Prepared in cooperation with the

U.S. DEPARTMENT OF ENERGY

NATIONAL NUCLEAR SECURITY ADMINISTRATION

NEVADA OPERATIONS OFFICE, under

Interagency Agreement DE-AI08-01NV13944

Probability Distributions of Hydraulic Conductivity for 

the Hydrogeologic Units of the Death Valley Regional 

Ground-Water Flow System, Nevada and California

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 02–4212
(Back of Cover)
Probability Distributions of Hydraulic Conductivity for 

the Hydrogeologic Units of the Death Valley Regional 

Ground-Water Flow System, Nevada and California

By Wayne R. Belcher, Donald S. Sweetkind, and Peggy E. Elliott

U.S. GEOLOGICAL SURVEY

Carson City, Nevada

2002

Prepared in cooperation with the

U.S. DEPARTMENT OF ENERGY

NATIONAL NUCLEAR SECURITY ADMINISTRATION

NEVADA OPERATIONS OFFICE, under

Interagency Agreement, DE-AI08-01NV13944

Water-Resources Investigations Report 02-4212
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

Any use of trade, product, or firm names in this publication is for descriptive

purposes only and does not imply endorsement by the U.S. Government

For additional information

contact:

District Chief 

U.S. Geological Survey

333 West Nye Lane, Room 203

Carson City, NV 89706–0866

email: GS-W-NVpublic-info@usgs.gov

URL: http://nevada.usgs.gov
CONTENTS III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 1

Location...................................................................................................................................................................... 2

Purpose and Scope ..................................................................................................................................................... 2

Limitations ................................................................................................................................................................. 2

Acknowledgments...................................................................................................................................................... 2

Hydrogeologic Setting........................................................................................................................................................... 4

Regional Overview..................................................................................................................................................... 4

Hydrogeologic Units .................................................................................................................................................. 4

Data Analysis and Synthesis.................................................................................................................................................. 4

Estimating Probability Distributions.......................................................................................................................... 5

Statistical Hypothesis Testing .................................................................................................................................... 5

Probability Distributions........................................................................................................................................................ 5

Basin-Fill Hydrogeologic Units ................................................................................................................................. 5

Bedrock Confining Hydrogeologic Units .................................................................................................................. 9

Carbonate Rock Hydrogeologic Units ....................................................................................................................... 9

Volcanic Rock Hydrogeologic Units.......................................................................................................................... 9

Applicability of Distributions to a Regional Model................................................................................................... 14

Principle of Parsimony ............................................................................................................................................... 14

Summary................................................................................................................................................................................ 17

References Cited.................................................................................................................................................................... 17

FIGURES

1. Location map of study area.................................................................................................................................... 3

2–8. Graphs showing hydraulic-conductivity distributions for:

2. Basin-fill hydrogeologic units............................................................................................................................ 8

3. Regional bedrock confining hydrogeologic units .............................................................................................. 10

4. Carbonate rock hydrogeologic units .................................................................................................................. 11

5. Tertiary volcanic rock hydrogeologic units........................................................................................................ 12

6. Tertiary volcanic rock lithologies....................................................................................................................... 13

7. Welding in ash-flow tuffs................................................................................................................................... 15

8. Alteration in ash-flow tuffs ................................................................................................................................ 16

TABLES

1. Horizontal hydraulic-conductivity estimates of hydrogeologic units 

in the Death Valley regional ground-water flow system................................................................................. 6

2. Horizontal hydraulic-conductivity estimates of volcanic rock hydrogeologic

units in the Death Valley regional ground-water flow system ........................................................................ 7

3–9. Results of hypothesis testing for:

3. Basin-fill hydrogeologic units............................................................................................................................ 8

4. Bedrock confining units ..................................................................................................................................... 10

5. Carbonate rocks.................................................................................................................................................. 11

6. Tertiary volcanic rock hydrogeologic units........................................................................................................ 12

7. Tertiary volcanic rock lithologies....................................................................................................................... 13

8. Welding in ash-flow tuffs................................................................................................................................... 15

9. Alteration in tuffs ............................................................................................................................................... 16
IV Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

CONVERSION FACTORS, VERTICAL DATUM, AND ACRONYMS

Multiply By To obtain

meter (m) 3.2808 foot

meter per day (m/d) 3.2808 foot per day

square kilometer (km2) 0.3861 square mile

Temperature: Degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) by using the formula oF = [1.8(oC)]+32. 

Degrees Fahrenheit can be converted to degrees Celsius by using the formula oC = 0.556(oF-32).

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929, formerly called “Sea-Level

Datum of 1929”), which is derived from a general adjustment of the first-order leveling networks of the United States and Canada.

ACRONYMS USED IN THIS REPORT

AA Alluvial aquifer NTS Nevada Test Site

ACU Alluvial confining unit OVU Older volcanics unit

BRU Belted Range unit PVA Paintbrush volcanic aquifer

CFBCU Crater Flat-Bullfrog confining unit SCU Sedimentary confining unit

CFPPA Crater Flat-Prow Pass aquifer TMVA Thirsty Canyon/Timber Mountain volcanic aquifer

CFTA Crater Flat-Tram aquifer TV Tertiary volcanics

CHVU Calico Hills volcanic unit UCA Upper carbonate aquifer

DVRFS Death Valley regional ground-water flow system UCCU Upper clastic confining unit

