U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5163

Groundwater Ages from the Freshwater Zone of the Edwards 

Aquifer, Uvalde County, Texas—Insights into Groundwater 

Flow and Recharge

Groundwater Ages from the Freshwater 

Zone of the Edwards Aquifer, Uvalde 

County, Texas—Insights into Groundwater 

Flow and Recharge

By Andrew G. Hunt, Gary P. Landis, and Jason R. Faith

Scientific Investigations Report 2015–5163

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Hunt, A.G., Landis, G.P., and Faith, J.R., 2016, Groundwater ages from the freshwater zone of the Edwards Aquifer, 

Uvalde County, Texas—Insights into groundwater flow and recharge: U.S. Geological Survey Scientific Investigations 

Report 2015–5163, 28 p., http://dx.doi.org/10.3133/sir20155163.

ISSN 2328-0328 (online)
iii

Acknowledgments

Primary funding for this project was provided by the Edwards Aquifer Authority (EAA), and 

U.S. Geological Survey Mendenhall Program. The authors would like to thank Edwards Aquifer 

Authority and San Antonio Water System’s field personnel for their help in sampling, and 

we would also thank the municipalities of Sabinal, Knippa, and Uvalde for access to production wells.
iv
v

Contents

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

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

Overview of Groundwater Age....................................................................................................................1

3

H-3He Age..............................................................................................................................................3

Uvalde County.................................................................................................................................................4

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

Geologic Setting...........................................................................................................................4

Hydrologic Setting .......................................................................................................................5

Sampling..........................................................................................................................................................6

Laboratory Analysis.......................................................................................................................................6

Data Analysis..................................................................................................................................................7

Results .............................................................................................................................................................8

Geophysical Logs..................................................................................................................................8

Electrical Conductance Logs.....................................................................................................8

Temperature Logs ........................................................................................................................8

Dissolved Gas in Groundwater.........................................................................................................10

Noble Gas Solubility ..................................................................................................................10

Excess Air....................................................................................................................................11

Calculated Recharge Temperatures................................................................................................15

Tritium Data and Apparent Age........................................................................................................17

Apparent 3

H-3He Age.................................................................................................................17

Vertical Apparent Age Distribution.........................................................................................19

Apparent Age and Flow Patterns.....................................................................................................19

Summary........................................................................................................................................................21

References Cited..........................................................................................................................................21

Appendix........................................................................................................................................................23

Figures

1. A, Location of study area with inset of Uvalde County, Texas. B, Sample locations, 

major faults with relative offsets, and approximate location of the Uvalde Salient..........2

2. Potentiometric surface of Edwards aquifer water levels as interpreted from data 

in Green, Bertetti, and others (2006)..........................................................................................7

3. Comparison of geophysical logs of temperature (in degrees Celsius [°C]) and 

specific conductance (in millisiemens per meter [mS/m]) from monitoring well 

East Uvalde 2 for November 2002 and November 2005........................................................10

4. Binary plots of A, F 20Ne/36Ar compared to 1/36Ar; B, F4He/20Ne compared to 

F20Ne/36Ar; and C, FN2

/

36Ar compared to 1/36Ar.......................................................................14

5. Binary plot of 3H+3He* (tritium units) for samples compared to the recharge 

year from the 3

H-3He age, superimposed on the historical 3H input curve from 

Michel (2007) ...............................................................................................................................18

6. Contour of apparent age (3H-3He) with flow vectors derived from the dataset of 

Green, Bertetti, and others (2006) from figure 2 ....................................................................20
vi

Tables

1. Sample location data ...................................................................................................................9

2. Dissolved gas compositions with analytical errors ..............................................................12

3. Closed equilibrium model parameters and apparent age data...........................................16

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L)

pint (pt) 0.4732 liter (L)

quart (qt) 0.9464 liter (L)

gallon (gal) 3.785 liter (L)

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

bar 100 kilopascal (kPa)

inch of mercury at 60 oF (in Hg) 3.377 kilopascal (kPa)

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

cubic centimeter (cm3

) 0.06102 cubic inch (in3)

liter (L) 61.02 cubic inch (in3

)

Flow rate

liter per second (L/s) 15.85 gallon per minute (gal/min) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Pressure

kilopascal (kPa) 0.009869 atmosphere, standard (atm)

kilopascal (kPa) 0.01 bar

kilopascal (kPa) 0.2961 inch of mercury at 60 °F (in Hg)
vii

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

°F = (1.8×°C)+32

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

°C = (°F–32)/1.8

Temperature in degrees Celsius (°C) may be converted to degrees Kelvin (°K) as follows:

°K =°C–273.15

Temperature in degrees Kelvin (°K) may be converted to degrees Celsius (°C) as follows:

°C =°K+273.15

Molar volume is defined at 1 atmosphere pressure and 0 °C as: 1 mole = 22.414 liters.

Abbreviations

AEW air equilibrated water

Ar argon

36Ar argon-36 

38Ar argon-38 

40Ar argon-40 

cm3 cubic centimeter

cm3/cm3 cubic centimeter per cubic centimeter

cm3STP cubic centimeter at standard temperature and pressure

cm3STP/gH2O cubic centimeter at standard temperature and pressure per gram of water

CFC chlorofluorocarbons

CH4 methane

H2 hydrogen

3

H tritium

He helium

3

He helium-3

4

He helium-4

Kr krypton

84Kr krypton-84

86Kr krypton-86

mL milliliter
viii

N2 nitrogen 

Ne neon

20Ne neon-20

21Ne neon-21

22Ne neon-22

NGRT noble gas recharge temperature

O2 oxygen

Patm atmospheric pressure

STP standard temperature and pressure (0 °C, 1 atmosphere)

Xe xenon

130Xe xenon-130

132Xe xenon-132

USGS NGL U.S. Geological Survey Noble Gas Laboratory

Symbols

> greater than

< less than

>> much greater than

⇔ equivalent

≈ approximately (nearly) equal to

° degree

°C degrees Celsius

°K degrees Kelvin

% percent

± plus or minus
Groundwater Ages from the Freshwater Zone of the 

Edwards Aquifer, Uvalde County, Texas—Insights into 

Groundwater Flow and Recharge

By Andrew G. Hunt, Gary P. Landis, and Jason R. Faith

includes municipal, agricultural, military, domestic, and recreational water use; Federal mandates also exist to protect listed 

endangered species that dwell in the spring discharges emanating from the aquifer. Geologic, geochemical, and geophysical 

studies combined with hydrologic models aid water-resource 

managers in developing long- and short-range strategies for 

optimal groundwater use and resource management that will 

balance increasing demands on the aquifer system. The aquifer 

consists of fractured limestone in the Balcones fault zone and 

associated karst features with fracture- and conduit-flow control 

of groundwater, a prominent downdip saline zone, and the presence of igneous hypabyssal alkalic stock intrusions.

Groundwater ages can provide a useful tool to aid traditional methods used for aquifer characterization. Traditional 

methods such as measurement of water levels, analysis of 

basic groundwater chemistry, and field or laboratory hydraulic 

conductivity tests can be used to assess horizontal and vertical 

hydraulic gradients, fluid flow directions, and to derive average linear velocities along flow vectors. Results of groundwater 

dating techniques and geophysical well-log analysis aid in the 

conceptual and computational modeling of groundwater flow. 

The results analyzed in this report are for the western part of the 

Edwards aquifer within Uvalde County, Texas (fig. 1B).

Overview of Groundwater Age

Groundwater age data commonly are misinterpreted 

when reporting groundwater flow studies; therefore, the data 

require an operational definition and definition of significance 

of age. The groundwater age is not the actual age of the water, 

but represents the mean residence time of a parcel of groundwater in an aquifer after isolation from the atmosphere. For an 

aquifer, mean residence time for the groundwater has implications for potential sustainability of the resource, understanding 

recharge, contaminant transport, and effects on groundwater 

quality. Young-age groundwaters typically imply high flow 

rates sustained by rapid recharge into the aquifer, and are an 

indicator that the resource is likely to be sustainable. Such 

groundwaters are also vulnerable to anthropogenic pollutants and other adverse effects to water quality. In contrast, 

Abstract

Tritium–helium-3 groundwater ages of the Edwards 

aquifer in south-central Texas were determined as part of a longterm study of groundwater flow and recharge in the Edwards 

and Trinity aquifers. These ages help to define groundwater 

residence times and to provide constraints for calibration of 

groundwater flow models. A suite of 17 samples from public 

and private supply wells within Uvalde County were collected 

for active and noble gases, and for tritium–helium-3 analyses 

from the confined and unconfined parts of the Edwards aquifer. Samples were collected from monitoring wells at discrete 

depths in open boreholes as well as from integrated pumped 

well-head samples. The data indicate a fairly uniform groundwater flow system within an otherwise structurally complex 

geologic environment comprised of regionally and locally 

faulted rock units, igneous intrusions, and karst features within 

carbonate rocks. Apparent ages show moderate, downward 

average, linear velocities in the Uvalde area with increasing age 

to the east along a regional groundwater flow path. Though the 

apparent age data show a fairly consistent distribution across the 

study area, many apparent ages indicate mixing of both modern 

(less than 60 years) and premodern (greater than 60 years) 

waters. This mixing is most evident along the “bad water” line, 

an arbitrary delineation of 1,000 milligrams per liter dissolved 

solids that separates the freshwater zone of the Edwards aquifer 

from the downdip saline water zone. Mixing of modern and 

premodern waters also is indicated within the unconfined zone 

of the aquifer by high excess helium concentrations in young 

waters. Excess helium anomalies in the unconfined aquifer 

are consistent with possible subsurface discharge of premodern groundwater from the underlying Trinity aquifer into the 

younger groundwater of the Edwards aquifer.

Introduction

The Edwards aquifer is one of the most permeable and 

productive aquifers in the world. The aquifer is a primary water 

source for an ever expanding population in south-central Texas 

(fig. 1A). Demand for water in this semiarid region of Texas 
2 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

Figure 1. A, Location of study area with inset of Uvalde County, Texas. B, Sample locations, major faults with relative 

offsets, and approximate location of the Uvalde Salient. The freshwater/saline water transition is indicated as concentration 

equipotentials for 1,000 and 3,000 milligrams per liter (mg/L) of total dissolved solids after Shultz (1994). See table 1 for sample 

location symbols.

extremely old groundwaters may be less vulnerable to anthropogenic contaminants, but unable to sustain use because of the 

long mean residence time between recharge and extraction. 

Such resources become depleted with time from use and lack 

of sustaining recharge.

