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. •The leakage of injected CO2 was successfully detected at the surface.•The decrease in δ13C after CO2 injection indicates detection of the released CO2.•Carbon isotope ratio of CO2 is an effective tool for monitoring CO2 leakage.•Soil heterogeneity affects the CO2 migration pattern and rate.•Soil properties regulate the vertical distribution of injected CO2.