science for a changing world

HYDROCHEMICAL INVESTIGATIONS IN 

CHARACTERIZING THE UNSATURATED ZONE AT 

YUCCA MOUNTAIN, NEVADA

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 98-4132

Prepared in cooperation with the NEVADA OPERATIONS OFFICE, 

U.S. DEPARTMENT OF ENERGY, under 

Interagency Agreement DE-AI08-97NV12033

Hydrochemical Investigations in 

Characterizing the Unsaturated Zone at 

Yucca Mountain, Nevada

By In C. Yang, 1 Pei Yu,2 Gordon W. Rattray, 1 Janine S. Ferarese, 1 and 

Joseph N. Ryan3

1 U.S. Geological Survey, Denver, Colorado 

2Taipower Company, Taipei, Taiwan

Department of Civil, Environmental, and Architectural Engineering, 

University of Colorado, Boulder, Colorado

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 98-4132

Prepared in cooperation with the

NEVADA OPERATIONS OFFICE,

U.S. DEPARTMENT OF ENERGY under

INTERAGENCY AGREEMENT DE-AI08-97NV12033

Denver, Colorado 

1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

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

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

For additional information write to:

Chief, Earth Science Investigations Program

Yucca Mountain Project Branch

U.S. Geological Survey

Box 25046, Mail Stop 421

Denver Federal Center

Denver, CO 80225-0046

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

Abstract.......................................................................................................................................~ 1

Introduction............................................................................................^ 1

Chemical and Radioactive-Isotope Compositions (3H and 14C) of Gas and Pore Water from the Unsaturated Zone.......... 4

Boreholes From Which Data Were Collected.............................................................................................................. 4

Sample Collection and Analysis.................................................................................................................................. 4

Collection of Gas Samples from Boreholes SD-7, SD-9, and SD-12 ............................................................... 6

Collection of Pore-Water Samples from SD-7, SD-9, SD-12, NRG-6, and NRG-7a ....................................... 6

Analyses of Gas and Water Samples ................................................................................................................. 7

Data and Interpretations of Chemical and Radioactive-Isotope Analyses................................................................... 7

Spatial Variabilities in Chemical Compositions................................................................................................ 7

Tritium Profiles in Boreholes............................................................................................................................. 16

Residence Times of 14CO2 and Pore Water....................................................................................................... 16

Stable-Isotope Compositions (SD and S 18O) of Pore Water from USW UZ-14................................................................... 23

Methods of Pore-Water Extraction for 8D and5 18O and Analysis............................................................................ 24

Toluene Distillation............................................................................................................................................ 24

Vacuum Distillation.......................................................^ 25

Compression Extraction..................................................................................................................................... 27

Water Analyses for 8 18O and 8D....................................................................................................................... 27

Inconsistency in 8D and 6 I8O Data with Different Extraction Methods..................................................................... 29

Review of Water-Extraction Methods Published in the Literature.................................................................... 29

Analytical Data from Methods Used in This Study........................................................................................... 30

Isotopic Data from Toluene-Azeotropic Distillation............................................................................... 30

Comparison of Vacuum-Distillation and Compression-Extraction Methods.......................................... 30

Laboratory Tests.......................................................................................................................................................... 34

Imbibing Dry Core with Known Isotope-Composition Water: Imbibing Test.................................................. 34

Extracting Imbibed Water by Vacuum Distillation.................................................................................. 34

Extracting Imbibed Water by Compression............................................................................................. 36

Isotopic Exchange Between Matrix Water and Percolating Water: Column Test............................................. 38

Zeolite Powder......................................................................................................................................... 39

Zeolite-Rich Core.................................................................................................................................... 41

Applicability of the Extraction Methods to Yucca Mountain Cores............................................................................ 44

The Pah Canyon and the Topopah Spring Tuffs: Vacuum Distillation Method................................................. 44

Bedded Tuff and the Calico Hills Formation: Compression Method................................................................ 44

Interpretation of the Stable-Isotope Data..................................................................................................................... 44

Source and Nature of Infiltrating Water in the Unsaturated Zone..................................................................... 48

x Large Dissolved-Solids Concentrations and 8D and 8 18O Values .................................................................... 51

Interactions of Old Matrix Water with Younger Percolating Water .................................................................. 53

Preglacial or Postglacial Pore Water in the Topopah Spring Tuff..................................................................... 53

Summary and Conclusions.................................................................................................................................................... 53

References.............................................................................................................................................................................. 54

FIGURES

1. Hydrogeologic units at Yucca Mountain, Nevada..................................................................................................... 3

2. Map showing locations of boreholes completed in the unsaturated zone, Yucca Mountain area, Nevada............... 5

3-5. Piper diagram showing:

3. USW UZ-14 pore-water compositions............................................................................................................... 9

4. USW NRG-6 and NRG-7a pore-water compositions........................................................................................ 11

5. USW SD-7, SD-9, and SD-12 pore-water compositions................................................................................... 14

CONTENTS III
6. Lithologic units, tritium, and I4C concentrations of samples from borehole USW UZ-14, (A) top 121.9 meters, 

(B) bottom 274.3 meters in Calico Hills Formation and Prow Pass Tuff................................................................. 17

7. Lithologic units, tritium, and 14C concentrations of boreholes (A) USW NRG-6 and

(B) USW NRG-7a, Yucca Mountain area................................................................................................................. 18

8. Lithologic units, tritium, and 14C concentrations of borehole USW SD-7, Yucca Mountain area .......................... 19

9. Lithologic units, tritium, and 14C concentrations of boreholes (A) USW SD-9 and (B) USW SD-12,

Yucca Mountain area................................................................................................................................................. 20

10. Lithologic units and S 13C (gas) values, borehole USW SD-12, Yucca Mountain area............................................ 22

11 14. Schematics showing:

11. Toluene distillation apparatus for pore-water extraction................................................................................. 25

12. Vacuum distillation apparatus for pore-water extraction................................................................................. 26

13. Apparatus for imbibing test............................................................................................................................. 35

14. Apparamsforcolimmtest..........................................................................................................................^ 40

15 19. Graphs showing:

15. Lithologic units and 8' 8O profile of compressed and distilled pore water from borehole USW UZ-14,

Yucca Mountain area ....................................................................................................................................... 46

16. Lithologic units and 5D profile of compressed and distilled pore water from borehole USW UZ-14,

Yucca Mountain area ....................................................................................................................................... 47

17. Relation of 5' 8O and 5D of USW UZ-14 unsaturated-zone pore water extracted by compression on 

bedded tuff and Calico Hills Formation and vacuum distillation on Topopah Spring Tuff, 

Yucca Mountain area ....................................................................................................................................... 49

18. Relation of 6 O and 5D 1984 storms, saturated-zone water, and perched water, Yucca Mountain area....... 50

19. Relation of 5 O and 5D data from samples containing large dissolved-solids concentrations in

pore waters of USW UZ-14 and USW NRG-6 at different depths, Yucca Mountain area.............................. 52

TABLES

1. Borehole designation, drilling date, depth, and perched-water occurrence of selected boreholes at

Yucca Mountain area............................................................................................................................................... 6

2. Chemical composition of pore-water samples from borehole USW UZ-14, Yucca Mountain area....................... 8

3. Chemical composition of pore-water samples from boreholes USW NRG-6 and NRG-7a,

Yucca Mountain area............................................................................................................................................... 10

4. Chemical composition of pore-water samples from boreholes USW SD-7, SD-9, and SD-12,

Yucca Mountain area............................................................................................................................................... 12

5. Gaseous-phase carbon-14, 8 13C, and fracture-frequency data from boreholes USW SD-7, SD-9,

and SD-12, Yucca Mountain area............................................................................................................................ 21

6. Stable-isotopic composition and water yield of water distilled from Yucca Mountain cores by use of

different distillation times ...................................................................................................................................... 28

7. Results of the toluene-distillation method in extracted water from imbibed cores, Yucca Mountain area............. 31

8. Mineralogic data from Yucca Mountain core samples used for comparing the vacuum-distillation and

compression-extraction methods for stable-isotope analysis.................................................................................. 31

9. Stable-isotope compositions of compressed water, remaining water extracted by vacuum distillation, and

water distilled from bedded tuff cores..................................................................................................................... 32

10. Stable-isotope compositions of compressed water and water distilled from the Pah Canyon Tuff cores............... 32

11. Stable-isotope compositions of compressed water, remaining water extracted by vacuum distillation,

and water distilled from the Calico Hills Formation cores ..................................................................................... 33

12. Stable-isotope compositions of compressed water, remaining water extracted by vacuum distillation,

and water distilled from the Prow Pass Tuff cores.................................................................................................. 33

13. Stable-isotope compositions of the vacuum-distilled water from the imbibed cores ............................................. 36

14. Mineralogic data of the imbibed cores used in the compression method............................................................... 36

15. Stable-isotope compositions of water extracted by the compression method from Pah Canyon Tuff and

Topopah Spring Tuff cores...................................................................................................................................... 37

IV CONTENTS
16. Stable-isotope compositions of water extracted by the compression method from the Yucca Mountain Tuff and

bedded tuff cores ................................................................................................................................................... 38

17. Stable-isotope compositions of water extracted by the compression method from Calico Hills Formation and

Prow Pass Tuff cores............................................................................................................................................... 39

18. Comparison of stable-isotope compositions of water distilled from clinoptilolite imbibed with

known-source water................................................................................................................................................ 41

19. Stable-isotope compositions of the effluents from the column experiment with clinoptilolite.............................. 42

20. Stable-isotope compositions of water from the imbibition experiment with two different source waters

for zeolite-rich core................................................................................................................................................. 43

21. Depths and mineral contents in each lithologic unit of borehole USW UZ-14, Yucca Mountain area.................. 45

CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obtain

nanometer (nm)

centimeter (cm)

kilometer (km)

meter (m)

meter per day (m/d)

micrometer (jam)

millimeter (mm)

square centimeter per second (cm2/s)

square meter (m2 )

cubic centimeter (cm3 )

cubic meter (m3 )

liter (L)

milliliter (mL)

gram (g)

gram per cubic centimeter (g/cm3 )

megapascal (MPa)

microgram per liter (ug/L)

milligram per liter (mg/L)

3.937 x

3.937 x

6.214 x

3.281

3.281

3.937 x

3.937 x

0.1550

10.76

6.102 x

6.102 x

0.2642

6.102 x

2.2 x 10

10'8

10-'

lo- 1

io-5

JO'2

io-2

IO4

io-2

-3

3.6 x P2

144.7

1

1

inch

inch

mile

foot

foot per day

inch

inch

square inch per second

square foot

cubic inch

cubic inch

gallon

cubic inch

pound, mass

pound per cubic inch

pound per square inch

part per billion

part per million

Degree Celsius (°C) may be converted to degree Fahrenheit (°F) by using the following equation:

°F = 9/5 (°C) + 32

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

CONTENTS

Hydrochemical Investigations in Characterizing the 

Unsaturated Zone at Yucca Mountain, Nevada

By In C. Yang, Pei Yu, Gordon W. Rattray, Janine S. Ferarese, and Joseph N. Ryan

ABSTRACT

Hydrochemical and isotopic investigations 

of ground water at Yucca Mountain, Nevada, site 

of a potential permanent national nuclear-waste 

repository, demonstrate that younger rocks are 

dominated by calcium-sulfate or calcium-chloride 

water and that older rocks contain sodiumcarbonate or sodium-bicarbonate water. Furthermore, unsaturated-zone pore water has significantly larger concentrations of major ions and 

dissolved solids than does the saturated-zone 

water. Recharge of perched or saturated-zone 

water, therefore, requires rapid flow through fractures or permeable regions in the unsaturated zone 

to avoid mixing with the chemically concentrated 

water in the unsaturated zone. This conceptual 

model is consistent with observations of rapidly 

moved post-bomb (post-1954) tritium and chlorine-36 in the deep unsaturated zone at Yucca 

Mountain. Presence of post-bomb tritium in 

matrix water away from fracture zones further 

indicates that parts of the fast-flow water that 

moves through fractures have been diverted laterally into nonwelded units.

Discrepancy in carbon-14 activities 

between gas and water phases from various 

boreholes within individual and from different 

lithologic units may be due to atmospheric 

contamination during drilling. Larger carbon-14 

activities (indicating younger ages) are seen in 

water from some parts of a nonwelded unit of 

Miocene age than in samples elsewhere in the 

same unit, and gaseous-phase carbon-14 activities 

in one unit are smaller (older ages) than porewater activities of the underlying unit. The

gaseous-phase carbon-14 age profiles across the 

entire unsaturated zone in one borehole increase 

in age with depth, confirming previous results 

from another borehole and demonstrating that gas 

transport in the Yucca Mountain unsaturated zone 

occurs through a diffusion mechanism.

Experimental data show that different lithologic units require specific water-extraction 

methods for stable-isotope analyses of hydrogen 

and oxygen to ensure accurate characterization. 

Vacuum-distillation and compression-extraction 

methods both can yield accurate data but must be 

used with specific lithologies. Column experiments demonstrate that percolating water can 

exchange with pore water of the core as well as 

water held in zeolite minerals in the core. 

Exchange rates range from days to months. Porewater samples from core, therefore, reflect the 

most recently infiltrated water but do not reflect 

percolating water of the distant past.

Stable-isotopic compositions of perched 

water generally plot close to the Yucca Mountain 

precipitation line, which indicates little evaporative loss before recharge, although exceptions do 

exist. Estimated evaporation under conditions of 

largest possible loss produced 6-percent volumetric loss. Winter precipitation is the likely 

source of recharge to the unsaturated zone.

INTRODUCTION

A thorough understanding of hydrochemical 

processes in the unsaturated zone (UZ) is essential to 

the site-characterization program for the potential 

permanent repository for nuclear waste at Yucca 

Mountain, Nevada. Chemical composition of pore

ABSTRACT
water reflects the extent of water-rock interaction 

between the recharge water and the rock matrix. 

Tritium (3H) and carbon-14 ( 14C) ages provide insight 

as to the nature of flow mechanisms and residence 

times of the unsaturated-zone water. Carbon ( 14C) 

concentrations measured in the gaseous phase can 

provide information relevant to the residence time of 

carbon dioxide (CC^) in this unusually thick unsaturated zone, and carbon isotopic data (5 13C) can help to 

identify carbon sources, a necessary step in 14C age 

estimation. Carbon isotopic data of CO2 as a function 

of depth provide information on gas-flow mechanisms 

through the unsaturated zone as well as interactions 

with water and secondary calcite. Stable-isotopic 

compositions of hydrogen and oxygen in fracture and 

matrix waters can help to identify fracture/matrix 

interactions and sources of water. From the present 

chemical and isotopic data, along with previously 

published data collected at Yucca Mountain (for 

example, Yang and others, 1993 and 1996), conceptual 

hydrologic flow at Yucca Mountain can be described. 

Such investigations are important because ground 

water is the most likely transport medium for radionuclides from a repository to the accessible environment.

The purpose of this study is to obtain hydrochemical data, such as chemical compositions of pore 

and perched waters, and to determine flow mechanisms and residence times of gas and water and extent 

of fracture/matrix interactions for water. All of these 

can be used to calibrate flow and transport models, 

which can be used to estimate repository performance 

and safety.

A method of pore-water extraction for chemical-composition analysis developed by Yang and 

others (1988) and later modified by Mower and others 

(1991, 1994) was used in this study. Methods of porewater extraction for stable-isotope analysis were 

investigated in this study. Lithologic units that contain 

different minerals require different methods of porewater extraction for stable-isotope analyses of 

hydrogen and oxygen, such as compression extraction 

or vacuum distillation. Validations of the methods that 

were applicable to each lithologic unit are presented in 

the section on the stable-isotope compositions of pore 

water.

Yucca Mountain, located near the western 

boundary of the Nevada Test Site in southern Nye 

County about 160 km northwest of Las Vegas, consists 

of a rugged series of north-trending fault-block ridges 

that are composed of volcanic rocks that generally

have an eastward tilt of 5° to 10° (Scott and Bonk, 

1984). A thick section of volcanic rocks of Tertiary 

age overlies sedimentary strata of Paleozoic age in the 

Yucca Mountain region (Byers and others, 1976), 

with units of the 12.8 to 12.7 million-year-old (Ma) 

Paintbrush Group (Tertiary age) (Sawyer and others, 

1994) forming most exposures (Christiansen and 

Lipman, 1965; Scott and Bonk, 1984). Two voluminous, densely welded ash-flow tuffs underlie Yucca 

Mountain (the Tiva Canyon and Topopah Spring 

Tuffs), separated by a much thinner interval of mostly 

nonwelded pyroclastic rocks. Stratigraphic nomenclature was recently revised and improved with many 

new isotopic ages (Sawyer and others, 1994) and that 

Stratigraphic nomenclature is used here (fig. 1). 

Numerous washes, generally underlain by alluvium, 

dissect the eastern side of the mountain. Nearly threefourths of the annual precipitation falls from October 

through April (Quiring, 1983). No perennial streams 

exist in the Yucca Mountain area. Surface runoff is 

infrequent and of short duration, occurring only as a 

direct result of intense precipitation or rapid snowmelt.

Numerous geologic and hydrologic investigations conducted in the extensive characterization effort 

at Yucca Mountain have resulted in overlapping 

nomenclature to describe aspects of the geologic 

setting. Obvious differences in the hydrologic character of the welded and nonwelded sequences led 

Montazer and Wilson (1984) and Ortiz and others 

(1985) to develop thermo-mechanical (or hydrogeologic) designations for these units (fig. 1). Because the 

hydrogeologic units are based on similarity of properties (see Luckey and others, 1996, p. 17 20 for discussion), they do not correspond exactly with the 

Stratigraphic units, in which physical properties 

(welding, degree of alteration, fracturing, permeability) vary significantly both laterally and vertically 

within single flow units. Note that both nomenclatures 

will be used in this report, but they are not interchangeable. For example, the Calico Hills nonwelded 

hydrogeologic unit (CHn) contains basal parts of the 

Topopah Spring Tuff, the entire Calico Hills Formation and the Prow Pass Tuff, and upper parts of the 

Bullfrog Tuff. Stratigraphic and hydrogeologic correlations are shown graphically in figure 1.

This report describes ongoing investigations of 

unsaturated-zone water chemistry and isotope 

hydrology at Yucca Mountain. New data collected in 

1996 include CO2 concentrations and carbon isotopic 

compositions ( 13C and I4C) of rock gas from several

2 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Stratigraphic

unit

Alluvium

a

3

O

cS

.c(/>3 i_

.Q+*

C

(0

a.

Tiva Canyon 

Tuff

Yucca Mtn

Tuff

Bedded Tuff

Pah Canyon

Tuff

Bedded Tuff

Topopah 

Spring

Tuff

Calico Hills

Formation

a

3

0

CB

LL

<5 

CO

0

Prow Pass

Tuff

Bullfrog 

Tuff

Tuff

lithology

MD

NP,B

NP,B

MD

(V) /

/ (D)

(in part

zeolitic)

MD,NP,B 

(undifferentiated)

i

Hydrogeologic

unit

Alluvium

Tlva Canyon 

welded unit (TCw)

Paintbrush 

nonwelded unit

(PTn)

Topopah Spring 

welded unit

(TSw)

Figure 1. Hydrogeologic units at Yucca Mountain, Nevada [modified from Montazer and 

Wilson (1984), table 1]. MD, moderately to densely welded; NP, nonwelded to partially 

welded; B, bedded; (V), vitric; (D), devitrified.

