Geochemistry of Ground Water 

Associated with Tuffaceous Rocks, 

Oasis Valley, Nevada

GEOLOGICAL SURVEY PROFESSIONAL PAPER 712 E.

Prepared on behalf "of the 

U.S. Department of Energy

Geochemistry of Ground Water 

Associated withTUffaceous Rocks, 

Oasis Valley, Nevada

By A. F. WHITE

HYDROLOGY OF NUCLEAR TEST SITES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 7 1 2 - E

Prepared on behalf of the 

U.S. Department of Energy

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

White, A. F.

Geochemistry of ground water associated with tuffaceous rocks, Oasis Valley, Nevada.

(Hydrology of nuclear test sites) (Geological Survey Professional Paper 712-E)

"Prepared on behalf of the U.S. Department of Energy."

Bibliography: p. E24 - E25.

1. Water, Underground Nevada-Oasis Valley. 2. Geochemistry-Nevada-Oasis Valley.

3. Volcanic ash, tuff, etc.-Nevada-Oasis Valley. I. United States. Dept. of Energy.

II. Title. III. Series. IV. Series: United States. Geological Survey. Professional

Paper 712-E. 

GB1025.N4W48 551.4'9'09793 79-707

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03164-7
CONTENTS

Page

Abstract ________________________________ E 1

Introduction _________ ___________________________________ 1

Previous work ___________________________________________ 1

Acknowledgments ______________________________________ 1

Methods of investigation ________________________________ 2

Well-numbering system_______________________________ 2

Geographic and geologic setting __________________________ 2

Rocks of Paleozoic age _________________________________ 2

Rocks of Tertiary age.------------____________________ 5

Alluvium of Pleistocene and Holocene age ________________ 5

Hydrology ______________ __________________________________ 6

Surface water ________________________________________ 6

Ground water _______________________________________ 6

Recharge and discharge__________-_________________-_____ 6

Geochemical environments _________________________________ 6

Chemical characteristics controlled by the tuffaceous aquifer __ 8

Equilibrium relationships----------________-_--------____ 8

Page

Chemical characteristics controlled by the tuffaceous aquifei

 Continued

Sources of dissolved species _____ __ Ell 

Silica __________________________ ___ ____ 11

Sodium, potassium, cakium, and magnesium____ 12 

Bicarbonate ____________ __ 12

Fluoride and chloride------______-__- 12

Chemical characteristics controlled by the alluvial aquifer 12 

Concentration increases ______________ 12

Ratios of solute components.--- __ 13 

Sodium, bicarbonate, and chloride- __ 13 

Potassium and fluoride ------ 15

Calcium and sulfate __-_____ ___ 17 

Equilibrium relationships_________ ___ 17

Mass balance of chemical species ___ _ ___ _ 20

Summary and conclusions ______ __ 23

References _________-_________ _ 24

ILLUSTRATIONS

FIGURE 1. Index map showing location of Oasis Valley drainage basin and study area __ ________ _ E2

2. Map showing geology and sampling sites _________________________________________________ 3

3. Map showing the approximate locations of the ground-water flow paths in the alluvium and the

regional distribution of dissolved solids in the alluvial and tuffaceous aquifers ___________ __ 7

4. Aqueous activity diagrams depicting montmorillonite stability ___________________________ 10

5. Diagrams showing changes in concentration of dissolved chemical constituents

as a function of distance along the alluvial flow path ______________________________ 13

6. Diagrams showing concentration trends of sodium, bicarbonate, and chloride---_______ - 14

7. Diagrams showing progressive increases in Pco2 with increases in bicarbonate ___________ __ 15

8. Diagrams showing nonlinear average concentration trends of fluoride and potassium relative to sodium _____ _ 16

9. Diagrams showing concentration trends of sulfate and calcium relative to sodium ________________ 18

10. Triangular diagrams depicting relative concentrations of selected cations and anions in the alluvial aquifer _ _ 19

11. Diagram showing ionic-activity products of calcium fluoride compared to

calcium carbonate. Saturation values are at 25°C ______________________________ _ 19

12. Map showing geographic distribution of calcite- and fluorite-saturated ground water _ ___________ 21

13. Diagram of ionic-activity products of magnesium carbonate compared to calcium carbonate __ _ 22

TABLES

P-4T*

TABLE 1. Description of sample sites _________________________________________________________ E4

2. Analyses of selected major chemical constituents ________________________ 4

3. Analyses of selected minor chemical constituents ______________________ - 5

4. Comparison between average ionic-activity products of Oasis Valley ground water and

solubility products of selected silicate phases_____________________________ 8

5. Comparisons of selected chemical components normalized to silica for montmorillonites

from tuffaceous rocks that are used in stability calculations__________________ 10

III
CONTENTS

Page

TABLE 6. Comparisons of relative molar percentages of selected cations found in the volcanic glass

and Oasis Valley ground water ___ _____ - ________---__------ -- E12

7. Comparisons of selected chemical species in ground water found at the mouth of Thirsty Canyon

and the exit of Amargosa Narrows _____ ______ _____ _____ _____ ___ __ 23

SYSTEM OF MEASUREMENT UNITS

The following report uses both the metric and the U.S. customary systems of units. In the text the metric units are given first, and the 

equivalent measurement in U.S. customary units is given in parentheses. The units are frequently abbreviated, using the notations shown 

below. The metric units can be converted to U.S. customary units by multiplying by the factors given in the following list.

Metric unit U.S customary unit 

Tb convert Multiply by Tb obtain

Kilometer (km) ___________________________ 0.622 Mile (mi).

Meter (m) _____________________________ 3.281 Foot (ft).

Square kilometer (km2) ______________________ .387 Square mile (mi2).

Meter per kilometer (m/km) ___________________ 5.280 Foot per mile (ft/mi).

Cubic hectometer (hm3 ) ______________________ 811.030 Acre-feet (acre-ft)

Liter per second (L/s) _______________________ 15.850 Gallon per minute (gal/min)
HYDROLOGY OF NUCLEAR TEST SITES

GEOCHEMISTRY OF GROUND WATER ASSOCIATED 

WITH TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA

By A. F. WHITE

ABSTRACT

Oasis Valley is an arid intermontane basin located in southern 

Nevada. The two principal aquifers are rhyolitic tuffs of Tertiary age 

in the highlands, and tuff-derived alluvium of Pleistocene and 

Holocene age in the valley bottom. Regional similarities and trends 

in the aqueous chemistry indicate that most of the recharge entering 

the Oasis Valley ground-water system probably is the result of 

ground-water inflow from Pahute Mesa, Gold Flat, and other areas to 

the north and east. Mass-balance calculations show that about half 

the water recharged to Oasis Valley discharges through evapotranspiration, and the rest exits by flow through the alluvium southward 

toward the Amargosa Desert.

Most of the water in the basin has moved from areas of recharge in 

the highlands through the fracture system in the tuffaceous rocks to 

the alluvial material in the valley floor. The solute concentrations of 

sodium and silica suggest that hydrolysis and incongruent dissolution of volcanic glass are the principal reactions in the tuffaceous 

aquifer. Chloride is also leached preferentially relative to fluoride. 

Stability calculations, based on recently reported solubility data for 

montmorillonite of a chemical composition similar to that found in 

the tuffs in areas adjacent to Oasis Valley, show that montmorillonite 

is a stable weathering product formed by the ground water. The 

water is thermodynamically saturated with respect to silica gel but 

undersaturated in terms of analcime, a mineral which occurs extensively in zeolitized zones within the tuffs. This lack of saturation 

suggests that zeolitization occurs in localized geochemical environments, which are not generally reflected in the ground-water composition found in Oasis Valley.

Ground water infiltrates the alluvial aquifer from the tuffs via 

springs and subsurface flow. The close proximity of the water table in 

the alluvium to the ground surface promotes evapotranspiration, and 

evapotranspiration causes a decrease in the volume of ground water 

and an increase in chemical concentration. Colinear increases in 

concentrations of selected solute species demonstrate that water contained in both the tuffaceous aquifer and the alluvium has the same 

generic origin. Such linearity also suggests that sodium and chloride 

are neither selectively added nor removed from the aqueous system, 

and that bicarbonate is only locally affected by dissolution and precipitation of calcite. Calcium concentrations are controlled by dissolution of calcium sulfate minerals present in lacustrine deposits or as 

oxidation products of hydrothermal alteration and by precipitation of 

calcite. Fluoride reaches a limiting concentration controlled by fluorite saturation. Aqueous potassium concentrations are probably controlled by adsorption and fixation by degraded clays common to arid 

soil zones and also by uptake by the vegetative cover.

INTRODUCTION

The study of the geology, geochemistry, and hydrology of Oasis Valley, Nye County, Nevada, was begun in

1967 as part of a program to determine the source and 

eventual destination of ground water under and surrounding the Nevada Test Site. This program was 

undertaken by the U.S. Geological Survey in cooperation with the U.S. Department of Energy (formerly the 

U.S. Energy Research and Development Administration). The geochemical characteristics discussed in this 

paper are meant to lend insight into the probable 

history of the ground water of Oasis Valley and to contribute to the general understanding of geochemical 

processes that govern the composition of ground water 

associated with tuffaceous aquifers.

PREVIOUS WORK

The geochemical nature of the ground water of Oasis 

Valley was described briefly by Malmberg and Eakin 

(1962); their description is based on samples collected 

from several wells and springs within the area. They 

classified the water as being a sodium bicarbonate type 

commonly found in areas of Nevada underlain by tuffaceous deposits of Tertiary age. Malmberg and Eakin 

(1964) later elaborated upon the geochemistry of 

fluoride, high concentrations of which are found in the 

domestic water supply of Beatty (fig. 1).

The economic geology of the Bullfrog Hills, parts of 

which are within the western part of the Oasis Valley 

drainage (fig. 1), has been described by Ransome, Emmons, and Garrey (1910). The geology and structural 

history of the Bullfrog Hills was enlarged upon by 

Cornwall and Kleinhampl (1964). Geologic quadrangle 

maps of parts of Oasis Valley, including Bare Mountain 

(Cornwall and Kleinhampl, 1961), Thirsty Canyon 

(O'Connor, Anderson, and Lipman, 1966), and Timber 

Mountain (Carr and Quinlivan, 1966), have been published.

ACKNOWLEDGMENTS

The author especially wishes to acknowledge the 

help of W. A. Beetem and R. A. Young who initiated the 

study of Oasis Valley and who carried out the sampling 

program. The author wishes to express thanks to B. F. 

Jones and H. C. Claassen for valuable comments and

El
E2 HYDROLOGY OF NUCLEAR TEST SITES

Las Vega's^ 

NEVADA

OASIS VALLEY 

DRAINAGE BASINx^**

AMARGOSA 

DESERT

n i 5 10 KILOMETERS

FIGURE 1. Location of Oasis Valley drainage basin and study area.

criticisms on the concepts presented in this paper and 

to P. P. Orkild for the use of unpublished geologic maps 

of the area. Appreciation is also expressed to the U.S. 

Department of Energy for support given to the project.

METHODS OF INVESTIGATION

Water samples were collected from 29 springs and 

wells in the Oasis Valley area. The locations of the 

sampling sites are shown in figure 2. Descriptions of 

the individual sites are given in table 1. Specific conductance, temperature, pH, and alkalinity were determined in the field. These parameters plus additional 

analyses performed in the laboratories of the U.S. 

Geological Survey are presented in tables 2 and 3.

The computer program, WATEQ (Truesdell and 

Jones, 1974), was used to calculate equilibrium relationships between solute species and various mineral

phases. Unless otherwise noted, thermodynamic values cited in this paper are those listed in WATEQ. 

