• The geology of the southern great basin
  • The hydrology of the southern great basin
  • The geology and hydrology of the Yucca Mountain site
  • The concept of using the unsaturated zone for geologic disposal
  • The natural and engineered barriers of a repository at Yucca Mountain

Overview of the Meteorology and Climatology of the Yucca Mountain Site:


Present-day climate and past climates in the region have been studied to provide a basis for estimating the range of future climate variability. Temperature and the occurrence, frequency, and duration of precipitation will affect infiltration and rates of water movement through the repository environment. Future wetter or cooler climates afford opportunities for increased infiltration, with corresponding  increased amounts of unsaturated zone flux.


Local and regional meteorological characteristics: Precipitation rates vary on a seasonal basis and are affected by local topographic features; extreme precipitation events generally occur episodically and are typically of short duration. In the southwestern United States, the wettest day typically results in about a quarter to a third of the annual mean precipitation. At a frequency of once every 50 to 100 years, the wettest annual day rivals the mean annual total precipitation in the driest locations. In the Yucca Mountain region, annual total precipitation ranges from 4.5 inches to almost 9 inches, depending on site location; the average annual total precipitation varied considerably from year to year, ranging from

1.5 inches to more than 14 inches.


Mean daytime and nighttime air temperatures in the vicinity of Yucca Mountain range typically from 93°F to 72°F in the summer and from 51°F to 36°F in the winter; the winter season is mild, although there are some periods of below-freezing temperatures. Temperature extremes were reported to range from 5°C in winter to 45°C (41°F and 113°F, respectively) in summer at a National Weather Service station approximately 25 miles southeast of Yucca Mountain near Mercury on the Nevada National Security Site. The elevation at Yucca Mountain, from 3,200 feet to 5,000 feet, is enough to prevent occurrence of the extremely hot temperatures reached at lower elevations in typical southwestern deserts. Surfaces cool efficiently in the dry and cloudless air, and daily temperature ranges can be large. Topography is the primary influence on local wind, with air flow toward higher terrain in the daytime and away from higher terrain at night; local wind patterns also have a strong diurnal cycle of daytime winds from the south and nighttime winds from the north.


Paleoclimatology: Projections of future climate change at Yucca Mountain were based on evaluations of paleoclimate data that span 800,000 years. Evidence of climate change for the past 568,000 years was interpreted from calcite vein deposits in Devils Hole, which is a water-filled fracture zone located approximately 40 miles southeast of Yucca Mountain; in addition, core samples from lake deposits in Owens Lake, a playa located 100 miles west of Yucca Mountain, contain paleoclimate indicators such as pollen, plant remains, and microfossils, which were used to establish a climate progression for the region. Investigators have identified evidence of four distinct climate states: interglacial, intermediate/ monsoon, glacial transition, and glacial. The present-day climate is the interglacial state, whereas hotter summers with increased summer rainfall relative to today characterize the intermediate/ monsoon state. The glacial-transition climate is characterized by cooler and wetter climatic conditions than today. Finally, the glacial climate state is characterized by lower mean annual temperatures and higher mean annual precipitation; relative to today there is also much greater effective moisture, which is defined as precipitation minus evaporation. Both the glacial-transition and the glacial climate states are expected to have much greater effective moisture conditions than today, which leads to higher infiltration rates. The glacial-transition state will be the prevalent future climate state, and interglacial and glacial states will be of shorter durations. Monsoon climate states are the shortest duration, lasting a few hundred to a few thousand years, and occur within the glacial-transition state. Analog sites located in western states were identified as surrogates for estimating mean annual temperature and mean annual precipitation values for each of the future climatic states. Present-day meteorological data for the lower-bound estimates were based on Yucca Mountain, while the upper-bound estimates were developed from a station in the Area 12 Mesa in the northern area of the Nevada National Security Site. Stations in Hobbs, New Mexico, and Nogales, Arizona, were selected to represent the upper-bound conditions for the monsoon climate state; the current dry conditions at Yucca Mountain were used to represent the lower-bound conditions. Spokane, Rosalia, and St. John, three meteorological stations in eastern Washington, were selected for the glacial-transition climate upper-bound conditions. Lower-bound glacial-transition climate conditions were established from meteorological data at stations in Delta, Utah, and Beowawe, Nevada. FIGURE



Overview of the Hydrology of the Yucca Mountain Site:


