3,000 mg/L

10,000 mg/L

10,000 mg/L

3,000 mg/L

1,000 mg/L

San Antonio

New 

Braunfels

Hondo

San 

Marcos

Uvalde

Devine

Brackettville

Kyle

BEXAR

ATASCOSA

COUNTY

WILSON

 COUNTY

MEDINA

COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY BANDERA COUNTY

COMAL

 COUNTY

KENDALL

COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO

COUNTY

KINNEY COUNTY GUADALUPE

COUNTY

CALDWELL

COUNTY

MAVERICK COUNTY

FH2

TC4

TC3

TC2

TC1

KY4

KY3

KY2 KY1

EU4 DV1

EU2 EU1

East Uvalde transect

Tri-County transect

Kyle transect

San Antonio transect SA1, SA2, SA3

SC1, SC2

SD1

Devine well

Fish Hatchery transect

(single well for this report)

A

A'

B B'

C

C'

D'

D

Sources of Groundwater Based on Helium Analyses in and 

near the Freshwater/Saline-Water Transition Zone of the 

San Antonio Segment of the Edwards Aquifer, South-Central 

Texas, 2002–03

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5030

In cooperation with the San Antonio Water System

Sources of Groundwater Based on Helium Analyses in and 

near the Freshwater/Saline-Water Transition Zone of the 

San Antonio Segment of the Edwards Aquifer, South-Central 

Texas, 2002–03
Front cover: Kyle 2 monitoring well, Hays County, Texas. 

Back cover: Collecting sample with Kemmerer sampler. 
Sources of Groundwater Based on Helium 

Analyses in and near the Freshwater/

Saline-Water Transition Zone of the San 

Antonio Segment of the Edwards Aquifer, 

South-Central Texas, 2002–03

By Andrew G. Hunt, Rebecca B. Lambert, and Lynne Fahlquist

In cooperation with the San Antonio Water System

Scientific Investigations Report 2010–5030

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Hunt, A.G., Lambert, R.B., and Fahlquist, Lynne, 2010, Sources of groundwater based on helium analyses in and near 

the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 

2002–03: U.S. Geological Survey Scientific Investigations Report 2010–5030, 15 p.
iii

Contents

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

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

Purpose and Scope .............................................................................................................................3

Approach ...............................................................................................................................................3

Sources of Groundwater Based on Helium Analyses ............................................................................5

General Principles ...............................................................................................................................5

Helium Concentrations .......................................................................................................................6

Helium Isotopic Composition .............................................................................................................7

Summary .......................................................................................................................................................13

References Cited .........................................................................................................................................14

Figures

1. Map showing areal extent of freshwater/saline-water transition zone of the San 

Antonio segment of the Edwards aquifer, south-central Texas, and location of 

monitoring wells from which dissolved gas data were collected in and near the 

transition zone, 2002–03 ..............................................................................................................2

2. Diagrammatic section showing generalized relation between the Edwards and 

Trinity aquifers and underlying pre-Cretaceous rocks ..........................................................6

3–5. Graphs showing: 

3. Relation of helium-4 concentration to salinity (computed dissolved solids 

concentration) in monitoring wells in and near the freshwater/saline-water 

transition zone of the San Antonio segment of the Edwards aquifer, southcentral Texas, 2002–03 .......................................................................................................7

4. Relation of sampling depth to helium-4 concentration in monitoring wells in 

and near the freshwater/saline-water transition zone of the San Antonio 

segment of the Edwards aquifer, south-central Texas, 2002–03 ...............................11

5. Relation of isotopic composition of excess helium-4 (R/RA

4

Heex) to 

excess helium-4 (4

Heex) concentration in monitoring wells in and near the 

freshwater/saline-water transition zone of the San Antonio segment of the 

Edwards aquifer, south-central Texas, 2002–03 ..........................................................12

Tables

1. Descriptive information for monitoring wells from which dissolved gas data were 

collected in and near the freshwater/saline-water transition zone of the San 

Antonio segment of the Edwards aquifer, south-central Texas, 2002–03 ...........................4

2. Results of analyses of samples for dissolved gases from monitoring wells in and 

near the freshwater/saline-water transition zone of the San Antonio segment of 

the Edwards aquifer, south-central Texas, 2002–03 ...............................................................8
iv

Conversion Factors, Datums, and Water-Quality Units

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

SI to Inch/Pound

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

Volume

liter (L) 0.2642 gallon (gal)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Water-Quality Units

Specific conductance is given in microsiemens per centimeter at 25 °C (μS/cm).

Concentrations of chemical constituents are given in milligrams per liter (mg/L).

Concentrations of dissolved gases in water are given in microcubic centimeters per kilogram 

at standard temperature (20 °C) and pressure (1 atmosphere) (μccSTP/kg) and in cubic 

centimeters per kilogram at standard temperature (20 °C) and pressure (1 atmosphere) (ccSTP/

kg).

Concentrations of uranium in soil are given in milligrams per kilogram (mg/kg).
Abstract

This report evaluates dissolved noble gas data, specifically helium-3 and helium-4, collected by the U.S. Geological 

Survey, in cooperation with the San Antonio Water System, 

during 2002–03. Helium analyses are used to provide insight 

into the sources of groundwater in the freshwater/saline-water 

transition zone of the San Antonio segment of the Edwards 

aquifer. Sixty-nine dissolved gas samples were collected from 

19 monitoring wells (categorized as fresh, transitional, or 

saline on the basis of dissolved solids concentration in samples 

from the wells or from fluid-profile logging of the boreholes) 

arranged in five transects, with one exception, across the 

freshwater/saline-water interface (the 1,000-milligrams-perliter dissolved solids concentration threshold) of the Edwards 

aquifer. The concentration of helium-4 (the dominant isotope 

in atmospheric and terrigenic helium) in samples ranged from 

63 microcubic centimeters per kilogram at standard temperature (20 degrees Celsius) and pressure (1 atmosphere) in a well 

in the East Uvalde transect to 160,587 microcubic centimeters 

per kilogram at standard temperature and pressure in a well 

in the Kyle transect. Helium-4 concentrations in the 10 saline 

wells generally increase from the western transects to the 

eastern transects. Increasing helium-4 concentrations from 

southwest to northeast in the transition zone, indicating 

increasing residence time of groundwater from southwest to 

northeast, is consistent with the longstanding conceptualization of the Edwards aquifer in which water recharges in the 

southwest, flows generally northeasterly (including in the 

transition zone, although more slowly than in the freshwater 

zone), and discharges at major springs in the northeast. 

Excess helium-4 was greater than 1,000 percent for 60 of the 

69 samples, indicating that terrigenic helium is largely present 

and that most of the excess helium-4 comes from sources 

other than the atmosphere. The helium data of this report 

cannot be used to identify sources of groundwater in and 

near the transition zone of the Edwards aquifer in terms of 

specific geologic (stratigraphic) units or hydrogeologic units 

(aquifers or confining units). However, the data indicate that 

the source or sources of the helium, and thus the water in 

which the helium is dissolved, in the transition zone are mostly 

terrigenic in origin rather than atmospheric. Whether most 

helium in and near the transition zone of the Edwards aquifer 

originated either in rocks outside the transition zone and at 

depth or in the adjacent Trinity aquifer is uncertain; but most 

of the helium in the transition zone had to enter the transition 

zone from the Trinity aquifer because the Trinity aquifer is the 

hydrogeologic unit immediately beneath and laterally adjacent 

to the transition zone of the Edwards aquifer. Thus the helium 

data support a hypothesis of sufficient hydraulic connection 

between the Trinity and Edwards aquifers to allow movement 

of water from the Trinity aquifer to the transition zone of the 

Edwards aquifer.

