Hydrochemical and Isotopic Characteristics of CO₂-rich Groundwater in the Gyeongsang Sedimentary Basin, South Korea: A Natural Analogue Study on the Potential Leakage of Geologically-stored CO₂

The potential of geologic carbon storage has been considered in the Gyeongsang sedimentary basin (GSB) at the southeastern part of South Korea. In order to understand the long-term behaviors of geologically-stored CO₂, hydrochemical and multi-isotopic data (δ¹⁸O-δD of water and δ¹³C of dissolved inorganic carbon) of groundwater samples were collected from naturally seeping CO₂-rich springs and ambient shallow groundwater wells in the GSB. The collected samples were classified into three hydrochemical groups: 1) dilute and acidic, CO₂-rich springs, 2) high TDS, CO₂-rich springs, and 3) CO₂-poor groundwater. CO₂-rich springs (i.e., Groups 1 and 2) were differentiated from Group 3 by their high carbon isotope values and partial pressure of CO₂, suggesting that the CO₂ in Groups 1 and 2 originates from a deep-seated (magmatic) source. Compared to the CO₂-rich springs of Group 1, major cations and HCO₃^-, together with Fe, Mn and Sr, are enriched in CO₂-rich springs of Group 2 and these constituents are highly correlated with TDS. This indicates that, during the ascent of CO₂-rich fluids via fractures, CO₂-rich springs of Group 2 experienced substantial CO₂-water-rock interactions, possibly through longer or restricted pathways, whereas CO₂-rich springs of Group 1 acquired the chemistry through lower degrees of water-rock interaction via the rapid ascent through more permeable pathways. The results of principal component analysis (PCA) with compositional data demonstrated that pH can be used as a prior monitoring parameter to detect the direct leakage of CO₂ as represented by Group 1. In addition, the changes of ionic composition can be used as a supplementary tool to monitor the indirect leakage of CO₂ accompanying a substantial CO₂-water-rock interaction.