Effect of soil textural characteristics on longitudinal dispersion in saturated porous media

Abstract

This study focuses on the effects of soil textural heterogeneity on longitudinal dispersion under saturation conditions. A series of solute transport experiments were carried out using saturated soil columns packed with two filter sands and two mixtures of these sands, having d(50) values of 95, 324, 402, and 480 mu m, subjected to four different steady flow rates. Values of the dispersion coefficient (D) were estimated from observed in-situ distributions of calcium chloride, injected as a short nonreactive tracer pulse, at four different locations (11, 18, 25, 36 cm). Analyses of the observed distributions in terms of the standard advection-dispersion equation (ADE) showed that D increased nonlinearly with travel distance and higher Peclet numbers+. The dispersion coefficient of sand sample S1 with its largest average particle size (d(50)) was more affected by the average pore-water velocity than sample S4 having the smallest d(50). Results revealed that for a constant velocity, D values of sample S1 were much higher than those of sample S4, which had the smallest d(50). A correlation matrix of parameters controlling the dispersion coefficient showed a relatively strong positive relationship between D and the Peclet number. In contrast, almost no correlation was evident between D and porosity as well as grain size. The results obtained with the four sandy matrices were consistent and proved that the dispersion coefficient depends mainly on the particle size.