A useful interpretation of groundwater age is to classify 

waters with respect to connection to present-day hydrologic 

cycling. If “modern” groundwaters are those recharged within 

the past few decades and are part of the active hydrologic 

cycle (Clark and Fritz, 1999), then an indicator of “modern” 

groundwater is the presence of tritium (3

H). Tritium is the 

radioactive isotope of hydrogen, mainly derived from atmospheric thermonuclear device testing from 1951 to 1976, 

and from a small natural background level of cosmogenic 

tritium produced in the upper atmosphere. The 3

H atom is 

incorporated into the atmospheric water molecule and introduced into recharging groundwaters with precipitation. Given 

the thermonuclear and cosmogenic production rates and a 

Edwards

aquifer

outcrop

BLANCO

COUNTY

KENDALL

COUNTY

TRAVIS

COUNTY

HAYS

COUNTY BASTROP

COUNTY

CALDWELL

COUNTY

GUADALUPE

COUNTY

MEDINA

COUNTY BEXAR

COUNTY

BANDERA

COUNTY

COMAL

COUNTY

EDWARDS

COUNTY

KERR

COUNTY

REAL

COUNTY

KINNEY

COUNTY

ATASCOSA

COUNTY

FRIO

COUNTY

ZAVALA

COUNTY

UVALDE

COUNTY

WILSON

COUNTY

TEXAS

A

Study area

Balcones

Fault

zone

Uvalde

Austin

New

Braunfels

San

Antonio

100°30' 99°30'

98°30' 97°30'

98°

99°

30°30'

30°

29°30'

Base from U.S. Geological Survey 

digital data, 2015

Geographic projection

North American Datum 1984

29°

100°

0

60 KILOMETERS

40

20

60 MILES

40

20

0
Overview of Groundwater Age 3

Figure 1. A, Location of study area with inset of Uvalde County, Texas. B, Sample locations, major faults with relative offsets, and 

approximate location of the Uvalde Salient. The freshwater/saline water transition is indicated as concentration equipotentials for 1,000 and 

3,000 milligrams per liter (mg/L) of total dissolved solids after Shultz (1994). See table 1 for sample location symbols.—Continued

B

29°05'25.3"

99°27'14.7"

29°32'21.8"

100°00'32.2"

We st Nueces River

Nu e ces River

Leona River

Frio River

Blanco Creek

Sabin al River

0 5 KILOMETERS

0 5 MILES

NUC

LVN

ANN

BOW

7MH

UV09 A&M

UV11

UV1

EU4

KPA

SAB

EU2

EU1

U

D

U

D

U

D

U

D

U

D

U

D

Approximate 

axis of 

Uvalde Salient

Uvalde

Knippa

Sabinal

127

55

90

83

Map Base from U.S. Geological Survey digital data

CONNOR FAULT

AGAPE FAULT

BLACK MOUNTAIN FAULT

BLUE MOUNTAIN FAULT

COOKS FAULT

EXPLANATION

Edwards aquifer outcrop

Igneous intrusions

Faults

Sampled wells

1,000 mg/L

3,000 mg/L

EU4

known 12.32 year half-life (Lucas and Unterweger, 2000), 

and recharge efficiency, concentrations of 3H diminish to 

below detectable limits after approximately 4 to 5 half-lives or 

49.3 to 61.7 years. As a convention, waters containing 3

H are 

modern groundwaters (less than 60 years of apparent age) and 

waters devoid of 3

H are submodern or premodern (greater than 

60 years of apparent age).

Techniques for the accurate measurement of groundwater 

age are varied. In general, the technique should address 

exactly what is being dated, how it is introduced into the 

groundwater flow system, when the “clock” is started for the 

technique, what affects the interpretation of the analytical 

components involved, and whether there are any interferences 

in the subsurface that affect the calculation of age. The basis of 

many of these techniques relies on knowing the rates of introduction of elements or chemical species to the groundwater 

and equating the apparent age by matching concentrations to 

known input concentrations (for example, chlorofluorocarbons 

(CFCs) and sulfur hexafluoride), or known input of a short- or 

long-lived radioisotope to the groundwater and then determining age based on measured activity and known decay rate (that 

is, 3

H, 14C, 39Ar, 81Kr). The conservative chemical nature of 

environmental tracers used for age dating (nonreactive, neither 

consumed nor produced by chemical reactions) allows for best 

estimates of an absolute age. However, integration of several 

sources of recharge with different ages through hydrodynamic 

mixing along convergent flow paths yields a composite and 

ambiguous groundwater age (Clark and Fritz, 1999), and the 
4 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

effect of a mixing process on the calculated age differs with 

each tracer. For these reasons, the age determined by any specific technique is an apparent age. An extreme example of this 

would be when a known amount of modern recharge water 

containing a fixed concentration of CFCs and 3

H mixes with 

premodern water, resulting in a dilution of the environmental tracers. The calculated age resulting from dilution causes 

the 3

H apparent age to be younger than the actual age of the 

modern water while the apparent age derived from the CFCs 

is older. This discrepancy in apparent ages is because the 3

H 

age is derived from an input curve that decreases through 

time while the CFC age is from an input curve that increased 

with time until the late 1990s. An apparent age attributed to 

groundwater is useful when the technique and conditions of 

determination are understood. Age discrepancies between 

different techniques are a result of incomplete accounting of 

hydrogeologic-geochemical processes affecting the sample.

3

H-3He Age

The 3H-3He method of dating modern groundwaters 

(Schlosser, 1992; Solomon and Cook, 2000) exploits the 

radioactive “parent-daughter” decay. The 3

H-3He method 

results from the introduction of naturally occurring 3

H (as part 

of the tritiated water molecule) to the aquifer during recharge. 

As the groundwater moves away from the water table with 

continued recharge, the 3

H contained in the water molecule 

decays to 3

He, a dissolved gas. The “clock” starts after isolation from the atmosphere (about 1 meter [m] below the water 

table because of possible back diffusion of 3

He) and the age 

obtained in this case represents the time from isolation from 

the atmosphere. The transport of both the 3

H in the form of 

water and 3

He as a dissolved gas in the groundwater is considered highly conservative, and because the age calculation 

is based on both the parent and daughter isotopes and known 

rate of decay, the method is fairly insensitive to mixing. In the 

case of mixing with a premodern end member (void in both 

parent and daughter isotopes), the daughter to parent ratio 

is preserved as the apparent age of the modern groundwater 

component. For the binary mixing of two modern waters, the 

calculated apparent age actually represents an average age of 

the two end-member components.

The 3H-3He method relies on the measurement of not 

only the 3

H contained in the groundwater, but also the amount 

of tritiogenically derived 3

He (3He*). Helium in the atmosphere has a homogeneous composition that is made up of two 

isotopes (3

He and 4He) with a concentration of 5.24 parts per 

million (ppm) and isotopic ratio of 1.384 × 10–6 for 3

He/4He. 

As recharged groundwater enters an aquifer, the concentration of helium in the groundwater is in equilibrium with the 

atmosphere (air saturated water [ASW]) at the temperature of 

recharge. This water may also have extra helium from excess 

atmospheric gas trapped during recharge (excess air), which 

is forced into solution with increasing hydrostatic pressure 

as isolated bubbles are entrained deeper into the zone of 

recharge. In order to resolve the tritiogenic 3

He from the total 3

He in a sample of groundwater, the initial concentration of 

3

He associated with recharge and excess gas is computed by 

measuring other dissolved atmospheric gases contained within 

the sample. Neon, argon, krypton, and xenon are measured, 

along with total helium and the helium isotopic ratio. The 

temperature and pressure of groundwater recharge is computed using known physiochemical solubility parameters for 

atmospheric gas, as well as parameters for excess air compositions. These calculated conditions define the amount of excess 

3

He (Stute and Sonntag, 1992, Aeschbach-Hertig, Peeters, 

and others, 1999; Ballentine and Hall, 1999). The remaining 

excess 3

He is a combination of the tritium-derived 3He and a 

terrigenic 3

He produced from thermal neutron fluxes derived 

from decay of radioactive nuclides within the crust, and from 

a flux of primordial helium released from the Earth’s interior. 

In most modern waters the terrigenic helium component is not 

present, but in older waters terrigenic helium can dominate 

the 3

He composition. Using similar methods to those used for 

resolving the dissolved atmospheric gas components, excess 

helium is calculated. Using a characteristic isotopic ratio for 

the excess helium component, terrigenic 3

He can be defined, 

and thus the remaining amount of 3

He from the total 3He measured is derived from tritium. This process of 3

He resolution 

for the groundwater dating method gives insight into recharge 

conditions and possible subsurface conditions that can affect 

the dissolved gas composition.

Uvalde County

The study area is located approximately 100 kilometers 

west of San Antonio, Texas in Uvalde County (fig. 1A). The 

area is largely scattered agricultural lands and the municipalities of Uvalde, Knippa, and Sabinal, with an estimated 

population of approximately 26,000 (U.S. Census Bureau, 

2008), most of which reside in Uvalde. The region is characterized as a semiarid climate and receives an average rainfall 

of approximately 60 centimeters per year with a mean annual 

temperature of 20 °C (National Oceanic and Atmospheric 

Administration, 2006). Carbonate bedrock units of the 

Edwards aquifer crop out in the northern part of the county 

and dip gently to the south, becoming a confined flow system 

in the southern part of the county (fig. 1B). The area lies in the 

Balcones fault zone, a series of east-west normal faults that 

are downthrown to the south. The bedrock units of interest 

in the area range from Lower Cretaceous units of the lower 

confining unit and the Edwards aquifer to Upper Cretaceous 

to Tertiary units of the upper confining units. For purposes of 

this report the downdip limit of unmixed, freshwater infiltration in the Edwards aquifer is defined by the 1,000 mg/L of 

total dissolved solids as recognized by Schulz (1994). This 

arbitrary demarcation is shown in Figure 1B as equipotential 

lines of TDS (total dissolved solids) concentration. South of 

this demarcation the groundwaters contained in the Edwards 
Uvalde County 5

aquifer are characterized by mixed saline formation waters 

that do not appear to be well connected to the hydraulically 

active, updip freshwater zone.

Hydrogeologic Setting

Geologic Setting

The carbonate-rich formations comprising the Edwards 

aquifer in Uvalde County consist of two depositional facies 

associated with early Cretaceous marine transgressive/

regressive sediment cycles that were deposited over the Glen 

Rose Limestone (early Cretaceous), a sequence of thinly 

bedded limestone and marl that comprise the confining unit 

below the aquifer (Clark and Small, 1997; Clark, 2003). In the 

western part of the study area, the aquifer consists of formations associated with the Maverick Basin, a restricted marine 

basin situated between the Stuart City reef complexes to the 

south. Early deposition in the Maverick Basin is characterized 

by the accumulation of lagoonal evaporites and organic-rich 

euxenic shales of the West Nueces and McKnight Formations. 

With time and sediment loading, the Maverick Basin subsided 

to become a deep open marine basin in which the sediments 

of the Salmon Peak Formation accumulated. The Salmon Peak 

Formation is characterized by thick massive lime mudstone, 

capped by a grainstone that grades into a mudstone.

In the eastern part of the study area, the Maverick Basin 

sediments interfinger with the synchronous Devils River 

Formation and terminate to the east against the San Marcos 

Platform. This formation is informally subdivided into the 

upper and lower Devils River, with the lower Devils River 

Formation stratigraphically equivalent to the West Nueces and 

McKnight Formations and the upper Devils River Formation 

equivalent to the Salmon Peak Formation. The lower Devils 

River Formation is characterized by dense, shaley, nodular 

mudstone that grades to a bioturbated mudstone to wackestone capped by a mudstone and collapsed breccia. The upper 

Devils River Formation represents carbonate banks of rudist 

bioherms and biostromes, composed of wackestones and 

miliolid grainstones.

A series of marine transgressive/regressive sediment 

cycles ended deposition of the carbonate units of the Maverick 

Basin and Devils River Formation. These units cap the 

Edwards aquifer with about 490 meters of confining layers 

of Del Rio Clay (Upper Cretaceous) and the Indio Formation 

(Tertiary) predominantly in the central and eastern parts of the 

Edwards aquifer system.

Scattered throughout Uvalde County also are a series of 

volcanic to subvolcanic intrusive rocks. The aphanitic to phaneritic ultramafic basalts occur as dikes, plugs, and hypabyssal 

irregular shaped diatremes within the Edwards aquifer. The 

age of these intrusions varies from 72 to 80 Ma (Miggins and 

others, 2004). Recent high-resolution aeromagnetic surveys 

from 2001 (Smith and others, 2002) detected more than 

200 shallow intrusive bodies with only 30 having a surface 

expression. These mafic diatremes and plugs are hypothesized 

to act as barriers to groundwater flow and as possible traps for 

natural gas in the subsurface.

