Impact of variable density on electrokinetic transport and mixing in porous media

Electrokinetic (EK) techniques can effectively mobilize amendments and contaminants in porous media and therefore have a great potential for in situ subsurface remediation of a wide variety of organic and inorganic contaminants. However, the fundamental mechanisms governing EK applications in porous media are complex and entail coupling between different flow, transport, electrostatic and geochemical processes. In this study we investigate the interactions between density driven effects and electrokinetic transport in permeable porous media and we compare the observed dynamics with the outcomes of analogous scenarios in which advection and dispersion are the only transport mechanisms. We perform a series of multidimensional experiments and we demonstrate the strong interplay between Coulombic interactions among charged species in pore water and density-driven effects during electrokinetics. Our study illuminates non-trivial interactions between the pore water composition and the macroscopic behavior of variable density dye tracer plumes, showing different sinking and floating patterns, distinct plume shapes and spreading, as well as different extent of mixing depending on the chemical composition of the pore water. A numerical model, based on the Nernst-Planck-Poisson formulation and on the coupling with the geochemical code PHREEQC, was developed to simulate variable density flow and transport and to update the density of the aqueous solution resulting from mixing and electrostatic interactions between all chemical species in the injected plume and in the surrounding pore water electrolyte. The model allowed us to interpret the experiments and to visualize the impact of small density gradients and Coulombic effects on the macroscopic transport of the dye tracer. The outcomes of this study highlight the importance of density-driven flow also during EK transport, where the electrostatic interactions not only influence the concentration of charged species but can also affect the density and result in complex flow patterns, plume shapes and mixing behavior, radically different from the ones generated during advective-dispersive transport.