HYDROCHEMICAL AND ISOTOPIC VARIATION OF CARBONATED SPRINGS AND GEOTHERMAL GROUNDWATERS IN KOREA – A BASELINE STUDY OF UNDERGROUND CO2 INJECTION

For adoption of CCS (Carbon dioxide Capture and Storage) technology, public concerns the safety issues related to the CO₂ leakage from the deep storage aquifers. As natural analogy of deep groundwater, in this study, geothermal waters and carbonated springs have been monitored for a year. The ranges of natural variation in hydrochemical and isotopic characteristics of the waters were drawn from the seasonal samples, and the effects of CO₂ introduction to shallow groundwater by the leakage were estimated from changes of water chemistry using the program PHREEQC.

According to the stable isotope analysis, geothermal and carbonated waters were plotted around the Local Meteoric Water Line (LMWL; δ²H = 6.16 δ¹⁸O - 6.04), implying that those waters be recharged from the rainfalls in the study area. Average isotope values of coastal geothermal water for δ¹⁸O and δ²H were -4.43 ‰ and -33.2 ‰, respectively, and they were significantly heavier than those of other samples, indicating the effects of seawater mixing.

Coastal geothermal waters show high Na, Cl, SO₄ concentration while inland geothermal waters Na-HCO₃ type by water-rock interaction in the granite aquifer, and carbonated waters Ca-HCO₃ type by dissolved CO₂. The geothermal waters show changes in saturation indices of carbonate minerals such as calcite and aragonite from about 0.3 to –1.0, when CO₂ in the storage formation becomes leaked into the upper groundwater more than 10^-5 moles/cm³. It means that the mineral condition move from equilibrium or slightly oversaturated to undersaturated, and thus water-rock interaction could keep in direction to the mineral dissolution, resulting in the increase of Ca²⁺ and CO₃^2- concentrations in the upper shallow aquifer. On the other hand, carbonated springs consistently show slightly acidic conditions, implying that CO2 from the geological source cause pH decrease and maintain undersaturated conditions for carbonated minerals. PHREEQC modeling results in the interpretation, based on the saturation indices, that present carbonate minerals could become unstable when CO₂ enter the upper groundwater. And thus, changes in ionic composition, specifically Ca²⁺ and carbonate species, could indicate groundwater quality change due to CO₂ leakage from the storage formations.