Introduction

The San Antonio segment of the Edwards aquifer, 

hereinafter the Edwards aquifer, is the primary source of 

water supply in south-central Texas and one of the most 

permeable and productive carbonate aquifers in the United 

States. The Edwards aquifer is about 180 miles long from 

west to east and ranges from 5 to 40 miles wide from north to 

south (Maclay, 1995) (fig. 1). The Edwards aquifer consists of 

regionally extensive, faulted and fractured carbonate rocks of 

the Georgetown Formation and Edwards Group that dip to the 

south and southeast. The less permeable and more clay-rich 

Trinity aquifer, composed primarily of Glen Rose Limestone, 

underlies and juxtaposes the Edwards aquifer to the north 

(Barker and Ardis, 1996, plate 3). To the west and east, the 

aquifer is bounded by groundwater divides and to the south 

and southeast by a zone of transition from freshwater to saline 

water. 

The freshwater zone of the Edwards aquifer extends north 

to the northern limit of the aquifer, which coincides with the 

northern limit of the recharge zone (outcrop) of the aquifer 

Sources of Groundwater Based on Helium Analyses in and 

near the Freshwater/Saline-Water Transition Zone of the 

San Antonio Segment of the Edwards Aquifer, SouthCentral Texas, 2002–03

By Andrew G. Hunt, Rebecca B. Lambert, and Lynne Fahlquist 
2 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03

Figure 1. Areal extent of freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, and location of monitoring wells from which dissolved gas data were collected in and near the transition zone, 2002–03 (modified from Lambert and others, 2009, fig. 1). 

3,000 mg/L

10,000 mg/L

10,000 mg/L

3,000 mg/L

1,000 mg/L

San Antonio

New 

Braunfels

Hondo

San 

Marcos

Uvalde

Devine

Brackettville

Kyle

BEXAR

ATASCOSA

COUNTY

WILSON

 COUNTY

MEDINA

COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY BANDERA COUNTY

COMAL

 COUNTY KENDALL COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO

COUNTY

KINNEY COUNTY

GUADALUPE

COUNTY

CALDWELL

COUNTY

MAVERICK COUNTY

FH2

TC4

TC3

TC2

TC1

KY4

KY3

KY2 KY1

DV1 EU4

EU2 EU1

East Uvalde transect

Tri-County transect

Kyle transect

San Antonio transect

SA1, SA2, SA3

SC1, SC2

SD1

Devine well

Fish Hatchery transect

(single well for this report)

A

A'

B

B'

C

C'

D'

D

Recharge zone (outcrop) (Ashworth and Hopkins, 1995)Freshwater zoneTransition zoneSaline-water zoneGround-water divide (modified from Maclay, 1995, plate 1; Mahler and others, 2006, fig. 1)Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994)Slightly to moderately saline interface (3,000 mg/L dissolved solids concentration) (Schultz, 1994)Moderately to very saline interface (10,000 mg/L dissolved solids concentration) (Schultz, 1994)Monitoring well transect Monitoring well (identifier) from which dissolved gas data collected (table 1) EU2

EXPLANATION

A A'

LOCATION MAP

Trace ofgeneralized section (fig. 2)

Balconesfault zone 

Balcones fault zone fromMaclay (1995, fig. 2)

TEXAS

Study area

0 100 200 MILES0 100 200 KILOMETERS

100°30'

100°00'

99°30'

99°00'

98°30'

98°00'

97°30'

28°30'

29°00'

29°30'

30°00'

30°30'

0 30 60 KILOMETERS 15 45

60 MILES 0 30 15 45 Base modified from U.S. Geological Survey

digital data—County lines 1:24,000 scale.Universal Transverse Mercator Projection,Zone 14, North American Datum of 1983
Introduction 3

(fig. 1). Freshwater is defined as water with a dissolved 

solids concentration of 1,000 milligrams per liter (mg/L) 

or less; thus the freshwater/saline-water interface is the 1,000-

mg/L dissolved solids concentration threshold. The transition 

zone of the aquifer is defined as the region with dissolved 

solids concentrations ranging from 1,000 to 10,000 mg/L 

(Schultz, 1994). Slightly saline water is defined as water with 

dissolved solids concentrations ranging from 1,000 to 3,000 

mg/L; moderately saline is defined as water with dissolved 

solids concentrations ranging from 3,000 to 10,000 mg/L; and 

very saline is defined as water with dissolved solids concentrations ranging from 10,000 to 35,000 mg/L (Winslow and 

Kister, 1956). On the basis of these definitions, the transition 

zone primarily contains water that is slightly or moderately 

saline, with very saline water in a few locations.

As the population increases throughout the region 

(U.S. Census Bureau, 2010), withdrawals from the aquifer 

likely will increase. Increased withdrawals lower water 

levels in the aquifer, which increases the potential for 

encroachment of saline water into the freshwater zone. In 

1985, the U.S. Geological Survey (USGS), the San Antonio 

Water System (SAWS), and other Federal, State, and local 

agencies began a series of studies to learn more about the 

interaction between the freshwater and saline-water zones 

of the Edwards aquifer. The main goal of the studies was 

to provide better understanding of the potential for movement of saline water into the freshwater zone of the aquifer 

(Pavlicek and others, 1987; Poteet and others, 1992; Groschen 

and Buszka, 1997). 

The objective of a more recent 9-year (1999–2007) 

study, done by the USGS in cooperation with SAWS, was 

to improve understanding of the hydrogeologic, hydraulic, 

and chemical characteristics of the transition zone of the 

Edwards aquifer. The study included drilling of additional 

monitoring wells by SAWS to expand the hydrogeologic and 

geochemical monitoring network in the aquifer across the 

freshwater/saline-water transition zone throughout the region. 

Monitoring wells were drilled or re-completed in and near the 

transition zone in Uvalde County (East Uvalde transect, four 

wells), in Medina County (South Medina well and Devine 

well), in Bexar County (Pitluck transect, three wells; Mission transect, three wells; San Antonio transect, seven wells), 

in Comal and Guadalupe Counties (Tri-County transect, five 

wells; New Braunfels transect, six wells), and in Hays County 

(Fish Hatchery transect, two wells; San Marcos transect, five 

wells; Kyle transect, four wells) (Lambert and others, 2009, 

fig. 1). Dissolved constituent and noble (inert) gas data were 

collected during 2002–03 from 19 of the 41 monitoring wells 

to further characterize geochemical and hydrologic processes 

occurring in and near the freshwater/saline-water transition 

zone.

Purpose and Scope

This report evaluates dissolved noble gas data, specifically helium-3 (3

He) and helium-4 (4He), collected by the 

USGS during 2002–03 as part of the larger, 9-year study of the 

freshwater/saline-water transition zone of the Edwards aquifer. 

Helium analyses are used to provide insight into the sources of 

water in the transition zone. Sixty-nine dissolved gas samples 

were collected from 19 monitoring wells arranged in five 

transects, except for the Devine well, across the freshwater/

saline-water interface of the Edwards aquifer. Other chemical, 

hydrologic, and geophysical data collected for the larger study 

are presented in Lambert and others (2009). 

