Fluid flow and solute transport processes in unsaturated heterogeneous soils: Preliminary numerical experiments

Abstract

Preliminary numerical experiments are conducted to investigate scale-dependent macrodispersivity, relative to solute transport, in unsaturated heterogeneous porous media. Synthetic heterogeneous soil columns are generated with the turning-bands approach. Fluid flow and solute transport are simulated with a three-dimensional finite-element model. Our numerical experiments confirm a three-phase development of macrodispersivity as a plume of contaminant moves through a heterogeneous soil. The initial phase grows linearly, then becomes nonlinear, and eventually reaches an asymptotic constant value. We find that the development of macrodispersivity in an unsaturated heterogeneous soil column can be predicted successfully by Dagan's formulas which were originally developed for heterogeneous aquifers. For the study of contaminant transport in unsaturated soils, the travel distance one would normally deal with is much shorter than that required to reach asymptotic macrodispersivity; thus, the two pre-asymptotic phases should be the principal concern in modeling contaminant transport in unsaturated soils. The first phase of the macrodispersivity development corresponds to what is normally referred to as the advection period during which no significant lateral mixing takes place. The linear system model for contaminant transport, which uses the travel time probability density function as the model's impulse response function, is valid only for the first phase and should therefore be applied only to the simulation of near-field contaminant transport within the advection period.