SATURATED-ZONE HYDROLOGY AT YUCCA MOUNTAIN

The saturated zone at Yucca Mountain, Nevada, is an important natural barrier to contaminant migration from the proposed high-level radioactive-waste repository, because the most likely pathway for radionuclide transport from the repository is through ground-water flow.

Yucca Mountain is near the center of the Death Valley regional flow system, in which ground water flows primarily through highly transmissive carbonate rocks from recharge areas in the east and northeast to discharge areas in the southwest. At Yucca Mountain, ground-water flow is primarily through fractured and faulted volcanic rocks and through alluvium south of the mountain. These rocks overlie the regional carbonate aquifer in much of the area, but they are hydraulically separated from it by a volcanic confining unit. Recharge at Yucca Mountain occurs as infiltration of precipitation on the mountain, infiltration of intermittent streamflow in Fortymile Wash, and underflow from the north, west, and east. Dishcarge occurs as underflow to the south, which ultimately discharges as evapotranspiration at Alkali Flat and Carson Slough, and possibly as springflow in Death Valley. Some ground water also discharges to pumping wells in Jackass Flats and Amargosa Desert. Ground water flows southeast and south from Yucca Mountain through the volcanic rocks, and then through the alluvium in southern Jackass Flats and Amargosa Desert. Depths to water in the Yucca Mountain area range from 280 to more than 750 meters below land surface. Potentiometric-surface maps indicate a large hydraulic-gradient area to the north, moderate hydraulic-gradient areas to the west and south, and a small hydraulic-gradient area beneath Jackass Flats. Potentiometric data indicate an overall upward gradient from the regional carbonate aquifer, although some downward gradients occur within the volcanic rocks south of Crater Flat.

The volcanic rocks at Yucca Mountain have been described as a sequence of three aquifers and three confining units, but they appear to be hydraulically connected. Transmissivity of the volcanic rocks ranges over several orders of magnitude from about 10-3 to 103 m2/d. Welded tuffs tend to be more permeable than nonwelded tuffs, and the alluvium also is very permeable.