?

?

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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
 
A Conceptual Model of the Hydrogeologic 

Framework, Geochemistry, and 

Groundwater-Flow System of the EdwardsTrinity and Related Aquifers in the Pecos 

County Region, Texas 

By Johnathan R. Bumgarner, Gregory P. Stanton, Andrew P. Teeple, Jonathan V. 

Thomas, Natalie A. Houston, Jason D. Payne, and MaryLynn Musgrove

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

Scientific Investigations Report 2012–5124

(Revised July 10, 2012)

U.S. Department of the Interior

U.S. Geological Survey
Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

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 EdwardsTrinity and related aquifers in the Pecos County region, Texas: U.S. Geological Survey Scientific Investigations Report 

2012–5124 (revised July 10, 2012), 74 p. 

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

(Revised July 10, 2012)

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

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

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

1-888-ASK-USGS

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

U.S. Government.
iii

Contents

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

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

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

Description of the Study Area ...........................................................................................................4

Geologic and Hydrogeologic Setting ................................................................................................4

Hydrogeologic Framework ..........................................................................................................................9

Interpretive Methods of the Hydrostratigraphic Analysis ............................................................9

Well Reports ................................................................................................................................9

Borehole Geophysics .................................................................................................................9

Natural Gamma Logs .........................................................................................................9

Electric Logs .......................................................................................................................9

Electromagnetic Induction Logs ...................................................................................10

Surface Geophysics .................................................................................................................10

Hydrostratigraphic Unit Interpretation ...........................................................................................10

Structural Interpretations .................................................................................................................17

Aquifer Tests and Hydraulic Properties .........................................................................................27

Geochemistry ...............................................................................................................................................27

Overview of Geochemical Data .......................................................................................................29

Specific Conductance ..............................................................................................................29

Hydrochemical Facies .............................................................................................................31

Sulfate and Chloride Concentrations ....................................................................................31

Silica Concentrations ...............................................................................................................31

Stable Isotopes .........................................................................................................................32

Strontium Isotopes ...................................................................................................................34

Environmental Tracers .............................................................................................................37

Organic Compounds and Nutrients .......................................................................................37

Geochemical Endmembers ..............................................................................................................40

Groundwater-Flow System ........................................................................................................................42

Regional Groundwater Flow ............................................................................................................43

Recharge .............................................................................................................................................43

Geochemical Mixing and Discharge ..............................................................................................45

Summary .......................................................................................................................................................47

References Cited .........................................................................................................................................48

Figures

1. Map showing location and physiographic provinces of the Pecos County region 

study area in the Trans-Pecos region, Texas ..........................................................................2

2. Map showing generalized boundaries of geologic structural features in the Pecos 

County region study area, Texas ...............................................................................................5

3. Map showing major aquifers in the Pecos County region study area, Texas ...................7

4. Map showing minor aquifers in the Pecos County region study area, Texas ...................8
iv

5. Example log for well 304711103003301 showing natural gamma, and induction 

conductivity/resistivity borehole geophysical properties, time-domain 

electromagnetic (TDEM) soundings, and stratigraphic layers for the Pecos County 

region study area, Texas ...........................................................................................................11

6. Example log for well 302630102503801 showing natural gamma borehole 

geophysical properties, audio-magnetotelluric sounding (AMT08), and 

stratigraphic layers in the Pecos County region study area, Texas ..................................12

7. Map showing locations where geological and geophysical data were used to 

determine the tops and bases (picks) of hydrostratigraphic subdivisions in the 

Pecos County region study area, Texas .................................................................................13

8. Example logs with geophysical properties of Permian, Triassic, and Cretaceousage stratigraphic layers in A, northern Pecos County and B, northwestern Pecos 

County in the Pecos County region study area, Texas ........................................................14

9. Schematic showing the procedure to develop layered-earth electrical scenarios 

for the audio-magnetotelluric smooth inverse modeling results for the Pecos 

County region study area, Texas .............................................................................................15

10. Schematic diagram of three (A, B, and C) layered-earth electrical scenarios for the 

Pecos County region study area, Texas .................................................................................16

11. Map showing the altitude of the top surface of the Edwards-Trinity aquifer 

estimated by interpolating the tops and bases of hydrostratigraphic subdivisions in 

the Pecos County region study area, Texas ..........................................................................18

12. Map showing the altitude of the top surface of the Trinity part of the EdwardsTrinity aquifer estimated by interpolating the tops and bases of hydrostratigraphic 

subdivisions in the Pecos County region study area, Texas ..............................................19

13. Map showing the altitude of the base surface of the Edwards-Trinity aquifer 

estimated by interpolating the tops and bases of hydrostratigraphic subdivisions 

in the Pecos County region study area, Texas ......................................................................20

14. Map showing the thickness of the Edwards-Trinity aquifer in the Pecos County 

region study area, Texas, calculated as the difference between the altitudes of the 

top and base surfaces of the Edwards-Trinity aquifer ........................................................22

15. Map showing the thickness of the Edwards part of the Edwards-Trinity aquifer in 

the Pecos County region study area, Texas, calculated as the difference between 

the altitudes of the top and base surfaces of the Edwards part of the EdwardsTrinity aquifer ..............................................................................................................................23

16. Map showing the thickness of the Trinity part of the Edwards-Trinity aquifer in 

the Pecos County region study area, Texas, calculated as the difference between 

the altitudes of the top and base surfaces of the Trinity part of the EdwardsTrinity aquifer ..............................................................................................................................24

17. Generalized map and detailed cross section A–A′ in the Pecos County region 

study area, Texas .......................................................................................................................25

18. Generalized map and detailed cross section B–B′ in the Pecos County region 

study area, Texas .......................................................................................................................26

19. Map showing the log of transmissivity values calculated and estimated using 

aquifer-test data from wells completed in the Edwards-Trinity aquifer in the Pecos 

County region study area, Texas .............................................................................................28

20. Map showing locations of groundwater-well and spring sites for geochemical 

data collection in the Pecos County region study area, Texas ..........................................30

21. Trilinear diagram showing relations between major cations and anions in 

groundwater and spring water collected in the Pecos County region study 

area, Texas ..................................................................................................................................32
v

22. Graph showing relation between sulfate concentrations and chloride 

concentrations for samples collected from groundwater and spring sites in the 

Pecos County region study area, Texas .................................................................................33

23. Map showing relation between oxygen-18/oxygen-16 and hydrogen-2/hydrogen-1 

isotopic ratios for samples collected from groundwater and spring sites in the 

Pecos County region study area, Texas, and the global meteoric water line .................35

24. Map showing oxygen-18/oxygen-16 isotopic ratios for samples collected from 

groundwater and spring sites in the Pecos County region study area, Texas ................36

25. Graph showing relation between strontrium-87/strontium-86 isotopic ratio and 

strontium concentration for groundwater and spring samples collected in the 

Pecos County region study area, Texas .................................................................................38

