60

70

Bracketville

Hondo

KINNEY COUNTY

EDWARDS COUNTY

 UVALDE COUNTY

Dripping

Springs

San

Marcos

Castroville

Lytle

Natalia

Devine

MEDINA COUNTY

Kyle

Buda

New 

Braunfels

San Antonio

BEXAR

COUNTY

COMAL

COUNTY

HAYS

COUNTY

Uvalde

Ground-water divide

1

Geologic map compilation

of the Edwards recharge zone

SIM 2005–2873

1

Infiltration potential map of

Comal County

OFR FY 07

2

HEM Survey of

Northern Bexar County

OFR 2005–1158

1

Hydrostratigraphic map of

Northern Bexar County 

OFR FY 07

1

Geologic map compilation of

Upper Seco Creek

OFR 2004–1430

4

3-D subsurface model of

Upper Seco Creek

SIR FY 07

2

HEM Survey of

Upper Seco Creek

OFR 2003–0226

2

Aeromagnetic survey of

Medina and Uvalde Counties

OFR 2002–0049

Occurrence of

Shallow Igneous Rocks

in Uvalde and

Medina Counties

SIR FY 07

3

Argon dating of

mafic intrusive rocks in

Uvalde and Kinney Counties

OFR 2004–1031

4

3-D subsurface

geologic model of the

Edwards recharge zone

SIR 2004–5226

2

AMT Surveys of

fresh-/saline-water interface

OFR 2002–0118

5

Noble gas analyses of the

fresh-/saline-water interface

SEE REFERENCES 

1

Geologic map of the

Edwards recharge zone in

Kinney County

SIM FY 07

1

Geologic map of

North Medina and 

NE Uvalde Counties

OFR 2006–1372

Rio Grande 

Medina

Lake

GuadalupeRiver Canyon

Lake

Braunig

Lake

Calaveras

Lake

Ground-water divide

29 ̊30’

30 ̊

100 ̊

99 ̊

98 ̊

1 GEOLOGIC MAPPING

2 GEOPHYSICAL SURVEYS

3 GEOCHRONOLOGY

4 THREE-DIMENSIONAL MODELING

5 NOBLE GAS GEOCHEMISTRY

PROJECT RESEARCH AREAS

Mapped recharge zone

Unmapped recharge zone

Balcones fault zone

Normal faults

County boundaries

Streams and rivers

Fresh-/saline-water 

interface

Mafic intrusive rocks

EXPLANATION

0 5 10 20 MILES

OFR USGS Open-File Report

SIR USGS Scientific

 Investigations Report

SIM USGS Scientific

 Investigations Map

1

2

3

4

5

2

3

4

5

UVALDE COUNTY MEDINA COUNTY

BEXAR COUNTY

COMAL COUNTY

HAYS

COUNTY

TRAVIS COUNTY

Balcones

 fault 

 zone

Uvalde

salient

Edwards recharge zone

SAN MARCOS PLATFORM

MAVERICK

BASIN

DEVILS RIVER TREND

100 ̊ 99 ̊

98 ̊

29 ̊30’

 Geologic Mapping 1 GEOLOGIC MAPPING

The complex geology of the recharge zone, as defined by the Texas 

Commission on Environmental Quality (TCEQ), includes lithologic 

units assignable to the Lower Cretaceous Edwards Group, which is 

underlain by the Glen Rose Limestone (lower confining unit) and 

overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and 

Eagle Ford and Austin Groups (upper confining units). 

The geology of the Edwards aquifer in the northeastern part of the 

recharge area is characterized by the Kainer, Person, and Georgetown 

Formations, which are subdivided into eight informal hydrostratigraphic units. In the area west of San Antonio in Medina, Uvalde, and 

Kinney Counties, significant facies changes (fig. 4) exist across the 

Devils River trend reefal facies (Devils River and Georgetown Formations) and into the deeper water Maverick Basin facies (West Nueces, 

McKnight, and Salmon Peak Formations). 