HGU Hydrogeologic unit VSU Volcaniclastics and sediments unit

ICU Intrusive confining unit WVU Wahmonie volcanic unit

LCA Lower carbonate aquifer YVU Younger volcanic unit

LCCU Lower clastic confining unit XCU Crystalline confining unit

LFU Lava flow unit
ABSTRACT 1

ABSTRACT

The use of geologic information such as 

lithology and rock properties is important to constrain conceptual and numerical hydrogeologic 

models. This geologic information is difficult to 

apply explicitly to numerical modeling and analyses because it tends to be qualitative rather than 

quantitative. This study uses a compilation of 

hydraulic-conductivity measurements to derive 

estimates of the probability distributions for several hydrogeologic units within the Death Valley 

regional ground-water flow system, a geologically 

and hydrologically complex region underlain by 

basin-fill sediments, volcanic, intrusive, sedimentary, and metamorphic rocks. Probability distributions of hydraulic conductivity for general rock 

types have been studied previously; however, this 

study provides more detailed definition of hydrogeologic units based on lithostratigraphy, lithology, alteration, and fracturing and compares the 

probability distributions to the aquifer test data. 

Results suggest that these probability distributions 

can be used for studies involving, for example, 

numerical flow modeling, recharge, evapotranspiration, and rainfall runoff. These probability distributions can be used for such studies involving the 

hydrogeologic units in the region, as well as for 

similar rock types elsewhere.

Within the study area, fracturing appears to 

have the greatest influence on the hydraulic conductivity of carbonate bedrock hydrogeologic 

units. Similar to earlier studies, we find that alteration and welding in the Tertiary volcanic rocks 

greatly influence hydraulic conductivity. As 

alteration increases, hydraulic conductivity tends 

to decrease. Increasing degrees of welding appears 

to increase hydraulic conductivity because welding increases the brittleness of the volcanic rocks, 

thus increasing the amount of fracturing.

INTRODUCTION

The U.S. Geological Survey (USGS) is developing a three-dimensional ground-water flow model of 

the Death Valley regional ground-water flow system 

(DVRFS). This area lies within the southern Great 

Basin section of the Basin and Range physiographic 

province and surrounds both the Nevada Test Site 

(NTS), where nuclear weapons tests have contaminated 

the ground water beneath some areas, and Yucca 

Mountain, which is being investigated for its suitability 

for permanent storage of high-level nuclear waste in a 

mined geologic repository. As a result of this area’s 

importance and intensive studies, an extensive geologic 

and hydrologic data set exists for a large, regional system. 

Bedinger and others (1989) produced a series of 

probability distributions for rock types common to the 

Basin and Range physiographic province. Data used to 

prepare these distributions consisted of published field 

and laboratory tests within the Basin and Range province, as well as general studies from rocks with similar 

characteristics from outside the Basin and Range province (Bedinger and others, 1989, p. 18). This work differs in that we use only compiled field data for the rock 

types found in the region. This study uses the hydrogeologic unit assignments based on Laczniak and others (1996) developed at the NTS, and thus is more 

detailed than the regional definitions provided by 

Bedinger and others (1989). Hydrogeologic units are 

rock units grouped according to their water-storage and 

PROBABILITY DISTRIBUTIONS OF HYDRAULIC 

CONDUCTIVITY FOR THE HYDROGEOLOGIC UNITS 

OF THE DEATH VALLEY REGIONAL GROUND-WATER 

FLOW SYSTEM, NEVADA AND CALIFORNIA

By Wayne R. Belcher, Donald S. Sweetkind, and Peggy E. Elliott
2 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

transmissive properties (Laczniak and others, 1996). 

Relations between secondary processes such as fracturing and alteration that affect measured hydraulic conductivity also are examined.

Location

The DVRFS is in southeastern California and 

Nevada (fig. 1). The DVRFS encompasses about 

45,000 km2

 within the Great Basin section of the Basin 

and Range physiographic province. The area for this 

study is considerably larger than the DVRFS in order to 

include areas that contain sites important to defining 

hydraulic-property estimates for units contained within 

the DVRFS. The topography typically consists of 

northerly and northwesterly trending mountain ranges 

surrounded by broad sediment-filled basins. The 

Spring Mountains, the highest topographic feature in 

the area, are about 3,600 m above mean sea level. Other 

prominent topographic features within the region 

include the Sheep Range, Pahute Mesa, the Funeral 

Mountains, and the Panamint Range. Basins generally 

decrease in altitude from north to south. The lowest 

altitude at 86 m below sea level in the study area is at 

Badwater in Death Valley National Park.

Purpose and Scope

The purpose of this report is to present statisticalprobability distributions that can be used by hydrologists to constrain hydraulic-conductivity estimates in 

their studies. These distributions could be useful for 

hydrologic studies involving numerical simulations of 

ground water, recharge, rainfall runoff, evapotranspiration, basin analyses, and water budgets. Other uses of 

the distributions could include contaminant-transport 

modeling, water-supply issues, and resource protection. The probability distributions also could be used to 

apply the principle of parsimony (Hill, 1998, p. 35) to 

simulation efforts. Specifically, the work presented in 

this report is for use in a transient numerical groundwater flow model of the DVRFS, but because of the 

diversity of rock types within the study area, these distributions may be useful to flow modelers in other 

regions and other types of studies for which hydraulicconductivity information is required. 

Limitations

The analyses in this report have several limitations:

1. The hydraulic-conductivity measurements presented in this report are based mostly on the results 

of field-scale tests and represent a very small part of 

an overall regional HGU. Lithologic factors that can 

affect hydraulic conductivity, such as facies changes 

in sedimentary rock, welding and alteration in volcanic rocks, and degree of fracturing can cause hydraulic properties to vary greatly, even over relatively 

short distances.

