Hydrochemical Parameters to Assess the Evolutionary Process of CO2-Rich Spring Water: A Suggestion for Evaluating CO2 Leakage Stages in Silicate Rocks

Eighteen water samples collected from eight CO2-rich springs in the northern part of the Gyeongsang sedimentary basin (GSB), South Korea showed distinct hydrochemistry, in particular, pH, total dissolved solids (TDS), and Na contents, and they were classified into four groups: (1) Group I with low pH (average of 5.14) and TDS (269.8 mg/L), (2) Group II with high TDS (2681.0 mg/L) and Na-enriched (202.9 mg/L), (3) Group III with intermediate Na content (97.5 mg/L), and (4) Group IV with Na-depleted (42.3 mg/L). However, they showed the similar partial pressure of CO2 (0.47 to 2.19 atm) and stable carbon isotope ratios of dissolved inorganic carbon (−6.3 to −0.6‰), indicating the inflow of deep-seated CO2 into aquifers along faults. In order to elucidate the evolutionary process for each group of CO2-rich springs, a multidisciplinary approach was used combining stable hydrogen (δD), oxygen (δ18O) and carbon (δ13C), and radioactive carbon (14C) isotopic, geophysical, and hydrochemical data. The highest δD and δ18O ratios of water and the relatively young 14C ages in Group I and the lowest δD and δ18O in Group II indicated the short and long residence time in Group I and II, respectively. The electrical resistivity tomography (ERT) survey results also supported the fast rising through open fractures in Group I, while a relatively deep CO2-rich aquifer for Group III. Group II had high contents of Mg, K, F, Cl, SO4, HCO3, Li, and As, while Group I showed low contents for all elements analyzed in this study except for Al, which exceeded the World Health Organization (WHO) guideline for drinking-water quality probably due to the low pH. Meanwhile Group IV showed the highest Ca/Na as well as Ca, Fe, Mn, Sr, Zn, U, and Ba, probably due to the low-temperature dissolution of plagioclase based on the geology and the ERT result. The levels of Fe, Mn, and U exceeded the WHO guidelines in Group IV, while As in Group II. The different hydrochemistry suggests a distinct evolutionary process for each group. Group I seems to represent a fast discharge from the CO2-rich aquifer to a discharge point, experiencing a low degree of water-rock interaction, while Group II seems to represent a slow discharge with a high degree of water-rock interaction. GSB is a potential site for geological carbon storage (GCS), and injected CO2 may leak through various evolutionary processes given heterogenous geology as CO2-rich springs. The study result suggests that the combined use of pH, Na, K, Li, and Ca/Na are effective hydrochemical monitoring parameters to assess the leakage stage in silicate rocks in GCS projects. Besides, aluminum (Al) can be risky at the early stage of CO2 leakage, while Fe, Mn, U, and As at the later stage of CO2 leakage.