CO2 leakage detection in the near-surface above natural CO2-rich water aquifer using soil gas monitoring

Conventional soil gas monitoring methods were applied above a natural low-level CO2 reservoir to locate natural CO2 leakage and assess effectiveness in monitoring geological carbon storage sites. The concentrations of the soil gas (N-2, O-2, and CO2) and carbon-13 isotopes of soil CO2 (delta(13) C-CO2) were determined for ninety-four soil gas samples that were collected at a depth of similar to 60 cm. The CO2 flux was also measured at the same sampling locations. The 93 soil gas samples were divided into two groups: Group A with low soil CO2 concentrations and high delta C-13(CO2), which was influenced by the atmospheric air, and Group B with high soil CO2 concentrations and low delta C-13(CO2), which originated from microbial processes. Sample M17, which was close to a CO2-rich water well, had an exceptionally high soil CO2 (36.0% v/v), delta C-13(CO2) (-5.7 parts per thousand), and flux (546.2 g/m(2)/d), indicating geogenic CO2 inflow to the soil layer and discharge through the surface. This study shows that conventional soil gas monitoring methods are useful for locating CO2 leakage. A dense grid soil CO2 sampling near wells and periodic investigations are crucial for further understanding of the CO2 flow paths in the soil layer.