The mobile–immobile (MIM) conceptual model attributes physical nonequilibrium to the presence of practically immobile water held in pore spaces and mass transfer between mobile, flowing water and the immobile water. The goal of this study was to explore the relationships between MIM transport parameters and state variables as a function of flow and porous medium characteristics. The investigation involved conducting a series of solute tracer experiments under steady-state and transient flow regimes across a range of pore velocities and water contents and in two unstructured porous media. Dispersivities obtained under transient conditions were higher than steady-state dispersivities only for the more uniform porous medium. There was no significant difference between scaled mobile water contents obtained for the transient experiments and those obtained for the steady-state experiments for either porous medium. Mass transfer coefficients for the transient experiments were consistently higher than those estimated for the steady-state experiments for the more uniform porous medium. These results indicate that the flow regime (transient vs. steady-state conditions) may be significant for physical nonequilibrium transport but depends on the porous medium characteristics. The results imply that conceptual models of physical nonequilibrium transport may need to be refined and, in some cases, models need to be developed to relate transport parameters to flow conditions.

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