Quantitative assessment of deep-seated CO_2 leakage around CO_2-rich springs with low soil CO_2 efflux using end-member mixing analysis and carbon isotopes

摘要

This study examined a mountainous area with two hydrochemically distinct CO_2-rich springs to understand the origin, flow, and leakage of CO_2, which may provide implications for precise monitoring of CO_2 leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO_2-rich water (δ~(13)C_(DIC)) and those of soil CO_2 (δ~(13)C_(CO_2)) indicated a deep-seated CO_2 supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO_2-rich springs separated by 7 m, despite similar δ~(13)C_(DIC) and partial pressure of CO_2, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO_2 preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO_2-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO_2 efflux was low compared to that in other natural CO_2 emission sites, and in particular it was noticeably low near the CO_2-rich springs, whereas δ~(13)C_(CO_2) was high close the CO_2-rich springs. The dissolution of CO_2 in the near-surface water body seemed to decrease the deep-seated CO_2 leakage through the soil layer, while δ~(13)C_(CO_2) imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO_2 to the low soil CO_2 efflux by assuming that atmospheric CO_2 and soil CO_2 (by respiration) as well as deep-seated CO_2 contribute to the soil CO_2 efflux. For each end-member, characteristic δ~(13)C_(CO_2) and CO_2 concentrations were defined, and then their apportionment to soil CO_2 efflux was estimated. The resultant proportion of deep-seated CO_2 was up to 8.8%. Unlike the spatial distribution of high soil CO_2 efflux, high proportions exceeding 3% were found around the CO_2-rich springs along the east-west valley. The study results indicate that soil CO_2 efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO_2 leakage detection because CO_2 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 CO_2 using the soil CO_2 concentration, soil CO_2 efflux, and δ~(13)C_(CO_2) at each measurement site.