2. Significant spatial bias may exist in the hydraulicconductivity measurements. Wells tested for aquifer 

properties were installed to meet the objectives of 

their parent studies or to provide an adequate water 

supply, not necessarily to provide adequate spatial 

coverage for a regional study.

3. Transmissivity measurements from aquifer tests 

were divided by a thickness value to obtain hydraulic conductivity. The length of the open interval of 

the well or borehole was used to calculate hydraulic 

conductivity from this transmissivity. This is a simplistic assumption. If the thickness of the rock or 

sediment contributing flow is less than the open 

interval, the hydraulic conductivity will be underestimated, and if the thickness is greater than the open 

interval, the hydraulic conductivity will be overestimated. 

4. Hydraulic-conductivity estimates in heterogeneous 

aquifers can be biased above the average hydraulic 

conductivity because many wells are screened preferentially across more productive intervals. 

Acknowledgments

We acknowledge the support of Mr. Robert 

Bangerter (U.S. Department of Energy, National 

Nuclear Security Administration Nevada Operations 

Office). This work was performed in cooperation with 

the U.S. Department of Energy, National Nuclear Security Administration Nevada Operations Office, under 

Interagency Agreement DE-AI08-01NV13944. 
INTRODUCTION 3

0 30 60 MILES 15

0 30 60 KILOMETERS 15

127

373

Base from U. S. Geological Survey digital data 1:100,000–scale, 1978–89

Universal Transverse Mercator Projection Zone 11

Shaded relief base from 1:250,000–scale Digital Elevation Model

Sun illumination from northwest at 45 degrees above horizon 

118 ̊ 116 ̊ 114 ̊

36 ̊

38 ̊

CALIFORNIA

NEVADA

UTAH ARIZONA

EXPLANATION

Well

Boundary of Death Valley

 Ground-Water System

Las

Vegas

Nevada

Test

Site

Yucca

Mountain

Death

Valley

Range

Pahute

Mesa

Badwater

Panamint

Range

Funeral Mountains

Sheep 

Spring

Mountains

NEVADA

NEVADA

CALIFORNIA

Las

Vegas

Tonopah

Nevada

Test

Site

Death

Valley

National

Park Yucca

Mountain

Death

Valley

Range

Pahute

Mesa

Badwater

Panamint

Range

Funeral Mountains

Sheep 

Spring

Mountains

NEVADA

NEVADA

CALIFORNIA

Figure 1. Location map of study area.
4 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

HYDROGEOLOGIC SETTING

Regional Overview

The study area includes a stratigraphically diverse 

and structurally complex region in which a thick Tertiary volcanic and sedimentary section unconformably 

overlies previously deformed Proterozoic through 

Paleozoic rocks. The stratigraphic framework of the 

DVRFS consists of a Late Proterozoic through Middle 

Cambrian wedge of continental siliciclastic rocks, 

overlain by a thick Middle Cambrian through Middle 

Devonian carbonate-dominated succession. These carbonate rocks form the major regional carbonate-rock 

aquifer where ground water flows from central Nevada, 

through the NTS toward discharge sites in Ash Meadows and Death Valley to the south (Winograd and Thordarson, 1975; Bedinger and others, 1989; Dettinger, 

1989). The carbonate sequence is interrupted by Upper 

Devonian through Mississippian synorogenic clastic 

and carbonate rocks that form a locally important confining unit near the NTS (Winograd and Thordarson, 

1975; Laczniak and others, 1996; D’Agnese and others, 1997). Mesozoic siliciclastic and intrusive rocks 

are only locally present in the region. Overlying the 

older rocks are locally thick Oligocene to Pliocene fluvial, paludal, and playa sedimentary rocks, a thick 

sequence of regionally distributed welded and nonwelded tuffs (which form important bedrock aquifers at 

the NTS), more locally distributed lava flows with 

associated intrusive rocks, and overlying Pleistocene to 

recent alluvium, eolian deposits, and spring discharge 

deposits (Grose and Smith, 1989, p. 10). All of these 

rocks have been deformed by complex Neogene extensional normal and strike-slip faults that are superimposed on late Paleozoic to mid-Mesozoic folds and 

thrusts (Stewart, 1978; Mifflin, 1988). The stratigraphic and structural complexity of the region results 

in a close spatial relation of diverse rock types and 

deformational styles; individual HGUs have widely 

varying physical properties and hydraulic conductivities as a result of variable primary and secondary porosity and permeability. 

In the southern Great Basin hydraulic connection 

between basins is maintained through unconsolidated 

sediments that were deposited across low topographic 

divides between the basins and by deep interbasin flow 

beneath valley floors and adjacent ranges through fractured Paleozoic carbonate rocks (Winograd and Thordarson, 1975; Prudic and others, 1995). Faults and 

related fractures typically enhance ground-water flow 

through bedrock aquifers (Faunt, 1997), however, 

faults also disrupt stratigraphic continuity, which can 

divert water in regional circulation to local and subregional outlets.

Hydrogeologic Units

The rocks and unconsolidated deposits that form 

the framework for a ground-water flow system are 

termed hydrogeologic units (HGUs). HGUs are 

assigned to a unit that has considerable lateral extent 

and reasonably distinct hydrologic properties because 

of its geological and structural characteristics. The distinction between aquifers and confining units in basinfill sediments is closely related to primary lithologic 

variations; whereas, the hydraulic properties of competent rocks often are related to observations and 

assumptions of the degree to which stratigraphic units 

are fractured. These physical characteristics were used 

to group geologic formations of hydrologic significance in the vicinity of the NTS into HGUs (Winograd 

and Thordarson, 1975). Winograd and Thordarson 

originally defined seven HGUs in the DVRFS region. 

