Investigating Geochemical Characteristics of South Korean Geothermal Waters and Their Reservoir Condition Using Combined Hydrogeochemical, Isotopic and Geothermometric Approaches

Geothermal resources have been emerged as a practical and cost-effective clean energy alternative to fossil fuels in many countries. The study of geothermal water can provide valuable insights for sustainable long-term utilization of geothermal energy and for understanding deep geologic environments for diverse emerging geo-energy technologies. For this study, chemical and isotopic tracers (δ²H, δ¹⁸O, δ¹³C, ¹⁴C) of thermal groundwaters over South Korean peninsula were investigated to evaluate their geochemical characteristics and reservoir parameters (i.e., temperature, recharge and circulation depth of groundwater, and circulation dynamics). Based on geochemical characteristics, geologic setting, and geographical location, South Korean thermal groundwaters were categorized into five groups (Kim et al., 2020): saline, CO₂-rich, high-pH alkaline, sulfate-rich, and diluted freshwater. These distinct hydrochemical characteristics are determined by complex geochemical processes which include calcite dissolution, plagioclase feldspar hydrolysis, CO₂ gas dissolution, cation exchange, precipitation of secondary minerals (clay, calcite, fluorite), gypsum dissolution, and seawater mixing. However, each group tends to show no systematic change among outflowing temperature, hydrochemistry, and stable isotopes over time (i.e., radiocarbon age), which may indicate that thermal waters have reached an equilibrium through deep circulation over very long periods of time (millennial scales). All groups of thermal water originated from deeply circulated meteoric water, and their stable water isotope data (δ²H, δ¹⁸O) show a systematic fractionation pattern depending on the recharge altitude (Choi et al., 2023). Temperatures of geothermal reservoirs at depths were estimated by using chemical geothermometers considering the ambient geology and measured outflow temperatures. The results obtained from K-Mg, Li-Mg, Na-K-Ca, Na-K, and Si geothermometers ranged widely from a minimum of 28°C to a maximum of 207°C, with an average of 61 to 148°C. The estimated depths of groundwater circulation to form South Korean thermal waters fall between about 1.0 and 3.3 km, if calculate from the calculated average reservoir temperature and geothermal gradient data in each location. The estimated circulation depths tend to increase with decreasing stable water isotope data (δ²H, δ¹⁸O) and increasing radiocarbon (¹⁴C) ages. This study, conducted across South Korea, provides important information about the origin and evolution of deep thermal water, which may be helpful to the efficient and sustainable management of geothermal resources and to the selection of suitable sites for geologic CO₂ storage or high-level radioactive waste disposal. Acknowledgements: This study was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and the BK Plus project in Korea.

References

Choi, J., Yu, S., Park, S., Yun, S.T., Lee, J., Park, J. (2023). Hydrochemical and Isotopic Assessment of Deep Groundwater: Residence Time, Circulation and Inter-Aquifer Mixing in South Korea. Goldschmidt 2023 Abstracts. https://doi.org/10.7185/gold2023.20028

Kim, K.H., Yun, S.T., Yu, S., Choi, B.Y., Kim, M.J., Lee, K.J. (2020). Geochemical pattern recognitions of deep thermal groundwater in South Korea using self-organizing map: identified pathways of geochemical reaction and mixing. Journal of Hydrology 589, 125202. https://doi.org/10.1016/j.jhydrol.2020.125202