26. Map showing strontium-87/strontium-86 isotopic ratios for samples collected from 

groundwater and spring sites in the Pecos County region study area, Texas ................39

27. Map showing national Land Cover Database 2006 land-cover categories in the 

Pecos County region study area, Texas .................................................................................41

28. Map showing potentiometric-surface map of the Edwards-Trinity aquifer developed 

using the average of the winter (November through April) groundwater-level data 

collected at each site for 1980–2010 in the Pecos County region study area, Texas .....44

Tables

1. Hydrostratigraphic section in the Pecos County region study area, Texas .....................52

2. Sites contributing well reports, borehole geophysical logs, and surface 

geophysical soundings used to determine the lithologies, hydrostratigraphic units, 

and tops and bases of the hydrostratigraphic units in the Pecos County region 

study area, Texas .......................................................................................................................54

3. Location of geophysical soundings and the nearby borehole geophysical log the 

sounding was compared to in the Pecos County region study area, Texas ....................65

4. Compiled transmissivity values using data from aquifer tests conducted at wells 

completed in the Edwards-Trinity aquifer in or near the Pecos County region study 

area, Texas ..................................................................................................................................66

5. Data-collection sites providing data for the geochemical analysis in the Pecos 

County region study area, Texas .............................................................................................67

6. Contributing-aquifer revisions for groundwater wells providing data for the 

geochemical analysis based on the comparison of well production depths with 

the hydrostratigraphy of hydrogeologic framework for the Pecos County region 

study area, Texas .......................................................................................................................68

7. Summary of selected physical properties and constituents measured in 

groundwater and spring water samples and hydrochemical facies, Pecos County 

region study area, Texas ...........................................................................................................69

8. Tritium concentrations and helium-4 screen interpretations for groundwater 

samples collected in the Pecos County region study area, Texas ....................................71

9. Average winter (November through April) groundwater-level data used for the 

1980–2010 compiled potentiometric-surface map of the Edwards-Trinity aquifer in 

the Pecos County region study area, Texas ..........................................................................72
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Specific capacity

gallon per minute per foot 

[(gal/min)/ft)]

0.2070 liter per second per meter 

[(L/s)/m]

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 

gallon per day per foot of drawdown (gal/d/ft)0.01242 meter squared per day (m2/d)

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

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

(NAVD88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, gallon per day per foot of drawdown (gpd/ft) 

is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).
vii

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

or micrograms per liter (μg/L).

Tritium concentrations are given in units of picocuries per liter (pCi/L) and Tritium Units (TU). 

Based upon a tritium half-life of 12.32 years 1 TU is equal to 3.22 pCi/L (Lucas and Unterweger, 

2000).

Explanation of Isotope Units

The values for stable-isotope ratios discussed in this report are referenced to the following 

standard materials: 

Element Ratio Standard identity and reference

Hydrogen Hydrogen-2/hydrogen-1 Vienna Standard Mean Ocean Water (VSMOW) (Casciotti and 

others, 2002)

Oxygen Oxygen-18/oxygen-16 Vienna Standard Mean Ocean Water (VSMOW) (Casciotti and 

others, 2002)

Abstract 

A conceptual model of the hydrogeologic framework, 

geochemistry, and groundwater-flow system of the EdwardsTrinity and related aquifers, which include the Pecos Valley, 

Igneous, Dockum, Rustler, and Capitan Reef aquifers, was 

developed as the second phase of a groundwater availability 

study in the Pecos County region in west Texas. The first 

phase of the study was to collect and compile groundwater, 

surface-water, water-quality, geophysical, and geologic data 

in the area. The third phase of the study involves a numerical 

groundwater-flow model of the Edwards-Trinity aquifer in 

order to simulate groundwater conditions based on various 

groundwater-withdrawal scenarios. Resource managers plan to 

use the results of the study to establish management strategies 

for the groundwater system.

The hydrogeologic framework is composed of the 

hydrostratigraphy, structural features, and hydraulic properties 

of the groundwater system. Well and geophysical logs 

were interpreted to define the top and base surfaces of the 

Edwards-Trinity aquifer units. Elevations of the top and 

base of the Edwards-Trinity aquifer generally decrease from 

the southwestern part of the study area to the northeast. 

The thicknesses of the Edwards-Trinity aquifer units were 

calculated using the interpolated top and base surfaces of the 

hydrostratigraphic units. Some of the thinnest sections of the 

aquifer were in the eastern part of the study area and some 

of the thickest sections were in the Pecos, Monument Draw, 

and Belding-Coyanosa trough areas. Normal-fault zones, 

which formed as growth and collapse features as sediments 

were deposited along the margins of more resistant rocks 

and as overlying sediments collapsed into the voids created 

by the dissolution of Permian-age evaporite deposits, were 

delineated based on the interpretation of hydrostratigraphic 

cross sections. The lowest aquifer transmissivity values were 

measured in the eastern part of the study area; the highest 

transmissivity values were measured in a faulted area of 

the Monument Draw trough. Hydraulic conductivity values 

generally exhibited the same trends as the transmissivity 

values.

Groundwater-quality data and groundwater-level data 

were used in context with the hydrogeologic framework to 

assess the chemical characteristics of water from different 

sources, regional groundwater-flow paths, recharge sources, 

the mixing of water from different sources, and discharge in 

the study area. Groundwater-level altitudes generally decrease 

from southwest to northeast and regional groundwater 

flow is from areas of recharge south and west to the north 

and northeast. Four principal sources of recharge to the 

Edwards-Trinity aquifer were identified: (1) regional flow that 

originated as recharge northwest of the study area, (2) runoff 

from the Barilla, Davis, and Glass Mountains, (3) return 

flow from irrigation, and (4) upwelling from deeper aquifers. 

Results indicated Edwards-Trinity aquifer water in the study 

area was dominated by mineralized, regional groundwater 

flow that most likely recharged during the cooler, wetter 

climates of the Pleistocene with variable contributions of 

recent, local recharge. Groundwater generally flows into 

the down-dip extent of the Edwards-Trinity aquifer where 

it discharges into overlying or underlying aquifer units, 

discharges from springs, discharges to the Pecos River, 

follows a regional flow path east out of the study area, or is 

withdrawn by groundwater wells. Structural features such as 

mountains, troughs, and faults play a substantial role in the 

distribution of recharge, local and regional groundwater flow, 

spring discharge, and aquifer interaction.

Introduction

The Edwards-Trinity aquifer is a vital groundwater 

resource for agricultural, industrial, and municipal uses in the 

Trans-Pecos region of west Texas (fig. 1) (Barker and Ardis, 

1996; Freese and Nichols, LBG-Guyton, 2010). A better 

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

By Johnathan R. Bumgarner, Gregory P. Stanton, Andrew P. Teeple, Jonathan V. Thomas, Natalie A. Houston, 

Jason D. Payne, and MaryLynn Musgrove 
Figure 1. Location and physiographic provinces of the Pecos County region study area in the Trans-Pecos region, Texas. 