Approach

Water-quality samples and water-quality monitoring 

data were collected from 19 monitoring wells in and near 

the freshwater/saline-water transition zone to characterize 

chemical properties and changes in chemical properties 

of water over time. The 19 monitoring wells are categorized as 

fresh, transitional, or saline on the basis of dissolved 

solids concentration in samples from the wells or from fluidprofile logging of the boreholes (Lambert and others, 2009). 

The wells were drilled during 1972–2001 by various Federal, 

State, and local agencies. Most of the wells were constructed 

with the casing extending from land surface downward into 

the upper 20 feet of the Edwards aquifer. The remaining vertical extent of each borehole was completed as open hole in the 

aquifer. Where possible, the open-hole section of each well 

was drilled through the entire thickness of the aquifer. Some 

wells do not penetrate the entire thickness because of depth 

limitations of the drilling rigs. The 19 wells, except the Devine 

well, are arranged in five transects across the freshwater/

saline-water interface; transects are distributed spatially from 

Uvalde County to Hays County (fig. 1). Three wells are in the 

East Uvalde transect (A–A' in fig. 1); the Devine well is alone; 

six wells are in the San Antonio transect (B–B'); four wells 

are in the Tri-County transect (C–C'); one well is in the Fish 

Hatchery transect; and four wells are in the Kyle transect 

(D–D'). Descriptive information for wells sampled for dissolved gases is listed in table 1 and in Lambert and others 

(2009).

Field-property measurement techniques, samplecollection procedures for inorganic and isotopic samples, and 

results are described in Lambert and others (2009). Field properties (pH, specific conductance, water temperature, depth to 

water) and samples for dissolved gases were collected at each 

well. For non-flowing wells in which the borehole was accessible, samples were collected at discrete depths in the uncased 

borehole using a 6-liter, stainless steel Kemmerer sampling 

flask attached to a portable cable winch fitted with a digital 

depth counter. The distances between depth intervals in a borehole water column where discrete samples were collected were 

based on changes in specific conductance with depth obtained 

from fluid salinity profiles (Lambert and others, 2009). Fewer 

samples were collected in water columns of relatively stable 

specific conductance than in water columns where specific 

conductance changed appreciably. After being lowered to the 

selected sampling depth, the Kemmerer flask was closed by 
4 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03 Table 1. Descriptive information for monitoring wells from which dissolved gas data were collected in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03 (modified from Lambert and others, 2009, table 1). [NAD 83, North American Datum of 1983; LSD, land-surface datum; NAVD 88, North American Vertical Datum of 1988. Well descriptor: Fresh, water with dissolved solids concentration less than 1,000 milligrams per liter; Saline, water with dissolved solids concentration greater than 1,000 milligrams per liter; Transitional, freshwater and saline water in stratified lenses] U.S. Geological Survey site number State well number Well name Well identifier (fig. 1) Latitude, NAD 83 (decimal degrees) Longitude, NAD 83 (decimal degrees) Year drilled Well depth (feet) Altitude of LSD (feet above NAVD 88) Open interval (feet below LSD) Well descrip- tor based on water type 291443099325801 YP–69–52–202 East Uvalde 1 EU1 29.245516 99.549772 1998 1,500 874.02 985 to 1,500 Fresh 291612099302001 YP–69–44–902 East Uvalde 2 EU2 29.270237 99.505604 1999 1,560 899.91 1,072 to 1,560 Fresh 291133099363801 YP–69–52–404 East Uvalde 4 EU4 29.192740 99.610885 1999 1,463 867.02 950 to 1,463 Saline 290955098562101 TD–68–49–813 Devine DV1 29.165778 98.939411 1972 3,200 663.20 2,570 to 3,194 Fresh 292505098254001 AY–68–37–521 San Antonio A1 SA1 29.418289 98.428071 1985 1,275 621.53 1,211 to 1,275 Saline 292505098254002 AY–68–37–522 San Antonio A2 SA2 29.418289 98.428071 1985 1,075 621.53 1,014 to 1,075 Saline 292505098254003 AY–68–37–523 San Antonio A3 SA3 29.418289 98.428071 1985 1,175 621.53 1,113 to 1,175 Saline 292546098260001 AY–68–37–524 San Antonio C1 SC1 29.429677 98.433627 1985 881 626.21 842 to 881 Fresh 292546098260002 AY–68–37–525 San Antonio C2 SC2 29.429677 98.433627 1986 1,150 625.19 1,087 to 1,150 Saline 292556098260701 AY–68–37–526 San Antonio D1 SD1 29.432455 98.435571 1986 1,223 643.60 1,220 to 1,223 Fresh 293610098152701 KX–68–30–314 Tri-County 1 TC1 29.602931 98.257508 1999 920 871.01 385 to 920 Fresh 293424098134701 KX–68–31–403 Tri-County 2 TC2 29.573501 98.229936 1999 1,050 709.08 486 to 1,050 Transitional 293245098121001 KX–68–31–511 Tri-County 3 TC3 29.545969 98.202656 1999 1,222 674.00 656 to 1,222 Saline 293058098110501 KX–68–31–808 Tri-County 4 TC4 29.516217 98.184996 2000 1,562 648.92 1,000 to 1,562 Saline 294946097574501 LR–67–09–401 Fish Hatchery 2 FH2 29.829250 97.962364 2001 1,030 642.51 510 to 1,030 Saline

295853097532901 LR–67–01–311 Kyle 1 KY1 29.981389 97.891389 1997 810 770.52 307 to 810 Fresh295858097521801 LR–67–02–104 Kyle 2 KY2 29.982995 97.871949 1998 975 674.32 427 to 975 Transitional295829097512601 LR–67–02–106 Kyle 3 KY3 29.974940 97.857504 1998 1,100 678.28 600 to 1,100 Saline295730097503201 LR–67–02–105 Kyle 4 KY4 29.958551 97.842504 1998 970 646.70 562 to 970 Saline
Sources of Groundwater Based on Helium Analyses 5

a sender weight and retrieved. For flowing wells in which the 

casing was accessible, a single composite sample was collected from a sampling spigot at the top of the casing after the 

well was purged and field properties had stabilized.

Dissolved gas samples were collected first to minimize 

atmospheric contamination; then the other inorganic and 

isotopic samples were collected. Dissolved gas samples were 

collected by connecting an approximately 1-foot length of 

0.375-inch-diameter copper tubing to Tygon tubing and allowing the water sample to flow from the Kemmerer sampler 

through the tubing. Once air bubbles were purged from the 

tubing, the copper tubing was crimped on both ends and sealed 

with refrigeration clamps. Care was taken to ensure that the 

samples were bubble-free and did not come into contact with 

the atmosphere. 

Sixty-nine dissolved gas samples were analyzed for 

helium (He), neon (Ne), argon (Ar), nitrogen (N2), oxygen 

(O2

), and methane (CH4) by the USGS Central Region 

Isotopic/Geochronology Core Operations Laboratory Support Project (Denver Noble Gas Laboratory) in Denver, Colo. 

Several wells were sampled at multiple depths. Isotopic 

ratios of 3He to 4He (3He/4He), neon-20 to neon-22 (20Ne/22Ne), 

argon-40 to argon-36 (40Ar/36Ar), and nitrogen to argon 

concentration (N2/Ar) also were determined. 3He/4He in a 

sample (R), the isotopic composition, is commonly expressed 

as a multiple of 3He/4He in the atmosphere (in air) (RA). 

