Sources and Mechanisms of 

Recharge for Ground Water 

in the West-Central 

Amargosa Desert, Nevada 

A Geochemical Interpretation

U.S. GEOLOGICAL SU.RVEY PROFESSIONAL PAPER 712WF

Prepared in cooperation with the 

U.S. Departnient of Energy

Sources and Mechanisms of 

Recharge for Ground Water 

in the West-Central 

Amargosa Desert, Nevada 

A Geochemical Interpretation

By HANS C. CLAASSEN

HYDROLOGY OF NUCLEAR TEST SITES

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 712-F

Prepared in cooperation with the 

U.S. Department of Energy

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1985
UNITED STATES DEPARTMENT OF THE INTERIOR

DONALD PAUL HODEL, Secretary

GEOLOGICAL SURVEY

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Claassen, Hans C.

Sources and mechanisms of recharge for ground water in the west-central Amargosa Desert, Nevada 

a geochemical interpretation.

(Geological Survey professional paper ; 712-F)

Bibliography: p. F30-F31

Supt. of Docs, no.: I 19.16:712-F

1. Water, Underground Amargosa Desert (Calif, and Nev.) 2. Geochemistry Amargosa Desert (Calif.

and Nev.) I. United States. Dept. of Energy II. Geological Survey (U.S.) III. Series.

GB1021.C58 1985 551.49'0979'87 84-600105

For sale by the Branch of Distribution

U.S. Geological Survey

604 South Pickett Street

Alexandria, VA 22304
CONTENTS

Page Page

Abstract........................................ F 1 Ground-water geochemistry continued

Introduction ..................................... 1 Water quality in the valley fill near the Amargosa River

Description of the study area ........................ 1 channel F20

Topography, vegetation, and climate .............. 1 Oasis Valley as a source of water in the valley fill

Geology and geomorphology ..................... 3 along the Amargosa River .................... 23

Hydrology ................................... 3 Paleohydrology of the west-central Amargosa Desert .... 23

Ground-water geochemistry ........................ 5 Interpretation of carbon-isotope data .............. 23

Water quality in the tuffaceous aquifers ............ 7 Age of ground water and paleoclimate ............. 27

The potentiometric high in the eastern one-third of Where is the water of middle Wisconsin age? ........ 27

T. 16 S., R. 49 E. ............................ 16 The problem of Ash Tree Spring .................. 29

Water-quality gradients near the eastern boundary of Summary and conclusions ......................... 30

the tuffaceous valley fill ...................... 19 References cited .................................. 30

ILLUSTRATIONS

Page 

FIGURES 1-7. Maps showing:

1. West-central Amargosa Desert and vicinity ............................................. F 2

2. Water-level altitudes .................................................................. 4

3. Location of ground-water sampling sites ................................................. 8

4. Dissolved sodium (Na + ) in ground water ........................................... 9

5. Dissolved calcium (Ca2+ ) in ground water ............................................. 10

6. Dissolved bicarbonate (HCOs) in ground water ............................................ 11

7. Dissolved sulfate (SO4~) in ground water...................................... 12

8. Diagram showing evolution of water in tuff, Nevada Test Site ...................................... 14

9. Diagram showing composition of water in tuffaceous valley fill .................................... 15

10. Map showing location of ground-water sampling sites peripheral to the west-central Amargosa Desert....... 17

11-13. Graphs showing:

11. Comparison of mixing percent calculated by two methods ................................. 20

12. Chloride and sulfate in precipitation and selected ground and surface waters, Colorado and Nevada 21

13. Hydrogen and oxygen isotopes in precipitation and selected ground and surface waters,

Colorado and Nevada ............................................ 22

14. Map showing unadjusted carbon-14 age of ground water in relation to major surface drainageways

and relative permeability ............................................ 25

15. Graph showing carbon-, hydrogen-, and oxygen-isotope data for water in tuff and tuffaceous valley

fill and their relationship to climate change..................................... 28

TABLES

Page

TABLE 1. Water-quality analyses ................................................... F 6

2. Relationship between dissolved solids in ground water and surface geology in a part of T. 17 S., R. 49 E. 13

3. Comparison of water quality from valley-fill (VF) and lower carbonate (C) aquifers east of the intersection

of the Gravity and Specter Range faults ....................................... 16

4. Comparison of the average water quality in valley fill near the intersection of the Gravity and Specter Range

faults with that of lower carbonate aquifer ................................... 19

5. Expression of ground-water quality between the center of the tuffaceous valley-fill aquifer and the Gravity fault

as a mixture of two component water qualities .................................. 20

6. Carbon-, hydrogen-, and oxygen-isotope data for the west-central Amargosa Desert and vicinity 24

III
METRIC TO INCH-POUND

Multiply SI units

meter (m)

square kilometer (km2 )

cubic meter per day per meter

[<mVd)/m]

cubic meter per second

(mVs)

kilopascal (kPa)

degree Celsius (°C)

by

3.281

0.386

10.764

15,842

0.145

°F = 1.8°C + 32

To obtain inch-pound units

foot (ft)

square mile (mi2 )

cubic foot per day per

foot [(ftVd)/ft]

gallon per minute

(gal/min)

pound per square inch

degree Fahrenheit ( ° F )
HYDROLOGY OF NUCLEAR TEST SITES

SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER IN THE

WEST-CENTRAL AMARGOSA DESERT, NEVADAA GEOCHEMICAL INTERPRETATION

By HANS C. CLAASSEN

ABSTRACT

Ground water in the west-central Amargosa Desert, Nevada, 

was recharged primarily by overland flow of snowmelt in or near 

the present-day stream channels, rather than by subsurface flow 

from highland recharge areas to the north. Geochemical arguments, 

including reaction mechanisms, are used to support these findings. 

Carbon-, hydrogen-, and oxygen-isotope data show that much of 

the recharge in the area occurred during late Wisconsin time. 

Absence of ground water recharged prior to late Pleistocene is 

considered to indicate that either climatic conditions were unfavorable for recharge or that ground-water velocities were such that 

they transported this earlier recharge away from the study area.

INTRODUCTION

Ground-water chemical data can help define groundwater flow systems and evaluate the relative importance 

of ground-water sources and pathways when combined 

with hydraulic data. Many attempts have been made to 

use ground-water chemical data in this manner, with 

limited success, primarily because of lack of understanding of the mechanisms responsible for dissolved constituents in ground-water samples and difficulty in obtaining 

samples that are representative of a definable part of the 

hydrologic environment.

Advances have been made during the last decade toward 

understanding the role of various mineral phases in dissolution and precipitation reactions, primarily in the context of thermodynamic equilibrium. However, this concept 

is limited for two reasons (1) Accuracy of the thermodynamic data for naturally-occurring minerals is suspect; 

and (2} use of equilibrium concepts to describe systems 

that are generally in disequilibrium is questionable.

In this report, geochemical concepts that recognize 

the chemically dynamic nature of ground water are applied 

to determine the sources and pathways of ground water 

in the west-central Amargosa Desert in southern Nevada. 

These concepts help evaluate migration potential for 

radioactive waste resulting from nuclear weapons testing or migration potential for radioactive wastes stored 

on the Nevada Test Site. The study area encompasses 

one of the possible migration paths.

This research was supported in part by the U.S. Department of Energy under Interagency Agreement DE-AIO 

8-76DP00474A between the U.S. Geological Survey and 

the U.S. Department of Energy.

DESCRIPTION OF THE STUDY AREA

The first comprehensive study made of water resources of the study area was by Walker and Eakin (1963); 

they discussed the climate, geologic history, lithology, 

hydrology, and some of the water-quality characteristics 

of the Amargosa Desert, particularly as they relate to 

water use and development potential. The primary study 

area in this report is shown in figure 1.

TOPOGRAPHY, VEGETATION, AND CLIMATE

The surface-drainage area of the Amargosa Desert is 

about 6,700 km2 (Walker and Eakin, 1963); a part is 

shown in figure 1. Elevations in the study area range 

from about 670 to 2,100 m above mean sea level, although 

a few higher peak elevations occur at about 2,400 m. 

Mean annual precipitation corresponding to these elevations is 50 to 380 mm (millimeters); mean annual temperatures range from 6° to 16°C.

The type of vegetation depends on degree of slope, 

slope aspect, and precipitation. Steep slopes, especially 

south- or west-facing, have little to no vegetation. Lower 

elevation, gently sloping alluvial fans support desert 

scrub; higher density cover exists at elevations near 

1,500 m and lower density cover exists at lower elevations. 

Scattered juniper and sage are the dominant woody 

plants from about 1,500 m to about 1,800 to 2,100 m, 

where pinyon pines begin to dominate.

Fi
HYDROLOGY OF NUCLEAR TEST SITES

30'

cVA^VsV^^>'/: '\\/^^'-* ^ » \' */ /-' 

'^vl> X 'x^*:4^

! J ' U \ ' S \ ' » - K \ - V» v^ 'v^';H^'

\V '^W^-' ki ^::Uy l%\ M^

ill.X ;\ h^;V: ^

^X i Xs 'VV ®

"^ V '"^'V

'% >x ^^ W..J V.VA-'

EXPLANATION

*.:.y.:.y.:A' CARBONATE AND CLASTIC 

v: ';'/Vi'-' ROCKS

. ;V^ J J TUFF AND TUFF DETRITUS

BASALT

MIXED LITHOLOGY

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM DF 1929

FIGURE 1. West-central Amargosa Desert and vicinity.
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA 

GEOLOGY AND GEOMORPHOLOGY HYDROLOGY

F3

Lithology of the bedrock outcrops in the basin is varied; 

the bedrock consists primarily of rhyolitic volcanic ash 

falls and flows of Tertiary age, and carbonate, quartzite, 

and argillite that range from Cambrian to Devonian in 

age (Cornwall and Kleinhampl, 1961; Walker and Eakin, 

1963; Burchfiel, 1964; Orkild, 1965; Barnes and Poole, 

1968; Ekren, 1968; Winograd and Thordarson, 1975; 

Byers and others, 1976). Valley fill is derived from these 

rocks, and areal and subsurface distribution of lithology 

in the valley fill is presumed to be related to the proximity of the outcrop as well as degree and orientation of 

slope below the outcrop. Reliable subsurface lithologic 

data for the valley fill in the study area is sparse.

The southeastern quadrant of the study area (fig. 1) 

has been mapped geologically by Denney and Drewes 

(1965). They differentiate playa deposits of salt, sand, silt, 

and clay from coarser valley fill; the playa deposits constitute more than one-half the valley fill material. These 

playa deposits have been dissected by later erosion and 

replaced in places by coarser, more permeable material.

Denney and Drewes (1965, p. 32-33) as well as Hunt 

(1960) further conclude that little disturbance of the 

valley-fill material has occurred during the last 2,000 

years and they propose that the most recent period constituting significant erosion and deposition occurred in 

late Pleistocene. During that period, the playa deposits 

were dissected "* * * to depths of at least 100 feet. Washes 

from volcanic mountains to the north spread a layer of 

gravel on top of the playa deposit * * *."

Hydrologic significance of the lithology and distribution 

of valley-fill material is twofold: (1) The effect that different 

materials have on distribution of horizontal and vertical 

hydraulic conductivity; and (2) the chemical quality that 

results from the interaction of water with different materials. 

Approximate areal distribution of valley-fill material by 

lithologic character is shown in figure 1. Outcrops of 

carbonate and other clastic rocks also are shown in figure 

1. From a consideration of chemical reactivity, these 

rocks represent the major exception to a primarily tuff aceous 

bedrock and valley fill north of the Amargosa Desert. 

The region of tuffaceous bedrock or valley fill has been 

estimated from consideration of outcrop lithology (Walker 

and Eakin, 1963; Denney and Drewes, 1965), slope, major 

drainage, drill-hole data (either drillers' logs or lithologic 

logs when available) (Walker and Eakin, 1963; F.M. 

Byers, U.S. Geological Survey, written commun., 1972), 

and water-quality data presented in this report.

The most detailed previous discussion of hydrology of 

the west-central part of the Amargosa Desert has been 

by Walker and Eakin (1963), although their study area 

was larger than the area considered in this report. They 

present a map of 1962 water-level altitude contours for 

the valley-fill aquifer for the part of the area containing 

the greatest density of wells. Measurements made in 

1974 indicate that probably not more than 0.3 m of 

change had occurred during 1962-74 throughout most of 

the area. A summary of their data is given in figure 2, 

modified to include information collected since their study. 

Water-level altitude contours show that the gradient is 

parallel to the surface-drainage pattern. Geologic, hydrologic, 

and chemical data indicate that a confined or semiconfined 

aquifer controls the potential distribution east of the 

Gravity fault (figs. 1 and 2). This is the regional carbonate 

aquifer described by Winograd and Thordarson (1975). 

