U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

ON THE MECHANISM OF THE LARGE HYDRAULIC GRADIENT 

UNDER YUCCA MOUNTAIN, NEVADA

by

Chris Fridrich 

U.S. Geological Survey, Box 25046, MS 913, Denver, CO 80225

Shemin Ge

Department of Geological Sciences, Campus Box 250, 

University of Colorado, Boulder, CO 80309

John Sass 

U.S. Geological Survey, 2255 Gemini Dr., Flagstaff, AZ 86001

Open-File Report 98-119

March 9, 1998

This report is preliminary and has not been reviewed for conformity with U. S. Geological 

Survey editorial standards (or with the North American Stratigraphic Code). Any use of 

trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U. S. Government.
Abstract

We use coupled fluid- and heat-flow modeling (the TOUGH2 code) to investigate 

three alternative hydrogeologic models for a steep 300-m water-table decline, known as 

the large hydraulic gradient (LHG), and a spatially coincident heat-flow low under Yucca 

Mountain, Nevada: (1) a fault-related dam within the volcanic tuff (water-table) aquifer, 

(2) a spillway created by abrupt southward increase in the depth-extent (thickness) and 

permeability of the tuff aquifer across a fault, and (3) a drain formed by a fault that allows 

the major flow through the tuff aquifer from the north to be captured by the underlying 

confined carbonate aquifer. All three models provide an adequate simulation of the 

observed water-table configuration under Yucca Mountain, as well as the shallow thermal 

regime. None of the models provides an adequate simulation of the deep thermal regime, 

and the drain model alone can be eliminated on that basis. Our simulations of the dam and 

spillway models yield strongly contrasting predictions about the pattern of vertical 

hydraulic gradients in the vicinity of the LHG, suggesting that a well-designed drilling and 

hydrologic testing program would yield definitive answers about the cause of this feature.

Introduction

Yucca Mountain, which straddles the western boundary of the Nevada Test Site in 

the southern Great Basin (Figures 1 and 2), has been under intensive study as a potential 

geological repository site for storing the nation's high-level radioactive waste and spent 

fuel from nuclear reactors. Because of the exceptionally deep water table under central 

Yucca Mountain, the potential repository can be constructed in volcanic tuffs 300-400 

meters beneath the surface, and yet be 200-400 meters above the current water table. The 

design concept is to create a "mummy's tomb" in which the waste would be isolated from 

the surface by the great depth of the repository, and isolated from the saturated-zone 

groundwater system by the thick unsaturated section between the repository and the water 

table, as well as by the current low flux of groundwater through the unsaturated zone in 

the arid climate of southern Nevada.

Whereas the "mummy's tomb" idea is an excellent concept for a radioactive waste 

repository, it is unclear how well the Yucca Mountain site actually conforms to this 

concept. For example, the water table abruptly rises at least 300 m immediately to the 

north of the potential repository area (Figure 2), and is higher than much of the design 

repository at a distance of 2 km to the north, and perhaps closer. Some paleohydrology 

studies indicate that the water table was about 100 m higher under central Yucca Mountain 

in the recent past [Marshall et aL, 1993]. Moreover, an extensive perched water zone is 

present throughout most of the area of the potential repository near the base of the 

proposed repository horizon [Rousseau et. al. , 1996; Hinds et aL, 1997], and the presence 

and behavior of the perched-water system may be genetically linked to the immediately 

adjacent steep decline in water-table elevation. A National Academy of Sciences panel 

concluded that "not until the source of the gradient [the > 300-m water-table decline] is 

known can the potential hazard that the repository may face due to climate changes and/or 

tectonic events be evaluated with a high level of confidence" [NRC, 1992].
This paper examines the hydrodynamics of the saturated-zone groundwater regime 

of Yucca Mountain with emphasis on the mechanism of the 300-m water table decline. 

Three alternative hypotheses for the cause of the water-table decline are considered. 

Based on the existing geologic, hydrological, and geophysical data, a two-dimensional 

cross-sectional conceptual model is constructed and is used to test these hypotheses 

quantitatively in a series of simulations using TOUGH2, a fully coupled fluid- and heatflow code [Pruess, 1991].

Hydrogeology of Yucca Mountain

Yucca Mountain is a multiple-fault-block ridge located on the south flank of the 

southwest Nevada volcanic field, within the Death Valley groundwater system (Figure 1). 

The configuration of the southeastward-sloping water table under Yucca Mountain is 

dominated by an abrupt decline of 300 m over a distance of 2 km or less (Figure 2). This 

northeast-striking zone of large hydraulic gradient (0.15 or more) separates an area of 

moderate gradient (of about 0.015) to the north from an area of very small gradient 

(0.0001) to the south. An additional feature of the water-table configuration is a 45-m 

decline in water-table elevation under western Yucca Mountain, immediately east of 

Solitario Canyon (Figure 2).

The north-striking 45-m step hi the water table coincides spatially with the Solitario 

Canyon fault. This step evidently results from the damming effect created by offset 

stratigraphy, specifically the juxtaposition of a confining unit against the water-table 

aquifer along this major fault [Fridrich et aL, 1994]. In contrast, the 300-m water-table 

decline under northern Yucca Mountain does not correspond to any exposed geologic 

feature.

Fridrich et al. [1994] showed, however, that the 300-m decline, which is 

commonly known as the large hydraulic gradient, coincides spatially with the northern 

margin of a trough-like 10-milligal gravity low, and that this gravity low can be explained 

by the greater thickness of Tertiary volcanic rocks encountered hi bore holes drilled within 

the low in relation to holes drilled to the north and south of it. The gravity anomaly thus 

reflects a buried depression filled with low-density rocks. The observed stratigraphic 

thickening into the gravity low is a step change, which indicates that this depression is 

bounded by growth faults in the deep (>0.5 km depth) Tertiary volcanic section. The 

large hydraulic gradient is thus interpreted as coinciding with the northern bounding fault 

of a buried graben as shown in Figure 3 [Fridrich et al., 1994].

The large hydraulic gradient may also coincide, at least roughly, with the north 

boundary of the deep carbonate aquifer under Yucca Mountain. Yucca Mountain consists 

of a thick (1.5-3 km) sequence of faulted and weakly tilted Miocene volcanic rocks of the 

southwest Nevada volcanic field (Figure 1). This Miocene volcanic sequence 

unconformably overlies a much thicker (> 10 km) section of strongly deformed Paleozoic 

and latest Proterozoic sedimentary rocks, consisting of highly permeable carbonate rocks 

(limestones and dolomites) and much less permeable clastic rocks (quartzites and 

argillites). Whereas the water table under Yucca Mountain is located in the upper part of 

the volcanic section, the major regional groundwater flow is presumed to pass through the
underlying Paleozoic carbonate aquifer [Winograd and Thordarson, 1975], The deep 

carbonate aquifer is separated from the volcanic water-table aquifer by the lower tuff 

confining unit, which consists of the Lithic Ridge Tuff, older tuffs, and underlying 

unnamed tuffaceous sedimentary rocks (Figure 3).