Dissolved gas samples were analyzed using procedures 

documented in Bayer and others (1989) and Solomon and 

others (1995). The dissolved gases were separated from 

the formation water in the laboratory and analyzed in an 

ultra-high vacuum extraction system. Total pressure of the 

extracted gases was measured on a capacitance manometer, 

and a split of the gas was run dynamically on a quadrapole 

mass spectrometer to obtain the concentrations of the major 

gas components (CH4, N2, Ar, O2). The extracted gases then 

were separated using an STS–101 separator heated to 300 

degrees Celsius, and another split of the gas was analyzed 

statically on a quadrapole mass spectrometer for isotopic 

measurements of Ar (36Ar, 38Ar, and 40Ar). He and Ne were 

further separated from the sample gas using a cryogenic cold 

trap and then were analyzed for their isotopic composition 

(3

He, 4He, 20Ne, 21Ne, and 22Ne) statically on a MAP–215–50 

mass spectrometer. The Ne and Ar isotopic ratios are reported 

as absolute ratios, unlike the He isotope ratios expressed as 

R/RA. 

No field quality-control dissolved gas samples were 

collected. For the Denver Noble Gas Laboratory, in-house 

quality-control standards were used to compute the percentage deviation from known values and to compute laboratory 

error for dissolved gas analyses. The in-house dissolved gas 

standards were cross-calibrated with air samples collected 

from Loveland Pass, Colo., and compared to known dissolved 

gas and bulk-gas compositions of U.S. Standard Atmosphere 

1976 (Weast, 1983). Analytical variation (2-sigma standard 

deviation) of He gas analyses typically was 1 percent or less of 

that measured in the samples. 

Sources of Groundwater Based on 

Helium Analyses

General Principles

As water recharges an aquifer, the water that is in equilibrium with the atmosphere commonly is referred to as airsaturated water (ASW). The concentration of a dissolved gas 

in the water can be altered during recharge by the addition of 

more gas that is trapped by the recharging waters and forced 

into solution. This additional component greater than atmospheric solubility is referred to as “excess air” and typically 

has an air-like composition. Dissolved gas data, including 

isotopic composition, can be used to determine the amount of 

excess air measured in water. High concentrations of excess 

air are common in fractured-rock aquifers, in aquifers that 

function like fractured rock such as karst, and in aquifers in 

semiarid areas, such as the Edwards aquifer (Cook and others, 

2006). 

Most of the He in the atmosphere is 4He; RA is 1.384 x

10-6 (Clark and others, 1976; Clark and Fritz, 1997). Although 

local variation in He concentration (typical atmospheric concentration is 5.24 parts per million [Mamyrin and Tolstikhin, 

1984]) and isotopic composition in the atmosphere might 

occur, the variation is presumed to be negligible. Concentrations of He in groundwater, much like other dissolved gases, 

initially conform to conditions of atmospheric solubility 

(recharge temperature, salinity [as indicated by dissolved solids concentration], and altitude) and excess air. Under typical 

recharge conditions, the atmospheric He component is about 

50 to 100 microcubic centimeters per kilogram at standard 

temperature (20 degrees Celsius) and pressure (1 atmosphere)1

(μccSTP/kg). The solubility of He at 20 degrees Celsius in 

freshwater (the ASW value of He) and 1,000 feet elevation is 

about 43.1 μccSTP/kg (Weiss, 1971), and the isotopic composition is 0.98 RA (Benson and Krause, 1980). These conditions 

represent the approximate mean annual air temperature and 

elevation of the study area, and they also are representative of 

recharge conditions.

Dissolved He in groundwater is derived from atmospheric 

and terrigenic (earth) sources. The atmospheric component 

includes air-soluble He and excess-air He (He derived from 

excess air) that results from dissolution of air bubbles trapped 

just below the water table. Another atmospheric source of 

He (as 3

He) is from decay of tritium in recent (less than 60 

years old) recharged water; tritium concentrations in rainfall, 

some of which became recharge, during the mid-20th century 

were increased substantially by atmospheric testing of nuclear 

weapons (Michel, 1989). 

Helium also can accumulate in groundwater in excess of 

the sum of ASW and excess-air concentrations because it is 

1

 Standard conditions for temperature and pressure per U.S. Environmental 

Protection Agency (Electronic Code of Federal Regulations, 2009).
6 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03

produced by the radioactive decay of uranium and thorium in 

crustal rocks and aquifer solids (crustal He) and from upward 

diffusion or advection of He from the mantle (mantle He). 

Together, crustal and mantle He are termed terrigenic He 

(Solomon, 2000). Likely sources of terrigenic He in the study 

area include igneous rocks intruded into the Edwards aquifer, 

oil and gas hydrocarbons that are widely present in Edwards 

Group rocks downdip from the transition zone of the Edwards 

aquifer, rocks composing the underlying and adjacent Trinity aquifer, and rocks underlying the Trinity aquifer (fig. 2). 

Uranium concentrations from selected Edwards aquifer cores 

collected in the San Antonio area ranged from less than 0.1 

to 1.7 milligrams per kilogram (mg/kg), and the median 

concentration was 0.6 mg/kg (K.M. Conko, U.S. Geological Survey, written commun., 2008), indicating that uranium 

concentrations in rocks of the Edwards aquifer typically might 

be relatively low. The concentration of natural uranium in soil 

is about 2 mg/kg (International Atomic Energy Agency, 2009). 

Rocks of the Trinity aquifer generally contain more clastic 

material (conglomerate, sand, silt, clay, shale) than rocks of 

the Edwards aquifer (Duffin and Musick, 1991), and therefore 

might contain more radionuclide sources. 

Except within active volcanic centers, terrigenic He is 

essentially all crustal He. Crustal He has a 3He/4He isotopic 

ratio of 2.77 x 10-8 (Mamyrin and Tolstikhin, 1984, p. 273); 

so as in atmospheric He, the dominant isotope by far in terrigenic He is 4

He. Relatively high (greater than 1,000 μccSTP/

kg [Solomon, 2000]) terrigenic He concentrations typically 

are observed in older groundwater with residence times of 

thousands of years or more because of the relatively slow 

accumulation rate of crustal He. Terrigenic He concentrations 

in relatively old water can exceed the atmospheric component 

by several orders of magnitude (Davis and De Wiest, 1966; 

Marine, 1979; Stute and others, 1992). 

Helium Concentrations

Because of the dominance of 4He in atmospheric and 

terrigenic sources, helium results will pertain to 4He unless 

otherwise noted. The concentration of 4He in samples ranged 

from 63 μccSTP/kg in well EU2 in the East Uvalde transect to 160,587 μccSTP/kg in well KY3 in the Kyle transect 

(table 2). The lowest 4He concentrations were measured in 

freshwater wells EU2 and KY1 and are more representative 

of atmospherically derived He. 4He concentrations in the 10 

saline wells generally increase from the western transects to 

the eastern transects, with the highest (and most variable) 4He 

concentrations occurring in the Kyle transect. Increasing 4He 

concentrations from southwest to northeast in the transition 

zone, indicating increasing residence time of groundwater 

from southwest to northeast, is consistent with the longstanding conceptualization of the Edwards aquifer: Water recharges 

in the southwest, flows generally northeasterly (including in 

the transition zone, although more slowly than in the freshwater zone), and discharges at major springs in the northeast 

(Lindgren and others, 2004).

In general, 4

He concentrations increase as salinity 

increases (fig. 3). Samples with the lowest salinities show 

typical ASW values of He coupled with some He from excess 

air. Because dissolved solids concentration was not measured 

in all samples, dissolved solids concentration was computed 

from a linear regression with specific conductance as follows: 

Dissolved solids = 0.658 x specific conductance, with a coefficient of determination of 0.997, which was determined from 

existing pairs of dissolved solids concentrations and specific 

conductance in table 2. Assuming larger 4

He concentrations 

represent older water, the relation between 4He and salinity indicates longer residence time for the more saline water. 