Hydraulic potential of the confined aquifer does not vary 

much throughout several townships in this area. The 

effect of the degree of confinement is evident at several 

locations. Some wells that are completed in the overlying 

alluvium have water levels similar to those of the confined aquifer, while other wells have significantly lower 

water levels. These lower values indicate that the degree 

of confinement in this area is significant.

The lack of potentiometric discontinuities west of the 

Gravity fault and north of the Ash Tree Spring (site 20) 

and the presence of numerous large springs just east of 

the fault indicate that the confined aquifer does not 

extend west of the Gravity fault into the west-central 

Amargosa Desert. However, the serpentine character of 

the 698-m contour in T. 16 S., R. 49 E. could be explained 

by leakage from the confined aquifer across the fault. 

This possibility is explored further in discussions of 

geochemistry of the ground water later in this report.

Ground-water potential gradient distribution is another 

feature of the potentiometric map as it relates to the 

water-quality data that are discussed in more detail in a 

later section. The southernmost region has a gradient of 

about 4.7 x 10 3 ; the central region near W/z T. 16 S., R. 

48 E., Wy2 T. 16 S., R 49 E. has a gradient of about 

1.3xl0 3 ; the northwestern region has a gradient of 

about 2.9 x 10 3 ; and the northeastern region, Fortymile 

Canyon, has a gradient of about 1.3 x 10 3 . Regions with 

less steep gradients probably represent more permeable 

or thicker aquifer materials; regions with steeper gradients 

represent less permeable or thinner aquifer materials. 

Although no permeability data exist for the area, a plot 

of the ratio of sand plus gravel divided by silt plus clay, 

using drillers'-log data from wells in the study area that
HYDROLOGY OF NUCLEAR TEST SITES

30'

Death Valley f ^ . 

Junction / \i \| ". 

\ \ V

EXPLANATION

WATER-LEVEL CONTOURShows altitude of water 

table 1962-74. Contour 

interval, in meters, is 

variable. Datum is NGVO 

of 1929

VALLEY FILL CONTAINING 

AT LEAST TWO-THIRDS 

SAND AND GRAVEL

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 2. Water-level altitudes (modified from Walker and Eakin, 1963).
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F5

were completed in the valley fill, provided corroboration 

for the relationship of gradient and permeability. A line 

of sand + gravel/silt + clay = 2 (arbitrarily chosen) is 

shown in figure 2 (William Thordarson, U.S. Geological 

Survey, written commun., 1975). The area within this 

line includes valley fill containing at least two-thirds 

sand and gravel, suggesting relatively high permeability. 

This area generally correlates well with the presence of 

both smaller ground-water potential gradients and presentday surface drainage, as indicated by the ephemeral 

stream channels on the topographic map base. It also is 

consistent with the limited well-performance data given 

in Walker and Eakin (1963, table 3). The east-trending 

knob beginning in the center of T. 16 S., R. 49 E. and 

ending near the mapped fault coincides with a significant 

change in orientation of the potentiometric contours, 

referred to earlier as the locus of possible leakage from 

the confined aquifer east of the fault.

The parts of the study area with larger and much 

smaller hydraulic gradients may be explained in several 

ways. Where no significant withdrawal of water is made, 

such as in T. 15 S., R. 48 and 49 E., the steepening of 

contours southward simply may reflect the change in 

saturated thickness of valley fill. The greater density of 

pumping present in T. 16 S., R. 48 E., however, may 

explain the very slight gradients in this township. This 

area had the greatest withdrawal rate of any of the 

townships in the study area from 1971 to 1974. In the 

northern part of the study area (Crater Flat and Yucca 

Mountain), ground water exists in a bedrock aquifer, in 

contrast to the valley-fill aquifer of the central part. 

Permeability of this bedrock aquifer depends on the 

degree of fracturing of the bedrock; fracturing is greater 

in the upper reaches of Fortymile Canyon than at Crater 

Flat. This statement is supported by comparison of relative 

specific capacities of wells J-12 [1,800 (m3/d)/m], J-13 

[400 (m3/d)/m] (Claassen, 1973), and that of well VH-1 

[350 (m3/d)/m] (L.W. Howells, U.S. Geological Survey, 

written commun., 1981). The steepening contours near 

the intersection of State Highway 29 and the NevadaCalifornia State line have been explained by Winograd 

and Thordarson (1975, p. C-78) as being due to the 

presence of Tertiary rocks with little permeability and 

thinning of the valley-fill aquifer.

GROUND-WATER GEOCHEMISTRY

Data from water samples collected by the U.S. Geological 

Survey in 1971, 1974, and 1979 were used to determine 

sources and mechanisms of recharge of ground water in 

the west-central Amargosa Desert (table 1, fig. 3). Samples 

collected by other agencies and not published previously,

as well as previously published data, are included in 

table 1. Location numbers are keyed to table 1 and figure 

3 and are used throughout the text.

Ground water of the west-central Amargosa Desert 

generally may be divided into categories that reflect: (1) 

Reaction with tuffaceous rocks or tuffaceous valley fill; 

and (2) water that has reacted with primarily carbonate 

rocks or carbonate valley fill; and (3) water reacting with 

a mixed lithology of carbonate and tuffaceous material. 

Other rocks, if present, are assumed to be considerably 

less reactive or of such low abundance that they contribute 

little to the water chemistry. Carbonate-derived waters 

may enter a region of tuffaceous valley fill (or vice versa), 

and the resulting water quality is indistinguishable from 

that resulting from reaction in the valley fill of mixed 

lithology.

In figure 1, the area in the center of the map is tuff 

outcrop and tuffaceous valley fill. The outcrops have 

been mapped by Walker and Eakin (1963) and Denney 

and Drewes (1965); composition of the valley fill has 

been inferred by proximity to outcrop, and degree and 

direction of slope away from outcrop and from the lithologic 

log of a deep test hole drilled by the U.S. Bureau of 

Reclamation in NE*4 sec. 27, T. 15 S., R. 49 E. about 5 

km southwest of Lathrop Wells, Nevada (F. M. Byers, 

Jr., U.S. Geological Survey, written commun., 1972). 

Bedrock is composed primarily of limestone, dolomite, 

quartzite, and occasionally shale; the remaining valley 

fill is composed primarily of limestone, dolomite, and 

quartzite detritus. Some tuffs probably occur near the 

tuffaceous valley fill boundary. Quaternary playa deposits 

have been mapped by Denney and Drewes (1965) in the 

southeastern corner of the study area; these deposits 

may be present in other areas farther north, but at 

greater depths. This possibility is indicated by the presence 

of such deposits at a depth of about 405 m in the deep 

test hole. Presence of these deposits in the zone of saturation 

generally has a significant effect on water quality, as will 

be discussed later.

Maps of the concentrations of the four major ions in 

the ground water, sodium (Na+ ), calcium (Ca2+ ), bicarbonate 

(HCO3 ), and sulfate (SOf), are shown in figures 4 through 

7. The sodium map is similar to one published by Naff 

(1973) but is more detailed. All these maps show a central 

region of small values of each ion bounded on the west by 

a very steep concentration gradient and on the east by a 

less steep, but still prominent, gradient. Gradients at the 

south end are intermediate. Smaller gradients generally 

are coincident with more permeable sands and gravels 

(fig. 2); they also are alined with the major drainageways 

leading from Fortymile Canyon. The reason for the almost 

90° change in direction of the surface drainage from
F6 HYDROLOGY OF NUCLEAR TEST SITES

TABLE 1. Water quality analyses

[m, meters; °C, degrees Celsius; mmol/L, millimoles per liter; Ca2 + , calcium ion; Mg2 + , magnesium ion; Na + , sodium ion; K + , potassium ion; HCOg, bicarbonate ion; 

Cl", chloride ion; SO2", sulfate ion; SiO2, silica]

Number 

( 1-60 in 

fig. 3; 

61-71 in 

fig. 10)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

Site designation 

or name

J-13

J-12

15S/49E-22dc

16S/49E-5acc

16S/49E-8abb

16S/49E-8acc

16S/49E-9cda2

16S/49E-9dcc

16S/49E-18dc

16S/49E-16ccc

16S/49E-19daa

16S/48E-24aaa

16S/48E-25aa

16S/48E-36aaa

17S/48E-lab

17S/49E-7bb

17S/49E-9aa

17S/49E-8ddb

17S/49E-15bbd

17S/49E-35ddd2

( Ash Tree Spring )

16S/49E-23add

16S/48E-15dda

16S/48E-15aaa

16S/48E-23bdb

16S/48E-10cba

16S/48E-36dcc

16S/50E-7bcd2

16S/49E-12ddd

16S/49E-15aaa

16S/49E-36aaa

16S/49E-36aba

16S/49E-35aaa

16S/49E-35baa2

15S/50E-18cdc

15S/50E-18ccc

17S/49E-llba

16S/48E-15ba

17S/49E-15bc

17S/49E-28bcd

18S/49E-laba

18S/49E-2cbc

18S/49E-llbbb

18S/50E-6dac

17S/49E-29acc

16S/48E-8ba

16S/48E-7bba

16S/48E-7cbc

16S/48E-18bcc

16S/48E-17ccc

16S/48E-18dad

16S/48E-8cda

16S/48E-17abb

16S/48E-23da

27N/4E-27bbb

VH-1

Approximate 

well 

depth 

(m)

1,060

350

150

90

60

90

90

60

110

 

90

150

50

50

60

150

 

100

110

0

__

50

100

 

120

60

 

120

 

__

 

100

 

__

50

 

 

0

160

 

 

80

__

 

 

 

 

__

90

140

90

760

Approximate 

depth 

to 

water 

(m)

283

226

78

21

45

45

46

49

33

 

30

29

26

21

16

12

 

15

17

0

__

 

29

29

 

13

43

 

51

 

__

 

26

 

 

__

30

 

 

0

22

 

 

 

34

__

 

 

 

 

__

31

24

14

184

Sample 

collection 

date

3-26-71

3-26-71

11-20-72

3-04-74

11-17-72

4-01-71

8-19-62

11-18-72

3-01-74

3-01-74

6-26-79

3-05-74

11-17-72

3-05-74

3-04-74

3-05-74

3-01-74

3-01-74

3-06-74

3-06-74

5-08-52

3-06-74

6-25-79

6-26-79

3-31-71

8-19-62

3-31-71

4-05-71

8-18-62

4-01-71

6-26-79

3-31-71

6-24-79

6-24-79

6-24-79

8-18-62

6-24-79

12-15-68

11-17-72

3-07-74

6-24-71

2-17-66

6-23-79

4-06-71

11-18-72

6-23-79

2-12-72

6-23-79

6-24-71

3-30-71

3-31-71

6-24-79

6-25-79

6-25-79

3-31-71

8-18-62

2-17-66

8-18-62

2-11-81

Source 

of 

data1

USGS-1

USGS-1

USGS-2

USGS-2

USGS-2

USGS-2

W&E

USGS-2

USGS-2

USGS-2

USGS-3

USGS-2

USGS-2

USGS-2

USGS-2

USGS-2

USGS-2

USGS-2

USGS-2

USGS-2

W&E

USGS-2

USGS-3

USGS-3

USGS-2

W&E

USGS-2

USGS-2

W&E

USGS-2

USGS-3

USGS-2

USGS-3

USGS-3

USGS-3

W&E

USGS-3

USGS-2

USGS-2

USGS-2

USER

USER

USGS-3

USGS-2

USGS-2

USGS-3

DRI

USGS-3

USER

USGS-2

USGS-2

USGS-3

USGS-3

USGS-3

USGS-2

W&E

USER

W&E

USGS-4

Wf.ter 

temperature 

<°C)

31

26.8

 

 

23

25.8

24

24

23.3

 

 

26.4

27

26.5

 

 

__

 

24.0

22.5

23

18.0

__

 

25.5

24

24.5

26.0

 

30.6

 

23.8

 

__

 

24

 

25.1

 

22

25

24

 

17.5

23.8

25.0

 

21.0

25

24.7

24.2

 

 

 

23.3

24

24

22

35.5

Dissolved constituents 

( mmol/L )