INTRODUCTION
boreholes and major-ion concentrations and isotopic 

compositions of water from those and additional boreholes. Pore-water chemical compositions and C data 

are from bedded tuff and the Calico Hills nonwelded 

unit (fig. 1). Limited 3H data were obtained from intervals of each of the boreholes.

This investigation was conducted by the U.S. 

Geological Survey (USGS) in cooperation with the 

U.S. Department of Energy under Interagency Agreement DE-AI08-97NV12033.

CHEMICAL AND RADIOACTIVE-ISOTOPE 

COMPOSITIONS (3H AND 14C) OF GAS 

AND PORE WATER FROM THE 

UNSATURATED ZONE

A vacuum-reverse air-circulation drilling 

method (Whitfield, 1985) was used to drill all boreholes in the UZ drilling program. This method is 

intended to prevent contamination of the unsaturated 

rocks with drilling fluids and thus allow collection of 

native pore water and gas from the UZ for hydrochemical analysis. Locations of boreholes where gas and 

pore-water samples were collected are shown in 

figure 2. Borehole designation, drilling date, depth, 

and perched-water detections are given in table 1. 

Abbreviated borehole names will be used throughout 

the text.

Boreholes From Which Data Were 

Collected

Drilling of geologic borehole NRG-6 was 

started on November 23, 1992, and the borehole was 

cored to a depth of 335.3 m, terminating in the 

Topopah Spring Tuff by March 3,1993. NRG-7a was 

drilled beginning October 21, 1993, and cored to a 

depth of 461.3 m in the upper Calico Hills Formation 

by May 6, 1994. Cores above the Topopah Spring Tuff 

from these two boreholes were used for water extraction. Drilling of borehole SD-7 in the systematic 

drilling program was started on October 3, 1994; SD-7 

was cored to 615.8 m with a 10-cm bit by September 

14, 1995, and completed to a depth of 815.3 m in the 

Tram Tuff on November 9,1995. The water-table level 

in this borehole was at 633.3 m on November 13, 

1995. Perched water was reached in this borehole 

between 479.7 m and 488.3 m. SD-9 was started on 

May 10, 1994, and completed to a depth of 677.6 m,

terminating in the Prow Pass Tuff on September 26, 

1994. Perched water also was reached in this borehole 

at 453.8 m. Borehole SD-12 drilling was started on 

January 23, 1994, and was completed to a depth of 

660.3 m, terminating in the Prow Pass Tuff on 

September 14, 1994. The static water table in SD-12 

was located between 610.8 m and 612.9 m. These 

SD holes yielded cores from both the Paintbrush 

nonwelded (PTn) and the Calico Hills nonwelded 

(CHn) hydrogeologic units for water extraction.

The UZ-14 borehole is located in Drill Hole 

Wash, 26.2 m from UZ-1. Drilling of the borehole was 

started on April 15, 1993, and it was cored to a depth 

of 672.6 m by April 29,1994, and completed to a total 

depth of 677.8 m on May 6, 1994. Chemical and 

radioactive-isotope data from UZ-14 have been previously reported from the interval from the surface to 

528.8 m (Yang and others, 1996). This report includes 

data from the depth interval of 528.8 to 640.1 m.

Gas samples were collected from the full depth 

range of SD-12 from 16 sampling ports. Boreholes 

SD-7 and SD-9 were not instrumented. Only one 

sample was collected from the Calico Hills nonwelded 

unit in SD-7, and two samples were obtained from 

SD-9 (one above and one below the Topopah Spring 

Tuff). For gas sampling from these open boreholes, an 

inflatable borehole liner was used.

Sample Collection and Analysis

Details of drilling were designed to enhance 

analysis. After a borehole was drilled, the atmospheric 

air that entered during the drilling process was 

removed by evacuation from the surface with a largecapacity pump. All drilling air was tagged with a 

tracer: sulfur hexafluoride (SF6). The tracer concentration in the evacuated air was continuously monitored, 

and disappearance of the SF6 [concentration less than 

0.1 part per million (ppm)] assured essentially 

complete removal of the drilling air. Packers were 

installed inside the open boreholes in SD-7 and SD-9 

for gas-sample collections. The diameter of the rotary 

core was 6 cm. Cores from the rock formation were 

obtained using a 1.5-m core barrel with a split inner 

tube. Core recovery typically was greater than 

90 percent. Three pieces of continuous, unfragmented 

solid core, each approximately 15 cm long, are needed 

to obtain enough water for age and chemical determinations.

4 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
O

Z

m 

o

o

33

>

g 

o

m

55 O

TJm 

o 

o

6

CO

o

o n

O

O

s

5 a

m 3 

o O

« S

O H

Z Z

m m

NEVADA

Carson City

116r 30'

36: 45' -

116C 20'

Base from U.S. Geological Survey 

1:250.000, Death Valley. California: 

Nevada. 1970

10 KILOMETERS

10 MILES

36^50'

UZ-14,

Boundary of

potential

repository

*UZ-1

UZ-6s

2 KIL

_l__

EXPLANATION

s°-7« BOREHOLE LOCATION A

Figure 2. Locations of boreholes completed in the unsaturated zone, Yucca Mountain area, Nevada.
Table 1. Borehole designation, drilling date, depth, and perched-water occurrence of selected boreholes at Yucca 

Mountain area

[m, meter; , not found (source of information: Yang and others, 1996; Rautman and Engstrom, 1996)]

Borehole 

designation

USW NRG-6

USW NRG-7a

USW SD-7

USW SD-9

USWSD-12

USWUZ-I4

Borehole 

abbreviation

NRG-6

NRG-7a

SD-7

SD-9

SD-12

UZ-14

Drilling date

Start Finish

1 1 -23-92 03-03-93

1 0-2 1 -93 05-06-94

10-03-94 11-09-95

05-10-94 09-26-94

01-23-94 09-14-94

04-15-93 05-06-94

Depth of 

borehole 

(m)

335.3

461.3

815.3

677.6

660.3

677.8

Perched water 

detected at depth 

(m)

460.2

479.7

453.8

384.6

Sample-handling procedures affect the success 

of extracting uncontaminated water from cores of tuff. 

Evaporation of pore water increases with sample exposure time in a dry climate, and strict precautions were 

taken to avoid loss of moisture or contamination of 

pore water in the cores (Striffler and Peters, 1993). 

Each core was wrapped in a thin plastic sheet, placed 

inside a Lexan liner, and capped on both ends of the 

Lexan liner. The Lexan-contained core was then 

placed inside a thick aluminum-foil bag and heatsealed for moisture protection. The sealed cores were 

packed carefully into plastic coolers and transported 

from the drill site to the Sample Management Facility 

(SMF) at Jackass Flats, Nevada, for log-in and storage. 

The storage temperature was kept at 6° to 9"C until the 

cores were removed for pore-water extraction.

Analyses were contracted out by the USGS 

except for tritium, gas composition, and CO2 concentration, which were analyzed by the USGS in Denver, 

Colorado.

Collection of Gas Samples from Boreholes SD-7, 

SD-9, and SD-12

The whole-gas (WG) method or batch sampling 

was used to collect samples for 8 l3C analysis. A gas 

stream was allowed to flow into a Mylar-balloon 

sample container to purge the balloon several times 

before sampling. When the gas had filled up the 

balloon, the sample container was removed from the 

flow line. The collection time was less than 5 minutes.

The molecular-sieve (MS) method was used to 

collect gas samples for 14C and 8 13C analyses. The

gas stream was initially passed continuously through a 

silica-gel tower to remove moisture, then through a 

300-mL stainless-steel cylinder containing a 

50-nanometer (50-nm), anhydrous (dehydrated under 

a vacuum at 350°C) MS to trap the CO2 gas. The CO2 

gas trapped in the MS was degassed in the laboratory 

to release the trapped CO2 . Released CO2 was 

collected in a cold trap, the volume of CO2 was 

measured, and the CO2 was transferred to storage 

cylinders.

Boreholes SD-7 and SD-9 were open boreholes. 

For gas sampling from these boreholes, an inflatable 

borehole liner was used. Gas samples were collected 

from individual tubes connected to open ports located 

along the length of an impermeable membrane liner. 

The liner is inflated to seal the formation wallrock 

from direct contact with the atmosphere. Two intervals 

were packed in SD-9: one from the surface to 453.5 m 

(lithologic units of Tiva Canyon and Topopah Spring 

Tuff), and the other from 453.7 to 560 m (Calico Hills 

nonwelded hydrogeologic unit). Only one interval was 

packed in SD-7, from 508 to 633 m (Calico Hills 

nonwelded unit). Gas data from SD-12 were obtained 

from the entire borehole, in contrast to the SD-7 and 

SD-9 data obtained from selected intervals.

Collection of Pore-Water Samples from SD-7, SD-9, 

SD-12, NRG-6, and NRG-7a

Pore water was extracted from the cores by 

using a high-pressure, one-dimensional (uniaxial) 

compression procedure (Mower and others, 1991, 

1994). The compression operations were described in

6 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
a previous report (Yang and others, 1996). A pressure 

of 103.4 megapascals (MPa) was initially applied and 

gradually increased to a final pressure of 827 MPa. 

Additional pore water was extracted by injecting dry 

nitrogen gas (greater than 99.99 percent pure) into the 

pore space and by forcing out pore water at the end 

of compression. The extracted water was filtered 

through 0.45-micrometer (0.45-um) Nuclepore filters 

into suitable bottles. Samples for major ions were 

stored in polyethylene bottles, and samples for stable 

isotopes and 14C were stored in glass bottles.

The aluminum (Al) concentration in pore water 

is mainly controlled by aluminum silicates. Except for 

extreme pH conditions, dissolved Al is usually below 

the detection limit (less than 1 milligram per liter 

(mg/L). Very fine filters (0.1 um) must be used to 

remove particulate Al to obtain trustworthy measured 

concentrations for dissolved Al. Because 0.45-um 

filter paper was used for pore-water filtration in this 

study, reported Al values for pore waters are total Al 

concentration, including dissolved and particulate 

components.

Analyses of Gas and Water Samples

Gas samples for C and 8 C analyses were 

sent to the Desert Research Institute (DRI) in 

Las Vegas, Nevada, for conventional liquid scintillation counting for 14C and mass spectrometry for S 13C. 

The precision of 8 C analysis was about 0.2 per mil 

[one standard deviation (la)]. Uncertainty in the 14C 

measurement was 0.7 percent modern carbon (pmc) 

(la).

Cation concentrations were measured using 

inductively coupled plasma emission spectroscopy, 

and anion concentrations were measured using ion 

chromatography at Huffman Laboratories, Inc., in 

Golden, Colorado. The analytical error is ±5 percent 

for all major ions except sulfate, for which the error is 

±10 percent. Charge balance is calculated by 

subtracting total milliequivalent anions from total 

milliequivalent cations divided by the total milliequivalents of cation and anion multiplied by 100. However, 

silica concentration is not considered in the charge 

balance because of difficulty in assessing charge on 

silicate species. The tritium was analyzed at the 

USGS Yucca Mountain Project Branch (YMPB) UZ 

Hydrochemistry Laboratory using procedures 

published by Thatcher and others (1977). Tritium 

values were calculated by regressing to the date of 

sample collection so that tritium decay is considered 

between sample collection and date of analysis. 

Tritium errors were approximately 4 tritium units (la) 

[1 tritium unit (1 TU) = 1 3H atom per 10 18 ] H atoms]. 

Carbon-14 and 8 I3C samples were sent to Beta 

Analytic, Inc., in Miami, Florida, for analysis by 

Tandem Accelerator Mass Spectrometer (TAMS). 

Uncertainty in 8 I3C values was 0.2 per mil and for 

I4C was 0.7 pmc. All uncertainties quoted are one 

standard deviation (la).

Data and Interpretations of Chemical and 

Radioactive-Isotope Analyses

This section presents hydrochemical data 

obtained to date and interpretations of those data 

related to possible flow mechanisms and residence 

times in the UZ. Data are chemical compositions 

(major cations and anions) and isotopic concentrations, which include 3H, 14C, and 8 C of pore water 

in cores obtained from boreholes in the UZ. Preliminary conceptual hydrologic flow models are derived 

from the present chemical and isotopic data, along 

with previously published data collected at Yucca 

Mountain (Yang and others, 1996).

Spatial Variabilities in Chemical Compositions

Recent analyses augment previously published 

data from major-ion chemical investigations. Majorion chemical compositions of pore water from UZ-14, 

from the surface to 528.7 m were published previously 

by Yang and others (1996). Additional data from 

waters obtained mostly below 528.7 m are presented 

in table 2 and plotted in figure 3. NRG-6 and NRG-7a 

major-ion chemical compositions are presented in 

table 3 and plotted in figure 4. Major-ion chemical 

compositions of samples from boreholes SD-7, SD-9, 

and SD-12 are presented in table 4 and plotted in 

figure 5. Several samples from the Topopah Spring 

Tuff were analyzed because not many samples of pore 

water could be extracted for chemical analysis. In the 

previous report by Yang and others (1996), only a few 

samples from the caprock unit (usage after Scott and 

Bonk, 1984) of the Topopah Spring Tuff from UZ-4 

and UZ-5 were analyzed.

CHEMICAL AND RADIOACTIVE-ISOTOPE COMPOSITIONS (3H AND 14C) OF GAS AND PORE WATER FROM THE 7

UNSATURATED ZONE
Table 2. Chemical composition of pore-water samples from borehole USW UZ-14, Yucca Mountain area

<D

3 

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il Investlgi

09

0

(0

5"

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10

N*

3

(D

S*

<D

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CD 

09

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20)

fc

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[- - -, data not available; 0, values below detection limit; m, meter; uS/cm, microsiemens per centimeter at 25 degreesi Celsius; rng/L, millijgrams per liter; charg<j bal

(meq cation + meq anion) x 100; PTn, Paintbrush nonwelded; CHn, Calico Hills nonwelded; PP, Prow Pass Tuff; YM, Yucca Mountain Tuff; CFu, Crater Flat unit]

Sample 

identification

UZ- 14/45.0-

45.4/PTn/YM

UZ-14/1735.3-

1735.53/CHn

UZ-14/1804.5-

1804.7CHn/PP

UZ-14/1825.8-

1826.0/CHn/PP

UZ- 14/1 854.89-

1855.1/CHn/PP

UZ-14/1865.7-

1865.9/CHn/PP

UZ- 14/20 14,7-

2014.9/CHn/PP

UZ-14/2015.2-

2025.7CHn/PP

UZ- 1 4/2025.1-

2025.3/CHn/PP

UZ-14/2095.6-

2095.8/CFu

UZ-14/2104.05/

bottom/CFu

Symbol 

in Piper 

diagram

(fig- 3)

1

a

b

c

d

e

f

g

h

n

*

Average 

depth 

(m)

13.78

528.95

550.04

556.53

565.40

568.70

614.11

615.85

617.28

638.71

641.33

Date

09-19-94

05-04-95

05-10-95

06-22-95

07-07-95

07-20-95

07-27-95

08-02-95

08-02-95

08-18-95

04-22-94

PH

8.6

9.0

8.7

9.4

9.2

9.1

9.3

9.2

9.3

9.0

9.4

Specific 

conductance

(uS/cm)

1,320

700

480

590

460

480

1,520

1,450

1,550

530

Calcium 

Ca 

(mg/L)

19.6

1.7

0.5

0.6

0.5

0.3

3.2

0.9

4.0

0.0

2.1

Magnesium 

Mg 

(mg/L)

6.1

0.0

0.2

0.1

0.2

0.0

0.2

0.0

0.2

0.0

0.7

Sodium 

Na 

(mg/L)

249.3

158

111

138

107

115

392

312.7

414

112.8

143

Silica 

SiO2 

(mg/L)

59.5

46.9

43.0

47.9

34.5

30.4

37.7

68.6

54.8

63.0

352

Aluminum 

Al 

(mg/L)

I

0.0

1.0

0.9

1.6

0.8

0.2

...

0.2

...

38

Bicarbonate 

HCO, 

(mg/L)

245

288

181

42

181

204

409

339

493

143

229

Carbonate 

CO, 

(mg/L)

18

18

12

112

23

18

219

176

189

29

19

Chlorine 

Cl 

(mg/L)

245

37.4

25.1

23.5

18.8

17.3

32.1

23.2

36.5

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10 Hydrochemical Investigations In Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
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12 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Table 4. Chemical composition of pore-water samples from boreholes USW SD-7, SD-9, and SD-12, Yucca Mountain area Contin

[m, meter; uS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; , data not available; 0, values below detection limit; -, less than; 

(meq cation - meq anion)/(meq cation + meq anion) x 100; PTn, Paintbrush nonwelded; CHn, Calico Hills nonwelded; PP, Prow Pass Tuff; BT, bedded tuff; TSw, Top

0

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MICAL AND1

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Symbol . 

Sample inPiper A^af 

identification diagram , pl" 

fig. 5 (m)

SD-9/1452.6-1452.8/TSw

SD-9/ 1 535.2- 1535.4/CHn

SD-9/1619.9-1661.4/CHn

SD-9/1661.1-1661.3/CHn

SD-9/1741.0-1741.2/CHn

SD-9/ 174 1.7-1 74 1.9/CHn

SD-9/1800.8/CHn/BT

SD-12/265.8-266. 1/PTn/BT

SD- 1 2/278.6-278.8/PTn/pc

SD-1 2/296. 1-296.6/TSnw

SD-12/1460.7-1461.0/CHn

SD-12/1495.5-1495.8/CHn

SD-12/1517.0-1517.4/CHn

SD-1 2/1 558.9- 1559.5/CHn

SD-12/1582.5-1582.7/CHn

SD-12/1600.6-1603.0/CHn

SD-12/1636.9/CHn/BT

SD-12/1901.5/CHn/PP

SD-1 2/1 938.8/CHn/PP

SD-12/1942.4/CHn/PP

7

8

9

X

Y

-

Z

A

B

C

D

E

F

G

H

I

J

K

L

M

442.78

467.96

500.09

506.33

530.69

530.90

548.88

81.08

84.95

90.34

445.28

455.89

462.44

475.24

482.38

488.23

498.93

579.55

590.93

592.04

Date

08-30-95

09-07-95

01-26-96

01-22-96

07-18-96

07-12-96

01-30-96

10-27-95

06-20-97

09-10-96

04-17-96

06-24-97

02-21-96

08-01-97

08-05-97

05-08-96

03-22-96

09-16-96

02-28-96

03-11-96

pH

7.5

7.4

8.9

9.1

9.1

8.8

9.3

7.2

7.3

7.1

8.5

8.5

8.3

8.5

8.5

9.1

9.0

9.1

8.8

9.1

Speflc Calcium Ma9ncconduc- . sium

tance a Mg 

(uS/cm) (m9/L) (mg/L)

520

530

610

670

520

310

790

470

580

490

490

550

660

640

420

540

535

470

625

520

6.9

0.8

0.4

0.7

0.2

0.2

0.8

48.9

74

75

31

16

14

4.4

1.2

2.6

3.8

1.3

1.4

1.3

0

0.1

0

0.0

0

0

0

7.6

12.7

8.0

0

0.2

0

0.1

0.1

0

0

0

0

0

Sodium 

Na

(mg/L)

112

112

136.6

164

125

74

180.7

28.6

27

21

89

108

150

155

97

129

129

122

165

140

Silica P<?tas" 

Si02 Sl£m

(m9/L) (mg/L)

62.5

54.9 7

55.6 4

48.8

52

53

59.6 6

62.7 3

71.2

72.2

65.5

87.3

56.9

76.1

71.0

65.7

64.6

90.7

55.0

60.1

Bicarbonate 

HC03 

(mg/L)

256

226

232

317

185

113

137

107

159

163

98

49

323

210

113

79

67

151

171

134

Carbonate 

C03 

(mg/L)

0

0

12

0

41

12

106

0

0

0

30

57

0

41

29

90

102

59

66

66

Chlorine 

Cl 

(mg/L

15.7

15.6

50.2

42.0

19

15

32.3

49.6

46

60

55.9

57

35.6

33

17

27.8

15.2

22

34.0

24.0
f 

5

n

(D

I

(0

(D* 

0>

O

CD

(Q

(D

2.

