Three-dimensional representation of the 

conceptual model output.

Davis

Mountains

Barilla

Mountains

Glass

Mountains

Grandfalls

Pecos River Pecos River

PECOS

COUNTY

BREWSTER

COUNTY

PRESIDIO

COUNTY

REEVES

COUNTY

TERRELL COUNTY

CROCKETT

 COUNTY

JEFF DAVIS

COUNTY

UPTON COUNTY

WARD

COUNTY CRANE

COUNTY

285

10

20

10

285

Balmorhea

Toyahvale

Alpine

Fort

Davis

Belding Fort Stockton

Pecos

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000

Albers Equal-Area Conic projection, 

Texas State Mapping System

North American Datum of 1983

Aquifer modified from Ashworth and Hopkins, 1995. Physiographic provinces from N.M. Fenneman

 and D.W. Johnson, 1946; E.G. Wermund, 1996. Lithologic and geophysical data from

Daniel B. Stephens and Associates, 2010; Railroad Commission of Texas, 2010; Meyer

 and others, 2011; Texas Water Development Board, 2011; University Lands, 2011;

 and U.S. Geological Survey, 2011

San

Solomon

Spring

Santa Rosa

Spring

Leon Springs

Comanche

Springs

Diamond Y

 Spring

0

0 10

10 20 MILES

20 KILOMETERS

104°00’ 103°30’ 102°30’

31°00’

30°30’

103°00’

HIGH PLAINS

MEXICAN

HIGHLAND

PECOS VALLEY EDWARDS (STOCKTON) PLATEAU

BASIN AND RANGE PROVINCE

GREAT PLAINS PROVINCE

BALMORHEA

STATE PARK

EXPLANATION

Method of hydrostratigraphic picks

 Pick reviewed from lithological

 descriptions in well reports

 Pick made from interpreted borehole

 geophysical log

 Pick made from interpreted geophysical

 audio-magnetotelluric sounding

Spring

Edwards-Trinity aquifer subcrop

Edwards-Trinity aquifer outcrop

Conceptual model study area boundary

Leon-Belding area boundary

Section boundary within physiographic

 province

7

TEXAS

Study area

PECOS COUNTY

REGION

Pecos River

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3025

July 2014

Introduction

The Edwards-Trinity aquifer is a vital groundwater resource for agricultural, industrial, and public supply uses in the Pecos 

County region of western Texas (Barker and Ardis, 1992; Freese and Nichols, Inc., and LBG-Guyton, Inc., written commun., 2010). 

Resource managers would like to understand the future availability of water in the Edwards-Trinity aquifer in the Pecos County region 

and the effects of the possible increase or temporal redistribution of groundwater withdrawals. To provide resource managers with that 

information, the U.S. Geological Survey (USGS), in cooperation with the Middle Pecos Groundwater Conservation District, Pecos 

County, City of Fort Stockton, Brewster County, and Pecos County Water Control and Improvement District No. 1, completed a threephase study of the Edwards-Trinity and related aquifers in parts of Brewster, Jeff Davis, Pecos, and Reeves Counties (fig. 1). 

Figure 1. Location of study area and hydrostratigraphic data used to identify tops and bases of the hydrostratigraphic 

contacts in the Pecos County region, Texas (modified from Bumgarner and others, 2012, figs. 1 and 7).

Prepared in cooperation with the Middle Pecos Groundwater Conservation District, Pecos County, City of Fort Stockton, 

Brewster County, and Pecos County Water Control and Improvement District No. 1

A Multiphased Approach to Groundwater Assessments

for the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Printed on recycled paper
100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