Ca2 +

0.30

.35

.67

.72

.75

.57

.76

.57

.50

.75

.60

.45

.47

.42

.47

.60

.62

.52

.52

.38

0.40

.50

.24

.23

.23

1.37

1.19

1.14

1.02

1.30

1.12

1.10

1.33

.30

.42

1.00

1.50

.54

1.07

.60

0.72

.85

.59

1.35

1.46

1.32

1.17

1.37

1.65

1.32

1.20

1.50

.55

1.45

.25

Mg2+

0.09

.09

.08

.09

.11

.10

.14

.11

.11

.08

.05

.03

.03

.08

.06

.07

.15

.11

.16

.19

0.07

.24

.13

.04

.16

.40

.72

.70

.31

.91

.82

.66

.74

.03

.02

.58

.32

.04

.41

.49

0.49

.35

.49

.62

.26

.39

.66

.45

.45

.35

.28

.32

.09

.78

.06

Na+

1.83

1.65

1.87

1.52

1.61

1.61

2.22

2.44

1.83

1.73

1.57

2.35

1.87

1.74

1.74

2.09

2.09

1.57

1.36

2.20

2.43

3.08

2.52

2.87

2.65

4.35

4.85

5.22

3.48

5.22

4.79

5.22

4.92

4.05

4.05

4.22

6.40

1.70

4.35

4.13

5.22

4.31

4.46

6.96

7.85

6.09

5.66

6.53

7.39

6.52

6.96

6.83

3.00

5.83

3.48

K +

0.13

.13

.12

.13

.14

.17

.22

.23

.23

.11

.21

.18

.19

.16

.18

.19

.25

.19

.21

.205

0.165

.19

.15

.17

.14

.33

.33

.11

.25

.46

.43

.41

.34

.10

.10

.36

.25

.17

.31

.49

0.25

.30

.35

.51

.33

.26

.24

.30

.31

.27

.26

.31

.17

.49

.05

HCO'3

2.03

1.95

2.44

2.21

2.49

2.26

2.35

2.31

2.46

2.17

2.20

2.41

2.18

2.18

2.21

2.51

2.15

2.02

1.97

2.58

2.08

2.88

2.51

2.56

2.72

4.92

4.78

4.73

3.20

5.15

4.80

4.45

4.96

2.51

2.57

3.44

4.33

2.00

4.83

4.31

5.77

3.68

3.77

4.52

4.85

4.11

3.92

4.45

3.92

3.87

4.33

4.95

2.20

7.18

2.60

cr

0.20

.21

.24

.17

.22

.17

.34

.28

.21

.23

.19

.22

.26

.19

.18

.27

.28

.18

.28

.19

0.25

.49

.21

.25

.23

.93

.82

.68

.65

.76

.68

.82

.88

.48

.37

.79

1.85

.30

.68

.51

0.56

.87

.58

1.97

2.25

1.78

1.75

1.72

2.34

1.78

1.89

1.95

.75

.90

.28

sof

0.18

.23

.34

.27

.31

.30

.67

.70

.29

.53

.34

.31

.29

.26

.26

.32

.72

.28

.36

.42

0.36

.39

.29

.28

.34

1.15

1.58

1.67

1.35

1.75

1.62

1.54

1.77

.81

1.04

1.67

2.07

.29

.93

1.04

0.77

.94

1.11

1.94

2.11

1.87

1.87

1.98

2.45

1.95

1.87

1.86

.70

1.11

.47

SiO2

0.95

.90

.82

1.03

.90

.97

1.09

1.20

.98

1.28

1.25

1.31

1.20

1.31

1.31

1.33

1.17

1.35

1.21

1.345

1.27

1.19

1.13

1.23

1.07

1.17

.48

.34

.77

.63

.71

.61

.63

.63

.57

.88

.62

 

1.17

1.21

0.98

1.30

1.34

1.20

.63

1.15

1.07

1.33

1.29

1.28

1.13

1.25

 

1.20

.83

Onsite 

pH

7.2

7.1

7.78

8.15

7.47

7.90

7.61

8.16

8.12

7.87

8.20

8.09

8.06

8.40

8.15

8.30

8.02

8.35

8.12

7.96

8.19

7.97

8.1

37.3

8.3

7.2

7.62

7.64

7.7

7.76

7.65

7.65

7.41

38.00

8.37

38.1

8.0

8.2

7.56

8.6

7.82

7.56

8.22

7.60

7.9

7.4

7.7

7.98

7.69

7.69

7.6

37.4

8.2

37.8

7.5
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F7

TABLE 1. Water quality analyses Continued

Number 

(1-60 in 

fig. 3; 

61-71 in 

fig. 10)

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

Site designation 

or name

H-l

UE25B-1

17S/50E-19aab

18S/50E-7aa

Nuclear Engr. Co.

Well 1

UEl9e2

Whiterock Spring

TW1

Well 8, Nevada

Test Site

UElSr

UEl9g-s2

UEl9b-l

Butte Spring

Rainier (typical

sample)

Oak Spring

Topopah Spring

Approximate 

well 

depth 

(m)

1,830

1,220

 

 

180

1,830

 

170

1,680

1,530

2,290

1,370

 

 

__

 

Approximate 

depth 

to 

water 

(m)

572

471

 

 

86

676

 

125

328

418

623

645

 

 

__

 

Sample 

collection 

date

10-01-80

9-01-81

12-27-71

12-28-71

12-16-68

3-19-71

10-06-71

4-10-72

10-01-60

3-24-71

1-29-68

3-20-71

10-06-71

10-13-64

11-10-60

 

4-28-58

3-25-58

Source 

of 

data 1

USGS-5

USGS-6

DRI

DRI

USGS-2

USGS-1

USGS-2

USGS-2

USGS-1

USGS-7

USGS-1

USGS-7

USGS-2

USGS-8

USGS-9

USGS-9

Water 

temperature

33

36

16

13

 

31.2

 

 

26.5

32.2

42.0

31.6

15.0

23

12.8

11.7

Dissolved constituents 

(mmol/L)

Ca2 +

.11

.42

.19

.64

1.37

0.00

.10

.05

.21

.65

.97

.60

.52

.37

.45

.18

Mg2+

.00

.02

.35

.39

.58

0.01

.01

.00

.05

.04

.01

.10

.10

.13

.20

.04

Na +

2.22

2.00

10.96

6.13

7.40

1.61

1.91

1.57

1.35

3.52

3.09

1.83

1.48

.99

.96

.61

K +

.06

.09

.70

.49

.26

0.02

.20

.03

.09

.08

.02

.08

.15

.12

.16

.16

HCO3

1.88

2.20

6.81

4.28

5.38

1.33

1.26

1.34

1.31

4.13

2.98

2.46

2.28

2.16

1.90

.79

cr

.16

.24

1.97

1.06

2.23

0.12

.31

.20

.21

.22

.27

.19

.34

 

.25

.08

so2-

4

.19

.23

1.83

1.53

1.98

 

.28

.12

.15

.25

1.04

.22

.12

 

.15

.16

SiO2

.78

.87

.71

.79

1.17

0.97

.77

.65

.68

.75

.86

.68

.50

 

.95

.83

Onsite 

pH

7.7

7.5

8.60

8.4

7.6

7.9

7.3

7.6

7.4

38.0

8.0

37.4

7.8

8.0

7.5

6.9

1 Reference key to data source: USGS-1 = Claassen (1973); USGS-2 = H.C. Claassen (unpublished data, 1960, 1968, 1971, 1972, 1974); USGS-3 = A.F. White (Lawrence 

Berkley Laboratories, written commun., 1980); W&E = Walker and Eakin (1963); USGS-4 = L.W. Howells (U.S. Geological Survey, written commun., 1981): USGS-5 = E.F. 

Rush and William Thordarson (U.S. Geological Survey, written commun., 1980); USGS-6 = M.S. Whitfield (U.S. Geological Survey, written commun, 1981); DRI = Naff 

(1973); USBR-U.S. Bureau of Reclamation (written commun., 1974), courtesy of E.A. Lundberg; USGS-7 = Blankennagel and Weir (1973); USGS-8 = Claassen and White 

(1979) and USGS-9 = Moore (1961).

2Average of two analyses.

laboratory pH.

Fortymile Canyon to the south end of the system may be 

related to an extension of the Stewart Valley fault (fig. 1) 

or to a thicker sequence of less permeable playa deposits 

in the south end of the system. The geologic map presented 

by Denney and Drewes (1965) includes a part of T. 17 S., 

R. 49 E., within which bouldery-to-pebbly gravel, pebbly 

gravel, and sand and playa deposits were mapped. Coarser 

materials (bouldery-to-pebbly gravel) coincide primarily 

with present-day stream channels; pebble gravel and 

sand commonly occupy the areas between present-day 

stream channels and undissected playa deposits. Dissolved solids of seven water samples within this township 

correlate well with surface-mapped materials (table 2). 

Although small dissolved-solids concentrations are 

related to coarser, more permeable materials, both surficially 

and in the subsurface, additional information is needed 

to determine the mechanism of recharge (source) of ground 

water in the study area. The central part of the area 

consists of tuffaceous-rock fragments containing abundant 

glass. The chemical character of water in contact with 

both vitric and devitrified tuffaceous-rock fragments 

primarily reflects results of reaction with vitric material 

(White and Claassen, 1979, 1980; Claassen and White,

1979; White and others, 1980). Water in the tuffaceous 

valley fill of the west-central Amargosa Desert might be 

expected to have a similar chemical character. However, 

the source of this water needs to be considered. The 

effects of reaction with tuffaceous valley fill probably are 

indistinguishable from those of reaction with bedrock; 

therefore, distinction between recharge directly to valley 

fill via surface runoff versus lateral flow from bedrock 

recharge areas to the north might not be possible. Water 

quality in tuffaceous valley fill and bedrock aquifers 

north of the study area was examined to determine if 

there were any characteristics that would permit its 

differentiation from ground water in the west-central 

Amargosa Desert.

WATER QUALITY IN THE TUFFACEOUS AQUIFERS

If the source of water in tuffaceous alluvium in westcentral Amargosa Desert is bedrock aquifers north of the 

study area, its chemical composition should reflect a 

stage of development equal to or beyond that of the 

bedrock aquifers. The change in water quality with time 

in the vitric tuffs of Rainier Mesa, Nevada, was described
HYDROLOGY OF NUCLEAR TEST SITES

30' 11BM5'

j // / ^-*^ 

/ / ' / ' Lathrop

EXPLANATION

GROUND-WATER SAMPLING 

SITES-Sites 61-71 located 

in figure 10 

Well and number

.20 « Spring and number

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 3. Location of ground-water sampling sites.
116°45'

SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA

30' 11P15'

F9

7.39

m \tfpl V. M

v|,\ ;VcNV,i£

";\\ ; X "^^^ ~-v ^

EXPLANATION

,4.35 GROUND-WATER SAMPLING 

SITE Number is dissolvedsodium concentration, in 

millimoles per liter

2 LINE OF EQUAL DISSOLVEDSODIUM CONCENTRATION Interval 2 millimoles 

per liter

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 4. Dissolved sodium (Na + ) in ground water.
HYDROLOGY OF NUCLEAR TEST SITES

30' 11B'15'

/ 2 #42 \ «!

SKELETON/ HILLS

>v >*" R°9?rs

,- ! /V Spring 

^ \|i\^i

EXPLANATION

GROUND-WATER SAMPLING 

SITE Number is dissolvedcalcium concentration, in 

millimoles per liter

LINE OF EQUAL DISSOLVEDCALCIUM CONCENTRATION Interval 0.5 millimoles 

per liter

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 5. Dissolved calcium (Ca2 + ) in ground water.
116°45'

SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA

30'

Fll

,2.15

EXPLANATION

GROUND-WATER SAMPLING 

SITE Number is dissolvedbicarbonate concentration, 

in millimoles per liter 

 2.50 LINE OF EQUAL DISSOLVED 

BICARBONATE CONCENTRATION-Interval, in millimoles per liter, is variable

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 6. Dissolved bicarbonate (HCO' ) in ground water.
HYDROLOGY OF NUCLEAR TEST SITES

30' ._ L16°15'

EXPLANATJON

1 -77 GROUND-WATER SAMPLING 

SITE Number is dissolvedsulfate concentration, in 

millimoles per liter

 0.60 LINE OF EQUAL DISSOLVEDSULFATE CONCENTRATION Interval, in millimoles 

per liter, is variable

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 7. Dissolved sulfate (SO2-) in ground water.
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F13

TABLE 2. Relationship between dissolved solids in ground water and surface geology in a

part of T. 17 S., R 49 E.

Site

19

18

17

16

36

Surficial

material

Bouldery-to-pebbly gravel

Pebble gravel and sand

do.

do.

do.

Calculated

dissolved solids

( milligrams

per liter)

302

305

366

355

616

Comments

In stream channel.

Near channel.

___

 

Close to both stream

44

39

Playa deposits 

do.

853

643

channel and playa 

deposits.

by Claassen and White (1979) and is summarized below:

1. Recharging water obtains carbon dioxide (CO2 ) by 

some nonequilibrium process.

2. Reaction of dissolved CO2 with vitric tuff occurs by 

both ion-exchange and ion-diffusion processes.

3. Contemporaneous with 2. above, chemical precipitation 

of authigenic phases occurs if suitable surfaces are 

available as nucleation sites.