£¤285

£¤285

¦§ ̈20

¦§ ̈10

BALMORHEA STATE PARK

Pecos River

Pecos River

Pecos River

Pecos River

DAVIS

MOUNTAINS

BARILLA

MOUNTAINS

GLASS

MOUNTAINS

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

HIGH PLAINS

MEXICAN

HIGHLAND

PECOS VALLEY

EDWARDS (STOCKTON) PLATEAU

"

"

"

"

"

Crane

Pecos

Alpine

Fort

Stockton

Grandfalls

Balmorhea

Toyahvale

Belding

Fort

Davis

San Solomon Spring

Santa Rosa

Spring

Comanche Springs

Diamond Y Spring

Leon Springs

Middle Pecos

Groundwater

Conservation Distri

TRANS-PECOS

REGION Stud

PEC

COU

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPTO

COUN

WARD 

COUNTY

CRANE 

COUNTY

0 5 10 0 5 10 15 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Physiographic prov

and D.W. Johnson

EXPLANATION

Physiographic province Basin and Range Great Plains

Study area boundaryLeon-Belding area boundarySection boundary within physiographic province

Spring
Introduction 3

understanding of the hydrogeologic setting and processes 

that control the distribution, quality, and availability of 

water in the aquifer is required for optimal resource 

management. In general, a comprehensive, integrated analysis 

of available scientific data facilitates a better understanding 

of an aquifer system, which enables water-resource managers 

to establish long-range and short-range aquifer management 

strategies that support present and projected aquifer uses. 

Resource managers would like to know more about the future 

availability of water in the Edwards-Trinity aquifer in Pecos 

County, Texas, and the effects of the possible increase or 

temporal redistribution of groundwater withdrawals. In order 

to provide resource managers with that knowledge, 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 

comprehensive, integrated analysis of available hydrogeologic 

data in order to develop a conceptual model of the EdwardsTrinity and related aquifers in the study area in parts of 

Brewster, Crane, Crockett, Jeff Davis, Pecos, Reeves, Terrell, 

and Ward Counties (hereinafter referred to as the Pecos 

County region study area) (fig. 1). The Edwards-Trinity and 

related aquifers (hereinafter referred to as the groundwater 

system) include the Pecos Valley, Igneous, Dockum, Rustler, 

and Capitan Reef aquifers. The Edwards-Trinity aquifer was 

the focus of the investigation presented in this report, and 

the related aquifers were studied in terms of how they 

potentially interact with and affect the Edwards-Trinity 

aquifer. 

Development of the conceptual model is the second 

phase of a three-phase groundwater-availability study being 

conducted in the Pecos County region by the USGS and 

the cooperators. 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). The data compiled 

in the first phase of the study were used in this report to 

develop the conceptual model. The third phase of the study 

involves a numerical groundwater-flow model of the EdwardsTrinity aquifer in order to simulate groundwater conditions 

based on various groundwater-withdrawal scenarios.

The conceptual model of the hydrogeologic framework, 

geochemistry, and groundwater-flow system in the 4,700 

square-mile (mi2) study area was developed in an effort to 

better understand the groundwater system and establish a 

scientific foundation for resource-management decisions. 

Lithologic information obtained from well reports and 

geophysical data was used to describe the hydrostratigraphy 

and structural features of the groundwater system and 

aquifer-test data were used to estimate aquifer hydraulic 

properties. Groundwater-quality data (hereinafter referred 

to as geochemical data) were used to evaluate groundwaterflow paths, water-rock interaction, aquifer interaction, and the 

mixing of water from different sources. Groundwater-level 

data were also used to evaluate aquifer interaction as well as 

to develop a potentiometric-surface map, delineate regional 

groundwater divides, and describe regional groundwater-flow 

paths.

Several scientific investigations have been done to 

collect and compile physical and chemical data, describe 

the hydrogeologic processes, and develop conceptual and 

numerical groundwater-flow models of the Edwards-Trinity 

aquifer in the Trans-Pecos region. Pearson and others (2012) 

documented the methods used to compile the available 

groundwater, surface-water, geochemical, geophysical, and 

geologic information used to develop the conceptual model 

described in this report. The data compiled by Pearson and 

others (2012) include existing data in the study area and data 

collected by the USGS. The Thornhill Group, Inc. (2008) 

and Daniel B. Stephens and Associates (2010) developed 

conceptual and numerical groundwater-flow models of the 

Edwards-Trinity aquifer that focused on 100 mi2 near Belding, 

Tex., referred to hereinafter as the Leon-Belding area (fig. 1). 

The Leon-Belding area is an agricultural area about 7 miles 

(mi) southwest of Fort Stockton that includes about 30 mi2

of cultivated land. Water primarily from the Edwards-Trinity 

aquifer is used for irrigation purposes in the Leon-Belding 

area. Using the conceptual and numerical groundwater-flow 

models, simulations to project future aquifer conditions in the 

Leon-Belding area based on various groundwater-withdrawal 

scenarios were published (Thornhill Group, Inc., 2008; Daniel 

B. Stephens and Associates, 2010). Anaya and Jones (2009) 

developed a 44,000-mi2 regional groundwater-flow model of 

the Edwards-Trinity and Pecos Valley aquifers in central and 

western Texas to determine the availability of groundwater 

based on projected needs. As part of the USGS Regional 

Aquifer Systems Analysis (RASA) program, Barker and 

Ardis (1996) described the hydrogeologic framework of the 

Edwards-Trinity aquifer system on a broad, regional scale in 

west-central Texas that included the Pecos Region study area. 

Small and Ozuna (1993) investigated the Edwards-Trinity 

aquifer in the Pecos County region; they broadly described 

the hydrogeology of the groundwater system, defined the 

groundwater-level and water-quality characteristics of the 

aquifer, determined post-development changes in water 

levels and water quality, and described relations between 

groundwater levels and flow from Comanche Springs in 

Fort Stockton, Tex.; Comanche Springs were historically 

important to this region for water supply and recreation but 

have not flowed continuously since the 1950s (Small and 

Ozuna, 1993). 

Purpose and Scope

This report documents the development of a conceptual 

model of the Edwards-Trinity and related aquifers in the 

Pecos County region, Tex., using data collected by the 

USGS during 2009–11 and historical data from 1930–2011 

collected by various State and local agencies and compiled 

by the USGS (Pearson and others, 2012). The parts of the 

conceptual model are described, including the hydrogeologic 
4 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

framework, geochemistry, and groundwater flow of the 

groundwater system in the study area. The methodologies used 

for analyzing the well-log and geophysical datasets and the 

construction of the hydrogeologic framework and the principal 

chemical properties of water in the study area and geochemical 

endmembers used to describe the chemical characteristics 

of water from different sources are described. Finally, 

the chemical properties of water and groundwater-level 

information are described in context with the hydrogeologic 

framework; regional groundwater flow, aquifer recharge, the 

mixing of water from different sources, and groundwater 

discharge are qualitatively assessed.