Water with longer residence time likely contains 4He from 

crustal or mantle sources.

A relation between 4He concentration and depth might 

contribute information useful for identifying sources of 

Figure 2. Diagrammatic section showing generalized relation 

between the Edwards and Trinity aquifers and underlying preCretaceous rocks. 

Edwards aquifer

Trinity aquifer

Not to scale

NORTHWEST SOUTHEAST

Modified from Barker and Ardis, 1996, fig. 5

Cenozoic (mostly alluvium) and upper Cretaceous rocks

(Edwards aquifer upper confining units)

Lower Cretaceous rocks (Edwards and Trinity aquifers)

Paleozoic rocks

Precambrian rocks

Fault—Arrows show relative movement

EXPLANATION
Sources of Groundwater Based on Helium Analyses 7

4

He and thus sources of groundwater in the transition zone. 

However, no discernible relation is apparent (fig. 4). 4He concentrations increase moderately with sample depth in TC2 and 

KY2, and slightly in KY3, but 4He concentrations generally do 

not appear to increase with sample depth in most of the wells.

Helium Isotopic Composition

The presence and amount of excess 4He (4Heex) can be 

useful for identifying possible sources of 4Heex. In this report, 4

Heex is computed as 

4

Heex = 4Hemeasured – 4Heair-water equilibrium (ASW), (1) 

where 

4

HeASW = 43.1 μccSTP/kg; 

and the percentage of 4Heex relative to 4He in air at equilibrium 

with water (D4Heex) is computed as 

D4Heex = (4Heex / 4He ASW) x 100. (2)

When Δ4Heex is near zero, little excess air is present. When 

Δ4Heex is greater than about 200 percent, excess Δ4Heex from 

Figure 3. Relation of helium-4 concentration to salinity (computed dissolved solids concentration) in monitoring wells in and near the 

freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03. 

Air-saturated water

Terrigenicsourcesdominate

100 1,000 10,000 100,000

COMPUTED DISSOLVED SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER 

0.00001

0.0001

0.001

0.01

0.1

1

HELIUM-4, IN CUBIC CENTIMETERS PER KILOGRAM AT STANDARD TEMPERATURE AND PRESSURE

Moderately to very saline interface

Freshwater/saline-water interface

FRESHWATER TRANSITIONAL WATER SALINE WATER

EU1

EU2

EU4

DV1

SA1

SA2

SA3

SC1

SC2

SD1

TC1

TC2

TC3

TC4

FH2

KY1

KY2

KY3

KY4

EXPLANATION

Well identifier 

(fig. 1; table 1)
8 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03 Table 2. Results of analyses of samples for dissolved gases from monitoring wells in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03—Continued. U.S. Geological Survey site number Well iden- tifier (fig. 1; table 1) Date sampled Sam- ple depth (ft below LSD) Depth to water (ft below LSD) Spe- cific con- duc- tance (μS/ cm) Dis- solved solids, residue on evapo- ration at 180 °C (mg/L) Dis- solved solids, sum of constit- uents (mg/L) Com- puted dis- solved solids (mg/L) Chlo- ride (mg/L) Helium-4 (μccSTP/ kg) Helium-4 (ccSTP/ kg) R/RA Delta excess helium-4 (percent) Excess helium-4 (μccSTP/ kg) Excess helium-4 (ccSTP/ kg) Helium-3 (μccSTP/ kg) (com- puted from helium-4 measured) Excess helium-3 (μccSTP/ kg) (com- puted from helium-3 in ASW) R/RA of excess helium (com- puted) 291443099325801 EU1 06/12/2003 1,020 -- 658 -- -- 433 -- 2,028 0.002028 0.18 4,605 1,985 0.001985 0.000491 0.000432 0.16 06/12/2003 1,140 -- 642 395 377 422 33.7 1,511 .001511 .18 3,406 1,468 .001468 .000385 .000325 .16 06/12/2003 1,250 -- 642 -- -- 422 -- 2,154 .002154 .18 4,898 2,111 .002111 .000543 .000483 .17 06/13/2003 1,360 -- 650 -- -- 428 -- 2,164 .002164 .17 4,921 2,121 .002121 .000521 .000461 .16 06/13/2003 1,470 -- 645 281 380 424 34.0 2,351 .002351 .18 5,355 2,308 .002308 .000579 .000520 .16 292015099351001 EU2 07/11/2002 1,100 123.67 489 -- 263 322 25.0 74 .000074 .89 72 31 .000031 .000091 .000031 .74 07/11/2002 1,200 123.67 491 -- 257 323 24.2 112 .000112 1.09 160 69 .000069 .000169 .000109 1.15 07/11/2002 1,350 123.67 523 -- 258 344 23.6 63 .000063 1.06 46 20 .000020 .000092 .000033 1.19 291133099363801 EU4 06/02/2003 1,000 115.86 3,240 -- -- 2,132 -- 30,699 .030699 .18 71,127 30,656 .030656 .007563 .007503 .18 06/02/2003 1,100 115.86 3,190 2,310 2,420 2,099 407 32,014 .032014 .19 74,178 31,971 .031971 .008285 .008226 .19 06/02/2003 1,160 115.86 3,140 -- -- 2,066 -- 34,188 .034188 .20 79,223 34,145 .034145 .009227 .009167 .19 06/02/2003 1,220 115.86 2,940 -- -- 1,935 -- 39,183 .039183 .18 90,812 39,140 .039140 .009544 .009485 .18 06/03/2003 1,300 -- 3,160 -- -- 2,079 -- 32,140 .032140 .19 74,471 32,097 .032097 .008452 .008392 .19 06/03/2003 1,400 -- 2,900 1,950 1,885 1,908 431 38,240 .038240 .19 88,624 38,197 .038197 .010003 .009943 .19 290955098562101 DV1 02/27/2003 -- -55.94 1,220 725 E645 803 185 837 .000837 .14 1,842 794 .000794 .000162 .000103 .09 292505098254001 SA1 07/12/2002 -- -- 5,850 -- 3,941 3,849 848 17,468 .017468 .27 40,429 17,425 .017425 .006407 .006347 .26 292505098254002 SA2 07/12/2002 -- -- 4,170 -- 2,731 2,744 575 13,052 .013052 .25 30,183 13,009 .013009 .004588 .004529 .25

292505098254003 SA3 07/12/2002 -- -- 5,900 -- 4,036 3,882 900 17,710 .017710 .26 40,990 17,667 .017667 .006299 .006240 .26

292546098260001 SC1 07/12/2002 -- -- 943 -- 540 620 71.9 1,843 .001843 .30 4,176 1,800 .001800 .000760 .000700 .28

292546098260002 SC2 07/12/2002 -- -- 6,720 -- 4,728 4,422 1,096 22,352 .022352 .26 51,761 22,309 .022309 .007888 .007829 .25

292556098260701 SD1 07/12/2002 -- -- 854 -- 479 562 67.0 1,004 .001004 .29 2,229 961 .000961 .000399 .000339 .26

Table 2. Results of analyses of samples for dissolved gases from monitoring wells in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03. [ft, feet; LSD, land-surface datum; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degrees Celsius; mg/L, milligrams per liter; Computed dissolved solids = 0.658 x specific conductance; μccSTP/kg, microcubic centimeters per kilogram at standard temperature (20 °C) and pressure (1 atmosphere); ccSTP/kg, cubic centimeters per kilogram at standard temperature (20 °C) and pressure (1 atmosphere); R/RA, (helium-3/helium-4) sample/(helium-3/helium-4) atmosphere; Delta excess helium-4, percentage of excess helium-4 relative to helium-4 in air at equilibrium with water; ASW, air-saturated water; 

--, not measured; E, estimated; no shade, freshwater well; pink shade, transitional well; gray shade, saline well]
Sources of Groundwater Based on Helium Analyses

9

Table 2. Results of analyses of samples for dissolved gases from monitoring wells in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03—Continued. 