Past and present 1:24,000-scale geologic mapping in the Edwards 

aquifer area by the USGS Texas Water Science Center was conducted county-by-county but was never compiled at a regional scale. 

Compilation efforts for this project began in late 2002 and resulted 

in the publication of U.S. Geological Survey Scientific Investigations 

Figure 3.  Project research

tasks and published products

through FY 07.

Figure 4.  Distribution of depositional

facies across the Edwards aquifer.

Map 2873 (http://pubs.usgs.gov/sim/2005/2873/). This compilation 

is the first effort of its kind to digitally synthesize the geology of the 

Edwards recharge zone. 

Geologic mapping of four-quadrangles along the upper Seco 

Creek area, Medina and Uvalde Counties (http://pubs.usgs.gov/

of/2004/1430/), represents a digital compilation of work by the Texas 

Bureau of Economic Geology and contains new geologic interpretations for the southern two quadrangles. This map, as well as 

geologic mapping to the east in Medina County (http://pubs.usgs.

gov/of/2006/1372), has been used to provide the digital geologic framework used in a helicopter electromagnetic (HEM) geophysical survey 

of the area as well as for ongoing fracture and three-dimensional (3-D) 

EarthVisionTM (EV) modeling. 

One of the primary goals of the project is to produce infiltrationpotential assessment maps for critical parts of the Edwards recharge 

area. An infiltration-potential assessment model for Comal County, 

which includes a 10-meter Digital Elevation Model (DEM) grid and 

numerical values for hydrostratigraphic units, faults and fractures, karst 

features, soils, vegetation, fracture density, and basin accumulation 

indices, is ongoing.

 Geophysical Surveys

The presence of volcanic intrusive rocks in south-central Texas has 

been known since the earliest surveys. A large exposure of volcanic rocks, 

sometimes called the Uvalde igneous field, is centered in Uvalde County and 

extends west into Kinney County, south into Zavala County, and north and 

east as far as the city of Austin. The rocks comprising the Edwards aquifer 

and the lower and upper confining units are essentially nonmagnetic, with 

magnetic susceptibilities on the order of 0-5 × 10-5 (SI units). In contrast, the 

igneous rocks have susceptibilities of 200–3,000 × 10-5 SI. New occurrences 

of intrusive bodies at both surface and subsurface levels were unveiled by the 

2001 aeromagnetic survey (http://pubs.usgs.gov/of/2002/ofr-02-0049/) which 

reveals how the intrusives may control the unique morphology of the Edwards’ 

major flow paths (fig. 5). 

Another geophysical method used in studying the subsurface geohydrology 

of the Edwards and Trinity aquifers is audio-magnetotelluric sounding or AMT 

(http://pubs.usgs.gov/of/2002/of02-118/). AMT is an electrical technique that 

uses either natural signals or a controlled transmitter to measure earth conductivity as a function of depth.

The AMT sounding locations were specifically selected to resolve a number 

of key geological issues, including a volcanic plug along the Frio River near the 

town of Knippa in Uvalde County. Additional AMT studies conducted in and 

around selected transect wells near Kyle, Texas, mapped the fresh-/saline-water 

interface (fig. 3). 

A helicopter electromagnetic and magnetic (HEM) survey (http://pubs.usgs.

gov/of/2003/ofr-03-226/) was completed in 2002 for a 209-square-kilometer (81-

square-mile) area in the Seco Creek drainage area, Medina and Uvalde Counties. 

The surface geology can be viewed at http://pubs.usgs.gov/of/2004/1430/. The 

survey area was centered on Woodard Cave (Valdina Farms sinkhole), a significant karst feature in northwestern Medina County. The primary objective of the 

survey was to image the subsurface electrical resistivity of select geologic features 

that control the area’s ground-water resources. 