Theoretical activity diagrams (Helgeson and others, 

1969) were also employed to facilitate prediction and 

interpretation of chemical equilibrium. The reader is 

referred to Garrels and Christ (1965) and Hem (1970) 

for a more complete discussion of geochemical concepts 

presented in this paper, including the use of ionactivity 

products, solubility products, mineral stability fields, 

and the significance of thermodynamic undersaturation and supersaturation.

WELL-NUMBERING SYSTEM

Numbers assigned to wells and springs in Nevada 

are based on the land net established by the General 

Land Office and its successor, the U.S. Bureau of Land 

Management. The locations are referred to the Mount 

Diablo baseline and meridian. Thus the well number 

10S/47-27aaal refers to a well in Township 10 South as 

indicated by the number and letter before the slash. 

The number after the slash, 47, is the Range east of the 

Mount Diablo meridian. Because all ranges in Nevada 

are east of that meridian, the letter E is disregarded. 

The number after the dash, 27, is the number of the 

section, and the letters refer to the quarter, quarterquarter, and the quarter-quarter-quarter section in 

which the well was found. The quarter sections are 

labeled a, b, c, and d counter-clockwise in the section.

GEOGRAPHIC AND GEOLOGIC SETTING

Oasis Valley (fig. 1) is defined topographically as the 

central part of an arid intermontane basin which contains the headwaters of the Amargosa River. Thirsty 

Canyon and Springdale tributaries (fig. 2) enter Oasis 

Valley from the north, Beatty Wash and Fluorspar 

Canyon from the east, and ephemeral stream drainage 

of the Bullfrog Hills from the west. The Amargosa 

River and these tributaries are ephemeral streams 

that carry surface flow only during periods of heavy 

runoff. The entire drainage basin contains approximately 1,200 sq km (460 sq mi) of mountainous and 

greatly dissected terrane. Timber Mountain, with an 

altitude of 2,270 m (7,450 ft), is the highest point in the 

area, and the Amargosa Narrows, with an altitude of 

975 m (3,200 ft), is the lowest point. Beatty, the principal town in the basin, is 185 km (115 mi) northwest of 

Las Vegas.

ROCKS OF PALEOZOIC AGE

The oldest rocks exposed in the Oasis Valley drainage basin are of Paleozoic age. They are exposed principally in the mountains in the extreme southern parts
II6°50' 

37°05'

GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA

45' II6°40'

E3

37°00' T36°55

EXPLANATION

Tertiary tuffs and 

related rocks

Paleozoic limestone,

dolomite, shale,and

related rocks

O 

Sample location

Location number in 

tables 1,2, and 3

Geology by H.Cornwall, 

F. Kleinhampl,l96l, 

l964,ROrkild,written 

communication

Base by U.S. Geological 

Survey. I954

5 KILOMETERS 

FIGURE 2. Geology and sampling sites.
E4 HYDROLOGY OF NUCLEAR TEST SITES 

TABLE 1. Description of sampling sites

[Qal, alluvium of Quaternary age; TV, volcanic rock of Tertiary age; samples collected July 3-6, 1967]

Sample 

site 

number

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

Site Site 

coordinates name

10S/47-14bab 

10S/47-27cba

10S/47-31aab

10S/47-32dda

10S/47-33aab

10S/47-30dcc

llS/46-26bbb Upper Indian Spring 

llS/46-26bcc Lower Indian Spring

HS/47-3cdb

HS/47-4cad

HS/47-10caa

HS/47-10bcc

HS/47-16dcd Burro Hot Springs 

HS/47-16bdc do

HS/47-18acd Crystal Springs

llS/47-21acc

HS/47-21dbb

HS/47-21aba

HS/47-21aba

HS/47-27cba

HS/47-28aac

HS/47-28dac Ute Spring

HS/47-33bac

HS/47-10ccb

12S/47-5cda Beatty Spring

12S/47-6cdd

12S/47-7dbd

12S/47-20bbb

12S/47-19adc

Sample 

source

Spring 

Well

Spring

do

do

Well

Spring 

do

do

do

do

do

do 

do

do

do

Well

Spring

do

Well

do

Spring

do

do

do

Well

do

do

do

Probable

aquifer 

type

Qal 

Qal

Qal

Qal

Qal

Qal

TV 

TV

TV

Qal

TV

TV

TV 

TV

TV

Qal

Qal

Qal

TV

TV

Qal

Qal

TV

TV

TV

Qal

Qal

Qal

Qal

Depth Discharg

m ft L/s

6 

2 6

.7

14

16

37 121 3

II II ~.5

2.5

.6

3.1

--

--

\\ II 1.6

17 55 1

8 25 .06

1.6

1.6

--

55 180 5

91 300 14

6

--

e

gal/ 

min

100

Ti

225

255

50

-8

40

10

49

-

"26

20

1

25

25

80

230

100

--

TABLE 2. Analyses of selected major chemical constituents

[Sampling-site numbers keyed to figures; samples collected July 3-6, 1967. Qal=Alluvium of Quaternary age; Tv= Volcanics of Tertiary age. Concentrations in millimoles per liter, 

except as indicated]

Dissolved constituents

Samplingsite 

number

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

26

26

27

28

29

Probable 

aquifer 

source

Qal

Qal

Qal

Qal

Qal

Qal

TV

TV

TV

Qal

TV

TV

TV

TV

TV

Qal

Qal

TV

TV

Qal

Qal

TV

TV

TV

Qal

Qal

Qal

Qal

Temperature

29.0

19.0

19.5

22.0

22.0

22.5

26.5

21.0

23.0

21.0

24.0

18.5

36.5

36.5

24.0

31.5

29.0

26.0

41.0

21.5

18.0

21.0

34.0

21.0

24.0

21.5

20.0

18.5

20.0

pH

8.1

7.7

7.6

7.6

7.8

7.8

8.7

7.9

8.2

7.7

8.1

7.6

7.8

7.8

7.7

7.7

7.7

7.9

7.6

8.0

9.1

8.2

8.3

8.2

7.9

7.9

7.7

7.8

7.7

calcium

(Ca)

0.18

.55

.58

.75

.75

.60

.00

.15

.40

.65

.35

.35

.45

.42

.55

.58

.62

.65

.32

.90

.25

.21

.30

.32

.80

.68

.63

.68

.95

magnesium 

(Mg)

0.01

.06

.18

.22

.19

.19

.01

.04

.04

.18

.03

.02

.02

.02

.15

.12

.13

.13

.02

.21

.19

.00

.03

.11

.18

.13

.15

.15

.23

sodium

(Na)

6.22

7.44

4.35

5.96

7.35

4.35

2.57

2.48

5.31

9.70

8.53

6.79

7.53

7.13

2.18

10.09

10.57

10.70

6.53

5.00

13.70

10.83

4.87

5.39

4.61

4.57

11.14

11.05

12.62

potassium 

(K)

0.21

.22

.20

.00

.23

.20

.04

.04

.12

.22

.06

.18

.20

.19

.09

.22

.21

.21

.20

.28

.23

.06

.12

.15

.19

.26

.26

.26

.26

bicarbonate 

(HCOs)

3.39

4.57

3.75

5.08

4.85

3.80

1.90

2.07

3.00

6.23

5.41

4.79

4.39

4.15

2.33

6.10

6.44

6.46

3.65

3.03

8.39

5.08

2.84

3.03

3.21

3.47

6.49

6.39

7.20

chloride 

(CD

1.44

1.83

1.18

1.04

1.92

1.13

.39

.42

1.27

2.26

1.52

1.18

1.33

1.21

.59

1.95

2.03

2.03

.99

2.62

1.92

.76

1.27

.73

1.07

2.06

2.06

2.17

2.82

fluoride

(F)

0.22

.99

.13

.12

.23

.09

.02

.02

.15

.27

.32

.24

.32

.30

.03

.32

.32

.32

.32

.37

.34

.20

.14

.20

.02

.32

.32

.31

.33

sulfate

(SO.)

0.86

1.06

.55

1.00

1.07

.61

.15

.18

.98

1.35

1.13

.95

1.32

1.25

.23

1.65

1.74

1.74

1.21

2.27

1.76

.73

.85

.73

.97

1.86

1.36

1.91

2.60

silica 

(SiO2)

0.95

1.03

1.18

1.03

.90

1.20

.73

.80

.78

1.03

.63

.85

1.00

1.07

.75

1.00

.93

.90

.90

.98

.75

.77

.83

1.00

.90

00

1.12

1.12

Total 

dissolved 

solids

(mg/L)

458

573

421

523

584

424

192

213

420

726

583

511

572

546

251

750

783

777

509

396

997

737

355

414

384

440

814

820

1,040
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E5

TABLE 3. Analyses of selected minor chemical constituents

[Sampling-site numbers keyed to figures; samples collected July 3-6, 1967. Concentrations in micromoles per liter]

Dissolved constituents

Samplingsite 

number

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

Boron

(Bl

2~L3

45.3

53.6

31.4

47.1

15.7

16.6

17.6

41.6

19.4

18?5

17.6

10.2

36.0

35.1

30.5

15.7

14.8

38.8

34.2

16^6

20.3

43.4

46.2

53.6

--

Copper

(Cu)

0.03

.01

.13

.03

.05

.02

.02

.02

.02

.02

.05

.03

.09

.05

.03

.03

.08

.03

.05

.05

.03

.05

.06

.02

.05

.03

.03

.06

--

Iron

(Fe)

0.36

.18

.20

.18

.32

.32

.07

.16

.36

.14

.27

.81

.43

1.52

.25

.89

1.07

.53

.18

1.07

1.25

8.41

.20

.16

.18

1.07

.89

.16

--

Aluminum 

(Al)

0.22

.26

.15

.074

.45

.37

.41

.55

.45

.26

.11

1.48

.26

.03

.15

3.34

.19

.26

.04

1.11

.26

.93

.30

.52

.26

.11

1.12

1.67

--

Strontium 

(Sr)

0.17

1.71

2.05

4.45

2.17

2.17

.10

.34

1.08

3.19

1.48

.46

1.37

1.37

.51

2.51

2.86

2.97

1.37

.23

4.56

1.03

.23

.68

.86

1.37

1.37

.11

Barium

(Ba)

0.03

.09

.09

.17

.18

.09

.01

.02

.06

.14

.04

.04

.17

.19

.02

.03

.03

.03

.20

.02

.11

.02

.01

.04

.02

.04

.06

.07

'No analyses for sample site 29.

of Oasis Valley (fig. 2) and to a lesser extent in the 

Bullfrog Hills southwest of Springdale. These rocks 

have been subjected to intense thrust faulting and folding. The formations range in age from Early Cambrian 

to Late Mississippian and have a composite thickness 

of about 7,600 m (25,000 ft). The sequence in Oasis 

Valley consists of stratified layers of shale, siltstone, 

quartzite, limestone, and dolomite. Carbonate rocks 

predominate in the upper half of the sequence, whereas 

shales and related rocks make up most of the lower 

half.

ROCKS OF TERTIARY AGE

Most of the bedrock exposed in Oasis Valley consists 

of ash-flow tuff, ash-fall tuff, rhyolite flows, and minor 

basalt flows of Tertiary age that emanated from local 

volcanic centers. The formations of most interest in the 

Oasis Valley drainage basin are the Paintbrush Tuff, 

the Timber Mountain Tuff, and the Thirsty Canyon 

Tuff. The formations are multiple-flow units ranging in 

rock type from rhyolite to quartz latite. The rocks 

range gradationally from nonwelded to densely

welded. Many exposures display extensive jointing and 

fracturing.

ALLUVIUM OF PLEISTOCENE AND HOLOCENE AGE

Much of the surface area of Oasis Valley is covered 

with alluvium. Cornwall and Kleinhampl (1961) and 

other workers have mapped this alluvial cover as two 

separate units. The older unit, classified as gravels of 

Pleistocene age, represents old dissected alluvial fans. 