The hydrology of Yucca Mountain is key to understanding repository performance and safety. Since the initial identification of volcanic tuffs in the unsaturated zone as potential media for repository development,31 understanding of the safety of a repository at Yucca Mountain has relied principally on how the limited amount of water present in the desert climate of the region could move through the unsaturated zone, contact waste  materials, and then be transported to locations where the local population could potentially be exposed to radiation. The underground facility at Yucca Mountain would be located about 650 to 1,650 feet below the surface, at a depth approximately 1,000 feet above the water table. REFER TO FIGURE The deep water table and thick unsaturated zone are due to a combination of low annual precipitation, low infiltration rates, and high rates of evaporation in an arid environment. The water table is in a volcanic tuff aquifer that flows to the south toward Amargosa Valley. Below the tuff aquifer is a regional carbonate aquifer. Confining units separate the aquifers. Features, events, and processes important to the hydrology of Yucca Mountain and radionuclide transport include stratigraphy, fractures, faults, rock properties, and characteristics of faults and fractures; and climate change, water table change, seepage into drifts under ambient conditions and thermal effects on seepage, flow characteristics, perched water characteristics, and colloid characteristics.


Hydrogeologic features of the Yucca Mountain site—unsaturated zone:


The hydrogeologic features and characteristics at Yucca Mountain are related to the volcanic origin of the tuffs that make up the thick unsaturated zone. The cooling of the tuffs influenced their mechanical and hydrologic properties, resulting in heterogeneous layers of fractured volcanic rock characterized by various degrees of welding, faulting, and hydrothermal alteration. FIGURE  The heterogeneity leads to variability in hydraulic parameters, which has important effects on the flow paths. The major geologic units are subdivided into unsaturated zone hydrogeologic units on the basis of properties that are important to flow and transport modeling. The units are the: Tiva Canyon welded; Paintbrush nonwelded; Topopah Spring welded; Calico Hills nonwelded; and Crater Flat undifferentiated. Flow through the unsaturated zone occurs in fractures and the matrix and can be redistributed by lateral flow down-dip along horizons generally defined by bedding planes. The Paintbrush nonwelded unit, which is the unit above the repository level, may, in some cases, laterally divert downward-percolating water. Below the Topopah Spring welded unit, in which the repository would be located, lateral diversion of unsaturated zone waters is associated with low permeability zeolites immediately above and within the Calico Hills nonwelded unit. This has led to development of perched water zones. Subsurface flow occurs in a heterogeneous system of layered, anisotropic, fractured welded and nonwelded volcanic rocks; spatially and temporally variable infiltration pulses move rapidly through the  fractures in the Tiva Canyon welded unit, with little attenuation by the matrix; attenuation effects within the Paintbrush nonwelded unit result in nearly steady-state liquid-water flow below this unit.


Fracture flow is dominant in the Tiva Canyon and Topopah Spring welded units, while matrix flow is dominant in the Paintbrush nonwelded unit; below the Paintbrush nonwelded unit, isolated, transient, and fast-flow paths may exist but are expected to transmit only a small amount of liquid water; and below the repository, low-permeability layers and perched water bodies in the Calico Hills nonwelded unit may channel some flow laterally to faults that can act as conduits for water flow to the water table. There are  also perched water bodies north of the repository, which are at a higher elevation than the repository footprint, along the base of the Topopah Spring welded unit.


Hydrogeologic features of the Yucca Mountain site—saturated zone:


At Yucca Mountain, the hydrogeologic features of the saturated zone are due to a combination of rock type, structural setting, geochemical history, and burial depth. The Topopah Spring tuff dips beneath the water table at several places east and south of the repository, comprising the upper volcanic aquifer. The remainder of the saturated zone beneath the repository is in the Calico Hills formation, and the Crater Flat group, which is called the lower volcanic aquifer. A deeper aquifer also exists beneath the site, referred to as the regional carbonate aquifer. Stratigraphic units are grouped into twenty-seven hydrogeologic units that are classified as either aquifers, having either relatively large permeabilities, or confining units, having relatively small permeabilities. These hydrogeologic units and the major faults control  groundwater flow. The source of most of the groundwater flow in the saturated zone is lateral flow from the west, north, and east. Approximately 10 percent of the total flux through the saturated zone near the site is thought to be from precipitation and surface runoff infiltrating along Fortymile Wash.


