Distribution of δ34S and δ18O in SO42- in Groundwater from the Ordos Cretaceous Groundwater Basin and Geological Implications

摘要

The Ordos Cretaceous Groundwater Basin,located in an arid-semiarid area in northwestern China,is a large-style groundwater basin.SO_4^(2-) is one of the major harmful components in groundwater.Dissolved SO_4^(2-) concentrations,andδ^(34)S-SO_4^(2-) andδ^(18)O-SO_4^(2-) in groundwater from 14 boreholes and in gypsum from aquifer were analyzed.Results show that SO_4^(2-) in shallow groundwaters originates from precipitation,sulfide oxidation,and dissolution of stratum sulphate,with a big range ofδ^(34)S values,from-10.7‰to 9.2‰,and addition of SO_4^(2-) in deep groundwater results from dissolution of stratum sulphate,with biggerδ^(34)S values,from 7.8‰to 18.5‰,compared with those in shallow groundwater.This research also indicates that three types of sulphate are present in the strata,and characterized by highδ^(34)S values and highδ^(18)O values-style,highδ^(34)S values and middleδ^(18)O valuesstyle, middleδ^(34)S values and lowδ^(18)O values-style,respectively.Theδ^(34)S-SO_4^(2-) andδ^(18)O-SO_4^(2-) in groundwater have a good perspective for application in distinguishing different groundwater systems and determining groundwater circulation and evolution in this area.