A single bore hole, UE25-p#l (Figures 2 and 3), was drilled into rocks underlying 

the Tertiary volcanic section of Yucca Mountain. This hole encountered the Paleozoic 

carbonate aquifer, demonstrating its presence under southern Yucca Mountain [Craig and 

Robison, 1984]. A long-wavelength aeromagnetic anomaly over northernmost Yucca 

Mountain [Boynton and Vargo, 1963] suggests that the volcanic rocks there are underlain 

by a strongly magnetic rock unit [Bath and Jahren, 1984]. Bath and Jahren interpreted this 

magnetic anomaly as an extension of a stronger anomaly to the east, which results from 

magnetite-bearing hydrothermally altered Paleozoic argillite (originally identified as the 

Eleana Formation (Argillite)) as shown in Figure 3, but recently reclassified as the 

Chainman Shale by J. C. Cole, USGS, oral comm., 1997). Bath and Jahren thus inferred 

that the volcanic rocks under northernmost Yucca Mountain are underlain by argillite 

instead of carbonate rocks. If their interpretation is correct, then the carbonate aquifer 

pinches out northward under Yucca Mountain approximately hi the position of the large 

hydraulic gradient [Fridrich et al., 1994].

In the saturated zone under Yucca Mountain, the effective permeable pathways for 

saturated-zone flow are apparently controlled dominantly by tectonic fracturing, especially 

the major fault zones that cut through Yucca Mountain [Fridrich et al., 1994]. This is not 

to say that stratigraphy is unimportant, however, because the development of fracture 

permeability along tectonic fracture zones is strongly dependent on lithology, especially on 

changes in welding, crystallization, and other lithologic features that vary through the 

volcanic section. The units that comprise the principal Tertiary aquifer under Yucca 

Mountain are the welded tuffs that underlie the water table, especially the Prow Pass, 

Bullfrog, and Tram Tuffs of the Crater Flat Group (Figure 3). South of the large 

hydraulic gradient, the densely welded Topopah Spring Tuff is also locally below the 

water table and is an important aquifer, as discussed below.

In addition to the lateral changes in potentiometric head under Yucca Mountain (the 

water-table configuration), vertical changes in head have been observed. For example, 

there is a downward hydraulic gradient under northern Yucca Mountain, based on 

observations in drill hole USW G-2 [Czarnecki, 1994; Czarnecki et al., 1995]. Thermal 

observations suggest that this downward gradient extends to a depth of at least 1.3 km 

[Sass et al., 1988; Czarnecki et al., 1995]. An upward hydraulic gradient has been 

observed under southern Yucca Mountain in drill hole UE25-p#l; specifically, the head in 

the carbonate aquifer is about 20 m higher than that in the overlying tuff aquifer in this 

well [Craig and Robison, 1984]. This southward transition from a downward gradient to 

an upward gradient across Yucca Mountain is located approximately in the position of the 

large hydraulic gradient, and is probably just part of the regional pattern as one moves 

away from the major recharge areas toward the major discharge areas of the hydrologic 

system.
Thermal Structure under Yucca Mountain

The Great Basin of the western United States is characterized by diverse and 

complex thermal and hydrologic regimes. The average heat flow is high (100 mWm"2) but 

there are large regions of both very high and anomalously low heat flow. One such region 

is the "Eureka Low," which occupies about 35,000 km2 of a terrain in southeast Nevada 

(Figure 2, inset map) in which the dominant aquifer consists of Paleozoic limestones and 

dolomites [Sass et al., 1995]. Conductive heat flows within the Eureka Low are mostly 

low (<60 mWrn 2). There are, however, large areas of high advective flux (e.g., Railroad 

Valley; Hot Creek Valley] within its boundaries, indicating that the average ambient heat 

flow for this terrain is similar to that of the rest of the Great Basin, and that the deviations 

from the average heat flow are the result of redistribution of heat by groundwater flow. 

The Eureka Low has been interpreted as a relatively shallow (3-4 km) hydrologic feature 

from both thermal and hydrologic observations [Sass et al.,1971; Winograd and 

Thordarson, 1975; Lachenbruch and Sass, 1977].

Yucca Mountain is located on the southern margin of the Eureka Low and exhibits 

considerable thermal variability on the scale of kilometers (Figure 2). In particular, the 

candidate repository site is within a local heat-flow low that extends from the vicinity of 

the large hydraulic gradient southward for about 10 km. The heat flows shown here 

(Figure 2) are from the unsaturated zone, but they are consistent with independent 

determinations in the saturated zone of drill holes that extend a significant distance below 

the water table and exhibit conductive thermal profiles [Sass et al., 1988].

Because the apparent width of the local heat-flow low at Yucca Mountain is only 

about 5 kilometers (Figure 2), we can assume that it originates at comparable or shallower 

depths. The major cause of low heat flow at Yucca Mountain relative to most of the Great 

Basin probably is related to processes operating within the carbonate aquifer, because 

these rocks provide the conduit for the interbasin flow responsible for the larger Eureka 

Low [Sass et al., 1995]; moreover, the very low temperature for the depth, at the bottom 

of well UE-25p#l (54 degrees C at 1800 m), indicates that the thermal anomaly extends 

into the carbonate aquifer under Yucca Mountain, at least locally.

Shallower processes, however, also play a role in creating the total heat flow 

anomaly at Yucca Mountain. For example, some unknown rate of groundwater flow 

presumably occurs southward through the water-table aquifer across the large hydraulic 

gradient, and the downward component of that flow contributes to the heat-flow anomaly. 

Evaporative cooling and downward percolation in the unsaturated zone may also 

contribute; however, existing data indicate that the unsaturated-zone contribution to the 

heat-flow anomaly is negligible [Sass et al., 1995].

Conceptual Models for the Large Hydraulic Gradient

Whereas the inferred buried graben and northward pinchout of the deep carbonate 

aquifer under Yucca Mountain discussed above are interpretations, the data that these 

interpretations are based on are ground truth. For example, even if the buried graben 

interpretation is wrong in some of its specifics, there must be a large geologic feature of
some kind under the large hydraulic gradient based on the objective data, namely the 

gravity anomaly and the observed step changes hi stratigraphic thicknesses across the 

boundaries of this anomaly. The most important question about the concealed geologic 

features that coincide spatially with the large hydraulic gradient is not what their precise 

geologic character is, but rather what their hydrologic effect is. The geologic 

interpretations discussed above do not lead to a unique conclusion about the hydrologic 

cause of the large hydraulic gradient; they do, however, provide a basis upon which to 

develop alternative conceptual models of the physical mechanism of this hydrologic 

feature.

