction,from the piedmont groundwater supply area in the south of the basin to the groundwater discharge area in the north of the Jinta-Heli mountain.The TDS and ion concentration gradually increase.The content of NO3-in shallow and deep groundwater is low in most areas,indicating that the groundwater in the study area is less affected by human activities.The hydrochemical type of shallow groundwater is gradually evolved from the HCO3·SO4-Mg·Ca type in the southern part of the basin(groundwater recharge runoff area)to the SO4·Cl—Na·Mg type in front of the Jintanan Mountain(groundwater discharge area)of the northern part of the basin.Similarly,deep groundwater evolves gradually from HCO3·SO4-Mg · Ca type to SO4·HCO3—Na·Mg type.The isotopic technique and analytical methods are used to explore the isotopic characteristics and supply sources of groundwater.The results show that the average values of ?18O and ?D in shallow groundwater are-9.60‰ and-60.78‰,respectively.The average values of ?18O and ?D in deep groundwater are-10.07‰ and-63.75‰,respectively.Both shallow and deep groundwater ?18O and ?D values are mainly close to the local meteorological precipitation line,and are mainly located above the meteorological precipitation line.It shows that the recharge sources of shallow and deep groundwater in the study area come from ice and snow melting water in the Qilian Mountains and local precipitation,and mainly from ice and snow melting water.In addition,there is a certain hydraulic relationship between shallow and deep groundwater.The hydrochemical data of groundwater are analyzed by correlation analysis,Gibbs diagram analysis,Piper diagram analysis and ion proportion coefficient analysis.The results show that in the groundwater recharge-runoff area in the south of the basin,the hydrochemical components in the shallow and deep groundwater are mainly derived from silicate rock weathering,evaporite rock dissolution and carbonate weathering.During the process of shallow groundwater from the southern part of the basin to the northern part of the basin,the evaporation concentration,the cation exchange and the adsorption gradually become the main controlling factors of hydrochemical composition.Deep groundwater mainly undergoes the leaching,the cation exchange and the adsorption during runoff.