Description of the Study Area

The study area covers about 4,700 mi2 of the TransPecos region of Texas west of the Pecos River, and its 

boundaries were defined to include the extent of the fieldcollected data gathered for this study (fig. 1) (Pearson and 

others, 2012). The southwestern and southern boundaries of 

the study area are rimmed by the Barilla and Davis Mountains 

in northeastern Jeff Davis County and southwestern Reeves 

County and the Glass Mountains in northeastern Brewster 

County and southern Pecos County. The northeastern 

boundary of the project study area is the Pecos River, and 

the southeastern and northwestern boundaries were aligned 

to the data cells of the Texas Water Development Board 

(TWDB) Groundwater Availability Model (GAM) of the 

area (Anaya and Jones, 2009). The southwestern boundary 

was modified using the “active” part of the GAM model. The 

western part of the Middle Pecos Groundwater Conservation 

District (MPGCD) management area (Pecos County is the 

MPGCD management area) is in the study area. Altitude 

ranges from 6,350 feet (ft) in the southwestern part of the 

study area in the Davis Mountains in Jeff Davis County to 

2,150 ft in the northeastern part of the study area near the 

Pecos River in Pecos County. The climate in the study area 

is arid, characterized by scant rainfall and large amounts of 

evaporation. The average annual rainfall during 1970–2000 at 

Fort Stockton was approximately 14 inches (National Weather 

Service, 2011). Rainfall, as recorded at Fort Stockton, annually 

is quite variable; during 2000–10, 2004 was the wettest year 

with an annual rainfall of about 26 inches and 2008 was the 

driest year with an annual rainfall of about 6 inches (National 

Weather Service, 2011). Potential annual evaporation of as 

much as 109 inches has been estimated (Boghici, 1997). 

Temperatures during 1970–2000 ranged from an average low 

of about 32 degrees Fahrenheit (oF) in January to an average 

high of about 96oF in July (National Weather Service, 2011). 

The study area is in the Pecos Valley, Edwards Plateau, 

and High Plains sections of the Great Plains physiographic 

province, and the Mexican Highland section of the Basin and 

Range Province (fig. 1) (Fenneman and Johnson, 1946). West 

of the Pecos River, the Edwards Plateau section of the Great 

Plains Province is also referred to as the Stockton Plateau 

(Wermund, 1996).

Geologic and Hydrogeologic Setting1

There were several periods of seawater inundation and 

erosion during the geologic history of the Trans-Pecos region 

of west Texas. Sedimentary rocks of Pennsylvania, Permian, 

Triassic, and Cretaceous age; Tertiary-age igneous rocks; and 

Cenozoic-age alluvium are present in the subsurface, and 

many are exposed at the surface in the study area (Texas Water 

Development Board, 1972). This study focuses on subsurface 

rocks deposited from the Permian to the Quaternary Period 

(table 1 at end of report). During the Permian Period, this 

region of western Texas was a shallow sea; marine sandstones, 

limestone, and shale were deposited in the basin. In the later 

part of the Permian Period, the basin became more isolated 

and the deposition of sediments changed to gypsum, anhydrite, 

halite, and associated salts. Several geologic structures also 

formed in the study area during the Permian Period (fig. 2). 

The Central Basin Platform is a structural high in the northern 

part of Pecos County that divides the Permian Basin into the 

Delaware and the Midland Basins (Ashworth, 1990). The 

Val Verde Basin was separated from the Delaware Basin 

by the development of a reef complex during the Permian 

Period (Small and Ozuna, 1993). Dissolution of Permian-age 

evaporite deposits that began at the time of deposition and 

continued through the Cretaceous Period caused the Permian 

beds to collapse and form a north-south depositional trough 

called the Belding-Coyanosa trough (fig. 2) (Armstrong and 

McMillion, 1961; Boghici, 1997). By the Triassic Period, the 

sea retreated and a sequence of nondeposition, erosion, and 

then deposition of fluvial and deltaic sediments took place. 

During the Jurassic Period, this region of western Texas was 

above sea level, erosion was the dominant process, and the 

land surface was tilted to the southeast (Barker and Ardis, 

1996). As a result, there are no Jurassic rocks in the geologic 

record of the study area. During the Cretaceous Period, 

sea level once again rose and the deposition of continental 

sediments changed to shallow marine sediments (Barker and 

Ardis, 1996). Cretaceous deposition included the filling of 

the structural troughs that had begun forming in the Permian 

and Triassic Periods, which resulted in thicker units in these 

areas. The Cretaceous Period was the last marine deposition 

event in the study area. Tertiary Period volcanism deposited 

extrusive igneous rocks following the Cretaceous marine 

deposition (George, and others, 2011). Continental sediments 

of sand and gravel were deposited during the Tertiary and 

Quaternary Periods (Texas Water Development Board, 1972). 

During the Cenozoic Era, two depositional troughs that 

roughly trend north-south formed in the central and western 

parts of the study area because of the continued dissolution 

1

This section modified from Pearson and others (2012, p. 3).
Figure 2. Generalized boundaries of geologic structural features in the Pecos County region study area, Texas. 

Pecos River

MIDLAND

COUNTY

ECTOR

COUNTY

WINKLER

COUNTY

LOVING

COUNTY

UPTON

COUNTY

CRANE

COUNTY

WARD

COUNTY

CULBERSON

COUNTY

JEFF

DAVIS

COUNTY

BREWSTER

COUNTY

TERRELL

COUNTY

PRESIDIO

COUNTY

PECOS

COUNTY

REEVES

COUNTY

San Solomon

Spring

Santa

 Rosa

 Spring

Comanche

Springs

Diamond

Y Spring

DELAWARE

BASIN

CENTRAL BASIN PLATFORM

DIABLO

PLATFORM

VAL VERDE

BASIN

MONUMENT DRAW TROUGH AND BELDING-COYANOSA TROUGH

PECOS

TROUGH

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

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

31°30'31°00'30°30'30°00'

EXPLANATION

Study area boundaryPermian geologic structure boundaryPermian to Cenozoic geologic structure boundary

Bel

Dia

Fau

Belding Fault System fr

Diamond Y Fault 

Belding-Coyanosa Trough m

Baumgardner and others, 1982; a

Permian geologic structure boundaries modified fr

Cenozoic geologic structure boundaries modified fro

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

TEXAS

Permian

Basin

Study area
6 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

of the Permian-age evaporite deposits and collapse of the 

overlying sediments (Armstrong and McMillion, 1961). 