The present-day structure of the Edwards aquifer formed 

along a crustal zone of weakness known as the Ouachita 

structural belt. This structural belt is a deep zone of weakness 

in the Earth’s crust which has seen several continental collision and separation episodes since the Precambrian. Superimposed on this ancient belt is the Balcones fault zone, which is 

a Miocene age, east-west trending zone of normal faults that 

are downthrown to the south. Notable faults associated with 

this structural trend in the study area include Cooks, Black 

Mountain, Blue Mountain, Agape, and Connor faults (fig. 1B) 

(Clark and Small, 1997; and Clark, 2003). The Agape fault is 

slightly upthrown in relation to the Blue Mountain fault with a 

graben being formed between the two.

The Uvalde Salient represents a structural high, trending 

northwest to southeast, which is not associated with faulting splays of the Balcones fault zone. This structural high 

brings confined units of the Edwards aquifer up from a depth 

of 275 meters to the surface over a horizontal distance of 

14.5 kilometers, then drops the units 92 meters below ground 

surface over a distance of 9.7 kilometers. The development 

of this structure is thought to have been synchronus with the 

emplacement of magma during the late Cretaceous (Ewing and 

Caran, 1982; Ewing and Barker, 1986) and then modified by 

Miocene-age faulting.

Hydrologic Setting

Recharge to the Edwards aquifer is mainly attributed to 

streamflow loss through channel bed infiltration for streams 

that cross the outcrop area of the Edwards aquifer. Other 

possible recharge mechanisms to the area include subsurface 

discharge from the Trinity aquifer, Buda Limestone, Austin 

Chalk and Leona Gravels; and distributed recharge associated 

with direct infiltration from precipitation. While the measurement of stream loss across the outcrop belt of the Edwards 

aquifer can give a limited estimate of recharge in a broad 

scale, the quantification of other sources of recharge is determined by a water-balance analysis.

In Uvalde County, three contributing watersheds (Frio, 

Dry Frio, and Nueces Rivers) flow from the northern end 

of the county into the outcrop belt of the Edwards aquifer. 

Channel bed infiltration from these rivers is recharged into the 

aquifer and migrates downdip into the confined region of the 

aquifer. Green, Bertetti, and others (2006) concluded that there 

is little contribution of groundwater flow from the western side 

of Uvalde County because of the presence of a low permeability zone separating higher permeability conditions to the 

west (the Kinney County “pool”) and high permeability zones 

in the eastern Uvalde County. The southern limit of freshwater 

flow is associated with the location of the “bad water line” 

(Shultz, 1994 [1,000 mg/L of total dissolved solids threshold]). The exact structure and position of this transition is not 

well understood.
6 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

 Water level altitude data from November 2006 are contoured in figure 2, generally the surface dips downward to the 

south. Elevations for the potentiometric surface south of the 

“bad water line” and west of State Road 55 are not provided 

because of lack of data for those regions. There is a topographic high to the east of the city of Uvalde. This topographic 

high creates two troughs to the east and west. One trough 

occurs north of the city of Uvalde, running through the city 

of Knippa. This trough runs through an area locally referred 

to as the “Knippa gap,” a structure that is a combination of 

groundwater flow caused by the Uvalde Salient and enhanced 

permeability associated with localized faulting (Green, 

Bertetti, and others, 2006). The other trough runs to the south 

between Blanco Creek and the Sabinal River, northwest of the 

city of Sabinal, and is not historically associated with known 

structures. The convergence of these two troughs appears to 

merge together south of the city of Sabinal and trend southeastward into Medina County.

Sampling

Sample sites were selected from a network of wells 

associated with the July 2004 water level synoptic conducted 

by Clark and Journey (2006). The sites included municipal, 

domestic, agricultural, and several monitoring wells present in 

the study area. Criteria used for selection of a well for sampling included (a) water production from the freshwater zone 

of the Edwards aquifer, (b) presence of an operating pump that 

could be used to sample for dissolved gases, (c) known well 

construction, (d) known depth and production intervals, and 

(e) permission of the well operator. Many possible agricultural wells were eliminated from consideration because of 

lack of use during the time of sampling. Four municipal, four 

domestic, three monitoring and one agricultural well were 

selected over an even distribution from up to downgradient 

within the study area (fig. 1B, table 1). Selected wells produce 

from the Maverick Basin facies (Salmon Peak and McKnight 

Formations) and the Devils River Formation. The production 

depth for the wells varied with location from 51 to 369 meters 

below ground surface. Data collected from two saline zone 

monitoring wells (East Uvalde 1 and East Uvalde 4) are 

included in this study. These monitoring wells are included to 

illustrate the differences between the dissolved gas content and 

age relations between the freshwater and saline zone transition 

wells. Samples and geophysical logs from these wells were 

taken from a prior investigation in the fall and winter of 2002.

Sampling consisted of two techniques depending on 

the conditions present at the wellhead (table 1). Municipal, 

domestic, and agricultural supply wells with working pumps 

installed in the well were sampled directly from a sampling 

port present at the wellhead. Upon confirmation with the well 

operator as to pump operation, wells were assumed to be 

sufficiently purged for sampling. After connecting the sample 

apparatus to the wellhead port, water was allowed to flow 

through a multiparameter meter equipped with a flow cell for 

approximately 5 to 10 minutes. During this initial purging of 

the sample apparatus, field parameters were measured with 

a calibrated multiparameter meter equipped with temperature, specific conductance, and pH probes. Upon stabilization 

of these field parameters, the water was routed through the 

dissolved gas sample apparatus (an inline copper tube with a 

back pressure valve). Water flowed through the copper sample 

tube with a minor back pressure being supplied to the system, 

to ensure that no bubbles formed in the water sampling stream. 

The copper tube was sealed at pressure using refrigeration 

clamps and then removed from the sampling apparatus. After 

the dissolved gas sample was collected, a tritium sample bottle 

(250 mL high density polyethylene bottle with polyseal cap) 

was completely filled and capped, allowing no headspace. 

A final set of field parameters were recorded to compare the 

initial measurements and ensure that water quality conditions 

were stable during sampling.

Monitoring wells with no pumps present were sampled 

using a discrete interval sampler at selected intervals within 

the open interval of the well. The wells were first logged to 

determine zones to be sampled using a portable geophysical 

logging system. Fluid conductivity, temperature, and fluid 

pressure readings were logged at 6-inch intervals at a rate 

of about 3.05 meters per minute for the length of the open 

borehole (about 152 meters). Specific sampling points were 

selected for discrete interval sampling on the basis of logs 

and spacing along the open well intervals. A U.S. Geological 

Survey- (USGS) developed discrete dissolved gas sampler 

was attached to the portable logging winch and lowered to 

the selected depth for sample collection. After acquiring the 

dissolved gas sample, a 500-mL bulk fluid sampler was then 

attached to the winch and a similar process was used to obtain 

enough water for the tritium sample. At the surface, the bulk 

sampler was open and the contents used to rinse and fill a 

tritium sample bottle. Since there was little sample water 

remaining in the bulk fluid sampler, no field parameters were 

measured for these samples. This process was repeated with 

successively deeper samples until the sampling was complete.

Laboratory Analysis

Dissolved gas samples were extracted on an ultra-high 

vacuum extraction line at the USGS Noble Gas Laboratory 

(USGS NGL) in Denver, Colorado. The gas extracted from 

the samples was analyzed for nitrogen, oxygen, methane, 

and argon using a quadrapole mass spectrometer in dynamic 

operation mode; noble gas isotopic concentrations and compositions (helium, neon, argon, krypton, and xenon) were 

measured using separate aliquots on a magnetic sector mass 

spectrometer run in static operation mode. All gas concentrations are reported in units of standard temperature and pressure ([STP] – 0 degrees Celsius [°C] at 1 atmosphere).
Data Analysis 7

Figure 2. Potentiometric surface of Edwards aquifer water levels as interpreted from data in Green, Bertetti, and others (2006).

29°05'25.3"

99°27'14.7"

29°32'21.8"

100°00'32.2"

We st Nueces River

Nu e ces River

Leona River

Frio River

Blanco Creek

Sabin al River

340

330

320

310

300

290

280

270

260

250

270

280

290

300

310

250

260

270

260

250

250

0 5 KILOMETERS

0 5 MILES

NUC

LVN

ANN

BOW

7MH

UV09 A&M

UV11 UV1

EU4

KPA

SAB

EU2

EU1

U

D

U

D

U

D

Uvalde

U

D

Knippa

U

D

U

D

Sabinal

127

55

90

83

Map Base from U.S. Geological Survey digital data

CONNOR FAULT

AGAPE FAULT

BLACK MOUNTAIN FAULT

BLUE MOUNTAIN FAULT

COOKS FAULT

EXPLANATION

Edwards aquifer outcrop

Igneous intrusions

Faults

Sampled wells

1,000 mg/L

3,000 mg/L

Equipotential based on data

 Green and others, 2006

EU4

Tritium concentrations were measured by the 3He ingrowth method (Bayer, Schlosser, and others, 1989; Clark, 

Jenkins, and Top, 1976). Approximately 170 mL of unfiltered sample water was completely degassed and sealed 

into a vacuum flask. Tritiogenically produced 3

He derived 

from the degassed sample was allowed to accumulate in 

the flask for approximately 100 days. The accumulated 3

He 

was extracted and measured using a magnetic sector mass 

spectrometer and 3

H concentration is then calculated using 

the known decay constant of 3

H (Lucas and Unterweger, 

2000) and time of accumulation. This procedure results in 

a lower detection limit of approximately 0.05 tritium units 

(TU) where 1 TU is equals to one atom of 3

H per 1018 atoms 

of hydrogen.

Data Analysis

For the determination of the amount of tritiogenic 3He 

(3

He*) in the dissolved gas phase of a sample, a number of 

parameters must be defined. Total amount of 3

He contained 

within a sample (3

Hemeasured) is a combination of 3He derived 

from the atmosphere in equilibrium with the groundwater 

(3

Hesolubility), 3He derived from excess gas trapped during 

recharge (3

Heea) and excess 3He (3Heexcess) from a combination 

of 3

He released to groundwater from radioactive production in 

geological materials and (or) an external flux (3

Heterr), and 3He 

generated from 3

H decay (3He*).

3

Hemeasured = 3Hesolubility + 3Heea + 3Heexcess
8 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

where

3

Heexcess = 3Heterr + 3He*

For the solution of the solubility and excess gas components the Closed Equilibrium (CE) model (Aeschbach-Hertig, 

Peeters, and others, 1999) was used. The CE model uses a 

chi-squared comparison of modeled to measured dissolved 

gas data for neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) 

and nitrogen (N2

) in order to determine recharge temperatures 

and amounts of excess gas present in waters. The amount 

of 3

Heterr is calculated by subtracting the modeled 4He from 

the CE-model from 4

He measured concentrations from the 

samples. The result represents the amount of excess 4

He 

associated with the terrigenic component. Assuming that the 

excess helium is a combination of in-situ production and an 

external flux of helium, a 3

He/4He ratio of 2.08 × 10–7 is multiplied by the concentration of excess 4

He to define the amount 

of 3

Heterr. This ratio of terrigenic helium is measured from

tritium absent samples that contained high excess helium concentrations sampled from East Uvalde 4. By subtracting the 

solubility, excess gas and terrigenic 3

He concentrations from 

the measured, 3

He* is determined. The apparent groundwater 

age is defined by the ratio of 3

He* to measured 3H values and 

by using a decay constant (l) of 0.05621 year–1 (half-life of 

12.32 years) (Lucas and Unterweger, 2000) and the equation:

Apparent age (in years) = 1/0.05621 * ln (1+3He*/3H).