The younger unit, presently forming alluvial fans, is 

mapped as alluvium of Holocene age. The gravel in 

these newer fans grades into sand and silt toward the 

topographic bottom of Oasis Valley. With the exception 

of alluvium associated with rocks of Paleozoic age 

found in the extreme southern part of Oasis Valley, 

both units are composed of detritus originating from 

the tuff formations. The exact thickness of the total 

alluvial cover is not known, but it is estimated by 

Malmberg and Eakin (1962) to be as much as several 

thousand feet in the central part of Oasis Valley. The 

alluvium is much thinner in the southern part of Oasis
E6 HYDROLOGY OF NUCLEAR TEST SITES

Valley. Based on unpublished geophysical data of the 

U.S. Geological Survey, the estimated thickness of alluvium resting on the Paleozoic bedrock within the 

Amargosa Narrows (fig. 2) is 4-5 m (12-16 ft).

HYDROLOGY

SURFACE WATER

The Amargosa River flows in a few reaches in the 

central part of Oasis Valley during part of the winter 

and early spring until evapotranspiration becomes 

sufficient to draw down the potentiometric surface 

below the stream bottom. Flow at other times is incidental with thunderstorms in the headwaters of 

Thirsty Canyon or Beatty Wash. Surface flow from 

springs also occurs along short reaches of the valley 

floor.

GROUND WATER

The two principal aquifers in the Oasis Valley basin 

are tuffaceous rocks in the highlands and alluvium beneath the valley floor. Blankennagel and Weir (1973) 

concluded that the ground water in the tuffaceous 

rocks related to those surrounding Oasis Valley was 

transmitted principally through extensive fracture 

systems produced by contractional cooling during solidification and by post-depositional faulting. Where 

such rocks are saturated, are broken by recent faulting, or intersect the land surface, ground water issues 

as springs and seeps. Such is the case for several 

sample locations shown in figure 2 along the east side 

of Oasis Valley between Beatty Wash and Thirsty 

Canyon.

Ground water infiltrates from springs and subsurface flow into the valley-fill alluvium which occurs 

principally along the Amargosa River and its tributaries. At most places, this valley fill is saturated to 

within several feet of the surface. Malmberg and Eakin 

(1962) suggested that where the cross-sectional area of 

the alluvial fill is decreased by underlying, less permeable material, water is forced to the surface and issues 

as springs, seeps, and swampy areas. Some artesian 

wells have been developed in alluvium where ground 

water is confined beneath layers of clay.

Figure 3 shows the approximate locations of the 

ground-water flow paths in the alluvium; the locations 

are based on topographic and geographic distributions 

of the valley fill.

RECHARGE: AND DISCHARGE

Malmberg and Eakin (1962) estimated an average 

annual recharge from precipitation within the Oasis 

Valley drainage basin of 0.31 hm3 (250 acre-ft), and an 

annual discharge from Oasis Valley by evapotranspiration and ground-water underflow of 3.0 hm3 (2,400 

acre-ft). Based on the above estimates, they postulated 

a significant inflow of ground water from outside the 

Oasis Valley drainage basin, probably as recharge from 

Pahute Mesa, Gold Flat, and other areas to the north 

and northeast (fig. 1). Blankennagel and Weir (1973) 

substantiated this conclusion by defining a southwestward-trending potentiometric surface under Pahute 

Mesa in the direction of Oasis Valley. The total underflow passing southwestward through tuffaceous rocks 

beneath Pahute Mesa was estimated at 10 hm3 (8,100 

acre-ft) annually only part of which probably recharges Oasis Valley. Of this total volume, perhaps 3.9 

hm3 (3,200 acre-ft) is recharge from precipitation on 

Pahute Mesa, and the remainder may be underflow 

from Gold Flat and other areas to the north. Altitudes 

of the water table indicate a gradient of approximately 

4 m/km (20 ft/mi) between Gold Flat and the upper 

spring in Oasis Valley, (sample location 1 in fig. 2). 

Winograd and Thordarson (1975), in defining the general hydrologic systems in the south-central Great 

Basin, also suggested that the Oasis Valley groundwater system was recharged by underflow through tuffaceous rocks from Pahute Mesa and Gold Flat.

Evapotranspiration accounts for much of the ground 

water discharged from Oasis Valley. The principal 

species of phreatophytes, including salt grass, Bermuda grass, greasewood, and salt bush, occur along the 

flood plain of the Amargosa River. The average depth to 

water below land surface in most phreatophyte zones is 

2-3 m (6-10 ft). Evaporation also occurs locally from 

spring pools and swampy or seep areas. Based on 

phreatophyte cover, open-water acreage, and assumed 

rates of evapotranspiration, Malmberg and Eakin 

(1962) estimated an evapotranspiration discharge of 

2.5 hm3 (2,000 acre-ft). They estimated the average 

ground-water discharge southward toward the Amargosa Desert (fig. 1) through the thin alluvium overlying 

the Paleozoic bedrock in the Amargosa Narrows to be 

0.5 hm3 (400 acre-ft) annually.

GEOCHEMICAL ENVIRONMENTS

The geochemical history of the Oasis Valley ground 

water can be broadly classified into two stages based on 

differing aquifer characteristics. Most ground water in 

Oasis Valley has presumably moved from areas of recharge through the fracture system of the tuffaceous 

aquifer. In this first stage, dissolved carbon dioxide 

originating in the soil zone of the recharge area reacts 

with mineral phases in the tuffaceous rocks. The 

specific chemical composition of the ground water at 

any given point along the flow path is a function of the 

kinetic rate of reaction, and hence the total residence 

time spent within the tuffaceous aquifer. Ground wa-
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E7

116° 50 37°05'

37°00' -

36°55 -

EXPLANATION

O

Sample site with 

probable ground- water 

source in alluvial aquifer

<§>

Sample site with probable 

ground water source 

in tuffoceous aquifer

(509)

Dissolved solids in 

milligrams per liter

SPRINGDALE

Approximate location of

named flow path in

valley alluvium

600 

Isograds showing increase

in dissolved solids along

main Thirsty CanyonOasis Valley

Base by U.S. Geological 

Survey. 1954

5 KILOMETERS

FIGURE 3. The approximate locations of the ground-water flow paths in the alluvium and the regional distribution of dissolved solids TrTtHe

alluvial and tuffaceous aquifers.
E8 HYDROLOGY OF NUCLEAR TEST SITES

ters from Indian Springs and Crystal Springs (fig. 3) 

that are recharged locally from the Bullfrog Hills 

probably have relatively short residence times within 

the tuffaceous aquifer. Dissolved solids in these ground 

waters are considerably lower than they are in ground 

water issuing from springs along the east side of Oasis 

Valley. Ground water from the springs on the east 

probably has much more distant recharge areas within 

the tuffaceous aquifer to the north and east of Oasis 

Valley.

The second stage in the geochemical history of Oasis 

Valley is the upward percolation of ground water by 

subsurface flow into the shallow alluvial aquifer. Figure 3 shows that the concentration of dissolved solids 

increases progressively along the main Thirsty 

Canyon-Oasis Valley flow path. Ground water in the 

alluvium at site 1, the uppermost sampling point on the 

flow path, contains 458 mg/L dissolved solids. Ground 

water in the alluvium at the lower end of the flow path 

contains 1,040 mg/L dissolved solids or more than twice 

the amount contained at site 1. Although some additional reaction may occur between the ground water 

and tuffaceous detritus in the alluvium, the principal 

mechanism appears to be an increase in chemical concentration caused by a decrease in the volume of 

ground water owing to the immediate proximity of the 

water table to the atmosphere and soil zone. The decrease is probably the result of both direct evaporation 

and transpiration through the vegetation cover.

CHEMICAL CHARACTERISTICS CONTROLLED 

BY THE TUFFACEOUS AQUIFER

Although much of the ground water in the alluvium 

may have undergone concentration increases and reactions due to evapotranspiration, the chemical composition of all ground water in Oasis Valley is assumed to 

generally reflect a solute composition determined by 

interaction with solid phases of the tuffaceous rock. 

This interaction consists of dissolution and hydrolysis 

of primary mineral phases and precipitation of secondary minerals.

EQUILIBRIUM RELATIONSHIPS

The percentages of primary mineral phases vary 

in the tuffs with changes in bulk chemistry and in degree of devitrification. The most common minerals 

are glass, alkali feldspar, plagioclase, quartz; lesser 

amounts of biotite, clinopyroxene, and opaque oxide 

occur (Cornwall and Kieinhampl, 1964; Lipman, and 

others 1966). The principal alteration minerals, which 

formed as a result of ground-water action, include clay 

minerals, zeolites, cristobalite, and possibly potassium 

feldspar. The clay minerals are principally montmorillonite and montmorillonite-illite mixed-layer clays.

The zeolites are primarily of the sodium-potassium 

types, dominantly clinoptilolite and analcime and, to a 

lesser extent, mordenite and chabazite (Hoover, 1968).

Table 4 shows the mean average thermodynamic 

saturation of both alluvial and tuffaceous ground waters with respect to a number of silicate minerals observed in the tuffaceous rocks of Oasis Valley. The 

term, IAP/KS, defines the ratio of the actual aqueous 

ionic activities products (IAP) to the expected solubility products in equilibrium with a given mineral 

phase (Ks). If IAP/KS is less than unity, the ground 

water is undersaturated and will tend to dissolve a 

particular phase. If IAP/KS is greater than unity, the 

water is supersaturated and may tend to precipitate a 

given phase out of solution. Whether a mineral does 

indeed form depends both on its stability with respect 

to other minerals and kinetic factors influencing 

nucleation and precipitation. The ground water is 

supersaturated with respect to the potassium feldspar, 

adularia, which has been observed as a ground-water 

alteration product (Hoover, 1968). The water is undersaturated in relation to albite and anorthite, and two 

end members of the plagioclase solid solution. Plagioclase is found only as a primary mineral. The ground 

water is also supersaturated in terms of three clay 

minerals; kaolinite, montmorillonite, and illite. The 

aqueous silica concentration displays near-saturation 

with respect to silica gel and supersaturation with respect to both cristobalite and quartz. Although data is 

not given in table 4, several of the springs emanating 

from tuffaceous rocks in the east central part of Oasis 

Valley are also saturated with calcite.

Table 4 is incomplete owing to the lack of thermodynamic data for a number of zeolite phases. Analcime, one zeolite for which solubility data exist, is undersaturated by more than two orders of magnitude. 

Such a high degree of undersaturation is of interest 

because analcime occurs extensively in some altered 

tuffs adjacent to Oasis Valley to the north and east 

(Hoover, 1968). Lack of saturation suggests that ongoing zeolitization, as documented by Benson (1978),

TABLE 4. Comparison between mean ionic-activity products of

Oasis Valley ground water and solubility products

of selected silicate phases

Mineral

Ionic Activity 

Product (LAP)

Solubility

Product (KJ

at 25°C

IAP/K,

Adularia ____1.2xlQ- 2o 2.7xlQ-21

Albite_______5.2xlO~19 l.OxlQ- 18

Anorthite ______9.8xlO-22 3.8xlQ-20

Kaolinite ___-_2.0xlO-37 1.2xlQ- 37

Montmorillonite S.OxlQ-28 2.0xlQ- 30

Illite __________3.8xlO-4o 5.4xlO~41

Potassium-Mica 5.5xlO~51 7.9xlQ- 50

Analcime ____1.6xlQ- 15 2.0xlQ- 13

Silica gel ____9.1xlQ-4 9.6xlO~4

Cristobalite __9.1 xlO'4 2.6xlO~4

Quartz _____9.1 xlO" 4 9.9xlO~5

4.4

.52

.026

1.7

250

7.0

.070

.0080

.95

3.5

9.2
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA £9

probably occurs in localized geochemical environments 

that are not reflected in the average composition of 

ground water entering Oasis Valley. The existence of 

such localized environments is supported by the work 

of Gibbons, Hinrichs, and Botinelly (1960), who found 

that many of the zeolites in southern Nevada occur 

above clay zones and some densely welded tuffs. These 

workers suggest that the confining effect of impermeable layers slow the rate of flushing of solute species 

from the aquifer. The increase in aqueous concentrations as a result of continuing reaction with the tuffaceous rocks causes zeolitization. At least in the case 

of analcime, most of the ground water contained in 

Oasis Valley never reaches a composition that permits 

zeolitization.

