Estimation of deep-reservoir temperature of CO₂-rich springs in Kangwon district, South Korea

Weakly acid to neutral CO₂-rich springs (pH=5.5 to 6.7) with high concentrations of dissolved ions (TDS=419 to 3091 mg/L) and bicarbonate (249 to 2470 mg/L) occur in Kangwon district, South Korea. They are grouped into three chemical types: Na-HCO₃, Ca-Na-HCO₃, and Ca-HCO₃. The Na-HCO₃ type waters correspond to mature or partially equilibrated waters, while the other two types show the hydrochemical characteristics of immature waters. In this study, we examined the applicability of chemical geothermometers and multiple mineral equilibrium approach to estimate potential deep-reservoir temperatures. The application of silica, Na-K, and Na-K-Ca thermometers to a representative Na-HCO₃ type water yielded a reasonable temperature estimate (115-157 °C) for a deep geothermal reservoir. The multiple mineral equilibrium approach yielded a similar temperature estimate (about 140-160 °C). On the other hand, the Ca-HCO₃ type waters showed a relatively low and wide range of temperature estimates (about 60-130 °C), which suggests that their disequilibrium state is possibly due to surface water mixing. We believe that deep-reservoir temperatures of CO₂-rich groundwaters in Kangwon district can be estimated from the chemistry of Na-HCO₃ type waters and possibly approach 140-160 °C.