Water chemistry controlled enhancement of electrokinetic transport and reactant delivery in low-permeability zones.

Electrokinetic (EK) transport and remediation represent a promising approach for delivering charged reactants into low-permeability zones (LPZs) in the subsurface, where contaminants often accumulate, and conventional hydraulic-based in situ remediation is limited by preferential flow through surrounding permeable regions. EK transport in physically heterogeneous porous media is governed by the coupled action of pressure-driven flow, electromigration, and electroosmosis, the latter of which depends substantially on pore water ionic composition and ionic strength and can significantly influence plume mixing and reactant delivery. This study aims to enhance permanganate delivery into poorly accessible low-permeability zones by investigating the influence of water chemistry on EK transport in heterogeneous porous media. We conducted a series of electroosmotic and EK transport experiments in multidimensional setups using both homogeneous and heterogeneous porous media, as well as different electrolyte solutions (NaCl and CaCl2) at varying concentrations and constant pH. Measured electroosmotic coefficients (keo) showed that both CaCl2 solutions and higher ionic strengths consistently produced lower values than NaCl or dilute electrolytes, resulting in reduced electroosmotic resistance and enhanced reactant delivery into LPZs. A logarithmic correlation between ionic strength and keo was established and applied in subsequent EK transport simulations, enabling spatially and temporally variable representation of keo in heterogeneous porous media. We show that pore water chemistry exerts a key control on EK-induced permanganate transport in low-permeability inclusions. Furthermore, we propose an approach, combining advection and electrokinetic transport through saline hydraulic flushing and reverse EK pre-treatment, to tune water chemistry to favorable background electrolyte conditions with low electroosmotic resistance, enabling effective reactant delivery into LPZ. Process-based numerical modeling allowed us to reproduce the experimental observations and to elucidate the complex interplay of coupled physical, chemical and electrostatic mechanisms governing EK transport in heterogeneous porous media.