Macroporal heterogeneity influence on solute transport process in porous media: experiments on model soil and modeling with Lattice-Boltzmann method

Flow and solute transport modeling in porous media is an important issue for the monitoring of pollutant contaminations in soils or retention ponds. It is often necessary to take into account macroporal type heterogeneities. For this purpose, several numerical models use double or multi permeability concepts in order to characterize different permeabilities of a porous medium. However classical models seem to underestimate the impact of the preferential path induced by a macropore. This impact has to be extended in the surrounding porous matrix for the good fitting of experimental data. This study proposes to understand hydraulic and mass exchange mechanisms between the macropore and the porous matrix, by using a model porous media made of glass beads, and by using the lattice-Boltzmann method for flow simulations. First results show the influence of flow rate on the apparition of an inflection on breakthrough curves. Results show that molecular diffusion, higher in this case, causes higher mass transfer from the macropore to the porous matrix. Moreover, simulations show that the preferential flow is extended to the porous matrix in a zone having the same dimension than the macropore. These results emphasize that the macropore influence has to be extented in the porous matrix for flow and the diffusive exchange zone depends on the molecular diffusion coefficient of the tracer.