N

O

CD 

0

O 

O

CO

o

c

(9

i

CD

Explanati

Q D D O

a D D o

a o D a

a H ea r> T

Total Dissolved Soli da 

IPcrte Par Million)

0 50-7mD.OFT/PTN/BT) 

b SD-7I37D.9FT/TSIW) 

c SHI119B.9FT/CN) 

d SD-7I1525.2F7/CHN) 

e 50-7ll538.5FT/CrN) 

r 8D-7I16D0.2/CHN) 

3 SD-7I1617.1FT/CKN) 

h SD-7I1890.9FT/CKN/PPI 

! 3h7llS3Z.6FT/CrN/PPI 

SHI20BB.1/PPI 

SD-7l2i7D.3/PP/BT) 

1 SD-712596.2FT/BF) 

"ED-7I259&.7/BFI

60 <*

Colciun (Co)

11 P-60 BD 

Chlcricfe ten

Cl

CATIONS %neq/l A N I 0 N S 

Figure 5. USW SD-7, SD-9, and SD-12 pore-water compositions.

PSD-7[S98.1Fr/BF) 

1 SD-9(94.3Ff/rWBTI

3 SD-9(135.2/PTN/Yh) 

< 80-8(151. 1FT/PTN/BT)

SO-9(i7G.3/PTN/PC)

8 30-3(1535. 3FF/CHN) 

9 SD-9(1JM0.7FT/CHN) 

x SD-S(lfi61.2Ff/CHN) 

T SD-9(17^ll.iFF/CHN) 

z SD-9QfiQ0.8Fr/CHN/BT)

%meq/l = percentage 

milliequivalents per li
In all tables in this paper, core samples are 

identified as "UZX-YYYY-ZZZZ/UPN/" or 

"UZX-YYYY-ZZZZ /T/UPN/D". The first three 

letters "UZX" are used to identify the unsaturatedzone borehole number; for example UZ-14. The next 

group of letters YYYY-ZZZZ indicates the depth 

interval (in feet unless noted otherwise) where core 

was obtained; UPN stands for uniaxial compression 

first stage (UP1) or second pressure stage (UP2), and 

so on; "D" stands for distillation; and "T" stands for 

imbibed test water (not the original pore water). For 

example, a sample identification of "......../UP 1/D"

indicates a vacuum-distilled water sample from core 

that has undergone first-stage uniaxial compression. If 

there is no "UPN" in front of "D" (such as "UZ14- 

YYYY-ZZZZ/D"), the water was extracted by vacuum 

distillation without compression.

Chemical compositions of water samples show 

clear variations. Locations of UZ-14, SD-9, and the 

NRG boreholes are in the northern part of Yucca 

Mountain, while boreholes SD-12 and SD-7 are in the 

southern part of the mountain. The pore waters 

extracted from bedded tuff are calcium-chloride- or 

calcium-sulfate-type waters which plot near the top 

part of the diamond in a Piper-type diagram (see figure 

3, for example). Three samples from Topopah Spring 

Tuff, one from each borehole, are shown in figure 5. 

Two of them, "b" (SD-7) and "C" (SD-12), are from 

the upper nonwelded unit and one, "7" (SD-9), is from 

the lower welded unit of the Topopah Spring Tuff. 

Chemical composition of upper and lower units also 

are different, with the upper unit closer to calciumchloride and calcium-sulfate type, and the lower unit 

closer to sodium-bicarbonate type. Chloride concentration in the lower unit is less concentrated than the 

upper unit, indicating dilution by the fracture water. 

The pore waters extracted from the Calico Hills 

nonwelded hydrogeologic unit are sodium-carbonateor sodium-bicarbonate-type waters which plot near the 

lower part of the diamond. Samples represented by 

"c", "d", "e", "f', "g", "D" and "E" in figure 5 are pore 

waters from borehole SD-7 and SD-12 in the Calico 

Hills nonwelded unit. These waters tend toward 

calcium-chloride- and calcium-sulfate-type waters 

instead of the sodium-bicarbonate- or sodiumcarbonate-type waters typical of the Calico Hills 

nonwelded unit in the other boreholes. This could be 

due to less Ca/Na cation exchange, with less sodium 

ion released into water in the SD-7 and SD-12 boreholes. Therefore, in spite of large distances separating

these boreholes, chemical compositions are, with few 

exceptions which will be described below, in general 

similar and vary according to the lithologic units in 

which they were obtained.

The dissolved solids concentrations of the porewater samples from the Yucca Mountain Tuff at 

13.78 m and from the Prow Pass Tuff at 614.11 m 

(table 2), at 615.85 m, and at 617.28 m in UZ-14 

(shown by points 1, f, g and h in figure 3) have very 

high concentrations of dissolved solids (about 

1,200 mg/L for the 13.78-m sample and 2,000 mg/L 

for samples in the Prow Pass Tuff). Also, pore-water 

sample G in figure 4 from the NRG-6 borehole at 

78.03 m has a total dissolved solids concentration of 

3,500 mg/L. Evaporative concentration and/or 

extended water-rock interactions can result in such a 

large concentration. If the waters were concentrated by 

evaporation, one possible mechanism could involve 

evaporation by vapor transport in the unsaturated zone.

The sample from UZ-14 at a depth of 13.78 m 

(UZ14/45.0-45.4/PTn/YM in table 2) has Na and Cl 

concentrations of 249.3 mg/L and 245 mg/L, respectively. Samples at slightly greater depth, such as 

UZ14/85.2-85.6/up 1UZ14/91.0-91.3/upl, and 

UZ14/95.5-95.9/upl (table 3, Yang and others, 1996), 

have an average Na concentration of 39 mg/L and Cl 

concentration of 60 mg/L. Sample UZ 14/45.0- 

45.4/PTn/YM is more concentrated in Na by a factor 

of 6 and in Cl by a factor of 4 than the latter. If evaporative concentration is the cause, both Na and Cl 

should be elevated by the same factor, a relation that is 

not observed. Further, the stable-isotope composition 

of this pore water (8D and 8 I8O values) plots close to 

the Yucca Mountain Precipitation Line (YMPL) in the 

8D versus 8 18O diagram (see fig. 19) indicating little 

moisture loss by evaporation. (A detailed explanation 

is given later in the section on the interpretation of 

stable-isotope data.) These two lines of evidence indicate that high total dissolved solids in this sample are 

the result of water-rock interactions with only a minor 

contribution from evaporative concentration. Similarly, for samples from the Prow Pass Tuff from UZ-14 

and the NRG-6 sample from a 78.0-m depth, the 

concentration factors for Na and Cl are different, again 

indicating extensive water-rock interaction. The 8D 

and 8 18O values of these pore waters, when plotted on 

the 8D versus 8 18O diagram, also show deviation from 

the YMPL, indicating loss of water by evaporation. 

Evaporative concentration does occur to some extent 

(less than 10 percent) but cannot account for the

CHEMICAL AND RADIOACTIVE-ISOTOPE COMPOSITIONS (3H AND 14C) OF GAS AND PORE WATER FROM THE

UNSATURATED ZONE

15
observed doubled concentration factor. (See detailed 

explanation later in the interpretations of stableisotope data.) Water-rock interactions thus appear to 

be the major cause of high dissolved-solid concentrations, and there is no evidence from these samples for 

major unsaturated-zone vapor transport.

The concentrations of major ions in the pore 

water are three to ten times larger than those in the 

perched or saturated-zone waters. These pore waters 

are from bedded/nonwelded tuffs of the Paintbrush 

nonwelded (PTn) and Calico Hills nonwelded (CHn) 

hydrogeologic units only, not from the large block of 

the welded Topopah Spring Tuff (TSw) hydrogeologic 

unit at Yucca Mountain. If the more dilute infiltrating 

waters provide recharge to the perched or saturatedzone waters, they must flow rapidly through fractures 

or permeable zones in the intervening PTn or CHn 

units to avoid mixing with the chemically concentrated 

pore waters in those units. The water flux through such 

a flow regime is small based on the narrow shape of 

the tritium peaks. This conceptual model is also 

supported by the detection of post-bomb 3H in the 

deep unsaturated zone of the mountain, indicating a 

fast flow path (Yang and others, 1996).

Tritium Profiles in Boreholes

Pre-1952 tritium concentrations in precipitation 

were around 10 TU (Davis and Bentley, 1982). In 

1954, atmospheric nuclear tests artificially increased 

the average annual tritium concentration in precipitation near Yucca Mountain to about 200 TU, with a 

monthly maximum of about 700 TU. By 1963, further 

nuclear tests in the atmosphere increased the average 

annual tritium concentration in precipitation to about 

2,000 TU, with a monthly maximum of about 

4,000 TU (International Atomic Energy Agency 

reports, 1969-86). After three half-lives of tritium 

decay from 1954 to 1990, the average annual tritium 

concentration in precipitation for 1954 input still 

remained at 25 TU, with a monthly maximum of about 

87 TU. For 1963 input, average annual tritium concentration in precipitation after two half-lives from 1963 

to 1987 still remained at 500 TU with a monthly 

maximum of 1,000 TU. Therefore, water samples 

extracted from rock core with tritium concentrations 

greater than 25 TU in 1990 would indicate mixtures 

derived from pre- and post-1954 precipitation. Tritium 

concentrations greater than 500 TU in 1987 would 

indicate mixtures derived from pre- and post-1963

precipitation. Assuming piston flow is dominant in the 

unsaturated zone of Yucca Mountain, an assumption 

described by Yang and others (1996), the high tritium 

concentrations of pore-water samples such as these 

can be attributed mainly to post-1954 or post-1963 

precipitation.

The purpose of the following interpretations of 

tritium concentrations is not to determine the age of 

pore water in the unsaturated zone but to isolate any 

pore-water samples that could have been derived 

mainly from post-1954 precipitation. Tritium profiles 

for pore waters in borehole UZ-14 are shown in 

figure 6. Tritium concentrations between 20 to 40 TU 

are observed in the intervals between 30 to 45 m in the 

Pah Canyon Tuff and at 385 m depth in the Topopah 

Spring Tuff. Only small intervals of the NRG-6 and 

NRG-7a cores were analyzed for pore-water tritium 

and 14C. The data are shown in figures 7A and 7B. 

High tritium concentrations of about 30 to 150 TU in a 

broad peak are observed in NRG-6 boreholes (fig. 7A) 

from 53.3 m to 74.7 m in the Pah Canyon Tuff and 

near the top of the Topopah Spring Tuff. One large 

tritium concentration of about 47 TU is observed at a 

depth of 108.7 m in NRG-7a. Tritium data for pore 

waters extracted from SD-7, SD-9, and SD-12 borehole cores are shown in figures 8, 9A, and 9B. The 

tritium concentrations are less than 10 TU in SD-7 

samples and as large as 14.3 TU in the CHn unit of 

SD-12; SD-9 core samples have tritium concentrations 

as high as 21 TU at 50-m depth. As mentioned previously, only limited samples were analyzed from each 

borehole. Therefore, some fast flow paths may not 

have been detected.

Residence Times of 14CO2 and Pore Water

Gaseous-phase 14C and 5 l3C data from SD-7, 

SD-9 and SD-12 are shown in table 5. The 6 13C 

samples collected by the WG balloon method varied 

from -10 to -25 per mil in 8 I3C values for the replicate 

samples. Therefore, these data are not listed here. 

Problems likely were in the balloon nozzle. Values of 

8 I3C values obtained by Desert Research Institute 

(DRI) are less variable, although they tend to be 

systematically more negative by a few units per mil 

when collected with the molecular sieve. The other 

possible explanation of 6 13C values less than -20 per 

mil could be due to Exploratory Study Facility (ESF) 

ventilation, which causes degassing of pore-waterdissolved CO2 that was collected during gas sampling

16 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
CHEMICAL ANDRADIOACTIVE-ISOTOPE COMPOSITIONS (3HAND14 C) OFGASPORE WATERFROM Tl

UNSATURATED ZOI

METEF 

0

20

I 40 

rr

C/)

<

£ 60

g

LU 

CO

Q_ 

LU

Q 80 

100

120

(

3S M- F

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) (water) x

/ (-17-1) 

C~^ (-15.0) .

~^^* (-13.0) #

Y o 

N. (-25.0) , 

V. (-10.5) 

~\ (-21.2) * o

\ Tritium data C-14 (water) C-14 (gas)

( 0

~~ , I I I ,9 I i I

Alluvium

Yucca Mtn 

Tuff

Bedded Tuff

Pah Canyon 

Tuff

Bedded Tuff

Topopah 

Spring 

Tuff -

I

) 20 40 60 80 100 120 14 

H-3 IN TU AND C-14 IN PERCENT MODERN CARBON

m m

FEET METERS 

0

B.

400

= 100

450 -

- 200

500

- 300 550

600 -

400

C-1 3 data 

(-15.8)** 

(-14.6) 

M0.3)

(-14-7)

(-is.?)

0 20 40 60 80 

H-3 IN TU AND C-14 IN PERCENT 

Figure 6. Lithologic units, tritium, and 14C (water) concentrations of borehole USW UZ-14, (A) top 121.9 m, (B) bottom 274.3 m in C

Pass Tuff.
81 |BO|iuaijoojpAH suo|ieB|isaAU| ! u Bu|Z|japejeu,3 ' epBA9N 

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Tuff ~

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Topopah 

Spring 

Tuff

/Bedded Tuff

Calico 

Hills 

Formation

Bedded Tuff 

Prow 

Pass 

Tuff

/Bedded Tuff

Bullfrog 

Tuff 

/ Tram Tuff '

1 ,

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200 

400 

600 

800 

1,000 

1,200 

1,400 

1,600 

1,800 

2,000 

2,200 

2,400 

2,600

0 20 40 60 80 100 120 140 160

CONCENTRATION IN TRITIUM UNITS (TU) AND 

PERCENT MODERN CARBON

Figure 8. Lithologic units, tritium, and 14C (gas and water) concentrations of borehole USW SD-7, Yucca Mountain area.

CHEMICAL AND RADIOACTIVE-ISOTOPE COMPOSITIONS (3H AND 14C) OF GAS AND PORE WATER FROM THE 19

UNSATURATED ZONE
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Table 5. Gaseous-phase carbon-14 (14C), 613C (gas), and fracture-frequency data from boreholes USW SD-7, SD-9, 

and SD-12, Yucca Mountain area

[m, meter; pmc, percent modern carbon; , data not analyzed]

Borehole

SD-7

SD-9

SD-9

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

SD-12

Depth 

(m)

508-633

0-450.5

450.5-537.7

24.7

43.9

65.2

91.7

107.0

128.9

171.0

208.2

236.8

256.6

285.0

322.5

385.6

407.2

435.9

Station with 

instrument

...

...

P

O

N

L

K

J

I

H

G

F

E

D

C

B

A

Lithologic unit1

Prow Pass Tuff

Tiva-Topopah Spring Tuff

Calico Hills nonwelded unit

Tiva Canyon Tuff

Tiva Canyon Tuff

Tiva Canyon Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Calico Hills nonwelded unit

i«C 

(pmc) 

DRI2

41.5

96.7

51.0

91.0

49.2

87.4

82.7

77.8

58.0

39.0

36.9

31.6

34.1

52.4

34.3

88.7

24.4

84.5

6I4C Published 

(per mil) 3.05-m 

DRI2 composite

-25.0

-16.1

-19.1

-22.6

-20.2

-22.0

-16.4

-21.6

-22.1

-23.6

-24.7

-25.6

-24.4

-21.9

-23.8

-22.9

-24.6

-17.3

...

...

...

8

4

15

...

5

6

5

18

62

6

13

35

48

7

...

Lithologic unit from Sandia National Laboratory Reports: SD-7, Rautman and Engstrom, September 1996; SD-9, Engstrom and 

Rautman, September 1996; and SD-12, Rautman and Engstrom, November 1996.

2Desert Research Institute (DRI), 8 13C values are measured from subsample of I4C collected in molecular sieve. Delta 13C is 

defined as relative 13C/I2C values of sample, expressed in part per mil (%o) relative to Pee Dee belemnite (PDB) standard.

(a kinetic fractionation effect). Carbon dioxide 

concentrations are not reported here due to variable 

data, with values ranging from 1,000 to 7,000 ppm.

The excess CO2 could come from dissolved 

CO2 in pore water of the Topopah Spring Tuff. The 

I4C activities of CO2 gas from boreholes SD-9 and 

SD-12 are plotted in figures 9A and B, and 5 13C 

values from SD-12 are plotted in figure 10. One 14C 

activity of gaseous-phase CO2 was derived from the 

interval between the top of the Calico Hills nonwelded 

unit and the water table of SD-9. The 14C activity of 

this sample is 51 pmc, significantly smaller than the 

I4C activities of pore waters (between 82.3 and 95.3 

pmc) in the same zone. If the CO2 in the gaseous phase 

were in isotopic equilibrium with CO2 in the aqueous 

phase of the pore water, the I4C activities in the two 

phases should be comparable. The fact that they are 

different possibly could be due to atmospheric 14CO2 

contamination to the pore water during drilling.

The I4C activities of gaseous-phase CO2 in 

instrumented borehole SD-12 (table 5) decreased from

surface to depth with some irregularly large C activities in stations E, C, and A and small 14C activity in 

station O. Anomalously large C activities in station 

C and A may indicate contamination of the samples. 

Samples from station C may have been affected by 

leakage through the borehole instrumentation support 

pipe (J.P. Rousseau, U.S. Geological Survey, oral 

commun., 1997). Station A lies in a region of strongly 

low pressure at depth (identified from monitoring 

sensors) with complete attenuation of the surface 

(synoptic) barometric-pressure signal. Gas-monitoring 

evidence indicates that station A likely was contaminated with atmospheric air drawn in due to the low 

pressure, prior to the instrumentation and sealing of 

the borehole (J.P. Rousseau, U.S. Geological Survey, 

oral commun., 1997). Moderately large C activity at 

station E may be related to the relatively high fracture 

frequency noted at that station (table 5). The majority 

of 14C activities in gas-phase CO2 from SD-12, 

however, are consistent with the data obtained from 

borehole UZ-1 and further are consistent with gas

CHEMICAL AND RADIOACTIVE-ISOTOPE COMPOSITIONS (3H AND 14C) OF GAS AND PORE WATER FROM THE

UNSATURATED ZONE

21
METERS 

0

100

200 

LU 

O 

<

LL

or

Z) 

V) 

Q 300

g

CD 400

Q_ 

LU 

Q

500

600 -

x Alluvium

Tiva

Canyon

Tuff

- Bedded Tuff

Pah Canyon Tuff

Topopah

Spring

Tuff

Calico

Hills

Formation

Bedded Tuff

Prow 

Pass 

Tuff

Bedded Tuff

FEET 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

-35 -30 -25 -20 -15 -10 -5

DELTA CARBON-13, IN PER MIL

Figure 10. Lithologic units and 8 13C (gas) values, borehole USW SD-12, Yucca Mountain area.

