Application of Inert Gas Tracers to Identify the Physical Processes Governing the Mass Balance Problem of Leaking CO2 in Shallow Groundwater System

The CO2, leaked from a deep storage reservoir, can arrive at the shallow groundwater system. In shallow aquifer, the leaked CO2 forms a multi-phase plume and its mass balance will be controlled by various physical, chemical and biological processes. Inert gases such as sulfur hexafluoride (SF6) and noble gases are biochemically stable in groundwater system which can separate and explain the physical process governing the mass balance of the multi-phase plume. In this study, two pilot tests were conducted using inter gas tracers to evaluate the influence of physical processes on multi-phase plume migration. First injection test was made in Wonju, Korea. Three different tracers including chlorine, helium and argon tracer were jointly injected in 7.5 – 10.5 m below water table and recollected at same point after 1 day drift time in groundwater system. The mass recovery of the Single-Well Tracer Test (SWTT) was 49.3% for SF6, 58.1% for helium, 78.2% for argon, and 73.1% for chlorine. The recovery of inert gas tracers was relative to their solubility. Second injection test was made in Eumseong, Korea. Four tracers such as chlorine, helium, argon, and krypton were released along an induced pressure gradient where 0.0135% of helium, 0.0137% of argon, 0.0602% of krypton, and 75.0% of chlorine were retrieved. In this Inter-Well Tracer Test (IWTT), the recovery of inert gas tracer was also relative to tracer’s solubility. This study suggested the applicability of inert gas tracers to evaluate the mass balance of leaking CO2 plume in shallow aquifer system.