These troughs subsequently filled with Cenozoic-age alluvium 

and are known as the Monument Draw (central) and Pecos 

(western) troughs (fig. 2). For simplicity, hereinafter, the name 

“Monument Draw trough” will be used to represent both the 

Cenozoic-age Monument Draw and Permian to Cretaceousage Belding-Coyanosa troughs because the spatial extents and 

separation of these structural features are not well defined. 

The geologic setting contributed to the formation of two 

major and four minor aquifers in the study area (figs. 3 and 4, 

table 1). The Pecos Valley aquifer is a major aquifer composed 

of Cenozoic-age alluvium consisting of unconsolidated silt, 

sand, gravel and clay (fig. 3) (Small and Ozuna, 1993). In 

the northern part of the study area, the Pecos Valley aquifer 

uncomformably overlies the Edwards-Trinity aquifer, the 

other major (and primary) aquifer in the study area (fig. 3). 

Minor aquifers include the Igneous, Dockum, Rustler, and 

Capitan Reef aquifers (fig. 4). The Igneous aquifer consists 

of Tertiary-age igneous and volcaniclastic rocks. Located in 

the southwestern part of the study area, the Igneous aquifer 

uncomformably overlies the Cretaceous-age Edwards-Trinity 

aquifer. The Edwards-Trinity aquifer is composed of lower 

Cretaceous-age rocks of limestone, marl, and clay of the 

Washita Group; limestone of the Fredericksburg Group; 

and sand, limestone, and shale of the Trinity Group (fig. 3, 

table 1). The Edwards part of the aquifer is composed of upper 

Cretaceous rocks of the Fredericksburg and Washita Groups, 

which locally are referred to as the Edwards and Sixshooter 

Groups (Brand and DeFord, 1958; Small and Ozuna, 1993; 

Smith and others, 2000). The Fort Lancaster Formation, the 

Burt Ranch Member, and the Fort Terrett Formation make up 

the Edwards Group and occur in the eastern part of the study 

area (Rose, 1972; Smith and Brown, 1983; Small and Ozuna, 

1993). The Boracho Formation, the University Mesa Marl, 

which is a facies change equivalent of the Boracho Formation, 

and the Finlay Formation make up the Sixshooter Group and 

occur in the western part of Pecos County (Brand and DeFord, 

1958; Small and Ozuna, 1993; Smith and others, 2000). The 

Buda Limestone, which overlies the Boracho Formation, 

is present east of Fort Stockton. Regionally, the Buda 

Limestone, the Fort Lancaster Formation, and the Burt Ranch 

Member form the Washita Group. The Fort Terrett and Finlay 

Formations form the Fredericksburg Group. The Trinity group 

is composed of the Maxon Sands, the Glen Rose Formation, 

and the Basal Cretaceous Sand (Anaya and Jones, 2009). The 

individual formations in the Trinity Group are not separated 

for the purposes of this report. Locally the Trinity Group is 

known as the Trinity Sands (Small and Ozuna, 1993; Rees and 

Buckner, 1980).

The Dockum aquifer is a minor aquifer and is composed 

of Triassic-age rocks of the Dockum Group (fig. 4) (Bradley 

and Kalaswad, 2003). The stratigraphic nomenclature of the 

Dockum Group has been updated and regionalized in the 

literature as better information became available (Lehman, 

1994a, b; Bradley and Kalaswad, 2003). In Pecos County, 

a sand unit within the Dockum aquifer is recognizable in 

some geophysical logs, but the individual formations of the 

Dockum Group are not separated for the purposes of this 

report. Locally, the Dockum aquifer is also known as the Santa 

Rosa aquifer (Small and Ozuna, 1993). The boundaries of the 

Dockum aquifer are not explicitly defined in this report and 

might extend beyond the general aquifer boundaries shown in 

figure 4.

The Rustler and Capitan Reef aquifers are minor aquifers 

composed of Permian-age rocks (fig. 4). The Rustler aquifer is 

composed of mostly dolomite, anhydrite, and some limestone 

of the Rustler Formation. A basal unit consists of sand, 

conglomerate, and some shale (Small and Ozuna, 1993; 

LBG-Guyton, 2003). The boundaries of the Rustler aquifer 

are not explicitly defined in this report and might extend 

beyond the general aquifer boundaries shown in figure 4. The 

Capitan Reef aquifer consists of reef, fore-reef, and backreef facies of dolomite and limestone of the older Capitan 

Limestone. 

Water is supplied to the region from groundwater; the 

Pecos River, which is the main surface-water drainage and 

forms the northeastern boundary of the study area; and springs 

that discharge from groundwater sources. San Solomon 

Spring in Balmorhea State Park in Reeves County near 

Toyahvale, Tex., is currently (2012) the largest spring in 

the Trans-Pecos region (Sharp, 2001). San Solomon Spring 

provides water for irrigation, recreation, and endangeredspecies habitat (fig. 1) (Texas Water Development Board, 

2005). Santa Rosa Spring is near Grandfalls, Tex., and at one 

time, this spring supplied water for irrigation. Santa Rosa 

Spring did not flow from the 1950s until the late 1980s, at 

which time flow resumed at a decreased rate (Freese and 

Nichols and LBG-Guyton, 2010). Until the 1950s, Comanche 

Springs were the largest springs in the Trans-Pecos region 

and sixth largest in the State (Sharp, 2001). Comanche Springs 

first went dry in 1955 and perennial flow ceased in 1961

(Texas Parks and Wildlife Department, 2012). According 

to Freese and Nichols and LBG-Guyton (2010), Comanche 

Springs have flowed occasionally since 1987. Diamond Y 

Spring, which are located north of Fort Stockton, support 

habitat for endangered species (Freese and Nichols and LBGGuyton, 2010). The San Solomon, Santa Rosa, Comanche, 

and Diamond Y Springs were sampled for this study (Pearson 

and others, 2012). Finally, the four springs are in faulted 

areas, and it is likely the presence of faults contributed to the 

formation of the springs (Armstrong and McMillion, 1961; 

Baumgardner and others, 1982; Small and Ozuna, 1993; 

Veni, 1991; Boghici, 1997; Sharp and others, 1999; Texas 

Water Development Board, 2005; Anaya and Jones, 2009). 

Furthermore, it is likely that structural features such as faults, 

joints, bedding planes, and fractures influence all of the 

groundwater-flow components of the groundwater system (that 

is, recharge, local and regional flow, and discharge), not just 

spring flow.
Figure 3. Major aquifers in the Pecos County region study area, Texas. 

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 0 5 10 15 

£¤285

£¤285

¦§ ̈20

¦§ ̈10

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifers modified from As

EXPLANATION

Major aquifers Pecos Valley Edwards–Trinity (outcrop) Edwards–Trinity (subcrop)Study area boundary
Figure 4. Minor aquifers in the Pecos County region study area, Texas.