Cretaceous P ermian

Comamche Ochoa Guadalupe

Fredericksbu

Trinity Whitehorse

Salado Yates Seven Rivers Queen Grayburg

Triassic

Tansill Edwards Trinity Sands

Limestone

Shale

Sandstone

Dolomitic 

anhydrite

 Salt and 

anhydrite

Sandstone 

 Anhydritic

 dolomite

Sandstone

 and shale

Anhydrite

Dolomitic

 shale

Dolomite

Sandstone 

 and shale Dockum

Rustler

Dewey Lake

Dolomitic

 shale

 MONUMENT 

DRAW TROUGH

PECOS

TROUGH

Glass Mountains

Davis Mountains

Barilla

Mountains

Pecos River

285

10

20

10

285

Base modified from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area Conic projection, Texas State Mapping System

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

31°00'

30°30'

102°00'

104°00' 103°00' 102°30' 103°30'

Base surface altitude of the Edwards-Trinity aquifer, in feet

 above North American Vertical Datum of 1988

High: 4,315 Low: 1,555

EXPLANATION

Edwards-Trinity aquifer 

 subcrop

Edwards-Trinity aquifer

 boundary

Conceptual model

 study area boundary

Fault zone—Represents

 numerous faults

Geologic structure

 boundary

County boundary

Pecos River

Glass Mountains

Barilla Mountains

FB01

FB02

FB03 FB04 FB05

FB06

FB09 FB08

FB10

FB07

Davis Mountains

285

10

20

10

285

Base modified from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area Conic projection, Texas State Mapping System

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

A

A'

B'

B

31°00'

30°30'

102°00'

103°00'

104°00'

102°30' 103°30'

EXPLANATION

Edwards-Trinity aquifer

 subcrop

Edwards-Trinity aquifer

 outcrop

Conceptual model

 study area boundary

Fault zone—Represents

 numerous faults

County boundary

Line of section

Fault block identifier

A A'

FB01

U.S. Geological Survey gage at Santa Rosa Spring near 

Grandfalls, Texas.

The first phase was to collect groundwater, surface-water, 

geochemical, geophysical, and geologic data in the study area 

and develop a geodatabase of historical and collected data 

(Pearson and others, 2012). Data compiled in the first phase 

of the study were used to develop the conceptual model in 

the second phase of the study (Bumgarner and others, 2012). 

The third phase of the study involved the development and 

calibration of a numerical groundwater-flow model of the 

Edwards-Trinity aquifer to simulate groundwater conditions 

based on various groundwater-withdrawal scenarios (Clark 

and others, 2014).

Analysis of well, geophysical, geochemical, and 

hydrologic data contributed to the development of the 

conceptual model in phase 1. Lithologic information 

obtained from well reports and geophysical data was used to 

describe the hydrostratigraphy and structural features of the 

groundwater-flow system, and aquifer-test data were used 

to estimate aquifer hydraulic properties. Geochemical data 

were used to evaluate groundwater-flow paths, water-rock 

interaction, aquifer interaction, and the mixing of water from 

different sources in phase 2. Groundwater-level data also were 

used to evaluate aquifer interaction, as well as to develop a 

potentiometric-surface map, delineate regional groundwater 

divides, and describe regional groundwater-flow paths. During 

phase 3, the data collected and compiled along with the 

conceptual information in the study area were incorporated 

into a numerical groundwater-flow model to evaluate the 

sustainability of recent (2008) and projected water-use 

demands on groundwater resources in the study area.

Hydrogeologic Framework

Subsurface data were obtained from well reports; natural 

gamma, electric, and electromagnetic induction borehole 

geophysical logs; and audio-magnetotelluric soundings. The 

subsurface data were used to map the top and base surfaces 

of the Edwards-Trinity aquifer (fig. 2), the top of the Trinity 

Group of the Edwards-Trinity aquifer, and the lateral and 

vertical relations of overlying and underlying aquifers where 

they occur in order to develop the hydrostratigraphy of the 

study area and evaluate structural features. 

In the study area, the Edwards-Trinity aquifer ranged in 

thickness from about 5 feet (ft) to about 797 ft. The aquifer 

is thinnest in the eastern part of the study area, near the 

northwestern slope of the Glass Mountains, and near the 

northeastern slope of the Davis Mountains. Some of the thickest 

sections of the Edwards-Trinity aquifer were in the Pecos, 

Monument Draw, and Belding-Coyanosa Trough areas.