These assignments formed the basis of HGUs used by 

subsequent regional modeling studies (D’Agnese and 

others, 1997; U.S. Department of Energy, 1997). A 

refinement of the HGU assignments by Laczniak and 

others (1996) form the basis for the 10 main HGUs 

(plus several subcategories of these HGUs) used in this 

report. The geologic units comprising the hydrogeologic units discussed in this report are fully discussed in 

Belcher and others (2001, table 1). 

DATA ANALYSIS AND SYNTHESIS

The 930 hydraulic-conductivity measurements 

used in this study were compiled by Belcher and others 

(2001) from published and some previously unpublished hydraulic-property measurements for hydrogeologic units within the study area. Only field-aquifer 

tests were considered in this compilation, excepting the 

quartzites of the lower clastic confining unit. The limited number of hydraulic-conductivity estimates from 

aquifer tests for this particular unit were augmented 

with estimates from permeameter results.
PROBABILITY DISTRIBUTIONS 5

Estimating Probability Distributions

The logarithmically transformed values of 

hydraulic conductivity were used for statistical calculations because this parameter tends to be normally distributed (Neuman, 1982). The Cunnane plotting 

position method was used to assess the normality of the 

logarithms of hydraulic-conductivity measurements 

used for each HGU or HGU subcategory (Helsel and 

Hirsch, 1992, p. 27–29). The assumption of a normal 

distribution for log-transformed hydraulic conductivity 

was true. The probability plots for the combined upper 

and lower clastic confining units and the quartzites of 

the lower clastic confining unit were influenced by high 

outliers in the lower clastic confining unit. These data 

are from rare pumping tests, probably in highly fractured areas with enhanced permeability, and likely 

atypical. Field tests sample a greater volume of rock 

types than laboratory tests, thus, these results are more 

appropriate for application to a regional-scale numerical model. The geometric and arithmetic means, range, 

and the 95-percent confidence intervals are listed in 

tables 1 and 2.

To compare the probability distributions of the 

log hydraulic conductivity for each HGU the data were 

normalized (Davis, 1986, p. 46–50). To compute the 

normalized values, the following equation (Davis, 

1986, p. 48) was used:

,

where

Z is the normalized value,

X is the observation (in this case log hydraulic

conductivity),

X is the mean of the log-transformed observations 

(or the geometric mean), and

s is the standard deviation of the log-transformed 

observations.

The normalized values represent how many standard deviations particular values of log hydraulic conductivity are away from the geometric mean. The log 

hydraulic conductivities were plotted as a lognormal 

distribution by plotting the resulting distributions as a 

straight-line plot on log-probability coordinates. The 

distribution for the HGUs (and subcategories) was 

drawn using the spread plus or minus three standard 

deviations for log hydraulic-conductivity values 

reported for each HGU. These values correspond to the 

spread from 0.1 to 99.9 percentiles of the range of values reported in Belcher and others (2001). The geometric mean of the log hydraulic conductivity is at the 

50-percentile value and the majority of the values were 

included in the range on either side of the mean (plus or 

minus two standard deviations).

Statistical Hypothesis Testing

Statistical hypothesis testing on the differences of 

the geometric means between the groupings of HGUs 

was performed. The large or small sample size with differing variances test was used (as appropriate), with the 

level of significance being 0.05 (Mendenhall and Sinich, 1988, p. 376–378). The null hypothesis, that there 

is no difference between the geometric means, and the 

alternate corollary hypothesis, that a statistically significant difference exists, were tested. The results of the 

hypothesis testing are listed in tables 3–9.

PROBABILITY DISTRIBUTIONS

Basin-Fill Hydrogeologic Units

The probability distributions of the hydraulic conductivity of the basin-fill hydrogeologic units are 

shown in figure 2. Units considered include: (1) a 

coarse, unconsolidated alluvial aquifer (AA), includes 

the older alluvial and younger alluvial aquifers 

(Belcher and others, 2001); (2) an alluvial confining 

unit (ACU) consisting predominantly of silt and clay 

playa deposits; and (3) undifferentiated basin filling of 

younger volcanic rocks (YVU) and Tertiary volcaniclastic and sedimentary rocks (VSU) are presented as 

combined data. The combined YVU/VSU unit shows a 

distinctly lower distribution of hydraulic conductivity 

than either the AA or the ACU (fig. 2). This result may 

be from the unconsolidated nature of the AA which 

gives it a greater hydraulic conductivity, coupled with a 

decrease in hydraulic conductivity of the older sedimentary and volcaniclastic rocks due to zeolitic alteration of the volcanic component. The hydraulicconductivity values for the ACU are problematic, 

because these values generally are greater than those of 

the AA. This is probably an indication that the wells in 

which aquifer tests were performed may be from more 

permeable parts of the ACU. The ACU as defined by 

Belcher and others (2001) includes lacustrine carbonates that are locally productive aquifers (Dudley and 

Larson, 1976). The hypothesis testing confirms that the 

geometric mean of the ACU is similar to the AA and 

that the YVU/VSU unit is distinct from both the ACU 

and the AA (table 3).

