Status of Understanding of the Saturated-Zone 

Ground-Water Flow System at Yucca Mountain, 

Nevada, as of 1995

by Richard R. Luckey, Patrick Tucci, Claudia C. Faunt, Elisabeth M. Ervin, 

William C. Steinkampf, Frank A. D'Agnese, and Gary L. Patterson

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 96-4077

Prepared in cooperation with the 

NEVADA OPERATIONS OFFICE, 

U.S. DEPARTMENT OF ENERGY, under 

Interagency Agreement DE-A108-92NV10874

Denver, Colorado 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

Chief, Hydrologic Investigations Program

Yucca Mountain Project Branch

U.S. Geological Survey

Box 25046, Mail Stop 421

Denver Federal Center

Denver, CO 80225-0046

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

Abstract....................................................................^ 1

Introduction........................................................................................................................................................... 2

Historical and Regulatory Background....................................................................................................................... 2

Puipose and Scope...........................................» 3

Previous Work............................................................................................................................................................. 6

Geologic Setting.................................................................^^^ 7

Geologic History....................................................^ 10

Stratigraphy...................................................................»^^^ 10

Stractore............................................................^^ 12

Regional Hydrologic Setting................................................................................................................................................. 13

Hydrology of the Saturated Zone at Yucca Mountain........................................................................................................... 16

HydrogeologicUmts......................................................^ 17

Potentiometric Surface................................................................................................................................................ 21

Large Hydraulic Gradient.................................................................................................................................. 21

Moderate Hydraulic Gradient............................................................................................................................ 25

Small Hydraulic Gradient.................................................................................................................................. 26

Vertical Hydraulic Gradients....................................................................................................................................... 27

Potentiometric-Level Fluctuations and Trends............................................................................................................ 29

Hydraulic Data................................................................................................................................^ 32

Aquifer Tests................................................._ 32

Flow Surveys..................................................................................................................................................... 37

Inflow to Volcanic Flow System.................................................................................................................................. 39

Upgradientmflow............................................^ 39

Fortymile Wash Recharge.................................................................................................................................. 39

Local Infiltration................................................................................................................................................ 40

Inflow From the Carbonate Aquifer.................................................................................................................. 40

Outflow From Volcanic Flow System.......................................................................................................................... 41

Downgradient Outflow...................................................................................................................................... 41

Pumpage............................................................................................................................................................ 41

Outflow to the Carbonate Aquifer..................................................................................................................... 42

Flow Into Unsaturated Zone.............................................................................................................................. 42

HydrochenucalData.......................................................^ 42

Paleohydrology............................................................................................................................................................ 44

Northern Discharge Deposits............................................................................................................................. 45

Southern Discharge Deposits............................................................................................................................. 45

Zeolites and the Water Table............................................................................................................................. 45

Local Hydrologic Setting (C-Hole Complex) ....................................................................................................................... 47

Discussion of the Saturated Zone at Yucca Mountain........................................................................................................... 51

Conceptual Uncertainties.................................................................................^^ 52

Data Uncertainties....................................................................................................................................................... 52

Recharge......................................................................^^ 53

Storage Properties................................................^ 53

Transmissive Properties..................................................................................................................................... 53

Discharge..................................................._ 54

Potentiometric Levels........................................................................................................................................ 55

Contents III
Proposed Simulations..............................................^ 55

Discrete Features.................................................,..........^^ 56

Hydraulic Gradients.......................................................................................................................................... 56

Climate and Recharge....................................................................................................................................... 57

Additional Data Needs................................................................................................................................................ 58

Summary........................................................................^^ 59

Selected References...................................................^ 60

FIGURES

1-3. Maps showing:

1. Locations of major geographic features in the Death Valley region. A, Death Valley groundwater basin, and B, Alkali Flat-Furnace Creek subbasin................................................................................... 4

2. Locations of southwestern Nevada volcanic field and selected volcanic features............................................ 8

3. Locations of major geographic features and selected faults in the Yucca Mountain area................................. 9

4. Chart showing selected stratigraphic units in the vicinity of Yucca Mountain that are important to

the hydrology......................................................................^ 10

5. Map showing locations of deep boreholes in the vicinity of Yucca Mountain......................................................... 11

6. Map showing potentiometric surface, Death Valley region..................................................................................... 15

7. Chart showing selected geologic, thermal/mechanical, and hydrogeologic units at

Yucca Mountain, Nevada.......................................................................................................................................... 18

8-11. Maps showing:

8. Preliminary potentiometric surface, Yucca Mountain....................................................................................... 22

9. Revised potentiometric surface of an area of small hydraulic gradient, Yucca Mountain................................ 23

10. Potentiometric levels in the lower volcanic confining unit and carbonate aquifer............................................ 26

11. Locations of boreholes in the vicinity of Yucca Mountain in which water levels are monitored..................... 30

12. Hydrographs showing potentiometric-level fluctuations, 1985-94, for boreholes USW WT-2 and

UE-25 p#l at Yucca Mountain................................................................................................................................. 31

13. Hydrographs showing response of the potentiometric levels in borehole USW H-6 to earthquakes

at Little Skull Mountain, Nevada, on June 29,1992................................................................................................ 33

14. Map showing locations of aquifer test data at Yucca Mountain............................................................................... 34

15. Flow surveys for selected boreholes in the Yucca Mountain area............................................................................ 38

16. Diagram showing diagenetic zones in the volcanic sequence at Yucca Mountain................................................... 47

17. Map showing layout of the C-hole complex............................................................................................................ 49

18. Chart showing stratigraphy and lithology at the C-hole complex............................................................................ 50

TABLES

1. Selected regional stratigraphic and hydrogeologic units in the vicinity of Yucca Mountain................................. 16

2. Altitudes of hydrogeologic units in deep boreholes at Yucca Mountain................................................................ 19

3. Potentiometric levels at selected depth intervals in boreholes at Yucca Mountain................................................ 27

4. Estimated apparent hydraulic-conductivity values obtained from single-borehole aquifer tests

in the vicinity of Yucca Mountain.......................................................................................................................... 35

5. Estimated transmissivity values obtained from single-borehole aquifer tests in the vicinity

of Yucca Mountain.................................................................................................................................................. 36

6. Boreholes in the vicinity of Yucca Mountain from which hydrochemical data were obtained.............................. 43

7. Estimates of ground-water discharge at Alkali Flat and Furnace Creek Ranch..................................................... 54

IV Contents
CONVERSION FACTORS AND VERTICAL DATUM

Multiply

centimeter (cm) 

cubic meter (m3) 

cubic meter per year (m3/yr)

kilometer (km)

liter per second (L/s)

meter (m)

meter per day (m/d)

meter per kilometer (m/km)

meter per second (m/s)

meter per year (m/yr) 

meter squared per day (m2/d)

millimeter (mm)

millimeter per year (mm/yr) 

square kilometer (km2)

By

0.3937 

264.2 

264.2

0.6214

15.85

3.281

3.281

5.280

3.281

3.281 

10.76

0.03937

0.03937 

0.3861

To obtain

inch 

gallon 

gallon per year

mile

gallon per minute

foot

foot per day

foot per mile

foot per second

foot per year 

foot squared per day

inch

inch per year 

square mile

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level 

nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Contents
Status of Understanding of the Saturated-Zone 

Ground-Water Flow System at Yucca Mountain, 

Nevada, as of 1995

^Richard R. Luckey, Patrick Tucci, Claudia C. Faunt, Elisabeth M. Ervin, 

William C. Steinkampf, Frank A. D'Agnese, ancfGary L. Patterson

Abstract

Yucca Mountain, which is being studied 

extensively because it is a potential site for a 

high-level radioactive-waste repository, consists 

of a thick sequence of volcanic rocks of Tertiary 

age that are underlain, at least to the southeast, by 

carbonate rocks of Paleozoic age. Stratigraphic 

units important to the hydrology of the area 

include the alluvium, pyroclastic rocks of 

Miocene age (the Timber Mountain Group; the 

Paintbrush Group; the Calico Hills Formation; the 

Crater Flat Group; the Lithic Ridge Tuff; and 

older ruffs, flows, and lavas beneath the Lithic 

Ridge Tuff), and sedimentary rocks of Paleozoic 

age. The saturated zone generally occurs in the 

Calico Hills Formation and stratigraphically 

lower units. The saturated zone is divided into 

three aquifers and two confining units. The upper 

volcanic aquifer consists of the densely welded 

section of the Topopah Spring Tuff of the Paintbrush Group. It is generally saturated only to the 

west, south, and east of Yucca Mountain. The 

upper volcanic confining unit consists of the lowermost part of the Topopah Spring Tuff, the 

Calico Hills Formation, and the uppermost part of 

the Crater Flat Group. The lower volcanic aquifer 

consists of most of the Crater Flat Group. The 

lower volcanic confining unit consists of the 

Lithic Ridge Tuff and older tuffs, flows, and 

lavas. The carbonate aquifer consists of a thick 

sequence of Paleozoic carbonate rocks that may 

be confined locally by an overlying clastic unit,

the Eleana Formation of Mississippian and Late 

Devonian age.

The flow system at Yucca Mountain is part 

of the Alkali Flat-Furnace Creek subbasin of the 

Death Valley ground-water basin. This subbasin 

is recharged by water from the higher mesas north 

of Yucca Mountain, and possibly by interbasin 

inflow, and discharges most, if not all, of its water 

at Alkali Flat. Variations in the gradients of the 

potentiometric surface provided the basis for subdividing the Yucca Mountain area into zones of: 

(1) large hydraulic gradient where potentiometric 

levels change at least 300 meters in a few kilometers; (2) moderate hydraulic gradient where 

potentiometric levels change about 45 meters in a 

few kilometers; and (3) small hydraulic gradient 

where potentiometric levels change only about 

2 meters in several kilometers. Vertical hydraulic 

gradients were measured in only a few boreholes 

around Yucca Mountain; most boreholes had little 

change in potentiometric levels with depth. The 

larger vertical gradients measured generally were 

upward.

Limited hydraulic testing of boreholes in 

the Yucca Mountain area indicated that the range 

in transmissivity was more than 2 to 3 orders of 

magnitude in a particular hydrogeologic unit, and 

that the average values for the individual hydrogeologic units generally differed by about 1 order 

of magnitude. The upper volcanic aquifer seems 

to be the most permeable hydrogeologic unit, but 

this conclusion was based on exceedingly limited 

data.

Abstract
INTRODUCTION

The Nuclear Waste Policy Act of 1982 directed 

the U.S. Department of Energy to conduct a program 

to find a suitable site to permanently dispose of highlevel radioactive waste in a mined geologic repository. 

In 1983, the U.S. Department of Energy identified nine 

potential sites, including Yucca Mountain in southern 

Nevada (U.S. Department of Energy, 1986, p. 1), 

which is about 150 km northwest of Las Vegas (fig. 1). 

The nine sites were later reduced to three, and in 

December 1987, the U.S. Congress designated Yucca 

Mountain as the sole site to be studied as a potential 

repository site. A 150-km2 area enclosing the Yucca 

Mountain site has been studied extensively since 1978 

(Waddell and others, 1984, p. 2) by the U.S. Geological Survey (USGS) and various national laboratories. 

Yucca Mountain is located in an arid environment; 

average annual precipitation ranges from about 

130 mm at the base of the mountain to about 170 mm 

at the northern part of the crest (D.S. Ambos, Foothill 

Engineering Consultants, oral commun., 1994). The 

remote location and the arid environment are two features that make the Yucca Mountain site attractive for 

disposal of radioactive waste.

Because water would be the primary mechanism 

for moving most radionuclides away from a geologic 

repository, a study of the hydrology of a potential 

site is critical. Only a few radionuclides would 

move away from a repository in a gaseous phase 

(Von Konynenburg and others, 1985). Radionuclides 

could move naturally in ground water, in dissolved, 

colloidal, or suspended form. Generally, the term 

"ground water" means water moving through the saturated zone; however, as used for the Yucca Mountain 

Project and in this report, ground water also includes 

liquid water moving through the unsaturated, or 

vadose, zone. The Yucca Mountain Project refers to 

the collection of various site-characterization and construction activities carried out by the U.S. Department 

of Energy at Yucca Mountain.

The U.S. Department of Energy is using numerous organizations to carry out scientific studies at 

Yucca Mountain. The organizations were already 

involved in scientific studies for the U.S. Department 

of Energy at the Nevada Test Site when the first boreholes were drilled at Yucca Mountain. These organizations include the USGS, Los Alamos National 

Laboratory, Lawrence Livermore National Laboratory, 

and Sandia National Laboratory. The USGS is the

lead organization for hydrologic and geologic studies, 

Los Alamos National Laboratory is the lead organization for geochemical studies, Lawrence Livermore 

National Laboratory is the lead organization for wastepackage studies, and Sandia National Laboratory is 

the lead organization for performance assessment 

studies. Lawrence Berkeley Laboratory is providing 

support to the USGS in site-scale numerical modeling 

the unsaturated zone and fracture-flow modeling of 

the saturated zone. The Bureau of Reclamation, 

U.S. Department of the Interior, is providing support 

to the USGS in the area of geologic studies and is 

providing direct engineering-geology support to the 

U.S. Department of Energy.

This report was formulated in cooperation with 

the Yucca Mountain Site Characterization Project, 

Office of the U.S. Department of Energy under 

Interagency Agreement DE-AI08-92NV10874.

Historical and Regulatory Background

In early attempts to find a suitable site for a 

high-level radioactive-waste repository, virtually all 

investigations were directed toward saturated sites 

(U.S. Department of Energy, 1986, p. 5) because the 

waste would have to be buried deeply. However, 

when the Great Basin and the area around the Nevada 

Test Site were evaluated in the 1970's, the thick unsaturated zone was noted. Winograd (1981) recognized 

that the unsaturated zone in the Great Basin might be 

suitable for disposal of high-level radioactive waste 

and discussed a potential area at the Nevada Test Site 

(not Yucca Mountain). He pointed out that groundwater movement in the unsaturated zone can be 

extremely slow, and disposal of waste in the unsaturated zone could provide long-term isolation. Another 

investigator (Roseboom, 1983) pointed out that an 

unsaturated-zone repository might eliminate many 

technical problems that could result from disposal in 

the saturated zone. He also argued that the waste in 

the unsaturated zone could be fully retrievable for a 

long period of time. The water table at Yucca Mountain is extremely deep (Robison, 1984, table 1) and the 

unsaturated zone is up to 750 m thick.

The concept of multiple barriers to radionuclide 

migration away from a repository is an important 

aspect of the study of Yucca Mountain. Multiple barriers include engineered and natural barriers. Engineered barriers include the waste form, the waste

2 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
canister, and any materials introduced in or around the 

canister. Natural barriers include the geologic units 

(and their geochemical waste-isolation properties) that 

radionuclides would have to move through to migrate 

from the site.

The saturated zone at Yucca Mountain provides 

the final transport pathway and the last natural barrier 

between the potential repository and the accessible 

environment . The saturated zone at Yucca Mountain 

is considered to be a secondary natural barrier to radionuclide transport (U.S. Department of Energy, 1988, 

sec. 6.1.5) because ground-water travel time in the saturated zone probably is much shorter than travel time 

in the unsaturated zone. Because the saturated zone is 

a secondary barrier, only limited characterization of it 

may be appropriate. However, flow and transport in 

the unsaturated zone might be difficult to characterize 

adequately or the uncertainty of its characterization 

may be large, whereas characterization of flow and 

transport in the saturated zone may have less uncertainty and it may be able to be done less expensively. 

Therefore, the degree to which the saturated zone at 

Yucca Mountain needs to be characterized remains 

(in 1996) an open question.

Two major regulatory issues can be addressed in 

part by characterizing the saturated zone at Yucca 

Mountain: (1) Ground-water travel time; and 

(2) releases of radionuclides to the accessible environment. The regulations for a geologic repository 

(U.S. Nuclear Regulatory Commission, 1986, 

p. 618-619) include (sec. 113):

The geologic repository shall be located so 

that pre-waste-emplacement groundwater 

travel time along the fastest path of likely 

radionuclide travel from the disturbed zone to 

the accessible environment shall be at least 

1,000 years or such other travel time as may 

be approved or specified by the [Nuclear 

Regulatory] Commission, 

and (sec. 112):

The geologic setting shall be selected and the 

engineered barrier system and the shafts, bore1The accessible environment is a regulatory term that 

includes the atmosphere, land surface, surface water, and all of the 

lithosphere that is beyond the controlled area. The controlled area 

is the surface location and the underground area beneath that location that encompasses no more than 100 km2 and extends horizontally no more than 5 km in any direction from the outer boundary 

of the original location of the radioactive wastes in a repository 

(U.S. Department of Energy, 1988, vol. 8, p. G-l and G-20).

holes and their seals shall be designed to 

assure that releases of radioactive materials to 

the accessible environment following permanent closure conform to such generally applicable environmental standards for radioactivity as may have been established by the 

U.S. Environmental Protection Agency with 

respect to both anticipated processes and 

events and unanticipated processes and 

events.

The exact nature of the standards to be set by the 

U.S. Environmental Protection Agency for the Yucca 

Mountain site have not yet been determined. The 

ground-water travel-time issue can be addressed 

directly by studying the hydrology of the unsaturated 

and saturated zones once such terms as "likely radionuclide travel" and "disturbed zone," which are part of 

the regulations, are adequately defined and understood. The issue of releases to the accessible environment, although dependent on hydrology, also is greatly 

affected by the rate of waste release from the engineered barriers, geochemical retardation by minerals 

in the rocks, and dispersive and diffusive processes.

Purpose and Scope

This report documents the status of understanding of the saturated-zone flow system at Yucca Mountain as of 1995. This understanding was developed 

using the work of a large number of investigators who 

have studied the saturated-zone hydrology of the 

Yucca Mountain and the Death Valley region for several decades. The many people who have contributed 

to the understanding of the flow system at Yucca 

Mountain are indicated in the "Selected Reference" 

section. The list of references is extensive, but not 

exhaustive. It does include all reports that were used 

in writing this report. In addition to the references, the 

authors relied heavily on discussions with many 

USGS investigators, especially IJ. Winograd, 

W.W. Dudley, Jr., J.H. Robison, M.P. Chornack, 

R.K. Waddell, and J.B. Czarnecki.

The concepts and information presented in this 

report are preliminary in that they were developed 

early in the formal characterization of the saturatedzone flow system at Yucca Mountain that began in 

1989. Formal characterization began after detailed 

planning for that characterization was completed. 

This report relies, in large part, on data collected in the 

1970's and 1980's before formal characterization

INTRODUCTION
118° 117° 115°

b-relief base from U.S. Geological Survey digital 

:250,000, 19B7; sun illumination from northwest 

above horizon

0 50

L NEVADA 

I.R.no

Vc \

\\ \-> \ ^ OF -as \ \, vr1 \ '1 MAP\ I V L^

I 

0 25

^mm

AD 

AF 

BT

Vegas DV 

FM 

LV 

MC

100 KILOMETERS

I

I 

50 MILES

EXPLANATION

 Boundary of Death Valley ground-water 

basin (from D'Agnese, 1994)

Amargosa Desert PH Pahrump 

Alkali Flat PM Pahute Mesa 

Beatty PV Pahranagant Valley 

Death Valley SM Spring Mountains 

Funeral Mountains SR Sheep Range 

Las Vegas YM Yucca Mountain 

Mercury

Los Angeles 

Figure 1. Locations of major geographic features in the Death Valley region. A, Death Valley 

ground-water basin, and B, Alkali Flat-Furnace Creek subbasin.

4 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
116*00'

36'30'

36W

Shaded-relief base from U.S. 

Geological Survey digital 

data, 1:250,000,1987; 

sun illumination from 

northwest at 30° above horizon

10 20

I

30 KILOMETERS

j

I 

10 15 MILES

EXPLANATION

Approximate boundary of the Alkali 

Flat-Furnace Creek subbasin of the 

Death Valley ground-water basin. 

Northern boundary probably is flow 

boundary with underflow from Pahute 

and Rainer Mesas.

AF Alkali Flat

AD Amargosa Desert

AV Amargosa Valley (town)

BM Bare Mountain

BT Beatty

BW Beatty Wash

CH Calico Hills

CF Crater Flat

DJ Death Valley Junction

DV Death alley

FC Furnace Creek Ranch

FM Funeral Mountains

JF Jackass Flats

LV Las Vegas

OV Oasis Valley

PH Pahrump

PM Pahute Mesa

PR Pinnacles Ridge

RM Ranier Mesa

SM Shoshone Mountain

TM Timber Mountain

YF Yucca Flat

YM Yucca Mountain

Figure 1. Locations of major geographic features in the Death Valley region. A, Death Valley 

ground-water basin, and B, Alkali Flat-Furnace Creek subbasin Continued.

INTRODUCTION
began in 1989 because only limited characterization of 

the saturated zone has been done since 1989.

The emphasis of this report is on the saturatedzone flow system in the loosely defined area discussed 

in the "Hydrology of the Saturated Zone at Yucca 

Mountain" section. Although the report emphasizes 

the site-scale flow system, it also describes the 

regional flow system in some detail to put the sitescale flow system into perspective. In addition, some 

small-scale (within tens of meters) effects observed at 

a multiple-borehole complex are described. This 

local-scale study will help define the complexities and 

variability of the flow system at individual points 

within the site area. Transport characteristics, including the associated geochemical, dispersive, and diffusive processes, are beyond the scope of this report.

Although some components of the ground-water 

flow system (such as the configuration of the potentiornetric surface in an area of small hydraulic gradient) 

are fairly well defined as of 1995, other components 

(such as the cause of a large hydraulic gradient) are 

not well understood. This report also documents the 

level of understanding and defines major uncertainties 

about the flow system. The report indicates areas 

where more information is needed for an adequate or 

even elementary understanding of the flow system. 

This report is intended to lay a foundation for the 

creation of numerical models of the saturated-zone 

ground-water flow system at Yucca Mountain, but it 

does not discuss detailed formulation of numerical 

models. It does, however, suggest how numerical 

models may be used to fully test various hypotheses 

about the flow system and to improve the understanding of the flow system.

Previous Work

The first comprehensive study of the hydrology 

of the Nevada Test Site and surrounding areas was 

reported by Winograd and Thordarson (1975). Their 

report included: (1) A study of regional flow through 

carbonate rocks that underlie much of southern 

Nevada; (2) a study of volcanic rocks in the area of 

underground nuclear-weapons testing at Yucca Flat; 

(3) a study of the hydrochemistry of water throughout 

the area; and (4) identification of potential hydrologic 

barriers and their effects on ground-water flow. A 

broader regional, although less intense, study is 

contained in Bedinger and others (1984). Montazer

and Wilson (1984) described a conceptual model of 

ground-water flow in the unsaturated zone beneath 

Yucca Mountain; Czarnecki (1989a) and Czarnecki 

and Wilson (1991) summarized regional ground-water 

flow germane to Yucca Mountain using conceptual 

models. Downey and others (1990) discussed various 

criteria for delimiting boundaries of the regional flow 

system. Other hydrologic reconnaissance studies that 

described the regional flow system include Eakin and 

others (1963), Walker and Eakin (1963), Rush (1970), 

Waddell (1982), Waddell and others (1984), Czarnecki

(1990), Kilroy (1991), and Oatfield and Czarnecki

(1991).

Several numerical models at various scales were 

developed to describe the saturated-zone flow system 

in the vicinity of Yucca Mountain. Modeling was 

done for the area of the Nevada Test Site and vicinity 

by Oberlander (1979), Waddell (1982), Rice (1984), 

and Sinton (1987). Waddell (1982) and Rice (1984) 

examined two-dimensional areal ground-water flow, 

whereas Sinton (1987) incorporated the third dimension (depth) in a quasi-three dimensional application 

of the MODFLOW computer program (McDonald and 

Harbaugh, 1988). D'Agnese (1994) constructed a 

three-dimensional numerical model of much of southern Nevada and adjacent eastern California using the 

hydrogeologic framework developed by Faunt (1994).

Czarnecki and Waddell (1984) developed a twodimensional parameter-estimation model to estimate 

values of transmissivity throughout the ground-water 

subbasin that includes Yucca Mountain. Czarnecki 

(1985) used the same finite-element grid to estimate 

the effects of increased recharge on water-table altitude and on direction of ground-water flow. Additional simulations by Czarnecki (1989b, 1991) 

analyzed the effects of abrupt changes in hydraulic 

conductivity in the vicinity of a large hydraulic gradient and the effects of increases in recharge on the flow 

system. A cross-sectional model was developed by 

Haws (1990) along a flow path constructed from the 

flow-field vectors of Czarnecki and Waddell (1984). 

Modeling by Carrigan and others (1991) analyzed 

potential excursions of the water table at Yucca Mountain that could be produced by seismic events. Models 

developed by Ahola and Sagar (1992,1993) and 

Dressel (1992) were used to analyze the effects of 

changes in hydraulic properties and recharge on watertable altitudes in the flow system.

Barr and Miller (1987) developed a model of 

ground-water flow in the vicinity of Yucca Mountain

6 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
to use for the saturated-zone facets of performanceassessment modeling to determine if the regulatory 

requirements for a geologic repository could be met. 

Buscheck and Nitao (1992,1993,1995) reported on 

the possible effects of heat from a repository on 

ground-water flow in the unsaturated and saturated 

zones. Barr and others (Chapter 11 of Wilson and 

others, 1994) developed a three-dimensional model at 

the site scale to support performance-assessment 

calculations.

