Coulombic effects in advection-dominated transport of electrolytes in porous media: Multicomponent ionic dispersion

We study the influence of Coulombic effects on transport of charged species in saturated porous media in advection-dominated flow regimes. We focus on transverse hydrodynamic dispersion and we performed quasi two-dimensional flow-through experiments to investigate transport of dilute electrolyte solutions. The experiments were repeated for two average flow velocities (1.5 and 6 m/day) representing advection-dominated and strongly advection-dominated flow conditions, respectively. Numerical transport simulations have been conducted to quantitatively interpret the experimental results. The adopted modeling approach is based on a multicomponent formulation and on the accurate description of transverse dispersion. The latter entails a non-linear dependence of the transverse dispersion coefficient on the flow velocity as well as a compound-specific dependence on the molecular diffusion of the transported solute. These dependencies hold true at low and also at high flow velocities. Our experimental and modeling results show that Coulombic cross-coupling of dispersive fluxes of charged species in porous media significantly affects the lateral displacement of charged species in flow-through systems. Such effects are remarkable not only in diffusion-dominated but also in advection-dominated flow regimes. Their accurate description requires a multicomponent modeling approach and the recognition of the key role of molecular diffusion for both the pore diffusion and the mechanical dispersion terms of hydrodynamic dispersion.