Linking Geophysical Signatures to Iron Mineral Dissolution in Acidified Aquifer Sands

The real-time monitoring of mineral dissolution processes would constitute a much-needed advancement in remediation strategies at contaminated aquifers. Recent advances in applied geophysical techniques have highlighted the potential of methods such as spectral induced polarization (SIP) for monitoring subsurface (bio)geochemical processes non-invasively. SIP's unique ability to trace geochemical changes stems from its capability to detect changes in the charge storage properties at the mineral-fluid interface in porous media. The unique link between geo-electrical and geochemical processes makes SIP a valuable tool for monitoring changes in redox-active minerals such as oxides and reduced minerals such as iron sulfides. Here, we present results from a flow-through column experiment, where we triggered the dissolution of naturally occurring reduced iron (sulfide) phases by infiltrating 0.01 M HCl into aquifer sand collected from the Fuhrberger Feld Aquifer (Hannover, Germany). We integrated our geochemical investigation with geophysical measurements collected over both space and time. We further developed a reactive transport model to simulate coupled chemical and physical processes. Our model, calibrated using observed breakthrough curves, accounted for both the dissolution of iron sulfides and the protonation of residual mineral surfaces under acidic conditions. The incorporation of mineral dissolution as well as protonation of the residual mineral surface at lower pH in our reactive transport model reveled a strong linear correlation between the dissolution front arrival time and the timing of a measured anomaly in the SIP signal. These results underscore the potential of SIP as a powerful tool for subsurface process monitoring and predictive environmental modeling.