The GoldSim Contaminant Transport Module provides the capability to model the diffusion of species through fluids, including fluids within porous media.
GoldSim's approach to simulating diffusion works well for saturated porous media, and through use of the Diffusivity Reduction Formula, it also works well for diffusion through a single unsaturated porous medium. However, in complex systems where multiple Solid media are present, it is difficult to simulate the system accurately. To address these cases, Neptune and Company has developed a number of suggested modifications to the way that diffusive inputs are defined in order to properly account for these complex diffusive cases. These modifications are discussed in detail in an excellent document describing modeling complex unsaturated diffusion in GoldSim. Anyone needing to model such processes is encouraged to read this document.