Downwelling from the tuff (water-table) aquifer into the deep carbonate aquifer, 

near the postulated northern limit of the carbonate aquifer, could by itself result hi a 

southward decline in the water table under Yucca Mountain. However, because of the 

presence of the lower tuff confining unit between the two major aquifers, the zone of 

downwelling would be spread out laterally across several kilometers unless some 

additional feature provided a transmissive pathway through the lower tuff confining unit to 

localize the downwelling into the observed narrow zone of water-table decline. The 

inferred buried fault that coincides with the large hydraulic gradient could provide the 

pathway and, if so, this fault is acting as a dram allowing flow hi the tuff aquifer, derived 

from recharge areas to the north, to be largely captured by the deep carbonate aquifer 

(Figure 4a). We call this the drain model.

Alternatively, the carbonate aquifer may not be involved and the buried fault may, 

by itself, create the large hydraulic gradient within the volcanic section. There are two 

major ways that this could work. First, the buried fault may form a barrier to flow hi the 

volcanic section. This is the dam model (Figure 4c).

A more complex, but geologically reasonable model is that the buried fault may 

act as a spillway. The major tuff aquifer under southern Yucca Mountain is the Crater Flat 

Group of tuffs. This volcanic group thins strongly and abruptly northward across the large 

hydraulic gradient. The abrupt thinning results hi a reduced degree of welding of the tuff 

units on the upthrown (northern) side and, consequently, in greater susceptibility to 

permeability-reducing hydrothermal alteration. The upthrown side is also closer to the 

Miocene heat source for the hydrothermal alteration, the caldera complex at the north end 

of Yucca Mountain, and the rocks under northern Yucca Mountain are, in fact, 

significantly more altered than those to the south [Bish, 1989]. If large-scale permeability 

is strongly reduced northward across this buried fault, then the buried fault under the large 

hydraulic gradient may be the effective northern boundary of the tuff aquifer under Yucca 

Mountain. The small flow of groundwater that comes through the tuffs to the north 

probably would flow dominantly through the less altered upper part of the tuff section, and 

then suddenly descend, in spillway fashion, into the tuff aquifer where it abruptly begins 

on the downthrown side of the fault (Figure 4b).

The drain, dam, and spillway models differ mainly hi the physical arrangement of 

the permeability structure and of the resultant flow pathways (Figure 4). In the case of the 

dam (Figure 4c), permeability and thus flow pathways extend to depth within the tuff 

section on both sides of the dam, and the fault-dam resists the flow, resulting in a higher 

water-table elevation on the upgradient side. In the case of the spillway (Figure 4b), the
water table abruptly drops because the effective base of the hydrologic system, and thus of 

possible flow pathways, abruptly drop within the volcanic section. The drain model 

(Figure 4a) differs from the spillway model in that the dominant flow is descending into 

the deep carbonate aquifer, rather than just descending within the volcanic section.

The dam, drain, and spillway models are simple end-member conceptions of the 

physical mechanism of the large hydraulic gradient under Yucca Mountain that we believe 

embrace the major range of possible mechanisms for the large hydraulic gradient. For 

example, the physical arrangements of the permeability structure and of the flow pathways 

in the spillway model provide a reasonable representation of a model proposed by 

Czarnecki [1994] and Czarnecki et al. [1995] that the large hydraulic gradient is created 

by the abrupt lateral termination of a perched water table that is 300 m higher than the true 

water table.

Summary of Past Modeling Efforts

Quantitative assessment of the groundwater flow system in the whole Death Valley 

basin, of which Yucca Mountain is a very small part (Figure 1), has been conducted 

previously by several researchers, including Waddell [1982] and Sinton [1989]. The 

emphasis of these models is on recharge and discharge patterns at a regional scale. The 

large hydraulic gradient of Yucca Mountain, however, is on a scale of several kilometers 

and therefore cannot be adequately examined by these regional models.

At a slightly more focused scale, Czarnecki and Waddell [1984] simulated 

groundwater flow in the hydrologic subbasin in which Yucca Mountain lies using a twodimensional (plan-view) finite-element model. They used a parameter-estimation 

technique to calibrate their model with observed hydraulic heads. Their estimated 

parameters include rock transmissivities and recharge and discharge fluxes at the 

boundaries of their computational domain. The two-dimensional areal model is restricted 

to horizontal flow and therefore cannot be applied to areas where vertical flow components 

are present. By assigning a low-permeability zone in the large hydraulic gradient area, 

they reproduced this feature as resulting from a dam.

In the same area of the Czarnecki and Waddell model, Haws [1990] performed 

two-dimensional finite-element modeling of a vertical cross section and concluded that a 

shallow low-permeability zone in the area of the large hydraulic gradient can produce the 

observed potentiometric pattern. To examine the effects of infiltration and permeability 

structure of water-table configuration, Jasek [1991] used a similar technique of inserting a 

low-permeability zone beneath the large hydraulic gradient to simulate the 300-m water 

table decline.

In a broad context, all of these previous modeling efforts thus assumed a dam 

model to explain the large hydraulic gradient.

Design of the Present Modeling Effort

Numerical modeling in this study was undertaken to test alternative hypotheses for 

the cause of the large hydraulic gradient under Yucca Mountain. Our approach can be
summarized in five points:

(1) We wanted to incorporate as many of the geologic constraints on the Yucca 

Mountain hydrologic system as possible, while allowing flexibility in associating different 

hydrologic processes with key geologic features, especially the postulated buried fault 

under the large hydraulic gradient.

(2) To achieve our goal, the uncertainties associated with the poorly constrained 

boundary conditions could not be avoided; those uncertainties must, therefore, be taken 

into account in the analysis of model results. Previous modeling efforts have extended 

over areas much larger than Yucca Mountain in order to overcome the problem of poorly 

constrained boundary conditions in the immediate vicinity of the potential repository area. 

However, exploring the physical processes requires modeling that is focussed on the area 

of the feature of interest, in this case the large hydraulic gradient.

(3) We were less concerned with achieving an exact calibration in our simulations 

than with developing an understanding of the physical processes that govern the system. In 

addition, we did not attempt to match features that we thought might just be local 

anomalies.

(4) We decided that including the thermal regime in our modeling was critical 

because it has been shown that the groundwater flow field under Yucca Mountain couples 

with heat transfer [Sass et al, 1995]. Hence, we used TOUGH2, a code that solves the 

fluid mass-balance equation and heat conduction-convection equation simultaneously using 

the integrated finite-difference method. The basic physics behind fluid and heat coupling is 

that fluid flow redistributes the heat while the temperature gradient modifies the fluid 

density and viscosity, and consequently the hydraulic conductivity and flow pattern.