Normal fault zones were delineated on the basis of 

interpretations of cross sections of the top and base surfaces 

of the Edwards-Trinity aquifer. Faults in the study area appear 

to have formed as growth and collapse features as sediments 

were deposited along the margins of more resistant rocks and 

structures, such as the Glass Mountains, and as overlying 

sediments collapsed into the voids created by the dissolution 

of Permian-age evaporite deposits. The fault zones delineate 

domains in the hydrogeologic framework that generally align 

with previously identified structural features. Each fault zone 

represents a series of parallel and transverse faults that result 

in an overall displacement between two adjacent fault blocks 

(fig. 3).

Figure 2. Base surface altitude of the Edwards-Trinity aquifer in the 

Pecos County region, Texas (modified from Bumgarner and others, 

2012, fig. 13).

Figure 3. Locations of fault zones and fault blocks within the 

study area, Pecos County region, Texas, delineated on the basis 

of the interpretation of multiple cross sections of interpolated 

hydrostratigraphic contacts (modified from Bumgarner and others, 

2012, fig. 17).
U.S. Geological Survey pump hoist truck and water-quality trailer.

Water-quality sampling by the U.S. Geological Survey, 

San Solomon Spring, Texas.

Geochemistry

Analysis of the geochemical and isotopic samples, from 

the study, provided insights into the chemical characteristics of 

water from different sources and different aquifers. Chemical 

characteristics of water from different sources were used 

to qualitatively evaluate aquifer interaction, groundwaterflow paths, water-rock interaction, and mixing of water from 

different sources and to identify likely source waters and 

geochemical endmembers (distinct water types).

Geochemical and isotopic results indicate that groundwater 

in the system likely is dominated by mineralized, regional 

groundwater flow that probably recharged during the cooler, 

wetter climates of the Pleistocene with variable contributions of 

more recent, local recharge. The mixing of water from multiple 

sources combined with water-rock interaction with various rock 

types, including siliciclastic, carbonate, evaporite, and igneous 

rocks, contributed to a groundwater chemistry that was complex 

and variable between and within aquifers.

Four endmembers were identified to use as part of 

the qualitative groundwater-flow and mixing analysis. The 

endmembers represented (1) mineralized groundwater that 

likely recharged northwest of the study area during the 

Pleistocene and is flowing through the Edwards-Trinity 

aquifer along regional groundwater-flow paths; (2) dilute, 

recent recharge from the Barilla and Davis Mountains with 

a composition indicative of interaction with igneous rocks; 

(3) dilute, recent recharge from the Glass Mountains with a 

composition indicative of interaction with carbonate rocks; 

and (4) mineralized groundwater that is likely a mixture of 

recharge under recent and Pleistocene climatic conditions and 

is flowing through the Edwards-Trinity aquifer along regional 

groundwater-flow paths east of the Monument Draw Trough. 

Inverse geochemical modeling (PHREEQC; Parkhurst and 

Appelo, 1999) was used to simulate the mixing of upgradient 

Edwards-Trinity aquifer groundwater, recharged in the Barilla, 

Davis, and Glass Mountains, with Rustler aquifer groundwater 

to approximate the composition of the groundwater. 

Geochemical model results indicate that Edwards-Trinity 

aquifer groundwater in the Leon-Belding and Fort Stockton 

areas is a mixture of groundwater recharged in the Barilla and 

Davis Mountains and groundwater that has upwelled from the 

Rustler aquifer. In the Leon-Belding area, the proportion of 

Rustler aquifer groundwater in the Edwards-Trinity aquifer 

increased downgradient from south (0–48.8 percent) to north 

(87.1–100 percent). 

