Hydrochemistry and genesis of CO₂-rich springs from Mesozoic granitoids and their adjacent rocks in South Korea

抄録

Many CO₂-rich springs are found in Mesozoic granitoids and surrounding rocks in South Korea. Their presence is locally restricted to three regions: the Kangwon region, the Chungcheong region and the Kyungpook region. Discharge of many CO₂-rich springs is mainly related to the geologic structures, i.e., the geologic boundaries, faults and dykes. The sixty-three CO₂-rich water samples can be classified into three chemical water types; Ca-HCO₃ water, Ca(Na)-HCO₃ water, and Na-HCO₃ water. Most of the soda waters show a high CO₂ concentration (P_CO2 0.12 atm to 5.21 atm), a slightly acid pH (4.8 to 6.76), and high ion concentrations. The microscopic observation and the chemical analysis of host rocks at CO₂-rich spring sites show that the carbonate minerals are secondary precipitates and interstitial fillings in Cretaceous sedimentary rocks that are the main sources of Ca, Mg, HCO₃ and Fe in the CO₂-rich waters of the Kyungpook region. The carbonate minerals locally present in the fractures of granite and gneiss would be one of the main sources of abundant Ca, Mg and HCO₃ in the CO₂-rich water of the Kangwon and the Chungcheong regions. The chemical composition of these CO₂-rich springs according to host rocks and discharge regions was compared by using the Box-Whisker diagram. Oxygen and hydrogen isotope data indicates that the CO₂-rich waters are of meteoric origin. The carbon isotope data (δ¹³C -6.6 to -0.9‰) suggest that the carbon of the soda waters is mainly derived from a deep-seated source, but is partly mixed with CO₂ derived from carbonate minerals. The formation process of the CO₂-rich springs can be summarized as follows. After the CO₂ gas derived from the deep-seated source enters the groundwater system along faults or geologic boundaries, the CO₂-rich water evolved into three chemical types depending on the aquifer rock types.