0

22 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
transport by a postulated diffusion mechanism as 

previously concluded (Yang and others, 1996).

Yang and others (1996) stated that the high 

pore-water 14C pmc values in the CHn from borehole 

UZ-14 could be related to (1) 14CO2 in the gaseous 

phase possibly exchanging with the bicarbonate 

species in the pore water; and/or (2) the large amounts 

of cement used below a perched-water zone at 383.1 m 

to seal off the borehole wall and prevent perchedwater leakage (Portland cement contains Ca(OH)2 

which will absorb atmospheric CO2 during mixing 

with water and release the CO2 during exothermic 

curing). In more recent pore-water data obtained from 

the Calico Hills nonwelded unit in SD-9, 14C values 

ranged from 82.3 pmc to 95.3 pmc between depths of 

467.9 m and 548.6 m. Those data are in the same 

range of 14C values obtained in the Calico Hills 

Formation from UZ-14. Both boreholes are located in 

the northern part of Yucca Mountain; SD-9 was drilled 

without using cement. Therefore, it is unlikely that the 

14C data in the Calico Hills nonwelded unit from 

UZ-14 were contaminated by cement.

With regard to exchange of young 14C in 

gaseous-phase C02 with the bicarbonate species in the 

pore water in the Calico Hills nonwelded unit of UZ14 and SD-9, the sparse data obtained so far are inconclusive. Gaseous-phase 14C data (table 5) from one 

analysis from the Calico Hills nonwelded unit in SD-9 

show 51.0 pmc (calculated age about 5,800 years), but 

one analysis from the Calico Hills nonwelded unit in 

SD-12 shows 84.5 pmc (about 1,340 years). From the 

14C data of gaseous-phase CO2 obtained in the UZ-1 

and SD-12 boreholes, true gaseous-phase 14C activities likely are about 50 pmc in the Calico Hills 

nonwelded unit (Yang and others, 1998). A possible 

explanation for the discrepancy between large 14C 

activities in the Calico Hills nonwelded unit in UZ-14 

and SD-9 is that pore waters were contaminated with 

atmospheric 14CO2 during drilling or mixing of young 

water with old water. However, the fact that 14C activities in the Calico Hills nonwelded unit in SD-7 and 

SD-12 are about 40 to 85 pmc would exclude large 

contamination of cores by the drilling air (with C 

about 120 pmc) because all these boreholes were 

drilled with air. The 8 13C values of bicarbonate carbon 

in pore water from the Calico Hills nonwelded unit in 

SD-9, UZ-14, SD-7, and SD-12 also do not show large 

contamination from atmospheric 14CO2 (8 13C of -7 to 

-8 per mil in atmospheric CO2). For confirmation of 

positive 14C values in pore water, 14C analyses should

be made on cores that are drilled with compressed 

nitrogen gas (which contains no carbon dioxide as a 

coolant) instead of atmospheric air. In contrast, 

perched water encountered in boreholes likely is 

uncontaminated by the 14CO2 in the drilling air 

because the mass of carbon in these water reservoirs is 

orders of magnitude larger than in the drilling air.

STABLE-ISOTOPE COMPOSITIONS (5D 

AND 818O) OF PORE WATER FROM 

USW UZ-14

Oxygen, deuterium (2H), and hydrogen isotopic 

ratios of water have great value in hydrologic studies 

because they are intimately dependent upon the nature 

of physical processes that govern water transport 

(Craig, 1961b; Gat, 1980). Stable-isotope ratios 

( 18O/ 16O and 2H/! H) expressed as 8 18O and SD can be 

used to indicate the origins of water, climatic and 

evaporative history of water, and flow paths through 

the unsaturated zone. They can also be used as indicators of transport properties or water interactions with 

other minerals. The lack of long-term direct measurements of infiltration requires proxy indicators of water 

movement through the unsaturated zone to extend the 

record into the past. The isotopes most commonly 

used in hydrology are those that are constituents of 

water molecules ( 18O, 2H, and 3H). Other environmental isotopes ( 14C, 13C, and 36C1) that occur in 

dissolved compounds are also valuable for studying 

ground-water cycles (Fontes, 1980) and have been 

applied in the previous section.

Hydrogen has two stable isotopes (2H and *H), 

and oxygen has three stable isotopes ( 16O, 17O, and 

18O). Because hydrogen and oxygen are components 

of the water molecule, isotopic fractionation of both is 

usually co-variant. Consequently, the isotope ratios of 

these two elements generally are discussed together. 

Natural variations of the abundance of these stable 

isotopes can be related to the history of the water 

molecules (Gat and Gonfiantini, 1981). Rather than 

express these variations as the absolute abundance of 

the isotopes, Vienna standard mean ocean water 

(VSMOW) is used as the standard against which the 

natural variations are measured.

STABLE-ISOTOPE COMPOSITIONS (5D and S18O) OF PORE WATER FROM USW UZ-14 23
Thus:

18 50(per mil) = [(R0-Rstd,o)/Rstd,o] * 1<> 

8D(per mil) = [(Rh-Rstd,h)/Rstd,h] * 103

where 8 18O (per mil) and 5D (per mil) are the deviation in O and D of the sample from the 

reference standard in units of per mil 

respectively, R0 = 18O/ 16O and Rh = D/ ! H 

of the sample being studied, and Rst(j 0 = 

1 8O/ 1 6O and Rstd h = D/ 1 H of the 

VSMOW.

After surface water enters the ground-water 

system, its original isotopic composition is typically 

conserved. Some factors, however, can cause the 6 18O 

and 8D concentrations of unsaturated-zone water and 

ground water to differ from local meteoric water. 

These factors include:

1 . Recharge from a water body of partially evaporated 

surface water. Evaporation from surface water 

bodies is a nonequilibrium process that enriches 

8 18O and 8D in the water such that the 8D/8 18O 

slope is less than 8 and usually between 3 and 6.

2. Recharge to an aquifer from an area of different 

altitude or latitude.

3. Mixing of two or more sources of water.

4. Recharge of either the ground water or UZ water 

from a different climatic regime in the past.

5. Interaction with aquifer carbonate or silicate rocks 

at high temperature. Exchange of oxygen in 

water and rock at high temperatures increases the 

8 I8O of the water (the 8 18O shift).

6. Fractionation between the ground or pore water 

and mineral-hydration water.

Methods of Pore-Water Extraction for 5D 

and 518O and Analysis

It is important that a pore-water extraction 

method extract only the hydrologically active water 

(percolating mobile water), or the stable-isotope data 

of the extracted water will be misinterpreted. Extraction methods using vacuum distillation and toluene 

azeotropic distillation to remove water from soil and 

clay-rich soil have been reported by Revesz and 

Woods (1990), Walker and others (1994), and 

Ingraham and Shadel (1992). However, none had been 

applied to tuffaceous volcanic rocks. The distribution

of minerals at Yucca Mountain differs from borehole 

to borehole, but certain commonalities are apparent. 

The major mineral groups of Yucca Mountain are 

zeolites, clays, alkali feldspar, silica polymorphs, and 

glass. The zeolites are primarily clinoptilolite, 

mordenite, and analcime. The clays are principally 

smectites and illite with lesser kaolinite (Broxton and 

others, 1986; Bish and Vaniman, 1985; Bish and 

Chipera, 1989). Zeolite and clay contain water of 

crystallization that can be released during the heatdistillation process. Three extraction methods 

toluene distillation, vacuum distillation, and compression were used on core samples of volcanic rocks 

from Yucca Mountain in order to establish a standard 

extraction procedure.

Toluene Distillation

Toluene distillation of water from soil for 

isotopic analysis was first used by Allison and Hughes 

(1983). The principle of the toluene distillation 

method is that the toluene forms an azeotropic mixture 

with water at 84.1°C but separates from water at room 

temperature. The boiling point of the toluene-water 

azeotrope is 84.1 °C, significantly lower than the 

boiling point of water (100°C) and toluene (110°C). 

Ingraham and Shadel (1992) found that the isotopic 

compositions of water extracted during toluene distillation will fractionate when the distillation process is 

not completed. This result is predicted by the Rayleigh 

distillation model. The results indicate that the toluene 

is simply a carrier of water. Thus, fractionation will 

occur during the toluene distillation if the extraction is 

not complete.

The toluene distillation method used in this 

study is similar in design to that of Revesz and Woods 

(1990). The Dean-Staark apparatus used for toluene 

distillation (fig. 11) consists of a laboratory heating 

element, a flask, a specially designed receiving trap, 

and a straight condenser. The funnel has a wide area 

above the joint to prevent loss of water by uneven, 

explosive boiling (Revesz and Woods, 1990). Enough 

water-free toluene is added so that the sample remains 

covered throughout the entire distillation process. 

During distillation, the temperature is maintained at 

the boiling point (84.1°C). The temperature of the 

solution is then raised to 110°C. At the beginning of 

the distillation, water and toluene are distilled 

together, forming a cloudy liquid. The distillation 

continues until the condensate turns clear and no water

24 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
To vacuum pump t

Condenser

Cooling water

Cooling water

Overflow of 

Hydrocarbon

Funnel 

Joint

Flask

Hydrocarbon 

Sample

Heating element

Figure 11. Toluene distillation apparatus for pore-water extraction.

Hydrocarbon

Receiving trap 

Water

Sampling bottle

drops remain on the glass wall of the entire apparatus. 

After collecting the water in a glass bottle with Teflon 

cap, any trace of toluene is removed by adding paraffin 

wax (enough to solidify after dissolving the toluene) to 

the water and warming it in the closed sample bottle to 

the melting point of the wax (60-70°C). The bottle is 

shaken and allowed to cool, and the wax solidifies. 

The water is decanted easily. The wax procedure can 

be repeated to ensure complete removal of the toluene. 

The wax procedure was repeated three to four times in 

this study.

Vacuum Distillation

The vacuum distillation technique uses a distillation apparatus consisting of vacuum glassware, 

nitrogen gas, dry ice, a set of heating elements, and a

vacuum pump (fig. 12). The procedure consists of four 

steps: (1) the core segment is weighed and placed 

inside the distillation apparatus; (2) a vacuum is 

applied to the distillation apparatus for a couple of 

hours and heated to 150°C (as measured at the heating 

mantle) resulting in a core temperature of about 110°C 

(the reason for choosing this temperature is explained 

below); (3) the core water is expelled into a flask 

cooled with a dry ice/alcohol mixture at a temperature 

of about -80°C; and (4) the total volume of water 

recovered is measured under nitrogen gas. The 

distilled core was placed inside an oven at 104°C for 

three days to check for any further loss of water. The 

third step may take around 24 hours to ensure 

complete extraction of core water. If the core water 

yield is less than 99.75 percent of the total measured 

water in the subsample (a small piece of core chipped

STABLE-ISOTOPE COMPOSITIONS (8D and §18O) OF PORE WATER FROM USW UZ-14 25
f >A Vacuum gage

High-vacuum tubing to vacuum pump

To Nitrogen

Sample chamber

Stopcock C

Water collection bulb

Cold trap 

Dry Ice/Alcohol

Figure 12. Vacuum distillation apparatus for pore-water extraction.

off and dried in the oven at 105°C to measure water 

content), there can be an appreciable amount of fractionation (Knowlton and others, 1989). If incomplete 

extraction was suspected, an additional core was 

vacuum-distilled for a longer time to make sure the 

water yield was greater than 99.75 percent.

The distillation temperature of the core sample 

will affect (1) the release of hydrated water,

(2) the water-rock interaction at high temperature, and

(3) the efficiency of extraction. Hydration water may 

mix with the pore water, thus masking the values of

stable isotopes of pore water. The exchange of 18O 

between water and rock at high temperature (greater 

than 150°C) will enrich the I8O of water (Panichi and 

Gonfiantini, 1981). Extraction efficiency increases 

with distillation temperature, but high temperature will 

cause problems from release of hydrated water and/or 

water-rock interaction. For clay-bearing soil, the 

temperature at which hydrated water is released is 

about 110°C, as reported by Ingraham and Shadel 

(1992). They postulated that the hydration water of 

clay soils may not be released at temperatures below

26 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
110°C; thus, an extraction method with a low temperature may be advantageous. Araguas-Araguas and 

others (1995) concluded that if the isotopic compositions of mobile water are of interest, the distillation 

temperature should be kept as low as possible to minimize the impact of removing the weakly bound hydration water. Feng and Savin (1993) found that some 

zeolite species (clinoptilolite, heulandite, and stilbite) 

will lose about 50 percent of hydrated (channel) water 

during overnight evacuation. Hydrated water is held in 

the structural channels of those minerals. However, the 

exchange of framework oxygen with bulk-water 

oxygen in zeolite was less than 5 percent at temperatures below 110°C. For low-temperature distillation, 

Wood (1990) used vacuum distillation at room temperature, but the extraction was very poor because of 

incomplete recovery of water.

The distillation temperature for this study therefore was chosen at 150°C at the heating mantle, 

resulting in a core temperature of about 110°C. From 

time-series measurements of mantle temperature and 

core temperature for different lithologic cores, the 

temperature distribution indicates that the temperature 

of the core increased with the temperature of mantle 

and equilibrated to core temperature of about 110°C 

when the heating-mantle temperature was at 150°C 

(Yu, 1996).

The time required for complete distillation of 

soil samples was reported by Ingraham and Shadel 

(1992) and Araguas-Araguas and others (1995) to be 

about 7 hours. However, other factors affect the time 

required for complete extraction, such as sample size, 

moisture, distillation temperature, and distillation 

system design. The optimum length of time for distillation was determined experimentally by comparing 

the stable-isotopic compositions of water distilled 

from a piece of selected Yucca Mountain core cut into 

five sections that were distilled for different time 

periods (4, 8, 12, 24, and 36 hours). Five different 

lithologic cores were selected for the test one core 

from the Pah Canyon Tuff, three cores from the 

Topopah Spring Tuff, and one core from the Calico 

Hills nonwelded hydrogeologic unit (table 6). The 

minimum distillation times are different, depending on 

the lithology. For Pah Canyon and Topopah Spring 

Tuff cores, at least 24 hours are required for complete 

extraction, but for Calico Hills nonwelded unit cores, 

more than 36 hours are required.

Compression Extraction

One-dimensional compression extraction 

(Higgins and others, 1997) was also tested. Compression operations were similar to the procedures 

described previously for pore-water extraction for 

chemical compositions and 14C analyses. Water 

samples were collected in glass syringes attached to 

the compression cell when an adequate volume was 

extracted. The extracted water was filtered through a 

0.45-nm filter into glass bottles for stable-isotope 

analysis. The effect of the compression process on 

the alteration of original stable-isotope compositions 

of pore water can be investigated by two methods: 

(1) water samples are collected at each pressure level 

during the compression procedure, and stable-isotope 

compositions at each pressure level are then 

compared; and (2) a water of known stable-isotope 

composition is imbibed into dry cores (from each tuff) 

after the core was oven-dried overnight. The isotopic 

values of known water can be compared with those of 

the water extracted then from the imbibed cores by the 

compression method. If there is no change in composition as a function of pressure in the first test, this 

would indicate that the compression method did not 

cause pore-water fractionation. The results of the 

second test may indicate which pool of water inside 

the core is extracted by the compression method. If 

there is no difference in isotopic composition between 

known water and compression-extracted water, this 

would also indicate that compression processes did not 

alter the original pore-water isotopic compositions. On 

the other hand, if the stable-isotope composition of 

compression-extracted water is different from the 

imbibed water, and the extracted water does not 

change with pressure, the possible explanation is that 

the water imbibed into the dried core has been fractionated. That can be investigated by comparing the 

stable-isotope compositions of residual water with 

those of compression-extracted water.

Water Analyses for 518O and 5D

Stable-isotope ratios 8 l8O and 8D were 

analyzed using mass spectrometers at the University of 

Colorado, Institute of Arctic and Alpine Research 

Stable Isotope Laboratory and at the U.S. Geological 

Survey Research Laboratory in Reston, Virginia. The 

5 l8O values were determined by isotopically equilibrating water with a small amount of pure CO2 in a 

constant-temperature bath. The equilibrated CO2 gas

STABLE-ISOTOPE COMPOSITIONS (5D and 518O) OF PORE WATER FROM USW UZ-14 27
Table 6. Stable-isotopic compositions and water yield of water distilled from Yucca Mountain cores by 

use of different distillation times

[6I8O, delta oxygen-18; 8D, delta deuterium; VSMOW, Vienna Standard Mean Ocean Water; NA, not analyzed. There was 

not enough sample to perform oxygen-18 analyses]

Step 

number

1

2

3

4

5

1

2

3

4

5

6

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

6

Collection time Water yield 

(hours) (percent)

8'°O 8D 

relative to relative to 

VSMOW VSMOW 

(per mil) (per mil)

Run A. Core ID: UZ1 4-156.2 from Pah Canyon Tuff

4 10.12 -15.1

8

12

24

36

Run B. Core ID

4

8

12

24

36

48

Run C. Core ID:

4

8

12

24

36

Run D. Core ID:

4

8

12

24

36

15.12

18.33

19.78

19.82

-13.2

-12.9

-12.2

-12.1

: UZ14-1020 from Topopah Spring Tuff

3.42 -14.9

4.51

5.20

5.21

6.03

6.03

-14.2

-13.3

-13.6

-13.4

-13.2

UZ14-1 187.8 from Topopah Spring Tuff

2.16 NA

3.71

3.96

4.08

4.08

-16.3

-16.5

-15.0

-15.1

UZ14-1391.2 from Topopah Spring Tuff

11.13 -16.2

17.55

18.43

21.90

21.90

-16.7

-16.5

-16.7

-16.7

Run E. Core ID: UZ1 4-1 430.2 from Calico Hills nonwelded hydrogeologic unit

4 4.28 -19.1

8

12

24

36

48

16.67

17.35

18.53

18.50

18.51

-16.3

-16.2

-16.0

-15.9

-15.8

-115

-104

-99

-96

-96

-110

-104

-99

-99

-99

-98

-106

-103

-103

-102

-102

-112

-109

-108

-109

-109

-140

-112

-108

-109

-106

-107

was measured in the mass spectrometer, and the 8 18O 

of the water was calculated from this value. Typically 

4 mL of water are used, although smaller samples 

(down to 2 mL) can be measured in this way. The 

equilibration system is automated and controlled by a 

computer that also controls the operation of the mass 

spectrometer. All critical parameters of the system 

(pressures and temperatures) are monitored by the 

computer, and any sample that experiences conditions

1 %

outside of acceptable ranges is rejected. The 8 O 

equilibration system is capable of measuring 

20 samples and 4 standards overnight.