1

2

3

4

5

4

5

600

-400

5,000

Kyle well no. 1

Edwards Aquifer

Edwards Aquifer

24°C

25°C

27°C

26°C

Kyle well no. 2 Kyle well no. 3 Kyle well no. 4

0

-200

Conductance

millisiemens/meter

0

0

500

1,000

1,500

2,000

2,500

3,000

3,500

400

10,000

HORIZONTAL DISTANCE (FEET)

ELEVATION IN FEET (0 IS APPROXIMATE MEAN SEA LEVEL)

15,000 20,000

200

Del Rio Clay

Glen Rose Limestone

Del Rio Clay

Glen Rose Limestone

5 NOBLE GAS GEOCHEMISTRY

2

3

4

5

3 GEOCHRONOLOGY

3255000

UTM COORDINATES

3260000

3265000

472000

464000

UTM COORDINATES

4 THREE-DIMENSIONAL MODELING

Figure 9.  Diagram showing conductance (millisiemens/meter) and fault displacement

near Kyle, Texas.

 Three-Dimensional Modeling

Three-dimensional (3-D) geologic modeling of aquifers can help 

to quantitatively evaluate the connectedness of hydrostratigraphic 

units across fault and fracture zones and to estimate the distribution 

of geologic units and structures in the subsurface. Geologic 3-D 

framework modeling is also useful for visualizing features within 

fault zones and the interactions of en echelon fault strands and 

flexed, relay ramps. All of these parameters are complex variables 

that reflect original depositional conditions and subsequent alteration 

and dislocation. The following 3-D models have helped project staff 

to evaluate some of the geologic processes controlling the Edwards 

and Trinity aquifers. 

An interactive 3-D EarthVisionTM (EV) model of the northern 

Bexar County area (http://pubs.usgs.gov/sir/2004/5226/) reveals the 

subsurface geology of the Edwards and Trinity aquifers where water 

wells are 200-1,000 feet or more in depth. This model is based on 

mapped geologic relationships that reflect the: (1) Balcones faultzone structures, (2) detailed interpretations of 40 principal wells, and 

(3) geometry of the Edwards Group hydrostratigraphic units.

A similar 3-D EV model of the north Seco Creek area (Medina 

and Uvalde Counties) is ongoing. This model (fig. 8) is being 

built using a variety of digital datasets, including: (1) the current 

geologic map (http://pubs.usgs.gov/of/2004/1430/), (2) detailed 

lithologic descriptions and interpretations from 40 drill holes, and 

(3) helicopter electromagnetic geophysical data.

 Noble Gas Geochemistry

Noble gases have proven to be excellent tracers 

for ground-water studies, and they can help to recognize ground-water origins and water/rock processes. 

The very rapid ground-water movement through the 

Edwards aquifer is controlled by two major flow 

paths: the western Medina flow path and the eastern 

flow path. Within both flow paths, the fresh-water 

zone of the aquifer is bounded to the south and 

southeast by the transitional downdip saline zone. 

This interface (fig 3), locally referred to as the “badwater line,” is arbitrarily defined where aquifer waters 

exceed 1,000 mg/L of total dissolved solids (TDS). 

Fluid logging and discrete isotope and noble gas 

sampling were conducted in four transect wells across 

the fresh-/saline-water interface in Uvalde, Medina, 

Bexar, Guadalupe, and Hays Counties (Lambert and 

others, 2003). The fluid logs obtained from the monitoring transect wells show a distinct interface in the 

eastern part of the study area, such as near Kyle (fig. 

9). In the western transect wells, the interface appears 

to be more gradational. The dissolved gas samples 

also indicate that the saline-zone water is much older 

than previously thought. 

The helium isotopic (3He and 4He) data from 

the Kyle transect wells tell an interesting story. 