Numerous geophysical surveys were done in the 

vicinity of Yucca Mountain (Oliver and others, 1995), 

largely for the characterization of its geology. Magnetic surveys were reported on by Kane and others 

(1981), Kane and Bracken (1983), Bath and Jahren 

(1984), Glen and Ponce (1991), Langenheim (1995), 

Ponce and Langenheim (1995), and Ponce and others 

(1995). Regional gravity surveys were reported by 

Healey and Miller (1971), Snyder and Carr (1982), 

Healey and others (1987), and Ponce and others 

(1988). Electrical-resistivity surveys were reported by 

Flanigan (1981), Greenhaus and Zablocki (1982), and 

Senterfit and others (1982). Seismic surveys were 

reported by Pankratz (1982) and Brocher and others 

(1993,1996). Borehole-geophysical surveys were 

described by Daniels and others (1981), Muller and 

Kibler (1984), Stock and others (1984), and Nelson 

and others (1991); this latter report contained geophysical logs from 40 boreholes. Boreholetemperature surveys, used to calculate crustal-heat 

flow and to infer ground-water flow and direction, 

were discussed by Sass and Lachenbruch (1980) and 

Sass and others (1988). Overviews of various geophysical surveys were discussed by Carr and Yount 

(1988) and Oliver and others (1991).

Characterizations of the geology from boreholes 

constructed at or near Yucca Mountain were discussed 

by Spengler and others (1979,1981, 1984), Spengler 

and Rosenbaum (1980), Carr (1982), Maldonado 

and Koether (1983), Carr and Parrish (1985), and 

Carr, M.D., and others (1986). Geologic syntheses 

were produced by Carr (1984), Scott and Bonk (1984), 

Scott and others (1984), Robinson (1985), Carr, W.J., 

and others (1986), Swadley and Carr (1987), Carr and 

Yount (1988), Swadley and Parrish (1988), Swadley 

and Hoover (1989), and Diehl and Chornack (1990). 

Sawyer and others (1994) published a revised stratigraphic nomenclature for the volcanic rocks of the 

area that includes Yucca Mountain; their nomenclature 

is used in this report.

Water-level surveys and measurements at or 

near Yucca Mountain were described by Fenske and 

Carnahan (1976), Robison (1984), Waddell and others 

(1984), Robison and others (1988), Gemmell (1990), 

Kilroy (1991), O'Brien (1991,1992), Luckey and 

others (1993), Boucher (1994a), Ciesnik (1995), 

Lobmeyer and others (1995), O'Brien and others 

(1995), and Tucci and others (in press).

Hydraulic-test results and analyses from boreholes at or near Yucca Mountain were reported by 

Bentley and others (1983), Craig and others (1983), 

Lobmeyer and others (1983), Thordarson (1983), 

Bentley (1984), Craig and Johnson (1984), Craig and 

Robison (1984), Rush and others (1984), Thordarson 

and others (1984), Waddell (1984), Whitfield and 

others (1984; 1985), Erickson and Waddell (1985), 

Galloway and Erickson (1985), Thordarson and others 

(1985), Lobmeyer (1986), Craig and Reed (1991), 

Robison and Craig (1991), and Geldon (1993).

Hydrochemical and isotopic data and analyses 

from boreholes at or near Yucca Mountain were 

reported by Winograd and Thordarson (1975), Benson 

and others (1983), Benson and McKinley (1985), 

Claassen (1985), Kerrisk (1987), McKinley and others 

(1991), Stuckless, Whelan, and Steinkampf (1991), 

Thomas and others (1991), and Perfect and others 

(1995).

GEOLOGIC SETTING

Yucca Mountain is located in the Great Basin 

section of the Basin and Range physiographic province (Snyder and Carr, 1982), which is characterized 

by generally linear, north-trending mountain ranges 

separated by intervening basins. Yucca Mountain is in 

the southern part of the southwestern Nevada volcanic 

field of Miocene age (Byers and others, 1976; Sawyer 

and others, 1994) (fig. 2). Yucca Mountain consists of 

a group of north-trending block-faulted ridges that are 

composed of volcanic rocks of Tertiary age that may 

be several kilometers thick (Snyder and Carr, 1984, 

table 1). Yucca Mountain extends northward to 

Pinnacles Ridge (fig. 1) at the southern end of the 

Claim Canyon caldera and southward to approximately U.S. Highway 95 (fig. 2) at the northern edge 

of the Amargosa Desert. Jackass Flats, an alluviumfilled basin with exterior drainage to the south, is east 

of Yucca Mountain and is separated from Yucca 

Mountain by Fortymile Wash (fig. 3). The basin to the

GEOLOGIC SETTING
117°00' 116°00'

37°30' -

37°00' 

36°30l 

Boundary of southwestern 

Nevada volcanic field

Stonewall Mountain 

caldera(?) complex

Silent 

Canyon 

caldera 

complex

Black 

Mountain 

caldera

? Ranier Mesa 

Timber Mountain caldera 

caldera complex

Timber Mountain caldera complex

Yucca Claim Canyon caldera 

Mountain 

I

Modified from Carr and others, 1986, 10 20 30 40 50 KILOMETERS

fig.1; Sawyer and others, 1994, fig 1

ii i r

0 5 10 15 20 25 MILES

Figure 2. Locations of southwestern Nevada volcanic field and selected volcanic features (modified from Carr and others, 

1986 and Sawyer and others, 1994).

8 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
36°55'

116°30' 116°25'

36°50'

36°45'

\

Base from Yucca Mountain Site Characterization 

Project Site Atlas, 1994, maps SA94-1-05 and 

SA94-2-03; Prepared by EG&G Energy 

Measurements, Inc.

10 KILOMETERS

_I \

5 MILES

Figure 3. Locations of major geographic features and selected faults in the Yucca Mountain area.

GEOLOGIC SETTING 9
west of Yucca Mountain is Crater Flat (fig. 1), which 

contains a thick sequence of Tertiary volcanic rocks, 

alluvium, and small basaltic lava flows of Quaternary 

age. Crater Flat is separated from Yucca Mountain by 

Solitario Canyon (fig. 3), which is formed along the 

trace of a steeply dipping normal-fault feature.

Timber Mountain (fig. 1), located 25 km to the 

north of Yucca Mountain, is the surface expression of 

a resurgent dome in a large caldera complex (Byers 

and others, 1976; Sawyer and others, 1994). Pahute 

Mesa, about 40 km to the north of Yucca Mountain, 

and Rainier Mesa, about 35 km to the north-northeast, 

are higher altitude plateaus that consist of a sequence 

of pyroclastic flows and fallout deposits and tuffaceous sedimentary rocks of Tertiary age (Byers and 

others, 1976). Approximately 25 km to the south is 

Alkali Flat (Franklin Lake Playa)2. Between Yucca 

Mountain and Alkali Flat is the alluvium-filled 

Amargosa Desert, a northwest-southeast-trending 

valley.

Geologic History

The topography of Yucca Mountain resulted 

from faulting and erosion of an extensive volcanic plateau created by the deposition of a thick sequence of 

pyroclastic rocks of Miocene age. Extensional tectonics occurring between 17 Ma (million years before 

present) and 11 Ma formed the basin and range topography that characterizes much of the surrounding area. 

The silicic volcanic rocks were erupted between 

15 Ma and 7 Ma from the southwestern Nevada volcanic field (fig. 2). Yucca Mountain is located in the 

southeastern part of the volcanic field. Except for 

surficial erosion and minor displacement on existing 

faults, the topography of Yucca Mountain has changed 

very little since the cessation of major faulting approximately 12 Ma (Carr, 1988, p. 40; Huber, 1988).

2The two names of this geographic feature are used in this 

report because both names have been used in hydrologic reports to 

identify this feature. Most modern maps call this feature Alkali 

Flat. However, because there are numerous places called Alkali 

Flat in the Great Basin, some authors have used the term "Franklin 

Lake Playa," which appeared on some very early maps. Both 

terms are used together throughout this report to avoid confusion.

Stratigraphy

The stratigraphic section at Yucca Mountain 

consists of a thick sequence of Tertiary pyroclastic 

rocks overlying sedimentary and possibly igneous 

intrusive rocks of Paleozoic age (fig. 4) (Snyder and 

Carr, 1982). Data from deep boreholes and geophysical studies indicated that Tertiary volcanic rocks were 

more than 2,000 m thick in much of the area around 

Yucca Mountain and, in some places, were as much as 

3,050 m thick (Snyder and Carr, 1984, p. 10193 and 

table 1). The Tertiary volcanic rocks at Yucca Mountain consist of pyroclastic flow and fallout deposits, 

lava flows, and volcanic breccias of Miocene age. The 

tuffs at Yucca Mountain are primarily nonwelded to 

densely welded, vitric to devitrified pyroclastic-flow 

deposits separated by nonwelded, vitric fallout deposits. These deposits are laterally continuous and fairly 

homogeneous throughout the Yucca Mountain area. 

Rhyolitic to dacitic lava flows, in addition to tuffs, 

were penetrated both above and beneath the Lithic 

Ridge Tuff in boreholes USW G-l and USW G-2 

(fig. 5), but are not laterally extensive throughout the 

area.

Period

Quaternary

Tertiary

Early Permian and 

Pennsylvanian

Mississippian and 

Late Devonian

Devonian to 

Cambrian

Cambrian

Statigraphic Unit

Alluvium

Timber Mountain 

Group

Paintbrush 

Group

Ranier Mesa Tuff

Tiva Canyon Tuff

Yucca Mountain 

Tuff

Pah Canyon Tuff

Topapah Spring 

Tuff

Calico Hills Formation

Crater Flat 

Group

Prow Pass Tuff

Bullfrog Tuff

Tram Tuff

Lithic Ridge Tuff

Older tuffs, lavas, and breccias

Tippipah Limestone

Eleana Formation

Undifferentiated, primarily 

carbonate rocks

Undifferentiated, primarily 

clastic rocks

Figure 4. Selected stratigraphic units in the vicinity of 

Yucca Mountain that are important to the hydrology (Tertiary 

nomenclature modified from Sawyer and others, 1994).

10 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
36°45'

NEVADA 

0 Carson City

Area of 

Map

Las 

Vegas

01234 5 KILOMETERS

I I I

I T 

1 2

T 

3 MILES

Alluvium (Quaternary or Tertiary) 

Bedrock (Tertiary or Paleozoic)

EXPLANATION

JF-3

 Borehole location and numberNumbers are preceded by 

either UE-25 or USW

Figure 5. Locations of deep boreholes in the vicinity of Yucca Mountain.

Borehole UE-25 p#l, situated on a gravity high 

that was interpreted to be a local minimum depth to 

pre-Tertiary rocks (Carr, M.D., and others, 1986, p. 7), 

was drilled to the east of Yucca Mountain. This borehole penetrated Paleozoic carbonate rocks at a depth 

of 1,244 m (Carr, M.D., and others, 1986, table 2). A 

fault contact separates the Tertiary and Paleozoic 

rocks in this borehole. Regional geologic and geophysical data compiled and interpreted by Robinson 

(1985, pi. 1 and fig. 9) indicated that Paleozoic sedimentary rocks subcrop on the pre-Tertiary surface 

beneath Yucca Mountain and dip generally northward. 

Robinson's interpretation indicated that the carbonate 

aquifer underlies the Yucca Mountain site and that the 

Eleana Formation of Mississippian and Late Devonian

age, a confining unit, underlies northern Yucca Mountain.

The rocks exposed at the surface of Yucca 

Mountain consist of formations of the Paintbrush 

Group with minor outcrops of the Rainier Mesa Tuff 

of the Timber Mountain Group (Scott and Bonk, 1984; 

Sawyer and others, 1994). The Paintbrush Group was 

erupted from the Claim Canyon caldera, approximately 12 km to the north of Yucca Mountain, 

between 12.7 Ma and 12.8 Ma (Sawyer and others, 

1994). The Rainier Mesa Tuff was erupted 11.6 Ma 

(Sawyer and others, 1994) from the Rainier Mesa 

caldera. The younger Rainier Mesa Tuff was deposited in the vicinity of Yucca Mountain in structurally

GEOLOGIC SETTING 11
controlled valleys that formed after the deposition of 

the Paintbrush Group.

The formations of the Paintbrush Group, in 

descending order, are the Tiva Canyon Tuff, the Yucca 

Mountain Tuff, the Pah Canyon Tuff, and the Topopah 

Spring Tuff. The Tiva Canyon Tuff is a compositionally zoned ash-flow tuff as defined by Ross and Smith 

(1961). It is laterally extensive near Yucca Mountain 

and includes an area in excess of 2,600 km2 (Byers 

and others, 1976). The Yucca Mountain Tuff and Pah 

Canyon Tliff are small-volume ash flows (Byers and 

others, 1976) and are not laterally extensive. Near 

Yucca Mountain, the distal edges of these units are 

included in the bedded tuffs. The bedded tuffs are 

exposed in many of the deeper washes and adjacent to 

faults. The Topopah Spring Tuff, a compositionally 

zoned ash-flow tuff (Lipman and others, 1966), is the 

thickest member of the Paintbrush Group in the Yucca 

Mountain area and is laterally extensive at Yucca 

Mountain.

The principal units composing the volcanic 

stratigraphy beneath the Paintbrush Group at Yucca 

Mountain are, in descending order, the Calico Hills 

Formation (principally nonwelded tuffs), the Prow 

Pass, Bullfrog, and Tram Tliffs of the Crater Flat 

Group, the Lithic Ridge Tliff, and older unnamed ashflow tuffs, lavas, and breccias (Carr, W.J., and others, 

1986). These units are mainly pyroclastic flow 

deposits that are separated by bedded, pyroclastic-fall 

deposits and occasionally by localized lava flows. 

Diagenetic alteration resulting from the interaction 

between the volcanic glass and ground water has produced zones of zeolitization and clay alteration in the 

vitric, nonwelded to partially welded pyroclastic 

rocks.

Structure

Structures of Cenozoic age having surface 

expressions at and near Yucca Mountain are: (1) normal faults, (2) strike-slip faults, and (3) volcanotectonic structures (Scott and others, 1983, p. 297). In 

addition to these Cenozoic structures, older low-angle 

faults have been identified at the land surface in the 

vicinity of Yucca Mountain (Maldonado, 1985).

Normal faulting, the first type of Cenozoic 

structure related to basin and range extensional 

tectonics, began approximately 18 Ma and major 

activity ended prior to or shortly after the deposition of

the Timber Mountain Group around 11.4 Ma (Noble 

and others, 1990). The normal faults at Yucca Mountain dip steeply to the west. The blocks bounded by 

these faults have been rotated to the east from 5 to 10° 

(Carr, 1984, p. 66). Displacements on the faults range 

from less than 1 m on individual fault traces in imbricate fault zones to more than 300 m on the Solitario 

Canyon Fault (fig. 3) (Carr, 1984, fig. 30). The spacing between major north-striking normal faults at 

Yucca Mountain ranges from 0.5 to 2.0 km (Carr, 

1984, p. 66; Frizzell and Shulters, 1990). The zone of 

disruption adjacent to the normal faults varies from 

thin brecciation along fault planes to broken and brecciated zones that are many tens of meters wide 

(Spengler and others, 1993; Spengler and others, 

1994). The geometry of the normal faults and of the 

strike-slip faults controls much of the topography and 

drainage development at Yucca Mountain.

Northwest-striking strike-slip faults that have 

right-lateral movement are the second type of 

Cenozoic structures present at Yucca Mountain (Scott 

and others, 1984). Strike-slip faulting has been linked 

to tectonic activity in the Walker Lane Belt, which is a 

large northwest-trending structural zone that parallels 

most of the southwest border of Nevada. The Las 

Vegas Valley shear zone is part of the Walker Lane 

Belt between Las Vegas and Indian Springs, Nevada 

(Carr, 1984, p. 9-12). Movement on the northweststriking strike-slip faults apparently diminished and, in 

some places, ceased after the deposition of the Timber 

Mountain Group (Scott and others, 1984, p. 19). The 

structural blocks near Yucca Mountain that are 

bounded by the strike-slip faults generally are tilted 

approximately 5° east-southeast to southeast. Vertical 

offset on these faults is minor (Scott and others, 1984, 

p. 18); no obvious offsets are present at the land surface. Major washes and drainage patterns, such as 

Yucca Wash and Pagany Wash (fig. 3), developed parallel to the strike-slip faults (Scott and others, 1984).

Volcano-tectonic features (Scott and others, 

1983; Carr, 1990) are the third type of Cenozoic structures in the Yucca Mountain area and are related to 

caldera formation and volcanic activity. Generally, 

circular or semicircular faults are coincident with the 

surficial expression of the boundary of a caldera. Near 

Yucca Mountain, these types of faults are located to 

the north in the vicinity of the boundary of the Timber 

Mountain caldera complex (fig. 2).

The evidence for low-angle faulting near Yucca 

Mountain is east in the Calico Hills and west at Bare

12 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
Mountain (fig. 1). In the Calico Hills, low-angle faults 

have been mapped in the Paleozoic-age rocks and 

between the Paleozoic sedimentary rocks and the 

Tertiary volcanic rocks (Maldonado, 1985). The lowangle fault contact between the Paleozoic and Tertiary 

rocks in the Calico Hills has been interpreted as an 

extensional feature by Simonds and Scott (1987). 

Extensional faulting is present between the Paleozoic 

and Tertiary rocks at the northern end of Bare Mountain (Carr and Monsen, 1988).

REGIONAL HYDROLOGIC SETTING

Although the geology, unsaturated-zone hydrology, and saturated-zone hydrology of the 150-km2 area 

around Yucca Mountain is being studied intensively, 

regional geologic and hydrologic studies also are 

needed. Understanding the regional saturated-zone 

flow system, at least in general terms, is necessary to 

understanding the saturated-zone flow system in the 

immediate vicinity of Yucca Mountain. General concepts learned from the regional flow system could be 

applied to the flow system at Yucca Mountain. More 

specifically, regional numerical flow models are 

needed to define boundary conditions for numerical 

models that need to be developed for the saturatedzone flow system at the scale of Yucca Mountain.

The saturated-zone flow system at Yucca Mountain is part of the Alkali Flat-Furnace Creek groundwater subbasin as described by Waddell (1982) and is 

a segment of the larger Death Valley ground-water 

flow system (Bedinger, Sargent, and Langer, 1989; 

D'Agnese, 1994) (fig. 1). In this report, the regional 

scale refers to the Death Valley flow system, an area of 

about 47,000 km2. The subregional scale refers to that 

part of the Death Valley flow system that discharges at 

Alkali Flat (Franklin Lake Playa) in the southern 

Amargosa Desert. The subregional flow-system 

boundary continues to evolve, and the boundary does 

not necessarily coincide with the boundary used by 

Waddell (1982). As will be discussed in the "Data 

Uncertainties Discharge" section, the subregional 

system may not include the Furnace Creek discharge 

area. The site scale for geologic and unsaturated-zone 

hydrologic studies refers to an approximately 150-km2 

area around the potential repository at Yucca Mountain. However, because there are no natural hydrologic boundaries in the saturated zone at the site scale, 

the discussion of the site-scale saturated-zone flow

system needs to extend beyond the 150-km2 area generally considered the site scale.

The Death Valley ground-water basin as defined 

by Bedinger, Sargent, and Langer (1989) includes several subbasins, each of which has a particular discharge area and generally is named after its discharge 

area. The boundaries of the Death Valley groundwater basin and its subbasins are not precisely known, 

and the subbasins may exchange water with other subbasins through subsurface underflow. The boundaries 

between the Death Valley ground-water basin and 

adjoining basins and boundaries between the subbasins were selected using inferred no-flow or limitedflow boundaries. No-flow boundaries were based on 

postulated ground-water divides or were placed along 

inferred flow paths because no flow was assumed to 

occur perpendicular to the flow paths. Limited-flow 

boundaries were selected where the flow across the 

boundary was assumed to be small compared to flow 

through the basin or subbasin.

Yucca Mountain is centrally located in the 

Death Valley ground-water basin and also is centrally 

located in the Alkali Flat-Furnace Creek subbasin 

(fig. 1). The subbasin receives water from recharge 

within its boundaries and probably also receives water 

as underflow from adjoining subbasins. Ground water 

leaves the subbasin as evapotranspiration at discharge 

areas or as interbasin outflow.

Recharge within the Death Valley ground-water 

basin probably occurs at higher altitudes where there 

is more precipitation. In the vicinity of Yucca Mountain, recharge from precipitation probably occurs at 

Timber Mountain, Pahute Mesa, Rainier Mesa, 

Shoshone Mountain, and the Spring Mountains 

(fig. 1). Some water may enter the Death Valley 

ground-water basin by subsurface flow from Pahranagant Valley (fig. 1) on the northeastern boundary of the 

basin (Winograd and Thordarson, 1975, p. 110-111). 

Closer to Yucca Mountain, infiltration of runoff in 

Fortymile Canyon and Fortymile Wash probably contributes recharge to the Death Valley flow system 

(Claassen, 1985; Savard, 1994). Ground water flows 

through Quaternary, Tertiary, and Paleozoic aquifers to 

discharge areas. The discharge areas for the Death 

Valley ground-water basin include, among others, Ash 

Meadows, Alkali Flat (Franklin Lake Playa), Oasis 

Valley, and Death Valley. Death Valley is the ultimate 

ground-water discharge area and is a closed basin; that 

is, no water leaves it as surface or subsurface flow.

REGIONAL HYDROLOGIC SETTING 13
Winograd and Thordarson (1975, pi. 1) constructed a partial potentiometric-surface map of the 

Nevada Test Site and vicinity. Their map allowed for 

flow from Pahute Mesa in the north of Yucca Mountain to the Amargosa Desert in the south of the mountain. However, they had no data for the intervening 

Timber Mountain area, and they omitted contours 

which might imply that they knew flow occurred 

through this area. In constructing the first numerical 

model of the Alkali Hat-Furnace Creek subbasin and 

the adjacent subbasins, Waddell (1982, fig. 3) produced a more complete potentiometric-surface map of 

the region although he also had no data for the Timber 

Mountain area. Waddell's (1982) potentiometricsurface map indicated southward flow from Pahute 

Mesa, beneath the Timber Mountain and Yucca Mountain areas, then southward beneath the Amargosa 

Desert to Alkali Flat (Franklin Lake Playa) and into 

Death Valley. Czarnecki and Waddell (1984, fig. 3) 

constructed a potentiometric-surface map of the Yucca 

Mountain area from Timber Mountain into the Amargosa Desert, using data not available to earlier investigators. They had a considerable number of data points 

in the immediate vicinity of Yucca Mountain, but they 

still had little data north of Yucca Mountain and no 

data for the Timber Mountain area. Czarnecki and 

Waddell (1984) also indicated a general flow from 

north to south with implied water levels more than 

1,140 m above sea level in the Timber Mountain area 

to less than 660 m at Alkali Flat. D'Agnese (1994, 

fig. 10-3), produced a potentiometric-surface map of 

the Death Valley ground-water basin using newer 

information not available to previous investigators. 

He too indicated flow from Pahute Mesa, beneath 

Yucca Mountain, and then to the Amargosa Desert and 

beyond. However, he also was hampered by a lack of 

data in the Timber Mountain area. A slightly modified 

version of D'Agnese's (1994) map is shown in 

figure 6.

Existing potentiometric data for the region 

around Yucca Mountain indicated a generally southerly flow of ground water from upland recharge areas 

of Pahute Mesa and Rainier Mesa (water-table altitudes of about 1,400 m) and southward to Yucca 

Mountain (water-table altitude of about 730 m). 

Numerical modeling results (Waddell, 1982; 

Czarnecki and Waddell, 1984; Rice, 1984; Sinton, 

1989; D'Agnese, 1994) have been consistent with this 

general direction of flow. Because of uncertainties due 

to a lack of data, alternative conceptual models of flow

from upland areas to the north to Yucca Mountain are 

discussed in the "Upgradient Inflow" section.

In the vicinity of Yucca Mountain, ground water 

flows in a southerly direction to basin-fill sediments 

(valley-fill aquifer of Winograd and Thordarson, 1975, 

table 1) and lacustrine carbonate rocks underlying the 

Amargosa Desert. Potentiometric and hydrochemical 

data indicated that ground water flows in a southerly 

direction beneath the Amargosa Desert toward Alkali 

Flat (Franklin Lake Playa). Discharge at the playa 

occurs primarily through evapotranspiration. Some 

ground water may flow beneath the mountain at the 

south end of the playa and continue on southward. 

Some ground water also may flow in a southwesterly 

direction from the Amargosa Desert beneath the 

Funeral Mountains to discharge as spring flow and 

evapotranspiration in the vicinity of Furnace Creek 

Ranch.

Potentiometric data also indicated that a groundwater divide may exist to the south of the Funeral 

Mountains in the Greenwater Range, between the 

Amargosa Desert and Death Valley (Czarnecki and 

Wilson, 1991, p. 18), at least for the relatively shallow 

hydrologic system. If a ground-water divide does 

exist beneath the Greenwater Range and the Funeral 

Mountains to the north, flow through the Alkali FlatFurnace Creek subbasin, and hence flow through the 

Yucca Mountain area, may be substantially less then 

postulated by previous modeling (Waddell, 1982, 

table 4; Czarnecki and Waddell, 1984, pi. 2).

Deep hydraulic connection through the carbonate aquifer may connect the Ash Meadows area on the 

east side of the Amargosa Desert to the Furnace Creek 

Ranch area of Death Valley. This possible connection 

is consistent with the observation that the hydrochemistry of water from springs that discharge at Furnace 

Creek Ranch is similar to the hydrochemistry of water 

discharging at some springs in the Ash Meadows area 

(Winograd and Thordarson, 1975, p. 95-97). This 

similarity in hydrochemistry allows the possibility of 

westward ground-water flow through deep aquifers 

beneath the Amargosa Desert whereas flow through 

the shallower aquifers seems to be predominately 

southward. However, carbonate rocks beneath the 

Funeral Mountains also might provide preferential 

conduits or drains for flow from the basin-fill sediments beneath the Amargosa Desert toward Death 

Valley.

