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

Abstract

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 (PCO2 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 o f 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 water 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 th e 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. Copyright © 2005 by The Geochemical Society of Japan.