SIMULTANEOUS TRANSPORT OF WATER AND REACTING SOLUTES THROUGH MULTILAYERED SOILS UNDER TRANSIENT UNSATURATED FLOW CONDITIONS

A one-dimensional unsteady-state numerical model for the simultaneous movement of water and multisolutes through multilayered soil systems under unsaturated flow conditions is presented. The multilayered system is represented as a series of homogeneous layers and a two-step implicit finite-difference method was used to solve the resulting set of flow equations. The solute equation was solved by the Crank-Nicolson method. For illustrative purposes one of the multisolute systems considered was an ammoniacal agricultural fertilization problem in which the sequential chemical reaction of ammonium going to nitrite, going to nitrate was described by first-order kinetics with linear adsorption of ammonium. In addition, a nonlinear adosption isotherm was used to illustrate the effects of single solute adsorption in a multilayer sequence. Of considerable interest to those involved in the disposal of wastes on land, is the effect of the layering sequence (coarse-fine-coarse vs. fine-coarse-fine) on the final outlet concentration. The pressure based model considers both infiltration, redistribution and evaporation. Cubic spline functions are used to define the soil hydraulic properties. © 1979.