Lesson 2: Reading Lesson

Yucca Mountain’s Natural Environment

View of the Yucca Mountain crest looking north. Inset photo shows a branched desert yucca, known as a Joshua tree.

A geologic repository is a system used for the permanent underground disposal of highly radioactive nuclear waste. A repository must keep this waste away from people and the environment for tens of thousands of years.

The nuclear waste will be in the form of solid metals, ceramics, and glass. After it is placed deep underground, and as long as it stays in solid form, it will not be harmful because layers of rock and soil will shield its radiation. However, if enough water comes in contact with the waste, over time it could break it down into microscopic radioactive particles (called radionuclides) and then carry the particles into the environment. Therefore, a repository must keep the waste as dry as possible for as long as possible.

The deep water tables and closed ground water systems of the Western United States are relatively rare in the world. At Yucca Mountain, the natural system allows for a repository to contain radioactive waste well above the water table (approximately 300 meters).

A repository cannot be built just anywhere. The natural environment is extremely important to repository safety. Features such as the area’s climate, the type of rock, and the depth of the water table will affect how long the waste will remain isolated.

To determine if Yucca Mountain is a suitable site for a repository, hundreds of scientists in many disciplines have performed thousands of scientific studies on the area’s natural environment.

These studies have focused on the mountain’s geology and hydrology as well as other physical aspects and processes that could affect a repository’s safety.

Yucca Mountain’s Geology

Western United States map showing the areas in the Basin and Range Province and areas in the Great Basin.

Yucca Mountain has changed little over the last several million years. There is little water present to erode and reshape the landscape. For millions of years, Yucca Mountain has remained the prominent set of ridges that we see today.

Yucca Mountain is within a geologic area called the Basin and Range Province. This area encompasses nearly all of Nevada and parts of Utah, Idaho, Oregon, and California, and extends into northern Mexico. The Basin and Range Province is characterized by mountain ranges separated by basins containing thick deposits of sand and gravel eroded from the surrounding mountain ranges.

The northern part of the Basin and Range is also referred to as the Great Basin. This is a physiographic area that contains most of Nevada and parts of Utah, Idaho, Oregon, and California. In the Great Basin, surface water and groundwater are trapped and isolated within closed basins. This means water (surface and underground) cannot escape and move to a river and travel to the ocean (Pacific). In the case of Yucca Mountain, the Death Valley Ground Water Province is the closed portion of the Great Basin in which Yucca Mountain is located.

The mountains and valleys surrounding Yucca Mountain formed over the past 10 million years from faults moving on one or both sides of the ranges. Rocks and sedimentary deposits surrounding Yucca Mountain range in age from more than 570 million years old in the mountains to as young as 10,000 years old in the valleys.

Yucca Mountain itself consists of a series of ridges made up of layers of rock formed by successive eruptions of gases and ash (small particles of molten rock) from nearby volcanoes between approximately 11 and 14 million years ago. When these volcanoes erupted, they spewed out hot ash, which eventually settled on the ground. Each time a volcano would erupt, it would form a layer of ash. With pressure and their residual heat, these layers of ash hardened into a type of rock called volcanic tuff.

Aerial photo of Yucca Mountain looking northwest. The equipment area known as the subdock is shown in the foreground. Yucca Mountain extends left to right across the middle of the photo. Crater Flat, Bare Mountain, and Death Valley appear in the background.

Yucca Mountain consists of over 1800 meters of layer upon layer of volcanic rock called “tuff”, which is made of compressed and fused ash. The tuff is either welded or nonwelded, depending on the temperature of the ash when it was deposited. If the temperature was high enough, the ash compressed and fused together, producing layers of welded tuff — a hard, glass-like rock with very little open pore space. Other times, the erupted material compacted and consolidated at lower temperatures, producing layers of nonwelded tuff — with less density and larger pore spaces. Sometimes ash fell in thinner, cooler layers and are then simply called ash layers.

The thick layers of volcanic rock will serve to protect and isolate the highly radioactive material from the groundwater below Yucca Mountain.

