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

Abstract

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.