Monitoring Proton-Promoted Iron Mineral Dissolution in an Aquifer Sand Using Spectral Induced Polarization (SIP)

Understanding and monitoring (iron) mineral dissolution in porous media is essential for predicting soil and groundwater geochemistry and guiding contaminant remediation efforts. The noninvasive geophysical technique, spectral induced polarization (SIP) measures the (changing) charge storage capacity at charged surfaces, linked with changes in geochemistry. Here, we present results from a flow-through column experiment, where we triggered Fe(II)-bearing mineral dissolution in a natural aquifer sand pack by infiltrating a 0.01 M HCl (pH = 2) solution. We combined breakthrough curve geochemical monitoring, with reactive transport modeling and simultaneous spatially and temporally resolved SIP measurements. The low pH front triggered mineral dissolution, and simultaneously, release of ionic species into solution as well as protonation and deprotonation of charged surfaces. These coupled parallel processes yielded distinct SIP signals. We detected a temporally variant and consistent V-shaped anomaly in the imaginary conductivity (σ″) signal. Despite the integrated contribution of coupled and parallel processes driving SIP signals, the joint interpretation with our reactive transport modeling results unequivocally linked the timing of the σ″ anomaly with the arrival and migration of the dissolution front. Our results highlight new ways to monitor subsurface geochemical reactions in real time and provide early warning tools for reactive transport applications.