U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3078

September 2013

Prepared in cooperation with the San Antonio Water System

Borehole Geophysical, Fluid, and Hydraulic Properties

Within and Near the Freshwater/Saline-Water Transition Zone, 

San Antonio Segment of the Edwards Aquifer,

South-Central Texas, 2010–11

Printed on recycled paper

Introduction

The freshwater zone of the San Antonio segment of the Edwards aquifer is used by residents of San Antonio and 

numerous other rapidly growing communities in south-central Texas as their primary water supply source (fig. 1). This 

freshwater zone is bounded to the south and southeast by a saline-water zone with an intermediate zone transitioning from 

freshwater to saline water (transition zone). As demands on this water supply increase, there is concern that the transition 

zone could potentially move, resulting in more saline water in current freshwater supply wells. Since 1985, the U.S. 

Geological Survey (USGS), San Antonio Water System (SAWS), and other Federal and State agencies have conducted 

studies to better understand the transition zone.

San Antonio

New

Braunfels

Hondo

San

Marcos

Uvalde

Devine

Brackettville

Kyle

Comal Springs

San Marcos Springs

A

A'

B

B'

C C'

D'

D

10,000 mg/L

3,000 mg/L

3,000 mg/L

10,000 mg/L

1,000 mg/L

TRINITY AQUIFER

(Updip of Edwards aquifer)

BEXAR

ATASCOSA

COUNTY

WILSON

 COUNTY

MEDINA

COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY BANDERA COUNTY

COMAL

 COUNTY

KENDALL

COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO

COUNTY

KINNEY

COUNTY GUADALUPE

COUNTY

CALDWELL

COUNTY

MAVERICK

COUNTY

EU4 EU3

EU2 EU1

East Uvalde transect 

TC5

TC4

TC3

TC2

TC1

Tri-County transect 

FH2 FH1

Fish Hatchery

transect

KY4 KY3

KY1KY2

Kyle transect 

Recharge zone (Ashworth and Hopkins, 1995)

Freshwater zone

Transition zone

Saline-water zone

Groundwater divide (modified from Maclay and Land, 1988, fig. 1; Maclay, 1995, pl. 1)

Freshwater/saline-water interface—1,000 milligrams per liter [mg/L] dissolved

 solids concentration (Schultz, 1994)

Slightly to moderately saline-water interface—3,000 mg/L dissolved solids

 concentration (Schultz, 1994)

Moderately to very saline-water interface—10,000 mg/L dissolved solids

 concentration (Schultz, 1994)

Line of section

Well and identifier

Spring and identifier

EXPLANATION

EU2

Comal Springs

A'A

T E X A S

OUACHITA STRUCTURAL BELT

Paleogeographic and structural

features modified from

Barker and Ardis, 1996, fig. 6

MAVERICK

BASIN

DEVILS RIVER

TREND

ANCESTRAL GULF OF MEXICO

PRESENT–DAY

GULF OF MEXICO

SAN MARCOS 

PLATFORM

BALCONES

FAULT ZONE

28°30'

29°00'

29°30'

30°00'

100°30'

100°00'

99°30'

99°00'

98°30'

98°00'

97°30'

0 15 30 45 60 MILES

0 3015 45 60 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:250,000

Universal Transverse Mercator projection, zone 14

North American Datum of 1927

Figure 1. Areal extent of the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer,

south-central Texas, and locations of monitoring wells within and near the transition zone from which data were collected for

this report, 2010–11 (modified from Lambert and others, 2010, fig. 1). 

Figure 1. Areal extent of the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central 

Texas, and locations of monitoring wells within and near the transition zone from which data were collected for this report, 2010–11 (modified 

from Lambert and others, 2010, fig. 1).