In tuffs of the Nevada Test Site, north of the study 

area, from which the valley-fill materials in the study 

area were derived, two precipitating phases probably are 

primarily responsible for development of the observed 

ground-water compositions. These are montmorillonite, 

which probably precipitates in all vitric ground-water 

systems, and clinoptilolite, which precipitates in quantities 

from none to about three times greater than the 

montmorillonite. The change in solution cation composition 

with time, or during the interval when ground water 

flows from its recharge location to the sampling point in 

the aquifer, is shown in figure 8 for several waters typical 

of the bedrock aquifers of the Nevada Test Site. Composition 

of the samples depends on the quantity of clinoptilolite 

precipitated relative to the quantity of montmorillonite 

(fig,. 8). Greater percentages of Na+ in ground water are 

associated with greater clinoptilolite/montmorillonite ratios. 

The Rainier, Topopah Spring, Butte Spring, and Oak 

Spring samples, representing perched ground water that 

has undergone montmorillonite precipitation only, have 

been included as examples; however, most bedrock water 

samples have undergone clinoptilolite precipitation also. 

If recharge to the west-central Amargosa Desert occurs 

in the highlands north of the study area, Na+ probably 

would comprise greater than 70 percent of the three 

major cations (Na+ , Ca2+ , and Mg2+ ) in ground water. 

This composition would result from infiltrating precipitation 

or surface runoff reacting with primarily vitric tuff, and 

precipitation of montmorillonite and clinoptilolite in varying 

quantities, depending on the lithology and flow path.

The total concentration of dissolved material in the ground 

water depends on the conditions of recharge. If direct 

infiltration is recharged, temperature and vegetation are 

most important; if surface runoff is recharged, the area 

of ground surface in contact with unit volume of overland 

flow is most important. The foregoing is true if the 

ground-water system is closed to entrance of CO2 after 

recharge; this condition has been assumed in the calculations 

and the discussion (unless stated otherwise). Larger 

concentrations of dissolved CO2 during recharge result 

in larger concentrations of dissolved constituents; hydrolysis 

of dissolved CO2 yields the hydronium ions that react to 

produce the solutes. The concentration of dissolved solids 

also depends on residence time of the ground water, but 

to a lesser degree than the concentration of CO2 .

The Na+-Ca2+-Mg2+ compositions of most of the Amargosa 

Desert water (in tuffaceous valley fill) are inconsistent 

with their source in bedrock aquifers to the north (fig. 9). 

A few of the presumed source water samples (Butte 

Spring, Rainier, Oak Spring, Topopah Spring; fig. 8) are 

similar to those in the Amargosa Desert, but these source 

waters all represent very young, generally perched ground 

water atypical of deeper ground water. It is assumed 

that the deeper ground water (UE19e, TWl, Site 55, 

Well 8, UE18r) represents the regional water table. These 

younger waters, named above, generally less than a few 

years old, have not evolved chemically to the degree that 

those of the regional aquifer have. In at least two instances (Topopah and Oak Springs), the water source 

has dried up due to short-term climatic variations, further 

supporting the contention that they are not representative 

regionally. However, the sources are representative of a 

point in the evolution of the composition of waters in 

tuffs of southern Nevada.

The preceding information indicates that water in the 

tuffaceous valley fill has not been derived from recharge 

in the highlands to the north via subsurface flow. A 

reasonable alternative is that surface runoff directly recharges
F14 HYDROLOGY OF NUCLEAR TEST SITES

Na +-

White rock Spring

EXPLANATION

All percentages as milliequivalents 

REGIONAL GROUND WATER 

PERCHED GROUND WATER 

CLINOPTILOLITE/MONTMORILLONITE RATIO

GROUND-WATER EVOLUTION PATH

IN RHYOLITE TUFF, NEVADA TEST SITE

Ca 2 +

FIGURE 8. Evolution of water in tuff, Nevada Test Site.

the valley fill. This source was suggested for sites 1 and 2 

first by Schoff and Moore (1964, p. 58) and later for 

points farther south by Winograd and Thordarson (1975, 

p. C112), but without detailed supporting evidence beyond 

the alinement of water containing less dissolved solids 

with the present-day stream channel. This recharge mechanism 

allows development of a different reaction extent than 

that followed by ground water recharged in the highlands 

to the north. One expected difference between highland 

recharge and desert runoff is the quantity of CO2 available 

for reaction. In highland recharge, direct infiltration 

requires that the dissolved CO2 be proportional to the 

quantity of CO? in the soil zone, where it results from soil 

biological activity; the greater the biological activity, the 

greater the quantity of CO2 dissolved. Surface runoff,

however, initially contacts only a few millimeters of 

surface material, which is probably in equilibrium with 

atmospheric CO2 . As runoff rivulets coalesce, greater 

scour occurs, but the biological activity probably is less 

than in highland areas because of the lack of established 

vegetation in arid-climate washes. Nevertheless, evidence 

from water-quality samples of surface runoff collected in 

arid environments indicates that the quantity of carbon 

dioxide dissolved during runoff is similar to that dissolved 

during direct infiltration. Data are from the Nevada Test 

Site (Alfred Clebsch, Jr., U.S. Geological Survey, written 

commun., 1958), New Mexico (U.S. Geological Survey, 

1979a), and Nevada (U.S. Geological Survey, 1979b). 

This condition is necessary because water in the tuffaceous 

aquifer in the Amargosa Desert contains HCOj in
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA

Na +

F15

EXPLANATION

All percentages as mi 11 {equivalents

FORTYMILE CANYON BEDROCK (f) 

AND VALLEY FILL

CRATER FLAT BEDROCK (0)

AND VALLEY FILL DERIVED FROM 

CRATER FLAT (?)

CLINOPTILOLITE/MONTMORILLONITE RATIO

FIGURE 9. Composition of water in tuffaceous valley fill.

concentrations similar to that in bedrock aquifers to the 

north. Bicarbonate is a direct result of the reaction of 

dissolved CO2 with the lithologic environment. This 

comparison applies to present-day conditions of infiltration 

and recharge via surface runoff; however, a problem 

occurs in the west-central Amargosa Desert. Age of the 

ground water, determined from carbon isotopes, ranges 

from about 7,000 to 20,000 yr B.P. (years before present). 

This timespan included changes not only in mean annual 

temperature and precipitation but also in vegetation 

type and density, thereby affecting the availability of 

C02 for dissolution during the recharge or runoff process. 

Insufficient data are available to resolve this problem. 

A trend of decreasing percent Ca2+ relative to Na+ 

occurs from east to west in that part of the area designated as primarily tuffaceous valley fill (figs. 4, 5, and 9). 

This trend is particularly evident in T. 16 S., R. 49 E. 

and the eastern one-half of T. 16 S., R. 48 E. Because 

the potentiometric map (fig. 2) shows a high hydraulic 

potential region in the eastern one-half of T. 16 S., R 49 

E., one explanation for the trend could be that Ca2+ 

enriched water is diffusing westward; then precipitation 

of calcium carbonate (CaCO3 ) could account for the increasing percent Na+ . This hypothesis is unlikely, because 

the Ca2+ enriched water from the east also is enriched 

in Na2+ (fig. 4). Sodium concentrations decrease to a 

minimum value somewhere near the western two-thirds 

of T. 16 S., R. 49 E.; therefore, greater Na+ concentrations in the eastern one-third of T. 16 S., R 48 E. cannot 

result from diffusion of ground water from the east,
F16 HYDROLOGY OF NUCLEAR TEST SITES

with loss of Ca2+ by precipitation of CaCO3 . Bicarbonate 

concentrations (fig. 6) support this argument also, because they, too, decrease to a minimum value in the 

same area.

The trend also could result from diffusion from the 

west of water more saline than that in the eastern onethird of T. 16 S., R. 48 E., and subsequent precipitation 

of CaCO3 ; however, the potentiometric contours do not 

support this argument, as the flow is primarily southerly, 

not from the west as desired (fig. 2; Walker and Eakin, 

1963). Furthermore, if the water quality in the eastern 

one-third of T. 16 S., R. 48 E. (as evidenced by the water 

quality at sites 21 through 25) resulted from a mixture 

of water from the center of the tuffaceous valley fill (the 

central trough shown on the water-quality maps) and 

water from the western one-third of the same township, 

a significant difference would occur in Cl concentration 

between water in the trough and water in the eastern 

one-third of the township. Although the contrast in Cl' 

concentration of the postulated source is almost a factor 

of 10, no significant difference exists between Cl in 

trough water and Cl' in water at sites 21 through 25.

Interdiffusion (mixing) of ground water of different 

chemistries and precipitation of CaCO3 also would result 

in decreases in the CaCO3 saturation index; that is, the 

ion-activity product divided by the equilibrium constant, 

as the percent Ca2+ decreases. If the increase in percent 

Na+ among the sites plotted in figure 9 resulted from 

CaCO3 precipitation, a corresponding decrease in CaCO3 

saturation should be observed. Although some of the 

chemical data may not be sufficiently accurate for precise calculation of CaCO3-saturation indices, no such 

trend is observed. Therefore, east-to-west changes in 

water quality in the primarily tuffaceous valley fill in 

the west-central Amargosa Desert are not due to simple 

ground-water diffusion processes accompanied by CaCO3 

precipitation. The decrease in Ca2+ relative to Na+ toward

the center of the trough is presumed to result from 

greater quantities of clinoptilolite precipitated from a 

ground-water composition evolved by reaction with vitric 

tuff and, therefore, represents a trend of increasing 

maturity of ground water.

THE POTENTIOMETRIC HIGH IN THE EASTERN 

ONE-THIRD OF T. 16 S., R. 49 E.

As first suggested by Winograd and Thordarson (1975, 

p. C84-C85), the proximity of a potentiometric high to 

the Gravity fault near its intersection with the Specter 

Range thrust fault indicates the possibility of a breach of 

the confining properties of the Gravity fault observed to 

the southeast along the Ash Meadows spring line (fig. 2). 

The permeable valley fill just west of the fault junction 

could be a drain for water transecting the fault from the 

east; this fact is consistent with the general hydrologic 

setting indicated by the potentiometric map. The large 

gradient from east to west across the southern end of the 

Gravity fault decreases significantly near the junction 

with the thrust fault. In the valley fill west of the fault, 

flow vectors change directions from a generally southerly orientation to approximately westerly in the vicinity 

of the fault juncture. The implication is that water is 

flowing from east to west across the Gravity fault; 

geochemical data were used to assess whether it is derived from the unconfined (valley fill) or the confined 

(lower carbonate) aquifer, discussed by Winograd and 

Thordarson (1975).

No water-quality data exist for the valley-fill aquifer 

just east of the Gravity fault, but a few samples from 

wells in T. 16 to 17 S., R. 51 to 52 E. (table 3, fig. 10) 

disclose an areal variability in water chemistry that 

would indicate that the valley fill is not a significant 

source of water for movement west of the Gravity fault. 

The potentiometric surface in the valley fill is about 10m

TABLE 3. Comparison of water quality from valley-fill (VF) and lower carbonate (C) aquifers east of the intersection of the Gravity

and Specter Range faults

[m, meters; °C, degrees Celsius; mmol/L, millimoles per liter: Ca2 + , calcium; Mg2 + , magnesium; Na + , sodium; K + , potassium; HCO3 , bicarbonate; Cl', chloride; SO2", sulfate;

SiC>2, silica]

Location

(fig. 10)

16S/51E-23

16S/52E-15a

17S/5lE-la

17S/5lE-23b

17S/5lE-23b

16S/51E-23

14S/52E-3d

Fairbanks Spring

(17S/53E-9a)

Aquifer

VF

VF

VF

VF

VF

C

C

C

Approximate 

well

depth

(m)

189

244

41

7

7

 

1,306

 

Water-level

altitude

(m)

 

721

714

710

710

720

729

 

Dissolved constituents 

(mmol/L)

Date of Temperature

collection

9-09-69

5-01-64

1-10-61

10-05-71

8-02-72

9-09-69

3-12-80

9-28-69

(°C>

28

29

23

20

21

31

64

27

Ca2 +

0.15

.82

.97

.75

.77

1.07

1.10

1.17

Mg2 +

0.31

.74

.82

.86

.82

.74

.66

.82

Na +

2.78

5.35

3.00

3.26

3.13

2.78

2.78

2.83

K +

0.25

.31

.26

.31

.31

.21

.25

.24

HCO;+

cof

2.20

3.44

5.74

4.85

4.62

4.67

4.00

4.95

cr

0.62

1.55

.17

.56

.57

.56

.56

.59

so2-

0.41

1.71

.55

.65

.72

.73

.78

.85

SiO2

0.03

.32

.30

.45

.45

.37

.63

.37

Onsite

pH

8.4

7.6

7.2

7.7

7.9

7.6

7.4

7.3
37"30

SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA

117W . 30' 116W

F17

37"00

36°30' fcBase from U.S. Geological 

Survey, California State 

base 1:500,000, 1968, end 

Nevada Stete base, 

1:500,000, 1965

\\ 

10 15 KILOMETERS

CONTOUR INTERVAL 500 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 10. Location of ground-water sampling sites peripheral to the west-central Amargosa

Desert.
F18 HYDROLOGY OF NUCLEAR TEST SITES

lower than that in the underlying carbonates; this difference provides a potential gradient for upward leakage. 