The 8D values were determined by reducing 

water to hydrogen gas and by measuring the 8D of the 

hydrogen in the mass spectrometer. Water was injected 

into a hot evacuated cell. The water vapor thus formed 

was passed through a capillary tube and over uranium 

metal at 600°C. The uranium metal reduced water

28 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
vapor to hydrogen gas. The injection of water, reduction to hydrogen, and measurement of 8D values in the 

mass spectrometer are all controlled by computer. The 

volume of water required is 2 mL. This system is 

capable of measuring 20 samples and 6 standards in 

about 20 hours. Samples prepared for this method 

should have a conductivity of 2,000 uS/cm or less to 

avoid salt buildup in the evacuated cell. Uncertainty in 

8 O values was 0.2 per mil and in 8D was 1.0 per mil. 

These uncertainties are one standard deviation (la). 

Relative 18O/ 16O (818O) and D/H (8D) values of water 

expressed in %o relative to VSMOW water 

(0 %o) on a scale normalized so that the 8 I8O and 8D 

of Standard Light Antarctic Precipitation (SLAP) 

water are -55.5 %o and -428 %o, respectively.

Inconsistency in 5D and 518O Data with 

Different Extraction Methods

The extraction of isotopically representative 

water from porous media for stable-isotope analysis 

has been sometimes difficult, particularly in the case 

of unsaturated core, in which not all pore spaces are 

filled with water. It is mandatory that no fractionation 

(change in isotopic composition) occurs during the 

extraction processes. Pore water extracted from 

samples from the same piece of core by different 

methods ideally should yield the same isotopic 

composition.

Review of Water-Extraction Methods Published in 

the Literature

Walker and others (1992, 1994) compared the 

stable-isotope data from 14 different laboratories on 

four continents for soil water derived by various 

extraction methods. They distributed soils rehydrated 

with water of known stable-isotope composition to the 

14 laboratories where the soil water was extracted and 

returned for analysis. Extraction methods included 

toluene-azeotropic distillation (six laboratories), 

vacuum distillation (seven laboratories), microdistillation (three laboratories), and centrifugation (one laboratory). The comparison showed large variation among 

laboratories in isotopic compositions of the extracted 

water (up to 30 per mil for 8D and 4.4 per mil for 

8 O). The measured compositions from extracted 

waters were commonly lighter than those of the introduced water by more than 10 per mil in 8D and 2 per

mil in 8 I8O. The variation increased as the water 

content of the soil decreased and was greater for clays 

than for sand at a comparable matrix suction force. 

The variation was caused by incomplete extraction of 

soil water from the soil samples. Centrifugation 

followed by toluene-azeotropic distillation gave the 

results closest to the known value.

Because few studies have examined extraction 

methods for stable-isotope analyses of hydrated 

mineral soil (soil containing certain clays or gypsum), 

Walker and others (1992, 1994) suggested that 

toluene-azeotropic distillation and vacuum distillation 

are unsuitable for gypsum media because gypsum is 

unstable above about 45 °C. At temperatures above 

45 °C, gypsum releases its hydrated water to form 

bassanite, which results in the fractionation. Centrifugation and compression at room temperature may be 

useful but are generally restricted to samples of high 

water content only.

Other studies examined these problems. 

Ingraham and Shadel (1992) did a comparative study 

of the toluene and vacuum distillation methods using 

Hanford loam soil. The stable-isotope compositions 

of water extracted by these two methods were significantly different from those of the added water 

when the water content of the soil was low (3.6 to 

5.2 percent). They postulated that some of the added 

water bound with minerals in the soil with an associated fractionation and was not released at extraction 

temperatures below 110°C. At low soil-moisture 

content, the volume of fractionated bound water is 

enough to affect the remaining extracted soil water. 

Leaney and others (1993) used the azeotropic distillation method with kerosene as a solvent for soils with 

high clay content (20 to 60 percent). They found a 

large discrepancy in the 8D composition between 

added water and extracted water. The extracted water 

was 3 to 10.8 per mil more depleted in deuterium than 

the added water, depending on the temperature of 

distillation and the content of clay. Only deuterium 

was measured in that experiment. When extrapolated 

to zero percent clay content, the extracted water was 

3 per mil more depleted than added water. They 

concluded that fractionation likely occurred during 

solvent extraction of soil and that the extracted water 

would be depleted by more than 3 to 4 per mil in 8D 

compared with the true value of soil water. AraguasAraguas and others (1995) performed the vacuumdistillation method on clay-bearing soil. The data 

indicated that the extracted water was depleted in

STABLE-ISOTOPE COMPOSITIONS (50 and 518O) OF PORE WATER FROM USW UZ-14 29
deuterium by about 5 to 10 per mil and 0.36 per mil in 

oxygen, when compared with mobile water.

Analytical Data from Methods Used in This Study

Eleven sections of cores, three from nonwelded 

bedded tuff, one from the Pah Canyon Tuff, four from 

the Calico Hills nonwelded unit, and three from the 

Prow Pass Tuff, were chosen to determine if there is 

any alteration of the stable-isotope compositions of 

pore water extracted by toluene-azeotropic distillation, 

vacuum distillation, and compression extraction. Clay 

or zeolite minerals occur in the nonwelded bedded tuff 

and the Calico Hills nonwelded unit but not in the Pah 

Canyon Tuff. Cores from the Prow Pass Tuff, from 

depths of 550.0 m and 625.1 m, contain zeolite, but 

core from a depth of 564.2 m does not. A comparison 

of data from the Topopah Spring Tuff was not possible 

because the water content of this formation is too low 

to provide enough water for analysis by the compression method.

Isotopic Data from Toluene-Azeotropic Distillation

Five sections of cores from Yucca Mountain 

Tuff, Pah Canyon Tuff, Topopah Spring Tuff, Calico 

Hills Formation, and Prow Pass Tuff were imbibed 

with water of known isotopic composition. The source 

water was Boulder, Colorado, tap water with an 

isotopic composition of-17.7 per mil for 8 I8O and 

-129 per mil for 8D. The imbibed cores were then 

subjected to toluene distillation to extract water for 

stable-isotope analyses. Table 7 shows the stableisotope compositions of distilled water, water 

recovery, and major mineralogy of cores. The data 

indicate that the isotopic compositions of recovered 

water were more depleted than the source water by 1.9 

to 3.7 per mil for 6^O and 9 to 23 per mil for 8D. The 

water recoveries are much less than 100 percent, indicating incomplete recovery of core water by toluene 

distillation. Small water yields may be due to the use 

of intact core instead of crushed core, which would 

yield more surface area for water distillation. Based on 

the observation of Ingraham and Shadel (1992), the 

variation of the stable isotopes may be caused by 

incomplete extraction. From the results of the imbibition tests, the toluene distillation method is evidently 

unsuitable for extracting water from the Yucca Mountain cores and is not considered further in this report.

Comparison of Vacuum-Distillation and 

Compression-Extraction Methods

In a comparison test, eleven sections of dried 

core were chosen to test effects of the vacuum-distillation and compression-extraction methods on the 

measured stable-isotope compositions of in-situ pore 

water. Different samples were used than for the 

toluene extraction (table 7), but samples covered a 

similar lithologic range (three from bedded tuff, one 

from the Pah Canyon Tuff, four from the Calico Hills 

nonwelded hydrogeologic unit, and three from the 

Prow Pass Tuff). Table 8 shows mineralogic data from 

the samples chosen for the vacuum-distillation and 

compression-extraction tests.

In the bedded tuff (table 9), three cores from 

depths of about 26 to 29 m indicate that the stableisotope compositions of the water collected by the 

compression method are heavier than the remaining 

water (water extracted by vacuum distillation after 

compression) in the range of 5 to 14 per mil for 8D and 

0.2 to 1.9 per mil for 5 O. The compressed water also 

is more enriched than water extracted from samples 

from nearby core by vacuum distillation in the range 

of 6 to 9 per mil for 8D and 0.1 to 1.1 per mil for 8 ]\). 

The stable-isotope compositions of compressed water 

collected at different pressure steps during the 

compression process changed little with pressure 

changes. Mineralogic data indicate that all of the 

bedded tuff cores contain clay minerals.

The tested cores of Pah Canyon Tuff from a 

depth of 49 m (table 10) indicate that the stableisotope compositions of water collected by different 

methods are not substantially different. The stableisotope compositions of the water collected at different 

pressure steps during the compression process are 

similar, with little variation as a function of pressure. 

These data imply that both distillation and compression methods have no effect on the stable-isotope 

compositions of pore water for cores from Pah Canyon 

Tuff, which contain few hydrated minerals and which 

especially lack zeolites (table 8).

Three cores were sampled from the depth 

interval of 446.7 m to 476.9 m from the Calico Hills 

nonwelded hydrogeologic unit (table 11). Mineralogic 

data (table 8) indicate that these cores contain abundant zeolitic minerals. The stable-isotope compositions of the water collected by the compression 

method are heavier relative to the remaining water 

extracted by vacuum distillation in the range of 2.6 to

30 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Table 7. Results of the toluene-distillation method in extracted water from imbibed cores, Yucca Mountain area

[VSMOW, Vienna Standard Mean Ocean Water; source water, water with known isotopic composition used for imbibing test; %, percent]

Sample identification, lithology, and mineralogy

Lithology (Mineralogy)

5D

(per mil) (per mil)

recovery 

/Dercent\ <Percem'

UZ 14-77.4/T16^o 52 60.5

Yucca Mountain Tuff: Amorphous 30-35%, montmorillonite 14%, plagioclase 20%

UZ14-215/T16/To -19.6 -138 80.2

Pah Canyon Tuff: Mineralogic data not available

UZ14-1271.7/T16Ab -21.4 -149 52.1

Topopah Spring Tuff: Plagioclase 34%, K-spar 20%, Amorphous material 10-15%

UZ14-1514.2/T16Ao -20.5 -141 67.8

Calico Hills Formation: Amorphous 15-20%, Zeolite 67%

UZ14-1943.6/T16Ab -19.7 -140 60.7

Prow Pass Tuff: Zeolite 28%, Plagioclase 1 1%, Amorphous material 25-30%)

Source water -17.7 -129

Table 8. Mineralogic data from Yucca Mountain core samples used for comparing the vacuum-distillation and

compression-extraction methods for stable-isotope analysis

Sample 

identification

UZ14-85.2

UZ 14-9 1.0

UZ14-95.5

UZ14-155.7

UZ 14- 1465.5

UZ14-1542.3

UZ 14- 1564.3

UZ14-1 734.5

UZ 14- 1804.8

UZ14-1 854.8

UZ 14-20 15.2

Zeolite 

(percent)

0

0

0

0

35

35

35

32

0

0

28

Clay 

(percent)

56

30

30

0

0

0

0

0

56

0

0

Plagioclase 

(percent)

5

20

20

30

10

10

10

15

5

30

11

Amorphous 

material

(percent)

30-35

20-30

20-30

30-35

30-35

30-35

30-35

20-30

30-35

5-10

25-30

Other 

(percent)

4-9

20-30

20-30

35-40

20-25

20-25

20-25

33-43

4-9

60-85

21-26

Lithology

Bedded tuff

Bedded tuff

Bedded tuff

Pah Canyon Tuff

CHn Calico Hills

nonwelded unit

CHn Calico Hills

nonwelded unit

CHn Calico Hills

nonwelded unit

CHn Calico Hills

nonwelded unit

Prow Pass Tuff

Prow Pass Tuff

Prow Pass Tuff

STABLE-ISOTOPE COMPOSITIONS (6D and 5180) OF PORE WATER FROM USW UZ-14 31
Table 9. Stable-isotope compositions of compressed water (/UP), remaining water extracted 

by vacuum distillation (/UP/D), and water distilled (/D) from bedded tuff cores

[Remaining water, water remaining in the core after compression; distilled water; the whole-core water extracted 

only by vacuum distillation; VSMOW, Vienna Standard Mean Ocean Water]

Sample identification

UZ14-85.2-85.6/UP1

UZ14-85.2-85.6/UP2

UZ14-85.2-85.6/UP5

UZ14-85.2-885.6/UP6

UZ14-85.2-85.6/UP/D

UZ14-84.3-65.6/D

UZ 14-91. 0-9 1.4/UP

UZ 14-91. 0-9 1.4/UP/D1

UZ 14-91. 0-9 1.4/UP/D2

UZ1 4-89.6-9 1.4/D

UZ14-95.5-95.9/UP1

UZ14-95.5-95.9/UP2

UZ14-95.5-95.9/UP3

UZ14-95.5-95.9/UP4

UZ14-95.5-95.9/UP/D

UZ14-94.8-95.9/D

6180 

relative to 

VSMOW 

(per mil)

-12.7

-12.4

-12.6

-12.7

-13.2

-12.8

-12.4

-13.9

-14.3

-13.5

-12.4

-12.3

-12.3

-12.7

-13.6

-13.4

8D 

relative to 

VSMOW 

(per mil)

-90

-89

-91

-91

-103

-97

-90

-103

-104

-97

-90

-90

-89

-91

-103

-98

Table 10. Stable-isotopic compositions of compressed water (/UP) and water distilled (/D) 

from the Pah Canyon Tuff cores

[VSMOW, Vienna Standard Mean Ocean Water]

Sample identification

6180

(per mil) 

relative to

VSMOW

5D

(per mil) 

relative to

VSMOW

Core ID: UZ14-155.7-156.2

UZ14-155.7-156.2/UP1 -12.5 

UZ 14-155.7-156.2/UP2 -11.7 

UZ14-155.7-156.2/UP3 -11.8 

UZ14-155.7-156.2/UP4 -11.6 

UZ14-155.7-156.2/D -12.2

32 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Table 11. Stable-isotope compositions of compressed water (/UP), remaining water extracted 

by vacuum distillation (/UP/D), and water distilled (/D) from the Calico Hills Formation cores

[VSMOW; Vienna Standard Mean Ocean Water]

Sample identification

UZ14-1465.5-1466.1/UP2

UZ 14- 1 465.5-1 466. 1/UP3

UZ14-1465.5-1466.1/UP4

UZ 14- 1465.5-1 466. 1/UP5

UZ 14- 1465.5-1 466. 1/UP6

UZ 14- 1465.5-1 466. 1/UP7

UZ 14- 1465.5- 1466.1 /UP/D

UZ 14- 1465.5-1 466. 1/D

UZ 14-1 542.3- 1452.4/UP

UZ 14-1 542.3- 1452.4/UP/D

UZ14-1564.3-1564.7/UP

UZ 14-1 564.3- 1564.7/UP/D

UZ 14-1 564.3- 1564.7/D

8180 

(per mil

relative to 

VSMOW

Core ID: UZ14-1455.5-1466.1

-13.6

-13.6

-13.5

-13.6

-13.6

-13.6

-16.1

-15.9

Core ID: UZ14-1542.3-1542.7

-13.4

-16.1

Core ID: UZ1 4-1 564.3-1 564.7

-13.7

-20.0

-18.0

5D 

(per mil) 

relative to 

VSMOW

-102

-102

-102

-102

-102

-102

-121

-111

-100

-117

-103

-125

-114

6.3 per mil for 8 18O and 17 to 22 per mil for 8D. The 

compressed waters also are heavier than the water 

extracted from the adjacent core by vacuum distillation in the range of 2.3 to 4.3 per mil for 8 18O and 9 to 

11 per mil for 8D. The stable-isotope compositions of 

the compressed water collected at different pressure 

steps during the compression process are similar. As 

with the bedded tuff samples (table 9), the differences 

in stable-isotope compositions may be attributed to 

extraction of isotopically depleted water from 

hydrated minerals during the distillation process.

Three cores also were sampled from the Prow 

Pass Tuff from depths of 550.0 m to 614.3 m 

(table 12). One core contains zeolitic minerals 

(UZ14-2015.2), and one does not (UZ 14-1854.8). For 

the clay-bearing core, the stable-isotope composition 

of compressed water is heavier than the remaining 

water by 20 per mil for 8D and 4.1 per mil for 8 18O 

and also is heavier than water extracted by whole-core 

vacuum distillation by 13 to 16 per mil for 8D and 

3.2 to 5.8 per mil for 8 18O. For the non-zeolite and 

none lay-bearing core (UZ 14-1854.8), the stableisotope compositions of compressed water are similar 

to the distilled water. These data indicate that if core

samples contain zeolites, distilled water and

1 ft compressed water will have different 8D and 80 

values.

Table 12. Stable-isotope compositions of compressed water 

(/UP), remaining water extracted by vacuum distillation 

(/UP/D), and water distilled (/D) from the Prow Pass Tuff cores

[VSMOW, Vienna Standard Mean Ocean Water]

6180

Sample Identification

VSMOW

5D

(per mil)

relative to

VSMOW

Core ID: UZ 14-1804.8-1805.3

UZ 14-1804.8-1805.3/UP -13.1 -98 

UZ 14-1804.8-1805.3/UP/D -17.2 -118 

UZ 14-1804.8-1805.3/D -16.3 -111

Core ID: UZ14-1854.8-1855.3

UZ14-1854.8-1855.3/UP -13.3 -100 

UZ 14-1854.4-1854.8/D -13.4 -101

Core ID: UZ14-2015.2-2015.6

UZ14-2015.2-2015.6/UP -12.5 -99 

UZ 14-2014.0-2014.4/D -18.3 -115

STABLE-ISOTOPE COMPOSITIONS (50 and 518O) OF PORE WATER FROM USW UZ-14 33
Laboratory Tests

From these data, it is clear that the stableisotope compositions of compressed water, the 

remaining water collected after compression by 

vacuum distillation, and the whole-core distillation of 

adjacent core are different for bedded tuff, the Calico 

Hills nonwelded hydrogeologic unit, and Prow Pass 

Tuff, which contain hydrated minerals (the 8D and 

8 18O of compressed water were heavier than those of 

distilled water). A laboratory test was designed to 

investigate why different results are obtained with 

different extraction procedures. The plan for this laboratory test was to imbibe the dried Yucca Mountain 

cores with known isotopic-composition water 

followed by extractions using different methods. This 

provided information on the types of pore water 

(hydrated water or channel water and pore water) 

extracted with each procedure. It also provided information on how the original infiltrating water was fractionated when it permeated pore spaces.

A column experiment also was conducted to 

evaluate whether the channel water of zeolite is 

exchangeable with bulk water. If it is exchangeable, 

what is the exchange rate? The details of this test 

provide information on the source of zeolite channel 

water. Does channel water represent the most recent 

percolation water or fluid percolated in the distant past 

at Yucca Mountain? Araguas-Araguas and others 

(1995) used a column experiment with clay soil to 

reveal that weakly bound water in clay is easily 

exchangeable with isotopically different mobile water.

Imbibing Dry Core with Known IsotopeComposition Water: Imbibing Test

The purpose of this test was to determine 

(1) whether the pore-water stable-isotope compositions are altered during the core-water extraction from 

different tuff lithologies, (2) if the compressed water 

represents percolating water, (3) if the distillation 

method removes hydration water, and (4) if the stableisotope compositions of hydrated water fractionates 

from bulk water (which includes pore water, residual 

water for the core sample, and intergranular water for 

the zeolite powder sample).