U.S. Geological 

Survey site number

Well 

identifier 

(fig. 1; 

table 1)

Date 

sampled

Sample 

depth 

(ft 

below 

LSD)

Depth to 

water (ft 

below 

LSD)

Specific 

conductance 

(μS/

cm)

Dissolved 

solids, 

residue 

on 

evaporation at 

180 °C 

(mg/L)

Dissolved 

solids, sum of 

constituents 

(mg/L)

Computed 

dissolved 

solids 

(mg/L)

Chloride 

(mg/L)

Helium-4 

(μccSTP/ 

kg)

Helium-4 

(ccSTP/ 

kg)

R/RA

Delta 

excess 

helium-4 

(percent)

Excess 

helium-4 

(μccSTP/ 

kg)

Excess 

helium-4 

(ccSTP/ 

kg)

Helium-3 

(μccSTP/

kg) (computed 

from 

helium-4 

measured)

Excess 

helium-3 

(μccSTP/

kg) (computed 

from 

helium-3 

in ASW)

R/RA of 

excess 

helium 

(computed)

293610098152701 TC1 02/19/2003 440 193.74 1,060 587 529 697 104 1,518 0.001518 0.25 3,422 1,475 0.001475 0.000521 0.000461 0.23

02/19/2003 510 193.74 1,050 -- -- 691 -- 1,364 .001364 .24 3,065 1,321 .001321 .000444 .000384 .21

02/19/2003 550 193.74 1,010 573 542 665 91.1 1,564 .001564 .25 3,529 1,521 .001521 .000530 .000471 .2202/19/2003 650 193.74 820 -- -- 540 -- 1,478 .001478 .24 3,329 1,435 .001435 .000499 .000439 .22

02/19/2003 750 193.74 1,010 -- -- 665 -- 1,784 .001784 .24 4,039 1,741 .001741 .000600 .000540 .22

02/19/2003 840 193.74 1,020 599 593 671 89.4 1,570 .001570 .25 3,543 1,527 .001527 .000537 .000477 .23

293424098134701 TC2 02/11/2003 520 23.46 1,090 550 -- 717 263 4,606 .004606 .21 10,587 4,563 .004563 .001351 .001292 .20

02/11/2003 575 23.46 2,980 -- -- 1,961 -- 19,782 .019782 .22 45,798 19,739 .019739 .006023 .005964 .22

02/12/2003 620 23.46 2,690 1,510 1,440 1,770 691 21,495 .021495 .22 49,772 21,452 .021452 .006604 .006545 .22

02/12/2003 700 23.46 4,100 -- -- 2,698 -- 27,527 .027527 .22 63,768 27,484 .027484 .008496 .008436 .2202/12/2003 780 23.46 4,150 2,690 2,540 2,731 807 27,726 .027726 .22 64,229 27,683 .027683 .008442 .008382 .2202/12/2003 820 23.46 10,000 -- -- 6,580 -- 72,585 .072585 .21 168,311 72,542 .072542 .021197 .021137 .2102/12/2003 900 23.46 10,200 -- -- 6,712 -- 71,548 .071548 .21 165,905 71,505 .071505 .020993 .020933 .2102/12/2003 1,000 23.46 10,200 6,140 6,140 6,712 1,950 71,502 .071502 .22 165,798 71,459 .071459 .021672 .021612 .22

293245098121001 TC3 03/11/2003 700 -2.39 11,000 8,030 7,680 7,238 2,620 78,911 .078911 .22 182,988 78,868 .078868 .024136 .024076 .22

03/11/2003 800 -2.39 11,100 -- -- 7,304 -- 77,723 .077723 .22 180,232 77,680 .077680 .023665 .023605 .2203/11/2003 900 -2.39 11,100 8,110 7,780 7,304 2,650 79,561 .079561 .23 184,496 79,518 0.079518 .024885 .024826 .23

03/12/2003 1,000 -2.39 11,200 -- -- 7,370 -- 79,565 .079565 .22 184,506 79,522 0.079522 .024226 .024166 .2203/12/2003 1,140 -2.39 11,200 8,020 7,760 7,370 2,650 80,535 .080535 .22 186,756 80,492 0.080492 .024633 .024573 .22

293058098110501 TC4 05/22/2003 1,050 -6.04 11,700 -- -- 7,699 -- 84,684 .084684 .22 196,383 84,641 0.084641 .026019 .025959 .22

05/22/2003 1,150 -6.04 11,600 8,360 7,980 7,633 2,840 83,550 .083550 .22 193,752 83,507 0.083507 .025092 .025033 .22

05/22/2003 1,250 -6.04 11,600 -- -- 7,633 -- 83,172 .083172 .22 192,874 83,129 0.083129 .025554 .025495 .2205/28/2003 1,350 -6.04 11,700 -- -- 7,699 -- 89,759 .089759 .23 208,158 89,716 0.089716 .028324 .028264 .2305/28/2003 1,450 -6.04 11,400 8,250 8,010 7,501 2,780 87,530 .087530 .23 202,986 87,487 0.087487 .027257 .027197 .22