Groundwater-Flow System

Groundwater-level and geochemical data were used in 

context with the hydrogeologic framework to assess regional 

groundwater-flow paths, recharge sources, and groundwater 

mixing and discharge in the study area. Groundwater-level 

altitudes used to generate the potentiometric-surface map of 

the Edwards-Trinity aquifer ranged from about 2,300 to about 

3,300 ft and generally decreased from southwest to northeast. 

Regional groundwater flow is from areas of recharge in the 

south and southwest to the north and northeast. Four principal 

sources of recharge to the Edwards-Trinity aquifer were 

identified: (1) regional groundwater flow in the EdwardsTrinity aquifer that originated as recharge northwest of the 

study area and enters the study area near the western corner; 

(2) runoff from the Barilla, Davis, and Glass Mountains that 

percolates through underlying rocks and into the gravels along 

the slopes of the mountains; (3) return flow from irrigation; 

and (4) upwelling from deeper aquifers. Although some of 

the groundwater appears to have recharged under conditions 

similar to the current climate, the only samples collected from 

the Edwards-Trinity aquifer that likely recharged during the last 

60 years (after the onset of atmospheric nuclear weapon testing) 

were collected from wells in mountain-front recharge areas and 

in areas receiving agricultural return flow.

Groundwater generally flows north into the downdip extent 

of the Edwards-Trinity aquifer or east out of the study area. 

Regional groundwater flow entering the study area from the 

northwest naturally discharges from springs or turns northward 

to flow into the Pecos Trough where it discharges into the 

Pecos Valley or Dockum aquifers at the downdip extent of 

the Edwards-Trinity aquifer. Recharge from the Barilla and 

Davis Mountains also predominantly flows toward the Pecos 

Trough and most likely naturally discharges to other aquifers 

in the groundwater system. Groundwater flow in the EdwardsTrinity aquifer in the Monument Draw Trough originated as 

recharge in the Glass Mountains, agricultural return flow, or 

upwelling groundwater from lower units. Edwards-Trinity 

aquifer groundwater generally flows north and northeast in 

the Monument Draw Trough and naturally discharges from 

springs or to other aquifers in the groundwater system at the 

downdip extent. Groundwater in the eastern part of the study 

area likely originated in the Glass Mountains, generally flows 

northeast, and flows out of the study area to the east or naturally 

discharges from springs to other aquifers in the groundwater 

system at the downdip extent or to the Pecos River.
PECOS

COUNTY 

BREWSTER

COUNTY 

PRESIDIO

COUNTY 

REEVES

COUNTY 

TERRELL COUNTY 

CROCKETT

 COUNTY 

JEFF DAVIS

COUNTY 

UPTON COUNTY 

WARD

COUNTY CRANE

COUNTY 

Pecos River

Leon-Belding area

Belding

Fort Stockton

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£¤285

¦§ ̈10

¦§ ̈20

¦§ ̈10

Glass

Mountains

Davis

Mountains

Barilla

Mountains

3,100

3,000

2,900

3,100

3,000

2,800

3,500

3,400

3,300

3,200

2,700

2,600

3,600

2,700

2,600

2,500

2,400

2,600

2,500

2,600

2,300

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area Conic projection, Texas State Mapping System

North American Datum of 1983

EXPLANATION

Simulated water level, in feet above North American

Vertical Datum of 1988 (NAVD 88)

 2,181.1 to 2,200.0

 2,200.1 to 2,400.0

 2,400.1 to 2,600.0

 2,600.1 to 2,800.0

 2,800.1 to 3,000.0

 3,000.1 to 3,200.0

 3,200.1 to 3,400.0

 3,400.1 to 3,600.0

Measured water level 1980 to 2010 

(Bumgarner and others, 2012)—

Dashed where approximately located. 