Borehole Geophysical, Fluid, and Hydraulic Properties

600

700

800

900

1,000

Formation

Person Formation

Kainer Formation

Glen Rose Limestone

15

Georgetown

Kirschberg

Evaporite

member 

Cyclic and

marine members 

Leached and

collapsed

members 

Regional

dense member

Grainstone

member

Dolomitic

member

Basal nodular

member

API, American Petroleum Institute; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; gal/min, gallons per minute; TDS, total dissolved solids; Calculated amb sp cond, Specific conductance logs calculated from fluid resistivity logs 

collected with the electromagnetic (EM) flowmeter probe under ambient conditions; Calculated pump sp cond, Specific conductance logs calculated from fluid resistivity logs collected with the EM flowmeter probe under pumped conditions; 

Calculated TDS, estimated total dissolved solids profile calculated from the specific conductance log; LSD, land-surface datum 

gal/min

Figure 15. Borehole geophysical data from Tri-County transect well TC2 (KX–68–31–403), San Antonio segment of the Edwards aquifer, south-central Texas, 2003–10.
During 2010 and 2011, the USGS, in cooperation 

with SAWS, conducted a study to further assess the 

potential for movement of the transition zone in 

part of the San Antonio segment of the Edwards 

aquifer (Thomas and others, 2012; fig. 1). Equivalent 

freshwater heads were computed to investigate the 

transition from freshwater to saline-water zones in the 

San Antonio segment and evaluate the potential for 

lateral flow at the freshwater/saline-water interface. 

Data were collected within and near the transition 

zone from 15 monitoring wells in four transects (East 

Uvalde, Tri-County, Fish Hatchery, and Kyle; fig. 1).

Hydrogeologic Setting

The San Antonio segment of the Edwards aquifer 

(the study area) is about 175 miles long and extends from 

the western groundwater divide near Brackettville in 

Kinney County to the eastern groundwater divide near 

Kyle in Hays County (fig. 1). From its outcrop (recharge 

zone), the Edwards aquifer dips to the southeast at about 

300–400 feet per mile and becomes buried and confined 

toward the present-day Gulf of Mexico coastline. From 

its outcrop immediately north of the Edwards aquifer 

recharge zone, the Trinity aquifer dips to the southeast 

beneath the Edwards aquifer, thus forming the northern 

lateral boundary and the underlying boundary of the 

Edwards aquifer. 

The present-day Edwards aquifer formed along a 

crustal zone of weakness known as the Ouachita structural 

belt (Maclay, 1995) and consists of Cretaceous-age 

carbonate rocks of varying lithologies that were deposited 

in three depositional environments, or depositional 

provinces: the Maverick Basin, the Devils River Trend, 

and the San Marcos Platform (fig. 1). These depositional 

environments in part influence the transmissive and 

storage properties of the aquifer. 

The direction of groundwater flow is controlled 

partially by regional faulting (Maclay and Land, 1988). 

Once in the aquifer, groundwater generally moves 

downdip and then is directed by faults to the east and 

northeast toward Comal Springs and San Marcos 

Springs, major springs in the northeastern part of the 

San Antonio segment of the aquifer (fig. 1; Groschen, 

1994; Maclay, 1995). 

Description of Transects and Monitoring Wells

The monitoring wells that provided data for this 

report were drilled during 1997–2001 by SAWS. The four 

transects (East Uvalde, Tri-County, Fish Hatchery, and 

Kyle; fig. 1) consist of 2–5 wells per transect and were 

configured approximately perpendicular to and across the 

expected trace of the freshwater/saline-water interface. 

A well descriptor was applied to each well on the basis 

of water type in the borehole (freshwater, saline water, or 

interface [freshwater atop saline water]; fig. 2). 
FEET FEET

NAVD 88 NAVD 88

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

NAVD 88, North American Datum of 1988

Freshwater

Saline water

LR–67–02–105

 KY4

LR–67–02–106

KY3

LR–67–02–104

KY2

LR–67–01–311

KY1 

Freshwater

Interface Saline water Saline water

LAND SURFACE D D'

Formation

Upper

Cretaceous

Stratigraphic unit

Person

Formation

Edwards aquifer

Upper

confining

unit

Trinity

aquifer

System

Hydrogeologic

unit

Lower Cretaceous

Kainer

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg Evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