05/28/2003 1,525 -6.04 11,600 -- -- 7,633 -- 81,779 .081779 .22 189,642 81,736 0.081736 .025353 .025293 .22
10 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03 Table 2. Results of analyses of samples for dissolved gases from monitoring wells in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03—Continued. U.S. Geological Survey site number Well iden- tifier (fig. 1; table 1) Date sampled Sam- ple depth (ft below LSD) Depth to water (ft below LSD) Spe- cific con- duc- tance (μS/ cm) Dis- solved solids, residue on evapo- ration at 180 °C (mg/L) Dis- solved solids, sum of constit- uents (mg/L) Com- puted dis- solved solids (mg/L) Chlo- ride (mg/L) Helium-4 (μccSTP/ kg) Helium-4 (ccSTP/ kg) R/RA Delta excess helium-4 (percent) Excess helium-4 (μccSTP/ kg) Excess helium-4 (ccSTP/ kg) Helium-3 (μccSTP/ kg) (com- puted from helium-4 measured) Excess helium-3 (μccSTP/ kg) (com- puted from helium-3 in ASW) R/RA of excess helium (com- puted) 294946097574501 FH2 01/15/2003 550 -- 13,700 9,750 -- 9,015 3,330 74,838 0.074838 0.26 173,538 74,795 0.074795 0.027240 0.027181 0.26 01/15/2003 650 -- 13,800 9,620 8,980 9,080 3,310 80,307 .080307 .26 186,227 80,264 .080264 .028342 .028282 .25 01/15/2003 750 -- 13,800 9,670 9,120 9,080 3,340 39,301 .039301 .26 91,086 39,258 .039258 .013870 .013810 .25 01/22/2003 850 -- 13,600 -- -- 8,949 -- 78,461 .078461 .26 181,944 78,418 .078418 .027908 .027848 .26 01/22/2003 950 -- 13,700 9,580 8,990 9,015 3,290 60,766 .060766 .25 140,888 60,723 .060723 .020605 .020545 .24 295853097532901 KY1 12/12/2002 320 167.88 1,110 432 459 730 10.7 141 .000141 .44 227 98 .000098 .000085 .000026 .19 12/12/2002 400 167.88 1,120 -- -- 737 -- 410 .000410 .41 851 367 .000367 .000230 .000171 .34 12/20/2002 500 167.52 1,140 870 797 750 11.4 374 .000374 .24 768 331 .000331 .000124 .000065 .14 12/20/2002 600 167.52 1,130 -- -- 744 -- 386 .000386 .27 796 343 .000343 .000142 .000082 .17 12/20/2002 700 167.52 1,130 854 771 744 11.9 366 .000366 .26 749 323 .000323 .000132 .000072 .16 12/20/2002 775 167.52 1,140 797 793 750 11.6 381 .000381 .27 784 338 .000338 .000141 .000081 .17 295858097521801 KY2 05/13/2003 475 106.36 1,630 -- -- 1,073 -- 3,766 .003766 .14 8,638 3,723 .003723 .000709 .000649 .13 05/13/2003 530 106.36 1,550 986 -- 1,020 206 3,702 .003702 .14 8,489 3,659 .003659 .000717 .000658 .13 05/13/2003 600 106.36 1,780 -- -- 1,171 -- 4,779 .004779 .14 10,988 4,736 .004736 .000926 .000866 .13 05/14/2003 700 106.22 6,480 1,170 1,120 4,264 283 24,106 .024106 .14 55,830 24,063 .024063 .004771 .004711 .14 05/14/2003 780 106.22 21,100 -- -- 13,884 -- 87,602 .087602 .15 203,153 87,559 .087559 .017944 .017884 .15 05/14/2003 900 106.22 22,000 -- -- 14,476 6,370 91,354 .091354 .15 211,858 91,311 .091311 .018459 .018400 .15 295829097512601 KY3 01/08/2003 615 88.12 1,490 758 -- 980 288 8,494 .008494 .23 19,608 8,451 .008451 .002657 .002597 .22 01/08/2003 660 88.12 2,480 1,360 1,350 1,632 585 18,359 .018359 .23 42,496 18,316 .018316 .005895 .005835 .23 01/08/2003 750 88.12 27,600 18,800 17,900 18,161 8,890 126,020 .126020 .23 292,290 125,977 .125977 .039243 .039183 .22

01/09/2003 950 87.77 27,900 18,500 17,900 18,358 8,860 160,587 .160587 .23 372,492 160,544 .160544 .050896 .050836 .2301/09/2003 1,050 87.77 28,100 19,100 18,100 18,490 9,000 84,358 .084358 .23 195,626 84,315 .084315 .026386 .026326 .23

295730097503201 KY4 04/01/2003 575 -- 24,900 17,100 16,500 16,384 7,720 127,656 .127656 .24 296,086 127,613 .127613 .042932 .042873 .24
Sources of Groundwater Based on Helium Analyses 11

terrigenic sources dominates (L.N. Plummer, U.S. Geological Survey, written commun., 2009). Δ4

Heex was greater than 

1,000 percent for 60 of the 69 samples (table 2), indicating 

that terrigenic He is largely present. The nine samples with 

the lowest Δ4Heex values (from freshwater wells EU2 and 

KY1) likely contain mostly atmospherically derived 4He. The 

predominance of very large quantities of Δ4Heex, much greater 

than could be derived from air-water equilibrium and excessair sources, indicates that groundwater residence times in the 

freshwater/saline-water transition zone are relatively long and 

that most of the Δ4

Heex comes from sources other than the 

atmosphere.

Excluding three EU2 samples influenced by atmospheric 

sources (R= 0.89 RA, 1.09 RA, and 1.06 RA [table 2]), the 

mean isotopic composition for the remaining 66 samples is 

0.22 RA. This relatively uniform isotopic composition might 

indicate a relatively homogeneous source of He in the freshwater/saline-water transition zone. Homogeneous isotopic 

composition indicates a uniform source of 4

Heex consisting 

mostly of crustal 4He, where crustal Ris about 0.02 RA (2.77 x

10-8 [the 3He/4He ratio of crustal He] and approximately equals 

0.02 times 1.384 x 10-6 [the 3He/4He ratio of atmospheric He]) 

and possibly includes some mantle 4He, where Ris about 8 RA

(Ozima and Podosek, 2002). Although some in situ production of He might occur in the Edwards aquifer, the Cretaceous 

carbonates are too young to have produced the large concentrations of 4

He observed. Mantle-derived igneous intrusions 

are present in and near the Edwards aquifer (Smith and others, 

2002) and might be a source of mantle-derived 4

He (Hunt and 

others, 2005).

The mean Rof the 32 samples from the 10 saline wells 

(0.23 RA, nearly the same as for the 66 wells noted in the 

previous paragraph) can be computed as a mixture of about 

97 percent crustal 4He (Requals about 0.02 RA) and 3 percent 

Figure 4. Relation of sampling depth to helium-4 concentration in monitoring wells in and near the freshwater/saline-water transition 

zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 2002–03. 

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

HELIUM-4, IN CUBIC CENTIMETERS PER KILOGRAM AT STANDARD TEMPERATURE AND PRESSURE

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

SAMPLING DEPTH, IN FEET BELOW LAND SURFACE

EU1

EU2

EU4

TC1

TC2

TC3

TC4

FH2

KY1

KY2

KY3

KY4

EXPLANATION

No data available

for San Antonio

transect wells and

Devine well.

Well identifier 

(fig. 1; table 1)
12 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03

mantle (Requals about 8 RA), assuming the source of the 4He is 

entirely terrigenic. The source of 4He to the saline water could 

be migration from deep crustal rocks (fig. 2), by way of faults 

or fractures, or rocks nearer the Edwards aquifer that contain 

or radiogenically produce He. Other noble gas (Ne, Ar) data 

(Lambert and others, 2009) indicate oil and gas hydrocarbons, 

which are present in relatively deep Edwards Group rocks, are 

a potential source of 4Heex. Those data indicate that there are 

fractionation pathways (models) between ASW and fluids of 

varying composition representative of hydrocarbon-bearing 

rocks (A.G. Hunt, U.S. Geological Survey, written commun., 

2009). No noble gas data were collected from the Trinity 

aquifer for this report, so that potential source could not be 

evaluated directly. Although Groschen and Buszka (1997) 

did not identify updip movement of saline water from relatively deep Edwards Group rocks to the shallower transition zone of the Edwards aquifer, a recent study (MaryLynn 

Musgrove, U.S. Geological Survey, written commun., 2009) 

indicates that, in localized areas, saline water is mixed with 

freshwater; and that the likely sources of saline water are 

relatively deep Edwards Group rocks, or the Trinity aquifer, or 

both. 

Figure 5. Relation of isotopic composition of excess helium-4 (R/RA

4

Heex) to excess helium-4 (4Heex) concentration in monitoring 

wells in and near the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 

2002–03. 

