WATER FLOW IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN

The unsaturated zone (UZ) at Yucca Mountain is being proposed as the host rock for a geological repository of high-level nuclear waste. Extensive characterization of the UZ has been conducted in over 60 deep boreholes and two large underground tunnels. The UZ rocks consist primarily of volcanic ash flows that vary in their degree of welding. Fractures provide the primary permeability in most units, with average large-scale permeability on the order of 10 ¹² m² (1 darcy). The average percolation flux is about 5 mm/year under current climate, but varies spatially. Only a very small fraction of the 10¹ total number of fractures in the mountain actually carry water; it is not known which fractures actually are conduits for water.

We have developed an active fracture model (AFM) that allows us to estimate the number of fractures that are conduits for flow. The number and spacing of water conduits is extremely important when seepage into drifts or radionuclide transport is being evaluated. Current best estimates suggests that the average spacing between water conduits is on the order of 10¹ meters, based on water saturation, water potential, and geochemical data. This estimate will be verified by further testing and analysis.

In this paper, we discuss the various tests and analysis that have been utilized to characterize the UZ at Yucca Mountain. The different geological, geophysical, geochemical, and hydrological approaches and data sources that have been successful in shedding light on various parameters and processes in the UZ are highlighted. Furthermore, the various methodologies that have been tried and found unsuccessful in characterizing this rock formation are also described.