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Capture and geological storage of CO₂ is emerging as an attractive means of economically abating anthropogenic CO₂ emissions from point sources. However, for the technology to be widely deployed it is essential that a reliable means to assess a site for both storage performance and regulation compliance exists. Hence, the ability to identify the origin of any CO₂ seepage measured at the near-surface and ground surface and determine if it originates from a deep storage site or a different source is critical. As an analogue for post-emplacement seepage, here we examine natural CO₂ rich springs and groundwater wells in the vicinity of the St. Johns Dome CO₂ reservoir located on the border of Mid-Arizona/New Mexico, USA. Extensive travertine deposits in the region document a long history of migration of CO₂ rich fluids to the surface. The presence of CO₂ rich fluids today are indicated by high levels of HCO₃ …