Although some water in the valley-fill is chemically similar to water in the carbonates, additional solutes are 

required for a reasonable mass balance. The most likely 

candidate water in the valley fill to have resulted from 

upward leakage is from well 17S/51E-23b (fig. 10). The 

most significant expected change in water quality during upward movement would be loss of CO2 from a 

decrease in pressure of about 100 kPa (kilopascals). The 

effect of this loss would be an increase in pH, with a 

consequent increase in carbonate (COf') activity and precipitation of CaCO3 . The temperature decreases about 

10°C as the water moves from the carbonate aquifer 

(~30°C) to the valley fill aquifer (~20°C). Considering 

only temperature effects, CO2 solubility should increase 

about 35 percent, and no loss of CO2 would be expected. 

A change in the CO2 equilibrium, therefore, would not be 

expected. However, this effect is opposed by the 50- 

percent decrease in pressure mentioned previously, from 

about 200 to 100 kPa.

Significant upward leakage and CaCO3 precipitation 

cannot readily explain the water quality of the other 

valley fill-derived samples, because they contain either 

significantly larger or smaller concentrations of presumably conservative constituents, such as Cl or SO2 . These 

waters could be derived in part from the carbonates; 

however, much of the valley fill from which these samples were derived consists of limestone and dolomite; 

relative proportions contributed from the two sources 

cannot be determined. The valley-fill water sample from 

16S/51E-23 yielded a carbon-14 ( 14C) activity of 90 percent modern (Schroder and others, 1978), indicating that 

there is some recharge to the valley fill in this area under 

present-day climatic conditions. This evidence supports 

the hypothesis that water in the valley fill east of the 

Gravity fault in T. 16 to 17 S., R. 51 to 52 E. is a mixture 

of recharge and upward leakage, proportions of which 

vary from one location to another.

The foregoing discussion of water quality in the valley 

fill is for the area located in a township next to (east of) 

the area just east of the intersection of the Gravity 

(normal) and Specter Range (thrust) faults. Results perhaps cannot be extrapolated because the presence of a 

major surface drainage, Rock Valley Wash (fig. 10), 

exists in T. 16 S., R. 50 E., whereas townships to the east 

do not contain a major surface drainage. An evaluation 

of water quality in the valley fill just west of the fault 

intersection may determine whether the effect of Rock 

Valley Wash must be considered to explain groundwater movement in this area.

Sites 27, 28, 30, and 31 (fig. 3) are those closest to the 

Gravity fault; chemical analyses of the water are included in table 1. The average of these four analyses is

presented in table 4 and compared with the average of 

the two nearest carbonate-aquifer sampling locations, 

16S/51E-23 and Fairbanks Spring (table 3). Similarity 

of the Ca2+ , Mg2+ , HCO3 + CO2 concentrations indicate 

development of both water qualities in carbonate terrane. 

The dissimilar Na+ , K + , Cl, SO2' concentrations suggest 

three possible mechanisms for development of the water 

quality in the valley fill just west of the fault intersection:

1. Upward leakage from carbonate aquifer and mixing 

with water recharged directly to the valley fill.

2. Upward leakage from carbonate aquifer and reaction 

with rock fragments, including evaporites, in the 

valley fill.

3. Water recharged primarily through and resident in

the valley fill.

Mechanisms 1 and 2 cannot be distinguished on the 

basis of chemical-quality data alone. Evidence of playa 

deposits in the area (W C Swadley, U.S. Geological 

Survey, written commun., 1981), which could contribute 

evaporites or small quantities of very saline paleowaters, 

further prevents distinction between mechanisms 1 and 

2. Mechanism 2 cannot be relegated to secondary importance by examination of the stable-isotope deuterium 

(D) and oxygen-18 ( 18O) contents of the carbonate and 

valley-fill water. In the carbonate aquifer, <5D = -103; 

<5 18O = -13.6 (I.J. Winograd, U.S. Geological Survey, 

written commun., 1982); whereas, in the valley fill at 

site 27, <5D = -105; and <5 18O = -13.8. The difference in 

analysis between the two water sources is about equal to 

standard analytical error reported for D and 18O analyses.

Water recharged directly through the valley fill 

(mechanism 3) also would be expected to reflect the 

presence of playa deposits. As previously mentioned, a 

large contrast occurs between ground water associated 

with playa deposits (for example, sites 39 and 44) and 

water only a few kilometers away (sites 16, 18, and 19) 

not associated with playa deposits. In the above example, 

the greatest contrast is in the Na+ and SO2 concentrations, 

with Cl and HCO3 next. If the source of the water at sites 

39 and 44 is the nearby more permeable valley fill, the 

observed differences in HCCh and Cl' concentrations o

would not be expected. In the case of HCO3 the source 

water already is saturated with respect to CaCO3 and 

would not be expected to dissolve carbonate f acies in the 

playa deposits and thereby increase its HCO3 concentration. 

A source of chloride enriched minerals, which could dissolve as water passed through the playa deposits, is 

unlikely, because the water associated only with playa 

deposits is only slightly saturated with respect to soluble chlorides. Therefore, the most probable source of 

water in the playa deposits in the vicinity of sites 39 and 

44 is direct recharge. This also may be the source of 

water in the playa deposits in the vicinity of sites 27, 28, 

30, and 31, and other sites associated with playa deposits.
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F19

TABLE 4. Comparison of the average water quality in valley fill near the intersection of the Gravity and Specter Range faults with

that of the lower carbonate aquifer

[mmol/L, millimoles per liter; Ca2 + , calcium; Mg2 + , magnesium; Na + , sodium; K + , potassium; HCOg, bicarbonate; Cl", chloride; SO2", sulfate; SiO2 , silica]

Averages

(mmol/L)

Location

Valley fill: Wells 27, 28, 30, and 31.

Lower carbonate aquifer: Fairbanks

Spring and 16S/51E-23.

Ca2 +

1.19

1.12

Mg2 +

0.79

.78

Na +

5.02

2.80

K+

0.33

.22

HC03

4.86

4.81

cr

0.74

.58

so2-

4

1.66

.79

SiO2

0.65

.37

pH

7.66

7.42

However, the relative homogeneity of water quality throughout the large area represented by sites 27, 28, 30, and 31 

(as opposed to the contrast within the short distance 

between sites 39 and 44) indicates some integrating effect, 

such as a water source in the carbonates. The Cl and 

HCO3 contrast between nearby ground water with relatively small dissolved-solids concentrations, and ground 

water associated with playa deposits observed near sites 

39 and 44, does not exist near sites 27,28, 30, and 31. The 

difference between these sites and the confined aquifer is 

less. Therefore, the carbonate aquifer cannot be ruled out 

as a contributor to the water in the playa deposits, using 

only major dissolved ions as indicators of water source. 

Two other types of evidence support the hypothesis 

that the carbonate aquifer rather than direct recharge is 

the source of ground water in the valley fill: water temperature and hydraulic potential. The water temperature 

at site 27 is 30.6°C and the well depth is 61 m; the 

temperature in the carbonate aquifer at 16S/51E-23 is 

31°C, and at Fairbanks Spring, it is 27°C. In contrast, 

average temperature of water from other wells completed to about the same depth (61 m) in the valley fill, but 

distant from sites 27, 28, 30, and 31, is 21°C (seven 

samples). Water-temperature data support the hypothesis that the carbonate aquifer is contributing water to 

the valley fill, at least at site 27.

The hydraulic-potential data (fig. 2) show a lesser 

contrast between the carbonate aquifer and the valley fill 

in the vicinity of site 27 than almost anywhere else in the 

west-central Amargosa Desert. At site 27, about a 6-m 

difference in hydraulic potential occurs (carbonate aquifer higher), whereas in the next township to the south 

near the Gravity fault, the difference is about 50 m. The 

smaller potential difference and the presence of more 

permeable sand and gravel in the valley fill in the vicinity of site 27 (previously discussed) support the presence 

of significant upward leakage from the carbonate aquifer. 

In summary, the mechanism that best explains the 

water-quality data in the vicinity of the potentiometric 

high is upward leakage from the lower carbonate aquifer 

into the valley fill in combination with water that has 

been recharged directly into the valley fill. This process 

allows the presence of recharged water in the valley fill,

known to exist at other locations, and it can explain the 

possibility of a small difference in D content between site 

27 and the carbonate aquifer. The effect of this westsouth westward moving water on the water quality in the 

tuffaceous valley fill is discussed in a subsequent section 

of the report.

WATER-QUALITY GRADIENTS NEAR THE EASTERN 

BOUNDARY OF THE TUFFACEOUS VALLEY FILL

The central part of the tuffaceous valley fill, which is 

the trough shown on the water-quality maps (figs. 4 

through 7), contains water that probably has originated 

as surface runoff. Infiltration of this runoff occurred 

primarily in the vicinity of present-day drainageways; 

reaction with primarily vitric tuff resulted in the observed 

water quality. Between the center of the trough and the 

area just west of Gravity fault, water quality is compositionally between that derived from vitric tuff (VT) and 

that resulting from a probable combination of water 

qualities of the lower carbonate aquifer (C) and valley fill 

aquifer (VF), as represented by water in the valley fill 

near the intersection of the Gravity and Specter Range 

faults. This compositional gradation may be expressed 

as a simple mixing of the water in the area of wells 27,28, 

30, and 31 (C + VF) with typical water from the center of 

the trough, as represented by the water in the area of 

wells 9,14, and 15 (VT). Component water qualities and 

the contribution of (C + VF) water needed to approximate the composition of the other intermediate groundwater samples are shown in table 5. Mass conservation is 

assumed in the mixing calculations. Negative numbers 

and numbers greater than 100 result from choice of 

composition for VT and (C + VF). The assumption was 

tested by comparison of the mixing percentage of (C + VF), 

using the average of eight major dissolved constituents, 

any of which might not be expected to react conservatively, 

with the mixing percentage of (C + VF) determined by 

using Cl only, an element expected to be conservative. 

Comparison of the mixing percentages calculated by 

each method (fig. 11) indicates that conservancy is approximated for percent mixing values less than 50, but a bias 

exists for percentages greater than 50, indicating the
F20 HYDROLOGY OF NUCLEAR TEST SITES

TABLE 5. Ground-water quality between the center of the tuffaceous valley-fill aquifer and the Gravity fault expressed as a 

mixture of two component water qualities

[VT, vitric tuff, expressed as an average of wells 9,14, and 15; C + VF, a combination 

of water qualities of the lower carbonate aquifer (C) and valley-fill aquifer (VF|, 

expressed as an average of wells 27,28,30, and 31; Ca2+ , calcium; Mg2 "1", magnesium; 

Na + , sodium; K + , potassium; HCOj, bicarbonate; Cl", chloride; SO2", sulfate; 

SiO2, silica; mmol/L, millimoles per liter]

Ca2+ Mg2+ Na K H HCO; cr so2- Sio,

VT

C + VF

Well 

Number

4

5

6

7

8

10

11

17

19

29

32

33

36

(mmol/L)

0.46 0.08 1.77 0.19 2.28 0.19 0.27

1.19 .79 5.02 .33 4.86 .74 1.66

Percent (C + VF) contribution

Average of eight dissolved constituents: 

Ca2+ , Mg2 + , Na + , K + , HC03, Cr.SO2;, SiO2

-3

9

5

20

11

-1

2

17

4

60

104

107

82

pH

1.20 8.21

.65 7.66

For Cr only

-4

5

-4

27

16

7

0

16

16

84

115

125

109

possibility of precipitation of a solid phase or ion exchange. 

However, the bias is insufficient for any meaningful 

estimate of the nature of the precipitate to be made.

WATER QUALITY IN THE VALLEY FILL NEAR 

THE AMARGOSA RIVER CHANNEL

The western-to-southwestern one-third of the study 

area is underlain by valley fill in and near the Amargosa 

River channel. Although water flows in the channel during floods, it is dry almost all the time under present 

climatic conditions. The downstream reach has been 

mapped by Denney and Drewes (1965) in the Ash Meadows quadrangle; they show a narrow area of gravel surrounding the present-day channel. The valley fill along 

the upstream reach of the river within the study area is 

unmapped, but probably contains gravels composed primarily of carbonate detritus derived from the carbonate 

outcrops north of, adjacent to, and south of the area (fig. 1).

