3-DIMENSIONAL ANALYSIS OF VARIABLY-SATURATED FLOW AND SOLUTE TRANSPORT IN DISCRETELY-FRACTURED POROUS-MEDIA

A discrete fracture, saturated-unsaturated numerical model is develope d where the porous matrix is represented in three dimensions and fract ures are represented by two-dimensional planes. This allows a fully th ree-dimensional description of the fracture network connectivity, Solu te advection and diffusion in the porous matrix are also directly acco unted for, The variably-saturated flow equation is discretized in spac e using a control volume finite-element technique which ensures fluid conservation both locally and globally. Because the relative permeabil ity and saturation curves for fractures may be highly nonlinear, and i n strong contrast to those of the matrix, the robust Newton-Raphson it eration method is implemented according to the efficient procedure of Kropinski (1990) and Forsyth and Simpson (1991) to solve the variably- saturated flow equation. Upstream weighting of the relative permeabili ties is used to yield a monotone solution that lies in the physical ra nge and adaptive time stepping further enhances the efficiency of the solution process, A time-marching Galerkin finite-element technique is used to discretize the solute transport equation. Although the method ology is developed in a finite-element framework, a finite-difference discretization for both groundwater flow and solute transport can be m imicked through a manipulation of the influence coefficient technique. The use of an ILU-preconditioned ORTHOMIN solver permits the fast sol ution of matrix equations having tens to hundreds of thousands of unkn owns. Verification examples are presented along with illustrative prob lems that demonstrate the complexity of variably-saturated flow and so lute transport in fractured systems.