CHARACTERIZING UNSATURATED HYDRAULIC PROPERTIES OF FRACTURES IN WELDED VOLCANIC TUFFS AT YUCCA MOUNTAIN, NEVADA

Hydraulic properties of fractures are most often assumed, or inferred from in situ measurements of air permeability. To provide a data set of measured hydraulic properties of fracture networks in a welded tuff, small benches were carved into the walls of tunnels in the underground Exploratory Studies Facility in the potential repository horizon (the Topopah Spring Tuff) at Yucca Mountain, Nevada. A tension infiltrometer was used to measure the bulk hydraulic conductivity of the fractured tuff at tensions between 0 and approximately 30 centimeters. The saturated hydraulic conductivity tended to be lower than in situ air permeability measured in the same rock unit. The unsaturated hydraulic conductivity stayed relatively high under tensions less than 15 centimeters and was reduced by almost two orders of magnitude under tensions greater than 30 centimeters. The decline in hydraulic conductivity was not smooth but occurred in a stair step manner. A fracture flow simulator (VSFRAC) was used to develop a variety of unsaturated hydraulic conductivity curves for simulating the measured data. The fracture apertures simulated were assumed to have a known mean fracture aperture, a known coefficient of variability of the fracture aperture, and a spherical variogram with a known correlation length for the variable fracture aperture. These variables were selectively changed to determine if one set of fracture properties could describe the measured unsaturated hydraulic conductivities. No single fracture property could reproduce the measured data, although a combination of progressively smaller fractures from a mean aperture of 150 micrometers to less than 25 micrometers could be combined to reproduce the data. This analysis suggests that a composite of fracture properties may be necessary to simulate a fracture network that would conduct water under varying tensions.