Edwards Group

Well—Numbers at top are State well number and identifier

Fault—Arrows show relative movement (Ewing, 1991;

 Collins and Hovorka, 1997)

Formation contact—Dashed where inferred

Freshwater/saline-water interface—Dotted pattern

 indicates saline water

EXPLANATION

VERTICAL SCALE GREATLY EXAGGERATED

0 3,125 6,250 FEET

0 1,000 2,000 METERS

Kyle

HAYS COUNTY

D

D'

0 6,250 FEET 3,125

0 2,000 METERS 1,000

LR–67–02–105

KY4

LR–67–02–106

KY3 

LR–67–02–104

 KY2 LR–67–01–311

KY1

97°54’ 97°52’30” 97°51’

29°59’

29°58’

Freshwater zone

Transition zone

Saline-water zone

Groundwater divide (Maclay and Land,

 1988, fig. 1)

Freshwater/saline-water interface—

 1,000 milligrams per liter (mg/L) dissolved

 solids concentration (Schultz, 1994)

Slightly to moderately saline-water interface—3,000 mg/L

 dissolved solids concentration (Schultz, 1994)

Moderately to very saline-water interface—10,000 mg/L

 dissolved solids concentrations (Schultz, 1994)

Line of section

Fault (Ewing, 1991; Collins and Hovorka, 1997)

Well, State well number, and identifier

LR–67–02–104

KY2

!

EXPLANATION

D D'

Figure 2. Hydrogeologic section of the Kyle transect (D–D’), San Antonio segment of the Edwards aquifer, south-central Texas

(modified from Lambert and others, 2010, fig. 8). 

Figure 2. Hydrogeologic section of the Kyle transect (D–D’), San Antonio segment of the Edwards aquifer, south-central Texas (modified from 

Lambert and others, 2010, fig. 8).

Borehole Geophysical, Fluid, and

Hydraulic Properties

Borehole geophysical data such as natural gamma, 

formation resistivity, and caliper are commonly used to 

characterize and identify stratigraphic units. These data 

were collected by the USGS at all 15 transect wells in 

the study area during a previous study (Lambert and 

others, 2009) and utilized to determine the stratigraphy 

of each well. Optical and acoustic televiewer logs were 

also collected and used to confirm the tops and bases 

of hydrostratigraphic subdivisions and assess voids and 

faulting identified in the rocks intersecting each well (fig. 

3). In 2010, to further assess the potential for movement 

of the transition zone, electromagnetic (EM) flowmeter 

and multiparameter fluid logs that directly measured 

specific conductance and temperature were collected from 

13 transect wells. The EM flowmeter logs were collected 

under ambient (nonpumping) and stressed (pumping) 

hydraulic conditions to assess the hydraulics of flow within 

the aquifer. Water-level data provided hydraulic-head data 

that were used to interpret borehole geophysical data. 

Fluid property logs were used as indicators of possible 

flow zones, as calculations of equivalent freshwater head, 

and as a characterization of the borehole fluid. Borehole 

fluid was classified on the basis of total dissolved solids 

(TDS) concentration. To correlate specific-conductance 

values with TDS concentrations, observations of specific 

conductance and TDS concentration from Lambert and 

others (2009) were related by regression to yield threshold 

values of specific conductance corresponding to the 

threshold values of TDS concentrations that describe 

freshwater and categories of saline water (slightly, 

moderately, or very saline; fig. 4). 

Changes between ambient and stressed calculated 

specific-conductance logs were analyzed to identify 

possible vertical or horizontal flow zones, to identify 

direction of flow, and to establish the relation between the 

ambient water in the borehole and the formation water 

entering the borehole during pumping. Flow-Log Analysis 

of Single Holes (FLASH) is a spreadsheet-based graphical 

user interface that supplies multilayered Thiem modeling 

results for steady-state flow of a borehole (Day-Lewis and 
0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 22,500 25,000 27,500 30,000

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

0

20,000

Total dissolved solids (TDS), in milligrams per liter (mg/L)

2,500

5,000

7,500

10,000

12,500

15,000

17,500

Freshwater zone

Transition zone

Saline-water zone

Observation

EXPLANATION

y = 0.6522x + 25.77,

 where

 y equals total dissolved solids;

 x equals specific conductance.