Although individual minerals listed in table 4 may 

be saturated or supersaturated with respect to the 

ground water, their existence in the natural system is 

not guaranteed because stability with respect to other 

solid phases is not considered. Activity diagrams developed by Garrels and Christ (1965), Helgeson, 

Brown, and Leeper (1969), and other workers depict the 

stability fields of mineral species coexisting with varying aqueous solutions. Compositions for the Oasis Valley ground water are plotted for the systems K2 O - 

MgO - H2 O - A12 O3 , Na2 O - MgO - H2 O - A12 O3 , 

and CaO - MgO - H2 O - A12 O3 in figure 4. Such 

stability diagrams are based on the free energy of formation for each solid phase considered.

Previous literature has cited the free-energy data for 

montmorillonite as estimated through indirect means 

by Helgeson (1969), Hemley, Meyer, and Richter (1961) 

and Marshall (1949). Those estimates were based on 

the assumption that the stability of montmorillonite 

is dependent on the nature of the absorbed ion (i.e., 

sodium montmorillonite, calcium montmorillonite, 

etc.). Recently, Kittrick (1971a, b) has shown that the 

solubility of montmorillonite depends upon ions common to the crystal structure and upon the magnitude of 

the exchange capacity, but it is independent of the type 

of exchangeable ion and its activity in solution. Kittrick measured experimentally the free energy of formation and the solubility products for montmorillonites from Aberdeen, Mississippi, and Belle Fourche, 

South Dakota. Table 5 shows selected chemical components normalized to silica for Kittrick's montmorillonites and two montmorillonites that occur in or adjacent to Oasis Valley; the latter are from the Grouse 

Canyon Member of the Belted Range Tuff (Indian Trial 

Formation of former usage) and the Ammonia Tanks 

Member of the Timber Mountain Tuff (Carr, 1974). The 

principal variations present in table 5 are due to substitutions of magnesium and iron for aluminum in the 

octahedral coordination sites and the replacement of

silica by aluminum in the tetrahedral sites. These substitutions are responsible for the charge imbalance 

which is neutralized by absorbed cations. A comparison 

between the montmorillonite from Aberdeen, 

Mississippi (Kittrick, 1971a, b) and the montmorillonites of the Grouse Canyon and Ammonia Tanks 

Members show that aluminum and magnesium are 

only slightly deficient in the latter samples. This deficiency may be partly due to the presence of unaltered 

silica glass in the samples. Montmorillonites of the 

Grouse Canyon and Ammonia Tanks Members show 

less iron by a factor of two in comparison to the Aberdeen sample. If montmorillonites of the Grouse Canyon 

and Ammonia Tanks Members represent the average 

composition of montmorillonite formed by Oasis Valley 

ground water, an assumption must be made that the 

substitution between aluminum and iron in the quantities shown in table 5 does not affect the free energy of 

formation of the Aberdeen montmorillonite. The degree of magnesium substitution in the montmorillonites of the Grouse Canyon and Ammonia Tanks 

Members is assumed to be equal to that found in the 

Aberdeen montmorillonite. A decrease in the iron concentration is balanced by an increase in aluminum. 

The charged montmorillonite structure is assumed to 

be balanced by hydrogen-ion absorption which permits 

expression of the chemical activities as ratios in terms 

of hydrogen-ion activities. Because the type of adsorbed cation does not greatly affect the free energy of 

formation of the montmorillonite structure (Kittrick, 

1971a, b), the exchange of hydrogen ion for sodium, 

potassium, or calcium should not significantly change 

the stability relations. The equation below, which defines the stability boundary between a montmorillonite 

of the average composition of samples from the Grouse 

Canyon and Ammonia Tanks Members and potassium 

feldspar, is used as an example. An equilibrium expression between the two minerals can be written in terms 

of the logarithms of the aqueous and solid activities 

and an equilibrium constant.

6.31 log aRAlSisOs (feldspar) + log EH+ +0.61 log ERJ+++

+ 1.73 log aMg++ + 12.61 log aHao - 6.31 log aK+

- 4.23 log a^sio*

- 3.81 log aH^Mg«Ftea6 Al

(Montmorillonite) 1.688

The expression on the right hand side of the equation is 

the logarithm of the equilibrium constant. Assuming 

that the activities of the solid phases and water are 

unity, the above expression can be rearranged to give:

1.73 log HT-+0.61 log b a2H+

- 6.31 log 6 an+

- 4.23 log a^sio* = 1.688
E10 HYDROLOGY OF NUCLEAR TEST SITES

In the above equation as well as in the other stability 

expressions, the iron-to-hydrogen activity ratio and the 

silica activity are assumed to equal constants denned 

by the average solute concentrations of the Oasis Valley ground water. Substituting 17.66 for log Fe+++ a3H+

and 3.04 for log a^sicu into the above equation gives 

the relationship:

1.73 log ^-6.31 log ^=21.943.

Variables in this equation are denned as coordinates 

in the first diagram of figure 4A. The resulting linear 

plot represents the stability boundary between montmorillonite and adularia as a function of varying magnesium, sodium, and hydrogen activities. Other stability boundaries shown in figure 4 are derived in an 

analogous manner.

Solid lines in figure 4 define the stability field for a 

montmorillonite with an iron concentration equal to 

the average concentration found in the montmorilTABLE 5. Comparisons of selected chemical components normalized to

silica for montmorillonites from tuffaceous rocks that are

used in stability calculations

Montmorillonites

Aberdeen, Miss1

Belle Fourche, S. D.2 __

Oasis Valley, Nev. 3 : 

Grouse Canyon Member of the 

Belted Range Tuff ___ ____ _ __

Ammonia Tanks Member of the 

Timber Mountain Tuff

atoms per unit cell

Mg

 0.45

___ .28

___ .42

___ .38

Fe

0.34

.23

.17 

.14

Al

1.49 

1.53

1.39 

1.29

Si

3.82 

3.82

3.82 

3.82

'Kittrick, 1971b. 

2Kittrick, 1971a. 

3Carr, 1974.

lonites of the Grouse Canyon and Ammonia Tanks 

Members. The more extensive montmorillonitestability field represented by dashed lines is for an iron 

concentration equivalent to the Aberdeen montmorillonite while the smaller field defined by dashed and 

dotted lines is for a montmorillonite that contains no 

iron. At a specific aqueous-iron concentration, the stability fields of montmorillonites containing succesLOG a K +/ a H+ LOG Q NaVa H+ 

B

LOGaCa++/a H

FIGURE 4. Aqueous activity diagrams depicting montmorillonite stability.
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA Ell

sively more iron expand at the expense of other minerals that do not contain iron.

Regardless of the lattice-iron concentrations, however, figure 4 depicts montmorillonite as the stable 

phase in equilibrium with Oasis Valley ground water. 

The stability of montmorillonite as a weathering 

product is documented by its occurrence in the tuffs 

surrounding Oasis Valley as well as in other tuffaceous 

rocks (Sheppard and Gude, 1968, 1973). Kaolinite occurs at lower ionic-activity ratios (fig. 4), and illite is 

unstable under all the conditions shown in the diagrams. Some of the clays in the tuff formations of Oasis 

Valley are described as mixed-layer minerals which 

represent a presumably continuous gradation between 

montmorillonite and illite. Reliable thermodynamic 

data exist only for the two end-member clay minerals; 

therefore, stability fields for mixed-layer clays, though 

not shown in figure 4, may represent stable phases.

Because of the lack of thermodynamic data, the stability fields for a number of zeolites commonly found in 

the tuffaceous rocks surrounding Oasis Valley (Hoover, 

1968) are missing from the diagrams. The metastable 

boundary between analcime, the only zeolite for which 

data exist, and an iron-free montmorillonite is shown 

for the Na2O - MgO - IrbO - A^Os system in figure 4 

as a dotted line.

Other zeolites such as clinoptilolite and mordenite 

have higher ratios of silica to aluminum in their structures than does analcime. These zeolites would have 

stability boundaries with slopes similar to that of analcime, but because of the relatively high silica concentrations found in Oasis Valley ground water, the 

boundaries would be shifted toward the upper lefthand corner of the diagrams in figure 4. These boundaries may encroach upon the montmorillonite stability 

field. The metastable analcime-montmorillonite 

boundary qualitatively demonstrates that water containing moderate amounts of magnesium leads to 

montmorillonite formation, whereas water containing 

sodium and potassium but deficient in magnesium favors analcime, clinoptilolite, and mordenite formation. 

Some water samples taken from tunnels driven into 

zeolitic tuffs at the adjacent Nevada Test Site have 

very low concentrations (<5 Xl0~6 moles/liter) of magnesium and may be in equilibrium with respect to zeolite minerals.

The variability in hydrogen activities appears to be a 

controlling factor in the linear data distributions for 

Oasis Valley ground water shown in figure 4. This dependency is only apparent because the maximum and 

minimum activities of magnesium, sodium, and potassium, when divided by hydrogen activities, do not 

necessarily correspond to maximum and minimum 

ratios shown in the stability diagrams. Also, the distribution of data in the magnesium-sodium system, and 

to a lesser extent in the magnesium-potassium system, 

have slopes of approximately 2 to 1. These slopes are 

the same as the logarithmic ratio of hydrogen activities 

that occur in the denominators of the two coordinates, 

log aMg++/a2H+ versus log aNa+/an+ and of the coordinates, log aMg++/a2H+ versus log aK+/an+. This variability in hydrogen-ion activities is expected because 

ground water in the alluvium is subjected to varying 

degrees of atmospheric and soil-zone interactions that 

affect the carbon-dioxide partial pressures and the pH. 

The distribution of data in the magnesium-calcium 

system, as shown in figure 4C, is controlled principally 

by equilibrium with calcite.

SOURCES OF DISSOLVED SPECIES

SILICA

As shown by the equilibrium data in table 3, the 

aqueous concentration of silica is apparently controlled 

by equilibrium with silica gel. This relationship is supported by silica gel observed in fractures in tuffaceous 

rocks. From thermodynamic considerations, silica gel 

represents a metastable phase in relation to both 

quartz and cristobalite. Secondary low cristobalite and 

quartz (Ransome, and others, 1910; Hoover, 1968) 

within the groundmass and fractures of the tuffs probably represent subsequent crystallization of silica gel.

A median value for silica concentrations in ground 

water is 0.6 millimole per liter (Davis, 1964) compared 

with the average concentration of 2.5 millimoles per 

liter silica in Oasis Valley ground water. Similarly high 

concentrations are found in ground water derived from 

other tuffaceous rocks (White and others, 1963).

The principal silicate phases that are present in the 

tuffs and that could be adequate sources for silica are 

feldspars, quartz, and volcanic glass. Feth, Roberson, 

and Polzer (1964) showed that the principal source of 

silica in ground water from granitic rocks in the Sierra 

Nevada, the bulk chemical composition of which is 

similar to the tuffs, was the dissolution of plagioclase 

and, to a lesser extent, of potassium feldspar. The silica 

concentration of these waters averaged one-third of the 

concentrations found in the Oasis Valley ground water. 

This difference suggests that volcanic glass and not 

feldspars is the major contributor of high silica concentrations in the Oasis Valley ground,water. Jones (1966) 

and other workers have recognized that glasses, which 

are much more soluble than crystalline silicate minerals, can produce high silica concentrations in ground 

water.