Winograd and Thordarson (1975, p. 14-46) 

divided the area around the Nevada Test Site into six

14 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
118' 117° 116° 115°

Shaded-relief base from U.S. Geological Survey digital 

data, 1:250.000. 1987; sun illumination from northwest 

at 30° above horizon

50 100 KILOMETERS

_I

25

I 

50 MILES

EXPLANATION

1500 Potentiometric contour Shows altititude of 

potentiometeric surface, 1994. Contour 

interval is 100 meters. Datum is sea level

-^ Boundary of Death Valley ground-water basin 

(from D'Agnese, 1994)

AF 

DV 

FM 

PM 

PV

Alkali Flat 

Death Valley 

Funeral Mountains 

Pahute Mesa 

Pahranagant Valley

SM 

SR 

YF 

YM

Spring Mountains 

Sheep Range 

Yucca Flat 

Yucca Mountain

Figure 6. Potentiometric surface, Death Valley region (from D'Agnese, 1994).

REGIONAL HYDROLOGIC SETTING 15
aquifers and four confining units (table 1). However, 

not all of these aquifers and confining units are present 

at or near Yucca Mountain. Only those units present 

near Yucca Mountain will be discussed in this section, 

and the nomenclature of Winograd and Thordarson 

(1975) will be used in this section only to be consistent 

with previous work. The valley fill around Yucca 

Mountain is above the saturated zone and thus is not 

an aquifer; however, it is an important aquifer south of 

Yucca Mountain and may be saturated in parts of 

Crater Flat. The lava-flow aquifer is not present at 

Yucca Mountain, but is present east of Yucca Mountain in well J-11 in Jackass Flats. The lower part of the 

welded-tuff aquifer is present at Yucca Mountain and 

is an important aquifer along Fortymile Wash and in 

Crater Flat. The bedded-tuff aquifer and lava-flow 

confining unit are not present at Yucca Mountain but 

may possibly be present in Jackass Flats. The tuff 

confining unit is present at Yucca Mountain, and parts 

of it yield substantial quantities of water to wells. In 

the "Hydrogeologic Units" section of this report, the 

tuff confining unit will be subdivided into two confining units and one aquifer. The upper carbonate aquifer 

is not present in the Yucca Mountain region and therefore is not listed in table 1. The upper clastic confining unit is not present in any boreholes drilled at Yucca 

Mountain, but geophysical evidence and some conceptual models conceive of it existing deeply beneath 

northern Yucca Mountain (see "Large Hydraulic Gradient" section). The lower carbonate aquifer was penetrated by borehole UE-25 p#l and may be present at 

least to the south and east of Yucca Mountain. The 

lower clastic confining unit would be too deeply buried to be penetrated by boreholes at Yucca Mountain.

HYDROLOGY OF THE SATURATED ZONE 

AT YUCCA MOUNTAIN

This section concentrates on the saturated-zone 

ground-water flow system in the immediate vicinity of 

Yucca Mountain and includes a smaller area than 

probably will be used for the site-scale numerical 

model. In this report, the site-scale hydrologic setting

Table 1. Selected regional stratigraphic and hydrogeologic units in the vicinity of Yucca Mountain 

(modified from Winograd and Thordarson, 1975, table 1)

[Remarks by authors of this report]

System

Quaternary and 

Tertiary

Tertiary

Mississippian to 

Upper Devonian

Devonian to 

Cambrian

Lower Cambrian to 

Precambrian

Series

Holocene, 

Pleistocene, 

and Pliocene

Miocene

Stratigraphic 

unit

Valley fill

Basalt of Kiwi Mesa 

and others

Timber Mountain 

Group and upper 

Paintbrush Group

Lower Paintbrush 

Group

Upper Wahmonie 

Formation

Lower Wahmonie 

Formation, Calico 

Hills Formation, 

Crater Flat Group, 

and others

Eleana Formation

Hydrogeologic 

unit

Valley-fill aquifer

Lava-flow aquifer

Welded-tuff aquifer

Bedded-tuff aquifer

Lava-flow confining 

unit

Tuff confining unit

Upper clastic confining 

unit

Lower carbonate 

aquifer

Lower clastic confining 

unit

Remarks

Unsaturated near 

Yucca Mountain

Exists east of Yucca 

Mountain in Jackass Flats

Lower part is upper volcanic 

aquifer of this report

Does not exist near 

Yucca Mountain

Does not exist near 

Yucca Mountain

Subdivided into upper 

confining unit and lower 

volcanic aquifer in this 

report

May exist beneath northern 

Yucca Mountain

Carbonate aquifer of this 

report

Probably exists deep beneath 

Yucca Mountain

16 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
refers to an area that extends north to about Pinnacles 

Ridge, east past Fortymile Wash, south to about the 

southern end of Yucca Mountain, and west past 

Solitario Canyon. These are only general boundaries 

for the purposes of this report and do not represent real 

hydrologic boundaries. When a numerical model of 

the site-scale flow system is developed, it need not 

necessarily have these same boundaries.

This section begins with a discussion of hydrogeologic units for the site-scale hydrologic setting. 

These hydrogeologic units will be used to organize 

later information. Potentiometric data (water levels) 

then are discussed. These data are quite abundant and 

reliable in the Yucca Mountain area but are sometimes 

difficult to interpret. Next, hydraulic data collected at 

various boreholes are discussed. These data are sparse 

and do not provide a clear picture of the site-scale setting. Concepts about inflows to and outflows from the 

system then are discussed. These subsections discuss 

only concepts because data on inflows and outflows 

are extremely limited. Hydrochemical data are discussed, but these data do not, in themselves, provide 

much information about the flow system. The section 

ends with a discussion of paleohydrology; that is, how 

the flow system may have been different in the recent 

geologic past.

Hydrogeologic Units

Hydrologists generally define hydrogeologic 

units by combining adjacent stratigraphic units that 

have similar hydrologic properties. This method is 

usually appropriate for sedimentary rocks in which the 

rock matrix tends to dominate hydrologic properties. 

For other rocks, in which secondary effects such as 

fracturing, tend to dominate hydrologic properties, 

hydrogeologic units are much more difficult to 

define. At Yucca Mountain, where the volcanic rocks 

may or may not be fractured and where the hydrologic 

properties can change significantly in a single 

stratigraphic unit, stratigraphic units are useful only in 

a very general sense for defining hydrogeologic units.

The Tertiary volcanic section at Yucca Mountain consists of a series of ash-flow and bedded tuffs 

that contain minor amounts of lava and flow breccia. 

Individual ash-flow tuffs may be as much as several 

hundred meters thick, whereas bedded tuffs generally 

are less than a few tens of meters thick. Ash-flow tuffs 

range from nonwelded to densely welded, and the

degree of welding varies both areally and vertically in 

a single flow unit. Nonwelded ash-flow tuffs, when 

unaltered, have moderate to small matrix permeability, 

but large porosity. Permeability is greatly decreased 

by secondary alteration, and fractures are infrequent 

and often closed in the low-strength nonwelded tuffs. 

Consequently, these rocks constitute laterally extensive saturated-zone confining units in the Yucca 

Mountain area. Partly welded tuffs have properties 

that are between the properties represented by fractured, welded tuffs and by altered, nonwelded tuffs. 

The densely welded tuffs generally have minimal primary porosity and water-storage capacity, but they can 

be highly fractured. Where interconnected, fractures 

can easily transmit water and highly fractured units 

function as aquifers. In general, the bedded tuffs have 

large primary porosity and can store large amounts of 

water. Their matrix permeability is moderate to small, 

depending on the degree of alteration. The bedded 

tuffs generally function as confining units, at least 

when compared to less porous but densely fractured 

ash-flow tuffs. Lavas, flow breccias, and other minor 

rock types are neither thick nor widely distributed in 

the Yucca Mountain area. Their hydraulic properties 

are probably as variable as the properties of the ashflow tuffs, but the relatively limited areal distribution 

of these minor rock types makes them generally unimportant to the hydrology of Yucca Mountain.

As depth increases, even fractured tuffs and 

lavas may not easily transmit water because lithostatic 

loading keeps the fractures closed. In addition, where 

volcanic glass has been partly replaced by zeolites and 

clays, particularly in the originally vitric nonwelded 

tuffs, these secondary minerals substantially decrease 

permeability and, thus, saturated ground-water flow 

through the rock. The degree of alteration can greatly 

affect the water-transmitting characteristics of the volcanic sequence. Alteration, particularly in the Calico 

Hills Formation, increases to the north at Yucca 

Mountain (Moyer and Geslin, 1995, p. 10) and probably accounts for the apparent decrease in hydraulic 

conductivity to the north. Alteration also tends to 

increase with depth and is pervasive below the Calico 

Hills Formation.

To summarize the understanding of the saturated-zone flow system at Yucca Mountain, the 

Tertiary volcanic sequence was divided into two aquifers and two confining units (fig. 7) that seem to have 

generally similar hydrologic properties. These hydrogeologic units are similar to the ones proposed by

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 17
TYF 

DE

ME" 

0

200 

400 

600 

800 

1,000 

1,200 

1,400 

1,600 

1,800 

2,000

>ICAL SELECTED SATURATED ZONE TYPICAL SELECTED THERMAL/ SATURATED ZONE 

PTH, GEOLOGIC HYDROGEOLOGIC DEPTH, GEOLOGIC MECHANICAL HYDROGEOLOGIC 

N UNITS UNITS IN UNITS UNITS UNITS 

FERS METERS (Ortiz and others, 

1985)

1 A MI-1 CM ID^A^C n

-

Q. 

O

O 

M Topopah Spring

I Tuff 

1

Calico Hills Formation

Prow Pass Tuff

Q.

2 Bullfrog Tuff 

ra

& 

0 Tram Tuff

Flow breccias 

and lavas

Lithic Ridge Tuff

Older tuffs, lavas, 

and breccias

Upper 

volcanic 

aquifer

Upper volcanic 

confining 

unit

Lower 

volcanic 

aquifer

Lower 

volcanic 

confining 

unit

100 

200 

300 

\ 400 

\ 500 

\ 600

? ? ? ? ? \ 700

Eleana Formation

Cambrian to 

Devonian 

formations

Proterozoic rocks

Upper clastic 

confining unit

Lower 

carbonate 

aquifer

Proterozoic 

confining 

unit

\ 800 

900 

1,000

-

-

Alluvium

Group Paintbrush 

Yucca Mountain 

Tuff

Pah Canyon 

Tuff

Topopah 

Spring 

Tuff

Calico Hills 

Formation

Q.

8 

o

Crater Fl

Prow Pass

Tuff

Bullfrog Tuff

Tram Tuff

UO

PTn

TSw1

TSw2

TSw3

CHn1

CHn2

CHn3

PPw

CFUn

BFw

CRMnl

CRMn2

CRMnS

TRw

Upper 

volcanic 

aquifer

Upper 

volcanic 

confining 

unit

Lower 

volcanic 

aquifer

EXANDED UPPER UNITS

Figure 7. Selected geologic, thermal/mechanical, and hydrogeotogic units at Yucca Mountain, Nevada. Only upper units 

shown.

18 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
Ervin and others (1994, p. 4-5). The definition of the 

hydrogeologic units may change as more field data are 

obtained or when a numerical flow model is developed. The exact stratigraphic locations of the hydrogeologic-unit contacts are vague in this report and may 

change as the hydrogeologic information is reinterpreted. The altitudes for the hydrogeologic units penetrated by selected boreholes and wells at Yucca 

Mountain are listed in table 2. Geologic and thermal/ 

mechanical units along with hydrogeologic units are 

shown in figure 7 because both were helpful in selecting the hydrogeologic units. The division of stratigraphic and thermal/mechanical units into aquifers 

and confining units was based on the relative permeability of the units. In general, the units designated as 

aquifers seem to be more permeable than the units designated as confining units. However, locally, an aquifer may function as a confining unit or vice versa. In 

this report, the Paleozoic sequence is divided into the 

same hydrogeologic units used by Winograd and 

Thordarson (1975).

The upper volcanic aquifer consists of the 

densely welded part of the Topopah Spring Tuff of the 

Paintbrush Group (fig. 7). It generally corresponds to 

the TSwl and TSw2 thermal/mechanical units defined 

by Ortiz and others (1985). It is saturated only to the 

east and south of Yucca Mountain and in Crater Flat.

This productive aquifer is the source of water for supply wells J-12 and J-13. At J-13, about 70 percent of 

the unit is saturated (Thordarson, 1983). Farther north 

near Fortymile Wash, at boreholes UE-25 WT#13, 

UE-25 WT#14, and UE-25 WT#15, only the lower 

part (6 to 25 percent) of the Topopah Spring Tuff is 

saturated. South of Yucca Mountain, only the lower 

part of the unit is saturated in boreholes USW WT-11 

(8 percent) and UE-25 WT#12 (15 percent). In Crater 

Flat, only the lower part of the Topopah Spring Tuff is 

saturated in borehole USW WT-10, but the Topopah 

Spring Tuff is fully saturated in borehole USW 

VH-1. Beneath the center of Yucca Mountain, the unit 

is above the saturated zone.

The upper volcanic confining unit consists of 

the basal vitrophyre of the Topopah Spring Tuff where 

it is not fractured or the fractures are plugged, the bedded tuff beneath the Topopah Spring Tuff, the Calico 

Hills Formation, and the uppermost, nonwelded part 

of the Prow Pass Tuff of the Crater Flat Group (fig. 7). 

The upper volcanic confining unit generally corresponds to the TSw3, CHnl, and CHn2 thermal/ 

mechanical units defined by Ortiz and others (1985). 

This confining unit can produce water, notably from 

boreholes UE-25 b#l (Lobmeyer and others, 1983, 

fig. 8), UE-25 c#3 (Geldon, 1993, fig. 33), and 

well J-13 (Thordarson, 1983, table 12). However,

Table 2. Altitudes of hydrogeologic units in deep boreholes at Yucca Mountain

[All altitudes are in meters above sea level; water level is for uppermost interval monitored; base of hydrogeologic unit is shown even if it is 

above the saturated zone; >TD indicates that at the total depth borehole was still in or had not reached this hydrogeologic unit]

Borehole 

name

USW G-l

USW G-2

USW G-3

USWG-4

USWH-1

USW H-3

USWH-4

USW H-5

USW H-6

UE-25 b#l

UE-25 c#l

UE-25 p#l

J-13

Altitude of 

water level

750

1,020

730

731

731

731

730

776

776

731

730

752

728

Altitude of - 

bottom of 

borehole

-503

-277

-53

355

-526

264

30

259

82

-19

216

-691

-52

Altitude of base of hydrogeologic unit

Upper 

volcanic 

aquifer

933

1,056

1,119

875

871

1,119

888

996

931

806

758

780

612

Upper 

volcanic 

confining unit

777

730

1,005

733

736

1,030

753

886

843

631

615

678

481

Lower 

volcanic 

aquifer

252

361

307

>TD

200

387

94

439

427

12

>TD

241

36

Lower 

volcanic 

confining unit

>TD

>TD

>TD

>TD

>TD

>TD

>TD

>TD

>TD

>TD

>TD

-130

>TD

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 1 9
production from the upper volcanic confining unit is 

small compared to production from either the upper or 

lower volcanic aquifer (see "Hydraulic Properties" 

section). The upper volcanic confining unit is above 

the saturated zone beneath much of the southern part 

of Yucca Mountain (Fridrich and others, 1994, fig. 5); 

in the remainder of the southern area, only the lower 

part generally is saturated. At borehole UE-25 b#l, 

about 67 percent of the unit is saturated, whereas at 

borehole USW WT-1, only 8 percent is saturated. 

Almost all of the upper volcanic confining unit is in 

the saturated zone at borehole USW G-2 at the north 

end of Yucca Mountain. The confining unit thickens 

to the northeast, and the thickness of the zeolitic facies 

increases from southwest to northeast (Montazer and 

Wilson, 1984, p. 17; Moyer and Geslin, 1995, p. 10). 

To the northeast, the increases in thickness and in 

zeolitization of this unit probably make it more effective as a confining unit.

The lower volcanic aquifer consists of most of 

the Prow Pass 1\iff and the underlying Bullfrog and 

Tram Tuffs of the Crater Flat Group (fig. 7). The 

uppermost part of the Prow Pass Tuff, if it is not fractured, is included with the upper volcanic confining 

unit. The lower volcanic aquifer underlies all of 

Yucca Mountain, and its units generally are completely or mostly in the saturated zone (table 2). The 

lower volcanic aquifer usually has a substantial number of fractures in it, but during hydraulic testing, most 

of the water came from only a small percentage of the 

fractures. The zones in this aquifer that yield water 

are highly variable and are difficult to predict (see 

"Flow Surveys" section). The lower volcanic aquifer 

tends to be less productive where it and the upper volcanic aquifer are present. For example, well J-13 

probably obtains, at most, a small percentage of its 

water from this aquifer (Thordarson, 1983, table 12). 

Because the lower volcanic aquifer is deeper, 

increased lithostatic load probably accounts for part of 

the difference between the two aquifers, but the lower 

aquifer also tends to be much less densely fractured 

than the upper volcanic aquifer (Spengler and others, 

1984, fig. 14). The lower volcanic aquifer also is more 

altered, which also accounts for the decreased permeability. Recent pneumatic testing of selected boreholes in the unsaturated zone confirmed that the lower 

volcanic aquifer is less permeable than the upper volcanic aquifer (G.D. LeCain, U.S. Geological Survey, 

written commun., 1994). The same testing also confirmed that the upper volcanic confining unit is less

permeable than either the upper or lower volcanic 

aquifer.

The lower volcanic confining unit consists of 

bedded tuffs, lava flows, and flow breccia beneath the 

Tram Tuff, and includes the Lithic Ridge Tuff and 

older flows and tuffs beneath the Lithic Ridge Tuff 

(fig. 7). This hydrogeologic unit is penetrated only by 

the deepest boreholes at Yucca Mountain (table 2). It 

is present in several geologic boreholes (USW G-l, 

USW G-2, and USW G-3) and was tested in hydrologic borehole USW H-l and in multipurpose borehole UE-25 p#l. The lower volcanic confining unit 

also was penetrated and tested in borehole UE-25 b#l 

and well J-13, but the tests included part of the overlying lower volcanic aquifer. Even though some zones 

in the lower confining unit are fractured, the unit may 

be buried so deeply that the fractures are closed and do 

not transmit substantial quantities of water.

Beneath the Tertiary volcanic section at Yucca 

Mountain are Paleozoic rocks that compose the upper 

Paleozoic confining unit(?) and the lower Paleozoic 

carbonate aquifer as defined by Winograd and Thordarson (1975, p. 14-30). The upper Paleozoic carbonate aquifer as defined by Winograd and Thordarson 

(1975, p. 30-31) is not present in the Yucca Mountain 

area. Within this report, the lower carbonate aquifer is 

referred to as simply the carbonate aquifer. The upper 

Paleozoic confining unit (Winograd and Thordarson, 

1975, p. 43) has not been found in outcrops or boreholes at Yucca Mountain. However, Robinson (1985) 

in his interpretation of pre-Cenozoic geology and 

structure, inferred, partly on the basis of a magnetic 

lineation, that the altered Eleana Formation that crops 

out in the Calico Hills to the east also exists beneath 

the volcanic section under northern Yucca Mountain. 

Part of the carbonate aquifer is penetrated by only one 

borehole in the Yucca Mountain area, UE-25 p#l. 

This borehole was drilled where geophysical data 

indicated a buried topographic high in the carbonate 

rocks. The carbonate aquifer exists well to the south 

and east of Yucca Mountain, but its presence beneath 

northern Yucca Mountain and in Crater Flat to the 

west has not been documented by drilling. If the carbonate aquifer does exist in these areas, it may be so 

deep that it is below the effective base of the hydrologic system.

An oil-test hole about 6 km south of the town of 

Amargosa Valley intersected the carbonate aquifer at 

depths from 670 to 1,524 m below land surface (Carr 

and others, 1995). The presence of the carbonate

20 Status off Understanding off the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as off 1995
aquifer at this site indicates, but does not demonstrate, 

that the carbonate aquifer extends from eastern Yucca 

Mountain southward into the Armagosa Desert.

Because so little is known about the Paleozoic 

rocks beneath Yucca Mountain, this report does not 

attempt to subdivide these rocks into aquifers and confining units beyond what was done by Winograd and 

Thordarson (1975). In the region, where the Eleana 

Formation is thick, the Paleozoic rocks are generally 

subdivided into an upper aquifer, a confining unit, and 

a lower aquifer. If the Eleana Formation is thin or 

absent, the Paleozoic rocks are generally lumped into 

a single aquifer. Although the Paleozoic rocks are 

subdivided into an aquifer and two confining units in 

figure 7, in the remainder of this report, when the 

Paleozoic rocks are discussed, they generally are 

treated as forming a single carbonate aquifer. Beneath 

the Paleozoic rocks is the Proterozoic confining unit.

Potentiometric Surface

Robison (1984, fig. 2) constructed a preliminary 

potentiometric-surface map for the immediate vicinity 

of Yucca Mountain using primarily 1983 water-level 

data. Robison's (1984) map is reproduced in this 

report as figure 8. This map shows contours for 1,000 

to 730 m above sea level and indicates a gradient 

toward the south and east in the immediate vicinity of 

Yucca Mountain. The contours indicate a large 

hydraulic gradient in the northern part of the area with 

a southward water-level decrease of 60 m per 0.5 km 

(gradient of 0.12) and 200 m in 1.5 km (gradient of 

0.13). A moderate eastward hydraulic gradient of 

20 m in 0.4 km (gradient of 0.05) is shown just west of 

the crest of Yucca Mountain in the vicinity of Solitario 

Canyon. Most of the area east of Solitario Canyon and 

south of borehole USW H-l has a very small hydraulic 

gradient, as indicated by the data points. There is only 

a single contour (730 m) in this area. The 1983 data 

were not sufficiently accurate to compute a meaningful hydraulic gradient in the small-gradient area.

Ervin and others (1993, 1994) revised the 

potentiometric-surface map in the area of the small 

hydraulic gradient (fig. 9). The large and moderate 

hydraulic gradients were identified and discussed, but 

were not contoured because no new data were available to update the previous (Robison, 1984) map. 

Ervin and others' (1993,1994) map (fig. 9) used primarily mean 1988 water-level data. These data were

more accurate than data that were available to earlier 

investigators because of improvements in measuring 

techniques (Boucher, 1994b). The map, which has a 

contour interval of 0.25 m, indicates a range of waterlevel altitudes from about 731.0 m beneath the crest of 

Yucca Mountain to about 728.5 m near Fortymile 

Wash. The predominant direction of the ground-water 

gradient shown in figure 9 is to the east-southeast.

Large Hydraulic Gradient

The potentiometric surface at the north end of 

Yucca Mountain is characterized by a large hydraulic 

gradient (contoured at 0.13 and possibly as large as 

0.15) between altitudes of approximately 1,030 to 

750 m; the surface is markedly steeper than the surface 

toward the south and east (fig. 8). Although relatively 

large hydraulic gradients have been observed in southern Nevada (Winograd and Thordarson, 1975, figs. 32, 

33, and pi. 2; Waddell and others, 1984, pi. 2), these 

large gradients generally were associated with known 

geologic or topographic features, such as edges of 

thick confining units, faults with major offset, caldera 

boundaries, or mountain range fronts. The large 

hydraulic gradient at the north end of Yucca Mountain 

is unique only in its lack of obvious geologic cause. 

However, the lack of an obvious geologic cause may 

be more a function of the scarcity of data than an 

inherent feature of this large hydraulic gradient.

Several hypotheses have been advanced to 

explain the large hydraulic gradient. One hypothesis 

is that the large hydraulic gradient is simply the result 

of flow through the upper volcanic confining unit. The 

1994 water-level altitude in borehole USW G-2 at the 

northern end of the large hydraulic gradient was 

1,020 m (table 2) and is in the upper volcanic confining unit. The base of the upper volcanic confining unit 

is at an altitude of about 730 m (table 2), so the 

saturated thickness of the confining unit is almost 

300 m. A large hydraulic gradient could be expected 

in a thick confining unit. Because the unit that composes the upper volcanic confining unit is not saturated 

to its full thickness at borehole USW G-2, the large 

hydraulic gradient may persist somewhat north to the 

area where the potentiometric surface is in the upper 

volcanic aquifer. The lower volcanic aquifer in this 

area also could have decreased hydraulic conductivity 

because of lithostatic pressure and hydrothermal alteration, which also would contribute to the large hydraulic gradient.

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 21
116°27'30"

36°521 30" -

36°50'

EXPLANATION 

ALLUVIUM (QUATERNARY) 

TUFF (TERTIARY)

WELL Upper number is well number 

assigned by U.S. Department of Energy. 

Lower number is altitude of water level 

measured during this study, in meters 

above sea level

 800 POTENTIOMETRIC CONTOUR shows 

altitude of potentiometric surface, 1983. 

Contour interval, in meters, is variable. 

Datum is sea level

3B°471 30' -

\

PERIMETER DRIFT OF UE-25 WT#14 

DESIGN REPOSITORY 729.3 -""'

0 1 2 KILOMETERS

0 1 2 MILES

Figure 8. Preliminary potentiometric surface, Yucca Mountain (from Robison, 1984, fig. 2).

22 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
116°27'30"

36°521 30" -

36°50' -

EXPLANATION 

ALLUVIUM (QUATERNARY) 

TUFF (TERTIARY)

WELL Upper number is well number 

assigned by U.S. Department of Energy. 

Lower number is altitude of water level, 

in meters above sea level

 730.50 POTENTIOMETRIC CONTOUR shows 

altitude of potentiometric surface. 

Contour interval is 0.25 meter. 

Datum is sea level

36°47'30" -

Q

PERIMETER DRIFT OF UE-25 WT #14 

DESIGN REPOSITORY

2 KILOMETERS

2 MILES

Figure 9. Revised potentiometric surface of an area of small hydraulic gradient, Yucca Mountain (from Ervin and others, 

1993, fig. 1).