Coefficient of determination R2 = 0.9914

Moderately to very saline-water

interface = 10,000 mg/L TDS

Slightly to moderately saline-water interface = 3,000 mg/L TDS

Freshwater/saline-water interface = 1,000 mg/L TDS

Figure 4. Observations of specific conductance and total dissolved solids (TDS) concentration related by regression to yield

threshold values of specific conductance corresponding to the threshold values of TDS concentrations that describe freshwater

and categories of saline water, 1999–2007.

Figure 4. Observations of specific conductance and total dissolved solids (TDS) concentration related by regression to yield threshold values 

of specific conductance corresponding to the threshold values of TDS concentrations that describe freshwater and categories of saline water 

(slightly, moderately, or very saline),1999–2007 (modified from Thomas and others, 2012, app. 2).

300

400

500

600

700

800

Georgetown Formation Kainer Formation

Glen Rose 

Limestone

0 API 100

2 inch 12 800 μS/cm 1,300

-15 15

-15 15

0°

22 degree Celsius 28

600 mg/L 850

800 μS/cm 1,300

800 μS/cm 1,300

0° 90° 180° 270° 0°

Increasing

-0.5 0.5

-0.5 0.5

0° 90° 180° 270° 0°

Person Formation

Kirschberg

Evaporite

member 

Cyclic and

marine members 

Leached and

collapsed

members 

Regional

dense member

Grainstone

member

Dolomitic

member

Basal nodular

member

API, American Petroleum Institute; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; gal/min, gallons per minute; TDS, total dissolved solids; Calculated amb sp cond, Specific 

conductance logs calculated from fluid resistivity logs collected with the electromagnetic (EM) flowmeter probe under ambient conditions; Calculated pump sp cond, Specific conductance logs 

calculated from fluid resistivity logs collected with the EM flowmeter probe under pumped conditions; Calculated TDS, estimated total dissolved solids profile calculated from the specific 

conductance log; LSD, land-surface datum referenced to the North American Vertical Datum of 1988; °, degree.

Caliper Specific conductance Natural gamma 

Temperature

Calculated amb sp cond

Calculated pump sp cond

Calculated TDS

Ambient flow model

Ambient flow stations

Pumping flow model

Pumping flow stations

Natural gamma Borehole Transmissive zone Stratigraphic unit Optical image Acoustic amplitude image Depth

(feet

below

 LSD)

gal/min

gal/min

gal/min

gal/min

Figure 3. Borehole geophysical data from Kyle transect well KY2 (LR–67–02–104), San Antonio segment of the Edwards aquifer,

south-central Texas, 2003–10 (modified from Thomas and others, 2012, fig. 19).

Figure 3. Selected borehole geophysical data from Kyle transect well KY2 (LR–67–02–104), San Antonio segment of the Edwards aquifer, southcentral Texas, 2003–10 (modified from Thomas and others, 2012, fig. 19).
595.33 584.85 590.69 588.00

T = 149

T = 23.6

T = 61.5

T = 62.8

T = 68.0

T = 218

T = 67.4

T = 17.7

T = 65.0

T = 36.4

T = 225

T = 39.2

T = 36.6

T = 150

T = 2,550

T = 166

T = 164

T = 562

D D'

Freshwater

Saline water

LAND SURFACE

LR–67-01-311

KY1

Freshwater

LR–67-02-104

KY2

Interface

LR–67-02-106

KY3

Saline water

KY4

LR–67–02–105

Saline water

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

FEET FEET

NAVD 88 NAVD 88

NAVD 88, North American Datum of 1988 0 3,125 6,250 FEET

0 1,000 2,000 METERS

VERTICAL SCALE GREATLY EXAGGERATED

Well—Numbers at top are State well number

 and identifier

Fault—Arrows show relative movement

 (Ewing, 1991; Collins and Hovorka, 1997)