Results

Geophysical Logs

The geophysical logs obtained prior to sampling of the 

monitoring wells are presented in Appendix 1. The logs were 

collected from the open intervals in the monitoring wells 

Seven Mile Hill, East Uvalde 2, Uvalde Index, East Uvalde 1, 

and East Uvalde 4.

Electrical Conductance Logs

The electrical conductance of water is defined as its 

ability to conduct electrical current. The electrical current is 

a function of the amount of charged ions dissolved in solution (total dissolved solids or TDS); therefore conductivity 

of a solution is proportional to its TDS. Applying the correlation between TDS concentrations and fluid conductance 

derived from previous data (TDS [mg/L] = (6.9146 × Specific 

Conductance [milliSiemens per meter (mS/m)] – 150.02) one 

can estimate TDS values for the borehole waters in Uvalde 

County. According to this correlation, a value of 166.3 mS/m 

correlates with the 1,000 mg/L TDS, the concentration of 

dissolved solids that distinguishes fresh from saline waters in 

the Edwards aquifer (Shultz, 1994). The logged monitoring 

wells have similar conductance values of approximately 47 to 

55 mS/m with the exception of increased conductance values 

in East Uvalde 1 (approximately 70 to 75 mS/m) and East 

Uvalde 4 (approximately 425 to 450 mS/m) associated with 

the freshwater/saline water transition zone in the southeastern 

part of Uvalde County. Though East Uvalde 1 satisfies the 

condition as a freshwater monitoring well (368 mg/L TDS), 

conditions present in the well show that this well represents a 

transitional well between the freshwater zone and saline water 

zone of the Edwards aquifer.

Temperature Logs

Temperature profiles from the monitoring wells show 

slightly more variation than the conductance profiles. East 

Uvalde 2 and Uvalde Index wells have almost no thermal 

gradient observed (less than 0.5 degrees Celsius per one hundred meters [°C/100 m]) with water temperatures measured 

between 23.5 °C and 24.5 °C. Seven Mile Hill also has a steep 

temperature gradient in the upper portion of the well; however, 

the gradient lessens in the lower 200 feet of the borehole with 

overall temperatures ranging from 23.0 to 24.0 °C. Both East 

Uvalde 1 and 4 show significant thermal gradients (1.3 and 

2.1 °C/100 m respectively) and temperatures ranging from 

29.5 to 35.0 °C. Although the increased temperatures observed 

in East Uvalde 1 and 4 could be attributed to greater depth 

when compared to the majority of the freshwater wells (see 

table 1), a comparison of East Uvalde 1 and 4 temperatures 

with East Uvalde 2 at similar depths demonstrates an approximate 8 to 10 °C increase associated with East Uvalde 1 and 4.

King and Simmons (1972) calculated the heat flow 

associated with a deep well in Uvalde County and showed that 

it was about 46 milliWatts per square meter (mW/m2

) with a 

thermal gradient of 2.16 °C/100 m. Other sources for thermal 

gradient data (Woodruff and Foley, 1985) indicate that deep 

thermal gradients along the Balcones fault zone (in the study) 

area also range from 2.05 to 2.46 °C/100 m. Assuming that the 

thermal conductivity is about 2.19 Watts per meter per degree 

kelvin (W/m/K) for a water-saturated limestone, the calculated 

heat flow in the study area is fairly constant from about 44.9 to 

53.9 mW/m2

. Temperature gradients measured in this investigation range from less than 0.5 °C/100 m in the freshwater 

wells to 1.3 to 2.1 °C/100 m in the saline-zone transition 

wells. The thermal and conductance data indicate that the 

groundwater flow is becoming more stagnant as it moves into 

the freshwater/saline water transition zone, not only mixing 

with a high TDS end member, but also coming into thermal 

equilibrium with the surrounding crustal heat fluxes. A direct 

result of this interpretation is that there must be a distinct 

difference in groundwater residence times (groundwater age) 

associated with the freshwater wells as compared to the saline 

transition zone wells.

Another example of this broad interpretation can be 

observed in comparison of the historical geophysical logs from 

East Uvalde 2 (fig. 3). The logs from November 2002 compared to the logs from November 2005 show a clear change in 

specific conductance and temperature profiles noted in the well 
Table 1. Sample location data.[mbgs, meters below ground surface; m, meter; not available, data not available for this publication]

Sample Description Type of well State well number U.S. Geological Survey station number Depth (mbgs) Sampled inverval (mbgs) Production formation Sample type 

Freshwater zone

EU2-1187 East Uvalde 2, 362 m Monitoring YP-69-44-902 291612099302001 476 362 Devils River Formation Discrete 

EU2-1293 East Uvalde 2, 394 m Monitoring YP-69-44-902 291612099302001 476 394 Devils River Formation Discrete 

EU2-1403 East Uvalde 2, 428 m Monitoring YP-69-44-902 291612099302001 476 428 Devils River Formation Discrete 

EU2-1503 East Uvalde 2, 458 m Monitoring YP-69-44-902 291612099302001 476 458 Devils River Formation Discrete 

KPA City of Knippa, north Production YP-69-43-606 291840099382601 213 213 Salmon Peak and McKnight 

Formations/Devils River Formation

Pumped 

SAB City of Sabinal 3 Production YP-69-45-405 291937099280501 369 369 Devils River Formation Pumped 

A&M Texas A&M AgExt Agricultural YP-69-51-114 291417099442901 Not

available

172 Salmon Peak and McKnight 

Formations

Pumped 

LVN Livingston Domestic YP-69-35-401 Not available 51 Not

available

Devils River Formation 

(upper to middle)

Pumped 

UV11 City of Uvalde 11 Production YP-69-50-503 291202099491001 161 161 Salmon Peak Formation Pumped 

NUC Nueces Agricultural YP-69-33-701 292346099594701 Not

available

Not

available

McKnight Formation 

(minor West Nueces)

Pumped 

UV09 City of Uvalde 9 Production YP-69-50-203 291407099473501 160 160 Salmon Peak Formation 

(near base, possibility of upper McKnight)

Pumped 

ANN Annandale Domestic YP-69-35-502 292652099401601 82 82 Devils River Formation

(middle to lower)

Pumped 

7MH-130 Seven Mile Hill, 40 m Monitoring YP-69-42-709 291623099514401 220 40 Salmon Peak Formation Discrete 

7MH-260 Seven Mile Hill, 79 m Monitoring YP-69-42-709 291623099514401 220 79 Salmon Peak Formation Discrete 

UVI-290 Uvalde Index, 88 m Monitoring YP-69-50-302 291237099471201 107 88 Salmon Peak Formation Discrete 

UVI-330 Uvalde Index, 101 m Monitoring YP-69-50-302 291237099471201 107 101 Salmon Peak Formation Discrete 

BOW Bowman Ranch Domestic YP-69-37-402 292711099282201 212 212 Devils River Formation Pumped 

Saline-water zone

EU1-1050 East Uvalde 1, 320 m Monitoring YP-69-52-202 291443099325801 457 320 Salmon Peak Formation Discrete 

EU1-1140 East Uvalde 1, 348 m Monitoring YP-69-52-202 291443099325801 457 348 Salmon Peak Formation

(faulted with facies change)

Discrete 

EU1-1250 East Uvalde 1, 381 m Monitoring YP-69-52-202 291443099325801 457 381 Devils River Formation Discrete 

EU1-1350 East Uvalde 1, 412 m Monitoring YP-69-52-202 291443099325801 457 412 Devils River Formation Discrete 

EU1-1470 East Uvalde 1, 448 m Monitoring YP-69-52-202 291443099325801 457 448 Devils River Formation Discrete 

EU4-1000 East Uvalde 4, 305 m Monitoring YP-69-52-404 291133099363801 446 305 Salmon Peak Formation

(faulted)

Discrete 

EU4-1100 East Uvalde 4, 335 m Monitoring YP-69-52-404 291133099363801 446 335 Salmon Peak Formation Discrete 

EU4-1160 East Uvalde 4, 354 m Monitoring YP-69-52-404 291133099363801 446 354 Salmon Peak Formation Discrete 

EU4-1300 East Uvalde 4, 396 m Monitoring YP-69-52-404 291133099363801 446 396 McKnight Formation Discrete 
10 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

Figure 3. Comparison of geophysical logs of temperature (in degrees Celsius [°C]) and specific 

conductance (in millisiemens per meter [mS/m]) from monitoring well East Uvalde 2 for November 2002 

and November 2005.

between 2002 and 2005. This observation could be indicative 

of the influx of warmer, more saline water into the freshwater 

zone of the Edwards aquifer possibly because of increased 

groundwater withdrawal or low amounts of recharge. More 

time series data would be required to confirm this hypothesis; 

however, it demonstrates that the transition zone between the 

fresh and saline zones in the aquifer is hydraulically connected 

and is dynamic and the interface responds to change in aquifer 

use or recharge.

Dissolved Gas in Groundwater

As meteoric water recharges an aquifer and becomes 

groundwater, it carries a dissolved gas composition that is in 

equilibrium with the atmosphere. This composition is called 

air saturated water (ASW) and results from the solubility of 

atmospheric gas in water while at a fixed temperature, salinity and partial pressure. Although the water is isolated from 

the atmosphere, the dissolved gas composition is assumed to 

remain fixed, reflecting conditions at the time of recharge. By 

measuring the dissolved gas composition of a groundwater 

sample and applying a solubility model, such parameters as 

temperature, pressure, and salinity of the recharge conditions 

can be resolved even though the physical conditions have 

changed (that is, temperature and salinity) since recharge. This 

modeling assumes that the gases used must be atmospherically 

derived, have a homogeneous composition in the atmosphere 

that does not change with time, have no subsurface sources, 

and are chemically inert.

Noble Gas Solubility

Noble gases, such as helium, neon, argon, krypton, and 

xenon represent a wide range of atomic masses and isotopic 

compositions that are thoroughly characterized in the atmosphere, have well understood thermodynamic and chemical 

properties, and are chemically inert. Many of the gases (that 

is, neon-20 and argon-36) have no subsurface sources and are 

reliable indictors of atmospheric or air saturated water interactions in isolated reservoirs. Other noble gases, like helium-4 

East Uvalde 2 logs

36 38 40 42 44 46 48 50 52 54 56 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0

Temperature,

in degrees Celsius

Specific conductance,

in millisiemens per meter

1,000

1,100

1,200

1,300

1,400

1,500

1,600

Depth, in feet below ground surfaceDepth, in feet below ground surface

1,000

1,100

1,200

1,300

1,400

1,500

1,600

EXPLANATION

November 2002

November 2005
Results 11

and argon-40, have well defined subsurface production 

mechanisms that allow the qualitative or quantitative interpretation of groundwater age. In general, the solubility of the 

noble gases is dependent on the atomic mass, with the heavier 

gases (that is, argon, krypton, and xenon) being more soluble 

in water than the lighter gases (that is, helium and neon). 

This difference in solubility that occurs with atomic mass and 

vastly different partial pressures in the atmosphere produces 

distinct ratios of the gases in air saturated water which reflect 

the physical conditions present at the time of equilibrium; 

therefore, at the time of recharge.