Tritiogenic influence

Crustal

Excess air

influence

Mean R/RA=0.22 (excluding tritiogenic outliers)

ISOTOPIC COMPOSITION OF EXCESS HELIUM-4

EXCESS HELIUM-4 , IN CUBIC CENTIMETERS PER KILOGRAM AT STANDARD TEMPERATURE AND PRESSURE

0.01

0.1

1

10

0.00001 0.0001 0.001 0.01 0.1 1

EU1

EU2

EU4

DV1

SA1

SA2

SA3

SC1

SC2

SD1

TC1

TC2

TC3

TC4

FH2

KY1

KY2

KY3

KY4

EXPLANATION

Well identifier 

(fig. 1; table 1)
Summary 13

A graph of R/RA of 4Heex (R 4Heex /RA) relative to the 

concentration of 4Heex in samples from all wells (fig. 5) might 

indicate sources of 4Heex. R 4Heex /RA is computed as

R 4Heex /RA = (3Heex / 4Heex)/1.384x10-6, (3)

where 3Heex is computed as

3

Heex = 3He – 3HeASW . (4)

3

He in the equation immediately above is computed from 

R/RA and 4He (table 2), and 3HeASW is computed as 43.1 

μccSTP/kg x 1.384 x 10-6. Excluding five outliers, the R 4Heex /

RA values for all samples are between 0.1 and 0.3 for all values 

of 4Heex. Whether the variation in that range among transects 

or wells within transects is meaningful relative to sources of 

He is unknown. However, because the 4He concentration in 

all but the freshest samples is almost entirely excess 4Heex, R/

RA of 4

Heex is about equal to R/RA of 4He. As previously noted 

in this section, relatively uniform isotopic composition might 

indicate a relatively homogeneous source of He in the freshwater/saline-water transition zone.

As for the outliers, two outliers have R 4Heex values 

greater than RA (EU2, R/RA greater than 1), indicating that 3He 

dominates the composition; 3He comes from radiogenic decay 

of tritium in recently recharged water. Two other outliers have 

R 4Heex values of 0.74 RA and 0.34 RA (EU2 and KY1, respectively). These two samples likely are mixtures containing He 

from recently recharged water and from crustal sources. The 

fifth outlier, DV1 (R 4Heex = 0.09 RA), is the lowest value, 

strongly reflecting the dominance of the crustal component of 

4

Heex in the sample. 

The He data of this report are problematic in that they 

cannot be used to identify sources of groundwater in and 

near the transition zone of the Edwards aquifer in terms of 

specific geologic (stratigraphic) units or hydrogeologic units 

(aquifers or confining units). However, the data indicate that 

the source or sources of the He, and thus the water in which 

it is dissolved, in the transition zone are mostly terrigenic in 

origin rather than atmospheric, and that the dominant terrigenic source likely is crustal rocks rather than deeper mantle 

rocks. As previously noted in this section, likely sources of 

crustal He are oil and gas hydrocarbons that are widely present 

in Edwards Group rocks downdip from the transition zone 

of the Edwards aquifer, and potentially rocks composing the 

underlying and adjacent Trinity aquifer. Whether most He in 

and near the transition zone of the Edwards aquifer originated 

either in rocks outside the transition zone and at depth or in 

the adjacent Trinity aquifer is uncertain; but most of the He 

in the transition zone had to enter the transition zone from the 

Trinity aquifer because the Trinity aquifer is the hydrogeologic 

unit immediately beneath and laterally adjacent to the transition zone of the Edwards aquifer. Thus the He data support 

a hypothesis of sufficient hydraulic connection between the 

Trinity and Edwards aquifers to allow movement of water 

from the Trinity aquifer to the transition zone of the Edwards 

aquifer.

Summary

This report evaluates dissolved noble gas data, specifically helium-3 (3

He) and helium-4 (4He), collected by the 

U.S. Geological Survey during 2002–03 as part of a larger, 

9-year study of the freshwater/saline-water transition zone 

of the Edwards aquifer done by the U.S. Geological Survey 

in cooperation with the San Antonio Water System. Helium 

analyses are used to provide insight into the sources of water 

in the transition zone. Sixty-nine dissolved gas samples were 

collected from 19 monitoring wells (categorized as fresh, 

transitional, or saline on the basis of dissolved solids concentration in samples from the wells or from fluid-profile logging 

of the boreholes) arranged in five transects, except for the 

Devine well in Medina County, across the freshwater/salinewater interface (the 1,000-mg/L dissolved solids concentration 

threshold) of the Edwards aquifer. 

Dissolved helium in groundwater is derived from atmospheric and terrigenic (earth) sources. The concentration of 

helium-4 (the dominant isotope in atmospheric and terrigenic 

helium) in samples ranged from 63 μccSTP/kg in well EU2 

in the East Uvalde transect to 160,587 μccSTP/kg in well 

KY3 in the Kyle transect. Helium-4 concentrations in the 10 

saline wells generally increase from the western transects 

to the eastern transects, with the highest (and most variable) helium-4 concentrations occurring in the Kyle transect. 

Increasing helium-4 concentrations from southwest to northeast in the transition zone, indicating increasing residence time 

of groundwater from southwest to northeast, is consistent with 

the longstanding conceptualization of the Edwards aquifer: 

Water recharges in the southwest, flows generally northeasterly (including in the transition zone, although more slowly 

than in the freshwater zone), and discharges at major springs 

in the northeast.

When excess helium-4 (the percentage of excess 

helium-4 [difference between measured helium-4 concentration and helium-4 concentration in air-saturated water]) is 

greater than about 200 percent, excess helium-4 from terrigenic sources dominates. Excess helium-4 was greater 

than 1,000 percent for 60 of the 69 samples, indicating that 

terrigenic helium is largely present and that most of the excess 

helium-4 comes from sources other than the atmosphere.

The ratio of 3

He/4He in a sample (R), the isotopic composition, is commonly expressed as a multiple of the ratio of 

3

He/4He in the atmosphere (in air) (RA). The mean Rof the 

32 samples from the 10 saline wells (0.23 RA) can be computed as a mixture of about 97-percent crustal helium-4 (R

equals about 0.02 RA) and 3-percent mantle (Requals about 8 

RA), assuming the source of the helium-4 is entirely terrigenic. 

The source of helium-4 to the saline water could be deep 

crustal rocks, by way of faults or fractures, or rocks nearer 

the Edwards aquifer that contain or radiogenically produce 

helium. 

The helium data of this report cannot be used to identify 

sources of groundwater in and near the transition zone of the 

Edwards aquifer in terms of specific geologic (stratigraphic) 
14 Sources of Groundwater Based on Helium Analyses, Edwards Aquifer, South-Central Texas, 2002–03

units or hydrogeologic units (aquifers or confining units). 

However, the data indicate that the source or sources of the 

helium, and thus the water in which the helium is dissolved, in 

the transition zone are mostly terrigenic in origin rather than 

atmospheric, and that the dominant terrigenic source likely is 

crustal rocks rather than deeper mantle rocks. Likely sources 

of crustal helium are oil and gas hydrocarbons that are widely 

present in Edwards Group rocks downdip from the transition 

zone of the Edwards aquifer, and potentially rocks composing the underlying and adjacent Trinity aquifer. Whether most 

helium in and near the transition zone of the Edwards aquifer 

originated either in rocks outside the transition zone and at 

depth or in the adjacent Trinity aquifer is uncertain; but most 

of the helium in the transition zone had to enter the transition 

zone from the Trinity aquifer because the Trinity aquifer is the 

hydrogeologic unit immediately beneath and laterally adjacent 

to the transition zone of the Edwards aquifer. Thus the helium 

data support a hypothesis of sufficient hydraulic connection 

between the Trinity and Edwards aquifers to allow movement 

of water from the Trinity aquifer to the transition zone of the 

Edwards aquifer.

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