Analytical solution for reactive solute transport in porous media with structural heterogeneity

Structurally heterogeneous porous media widely exist in subsurface environments. They commonly give rise to preferential flow that provides a fast path for water and solute transport in the subsurface environment, which causes classic advection-dispersion equations to fail to describe the behaviors of the solute transport in the heterogeneous media. One-dimensional (1-D) double-region models are frequently used to simulate solute transport in heterogeneous media by adopting a mass transfer coefficient to represent the interaction between two regions while neglecting transverse dispersions. A two-dimensional (2-D) mathematic model describing solute transport in the layered heterogeneous media where water in the two regions is mobile was presented. The analytical solutions were derived using the Green's function method . The solution was tested using a finite-element numerical solution built with COMSOL Multiphysics. The solution was applied to laboratory experiments and was compared to the 1-D model as well. The results indicated that the mass interaction between the two regions induced from the heterogeneity of the porous media was determined by the transverse dispersion across the interface. When solutes pass through a structurally heterogeneous medium, part of the solutes in fast flow zones will migrate into slow flow zones in the beginning, and this mass interactive pattern will later become inverse. The magnitude of the mass interaction was determined by the heterogeneity of the porous media. The 2-D model matched the observed breakthrough curves better than the 1-D model.