A convective-dispersive-adsorptive flow model for solute transport in soils. I. Model description and some simulations

Abstract

Solute transport predictions are usually based on mathematical (analytical and numerical) solutions of partial differential equations describing convective-dispersive-adsorptive flow and ion-soil interactions. Computer simulations based on a numerical model of solute transport in soil columns were used to illustrate the significance of flow dynamics and adsorption-desorption parameters in determining the shape and position of solute breakthrough curves. Approximating non-linear isotherms of the form S = KCM, where S is adsorption phase concentration, C is solution phase concentration, and K and M are constants, by linear isotherms of the form S = KC + .epsilon., where .epsilon. is the intercept, to facilitate the use of analytical solutions, resulted in significant errors when the retardation factor failed to account for the intercept .epsilon. (when .epsilon. is zero, the retardation factor equals 1 + .beta.K/.theta., where .beta. is the bulk density and .theta. is water content). Using the dynamic form of Freundlich adsorption, the importance of non-linearity in the equilibrium isotherms is demonstrated. Computer simulations were also used to demonstrate the extreme importance of intermittent flow, the simultaneous occurrence of adsorption dynamics, ionic fixation and hysteresis during instantaneous adsorption and desorption in determining the transport of solutes in soils.