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Minor aquifers Igneous Dockum Rustler Capitan ReefStudy area boundary

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital data EXPLANATIONAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifers from 

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 0 5 10 15 

£¤285

£¤285

¦§ ̈20

¦§ ̈10
Hydrogeologic Framework 9

Hydrogeologic Framework

The hydrogeologic framework of the conceptual model 

comprises the hydrostratigraphy, structural features, and 

hydraulic properties of the groundwater system. Well-log data 

were compiled and supplemented with data gathered through 

borehole and surface geophysics. The data were then analyzed 

to map the tops and bases of the hydrostratigraphic units 

that compose the Edwards-Trinity aquifer and the lateral and 

vertical relations of overlying and underlying aquifers in order 

to develop the hydrostratigraphy of the study area (table 1). 

The data also were used to evaluate the structural features such 

as bed orientation, unit thickness, and fault zones. The top, 

base, and thickness of the aquifer, as discussed in this report, 

refer to the hydrostratigraphic units that compose the aquifer, 

not just the saturated portion of the hydrostratigraphic units. 

Aquifer tests were performed and supplemented by historical 

aquifer-test data to assess the hydraulic properties of the 

Edwards-Trinity aquifer. Pearson and others (2012) detail the 

methods used for collection, analysis, and quality control of 

most of the data used in this report.

Interpretive Methods of the Hydrostratigraphic 

Analysis 

Well reports, borehole geophysical logs, and surface 

geophysical soundings (Pearson and others, 2012) were 

evaluated to determine the lithologies, hydrostratigraphic 

units, and tops and bases of the hydrostratigraphic units, 

creating datasets for characterizing vertical and lateral 

hydrostratigraphic extents. More than 2,000 data records for 

wells in or near the study area were acquired from various 

sources and evaluated for applicability to the study. A total of 

662 records were found to contain pertinent data of applicable 

vertical extent within the study area (table 2 at end of report). 

These records were supplemented by 44 geophysical logs, 

4 time-domain electromagnetic (TDEM) soundings, and 

13 audio-magnetotelluric (AMT) soundings collected by the 

USGS as part of the first phase of this study (Pearson and 

others, 2012). Drilling and well-completion log data from 

500 sites were used to help identify the depths to tops and 

bases of the formations. Depths to the tops and bases of 

the formations and structural features were entered into 

a geospatial database by altitude and spatial location for 

correlation among neighboring wells creating a regional 

network of correlated points. Geophysical logs are typically 

reliable sources for subsurface information. Incorrect or 

missing information including incorrect location information, 

missing or incorrect header information, unknown well 

completion, poor tool calibrations, and unsuitable borehole 

environments can introduce errors during the interpretation 

of hydrostratigraphic information. Geophysical logs with 

incorrect or missing information were not used to interpret the 

hydrostratigraphy.

Well Reports

Producing zones, well yield, and lithologic characterization were interpreted from 296 well reports. All well 

reports used in this study have been compiled from Daniel B. 

Stephens and Associates (2010; 201 data points) and Meyer 

and others (2011; 95 data points).

Borehole Geophysics

Borehole geophysical data such as natural gamma, 

formation resistivity, and caliper are commonly used to 

characterize and identify stratigraphic units; these data exist 

for many wells in the study area and were collected during 

previous scientific investigations, petroleum explorations, or 

both. The Railroad Commission of Texas (RRC) Geophysical 

Log Database (Railroad Commission of Texas, 2010) contains 

1,979 publicly available well logs in or near the study 

area; however, most of the well logs did not meet project 

requirements because the types of geophysical tools that 

were used did not yield data useful for modeling purposes 

or the depth interval studied did not include the waterbearing formations. Each geophysical log was evaluated to 

determine if the log penetrated the desired stratigraphic units 

and provided useful data for determining the tops and bases 

of stratigraphic units or data for identifying other structural 

features. Information used to supplement well reports were 

230 borehole geophysical logs (28 from University Lands 

(2011), 23 from USGS, 51 from RRC, and 128 from TWDB). 

The borehole geophysical logs used to help determine the 

tops and bases of hydrostratigraphic formations were natural 

gamma, electric, and electromagnetic induction logs. 

Natural Gamma Logs

Natural gamma logs provide a record of gamma radiation 

detected at depth in a borehole. Fine-grained sediments 

that contain abundant clay tend to be more radioactive than 

quartz-grain sandstones or carbonates (Keys, 1997). The 

natural gamma, electric, and electromagnetic induction logs 

collectively can be useful to identify lithologies and contact 

depths of the strata penetrated in the borehole.

Electric Logs

Electric logs use a series of electrodes mounted on the 

downhole probe and a surface electrode in the ground to 

measure potential (or voltage) that varies with the electrical 

properties of fluids and rock materials. Electric logs require 

an uncased, fluid-filled hole to allow the current to flow into 

the formation. Electric logs include the following data: normal 

resistivity, lateral resistivity, spontaneous potential, and singlepoint resistance. 

Normal resistivity logs are useful for determining and 

correlating various lithologies but also are affected by the 

resistivity of the fluids in the borehole and formation (Keys, 
10 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

1997). The lateral resistivity log increases the resolution and 

decreases the effect of adjacent beds in comparison with the 

normal resistivity logs (Keys, 1990). Spontaneous potential 

(SP) is one of the oldest logging techniques and uses a very 

simple method of measuring the potentials produced by 

various salinity conditions (Keys, 1990). SP is a function of 

the chemistry of fluids in the borehole and adjacent rocks, the 

temperature, and the clay present and is not related directly 

to porosity and permeability (Keys, 1997). The single point 

resistance (SPR) log uses the same circuitry as SP and shows 

the resistance measured between the electrode in the well and 

an electrode at the land surface (Keys, 1990).

Electromagnetic Induction Logs

Electromagnetic (EM) induction probes measure 

conductivity in air- or water-filled holes and perform well 

in open holes or polyvinyl chloride (PVC) cased holes. The 

measurement of conductivity commonly is reciprocated to 

provide logs with curves of both resistivity and conductivity 

(Keys, 1997). Conductivity is affected by the salinity of 

borehole and formation fluids and the type of lithology 

encountered. Generally, pure carbonates, sands, and gravels 

have lower conductivity (thus higher resistivity) than clays or 

shales (Keys, 1997).

Surface Geophysics

Surface geophysical resistivity methods can be used to 

detect changes in the electrical properties of the subsurface 

using noninvasive surface-based instrumentation (Zohdy 

and others, 1974). These methods are useful in order to fill 

data gaps in areas where borehole methods cannot be used. 

The electrical properties of soil and rock are determined by 

water content, porosity, clay content and mineralogy, and 

conductivity (or reciprocal of electrical resistivity) of the pore 

water (Lucius and others, 2007). Resistivity measurements 

can be used to construct graphical images of the spatial 

distribution of electrical properties of the subsurface. 

Comprehensive descriptions of the theory and application of 

surface geophysical resistivity methods, as well as tables of 

the electrical properties of earth materials, are presented in 

Keller and Frischknecht (1966) and Lucius and others (2007).