Z X X–

s = ------------
6 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California Table 1. Horizontal hydraulic-conductivity estimates of hydrogeologic units in the Death Valley regional ground-water flow sy

[Acronyms: AA, alluvial aquifer; ACU, alluvial confining unit; BRU, Belted Range unit; CFBCU, Crater Flat-Bullfrog confining unit; CFPPA, Crater FlaCFTA, Crater Flat-Tram aquifer; CHVU, Calico Hills volcanic unit; ICU, intrusive confining unit; LCA, lower carbonate aquifer; LCCU, lower clastic coLFU, lava flow unit;NA, not applicable; OVU, older volcanic rocks unit; PVA, Paintbrush volcanic aquifer; SCU, sedimentary rocks confining unit; TMVACanyon/Timber Mountain volcanic aquifer; TV, Tertiary volcanic rocks; UCA, upper carbonate aquifer; UCCU, upper clastic confining unit; VSU, volcansedimentary rocks unit; YVU, younger volcanic rocks unit. Note: Geometric mean and standard deviation are back-transformed from logarithmic values]

Hydrogeologic 

unit or subunit

Hydraulic conductivity, in meters per day 95-percent 

confidence 

interval

Geometric m

mean

Arithmetric 

mean

Minimum K Maximum K

AA 1.5 10.8 0.00006 130 0.005 - 430 ACU 3 10.5 0.003 34 0.02 - 470 LFU NA NA 0.002 4 NA YVU/VSU 0.06 1.5 0.00004 6 0.00005 - 80 TV 0.12 3.9 0.000002 180 0.0002 - 78 TMVA 0.01 2 0.0002 20 0.00001 - 18 PVA 0.02 4 0.000007 17 0.0000003 - 1,300 CHVU 0.2 0.55 0.008 2 0.007 - 5 BRU 0.3 1.03 0.01 4 0.006 - 17 CFTA 0.05 0.4 0.003 2 0.0004 - 5.3 CFBCU 0.4 6.8 0.0003 55 0.0006 - 240 CFPPA 0.3 13 0.001 180 0.000006 - 2.4 OVU 0.004 0.07 0.000001 1 0.00002 - 5 ICU 0.01 0.3 0.0006 1.4 0.00002 - 5 SCU 0.002 0.02 0.0002 0.3 0.00004 - 0.09 UCA and LCA 2.5 90 0.0001 820 0.0008 - 7,700 

fractured 19 150 0.01 820 0.03 - 11,000 unfractured 0.1 1.6 0.0001 14 0.0002 - 70 

UCCU and LCCU 0.00002 0.2 0.00000003 5 0.0000000001 - 3 

shale 0.01 0.07 0.0002 0.4 0.0001 - 1.4 quartzite 0.000001 0.24 0.00000003 5 0.0000000001 - 0.006 
PROBABILITY DISTRIBUTIONS 7

Table 2. Horizontal hydraulic-conductivity estimates of volcanic rock hydrogeologic units in the Death Valley regional ground-water flow s[Geometric mean and standard deviation are back-transformed from logarithmic values]

Hydrogeologic unit or subunit

Hydraulic conductivity, in meters per day 95-percent 

confidence 

interval

Num

Geometric measu

mean

Arithmetric 

mean

Minimum K Maximum K

Lava flows 0.13 0.63 0.000007 4 0.0005 - 31 Ash-flow tuff 0.12 5.3 0.000002 180 0.0002 - 97 1

Non-welded to partially welded 0.06 6.6 0.0003 180 0.0002 - 24 Partially to moderately welded 0.04 1.1 0.000002 19 0.00003 - 50 Moderately to densely welded 1.6 13.3 0.02 55 0.005 - 540 

Tuff breccia 0.31 4.2 0.0008 15 0.0002 - 550 Bedded tuff 0.14 2 0.00009 15 0.0003 - 57 Unaltered tuffs 0.4 8.1 0.00002 180 0.0006 - 260 Altered tuffs 0.04 1.3 0.000002 25 0.0001 - 15 
8 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

STANDARD DEVIATONS FROM GEOMETRIC MEAN

Alluvial aquifer (AA)

Alluvial confining unit (ACU)

Younger volcanic unit and volcaniclastic

 sedimentary rocks unit (YVU/VSU)

0.005

0.01

0.05

0.1

0.25

0.5

0.025

0.75

0.9

0.95

0.975

0.995

0.99

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Table 3. Results of hypothesis testing for 

basin-fill hydrogeologic units

[Acronyms: AA, alluvial aquifer; ACU, alluvial confining 

unit; VSU, volcaniclastic and sedimentary rocks unit; 

YVU, younger volcanic rocks unit. Abbreviations: NR, 

non-rejection of the null hypothesis; R, rejection of the 

null hypothesis. Note: Hydrogeologic units in column 

heading are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

AA ACU YVU/VSU

AA -- NR R

ACU NR -- R

YVU/VSU NR R --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 2. Hydraulic-conductivity distributions for basin-fill hydrogeologic units.
PROBABILITY DISTRIBUTIONS 9

Bedrock Confining Hydrogeologic Units

Regional bedrock confining units include (1) a 

lower clastic confining unit (LCCU) of Late Proterozoic through Lower Cambrian fine- to coarse-grained 

quartzite, siltstone, and conglomeratic sandstone; (2) 

an upper clastic confining unit (UCCU) consisting of 

Upper Devonian through Mississippian fine-grained 

clastic and carbonate rocks; (3) an intrusive confining 

unit (ICU) consisting of Cretaceous and Tertiary granitic rocks; and (4) a sedimentary rock confining unit 

(SCU) consisting of Mesozoic cratonic sedimentary 

rocks (Winograd and Thordarson, 1975; Belcher and 

others, 2001). The thick, regionally distributed rocks of 

the LCCU show some of the lowest hydraulic-conductivity measurements (fig. 3). As suggested by the 

hypothesis testing, the LCCU is distinct from the other 

confining units. The hydraulic conductivities of the 

UCCU, SCU, and ICU all have similar distributions 

(fig. 3), as confirmed by the hypothesis testing (table 

4). Greater than expected hydraulic conductivities of 

the crystalline rocks of the ICU may be due to significant fracture permeability in these granites. Data for 

the distribution of the crystalline confining unit (XCU) 

were unavailable (Belcher and others, 2001).