Formation contact—Dashed where inferred

Freshwater/saline-water interface—Dotted pattern

 indicates saline water

Equivalent freshwater head and altitude

 (North American Vertical Datum of 1988)

Gradient of equivalent freshwater heads

Transmissive zone of well—Value is transmissivity

 of the zone, in feet squared per day

Ambient borehole flow and direction

T = 250

598.01

Formation

Stratigraphic unit

Person

Formation

Kainer

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg Evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

Edwards Group

Edwards

aquifer

Trinity

aquifer

Upper

confining

unit

Upper

Cretaceous

System Hydrogeologic

unit

Lower

Cretaceous

EXPLANATION

LR–67–02–104

KY2

EXPLANATION

D D'

Freshwater zone

Transition zone

Saline-water zone

Groundwater divide (Maclay and Land,

 1988, fig. 1)

Freshwater/saline-water interface—

 1,000 milligrams per liter (mg/L)

 dissolved solids concentration 

 (Schultz, 1994)

Slightly to moderately saline-water interface—

 3,000 mg/L dissolved solids concentration 

 (Schultz, 1994)

Moderately to very saline-water interface—

 10,000 mg/L dissolved solids concentration 

 (Schultz, 1994)

Line of section

Fault (Ewing, 1991; Collins and Hovorka, 1997)

Well, State well number, 

 and identifier

97°54' 97°52'30" 97°51'

29°59'

29°58'

D

D' LR–67–02–105

KY4

LR–67–02–106

KY3

LR–67–02–104

KY2

LR–67–01–311

KY1

Kyle

HAYS COUNTY

0 3,125 6,250 FEET

0 1,000 2,000 METERS

Figure 5. Conceptual diagram showing ambient flow, transmissive zones, and equivalent freshwater heads in monitoring wells in the

Kyle transect, San Antonio segment of the Edwards aquifer, south-central Texas, 2010 (modified from Thomas and others, 2012, fig. 26).

Figure 5. Conceptual diagram showing ambient flow, transmissive zones, and equivalent freshwater heads in monitoring wells in the Kyle 

transect, San Antonio segment of the Edwards aquifer, south-central Texas, 2010 (modified from Thomas and others, 2012, fig. 26).

others, 2011). FLASH modeling results provided estimates 

of the differences between the open-hole water level under 

ambient and stressed conditions, transmissivities, and 

hydraulic heads for two or more water-producing (flow) 

zones intersecting a single borehole.

Hydraulics of Lateral Flow

Hydraulic heads in the aquifer primarily change in 

response to changes in recharge from rainfall and changes in 

nearby groundwater pumping. In karst systems such as the 

Edwards aquifer, changes in hydraulic heads can be abrupt, 

prolonged, or both (Wong and others, 2012). These changes 

in hydraulic heads were assessed to indicate to what extent 

transect wells were hydraulically connected to each other.

Equivalent freshwater heads define hydraulic gradients 

horizontally, and environmental-water heads define 

hydraulic gradients vertically (Lusczynski, 1961). Changes 

in hydraulic heads were used to evaluate lateral-head 

gradients and thus the potential for movement of water 

from the saline zone into the freshwater zone (fig. 5). 

Because saline water is slightly denser than freshwater, 

higher salinity of water causes a greater difference 

between the environmental-water head relative to the 

equivalent freshwater head. This correction was then used 

to convert measured water-level altitudes to equivalent 

freshwater-head altitudes. The direction of lateral-head 

gradients across the freshwater/saline-water interface was 

used to evaluate the potential for lateral flow across the 

freshwater/saline-water interface relative to the freshwater 

zone in the East Uvalde, Tri-County, and Kyle transects 

(into the freshwater zone, out of the freshwater zone, 

or mixed with regard to direction [head higher or lower 

at the freshwater/saline-water interface than on either 

side]). Lateral-head gradients were not computed for the 

Fish Hatchery transect because of the relatively large 

difference in altitude between the open-hole sections 

of wells FH1 and FH2, caused by fault offset, and the 

relative shallowness of well FH1.
This fact sheet is based on Thomas and others, 2012.