The sample cores for imbibition were dried in 

an oven at 250°C for approximately 2 days. Dried 

samples were then evacuated and cooled in a sealed 

jar. While still under vacuum, water with a known

stable-isotopic composition (source water) was 

imbibed into sample cores (shown in fig. 13). Various 

amounts of water were used to provide cores representing saturated and unsaturated conditions. The 

imbibed cores were kept under vacuum at room 

temperature for 10 days to allow the sample and water 

to equilibrate. After equilibration, the core water was 

extracted by various methods, and the stable-isotope 

compositions of extracted water and the source water 

were measured for 8 18O and 8D.

Extracting Imbibed Water by Vacuum Distillation

Five cores of Yucca Mountain Tuff, Pah Canyon 

Tuff, Topopah Spring Tuff, the Calico Hills nonwelded 

unit, and Prow Pass Tuff were selected for this 

imbibing test. Each test was performed in duplicate. 

Each core was imbibed with water of known isotopic 

composition (source water). Two sources of water 

(two samples of Boulder, Colorado, tap water) with 

isotopic compositions of-16.7 and -16.1 per mil for 

8 18O and -123 and -121 per mil for 8D were used. The 

imbibed core was then placed in the vacuum distillation line to extract water for stable-isotope analyses. 

The stable-isotope compositions of distilled water, 

water recovery, and lithology are shown in table 13.

The data indicate that the stable-isotope compositions of distilled water were more depleted than 

corresponding source waters by 0 to 11 per mil for 8D 

and 0 to 2.2 per mil for 8 18O, and that the extent of 

depletion is strongly dependent on the lithology. The 

water recoveries were close to 100 percent except for 

the Calico Hills nonwelded unit core. The core of 

Topopah Spring Tuff does not contain hydrated 

minerals, and the 8 18O and 8D values of extracted 

water were close to those of the source water. The 

cores from Yucca Mountain and Pah Canyon Tuffs

18 contain clay minerals, and the 8 O and 8D compositions were more depleted than source water by approximately 0.6 per mil and 5 per mil, respectively. These 

results agree with those of Araguas-Araguas and 

others (1995) who reported depletions of 5.2 to 9.6 per 

mil in 8D and 0.36 to 0.47 per mil in 8 18O for clay-rich 

soil water extracted using vacuum distillation. For the 

core from the Calico Hills hydrogeologic unit, which 

is rich in zeolite, water recovery was less than for 

other cores, and the stable-isotope compositions also 

were more depleted than others (averaging about 10 

per mil in 8D and 2.0 per mil in 8 18O). These data 

indicate that it is difficult to extract all water from the

34 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Syringe (to inject source water)

Stopcock

Glass-container cap

Source water

Figure 13. Apparatus for imbibing test.

0-Ring

Glass core-container

STABLE-ISOTOPE COMPOSITIONS (8D and 818O) OF PORE WATER FROM USW UZ-14 35
Table 13. Stable-isotope compositions of the vacuum-distilled water from the imbibed cores

[VSMOW, Vienna Standard Mean Ocean Water]

Sample Identification

UZ14-77.4AM/D

UZ14-77.4/T2/D

UZ14-215/T1/D

UZ14-215/T2/D

UZ14-1271.7/T1/D

UZ14-1271.7/T2/D

UZ14-1514.2AT1/D

UZ14-1514.2/T2/D

UZ14-1864.8/T1/D

UZ14-1864.8/T2/D

Source water (T I )

Source water (T2)

6180 

(per mil) 

relative to 

VSMOW

-16.8

-17.0

-17.6

-1 6.4

-16.8

-17.0

-17.7

-18.3

-16.7

-16.4

-16.7

-16.1

5D 

(per mil) 

relative to 

VSMOW

-127

-127

-128

-128

-127

-124

-133

-132

-123

-124

-123

-121

Water 

recovery 

(percent)

98.6

99.0

99.0

99.3

99.3

99.6

98.2

96.4

99.2

99.6

Lithologic unit

Yucca Mountain Tuff

Yucca Mountain Tuff

Pah Canyon Tuff

Pah Canyon Tuff

Topopah Spring Tuff

Topopah Spring Tuff

Calico Hills nonwelded unit

Calico Hills nonwelded unit

Prow Pass Tuff

Prow Pass Tuff

zeolite-rich cores using vacuum distillation. The Prow 

Pass samples (table 13) showed no depletion. A 

zeolite-free Prow Pass sample (UZ 14-1854.8) also 

showed no depletion effects (table 12).

The stable-isotope compositions of distilled 

water are generally more depleted than the source 

water or compressed water. For the nonhydrated 

mineral cores, such as the Topopah Spring Tuff, the 

8D and 8 18O of distilled water are typically close 

to those of the source water. Therefore, the vacuumdistillation technique can be applied to extract core 

water for stable-isotope analyses on nonhydrated 

mineral cores. The 6D and 6 O values of water 

extracted by the vacuum-distillation method from 

zeolite- or clay-bearing cores, however, are more 

depleted than those of source water by about

10 per mil in 8D and 1.0 to 2.2 per mil in 6 18O for 

zeolite, and by about 5 per mil in 6D and 0.8 per mil in 

6 18O for clay. This may be caused by incomplete 

removal of hydrated water by the distillation method.

Extracting Imbibed Water by Compression

Six cores from the Yucca Mountain Tuff, 

bedded tuff, the Pah Canyon Tuff, the Topopah Spring 

Tuff, the Calico Hills nonwelded unit, and the Prow 

Pass Tuff were selected for this imbibing test. Mineralogic data are shown in table 14. Hydrated minerals 

(clay or zeolite) occur in Yucca Mountain Tuff, bedded 

tuff, Calico Hills nonwelded unit, and the Prow Pass 

Tuff but do not occur in the Pah Canyon Tuff and the 

Topopah Spring Tuff. Each core was imbibed with

Table 14. Mineralogic data of the imbibed cores used in the compression method

Sample Identification

UZ 14-77.4

UZ 14-78.6

UZ 14- 144.7

UZ 14-869.1

UZ16-1431.3

UZ 14- 1764.2

Zeolite 

(percent)

0

0

0

0

46

32

Clay 

(percent)

56

22

0

0

1

0

Plagioclase 

(percent)

5

13

30

38

5

15

Amorphous 

(percent)

30-35

45-50

30-35

10-15

20-25

20-25

Other 

(percent)

28-38

15-20

35-40

47-52

23-28

28-33

Llthology

Yucca Mountain

Bedded tuff

Pah Canyon

Topopah Spring

Calico Hills

nonwelded unit

Prow Pass

36 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
water of known isotopic composition. Boulder, Colorado, tap water with isotopic composition ranging 

from -16.1 to -16.7 per mil for 8 l8O and -121 to 

-125 per mil for 8D was used as a source water. The 

stable-isotopic composition of Boulder, Colorado, tap 

water changed little as a function of time. The imbibed 

cores were compressed, and extracted core water was 

collected at different pressure steps for stable-isotope 

analyses. The conditions and results of the stableisotope compositions of extracted core waters are 

listed in tables 15 through 17.

These Pah Canyon Tuff and Topopah Spring 

Tuff cores do not contain hydrated minerals. The 

stable-isotope compositions of compressed water from 

these cores change little with pressures and are very 

close to those of the source water (table 15). The variation is typically near the precision of measurement 

(1 per mil for 8D and 0.2 per mil for 8 18O). The results 

indicate that fractionation did not occur when the 

source water was imbibed into these dried cores.

The data from high clay-content cores from 

Yucca Mountain Tuff and bedded tuff are shown in

table 16. The stable-isotope compositions of the 

compressed water do not change with pressure, as also 

was true of Pah Canyon and Topopah Spring Tuffs. 

The 8D of compressed water is heavier than the source 

water by 1 to 3 per mil, but the 818O of compressed 

water is not significantly different from that of the 

source water. These observations agree with those 

reported by Stewart (1972) that little, if any, isotopic 

fractionation occurs between hydrated water and bulk 

water in 8 18O and that the hydrated water is depleted 

in deuterium relative to the bulk water for clay-rich 

soils.

For the high zeolite-content cores from the 

Calico Hills Formation and the Prow Pass Tuff, the 

data show that the stable-isotope compositions of 

compressed water do not change as a function of pressure (table 17). The 8D and 8 18O of compressed water 

are about the same as those of residual water (source 

water remaining in the container after the core was 

imbibed) but lighter than those of the source water by 

6 to 12 per mil for 8D and 2 to 3 per mil for 8 18O. The 

most likely reason for the difference between source

Table 15. Stable-isotope compositions of water extracted by the compression method from Pah Canyon Tuff 

and Topopah Spring Tuff cores (nonhydrated water-bearing cores)

[VSMOW, Vienna Standard Mean Ocean Water; Ksi, thousand pounds per square inch; *, not enough water for analysis]

Sample identification (per mil)

relative to

VSMOW

5D

(per mil)

relative to

VSMOW

Pressure

(Ksl, 103 pounds per square 

inch)

T/source water

UZ14-144-l44.7/T/up/l

UZ14-144-144.7/T/up/2

UZ14-144-144.7AT/up/3

UZ14-144-144.7/T/up/4

UZ14-144-144.7AT/up/5

UZ14-144-144.7/T/up/6

UZ14-144-144.7/T/up/7

UZ14-144-144.7/T/up/8

TESTlO/source

UZ14-144-144.7/T 10/residue 1

UZ14-144-144.7/T10/upl

UZ 14-144-144.7/T 10/up2

UZ14-144-144.7/T10/up3

TEST A: Core From Pah Canyon ID: UZ14-144.0-144.7

-16.5 -123

-16.4 -123

-16.4 -122

-16.2 -123

-16.4 -122

-16.2 -122

-16.4 -123

-16.2 -123

-16.3 -123 

TEST B: Core From Topopah Spring ID: UZ14-869.1-869.7

-16.3 -124

-17.0 -126

-16.8 -125

* -125

* -123

0-6

0-15

6-15

15-45

15-30

30-45

45-75

75-120

30-75

0-120

75-120

1 Water remaining in the container after core was imbibed.

STABLE-ISOTOPE COMPOSITIONS (5D and 5180) OF PORE WATER FROM USW UZ-14 37
Table 16. Stable-isotope compositions of water extracted by the compression method from the Yucca Mountain Tuff 

and bedded tuff (clay-bearing) cores

[VSMOW, Vienna Standard Mean Ocean Water; Ksi, thousand pounds per square inch; NA, not applicable; *, not enough water for analysis]

Sample Identification

TEST 6/source

UZ14-77.4-78.0/T6/up/l

UZl4-77.4-78.0AT6/up/2

UZl4-77.4-78.0/T6/up/3

UZl4-77.4-78.0/T6/up/4

UZl4-77.4-78.0/T6/up/5

UZ14-77.4-78.0/T6/UP/6

UZl4-77.4-78.0/T6/up/7

UZ14-77.4-78.0/T6/up/8

UZl4-77.4-78.0/T6/up/9

8180 

(per mil) 

relative to 

VSMOW

5D 

(per mil) 

relative to (Ksi, 

VSMOW

TEST C: Core From Yucca Mountain ID: UZ 14-77.4-78.0

-16.2 -122

-16.1

-16.2

-16.4

-16.3

-16.3

-16.2

-16.2

-16.2

-16.1

*

-121

-121

-120

-120

-121

-121

-121

-121

Pressure 

103 pounds per square inch)

N/A

3-6

6-10

6-10

10-30

10-30

30-40

40-75

75-120

75-120

TEST D: Core From Bedded 1\iff ID: UZ1 4-94.8-95.4

TEST 10/Source

UZ14-94.8-95.4/T10/UP1

UZ14-94.8-95.4/T10/UP2

UZ14-94.8-95.4/T10/UP3

UZ14-94.8-95.4/T10/UP4

UZ14-94.8-95.4/T10/UP5

UZ14-94.8-95.4/T10/UP6

UZ14-94.8-95.4/T10/UP7

UZ14-94.8-95.4/T10/UP8

UZ14-94.8-95.4/T10/UP9

UZ14-94.8-95.4/T10/UP10

-16.3

-16.1

-16.3

-16.2

-16.3

-16.4

-16.4

-16.4

-16.3

-16.3

-16.1

-124

-121

-122

-122

-121

-122

-121

-122

-122

-122

-121

NA

4-6

4-6

5-8

8-12

8-12

12-15

15-30

15-30

30-60

60-120

water and compressed water is that fractionation 

between the channel water of zeolite and bulk water 

occurred when the source water was imbibed into the 

dry core. The water that was more depleted went into 

channel spaces, resulting in enrichment of the bulk 

water (pore and residual water). These observations 

agree with the Karlsson and Clayton (1990) and Feng 

and Savin (1993) observations that the oxygen-18 of 

zeolite channel water is more depleted than bulk water. 

The compression method does not extract hydration 

waters of minerals and thus may be representative of 

the percolation water.

It is apparent that the compression method does 

not extract hydration water because the stable-isotope 

compositions do not change with pressure. The water 

extracted by the compression method is mainly drawn 

from large pore spaces. The stable-isotope composition of pore water in large pore spaces is similar to the 

residual water.

Isotopic Exchange Between Matrix Water and 

Percolating Water: Column Test

A column experiment was conducted to investigate the exchangeability of channel water in zeolite 

with the bulk water and to determine the exchange 

rate. The results also provide information on sources 

of zeolite channel water. Possible sources of water in 

zeolite channels include the most recent percolation 

water or paleowater. Araguas-Araguas and others 

(1995) used a column experiment with clay soil to 

reveal that weakly bound water in clay is easily 

exchangeable with isotopically different mobile water.

38 Hydro-chemical Investigations In Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Table 17. Stable-isotope compositions of water extracted by the compression method from Calico Hills Formation and 

Prow Pass Tuff (zeolite-bearing) cores

[VSMOW, Vienna Standard Mean Ocean Water; Ksi, thousand pounds per square inch; NA, not applicable; *, not enough water for analysis]

Sample Identification

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TST-1431.

TEST E:

-1431.6/source

-1431.6/residual

-1431.6/upl

-1431.6/up2/A

-1431.6/up2/B

-1431.6/up3/A

-1431.6/up3/B

-1431.6/up4

-1431.6/up5

-1431.6/up6

-1431.6/up7

TESTF

TESTlO/source

UZ 1 4- 1 764.2- 1 764.7AT 1 0/residual

UZ 14-1 764.2- 1764.7AT10/upl

UZ 14-1 764.2- 1764.7/T10/up2

UZ 14-1 764.2- 1 764.7m 0/up3

UZ 14-1 764.2- 1764.7/TlO/up/D

8180 

(per mil) 

relative to 

VSMOW

Core From Calico Hills ID

-16.7

-14.3

*

-1.4.1

*

*

*

-13.6

-14.0

*

*

Core From Prow Pass ID:

-16.3

-13.0

-13.0

-13.0

-13.0

-17.2

Pressure 

rSvTto (Ksl, 103 pounds per square 

VSMOW lnch)

: UZ16-1431.1-1431.6

-125

-118

119

-119

-119

-119

-118

-119

-119

-119

-119

UZ1 4-1 764.2-1 764.7

-124

-114

-112

-112

-112

-134

N/A

N/A

0-15

15-30

15-30

30-45

30-45

45-60

60-75

75-105

105-120

N/A

N/A

31-60

60-120

60-120

N/A

Zeolite Powder

The column experiment was performed with 

pure natural zeolite powder (95 percent clinoptilolite). 

Three kinds of eluant waters with different isotopic 

compositions were used: (1) Boulder (Colorado) 

tap water, (2) depleted Greenland ice water, and 

(3) enriched water. In the laboratory, the zeolite was 

placed in an oven at 250°C to dehydrate. It is important to note that clinoptilolite will rehydrate reversibly 

below 450°C (Feng, 1991). After dehydration, the 

zeolite was cooled in a desiccator. The dried and 

cooled zeolite was packed into the column (50 cm in 

height, 8 cm in diameter) and eluted with Boulder tap 

water at a rate of approximately nine column-volumes 

per day. The effluent was collected at the outlet. (The 

apparatus is shown in fig. 14.) The effluent was periodically sampled for isotope analysis. After elution, the 

rehydrated zeolite was removed from the column and 

divided into three parts one part was taken for 

vacuum distillation to obtain the isotopic composition

of zeolite water, and the other two parts were packed 

into two separate columns. The second part was eluted 

with depleted Greenland ice water and the third part 

with enriched water. Each effluent was periodically 

sampled for isotope analysis. Finally, the zeolite 

samples were removed from each column and vacuum 

distilled to obtain the isotopic composition of zeolite 

water.

The isotopic compositions of channel water 

were calculated by mass balance from the analytical 

data. Calculations were based on two assumptions 

(1) no 18O exchange occurs between channel water 

and framework silicate minerals; and (2) the distillation method can extract water completely. In support 

of the first assumption, O'Neil (1987) reported that 

little oxygen isotope exchange occurred between silicate minerals and fluid water at temperatures below 

100°C. To verify the second assumption, the dehydrated zeolite was imbibed with water of known 

isotopic composition (source water) and subsequently 

distilled under vacuum. If the recovery is 100 percent,

STABLE-ISOTOPE COMPOSITIONS (8D and 5180) OF PORE WATER FROM USW UZ-14 39
Leached by source water 

into dehydrated sample

End cap -,

Stainless steel --, 

porous end plate

^ Source water ^

^ 

^

 M

 

^

1H

0-ring groove

Sample

Effluent water collection

Figure 14. Apparatus for column test.

40 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
the distillation is complete. If not, corrections are 

needed for fractionation. Table 18 shows differences 

between the distilled water and source water of about 

3 to 7 per mil for 6D and 0.6 to 1.0 per mil for 5 18O. 

The correction factors are, therefore, about 5 per mil 

for 8D and 0.8 per mil for 8 18O.

The results of column experiments are summarized in table 19. Effluents #1 to #8 are the periodic 

water samples collected when Boulder, Colorado, 

tap water (percolate I) was leached through the dehydrated sample. After the leaching, column packing 

was split into two parts and packed in separate 

columns. Effluents #1 to #9 in the middle of table 19 

are the periodic water sample collected when the 

depleted Greenland ice water (percolate II) was 

flushed over the column leached with percolate I. 

Effluents #1 to #8 at the bottom of table 19 are the 

periodic water samples collected when more enriched 

water (percolate III) was flushed over the other column 

previously leached with percolate I.

The column experiments reveal several interesting relations. The isotopic compositions of effluents 

#1 to #6 leached with percolate I are more enriched 

than the source water, with the effluent #1 aliquot 

being the isotopically heaviest. The observed effect is 

due to isotope fractionation caused by uptake of 

lighter-water molecules by empty channels of clinoptilolite, leaving enriched-water molecules as effluent. 

As the leaching progressed, enriched isotopic water in 

the bulk water (pore space and intergranular spaces)

was replaced by the leaching source water (effluents 

#7 and #8). This is evident by the isotopic compositions of effluents #7 and #8, which are the same as the 

bulk water collected by centrifugation. Source water in 

percolate I is more enriched than the percolate II 

source water (Greenland ice water). Leaching with the 

more depleted source water of percolate II on the 

column previously leached with percolate I resulted in 

a gradual decrease in 8D and 8 18O values as the 

leaching progressed. Effluents #8 and #9 of percolate

II in table 19 indicate the column is near equilibration

1 ft with little change in SO and 8D values from the 

source water. The isotopic compositions of channel 

water, as indicated by the vacuum-distilled water, 

changed significantly by leaching from percolate I to 

percolate II. The percolate III case is very similar to 

that of percolate II, but the source water of percolate

III is more enriched than the source water of percolate 

I. The 8D and 8 18O of effluent waters became more 

enriched toward the source water of percolate III 

(effluents #7 and #8). The stable-isotope compositions 

of vacuum-distilled water of percolate I, II, and III are 

more depleted than their source waters by 6 to 8 per 

mil in SD and 0.9 to 1.7 per mil in 8 18O.