Scientists have also studied the geologic features in the Yucca Mountain area to determine the likelihood of future volcanic eruptions and earthquakes occuring in the area and their size and magnitude. Experts have used these studies to assess how such events could affect the repository’s safety.

The Possibility of Future Volcanoes at Yucca Mountain

Geologic samples of Yucca Mountain’s tuff are stored on shelves at the Project’s Sample Management Facility.

Some of the world’s foremost experts in such fields as volcanology, geophysics, and geochemistry have studied the possibility of future volcanic activity disrupting a repository at Yucca Mountain. They began with studying the location, age, and volume of past volcanic activity in the Yucca Mountain area. Using the data from these studies, along with information from studies of both modern and ancient volcanoes throughout the world, the scientists evaluated the likelihood of magma entering the potential repository area. Their analysis also evaluated the possibility of magma intersecting the repository and erupting up through the mountain’s surface.

One of several small cinder cones located in Crater Flat, several miles west of Yucca Mountain. Bare Mountain is shown behind the cone.

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Based on these studies, experts estimate that the probability of a volcanic event disrupting the proposed repository is very low - about one chance in 60 million per year (1.7 X 10^-8 per year), or about 1 chance in 6,000 in 10,000 years. Put another way, it means there is about a 99.9999984 percent chance per year that a volcanic event will not disrupt the repository.

Although the likelihood of a volcano in the vicinity of Yucca Mountain is very low, scientists have performed analyses to understand how such an event could affect a repository and the impacts on people’s safety and on the environment.

Earthquakes at Yucca Mountain

Scientists cannot predict the exact location and timing of future earthquakes. However, by studying an area’s seismic history, and surface and underground geology, scientists can estimate the frequency and size of future earthquakes, the potential intensity of ground movement, and the possible effect from earthquakes on the area’s geologic features and man-made structures.

The chart depicts the characteristic drop off in earthquake intensity with depth of rock cover.

Top photos show precariously perched boulders in the Yucca Mountain area. These boulders show that large earthquakes have not occurred in the area in recent times.

Large earthquakes occurred in the Yucca Mountain area many millions of years ago and can still occur in regions adjacent to Yucca Mountain. The Yucca Mountain area has also experienced less severe earthquake events (shaking) in the last few million years. However, the shaking was not intense enough to dislodge tenuous boulders that have remained in position for thousands of years (see photo on previous page). Studies of mineral deposits on the rock tell scientists approximately how long the rocks have been in their current positions.

Although Yucca Mountain has apparently not experienced many great earthquakes in the past several thousands of years, experts cannot rule out that they could not happen in the future. However, by locating the repository deep underground, the waste would be protected from the impacts of moderate earthquakes. Damaging ground movement is the most intense at the earth’s surface and decreases with the depth underground. Severe earthquakes are unlikely, but their potential affects on the repository are being evaluated.

Yucca Mountain’s Hydrology

Hydrology is the study of the chemical properties, distribution, and circulation of water on the surface of the land, in the soil, and in the underlying rocks.

When hydrologists study the water movement at Yucca Mountain, they are trying to understand the complex plumbing of the mountain. They have conducted studies to determine how much water penetrates the surface and how it moves through hundreds of meters of rock to the water table. They have also studied where the water goes after it reaches the water table and how much of it moves to areas where people live and use water from the groundwater system.

Project scientist collects water samples from Alcove 1 hydrology test inside the Exploratory Studies Facility at Yucca Mountain.

In studying Yucca Mountain’s hydrology, experts have found that very little water penetrates the surface of the mountain. This is because of the area’s dry climate, hilly terrain, native vegetation, and lack of surface soils. Since Yucca Mountain is in the desert southwest, it receives very little rainfall—less than 19 centimeters of precipitation on average per year. Most of this precipitation runs off the mountain or evaporates. Only a very small percentage of the precipitation infiltrates or moves into the underground.

The roots of plants take up most of the water that does penetrate into the soil or ground. Any remaining water moves very slowly as a thin film or in the form of tiny droplets through an area of rock called the unsaturated zone. The unsaturated zone extends from the top of Yucca Mountain down about 600 meters to the water table. This area is called the “unsaturated zone,” because the pores and fractures in the rock are not completely filled with water.