(5) The model cross section used was taken from Fridrich et al. [1994] and extends 

in a north-northwest to south-southeast direction across the large hydraulic gradient 

(Figures 2 and 3). Four wells along this cross section were drilled deep enough to extend 

through the tuff aquifer into the lower tuff confining unit and have measured thermal 

profiles: USW G-2, USW G-l, UE25-b#l, and UE25-p#l. With this geologic section as a 

basis, a mesh was generated with 33 rows and 54 columns over a cross-sectional area that 

is 3 km deep and 10 km long.

One possible shortcoming associated with the selection of this particular cross 

section (Figure 3) is that the Topopah Springs Tuff is everywhere above the water table on 

this section, whereas it locally lies below the water table east and west of this section 

southward of the large hydraulic gradient (Fridrich et al. , 1994). Whereas the Crater Flat 

Tuffs are fully saturated under all but a very small area under Yucca Mountain, and are 

therefore considered the major Tertiary aquifer, the Topopah Springs Tuff is only locally 

saturated, but is significantly more permeable than the Crater Flat Tuffs. A potentially 

important geologic element that may affect the permeability structure of Yucca Mountain 

is thus absent in our model. Given the complex three-dimensional character of the 

structure of Yucca Mountain, there is no single cross section that can totally represent the 

permeability structure. We address this problem below in interpreting our simulation 

results.
Hydrostratigraphy

Using the hydrologic features of different lithologic units as a basis, we adopt five 

major hydrostratigraphic units, as suggested by previous researchers [Winograd and 

Thordarson, 1975; Fridrich et al. , 1994; Waddell et al., 1984]. These include the major 

tuff aquifer, namely the Crater Flat Group (the Prow Pass, Bullfrog, and Tram Tuffs), and 

the lower tuff confining unit, which consists of the Tertiary section underlying the Tram 

Tuff, as discussed above. The Crater Flat aquifer is overlain by the Calico Hills confining 

unit and by the Tiva Canyon/Topopah Spring aquifer. This uppermost aquifer is almost 

entirely unsaturated in the area of our cross section (Figure 3). The lower tuff confining 

unit is underlain by the Paleozoic carbonate aquifer.

In general, the development of fracture permeability in the Tertiary hydrologic 

units increases with the intensity of welding in the tuffs. In contrast, alteration decreases 

permeability, especially for the nonwelded tuffs. Some lavas that are locally present under 

the Tram Tuff under northern Yucca Mountain (Figure 3) are little fractured and are 

included here as part of the lower tuff confining unit. In general, the carbonate aquifer is 

highly fractured and is highly permeable [Winograd and Thordarson, 1975].

Numerous faults cut across Yucca Mountain in the model area [Scott and Bonk, 

1984]. We treat the major faults as distinct hydrologic units. In general, faults at Yucca 

Mountain are conduits for flow along their strikes owing to fracturing and brecciation in 

the fault zones. Some faults may also be barriers to flow perpendicular to their strikes 

owing to offset stratigraphy or to fracture fillings. In the model area, we represented all 

of the faults as more permeable zones except for the inferred fault under the large 

hydraulic gradient, which is represented differently in each of the four conceptual models.

Hydrologic and physical properties

Hydrologic and physical properties (Table 1) were based primarily on laboratory 

and field data collected at Yucca Mountain. For porosity estimates, we collated data from 

a number of reports on core-sample measurements and well logs [Geldon, 1993; Nelson 

and Schimschal, 1993; Muller and Kibler, 1986; Waddell et al., 1984; Lobmeyer et al., 

1983], and in some cases, we computed porosity from the difference between 

measurements of bulk density and grain density. The thermal conductivities chosen for the 

five hydrostratigraphic units are from Sass et al. [1988] who made 204 measurements of 

this parameter on core samples from boreholes in the Yucca Mountain area. 

Permeabilities (Table 1) were based on in-situ hydrologic tests conducted in bore holes 

[LeCain, 1996; Geldon, 1993; Montazer and Wilson, 1984; Waddell etal., 1984]; 

however, the measured values cover a large range for each hydrologic unit and the number 

of measurements is statistically inadequate to establish reliable estimates of large-scale 

permeability, especially for the confining units. We therefore concentrated on choosing 

reasonable relative values for the different hydrologic units.

One of the main uncertainties in the hydrologic parameters used is the permeability
of fault zones because few direct field measurements are available. We estimated the 

permeability of the major exposed faults indirectly using the observed heat-flow anomalies 

along the Bow Ridge and Solitario Canyon faults [Fridrich et al., 1994] and a solution 

from the one-dimensional heat conduction-convection theory Predehoeft and 

Papadopulos, 1967; Bodvarrson et aL, 1982]. We assumed that the heat flow low under 

northern Yucca Mountain (Figure 2) results from focused descent of relatively cool 

groundwater along some preferential pathways such as a fault. As the water migrates 

laterally at depth, it is heated by ambient geothermal flux. The warmed groundwater fluid 

then upwells along major exposed faults to the south, resulting in the observed relatively 

high heat flow under southern Yucca Mountain (Figures 2 and 3). We applied this 

conceptual model and derived the one-dimensional heat conduction-convection solution 

from Bredehoeft and Papadopulos [1965]:

' '. a)

where / is the heat flux, Kt is the thermal conductivity of the rock-fluid complex, T is 

temperature, z is the vertical coordinate, J0 is the background geothermal heat flux at a 

datum ZQ, parameter D is defined as c0 p0 v/Kt, c0 is the specific heat of the fluid, p0 is the 

density of the fluid, and v is the vertical groundwater velocity. Assuming a natural 

background temperature gradient of 30 degrees C/km, J0 = 63 mWm'2, £,=2.1 

Watt/(meter °C), Z0 =0, and z= 100 m, for a velocity v of -3.5 x 10"9 m/s, the heat flux / 

will be 31 mWm"2 . This calculation suggests that a downward flow at the rate of 3.5 x 10'9 

m/s will produce an apparent heat flow of 31 mWm"2 , which is lower than the background 

heat flow. Similarly, an upward flow velocity at 8 x 10" 10 m/s will produce a heat flow 

high of 74 mWm"2 , similar to that observed in the southernmost part of the study area 

(Figure 2). Sass et al. [1988] estimated a vertical flow velocity of 3.2 x 10"9 m/s from the 

temperature profile in well USW G-l. From the estimated velocity and an assumed 

hydraulic gradient, one can estimate the hydraulic conductivity using Darcy's Law. If we 

assume a vertical hydraulic gradient of 0.01 in the recharge area, then the estimated 

vertical permeability is 3 x 10" 14 m2 in the fault. For a vertical hydraulic gradient of 0.001 

in the discharge areas, the estimated permeability is 8 x 10" 14 m2 in the south.