Zeolite-Rich Core

Experiments with intact core were performed to 

further test the isotopic exchange characteristics of 

pore and channel waters. A zeolite-rich core from the 

Calico Hills Formation of Yucca Mountain (sample ID

Table 18. Comparison of stable-isotope compositions of water distilled from clinoptilolite imbibed with source water of 

known isotope composition

[VSMOW, Vienna Standard Mean Ocean Water]

Source water

Sample 

zeolite

Zeolite # I

Zeolite #2

Zeolite #3

Zeolite #4

Average

8180 

(per mil) 

relative to 

VSMOW

-I7.l

-25. 1

-9. 1

-39.3

5D 

(per mil) 

relative to 

VSMOW

-1 26

-193

-98

-306

Distilled water

6180 

(per mil)

relative to 

VSMOW

-18.1

-26.0

-9.7

-40.0

5D 

(per mil) 

relative to 

VSMOW

-133

-198

-102

-309

4.75 (a =1.5)

Difference

8180 

(per mil)

relative to 

VSMOW

1.0

0.9

0.6

0.7

0.8(a = 0.16)

5D 

(per mil) 

relative to 

VSMOW

7

5

4

3

The correction factors: 5%o for 8D, 0.8%o for 8 18O.

STABLE-ISOTOPE COMPOSITIONS (8D and 8180) OF PORE WATER FROM USW UZ-14 41
Table 19. Stable-isotope compositions of the effluents from 

the column experiment with clinoptilolite

[VSMOW, Vienna Standard Mean Ocean Water; , no data]

Sample ID

6D 818O

Volume (per mil) (per mil)

(meter) relative to relative to

VSMOW VSMOW

Percolate I. (leached by Boulder, Colorado, tap water into 

dehydrated sample)

Source water --- -129 -17.6

Effluent #1 240 -127 -17.2

Effluent #2 240 -127 -17.3

Effluent #3 200 -128 -17.4

Effluent #4 60

Effluent #5 180 -128 -17.5

Effluent #6 60

Effluent #7 60 -129 -17.5

Effluent #8 60 -129 -17.5

Water extracted by:

centrifugation -129 -17.6

vacuum distillation -137 -18.5

Percolate II: (flush percolate I sample by the more depleted 

Greenland ice water)

Source water -310 -39.6

Effluent #1 60 -186 -24.5

Effluent #2 60 -233 -30.2

Effluent #3 60 -268 -34.6

Effluent #4 60

Effluent #5 100 -299 -38.2

Effluent #6 80

Effluent #7 100 -306 -39.0

Effluent #8 60 -307 -39.1

Effluent #9 60 -308 -39.1

Water extracted by:

vacuum distillation -316 -40.6

Percolate III: (flush percolate I sample by the more enriched 

water)

Source water -79 -2.7

Effluent #1 60 -109 -10.0

Effluent #2 60 -90 -6.1

Effluent #3 60

Effluent #4 60 -82 -3.8

Effluent #5 100 -80 -3.1

Effluent #6 80

Effluent #7 100 -80 -3.8

Effluent #8 60 -79 -3.1

Water extracted by:

vacuum distillation -86 -4.4

UZ-14-1422.4-1423), which contains 45 percent 

zeolite (mostly clinoptilolite), was selected for this 

experiment. The selected core was dehydrated. After 

dehydration, 64.4 grams (g) of stable isotope water of 

known composition (source water I: Boulder, Colorado, tap water, 6 18O: -17.2 per mil and 8D: -128 per 

mil), was imbibed into the core to saturation, with 

some residual water remaining inside the container. 

The imbibed core was kept inside the container under 

vacuum for 10 days to attain equilibrium. At the end of 

7 days, the residual water was sampled for isotope 

analysis. After 7 additional days, the residual water, 

7.1 g, was completely removed and 37.0 g of a new 

source water (Greenland ice source water II, 8 O: 

-35.6 per mil and 6D = -272 per mil), was injected into 

the container containing the core imbibed with source 

water I. Source water II did not enter the core because 

the core already was saturated. The residual water was 

sampled periodically for stable-isotope analysis to 

record the extent of exchange between pore water and 

residual water. Three samples were collected from the 

residual water for analyses after the fourth (8.1 g 

sampled), fifth (8.8 g sampled), and sixth days (19.0 g 

remaining) of equilibration. Finally, imbibed core was 

removed from the container, and core water was 

extracted using the compression method (18 g 

extracted). The remaining water, after compression, 

was extracted by vacuum distillation (38.8 g obtained) 

for isotope analysis. The data are shown in table 20.

Water mass balance is as follows: Boulder, 

Colorado, tap water (source water I) added to the 

container, 64.6 g; Greenland ice water (source water 

II) added to the container, 37.0 g, for a total of 101.6 g 

added to container; water removed (7.1 g + 8.1 g + 

8.8g+19g+18g + 38.8 g =)99.8 g; andunrecovered, 1.8 g of water (held tightly inside the core by 

matrix potential). The stable-isotope compositions of 

periodically sampled residual water were gradually 

enriched. This implies that the pore water exchanged 

with the residual water through a diffusion mechanism. The exchange rate can be estimated by 

Einstein's random walk equation (Einstein, 1905; 

Ames, 1965):

X2 = 2Dt

where X is mean travel distance, D is the diffusion 

coefficient, and t is travel time.

42 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
The diameter of core is about 6.1 cm. The self-diffuf+++ C 01 sion coefficient of water is about 2.3 x 10 cm s 

at 25 °C (Harris and Woolf, 1980). Calculations based 

on this equation indicate that the pore water in a 

6-cm-diameter core will exchange in about 5 days. In 

this experiment, if the system reached equilibrium 

between pore water and residual water, the stableisotope composition of pore water (TESTlO/up) 

should equal the last sampled residual water 

(TEST10/R4). However, the stable-isotope compositions of compressed water (TESTlO/up) are enriched 

relative to the last sampled residual water 

(TEST10/R4) and the compositions of compressed

water changed slightly as a function of pressure. This 

indicates that the pore water and residual water have 

not reached equilibrium. The equilibration time of 

source water II with core sample was 6 days, apparently not long enough for equilibration. The disagreement between calculated and experimental 

equilibration times may be due to neglect of geometric 

effects (porosity and tortuosity) in the calculation. In 

any case, the results of the experiment indicate that the 

pore water is exchangeable with the residual water, 

and the exchange rate depends on the grain size of the 

core sample.

Table 20. Stable-isotope compositions of water from the imbibition experiment with two different source 

waters for zeolite-rich core

[%, percent; psi, pounds per square inch; VSMOW, Vienna Standard Mean Ocean Water]

Sample Identification 

(UZ14/1422.4-1423.0)

(per mil)

relative to

VSMOW

5D

(per mil)

relative to

VSMOW

Pressure 

(103 psi)

Amount of

water

(grams)

Calico Hills Formation (zeolite 45%, amorphous material 35%, others 20%)

TESTlO/sourcel -17.2 -128 64.6 

TEST10/R1 8 -16.4 -125 7.1

Source water II

TESTlO/sourcel I

TEST10/R2 b

TEST10/R3 C

TEST10/R4 d

TESTlO/upl

TEST10/up2

TEST10/up3

TEST10/up4

TESTlO/up/D

-35.6

-27.6

-26.2

-25.3

-22.9

....

-22.1

-22.5

-21.6

-272

-214

-202

-198

-183

-180

-178

-180

-169

29-60

60-75

75-120

75-120

37.0

8.1

8.8

1 9.0

Mass balance:

Total source water I added to container: 64.6 grams (g), residual water in container removed: 7.1 g. 

Added source water II to container: 37.0 g, residual water in container: 8.1+8.8+19.0 g. 

Compressed water from imbibed core: 18 g extracted. 

Distilled water after compression: 38.8 g obtained. 

Mass balance: 1.8 g of water unrecovered. 

Oxygen-18 balance: 1.1 per mil. 

Deuterium balance: 1.1 per mil.

a : imbibing source water I, sitting for 7 days, removed all the residual water (7.1 g). 

1 c> d :adding source water II into above saturated cores container, sampled 8.1, 8.8, and 19.0 g water for analysis.

STABLE-ISOTOPE COMPOSITIONS (50 and 5180) OF PORE WATER FROM USW UZ-14 43
Applicability of the Extraction Methods to 

Yucca Mountain Cores

by addition of about +5%o for 5D and +1.0 %o for 

5 18O.

From the results of the vacuum-distillation and 

compression methods for extracting water from 

imbibed cores, it is apparent that the stable-isotope 

compositions of distilled water extracted by distillation are more depleted in 6D and 8 18O than water 

compressed from core containing hydrated minerals. 

For the zeolite-bearing core, the isotopic composition 

of vacuum-distilled water is depleted in 6D and 5 18O 

relative to bulk water by 9 to 15 per mil and 0.9 to 

2.8 per mil, respectively. For the core containing clay 

minerals, the hydrated water is depleted about 5 per

1 o

mil in 8D and not depleted in 5 O relative to bulk 

water. These data demonstrate that different waterextraction methods should be used for samples of 

different mineral content. For UZ-14 core-sample 

analysis, different water-extraction methods were used 

as appropriate for various lithologic units.

The Pah Canyon and the Topopah Spring Tuffs: 

Vacuum Distillation Method

Vacuum distillation is good for samples 

containing few hydrated minerals, such as the Pah 

Canyon Tuff. Complete distillation is necessary, 

however, to avoid fractionation. For the Topopah 

Spring Tuff, vacuum distillation also is the recommended method (water content of cores is typically 

less than 5 percent). Zeolite-bearing minerals occur 

above the basal vitrophyre (between the devitrified 

zone and the vitrophyre) within the Topopah Spring 

Tuff in a zone about 2 to 3 m thick (Bish and Vaniman, 

1985). The vacuum distillation extraction is not a suitable technique for this zone.

Bedded Tuff and the Calico Hills Formation: 

Compression Method

For zeolite-bearing cores, vacuum distillation is 

not a suitable water-extraction method for stableisotope analyses because the channel (hydrated) water 

will be partially removed during the distillation 

process. The compression method should be used. For 

the clay-bearing cores, the compression method is 

again better than the vacuum-distillation method. If 

the compression method cannot successfully extract 

sufficient amounts of water for analysis from lowmoisture-content cores, however, then the distillation 

method can be used, but the data need to be corrected

Interpretation of the Stable-Isotope Data

Appropriate pore-water extraction methods for 

stable-isotope analysis are dependent on the type of 

minerals present in the cores. Therefore, it was important to determine the mineralogy of the major stratigraphic units penetrated in borehole UZ-14. Table 21 

shows the thickness and mineralogic data for each 

lithologic unit analyzed by the geochemistry laboratory of the U.S. Geological Survey in Denver, Colo., 

and by the Los Alamos National Laboratory (David 

Bish, oral commun., 1995).

The depth profiles of 5 18O and 8D values of 

pore water extracted from UZ-14 cores by vacuum-. 

distillation and compression methods are shown in 

figures 15 and 16. The 6 18O and 6D data of pore water 

obtained by the compression method are limited one 

measurement from Yucca Mountain Tuff, five from 

bedded tuff and the Pah Canyon Tuff, seven from the 

Calico Hills hydrogeologic unit, five from the Prow 

Pass Tuff, and one from the Bullfrog Tuff. These limitations arise because the compression method cannot 

extract water from the Topopah Spring Tuff or other 

lithologic units with a moisture content of less than 

7 percent by weight. The 8D profile (fig. 16) shows 

enrichment at shallow depth of the pore waters relative 

to the compositions of deeper samples. The figures 

exhibit more scatter in both 8 18O and 8D values 

obtained by the vacuum-distillation method than by 

the compression method. Data for the Topopah Spring 

Tuff in both figures are less scattered and clustered in a 

narrow band.

The stable-isotope compositions of water 

distilled from the Calico Hills hydrogeologic unit, the 

Prow Pass Tuff, and Bullfrog Tuff cores are more 

depleted than those of other tuffs (for example, the 

Topopah Spring Tuff and Pah Canyon Tuff). The 

depletion is roughly proportional to the zeolite 

contents of core samples. At a depth of about 460 m in 

UZ-14, the zeolite content is about 67 percent, 

resulting in a significantly more depleted 8 18O value 

of-17.9 per mil and a 8D value of-120 per mil. Also, 

water distilled from depths of 559.3 m to 575.5 m, 

where there is no zeolite in the cores, is more enriched 

than that from other adjacent zeolite-bearing cores. 

It previously has been shown that the vacuum44 Hydrochemlcal Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Table 21. Depths and mineral contents in each lithologic unit of borehole USW UZ-14, Yucca Mountain area

[%, percent; m, meter; undifferentiated amorphous material includes volcanic glass, gels, and silica species such as opal]

Lithology 

(depth) Sample Identification Major mineralogy

Yucca Mountain Tuff 

(12.1 to 23.8m)

Bedded tuff 

(23.8 to 31.2m)

Pah Canyon Tuff 

(31.2 to 73.3m)

Topopah Spring Tuff 

(80.9 to 427.6 m)

Calico Hills Formation 

(427.6 to 531.6m)

Prow Pass Tuff 

(531.6 to 623.9m)

Bullfrog Tuff 

(623.9 to 672.4 m)

UZ 14-69.0-69.5* 

UZ 14-77.4-78.0*

UZ 14-78.6-79.0* 

UZ 14-94.8-95.4*

UZ 14-144.0-144.7*

UZ 14-869.1-869.7* 

UZ 14-1049.4-1049.8* 

UZ14-1271.7-1271.9* 

UZ14-1277.4** 

UZ 14-1298.6** 

UZ14-1312.5-1313.2*

UZ14-1417.6**

UZ 14-1422.4-1423.0*

UZ14-1422.4-1423.0*

UZ 14-1465.5-1466.1*

UZ14-1514.8-15I5.2*

UZ 14-1666.3-1666.9*

UZ 14-1764.2-1764.7* 

UZ14-1854.4-1854.8* 

UZ 14-1904.3-1904.8*

montmorillonite 39%, amorphous material 50% 

smectite 22%, amorphous material 40-50%

smectite 22%, amorphous material 45-50%

montmorillonite 30%, amorphous material 30%, plagioclase 20%

plagioclase 30%, K-spar 19%, amorphous material 30%

plagioclase 38%, K-spar 24%, amorphous material 15%

plagioclase 36%, amorphous material 15%, K-spar 19%

plagioclase 34%, K-spar 20%, amorphous material 15%

zeolite 58%, smectite 20%, opal-CT 20%

zeolite 32%, smectite 50%, opal-CT 9%

magnetite 18%, plagioclase 17%, amorphous material 50%

zeolite 65%, opal-CT 12%, feldspar 12% 

zeolite 45%, amorphous material 35% 

zeolite 45%, amorphous material 35% 

zeolite 35%, amorphous material 35% 

zeolite 67%, amorphous material 20% 

zeolite 54%, amorphous material 25%

zeolite 32%, amorphous material 30%, plagioclase 15% 

plagioclase 30%, K-spar 28%, quartz 12%, pyroxene 8% 

zeolite 28%, plagioclase 11%, pyroxene 10%, amorphous material 

30%

UZ 14-2154.3-2154.8* plagioclase 33%, amorphous material 15%, zeolite 6%

* Mineralogic analysis by Geochemistry Laboratory, Branch of Geochemistry, U.S. Geological Survey. 

** Data From Los Alamos National Laboratory (David Bish, oral commun., 1995).

distillation method on zeolite-bearing cores can 

remove depleted channel water that does not represent 

the percolating water. Mineralogic data indicate large 

zeolite abundances in the Calico Hills Formation and 

parts of the Prow Pass Tuff. In comparison, the stableisotope compositions of core water extracted by the 

compression method on these zeolite-bearing cores are 

in the range of-12.5 to -13.7 per mil for 5 18O and -98 

to -103 per mil for 8D, comparable to Topopah Spring 

Tuff values. The reason is that the compression 

process does not extract the channel water.

The shape of the vertical profiles of 8D values 

relative to depth in figure 16 can be explained as 

follows. A series of precipitation events, all with the 

same 5D composition, infiltrate episodically into the 

soil. Between rainfall events, the soil water has a 

chance to evaporate and, hence, to become more 

enriched in 8D value near the surface. The next precipitation occurrence displaces this heavier water downward, and waters mix. The downward movement of 

the fluid in the matrix is controlled by two mechanisms a concentration gradient of 8D composition 

and a thermal gradient. Under quasi-steady-state 

conditions, with no appreciable temperature influence, 

an exponentially decaying 8D profile would develop 

due to concentration-driven diffusion (Allison and 

others, 1984). The observed decaying 8D compositions represented by four solid circles between depths 

of 45.7 m and 76.2 m (fig. 16) are products of this 

liquid-phase diffusion. This is consistent with the 

conclusion made by Yang and others (1996) on the 

UZ-1 I4C gas data that the gas transport in the unsaturated zone beneath Pagany Wash was by a gas-diffusion 

mechanism.

The enrichment of S I8O and 8D values of 

compressed water from Prow Pass and Bullfrog Tuff

STABLE-ISOTOPE COMPOSITIONS (5D and 6180) OF PORE WATER FROM USW UZ-14 45
METERS

0

100

200

UJ

O 

u_

oc

=>

300

g

CO

X

o_

LJJ 

Q

400

500

600

Alluvium

Yucca Mtn Tuff

Bedded Tuff

Pah Canyon 

Tuff

Bedded Tuff

Topopah

Spring

Tuff

Bedded Tuff

Calico

Hills

Formation

Prow 

Pass 

Tuff

Bullfrog 

Tuff

Distilled . 

Compressed 

FEET 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

-26 -24 -22 -20 -18 -16 -14 

DELTA O-18, IN PER MIL (VSMOW)

-12 -10

2,200

Figure 15. Lithologic units and 618O profile of compressed and distilled pore water from borehole USW UZ-14, 

Yucca Mountain area.

46 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
METER 

0

100

200 

LU 

O

LLtr

=>

0 300

_i

§

O

LLJ 

CO 400

IE

Q_ 

LLJ 

Q

500

600

S

Alluvium

Yucca Mtn Tuff

Rerirififi Tuff

Pah Canyon

Tuff

s

\Bedded Tuff

Topopah

Spring 

Tuff

/ Bedded Tuff

s

Calico

Hills 

 Formation

~ Prow

Pass

Tuff

Bullfrog

Tuff

. < »* :

 , 

 '' 

 

\ *

I* 9 ~

. -: 

-

H

Distilledx,^^ .*

\/ '

* *^ 9 

 .* _

;"* Compressed ~

* ~ 

 _ 

 ^

f

I

* . i i . i I i i i

FEET 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

o onn

-160 -150 -140 -130 -120 -110 -100 -90 -80 -70

DELTA D, IN PER MIL (VSMOW)

Figure 16. Lithologic units and 8D profile of compressed and distilled pore water from borehole USW UZ-14, 

Yucca Mountain area.