TDEM soundings were collected at four different 

locations (Pearson and others, 2012). Each of the locations 

was near a well where borehole geophysical logs also had 

been collected by the USGS. These locations were selected 

so that the TDEM results could be compared to the nearby 

borehole geophysical logs to determine if this geophysical 

method would supply useful data. Figure 5 shows an example 

comparison of a TDEM sounding and a nearby borehole 

geophysical log. The TDEM data collected throughout the 

area show good inversion results with the root mean squared 

(RMS) errors for all soundings less than 4 percent (Pearson 

and others, 2012), but few TDEM results were available at the 

depths needed to determine the tops and bases of formations. 

Consequently, AMT was determined the better method to 

obtain information necessary to determine the tops and bases 

of formations.

Thirteen AMT soundings (AMT01 through AMT13) 

were collected in the study area (Pearson and others, 2012). 

Four of the 13 soundings were collected near wells (AMT 

07, 08, 11, 12) from which borehole geophysical logs were 

collected by the USGS. These locations were selected so that 

the AMT soundings could be compared to the nearby borehole 

geophysical logs, which aided in the interpretation of the 

AMT soundings. Figure 6 shows an example comparison of 

an AMT sounding and a nearby borehole geophysical log. 

The remaining nine sounding locations (table 2) were selected 

where little or no other compiled data were available. An 

explanation of the processing and inversion methodologies 

performed on the TDEM and AMT data is provided by 

Pearson and others (2012).

Hydrostratigraphic Unit Interpretation

Lithologic descriptions and borehole geophysical logs 

obtained from existing reports were interpreted to identify 

the vertical extents (tops and bases, hereinafter referred to 

as picks) of hydrostratigraphic units. Once all of the existing 

data was compiled, a geospatial analysis was done to identify 

data gaps. Using picks from all the existing reports, the 44 

additional borehole logs and 9 AMT soundings (fig. 7), threedimensional surfaces were interpolated to represent the tops 

and bases of applicable hydrostratigraphic units.

Hydrostratigraphic picks were made from stratigraphic 

and lithologic descriptions and were compiled from existing 

reports. Logs with information concerning the tops and bases 

of various geological stratigraphic units were selected from 

published reports and databases as the basis for subsequent 

hydrostratigraphic picks on geophysical logs (fig. 8) (Meyer 

and others, 2011; Herald, 1957). Multiple types of geophysical 

logs were compiled for each borehole geophysical site in the 

study area and evaluated for use; however, the majority of 

the logs that fit project requirements regarding the type of 

geophysical data and depth interval were natural gamma logs. 

Natural gamma logs were most useful, in part, because of 

their versatility; they are one of the few geophysical tools 

that can be run in steel casing, which is present in many wells 

in the area, and they typically provide a good indication of 

clay content. 

Picks for the Edwards part of the Edwards-Trinity aquifer 

(Fredericksburg and Washita Groups) and the Trinity part 

of the Edwards-Trinity aquifer (Trinity Group) were made 

because, although they are both units of the same aquifer, 

they have different hydrologic and lithologic characteristics 

(table 1). In order to determine stratigraphic picks for the 

Trinity Group, surface geophysical inverse modeling results 

were interpreted with layered-earth electrical scenarios in 

which each layer represents a separate electrical layer (Pearson 

and others, 2012). These electrical layers were then associated 

with the stratigraphic layers, which in turn were used to 
Hydrogeologic Framework 11

identify water-bearing hydrogeologic units. For the TDEM 

inverse modeling results, a layered-earth resistivity model 

was created where this model was used as the layered-earth 

electrical scenarios (Pearson and others, 2012). Because the 

AMT inverse modeling software does not iterate a layeredearth resistivity model, another method to obtain the layeredearth electrical scenarios was developed. The layered-earth 

electrical scenarios for the AMT soundings were interpreted 

based on electrical changes in the smooth inverse modeling 

results (fig. 9). The resistivity values of these layers were 

selected at peak points along the smooth inverse modeling 

results, and the corresponding resistivity value was assigned 

to that layer. The top and bottom depths of each layer were 

interpreted by making the difference of the areas between 

the layered-earth electrical scenario layer and the smooth 

inverse modeling results directly above and below the depth 

pick equal to zero. Most of the AMT soundings were laterally 

anisotropic at the sounding location. For these soundings, 

a resistivity curve was made for the magnetic response 

(TM), the electrical response (TE), and a combination of 

the two responses (TMTE). Each of the three responses 

was interpreted individually to develop three layered-earth 

electrical scenarios followed by an average layered-earth 

electrical scenario. For the soundings that had little or no 

lateral variation in the subsurface, the TMTE response was 

calculated and interpreted. One disadvantage of surface 

geophysical techniques is the vertical resolution decreases 

with depth. The accuracy of the picks made from the surface 

geophysical methods was considered to be about plus or minus 

10 percent of the depth.

Figure 5. Example log for well 304711103003301 showing natural gamma, and induction conductivity/resistivity borehole geophysical 

properties, time-domain electromagnetic (TDEM) soundings, and stratigraphic layers for the Pecos County region study area, Texas.

Depth Natural gamma

0 API counts 150

TDEM sounding - smooth

0.2 Ohm-meter 200

 Conductivity

0 millimho per meter 200

Induction resistivity

0.2 Ohm-meter 200

100

200

400

Trinity

Group

Dewey Lake

Red Beds

Comanchean Series

Ochoan

Series

Cretaceous Permian

Quaternary

Fredericksburg

Group

300

 System

 Series

 Group

 Stratigraphic

 Lithology

Natural gamma

Triassic

TDEM sounding - layered

0.2 Ohm-meter 200

Edwards

Group

Trinity

Sands

Dockum

Group

Middle

and Lower

(undifferentiated)

Alluvium

 (feet below 

land-surface 

 datum)

Sandstone  and shale

Increasing

Sandstone and shale Sandstone Shale Limestone Alluvium

Note: Land surface datum, 3;238.70 feet above North American Vertical Datum 1988; API counts, American Petroleum Institute counts;

TDEM Sounding, Time-domain electromagnetic sounding; TDEM Sounding - Smooth, smoothed inverse resistivity modeling results; 

TDEM Sounding - Layered, layered-earth resistivity modeling scenario 

 or

unit
12 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Figure 6. Example log for well 302630102503801 showing natural gamma borehole geophysical properties, audio-magnetotelluric 

sounding (AMT08), and stratigraphic layers in the Pecos County region study area, Texas. 

