Unintended releases of CO 2 from storage sites present a risk to groundwater, surface water and atmospheric re-emission of stored CO 2 . Robust monitoring methodologies must detect anthropogenic CO 2 in the shallow subsurface and delineate it from naturally-present CO 2 . Noble gases have been used an effective tool for source delineation, leakage detection, and CO 2 migration owing to their non-reactivity and predictable partitioning behaviour. However, interpreting noble gas signals in the context of multiphase, multicomponent mass transfer remains a challenge, limiting the degree to which noble gases can elucidate injected CO 2 behaviours. This study addresses this knowledge gap by simulating CO 2 injections with realistic noble gas mixtures under varied groundwater flow and subsurface heterogeneity conditions. Results demonstrate heterogeneity and groundwater velocity influence the vertical migration of CO 2 by impacting the proportion of CO 2 which dissolves during migration. Furthermore, the presence of trapped gas can impact noble gas ratios, as less soluble noble gases such as He will preferentially partition to the gaseous phase, leading to changes in noble gas ratios and impacting both dissolved gas concentrations and gas surface flux. These findings suggest that the low solubility of noble gases make them valuable monitoring tools, however gaseous interactions with cm-scale heterogeneities and multicomponent partitioning drives compositional changes that must be considered when interpretating noble gas signals for monitoring CO 2 migration.
Ashmore et al. (Mon,) studied this question.