A generally southeasterly and southerly flow direction from Yucca Mountain has been defined by hydraulic heads up gradient and down gradient from the repository. Outflow from the region is generally toward the Amargosa Valley; a small amount of water is pumped from the aquifer at wells located in the valley. Groundwater would first flow through a series of welded and nonwelded volcanic tuffs with relatively low specific discharge as it moved away from the repository, and would eventually pass into alluvium. FIGURE The saturated zone is the potential pathway for radionuclide transport to the accessible environment.


Flow through the fractured tuff aquifers is expected to be confined to isolated fracture intervals, whereas the alluvial aquifer, which would be encountered 6 to 12 miles along the down gradient flow path, has dispersed flow through the porous material. FIGURE  The principal hydrogeologic unit at the accessible  environment is saturated alluvium. In the saturated zone, the saturated volcanic tuffs near Yucca Mountain can be treated as an equivalent porous medium; the fracture networks in the tuffaceous rocks likely are connected well enough that hydraulic responses are similar to those observed in porous media; larger-scale hydraulic characteristics of the saturated tuffs are strongly influenced by structural features such as faults; anisotropy in horizontal hydraulic conductivity in the saturated fractured volcanic tuffs results in the direction of greatest conductivity being oriented roughly north-south; data suggest that laboratory measured sorption parameters will tend to overestimate radionuclide transport rates in the tuffs and filtration processes will effectively attenuate a large percentage of small particles over relatively short distances; and at locations south of Yucca Mountain, hydraulic testing in the saturated alluvium has indicated that the alluvium behaves as an unconfined aquifer at shallow depths, and then transitions to a leaky-confined or confined aquifer system beneath the first significant confining layer.


Yucca Mountain is located in the Alkali Flat–Furnace Creek groundwater basin of the Death Valley

regional groundwater flow system. FIGURE The regional groundwater flow system conceptual model  suggests a partially enclosed groundwater basin composed of Quaternary surficial alluvial and valley-fill deposits, Tertiary volcanic tuff rocks, Mesozoic volcaniclastic and sedimentary rocks, and Paleozoic carbonate and clastic rocks that overlie Precambrian granitic and metamorphic basement rocks. Water  recharge is locally in the northern volcanic highlands and moves laterally southward toward Yucca Mountain through the underlying fractured volcanic rock aquifers and then into the valley-fill deposits that occupy the adjacent basins. Discharge occurs ultimately as spring flow or evapotranspiration in the

southern part of the basin. In the northwestern part of the basin, groundwater flow is downward into the underlying regional lower carbonate aquifer. Near Yucca Mountain, however, data indicate that the hydraulic gradients can be upward from the lower carbonate aquifer into the overlying volcanic units. This implies that water could tend to move upward from the carbonate rocks into the overlying volcanic

rocks if they were connected.

Surface water hydrology:


Yucca Mountain is located in the Amargosa River drainage basin. FIGURE Surface flow from Yucca Mountain can extend from local drainages to the Amargosa River and then to Death Valley. The stream channels of the Amargosa River and its tributaries are ephemeral and rarely exhibit streamflow, except in direct response to precipitation, or for short distances where groundwater discharges at springs into the channel system. During infrequent flood events, however, flow occurs throughout the Amargosa River and has filled many square miles of the Death Valley salt pan to depths of 1 foot or more. The Amargosa River flows southward through Oasis Valley, where the flow largely infiltrates into stream deposits and is dissipated by evapotranspiration. The channel is then joined by Beatty Wash and passes through  Amargosa Narrows into the Amargosa Desert. The channel trends southeasterly along the southwestern flank of the Amargosa Desert about 50 miles to Alkali Flat, which is also known as Franklin Lake Playa. There it passes through a bedrock narrows by Eagle Mountain to enter the Lower Amargosa Valley. The

channel continues 40 miles farther south past the southern end of the Black Mountains, turns westward where it is joined by Salt Creek, and then enters   Death Valley. The Amargosa River channel then extends northwesterly 50 miles, terminating at Badwater Basin and the lowest point in Death Valley.