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 23
Ervin and others (1994, p. 9-11) advanced the 

hypothesis that the large hydraulic gradient represents 

a semiperched system (Meinzer, 1923, p. 41). In a 

semiperched system, flow in the upper and lower aquifers would be predominantly horizontal, whereas flow 

in the confining unit would be predominantly vertical. Winograd and Thordarson (1975, p. 50) reported 

that semiperched water is not uncommon at and in the 

vicinity of the Nevada Test Site. The water levels to 

the north of the large hydraulic gradient represent levels in the upper volcanic aquifer, whereas water levels 

to the south represent levels in the lower volcanic 

aquifer (table 2). The upper volcanic confining unit 

separates these two systems. Under this hypothesis, 

the large hydraulic gradient may simply be an artifact 

of attempting to contour points from two different and 

widely separated surfaces. Water levels from boreholes completed in the confining unit between these 

two different surfaces (such as UE-25 WT#6), where 

flow is essentially vertical and water levels change 

dramatically with depth, would be difficult to interpret. To the north of the large hydraulic gradient, flow 

in the upper volcanic aquifer probably is limited with 

most of the water being lost to downward seepage into 

the lower volcanic aquifer. What little water remains 

in the upper volcanic aquifer at the north end of the 

large hydraulic gradient probably is lost in a short distance as it moves vertically through the upper volcanic 

confining unit and ultimately reaches the lower volcanic aquifer (Ervin and others, 1994, p. 9-11). The permeability of the lower volcanic aquifer at the north 

end of Yucca Mountain may be extremely limited due 

to lithostatic loading and to alteration. Water in the 

upper volcanic aquifer also may be diverted around 

the large hydraulic gradient into Crater Flat or along 

Fortymile Wash. In this hypothesis (Ervin and others, 

1994, p. 9-11), the hydraulic gradient in the lower volcanic aquifer probably increase to the north as hydraulic conductivity decreases, but does not increase as 

dramatically as shown on the potentiometric-surface 

map (fig. 8). Borehole USW G-2 in this area produced 

little, if any, water from the lower volcanic aquifer 

(see "Flow Surveys" section).

Fridrich and others (1991, 1994) proposed the 

hypothesis that the large hydraulic gradient represents 

a drain and is part of a regional large hydraulic gradient that extends many kilometers to the northeast of 

Yucca Mountain. They argued, based on gravity and 

stratigraphic indicators, that a fault in the vicinity of 

the large hydraulic gradient is buried beneath the

Calico Hills Formation and is the northern-bounding 

fault of a buried graben. There is no surface expression of the postulated fault. In this hypothesis, 

Fridrich and others (1994) suggested that this fault 

marks the northern extent of active flow in the carbonate aquifer and provides a drain that moves water from 

the volcanic aquifers into the carbonate aquifer. The 

drain removes most of the water from the volcanic 

aquifers and greatly decreases flow through the volcanic aquifers to the south, which accounts for the small 

hydraulic gradient to the south and east of Yucca 

Mountain. Part of the water returns to the lower volcanic aquifer south of the large hydraulic gradient 

through faults, most notably the Solitario Canyon, 

Bow Ridge, and Paintbrush Canyon faults (fig. 3). 

This hypothesis is intriguing in that it also accounts for 

the anomalously low heat flow observed at Yucca 

Mountain (Sass and others, 1988), but the hypothesis 

must rely heavily on aeromagnetic and gravity data to 

define the buried graben because other geologic data 

are sparse.

Fridrich and others (1994) proposed a second 

hypothesis, the spillway hypothesis, to explain the 

large hydraulic gradient. This explanation requires the 

same deeply buried fault; but, in this spillway hypothesis, the fault marks the effective northern limit of the 

lower volcanic aquifer. Fridrich and others (1994) 

speculated that, as a result of the fault, the Crater Flat 

Group is thinner to the north of the graben, is more 

altered, and is much less permeable than it is to the 

south. This explanation envisions very little flow from 

the north, and what little flow there is drops abruptly at 

the northern-bounding fault of the buried graben.

Another hypothesis is that the large hydraulic 

gradient is the result of the presence, at depth, of the 

Eleana Formation, which is part of the Paleozoic 

upper clastic confining unit described by Winograd 

and Thordarson (1975, p. 43). This possibility may 

have been considered when the large hydraulic gradient was discovered in 1981 with the drilling of borehole USW G-2 but has not been previously discussed 

in detail in publications although Fridrich and others 

(1994, p. 150) noted the regional large hydraulic gradient appeared to follow the contact between the upper 

clastic confining unit and the carbonate aquifer. The 

Eleana Formation is responsible for a large hydraulic 

gradient on the west side of Yucca Flat in the northeastern part of the Nevada Test Site (Winograd and 

Thordarson, 1975, p. 66). The Eleana Formation 

could be deeply buried beneath northern Yucca

24 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
Mountain and could be affecting the flow system in 

that area in the same way it affects the flow system on 

the west side of Yucca Flat. No borehole has penetrated deeply enough to verify the existence or absence 

of the Eleana Formation to the north of Yucca Mountain. More than 5 km south of the large hydraulic gradient, the Eleana Formation is not present in borehole 

UE-25 p#l, which penetrates the pre-Cenozoic section 

in carbonates of Silurian age beneath a fault contact 

(Carr, M.D., and others, 1986). However, the Eleana 

Formation is present as an altered, strongly magnetic 

argillite in the Calico Hills, about 15 km east of Yucca 

Mountain. Bath and Jahren (1984) attributed a prominent magnetic high to this argillite and suggested that 

the westward extension of the magnetic high beneath 

northern Yucca Mountain also indicated the presence 

of the Eleana Formation. Robinson (1985, fig. 9) projected the approximate location of the southern extent 

of the superposition of the confining unit on the carbonate aquifer, a location that correlates with the general locality of the large hydraulic gradient. The 

absence of the Paleozoic upper clastic confining unit 

south of the large hydraulic gradient could provide 

ground-water access to the carbonate aquifer, but the 

overlying lower volcanic confining unit also could 

prevent this access.

Moderate Hydraulic Gradient

The moderate hydraulic gradient is defined by 

boreholes with the altitudes of the potentiometric surface ranging from about 732 m to about 775 to 780 m 

(fig. 8) and is west of the crest of Yucca Mountain 

(Robison, 1984, fig. 2; Ervin and others, 1994, p. 8-9). 

Boreholes USW H-6, USW WT-7, and USW WT-10 

(water-level altitudes of about 776 m) (figs. 9 and 10) 

are located on the west side of the Solitario Canyon 

Fault. Borehole USW H-5 (water-level altitude of 

about 775 m) (figs. 8 and 9) is located to the east of the 

Solitario Canyon Fault but on the downthrown (west) 

side of a major northeast-trending splay of the 

Solitario Canyon Fault. Boreholes USW H-l and 

USW H-3 (water-level altitude about 731 m) (figs. 8 

and 9) are located on the east side of the Solitario 

Canyon Fault. The Solitario Canyon Fault is a major 

north-striking scissors fault, which to the south is 

downthrown on its western side and to the north is 

downthrown on the eastern side (Scott and Bonk, 

1984). The hinge line of the fault, where the displacement changes, is perpendicular to the fault plane and is

located approximately 1 km southwest of borehole 

USW G-2. Offset on the fault may be as much as 

250 m (U.S. Geological Survey, 1984, p. 50). Toward 

its southern end, the Solitario Canyon Fault seems to 

widen and have more splays. Fault gouge and secondary siliceous infillings are present along the fault 

(M.P. Chornack, U.S. Geological Survey, oral commun., 1992). Samples of this material indicate a low 

matrix porosity (A.L. Flint, U.S. Geological Survey, 

written commun., 1992). Although fault gouge and 

siliceous infillings are surface features, they indicate 

that the fault, in the saturated zone, could have less 

permeability than the surrounding rock.

The Solitario Canyon Fault seems to function as 

a barrier to flow from west to east. Boreholes west of 

the fault have higher water levels than boreholes east 

of the fault, except for borehole USW H-5. However, 

borehole USW H-5 may be connected hydraulically 

with the boreholes west of the Solitario Canyon Fault 

(Ervin and others, 1994, p. 9). According to Bentley 

and others (1983, p. 20) and Robison and Craig (1991, 

p. 12-13), much of the flow in borehole USW H-5 

originates from the part of the Bullfrog Tuff located at 

an altitude of between 720 to 780 m (see "Flow Surveys" section). Thus, most of the flow to the borehole 

comes from an interval that is above the interval where 

the fault splay probably intersects the borehole. The 

fault splay may be less permeable as a result of fault 

gouge. This low-permeability interval may tend to 

mound water against the fault splay.

The Solitario Canyon Fault could also function 

as a barrier because of offset of stratigraphic units, 

which places more permeable units against less permeable units, especially to the south where offset in 

the fault is greater (Ervin and others, 1994, p. 9). 

Some flow undoubtedly crosses the Solitario Canyon 

Fault because there is a large difference of hydraulic 

potential (45 m) across it. However, most of the 

ground water west of the Solitario Canyon Fault probably flows south, either along the fault or through an 

aquifer in Crater Flat. A test to determine the hydrologic significance of the fault at one locality has been 

described by the U.S. Department of Energy (1990a, 

sec. 3.1).

The moderate hydraulic gradient could be interpreted as a southward extension of the large hydraulic 

gradient, with or without the Solitario Canyon Fault 

causing the moderate gradient. This extension of the 

large gradient could be implied by information presented by Robison (1984, fig. 2) and Fridrich and

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 25
116'35' 

36°55'

36°45'

5 KILOMETERS

I

T

3 MILES

EXPLANATION

Area of 

Map

Alluvium (Quaternary or Tertiary) 

Bedrock (Tertiary or Paleozoic)

P#1 ,

751.3' Borehole location-Upper number is borehole number and 

is preceded by either UE-25 or USW. Lower number is 

water-level altitude in meters above sea level

Figure 10. Potentiometric levels in the lower volcanic confining unit and carbonate aquifer.

others (1994, fig. 3). As was discussed in the "Large 

Hydraulic Gradient" section, Fridrich and others 

(1994) suggested that there could be substantial 

upwelling of water from the carbonate aquifer along 

the Solitario Canyon Fault, as well as along other 

north-striking faults. If such upwelling along the 

Solitario Canyon Fault is larger compared to along 

other faults, or if the hydraulic conductivity east of the 

Solitario Canyon Fault is less than elsewhere around 

Yucca Mountain, the upwelling could cause the moderate hydraulic gradient. However, if upwelling is the 

cause of the moderate gradient, the Solitario Canyon 

Fault seems to be unique in this respect because similar gradients are not observed across other northstriking faults in the Yucca Mountain area.

Small Hydraulic Gradient

The small hydraulic gradient is present beneath 

much of eastern and southern Yucca Mountain where 

water levels ranged from 728 to 732 m above sea level 

(fig. 9); this area includes most of the boreholes at 

Yucca Mountain. The small hydraulic gradient is 

bounded on the north by boreholes USW H-l and 

UE-25 WT#14 (fig. 9) and on the west by boreholes 

USW H-l, USW H-3, USW G-4 and USW WT-11. 

The small hydraulic gradient extends east as far as 

Fortymile Wash where the water-level altitude is about

26 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
728 m and probably extends well into Jackass Flats 

where the water-level altitude at well J-l 1 was about 

732 m (Boucher, 1994a; LaCamera and Westenburg, 

1994). The southern extent of the small hydraulic gradient includes boreholes USW WT-11 and UE-25 

WT#12, but the small hydraulic gradient may extend 

still farther to the south. The small hydraulic gradient 

represents a water-level change of 0.1 to 0.3 m/km 

(gradient of 0.0001 to 0.0003). Ervin and others 

(1993, p. 1558) stated that the small hydraulic gradient 

could indicate highly transmissive rocks, limited 

ground-water flow through the system, or a combination of both phenomena. The actual cause cannot be 

determined until further data are available.

The potentiometric surface in the area of the 

small hydraulic gradient slopes generally eastward

from Yucca Mountain toward Fortymile Wash; a 

smaller component slopes to the south. Limited data 

east of Fortymile Wash (Waddell, 1982; Boucher, 

1994a; LaCamera and Westenburg, 1994) indicated 

that the potentiometric surface in Jackass Flats also is 

relatively flat and probably slopes westward toward 

Fortymile Wash; again, a smaller component slopes to 

the south.

Vertical Hydraulic Gradients

Potentiometric levels have been measured at 

multiple depth intervals at 10 sites in the vicinity of 

Yucca Mountain (table 3). Differences in potentiometric levels at different depth intervals in the same

Table 3. Potentiometric levels at selected depth intervals in boreholes at Yucca Mountain

[Depth interval in meters; Pot. level, potentiometric level in meters above sea level]

Borehole 

name

UE-25 b#l

UE-25 c#3

UE-25 p#l

USWG-4

USWH-1

USWH-3

USWH-4

USWH-5

USWH-6

J-13

Depth 

Interval

488-1,199

1,199-1,220

692-753

753-914

384-500

1,297-1,805

615-747

747-915

573-673

716-765

1,097-1,123

1,783-1,814

762-1,114

1,114-1,219

525-1,188

1,188-1,219

708-1,091

1,091-1,219

533-752

752-1,220

533-1,193

1,193-1,220

282-^51

471-502

585-646

820-1,063

Primary hydrogeologic unit

Lower volcanic aquifer

Lower volcanic confining unit

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Carbonate aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic confining unit

Lower volcanic aquifer

Lower volcanic confining unit

Lower volcanic aquifer

Lower volcanic confining unit

Lower volcanic aquifer

Lower volcanic confining unit

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic aquifer

Lower volcanic confining unit

Upper volcanic aquifer

Upper volcanic confining unit

Lower volcanic aquifer

Lower volcanic aquifer

Pot. 

level

730.71

729.69

730.22

730.64

729.90

751.26

730.3

729.8

730.94

730.75

736.06

785.58

731.35

753.05

730.41

730.56

775.43

775.65

775.99

775.91

776.09

778.18

728.8

728.9

728.9

728.0

Remarks

1991 mean level

1991 mean level

1990 mean level

1990 mean level

Craig and Robison (1984)

Average value; Craig and Robison (1984)

Average value; Bentley (1984)

Average value; Bentley (1984)

1991 mean level

1991 mean level

1991 mean level

1991 mean level

1991 mean level

!Mean for 12/91

1991 mean level

1991 mean level

1991 mean level

1991 mean level

1991 mean level

1991 mean level

Jan. - May 1984 mean level

Jan. - May 1984 mean level

Thordarson (1983)

Thordarson (1983)

Thordarson (1983)

Thordarson (1983)

'Potentiometric levels rising throughout 1991 in response to reconfiguration of the packer that isolates the lower interval.

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 27
borehole ranged from about zero to nearly 55 m 

(table 3).

Potentiometric levels generally were higher in 

the lower intervals of the volcanic rocks than in the 

upper intervals, indicating a potential for upward 

ground-water movement. However, at four boreholes 

(USW G-4, USW H-l, USW H-6, and UE-25 b#l), 

potentiometric levels in the volcanic rocks were higher 

in the uppermost intervals than in the next lower intervals (table 3). At borehole USW G-4, there was a 

downward hydraulic gradient with a potentiometric 

difference of 0.5 m between the upper and lower intervals. Both intervals are in the lower volcanic aquifer. 

At borehole USW H-l, the potentiometric level was 

about 0.2 m higher in the uppermost interval than in 

the next lower interval. The potentiometric level in 

the third interval in borehole USW H-l was about 5 m 

higher than that in the uppermost interval. The upper 

three intervals are all in the lower volcanic aquifer. 

The potentiometric level in the lowermost interval of 

borehole USW H-l, completed in the lower volcanic 

confining unit, was nearly 55 m higher than in the 

uppermost interval. At borehole USW H-6, there was 

a downward gradient with a potentiometric difference 

of about 0.1 m between the intervals that were isolated 

in the lower volcanic aquifer. However, based on data 

from 1984 (Robison and others, 1988, p. 122-123), 

there seemed to be an upward potentiometric difference of about 2 m between the lower volcanic confining unit and the lower volcanic aquifer in borehole 

USW H-6. At borehole UE-25 b#l, there was a downward potentiometric difference of about 1 m between 

the lower volcanic aquifer and the lower volcanic confining unit.

Potentiometric levels in the Paleozoic carbonate 

aquifer at borehole UE-25 p#l are about 21m higher 

than levels in the lower volcanic aquifer (Craig and 

Johnson, 1984, p. 12) (table 3). A potential for 

upward ground-water movement from the Paleozoic 

rocks to the volcanic rocks was, therefore, indicated. 

Because of the large difference in potentiometric levels in these two aquifers, they seem to be hydraulically 

separate. However, analyses of hydraulic-test data at 

the C-hole complex indicated a possible hydraulic 

connection between the lower volcanic aquifer and the

3The C-hole complex refers to three boreholes, UE-25 c#l, 

UE-25 c#2, and UE-25 c#3, drilled in a triangular pattern and used 

to conduct cross-hole hydraulic and tracer tests. See "Local 

Hydrologic Setting" section.

carbonate aquifer at the C-hole complex (Geldon, 

1996). During the March 1984 test of borehole 

UE-25 c#2, the water level in borehole UE-25 p#l 

apparently declined; Geldon (1996) attributed this 

decline to a connection between the lower volcanic 

aquifer and the carbonate aquifer. However, waterlevel monitoring instruments at that time may not have 

been reliable, and any conclusions from data collected 

would be tenuous. Testing at the C-hole complex in 

1995 and 1996 did not result in any water-level 

decline in borehole UE-25 p#l (A.L. Geldon, U.S. 

Geological Survey, written commun., 1996). Further 

testing at the C-hole complex (U.S. Department of 

Energy, 1990a, sec. 3.4) using more reliable waterlevel monitoring instruments may help to resolve the 

question of hydraulic connection between the lower 

volcanic aquifer and the carbonate aquifer at this site. 

Measurement of potentiometric levels at various 

depth intervals in borehole UE-25 p#l shortly after the 

borehole was drilled indicated that:

1. Potentiometric levels gradually increased from 

729.9 m to 734.5 m above sea level between the 

water table (depth of about 384 m) and deep 

within the lower volcanic confining unit, 1,114m 

below land surface;

2. Potentiometric levels abruptly increased to about 

752 m above sea level in the "older tuffs" 

between depths of 1,110 and 1,180 m below land 

surface; and

3. Potentiometric levels remained at about 751 m

above sea level throughout the Paleozoic carbonate aquifer to a total depth of 1,805 m (Craig and 

Robison, 1984, p. 6-9). 

The lowermost 70 m of the older tuffs of the 

lower volcanic confining unit in the borehole had 

potentiometric levels that were very similar to the levels in the carbonate aquifer, indicating that the lowermost part of the lower volcanic confining unit and the 

carbonate aquifer were hydraulically connected. Such 

a connection could be expected in the hanging-wall 

rocks adjacent to a fault, and such a connection is supported by calcification of the basal tuffs in the borehole (Carr, M.D., and others, 1986, p. 23-25). The 

remaining 237 m of the lower volcanic confining unit 

had a potentiometric level similar to that of the lower 

volcanic aquifer (Craig and Robison, 1984, table 2). 

No unambiguous area! patterns in the distribution of vertical hydraulic gradients around Yucca 

Mountain are apparent; however, some generalizations 

can be made as to the distribution of potentiometric

28 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
levels in the lower sections of the volcanic rocks. 

Potentiometric levels in the lower volcanic confining 

unit are relatively high (altitude greater than 750 m) in 

the western and northern parts of Yucca Mountain 

(fig. 10) and are relatively low (altitude about 730 m) 

in the eastern part of Yucca Mountain. Based on 

potentiometric levels that were measured in borehole 

UE-25 p#l, the potentiometric levels in the lower 

volcanic confining unit in boreholes USW H-l, 

USW H-3, USW H-5, and USW H-6 may reflect the 

potentiometric level in the carbonate aquifer. Boreholes UE-25 b#l and USW H-4 do not seem to fit the 

pattern established by the other boreholes (fig. 10). 

These two boreholes penetrated only 31 and 64 m 

respectively into the lower volcanic confining unit and 

had potentiometric levels (about 730 m), which were 

similar to potentiometric levels in the lower volcanic 

aquifer. Penetration of the other four boreholes into 

the lower volcanic confining unit ranged from 123 m 

at USW H-3 to 726 m at USW H-l (table 2). Perhaps 

only in boreholes USW H-l, USW H-3, USW H-5 and 

USW H-6 are the potentiometric levels in the lower 

volcanic confining unit influenced by the potentiometric level in the carbonate aquifer.

Vertical hydraulic gradients could have an 

important impact on the analysis of the effectiveness 

of the saturated zone as a barrier to radionuclide transport. Based on a very limited data set (five boreholes), 

an areally extensive upward gradient can be inferred 

between the carbonate aquifer and the volcanic aquifers, which may indicate that, at least for the immediate Yucca Mountain area, radionuclide transport would 

be restricted to the volcanic system.

Potentiometric-Level Fluctuations and 

Trends

Potentiometric levels have been measured since 

1983 at Yucca Mountain at various frequencies ranging from occasional to continuous. The network of 

monitoring boreholes (fig. 11) has evolved to one that, 

as of the end of 1994, consisted of 15 boreholes in 

which 16 zones were monitored on a monthly basis, 

and of 12 boreholes in which 12 zones were monitored 

on an hourly basis and 4 zones were monitored on a 

continuous basis. Borehole USW H-l has 2 zones that 

were monitored monthly and 2 zones that were monitored hourly. The four zones in boreholes USW H-5 

and USW H-6 were equipped to continuously monitor

water levels and record extremely short-term waterlevel and fluid-pressure changes in response to phenomena such as earthquakes and underground nuclear 

explosions (O'Brien, 1992, 1993).

Potentiometric levels in boreholes at Yucca 

Mountain seem to be stable with time (Ervin and 

others, 1994, p. 11-13). After the potentiometric levels have equilibrated following drilling or reconfiguration of packers, they generally change very little with 

time. Annual fluctuations are on the order of a few 

tenths of a meter, primarily in response to barometric 

changes and earth tides (fig. 12). Potentiometric levels tend to fluctuate more on a day-to-day basis during 

the winter in response to the larger barometric fluctuations associated with winter-storm fronts.

Examples of long-term potentiometric-level 

fluctuations and trends are shown in figure 12. Potentiometric levels for borehole USW WT-2 (fig. 12) are 

representative of potentiometric-level fluctuations in 

the upper part of the lower volcanic aquifer. Daily 

mean potentiometric levels from 1986 through 1992 

ranged from about 730.5 to about 731.0 m above sea 

level and averaged about 730.7 m. Potentiometric levels for borehole UE-25 p#l (fig. 12) represented 

potentiometric levels in the Paleozoic carbonate aquifer. Potentiometric levels were relatively stable at 

about 752 m above sea level prior to 1985 (Robison 

and others, 1988); however, potentiometric levels rose 

from about 751.8 to 752.6 m between 1985 and 1987 

(fig. 12) and averaged about 752.5 m from 1987 

through June 1992. The abrupt decline in potentiometric levels on June 28-29,1992, and the subsequent 

rise in potentiometric levels, was in response to earthquake activity and has been discussed by O'Brien 

(1993, p. 10).

Lehman and others (1990) examined potentiometric-level fluctuations around Yucca Mountain for 

1983-88 and fit cosine functions to the data. They 

found periodic components of 2- to 3-year duration in 

nine records. They reported that potentiometric levels 

in boreholes east of Yucca Mountain had a similar 

period and it was different from the period in boreholes west of Yucca Mountain. Lehman and others 

(1990) believed the periodic behavior of potentiometric fluctuation was related to periodicity in precipitation. Lehman and Brown (L. Lehman and Assoc., 

written commun., 1994) later extended their analysis 

to include structural controls on ground-water flow.

Short-term fluctuations in potentiometric levels 

in response to earthquakes are known to occur at

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 29
36°45'

01234 5 KILOMETERS

0 1 2 3 MILES

EXPLANATION 

Alluvium (Quaternary or Tertiary)

Bedrock (Tertiary or Paleozoic) v '

Borehole location and number Numbers are 

preceded by either UE-25 or USW

^ Monitored hourly or more frequently

_

O Monitored monthly

H~ 1 ® Zones monitored at both frequencies

Figure 11. Locations of boreholes in the vicinity of Yucca Mountain in which water levels are monitored.

30 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
731.5

731.0

730.5

111

111

730.0

HI 

CO 

LU

O 729.5 CD

<

CO 

DC 

HI

LU 753.0

HI 

Q

ID

< 752.5

n i i r

Borehole USW WT-2 

Interval 571 -628 meters

752.0

751.5

751.0

Borehole UE-25 p#1 

Interval 1,297-1,805 meters

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

Figure 12. Potentiometric-level fluctuations, 1985-94, for boreholes USW WT-2 and 

UE-25 p#1 at Yucca Mountain.

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 31
Yucca Mountain (fig. 13). Maximum peak-to-peak 

fluctuations in response to nearby (26 km) earthquakes 

in late June 1992 were estimated to be 0.9 m for water 

levels and 2.2 m for fluid pressures (O'Brien, 1993, 

p. 8). These fluctuations generally lasted only 1 to 

2 hours. Potentiometric levels in borehole UE-25 p#l, 

however, declined a total of 0.5 m in response to these 

earthquakes (O'Brien, 1993, p. 10), and slowly recovered to pre-earthquake levels during the next 6 months 

(fig. 12).

Trend analyses were performed on potentiometric-level data for 1986 through 1989 by Ervin and 

others (1994). Earlier data were not used because of 

inconsistencies in measuring techniques in the waterlevel network. Results of their analyses need to be 

considered preliminary because of the relatively short 

period of record used. Four boreholes (US W WT-2, 

USW H-5, UE-25 WT#6, and UE-25 WT#16) had 

potentiometric-level changes that exhibited statistically significant trends. For 1986 through 1989, the 

trend was positive, and ranged from 0.07 to 0.6 m/yr, 

indicating an upward change in potentiometric level. 

