Solute Transfer Through Columns of Glass Beads

Miscible displacement experiments were conducted in a 30-cm-long column filled with glass beads that had diameters in the range of 74–125 µm. Breakthrough curves were determined at various leaching rates for saturated or unsaturated conditions. The breakthrough curves of the unsaturated experiments showed early breakthrough and tailing. These could be described with the classical dispersion equation, provided dispersion coefficients were used, which were about 20 times larger than for saturated columns leached at comparable pore water velocities. A better description of these breakthrough curves was obtained with a model accounting for mobile and immobile water. With this model the dispersion coefficients, as obtained from the saturated experiments, could be used directly. The immobile water content was found to increase linearly with the total water content, while the transfer coefficient for solute transport into the immobile water increased proportionally with the pore water velocity. It is shown, based on the mobile and immobile water and solute transport model, that breakthrough curves of long unsaturated columns should exhibit no early breakthrough and tailing but that the presence of immobile water will result in predictably larger dispersion coefficients than those expected on the basis of saturated flow experiments alone.

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