Geochemical Characteristics of the Yucca Mountain Site:


The radionuclides that may become available for transport from a repository  at Yucca Mountain have varying geochemical and retardation characteristics. Several of the radionuclides that are dominant contributors to the total inventory are significantly retarded in the unsaturated zone if there has been significant fracture-matrix exchange by diffusion or advection. Sorption of the radionuclides that diffuse or advect into the matrix, in combination with radioactive decay, prevents or significantly reduces the movement or the rate of movement of those radionuclides from the repository to the accessible environment. FIGURE Concentrations of radionuclides in water moving away from the repository’s emplacement drifts can be limited by three mechanisms: slow dissolution rates of the waste form solids; the solubility of individual radionuclides; and the sorption of the radionuclides onto geologic media. Important geochemical characteristics affecting the ability of the natural barrier beneath the repository to prevent or substantially reduce the rate of movement of radionuclides to the accessible environment include the sorption capacities of the geologic materials along potential flow paths, the solubilities of the  radionuclides present in the disposed wastes, thermally induced geochemical and geomechanical changes or coupled processes expected to occur in the near-field environment, and the chemistry of fluids that come in contact with the waste package and drip shield.


The chemical composition of pore water within the rock units above and at the repository level influences the composition of seepage water that potentially could come into contact with the drip shield and waste packages. The emplaced nuclear wastes generate heat that can affect the near-field environment, which is manifested through coupled thermal, hydrologic, mechanical, and chemical processes. FIGURE  Coupled processes in the repository environment are relevant to repository performance because they have the potential to affect the chemical environment, which in turn affects corrosion of the drip shield or waste package and mobilization of radionuclides. The chemical evolution of waters, gases, and minerals is closely coupled to the thermal-hydrologic processes of boiling, condensation, and drainage. The presence of liquid water, and where it is distributed determines where mineral dissolution and precipitation can occur, and where  direct interaction can occur between matrix pore waters and fracture waters through diffusion. Evaporation will concentrate aqueous species in solution, while mineral precipitation and ion-exchange reactions can deplete the individual chemical components. The spatial extent of above boiling temperatures is related to the thermal-hydrologic behavior in the near-field host rock. The majority of heat generated during the preclosure period would be removed through ventilation, so rock temperature at the drift wall are not expected to reach the boiling point of water during this time period. The rock surrounding the emplacement drifts will begin to dry out during the preclosure phase because the relative humidity of the ventilation air in the drift will be low compared to that of the rock pore space. FIGURE When  forced ventilation ceases at repository closure, the rock temperatures will increase and the dry-out zone will expand into the rock around each emplacement drift. When the drift  wall temperature cools below boiling point, liquid water can enter the drift; however, capillary diversion can cause liquid to flow away from the drift opening. The bulk of the condensate water is expected to flow through the pillars and bypass the drifts, limiting the contribution of condensate water to seepage.


The in-drift chemical environment will evolve through dry-out, transition, and low temperature stages. Dry-out is expected to occur immediately after repository closure, when the near-field rock temperature is expected to increase above the boiling point of water. Vaporization and capillary diversion effects in the host rock surrounding the drift will prevent seepage from occurring during dry-out. Deliquescence of dust deposits on the waste package and drip shield and the acquisition of moisture from the surrounding atmosphere, may produce very small quantities of brines. Seepage water that drips onto the drip shield

or waste package surfaces may flow downward over the metal surface. With moving water and significant

evaporation occurring, spatial separation of components is possible; transport of the more soluble aqueous components may leave behind less soluble precipitates. A transition stage is expected to occur as the

host rock temperature cools below the boiling point of water, and the waste package temperature cools

to below approximately 105°C (about 220 °F). Vaporization will no longer prevent seepage; however capillary diversion is expected to remain active. Evaporative concentration of water that drips onto the waste package surface may occur during this phase. The low-temperature stage will persist after the transition period, through the remainder of the regulatory compliance period, with an ultimate return to ambient conditions.


Capillary diversion will remain effective, and relative humidity within the drift will increase, approaching 100 percent. Higher humidities and lower temperatures are expected to produce progressively more dilute

brines on the waste package and drip shield surfaces.





Overview of the Geology of the Yucca Mountain Site:


Physical setting of the repository site: Yucca Mountain is located in the north-central part of the Basin and Range Physiographic Province.  FIGURE The mountain ranges of this part of the Great Basin are mostly tilted, fault-bounded blocks more than about 50 miles in length, and generally about 5 to 15 miles wide. Yucca Mountain is composed of fine-grained volcanic rocks that are tilted toward the east. Fault-bounded west-facing slopes are generally high, steep, and straight, in contrast to the gentle and commonly deeply dissected east-facing slopes. PICTURE They rise from approximately 1,000  feet to more 5,000 feet above the floor of intervening basins, and occupy approximately 40 to 50 percent of the total land area. The basins are deep structural depressions containing alluvial sediments of late Tertiary and Quaternary ages that range in thickness from a few thousand feet to approximately 2 miles. Erosional processes are concentrated in the high, steep uplands; deposition and depositional processes are generally concentrated in the relatively arid lowlands.28


