Measurement and simulation of miscible displacement in soils and the prediction of dispersion coefficients

Abstract

Experimental and numerical simulation studies of solute transport processes in porous media were conducted to gain increased understanding of the phenomena that occur as solutes and water move through porous media. A numerical simulation model was developed to efficiently simulate the movement of conserved ions during one-dimensional vertical flow in a porous medium. The water flow equations were solved by an implicit finite difference technique, while the method of characteristics was used to solve the dispersion equations and eliminate numerical dispersion. The accuracy of this model was verified by comparing its simulated results with those of other simulations, laboratory and field studies, and analytical solutions from the literature. The longitudinal dispersion coefficient was investigated as a function of velocities under both saturated and unsaturated flow conditions for three different soil textures in the laboratory. It was found that dispersion coefficients increased with pore velocity in all cases but that there was no correlation with texture. It was concluded that average media properties or flow conditions are not good indicators of the magnitudes of dispersion coefficients. A numerical model was developed to predict the magnitudes of dispersion coefficients as functions of soil physical and hydrologic properties and solution velocities. A capillary bundle model was used, and the bundle hydraulic properties were determined from the hydraulic conductivity function.