The trends in boreholes UE-25 WT#6 and 

UE-25 WT#16 were probably attributable to postdrilling equilibration, based on the observation that 

hydrographs of potentiometric levels in these boreholes have shapes characteristic of recovery following 

pumping. The drilling process essentially pumped the 

boreholes dry. The long-term nature of the recovery 

trend indicated that the hydraulic conductivity in the 

vicinity of these boreholes was very small. The trends 

in the other two boreholes (USW WT-2 and 

USW H-5) are probably due to natural causes. As 

shown in figure 12, potentiometric levels in borehole 

USW WT-2 continued a slight overall upward trend 

through 1989, were relatively stable in 1990-91, and 

began a slight downward trend through 1992. An 

updated trend analysis, using a longer period of 

record, might have different results than the results 

reported by Ervin and others (1994).

Hydraulic Data

Knowledge of hydraulic properties, such as 

hydraulic conductivity, transmissivity, and storativity, 

is critical to understanding the hydrogeology of Yucca 

Mountain and demonstrating compliance with regulatory requirements for a mined geologic repository. 

Such information also is required for numerical models that can be used to simulate ground-water flow and 

contaminant transport. The following sections 

describe hydraulic data obtained from aquifer tests and 

from flow surveys in boreholes. A wide variety of 

methods were used to conduct and analyze these tests, 

with mixed success and results. No attempt is made in 

this report to evaluate or rank the results of the tests, 

and the reader is referred to the cited references for 

details.

Aquifer Tests

More than 150 individual aquifer tests were 

conducted at 13 boreholes (fig. 14) on and around 

Yucca Mountain in the early 1980's. Almost all the 

tests were single-borehole tests in specific depth intervals and included constant-discharge, fluid-injection, 

pressure-injection, borehole flow-meter, and radioactive-tracer tests. Multiple-borehole tests were conducted only at the C-hole complex (see "Local 

Hydrologic Setting" section). Results of aquifer tests 

usually were reported as transmissivity and hydraulic 

conductivity of intervals isolated with packers, but 

pumping-type aquifer tests were conducted in some 

boreholes for the entire saturated intervals. Storativity 

or specific yields were reported only for a few boreholes. Potential limitations of data collected during 

aquifer tests is discussed in the "Data Uncertainties" 

section.

Apparent hydraulic-conductivity values for the 

hydrogeologic units in the vicinity of Yucca Mountain 

are listed in table 4. These values were based on 

reported single-borehole aquifer tests and generally 

were calculated by dividing the total reported transmissivity of the tested interval by the thickness of that 

interval in the borehole. Hydraulic-conductivity values for individual intervals in a borehole could vary by 

several orders of magnitude, depending on whether or 

not water-bearing fractures were present. In intervals 

that contained no open fractures, hydraulic conductivity tended to be very low generally less than 

0.01 m/d reflecting the hydraulic conductivity of the 

rock matrix or of very small fractures. In tested intervals that contained water-bearing fractures, the apparent hydraulic-conductivity values listed in table 4 may 

be somewhat misleading; most, if not all, of the water 

produced in such an interval could have been produced by a few thin, highly conductive fractures in an 

otherwise thick, essentially nonproductive rock

32 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
o>

CVJ 

0)

o

2

1

CO

0)

(0 

CO

I

cr

CO 

0)

£ 

<p 

i

I

_ 

0)

o> .2*

I I I I

uoisiAip jed sjeieiuijueo 88"0)

U31VM dO Sy313IAILLN30 Nl

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U31VM dO SU313IAJI1N30 Nl 

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HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 33
36'45'

NEVADA 

. Carson City

Area of 

Map

Las 

Vegas

01234 5 KILOMETERS

I II II I

I i n r

0 1 2 3 MILES

Alluvium (Quaternary or Tertiary) 

Bedrock (Tertiary or Paleozoic)

EXPLANATION

J-13

Borehole location and numberNumbers are preceded by 

either UE-25 or USW

Figure 14. Locations of aquifer test data at Yucca Mountain.

34 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
Table 4. Estimated apparent hydraulic-conductivity values obtained from single-borehole aquifer tests in the vicinity of 

Yucca Mountain

[~, no data available; <, less than]

Apparent hydraulic conductivity, in meters per day

Borehole 

name

USWH-1

USWH-3

USWH-4

USWH-5

USWH-6

USWG-4

UE-25 b#l

C-hole complex

UE-25 p#l 

J-13

Upper 

volcanic 

aquifer

~

~

-

~

~

~

~

-

1.0

Upper 

volcanic 

confining 

unit

-

~

~

~

-

~

2.6 X lO-1

2.0 X10-2

h^xio-1

Lower 

volcanic 

aquifer

4.3 X 10-1

<3.7 X 10-3

4.3 X 10-1

6.0 X10-1

8.0 X10-1

1.4

4.9 X 10-1

7.0 X10-2

3.3 X 10-2 

7.6 X 10-3

Lower 

volcanic 

confining 

unit

5.5 X 10-6

<3.2 X 10-3

1.1 X 10-1

--

1.8 X10-4

~

<1.0X10^

"

5.8 X 10-3 

22.6 X 10-3

Carbonate Reference 

aquifer

Rush and others (1984)

 Thordarson and others (1985)

- Whitfield and others ( 1 985)

Robison and Craig (1991)

Craig and Reed (1991)

Lobmeyer(1986)

Lahoud and others ( 1 984); 

Moench (1984)

Geldon (in press)

1.9 X 10-1 Craig and Robison (1984) 

-- Thordarson (1983)

'Average determined from two tests. 

Includes part of the lower volcanic aquifer.

matrix. Therefore, hydraulic conductivity is termed 

"apparent" in this section.

Most hydraulic data were from tests conducted 

in the lower volcanic aquifer and in the lower volcanic 

confining unit. Very few data were available for the 

upper volcanic aquifer, the upper volcanic confining 

unit, and the carbonate aquifer.

Hydraulic data for the upper volcanic aquifer 

were available only for well J-13. Thordarson (1983) 

reported transmissivity (table 5) for geologic units that 

correspond to the upper volcanic aquifer of 120 m2/d, 

which resulted in an apparent hydraulic-conductivity 

value of about 1.0 m/d (table 4). The upper volcanic 

aquifer also was tested at borehole USW VH-1 

(Thordarson and Howells, 1987); however, the tests 

were conducted over the entire saturated interval, 

which includes the lower volcanic aquifer. Most of 

the water produced by the borehole may be from a 

small part of the lower volcanic aquifer (Thordarson 

and Howells, 1987, p. 9). Reported transmissivity 

values ranged from 450 to 2,400 m2/d (Thordarson 

and Howells, 1987, p. 14-18), and apparent hydraulicconductivity values range from 0.8 to 4.2 m/d for five 

tests at borehole USW VH-1.

Hydraulic data for the upper volcanic confining 

unit were available from tests conducted at the C-hole 

complex, borehole UE-25 b#l, and well J-13. 

Reported transmissivity values for the upper volcanic

confining unit ranged from 2.0 to 26 m2/d (table 5), 

and apparent hydraulic-conductivity values from tests 

at these boreholes ranged from 0.02 to 0.26 m/d 

(table 4).

Reported transmissivity values of the lower volcanic aquifer ranged from less than 1.1 to 589 m2/d 

(table 5). The arithmetic mean of the 10 transmissivity values for the lower volcanic aquifer was about 

152 m2/d, and the geometric mean was about 43 m2/d. 

Apparent hydraulic-conductivity values for the lower 

volcanic aquifer ranged from less than 3.7 x 10'3 to 

2.0 m/d (table 4). The arithmetic mean of the 10 values in table 4 was 0.49 m/d and the geometric mean 

was 0.15 m/d. In borehole USW H-l, apparent 

hydraulic conductivity of individual intervals tested in 

the lower volcanic aquifer ranged from 4 x 10-5 to 

18 m/d (Rush and others, 1984) in single-borehole 

tests.

Reported transmissivity values of the lower volcanic confining unit ranged from less than 3.0 x 10~3 to 

23 m2/d (table 5). The arithmetic mean of the seven 

transmissivity values for the lower volcanic confining 

unit was 3.7 m2/d and the geometric mean was 0.20 

m2/d. Apparent hydraulic-conductivity values for the 

lower volcanic confining unit ranged from 5.5 x 10"6 to 

0.11 m/d (table 4). The arithmetic mean of the seven 

values in table 4 was 1.7 x 10-2 m/d, and the geometric 

mean was 1.3 x 10'3 m/d.

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 35
Table 5. Estimated transmissivity values obtained from single-borehole aquifer tests in the vicinity of Yucca Mountain

[-, no data; <, less than]

Transmissivity, In meters squared per day

Borehole 

name

USW H-l

USW H-3

USWH-4

USW H-5

USW H-6 

USWG-4

UE-25 b#l

C-hole complex

UE-25 p#l 

J-13

Upper 

volcanic 

aquifer

-

-

-

~

mm

~

120

Upper 

volcanic 

confining 

unit

-

-

-

-

~

26

2.0

23.7

Lower 

volcanic 

aquifer

152

<1.1

178

35

229 

*589

297

21

15 

1.4

Lower 

volcanic 

confining 

unit

5.0 X10-3

<4.1 X 10-1

23

-

6.3 X 10-2

<3.0X10-3

-

2.0 

36.3 X 10-1

Carbonate Reference 

aquifer

Rush and others (1984)

Thordarson and others ( 1 985)

- Whitfield and others ( 1 985)

- Robison and Craig (1991)

Craig and Reed (1991) 

- Lobmeyer (1986)

Lahoud and others (1984); 

Moench (1984)

Geldon (in press)

1 1 8 Craig and Robison (1 984) 

- Thordarson (1983)

'Average determined from four tests. 

2Average determined from two tests. 

Includes part of the lower volcanic aquifer.

Data were available for the carbonate aquifer in 

the vicinity of Yucca Mountain only for borehole 

UE-25 p#l. Hydraulic data presented by Craig and 

Robison (1984) indicated a transmissivity of 118 m2/d 

(table 5) and apparent hydraulic-conductivity values 

for specific tested intervals that ranged from 0.1 to 

0.2 m/d and averaged 0.19 m/d.

In fractured-rock aquifers, such as those at 

Yucca Mountain, hydraulic conductivity probably is 

anisotropic that is, greater in a direction parallel to 

the predominant fracture orientation than in other 

directions. Although this information is critical to 

determining the ground-water flow paths from the 

potential repository, only one test has been conducted 

at Yucca Mountain to determine whether or not the 

aquifers actually are anisotropic. Erickson and 

Waddell (1985, p. 24-29) reported that, at borehole 

USW H-4, the maximum hydraulic conductivity was 

from 5 to 7 times greater than the minimum hydraulic 

conductivity; the maximum hydraulic conductivity 

was oriented in a direction 23° east of north. 

Czarnecki and Waddell (1984, p. 27-28), however, 

reported that their subregional model duplicated measured water levels more accurately when the aquifer 

was simulated as isotropic rather than anisotropic. 

Therefore, the question of anisotropy remains unresolved. Future tests at the C-hole complex may help 

resolve this issue.

Vertical hydraulic-conductivity values were 

reported only for borehole USW H-5 and the C-hole 

complex. Robison and Craig (1991, p. 26) reported 

an apparent vertical hydraulic-conductivity value of 

62 m/d for the lower volcanic aquifer at borehole 

USW H-5. This value was about 100 times greater 

than the reported value for horizontal hydraulic conductivity of 0.6 m/d (table 4). Data for the C-hole 

complex were in direct contrast with the data from 

borehole of USW H-5. The vertical hydraulicconductivity values for the lower volcanic aquifer 

were estimated to range from 1 to 2 m/d (Geldon, 

1996), and were from 0.01 to 0.17 times the values for 

the horizontal hydraulic conductivity (6 to 100 m/d) 

obtained over the same intervals using multipleborehole aquifer-test analyses.

Geldon (1996) reported that transmissivity and 

apparent hydraulic-conductivity values determined 

using multiple-borehole hydraulic tests tend to be 

much higher about 2 orders of magnitude than values reported for single-borehole tests conducted at the 

same borehole. For example, a single-borehole test at 

borehole UE-25 c#3 resulted in a transmissivity value 

of 27.9 m2/d, but results of a multiple-borehole test for 

the same test interval in the borehole indicated a

36 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
transmissivity value of 1,860 m2/d. Geldon (1996) 

concluded that multiple-borehole tests generally sample a much larger volume of the aquifer material and 

incorporate a larger number of water-bearing fractures 

than single-borehole tests. Because most transmissivity and hydraulic-conductivity values at Yucca Mountain were obtained from single-borehole tests, the 

values listed in tables 4 and 5 may not be representative values appropriate for scales of tens to hundreds 

of meters.

The hydraulic characteristics of a particular 

interval at a borehole are highly dependent on the fracture characteristics of that interval. A large conductive 

feature, such as the fault penetrated in the lower part 

of borehole UE-25 c#l, can contribute a large percentage of the total flow to a borehole. The tested interval 

containing such a feature can have a disproportionately large transmissivity and apparent hydraulic conductivity compared to other intervals in the same 

borehole. Intervals that have few conductive fractures 

generally contribute little water to the borehole and 

may have small transmissivity compared to other 

intervals in the borehole.

The storativity reported for tests at the C-hole 

complex was 0.0005, and specific yields ranged from 

0.003 to 0.07 (Geldon, 1996). The boreholes at this 

complex tap the lower volcanic aquifer. Storativity for 

borehole USW H-l was estimated by Rush and others 

(1984, table 12) to range from 6 x 1Q-6 to 6 x 1Q-3 for 

the lower volcanic aquifer and from 6x10^ to 2x10~3 

for the lower volcanic confining unit. Barr (1985) 

used the same data and obtained similar results. 

Thordarson and others (1985, table 2) reported storativities for borehole USW H-3 that ranged from 

4 x lO^6 to 7 x 10-6 for the entire saturated interval of 

the borehole. Specific yields for borehole USW H-5 

ranged from 0.15 to 0.28 (Robison and Craig, 1991, 

p. 24) for the entire saturated interval of the borehole; 

however, more than 95 percent of the water in the 

borehole was produced from fractures in the upper 

part of the lower volcanic aquifer (Robison and Craig, 

1991, fig. 6). Barr (1991, table 3) estimated storativity 

for nine intervals in the lower volcanic aquifer in borehole USW G-4. The values ranged from 1 x 1Q-5 to 

SxlO-6.

Flow Surveys

Flow surveys were conducted in most of the 

deeper boreholes drilled at Yucca Mountain. Flow

surveys are useful to determine which intervals of the 

borehole, and possibly which fractures, produced 

water. The flow surveys were conducted using a tool 

developed for oil-field use that releases small quantities of radioactive iodine-131 (Blankennagel, 1967, 

p. 15-19). As the iodine moves up or down the borehole, it is sensed by gamma-ray detectors. Most surveys are done while water is being pumped from or 

injected into the borehole. Static tests also were occasionally done.

Flow surveys are useful in determining which 

parts of the borehole produce (or accept) most of the 

flow. The information is useful when subdividing a 

system into aquifers and confining units. Minor producing intervals are not readily detected by this 

method. Flow surveys generally report velocities to 

about 0.03 rn/s, but the variability between successive 

measurements is considerably larger than 0.03 m/s. 

Therefore, for borehole diameters generally found at 

Yucca Mountain, intervals producing less than 0.1 L/s 

can be easily missed. Flow surveys for seven boreholes are shown in figure 15. For comparison purposes, the depths for all surveys were adjusted to a 

standard thickness of 524.3 m for the lower volcanic 

aquifer, which is the estimated thickness of this aquifer in borehole USW G-l, but is not necessarily representative of the thickness of the aquifer elsewhere.

The flow surveys generally indicated that production (or acceptance) of water occurred in a few 

intervals. Boreholes USW H-l and USW H-5 are the 

most dramatic examples of this generalization. In 

borehole USW H-1,64 percent of the production came 

from a 1-m interval, whereas in borehole USW H-5, 

73 percent of the production came from a 42-m interval. Boreholes USW H-4 and UE-25 b#l had production from larger intervals. All of the boreholes had 

little production (or acceptance) of water from their 

lower intervals.

None of the boreholes for which flow surveys 

were done had major production from the lower volcanic confining unit, but borehole USW H-4 produced 

about 12 percent of its flow from the uppermost part of 

that unit. The upper volcanic confining unit is saturated only in boreholes UE-25 b#l and USW G-2 

(fig. 15). The upper confining unit produced 

approximately one-third of the water in borehole 

UE-25 b#l and accepted virtually all the water in the 

test at borehole USW G-2. The lower volcanic aquifer 

did not accept any water in the test at borehole 

USW G-2 despite being fully penetrated. At the

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 37
Hydrogeologic

Units

(Thickness 

from USW G-1)

Upper 

Volcanic 

Aquifer

Upper Volcanic 

Confining Unit

Lower 

Volcanic 

Aquifer

Lower Volcanic 

Confining Unit

USWH-4. Pumping, 

17.2 liters per second USWH-3. Injection, 

2.7 liters per second

USWH-6. Pumping, 

26.6 liters per second

UE-25b#1. Pumping, 

13.4 liters per second

USWH-1. Pumping, 

1.4 liters per second

USWH-5. Pumping, 

7.6 liters per second

50

I

PERCENTAGE OF PUMPED 

OR INJECTED WATER

100 PERCENT

_J

» Figure 15. Flow surveys for selected boreholes in the Yucca Mountain area. Flow surveys adjusted to a "standard thickness" of 

& and a "standard thickness" of the upper confining unit of 121.3 meters, the thickness of the units in borehole USW G-1. All flow s

» pumping or injection.

a
C-hole complex, flow surveys indicated that the lower 

volcanic aquifer produced virtually all the flow, but 

temperature surveys indicated that the upper volcanic 

confining unit produced some water in borehole 

UE-25 c#3 and possibly in borehole UE-25 c#2 

(Geldon, 1993, figs. 31-33). The upper volcanic aquifer is above the saturated zone in all of the boreholes 

shown in figure 15. No flow surveys were available 

from boreholes that penetrated the upper volcanic 

aquifer in the saturated zone, but hydraulic tests at 

well J-13 indicated that most of the production was 

from the upper volcanic aquifer (Thordarson, 1983, 

table 12). At borehole USW VH-1 in Crater Flat, most 

of the production came from the lower volcanic aquifer (Thordarson and Howells, 1987, p. 9), whereas the 

upper volcanic aquifer produced little water. The 

upper volcanic confining unit is absent at this borehole.

Inflow to Volcanic Flow System

Sources of water that enter the volcanic aquifers 

and confining units in the vicinity of Yucca Mountain 

potentially include inflow from upgradient volcanic 

aquifers and confining units, local recharge from 

Fortymile Wash, precipitation that infiltrates the surface of Yucca Mountain, especially at higher altitudes 

at the northern end of Yucca Mountain, and upward 

flow from the underlying carbonate aquifer. The magnitudes of most of the inflows to the volcanic system 

have not been quantified.

Upgradient Inflow

Potentiometric data from widely spaced boreholes upgradient from Yucca Mountain indicate that 

ground water probably flows south from upland 

recharge areas in the volcanic terrain of Pahute and 

Rainier Mesas, beneath Umber Mountain, under the 

topographic divide near Pinnacles Ridge that separates 

Beatty Wash from Crater Flat and Yucca Mountain, 

continuing southward beneath the Yucca Mountain 

area (fig. 1). However, the concept of inflow from 

upgradient regions is based on limited data, particularly between Yucca Mountain and Pahute Mesa. The 

Timber Mountain caldera complex (fig. 2) lies 

between Pahute Mesa and Yucca Mountain. No data 

are available in the Umber Mountain area and its 

hydrologic characteristics are poorly known. The

Umber Mountain caldera complex could act as a barrier and divert upgradient flow away from the Yucca 

Mountain region and into Beatty Wash and the Oasis 

Valley ground-water subbasin. It is also possible, 

although not probable, that a ground-water divide 

exists between the Crater Flat-Yucca Mountain area 

and Beatty Wash. This possibility also would result in 

recharge from Pahute and Rainier Mesas being 

diverted into the Oasis Valley ground-water subbasin. 

Hydrochemical data (Benson and others, 1983, 

table 1) indicated that water in the volcanic aquifer 

beneath Yucca Mountain and Crater Flat was 

recharged during wetter climatic conditions approximately 13,000 to 18,000 years ago based on apparent 

carbon-14 ages. However, these data do not preclude 

that some modern recharge occurs and in fact water 

from borehole UE-29 a#2 in upper Fortymile Wash 

appeared to have been recharged about 4,000 years 

ago, whereas water from wells J-12 and J-13 have 

apparent ages of 9,000 and 10,000 years respectively. 

Actual ages may be older than apparent ages, and the 

water probably is a mixture from recharge events that 

spanned at least a number of millennia. The data do 

not indicate whether the recharge occurred far upgradient or in Fortymile Wash. If most of the ground 

water beneath Yucca Mountain was recharged in the 

distant past, the flow system may still be equilibrating 

from an ancient recharge pulse, resulting in a gradual 

decline in water levels beneath Crater Flat and Yucca 

Mountain over time (Czarnecki and others, 1990). 

This concept is consistent with the observation of 

Winograd and Szabo (1986) that water-levels apparently progressively declined in the southern Great 

Basin during the Quaternary Period, at least in the 

southern Amargosa Desert and in Death Valley.

Fortymile Wash Recharge

Fortymile Wash is a major southward-draining 

ephemeral channel located northeast, east, and southeast of Yucca Mountain (fig. 3). It is a long, linear feature that may be structurally controlled (note the long, 

straight canyon between Yucca Mountain and Calico 

Hills on the second page of fig. 1). Fortymile Wash 

begins to the north of Yucca Mountain in the highlands 

of Pahute Mesa and ends in the Amargosa Desert to 

the south of the mountain. During extreme runoff, 

Fortymile Wash would be tributary to the Amargosa 

River. In various numerical ground-water flow models (Czarnecki and Waddell, 1984; Rice, 1984;

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 39
Czarnecki, 1985; Sinton, 1987), recharge had to be 

specified in Fortymile Wash to replicate potentiometric-level measurements in observation boreholes near 

Yucca Mountain. Czarnecki (1985) specified about 

40 percent of the total recharge to his model as occurring in Fortymile Wash. Simulated potentiometric levels beneath Yucca Mountain were quite sensitive to 

this specified recharge (Czarnecki, 1985). Osterkamp 

and others (1994) estimated average annual recharge 

along the entire 95-km length of Fortymile Wash 

(including Fortymile Canyon) to be about 4.22 x 

1C)6 m3, based on channel-geometry measurements and 

analyses with a precipitation/runoff simulator.

Depths to water beneath upper Fortymile Wash 

were as small as 16 m in borehole UE-29 UZN#91 

(Savard, 1994, p. 1810-1812) compared to about 

283 m at well J-13 directly east of Yucca Mountain 

(Robison and others, 1988, p. 129). Savard (1994) 

indicated that recharge occurred in 1992 and 1993 at 

borehole UE-29 UZN#91 and at nearby boreholes 

UE-29 a#l and UE-29 a#2 based on rising water levels 

in each borehole following local precipitation and runoff.

Waddell (1984, table 4) obtained hydrochemical 

samples from boreholes UE-29 a#l and UE-29 a#2. 

Tritium and carbon-14 values from these samples indicated that apparently younger water was present at 

shallower depths in borehole UE-29 a#l (65.5 m deep) 

compared to borehole UE-29 a#2 (421.5 m deep). 

Younger water at shallow depth with deeper older 

water indicated that recharge was occurring at or near 

this site. In addition, the potentiometric level was 

about 4 m higher in the shallower borehole 

(UE-29 a#l) than in the deeper borehole (UE-29 a#2) 

which was consistent with recharge in this area.

Infrequently, Fortymile Wash has carried water 

to western Jackass Flats and even less frequently into 

the Amargosa Desert (Squires and Young, 1984). 

Water flowing down Fortymile Wash quickly infiltrates into the channel deposits; therefore, major flows 

need to occur for water to reach the lower sections. 

Such a major flow has not occurred during the site 

characterization of Yucca Mountain4; the last major

4Just as this report was nearing completion in March 1995, 

flow occurred in Fortymile Wash at least as far downstream as 

well J-13. No definitive analysis of this recharge event has been 

completed although boreholes UE-29 a#l, UE-29 a#2, and 

UE-29 UZN#91 showed significant water-level rise (C.S. Savard, 

U.S. Geological Survey, written commun., May 1995).

flow occurred in February 1969. No estimates of the 

volume of water recharged or the effects on water levels are available. As noted earlier in this section, 

Savard (1994) documented five small recharge events 

in 1992 and 1993 in Fortymile Wash or in its tributaries upstream from Jackass Flats.

Local Infiltration

Local infiltration from precipitation probably 

occurs in the Yucca Mountain area, but the amount of 

infiltration that reaches the water table may be inconsequential compared to the upgradient inflow and 

recharge from Fortymile Wash. Flint and Flint (1994) 

estimated that near-surface infiltration ranged from 

0.02 to 13.4 mm/yr and averaged 1.4 mm/yr. Higher 

infiltration tended to occur on the northern part of 

Yucca Mountain. Bodvarsson and others (1994), in a 

series of simulations of flow in the unsaturated zone, 

assumed average infiltration below the root zone over 

Yucca Mountain to be from 0.001 to 0.5 mm/yr. Deep 

infiltration would be less than shallow infiltration 

because air flow through the mountain would remove 

some moisture. Although accurate determination of 

the rate and distribution of percolation deep in the 

unsaturated zone at Yucca Mountain is important to 

predicting potential repository performance, this rate 

may be small enough under modern climatic conditions not to have a substantial effect on the saturatedzone flow system. If Flint and Flint's (1994) average 

shallow infiltration of 1.4 mm/yr was assumed to 

reach the water table beneath the entire 150-km area of 

intensive unsaturated-zone hydrologic study, recharge 

would amount to about 2 x 105 m3 . While it is unlikely 

that all of the shallow infiltration becomes recharge, 

local infiltration to the saturated zone cannot be 

neglected until either better estimates of deep percolation are available or numerical models are shown to be 

insensitive to the assumed range in values of local 

recharge.

