Solute dispersion in a variably saturated sand

Abstract

The understanding of solute dispersion in unsaturated porous media is still nascent. This work focuses on the measurement of solute dispersion in porous media at various levels of pore-volume saturation and on the parameterization of longitudinal dispersivity in terms of the saturation ratio and a soil's textural characteristics. The dispersion coefficient was determined in partly saturated columns packed with Toyoura sand or glass beads under steady state flow. A 0.0282 N sodium chloride solution was used as a nonreactive tracer in a series of laboratory experiments in which, for a fixed water content, the average pore-water velocity was varied. The column experiments showed that the dispersion coefficient increases linearly with increasing water velocity for a fixed water content and that the slope of the dispersion coefficient versus water velocity relationship diminishes as the water content is increased. The Peclet number (Pe) was fitted to the experimental tracer data by a nonlinear function Pe = aSrb, in which Sr is the saturation ratio and a and b are textured-dependent parameters. The dispersivity (α, in centimeters) of the test columns' porous media was then estimated in terms of the saturation ratio and the median grain size (d50, in centimeters) by the power law α = d50/Pe = d50/aSrb, which provides a specific functional relationship among a porous medium's dispersion properties, the saturation ratio, and the medium's textural characteristics.