Excess Air

Sometimes the concentrations of the measured dissolved gases are higher than what could be explained from 

just atmospheric sources. Heaton and Vogel (1981) noted that 

the composition of the excess gas component (above that of 

solubility) of a suite of groundwater samples was similar to 

that of atmospheric composition. The dissolved excess gas in 

the samples was attributed to the trapping of bubbles of air 

during recharge and complete dissolution of the bubbles with 

downward flow and increasing hydrostatic load. This excess 

gas component is known as excess air.

Later work by Stute and Sonntag (1992), AeschbachHertig, Peeters, and others (1999), and Ballentine and Hall 

(1999) showed that this excess air component could vary 

in composition depending on the degree of equilibration 

of the trapped air and possible loss, fractionating the original air-like component. This type of fraction of the air-like 

excess component is referred to as the closed equilibrium 

(CE) model (Aeschbach-Hertig, Peeters, and others, 1999). 

The CE model assumes that gas equilibration occurs in a 

closed system between air saturated groundwater and small 

bubbles being trapped in the quasi-saturated zone (Kipfer, 

Aeschbach-Hertig, and others, 2002). The free gas component 

in the quasi-saturated zone has a fixed volume that does not 

easily exchange with the free unsaturated zone air. Because of 

relatively slow groundwater velocities and slightly elevated 

pressure above that of atmospheric, there is a repartitioning of the trapped gas in groundwater, thereby establishing a 

new equilibrium concentration that is more indicative of the 

amount of free gas remaining in solution. If all of the free gas 

is forced into solution, the excess gas component will have an 

air-like composition (excess air). However, if free gas remains 

after equilibration, the dissolved excess gas component will 

be enriched in the more soluble gas components, giving a 

fractionated excess gas composition that favors the more 

soluble (greater mass) molecular gases. For implementation 

of the CE model, measured dissolved noble gas components 

are compared to modeled concentration values using estimated 

parameters of temperature, initial concentration of entrapped 

air (Ae), and the reduction of entrapped volume by dissolution 

and compression (F), with the assumption that the water was 

fresh (or low salinity) and the partial pressure of the gases is 

associated with atmospheric pressure at the elevation of the 

water table. An error weighted least squares fit is used to solve 

for free model parameters (T, Ae, and F) by minimizing chisquared values (X2

).

Measured noble gas compositions (dissolved gas) in 

cubic centimeters at standard temperature and pressure per 

gram of water (cm3

(STP)/gH2O) for helium (He), neon (Ne), 

argon (Ar), krypton (Kr), and xenon (Xe), for both the wells 

in the freshwater and saline-water zones are listed in table 2 

along with the dissolved nitrogen (N2) and methane (CH4) 

concentrations. Isotopic compositions of helium (R/RA, where 

R is the 3

He/4He of the sample normalized to the atmospheric 

ratio [RA] of 3

He/4He [1.384 × 10–6]), neon (20Ne/22Ne) and 

argon (40Ar/36Ar) are also presented in table 2. In general, 

the concentrations for the conservative atmospherically 

derived gas components (Ne, Ar, Kr, Xe and N2

) are close 

to that of air saturated water at 20 °C (305 meters elevation). Because of the lack of subsurface production of 20Ne 

and 36Ar isotopes, the concentrations of these gases provide 

insight into parameters involved with recharge and excess 

atmospheric gas. To further distinguish the atmospheric origin 

of the waters a binary plot of F 20Ne/36Ar values compared to 

1/36Ar ( [cm3

(STP)/gH20 × 10–3]) is presented in figure 4A. The 

F value here is defined as the measured i/36Ar (where i is the 

species of gas in question; for example, N2

, 20Ne, 4He) of the 

sample, normalized to i/36Ar of ASW at 20 °C at an elevation 

of 305 meters. Included on the plot are data for ASW at temperatures of 15 °C, 20 °C and 25 °C, as well as the addition of 

excess air (unfractionated) and fractionated air associated with 

a sample of 20 °C ASW at an elevation of 305 meters.

F 20Ne/36Ar values greater than one would indicate that 

there is an increase in the concentration of 20Ne relative to 36Ar. This increase in F 20Ne/36Ar values along with decreasing 

1/36Ar (increasing 36Ar concentration) is an indicator of excess 

gas above that of the normalizing ratio. A majority of the dissolved gas data falls into this classification (fig. 4A). Though 

there is a subtle variation in the values caused by differences 

in the recharge temperature and amount of excess air of the 

sample compared to the normalizing ratio, the general trend is 

for the majority of the dissolved gas sample to be derived from 

air equilibrated waters, with some component of excess gas 

associated with recharge.

One group of samples deviates from this trend. Samples from East Uvalde 4 plot opposite to the trend noted in 

respect to air saturated waters with excess dissolved gas. The 

F20Ne/36Ar values are less than one with correspondingly high 

1/36Ar values. This represents a loss of 36Ar relative to solubility values, along with decreased concentrations of 20Ne relative 

to 36Ar. The loss of dissolved gas from the system favors the 

lighter noble gas isotopes relative to 20 °C ASW. The pattern 

of loss is associated with liquid-gas phase partitioning occurring in the subsurface (Ballentine, Burgess, and Marty, 2002; 

Bosch and Mazor, 1988). As a parcel of air saturated groundwater comes into contact with a subsurface, free gas phase 

that contains low concentrations of atmospherically derived 

noble gas components, new dissolved gas exchange equilibrium within the groundwater will occur to yield a groundwater 
Table 2. Dissolved gas compositions with analytical errors. [NM, not measured; cm3(STP)/gH2O, cubic centimeters per gram at standard temperature and pressure per gram of water; R/RA, ratio helium-3 to helium-4 normalized to atmosph

ASW 20 °C, 305 meters, air saturated water equilibrated at twenty degrees Celsius at an elevation of 305 meters; ±, plus or minus] Sample Helium cm3(STP)/gH2O ± Neon cm3(STP)/gH2O ± Argon cm3(STP)/gH2O ± Krypton cm3(STP)/gH2O ± Xenon cm3(STP)/gH2O ± Nitrogen cm3(STP)/g

Freshwater zone EU2-1187 8.852E-8 6.1E-10 2.170E-7 5.6E-9 3.330E-4 5.0E-6 7.490E-8 1.5E-9 9.459E-9 2.8E-10 1.371E-

EU2-1187 7.177E-8 4.9E-10 1.990E-7 5.2E-9 3.396E-4 5.1E-6 7.513E-8 1.5E-9 9.552E-9 2.9E-10 1.411E-2 

EU2-1403 6.344E-8 6.3E-10 2.309E-7 6.9E-9 3.423E-4 1.0E-5 7.442E-8 2.2E-9 9.685E-9 2.9E-10 1.441E-2 

EU2-1503 6.354E-8 6.4E-10 2.499E-7 7.5E-9 3.345E-4 1.0E-5 7.437E-8 2.2E-9 9.370E-9 2.8E-10 1.376E-2 

KPA 6.199E-8 6.2E-10 2.278E-7 8.7E-9 3.392E-4 6.8E-6 7.226E-8 1.4E-9 9.948E-9 2.0E-10 1.412E-2 

SAB 8.298E-8 8.3E-10 3.306E-7 7.6E-9 3.920E-4 7.8E-6 7.948E-8 1.6E-9 1.054E-8 2.1E-10 1.763E-

A&M 5.616E-8 5.6E-10 2.195E-7 7.7E-9 3.324E-4 6.6E-6 7.125E-8 1.4E-9 9.734E-9 1.9E-10 1.372E-

LVN 1.887E-7 1.9E-9 1.858E-7 5.4E-9 3.013E-4 6.0E-6 6.613E-8 1.3E-9 9.049E-9 1.8E-10 1.229E-2 

UV11 6.601E-8 6.6E-10 2.279E-7 4.6E-9 3.238E-4 6.5E-6 6.901E-8 1.4E-9 9.243E-9 1.8E-10 1.344E-2 

NUC 5.950E-8 6.0E-10 2.231E-7 4.5E-9 3.384E-4 6.8E-6 7.188E-8 1.4E-9 9.919E-9 2.0E-10 1.381E-2 

UV09 5.351E-8 5.4E-10 2.221E-7 4.4E-9 3.202E-4 6.4E-6 6.801E-8 1.4E-9 9.150E-9 1.8E-10 1.312E-2 

ANN 4.770E-8 4.8E-10 1.919E-7 3.8E-9 3.461E-4 6.9E-6 7.686E-8 1.5E-9 1.099E-8 2.2E-10 1.325E-2 

7MH-130 5.052E-8 5.1E-10 2.189E-7 4.4E-9 3.206E-4 6.4E-6 6.799E-8 1.4E-9 9.125E-9 1.8E-10 1.326E-2 

7MH-260 5.120E-8 5.1E-10 2.072E-7 4.1E-9 3.115E-4 6.2E-6 6.498E-8 1.3E-9 8.690E-9 1.7E-10 1.273E-2 

UVI-290 5.799E-8 5.8E-10 2.253E-7 4.5E-9 3.128E-4 6.3E-6 6.388E-8 1.3E-9 8.329E-9 1.7E-10 1.296E-2 

UVI-330 5.745E-8 5.7E-10 2.238E-7 4.5E-9 3.122E-4 6.2E-6 6.490E-8 1.3E-9 8.391E-9 1.7E-10 1.527E-

BOW 8.390E-8 8.4E-10 3.311E-7 6.6E-9 3.862E-4 7.7E-6 7.805E-8 1.6E-9 1.038E-8 2.1E-10 1.854E-2 

Saline-water zone EU1-1050 2.028E-6 1.5E-8 1.752E-7 3.5E-9 3.016E-4 6.0E-6 NM NM 1.487E-2 

EU1-1140 1.511E-6 2.2E-8 2.139E-7 4.3E-9 2.971E-4 5.9E-6 NM NM 1.464E-2 

EU1-1250 2.154E-6 2.0E-8 1.786E-7 3.6E-9 3.238E-4 6.5E-6 NM NM 1.534E-2 

EU1-1350 2.164E-6 2.2E-8 1.778E-7 3.6E-9 2.917E-4 5.8E-6 NM NM 1.439E-2 

EU1-1470 2.351E-6 2.4E-8 2.127E-7 4.3E-9 3.333E-4 6.7E-6 NM NM 1.627E-2 

EU4-1000 3.070E-5 3.1E-7 1.473E-7 2.9E-9 3.010E-4 6.0E-6 NM NM 1.449E-2 

EU4-1100 3.201E-5 3.2E-7 1.443E-7 2.9E-9 2.727E-4 5.5E-6 NM NM 1.372E-2 

EU4-1160 3.419E-5 3.4E-7 1.429E-7 2.9E-9 2.835E-4 5.7E-6 NM NM 1.400E-2 

EU4-1300 3.214E-5 3.2E-7 1.371E-7 2.7E-9 2.789E-4 5.6E-6 NM NM 1.409E-

ASW 20oC, 305 meters 4.311E-8 1.783E-7 3.004E-4 6.709E-8 9.165E-9 1.148E-
Table 2. Dissolved gas compositions with analytical errors.—Continued[NM, not measured; cm3(STP)/gH2O, cubic centimeters per gram at standard temperature and pressure per gram of water; R/RA, ratio helium-3 to helium-4 normalized to atmosph

ASW 20 °C, 305 meters, air saturated water equilibrated at twenty degrees Celsius at an elevation of 305 meters; ±, plus or minus]