Inflow From the Carbonate Aquifer

As discussed in the "Vertical Hydraulic 

Gradients" section, the potentiometric level relation 

between the volcanic flow system and the carbonate 

aquifer is known only by measurement at borehole 

UE-25 p#l in the Yucca Mountain area. At a depth of 

about 1,100 m in the borehole, there was an upward 

gradient of about 18 m in 66 m between the lower

40 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
volcanic confining unit and the carbonate aquifer, indicating the potential for upward flow. This potential 

does not necessarily mean that upward flow occurs, 

but the presence of calcified tuff in the lower part of 

the lower volcanic confining unit strongly indicates 

that upward flow actually occurred, at least in the past 

(Carr, M.D., and others, 1986, p. 24).

The "Vertical Hydraulic Gradients" section also 

discusses a substantial upward gradient between the 

deeper part of the lower volcanic confining unit and 

the overlying volcanic hydrogeologic units at four 

boreholes at Yucca Mountain, USW H-l, USW H-3, 

USW H-5, and USW H-6. The potentiometric difference exceeds 50 m in borehole USW H-l. In the 

"Vertical Hydraulic Gradients" section, the potentiometric levels in these boreholes are indicated as being 

a possible reflection of potentiometric levels in the 

carbonate aquifer, which is additional evidence for 

potential inflow from the carbonate aquifer.

Temperature measurements from the upper saturated zone indicated anomalies (elevated temperatures) around Solitario Canyon Fault and in the area 

between the Bow Ridge and Paintbrush Canyon Faults 

(Fridrich and others, 1994, fig. 8). These anomalies 

have been interpreted as evidence of upward flow 

from the carbonate aquifer (W.W. Dudley, Jr., 

U.S. Geological Survey, written commun., 1991; 

Fridrich and others, 1991,1994). However, no quantitative estimates of rates for the postulated flow have 

been made.

Strontium isotopic data gives some evidence of 

upward flow from the carbonate aquifer into the volcanic system (Spengler and Peterman, 1991, p. 1420). 

This evidence, by itself, is not definitive, and no quantitative estimate of flow rates has been made.

Outflow From Volcanic Flow System

Pathways by which water may leave the volcanic aquifers and confining units in the vicinity of 

Yucca Mountain include outflow to downgradient volcanic aquifers and confining units and to the alluvial 

aquifer, pumpage, downward flow to the underlying 

carbonate aquifer, and upward flow into the unsaturated zone. The magnitudes of most of the outflows 

from the volcanic system have not been quantified.

Downgradient Outflow

Downgradient from Yucca Mountain in the 

Amargosa Desert, the potentiometric surface slopes 

gently toward the south (Czarnecki and Waddell, 

1984, fig. 3; D'Agnese, 1994, fig. 10-3), indicating 

southerly ground-water flow. The potentiometric surface steepens in the vicinity of the Nevada-California 

State line, but flattens toward Alkali Flat (Franklin 

Lake Playa), as it approaches and possibly even rises 

above the land surface. Much of the downgradient 

flow probably discharges at this playa, although some 

flow probably continues southward beyond the playa.

Pumpage

Water is routinely pumped from wells J-12 and 

J-13 in Jackass Flats for use in the Yucca Mountain 

area and the southwestern part of the Nevada Test Site. 

Well J-12 was completed in 1957 and was placed in 

service in 1960, and well J-13 was placed in service 

shortly after it was completed in 1963. A third well in 

Jackass Flats, J-ll, was completed in 1957, but was 

removed from service in 1962 because of poor water 

quality and casing problems (Young, 1972, p. 17). 

Young (1972, p. 13) estimated that about 6.9 x 

105 mVyr of water was pumped from these wells during 1962-67. LaCamera and Westenburg (1994, 

table 6) estimated that about 1.8 x 105 mVyr of water 

was pumped from these wells during 1981-92. The 

decrease in pumpage with time was due to a decrease 

in activity in this part of the Nevada Test Site.

Water was pumped from various boreholes in 

the Yucca Mountain area during 1980-84 to determine 

the hydraulic characteristics of the flow system. The 

largest volume test occurred at borehole UE-25 c#3 in 

November 1984 (Geldon, 1993, p. 25) where about 

3.2 x 104 m3 of water was pumped. In March 1992, 

borehole JF-3 south of well J-12 was tested. About 

2 x 103 m3 of water was pumped during that test 

(R.J. LaCamera, U.S. Geological Survey, oral commun., 1994).

Water is occasionally pumped from borehole 

USW VH-1 in Crater Flat, but the volume pumped is 

small. Elsewhere in Crater Flat, water is pumped to 

support mining operations. LaCamera and 

Westenburg (1994, table 6) estimated that about 

6.9 x 104 mVyr was pumped in Crater Flat during 

1989-92. Water was pumped during various tests of 

borehole USW H-6 in 1982 and 1984 (Craig and

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 41
Reed, 1991, p. 9-17). The total volume pumped was 

about 2.5 x 104 m3 .

By far, the largest amount of pumpage in the 

area occurs in the Amargosa Desert south of Yucca 

Mountain. Water is pumped for irrigation, mining, 

industrial, commercial, stock, and domestic use. 

LaCamera and Westenburg (1994, table 6) compiled 

pumpage inventories by the Nevada Division of Water 

Resources for the Amargosa Desert for various years. 

Average pumpage for 1985-92 was 8.1 x 106 mVyr 

and ranged from 4.8 x 106 to 11.9 x 106 mVyr. Pumpage in the Amargosa Desert is more than 40 times as 

large as pumpage in the Yucca Mountain area. This 

pumpage probably does not have a direct effect on the 

flow system at Yucca Mountain because of the large 

distances involved, but probably needs to be 

accounted for during any modeling of the area. If a 

model of the Yucca Mountain area extends well into 

the Amargosa Desert, the pumpage needs to be 

accounted for directly. If a model does not extend that 

far south, the pumpage may be accounted for with 

boundary conditions.

Outflow to the Carbonate Aquifer

Limited evidence discussed in the "Inflow from 

the Carbonate Aquifer" section indicated that the carbonate aquifer may supply at least some inflow to the 

volcanic system at the south end of Yucca Mountain. 

There is no direct evidence that the volcanic system 

supplies outflow to the carbonate aquifer in the vicinity of Yucca Mountain. However, one explanation for 

the large hydraulic gradient at the northern end of 

Yucca Mountain assumes a major outflow from the 

volcanic system into the carbonate aquifer (see the 

"Large Hydraulic Gradient" section). Because of the 

sparseness of the data, it cannot be determined if there 

is any outflow from the volcanic system to the carbonate aquifer. If there is an outflow to the carbonate 

aquifer, it still cannot be determined whether there is a 

net outflow from the volcanic system to the carbonate 

aquifer or whether there is a net inflow to the volcanic 

system from the carbonate aquifer.

Flow Into Unsaturated Zone

The interaction between the water table and the 

overlying unsaturated zone at Yucca Mountain is 

being intensively studied. A conceptual model of the 

unsaturated zone (Montazer and Wilson, 1984) indicated a very small downward flux of water deep in the 

unsaturated zone. Ross (1984) studied vapor movement in the unsaturated zone where little recharge 

occurred and concluded that vapor movement could 

dominate the system, and upward flow could occur 

where net recharge was less than 0.03 mm/yr. Whatever the interaction between the saturated zone and the 

unsaturated zone at Yucca Mountain, the volume probably is small compared to downgradient outflow.

Hydrochemical Data

Hydrochemical data describing saturated-zone 

ground water at and in the vicinity of Yucca Mountain 

date from 1971. Benson and McKinley (1985) 

reported chemical and isotopic information from 12 

boreholes drilled between October 1980 and July 1984 

(table 6). They also reported analyses of 1971 samples from two water-supply wells near Yucca Mountain. Matuska (1989) reported analyses of samples 

collected in 1988 from five boreholes, which 

penetrated the uppermost part of the flow system. 

Additional data for samples from more than 250 sites 

in the adjacent region were selectively compiled by 

McKinley and others (1991). Thomas and others 

(1991) compiled new and existing data for 209 sites in 

southern and eastern Nevada and adjacent southeastern California. The records for these sites, some of 

which are included in McKinley and others (1991), are 

fairly comprehensive and include all of the chemical 

and isotopic data available for each site until about 

1989. Peterman and Stuckless (1993) and Ludwig and 

others (1993) presented and discussed strontium and 

uranium isotopic data, some of which are from Yucca 

Mountain area boreholes and springs. Perfect and others (1995) compiled data for more than 3,000 sites in 

the region between latitudes 35° and 38°N. and longitudes 115° and 118°W. This data set contains data for 

all the sites identified in the preceding references, but 

individual analyses reported may be less complete 

than in the less extensive compilations.

Data reported by Benson and McKinley (1985) 

included values of pH, water temperature, and specific 

conductance; concentrations of major ions, lithium 

(Li), strontium (Sr), and dissolved solids; activities of 

tritium (3H) and carbon-14 (14C); and ratios of stable 

carbon, hydrogen, and oxygen isotopes. The Matuska 

(1989) data included the same suite, but without Sr 

and dissolved solids, and added bromide (Br) and

42 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
Table 6. Boreholes in the vicinity of Yucca Mountain from which hydrochemical data were obtained 

[All depths are in meters below land surface]

Borehole 

name

J-12

J-13

UE-25 b#l

UE-25 c#l

UE-25 c#2

UE-25 c#3

UE-25 p#l

UE-29 a#2

USW H-l

USW H-3

USWH-4

USWH-5

USW H-6

USW VH-1

Latitude

36'45'54"

36'48'28"

36'51'08"

36'49'47"

36'49'45"

36'49'45"

36'49'38M

36'56'29"

36'51'57"

36'49'42"

36*50'32"

36*5 1'22"

36*50'49"

36'4732"

Longitude

116'23f24M

116'23'40"

116'2ff23*

116'25'43"

116'25'43M

116'25'44"

116'25'2r

116'22'26"

116*2712"

116'28'00"

116'26'54"

116'27'55"

116'28'55"

116'33'07"

Total depth

347

1,060

1,220

914

914

914

1,805

422

1,829

1,219

1,219

1,219

1,220

762

Depth to water

226

283

470

401

401

402

382

28

572

751

519

704

527

184

Interval 

sampled

226-347

304-424; 

820-1,010

470-1,220

853-914

401-914

401-914

402-914

382-1,173

1,298-1,792

247-355

87-213

572-687

687-1,829

754-1,219

519-1,219

707-1,219

527-1,220

600-650

753-835

278-762

Date 

sampled

03/26/71

03/26/71

09/01/81

07/20/82

09/30/83

03/13/84

05/09/84

02/11/83

05/12/83

01/08/82

01/15/82

10/20/80

12/08/80

03/14/84

05/17/82

07/03/82

10/16/82

07/06/84

06/20/84

02/11/81

some nitrate (NO3) and detergent [methylene blue activated substance (MBAS)]. Duplicates of most of the 

Matuska (1989) samples were obtained by the USGS 

and independent analytical results were essentially the 

same.

The major-ion data of Benson and McKinley 

(1985) and Matuska (1989) may be subject to question, because Li concentrations in all except one of the 

samples ranged from 1.5 to more than 20 times the 

concentration in samples from nearby, heavily pumped 

water-supply well J-13. The concentration of Li may 

indicate the presence of contamination because LiBr 

was added as a tracer to air-foam fluids used in drilling 

the sample borehole. This information, and field 

observations of sample turbidity and foaming, indicated incomplete removal of drilling fluid prior to 

sample collection. This possible contamination also 

raised questions about the carbon-isotope data because 

the drilling fluid used in drilling the boreholes was an 

air-water-detergent mixture.

Stuckless, Whelan, and Steinkampf (1991) used 

the Benson and McKinley (1985) and Matuska (1989)

carbon-isotope data in mixing calculations to interpret 

measured variations in carbon-isotopic data and indicated the presence of at least three end-member 

waters. The presence of these end-member waters is 

not supported by ground-water flow that can be 

inferred from potentiometric data, nor do similar mixing calculations using nonreactive ionic concentrations or hydrogen- or oxygen-isotopic ratios provide 

support for these three end members. Peterman and 

Stuckless (1993) reported a general north-to-south 

increase in borehole and spring strontium isotopic values (587Sr) from Pahute Mesa to the southern Amargosa Desert, but could not describe regional flow paths 

beyond the flow paths that can be inferred from 

regional potentiometric data. The 887Sr data provided 

no insight as to flow paths at and in the immediate 

vicinity of Yucca Mountain. Peterman and Stuckless 

(1993) attributed this inability to determine flow paths 

to the isotopic stratification identified in the lithologic 

units and inhomogeneities along the flow paths due to 

structural and lithologic variations.

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 43
The chemical characteristics of ground water in 

the Yucca Mountain area have evolved primarily from 

rock/water interactions. Ground-water composition is 

a function of recharge water chemistry and the materials with which the water interacts along its flow path. 

Ground-water cations in the volcanic system derive 

from reaction with volcanic glass, primary minerals, 

soils, and probably to some extent, secondary phases 

such as calcite. In addition to being derived from 

rock/water interaction, anions derive from atmospheric, soil-zone and unsaturated-zone gases, and 

precipitation. From the available information (Benson 

and McKinley, 1985; Matuska, 1989; Thomas and others, 1991), water in the volcanic aquifers and confining units is a relatively dilute sodium bicarbonate type 

that would be expected to evolve in a volcanic geohydrologic system (Jones, 1966).

The Benson and McKinley (1985) and Matuska 

(1989) data indicated that the calcium sodium ratio in 

the water increased by an order of magnitude from 

west to east at Yucca Mountain, the lowest values 

being west of and near the Solitario Canyon Fault. 

This increase in ratio indicated that the chemical evolution of the water had progressed farther in the water 

to the west. This evolution to the west could be the 

combined effect of the Solitario Canyon Fault acting 

as a barrier to horizontal flow from the west, the cause 

of the large hydraulic gradient acting as a barrier to 

horizontal flow from the north, and less evolved water 

moving from in the vicinity of Fortymile Wash mixing 

with more evolved water moving from the west. This 

evolution also could indicate a contribution of water 

from the underlying Paleozoic aquifer to the volcanic 

system through faults on the east side of Yucca Mountain, without substantial upwelling of deeper water 

through Solitario Canyon Fault. Apparent carbon-14 

(I4C) ages from Benson and McKinley (1985) indicate 

the oldest water occurs beneath the crest of Yucca 

Mountain and the youngest water occurs beneath Fortymile Wash. The Benson and McKinley (1985) data 

were from samples obtained generally from considerably below the water table, whereas the Matuska 

(1989) data were from water that was from the uppermost 100 m of the saturated zone.

The variation in water chemistry is indicated in 

the exchange-ion compositions of clinoptilolites, a 

zeolite, examined by Broxton and others (1987, p. 98). 

In their Zone II (see "Zeolites and the Water Table" 

section), the uppermost part of which contains the 

modern-day potentiometric surface, sodium (Na) and

potassium (K) occupy most of the zeolite-exchange 

sites from the west part of Yucca Mountain, grading to 

a predominance by calcium (Ca), magnesium (Mg), 

and potassium (K) in zeolite-exchange sites to the 

east. Most of the clinoptilolites Broxton and others 

(1987) examined were from below the water table. 

Those clinoptilolites from above the water table 

tended to contain K in the zeolite-exchange sites. 

Broxton and others (1987, p. 96) also pointed out that 

zeolites in Zone I, all in or above the Topopah Spring 

Tuff, have primarily Ca in the zeolite-exchange sites, 

whereas the parent rock is relatively Ca poor. If these 

zeolites formed because of stratigraphically-related 

changes in permeability that could result in liquid 

accumulations parallel to the contrasts, the calcic 

nature of the zeolites could be attributed to dissolution 

of secondary calcite caused by zeolite formation that 

began once percolating water attained sufficiently high 

K, Na, and silicon (Si) activities (Broxton and others, 

1987, p. 105).

Hydrochemical and isotopic data, where adequate data are available, can provide qualitative information for checking numerical flow models. The 

numerical models can be used to identify potential 

flow paths through and between the volcanic and carbonate flow systems. The hydrochemical data then 

can be used to support or refute the potential flow 

paths by analyzing the evolution of the water along the 

potential flow path. The numerical models can be 

used to calculate residence time of water in the flow 

system. The residence time can be checked by comparing estimated and actual concentrations of selected 

hydrochemical constituents and isotopic ratios.

Paleohydrology

Previous sections of this report have described 

the ground-water flow system as it existed during 

modern times. Modern ground-water conditions are 

the result of the modern climate or the climate of the 

recent geologic past (tens of thousands of years). 

However, climate is dynamic and different climates in 

the past would have resulted in differences in the flow 

system.

Spaulding (1985) and Forester and Smith (1994) 

stated that the modern climate of southern Nevada is 

very atypical and that wetter and cooler climates have 

been normal in the geologic past. Evidence of the 

effects of wetter and cooler climates on the ground44 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
water flow system is preserved in discharge deposits 

around Yucca Mountain (Winograd and Doty, 1980; 

Paces and others, 1993) and, to some extent, also in 

some types of geochemical alteration of tuffs in Yucca 

Mountain.

In addition to climate-driven changes, 

Szymanski (1989) has suggested that stress- and 

thermally-driven effects [seismic pumping or Sibson 

pumping (Sibson and others, 1975)] have dramatically 

altered the flow system in the geologic past. Szymanski (1989) suggested that the water table at and in the 

vicinity of Yucca Mountain has been dramatically 

higher, even reaching the land surface, in the past as a 

result of cyclical tectonism. To address this concern, 

geologic, hydrologic, isotopic, tectonic, and thermal 

information has been examined by numerous investigators, including Quade and Cerling (1990), Carrigan 

and others (1991), Marshall and Mahan, (1991), 

National Research Council (1992), Stuckless, 

Peterman, and Muhs (1991), and Whelan and 

Stuckless (1992). These investigators studied the 

Szymanski (1989) model in considerable detail and 

rejected its important aspects. Dudley and others 

(1989) provided a summary of all of the reviews of the 

Szymanski model. Because this complex and controversial model was examined and rejected by previous 

investigators, it is not considered further in this report.

Northern Discharge Deposits

Three carbonate-rich, apparent spring discharge 

deposits are present in the southern end of Crater Flat 

and the northern end of the Amargosa Desert, approximately 18 km southwest of the potential repository at 

Yucca Mountain (Paces and others, 1993). These 

deposits are termed the northern discharge deposits 

only to distinguish them from the deposits discussed in 

the next section. Data indicated that these discharge 

deposits were partly contemporaneous and were most 

likely part of the same hydrologic system. The deposits range widely in ages, but probably have resulted 

from active spring discharge within the last 19,000 to 

150,000 years (E.M. Taylor, U.S. Geological Survey, 

oral commun., 1993; Paces and others, 1993). Uranium- and strontium-isotopic data indicated that the 

water in the volcanic aquifers (or alternately, alluvium 

derived from these volcanic rocks) provided the 

source for these spring discharge deposits. The water 

table may have been 80 to 115 m above its modern 

level at various times during the Quaternary Period

(Paces and others, 1993); however, the position of the 

water table beneath these deposits is not well known 

and needs to be determined. Even under modern climatic conditions, these spring discharge deposits frequently appear to be moist, which may indicate that 

they represent a perched-system discharge area.

These spring discharge deposits provide indirect 

information against which the numerical flow model, 

once it is developed, can be tested. If a numerical 

model predicts surface flow in these areas when wetter 

and cooler climatic conditions are imposed, confidence in the model would be increased. If surface 

flow cannot be reproduced under reasonable climatic 

conditions, then either the model has to be inadequate 

or the interpretation of the cause of these spring discharge deposits is in error.

Southern Discharge Deposits

Czarnecki (1985) simulated wetter climatic conditions using a subregional ground-water flow model 

of the Yucca Mountain area. According to the simulation, new or increased ground-water discharge as a 

result of wetter climatic conditions would occur: 

(1) South and west of Timber Mountain in Beatty 

Wash; (2) along Fortymile Canyon, resulting in perennial streamflow; (3) in the Amargosa Desert near the 

town of Amargosa Valley, about 10 km north of the 

town of Death Valley Junction, and at Alkali Flat 

(Franklin Lake Playa); and (4) near Furnace Creek 

Ranch in Death Valley. Field investigations near the 

predicted discharge areas in the Amargosa Desert indicated the presence of areally extensive marsh or spring 

discharge deposits at the land surface north of Death 

Valley Junction. These marsh or spring discharge 

deposits are being studied and dated and seem to have 

been active in the recent geologic past (Z.E. Peterman, 

U.S. Geological Survey, oral commun., 1995), but the 

deposits have not yet provided data to confirm the simulated ground-water discharge areas during wetter and 

cooler climatic conditions.

Zeolites and the Water Table

Many studies concentrated on the presence and 

the chemical and isotopic attributes of the primary and 

secondary mineral phases in the upper volcanic 

sequence in the vicinity of Yucca Mountain. Bish and 

others (1981) and Smyth and Caporuscio (1981), 

using analyses of core from borehole USW G-l,

HYDROLOGY OF THE SATURATED ZONE AT YUCCA MOUNTAIN 45
subdivided the volcanic sequence vertically into four 

zones that were based on the vertical distribution of 

diagenetic minerals. The lowermost zones (III and 

IV) are at least 500 m below the potentiometric surface and are not relevant to this discussion.

Broxton and others (1987) extended the examination of secondary minerals in the Yucca Mountain 

area. They reported that Zone I ranges in thickness 

from about 550 m at borehole USW G-2 on the northern end of Yucca Mountain and at borehole USW G-3 

on the southern end to about 170 m near the northern 

end of Yucca Mountain (fig. 16). At borehole USW 

G-l, Zone I is about 380 m thick. The base of Zone I 

essentially coincides with the top of the Calico Hills 

Formation at the northern end of Yucca Mountain and 

is in or near the base of the formation elsewhere. 

Zone I is above the water table everywhere and contains a large percentage of unaltered volcanic glass in 

vitric tuffs. Broxton and others (1987) reported that 

secondary mineralization in Zone I is only incipient 

and consists primarily of smectite and opal. Alteration 

of pumice and glass shards is limited and, where 

present, is restricted to particle margins. The predominance of glass indicates that no part of Zone I has ever 

been below the water table because the glass would 

have altered rapidly and completely to secondary 

phases had it been in the saturated zone.

In Zone I, the presence of clinoptilolite, the first 

zeolite likely to form during post-depositional evolution, is restricted to localized fractures (Carlos, 1985) 

and to thin, discontinuous areas in mostly vitric tuffs 

that are parallel to the bedding, which indicate that 

these fractures and areas were the only sites of substantial and persistent liquid accumulation. These 

bedding-parallel zeolitic zones tend to be horizontally 

discontinuous, except for an ubiquitous smectitezeolite interval at the top of the basal vitrophyre of the 

Topopah Spring lYiff of the Paintbrush Group 

(D.L. Bish, Los Alamos National Laboratory, oral 

commun., 1993). Levy (1984a,b) suggested that this 

interval is the product of epithermal diagenesis that 

occurred soon after deposition. Alternatively, the 

interval could be the result of the accumulation of fluid 

at the large permeability contrast provided by the basal 

vitrophyre, which, in the absence of through-going 

conductive fractures, could essentially stop the vertical 

movement of water.

Another exception to limited alteration in Zone I 

is a 130-m interval in the lower part of the Yucca 

Mountain lYrff, the underlying Pah Canyon lYiff, and

the upper part of the Topopah Spring TYiff. This interval has been identified only in borehole USW G-2. 

The base of the interval is about 300 m above the modern water table. Smectite is the predominant mineral 

in the upper one-half of this largely vitric interval; 

smectite- and clinoptilolite-group minerals are common in the lower one-half, and zeolites become predominant with increasing depth (Broxton and others, 

1987, p. 93). This gradation of minerals indicates that 

permeability decreases with depth in this interval, and 

that the downward flow of water tends to be restricted. 

This interval probably was not associated with a 

regional water table, even under wetter and cooler 

climatic conditions.

Zone II (fig. 16) thickens from about 480 m in 

the vicinity of borehole USW G-2 to about 700 m in 

the vicinity of borehole USW G-3. It contains little 

volcanic glass, most of the glass has been altered to 

clinoptilolite- and mordenite-group minerals. The top 

of Zone II is essentially in the Calico Hills Formation, 

slopes downward south from borehole USW G-2, 

where it roughly coincides with the modern water 

table. South of borehole USW G-2, the water table is 

above the top of Zone II, about 150 m at boreholes 

USW G-l and USW H-5, and 190 m at borehole 

USW G-3 (fig. 16). From borehole USW G-3 to boreholes USW H-5 and USW G-l, the top of Zone II 

essentially maintains an altitude of about 900 m. At 

borehole UE-25 b#l (fig. 16), about 2 km east of a line 

between boreholes USW G-2 and USW G-3, the top 

of Zone II is at an altitude of about 800 m, roughly 

70 m above the modern water table (Broxton and 

others, 1987, fig. 3).

Broxton and others (1987) discussed two scenarios that could explain the presence of zeolites 

above the water table. One scenario is the formation 

of zeolites in the unsaturated zone by downwardpercolating water, the effect of which would be 

enhanced by the formation of perched water because 

of horizontally continuous permeability contrasts 

(Hoover, 1968). Another scenario is formation of zeolites below the water table followed by uplift of units 

by basin and range faulting into the unsaturated zone.

