The isotopic and chemical compositions of the CO2-rich waters in Korea

Abstract

The chemical and isotopic compositions of CO₂-rich water (Pco₂ â‰ˆ 1 atm) in Korea mainly occurring in granitic terrain were investigated. CO₂-rich waters can be divided into three types based on chemical compositions: Na-HCO₃, Ca-Na-HCO₃ and Ca-HCO₃ types. Water chemistry indicates that these waters were evolved through a strong reaction of the host rocks with a supply of deep-seated CO₂ during deep circulation. The dissolution process of plagioclase is important in water/granite interactions and its solubility changes with reaction temperature played an important role in the determination of chemical composition. Multi-com-ponent mineral/water equilibrium calculations indicate that the CO₂-rich water reaches temperature of 115-140°C.
δ¹⁸O and δD data indicate that CO₂-rich waters were derived from local meteoric water and have a lighter isotope composition than other natural waters. The tritium contents show a long residence time and the possibility of CO₂ flux in an aquifer at great depth. There is a strong indication of mixing between deep CO₂-rich water and recent fresh water during ascent to the surface. Carbon isotope analysis reveals that the carbon composition is associated with mantle-derived CO₂. The preliminary helium isotope ratio also supports that the CO₂ gas was derived from mantle origin. The strontium isotope ratios indicate that the chemistry of the CO₂-rich water is determined by reaction with local granite.