Sample RRA ± 20Ne 22Ne ± 40Ar 36Ar 

Freshwater zone

EU2-1187 0.813 0.006 9.862 0.049 294.8 

EU2-1293 1.023 0.008 9.846 0.049 295.1 

EU2-1403 1.008 0.005 NM 0.000 296.7 

EU2-1503 0.991 0.005 9.914 0.050 299.4 

KPA 1.002 0.008 9.867 0.049 294.1 

SAB 1.068 0.009 9.852 0.049 295.5 

A&M 1.035 0.008 9.893 0.049 293.4 

LVN 0.348 0.005 9.846 0.049 295.2 

UV11 1.068 0.009 9.883 0.049 294.1 

NUC 0.983 0.008 9.860 0.049 294.0 

UV09 1.105 0.009 9.904 0.050 295.6 

ANN 0.973 0.008 9.922 0.050 295.5 

7MH-130 1.043 0.008 9.894 0.049 294.0 

7MH-260 1.085 0.009 9.901 0.050 293.6 

UVI-290 1.119 0.009 9.848 0.049 295.1 

UVI-330 1.127 0.009 9.835 0.049 294.6 

BOW 1.042 0.008 9.834 0.049 295.6 

Saline-water zone

EU1-1050 0.175 0.005 9.883 0.050 299.7 

EU1-1140 0.184 0.005 9.904 0.049 298.7 

EU1-1250 0.182 0.005 9.888 0.049 300.0 

EU1-1350 0.174 0.005 9.883 0.049 299.0 

EU1-1470 0.178 0.005 9.886 0.049 297.8 

EU4-1000 0.178 0.005 9.890 0.049 302.6 

EU4-1100 0.187 0.005 9.900 0.050 302.0 

EU4-1160 0.195 0.005 9.888 0.049 302.7 

EU4-1300 0.190 0.005 9.917 0.050 301.2 

ASW 20°C,

305 meters

0.980 9.800 295.5
14 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

Figure 4. Binary plots of A, F 20Ne/36Ar compared to 1/36Ar; B, F4He/20Ne compared to F20Ne/36Ar; and C, FN2/

36Ar compared to 1/36Ar. F values 

are defined as the concentration of gas species i of the sample divided by the concentration of 36Ar of the sample normalized to i/36Ar of air 

saturated water at a temperature of 20 degrees Celsius (oC) and altitude of 305 meters above sea level. Concentration values for 36Ar are 

presented in cubic centimeters at standard temperature and pressure per gram of water multiplied by 0.001 (cm3

(STP)/gH2O × 10–3).

0.010 cm3(STP)/g

0.005 cm3

(STP)/g

UVI

EU4

Terrigenic helium

EU1

LVN

Air-saturated

water at 20 °C

ANN

UVI

t = 25 °C

t = 15 °C EU4

f = 0.9

f = 0.8

ANN

Rayleigh

fractionation

Excess air addition

Excess nitrogen

Excess air

EU4

EU1

UVI

Air saturated

water at 20 °C ANN

700 800 900 1,000 1,100 1,200 1,300

1/36Ar, in cm3(STP)/g (×10 –3)

700 800 900 1,000 1,100 1,200

1/36Ar

0.8 0.9 1.0 1.1 1.2 1.41.3 1.5

F20Ne/36Ar

A

C

B

1.4

0.4

0.6

0.8

1.0

1.2

1.4

1.6

F20Ne/36Ar

F4He/20Ne

1,000

10,000

100

10

1

0.1

FN2

/36Ar

1.3

1.2

1.1

1.0

0.9

EXPLANATION

Freshwaters

Saline waters

Rayleigh fractionation

Excess air addition at 20° C 

Air saturated water line

Closed equilibrium model at 0.010 cm3

(STP)/g at 20 °C
Results 15

depleted in gas relative to its original atmospheric equilibrated 

value. Solubility relations fractionate the dissolved gases in 

the groundwater with the lighter, less soluble gases partitioned 

into the free gas phase and the heavier, more soluble gases 

tending to remain in the liquid phase. Presented on the plot is 

a simple Rayleigh fractionation model (Ballentine, Burgess, 

and Marty, 2002) to show the effects of gas partitioning. The 

fractional value (f) for the Rayleigh model presented represents the fraction of 36Ar left in solution. The model predicts 

that approximately 8 percent of the original 36Ar has been lost; 

however, this could be much greater if the starting gas concentration for the model contained excess atmospheric gases. 

The exact mechanism that is causing the atmospheric-derived 

gases to partition out of solution is unclear.

Further evaluation of the dissolved gas compositions with 

respect to He and N2

 concentrations are presented in figures 4B

and C. The binary plots of F 4

He/20Ne versus F 20Ne/36Ar 

(fig. 4B) and F N2

/

36Ar versus 1/36Ar (fig. 4C) illustrate gas 

partitioning and addition of excess air to the samples. The 

F 4

He/20Ne value for 4He is coupled with 20Ne and not 36Ar, in 

order to match the similar solubility of He and Ne.

For both plots (fig. 4B and 4C) the freshwater data correlate with the excess air trend while the saline waters trend 

away from assumed atmospheric values. In the F N2

/

36Ar 

compared to 1/36Ar plot, saline water data show an increasing 

F N2

/

36Ar value (N2

 increasing relative to 36Ar) while the 1/36Ar 

increases. This trend is not predicted by liquid-gas phase partitioning, where the F N2

/

36Ar should decrease with decreasing quantities of 36Ar. The trend is interpreted as an addition 

of excess N2

 above solubility, excess air, and the liquid-gas 

phase partitioning.

The same excess gas addition is also observed in the He 

data. The F 4

He/20Ne increases as the F 20Ne/36Ar decreases, 

and reveals a source of excess 4

He associated with the saline 

waters. This excess He is called terrigenic He. The measured 

concentration of terrigenic He increases to almost 1,000 times 

that of the atmospheric solubility values, with a homogeneous 

isotopic composition of 0.19 R/RA (table 2). Terrigenic He 

generation in the crust is dominated by 235U, 238U and 232Th 

decay chain alpha-production but also can be coupled with 

a flux of helium from the planetary interior. The increase of 

He with time in groundwaters has been studied as a means 

of dating young groundwaters (Marine, 1979; Mazor and 

Bosch, 1992; Solomon, Hunt, and Poreda, 1996; Torgersen 

and Clarke, 1985; Torgersen and Ivey, 1985). The relative 

contributions from each source can vary greatly, producing 

distinctive helium isotopic compositions and variable helium 

accumulation rates in an area.

Excess N2

 and terrigenic He correlate with groundwater salinity. The strong correlation may be associated with a 

uniform reservoir of terrigenic He and N2

 (crustally derived 

N2

) in the saline zone, while variations in the concentration 

are the result of mixing along the transition zone. While East 

Uvalde 4 has a fairly uniform gas composition, data suggest 

that samples from East Uvalde 1 represent a mixing between 

the fresh and saline waters. The saline waters are characterized 

by the uniform composition of atmospherically derived water 

that experienced some liquid-gas phase partitioning and are 

enriched in terrigenic He and excess N2

 while the freshwaters 

are constrained by solubility concentrations associated with air 

saturated waters with some additional dissolved gas component entrained during recharge.

The mixing of fresh and saline waters, as well as the 

presence of fractionated dissolved gas components, prohibit 

the application of the CE model to determine the recharge 

conditions that are associated with the saline and saline transition zone wells. The presence of excess N2

, which correlates 

with terrigenic helium, suggests that an external flux of He and 

N2

 dominates the saline zone. The concentrations of terrigenic 

helium are high enough to indicate that the saline zone waters 

are much older than modern groundwaters. The thermal data 

obtained corroborate this postulate that the age of the saline 

waters is greater than the fresh water. Typical estimates of He 

crustal flux with production for the area would generate apparent groundwater ages on the order of hundreds to thousands of 

years for the saline zone. Apparent ages based on 4

He concentration are heavily model dependent and are beyond the scope 

of this report.

Almost all of the freshwater samples plot well with ASW 

and excess gas concentrations except LVN, UVI-330 and ANN 

(fig. 4A, B and C). LVN has a minor amount of terrigenic 4

He 

that is not associated with excess N2

, UVI-330 has excess 

N2

 not associated with terrigenic He, and ANN does not plot 

well with the rest of the data. The data anomalies from ANN 

are explained as a difference in recharge temperature that 

is lower than 20 °C ASW (normalizing ratio for the F value 

[CE model]). The excess N2

 in UVI-330 can be attributed to 

denitrification of dissolved nitrogen compounds in the groundwater, but LVN concentration of excess He is more complex. 

The LVN sample plots as atmospheric-equilibrated groundwater with a slight amount of excess air, but the amount of 

excess helium observed in the sample cannot be explained by 

solubility and recharge conditions alone. The sample may be 

a mix between modern and premodern groundwaters, with the 

premodern component containing excess He. This important 

mixing relation can be further defined more clearly by measuring the concentration of 3

H in the sample.

Calculated Recharge Temperatures

The calculated recharge temperatures along with values 

of Ae and F parameters from the CE model (Aeschbach-Hertig 

and others, 1999) and the C2

 value for the fit test are presented in table 3 for the samples from the freshwater zone. All 

samples met the criteria for appropriateness of fit of the X2

 test 

with X2 values of less than four. Ae values all seem reasonable 

except for EUV2-1293 which has an extremely high Ae value 

of 0.147 cm3

 (STP)/gH2O associated with high F value of 0.870. 

The derived recharge temperature from EUV2-1293 is similar 

to others sampled at the same time and may possibly represent 

a sampling problem that is corrected through application of the 

CE model.
Table 3. Closed equilibrium model parameters and apparent age data. [cm3(STP)/gH2O, cubic centimeters per gram at standard temperature and pressure per gram of water; *, error is calculated by instrumental error of helium and tritium measurements; calculated; <0.5, less than 0.5; Ae, initial concentration of trapped air; F, fractional reduction of entrapped volume by dissolution and compression; TU, tritium unit, °C, degrees Cels

Sample Recharge temperature (°C) Ae cm3(STP)/gH2O F χ2 Tritiogenically derived helium-3 (TU) ± Tritium (TU) ± Apparent age (years) Recharge date (year) 

Freshwater zone EU2-1187 19.3 0.018 0.735 3.53 8.12 0.28 1.94 0.04 29.3 1977 

EU2-1293 20.0 0.147 0.870 3.25 12.05 0.29 1.76 0.04 36.7 1969 

EU2-1403 19.7 0.018 0.645 0.28 3.83 0.28 1.76 0.04 20.6 1985 

EU2-1503 20.1 0.005 0.202 1.90 0.21 0.28 1.45 0.03 2.4 2004 

KPA 18.5 0.005 0.364 0.67 2.95 0.31 2.39 0.05 14.3 1992 

SAB 18.1 0.010 0.134 2.01 3.46 0.45 2.11 0.04 17.3 1989 

A&M 18.8 0.003 0.207 0.41 2.20 0.29 2.32 0.05 11.8 1994 

LVN 20.7 0.000 0.044 0.11 <0.5 0.45 2.50 0.05 <0.5 2006 

UV11 20.6 0.003 0.014 0.09 6.91 0.36 1.98 0.04 26.7 1979 

NUC 18.3 0.003 0.122 1.09 1.64 0.29 2.00 0.04 10.7 1995 

UV09 20.7 0.003 0.217 3.29 3.34 0.30 2.49 0.05 15.1 1991 

ANN 14.4 0.000 0.204 1.58 0.58 0.23 3.00 0.06 3.2 2003 

7MH-130 21.3 0.007 0.525 0.17 1.71 0.27 2.27 0.05 10.0 1996 

7MH-260 23.2 0.016 0.750 0.62 2.83 0.28 2.13 0.04 15.1 1991 

UVI-290 25.9 0.020 0.650 0.82 5.05 0.33 2.56 0.05 19.4 1987 

UVI-330 24.7 0.012 0.587 0.00 4.97 0.33 2.12 0.04 21.5 1984 

BOW 18.6 0.008 0.000 0.62 0.75 0.44 2.64 0.05 4.5 2001 

Saline-water zone EU1-1050 ** 0.64 0.02   EU1-1140 **  0.21 0.02 ** ** EU1-1250 ** <0.13 ** **
Results 17

The average recharge temperature of the area is 19.8 

plus or minus 2.3 °C, which correlates well with the mean 

annual temperature of 20 °C for Uvalde County. This matches 

the concept that noble gas recharge temperatures are closely 

associated with mean annual air temperatures (soil temperatures) for a geographic area (Mazor, 1993; Stute and Sonntag, 

1992). The only outlier for recharge temperature is the sample 

ANN. The computed recharge temperature for the well is 

consistent with the relations derived from dissolved gas concentration plots (fig. 4A, B, and C), but the measured temperature is six degrees lower than what would be expected for 

the area. ANN is located in the recharge zone of the Edwards 

aquifer and is closely associated with the Frio River watershed 

(fig. 1B). The measured recharge temperature is close to that 

of average air temperatures for February in Uvalde County 

(13 °C), which coincides with the time the well was sampled. 

