Study On The Chemical Evolution Of Groundwater In Ordos Plateau Based On Multiple Isotopes

Various hydrochemical reactions occur during groundwater circulation,the fractionation of stable isotope in this process provides an effective way to identify these hydrochemical reactions.Based on a comprehensive analysis of the pattern of groundwater circulation and possible hydrochemical reactions in the Cretaceous aquifer of the Ordos Plateau,this paper chooses appropriate isotopes to promote the identification and quantitative calculation of the hydrochemical reactions in different parts of the groundwater circulation.In order to reveal the mechanism that HCO₃^-is the dominant anion in most groundwater samples,Sr isotope was selected to analyze its source.Rainwater,silicate minerals and calcite are selected as end-members in the recharge areas,while groundwater in recharge areas and calcite are selected as end-members in discharge areas,then,the contribution of each end-member to Sr isotopes is calculated quantitatively.The results show that Sr²⁺in recharge areas are mainly from silicate weathering,while Sr²⁺in discharge areas are mainly from carbonate dissolution.In order to reveal the cause of the significant decrease of Mg²⁺,K⁺and other cations from recharge to discharge areas,the removal mechanism is analyzed with the help of Mg isotopes.By selecting groundwater in recharge areas and calcite as end-members,the influence of calcite dissolution on?²⁶Mg is calculated quantitatively.Based on the Rayleigh fractionation process,the removal of Mg²⁺and the change of?²⁶Mg caused by the formation of clay minerals and decarbonation are quantitatively calculated.The formation of clay minerals also results in the removal of K⁺.By comparing the residence times of groundwater samples from different groups in discharge area,it was found that the residence times controlled the degree of fractionation of Mg isotopes.The contents of SO₄^2-,Ca²⁺and Na⁺increase significantly with depth in the deep part of the discharge areas.By analyzing the S isotopes of the discharge areas,groundwater in the middle parts of discharge areas and the underlying Jurassic groundwater are selected as end-members to identify the leakage of Jurassic groundwater.Based on the equation of Rayleigh fractionation,the influence of sulfate reduction on SO₄^2-and?³⁴S in groundwater in the discharge areas is calculated.In order to reveal the control of hydrodynamic field on hydrochemical field,reverse particle tracing of groundwater sampling points is carried out.And the distance between flowline and the floor of Cretaceous strata is determined.By comparing the distance and?³⁴S of groundwater samples in discharge areas,the influence of Jurassic leakage on deep part of Cretaceous is further confirmed.This paper analyzes the hydrochemical reactions in different parts of groundwater circulation by multiple isotopes.This paper not only deepens the understanding of hydrodynamic process and hydrochemical evolution process during groundwater circulation,but also have important reference value for the study of groundwater circulation in other areas.