Boundary conditions

The boundary conditions used in our models are in some cases artificial because 

insufficient data are available to determine the actual boundary conditions. At the upper 

boundary of our model, the ground surface, the pressure was set at the atmospheric value 

of 89,000 Pa, temperature was set at the annual mean temperature of 20 °C, and saturation 

was set at 20%. No local infiltration was assumed because existing data indicates it is 

minor relative to the lateral flow through the saturated-zone system. The base of our cross 

section is a no-flow boundary at which a constant heat flux of 53 mWm"2 [Sass et aL , 

1988] was assigned; this is the average value over the study area (Figure 2). The sides of 

the model were set as constant head boundaries using water-table elevations that were
based on extrapolations from the observed water levels in wells USW G-2 and UE25p-l. 

On the sides, we assumed linear temperature profiles that were extrapolations from these 

same wells.

Quantitative Testing of Hypotheses

To test the hypotheses discussed earlier, we conducted numerical simulations, 

employing the parameters shown in Table 1 as a base case. The features distinguishing the 

three hypotheses are straightforward variations of permeability (Figure 4). Below, we 

compare the results of the different simulations, using the computed hydraulic gradients 

and water levels, as well as the calculated temperature profiles in the positions of the four 

wells along the model cross section.

Base model

To establish a starting point for testing different hypotheses, we set up a base 

model that contains the well-known hydrologic features of Yucca Mountain, but that omits 

all of the features that are hypothesized to cause the large hydraulic gradient. The 

hydrogeologic units that comprise the base model are discussed in descending order as 

follows (see Figure 3 and Table 1). The uppermost layer of the model is the Topopah 

Springs/Tiva Canyon formation, in which the water table is located. In the simulations, 

the position of the water table was computed using an exponential characteristic function 

(/. e. , van Genutchen model) describing unsaturated hydraulic permeability in this top 

layer. The underlying Calico Hills unit is subdivided into two layers because the lower 

part of this formation is more altered and therefore less permeable than the upper part. 

The next unit, the Crater Flat Group consists of the Prow Pass, Bullfrog, and Tram Tuffs 

and is the major Tertiary aquifer. This unit was given a southward-increasing 

permeability because hydrothermal alteration of these rocks increases to the north, as 

discussed above [Bish, 1989]. The underlying older tuff aquitard includes the Lithic 

Ridge Tuff, older tuffs, and an unnamed sedimentary section. This unit is a confining unit 

that separates the Tertiary tuff aquifer from the Paleozoic deep carbonate aquifer. The 

carbonate aquifer, at the bottom of the section, is divided into upper and lower layers. The 

Paleozoic carbonates were exposed at the surface prior to deposition of the Tertiary rocks. 

The top of the carbonates was therefore subjected to surficial weathering and fracturing, 

and is assigned greater permeability than the lower part. We assign all of the faults 

identified in the cross section with a moderate permeability (Table 1).

The results of the base model simulation are that: (1) Almost all of the groundwater 

flow is through the tuff aquifer under Yucca Mountain, because no mechanism was 

incorporated to introduce flow into the carbonate aquifer. (2) A zone of steeper hydraulic 

gradient is present in the calculated water-table configuration (Figure 5a) owing to the 

assumed southward increase in permeability of the Crater Flat tuff aquifer, but it is a poor 

fit to the observed water-table configuration. Specifically, the hydraulic gradients in the 

simulation are 0.07 in the north (between USW G-2 and USW G-l) and 0.02 in the south 

(between UE25-b#l and UE25-p#l), as compared to the measured values of 0.15 and

V

10
0.001 in the same locations. (3) The downwelling that occurs under northern Yucca 

Mountain results in a reduced thermal gradient under the central part of the mountain, but 

the simulated thermal gradients are significantly different in detail from the profiles 

observed in the four Yucca Mountain wells in the cross section (not shown but similar to 

those of the spillway and dam models; Figure 6). Our conclusion from the base model 

simulation is that an additional feature(s) is needed in the simulation to create the observed 

water-table configuration and thermal structure of Yucca Mountain.

Spillway model

The spillway hypothesis assumes an abrupt southward increase in the magnitude 

and depth-extent of permeability in the Crater Flat tuff aquifer across the buried fault that 

underlies the large hydraulic gradient (Figure 4b). We tested this model by strongly 

reducing the permeability of the Crater Flat unit north of the buried fault relative to the 

base model, while all other parameters were kept the same. In the spillway-model 

simulation, the major flow is still through the tuff aquifer, as in the base model; however, 

the calculated water-table configuration is much closer to the observed pattern (Figure 5b). 

Specifically, the hydraulic gradients in the simulation are 0.16 in the north and 0.03 in the 

south, as compared to the measured values of 0.15 and 0.001.

The order-of-magnitude disagreement between the calculated and observed 

hydraulic gradient in the south can be explained by the fact that, to the east and west of the 

model cross section, the Topopah Spring Tuff becomes a significant part of the Tertiary 

aquifer system on the downgradient side of the large hydraulic gradient [Fridrich et al. , 

1994]; hence, the average transmissivity under southern Yucca Mountain is greater than 

that in the model cross section. We thus consider that the large hydraulic gradient is 

successfully reproduced by the spillway model.

The simulated thermal regime for the spillway model is similar to that of the base 

model. When the observed temperature profiles in the four wells of the cross section are 

compared with those computed in the spillway model simulation (Figure 6), they agree 

well in the upper part of the section, but diverge significantly at depth. The simulated 

temperatures are too high within the carbonate aquifer (in well UE25-p#l) and in the 

lower, confming-unit part of the volcanic section.

Drain model

The drain model postulates that the large hydraulic gradient is caused by focussed 

downwelling of the majority of the flow passing through the tuffs under northern Yucca 

Mountain into the carbonate aquifer under central and southern Yucca Mountain (Figure 

4a). We tested this scenario by increasing the permeability of the buried fault under the 

large hydraulic gradient by one order of magnitude, while all other parameters were kept 

the same as in the base run. In our simulation, approximately half of the flow through the 

tuffs is captured by the carbonate aquifer at the large hydraulic gradient. The computed 

hydraulic gradient is 0.08 in the north and 0.015 in the south (Figure 5c), as compared to 

the measured values of 0.15 and 0.001. As with the spillway model, we consider this to

11
be an adequate fit to the water-table configuration, given the limitations of a twodimensional model.

The drain model predicts the presence of a thermal low on the downgradient side of 

the the large hydraulic that extends several kilometers to the south. The fit between the 

calculated and observed thermal profiles in the four wells on the cross section is 

reasonably good in the shallow parts of the section (Figure 6c), but diverges significantly 

at depth. Specifically, the calculated temperatures are too low in the deeper part of section 

at the bottom of the large hydraulic gradient (in the location of well USW G-l) and too 

high in the carbonate aquifer in the location of well UE25-p#l (Figure 6c).