Water can alter volcanic glass by hydration and 

leaching. Both of these processes can contribute silica 

to the aqueous system. Assuming constant aluminum
E12 HYDROLOGY OF NUCLEAR TEST SITES

concentrations, Lipman (1965) estimated that volcanic 

glass in the tuffs of southern Nevada had been leached 

of several percent more silica than had the crystalline 

rocks of the same composition and origin.

SODIUM, POTASSIUM, CALCIUM, AND MAGNESIUM

The source of the major cations in the ground water 

in the tuffaceous aquifer must be potassium-feldspar, 

plagioclase, and volcanic glass. Comparison between 

Oasis Valley ground water and water derived from the 

springs in Sierra Nevada rocks with about the same 

bulk composition (Feth, and others, 1964) shows a 

major difference in solute compositions. In the case of 

water from the granitic terrane, sodium and calcium 

are roughly equal as dominant ions whereas water 

from the Nevada tuffs exhibits a sole dominance of 

sodium, which comprises roughly 90 percent of the cations present.

The dissolution of plagioclase in the granitic rocks of 

the Sierra Nevada is well documented as the prime 

source of aqueous sodium and calcium by Garrels 

and MacKenzie (1967). Although the plagioclase in 

the tuffaceous rocks is slightly deficient in calcium 

(AN25 ANso) as compared with the plagioclase in 

the granitic rocks (AN26-AN4o), breakdown of plagioclase is unlikely to produce the much higher sodiumto-calcium ratios found in Oasis Valley ground water.

A major source of high aqueous sodium concentrations appears to be volcanic glass. Table 6 shows a 

comparison between the relative molar percentages of 

sodium, potassium, calcium, and magnesium found in 

the glass and aqueous phases. The composition of the 

glass represents an average of three analyses reported 

by Lipman (1965) for tuff formations found in or around 

Oasis Valley. The aqueous composition is the average 

analyses for ground water in the tuffaceous aquifer of 

Oasis Valley not saturated with respect to calcite.

As is apparent from table 6, the Oasis Valley ground 

water cannot be described by a simple congruent dissolution of the glass phase. The percent of sodium is increased by a factor of two in the aqueous state, magnesium is decreased by a factor of two, and potassium is 

decreased by a factor of fourteen.

Relationships between the glass and aqueous phases 

shown in table 6 have been predicted by other workers. Lipman (1965), in a chemical comparison of volcanic rocks of southern Nevada, showed that hydrated 

glasses are characterized by higher K2O/Na2O ratios 

than crystallized rocks. As an approximation, Lipman 

suggested that about 0.5 percent Na2O has been removed by hydration and leaching of the glass phase 

while KaO remains relatively immobile. Truesdell 

(1966) determined the ionic-exchange constants of sevTABLE 6. Comparisons of relative molar percentages of

selected cations found in the volcanic glass

and Oasis Valley ground water

Volcanic 

glass'

Ground 

water2

Ca.

Na. 

Mg. 

K .

46

4

42

87

2

3

'Average of three analyses (Lipman, 1965) for tuffs in or around Oasis Valley. 

"Average of all water analyses (this paper).

eral natural and synthetic glasses by use of an electrode method, and he found that glasses preferentially 

absorbed hydrogen, potassium, and, to a lesser extent, 

calcium relative to sodium. He predicted that waters in 

the vitric tuffs would be sodium-rich.

BICARBONATE

The dominant anion present in the tuffaceous 

aquifer is bicarbonate. The occurrence of carbonate 

minerals is minimal in the tuffaceous rocks (Lipman 

and others, 1966). The principal source and control of 

bicarbonate is the reaction of dissolved, soil-zone carbon dioxide with various mineral phases. In the case of 

glass, Budd (1961) has demonstrated that in neutral 

solutions similar to Oasis Valley ground water, electrophilic reactions occur in which hydrogen enters the 

glass structure as hydronium ion and exchanges with 

sodium or other cations bonded to lattice oxygen atoms. 

As hydrogen atoms are consumed in this exchange 

reaction, dissolved carbon dioxide will disassociate to 

produce bicarbonate ions.

FLUORIDE AND CHLORIDE

Ratios of fluoride to chloride in tuffaceous rocks 

northeast of Oasis Valley vary between 0.5 and 2.0 

(Noble and others, 1967). Ground water entering Oasis 

Valley through such rocks has ratios generally an order 

of magnitude less. This lack of fluoride mobility in 

comparison to chloride agrees with observations that, 

during alteration of volcanic glass, fluoride remains in 

the solid phase. Noble and others (1967) compared 

fluoride and chloride contents of a number of nonhydrated and hydrated glasses of the same origin and 

found that four-fifths of the chloride and less than half 

of the fluoride were lost during hydration.

CHEMICAL CHARACTERISTICS CONTROLLED 

BY THE ALLUVIAL AQUIFER

CONCENTRATION INCREASES

Figure 5 illustrates trends in concentration as a 

function of a distance of approximately 24 kilometers
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E13

1.3 

1.2 

I.I 

1.0

0.9

 0.8 

O

x 0.7 

K 

< 0.6

O

s 0.5

0.4 

0.3 

0.2 

O.I 

0

KILOMETERS 

10 15 20

"02$

13.0 

12.0 

II.0 

10.0 

9.0

x

>~

< 6.0

O

5 5.0

4.0 

3.0

2.0 

1.0

KILOMETERS 

10 15 20

I i i i i

5 10 

MILES

15 5 10 

MILES

15

Sample numbers are keyed to tables 1,2, and 3 

FIGURE 5. Changes in concentration of dissolved chemical constituents as a function of distance along the alluvial flow path.

(15 miles) along the Thirsty Canyon-Oasis Valley flow 

path. The data on the far left-hand side of the figure are 

from the northernmost sampling point (site 1) located 

near the mouth of Thirsty Canyon. The data on the far 

right-hand side are from the southernmost point (site 

29) in the flow path located at the outlet of the Amargosa Narrows.

Sodium, bicarbonate, fluoride, and chloride show 

rapid increases in concentrations along the initial 8 

kilometers (5 miles) of the flow path in the alluvium. 

Throughout the remaining distance, except for the 

final sampling point, these species show a more 

moderate increase. Calcium and magnesium display 

even greater concentration increases along the initial 

part of the flow path; silica and potassium show only 

minor enrichment.

RATIOS OF SOLUTE COMPONENTS

SODIUM, BICARBONATE, AND CHLORIDE

Figure 6 shows the aqueous concentrations of bicarbonate and chloride plotted against sodium. Included 

in the diagrams are data for ground water in the tuffaceous and alluvial aquifers of Oasis Valley. Numbered data points correspond to samples listed in tables 

1, 2 and 3. Also shown in figure 6 are additional 

geochemical data for ground water from the tuffaceous 

aquifer under Pahute Mesa (Blankennagel and Weir, 

1973) and Gold Flat (unpublished data from files of 

U.S. Geological Survey). The latter two areas represent 

probable recharge areas for Oasis Valley ground water.

The most significant observation is that nearly all 

the data presented in figure 6 plot on the same linear
E14 HYDROLOGY OF NUCLEAR TEST SITES

ro 

O

9.0

8.0

7.0

6.0

UJ

S

I 50 

or

o

5 4.0

3.0

2.0

1.0

0.0

Thirsty Canyon - 

Oasis ValleySpringdale

A

0.0 I.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 IO.O I I.O 12.0 13.0 14.0

MOLALITY SODIUM x I03

0.0 I.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 IO.O 11.0 12.0 13.0 14.0 

3.5

3.0

ro 

O

2.5

UJO

a: 2.0

>- 

H

<

I. 5

I.O

0.5

0.0

Thirsty Canyon - 

Oasis Valley

A

Indian Springs 

15

20 Springdale

EXPLANATION

Oasis Valley alluvial aquifer

/^\

( } Oasis Valley tuffaceous aquifer 

.. 

/\ Pahute Mesa

D Gold Flat

(*) ^ Variation in concentration along specified 

alluvial flow path

 Average concentration trend for all data

Sample numbers are keyed to tables 1,2, and 3

FIGURE 6. Concentration trends of sodium, bicarbonate, and chloride.
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E15

trends. The data for the tuffaceous aquifer, both in the 

immediate vicinity of Oasis Valley as well as for Pahute Mesa and Gold Flat, plot in the more dilute region 

of the diagrams. Alluvial water compositions represented for specific flow paths in figure 6 range from 

overlapping the dilute region at the initial stages along 

the flow paths to considerably higher concentrations 

farther downgradient. The similarity in compositions 

supports the suggestion of Blankennagel and Weir 

(1973) that water beneath Pahute Mesa is chemically 

related to Oasis Valley ground water. The linearity of 

the data suggests that the water in the alluvium can be 

produced from tuffaceous compositions by concentration increases due to evapotranspiration. Sodium and 

chloride appear to be neither selectively added nor removed from the water during this process. Some variations in bicarbonate composition may be expected due 

to local dissolution or precipitation of carbonate minerals. The Springdale flow path appears anomalously 

high in bicarbonate, probably due to the dissolution of 

locally occurring Paleozoic carbonates (fig. 2). The 

slightly larger increase in bicarbonate along the initial 

part of the Thirsty Canyon-Oasis Valley system (fig. 6) 

may also represent dissolution of carbonate minerals 

in the alluvium, and the slight decrease along the final 

portion may represent precipitation after calcite saturation has been achieved. However, bicarbonate, like 

sodium and chloride, seems to be principally controlled 

initially by interaction with tuffaceous rocks and by 

later modification due to evapotranspiration.

Figure 7 shows the relationship between bicarbonate 

concentrations and the partial pressure of carbon 

dioxide in equilibrium with Oasis Valley ground water 

calculated from the expression

log Pco2 = log aHCO, - pH - log Ks

where log PCOZ and log aH coa ~ are the logarithms of the 

partial pressure of carbon dioxide (PcoJ and the activity of bicarbonate respectively. Log Ks is the 

logarithm of the solubility constant. Pco2 increases 

rapidly along the Thirsty Canyon-Oasis Valley and the 

Springdale flow paths relative to that associated with 

ground water in the tuffaceous aquifer. Increases in 

Pco2 with increasing bicarbonate are reverse to the relationship previously discussed for the tuffaceous 

aquifer, in which bicarbonate is produced and CO2 is 

consumed during the reaction with silicate minerals in 

a closed system. Rapid increases in dissolved CO2 in 

the alluvial aquifer suggest that the hydrologic system 

interacts with the atmosphere through the root-zone 

transpiration of CO2 by the relatively dense vegetation 

along the valley bottom.

-i .o

-2.0

-2.2

UJ

CC

S -2-« 

CL 

CO 

O

s _? fi 

<r

CJ

0 -2.8

CL

O -3.0

I

-3.2

-3.4

i i i * i i i

***"""-

^'^£&L 29

 \Q&^^ + * s/].f!\7W28

SPRINGDALE 5 ^<jp< x\ V

^1 f ' 5 \ ^1 6

^ 3 7 ®12 THIRSTY CANYON' 

015 / / n® OASIS VALLEY

/ /

/ *

f 230^24

^ 26 ©9 ®20

/ / 025

^ /

1 /

1 /

f / INDIAN SPRINGS

1 /

1 /

i 1 ®7i i i . > i

01 234 56 78

26,

HCOg MOLES * 10 

EXPLANATION

Oasis Valley alluvial aquifer 

7 © Oasis Valley tuffaceous aquifer 

-» Data points along specified flow path 

mm mm um General trend of increases in Oasis Valley

Sample numbers are keyed to tables 1,2,and 3

FIGURE 7. Progressive increases in PC02 with increasing 

bicarbonate.