Measured isotopic compositions of the excess

helium in the saline waters of the Edwards

aquifer have a composition of 0.22 ± 0.02 R/RA (R/RA is the sample 3

He to 4He ratio normalized to the present-day atmospheric ratio of helium). This homogeneous isotopic 

composition suggests a uniform source reservoir of 

excess helium associated with the aquifer. Measured 

helium compositions from Kyle 3 and 4 wells are 

similar to those of the other saline samples of the 

Edwards aquifer (0.23 ± 0.01 R/RA), but excess 

helium values from Kyle 1 and 2 wells (freshwater and 

fresh-/saline-water transition zone) are on the order 

of 0.14 R/RA (Hunt and others, 2003). This difference in isotopic composition suggests that the waters 

have a different origin and may be attributed to subsurface discharge of groundwater from the underlying Trinity aquifer. Major ion and stable isotope data 

also corroborate the

existence of a subsurface communication between the

aquifers as well as

dramatic salinity

changes in the

Kyle 2 well following a major recharge

event in the spring

of 2003.

Figure 8.  Three-dimensional EarthVisionTM model of the North Seco Creek area. Note the multiple faults

(shown in red) and the electromagnetic geophysical profiles (shown in blue) used to construct the model. 

Acknowledgments

Many thanks to W.R. Page, D.V. Smith, and 

T.W. Judkins who improved this Fact Sheet with 

thorough reviews.

References

Lambert, R.B., Hunt, A.G., Landis, G.P., and Waugh, J.R., 2003, Fluid logging 

and discrete sampling of the freshwater/saline-water interface of the 

Edwards aquifer, south-central Texas [abs.]: Implications for groundwater 

flow and origin of salinity (Part 1—Fluid logging): Geological Society of 

America, Abstracts with Programs, v. 35, no. 6, p. 280.

Hunt, A.G., Lambert, R.B., Waugh, J.R., and Landis, G.P., 2003, Fluid logging 

and discrete sampling of the freshwater/saline-water interface of the 

Edwards aquifer, south-central Texas [abs.]: Implications for groundwater 

flow and origin of salinity (Part 2—Geochemistry): Geological Society of 

America, Abstracts with Programs, v. 35, no. 6, p. 280.

By Charles D. Blome,

1 Jason R. Faith,2 and George B. Ozuna 3

1

U.S. Geological Survey, Denver, CO

2

U.S. Geological Survey, Stillwater, OK

3

U.S. Geological Survey, San Antonio, TX

Contacts: 

Charles D. Blome     George B. Ozuna

U.S. Geological Survey USGS Water Science Center

MS 980, Box 25046, DFC 5563 DeZavalla Road, Suite 290

Denver, CO 80225     San Antonio, TX 78249

cblome@usgs.gov

This fact sheet is available online at http://pubs.usgs.gov

Edited by F. Craig Brunstein

Publication design and digital layout by Carol A. Quesenberry

 Geochronology

The Uvalde igneous field consists of fine- to coarse-grained 

ultramafic and hypabyssal rocks that exist as dikes, plugs, and shallow intrusions. The five rock types identified in this field include 

alkali basalt, melilite-olivine nephelinite, olivine nephelinite, 

nepheline basanite, and phonolite.

A 2001 aeromagnetic survey (http://pubs.usgs.gov/of/2002/ofr02-0049/) detected over 200 shallow, igneous intrusive bodies; 

fewer than 20 had previously been mapped. The apparent random 

distribution of these igneous bodies raised questions as to whether 

they represent a single intrusive episode or multiple episodes.

A preliminary 40Ar/39Ar geochronologic study (http://pubs.usgs.

gov/of/2004/1031/) of various mineral separates and groundmass 

concentrates from a variety of rock types exposed in Uvalde County 

revealed two distinct age groups, one at approximately 82 to 80 Ma 

(million years ago) and the other at 74 to 72 Ma. Previous K/Ar 

ages had suggested that igneous activity spanned a range in age 

from 90 to 60 Ma. 

Figure 7.  Columnar joints in 80 to 82 million-year-old melilite-olivine nephelinite exposed at the Knippa Traprock Quarry east of Uvalde. USGS photograph.