Diagram of Yucca Mountain illustrating the position of the repository below ground surface and above the water table.

The unsaturated zone consists of layer upon layer of volcanic tuff. Some of these layers contain rocks that are non-welded and relatively porous and therefore absorb water like a sponge. Other layers are welded with lower porosity, therefore water does not penetrate through the pore space.

Most water droplets move in Yucca Mountain through a system of fractures in the rock. These fractures provide pathways for water movement and limit the amount of water available to enter the repository. A phenomenon called “capillary force” causes the droplets to stay in the rock’s pores or fractures rather than move into large openings, such as tunnels.

By studying the rock in the unsaturated zone, scientists have found that on average, water moves a little over a centimeter per year through the rock. Only about 5 percent of any precipitation on the surface would ever reach the repository depth 300 meters below the surface.

Although water moves slowly through Yucca Mountain, there is a significant but small fraction that moves quite rapidly though fractures. It is a small fraction, but significant in that it can lead to transport of radioactivity after engineered systems begin to experience failures.

Over time, water may eventually seep into the repository and may become available to degrade the waste packages. A thick layer of unsaturated rock below the repository level would then slow and restrict the movement of any water-borne radioactive particles from the repository to the water table.

One of the tuff layers in the unsaturated zone below the repository contains zeolites. Zeolites are minerals that can adsorb radioactive particles onto their crystal structure. If water were to eventually carry any radioactive particles from the repository, this layer of tuff may act as a natural filter by attracting and holding some particles thus keeping them from moving down to the water table. Not all downward moving water will pass through zeolites, however, and not all radionuclides would be held by zeolites.

The Saturated Zone

The area of rock that begins at the water table about 300 meters below the repository is called the saturated zone. It is called this because the rock’s pores and fractures are filled with water.

The water in the saturated zone moves slowly through the rock, largely through fractures. On average it takes thousands of years for any water to move from under Yucca Mountain to an area where someone would be likely to access it through a well. Therefore, the rock in the saturated zone could also work to slow and restrict the movement of any water-borne radioactive particles. As in the case of the unsaturated zone, however, it is not the average groundwater velocity that matters, but the fraction that may arrive early and carry radionuclides. Statistics and probabilities are used to evaluate safety impacts for different flow rates.

Closed Water Basin

Over the last 30 million years, the region surrounding Yucca Mountain has stretched, causing basins to drop down relative to the mountain ranges. Yucca Mountain is located in such a basin, which is an area completely surrounded by higher land. With the mountainous terrain enclosing the basin, the groundwater drains inward toward the lower elevations. In other words, water largely recharges to the saturated zone in the higher elevations and moves toward discharge points (springs, dry lakes, or water wells) in lower elevations. Groundwater does not move uphill beyond the boundaries of the basin or against the direction of flow as depicted on the map below.

Yucca Mountain is located within the Death Valley Ground Water Province. The arrows show the direction of ground water movement towards Amargosa Valley.

This means that the groundwater beneath Yucca Mountain is isolated within the Death Valley Ground Water Province and does not move into other groundwater systems (such as the aquifer under Las Vegas). In addition, the groundwater in this basin does not flow into any rivers or oceans. This province will continue to isolate the groundwater for millions of years.

A few thousand people live within the Death Valley Ground Water Province. The groundwater beneath Yucca Mountain moves slowly in the direction of Amargosa Valley— which has a small farming community about 40 kilometers south of Yucca Mountain. The groundwater either evaporates at the valley’s south end where there is a discharge point, moves to Death Valley along underground flow paths, or is recovered and consumed by residents in Amargosa Valley.

A Suitable Geologic Setting for a Repository

With its desert climate, deep water table, and thick layers of stable rock, Yucca Mountain provides a suitable geologic setting for a repository. In the following lesson in this unit, entitled, “How Will a Repository Work,” you will learn how experts are designing a repository that takes advantage of Yucca Mountain’s natural attributes to isolate nuclear waste for tens of thousands of years.

There is a thickness of about 1,829 meters of volcanic rock at Yucca Mountain.