POTASSIUM AND FLUORIDE

Figure 8 shows the aqueous concentrations of potassium and fluoride plotted against sodium. No data exist 

for fluoride concentrations in ground waters of Pahute 

Mesa and Gold Flat. Both potassium and fluoride show 

concentration increases in the alluvial aquifer but at a 

progressively decreasing rate relative to sodium. Assuming that sodium concentration increases reflect 

relative volume decreases of ground water by evapotranspiration, the decrease of potassium and fluoride 

concentrations relative to sodium suggests that their 

absolute amounts in the aqueous system are being diminished.

The mechanism for the removal of potassium is not 

certain. Table 4 shows that the average aqueous system is saturated by a factor of more than two with 

respect to potassium feldspar. Proven difficulties in 

nucleation (Berner, 1971) plus the instability of potassium feldspar with respect to montmorillonite (fig. 4) 

tend to discount potassium feldspar as a likely sink for 

potassium. A possible mechanism for removing potassium is by absorption and incorporation into the lattice
E16 HYDROLOGY OF NUCLEAR TEST SITES

4.0

3.5

10 

O

x

LJ

9

a: 

o

ID

2.5

2.0

1.0

0.5

0.0

0.0

3.0°-°

Thirsty Canyon-Oasis Valley

29 21'

Indian Springs

I_____I_____I_____L

I.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 IO.O II.O I2.0 I3.0 I4.0

MOLALITY SODIUM x I03

I.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 IO.O II.O I2.0 I3.0 I4.0

ro

2 2.5 

x

5

| 2.0

CO

O 1.5 Q_

0.5

0.0

'28

>18 

Thirsty Canyon-Oasis Valley

j____i i____L

EXPLANATION

26, Oasis Valley alluvial aquifer 

( ) Oasis Valley tuffaceous aquifer 

/\ Pahute Mesa

Gold Flat

 ^ Variation in concentration along specified 

alluvial flow path

 Average concentration trend for all data

Sample numbers are keyed to tables 1,2, and 3

FIGURE 8. Nonlinear average concentration trends of fluoride and potassium relative to sodium.
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E17

of clay minerals. Although large quantities of clay 

were deposited in the alluvium of Oasis Valley under 

subaerial and lacustrine conditions by erosion and 

weathering of the surrounding tuffs (Malmberg and 

Eakin, 1962), little is known concerning their specific 

mineralogy. However, degraded illites and montmorillonites, which are common clay minerals in most arid 

soil zones, adsorb and fix potassium. These minerals 

are known weathering products of the tuffs in and 

around Oasis Valley. Also the vegetation cover in Oasis 

Valley may remove significant amounts of potassium. 

As will be shown in the section entitled "Equilibrium 

Relationships," the mineral fluorite appears to be the 

major sink for the removal of fluoride from the ground 

water in the alluvium.

CALCIUM AND SULFATE

Both calcium and sulfate show anomalous increases 

in relation to sodium, chloride, and bicarbonate that 

cannot be explained by increases due to evapotranspiration. Figure 9 shows the concentrations of calcium 

and sulfate plotted against sodium. Along the initial 

part of the main flow path labeled Thirsty CanyonOasis Valley, the calcium concentration increases by a 

factor of four, which is double the increase observed for 

sodium along the entire system. Figure 10 shows a 

comparison between calcium, sodium, and magnesium 

distributions for the Thirsty Canyon-Oasis Valley and 

Springdale flow paths. Even higher calcium percentages in the Springdale system are apparent. This 

high calcium content, coupled with the anomalously 

high bicarbonate-to-sodium ratios shown in figure 6, 

suggest that calcium carbonate is being dissolved in 

the Springdale system. The source of calcium carbonate is probably in the Bullfrog Hills. Carbonate rock 

crops out locally in the northern Bullfrog Hills west of 

Springdale. Ransome, Emmons, and Garrey (1910) also 

reported calcite in fractures and mineralized veins 

within the hydrothermally altered zones of the Bullfrog Hills. The calcite displays cavernous structure 

indicating dissolution by ground water. Elsewhere, 

such as in the Thirsty Canyon-Oasis Valley system, 

both calcium carbonate and calcium sulfate may occur 

in lacustrine beds within the alluvium.

The distribution of sulfate plotted against sodium is 

also shown in figure 9. The slope of the sulfate-tosodium ratio is greatest in both the tuffaceous and alluvial aquifers of Oasis Valley relative to the Pahute 

Mesa and Gold Flats data. The slope is also greater for 

sulfate to sodium than for bicarbonate or chloride to 

sodium in the flow system in the alluvium. Like calcium, an additional source of sulfate must be contributed to the Oasis Valley ground water. Unlike calcium, 

however, figure 10 shows that the highest percentage of 

sulfate relative to bicarbonate and chloride in the alluvial aquifer is concentrated in the main Thirsty 

Canyon-Oasis Valley flow path.

The primary source of most of the additional sulfate 

seems to be hydrothermal. Hot Springs (sample site 

13), a major thermal (41 °C) spring in the area, shows an 

abnormally high sulfate-to-chloride ratio. Also, the 

mineralogy of the hydrothermally altered tuffaceous 

rocks in the Bullfrog Hills suggests that sulfate may 

have been contributed locally from sulfide minerals. 

Although pyrite is not now abundant near the surface, 

the large amounts of limonite (Ransome and others, 

1910) suggest that pyrite has been oxidized with the 

concurrent production of sulfate.

EQUILIBRIUM RELATIONSHIPS

As discussed in the section dealing with equilibrium 

relationships, the ground water in the tuffaceous aquifer is stable with respect to silica gel, montmorillonite, 

and possibly mixed-layer clay minerals. When the 

ground water moves from this aquifer into the alluvial 

fill of Oasis Valley, stability is maintained between this 

mineral assemblage and the aqueous phase. Silica concentrations remain constant throughout the flow system in the alluvium and appear to be continually controlled by silica-gel equilibrium. Montmorillonite stability appears to be maintained with increasing solute 

concentrations as the reaction paths advance across 

the montmorillonite stability field toward the chlorite 

boundary (fig. 4).

However, increasing chemical concentrations in the 

ground water of the Oasis Valley alluvium produce 

saturation with respect to several mineral phases in 

addition to those generally associated with the tuffaceous aquifer. Figure 11 shows the ionic-activity product 

of calcium fluoride plotted against the ionic-activity 

product of calcium carbonate. The figure also includes 

the stability fields for calcite and fluorite. Fluorite 

solubility is that given by Smyshlyaev and Edeleva 

(1962). Note that all ground-water samples associated 

with the tuffaceous aquifer are undersaturated with 

respect to fluorite and that the majority of samples are 

undersaturated with respect to calcite. Those waters 

that are calcite-saturated are confined to springs 

emanating along the east-central side of Oasis Valley 

and may be associated with carbonate deposition during local hydrothermal alteration of the tuffaceous 

rocks (Ransome and others, 1910).

Ionic-activity products for both calcium carbonate 

and calcium fluoride become progressively greater
E18 HYDROLOGY OF NUCLEAR TEST SITES

2.50

2.25

2.00

O 1.75

x

UJ

< 

O

1.50

1.25

1.00

0.75

0.50

0.25

i F

8.

Thirsty Canyon13 Oasis Valley

22

I_____I I_____I

0.25

ro 

O

x 0.20

5

2

y O.I5

< 

O

> O.IO

U

3 0.05

0.0

I.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 IO.O II.O I2.0 I3.0 14.0

MOLALITY SODIUM x I03

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0

Indian Springs

fr A

Springdale

A

Thirsty Canyon5 Oasis Valley

28

29

22 .21

EXPLANATION

9 Oasis Valley alluvial aquifer 

( ) Oasis Valley tuffaceous aquifer 

/\ Pahute Mesa 

Gold Flat

( ) £ Variation in concentration along specified 

alluvial flow path in Oasis Valley

 Average concentration trend for Pahute Mesa 

and Gold Flat data

Sample numbers are keyed to tables 1,2,and 3

FIGURE 9. Average concentration trends of sulfate and calcium relative to sodium.
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E19

downgradient along the Thirsty Canyon-Oasis Valley 

and Indian Springs alluvial flow paths. Ionic-activity 

products increase until they intersect the stability 

fields of both calcite and fluorite. Beyond this point in 

the flow path, the ground water shows only minor and 

erratic variations in the ionic-activity products. This 

would indicate that calcium carbonate and calcium 

fluoride are being precipitated from the alluvial system 

as the volume of ground water is being reduced by 

evapotranspiration. Ground water originating from

/ Thirsty CanyonOasis Valley

50%

50% CL~

50%

HC0 3 

100%

50%

Indian Springs in the Bullfrog Hills shows a rapid increase in the ionic-activity product of calcium carbonate but only a minor increase in calcium fluoride. This

-10.2

-10.4

-10.6

-10.8

-11.0

-11.2

-11.4

8 -

0.

O -M.8 

O

-12.0

-12.2

-12.4

-12.6

-12.8

-13.0

FLUORITE SATURATION

13,

\19

Thirsty CanyonOasis Valle

UNSATURATED AQUEOUS SOLUTION

®

,15

Indian Spring

.26

FIGURE 10. Triangular diagrams depicting relative concentrations 

of selected cations and anions.

-I3.2

-9.4 -9.2 -9.0 -8.8 -8.6 -8.4 -8.2 -8.0

LOG IAP CaCo 3

EXPLANATION

26^ Oasis Valley alluvial aquifer 

15 ® Oasis Valley tuffaceous aquifer

Sample numbers are keyed 

to tables 1,2, and 3

FIGURE 11. Ionic-activity products of calcium fluoride compared to 

calcium carbonate. Saturation values are at 25°C.
E20 HYDROLOGY OF NUCLEAR TEST SITES

rapid saturation with respect to calcite but not fluorite 

suggests again that calcium carbonate is being dissolved along the Indian Springs flow path.

Figure 12 shows the approximate geographic distribution of the saturation boundaries for calcite and 

fluorite. Equilibrium with respect to calcite occurs in 

the lower three-fourths of the main alluvial flow path 

that originates at the mouth of Thirsty Canyon. 

Equilibrium is also achieved throughout the Springdale flow path and the lower part of the Indian Springs 

flow path. Both of these systems have recharge areas 

in the Bullfrog Hills and show abnormally high calcium carbonate concentrations as compared with the 

dissolved-solids concentrations. Fluorite saturation is 

limited to a narrow strip in the lower center of the 

valley; it coincides with the area in which the ground 

water would be subjected to maximum evapotranspiration. Most of the springs issuing from the tuffaceous 

aquifer, with the exception of those in the central part 

of Oasis Valley, are not saturated with calcite or fluorite.

The concentration of aqueous magnesium also appears to be controlled by an equilibrium relationship 

although the exact mechanism is less well defined than 

for calcium and fluoride. Initially, the percentages of 

magnesium increase downgradient in the water in the 

alluvium at a much faster rate than do the percentages 

of sodium, chloride, and bicarbonate (fig. 5). This rapid 

increase suggests that, like calcium, magnesium is 

being added to the system from an additional source. 

Figure 13 shows the ionic-activity product of magnesium carbonate plotted against calcium carbonate. 

