Monitoring the movement of artificially injected CO₂ at a shallow experimental site in Korea using carbon isotopes

The greenhouse effect is closely related to elevated atmospheric CO₂ concentrations and therefore, carbon capture and storage (CCS) has attracted attention worldwide as a method for preventing the release of CO₂ into the atmosphere, which highlights the importance of monitoring CO₂ released from subsurface deposits. In this study, CO₂ gas with a δ¹³C value of −30‰ was injected into soil through pipes installed at a depth of 2.5 m, and samples of CO₂ gas released from the soil surface and three soil depths were collected from September 2015 to March 2016 to estimate subsurface CO₂ movement. Before and after CO₂ injection, the δ¹³C values of CO₂ released from the soil surface ranged from −24.5 to −13.4‰ (average −20.2 ± 2.1‰, n = 25) and from −31.6 to −11.9‰ (average −23.2 ± 4.3‰, n = 49), respectively. The results indicated that the leakage of injected CO₂ was successfully detected at the surface. The δ¹³C values were visualized using an interpolation map to estimate the subsurface CO₂ distribution, which confirmed that diffusion of the injected CO₂ gas extended to the soil zone where CO₂ was not injected. Additionally, variation in δ¹³C for soil CO₂ was detected at the three soil depths (15, 30, and 60 cm), where the values were −16.1, −20.0, and −22.1‰, respectively. Different δ¹³C values horizontally and vertically indicated that soil heterogeneity led to different CO₂ migration pathways and rates. We suggest that the carbon isotope ratio of CO₂ is an effective tool for concurrently monitoring CO₂ leakage on and under surface in a soil zone if a thorough baseline study is carried out in the field.