Dam model

In the dam model, the large hydraulic gradient is produced by inserting a narrow, 

high-angle planar zone of low permeability in the position of the buried fault under 

northern Yucca Mountain (Figure 4c). The flow regime is similar to that of the spillway 

and base models. The computed hydraulic gradients are 0.07 in the north and 0.03 in the 

south (Figure 5d), as compared to the measured values of 0.15 and 0.001. As with the 

previous models, this is considered an adequate fit for our purposes. The thermal regime 

and the calculated thermal profiles in the positions of the four wells in the model cross 

section are almost identical to those calculated in the spillway model (Figure 8).

Discussion and Conclusions

All three of the hypotheses we considered for the large hydraulic gradient - dam, 

drain, and spillway - provide an adequate simulation of the observed water-table 

configuration under Yucca Mountain, given the limitations of a two-dimensional model. It 

is probable that exact fits could be achieved by adjusting the permeability structures in the 

models; however, our purpose was testing of hypotheses rather than achieving exact fits.

All three models also provide an adequate simulation of the shallow thermal regime 

under Yucca Mountain. None of the models, however, provide an adequate simulation of 

the deep thermal regime, and the drain model alone can be eliminated on that basis. The 

simulations of the drain model show that, if the thermal low observed at northern Yucca 

Mountain was caused by downwelling from the tuff aquifer into the carbonate aquifer 

along the buried fault between drill holes USW G-l and USW G-2, then the thermal 

gradient at the base of the large gradient (in the vicinity of USW G-l) should be extremely 

low, much lower than was observed (Figure 6). We consider the drain model to be 

disproven by this result because of the very large difference between the observed and 

simulated value, and because the physical process that underlies the simulated value is 

inherent to the concept of the drain model.

The deep thermal regimes simulated for the dam and spillway models do not match 

the observed regime either; however, this does not present the same problem because these 

two models include no prediction about groundwater flow processes in the carbonate 

aquifer. For the dam and spillway models, the misfit can be resolved by invoking an

external process ~ cool underflow through the carbonate aquifer. That invokation can be i

12
justified because the heat flow anomaly under Yucca Mountain is part of the regional-scale 

Eureka Low, and because the Eureka Low as a whole is considered to be caused by cool 

underflow through the carbonate aquifer, as discussed above.

The model results demonstrate that downwelling within the tuff aquifer at the large 

hydraulic gradient, and related processes, can explain the short-wavelength details of the 

thermal anomaly under Yucca Mountain, and the dam and spillway models appear equally 

acceptable in this regard. The spatial coincidence of the lowest point in the heat-flow 

anomaly with the large hydraulic gradient (Figure 2) is strong circumstantial evidence for 

a causative relation between these two features. The next lowest part of the thermal 

anomaly is a trough aligned along the major fault block that comprises Yucca Mountain, 

with relative thermal highs along the major faults on either side (in Solitario Canyon and 

on the east side of Midway Valley; Figure 2). The geometry of the anomaly is consistent 

with the concept that, across the large hydraulic gradient, the potentiometric head in the 

tuff aquifer drops below that in the underlying confined carbonate aquifer, so that some 

upward leakage occurs along the major faults that cut the intervening confining unit to the 

south [Fridrich et al., 1994]. The gradual increase in the heat flow to the south reflects 

the gradual heating of the water in tuff aquifer, southward of the point of downwelling, 

owing to both ambient geothermal heating and to inmixture of warmer water upwelling 

from the carbonate aquifer. Isotopic discontinuities in the groundwater provide 

independent evidence for the upwelling of water from the carbonate aquifer into the tuff 

aquifer under southern Yucca Mountain [Stuckless et al., 1991].

The fact that the short-wavelength details of the thermal anomaly under Yucca 

Mountain are well explained by the flow regime in the tuff aquifer does not prove that the 

entire heat-flow low is caused by flow processes in the tuff aquifer. In all of our 

simulations, we assumed that the heat flow at the base of the models was 53 mWm-2, a 

value that was derived by averaging the heat flow observed in the unsaturated zone over 

the study area (the area with heat flow contours in Figure 2). The 53 mWm-2 value thus 

represents a reasonable estimate of the heat flow at the water table, and probably is a close 

estimate of the heat flow at the base of the Tertiary volcanic section. It may, however, be 

a poor reflection of the heat flow at the base of the hydrologic system, which is typically 

about 100 mWm-2 in most of the Great Basin. The lower heat flows observed in the near 

surface in most of the area of the Eureka Low, relative to the rest of the Great Basin, have 

been explained as being caused by removal of heat by groundwater flow within the 

carbonate aquifer. The abundance of hot springs and other evidence of high advective heat 

flows in the discharge areas in the Death Valley groundwater system (Winograd and 

Thordarson, 1975), as well as in other parts of the Eureka Low, suggests that a significant 

amount of heat is redistributed by the hydrologic system and that the heat flows observed 

in wells throughout much of the rest of the system (outside of the discharge areas) are 

depressed relative to the ambient heat flow at the base of the system.

The inverted slope in the thermal profile observed in the deeper part of drill hole 

UE-25p#l (Figure 6) indicates either (1) that significant vertical flow was occurring in the 

bore hole during measurement of the profile or (2) that there is strong lateral flow within 

the carbonate aquifer in the vicinity of this bore hole. Either way, this profile is not

representative of the thermal regime in the carbonate aquifer under Yucca Mountain as a

*

13
whole. Moreover, the very low bottom-hole temperature in UE-25p#l is almost certainly 

a local anomaly and is probably associated with channelized flow through a fracture 

system that was encountered in the deeper part of this hole. Although local in nature, the 

magnitude of this anomaly nonetheless supports the interpretation that flow in the 

carbonate aquifer under Yucca Mountain significantly modifies the ambient heat-flow.

The dominant process controlling the long-wavelength part of the heat-flow 

anomaly at Yucca Mountain is probably flow within the deep carbonate aquifer and is 

evidently unrelated to the large hydraulic gradient. This conclusion is appealing for two 

reasons. First, the heat flow low at Yucca Mountain is a very small part of a regional 

feature, the Eureka Low (Figure 2, inset map). A regional process - groundwater flow 

through the carbonate aquifer - thus appears to provide a better explanation than one that is 

based on a local hydrologic feature. Second, hydrochemical data from Yucca Mountain 

indicate that the water in the carbonate aquifer has about ten times the chlorine abundance 

of the water in the tuff aquifer [La Camera and Westerburg, 1994]. These data do not, by 

themselves, disprove the drain model; however, they can be reconciled with it only 

through special pleading.

