CHARACTERIZATION OF INTERACTIONS BETWEEN CARBON DIOXIDE AND GROUNDWATER AT A CO2 LEAK ANALOGUE STUDY SITE IN SODA SPRINGS IDAHO

Migration of CO₂ stored in deep geologic formations into overlying freshwater aquifers may have significant environmental impacts, including the transport of undesirable metals into a groundwater resource. Because of the timescales and volumes involved, it is not possible to conduct field scale experiments to measure the actual phenomenon. Geochemical transport models are useful, but without field-derived data, these models cannot provide unique solutions. Natural analogue sites for the study geologic carbon dioxide sequestration provide opportunities to bridge this gap by examining CO₂ systems that have operated on time scales of centuries to millenia. Such sites provide a perspective that laboratory and field experiments (e.g., time scales of days to decades) cannot provide. One such Site is the CO₂ Leak Analogue Study Site (CLASS) located in Soda Springs, Idaho, USA. At this site, CO₂ and formation fluids generated by the dissolution of Paleozoic carbonates at depth are migrating and reacting with a series of shallower thoelitic basalt flows that host a fresh water aquifer. In many cases the CO₂ escapes to the surface in large volumes. But in most cases the CO₂ is trapped by mineralization reactions with in the aquifer and by dissolution into the groundwater. These mineralization reactions have generated a specific chemical signature within the formation fluid. This signature can be used to assess the relative impact of leaking CO₂ on water-rock interactions, and provides a basis for interpreting results from field and laboratory experiments in the context of a large-scale CO₂ system interacting with an overlying aquifer. Lessons learned from the work at the Soda Springs analogue site have provided insight into how reactive in situ tracers can track the progress of CO₂-water-rock interactions, and preliminary results have been integrated into the risk management plan for the Big Sky Carbon Sequestration Partnership’s project at Kevin Dome.