Application of noble gas tracers to identify the retention mechanisms of CO₂ migrated from a deep reservoir into shallow groundwater

Carbon Capture and Storage (CCS) is a valuable climate-mitigation technology, which offers the potential to cost-effectively reduce the emissions associated with the burning of fossil fuels. However, there is a potential risk of a small portion of the stored CO₂ unintentionally migrating from a storage site to a shallow groundwater aquifer which is the final retaining zone for any migrated CO₂ before it escapes to the atmosphere. Hence, it is imperative to identify the physical retention mechanisms of CO₂ within a shallow aquifer. In this study 1.70 × 10² kg of CO₂ and noble gas tracers (He, Ar and Kr) were continuously injected into a groundwater aquifer over 28 days with the aim of identifying the mechanisms and amount of CO₂ retention. Among the tracers, Kr was found to be the earliest indicator of CO₂ migration. The other tracers – He and Ar – arrived later and exhibited diluted signals. The diluted signals were attributed to degassing of the plume mass (1.6 % of CO₂) during the early stages of CO₂ migration. Diffusion accelerated the dilution of the lighter elements at the plume boundaries. Consequently, the clear relation of the noble gases with the CO₂ proved that degassing and mixing primarily control the mass retention of CO₂ in shallow groundwater, and the relative importance of these processes varies along the evolving path of migrating CO₂.