References

Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: Texas 

Water Development Board Report 345, 69 p.

Barker, R.A., and Ardis, A.F., 1996, Hydrogeologic framework of the 

Edwards-Trinity aquifer system, west-central Texas: U.S. Geological 

Survey Professional Paper 1421–B, 61 p.

Collins, E.W., and Hovorka, S.D., 1997, Structure map of the San 

Antonio segment of the Edwards aquifer and Balcones fault zone, 

south-central Texas—Structural framework of a major limestone 

aquifer, Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties: 

Austin, The University of Texas, Bureau of Economic Geology 

Miscellaneous Map 38, scale 1:250,000.

Day-Lewis, F.D., Johnson, C.D., Paillet, F.L., and Halford, K.J., 2011: 

A computer program for flow-log analysis of single holes (FLASH): 

Ground Water, v. 49, no. 6, p. 926–931, accessed December 

8, 2012, at http://onlinelibrary.wiley.com/doi/10.1111/j.1745-

6584.2011.00798.x/pdf.

Ewing, T.E., 1991, The tectonic framework of Texas—The tectonic 

map of Texas: Austin, The University of Texas, Bureau of Economic 

Geology, 36 p. 

Groschen, G.E., 1994, Analysis of data from test-well sites along 

the downdip limit of freshwater in the Edwards aquifer, San 

Antonio, Texas, 1985–87: U.S. Geological Survey Water-Resources 

Investigations Report 93–4100, 92 p.

Lambert, R.B., Hunt, A.G., Stanton, G.P., and Nyman, M.B., 2009, 

Water-level, borehole geophysical log, and water-quality data from 

wells transecting the freshwater/saline-water interface of the San 

Antonio segment of the Edwards aquifer, south-central Texas, 1999–

2007: U.S. Geological Survey Data Series 403 [variously paged].

Lambert, R.B., Hunt, A.G., Stanton, G.P., and Nyman, M.B., 2010, 

Lithologic and physicochemical properties and hydraulics of flow 

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

segment of the Edwards aquifer, south-central Texas, based on 

water-level and borehole geophysical log data, 1999–2007: U.S. 

Geological Survey Scientific Investigations Report 2010–5122, 69 p.

Lusczynski, N.J., 1961, Head and flow of groundwater of 

variable density: Journal of Geophysical Research, v. 

66, no. 12, p. 4247–4256.

Maclay, R.W., 1995, Geology and hydrology of the Edwards aquifer 

in the San Antonio area, Texas: U.S. Geological Survey WaterResources Investigations Report 95–4186, 64 p.

Maclay, R.W., and Land, L.F., 1988, Simulation of flow in the 

Edwards aquifer, San Antonio region, Texas, and refinement of 

storage and flow concepts: U.S. Geological Survey Water-Supply 

Paper 2336–A, 48 p.

Schultz, A.L., 1994, Review and update of the position of the 

Edwards aquifer freshwater/saline-water interface from Uvalde to 

Kyle, Texas: Edwards Underground Water District Report 94–05, 

31 p.

Thomas, J.V., Stanton, G.P., and Lambert, R.B., 2012, Borehole 

geophysical, fluid, and hydraulic properties within and surrounding 

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

of the Edwards aquifer, south-central Texas, 2010–11: U.S. 

Geological Survey Scientific Investigations Report 2012–5285, 65 

p., 3 apps., http://pubs.usgs.gov/sir/2012/5285/.

Wong, C.I., Mahler, B.J., Musgrove, M., and Banner, J.L., 2012, 

Changes in sources and storage in a karst aquifer during a 

transition from drought to wet conditions: Journal of Hydrology, v. 

468–469, p. 159–172.

—By Jonathan V. Thomas and Gregory P. Stanton

For additional information, contact

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, Texas 78754–4501

http://tx.usgs.gov/