Immobile water during solute transport in unsaturated sand columns

Solute transport experiments were carried out to investigate the presence of immobile water in an unsaturated fine sand under different flow regimes. Both steady and unsteady flow conditions were imposed, but the average pore water velocity and water content were the same in each. Evidence for the presence of an immobile water fraction was sought by fitting the concentration distributions using models both with and without an explicit term accounting for solute exchange with immobile water. Strong evidence was found for the presence of an immobile water fraction affecting the solute concentration distribution under steady flow conditions but not for unsteady flow. These apparently conflicting results are explained in terms of different water flow patterns arising from the two flow regimes.

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