Modeling contaminant migration with linear sorption in strongly heterogeneous media

A triple-porosity model is presented to evaluate transport behavior in porous media with a structure comprising a spectrum of pore sizes, represented discretely as macro-, meso-, and micropores. Characterizations are completed to provide adequate semianalytical solutions for the validation of codes representing discrete distributions of pore geometry and to adequately describe extended tailing and multicomponent solute front breakthroughs apparent in field and laboratory data. Semianalytical solutions are derived for a one dimensional flow geometry by using Laplace transforms under the assumption that solute transport in the two interactive mobile-transport regions (i.e., macro- and mesopores) is affected by exchange with immobile solutes in the micropore region. Sensitivity analyses are conducted to identify the propensity for extensive tailing in the breakthrough response, over single-porosity approaches, and the development of multiple breakthrough fronts with reverse diffusion. Both behaviors result from the strongly heterogeneous nature of the transport processes, accommodated in the multiporosity model, and are well suited to the representation of "real" porous and porous-fractured disordered media.