Water quality associated with the valley fill along the 

upstream reach is typified by samples from wells 37, 45 

through 52, and 60, and the downstream reach by samples from wells 39, 41, 42, 44, 54, 55, and 57. Generally, 

ground water along the upstream reach contains more 

dissolved solids than ground water along the downstream 

reach. Water-quality maps indicate large gradients between 

the valley fill containing principally carbonate detritus 

near the Amargosa River and the valley fill to the northeast, 

in the central part of the study area, presumed to be

O

9

I 

o

LU

O 

ec

130

120

110

100

90

80

70

60

50

40

30

20

10

0

-10 I I J__I

-10 0 10 20 30 40 50 60 70 80 90 100110120 

MIXING PERCENTAGE (AVERAGE OF EIGHT MAJOR IONS)

FIGURE ll. Comparison of mixing percent calculated by two 

methods.

principally tuffaceous. Farther south, gradients are smaller. 

If Cl and SO2; may be treated as conservative components of the aqueous phase, a hypothesis regarding the 

source of the water near the Amargosa River may be 

formulated.

The relationship of some ground water in the Amargosa 

Desert to present-day (1980) precipitation, and evaporative 

concentration of that precipitation, is shown in figure 12. 

Ground water in the Amargosa Desert contains less 

sulfate (relative to chloride) than present-day precipitation 

concentrates. Water in the tuffaceous valley fill has been 

concentrated by a factor of about 25; water in the valley 

fill along the upstream reach of the Amargosa River has 

been concentrated by a factor greater than 200; whereas, 

water in the valley fill along the downstream reach of the 

Amargosa River has been concentrated by a factor of 

about 100. If it is assumed the source of all Cl and SO2" is 

precipitation, which appears to be verifiable for the tuffs, 

and further assumed that recharge is through the valley 

fill during flooding, it must be concluded that either: (1) 

A greater number of floods which result in recharge 

occur in Fortymile Canyon than in the Amargosa River, 

resulting in a greater fraction of runoff recharge in the 

tuffaceous valley fill; or (2) floods in the Amargosa River 

are smaller than those in Fortymile Canyon.

Conclusion 2 is not reasonable, because the Amargosa 

River drainage basin is larger (~2,600 km2 ) than that of 

Fortymile Canyon (~ 700 km2 ) (Nevada State Engineer's 

Office, 1971). Therefore, conclusion 1 probably is correct.
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F21

O

3.0

2.5

2.0

1.5

1.0

0.5

49.

concentration 

factor of 2009%^.,

Ml j|««inf

concentration 

factor of 100

^Valley fill along 

downstream reach 

of Amargosa River

f

\ Tuffaceous valley fill in and 

*'^ fT be low Fortymile Canyon

/ Yucca Lake, Nevade Test Site 

Present day precipitation, Colorado and Nevada.

W25 

O

EXPLANATION

SURFACE OR GROUND WATER, COLORADO

WATER FROM VALLEY FILL ALONG 

AMARGOSA RIVER-Number is 

site number is figure 3

WATER FROM TUFFACEOUS VALLEY 

FILL ALONG AND BELOW FORTYMILE 

CANYON Number is site number 

in figure 3

WATER FROM VALLEY FILL IN OASIS 

VALLEY NEAR BEATTY, NEVADA - 

Number is site number from White(1979); 

prefix W added for this report

0.5 1.0 1.5 2.0 2.5 

CHLORIDE CONCENTRATION, IN MILL I MOLES PER LITER

3.0

FIGURE 12. Chloride and sulfate in precipitation and selected ground and surface waters, Colorado and Nevada.

In Fortymile Canyon, even small discharges can saturate 

significant depths of vadose zone, because of the confined 

nature of the stream channel in and near the area of 

recharge. In contrast, the Amargosa River flood plain is 

large and has little relief. A given discharge probably 

covers a larger area to a shallower depth than in Fortymile 

Canyon, resulting in a decreased potential for recharge.

As previously discussed, drillers' logs show the presence 

of coarser valley fill below Fortymile Canyon (fig. 2), 

whereas lakebeds are indicated in at least parts of the 

valley fill along the Amargosa River. Coarser materials 

probably indicate greater vertical permeability and, therefore, 

greater ease of recharge for all surface runoff.

Although the greater salinity of ground water in the 

vicinity of the Amargosa River may be rationalized by 

the preceding arguments, the fact that salinity along the 

upstream reach is greater on the average than salinity 

along the downstream reach needs further explanation. 

One explanation may be that valley fill along the downstream 

reach of the Amargosa River received recharge from 

floods occurring in both the Amargosa River and Fortymile 

Canyon. This source seems likely, based on present-day 

topography, which shows coalescing of surface drainageways

of the two systems in the vicinity of the sampling sites 

along the downstream reach of the Amargosa River.

Hydrogen- and oxygen-isotope data support the preceding chemical arguments. Hydrogen- and oxygenisotope relationships among coastal precipitation, 

continental precipitation, and various ground and surface waters are shown in figure 13. Craig (1961) and 

Dansgaard (1964) have shown that near-oceanic precipitation has a predictable relationship between hydrogen 

and oxygen isotopes, and that, as the precipitation contains an increasing proportion of continental moisture, 

oxygen-isotope fractionation causes the heavy isotope in 

rain and snowfall to be depleted. This relationship is 

illustrated by the displacement of line Y from line X 

in figure 13. The precipitation line (Y) from Craig (1961) 

has present-day Colorado and Nevada precipitation data 

superimposed. Ground- and surface-water data from 

Colorado plot to the right of the precipitation line, indicating a small f ractionation during the recharge process. 

This fractionation is most likely due to evaporation in a 

climate with little humidity, as suggested by Fontes and 

others (1979) and by other investigators. Assume that 

during the paleoclimate that existed at the time recharge
F22 HYDROLOGY OF NUCLEAR TEST SITES

-70

-80

CO

O

X

ffi

D_

O.

Z

s

D 

oc

UJ

§

-90

-100

-110

-120

-130 

-20

.Summer

Deuterium

«4-« Oxygen-18 

Error bars for data

I 3350

ID

Winter 

2440

Fall 

1520

2680

47

.14

.W28

-15 

DEL OXYGEN-18, IN PARTS PER THOUSAND

 10

EXPLANATION

PRECIPITATION, NEVADA-Season sample 

was collected shown above line. Elevation 

of sampling site, in meters above sea level, 

shown below line

PRECIPITATION, COLORADO-Season 

sample was collected shown above line. 

Elevation of sampling site, in meters above 

sea level, shown below line

SURFACE OR GROUND WATER, COLORADO Number is elevation of sampling 

site, in meters above sea level

WATER FROM VALLEY FILL ALONG 

AMARGOSA RIVER-Number is site 

number in figure 3

WATER FROM TUFFACEOUS VALLEY FILL 

ALONG AND BELOW FORTYMILE 

CANYON. Number is site number 

in figure 3

WATER FROM VALLEY FILL IN OASIS 

VALLEY NEAR BEATTY, NEVADA-Number is site number assigned by White 

(1979), prefix W added for this report

FIGURE 13. Hydrogen and oxygen isotopes in precipitation and selected ground and surface waters, Colorado and Nevada.

occurred in the Amargosa Desert, precipitation had isotopic characteristics similar to contemporary precipitation. 

A comparison of isotopic composition of ground water in 

the Amargosa Desert with that of precipitation shows 

appropriate shifts toward heavier oxygen-isotope content in the ground water, and the range of values indicates snowmelt as the runoff source. This shift is similar 

to that observed for ground and surface waters in Colorado, 

whose source is in a colder climate than the probable 

paleoclimate of the Amargosa Desert. The isotopic shift 

occurs as a result of different fractionation factors for D

and 18O during evaporation. Furthermore, no difference 

in isotopic composition exists between ground water in 

the tuffaceous valley fill and that near the Amargosa 

River. This similarity indicates similar recharge mechanisms occurring under similar climatic regimes. Because 

water in valley fill along the Amargosa River and in 

tuffaceous valley fill below Forty mile Canyon is dissimilar in Q- and SO^, larger salt accumulations corresponding to complete evaporation, as opposed to concentration 

by evaporation, must be responsible. The concentrationby-evaporation process should result in a parallel increase
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F23

in heavy-isotope content as solute concentration increases. 

This increase is not evident, because there is a similarity 

in isotopic composition between the two water types 

with as much as a factor of 8 difference in salinity. The 

source of greater salinity in valley fill along the Amargosa 

River may be residual salt from evaporation of a Pleistocene 

lake, or evaporation of precipitation or periodic small 

flood flows that do not result in recharge. Because climatic regimes of the two recharge areas are identical, only 

the areal distribution of flow during runoff and the permeability difference between Fortymile Canyon and 

Amargosa River can account for existing water quality.

OASIS VALLEY AS A SOURCE OF WATER IN THE 

VALLEY FILL ALONG THE AMARGOSA RIVER

The potentiometric surface in the valley fill along the 

Amargosa River indicates that subsurface flow downriver 

is possible. The source of such flow likely would be the 

valley fill in Oasis Valley, near Beatty, Nevada. White 

(1979) provides a comprehensive discussion of the hydrology and geochemistry of Oasis Valley. The Cl and SO^ 

data (White, 1979) and the hydrogen- and oxygen-isotope 

data (A.F. White, U.S. Geological Survey, written commun., 

1981) for the five sites near the Narrows below Beatty 

were plotted in figures 12 (Cl and SO^) and 13 (hydrogen 

and oxygen isotopes) for comparison with water in the 

valley fill of the Amargosa Desert.

Water quality in Oasis Valley is quite diverse (figs. 12 

and 13) in an area of only a few square miles and makes 

selection of a typical water chemistry difficult. Well W27 

has the greatest reported discharge and penetrates the 

valley fill to the greatest depth; therefore, it might be 

chosen as representative of water leaving the Narrows. 

The Cl and SO^ concentrations and isotopic composition of this water are quite unlike those in water from the 

valley fill along the entire Amargosa River, and these 

differences would indicate that the ground waters farther south are not derived from subsurface flow originating in the Beatty area. A contrast to this conclusion 

results if the water chemistry in Oasis Valley is represented by water from wells W26 or W29. The chemistry of 

these waters is quite similar to that in the valley fill 

along the upstream reach of the Amargosa River. Thus, 

two mechanisms may account for the ground-water quality along the upstream reach of the Amargosa River: 

recharge of surface runoff or underflow of ground water 

from Oasis Valley. Because the ground-water chemistry 

along the downstream reach of the Amargosa River can 

be explained only by recharge of surface runoff, the 

author favors this mechanism as dominant along the 

entire reach of the Amargosa River.

The data also are consistent with at least some of the 

ground water being derived from infiltration of overland

flow in Oasis Valley. Carbon-14 estimates of groundwater age in Oasis Valley are consistent with this hypothesis (A.F. White, U.S. Geological Survey, written commun., 

1981).

PALEOHYDROLOGY OF THE WEST-CENTRAL 

AMARGOSA DESERT

Carbon-, hydrogen-, and oxygen-isotope data from 

ground water of the west-central Amargosa Desert support the hypothesis that water recharged the valley fill 

primarily through runoff infiltration, with upward leakage from the regional carbonate aquifer in one area. The 

hydrogen- and oxygen-isotope data have afforded simple 

interpretation, but, as discussed below, the carbon-isotope 

data are more difficult to interpret.

INTERPRETATION OF CARBON-ISOTOPE DATA

Carbon-isotope data are given in table 6; distribution 

in the study area of unadjusted 14C ages determined 

for selected ground-water samples is shown in figure 14. 

Values represented by solid circles indicate samples that 

probably were derived from greater than 90-percent reaction with tuff. Values represented by solid squares represent samples derived from 80- to 90-percent reaction 

in tuff; those represented by solid triangles probably were 

affected by significant carbonate dissolution. Examples 

of the latter are those samples derived from water in 

contact with valley fill containing carbonate detritus.

Little or no radiocarbon dilution from carbonate rocks 

probably occurred during the evolution of water in tuff 

or tuffaceous valley fill; therefore, the unadjusted ages 

are taken as true ages. Ground waters with as much as 

20-percent carbonate-derived contribution (all recharged 

contemporaneously) may contain only about 90 percent 

of the 14C activity they would have had if they were not 

diluted by carbonate-derived ground water; therefore, 

their unadjusted age is as much as 1,000 years too old. 

This latter estimate has assumed that no 12C, 13C,^14C 

exchange takes place. Those age data derived from groundwater samples suspected to have evolved in carbonatedetritus valley fill present a serious problem in evaluating 

the meaning of the unadjusted age. For example, at sites 

47 or 60, if all of the reaction has been with carbonate 

detritus, the actual age of the water is about 5,600 years 

younger than the unadjusted age of about 9,000 to 10,000 

yr B.P. The paucity of data and the complexity of the 

problem preclude further discussion of this example.