Peterman and others (1992) measured the relative abundances of strontium isotopes (87Sr and 86Sr) in 

calcite samples from five boreholes (USW G-l, 

USW G-2, USW G-3, USW G-4, and UE-25 b#l) 

and found three populations of isotopic ratios. Samples from within 400 m of land surface had values 

similar to samples of surficial pedogenic calcites.

46 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
SOUTH 

USW G-3 USW H-5

1,000 m -

500m -

Sea Level 

NORTH 

USW G-2

1,000 m

- 500 m

 Sea Level

USWH-4 s

Sea Level 

UE-25 b#1

0 1 MILE 

VERTICAL EXAGGERATION X 2

2 KILOMETERS

J

Figure 16. Diagenetic zones in the volcanic sequence at Yucca Mountain (modified from Broxton and others, 1987).

Samples obtained from within 100 m of the modern 

water table, all four of which came from borehole 

USW G-2, had values very close to the mean value for 

ground-water samples obtained from the volcanic flow 

system. Samples obtained from below the water table 

had isotopic ratios that were very close to the calculated initial isotopic values of the host volcanic rocks, 

but the values differed from the mean ground-water 

value and from values for calcites from above the 

water table. To explain these distinct differences, 

Peterman and others (1992) offered two scenarios. 

The first scenario is a higher water table in the past, 

which was supported by Marshall and others (1993). 

Marshall and others (1993) offered evidence for a 

higher water table at a site in southern Crater Flat 

about 15 km southwest of borehole USW G-3 (see 

"Northern Discharge Deposits" section). The second

scenario is upward aqueous diffusion of Ca and Sr 

from the saturated zone. Peterman and others (1992) 

also suggested the possibility of contribution of Ca 

and Sr from underlying Paleozoic carbonates to the 

lower part of the volcanic sequence.

LOCAL HYDROLOGIC SETTING 

(C-HOLE COMPLEX)

On a local scale (ten to hundreds of meters), 

ground-water flow in the volcanic rocks beneath 

Yucca Mountain is likely to be controlled by fractures. 

Although many fractures exist in the various sections 

of the volcanic rocks, only a small percentage of the 

fractures actually conduct large quantities of water to 

most of the boreholes. Flow in these fractures

LOCAL HYDROLOGIC SETTING (C-HOLE COMPLEX) 47
probably occurs via the fortuitous intersection and 

interconnection of conductive pathways. Effects at the 

scale of as much as 100 m are being studied at the 

C-hole complex (U.S. Department of Energy, 1990a, 

sec. 3.3 through 3.5), which is the only multihole complex in the saturated zone at Yucca Mountain.

The C-hole complex consists of three boreholes 

(UE-25 c#l, UE-25 c#2, and UE-25 c#3) located 

about 2.4 km east of the crest of Yucca Mountain, 

between Bow Ridge and Fran Ridge (fig. 17). The 

boreholes are located in a triangular pattern; boreholes 

UE-25 c#l and UE-25 c#2 are along a line estimated 

to be the major semiaxis of the permeability tensor. 

Boreholes UE-25 c#2 and UE-25 c#3 are along a line 

estimated to be the minor semiaxis of the permeability 

tensor (U.S. Department of Energy, 1990a, p. 3.3-4). 

Each borehole was drilled to a depth of approximately 

914 m. The water table is approximately 402 m below 

the land surface, and the geologic units penetrated 

beneath the water table are the Calico Hills Formation 

and the Prow Pass, Bullfrog, and Tram Tuffs of the 

Crater Flat Group (fig. 18).

Understanding of ground-water flow at the 

C-hole complex is based primarily on dynamic (tracejector) and static (heat-pulse) intraborehole-flow surveys, borehole television and acoustic televiewer logs, 

and borehole temperature logs (Geldon, 1993). In 

general, flow at the complex is controlled by fractures, 

fracture zones, and faults. Although the entire tested 

thickness contains numerous fractures, only a small 

percentage of those fractures contribute measurable 

quantities of flow.

Intraborehole-flow surveys in response to natural gradients indicated that flow in the upper part of 

borehole UE-25 c#l was downward at a rate of 

approximately 0.01 L/s to a depth of 768 m. Below 

768 m, flow was upward at a rate of approximately 

0.1 L/s. In borehole UE-25 c#2, flow in the upper part 

of the borehole was downward at a rate of approximately 0.01 L/s to a depth of about 823 m. Below 

823 m, flow was upward at a rate of approximately 

0.01 L/s. Flow in the upper part of borehole 

UE-25 c#3 was upward at a rate of approximately 

0.003 L/s to a depth of about 701 m. Below 701 m, 

flow was upward at a rate of approximately 0.25 L/s, 

and below 870 m, it was upward at a rate of approximately 0.13 L/s.

Although there are differences in flow rates in 

response to natural gradients, these surveys indicated 

several similarities among the three boreholes. Two

features, a fault and a fracture zone, seemed to be controlling flow. The fault, interpreted by Geldon (1993, 

p. 39-42) to be the Paintbrush Canyon Fault or a splay 

of the fault, intersects each of the boreholes in the 

Tram Tuff, at a depth of about 853 m. Warm water, 

indicating that the water may be coming from deeper 

in the flow system, seems to enter each borehole 

through this fault and flows upward until it reaches a 

fracture zone that intersects each borehole in the Bullfrog Toff (Geldon, 1996). Most of the intraborehole 

flow exits the borehole through the fracture zone in the 

Bullfrog TYiff. Small amounts of water enter the upper 

part of boreholes UE-25 c#l and UE-25 c#2 through a 

fracture zone near the contact between the Calico Hills 

Formation and the underlying Prow Pass Tuff of the 

Crater Flat Group. This water moves downward and 

also exits the borehole in the Bullfrog Tuff.

The differences in flow rates among the boreholes can be explained by assuming that borehole 

UE-25 c#3 intersected the fault at a point where the 

fault zone had a large transmissivity. As a result of 

higher potentiometric head at depth, possibly derived 

from the underlying Paleozoic carbonate aquifer 

(Geldon, 1996), water moved upward along the fault. 

Although most of the water exited the borehole 

through the Bullfrog Tuff, the flow (0.25 L/s) was too 

large for all the water to exit; thus, a residual upward 

flow (0.003 L/s) occurred in the upper part of the borehole. In borehole UE-25 c#l, water also entered 

through the same fault, but at a smaller rate 

(0.1-0.2 L/s). At this smaller flow rate, the fracture 

zone in the Bullfrog TYiff had sufficient transmissivity 

to allow all water from below the zone to exit; as a 

result, flow above the fracture zone was downward 

from the flow zone in the Prow Pass Tuff. Flow in 

borehole UE-25 c#2 was similar to that in borehole 

UE-25 c#l, ecept that borehole UE-25 c#2 intersected 

the fault where the transmissivity was smaller. 

Because of the smaller transmissivity, a much smaller 

flow (0.01-0.02 L/s) entered through the fault and 

moved up the borehole.

Two conceptual models could be used in designing and interpreting hydraulic and tracer tests at the 

C-hole complex. One conceptual model would treat 

each flow zone as an isolated permeable zone separated by zones of relatively small transmissivity. 

However, this model may not be appropriate because 

of the presence of the fault intersected by the boreholes at about 853 m, but the possibility that the transmissivity of the fault may decrease east of the C-hole

48 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
USW H-4

UE-25WT#14,

l 

0.5 MILE

1 KILOMETER

C-hole complex

USW WT-1

LAYOUT OF THE C-HOLE COMPLEX

230,770

230,670

173,570 173,590 173,610 173,630 173,650 173,670 

EASTING, IN METERS

Figure 17. Layout of the C-hole complex.

LOCAL HYDROLOGIC SETTING (C-HOLE COMPLEX) 49
0 -

50 -

100 -

200 -

300 -

CO

UJ

UJ 350 -

Z 

U-f 400 -

SURFAC ND 

ft o

I

^ 500 -

2

UJ 

m 550 -

X

CL 

UJ

Q 600 -

ccn

700 - 

750 -

800 -

850 - 

Qnn 

a-\AA

GROUP

o

O

.c

c

0.

!f 

8

T3

O

3=

I03

EC

0 

0)

O

FORMATION

c

c »_ 03 *=

OH 

§

*:

IOJ

|

'ormation

^

Prow Pass

*:

H 

D)

2

m

Tuff Tram 

LITHOLOGY

^<5< \A/V <><>c kxx; \x\\

/////

~~ ~^~\ 1 1

A>vX

XXX xVx

i1

d c) c

tp1r 1 <^ c c 1 1

JVj

- ." ' -.-'. : ' '. '":-

 .-':/:;- - ' ". -. " 

:--: :-". '.'; : ; 

';'.- : ''.'/:'.' ''. '"' 

^KSm 

^mm all ii

ji1ii:; ;';.'jir7j^.[;

SJJJS

\XN/N/

^\^>

C^X\X\>'> 

xv/V^X

/x >;>^x

^X^ ry^y^

 .: '. . : . .":. :-: .." 

j?' f' S.- /" > s

/'////

y, EXPLANATION

x ^^^^H ALTERED TUFF

^ : fiup^ BEDDED TUFF

« BEDDED TUFF NONWELDED TUFF

- IMV-/IM v» ti_L/t-L-' i urn

x /////// NONWELDED TO PARTIALLY

K x//l WELDED TUFF

f\\S\- MODERATELY WELDED TUFF

»?5<5<5 MODERATE TO DENSELY

Iy Y yi WELDED TUFF

\ j DENSELY WELDED TUFF

£§£>* TUFF BRECCIA

°0 °0 °0 LITHOPHYSAL ZONE

:

<

' ^ BEDDED TUFF

Figure 18. Stratigraphy and lithology at the C-hole complex (modified from Geldon, 1993, 

fig. 8).

50 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
complex requires that this model be considered. 

Another reason that this conceptual model may not be 

appropriate is the probable presence of subvertical 

fractures that would create hydraulic connections 

between the major permeable zones. During hydraulic 

tests at the C-hole complex, each of the major permeable zones could be isolated by packers and pumped. 

If no response was detected in the nonpumped zones, 

then this model would be more plausible. In this case, 

the tests may be analyzed using several equivalentporous-medium layers.

The second conceptual model would treat the 

entire saturated thickness as an interconnected network of a limited number of fractures, fracture zones, 

and faults. In this model, subvertical fractures or the 

fault, or both, provide hydraulic connections between 

the fracture zones identified in the boreholes. If this 

model is correct, pumping from any one zone would 

create measurable drawdown in most or all other 

water-producing fracture zones. If this is an appropriate conceptual model, the hydraulic and tracer tests 

could be usedfrif possible, to map all of the major connections and pathways. Analysis of hydraulic and 

tracer tests would then be much more complex and 

might require the application of fracture-network 

modeling techniques.

DISCUSSION OF THE SATURATED ZONE 

AT YUCCA MOUNTAIN

The flow system at Yucca Mountain occurs in a 

sequence of fractured volcanic rocks and underlying 

carbonate rocks. In this report, the volcanic sequence 

is divided into two aquifers (the upper and lower volcanic aquifers) and two confining units (the upper and 

lower volcanic confining units). The division does not 

correlate exactly with either the stratigraphic units or 

the thermal/mechanical units and is useful only as a 

general guideline as to where flow might occur. Generally, only a small number of intervals in the aquifers, 

usually associated with fractures or faults, produce 

water. The confining units also may transmit water, 

but to a lesser extent than the aquifers. In both the 

aquifers and the confining units, fractures probably 

transmit more water than the rock matrix does. The 

less transmissive units probably have a much larger 

proportion of their flow occurring in the matrix. The 

lower volcanic confining unit may transmit little

water, primarily because lithostatic pressure keeps the 

fractures closed.

Little is known about the carbonate rocks 

beneath Yucca Mountain, but they constitute a major 

aquifer in the region. The carbonate aquifer probably 

underlies the lower volcanic confining unit beneath all 

of Yucca Mountain. Several lines of evidence indicate 

that the lower volcanic confining unit effectively isolates the carbonate aquifer from the volcanic aquifers. However, the altered mineralogy of the tuffs, 

particularly in borehole UE-25 p#l, indicates past 

influx of deeper water, possibly localized along faults. 

Higher water-table temperatures aligned along existing faults may indicate upwelling, probably from the 

carbonate aquifer. Beneath the northern part of Yucca 

Mountain, the carbonate aquifer may additionally be 

confined by the Eleana Formation. If the volcanic 

aquifers are essentially isolated from the carbonate 

aquifer, then the potentiometric surface in the volcanic 

aquifer would be independent of the potentiometric 

surface in the carbonate aquifer. If, however, the confinement is breached by faults and, particularly if flow 

through one or the other aquifers is not large compared 

to upwelling or downwelling between aquifers, the 

potentiometric surface in one system may affect the 

potentiometric surface in the other system.

On a regional scale, flow is generally from the 

higher plateaus in the north to the Amargosa Desert in 

the south, probably continuing southward or westward, or both, to still lower areas of discharge. How 

flow moves through the Timber Mountain area is not 

clear because of lack of data. In the immediate vicinity of Yucca Mountain, the flow direction seems to be 

more eastward as indicated by the potentiometric contours, although anisotropy may deflect part of the flow 

southward. Whatever part of the flow that persists 

eastward to reach the Fortymile Wash area then would 

flow southward through the trough of the potentiometric surface.

In the area of small hydraulic gradient at Yucca 

Mountain, the potentiometric surface is a continuous, 

almost flat surface that does not change noticeably as 

it passes through the various faults east of the Solitario 

Canyon Fault. The potentiometric surface slopes gently to the east at a relatively constant gradient (fig. 9). 

However, because the transmissivity of the rocks 

probably is not isotropic, flow is not necessarily perpendicular to the contours. Flow may be more southerly because the major faults and fractures in the area

DISCUSSION OF THE SATURATED ZONE AT YUCCA MOUNTAIN 51
tend to be north-trending and may divert flow to the 

south.

The potentiometric surface in the area of small 

hydraulic gradient also does not seem to change as it 

transects aquifers and confining units. Beneath the 

crest of Yucca Mountain, flow is entirely in the lower 

volcanic aquifer and deeper units. However, at Fortymile Wash, the upper volcanic aquifer dips beneath 

the water table and dominates the flow system. There 

is no obvious break in the slope of the potentiometric 

surface in the transition area, which must be somewhere in Midway Valley (fig. 9). In the vicinity of the 

large hydraulic gradient in the northern Yucca Mountain area, the potentiometric surface may or may not 

reflect a change in hydrogeologic units, depending on 

the interpretation of the cause of the large hydraulic 

gradient.

The volume of ground water that flows beneath 

Yucca Mountain is unknown. If the large hydraulic 

gradient to the north of Yucca Mountain and the moderate hydraulic gradient to the west of the crest of the 

mountain effectively isolate the volcanic flow system 

from the regional flow system and, if the lower volcanic confining unit effectively isolates the volcanic and 

carbonate flow systems from each other, little water 

could be moving beneath the mountain. If these barriers are not particularly effective, then ground-water 

flow beneath Yucca Mountain could be much greater. 

The small hydraulic gradient can be interpreted as an 

area of either high transmissivity or small groundwater flux (Ervin and others, 1993, p. 1558), or some 

combination of the two. Because there is no direct 

way to measure ground-water flow, resolution of the 

possible causes of the small hydraulic gradient would 

depend on both direct and indirect evidence. Sufficient testing to determine transmissivity and 

anisotropy on scales of tens to hundreds of meters, 

combined with measured hydraulic gradients (fig. 9), 

would allow a direct estimate of flow. Detailed hydrochemical characterization, combined with analysis of 

rock-water interaction and water-chemistry evolution, 

would provide indirect evidence of the rate of flow. 

Likewise, water ages estimated from isotopic data 

would provide indirect evidence. Understanding the 

inflows to the volcanic aquifers across the large and 

moderate hydraulic gradients and from the carbonate 

aquifer may influence judgments as to the amount of 

flow in the area of the small hydraulic gradient. However, there is no direct method to measure these 

inflows either.

Conceptual Uncertainties

Although the flow system at Yucca Mountain 

has been divided into a series of aquifers and confining 

units, and the general direction of flow is known, substantial conceptual uncertainties remain. Fractures 

probably transmit most of the water through the aquifers, but at individual boreholes, only a small percentage of the fractures contribute flow and faults 

frequently dominate the flow. Conceptually, flow in 

the saturated zone occurs predominately in a few fractures, but as a practical matter, numerical models 

would have to simulate the system either as a porousmedia continuum or as a finite number of discrete features. At the scale of the Yucca Mountain site, it is not 

yet known whether fractures are so pervasive that the 

flow system can be simulated as a classical porous 

medium or if discrete features need to be simulated.

The discussions about the large hydraulic gradient, and to a lesser extent the moderate hydraulic gradient under the "Potentiometric Surface" section, 

indicated that there is uncertainty aboutj^ow the flow 

system behaves in these areas. This uncertainty would 

translate into uncertainty about either external or internal boundary conditions when a numerical model is 

developed. The uncertainties about potential groundwater divides also would translate into uncertainty 

about the numerical-model boundary conditions.

Although recharge certainly is related to climate, topography, flora, soils, and unsaturated-zone 

flow, major uncertainties exist about the interrelation 

and this uncertainty translates into a general conceptual uncertainty about recharge. Perhaps equally 

important, the time scale at which the flow system 

comes into equilibrium with climate, and hence 

recharge, is a conceptual uncertainty.

Tests to determine flow-system parameters are 

conducted at scales that are much smaller than the 

scales that ultimately need to be simulated. How to 

translate data from a test scale to a model scale is a 

major conceptual uncertainty. It may not be clear even 

whether the test scale underestimates or overestimates 

the parameter required at the model scale.

Data Uncertainties

No matter how well the saturated-zone flow system at Yucca Mountain and vicinity is described, 

uncertainty will always remain. The data needed to

52 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
simulate the ground-water flow system (in order 

according to the assumed uncertainty from largest to 

smallest) would be: (1) recharge; (2) storage properties; (3) transmissive properties; (4) discharge; and 

(5) hydraulic head. A discussion of each of these 

uncertainties follows.

Recharge

Recharge in arid environments is perhaps the 

most difficult model variable to quantify because: 

(1) the uncertainty in the relation between precipitation and resultant recharge; (2) the sporadic and infrequent nature of major precipitation events; and (3) the 

uncertainty in the location and distribution of subsurface barriers and conduits (such as fractures and 

faults) that could retard or enhance recharge. In various numerical models of the regional flow system 

(Oberlander, 1979; Waddell, 1982; Czarnecki and 

Waddell, 1984; Sinton, 1987; D'Agnese, 1994), 

recharge to the flow system was estimated and was 

assumed to be in equilibrium with discharge, a necessary but somewhat questionable, assumption.

Despite the difficulties and uncertainties, 

recharge was estimated in these studies based on 

where recharge was likely to occur (using topographic 

and climatic information) and where recharge was 

required by the model to make the model adequately 

simulate measured potentiometric levels. An example 

of the latter was recharge along Fortymile Wash. In 

the models developed by Czarnecki and Waddell 

(1984) and Sinton (1987), recharge had to be specified 

along Fortymile Wash for the models to accurately 

simulate measured potentiometric levels in the vicinity 

of Yucca Mountain.

In contrast, the northernmost boundary in the 

Czarnecki and Waddell (1984) model consisted of a 

series of constant-head nodes that supplied the appropriate amount of inflow across the boundary to make 

total inflow equal total outflow. This inflow simulated 

the throughflow of recharge from Pahute and Rainier 

Mesas, areas that are topographically higher and climatically wetter than Yucca Mountain. This northern 

boundary was set sufficiently distant from Yucca 

Mountain so the boundary would not overly affect 

simulated ground-water conditions at Yucca Mountain.

Storage Properties

Storage properties of the saturated-zone flow 

system at Yucca Mountain were not discussed in great 

detail in the "Aquifer Tests" section because so little 

data exist for storage properties. Estimates for storage 

properties are uncertain even when aquifer tests are 

conducted with nearby observation holes; storage 

properties are particularly unreliable for single-hole 

tests, which is the usual type for Yucca Mountain. 

Therefore, storage properties were estimated or 

assumed only for a few boreholes in the Yucca Mountain area, and these estimates were quite different from 

borehole to borehole.

Storage properties are not required for steadystate simulations of ground-water flow, but are 

required for time-dependent simulations and are critical in transport simulations. The range of values for 

storativity for water-table conditions (unconfined 

flow) may be large because effective porosity may be 

as small as 0.001 for fractured tuff and may exceed 

0.20 for alluvium or nonwelded tuff. For confined 

flow or deep unconfined flow within the saturated 

zone, the storativity could be an order of magnitude or 

more smaller. This wide range of uncertainty about 

the value of storativity would then result in wide 

ranges of uncertainty in estimates of ground-water 

travel times and transport characteristics.

Transmissive Properties

Transmissivity or hydraulic conductivity estimates based on aquifer tests in boreholes are available 

from a variety of sources, which used a variety of 

methods for the Yucca Mountain area, but the analysis 

of the tests require assumptions that frequently cannot 

be demonstrated to be even approximately correct. 

The reliability of the commonly used slug tests for 

estimating values of transmissivity in fractured tuff at 

Yucca Mountain is suspect in many cases, in part 

because the testing could have caused either elastic 

fracture dilatation or fracture-deformation by hydraulic fracturing of the rock mass (Thordarson and others, 

1985, p. 19; Stock and Healey, 1988). Additionally, 

the results of many of these slug tests could not be 

analyzed reliably because conditions in the boreholes 

were not appropriate for the porous-medium type of 

analysis required of slug tests.

The geometric mean value of the transmissivities listed in table 5 was about 70 m2/d, but the range 

was from 1.5 to 3,250 m2/d. Transmissivity probably

DISCUSSION OF THE SATURATED ZONE AT YUCCA MOUNTAIN 53
varies considerably around Yucca Mountain, but part 

of the range in values probably is due to uncertainty in 

the estimates. There is uncertainty in how to interpret 

the results of individual aquifer tests. Craig and Reed 

(1991, p. 9-13) analyzed one aquifer test in borehole 

USW H-6 four different ways and got values of 240, 

480, 210, and 370 m2/d. They chose 240 m2/d as the 

most reasonable value. The need to rely principally on 

single-borehole tests is a major cause of uncertainty in 

estimating both transmissive and storage properties. 

As discussed in the "Aquifer Tests" section, Geldon 

(1996) concluded that single-borehole tests did not 

provide reliable estimates of large-scale transmissivity 

at the C-hole complex and yielded transmissivity values that were about 100 times smaller than multipleborehole tests. Unfortunately, the C-hole complex is 

the only place at Yucca Mountain where multipleborehole tests can be conducted using existing boreholes.

Ground-water flow models have been used to 

estimate transmissivity over large areas. Waddell 

(1982), in his ground-water flow model of the Death 

Valley flow system, had two zones that included the 

Yucca Mountain area. Zone 1, which included the 

western and central part of the Yucca Mountain area, 

had an estimated transmissivity of about 700 m2/d, 

whereas Zone 8, which included the eastern part of the 

Yucca Mountain area, had an estimated transmissivity 

of about 7 mVd. The zones were quite large, and the 

model was not intended to be used to estimate transmissivity at Yucca Mountain, so these values need to 

be used with discretion. Czarnecki and Waddell 

(1984) modeled the subbasin of the Death Valley flow

system that includes Yucca Mountain. Their Zone 10, 

which included the western part of Yucca Mountain, 

had an estimated transmissivity of about 80 m2/d, 

whereas Zone 6, which included the eastern part of 

Yucca Mountain, had an estimated transmissivity of 

more than 3,000 m2/d. Transmissivity estimates from 

field tests and from simulations seem to be generally 

consistent, both in the order of magnitude of the values and the range in values. However, the field tests 

and simulations have a large uncertainty associated 

with them.

Vertical hydraulic conductivity and anisotropy 

are parameters that need to be incorporated into a 

three-dimensional flow model, but frequently are not 

simply because no data on them are available. Tests to 

estimate vertical hydraulic conductivity were conducted only at borehole USW H-5 (Robison and 

Craig, 1991, p. 26) and at the C-hole complex 

(Geldon, 1996). Data from borehole USW H-5 should 

be used with discretion because it may not be representative of the area. As was discussed in the 

"Potentiometric Surface" section, USW H-5 has a 

potentiometric level consistent with boreholes west of 

the Solitario Canyon Fault, even though the borehole 

is east of the fault. Anisotropy was estimated only for 

borehole USW H-4 (Erikson and Waddell, 1985, 

p. 24-29). These parameters are uncertain, both 

because of the few estimates available and because the 

estimates themselves are uncertain.

Discharge

Discharges at Alkali Flat and Furnace Creek 

Ranch (table 7) have been quantified on the basis of

Table 7. Estimates of ground-water discharge at Alkali Flat and Furnace Creek Ranch

Reference

Discharge, in millions of 

cubic meters per year

Alkali Flat Furnace 

Creek Ranch

Walker and Eakin, 1963 (table 7) 

Pistrang and Kunkel, 1964 (table 4) 

Hunt and others, 1966 (table 25)

'<29

5.0 

6.3

Winograd and Thordarson, 1975; Waddell (1982, p. 17-18) Summarized previous estimates; did not make new

estimates.

Czarnecki and Waddell, 1984 (table 3) 

Czarnecki, 1990 (p. 78) 

D'Agnese, 1994 (p. 298)

9.3 

8.3 

6.3

5.3

4.1

1 Also includes evapotranspiration of water from Ash Meadows and Carson Slough area. Text indicates that Alkali Rat contributes minority of this total discharge.

