Geochemical impacts of leaking CO2 from subsurface storage reservoirs to an unconfined oxidizing carbonate aquifer

• We evaluated the impact of leaking CO2 from subsurface reservoirs to aquifers.
• Aquifer samples contained potential contaminants which may be mobilized.
• Results confirmed the release of major elements (Ca, Mg, Ba, Sr, Si, Na, and K).
• Mobilization and possible rapid immobilization of Fe, Al, and Mn was observed.
• Sporadic mobilization of low concentrations of As, Cd, Pb, Cu, Zn, and Mo occurred.

A series of batch and column experiments combined with solid phase characterization studies was conducted to evaluate the impacts of the potential leakage of carbon dioxide (CO2) from deep subsurface storage reservoirs to overlying potable carbonate aquifers. The main objective was to gain an understanding on CO2 gas-induced changes in aquifer pH and mobilization of major, minor, and trace elements from dissolving minerals in rocks representative of an unconfined, oxidizing carbonate aquifer within the continental US. Samples from the unconfined portion of the Edwards limestone aquifer in Texas were exposed to a CO2 gas stream or were leached with a CO2-saturated influent solution simulating different leaking scenarios [i.e., sudden, fast, and short-lived release of CO2 (batch experiments) and gradual release (column experiments)]. The results from the batch and column experiments confirmed that exposure to excess CO2 gas caused significant decrease in pH (about two pH units); the release of major chemical elements into the contacting aqueous phase (such as Ca, Mg, Ba, Sr, Si, Na, and K); the mobilization and possible rapid immobilization of minor elements (such as Al and Mn), which are able to form highly reactive secondary phases; and sustained but low-concentration releases of some trace elements (such as Mo, Cs, Sn) in some samples. Spikes of low concentrations of other trace elements (such as As, Cd, Pb, Cu, Zn, Se, etc.), were observed sporadically during these experiments. The results help in developing a systematic understanding of how CO2 leakage is likely to influence pertinent geochemical processes (such as dissolution/precipitation and sorption/desorption) in the aquifer sediments and will support site selection, risk assessment, policy-making, and public education efforts associated with geologic CO2 sequestration.