Those ground-water samples believed to be chemically 

derived primarily by reaction with tuff, are amenable to 

useful interpretation. The assumption that these samples contain little or no CaCO3 -derived dissolved inorgan-
F24 HYDROLOGY OF NUCLEAR TEST SITES

TABLE 6. Carbon-, hydrogen-, and oxygen-isotope data for west-central Amargosa Desert and vicinity

1C, carbonate; T, tuff; °/oo, parts per thousand; SMOW, standard mean ocean water; PDB, Peedee belemnite; yr B.P., years before present; mmol/L, millimoles per liter;

IAP/K, ion-activity product divided by equilibrium constant; CaCO3 , calcium carbonate]

Location name

Fairbanks Spring1

Well 16S/5 IE-231 2

Amargosa well

3

Well 8, Nevada

Test Site.

1

2

3

4

5

8

9

10

11

13

14

15

16

17

18

19

20

21

23

25

27

29

30

47

50

55

56

57

60

Probable 

lithology

c

c

T

T

T, C?

T

T, C?

T, C?

T

T, C?

T

T

T

T

T, C

T, C?

T

T, C

T?

T

T

T

C, T?

C, T

C, T?

C, T?

C, T?

T

T

T

C, T

T

<5D 

%o SMOW

-103

 

-97.5

-97.5

-102

-103

-99.5

-103

-102

-97.5

-101

-102

-98.5

-104

-104

-105

-102

 

-102

-99

-103

-102

-105

-105

-104

-102

-104

-108

-102

-101

 

-104

6 ISO 

%o SMOW

-13.6

-13.6

-13.0

-12.8

-12.8

-13.2

-13.2

-13.4

-12.6

-13.2

-13.1

-13.0

-12.6

-13.0

-12.7

-12.8

-13.0

 

-12.4

-13.2

-13.4

-13.4

-13.8

-13.8

-13.7

-13.1

-13.6

-14.2

-13.4

-13.4

 

-13.0

6^C 

%o PDB

-5.0

-4.6

-7.3

-7.9

-7.1

-6.8

-7.3

 

-5.2

 

 

 

 

 

 

 

 

-8.4

-7.1

-5.6

-3.6

-3.4

-4.4

-6.2

-5.7

-8.5

-11.0

-10.4

-5.9

-12.1

14C

( percent 

modern )

1.9

1.8

29.2

32.2

15.6

19.3

21.4

21.9

28.4

24.8

20.8

19.3

 

18.4

10.0

18.9

27.8

40.3

13.8

27.4

17.1

15.6

7.0

 

10.3

31.4

 

12.0

19.8

17.3

28.8

25.4

14C

apparent 

age(yrB.P.)

31,800

32,300

9,900

9,100

14,900

13,200

12,400

12,200

10,100

11,200

12,600

13,200

 

13,600

18,500

13,400

10,300

7,300

15,900

10,400

14,200

14,900

21,400

 

18,300

9,300

 

17,000

13,000

14,100

10,000

11,000

Total dissolved 

inorganic 

carbon 

(mmol/L)

5.20

5.05

2.27

2.20

2.49

2.26

2.67

2.36

2.52

2.23

2.25

2.21

2.23

2.26

2.57

2.20

2.06

2.01

2.58

2.13

2.57

2.59

5.01

3.34

5.17

4.07

4.02

2.78

2.14

2.34

5.36

1.43

IAP/K 

CaCO3

 

 

0.12

.11

1.96

2.04

.47

1.51

1.46

1.06

1.95

.65

2.06

1.36

2.69

1.19

2.04

1.13

.66

1.15

.71

1.06

1.74

1.18

2.57

1.51

1.79

^rom Winograd and Pearson (1976).

2Amargosa tracer well 2.

3From Schroder and others (1978).

ic carbon is supported by major dissolved-ion geochemistry; 

this assumption should be supported further by stable 

carbon-isotope data. Several approaches have been used 

to correct 14C ages for CaCO3 dilution through understanding the reactions of 13C. These approaches involve 

either simple isotope-dilution models, equilibrium-exchange 

models, or combinations of both (Wigley and others, 

1978); for examples of applications of these models, see 

Shampine and others (1979), Pearson and Swarzenki 

(1974), Deines and others (1974), and Rightmire and 

Hanshaw (1973). Successful application of most techniques has been hampered by insufficient data to make 

precise calculations, from lack of data from a suitable 

hydrologic system to test the model, or from unsuitability of laboratory experimental design for extrapolation of

results to natural hydrologic systems. This study also 

has similar limitations, but the probable hydrologic mechanisms that have already been proposed will be tested 

against the evidence of stable carbon isotopes.

The <5 13C composition of water from the tuff is -7.3 

(median) or -8.0 °/oo (mean). For reaction with silicate 

rock only, and without precipitation of carbonate species, 

the carbon-isotope content of the ground water must 

equate to the carbon-isotope composition of recharging 

water. Because surface runoff is proposed as the principal recharge mechanism, the dissolved CO9 source responsible for the dissolved inorganic carbon in ground water 

might be expected to be primarily the atmosphere. Atmospheric CO., has been reported to range from -6.4 to -7.6 

%o (Wallick, 1976) and -6.7 to -7.4 (Keeling, 1961).
116°45'

SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA

30'

F25

Z ^5(12,4001? A7(21,400)/ 

8(12,200)?! / ±7

EXPLANATION

2(9100) GROUND-WATER SAMPLING 

SITE First number is that 

of site; number in parentheses is 14C age, in years, of 

water in material sampled

 Tuff or tuffaceous valley fill

 Primarily tuffaceous valley fill, 

possibly containing some 

carbonate detritus; query 

indicates contribution of 

carbonate is uncertain

Primarily carbonate-detritus 

valley fill

VALLEY FILL CONTAINING 

AT LEAST TWO-THIRDS 

SAND AND GRAVEL

10 KILOMETERS

CONTOUR INTERVAL 1000 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

FIGURE 14. Unadjusted carbon-14 age of ground water in relation to major surface drainageways and relative permeability.
F26 HYDROLOGY OF NUCLEAR TEST SITES

Although reacting CO2 may be primarily of atmospheric 

origin, the runoff mechanism involves interaction with 

an usually small but finite thickness of surficial material 

and, therefore, some component of root-respired CO2 

from native vegetation. For environments similar to this 

study area, 613C of soil-zone CO2 has been reported to 

range from -15 to -19 (Rightmire, 1967), about -18 (Galirnov, 

1960), -12.0 to -18.4 (Wallick, 1976), and an average of 

organic detritus from juniper woodland near the study 

area of about -21 %>o (W. G. Spaulding, University of 

Washington, written commun., 1980). Although PC02 in 

desert soils [~0.3 kPa (Wallick 1976)] is much greater 

than atmospheric ( ~0.03 kPa), the top few centimeters 

of a permeable soil or alluvium would be expected to 

have near-atmospheric PC02 , but perhaps a slightly more 

negative <S 13C than atmospheric CO2 . The latter phenomenon would result from diffusion of soil gas having a <S 13C 

of about -18 °/oo toward the atmosphere, coupled with 

mixing with atmospheric CO2 containing ^13C~ -7 °/oo. 

The reaction of runoff with surficial materials takes place 

under open-system conditions with respect to CO2 . As 

CO2 dissolves in runoff water, it reacts with soils, sands, 

and gravels, and additional CO2 dissolves to replace that 

used. Whether the dissolution and hydrolysis of CO2 :

C02 + H2 0-+H2C(VH ,,_3

dissolution hydrolysis rock

takes place under equilibrium conditions or not is not 

known, but it is almost certain that the resulting HCO3 

reflects the isotopic composition of the carbonic acid 

(H2CO3 ), because the reaction with rock is irreversible. 

That is, the runoff is a sink for HCO3 , and reacted H2CO3 

is not expected to exchange with gaseous CO2 under 

equilibrium conditions. The importance of this mechanism lies in the effect that the large fractionation of 13C 

bicarbonate under equilibrium conditions (Hendy, 1969; 

Deines and others, 1974) would have on the bicarbonate 

<S 13C. If the reaction of dissolved CO2 with surficial 

material took place under equilibrium conditions, a positive fractionation of between 8.2 %o (20°C) and 9.6 %o 

(5°C) might take place. This would result in <S 13C values 

of +1.2 to +2.6 °/oo for runoff and would be reflected in 

a similar positive <S 13C values for ground water. Ground 

water in the tuffaceous valley fill (and bedrock) has a 

median <S 13C of -7.3 °/oo (mean -8.0 °/oo) inconsistent 

with its development under equilibrium conditions.

At some point, runoff either evaporates or becomes 

recharge. Evaporation results in deposition of readily 

resoluble or sparingly soluble salts. Recharge of runoff 

requires that soluble salts be carried to the water table, 

but what of the inorganic carbon? If pH of the evaporating runoff is greater than 9, significant fraction of inorganic carbon will be in the form of COg ion; whereas, if

pH is below that, HCO3 will predominate. If the reaction 

rate during runoff is slow compared to the rate of incorporation of atmospheric CO2 , pH will tend to stay relatively low; whereas, pH will increase if the reaction rate is 

relatively fast. In the latter instance, evaporation may 

result in formation of CaCO3 , presumably as caliche; in 

the former case, readily soluble salts will deposit. Lattman 

(1973) observed that cementation in alluvial fans progresses most readily in carbonate or basic igneous rock 

terrane, and least readily in silicic igneous detritus. Because 

weathering rates (reaction rates) are greatest in carbonate and basic igneous rocks, pH of runoff in these terranes is expected to be greater; therefore, it would result 

in greater probability of CaCO3 formation. In the tuffs, 

pH of average runoff is expected to be less, and evaporation results in more soluble salts being deposited. This 

result also is promoted by ion-pair formation with Ca2+ , 

which decreases Ca2+ activity. These soluble salts either 

are carried farther down the surface gradient or recharged 

during subsequent runoff. All the steps occurring during 

runoff and recharge are non-equilibrium and, therefore, 

not subject to any significant carbon-isotope fractionation. 

This results in recharge water having a carbon-isotope 

composition identical to average runoff. This has been 

estimated to be somewhat more negative in <S 13C than 

atmospheric CO2 (-7 °/oo), the deviation from the atmospheric value depending on the quantity of vegetationderived CO2 that has been included. Runoff that recharges 

at higher elevations, closer to the heads of drainages, 

might be expected to have more negative <S 13C values, 

because the vegetation is more dense and contributes a 

greater proportion of CO2 compared to runoff at lower 

elevations. The runoff at higher elevations also contains 

less total inorganic carbon, because the time that opensystem conditions prevail is shorter. As the reach of 

surface runoff before recharge increases, a greater proportion of atmospheric CO2 is incorporated into the runoff, 

because vegetation is sparse at lower elevations, and 

contributes less to the total CO2, and total inorganic 

carbon increases because of increased reaction time. This 

can be seen by comparing the <S 13C and total-inorganiccarbon values for several wells in the main Fortymile 

Canyon drainage (table 6). Well 8, Nevada Test Site (site 

64) (elevation 1,700 m), located near the head of the 

drainage, has a <S 13C of -12.1 °/oo and total-inorganiccarbon concentration of 1.43 mmol/L; wells J-13 and J-12 

(sites 1 and 2) (elevation 980 m) are located about midway 

in the drainage reach and average <S 13C = -7.6 °/oo, with 

total-inorganic-carbon concentration =2.24 mmol/L; 

sites 4 and 5 (elevation 730 m) are located near the 

terminus of the drainage and average <S 13C = -7.0 °/oo, 

with total-inorganic-carbon concentration = 2.46 mmol/L. 

Almost all geochemists studying arid or semiarid hydrologic environments, who use carbon-isotope data to aid
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F27

in understanding their systems, have been concerned 

with the effect of caliche on carbon-isotope composition 

of ground water. This concern necessitates an evaluation 

of the importance of caliche in determining the isotope 

compositions for the Amargosa Desert and vicinity. 

Lattman's (1973) observation regarding caliche formation in different lithologies indicates that it may not be 

important in silicic tuffs, but nevertheless caliche does 

cement alluvium in the study area (W C Swadley, U.S. 

Geological Survey, written commun., 1981). This fact is 

still consistent with Lattman's concept that eolian carbonate dust can have an effect in calichification. Carbonate outcrops are present upwind (west) of the study area. 

Details of the occurrence of caliche in the stream channels diminish the concern about its effect on groundwater quality. W C Swadley (U.S. Geological Survey, 

written commun., 1981) also reported that the Fortymile 

Canyon floor probably did not contain caliche. Caliche is 

not known to occur in surficial materials in the head of 

the Fortymile Canyon drainage at an elevation of about 

2,100 m. From a mass-balance perspective, dissolution 

of caliche in a tuffaceous environment should result in a 

positive correlation between Ca2+ and HCO3 if no selective removal mechanism exists; no such correlation is 

observed for the Amargosa Desert and vicinity. The 

median total-inorganic-carbon concentration in tuff-derived 

water is about 2.6 mmol/L; whereas, the median totalinorganic-carbon concentration in carbonate water is about 

5.1 mmol/L, approximately the value that might be 

expected for simple dissolution under similar conditions 

in limestone. The <5 13C data are similarly supportive: 

tuffaceous ground water = 8.0 %o; carbonate ground 

water = 4.1 %>o, the latter value reflecting about 50-percent 

isotope dilution, with marine limestone having a <5 13C of 

~0 °/oo. Although the foregoing arguments do not rule 

out the possible dissolution of caliche, it needs to be 

designated a minor factor in the determination of groundwater chemistry. This concept is consistent with the 

hypothesis that, if recharge occurs, there can be no net 

accumulation of caliche, as such accumulation would 

tend to decrease permeability and decrease recharge. If 

recharge does occur, net removal of caliche must take 

place; permeability then increases, and recharge is increased.

