The effect of water content on solute transport in unsaturated porous media

The effect of water content on NaCl transport in unsaturated porous media w as investigated under steady state flow conditions for water contents rangi ng between full saturation and 15% by volume. The experiments were conducte d in a 25 cm column packed with homogeneous sand. Results of the experiment s indicate that solute transport in unsaturated porous media is subject to greater velocity variations and slower solute mixing than one in saturated media. As a result, NaCl breakthrough curves (BTCs) show earlier initial ar rival and greater tailing and variance as the average water content decreas es. These results suggest that transport processes in our experiments have not fully developed to the Fickian regime at lower water contents. Because the classical convection-dispersion equation does not adequately describe t he movement of solutes under the pre-Fickian regime, a mobile-immobile mode l was employed to reproduce the BTCs obtained under unsaturated conditions. In general, the results indicate that at lower water contents the medium h as a greater fraction of immobile water, higher dispersion, and slower mass transfer between the mobile and immobile regions. A power law relationship between dispersion and water content-normalized velocities was found to ex ist for our experiments and other experiments reported in the literature us ing different porous media. Thus we suggest dispersivity is not only a func tion of properties of the media but also of water content.