Abstract The success of carbon capture and storage (CCS) projects depends on the ability of storage sites to contain CO 2 without leakage; and the ability to convince regulators and the general public in the safety of the technology. If leakage were to occur after formal closure of the injection site, this could be over small areas from discrete point sources, such as abandoned wells, resulting in localised high concentrations of CO 2 in near-surface ecosystems. Consequently, studies of the potential environmental consequences of CCS have focused on near-surface ecosystems and, as a result, environmental impacts of localised elevated CO 2 on terrestrial and marine ecosystems are areas of active research. However, a CO 2 storage site, in itself, could also impact on deep subsurface microbial ecosystem and biogeochemical processes, potentially affecting groundwater quality. Using a microbial energetics approach, the significance of these impacts can be scoped with a simple evaluation tool.