Evaluating impacts of CO₂ intrusion into an unconsolidated aquifer: I. Experimental data

Capture and deep subsurface sequestration of CO₂ has been identified as a potential mitigation technique for rising atmospheric CO₂ concentrations. Sequestered CO₂ represents a potential risk to overlying aquifers if the CO₂ leaks from the deep storage complex. Batch and column experiments combined with wet chemical extractions were conducted to evaluate these risks to groundwater quality and to understand effects of unintentional release of CO₂ on groundwater chemistry and aquifer mineralogy. Sediments from the High Plains aquifer in Kansas, a largely unconsolidated aquifer, were used to study time-dependent release of major, minor and trace elements when exposed to CO₂ gas. Results showed that Ca, Ba, Si, Mg, Sr, Na, and K increased either within the first 4 h or followed nonlinear increasing trends with time, indicating that dissolution and/or desorption reactions controlled their release. In addition, other elements (e.g., Fe and Mn) and trace elements (e.g., As, Cu, Cr, Pb) were released during batch and column experiments, demonstrating the possibility for changes in mineralogy and groundwater quality degradation due to exposure to seepage of sequestered CO₂. National drinking water regulations were exceeded for As and Mn in the batch experiments, and As, Se, Mn, Pb and Hg in the column experiments, despite low levels of these contaminants found in the sediments. In addition, the concentration of another potential contaminant, i.e., Mo, was consistently higher in the control batch experiments (i.e., absence of CO₂) but was below detection in the presence of CO₂ indicating a potential for removal of elements by CO₂ gas exposure. Although results will be site specific for the High Plains aquifer and other mostly unconsolidated aquifers, these investigations will provide useful information to support site selection, risk assessment, and public education efforts associated with geological CO₂ storage and sequestration.