Interval 25 feet. Datum is NAVD 88

 Minor contour

2,600 Major contour

Groundwater divide

Groundwater-flow model 

boundary

Conceptual model study 

area boundary

Fault zone—Represents 

numerous faults

104°00’ 103°30' 103°00’ 102°30' 102°00’

31°00’

30°30'

Groundwater-Flow Simulation

The modular finite-difference code USGS MODFLOW-2005 (Harbaugh, 2005) was used to develop equations governing threedimensional groundwater flow. The model has five layers representing the Pecos Valley aquifer (alluvial layer), the Edwards part 

of the Edwards-Trinity aquifer (Edwards layer), the Trinity part of the Edwards-Trinity aquifer (Trinity layer), the Dockum aquifer 

(Dockum layer), and the Rustler aquifer (Rustler layer). The calibration period of the simulation extends from 1940 to 2010 for a 

total of 70 years and 144 stress periods, each one approximately 6 months in length. The hydrologic boundaries of the model include 

specified flux (areal recharge, pumping, and no-flow), specified heads (Rustler aquifer), and head-dependent flux (general-head 

and river boundaries). Each boundary was included to represent a specific aspect of the groundwater-flow system. The measured 

potentiometric surface for 1980–2010 (fig. 4) and the simulated potentiometric surface for 2010 of the Edwards-Trinity aquifer 

generally are in agreement.

Groundwater pumping in the model represents public supply, manufacturing, mining, power generation (industrial), and 

irrigation, with irrigation being the largest water-use category. Groundwater-pumping data were compiled from multiple sources 

to develop a pumping record for 1940–2010. Site-specific pumping data were used when available, though much of the record for 

irrigation pumping contained only aggregated amounts of withdrawals by county, aquifer, and year. Wells were assigned a model 

layer if the screen interval of the well was contained within the top and bottom of a given model layer (fig. 5).

Hydraulic properties in the model were assigned as discrete zones (large areas possessing the same property value) and as 

distributed properties by using pilot points (Doherty, 2005). The value of each pilot point, as well as other discrete zone parameters, 

was adjusted through manual and automated methods to achieve a best fit to measured values of hydraulic head and spring flow. 

Figure 4. Measured potentiometric surface (1980–2010) and simulated groundwater-level altitudes for the Edwards-Trinity 

aquifer in the model area of the Pecos County region, Texas, 2010 (modified from Clark and others, 2014, fig. 12).
U.S. Geological Survey pump hoist 

truck deploying pump in windmill well 

for aquifer testing and electromagnetic 

flowmeter logging.

Development of Groundwater-Pumping Scenarios

The model was used to simulate groundwater levels resulting from prolonged pumping to evaluate the sustainability of recent (2008) 

and projected water use. Each of three scenarios is a continuation of the 70-year calibration period and simulates a 30-year period from 

2010 to 2040. In scenario 1, recent (2008) irrigation and non-irrigation pumping rates are extended for each year of the 30-year simulation 

period. Scenario 2 evaluates the effects of 2008 pumping rates applied as continuous, year-round maximum permitted groundwater-pumping 

rates in the Leon-Belding area for the 30-year simulation period. Scenario 3 evaluates the effects of periodic increases in pumping rates 

over the 30-year simulation period. For each scenario, the change in groundwater level from 2010 to 2040 was extracted from the model 

for comparison with regard to effects of changes in pumping. The 30-year period is discretized into sixty 6-month stress periods generally 

representing seasonality of irrigation water demand.