The dam and spillway models both appear acceptable as physical-process 

explanations for the large hydraulic gradient, given existing hydrologic and heat-flow 

data. However, these two models yield strongly contrasting predictions about the pattern 

of vertical hydraulic gradients in the immediate vicinity of the large hydraulic gradient 

(Figure 5, A and D). It appears likely that the details of the potentiometric regimes 

calculated in these simulations could change significantly with the addition of the third 

dimension, and with changes in boundary-condition assumptions that currently are poorly 

constrained. Nonetheless, the very large difference in the simulated potentiometric 

patterns for these two models suggest that a well-designed drilling and hydrologic testing 

program that targets the immediate area of the large hydraulic gradient would yield data 

that could provide definitive answers about the cause of this feature.

14
References

Bath, G.D. and C.E. Jahren, 1984, Interpretations of magnetic anomalies at a potential

repository site located in the Yucca Mountain area, Nevada Test Site, U.S.

Geological Survey Open-File Report, 84-120, 40pp. 

Bish, D. L., 1989, Evaluation of past and future alterations in tuff at Yucca Mountain,

Nevada, based on clay mineralogy of drill cores USW G-l, G-2, and G-3: Los

Alamos National Laboratories, Los Alamos, NM, Report LA-10667-MS, 42 p. 

Bodvarrson, G., S.M. Benson, and P.A. Witherspoon, 1982, Theory of the development

of geothermal systems charged by vertical faults, Jour. Geophys. Res., 87(B11),

9317-9328. 

Boynton, G.R. and J.L. Vargo, 1963, Aeromagnetic map of the Topopah Spring

Quadrangle and part of the Bare Mountain Quadrangle, Nye County, Nevada, U.S.

Geological Survey Geophysical map GP-440, 1 sheet, 1:62 500. 

Bredehoeft, J.D. and S.S. Papadopulos, 1965, Rates of vertical groundwater movement

estimated from the earth's thermal profile, Water Resources Research, 1(2), 325-

328. 

Craig, R.W. and J.H. Robison, 1984, Geohydrology of the rocks penetrated by well UE25pl, Yucca Mountain area, Nye County, Nevada, U.S. Geological Survey WaterResources Investigation Report, 84-4248, 57pp. 

Czarnecki, J.B. andR.K. Waldell, 1984, Finite-Element Simulation of Ground-Water

Flow in the Vicinity of Yucca Mountain, Nevada-California, U.S. Geological

Survey Water-Resources Investigation Report, 84-4349, 38pp. 

Czarnecki, J., 1994, Is there perched water under Yucca Mountain in borehole USW G-2?

EOS, American Geophysical Union Fall Meeting, abs., 249-250. 

Czarnecki, J.B., P.H. Nelson, G.M. O'Brien, J. Sass, B. Thapa, Y. Matsumoto, andO.

Murakami, 1995, Testing in borehole USW G-2 at Yucca Mountain: the saga

continues, EOS, American Geophysical Union Fall Meeting, abs., 190. 

Ervin, E.M., R.R. Luckey, and D.J. Burkhardt, 1994, Revised Potentiometric-Surface

Map, Yucca Mountain and Vicinity, Nevada, U.S. Geological Survey WaterResources Investigations Report 93-4000, 17pp. 

Fridrich, C.J., W.W. Dudley, Jr., and J.S. Stuckless, 1994, Hydrogeologic analysis of the

saturated-zone ground-water system, under Yucca Mountain, Nevada, Jour.

Hydrology, 154, 133-168. 

Geldon, A.L., 1993, Preliminary hydrogeologic assessment of boreholes UE-25c #1, UE25c#2, UE-25c#3, Yucca Mountain, Nye County, Nevada, U.S. Geological Survey

Water-Resources Investigation Report, 92-4016. 

Haws, S.J., 1990, The Distribution of Vertical Groundwater Flow in the Saturated Zone

of the Yucca Mountain Area: a Cross-Sectional Finite Element Model, Master

Thesis, University of Las Vegas, 120pp. 

Hinds, J., C. Fridrich, and S. Ge, 1997, Perched water dynamics at Yucca Mountain,

Nevada: a numerical approach, in Hydrology Days, Proceedings of the Seventeenth

Annual American Geophysical Union, 127-140. 

Jasek, N.A., 1991, Simulated Effects of Changes in the Infiltration Rate and the Hydraulic

V

15
Conductivity Structure on the Location and Configuration of the Water Table at 

Yucca Mountain, Nevada, Master Thesis, Texas A&M University, 70pp.

LeCain, G.D., 1996, Air-injection testing in vertical boreholes in welded and nonwelded 

tuff, Yucca Mountain, Nevada, U.S. Geological Survey Water-Resources 

Investigation Report 96-4262.

La Camera, R. J., and Westerburg, C. L., 1994, Selected ground-water data for Yucca 

Mountain region, southern Nevada and eastern California, through December, 

1992: U. S. Geological Survey Open-File Report 94-54, 161 pp.

Lachenbruch, A.H. and J.H. Sass, 1977. Heat flow in the United States and the thermal 

regime of the crust, in The Earth's Crust, edited by J.G. Heacock, Geophysical 

Monograph 20, American Geophysical Union, Washington, D.C., 626-675.

Lobmeyer, D.H., M.S.Whitfield, Jr, R.G. Lahoud, and L. Bruckheimer, 1983,

Geohydrologic data for test well UE-25b-l, Nevada Test Site, Nye County, 

Nevada, U.S. Geological Survey Open-File Report, 83-855, 52pp.

Marshall, B.D., Z.E. Peterman, and J.S. Stuckless, 1993, Strontium isotopic evidence for 

a higher water table at Yucca Mountain: Proceedings of the Fourth International 

Conference on High-Level Radioactive Waste Management, American Nuclear 

Society, 2, 1948-1952.

Montazer, P. and W. E. Wilson, 1984, Conceptual hydrologic model of flow in the 

unsaturated zone, Yucca Mountain, Nevada, U.S. Geological Survey WaterResources Investigation Report, 84-4345.

Muller, D.C. and I.E. Kibler, 1986, Preliminary analysis of geophysical logs from the 

WT series of drill holes, Yucca Mountain, Nye County, Nevada, U.S. Geological 

Survey Open-File Report, 86-46, 30 pp.

National Research Council (NRC), 1992, Ground Water at Yucca Mountain, How High 

Can it Rise, National Academy Press, Washington D.C., 231pp.

Nelson P.H. and U. Schimschal, 1993, Assessment of geophysical logs from borehole 

USW G-2, Yucca Mountain, Nevada, U.S. Geological Survey Open-File Report, 

92-572.

Pruess, K., 1991, TOUGH2 - A General-Purpose Numerical Simulator for Multiphase 

Fluid and Heat Flow, Lawrence Berkeley Laboratory, CA, 102pp.

Robison, J.H., 1984, Ground-water Level Data and Preliminary Potentiometric Surface 

Maps, Yucca Mountain and Vicinity, Nye County, Nevada, U.S. Geological 

Survey Water-Resources Investigation Report 84-4197, 8pp.

