Simultaneous Transport of Water and Adsorbed Solutes Through Soil Under Transient Flow Conditions

Abstract

ABSTRACT ASET of differential equations for water and interacting and noninter-acting solute transport were solved si-multaneously for transient and steady state soil water conditions using a finite difference scheme. The solutions used independently measured soil and soil-solute adsorption-desorption character-istics to describe the movement of a solute in a soil profile. Numerical disper-sion in the finite difference solution of the solute transport equation was consi-dered and a correction included in the solution. Experimental results from a laboratory study were used to test the numerical solution's ability to describe the movement and distribution of a her-bicide in a soil profile with time. A nat-ural field problem involving infiltration and evaporation was simulated and dis-cussed. The agreement between labora-tory and calculated water and herbicide distributions was good. The simultaneous transfer of water and solutes through a soil under natural field conditions is generally a transient process. The direction and rate at which each of these phases move are depen-dent upon many factors, some of which are the phydical characteristics of the soil, climatic conditions, plant and mi-crobial activity, and interactions be-tween the solute and soil matrix. An un-derstanding of how these processes in-fluence the transfer of each phase is es-sential in the development of agricultur-al management practices for minimum contamination to soil and/or water sup-plies. Also, the efficient use of irrigation water and fertilizers can best be achieved through a quantitative under-standing of these processes. Numerous steady state miscible dis-placement experiments involving water saturated porous materials and solutes which do not interact with the solid matrix have been conducted (Rifai et al. 195b, Day and Forsythe 1959, Nielsen and Biggar 1962, and Rose and Passioura 1971). Similar studies have been carried out using solutes that were absorbed by the solid matrix (Biggar and Nielsen 1963, Kay and Elrick 1967, and Huggenberger et al. 1972). Although of interest, none of these studies describe the more general case in which water and solutes are displaced simultaneously as a transient phenomena. Numerical techniques to simulate the transient behavior of water and solutes in a soil (noninteracting) have been pre-sented by Bresler and Hanks (1969), Bresler (1973) and Kirda et al. (1974). The numerical approach by Bresler (1973) is of particular interest in that numerical dispersion associated with the finite difference solution of the solute equation was considered and a correc-tion made for this phenomena. To sim-plify the solution procedures of the above cases, the solute was assumed to not interact with the soil matrix. The mathematical model (plate theory) de-scribed by Dutt, et al. (1972) is one of the few systems capable of describing the transport of an adsorbed solute in the presence of transient water move-ment.