Field-scale time-domain spectral induced polarization monitoring of geochemical changes induced by injected CO₂ in a shallow aquifer

Contamination of potable groundwater by leaking CO₂ is a potential risk of carbon sequestration. With the help of a field experiment, we investigated whether surface monitoring of direct current (DC) electric resistivity and induced polarization (IP) could detect geochemical changes induced by CO₂ in a shallow aquifer. For this purpose, we injected CO₂ at depths of 5 and 10m and monitored its migration using 320 electrodes on a 126 × 25 m surface grid. Measured resistances and IP decay curves found a clear signal associated with the injected CO₂ and rebounded to preinjection values after the end of the injection. Full-decay 2D DC-IP inversion was used to invert for the subsurface distribution in Cole-Cole parameters and changes to these parameter fields over time. The time-lapse inversions found plumes of decreased resistivity and increased normalized chargeability. The two plumes were of different shapes, with the resistivity anomaly being larger. Comparison with measurements of electric conductivity and aluminum (Al) concentrations indicated that two geochemical processes were imaged. We interpreted the change in resistivity to be associated with the increase in free ions directly caused by the dissolution of CO₂, whereas the change in normalized chargeability was most likely linked to persistent acidification and best indicated by Al concentrations. The results highlight the potential for monitoring of field scale geochemical changes by means of surface DC-IP measurements. Especially the different developments of the DC resistivity and normalized chargeability anomalies and the different associated geochemical processes highlight the added value of IP to resistivity monitoring.