Investigating solute transport and reaction using a mechanistically coupled geochemical and geophysical modeling approach: application to calcite dissolution

This study investigates the promising use of geoelectrical methods for monitoring groundwater contamination and mineral reactivity. Geoelectrical methods are mostly used as qualitative detection tools for static subsurface characterization. However, we show that geoelectrical signals are complementary tools for the quantitative characterization of chemical species transport and reaction in the porous matrix by developing a coupled mechanistic model. We examine calcite dissolution as an effective proof-of-concept since calcite dissolution is a chemical process occurring ubiquitously in the Earth’s subsurface. Our investigation focuses on the impact of the reactive zone’s position, extent, and intensity of geoelectrical signals under various inlet conditions generating contrasted dissolution regimes. We conducted five experiments on flow-through columns filled with calcite grains and equipped with geoelectrical monitoring on sequential channels along the column. Three experiments explore the self-potential method and two others monitor the complex electrical conductivity from the spectral induced polarization method. pH in the column at two locations is also monitored. Additionally, the outlet fluid is sampled to monitor major ion concentrations, pH, and electrical conductivity. Thus, the study presents a unique dataset that combines traditional physicochemical monitoring of water samples with geoelectrical acquisition on multiple channels along the column. The quantitative analysis of the geoelectrical signals is achieved through their prediction using a 1D numerical workflow that combines reactive transport simulation with petrophysical modeling based on the evolution of the pore space and the geochemistry. Reactive transport is simulated by developing a CrunchFlow code, which well-reproduces the outlet pore water concentrations and pH. The comparison of the predicted geoelectrical signals with the experimental data clearly shows the characterization of the spatial and temporal distributions of the reaction rates, whatever the reaction rate and the reactive zone extent. Self-potential monitoring allows the spatialization of the reactive zone from the electrodiffusive coupling and enables the detection of a low dissolution regime contrary to what is observed from the outlet water electrical conductivity monitoring. The complex electrical conductivity shows significant variations during the intense dissolution regime. Water electrical conductivity, porosity, and the real electrical conductivity of the sample are successfully retrieved from petrophysical computation. This innovative study in which geophysical and geochemical methods are intrinsically intertwined paves the way to broader and more interdisciplinary studies of solute transport and reactivity in porous media and in a more general perspective, the presented methodology applies to contaminant transport.