Projected groundwater-level changes in and around 

the Fort Stockton area indicate little if any change from 

current conditions, indicating that the groundwater system 

is near equilibrium with respect to recent (2008) pumping 

stress. Projected groundwater-level declines (from 15.0 

to 31.0 ft) occurred in localized areas by the end of the 

scenario in the Leon-Belding area. Results of scenario 1 

indicate relatively stable water levels ranging from -5.0 to 

5.0 ft throughout most of the model area during the 30-year 

simulation using pumping amounts as specified for the year 

2008. In scenario 2, the maximum projected groundwaterlevel declines in the Leon-Belding irrigation area were 

approximately 31.3 ft in small isolated areas, which are 

depicted as water-level changes ranging from 25.0 to 32.0 

in. The remaining area and magnitude of groundwater-level 

decline are almost identical to that of scenario 1. Results 

of scenario 3 (fig. 6) indicate that the maximum projected 

groundwater-level decline in the Leon-Belding area was 

greater than either scenario 1 or 2 at approximately 34.5 

ft, and the extent of the decline is larger in area (about 17 

percent increase) than that of scenario 2. Additionally, the 

area of projected groundwater-level declines in the eastern 

part of the model area increased as compared to scenario 

2. The lack of differences in the remaining areas associated 

with the results of scenarios 2 and 3 might be attributed to 

the low magnitude of pumping in 2008 and the relatively 

small total increase in water use of about 15 percent over 

the 30-year period, which together produce small increases 

in pumping amounts.

VERTICAL SCALE GREATLY EXAGGERATED

Figure 5. Oblique view of the Pecos County region, Texas, with selected wells 

extending into the simulated water table of the Edwards-Trinity aquifer (vertically 

exaggerated) (modified from Clark and others, 2014).

£¤285

£¤285

§ ̈¦10

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

M

BARILLA

OUNTAINS

DAVIS

MOUNTAINS

Leon-Belding area

PECOS COUNTY

BREWSTER

COUNTY

REEVES COUNTY

TERRELL

COUNTY

CROCKETT

COUNTY

JEFF DAVIS

COUNTY

UPTON

COUNTY

WARD

 COUNTY

CRANE

 COUNTY

Belding

Fort Stockton

104 ̊00’ 103 ̊30' 103 ̊00’ 102 ̊30' 102 ̊00’

31 ̊00’

30 ̊30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital data

Albers Equal Area Projection, Texas State Mapping System

North American Datum of 1983

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Groundwater-flow model boundary

Conceptual model study area boundary

Fault zone—Represents numerous faults -5.1 to -13.5

4.9 to -5.0

14.9 to 5.0

 24.9 to 15.0

34.5 to 25.0

Water-level change from 2010 to 2040,

in feet (2010 minus 2040)

Pecos River

Figure 6. Groundwater-level difference for the Edwards-Trinity aquifer 

in the model area of the Pecos County region, Texas, from 2010 to 2040 for 

groundwater-pumping scenario 3 (modified from Clark and others, 2014, fig. 20).
U.S. Department of the Interior

U.S. Geological Survey

Data Series 678

Prepared in cooperation with the Middle Pecos Groundwater Conservation District, 

Pecos County, City of Fort Stockton, Brewster County, and Pecos County Water Control 

and Improvement District No. 1

Data Collection and Compilation for a Geodatabase of 

Groundwater, Surface-Water, Water-Quality, Geophysical, 

and Geologic Data, Pecos County Region, Texas, 1930–2011

?

?

?

?

?

?

?

?

?

? ?

WEST EAST SECTION B–B'

A A'

1,500

2,000

2,500

3,000

3,500

Altitude, in feet

above North American Vertical Datum of 1988

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5124

(Revised July 10, 2012)

Prepared in cooperation with the Middle Pecos Groundwater Conservation District,

Pecos County, City of Fort Stockton, Brewster County, and Pecos County Water Control

and Improvement District No. 1

A Conceptual Model of the Hydrogeologic Framework, Geochemistry, 

and Groundwater-Flow System of the Edwards-Trinity and Related 

Aquifers in the Pecos County Region, Texas

Water-level change 2010–2040 for Scenario 3

Water-level decline

Water-level rise

Leon-Belding area

PECOS COUNTY

BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT

COUNTY

JEFF DAVIS COUNTY

UPTON COUNTY

WARD

COUNTY

CRANE COUNTY

Belding Fort Stockton

Pecos River

Oblique view of the Pecos County region with selected wells extending into the simulated water table of the Edwards-Trinity aquifer (vertically exaggerated)

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5228

Revised July 2014

Prepared in cooperation with the Middle Pecos Groundwater Conservation District, Pecos 

County, City of Fort Stockton, Brewster County, and Pecos County Water Control and 

Improvement District No. 1

Simulation of Groundwater Flow in the Edwards-Trinity and 

Related Aquifers in the Pecos County Region, Texas

Railroad Commission of Texas, 2010, Geophysical log database: 

Accessed September 30, 2011, at http://rrcsearch.neubus.com/

esd-rrc/#results.