This extremely low recharge temperature can be explained by 

the hydraulic connection of the well to the Frio River. A close 

hydraulic connection to the river would cause the noble gas 

temperatures (NGT) data to mimic the seasonal temperature 

fluctuation observed in the river. This hydraulic connection 

would imply that the groundwater age from the well should 

be on the order of days to weeks. If ANN is not considered, 

the average recharge temperature of 20.2 plus or minus 1.9 °C 

is observed.

Tritium Data and Apparent Age

Tritium data are presented in table 3; concentrations are 

given in tritium units (TU, 1 TU is defined as 1 atom of 3

H per 

1018 atoms of H). All of the freshwater samples contain 3

H, 

as do two samples of the upper zone of East Uvalde 1, while 

the remaining samples in East Uvalde 1 and East Uvalde 4 

have less than 0.05 TU (3

H dead). The range of 3H values in 

the freshwater samples is narrow, because of a combination of 

low 3

H input in precipitation during the last 15 years and the 

normal decay of 3

H associated with residence time. Tritium 

by itself could be used to quantitatively date the groundwaters using a known input curve (International Atomic Energy 

Agency, 2004). Because of the narrow range in concentration 

of recent data and low levels of 3

H input, the error associated 

with such a method is too large to be useful. By definition 

the freshwaters are classified as modern (if less than 60 years 

in age) and the 3

H dead samples as premodern. This simple 

classification cannot account for mixing of older tritium dead 

components into modern waters.

The dissolved gas data from East Uvalde 1 coupled with 

the tritium data support the interpretation that the samples 

represent a mixing along the transition zone between the 

freshwater and saline water zones of the Edwards aquifer. 

The presence of tritium in the upper samples of the well show 

that there is a modern component mixing with the premodern 

component as characterized by high excess He concentrations. 

The exact degree of mixing is difficult to define though it was 

observed from historical geophysical log data. There are no 

end members to determine the scale of mixing beyond saying that a mix between two waters of different apparent ages 

is occurring.

Apparent 3H-3He Age

Using recharge conditions computed from the CE model 

and subtracting excess helium component with an isotopic 

composition of 0.15 R/Ra, the amount of tritiogenically 

derived 3

He (3He*) is calculated (table 3). For samples from 

East Uvalde 1 and East Uvalde 4 monitoring wells, no 3

He* is 

calculated for the data. The excess He values in these samples 

were too high to make an accurate determination of 3

He* 

(within the analytical precision of this procedure) and the 

dissolved gas compositions do not allow the application of the 

CE model. The 3

He* values are presented as TU units for ease 

of apparent age calculation. Unlike the tritium data, there is a 

wide range in 3

He* concentrations because of a wide distribution in apparent 3

H-3He ages. The amounts of 3He* ranging 

from less than 0.5 TU in LVN to 12.05 TU in EU2-1293.

Calculated apparent ages range from less than 0.5 years 

to a maximum of 36.7 years for the samples that contain 3

He*, 

which corresponds to calendar ages of 2006 to 1971. One way 

to evaluate the validity of the dataset is to plot the value of 3

H 

plus 3

He* (original 3H concentration in initial recharge) compared to the recharge year of the sample and known 3

H input 

values for the region. Historical 3

H concentrations in precipitation from the Waco, Texas dataset of the International Atomic 

Energy Agency (2004) have compiled a full set of 3

H input 

curves for the United States. Plotted in figure 5 is the average historical 3

H input curve and the 3H plus 3He* values and 

associated recharge year for the sample dataset. The data correlate well to the historical input curve, with only five samples 

plotting significantly below the input curve, implying 3

He* and 3

H are lower than expected in these samples. This finding can 

be explained by a dilution of the groundwater with a premodern 

component that contains low to no initial concentrations of 3

H. 

This premodern water dilutes the 3

He* and 3H concentrations, 

while the 3

He*/3H ratio of the sample preserves the age of the 

modern component. Samples UV11, EU2-1187, EU2-1293, 

EU2-1503 and LVN plot below the inferred 3

H input curve. The 

historical input curve has some amount of variation associated 

with different sources of precipitation which contain different 

3

H concentration within each year, but these five samples show 

definitive trends away from the expected variation of young or 

old samples.

The implication for mixing of groundwaters from East 

Uvalde 2 and East Uvalde 1 is expected. The proximity of the 

wells to the freshwater/saline water transition zone coupled 

with our understanding of the dissolved gas data and mixing 

relations noted in East Uvalde 1, allows us to predict some 

form of mixing between the modern and premodern components. Unlike the mixing noted in East Uvalde 1, the samples 
18 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

EU2-1186, EU2-1293, EU-1503 and UV11 do not contain 

high amounts of excess He. The amount of excess He ranges 

from less than 0.5 to 35.7 (cm3

(STP)/g × 10–9) (table 3). 

The amount of excess He does not correlate with salinity, 

unlike the correlation mixing observed in East Uvalde 1. The 

mixed end members for these samples could be derived from 

modern waters mixing with premodern waters that are not 

associated with the freshwater/saline-water interface. The premodern end member may be associated with younger, fresher 

waters than those currently found in the saline zone, but would 

have long flow paths (mean residence times greater than 

60 years) in the freshwater zone of the Edwards aquifer. The 

longer flow paths would still eventually merge with a modern 

freshwater system along the freshwater/saline-water transition zone, generating a mixing zone of relatively freshwater 

of varying apparent ages.

The sample plots are just slightly less than what is 

expected for recent 3

H input and could represent seasonal 

variability in 3

H input values; however, the LVN sample also 

contains elevated amounts of excess He (143.2 mcm3

/kg). LVN 

was sampled from the midpart of the recharge zone of the 

Edwards aquifer. The apparent age of the sample was less than 

0.5 years and correlates well with the conceptual understanding that young waters are recharging along the outcrop area 

of the Edwards aquifer. The amount of excess He measured 

in the sample must be associated with premodern end member. This mixing pattern could be attributed to an unknown 

interaction between the Edwards and Trinity aquifers. Groundwaters recharged in the Trinity aquifer are assumed to have 

a longer residence time (tritium dead and some amount of 

excess He) than those associated with the groundwaters within 

the recharge zone (ASW He concentrations and modern 3

H). 

EU2-394

EU2-362

UV11

EU2-428

EU2-458

LVN

EXPLANATION

Tritium input curve

One Sigma error in tritium input curve

Modern waters (tritiogenic helium-3 

 plus tritium)

1950 1960 1970 1980 1990 2000 2010

Calendar year

1

10

100

1,000 Tritium, in tritium units

Figure 5. Binary plot of 3H+3He* (tritium units) for samples compared to the 

recharge year from the 3

H-3He age, superimposed on the historical 3H input 

curve from Michel (2007). A one Sigma error (shaded area surrounding the 

3

H average curve) is reported for the 3H historical data as representing the 

variation of 3

H for an individual calendar year (January through December).
Results 19

Similar to findings from East Uvalde 2, the mixing of the two 

component waters produce a sample with a modern apparent age (measurable 3

H) but contain appreciable amounts of 

excess He. The amount of premodern water associated with 

the LVN sample is difficult to quantify with just the 3

H-3He 

data set because of the low concentrations and uncertainties 

in the 3

H input curve. The assumption that the premodern 

component is derived from the Trinity aquifer is circumstantial 

in nature and would need to be tested with more samples from 

the Trinity aquifer to further substantiate this observation.

Vertical Apparent Age Distribution

As water recharges an aquifer, it displaces earlier 

recharged waters away from the recharge zone. Apparent age 

also reflects this displacement of groundwater movement. The 

apparent age distribution of a set of vertically sampled groundwater samples will reflect the amount of recharge entering an 

aquifer for a specific area. Normal age distributions should 

increase with respect to apparent age with depth in an aquifer. 

The amount of water recharging the system can be quantified 

using simple models that described aquifer characteristics 

and the vertical distribution in apparent ages (Vogel, 1967; 

Solomon, Poreda, and others, 1995). Several wells were 

selected for multiple samples from different vertically distributed zones in the study area in an attempt to define the magnitude and distribution of groundwater recharge for this area.

The sampling results from the monitoring wells were 

problematic. Sampling from Seven Mile Hill encountered an 

obstruction at 353 feet below ground surface that limited data 

to two intervals. The short screened interval of Uvalde Index 

limited the number of possible samples to be taken. Only East 

Uvalde 2 had at least 4 samples taken from different depths.

The results from both Seven Mile Hill and Uvalde Index 

show increasing apparent age with increasing depth. Calculating the average linear velocity of the groundwater from the 

change in vertical distance with each sample along with the 

change in apparent age derives 8.6 meters per year (m/yr) 

downward for Seven Mile Hill and 5.1 m/yr downward for 

Uvalde Index. The downward velocity from these two wells 

does not reflect the downward velocity at the location of the 

wells, but is linked to the average linear velocity retained 

when the groundwaters were originally recharged in the 

aquifer. A conservative estimate of the amount of recharge 

it would take to generate this apparent vertical velocity is 

between 1.53 and 2.58 m/yr (assuming 0.3 for porosity of 

the aquifer and the vertical velocity component represents 

the velocity of recharging water). This amount of water far 

exceeds the annual precipitation for the area (approximately 

0.60 m/yr), as such, it must be related to a point source infiltration as a significant driving factor for recharge to the system. 

To understand this finding in detail, more data are needed to 

fully quantify recharge estimates.

The East Uvalde 2 data are slightly more problematic. 

Apparent age initially increases with depth for the upper two 

samples from the well (25.6 to 35.4 years), the apparent ages 

reverse and begin to decrease in age to an apparent age of 

1.2 years at the bottom of the well. In principle, this effect of 

reversed age gradients could be happening hydraulically in 

the system, but is improbable in porous media interpretation. 