Yucca Mountain is an irregularly shaped upland area, approximately 2 to 5 miles wide and about 22 miles long. The crest of the mountain reaches elevations of 5,000 to 6,250 feet, or about 400 to 900 feet higher than the floor of adjacent washes and lowlands. The ridges and valleys that characterize the uplands of the Yucca Mountain area trend north and are bounded by high-angle normal faults with vertical displacements ranging from a few tens of feet to several hundred feet. Yucca Mountain consists of layers of volcanic tuffs, successively laid down approximately 14 to 11.5 million years ago by eruptions of volcanic ash from calderas to the north of the mountain. Late-Tertiary volcanic rocks dominate the near-subsurface stratigraphic sequence at Yucca Mountain. These rocks consist mostly of pyroclastic ash flow and ash fall tephra deposits with some reworked materials and minor lava flows. Welded tuffs result from compression and consolidation under high-temperature conditions, while nonwelded tuffs involve lower temperatures, are less dense and brittle, and have a higher porosity. In the Yucca Mountain area, Cenozoic rocks overlie complexly deformed Paleozoic and Precambrian rocks along an erosional unconformity. Cenozoic rocks of the Yucca Mountain region fall into three general groups: (1) pre–Middle Miocene sedimentary rocks (including volcaniclastic rocks) that predate the southwestern Nevada volcanic field; (2) a Middle to Late Miocene volcanic suite that constitutes the southwestern Nevada volcanic field; and (3) Plio-Pleistocene basalts and basin-fill sediments.


The physical properties of the tuffs and lava units that make up Yucca Mountain tend to be relatively uniform over broad lateral areas; however, they often show contrasts across depositional contacts, and some contrasts within units. These characteristics have resulted from rapid deposition of large volumes of homogenous material over large areas as ash flows; differences in the composition of each eruptive batch and sometimes differences in the composition of first-erupted and last-erupted material in a single eruptive batch; and differences in post-depositional welding, vapor phase crystallization, alteration, and gas dispersion.


The surface of Yucca Mountain principally comprises  volcanic rocks of the approximately 12.7 to 12.8 million years old Paintbrush Group, consisting of four lithostratigraphic units, each primarily composed of pyroclastic flow deposits interstratified with small-volume pyroclastic flow and fallout tephra deposits. In descending order, in the area of the repository footprint, these units are the Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring tuffs. The Topopah Spring Tuff unit has a maximum thickness of about 1,250 feet in the Yucca Mountain vicinity, and is divided into a lower crystal-poor rhyolitic member and an upper crystal-rich quartz-latitic member. The repository was to have been located in the densely welded and crystallized rocks in the crystal-poor member of the Topopah Spring tuff. Beneath the Paintbrush Group, the roughly 12.9-million-year-old Calico Hills formation is a complex series of rhyolite tuffs and lavas that thins southward from thicknesses of as much as 1,500 feet.



Tectonic setting and structural framework: Yucca Mountain is located in the Walker Lane domain portion of the Basin and Range Province, an area characterized by high topography, block and detachment faulting, thin crust, west-northwest extension, and basaltic volcanism. The Yucca Mountain area is characterized by closely spaced, west-dipping normal faults. Several tectonic models have been proposed for the vicinity; these models were evaluated as part of both the probabilistic seismic and  volcanic hazard analyses. Basin and Range Province tectonic deformation results from forces related to  the inter-action of the Pacific Plate with the North American Plate and internal buoyancy forces, which may exist because of local density contrasts and the high topographic elevation of the province. Geodetic and satellite data indicate that the Pacific Plate is moving northwest, relative to the North American Plate. The San Andreas Fault system along the plate boundary accounts for the majority of this differential motion, with the remainder distributed through the Inyo-Mono Domain and through the Walker Lane Domain, which includes Yucca Mountain. The existence of numerous precariously balanced rocks at Yucca Mountain is considered to indicate that strong seismic motion has not occurred for tens of thousands of years.    PICTURE


The tectonic features of the Yucca Mountain area include faults and fractures. The dominant element of the structural framework of the site area is major block-bounding faults, spaced about 0.5 to 3 miles apart, which define separate, large, fairly intact blocks of generally east-dipping volcanic strata. In the Yucca Mountain area, from west to east, these faults include: the Windy Wash, Fatigue Wash, Solitario Canyon, Bow Ridge, and Paintbrush Canyon faults. FIGURE  The faults generally dip about 50° to 80° to the west. Within the structural blocks, small strains are accommodated along intra-block faults—such as the Ghost Dance Fault, a prominent intra-block fault at Yucca Mountain. Intra-block faults may accommodate small structural adjustments in response to displacements along the block-bounding faults. Data on geologic structure support an understanding of the volume and quality of rock available for the underground construction of a repository, the delineation of hydrologic flow paths, and the assessment of ground motion and fault displacement hazards.