Carbonate Rock Hydrogeologic Units

The distributions for the carbonate rocks, which 

form a regional aquifer in the DVRFS are shown in figure 4. Outcrop and drill-hole observations and hydraulic-test data indicate that large hydraulic conductivities 

within the regional carbonate aquifer are often the 

result of secondary fracturing and dissolution (Winograd and Thordarson, 1975, p. C14–C30). Aquifer-test 

data from the carbonate rocks were subdivided and statistically summarized to evaluate differences in hydraulic conductivity for rocks with extensive fracturing and 

rocks without extensive fracturing (fig. 4). These designations were assessed from descriptions given on lithologic logs from wells included in the aquifer tests (fig. 

4). Extensive fracturing can increase hydraulic conductivity values in the carbonate rocks. The hypothesis 

testing confirms that the geometric means of the fractured and unfractured carbonate are not equal; the differences between all carbonates and unfractured 

carbonates also are not equal (table 5).

Volcanic Rock Hydrogeologic Units

The Tertiary volcanic rocks were divided on the 

basis of lithostratigraphy (Sawyer and others, 1994; 

Slate and others, 2000) into the following units: Thirsty 

Canyon/Timber Mountain volcanic aquifer (TMVA), 

Paintbrush volcanic aquifer (PVA), Calico Hills volcanic unit (CHVU), Wahmonie volcanic Unit (WVU), 

Belted Range unit (BRU), Crater Flat-Bullfrog confining unit (CFBCU), Crater Flat-Prow Pass aquifer 

(CFPPA), Crater Flat-Tram aquifer (CFTA), and older 

volcanic unit (OVU). Each of these hydrogeologic 

units constitutes a variety of volcanic rocks with widely 

differing material properties such as lithology, degree 

of alteration, and degree of welding that vary both vertically and spatially (Blankennagel and Weir, 1973; 

Winograd and Thordarson, 1975). The hydraulic-conductivity distributions of these volcanic units overlap 

(fig. 5), as expected due to variations in lithology, alteration, and welding within individual HGUs. The 

hypothesis testing indicates that various units appear to 

have statistically significantly different geometric 

means while others are not statistically different (table 

6). The WVU is not in figure 5 or in table 6, because no 

aquifer tests were reported for this unit (Belcher and 

others, 2001).

The Tertiary volcanic rocks were examined for 

the influence of lithology on hydraulic conductivity 

(table 2; fig. 6). Four lithologic groups were considered: (1) ash-flow tuffs; (2) lava flows of rhyolite, rhyodacite, and trachyte; (3) tuff breccia; and (4) bedded 

tuff. The distribution shown in figure 6 suggests that 

lithology of the Tertiary volcanic units, as defined in 

lithologic logs, is not an important factor for controlling hydraulic conductivity. The hypothesis testing 

confirms that the geometric means are not statistically 

different between any of the lithologies of the Tertiary 

volcanic rocks (table 7).

Hydraulic-conductivity measurements for ashflow tuffs were divided into three categories to assess 

the effect of welding (table 2) based on descriptions 

from borehole lithologic logs (Warren and others, 

1998). These three categories are (1) non-welded to 

partially welded tuff; (2) partially to moderately 

welded tuff; and (3) moderately to densely welded tuff. 

Degrees of welding straddling these categories, such as 

non-welded to densely welded tuff, were omitted from 

this analysis. Descriptions of welding including only a 

single category were placed in the lower category (for 

example, all intervals described as “partially welded” 
10 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

STANDARD DEVIAITONS FROM GEOMETRIC MEAN

Sedimentary rocks confining unit (SCU)

Intrusive confining unit (ICU)

Upper and lower clastic confining units (UCCU/LCCU)

Upper clastic confining unit (UCCU)–Shales

Lower clastic confining unit (LCCU)–Quartzites

0.995

0.99

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Table 4. Results of hypothesis testing for bedrock confining units

[Acronyms: ICU, intrusive confining unit; LCCU, lower clastic confining unit; SCU, 

sedimentary rocks confining unit; UCCU, upper clastic confining unit. Abbreviations: 

NR, Non-rejection of null hypothesis; R, Rejection of null hypothesis; Note: 

Hydrogeologic units in column heading are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

SCU ICU UCCU/LCCU UCCU LCCU

SCU -- NR R NR R

ICU NR -- R NR R

UCCU/LCCU R R -- R R

UCCU NR NR R -- R

LCCU R R R R --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 3. Hydraulic-conductivity distributions for regional bedrock confining hydrogeologic units.
PROBABILITY DISTRIBUTIONS 11

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

STANDARD DEVIAITONS FROM GEOMETRIC MEAN

Carbonate aquifer

Fractured carbonate aquifer

Unfractured carbonate aquifer

0.995

0.9

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Table 5. Results of hypothesis testing for carbonate rocks

[Abbreviations: NR, non-rejection of the null hypothesis; R, rejection of the null hypothesis. 

