A 3-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 m edia. The nonlinear flow equation is solved using the Galerkin finite element technique with the Picard iteration scheme and a continuous ve locity field is obtained by separate application of the Galerkin techn ique 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 s olved with one-step backward particle tracking while the dispersive pa rt is solved by the regular Galerkin finite element technique. Precond itioned conjugate-gradient-like method is used for the iterative solut ion of the systems of linear simultaneous equations to save on compute r memory and execution time. The model is applied to a few situations and the numerical results are compared with observed and analytic valu es. The model is found to work quite well, even near very sharp fronts .