STABLE-ISOTOPE COMPOSITIONS (5D and 5180) OF PORE WATER FROM USW UZ-14 47
cores compared with the Calico Hills Formation cores 

could be the results of (1) large amounts of zeolite in 

these tuffs, causing the fractionation between the 

channel water and the pore water, producing enriched 

pore water, or (2) mixing with other source water. It is 

not clear from the limited data which factor is causing 

this enrichment.

Source and Nature of Infiltrating Water in the 

Unsaturated Zone

The stable-isotope compositions of the Yucca 

Mountain UZ pore waters (UZ-water) from UZ-14 

(using only those data that were obtained using a valid 

method on lithologic cores, that is, the compression 

method for the Calico Hills hydrogeologic unit and 

bedded tuff, and the vacuum-distillation method for 

the Topopah Spring Tuff) are plotted as 5D compared

1 ft

to 8 O in figure 17 along with the saturated-zone 

ground water (SZ-water) compositions beneath Yucca 

Mountain. The UZ-water data are scattered over a 

larger region than the SZ-water data but do overlap 

with part of the SZ-water data. This overlap probably 

occurs because the infiltrated pore water moved downward by piston flow representing various infiltration 

events (that is, variable 8D and 6 O values depending 

on the conditions of precipitation and evaporation 

history before infiltration), while the SZ water represents time-averages of well-mixed water from yet-tobe-determined sources. The SZ water is deep under 

Yucca Mountain. Therefore, this water could be a 

mixture of several source waters of different ages. In 

contrast, the shallow UZ water must come from local 

precipitation, either summer or winter precipitation or 

a combination of both.

Ingraham and others (1991) measured the 

stable-isotope compositions of precipitation in 

southern Nevada. They reported that the yearly 

weighted average ranged from -10.0 to -12.3 per mil 

for 8 18O and from -75 to -90 per mil for 8D, while the 

winter precipitation (November to May) ranged from - 

12.5 to -14.5 per mil for 8 18O and from -92 to -106 per 

mil for 8D. Milne and others (1987) measured the 

stable-isotope compositions of precipitation specifically at Yucca Mountain from 1984 to 1985. They 

reported that the yearly weighted average ranged from

-9.4 to -11.8 per mil for 8 !8O and -77 to -89 per mil for 

8D, while the winter precipitation ranged from -13.2 to

-15.8 per mil for 8 18O and -93 to -114 per mil for 8D. 

The UZ-water compositions ranged from -11.2 to

-14.0 per mil for 8 18O and -87 to -104 per mil for 8D 

(fig. 17). These data show that the stable-isotope 

compositions of UZ water are closer to the winter 

precipitation values and thus suggest that winter 

precipitation is the major source of recharge to the 

unsaturated zone at Yucca Mountain.

The stable-isotope composition of perched 

water, SZ water, and 1984 summer storms at Yucca

1 8 Mountain are plotted as 8 O relative to 8D in 

figure 18. Four major storms that occurred in July and 

August of 1984 have 8D and 8 18O values ranging from

-67 per mil to -97 per mil for 8D and from -10.5 per 

mil to -13.0 per mil for 8 18O (Milne and others, 1987). 

Waters from three of these four storm events are far 

heavier than the perched water or the UZ water values. 

Thus, summer rains appear to be a minor contribution 

to the UZ waters.

Stable-isotope data for UZ waters all plot to 

the right of the Yucca Mountain precipitation line 

(YMPL) but on a line that is approximately parallel to 

the YMPL (fig. 17). This offset is caused by nearsurface evaporation resulting in enrichment of the 

remaining water. When this enriched water is 

displaced downward by a subsequent rainfall event of 

sufficient magnitude to remove it from the influence of 

evaporation, this parcel of water will move farther 

downward by piston flow. A following recharge event 

will repeat this cycle but with different initial isotopic 

composition. In this way soil water at depth (the 

recharge water) will fall on a line of a slope parallel to 

the YMPL, but displaced from it. The amount of this 

displacement is determined by a balance between the 

isotopic enrichment attained in the upper layers of the 

soil and the dilution of the rainfall. In arid conditions, 

low recharge rate and preferential winter precipitation 

may cause large displacement from the local precipitation line regardless of the climatic conditions under 

which the precipitation fell (Allison and others, 1984).

The stable-isotope data for perched waters also 

plot to the right of the YMPL (fig. 18) but are generally closer to the YMPL than the UZ waters, indicating 

little evaporative loss before recharging to perchedwater bodies. This supports the conclusion of Yang 

and others (1996) based on chloride and dissolvedsolid concentrations that perched waters were derived 

from rapid fracture flows instead of matrix flows. The 

SZ waters are isotopically lighter than the UZ waters, 

although there is some overlap between the two clus10

ters. Although the average of all SO and 8D values in 

UZ waters may be different from the SZ 8 18O and 8D

48 Hydrochemical Investigations in Characterizing the Unsaturated Zone at Yucca Mountain, Nevada
Tl

m

8

 o

i

5

*> 

O

0) 

Q.

O

o n

TJ 

O 

30

m

m

3D

31

O

i

N

DC

LU 

Q

-60

-70

-80

-go

-100

-110

-120

-130

YUCCA MOUNTAIN 

PRECIPITATION LINE WORL

WA

UZ-pore H2O 

Distilled

UZ-14 

Squeezed

SZH2O

+

-18 -16 -14 -12 

DELTAO-18, IN PER MIL

-1

Figure 17. Relations of 618O and 8D plot of USW UZ-14 unsaturated-zone pore water extracted by compression on bedded tu

tion and vacuum distillation on Topopah Spring Tuff, Yucca Mountain area.
to

to

5

a

i 

&

s 

s

-60

-70 -

-80

tr -90

LJJ 

CL

Q -100

__j

LJJ 

Q

-110

-120

-130

-18

YUCCA MOUNTAIN 

PRECIPITATION LINE WORL

WA

UZ-14 SZH2O NRG-7 USW/G-2 SD-9

° -f * +

'84 Summer 

storms

0

-16 -14 -12 

DELTA O-18, IN PER MIL

Figure 18. Relation of 818O and 8D plot of 1984 storms, saturated-zone water, and perched water, Yucca Mountain area.

-10 
values, this overlap indicates matrix recharge to the 

SZ waters might be occurring to some extent from 

UZ waters.

These conclusions about Yucca Mountain 

hydrology are based on data from only a single borehole. Further investigations and additional data from 

other boreholes are needed to make generalized statements about the UZ water flow at Yucca Mountain.

Large Dissolved-Solids Concentrations and 5D 

and 518O Values

In this section, the cause of the large dissolvedsolids concentrations observed in three samples previously mentioned [NRG-6 (depth of 78 m), UZ-14 

(3.8 m), and UZ-14 (614 m)] is reexamined with 

respect to their pore-water 8D and 5 18O values. The 

8D versus 8 18O plot of these three samples is shown in 

figure 19. NRG-6 (78 m) and UZ-14 (614 m) plot 

farther away from the YMPL than the UZ-14 (13.8 m) 

sample, indicating more enrichment by evaporation. 

(Enrichment by high-temperature exchange between 

oxygen atoms of silicate and pore water is eliminated 

because 8D values also are enriched, and there is no 

source of deuterium in the rock.) The question here is 

to what extent evaporation had occurred. The extent of 

evaporation for sample NRG-6 (78 m), which has the 

largest apparent loss of moisture by evaporation, is 

calculated here.

The 8D and S 18O values of precipitation that fell 

at Yucca Mountain were reported by Ingraham and 

others (1991) and Milne and others (1987). Their 

average 8D and 8 18O values for winter precipitation 

are -103 per mil and -14.0 per mil. The NRG-6 (256 ft) 

sample pore-water values are -93.8 per mil for 8D and

1 ft -10.7 per mil for 8 O. The winter precipitation values 

were used because it is the apparent source of infiltrating water at Yucca Mountain (as explained in the 

previous section). Evaporation under Rayleigh conditions is given by the following equation (Fontes and 

Gamier, 1979, p. 8):

8- 8° =

where

8 = the composition of the remaining fraction of the 

pore water, 8D(r)/1,000 or 8 l8O(r)/l,000, 

at time t;

8° = the initial composition of pore water, 8D(i)/l,000 

or 8 18O(i)/l,000

a = fractionation factor; and 

f = fraction of remaining water

At 20°C, <x I8o = 1.00925 and aD = 1.08. When 

these values are substituted into the above equation 

along with precipitation values of-0.103 for 8° (D) 

and -0.014 for 8° ( 18O), and pore-water values of 

-0.0938 for 8D and -0.011 for 8 18O, the calculated 

value for f is about 0.88. That is, only about 12 percent 

of the original water volume has evaporated. This is 

the maximum value because Rayleigh conditions 

assume equilibrium between liquid water and vapor. 

This is one of several large f values because the pore 

water 8D and 18O data used in Rayleigh equation were 

affected by the kinetic evaporation, and this pair of 8D 

and ' 8O data has a very positive value. For more negative 8D and 18O data (less kinetic effect) the f value 

will be smaller. For the UZ-14(13.8 m) sample, the 8D 

and 8 I8O values are -90.5 per mil and -12.4 per mil, 

which are heavier than the UZ-14(614 m) sample (8D 

and 8 18O values of-99.0 per mil and -12.5 per mil), 

yet closer to the YMPL. Sources of precipitation that 

recharged this UZ water must be heavier in 8D and 

8 18O values, possibly from mixture of winter precipitation and spring or summer precipitation.

Did evaporation occur before infiltration or after 

infiltration? Most of the stable-isotope data presented 

here are from pore waters compressed from cores 

obtained from depths greater than 13.7 m, representing 

matrix water in the intact rock (not alluvium). Field 

investigation of core-moisture loss during core 

processing inside the field trailer is about 6 percent of 

total pore water (I.C. Yang, U.S. Geological Survey, 

unpub. data, 1993). Thus, about one-half of the 

12 percent evaporation was lost in sample handling.

_ i o

Therefore, all 8D and 8 O values are systematically 

in error by a maximum of 5 per mil due to sample 

handling (12 percent evaporation causes 10 per mil 

difference in 8D: -103 per mil - (-93 per mil) = 

10 per mil). It may be concluded (fig. 16) that vapor 

transport in matrix water is not occurring. Therefore, 

the 6 percent pore-water loss can be attributed to evaporation before infiltration. This supports the previous 

conclusion that large dissolved-solids concentrations 

in the three samples are the result of water-rock interactions and not of evaporative concentration.

STABLE-ISOTOPE COMPOSITIONS (8D and 818O) OF PORE WATER FROM USW UZ-14 51
I

to 

to

O 

(0

O

tt

CD 

NJ 

(0

(D

c

ta 

n

N 

O

O

n 

o?

o

9

i

a:

-60

-70

-80

-90

Q

< -100

LU 

Q

-110

-120

-130

-18

YUCCA MOUNTAIN 

PRECIPITATION LINE WORL

WA

NRG6(78m) UZ14(13.8m) UZ14(614m)

D + 

-16 -14 -12 

DELTA O-18, IN PER MIL

-10 

Figure 19. Relation of 518O and 5D plot of data from samples containing large dissolved-solids concentrations in pore waters of U

USW NRG-6 at different depths, Yucca Mountain area.
Interactions of Old Matrix Water with Younger 

Percolating Water

Rhyolitic volcanic ash, as erupted, contains 0.1 

to 0.3 percent water (Friedman and others, 1993). 

After eruption, however, ash begins to react with infiltrating water that is incorporated into the tuff. If this 

secondary water of hydration does not undergo further 

isotopic exchange with the subsequently infiltrated 

water, it will represent the old matrix water. On the 

contrary, if this secondary water undergoes isotopic 

exchange, then the pore water will lose its old water 

signal and be replaced by a series of subsequently 

infiltrated waters. As stated previously in the section 

on column experiments, tuffs at Yucca Mountain do 

undergo isotopic exchange. Therefore, stable-isotope 

compositions of extracted pore water are representative of the most recently infiltrated percolating water.

The channel water of pure clinoptilolite powder 

exchanges with the percolating water within several 

hours to days. For crushed and ground tuffs from 

Yucca Mountain, the exchange rate also requires 

several hours to days, while intact cores take longer 

depending on the core size and grain size. The 

exchange processes for zeolite-bearing core can be 

divided into two steps: (1) the exchange between the 

pore water and the percolating water, and (2) the 

exchange between the pore water and channel water. If 

the system reached equilibrium, the stable-isotope 

compositions of pore water should equal those of 

percolating water, and those of channel water will 

differ from pore water by a fractionation factor. The 

first process appears to be diffusion-controlled. The 

equilibration time depends on the mean travel distance 

and the diffusion coefficient (Yu, 1996). The mechanism controlling the second step is unknown, but the 

reaction is rapid, as seen from column experiments. 

Therefore, the equilibration time is determined by the 

first process (Yu, 1996). For Topopah Spring Tuff that 

contains no zeolite, stable isotopes of water molecules 

exchange by diffusion mechanisms.

Preglacial (greater than 10,000-year ages) or 

Postglacial (2,000- to 10,000-year ages) 

Pore Water in the Topopah Spring Tuff

Both UZ-water and perched-water 6D and 6 I8O 

values are heavier than SZ water values. The SZ water 

uncorrected 14C ages range from 9,000 to 18,000 years 

at Yucca Mountain (Benson and McKinley, 1985). 

Most UZ-water and perched-water stable-isotope

values in figures 17 and 18 are larger than -99.8 per 

mil in 8D and heavier than the last ice-age water 8D 

values of-101 to -103%o (uncorrected '*C ages 

between 12,000 and 18,000 years) collected in the 

deeper SZ ground water of borehole H-l, which is 

near the UZ-14 borehole (Benson and Klieforth, 

1989). If mixing of the last ice-age water (more than 

10,000 years old) were involved in the matrix water of 

the Topopah Spring Tuff, the data would shift toward 

the left side in figures 15 and 16 (more negative) 

instead of remaining such constant values. Therefore, 

the pore waters of the Topopah Spring Tuff are interpreted to be consistent with the postglacial (2,000- to 

10,000-year) water.

SUMMARY AND CONCLUSIONS

Pore-water compositions indicate that, in spite 

of large distances separating boreholes, chemical 

compositions are similar among boreholes and vary 

according to the lithologic units from which they were 

obtained. Chemical compositions are of the calciumsulfate or calcium-chloride types in the Paintbrush 

Group (Tiva Canyon Tuff, Yucca Mountain Tuff, Pah 

Canyon Tuff, and bedded tuffs), and sodium-carbonate 

or sodium-bicarbonate types in the Calico Hills 

nonwelded unit. Several samples from the Calico Hills 

nonwelded unit in SD-7 and SD-12, however, are not 

true sodium-carbonate or sodium-bicarbonate types 

but rather trend toward calcium-sulfate or calciumchloride types. Unsaturated-zone pore water has 

significantly larger concentrations of major ions and 

dissolved solids than do the saturated-zone water or 

the perched water. If the chemically dilute infiltrating 

waters were to recharge the perched or saturated-zone 

water, these waters must flow rapidly through fractures 

or permeable zones in the intervening PTn or Calico 

Hills nonwelded hydrogeologic units to avoid mixing 

(enrichment) with the chemically concentrated pore 

water in those units. This conceptual model is consistent with the observations of post-bomb tritium and 

36C1 in the deep UZ of Yucca Mountain that is, 

consistent with rapid transport to the deep UZ.

Carbon-14 activities in the pore water of the 

Calico Hills nonwelded unit in SD-9 are between 82.3 

to 95.3 pmc, with similar activities from the Calico 

Hills nonwelded unit in UZ-14, and activities of 50 to 

60 pmc in the Calico Hills nonwelded unit of SD-7 

and SD-12 considerably smaller (or older) than the

SUMMARY AND CONCLUSIONS 53
UZ-14 and SD-9 pore waters. The water with larger 

14C activities in UZ-14 and SD-9 could have been 

partly contaminated by the atmospheric 14CC>2 during 

drilling. More gaseous-phase 14C data from the Calico 

Hills nonwelded unit are needed to confirm this possibility. A general trend is apparent from the distribution 

of borehole data, however, that younger water underlies the northern part of Yucca Mountain and older 

water underlies the southern part of the mountain. 

Perched waters reached in boreholes had large water 

masses, which are more likely to preserve their original chemical signatures than pore water.

Large tritium concentrations of about 30 to 

150 TU were detected in the NRG-6 borehole at 

depths from 53.3 m to 74.7 m in the Pah Canyon Tuff 

and near the top of the Topopah Spring Tuff. One large 

tritium concentration of about 47 TU was detected in 

water from a depth of 108.7 m in NRG-7a. Only 

limited numbers of samples were analyzed from the 

SD-7, SD -9, and SD-12 boreholes, but no large 

tritium concentrations (greater than 30 TU) were 

detected.

The gaseous-phase 14C activities of CO2 gas 

from the entire unsaturated zone of borehole SD-12 

show a trend similar to that observed at UZ-1; that is, 

14C activities decrease with depth, indicating gas 

transport by a diffusion mechanism. Gaseous-phase 

14C values in the Calico Hills nonwelded unit are, 

from SD-7 and from SD-9, respectively, 41.5 and 51.0 

pmc.

Experiments showed that vacuum distillation of 

pore water for stable-isotope analysis produced reliable results from tuffs free of clay and zeolite minerals 

but not from tuffs with large hydrated mineral 

contents. Compression extraction of pore water from 

hydrated, mineral-rich tuffs did, however, produce 

fluids with isotopically unfractionated waters. Extraction methods need to be carefully chosen with regard 

to the mineralogic and lithologic composition of 

samples.

From the column experiments with pure clinoptilolite powder, the channel water was shown to be 

exchangeable with the bulk water, and the exchange is 

complete within hours to days. For core samples from 

Yucca Mountain containing large clinoptilolite 

components, the exchange is slower, ranging from 

days to months. This slower exchange rate is determined by the diffusion rate between the bulk water and 

pore water. Exchange between pore water and channel 

water is fast and is not a rate-determining factor. The

isotopic compositions of the bulk pore water in the 

unsaturated-zone tuff at Yucca Mountain, pertaining to 

zeolite-rich core, will reflect the most recently infiltrated water. The channel water in zeolite is fractionated according to Rayleigh distillation.

Stable-isotopic composition of perched water 

generally plots close to the Yucca Mountain precipitation line (YMPL) indicating little evaporative loss 

before recharge of the perched-water bodies. Stableisotopic composition of UZ-14 pore water, however, 

plots to the right of the YMPL, indicating slight evaporation before infiltration. Estimated quantities of 

evaporation under the Rayleigh conditions (the largest 

possible loss) were 12-percent volumetric loss, of 

which one-half is due to sample handling. Winter 

precipitation is likely the source of recharge to the UZ 

at Yucca Mountain.

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 H U.S. GOVERNMENT PRINTING OFFICE: 1998 673-083 / 20040 Region No. 8

Yang and others HYDROCHEMICAL INVESTIGATIONS IN CHARACTERIZING THE UNSATURATED ZONE AT YUCCA MOUNTAIN, NEV