Rousseau, J.P., E.M. Kwicklis, and D.C. Gillies, 1996, Hydrogeology of the unsaturated 

zone, north ramp area of the exploratory studies facility, Yucca Mountain, Nevada, 

U.S. Geological Survey Water-Resources Investigation Report, 272pp.

Sass, J.H., W.W. Dudley, Jr., A.H. Lachenbruch, 1995, Chapter 8: Regional thermal 

setting, in Oliver, H.W., Ponce, D.A., and Hunter, W.C., Major Results of 

Geophysical Investigations at Yucca Mountain and Vicinity, Southern Nevada: 

U.S. Geological Survey Open-File Report 95-74, 199-218.

Sass, J.H., A.H. Lachenbruch, W.W. Dudley, Jr., S.S. Priest, andR.J. Munroe, 1988, 

Temperature, thermal conductivity, and heat flow near Yucca Mountain, Nevada: 

some tectonic and hydrologic implications, U.S. ^Geological Survey Open-File

16
Report 87-649, 118pp.

Sass, J.H., A.H. Lachenbruch, RJ. Munroe, G.W. Greene, andT.H. Moses, Jr., 1971, 

Heat flow in the western United States, Jour. Geophys. Res., 76, 6376-6413.

Scott, R.B. and J. Bonk, 1989, Preliminary geologic map of Yucca Mountain with 

geologic sections, Nye County, Nevada, U.S. Geophysical Survey Open-File 

Report 84-494, 3 sheets, 1:12000.

Sinton, P.O., 1989, Three-Dimensional, Steady-State, Finite-Difference Model of the 

Groundwater Flow System in the Death Valley Groundwater Basin, NevadaCalifornia, Master thesis, Colorado School of Mines, 145pp.

Stuckless, J. S., Whelan, J. F., and Steinkampf, W. C., 1991, Isotopic discontinuities in 

ground water beneath Yucca Mountain, Nevada: High-Level Radioactive Waste 

Management, 2nd Int. Symp., Las Vegas, NV, 28 April - 1 May, 1991, v. 2, p. 

1410-1415, American Nuclear Society, La Grange, IL.

Waddell, R.K., 1982, Two-dimensional, steady-state model of ground-water flow, Nevada 

Test Site and vicinity, Nevada-California, U.S. Geological Survey WaterResources Investigation, 82-4085, 72pp.

Waddell, R.K., J.H. Robinson, and R.K. Blankennagel, 1984, Hydrology of Yucca

Mountain and vicinity, Nevada-California - Investigation results through mid-1983, 

U.S. Geological Survey Water-Resources Investigation Report 84-4267, 72pp.

Winograd, I.J. and W. Thordarson, 1975, Hydrogeologic and hydrochemical framework, 

south-central Great Basin, Nevada-California, with special reference to the Nevada 

test Site, U.S. Geological Survey Professional Paper 712-C, 126pp.

17
Figure 1. Location map of Yucca Mountain (YM) in the southern Great Basin in relation 

to the boundaries of the southwest Nevada volcanic field and the Death valley 

ground-water system.

Figure 2. Map of the Yucca Mountain area with the water-table configuration, the

pattern of heat flow in the unsaturated zone, and the locations of deep bore 

holes as black dots (modified from Robison [1984], Ervine et al. , [1994] and 

Sass, et al. [1988; 1995]. Bedrock area is shown by light shading. Yucca 

Mountain is the dendriform bedrock area occupying the west side of the figure. 

Line A-A' is the line of section shown in Figure 3.

Figure 3. North-northwest ~ south-southeast cross section across Yucca Mountain based 

on subsurface geology and gravity data; taken from Fridrich et al. [1994]. 

Since this figure was drafted, the Prow Pass, Bullfrog, and Tram Members of 

the Crater Flat Tuff have been formally renamed as the Prow Pass, Bullfrog, 

and Tram Tuffs of the Crater Flat Group (Sawyer and others, 1994). The 

Eleana Argillite (Formation), projected into this cross section from the Calico 

Hills based on an inteterpretation of aeromagnetic data (Bath and Jahren, 

1984), has since been reclassified as Chainman Shale (J. C. Cole, USGS, oral 

comm., 1997).

Figure 4. Schematic illustrations of the three hypotheses for the cause of the large 

hydraulic gradient: (a) drain model, (b) spillway model, (c) dam model.

Figure 5. Simulated potentiometric regimes for: (A) the base model, (B) the spillway 

model, (C) the drain model, and (D) the dam model.

Figure 6. Comparison of the simulated (lines) and observed (boxes) thermal profiles for 

the (A) spillway model, (B) dam model, and (C) drain model, in the four wells 

that lie along the cross section used.

18
Table 1. Summary of hydrologic and physical properties used in the model

Hydrostratigraphic Unit

Tiva Canyon/Topopah Spring

Calico Hills upper 

lower

Crater Flat Tuff Aquifer 

north & central 

south

Tuff Aquitard

Carbonate Aquifer upper 

lower

Buried fault

Faults in the south

Permeability 

m2

io-15

io-15

10-ie

io-15 

io-14

io-16

io-13 

io-14

io-14

io-14

Density 

kg/m3

2600

2400 

2400

2600 

2600

2700

2700 

2700

2700

2700

Porosity

0.2

0.35 

0.35

0.20 

0.20

0.2

0.20 

0.20

0.2

0.2

Thermal Conductivity 

W/m °C

1.87

1.22 

1.22

1.59 

1.59

1.96

4.90 

4.90

1.96

1.96

19
iir 115*30-

37'30-f36'30'h-

 "»/" » Southwest Nevada 

' -.. ' Volcanic Field (SNVF)

Calderas (TMCC = Timber 

Mountain CaJdera Complex)

Nevada Test Site (NTS)

 ^ Death Vafley groundwater

/ system of Waddel, 1982 (DVGS)

Alkali Flat Furnace Creek 

subbasin (AFFCSB) of Czamedd O and Wadded, 1984 D

Recharge areas 

Discharge areas

Area of Figure 2

(YM = Yucca Mountain)

FIGURE 1

20
SaM/Yuccarth-3

NEVADA

Eureka 

Carson

'x'x'x'x' 

iVx'x'x'

,'x'x'x'x'x'x' 

tttffttt

VUSW H-4-

X '/ > > «' '/ X

\ \ 

xx

Water-table contour, 

meters above sea-level

Proposed repository

FIGURE 2

21
A'

ItOUTHCAtn

UE26-P91

500

1000

FIGURE 3

22
Sass/Yucca/tlg 4

Confining Units 

Flow

Tuff Aquifer

Deep Carbonate Aquifer 

Water Table (WT)

FIGURE 4

23

Depth below land surface (m)

§ 080 

o o o

ro 

un

 n

> t

O 

G

Oeplh below land surface (m)