A three-dimensional numerical model for water flow and transport of chemically reactive solute through porous media under variably saturated conditions

A three-dimensional numerical model is developed for the simulation of water flow and chemical transport through variably saturated porous media. The nonlinear flow equation is solved using the Galerkin finite element technique with the Picard iteration scheme and a continuous velocity field is obtained by separate application of the Galerkin technique to the flux equation. A two-site adsorption-desorption model with a first-order loss term is used to describe the chemical behavior of the reactive solute. The advective part of the transport equation is solved with one-step backward particle tracking while the dispersive part is solved by the regular Galerkin finite element technique. Preconditioned conjugate-gradient-like method is used for the iterative solution of the systems of linear simultaneous equations to save on computer memory and execution time. The model is applied to a few situations and the numerical results are compared with observed and analytic values. The model is found to work quite well, even near very sharp fronts.