Included in the figure are the stability fields for calcite, 

magnesite, and dolomite. Both Thirsty Canyon and Indian Springs flow paths display initial rapid increases 

in the ionic-activity products of magnesium carbonate 

and calcium carbonate. However, both systems reach 

steady states downgradient, after which the ionicactivity products show only minor erratic increases 

and decreases. As in figure 11 for the calcium carbonate 

versus calcium fluoride system, such a steady-state 

situation suggests equilibrium conditions between 

aqueous and solid phases. The limiting value for calcium carbonate activities is controlled by calcite equilibrium (fig. 11). The distribution of the data suggests 

that calcium carbonate and magnesium carbonate activities also approach the solubility of dolomite from 

undersaturation. The inability of many, even highly 

supersaturated, waters to precipitate dolomite is well 

documented (Berner, 1971). Therefore, dolomite precipitation as a means of controlling magnesium concentrations in saturated or near saturated systems such as 

Oasis Valley is very improbable. However, waters that 

are dissolving dolomites exhibit solute concentrations 

which approach saturation with respect to dolomite

(Hem, 1970). Figure 13 suggests that the limiting value 

for magnesium concentrations in the Oasis Valley system is based on the ability of the ground water to dissolve dolomite. Once the system is almost saturated 

with dolomite, little more magnesium is introduced 

into the system. This theory implies that dolomite dissolution and not evapotranspiration is the dominant 

control on the maximum magnesium concentrations 

found along the flow paths in the alluvium. Dolomite 

crops out extensively in the hills southwest and southeast of Beatty and is present at depth under the 

Bullfrog Hills (Ransome and others, 1910). Oasis Valley 

ground water could come in contact with these rocks or 

with alluvium derived from them.

The Indian Springs system, which is initially greatly 

undersaturated with respect to both calcite and dolomite, shows a calcium to magnesium slope of 1 to 1 (fig. 

13). This slope would be expected from a congruent 

dolomite dissolution. The Thirsty Canyon system is 

initially much closer to calcite saturation. If dolomite 

reacts similarly to magnesian calcites, as proposed by 

Plummer and Mackenzie (1974), dissolution becomes 

incongruent near calcite saturation. Incongruent dissolution produces dominantly aqueous magnesium carbonate. This mechanism would explain why the 

magnesium-to-calcium ratios in the Thirsty CanyonOasis Valley system are higher than those in the Indian Springs system.

MASS BALANCE OF CHEMICAL SPECIES

As a means of summarizing the behavior of aqueous 

chemical species in the alluvial aquifer, a mass balance 

comparison is made in table 7 between chemical compositions of ground water recharging and discharging 

the alluvium. Line 1 in table 7 lists the chemical composition of ground water at site 29 below the mouth of 

the Amargosa Narrows (fig. 2). Assuming that the 

Paleozoic carbonate basement within the Amargosa 

Narrows is relatively impermeable, all ground water 

discharging from the alluvium in Oasis Valley must 

pass through the 3-4 meter (9-12 ft) thickness of alluvium at this point. Sample 29, immediately downgradient from the Amargosa Narrows should, therefore, be representative of the composition of ground 

water discharging the alluvium.

An estimate of the chemical composition of water 

recharging the alluvium from the tuffaceous aquifer is 

more difficult owing to the number of springs and seeps 

as well as probable subsurface flow. As discussed by 

Malmberg and Eakin (1964) and Blankennagel and 

Weir (1973), most ground water that recharges the alluvium probably moves considerable distances through 

the tuffaceous aquifer from areas north and east of
116° 50' 

37°05'

GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA

45' II6°40'

E21

37°00' -

36°55' -

Vs Q Indian 

\Springs

EXPLANATION

Calcite saturation

Fluorite saturation

o2

Water sample location

Sample numbers are keyed 

to tables 1,2, and 3

Base by U.S. Geological 

Survey. 1954

5 KILOMETERS

FIGURE 12 Geographic distribution of calcite- and fluorite-saturated ground water.
E22 HYDROLOGY OF NUCLEAR TEST SITES

ro 

O 

O 

o 

O

Q.

<

O

-8.0

-8.2 -

-8.4 -

-8.6 -

-8.8 -

-9.0 -

-9.

hirsty Canyon 

Oasis Valley

UNSATURATED AQUEOUS SOLUTION

-10.0 -9.8 -9.6 -9.4 -9.2 -9.0 -8.8 -8.6 -8.4 -8.2

LOG IAP Mg co3

EXPLANATION

£ Alluvial aquifer 

( ) Tuffaceous aquifer

Sample numbers are keyed 

to tables 1.2,and 3

FIGURE 13. Ionic-activity products of magnesium carbonate compared to calcium carbonate.

-8.0 -7.8

Oasis Valley. Chemical composition would be expected 

to be similar at individual points where this regional 

tuffaceous aquifer recharges the alluvium in Oasis Valley. The general similarity in chemical compositions of 

ground water in the tuffaceous aquifer is shown in 

table 2. The principal exceptions are samples 7, 8, and 

9 from springs in tuffaceous rocks to the west of Oasis 

Valley (fig. 3) which represent local recharge from the 

Bullfrog Hills. The average composition of the remaining 11 water samples from the tuffaceous aquifer (table 

3) is, therefore, assumed to represent the chemistry of 

water recharging the alluvium and is shown on line 2 

in table 7.

In the preceding discussion of chemical behavior, 

sodium and chloride appeared to be the two species 

that were neither added to nor removed from the 

ground-water system in the alluvium. The increases in 

sodium and chloride concentrations between lines 1 

and 2 in table 7 must, therefore, reflect the corresponding decrease in the volume of water as the result of 

evapotranspiration. Assuming a volume reduction of 

2.13 overestimates the sodium-discharge concentration

by 0.37 millimole (line 4), or 3 percent (line 5), and 

underestimates the chloride concentration by 0.11 

millimole, or 4 percent.

Evapotranspiration probably affects only the shallow ground water directly adjacent to the root zone and 

atmospheric interface. However, chemical trends along 

the flow paths are preserved irrespective of the sampling depth (0-100 m, 0-300 ft, table 1). Dispersivity 

along the flow path apparently produces chemical 

homogeneity within the zone sampled. If the alluvium 

thickness is more than 660 m (2,000 ft) in the central 

part of Oasis Valley, as suggested by Malmberg and 

Eakin (1964), concentrations may be more dilute at 

greater depth. However, no mixing of more dilute 

ground water with the shallow alluvial water occurs as 

the flow system approaches the hydrologic constriction 

at the Amargosa Narrows. The mass-balance calculation involving only discharge of shallow ground water 

appears to describe the hydrologically-active system in 

the alluvium.

The estimate in this report that slightly more than 

half (53 percent) of the ground water contained in
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E23

TABLE 7. Comparisons of selected chemical species in ground water from the mouth 

of Thirsty Canyon and the exit ofArnargosa Narrows

[Constituents in millimoles per liter]

1. Water composition at 

Amargosa Narrows 

2. Average water composition of tuffaceous 

aquifer1 

3. Average water composition of tuffaceous 

aquifer x 2.13

4. Molality 

difference2 

5. Percentage 

gain or loss3

Na+

12.62 

6.10 

12.99

- .37 

-3

K+

0.26 

.17 

.36

- .10 

-28

Ca++

0.95 

.45 

.96

- .01 

-1

Mg+ +

0.23 

.07 

.15

+ .08 

+ 53

HCO7

7.20 

3.68 

7.84

- .64 

-8

SOf

2.60 

1.12 

2.39

+ .21 

+ 9

cr

2.82 

1.27 

2.71

+ .11

+4

F~

0.33 

.24 

.51

- .18 

-35

H4SiO4

1.12

.90 

1.92

- .80 

-42

'Mean average of water compositions from tuffaceous aquifer (table 3) excluding locations 7, 8, and 15.

2Line 1 minus line 3.

"Line 4, divided by line 3, times 100.

Oasis Valley is discharged by evapotranspiration is in 

variance with the findings of Malmberg and Eakin 

(1962), who estimated that more than 80 percent of the 

ground water was lost by this process. Malmberg and 

Eakin based their value on estimates of total areal coverage by phreatophytes, average depth of ground water, and rates of evaporation.

Major variations exist between lines 1 and 3 (table 7) 

for the other chemical species that cannot be explained 

by concentration due to evapotranspiration. Silica, 

fluoride, and potassium show decreases and magnesium and sulfate show large increases. An estimate of 

the sources and sinks for the concentration excesses 

and depletions shown in lines 4 and 5 can be made by 

considering mass-balance relationships.

Based on an initial volume of 1,000 L and assuming a 

volume-reduction factor of 2.13, 0.80 mole (line 4) or 

42 percent (line 5) of the total silica is lost from the 

ground water in the alluvium. Ground water recharging the alluvium from the tuffaceous aquifer is nearly 

saturated with respect to silica gel (table 4). The loss of 

42 percent of the total silica can thus be explained by 

silica gel precipitation as silica concentrations are increased in excess of saturation by evapotranspiration. 

Similarly, 0.18 mole or 35 percent of the dissolved 

fluoride is lost by fluorite precipitation (fig. 11);

2F CaF22(fluoritel.

A small amount, 0.09 mole, of dissolved calcium is also 

lost during the formation of fluorite. This amount, 

when subtracted from a net calcium deficit of 0.01 mole, 

produces a deficit of 0.10 mole.

The most likely source of excess magnesium (0.08 

mole or 64 percent of the total) is from the incongruent 

dissolution of dolomite.

MgCOs (dolomite) + H + -» Mg++ + HCOs".

The excess of magnesium must be balanced by 0.16

mole of excess bicarbonate. This reaction would 

further increase the bicarbonate deficit to 0.80 mole. 

Concentration increases in calcite-saturated ground 

water (fig. 12) results in calcite precipitation and the 

loss of bicarbonate;

Ca CaCOs (calcite)

However, the loss of 0.80 mole of bicarbonate in the 

above reaction is not balanced by the loss of only 0.10 

mole of calcium. The excess sulfate (0.24 mole or 9 

percent) suggests that the dissolution of calcium 

sulfate,

CaSO4 (gypsum) -+ Ca++ + SO4=

may be a source of both calcium and sulfate.

The net reaction for calcium sulfate dissolution and 

calcium carbonate precipitation becomes

CaSO4 (gypsum) + HCO3~-+ CaCOs (calcite) + H+ + SO4=

in which calcium is conserved. Net calcium 

conservation is demonstrated in table 7. The deficit of 

0.60 mole of bicarbonate is approximately balanced in 

the above reaction by an increase of 0.48 mole of 

sulfate.

SUMMARY AND CONCLUSIONS

The abundance of springs and shallow wells in Oasis 

Valley provides an excellent opportunity for a detailed 

geochemical investigation of ground water associated 

with tuffaceous rocks. These geochemical data, in turn, 

permit additional insight into the hydrologic features 

that control the volume and movement of ground water 

in Oasis Valley.

Most of the dissolved solids, represented principally 

by sodium, bicarbonate, and silica, are derived from 

reaction with the tuffaceous rocks. Comparison of these 

high solute concentrations with reported analyses for
E24 HYDROLOGY OF NUCLEAR TEST SITES

intruded crystalline rocks of the same chemical composition suggests that hydrolysis and incongruent dissolution of the volcanic glass phase are the principal 

reactions between the water and the tuffaceous rock. 

The relative deficiencies in silica and sodium in hydrated glasses reported by other workers support this 

conclusion. Bicarbonate is the product of the disassociation of soil-derived carbonic acid by hydrolysis and of 

ion exchange involving the glass phase. Chloride is 

also leached preferentially relative to fluoride in the 

glass phase. The kinetic-reaction rates of tuffaceous 

rocks appear to be significant in determining chemical 

concentrations because relative concentrations are at 

least partially a function of distance and presumably of 

time spent in movement between recharge and discharge areas.

The ground water infiltrates from the tuffaceous 

rocks into the valley alluvium through springs and 

subsurface flow. Direct exchange with the atmosphere 

as well as transpiration through the vegetation cover 

occurs as the result of the immediate proximity of the 

water table to the land surface and soil zone. The resulting decrease in the volume of ground water leads to 

an increase in chemical concentration. Increases of 

sodium, chloride, and bicarbonate concentrations plot 

on the same linear reaction paths as do initial concentration increases attributed to chemical reaction with 

the tuffaceous rocks. Mass-balance calculations indicate, however, that more than 9 percent of the total 

sulfate and 53 percent of the magnesium can be attributed to sources outside those in the tuffaceous rocks. 

