Modeling solute transport in structured soils: a review

Solute transport in structured soils is and has been the focus of a significant research effort. Transport in such systems is often characterized by nonideal phenomena. These phenomena have usually been ascribed to the presence of immobile domains within the porous medium, which results in physical nonequilibrium. The phenomena can also, in some cases, be attributed to rate-limited sorption (i.e., sorption nonequilibrium). These processes are often modeled using a bicontinuum approach. It might be expected, for fieldscale problems, that nonequilibrium may be the result of more than one contributing process. A model that explicitly accounts for multiple sources of nonequilibrium would be preferable for such cases. The various models developed for simulating solute transport under nonequilibrium conditions are reviewed.