AGE OF GROUND WATER AND PALEOCLIMATE

The distribution of ground-water ages (fig. 14) supports the arguments previously presented regarding recharge 

via overland flow. The youngest ages are located in or 

near present-day drainageways, which, therefore, are 

assumed to be the paleodrainageways. The frequency 

distribution of ground-water ages in tuff is shown in 

figure 15. No correction for non-radioactive carbon is 

believed necessary for these ages; therefore, they should

represent true ground-water ages. The frequency distribution of 14C ages coincides with a cool period that ended 

in the early Holocene. The mean annual temperature trend 

shown in figure 15 is taken from Stuiver and others (1978) 

for the Pacific Northwest. The trend is appropriate to 

the study area, although perhaps not representative of 

the actual magnitude of the temperature changes, as 

demonstrated by Van Devender and Spaulding (1979) and 

Betancourt and Van Devender (1981) for the southwestern 

United States, using packrat middens as chronicles of 

changes in vegetation and therefore changes in climate. 

Changes in hydrogen- and oxygen-isotope data for the 

same ground waters corroborate the temperature trend.

WHERE IS THE WATER OF MIDDLE WISCONSIN AGE?

The absence of ground-water dates older than 17,000 

yr B.P. is curious, because the temperature record presented by Stuiver and others (1978) (a part of which is 

reproduced in fig. 15) shows a persistent cool period 

throughout the Wisconsin. One explanation is that the 

absence of ages greater than 17,000 yr B.P. is an artifact 

of well location and completion. Most of the wells penetrate only the top few hundred meters of saturated thickness, 

that part of the aquifer expected to contain the youngest 

(most recently recharged) water. The data were examined to determine whether this was the controlling factor 

in determining ground-water age, but no such correlation was obtained; rather the position of the well relative 

to surface drainage dominated. Deeper wells did not 

yield older waters, but wells farther removed from surface drainageways did. A few of the wells penetrated 

significant thickness of saturated valley fill or bedrock, 

but no great age differences were apparent: for example, 

site 64 (well 8, Nevada Test Site), depth 1,670 m, saturated thickness ~ 1,350 m; Amargosa wells: site 2 (well 

J-12), depth 350 m, saturated thickness 120 m; site 4, 

well depth 90 m, saturated thickness 46 m; site 9, well 

depth 105 m, saturated thickness 72 m; site 18, well 

depth 100 m, saturated thickness 84 m. Water from all 

these wells had ages between 9,100 and 13,200 yr B.P. 

Water from the well with the least saturated thickness 

had the oldest age; water from the well with the greatest 

saturated thickness had a younger age (table 6). Probably the major producing zones of each of these wells are 

near the water table, so that only water from the top of 

the aquifer is removed; this upper part of the section 

would be expected to contain the youngest water. Further indication that this relationship between well depth 

and major producing interval may be true in bedrock 

aquifers can be obtained by comparing water ages and 

construction of wells J-12 and J-13 (sites 2 and 1) (Claassen, 

1973). Both wells penetrate the same major producing
00

03 i-h

3 g"

? 5'

0) I

CARBON -14 AGEDATE FREQUENCY DEL DEUTERIUM, IN PARTS PER THOUSAND

i _> -» -» _> -» 

$ 8 8 2

DEPARTURE FROM MEAN ANNUAL TEMPERATURE IN

THE PACIFIC NORTHWEST, IN DEGREES CELSIUS

(from Stuiver, M. et al.. Science Vol. 200, 16-20,

fig 1., 7 April 1978, copyright 1978 by the American

Association for the Advancement of Science)

m

00

m

Tl

§

TJ 3Dm

Nm L<

c>srDEL OXYGEN-18. IN PARTS PER THOUSAND
SOURCES AND MECHANISMS OF RECHARGE FOR GROUND WATER, NEVADA F29

zone, but J-13 is 710 m deeper. Well-performance records 

indicate that the greater depth of well J-13 does not 

result in increased capacity, so that very little water is 

being obtained from its greater depth interval. The older 

ground-water age at J-13 (about 800 years older than 

that at J-12) may indicate that some older water is being 

produced from below the interval penetrated by J-12, 

although the age difference is of borderline significance 

at the 95-percent confidence level.

Another possible explanation for the lack of groundwater ages in tuff greater than 17,000 yr B.P. is that 

permeability characteristics of the aquifers sampled are 

such that the older ground water has moved beyond the 

study area. If it is assumed that significant recharge also 

occurred during 17,000 to about 27,000 yr B.P. (the end 

of the next nearest significant interstade), then those 

ground waters may have left the region that was sampled in this study. For example, if ground-water recharge 

occurred near well 8, Nevada Test Site (site 64), representing the head of Fortymile Canyon, and downgradient 

flow occurred in the most permeable part of the Canyon's 

subsurface toward site 18, its velocity would have to be 

between 7 m/a (meters per annum) (if recharged 17,000 

yr B.P.) and 4 m/a (if recharged 27,000 yr B.P.). These 

velocities agree approximately with those calculated from 

limited hydraulic data (R.K. Waddell, Jr., U.S. Geological Survey, written commun, 1981). This agreement may 

not add credence to the argument, because a large range 

of parameter estimates results in similar velocities.

A third explanation, and that favored by the author, is 

based on a recently completed (1982) study of past climates at the Nevada Test site (W.G. Spaulding, University of Washington, written commun., 1982). Using 

arguments based primarily on changes in vegetation at 

and near the Nevada Test Site, Spaulding concludes that 

until about 23,000 yr B.P., average annual precipitation 

was only about 20 percent greater than current (1982) 

precipitation, although much of this increase was during 

the winter months, during the late Wisconsin, however, 

average annual precipitation increased to about 40 percent greater than current (1982) precipitation, with 

70-percent increases for the winter component. Although 

both the middle and late Wisconsin represent significant 

cooling, only the late Wisconsin yields evidence for significant increase in winter precipitation. This author 

then concludes that snowfall earlier than about 20,000 yr 

B.P. was insufficient to result in snowmelt recharge, 

while subsequent climatic conditions caused such recharge. 

This interpretation results in estimates of ground-water 

velocities in the Amargosa Desert considerably slower 

than the 4 to 7 m/a previously stated. If no significant 

recharge occurred prior to 17,000 yr B.P., its disappearance need not be accounted for. For example, assume 

recharge near the head of the Fortymile Canyon occurred

17,000 years ago and flowed to the lower end of the 

Canyon, where it is sampled. The velocity that is calculated, 

7 m/a, must be a maximum. Because recharge may occur 

anywhere but not necessarily everywhere along surface 

drainage ways, no probable minimum velocity can be 

calculated. The absence of water older than about 10,000 

yr B.P., even near the head of Fortymile Canyon, would 

favor velocities slower than 4 m/a.

THE PROBLEM OF ASH TREE SPRING

Ash Tree Spring, site 20, is located approximately in 

line with the trend of other tuff-derived water of the 

central part of the study area (the trough in the waterquality maps); water from the spring was believed by 

Naff (1973) and Winograd and Thordarson (1975) to 

have been derived by a similar mechanism: surface runoff and infiltration through valley fill. Dissolved-ion chemical data generally support this hypothesis, although 

smaller calcium and larger magnesium concentrations 

are small deviations. However, the isotope and hydraulic 

data contradict the chemical data. The 14C age is 15,900 

yr B.P., in marked contrast to the much younger ages of 

the upgradient ground water in most other parts of the 

valley fill downgradient from Fortymile Canyon. 

Furthermore, the water-level altitude of the spring is 

approximately 21 m higher than water levels in wells 

within 1 km of the spring. The nearest wells also contain 

water of a very different quality from the water of Ash 

Tree Spring (see analysis for sites 40 and 41, as examples, 

in table 1). This difference was noted by both Naff (1973) 

and Winograd and Thordarson (1975).

Two hypotheses offer reasonable explanations of the 

origin of water at Ash Tree Spring. The first hypothesis 

requires that the water-level altitude at Ash Tree Spring 

represents recharge in the valley fill at a time when the 

land surface was higher than it is today. Subsequent 

erosion or subsidence then left the (presumably more 

resistant) aquifer material of Ash Tree spring topographically higher than the surrounding valley fill, and the 

formerly saturated material simply has been draining. If 

the flow were constant at the present-day (1972) discharge of 6.7 x 10'4 m3/sfor 15,900 years, approximately 

1.1 x 109 m3 (cubic meters) of aquifer with 30-percent 

porosity would have been dewatered. If the original greater thickness of aquifer is estimated by the difference in 

elevation between Ash Tree spring and the present-day 

(1972) water level, 21 m, the area dewatered is 5.2 x 107 

m2 (square meters), or a square 7.2 km on each side. 

Surficial materials are variable throughout such an area, 

and it seems unlikely that they could contain what appears 

to be a simple tuff-derived water quality; however, this 

possibility cannot be ruled out.

The second hypothesis requires that the recharge to
F30 HYDROLOGY OF NUCLEAR TEST SITES

Ash Tree Spring originate from a different source than 

the source that recharged valley fill to the northwest. A 

conduit providing a flow path more or less isolated from 

the lakebeds and valley fill surrounding Ash Tree Spring 

would be required; such a conduit may be provided by 

the sandstone and claystone mapped by Denney and 

Drewes (1965) in the vicinity and southeast of the spring. 

This formation dips approximately northwest and probably is continuous from the spring to near the summits of 

the Resting Spring Range. It is possible that recharge 

occurring through the quartzite that comprises the summits of the range travels downdip through the (fractured?) 

sandstone toward Ash Tree spring, discharging where 

the aquifer abuts less permeable lakebeds. Lithology of 

the sandstone and claystone includes conglomerate 

(quartzite, limestone, dolomite, tuff); Denney and Drewes 

(1965, p. L18) state that "* * * layers of white tuff 

containing pumice fragments are common. One of two 

beds of * * * very fine grained limestone occur in the 

lower part of the formation * * *." The common presence 

of volcanic glass generally supports the tuff-like characteristics of water in this formation, but the effect of 

interaction with some limestone or dolomite cannot be 

dismissed. The predominance of tuff compared to carbonate in this unit and the similarity of HCO3 concentration 

in Ash Tree Spring water to water found in other tuffaceous rocks would tend to minimize the effect of carbonate rocks in determining water quality.

SUMMARY AND CONCLUSIONS

All available data on ground-water quality for the 

west-central Amargosa Desert were applied to development of a conceptual geochemical model of that hydrologic regime. Hydraulic, geologic, and lithologic data for the 

region also were incorporated as needed.

Results indicate that ground water was recharged to 

the valley fill primarily by overland flow in or near 

present-day stream channels. The flow probably was 

derived from snowmelt. Numerous small floods occur 

that do not result in recharge. Frequency of effective 

recharge depends primarily on channel geometry and 

vertical permeability of the valley fill. The major-ion 

chemical composition of the resulting ground water is 

determined by the lithology of rock fragments that comprise the stream channel and the subsurface, as well as 

soluble salts that remain from evaporation of numerous 

non-recharge events. Recharge through valley fill composed of fragments of tuff or carbonate, or mixtures of 

the two, and through playa deposits, results in distinctly 

discernible water qualities. The central part of the study 

area is underlain primarily by tuff, while the eastern and 

western flanks are underlain primarily by carbonate rocks.

Each area occasionally shows the effect of playa deposits; 

that is, ground water in the vicinity of these deposits 

contains greater concentrations of the conservative SOf 

and Cl" ions than does the water not associated with the 

deposits. In a small area in the eastern part of the study 

area, the effect of upward leakage from a semiconfined 

regional carbonate aquifer is evidenced by differences in 

water temperature, hydraulic potential, and water quality from other sites in the vicinity.

Carbon-, hydrogen-, and oxygen-isotope data indicate 

that major recharge occurred in the area at the end of 

Pleistocene and through early Holocene time. The absence 

of ground water older than about 17,000 yr B.P. is taken 

to mean that no recharge occurred prior to that time, 

and ground-water velocities are slower than 7 m/a and 

probably slower than 4 m/a. It is also possible that the 

slower velocity, presumably less permeable, flow paths 

contain older water; however, until these are sampled 

separately from those of greater velocity, at shallower 

depth, there can be no verification of this hypothesis.

REFERENCES CITED

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