It is clear from numerous field and laboratory experiments that solute movement is often poorly described by the classical advection-dispersion model. Rather, solute breakthrough curves frequently exhibit earlier arrival and more pronounced tailing than predicted by this model. These observations have spurred the development of conceptual models that specifically include physical nonequilibrium to more accurately depict solute movement. In the simplest version of these models, the waterfilled pore space is partitioned into two domains, a mobile domain, where water is free to move and solute movement is by advection and dispersion, and an immobile domain, where water is stagnant and solute moves only by diffusion.