54 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
field observations or simulations by previous investigators (Walker and Eakin, 1963; Pistrang and Kunkel, 

1964; Hunt and others, 1966; Winograd and 

Thordarson, 1975; Waddell, 1982; Czarnecki and 

Waddell, 1984; Czarnecki, 1990; D'Agnese, 1994). 

The uncertainty in the estimates of discharge was less 

than the uncertainty for the other variables needed to 

simulate ground-water flow in the vicinity of Yucca 

Mountain. However, the uncertainty in discharge may 

still be large enough to warrant further refinement, 

particularly if, as the numerical models are updated, 

flow at Yucca Mountain appears to be sensitive to this 

parameter. An important factor in estimating discharge will be determining the area over which evapotranspiration occurs at Alkali Flat (Franklin Lake 

Playa). In addition to the discharge at Alkali Flat and 

Furnace Creek Ranch, some flow could continue 

southward beyond Alkali Flat and enter Death Valley 

from the south.

Czarnecki and Wilson (1991) developed a conceptual model that excluded Furnace Creek Ranch as a 

discharge area for the shallow flow system that 

includes Yucca Mountain. They reported that sparse 

potentiometric data indicated that a ground-water 

divide could exist in the Greenwater Range between 

the southern Amargosa Desert and Death Valley. 

Potentiometric levels beneath the Greenwater Range 

were as much as 875 m above sea level, whereas 

potentiometric levels in the Amargosa Desert were 

about 620 m above sea level and were at or below sea 

level at the floor of Death Valley. The nearby Funeral 

Mountains are higher than the Greenwater Range, 

receive more precipitation, and may have a similar 

ground-water divide (Czarnecki and Wilson, 1991, 

p. 22). Such a divide, if it extends farther north 

beneath the southern Funeral Mountains, could limit 

discharge from the shallow flow system in the Amargosa Desert to the Furnace Creek Ranch area. Such a 

divide, if it does exist, would not necessarily affect the 

deeper flow system that also may contribute discharge 

to the Furnace Creek Ranch area.

Potentiometric Levels

Potentiometric levels are the most numerous 

and the easiest of the model variables to obtain. 

Numerous boreholes where potentiometric levels can 

be measured are located throughout the flow system, 

although most are clustered in the vicinity of Yucca 

Mountain and in the Amargosa Desert. Uncertainty in 

potentiometric levels is small where data are common,

but can be large where data are sparse. Although values of potentiometric levels obtained from field measurements probably are accurate only to within a few 

meters of true values in many areas, the accuracy of 

measured potentiometric levels at Yucca Mountain is 

about 0.11 m (Boucher, 1994b, p. 17). The accuracy 

of measured potentiometric levels can be somewhat 

offset by the manner in which these data are used as a 

calibration standard for numerical models. The error 

associated with comparing measured potentiometric 

levels to model estimates can be larger than the 

measurement error because of the spatial distance 

between the field and model points. Where numerous 

boreholes are clustered in an area, an average value of 

the potentiometric level frequently is calculated and 

compared against the model node that represents the 

area. The potentiometric level measured in some 

boreholes may represent a composite level from multiple hydrogeologic units. Composite levels are frequently difficult to interpret and may be difficult to use 

as a calibration standard.

The differences between measured potentiometric levels and those calculated using a model need to 

be considered in the context of the range of potentiometric levels in the part of the flow system where the 

level is being calculated. In the area of the large 

hydraulic gradient at Yucca Mountain, potentiometric 

levels change 300 m in just a few kilometers; a small 

difference between measured and calculated potentiometric levels in this area would be insignificant. However, in the area of the small hydraulic gradient, where 

potentiometric levels change less than 1 m over several kilometers, small differences between measured 

and calculated potentiometric levels would be important.

Little is known about the distribution of potentiometric levels with depth within the flow system. 

Potentiometric levels have been measured at various 

depths in only a few boreholes, and these boreholes 

were generally concentrated around Yucca Mountain. 

Potentiometric-level data in the vertical dimension 

may be critical for calibrating three-dimensional 

ground-water flow models.

Proposed Simulations

This report has discussed various alternative 

concepts or components of conceptual models of the 

saturated-zone flow system at Yucca Mountain. This

DISCUSSION OF THE SATURATED ZONE AT YUCCA MOUNTAIN 55
section discusses some of the more important alternate 

concepts about the flow system that can be addressed, 

at least in part, with the numerical models of the flow 

system. The "Additional Data Needs" section discusses field investigations needed to adequately characterize the saturated-zone flow system. Some 

hypotheses and interpretations may quickly prove 

untenable during numerical simulation and may not 

need additional data. For those hypotheses that cannot 

be eliminated through numerical simulation, field 

investigations may be necessary to choose between 

alternative hypotheses.

Discrete Features

The degree to which faults and fractures dominate large-scale permeability in the flow system at the 

scale of Yucca Mountain is important to predictions of 

ground-water travel times and radionuclide transport. 

At individual boreholes, most water production comes 

from a few fractures. Where fault zones are present in 

boreholes, these zones frequently produce much of the 

water. This does not mean that all faults produce 

water, but rather that, when faults are present, much of 

the production seems to come from the faults. On the 

scale of Yucca Mountain, the potentiometric surface 

appears fairly smooth and water chemistry seems relatively consistent and does not vary dramatically over 

short distances. The borehole-scale and the site-scale 

observations result in conflicting information about 

the importance of faults to the ground-water flow system. Existing numerical flow models at the regional 

and subregional scales have been developed assuming 

that faults and fractures are so pervasive in the area 

that, at the scale simulated, the flow system can be 

modeled using a classical porous-medium approach. 

These regional and subregional flow models have been 

successful in duplicating the large-scale behavior of 

the flow system. Barr and others (Wilson and others, 

1994, chapter 11) constructed a site-scale flow model 

to do performance-assessment calculations that explicitly accounted for the Solitario Canyon Fault and the 

Drillhole Wash Fault. Constant-head boundary conditions were assigned around the entire boundary. Such 

boundary conditions are reasonable for doing performance-assessment calculations, but may have made 

the model too insensitive to internal conditions to adequately investigate the effect of the faults. Therefore, 

at the scale of Yucca Mountain, it is not clear whether 

individual faults and fracture zones will predominate

the flow system or if the flow system will behave 

approximately as an extensive porous medium.

Investigations as to whether the system can be 

treated as an equivalent porous medium or if discrete 

features need to be accounted for can best be carried 

out using a series of numerical simulations. The best 

simulation of the flow system as a continuous porous 

medium would be the baseline case for the investigation. The baseline case would not necessarily be 

either a homogeneous or an isotropic simulation. The 

first alternative case would treat the major faults and 

fracture systems as interconnected drains of different 

characteristics that carry most of the flow. Flow in the 

first alternative case would be analogous to flow 

through a series of interconnected parallel plates; the 

hydraulic characteristics of the faults and fracture systems would be varied according to their orientation, 

type, and other characteristics. If the first alternative 

case could reasonably reproduce known features of the 

flow system, subsequent simulations could reduce the 

number of faults and fracture systems that actively 

participate in the flow system, until the simulation no 

longer reasonably reproduces the important features of 

the flow system. A second alternative case would treat 

the flow system as a porous medium in which a few 

important faults and fracture systems are treated discretely. The base case and the alternative cases would 

be compared to determine the degree to which discrete 

features dominate the site-scale flow system.

Hydraulic Gradients

While awaiting new field data to characterize 

the large hydraulic gradient, simulations could be used 

to test whether any of the hypotheses related to the 

cause of the large hydraulic gradient can be discarded 

based on existing data. Simulations could investigate 

whether or not the upper volcanic confining unit could 

be enough of a hydraulic barrier to support the large 

hydraulic gradient. Simulations also could test if the 

drain hypothesis or the semiperched hypothesis discussed in the "Large Hydraulic Gradient" section are 

consistent with the existing data set. Given the large 

uncertainty in parameters, boundary conditions, and 

geometry of hydrogeologic units north of Yucca 

Mountain, the existing data set may not rule out any of 

these hypotheses. However, the simulations might 

provide the basis for discarding hypotheses once new 

field data are available. The simulations also may

56 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
guide further data collection for the characterization of 

the large hydraulic gradient.

Simulations also could be made to determine if 

the Solitario Canyon Fault associated with the moderate hydraulic gradient should be treated as a no-flow 

boundary for the Yucca Mountain flow system or if it 

needs to be treated as a leaky boundary. Such simulations could be used to make predictions about the 

results of the proposed Solitario Canyon Fault test 

(U.S. Department of Energy, 1990a, sec. 3.1) in the 

moderate hydraulic gradient area. The simulations, 

when combined with field investigations, could provide the basis for accepting the fault as a no-flow 

boundary, a leaky boundary, or a hydrologically 

transparent feature.

Numerical models can be used to test the relative sensitivity of the flow system to changes in 

inflows and outflows that might result from climatic or 

tectonic changes. Because most of the inflow to the 

flow system at Yucca Mountain is through the upper 

volcanic confining unit or the lower volcanic aquifer, 

both of which are poorly to moderately permeable, 

and most of the outflow is through the upper volcanic 

aquifer, which seems to be highly permeable, the flow 

system could be inflow limited. "Inflow limited" 

means that water can leave the flow system more easily than it can enter the flow system and that the outflow part of the system possibly could accommodate a 

much larger change in flow with a smaller change in 

potentiometric level than could the inflow part. 

Czarnecki (1985, p. 20-27) probably experienced an 

inflow-limited condition when he simulated the effects 

of a 100-percent increase in precipitation in the 

recharge areas for the Yucca Mountain flow system. 

The simulation indicated that upper Fortymile Wash 

would become a perennial stream and the northern 

model boundary would reject some potential inflow 

because increased recharge from precipitation could 

not enter the ground-water flow system. Tectonic or 

climate changes would have less effect on an inflowlimited saturated-zone flow system because any additional water entering the flow system could easily 

leave the system, and water levels beneath Yucca 

Mountain would change much less than they would to 

the north of the mountain.

Climate and Recharge

Regional modeling of the Yucca Mountain flow 

system has assumed that the system is in steady state,

that is, in equilibrium with the modern climate. However, some investigators (Winograd and Doty, 1980; 

Claassen, 1985) have suggested that recharge in the 

Yucca Mountain region has occurred mainly during 

pluvial cycles and the system is not in steady state. 

The assumption of equilibrium conditions can be 

investigated using numerical simulations. A simulation could be made that begins with potentiometric 

levels in the system at least as high as permitted by 

any reasonable data or analyses. The flow system then 

would be allowed to drain, without additional 

recharge, to determine if the modern flow system 

could simply be a relic of a flow system that was filled 

to capacity during the last glacial period. This simulation would be similar to one done by Luckey and others (1986, p. 11) for the southern High Plains. The 

simulation probably would assume that Fortymile 

Wash and some of the washes in Crater Flat were 

perennial streams that drained the flow system when it 

was filled to capacity. Initial potentiometric levels in 

this simulation could be as much as several hundred 

meters above the modern water table. If the flow system drains significantly below modern potentiometric 

levels in only a few millennia, this simulation would 

indicate that the modern flow system is not simply a 

relic of glacial-period recharge because the apparent 

age of the water is older than a few millennia (Benson 

and others, 1983, table 1). The simulation then would 

be modified to include some post-glacial recharge 

until the flow system was reasonably simulated. If the 

flow system could be reasonably depicted, these simulations could yield information about the relative 

importance of glacial and post-glacial recharge.

A different approach to determine if the groundwater flow system were in equilibrium with the postglacial climate would involve a series of simulations 

in which, at episodic intervals, major flooding in Fortymile Wash added large quantities of recharge to the 

flow system and, between these intervals, the flow system drained. As an extreme case in these simulations, 

the large hydraulic gradient and the Solitario Canyon 

Fault could be assumed to be no-flow boundaries. 

This assumption would cause the ground-water flow 

system at Yucca Mountain to be effectively isolated 

and would allow the flow system to drain more 

quickly. Simulations could be made for floods with 

recurrence intervals of perhaps 500 or 1,000 years (or 

longer). Squires and Young (1984) estimated various 

characteristics of peak flow for 100-year, 500-year, 

and maximum regional floods for several sections of

DISCUSSION OF THE SATURATED ZONE AT YUCCA MOUNTAIN 57
Fortymile Wash. However, estimating the volumes of 

such rare floods, as well as the recharge associated 

with them would be difficult. Therefore, although 

such simulations may provide useful insights, they 

probably would not provide definitive results because 

of the major unsubstantiated assumptions that need to 

be made.

Additional Data Needs

The "Proposed Simulations" section indicated 

that additional data are needed to resolve various questions about the saturated-zone ground-water flow system at Yucca Mountain. This section will be more 

specific about data that need to be collected to resolve 

some of the most important questions. The data needs 

that are discussed could improve the saturated-zone 

flow model and its application to performanceassessment issues, such as ground-water travel time, 

contaminant transport, and contaminant mixing or 

dilution.

Additional data are needed to resolve the cause 

of the large hydraulic gradient, the understanding of 

which could substantially alter the conceptual model 

of the saturated-zone flow system at Yucca Mountain. 

Proposed boreholes USW WT-23 and USW WT-24 

(U.S. Department of Energy, 1990a, p. 3.2-7 to 3.2-9) 

could further define the extent and shape of the large 

hydraulic gradient. More recent proposals include 

drilling these two boreholes through the upper confining unit and into the lower volcanic aquifer to test if 

the semiperched explanation for the large hydraulic 

gradient is viable (TRW Environmental Safety Systems Inc., written commun., 1993, as documented in 

"Catalog of Planned Boreholes for Surface-Based 

Testing Site Characterization Activities," Rev. 0, 

p. 167). If proposed borehole USW G-5 (TRW Environmental Safety Systems Inc., written commun., 

1993, p. 137) penetrates into the carbonate aquifer, it 

may provide information on the drain theory developed by Fridrich and others (1994).

Logging and testing of existing borehole 

USW G-2 would provide inexpensive, but potentially 

valuable information about the large hydraulic gradient. This borehole penetrated the upper volcanic confining unit and the lower volcanic aquifer and reached 

total depth in the lower volcanic confining unit. Previous information indicated that this borehole produced 

no water from the lower volcanic aquifer (fig. 15).

This lack of production needs to be replicated by 

pumping discrete intervals of the borehole. Hydrochemical samples need to be obtained when pumping 

is done to determine the source of water to the various 

intervals of the borehole.

If future drilling indicates that flow in the saturated zone through or beneath the large hydraulic 

gradient is large, additional drilling to provide potentiometric level data north of Yucca Mountain may be 

needed. The drilling would not only better define the 

potentiometric surface to the north, but it also would 

help define the nature of the northern boundary of the 

Yucca Mountain flow system. The drilling might provide insight as to how the northern boundary condition 

might change in response to future climatic or tectonic 

conditions.

The cause of the moderate hydraulic gradient 

to the west of Yucca Mountain needs to be resolved. 

Results of the Solitario Canyon Fault study 

(U.S. Department of Energy, 1990a, sec. 3.1) possibly 

would resolve the issue. Three additional boreholes 

are proposed for that study and a long-term hydraulic 

test is proposed to determine the hydrologic importance of the fault. The test also may contribute to 

understanding the hydrologic nature of other nearby 

faults.

Tests at the C-hole complex (U.S. Department 

of Energy, 1990a, sec. 3.4 and 3.5) would determine 

the hydraulic and transport properties at this site. The 

C-hole complex is the only site at which the borehole 

configuration allows the critical transport parameter, 

effective porosity, to be measured. Data from this site 

also may help resolve the issue of anisotropy. Additional data about hydraulic and transport properties 

certainly will be needed elsewhere; how to collect 

such data most efficiently could be one of the most 

important results of tests at the C-hole complex. The 

work at the C-hole complex needs to be followed by 

studies at additional multihole sites.

A detailed hydrochemical characterization of 

the saturated zone as described by the U.S. Department of Energy (1992a) could refine the conceptual 

model of flow at Yucca Mountain and could provide 

data that will help in the calibration of the numerical 

model.

The frequency and amount of recharge that 

occurs in Fortymile Wash warrants further study. 

Savard (1994) documented that runoff in upper 

Fortymile Wash caused an almost immediate rise in 

water levels in nearby boreholes. Results of this study

58 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
could help constrain the simulations of recharge in 

Fortymile Wash that were discussed in the "Climate 

and Recharge" section.

More definitive information is needed about the 

hydrologic effects of wetter and cooler climates in the 

recent geologic past. The deposits discussed in the 

"Northern Discharge Deposits" section may provide 

important data for calibrating a numerical flow model. 

The modern depth to water beneath the northern discharge deposits needs to be determined. The possibility that some local geologic structure caused the water 

table to rise to the land surface during wetter periods 

of the recent geologic past also could be investigated.

The relation of precipitation to recharge needs 

to be better understood so that the effects of climate 

changes can be realistically simulated. Total recharge 

is frequently estimated from total discharge, and it is 

generally distributed based on either precipitation and 

altitude or plant communities. Such a relation may not 

be appropriate under different climatic scenarios.

Unless the carbonate aquifer is demonstrated 

not to have a major effect on the volcanic flow system 

at Yucca Mountain, much more information is needed 

about the extent of this aquifer in the vicinity of the 

mountain. If the carbonate aquifer potentially has a 

major effect on flow in the volcanic aquifers, the 

hydrologic properties of the carbonate aquifer need to 

be determined. This need for data could be particularly expensive because the carbonate aquifer probably is quite deep under much of the Yucca Mountain 

area.

The development and calibration of a numerical 

model may indicate further data needs that were not 

anticipated in this report. Conversely, sensitivity analysis using a numerical model could indicate that some 

of the data needs described in this report do not greatly 

affect the numerical flow model; in which case, the 

data would not need to be collected.

SUMMARY

The area around Yucca Mountain in southern 

Nevada is being studied to determine if the site is suitable for permanent disposal of high-level radioactive 

waste in a geologic repository. The study of the 

hydrology of any potential geologic repository site is 

important because ground water is the primary 

medium for most radionuclides to move away from a 

geologic repository. This report summarizes the

understanding of the saturated-zone flow system at 

Yucca Mountain as of 1995.

Yucca Mountain consists of a thick sequence of 

Tertiary volcanic rocks that are underlain, at least to 

the southeast, by Paleozoic carbonate rocks. Stratigraphic units important to the saturated-zone hydrology of the area include the Topopah Spring Tuff; the 

Calico Hills Formation; the Crater Flat Group; the 

Lithic Ridge Tuff; older tuffs, flows, and lavas beneath 

the Lithic Ridge Tuff; and Paleozoic rocks, particularly the carbonate aquifer. The alluvium, the Rainier 

Mesa Tuff, and the upper units of the Paintbrush 

Group are above the water table in the Yucca Mountain area, and only the lowermost member of the 

Paintbrush Group, the Topopah Spring Tuff, is beneath 

the water table, and then only in selected areas. 

Saturated-zone flow usually occurs in the Calico Hills 

Formation and in lower units because of the generally 

great depths to the water table.

The saturated-zone ground-water flow system at 

Yucca Mountain is part of the Alkali Flat-Furnace 

Creek subbasin of the Death Valley ground-water flow 

system. This subbasin is recharged in the higher 

mesas north of Yucca Mountain and discharges most 

of its water at Alkali Flat (Franklin Lake Playa). In 

this subbasin, water moves generally from north to 

south.

The geologic section at Yucca Mountain is 

divided for hydrologic purposes into three aquifers 

and two confining units. The upper volcanic aquifer 

consists of the densely welded section of the Topopah 

Spring Tuff. It is generally saturated only to the west, 

south, and east of Yucca Mountain. The upper volcanic confining unit consists of the lower part of the 

Topopah Spring Tuff, the Calico Hills Formation, and 

the uppermost part of the Crater Flat Group. The 

lower volcanic aquifer consists of most of the Crater 

Flat Group. The lower volcanic confining unit consists of the Lithic Ridge Tuff and older tuffs, flow 

breccia, and lavas beneath the Crater Flat Group. 

Paleozoic argillites may exist beneath the volcanic 

rocks beneath Yucca Mountain and may effectively 

thicken the lower volcanic confining unit. Beneath the 

volcanic aquifers and confining units is the carbonate 

aquifer, which consists mainly of Paleozoic limestones 

and dolomites.

The potentiometric surface in the Yucca Mountain area can be divided into the three general areas: 

(1) The area at the northern end of Yucca Mountain, 

where potentiometric levels exceed 1,000 m above sea

SUMMARY 59
level; (2) the area generally west of Solitario Canyon, 

where potentiometric levels are about 775 m above sea 

level; and (3) the area at Yucca Mountain and to the 

south and east, where potentiometric levels are about 

730 m above sea level. The boundary between the 

first and third areas contains a large lateral hydraulic 

gradient over which potentiometric levels changed 

more than 260 m in less than 3 km. The boundary 

between the second and third areas contains a moderate hydraulic gradient over which potentiometric levels changed about 45 m in 1 to 2 km. The third area is 

characterized by a small hydraulic gradient; potentiometric levels changed only about 2 m over several 

kilometers.

Potentiometric levels in boreholes around Yucca 

Mountain generally change little with depth or potentiometric levels indicate a general upward gradient. 

Large vertical hydraulic gradients were measured only 

in a few boreholes. The largest differences in potentiometric levels, within the depths of exploration, were 

about 55 m at borehole USW H-l, 22 m at borehole 

USW H-3, and 21 m at borehole UE-25 p#l.

Limited hydraulic testing of boreholes in the 

Yucca Mountain area was done in the early 1980's. 

These tests were generally single-borehole tests and 

frequently tested large intervals of a borehole. 

Reported transmissivity values ranged over two or 

three orders of magnitudes within a particular hydrogeologic unit and the average values between hydrogeologic units generally differed by about an order of 

magnitude (table 5). The upper volcanic aquifer probably is the most permeable hydrogeologic unit, but 

this conclusion is based on exceedingly limited data.

Major sources of inflow to the hydrologic system at Yucca Mountain include upgradient inflows and 

recharge from Fortymile Wash, a major ephemeral 

surface drainage on the east side of Yucca Mountain. 

However, there is considerable uncertainty as to the 

magnitude of inflows from the northern and the western upgradient areas. Major sources of outflow from 

the hydrologic system are downgradient outflow and 

pumpage. Although some estimates have been made, 

the inflows and outflows are not well quantified.

Construction of numerical models would enable 

testing of various concepts about the saturated-zone 

flow system at Yucca Mountain; for example, the relative importance of major faults and fracture systems 

on the flow system, whether the flow system is in equilibrium with modern climate, and the various theories 

about the cause of the large hydraulic gradient.

Numerical models may have large uncertainties 

because of the complexity of the flow system at Yucca 

Mountain and insufficient data. The most important 

uncertainties among the numerical model components 

will be recharge, storage properties, hydraulic properties, discharge, and hydraulic head. To adequately 

understand the flow system at Yucca Mountain, additional data needs to be collected to resolve critical 

issues. The cause of the large hydraulic gradient is 

identified in this report to be a major issue that needs 

to be resolved. The cause of the moderate hydraulic 

gradient also needs to be resolved. The tests described 

to resolve the moderate-gradient issue could yield 

information on the importance of faults in the flow 

system. The proposed tests at the C-hole complex 

may determine hydraulic, storage, and transport properties at this site. Additional data about these properties are needed at other sites at Yucca Mountain. The 

proposed hydrochemical characterization could provide additional information that could help calibrate 

the numerical model. Once the numerical model is 

developed and calibrated, a sensitivity analysis could 

indicate other data needs and might indicate that some 

of the data are not critical.

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well USW H-l, adjacent to Nevada Test Site, Nye 

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60 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
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Barr, G.E., and Shannon, S.A., 1994, Constraining local 

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1370-F,49p.

Bedinger, M.S., Sargent, K.A., Langer, W.H.,

Sherman, F.B., Reed, I.E., and Brady, B.T., 1989, Studies of geology and hydrology in the Basin and Range 

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the Basin and Range province relative to the disposal 

of high-level radioactive waste Part I, introduction 

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48 p.

Bentley, C.B., Robison, J.H., and Spengler, R.W., 1983, 

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Mountain area, Nye County, Nevada: U.S. Geological 

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Bish, D.L., Caporuscio, F.A., Copp, J.F., Crowe, B.M., 

Purson, J.D., Smyth, J.R., and Warren, R.G., 1981,

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 1994b, Precision and accuracy of manual water-level 

measurements taken in the Yucca Mountain area, Nye 

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Brocher, T.M., Carr, M.D., Fox, K.F., Jr., and Hart, P.E., 

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Brocher, T.M., Hart, P.E., Hunter, C.E., and

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53 p.

 1982, Volcano-tectonic history of Crater Flat, southwestern Nevada, as suggested by new evidence from 

drill hole USW VH-1 and vicinity: U.S. Geological 

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 1984, Regional structural setting of Yucca Mountain, 

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 1988, Volcano-tectonic setting of Yucca Mountain and 

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 1990, Styles of extension in the Nevada Test Site

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Craig, R.W., and Johnson, K.A., 1984, Geohydrologic data 

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Craig, R.W., and Reed, R.L., 1991, Geohydrology of rocks 

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62 Status of Understanding of the Saturated-Zone Ground-Water Flow System at Yucca Mountain, Nevada, as of 1995
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-1991, Preliminary simulations showing potential

effects of a wetter future climate coupled with a localized increase in hydraulic conductivity on the groundwater flow system of Yucca Mountain and vicinity, 

Nevada-California [abs.]: American Geophysical 

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Czarnecki, J.B., and Waddell, R.K., 1984, Finite-element 

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Czarnecki, J.B., Steinkampf, W.C., and Kroitora, L., 1990, 

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characterization of hydrologic properties of fractured 

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 1994, Revised potentiometric-surface map for Yucca 

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 1986c, NNWSI hole history UE-25c #1, UE-25c #2, 

UE-25c #3: U.S. Department of Energy Report DOE/ 

NV/10322-14,61 p.

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