Enhanced hydrogeochemical baseline of a CO2 injection facility in southern Alberta, Canada

• Multi-year geochemical monitoring of shallow CO2 injection site shows no sign of CO2 at monitoring well during study period. • Changes δ2H and δ13C of CH4 have been observed over the study period but cannot be directly related to CO2 injection. • Hydraulic head has been observed to decrease in some groundwater zones but is interpreted to the result of drought and not related to CO2 injection. Geological storage of CO 2 is anticipated to play a significant role in the management and reduction of greenhouse gas emissions. Monitoring of CO₂ injection facilities is essential to provide reassurance of the containment of the injected CO 2 . Here, we report results over six years (2018–2023) for a hydrogeological and geochemical (gas compositions, δ 13 C CH4, δ 13 C CO2, δ 2 H CH4 and noble gas concentration and isotopes) monitoring program at a small-scale CO 2 injection facility located near Brooks, Alberta, Canada with injection ∼300 m below ground. The results provide a comprehensive record of the subsurface hydrological and geochemical conditions over the six-year period. Injected CO 2 was not detected in samples from the injection zone. There was also no indication of injected CO 2 in samples collected from surface casing vents of the three ∼300 m deep wells, nor was injected CO 2 observed in samples from the six shallow groundwater wells (<105 m deep). Various compositional and isotopic changes have been observed over time which are interpreted to either be indirectly related to CO 2 injection or completely unrelated indicating non-CO 2 injection related variability in the baseline conditions of the site. Additionally, a progressive reduction in hydraulic head has been observed in some shallow aquifers consistent with drought conditions in the region. Our study implies that complex subsurface changes may occur at CO 2 storage sites which may be unrelated to human activity, complicating the monitoring of CO 2 injection.