Texas Water Development Board, 2011, Groundwater database 

reports: Accessed December 19, 2011, at http://www.

twdb.state.tx.us/publications/reports/GroundWaterReports/

GWDatabaseReports/GWdatabaserpt.asp.

Texas Water Development Board, 2013, 2011 regional water 

plan: Accessed June 6, 2013, at http://www.twdb.state.

tx.us/waterplanning/data/projections/2012/doc/Population/

PopulationByRWPG/4PopulationF.pdf.

University Lands, 2011, Well library: Accessed July 1, 2011, at 

http://www.utlands.utsystem.edu/techdata.aspx.

U.S. Geological Survey, 2011, National Water Information 

System (NWISWeb) data: Accessed December 19, 2011, at 

http://waterdata.usgs.gov/tx/nwis/nwis.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143025

This fact sheet is based on the following USGS reports:

Publishing support provided by

Lafayette and Rolla Publishing Service Centers

For additional information, contact

Director, Texas Water Science Center 

U.S. Geological Survey 

1505 Ferguson Lane 

Austin, Texas 78754–4501 

http://tx.usgs.gov/

References Cited

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., 1992, Configuration of the base of 

the Edwards-Trinity aquifer and hydrogeology of the underlying 

pre-Cretaceous rocks, west-central Texas: U.S. Geological 

Survey Water-Resources Investigations Report 91–4071, 

25 p., 1 pl.

Doherty, J.E., 2005, PEST—Model-independent parameter 

estimation, user manual (5th ed.): Watermark Numerical 

Computing, 336 p.

Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological 

Survey modular ground-water model—The ground-water 

flow process: U.S. Geological Survey Techniques and 

Methods 6–A16, variously paged.

Meyer, J.E., Wise, M.R., and Kalaswad, Sanjeev, 2011, 

Pecos Valley aquifer, west Texas—Structure and brackish 

groundwater: Texas Water Development Board Brackish 

Resources Aquifer Characterization System (BRACS), 92 p.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (version 2)—A computer program for speciation, 

batch-reaction, one-dimensional transport, and inverse 

geochemical calculations: U.S. Geological Survey WaterResources Investigations Report 99–4259, 312 p.

Pearson, D.K., Bumgarner, J.R., Houston, 

N.A., Stanton, G.P., Teeple, A.P., and 

Thomas, J.V., 2012, Data collection 

and compilation for a geodatabase of 

groundwater, surface-water, waterquality, geophysical, and geologic 

data, Pecos County region, Texas, 

1930–2011: U.S. Geological Survey 

Data Series 678, 67 p., http://pubs.usgs.

gov/ds/678/.

Bumgarner, J.R., Stanton, G.P., Teeple, 

A.P., Thomas, J.V., Houston, 

N.A., Payne, J.D., and Musgrove, 

MaryLynn, 2012, A conceptual model 

of the hydrogeologic framework, 

geochemistry, and groundwater-flow 

system of the Edwards-Trinity and 

related aquifers in the Pecos County 

region, Texas: U.S. Geological 

Survey Scientific Investigations 

Report 2012–5124, 74 p., http://pubs.

usgs.gov/sir/2012/5124/.

Clark, B.R., Bumgarner, J.R., Houston, 

N.A., and Foster, A.L., 2014, 

Simulation of groundwater flow in the 

Edwards-Trinity and related aquifers 

in the Pecos County region, Texas: 

U.S. Geological Survey Scientific 

Investigations Report 2013–5228, 56 p., 

http://dx.doi.org/10.3133/sir20135228. 

(Revised July 2014).

— Jonathan V. Thomas