Note: Hydrogeologic units in column heading are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

Carbonate (all) Fractured carbonates Unfractured carbonates

Carbonates (all) -- NR R

Fractured carbonates NR -- R

Unfractured carbonates NR R --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 4. Hydraulic-conductivity distributions for carbonate rock hydrogeologic units.
12 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

STANDARD DEVIAITONS FROM GEOMETRIC MEAN

0.995

0.99

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Belted Range Unit (BRU)

Calico Hills volcanic unit (CHVU)

Paintbrush volcanic aquifer (PVA)

Thirsty Canyon/Timber Mountain volcanic aquifer (TMVA)

Crater Flat–Bullfrog confining unit (CFBCU)

Crater Flat–Prow Pass aquifer (CFPPA)

Crater Flat–Tram aquifer (CFTA)

Older volcanic rocks unit (OVU)

Table 6. Results of hypothesis testing for Tertiary volcanic rock hydrogeologic units

[Acronyms: BRU, Belted Range unit; CFBCU, Crater Flat-Bullfrog confining unit; CFPPA, Crater Flat-Prow Pass aquifer; CFTA, Crater Flat-Tram aquifer; 

CHVU, Calico Hills volcanic unit; OVU, older volcanic rocks unit; PVA, Paintbrush volcanic aquifer; TMVA, Thirsty Canyon/Timber Mountain volcanic 

aquifer. Abbreviations: NR, non-rejection of null hypothesis; R, rejection of null hypothesis. Note: Hydrogeologic units in column heading are being 

compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

BRU CHVU PVA TMVA CFBCU CFPPA CFTA OVU

BRU --- NR NR R NR NR NR R

CHVU NR --- NR R NR NR NR R

PVA NR NR --- NR NR NR NR NR

TMVA R R NR --- R R NR NR

CFBCU NR NR NR R --- NR R R

CFPPA NR NR NR R NR --- NR R

CFTA NR NR NR NR R NR --- R

OVU R R NR NR R R R --- 

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 5. Hydraulic-conductivity distributions for Tertiary volcanic rock hydrogeologic units.
PROBABILITY DISTRIBUTIONS 13

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

0.995

0.99

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

STANDARD DEVIAITONS FROM GEOMETRIC MEAN

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Ash-flow tuffs

Bedded tuffs

Lava flows

Tuff breccias

Table 7. Results of hypothesis testing for Tertiary volcanic rock lithologies

[Abbreviation: NR, non-rejection of the null hypothesis. Note: Hydrogeologic units in column heading 

are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

Ash-flow tuffs Bedded tuffs Lava flows Tuff breccias

Ash-flow tuffs -- NR NR NR

Bedded tuffs NR -- NR NR

Lava flows NR NR -- NR

Tuff breccias NR NR NR --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 6. Hydraulic-conductivity distributions for Tertiary volcanic rock lithologies.
14 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

only were included in the non-welded to partially 

welded tuff category). The hydraulic conductivity of 

ash-flow tuffs generally increases as the degree of 

welding increases (fig. 7). The non-welded to partially 

welded and the partially welded to moderately welded 

tuff categories appear to have lower overall values of 

hydraulic conductivity than the moderately welded to 

densely welded tuff category. The overlap of the 

hydraulic conductivity distributions for the two lesserwelded tuff groups may be an artifact of overlaps 

occurring in the reporting of the degree of welding 

present in the tested interval of the aquifer tests. 

Hypothesis testing confirms that the geometric means 

are not statistically different between the non-welded to 

partially welded and partially welded to moderately 

welded tuff categories, but that the geometric mean of 

the non-welded to partially welded and the partially 

welded to moderately welded tuff categories are statistically different from the moderately welded to densely 

welded tuff category (table 8).

Ash-flow tuffs, bedded tuffs, and tuff breccias 

were divided into two tuff categories, unaltered and 

altered (zeolitized or argillized), to assess the effect of 

alteration on hydraulic-conductivity measurements 

(table 2; fig. 8). Categorization was based on qualitative descriptions in borehole lithologic logs. Intervals 

of partly altered tuffs were omitted from this analysis. 

Clay minerals from the alteration of tuff tend to reduce 

permeability (Flint, 1998). The hydraulic conductivities of altered ash-flow tuffs are less than those for the 

unaltered tuffs (fig. 8). The geometric mean of the horizontal hydraulic conductivity of the unaltered tuff is 

greater than altered tuff by about an order of magnitude 

(table 2). Hypothesis testing of the geometric mean 

confirms that the unaltered and altered tuffs are distinct 

from each other (table 9).

Applicability of Distributions to a Regional 

Model

Values of hydraulic conductivity and transmissivity are dependent on the scale of the tests conducted to 

obtain these properties (Neuman, 1990). This scale 

effect generally is attributed to increasing access to a 

network of conduits for fluid flow as the volume of the 

medium encompassed by the test increases. In permeameter tests of core samples done in the laboratory, 

the hydraulic conductivity of the rock matrix is determined, because these tests require unfractured core for 

successful results. Permeameter-test results generally 

are not useful for regional-scale ground-water flow 

models. Thus, results for permeameter tests of core 

samples are not utilized in the descriptive statistical 

calculations of the hydraulic parameters (with the 

exception of the LCCU). If a single-well aquifer test is 

of short duration, in a formation with low transmissivity, or performed with minimal pumping rates, it typically determines hydraulic properties, only in the nearborehole environment. The accuracy of these tests can 

be decreased by inefficient borehole construction, convergence of flow lines and related head losses as water 

flows into or out of sections of perforated casing, and 

head loss as water moves between the test-interval 

depth and the pump-intake depth. As such, transmissivity estimates derived from single-well tests tend to be 

less than those of multiple-well tests. Storage-coefficient estimates from single-hole tests have lesser reliability than those from multiple-well tests. Multiplewell aquifer tests manifest the influence of field-scale 

features, such as faults and fractures, as well as the 

water-transmitting properties of the rock matrix.

