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

The greenhouse effect is closely related to elevated atmospheric CO concentrations and therefore, carbon capture and storage (CCS) has attracted attention worldwide as a method for preventing the release of CO into the atmosphere, which highlights the importance of monitoring CO released from subsurface deposits. In this study, CO gas with a δC value of -30‰ was injected into soil through pipes installed at a depth of 2.5 m, and samples of CO gas released from the soil surface and three soil depths were collected from September 2015 to March 2016 to estimate subsurface CO movement. Before and after CO injection, the δC values of CO 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 CO was successfully detected at the surface. The δC values were visualized using an interpolation map to estimate the subsurface CO distribution, which confirmed that diffusion of the injected CO gas extended to the soil zone where CO was not injected. Additionally, variation in δC for soil CO 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 δC values horizontally and vertically indicated that soil heterogeneity led to different CO migration pathways and rates. We suggest that the carbon isotope ratio of CO is an effective tool for concurrently monitoring CO leakage on and under surface in a soil zone if a thorough baseline study is carried out in the field.