Influence of NaHCO3 diffusion-driven pH changes on the electrical relaxation behaviour of sandstones

SUMMARY Fluid chemistry in the vadose zone absent of organic life is governed by the interactions between rock mineral surfaces, water and atmospheric carbon dioxide [CO$_2$] since carbon and its aquatic species control the system pH. Even though the effects of high carbon concentrations in the pore fluids of rocks and soils can be controlled ex-situ, their non-invasive monitoring and control still face difficulty, due to the inaccessibility of pore spaces. We propose monitoring the effect of carbon-rich solutions via their influence on the electrical relaxation behaviour, using the spectral induced polarization (SIP) method. Generally, the SIP response is determined by the rock’s texture and the chemical composition of the electrical double layer (EDL) forming at the mineral–water interface. The understanding of how the relaxation behaviour of rocks and soils is controlled by pore water salinity and pH and how fast the EDL adapts to changes in pore fluid chemistry, however, is still limited. In this study, we conducted a series of controlled experiments where the diffusion of sodium hydrogencarbonate solution into quartz-rich sandstones was monitored with SIP at high temporal resolution. To identify the underlying relaxation processes, we analysed the obtained complex conductivity spectra by performing a Debye decomposition, yielding the system’s relaxation-time distribution. Our results show that increasing pH leads to increased imaginary conductivities and systematic shifts in the peak relaxation time of the system. The observed temporal dynamics of the peak relaxation time can be described with a diffusion-type equation. We find that the dynamics are not governed by the permeability or porosity of the rock. In one experiment, salinities high enough to diminish the polarization of the system were reached. This reduction in polarization at peak frequency cannot be explained through simple Stern layer polarization models alone, suggesting that diffusive layer polarization plays an important role with increasing pH. While polarization magnitude reduces significantly, peak spectral position shifts towards larger relaxation times suggesting a reduction in the mobilities of the surface ions. Due to the increased salinity, the double layer thickness decreases, in turn decreasing the relaxation length scale, and lowering the diffuse layers screening effect. This study shows that the SIP method can capture the dynamic changes at the mineral–water interfaces in rocks in response to changing pH over a broad range of salinities, making SIP a promising method for the monitoring of subsurface processes associated with changes in the inorganic carbon concentration of the pore fluid.