Because of these variables involving variously 

scaled tests, how to quantitatively scale the hydraulicconductivity measurements among permeameter, slug, 

single-well, and multiple-well aquifer tests is currently 

unknown; only general comments can be made.

Principle of Parsimony

In the application of numerical flow model calibration, Hill (1998) introduces the concept of “parsimony.” The principle of parsimony (as applied to 

numerical models of ground-water flow) means to 

“start simple and add complexity as warranted by the 

hydrogeology and the inability of the model to reproduce observations” (Hill, 1998, p. 35). The probability 

distributions presented in this report enable investigators to apply the principle of parsimony. Both statistical 

hypothesis testing and visual examination of the probability distributions indicate that several of the units in 

each category can be grouped together. The initial four 

groups of basin-fill units, bedrock confining units, carbonate rock units, and volcanics represent an initial, 

practical grouping. Within these groups, the hypothesis 

testing and probability distributions provide further 

guidance for adding detail to the flow modeling if 
PROBABILITY DISTRIBUTIONS 15

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

0.995

0.99

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

STANDARD DEVIATIONS FROM GEOMETRIC MEAN

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Ash-flow tuffs (all)

Non-welded to partially welded tuffs

Partially welded to moderately welded tuffs

Moderately welded to densely welded tuffs

Table 8. Results of hypothesis testing for welding in ash-flow tuffs

[Abbreviations: DW, densely welded; MW, moderately welded; NR, non-rejection of null

hypothesis; NW; non-welded; PW, partially welded; R, rejection of null hypothesis. 

Note: Hydrogeologic units in column heading are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

NW/PW PW/MW MW/DW

NW/PW -- NR R

PW/MW NR -- R

MW/DW NR R --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 7. Hydraulic-conductivity distributions for welding in ash-flow tuffs.
16 Probability Distribution of Hydraulic Conductivity for the Hydrogeologic Units of the DVRFS, Nevada and California

-3

-2

-1

0

1

2

3

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

STANDARD DEVIATIONS FROM GEOMETRIC MEAN

0.995

0.99

0.975

0.95

0.9

0.75

0.5

0.25

0.1

0.05

0.025

0.01

0.005

PERCENT LESS THAN OR EQUAL TO INDICATED VALUE

Tuffs (all)

Unaltered tuffs

Altered tuffs

Table 9. Results of hypothesis testing for alteration in tuffs

[Abbreviation: R, rejection of null hypothesis. Note: Hydrogeologic units in column 

heading are being compared to those in row heading]

Null hypothesis: 

Alternate hypothesis: 

Tuffs (all) Unaltered Altered

Tuffs (all) -- R R

Unaltered R -- R

Altered R R --

μ1 μ2 – = 0

μ1 μ2 – ≠ 0

Figure 8. Hydraulic-conductivity distributions for alteration in ash-flow tuffs.
SUMMARY 17

required during calibration. Within the basin-fill units, 

the AA and the ACU are similar enough that they initially could be considered the same unit, with the 

YVU/VSU being distinct. Within the bedrock confining units, the UCCU and the ICU could be combined 

and the LCCU would exist as a separate unit. Fractured 

and unfractured carbonates appear to be distinct from 

one another and could be separated on that basis. The 

hydrogeologic units of the Tertiary volcanic rocks 

could be divided into three separate units: (1) TMVA 

and PVA; (2) BRU, CFBCU, CFPPA, and CFTA; and 

(3) CHVU and WVU (although no aquifer test data for 

the WVU exist, it is geologically similar to the CHVU). 

Alteration of all tuffs and welding in the ash-flow tuffs 

also appears to be a mechanism for adding complexity 

to a numerical model.

SUMMARY

The probability distributions of hydraulic conductivity were estimated to support regional-scale simulation of ground-water flow in the Death Valley 

regional ground-water flow system. Fracturing appears 

to have the greatest influence on the permeability of 

bedrock hydrogeologic units, within this region. The 

degree of alteration and welding in the Tertiary volcanic rocks also influences hydraulic conductivity. As the 

degree of alteration increases, hydraulic conductivity 

decreases. Increasing welding appears to increase 

hydraulic conductivity because degrees of welding 

increases the brittleness of the volcanic rocks, thus 

increasing the amount of fracturing.

Probability distributions can be used to apply the 

principle of parsimony for combining hydrogeologic 

units. Visual examination of the probability distributions and the use of statistical hypothesis testing allows 

groupings of the hydrogeologic units to be made, generalizing the units contained within a ground-water 

flow model. If warranted, complexity can be made by 

dividing units, either along hydraulic properties (for 

example welding or alteration in tuffs) or hydrogeologic units.

The hydraulic-conductivity distributions presented in this report have a greater use beyond that 

associated with the regional ground-water flow model 

being developed by the USGS. The probability of 

hydraulic-conductivity distributions could be used for 

many purposes including contaminant-transport modeling, water-supply issues, and resource protection. 

The distributions also could be used for similar rock 

types in areas outside of the southern Great Basin 

because volcanics, carbonates, and clastics rock types 

that were analyzed occur worldwide. 

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