Interplay between Molecular Diffusion and Advection during Solute Transport in Macroporous Media

Solute transport in soils is known to differ from solute transport in homogeneous porous media. Nonequilibrium processes, like those induced by the presence of macropores, can strongly influence the breakthrough of solute in soils. Breakthrough experiments and effective models are often combined to study the physicochemical processes involved in solute transport. However, the complexity of flow pathways and the diversity of possible processes is challenging. In this work, the influence of flow rate and viscosity of the carrying liquid on nonreactive solute transport is investigated under saturated conditions in a macroporous synthetic medium. As expected, solute transport is strongly affected by physical nonequilibrium induced by the preferential flow within the macropore. Breakthrough occurs early, and the shape of the breakthrough curve is influenced both by the flow rate and the coefficient of molecular diffusion of the solute. We show that when the mean residence time of the solute in the macropore is small enough, solute transport in a macroporous column can be considered as isolated within the macropore. The increase of the residence time strongly affects the shape of the breakthrough, and, eventually, a plateau appears during the ascent of the breakthrough curve. We demonstrate experimentally that the existence of this plateau, which is not predicted by classical effective models, is related to the relative importance of molecular diffusion versus advection. Indeed, this plateau can become unobservable if the coefficient of molecular diffusion is reduced through the use of a sufficiently viscous carrying liquid.