Impacts of CO2 leakage into shallow formations on groundwater chemistry

• Injecting CO2 into a shallow geological formation to study CO2 leakage effects.
• CO2 dissolution changes pH, TDS, Ca, Mg, HCO3, and F pronouncedly.
• Calcite and dolomite (or Mg-rich calcite) dissolution contributes to Ca and Mg increase.
• Fe has erratic behavior in response to CO2 dissolution.
• Concentrations of F vs. HCO3, and F vs. Ca show almost linear correlations.

Geological storage of CO2 is one option for mitigating atmospheric emissions of carbon dioxide. However, the injected CO2 has the possibility of leakage. The leaked CO2 may move upward into shallow formations and thereby affects shallow groundwater. To investigate this effect, 27 tonnes of gaseous CO2 was injected into an aquifer about 180 m below a surface. Periodic groundwater samples were studied to identify shift of hydro-chemical parameters, including pH, TDS, ORP, Ca, Mg, Na, HCO3, Cl, SO4, Pb, Fe, and F. The results indicated that CO2 dissolution and reaction concurrently reduced aquifer pH levels and increased concentrations of TDS, Ca, Mg, HCO3, and F. ORP level and SO4 concentration remained static. Pb and Fe concentrations were lower than the detection limit 1 mg/L and 0.02 mg/L, respectively. After the CO2 breakthrough, Cl concentration increased to a stable level slightly higher than the background concentration. The shift in groundwater levels of Ca and Mg was attributed to dolomite (or Mg-rich calcite) and calcite-dominant dissolution processes. Results indicated linear approximations between concentrations of F vs. HCO3 (slope 0.0036) and F vs. Ca (slope 0.013). Additionally, the TDS and the total molar concentration of Ca and Mg showed linear correlations with increased levels of HCO3.

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