Monitoring of $CO_2$ release through the ground surface is essential to confirm the safety of carbon storage projects. We conducted a feasibility study of the multi-channel surface-soil $CO_2$-concentration monitoring (SCM) system as a soil $CO_2$ monitoring tool with a small scale injection test. The background concentrations showed the distinct diurnal variation. The negative relation of $CO_2$ with temperature and the low $CO_2$ concentrations during the day imply that surface-soil $CO_2$ depends on photosynthesis and respiration. After 4.2 kg of $CO_2$ injection (1 m depth for 29 minutes), surface-soil $CO_2$ concentrations increased in the all five chambers, which were located less than 2.8 m of distance from each other. The $CO_2$ concentrations seem to be recovered to the background around 4 hours after the injection ended. To determine the leakage, the data from Chamber 2 and 5 with low increase rates were used for statistical analyses. Coefficient of variation for 30 minutes ($CV_{30min}$.) is efficient to determine a leakage signal, with reflecting the fast change in $CO_2$ concentrations. Consequently, SCM and $CV_{30min}$ could be applied for an efficient monitoring tool to detect $CO_2$ release through the ground surface. Also, this study provides ideas for establishing action steps after leakage detection.

Keywords: Geologic $CO_2$ storage;soil $CO_2$ monitoring;injection test;