Calcium and magnesium concentrations in the ground 

water are higher in areas to the south and west of 

Oasis Valley where dolomites and limestones locally 

crop out. Mass-balance equations indicate that substantial sulfate must be associated with increases in 

calcium; this relation suggests local dissolution of calcium sulfate minerals either in lacustrine deposits in 

the valley floor or as hydrothermal alteration products 

of sulfide minerals.

One of the more interesting geochemical features of 

Oasis Valley is the aqueous concentration controls exhibited by various mineral phases. Dissolved silica remains constant, corresponding to saturation with respect to amorphous silica. Excess silica produced by 

chemical reaction and evapotranspiration is removed 

from the aqueous system as silica gel. Silica gel is observed in fracture fillings at numerous locations in the 

tuffaceous rocks. Montmorillonite represents the stable clay weathering product formed by Oasis Valley 

ground water. Although the mineral occurs in the tuffs 

and alluvium, an aqueous supersaturation of more 

than two orders of magnitude suggests that formation 

is slow and montmorillonite does not significantly influence the bulk ground-water composition. Analcime 

is another common mineral occurring extensively in 

zeolitized zones within the tuffs. However, undersaturation of an order of magnitude with respect to the 

aqueous system indicates the zeolitization may occur 

in localized geochemical environments that are not reflected in the bulk ground-water composition found in 

Oasis Valley.

With increases in concentration in the alluvium, 

owing to evapotranspiration, the ground water becomes saturated with calcite, barite, and fluorite. For 

all three minerals, the aqueous concentration does not 

exceed saturation even with additional decreases in 

the volume of ground-water. This situation implies 

active precipitation of these mineral phases. Approximately 35 percent of the initial fluoride is precipitated 

in the valley alluvium.

Magnesium initially exhibits large increases in 

concentration in the alluvial aquifer, but downgradient 

magnesium reaches a constant concentration that is 

slightly undersaturated with respect to dolomite. The 

maximum magnesium concentration seems directly related to the inability of the ground water to dissolve 

dolomite in excess of the saturation value. The initial 

amount of potassium is decreased by approximately 28 

percent in the valley alluvium; this decrease is probably a result of adsorption and fixation by clay minerals present in the alluvium and soil zones.

Two geochemical contributions can be added to the 

understanding of the hydrologic processes in Oasis Valley. The geochemical history of the ground water in 

Oasis Valley seems to be generically related to ground 

water contained under Pahute Mesa and Gold Flat as 

suggested by previous workers. Concentrations of 

three major conservative components, sodium, bicarbonate, and chloride, plot on a well-defined linear 

trend. Water in the recharge areas of Pahute Mesa and 

Gold Flat plots in the dilute portion of the trend, and 

the water of Oasis Valley, representing a more advanced stage in the reaction sequence farther downgradient, plots in the more concentrated part. Secondly, 

mass balance calculations show that slightly more 

than half the ground water entering the alluvium in 

the floor of Oasis Valley is lost as a result of evapotranspiration. This value is considerably lower than had 

been previously estimated by more indirect methods; 

consequently, considerably more ground water is discharged by underflow to the Amargosa Desert than had 

been previously anticipated.

REFERENCES

Benson, L., 1978, Mass transport processes in vitric tuffs of 

Rainier Mesa, Nevada: Geol. Soc. America Bull, (in press).
GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E25

Berner, R. A., 1971, Principles of chemical sedimentology: New York, 

McGraw Hill Book Co., 328 p.

Blankennagel, R. K., and Weir, J. E., Jr., 1973, Geohydrology of the 

eastern part of Pahute Mesa, Nevada Test Site, Nye County, 

Nevada: U.S. Geol. Survey Prof. Paper 712-B, p. Bl-35.

Brown, Eugene, Skougstad, M. W, and Fishman, M. J., 1970, 

Methods for collection and analysis of water samples for dissolved minerals and gases: U.S. Geol. Survey Tech. WaterResources Inv, Book 5 Chap. A-l, 160 p.

Budd, S. M., 1961, The mechanisms of chemical reaction between 

silicate glass and attacking agents Part 1. Electrophilic and 

nucleophilic mechanisms of attack: Phys. Chem. Glasses, v. 2, 

p. 111-114.

Carr, W. J., 1974, Structure and clay alteration, in Results of exploration of the Baneberry site, early 1971: U.S. Geol. Survey rept. 

USGS-474-145, p. 7-18; available only from U.S. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, VA 22161.

Carr, W J., and Quinlivan, W. D., 1966, Geologic map of the Timber 

Mountain quadrangle, Nevada: U.S. Geol. Survey Quad. Map 

GQ-503, 1:62,500.

Cornwall, H. R., 1962, Calderas and associated volcanic rocks near 

Beatty, Nye County, Nevada, in Petrologic studies: a volume in 

honor of A. F. Buddington; A. E. J. Engle, H. L. James, and B. F. 

Leonard, eds., Geol. Soc. America, p. 357-372.

Cornwall, H. R., and Kleinhampl, F. J., 1961, Geology of Bare 

Mountain quadrangle, Nevada: U.S. Geol. Survey Quad. Map 

GQ-157, 1:62,500.

 1964, Geology of the Bullfrog quadrangle and ore deposits 

related to the Bullfrog Hills caldera, Nye County, Nevada and 

Inyo County, California: U.S. Geol. Survey Prof. Paper 454-J, p. 

J1-J25.

Davis, S. N., 1964, Silica in streams and ground water: Am. Jour. Sci., 

v. 262, no. 7, p. 870-891.

Feth, J. H., Roberson, C. E., and Polzer, W. L., 1964, Sources of mineral 

constituents in water from granitic rocks, Sierra Nevada, 

California and Nevada: U.S. Geol. Survey Water-Supply Paper 

1535-1, p. 11-1170.

Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals and 

equilibria: New York, Harper and Row, Inc., 450 p.

Garrels, R. M., and Mackenzie, F. T, 1967, Origin of the chemical 

compositions of some springs and lakes, in Equilibrium concepts 

in natural water systems: Am. Chem. Soc., Advances in Chemistry Series, v. 67, p. 222-242.

Gibbons, A. B., Hinrichs, E. N., and Botinelly, T, 1960, The role of 

impermeable rocks in controlling zeolitic alteration of tuffs, in 

Geological Survey Research 1960: U. S. Geol. Survey Prof. Paper 

400-B, p. B473-475.

Helgeson, H. C., 1969, Thermodynamics of hydrothermal systems at 

elevated temperatures and pressures: Am. Jour. Sci., v. 367, no. 

7, p. 729-804.

Helgeson, H. C., Brown, T. H., and Leeper, R. H., 1969, Handbook'of 

theoretical activity diagrams depicting chemical equilibria in 

geologic systems involving an aqueous phase at one atmosphere 

and 0 > to 300 >C: San Francisco, Freeman, Cooper and Co., 

254 p.

Hem, J. D., 1970, Study and interpretation of the chemical characteristics of natural water: U. S. Geol. Survey Water-Supply Paper 

1473, 362 p.

Hemley, J. J., Meyer, C., and Richter, D. H., 1961, Some alteration 

reactions in the system Na2O-Al2Os-SiO2-H2O, in Geological 

Survey Research 1961: U.S. Geol. Survey Prof. Paper 424-D, p. 

D338-D340.

Hoover, D. L., 1968, Genesis of zeolites, Nevada Test Site, in Nevada

Test Site, Eckel, E. B., ed.: Geol. Soc. America Mem. 110, p.

275-284. 

Jones, B. F., 1966, Geochemical evolution of closed basin water in the

western Great Basin, in Symposium on salt, 2d, v. 1, Geology,

geochemistry, mining: Cleveland, Ohio, Northern Ohio Geol.

Soc., p. 181-200. 

Kittrick, J. A., 1971a, Stability of montmorillonites, I. Belle Fourche

and Clay Spur montmorillonites: Soil Sci. America Proc., v. 35,

no. 1, p. 140-145.

 1971b, Stability of montmorillonites, II, Aberdeen montmorillonite: Soil Sci. America Proc., v. 35, no. 1, p. 820-823.

Lipman, P. W, 1965, Chemical comparison of glassy and crystalline 

volcanic rocks: U.S. Geol. Survey Bull, 1201-D, 24 p.

Lipman, P. W, Christiansen, R. L., and O'Connor, J. T, 1966, A compositionally zoned ash-flow sheet in southern Nevada: U.S. 

Geol. Survey Prof. Paper 524-F, 47 p.

Malmberg, G. T, and Eakin, T. E., 1962, Ground-water appraisal of 

Sarcobatus Flat and Oasis Valley, Nye and Esmeralda Counties, 

Nevada: Nevada State Engineer, Ground-Water Resources Reconn. Ser. rept. 10, 39 p.

 1964, Relation of fluoride content to recharge and movement 

of ground water in Oasis Valley, southern Nevada, in Geological 

Survey Research 1964: U.S. Geol. Survey Prof. Paper 475-D, p. 

D189-D191.

Marshall, C. E., 1949, The colloid chemistry of silicate minerals: 

Agronomy Monograph, v. 1IX, New York, Academic Press, Inc., 

195 p.

Noble, D. C., 1967, Sodium, potassium and ferrous iron contents of 

some secondary hydrated natural silica glasses: Am. Mineralogist, v. 52, no. 2, p. 280-286.

Noble, D. C., Smith, V C., and Peck, L. C., 1967, Loss of halogens from 

crystallized and glassy silicic volcanic rocks: Geochim. et Cosmochim, Acta, v. 31, no. 2, p. 215-223.

O'Connor, J. T, Anderson, R. E., and Lipman, P. W, 1966, Geologic 

map of the Thirsty Canyon quadrangle, Nevada: U.S. Geol. 

Survey Quad. Map GQ-524.

Plummer, L. N., and Mackenzie, F. T, 1974, Predicting mineral solubility from rate data: Application to the dissolution of magnesian calcites: Am. Jour. Sci., v. 274, p. 61-83.

Ransome, F. L., Emmons, W. H., and Garrey G. H., 1910, Geology and 

ore deposits, Bullfrog District, Nevada: U.S. Geol. Survey Bull. 

407, 129 p.

Sheppard, R. A., and Gude, A. J., 1968, Distribution and genesis of 

authigenic silicate minerals in tuffs of Pleistocene Lake Tecopa, 

Inyo County, California: U.S. Geol. Survey Prof. Paper 579, 38 p.

 1973, Zeolites and associated authigenic silicate minerals in 

tuffaceous rocks of the Big Sandy Formation, Mohave County, 

Arizona: U.S. Geol. Survey Prof. Paper 830, 35 p.

Smyshlyaev, S. J., and Edeleva, N. P., 1962, Determination of the 

solubility of minerals, Solubility product for fluorite: Izv. Vysshikh Vchebn. Zavedenii Khim i Khim. Tekhnologiya, v. 5, p. 

871-874.

Truesdell, A. H., 1966, Ion exchange of natural glasses by the electrode method: Am. Mineralogist, v. 51, nos. 1 and 2, p. 110-122.

Truesdell, A. H., and Jones, B. F., 1974, WATEQ, a computer program 

for calculating chemical equilibrium of natural waters: Jour. Research, v. 2, no. 2, p. 233-248.

White, D. E., Hem, J. D., and Waring, G. A., 1963, Chemical composition of subsurface waters: U.S. Geol. Survey Prof. Paper 440-F, 

p. F1-F66.

Winograd, I. J., and Thordarson, William, 1975, Hydrogeologic and 

hydrochemical framework, south-central Great Basin with special reference to the Nevada Test Site: U.S. Geol. Survey Prof. 

Paper 712-C, p. C1-C126.

GPO 689-035