Yucca Mountain Project

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Unsaturated-Zone Hydrology

image of Infiltration experiment near Fortymile Wash, Nevada The soil and water in the unsaturated-zone (above the water table) contain both air and water in the pores. The study of the unsaturated zone at Yucca Mountain has presented numerous technical challenges to USGS scientists. One of the largest challenges was to design and construct equipment for long-term monitoring, testing, and sampling in the corrosive high humidity of the unsaturated zone. Investigations included:

Characterizing gas flow and transport through fractured and faulted volcanic rocks
Estimating infiltration rates in a desert environment
Sampling of minerals, gas, and water to estimate travel times and sample ages.

Infiltration Studies

Infiltration studies at Yucca Mountain quantify the movement and distribution of precipitation from the surface and recharge to the ground-water system. The infiltration rate and distribution control, in part, the movement of water down to the proposed repository level, and potentially, radionuclides from the repository to the underlying water table.

Surface-Based Monitoring Program

Five unsaturated-zone boreholes were instrumented with sensors that provided data on the distribution and fluctuations of pressure, temperature, and water vapor associated with natural and human-induced phenomena.

image of Southern entrance to the underground Exploratory Studies Facility at Yucca Mountain, Nevada, where many of the unsaturated-zone studies were conducted. Air-Permeability Program

Air-permeability testing measured the ability of the unsaturated-zone rocks to conduct air and water. The flow of air and water in the unsaturated zone will control the distribution of infiltrating water, and the transport of gas and liquids at the repository.

Map of Major Fault Locations

Major faults bound and intersect Yucca Mountain and the proposed repository area. A comprehensive understanding of the influence of faults in the movement of gas and water through Yucca Mountain is needed. The faults may function as fast pathways from the surface to depths below the proposed repository and(or) as no-flow boundaries.

Matrix Properties

Because the fractures at Yucca Mountain occupy a small percentage of the rock volume, the tuff matrix may control, in part, the flow and distribution of gas and liquid in Yucca Mountain. Matrix characteristics of the welded and nonwelded tuff were studied in the field and laboratory.

image is a schematic overlay of the Exploratory Studies Facilitiy's (ESF) locaton at the Yucca Mountain site.

Image from U. S. Department of Energy - Office of Civilian Radioactive Waste Management.

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