Quantitative assessment of deep-seated CO₂ leakage around CO₂-rich springs with low soil CO₂ efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO₂-rich springs to understand the origin, flow, and leakage of CO₂, which may provide implications for precise monitoring of CO₂ leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO₂-rich water (δ¹³C_DIC) and those of soil CO₂ (δ¹³C_CO2) indicated a deep-seated CO₂ supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO₂-rich springs separated by 7 m, despite similar δ¹³C_DIC and partial pressure of CO₂, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO₂ preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO₂-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO₂ efflux was low compared to that in other natural CO₂ emission sites, and in particular it was noticeably low near the CO₂-rich springs, whereas δ¹³C_CO2 was high close the CO₂-rich springs. The dissolution of CO₂ in the near-surface water body seemed to decrease the deep-seated CO₂ leakage through the soil layer, while δ¹³C_CO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO₂ to the low soil CO₂ efflux by assuming that atmospheric CO₂ and soil CO₂ (by respiration) as well as deep-seated CO₂ contribute to the soil CO₂ efflux. For each end-member, characteristic δ¹³C_CO2 and CO₂ concentrations were defined, and then their apportionment to soil CO₂ efflux was estimated. The resultant proportion of deep-seated CO₂ was up to 8.8%. Unlike the spatial distribution of high soil CO₂ efflux, high proportions exceeding 3% were found around the CO₂-rich springs along the east-west valley. The study results indicate that soil CO₂ efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO₂ leakage detection because CO₂ dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO₂ using the soil CO₂ concentration, soil CO₂ efflux, and δ¹³C_CO2 at each measurement site.