Reactive transport modeling to assess geochemical monitoring for detection of CO2 intrusion into shallow aquifers

Abstract The hypothesis is tested if changes in electric conductivity of groundwater (EC) in response to gaseous CO 2 intrusion are sufficient to be detected using probe measurements and geophysical electromagnetic measurements, e.g. airborne electromagnetic measurements. Virtual reactive scenario modelling is used to simulate the effects of the presence of calcite, CO 2 intrusion rates, depth of the aquifer formation, initial salinity of groundwater and CO 2 intrusion time on changes in EC. In all simulations, EC rises rapidly in response to CO 2 intrusion, however in different magnitudes. When calcite is present, EC changes are strong (+1.11 mS/cm after 24 hours of CO 2 intrusion) mainly due to calcite dissolution, whereas in aquifers without calcite changes are very low (+0.02 mS/cm after 24 hours) and close to the resolution range of probes. Increased depth (250 m / 500 m), i.e. higher temperature and pressure, and higher intrusion rates (up to full saturation) result in stronger rises in EC (+5.08 mS/cm in 500 m depth and 100 % saturation), and initial salinity has a negligible influence on changes in EC. Temporally limited CO 2 intrusion leads to EC values close to pre- CO 2 -intrusion-levels in the long-term. Measurement resolution of commercial EC probes is sufficient to detect CO 2 intrusion in almost all cases. In terms of geophysical electromagnetic measurements, applications in the field of monitoring saltwater-freshwater interfaces indicate a sufficient measurement resolution to detect changes in all simulations. However, practical limitations are expected due to the dependence of measurement resolutions on the applied measurement devices and site-specific geological settings.