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 (N₂, O₂, and CO₂) and carbon-13 isotopes of soil CO₂ (δ¹³C_CO2) were determined for ninety-four soil gas samples that were collected at a depth of ∼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 δ¹³C_CO2, which was influenced by the atmospheric air, and Group B with high soil CO₂ concentrations and low δ¹³C_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), δ¹³C_CO2 (-5.7‰), and flux (546.2 g/m²/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.