A NUMERICAL-MODEL FOR TRANSIENT-HYSTERETIC FLOW AND SOLUTE TRANSPORT IN UNSATURATED POROUS-MEDIA

A two-dimensional flow and transport model was developed for simulatin g transient water flow and nonreactive solute transport in heterogeneo us, unsaturated porous media containing air and water. The model is co mposed of a unique combination of robust and accurate numerical algori thms for solving the Richards', Darcy flux, and advection-dispersion e quations. The mixed form of Richards' equation is solved using a finit e-element formulation and a modified Picard iteration scheme. Mass lum ping is employed to improve solution convergence and stability behavio r. The flow algorithm accounts for hysteresis in the pressure head-wat er content relationship. Darcy fluxes are approximated with a Galerkin and Petrov-Galerkin finite-element method developed for random hetero geneous porous media. The transport equation is solved using an Euleri an-Lagrangian method. A multi-step, fourth-order Runge-Kutta, reverse particle tracking technique and a quadratic-linear interpolation schem e are shown to be superior for determining the advective concentration . A Galerkin finite-element method is used for approximating the dispe rsive flux. The unsaturated flow and transport model was applied to a variety of rigorous problems and was found to produce accurate, mass-c onserving solutions when compared to analytical solutions and publishe d numerical results. (C) 1998 Elsevier Science B.V.