SOLUTION OF MULTISPECIES TRANSPORT IN THE UNSATURATED ZONE USING A MOVING POINT METHOD

Strongly advective contaminant transport can occur in highly permeable , unsaturated media such as gravel, fractured clay and soil-containing macropores. Standard numerical solution techniques are both highly in accurate and unstable under such fast flow regimes. The moving point t echnique has been applied to contaminant transport in groundwater and shown to provide an accurate solution to the convection-dispersion equ ation for advection-dominated transport. In this paper, the technique has been adapted for the simulation of transport of multiple species t hrough sorbing, dual porosity, unsaturated media. In order to model on e-dimensional vertical transport, moving points are translated at the effective retarded velocities, simulating convection and retardation o f each species. The contaminant concentrations in the immobile region and the changes in mobile concentrations due to diffusion, dispersion and chemical reactions are then calculated by finite difference method s on a fixed grid. The moving point model has been validated against a vailable analytical solutions for solute transport. Comparisons have b een made with standard finite difference solutions. The moving point m ethod is shown to be considerably more accurate than the finite differ ence solutions for convection-dominated flow.