Distribution of downward flux in unsaturated heterogeneous hydrogeology

Finite-element computer program UNSAT2 used to quantify the horizontal distribution of unsaturated downward flux in porous tuff as a function of hydrogeologic heterogeneities under two different recharge rates. The distribution of downward flux is important because it influences ground-water travel time at a potential geologic repository for high-level radioactive wastes. Hydraulic properties of the Topopah Spring Member of the Paintbrush Tuff Formation at Yucca Mountain, Nevada, were selected for use in the eight simulations that were conducted. All simulations were run essentially to study state (referred to herein as quasi-steady state). The simulations show that (1) heterogeneities in an isotropic porous rock matrix will cause downward flux to be distributed nonuniformly, (2) zones in which saturated matrix hydraulic conductivity is less than the downward flux tend to develop positive pressures that may divert flow into fractures if they exist, (3) one-dimensional analysis of vertical flow in the unsaturated zone is insufficient to ensure that fracture flow does not occur even if the true magnitude of vertical flux is known, and (4) the true spatial distribution of hydraulic conductivity above the regional water table must be determined in order to obtain the true spatial distribution of downward flux.--Modified journal abstract.