Geochemical Impacts of Leaking CO2 from Subsurface Storage Reservoirs on the Fate of Metal Contaminants in an Overlaying Groundwater Aquifer

Abstract:

The leakage of CO₂ and the concomitant upward transport of brine solutions and contaminants from deep storage reservoirs to overlaying groundwater aquifers is considered one of the major risks associated with geologic carbon sequestration (GCS). A systematic understanding of how such leakage would impact the geochemistry of potable aquifers is crucial to the maintenance of environmental quality and the widespread acceptance of GCS.

A series of batch and column experiments studies were conducted to understand the fate (mobilization and immobilization) of trace metals, such as Cd and As in the groundwater aquifer after the intrusion of CO₂ gas and CO₂-saturated fluids containing leached metals from deep subsurface storage reservoirs. Sediments from the High Plains aquifer in Kansas, United States, were used in this investigation, which is part of the National Risk Assessment Partnership Program sponsored by the US DOE. This aquifer was selected to be representative of consolidated sand and gravel/sandstone aquifers overlying potential CO₂ sequestration repositories within the continental US. The experiments were conducted at room temperature and atmospheric pressure.

The results demonstrated that Cd and As that intrude into groundwater aquifers with the leaking CO₂ at initial concentrations of 40 and 114 mg/L, respectively, will be adsorbed on the sediments, in spite of the acidic pH (between 5 and 6) due to CO₂ dissolution in the groundwater. Cd concentrations were well below its MCL in both the aqueous solution of the batch study and the effluent of the column study, even for one of the sediment samples which had undetectable amount of carbonate minerals to buffer the pH. Arsenic concentrations were also significantly lower than that in the influent, suggesting that natural sediments have the capacity to mitigate the adverse effects of the CO₂ leakage. However, the mitigation capacity of sediments is influenced by its geochemical properties. When there are anions such as phosphate in the sediment, competitive adsorption may occur and result in higher concentrations of toxic metals in the aqueous phase. Results from these investigations will provide useful information to support site selection, risk assessment, and public education efforts associated with geological CO2 storage and sequestration.