Liquid-release tests in unsaturated fractured welded tuffs: II. Numerical modeling

We have carried out a numerical modeling study in conjunction with the in situ liquid-release experiment described in Salve et al. [J. Hydrol. 257 (2001)]. The experiment site is a highly fractured welded tuff accessed from the Exploratory Studies Facility, an underground laboratory in the unsaturated zone at Yucca Mountain, Nevada. The experiment examines the effect of the rock matrix on liquid flow and transport that occurs primarily through the fracture network. The purpose of the modeling is to aid in experimental design, predict experimental results, and study the physical processes accompanying liquid flow through unsaturated fractured welded tuff. The model uses cubic elements arranged in a regular three-dimensional grid to represent a 24 m3 block of fractured tuff. High-permeability fracture elements located deterministically preserve the connectivity of the fracture network, which is crucial to its ability to conduct fluid. Because element thickness is much greater than fracture aperture, fracture elements are assigned properties of a fracture continuum rather than of an individual fracture. The fracture network is constructed using fracture geometry data taken from a fracture map of the walls and ceiling of the alcove adjacent to the field test site. The network is then refined using the results of air-permeability tests. Model results suggest that it may not be sufficient to conceptualize the fractured tuff as consisting of high-permeability fractures embedded in a low-permeability matrix. The need to include a secondary fracture network (with distinct characteristics from the network of larger mapped fractures) is demonstrated by comparison to the liquid flow observed in the field.