Chemical Evolution Mechanism And Water Quality Evaluation Of Groundwater In Southwest Ordos Basin

The research area is located in the northwest of China,where the climate is dry and evaporation is large,and groundwater plays a vital role in this area.Under the background of data collection,field investigation and water sample analysis,SPSS statistical analysis,Gibbs diagram,Piper trigram,Gaillardet hydrochemistry diagram,ion proportion relationship,reverse simulation of water-rock interaction with PHREEQC software and BP neural network analysis were used to study the phreatic aquifer and confined aquifer in the study area.The following understandings have been obtained:(1)In groundwater,SO₄^2-,HCO₃^-and Cl^-are the dominant anions,and Mg²⁺,Na⁺and Ca²⁺are the dominant cations.The phreatic aquifer environment in the study area is weakly alkaline with a mean p H of 7.81,while the confined aquifer environment is weakly alkaline with a mean p H of 7.89.The results of Piper trigram show that the hydrochemical types of the aquifer in the study area are mainly Ca-HCO₃ type,followed by mixed type and partial Ca-SO₄ type.The hydrochemical types of confined aquifers are mainly Na-Cl type,followed by mixed Na-SO₄ type.(2)Gibbs diagram shows that the chemical composition of groundwater in the study area is changed by evaporation and concentration,and weathering also plays a certain role.Gaillardet hydrochemical diagram shows that most of the submerged water samples and a small part of the confined water samples fall in the silicate and carbonate end areas.The leaching minerals are mainly calcite and weathering products of silicate aluminate.There are many confined water sampling points and a few phreatic sampling points in the silicate and evaporite areas,which indicates that the minerals that take place leaching are mainly chloride,sulfate,nitrate and silicate weathering products.(3)The calculation of saturation index and reverse simulation of water-rock interaction indicate that the hydrochemical interaction between groundwater and surrounding rocks is closely related to the change of the number of ions in groundwater.(4)The water quality of the phreatic aquifer in the study area is relatively good and meets the standard of domestic water as required by the regulations.The water quality of the confined aquifer is relatively poor.The?type water is mainly distributed in the northwest and northeast of the study area,and the primary hydrogeological environment is poor.