Implying a karst conceptual model of driving young water 

below the older water in the confined aquifer could explain 

the results. This explanation is problematic in explaining the 

observed heat flow from the geophysical logs but cannot be 

ruled out. Another explanation can be that there is vertical 

flow in the well itself. The 3

H-3He data showed that there was 

mixing of modern and premodern components associated with 

the two upper samples, but this was not observed in the two 

deeper samples. Other evidence that further clouds the interpretation of the data is the existence of excess He in the upper 

two samples which diminishes to no detectable excess He in 

the bottom of the hole. This pattern in the dissolved gas data 

and 3

H concentrations may be a result of vertical mixing of the 

groundwaters within the annulus of the well. Mixed modern 

and premodern waters containing a small amount of excess He 

and 3

H can be mixing with a premodern, fresh component with 

no excess He in the lower part of the well. The 3

H remains in 

the annulus while the excess He diffuses into the aquifer in the 

upper portion of the well. This would effectively separate the 

3

He* and 3H giving a false apparent age that would decrease 

with greater amount of 3

He loss. Apparent age interpretation 

for this well should be questioned.

Apparent Age and Flow Patterns

Apparent age distribution across the study area generally 

follows a consistent pattern with the youngest samples coming 

from the recharge zone of the Edwards aquifer and the older 

samples associated with the confined section of the aquifer. 

From the four samples in the recharge zone, the apparent age 

distribution is from less than 0.5 years to 10.9 years and within 

the confined zone age ranging from 10 to 34.7 years.

A contour of the age data is superimposed on the groundwater flow vectors obtained from the dataset in figure 2. The 

apparent age contour is constructed from the pumped samples, 

as well as the uppermost sample taken from the discretely 

sampled monitoring wells (fig. 6). Figure 6 shows the relations 

between the apparent age data and assumed groundwater flow 

pattern in a regional context. Vectors from the potentiometric 

surface should parallel apparent age gradients, indicating that 

the longer the flow path, the longer the residence time, and the 

older the water.

One notable feature within the age contour is a trend 

of young apparent ages along the axis of the Uvalde Salient. 

Apparent ages from samples LVN and A&M generate a ridge 

of young groundwater in the contour that runs in a north-south 

trend. This incursion of younger water with the salient may 
20 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

Figure 6. Contour of apparent age (3H-3He) with flow vectors derived from the dataset of Green, Bertetti, and others (2006) from figure 2. 

Age contours intervals (bold and normal solid lines) are in two year intervals.

CONNOR FAULT

AGAPE FAULT

COOKS FAULT

BLUE MOUNTAIN FAULT

CONNOR FAULT

COOKS FAULT

BLACK MOUNTAIN FAULT

BLUE MOUNTAIN FAULT

U

D

U

D

U

D

U

D

U

D

U

D

U

D

U

U

D

D

We st Nueces River

Nu e ces River

Leona River

Frio River

Blanco Creek

Sabin al River

7MH

UV09 A&M

EU4

KPA

EU1

EU2

SAB

UVI

7MH

UV09

UV11

A&M

EU4

KPA

ANN

LVN

EU1

EU2

SAB

BOW

NUC

UVI

2 6

10

14

18

10

22

14

26

6

22

14

26

30

10

2 6

10

14

18

10

22

18

14

26

6

22

14

26

18

30

10

127

55 90 83

EXPLANATION

Edwards aquifer outcrop

Igneous intrusions

Faults

Sampled wells

1,000 mg/L

3,000 mg/L

Groundwater flow vector

 based on data from

 Green and others, 2006 EU4

Map base from U.S. Geological Survey digital data 0 5 KILOMETERS

0 5 MILES

Uvalde

Knippa

Sabinal

29°05'25.3"

99°27'14.7"

29°32'21.8"

100°00'32.2"

be related to enhanced hydraulic conductivity associated with 

an unknown structure. Flow vectors mimic this trend, flowing 

southward until reaching a potentiometric trough locally known 

as the “Knippa Gap” north of the city of Uvalde. Generally flow 

vectors follow the apparent age gradient; however, the presence of a groundwater high east of the city of Uvalde reverses 

groundwater flow direction against the apparent age gradient. 

The resultant vectors indicate that the groundwater is becoming younger with increasing flow path. This is not possible and 

probably results from a lack of data points in the groundwater 

trough. In terms of the regional scale flow system, the apparent 

age values correlate well to the groundwater flow representation 

as determined from the potentiometric surface.

Samples from the recharge zone of the Edwards aquifer 

show a consistent older age component (about 4 to 10 years) 

where samples are expected to be extremely young, that is 

associated with streambed infiltration. As noted previously, 

the ANN recharge temperature was cooler than expected for 

the area. The anomalous temperature was attributed to rapid 

infiltration associated with the nearby Frio River. The age 

would be expected to be less than 0.5 years (no resolvable 

3

He*) but instead the data for the sample give an apparent 

age of 3.2 years. Similar older ages are observed in NUC and 

BOW samples that share close proximity to the Nueces and 

Sabinal Rivers. This age anomaly could be explained by localized, subsurface discharge of modern groundwater from the 
References Cited 21

Trinity aquifer, providing an apparent mixed age associated 

with the interaction between the aquifers. Another explanation is that the waters associated with streambed infiltration 

are a result of groundwater discharge of modern groundwater 

in the catchments north of the Edwards aquifer recharge zone. 

The discharge waters do not re-equilibrate with the atmosphere rapidly and end up recharging the Edwards aquifer with 

excess 3

He*. Both arguments are probable but cannot be fully 

addressed with the present dataset.

Even though the data are sparse, a mass balance model 

can be inferred. Groundwater flow from the northwestern 

part of the study area is considered to be a part of the 

Uvalde “pool” of the Edwards aquifer (Green, Bertetti, 

and others, 2006). The “pool” is characterized by recharge 

associated with the Nueces, Frio, and Dry Frio River Basins 

and discharges into the San Antonio “pool” through the 

Knippa Gap. Green and others (2006) predicted that the 

amount of water moving through the gap was on the order 

of 270,000 acre-feet per year (acre-ft/yr) (approximately 

3.3 × 108

 m3/yr). Assuming the apparent age from sample KPA 

(14.3 years) represents the residence time of the water in the 

reservoir (Uvalde “pool”), the reservoir size is estimated to be 

3,861,000 acre-ft (approximately 4.7 × 109

 m3). Equating this 

value to saturated volume in the aquifer is difficult because 

of the sensitivity of the calculation to effective porosity and 

thickness of the aquifer, but does give a scale of storage associated within the segment of the aquifer. Beyond this type of 

simple calculation, further data would be needed to equate the 

validity of flux estimates as well as other possible sources of 

discharge from the aquifer.

Summary

3

H-3He ages of groundwater within the freshwater zone 

of the Edwards aquifer within Uvalde County show a strong 

modern component. The modern ages and dissolved gas 

compositions of the fresh groundwater reflect an active flow 

system, bounded by a more stagnant saline-water zone that 

is considerably older in age. Dissolved gas, thermal, and 

conductivity data support a zone of mixing occurring between 

the freshwater and saline-water zone, as well as mixing of 

premodern freshwater components of the aquifer. The nature 

of groundwater flow along this transition zone is complex 

because of structural controls and karst features of the aquifer. 

The older ages and presence of excess He in wells within the 

northern recharge zone suggest the possibility of subsurface 

discharge from the Trinity aquifer, and could be a factor in 

determining total recharge for the area. Vertical velocities 

from single monitoring wells support a point-sourced recharge 

from streambed infiltration. Overall, the trend in age distribution across the aquifer follows inferred flow patterns from 

the potentiometric surface and appears to support conceptual 

models of groundwater flow.

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Aquifer, northern Medina and northeastern Uvalde Counties, 

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Clark, A.K., 2003, Geologic framework and hydrogeologic 

characteristics of the Edwards Aquifer, Uvalde County, 

Texas: U.S. Geological Survey Water-Resource Investigation Report 2003–4010, p. 17.

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the Edwards Aquifer and upper confining unit, and hydrogeologic characteristics of the Edwards Aquifer, southcentral Uvalde County, Texas: U.S. Geological Survey 

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South Texas Geological Society Guidebook, p. 1–5.
22 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

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Appendix 23

Appendix

Geophysical logs from East Uvalde 2 well from November 2002 and November 2005, Uvalde Index well, Seven Mile Hill 

well, East Uvalde 1 well, and East Uvalde 4 well. Bottom of well casing (BOC) and water level depths are given in feet below 

ground surface. Logging parameters of temperature and specific conductance are in units of degrees Celsius (°C) and millisiemens per meter (mS/m), respectively. 

East Uvalde 2 logged 11–07–2002

Water level - 90.5 FBGS

Bottom of casing

30 35 40 45 50 55

Specific conductance,

in millisiements per meter

22.0 22.5 23.0 23.5 24.0 24.5 25.0

Temperature,

in degrees Celsius

1,000

1,100

Depth,

in feet below ground surface

1,200

1,300

1,400

1,500

1,600

1,000

1,100

Depth,

in feet below ground surface

1,200

1,300

1,400

1,500

1,600
24 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

East Uvalde 2 logged 11–15–2005

Bottom of casing

Water level 96.6 FBGS

Bottom of casing

Temperature,

in degrees Celsius

22.0 22.5 23.0 23.5 24.0 24.5 25.0

Specific conductance,

in millisiemens per meter

30 35 40 45 50 55

1,000

1,100

1,200

1,300

1,400

1,500

1,600

Depth,

in feet below ground surface

Depth,

in feet below ground surface

1,000

1,100

1,200

1,300

1,400

1,500

1,600
Appendix 25

Uvalde Index Well logged 11–16–2005

0 0

100

200

300

Depth,

in feet below ground surface

Water level - 22 FBGS

Bottom of casing

30 35 40 45 50 55

Specific conductance,

in millisiemens per meter

22.0 22.5 23.0 23.5 24.0 24.5 25.0

Temperature,

in degrees Celsius

100

200

300

400

Depth,

in feet below ground surface

400
26 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

Seven Mile Hill logged 11–17–2005

Water level - 99 FBGS Water level

30 35 40 45 50 55

Specific conductance,

in millisiemens per meter

22.0 22.5 23.0 23.5 24.0 24.5 25.0

Temperature,

in degrees Celsius

0

100

200

300

400

500

600

700

800

Depth,

in feet below ground surface

0

100

200

300

400

500

600

700

800

Depth,

in feet below ground surface
Appendix 27

East Uvalde 1 logged 11-07-2002

Water-level 65.5 FBGS

Bottom of casing

65 70 75 80 85 90 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5

Temperature,

in degrees Celsius

Specific conductance,

in millisiemens per meter

800

900

1,000

1,200

1,300

1,400

1,500

1,600

1,700

Depth,

in feet below ground surface

Depth,

in feet below ground surface

800

900

1,000

1,200

1,300

1,400

1,500

1,600

1,700
28 Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge

East Uvalde 4 logged 11–08–2002

Water level - 72 FBGS

Bottom of casing

200 250 300 350 400 450 500

Specific conductance,

in millisiements per meter

31 32 33 34 35 36

Temperataure,

in degrees Celsius

900

1,000

1,100

1,200

1,300

1,400

1,500

Depth,

in feet below ground surface

900

1,000

1,100

1,200

1,300

1,400

1,500

Depth,

in feet below ground surface
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For more information concerning this publication, contact:

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Or visit the Crustal Geophysics and Geochemistry Science Center Web site at:

http://crustal.usgs.gov/

This publication is available online at:

http://dx.doi.org/10.3133/sir20155163
Hunt and others—Groundwater Ages from the Freshwater Zone of the Edwards Aquifer, Uvalde County, Texas—Insights into Groundwater Flow and Recharge—SIR 2015–5163

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155163