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

• CO2 from storage complexes can reach overlying groundwater aquifers.
• Lab experiments were used to determine the effect of CO2 on aquifer chemistry.
• CO2 seepage can mobilize contaminants of concern from the aquifer sediments.
• Regulatory limits were exceeded for several trace metals (As, Se, Mn, Pb and Hg).
• Variability in samples shows the importance of site specific analysis.

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.