f SPECTRAL INDUCED POLARIZATION SIGNATURES OF MINERAL PRECIPITATION AND HYDROXIDE ADSORPTIONIN IN POROUS MEDIA

There is a growing interest in the application of spectral induced polarization (SIP) technique for non-invasively characterizing subsurface physical and chemical property changes resulting from biogeochemical processes. To facilitate an improved understanding of the chemical reaction fronts associated with in situ urea hydrolysis and carbonate mineral precipitation in porous media, we investigated spatiotemporal variations in complex conductivity during evolution of urea hydrolysis and calcite precipitation reaction fronts within a flow-through silica gel column. The real and imaginary parts of complex conductivity were shown to be sensitive to changes in both solution chemistry and calcium carbonate precipitation. Distinct changes in imaginary conductivity coincided with increased hydroxide ion concentration during urea hydrolysis. In a separate experiment focused on the effect of hydroxide concentration on interfacial polarization of silica gel and well-sorted sand, both SIP and geochemical measurements were applied to a saturated column composed of sequential zones of Ottawa sand and silica gel. We found a significant dependence of the polarization response on pH changes of the solution. We propose a conceptual model describing hydroxide ion adsorption behavior in silica gel and its control on interfacial polarizability. Our results demonstrate the utility of SIP for non-invasive monitoring of reaction fronts, and indicate its potential for quantifying geochemical processes that control the polarization responses of porous media at larger spatial scales in the natural environment.