Fractures are ubiquitous in the rocks at Yucca Mountain; orientation, length, smoothness, connectivity, and other attributes are variable and strongly dependent on stratigraphic position. Depending on both the lithostratigraphic unit that an access tunnel or emplacement drift would encounter and the orientation and size of the tunnel or drift in each lithostratigraphic unit, the hazard from fractures and associated rockfall will be location-specific, and bounded by the hazard from fractures and rockfall under seismic loading conditions. The lithophysal  and nonlithophysal zones within the welded tuffs of the repository horizon exhibit distinctive fracturing patterns and characteristics. PICTURE The nonlithophysal units are hard, strong, fractured rocks with matrix porosities typically 10 percent or less. The fractures that formed during cooling are the primary structural features in these units. The lithophysal units have significantly fewer fractures, with trace lengths greater than a few feet, but the porosity of the lithophysal cavities is fairly uniform.


Geomorphic processes and erosion rates: Evidence  indicates that erosion at Yucca Mountain has occurred at very slow rates for the past several million years. Major erosion and deposition occurred primarily during climatic transitions; periods of relative landscape stability appear to be associated with more stable climate times. The greater magnitude of temperature and precipitation fluctuations during these transitions resulted in greater landscape response. Today, the landscape is dominated by warm temperatures and eolian processes, with infrequent storms producing localized runoff. During cooler and wetter climates there were changes in the type and density of vegetation, increases in runoff and streamflow, and the potential for longer periods of freezing. Slip rates on the local faults are relatively low in comparison with slip rates for significant regional faults. Recurrence intervals, or return times, for earthquakes on the local faults are long. Low-faulting rates have resulted in subtle landforms, which are reflected in the preservation of early and middle Pleistocene deposits on hill slopes, as well as the lack of well-defined alluvial fans at the base of the slopes.


Seismicity and seismic hazard: The seismic hazard at Yucca Mountain is related to potential ground motion and fault displacement that could be associated with earthquake activity in the vicinity of the site. FIGURE Seismic hazards at Yucca Mountain were assessed based on probabilistic evaluations of known seismic sources in the region, including maximum earthquakes, source geometry, and earthquake recurrence. The seismic hazard assessment considered the amounts and patterns of fault displacement and explicitly incorporated uncertainties in the characterization of seismic sources, fault displacement, and ground motion. Seismic hazard studies also examined longer and more active faults at distance from Yucca Mountain; a catalog of historical earthquakes was compiled for the region about 200 miles around the repository site. The area immediately beneath Yucca Mountain has a very low rate of seismicity, in contrast to the southern portion of the Nevada National Security Site, located to the southeast of Yucca Mountain, which is one of the more seismically active regions in the southern Great Basin. Modeling suggests that the Yucca Mountain area is an isolated block within the structural framework of the southern Great Basin.


Volcanism and Volcanic Hazard: Detailed field studies at volcanic centers, including trenching  and geologic mapping, and multiple dating methods for age determinations, have been conducted in the Yucca Mountain region. Basalts erupted in the region during two major episodes beginning about 11.3 million years ago. Quaternary and Pliocene basalts were formed during at least six volcanic events that occurred within about 30 miles of the repository. Three of these events occurred in the Crater Flat region at approximately 3.7 million, 1 million, and 0.08 million years ago, and are within approximately 12 miles of Yucca Mountain. The Quaternary volcanoes in the region occur in a roughly linear zone, called the Crater Flat volcanic zone, to the south, west, and northwest of Yucca Mountain. PICTURE The result of the Probabilistic Volcanic Hazard Analysis was a quantitative probability distribution of the annual probability of a basaltic dike intersecting the repository footprint. An aggregate probability distribution was computed that reflected the uncertainty across the entire expert panel. For the License Application repository footprint the mean annual frequency of intersection of the repository by a volcanic event such as a dike, was 1.7 × 10-8 and 1.3 × 10-8 for an eruption through the repository