Effect of dissolved CO₂ on a shallow groundwater system: A controlled release field experiment

Capturing carbon dioxide (CO₂) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO₂ concentrations in the atmosphere. Sequestering CO₂ underground has its own set of environmental risks, including the potential migration of CO₂ out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO₂ was initiated to investigate potential groundwater impacts. Dissolution of CO₂ in the groundwater resulted in a sustained and easily detected decrease of ∼3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.