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.