100

200

300

400

500

600

700

800

900

1,000

Trinity

Group

Dewey

Lake

Red Beds

Salado

Formation

Fredericksburg

Group

Rustler

Formation

Trinity

Sands

 

 

Edwards

Group

Dockum

Group

Natural gamma Depth

API counts

 System

Series

Group

 Stratigraphic

 Lithology

Natural gamma

 (feet below 

land-surface 

 datum)

Increasing

Caliper

Inch

AMT sounding - smooth

Ohm-meter

Ohm-meter

AMT sounding - layered 0 .2 2 0 0

Comanchean Series

Cretaceous Triassic

Ochoan Series

Permian

Limestone Shale Sandstone Sandstone and shale

Sandstoneand shale

Dolomiticanhydrite Salt and anhydrite

0 150

0 10 0 .2 2 0 0

Note: land surface datum, 4;354.39 feet above North American Vertical Datum of 1988; API Counts, American Petroleum Institute counts; AMT Sounding, audio-magnetotelluric sounding;

AMT Sounding - Smooth, smoothed inverse resistivity modeling results; AMT Sounding - Layered, layered-earth resistivity modeling scenario 

or

 unit

Middle

and Lower

(undifferentiated)
Figure 7. Locations where geological and geophysical data were used to determine the tops and bases (picks) of hydrostratigraphic subdivisions in study area, Texas.

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

£¤285

£¤285

¦§ ̈20

¦§ ̈10

Aquifer modified from A

Lithologic 

Daniel B. Step

Texas Water 

Railroad C

and U

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Edwards-Trinity aquifer (subcrop)

Study area boundary

Method of hydrostratigraphic picks

Pick reviewed from lithological descriptions in well reportsPick made from interpreted borehole geophysical logPick made from interpreted audio-magnetotelluric sounding

Edwards-Trinity aquifer (outcrop)

EXPLANATION
14 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Figure 8. Example logs with geophysical properties of Permian, Triassic, and Cretaceous-age stratigraphic layers in A, northern Pecos 

County and B, northwestern Pecos County in the Pecos County region study area, Texas (modified from Herald, 1957).

Depth

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 Permian

Comamchean Series Guadalupian Series

Fredericksburg Group Trinity Group Whitehorse Group

Salado Formation Yates Formation Seven Rivers Formation Queen Formation Grayburg Formation

Triassic

Tansill

Formation

 System

 Series

 Group

 Stratigraphic unit 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

 Group

Rustler

Formation

Dewey Lake

Red Beds

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

Cretaceous Triassic Permian

Comanchean Series

Ochoan Series

Trinity Group

Dewey Lake Red Beds Rustler Formation Salado Formation Trinity Sands

Salt

Salt

Lithology

Natural gamma

Increasing

Dolomitic

 shale

Depth

 System

 Series

Group

 Stratigraphic unit

A B

Dolomite

Anhydritic

 dolomite

Sandstone

Sandstone 

 and shale

Sandstone 

 and shale

Anhydritic

 dolomite

Anhydritic dolomite

Anhydrite

Anhydrite

Anhydrite

Salt

Salt

Salt

Salt

Sandstone 

 and shale

Limestone

Anhydritic dolomite

 (feet blow

land surface 

 datum)

 (feet below

land surface 

 datum)

Natural gamma Neutron

Increasing Increasing

Lithology

Natural gamma

Increasing

Natural gamma Neutron

Increasing Increasing

or or

Dockum Group

Note: land surface datum; North American Vertical Datum 1988 Ochoan Series
Hydrogeologic Framework 15

The layered-earth electrical scenarios obtained for 

applicable AMT soundings were compared to nearby well 

data in order to interpret the electrical signature of the Trinity 

Group compared to the borehole geophysical log picks (fig. 7, 

table 3 at end of report). Based on the AMT-log comparison, 

the Trinity Group was determined to be moderately resistive 

(approximately 50 ohm-meters) and was generally the 

first resistive layer below the first conductive layer. This 

information was used to interpret the depth of the Trinity 

Group in the nine AMT soundings that were not near a well.

The layered-earth electrical scenarios for the four AMT 

soundings near wells (AMT07, AMT08, AMT11, and AMT12) 

resulted in the identification of different electrical layers. Each 

layer was identified as electrically conductive (referred to as 

a conductor) or electrically resistive (referred to as a resistor). 

Four scenarios of conductors and resistors were identified. 

The scenarios from AMT07 and AMT08 were similar with 

three (AMT07) or four layers (AMT08) identified; for both 

soundings, the four layers (in top-down order from the 

surface) were sequentially conductor, resistor, conductor, and 

resistor (fig. 10a). The second scenario (AMT12) consisted 

of five layers with a near-surface resistor followed by a 

conductor, which was followed by three layers progressively 

becoming more resistive (fig. 10b). The last scenario (AMT11) 

consisted of five or six layers with alternating conductive and 

resistive layers where the five-layer scenario had a resistor at 

surface and the six-layer scenario had a conductor at surface 

(fig. 10c). Three of the four soundings (AMT07, AMT08, and 

AMT11) resulted in layers that matched closely (within the 

accuracy of AMT measurements; Pearson and others, 2012) to 

the picks made from the borehole geophysical logs. The one 

exception was the pick for the bottom of the Trinity Group 

at AMT08 which was substantially lower (greater than 150 

ft lower or 50 percent of the AMT depth pick) than the pick 

made from the borehole geophysical log. The fourth sounding 

(AMT12) did not resolve the Trinity Group likely because the 

unit was too thin to identify.

The layered-earth electrical scenarios for the remaining 

nine AMT soundings consisted of similar scenarios as the 

AMT soundings located near the wells. These scenarios 

are grouped into three pairs of scenarios. The first pair of 

scenarios, which was equivalent to the layer-earth electrical 

scenarios found at AMT07 and AMT08 (fig. 10a), had three 

(AMT06) and four layers (AMT13); in top-down order from 

the surface, conductor, resistor, and conductor electrical layers 

were identified with an additional resistor below the bottom 

conductor in the fourth layer. The second pair of scenarios, 

which was equivalent to the layered-earth electrical scenario 

found at AMT12 (fig. 10b), consisted of four (AMT10) or 

five layers (AMT04 and AMT05) with a near-surface resistor 

Figure 9. Schematic showing the procedure to develop layered-earth electrical scenarios for the audio-magnetotelluric smooth 

inverse modeling results for the Pecos County region study area, Texas. Depth, in feet below land surface

Resistivity, in ohm-meters

Increasing resistivity

Increasing depth

Resistivity value

of layered-earth

electrical scenario

(blue line) set to 

peak points on the

smooth inverse 

modeling results

(red line)

Depth of the layered-earth 

electrical scenario (blue line)

adjusted so the areas (filled

in with black) between the 

layered-earth electrical 

scenario (blue line) and the 

smooth inverse modeling 

results (red line) directly 

above and below the 

selected depth have an

equal area
Figure 10. Schematic diagram of three (A, B, and C) layered-earth electrical scenarios for the Pecos County region study area, Texas.

ConductorConductorResistorResistor

Depth, in feet below land surface

Resistivity, in ohm-meters

Increasing resistivity

Increasing depth

ResistorConductor

Progressively 

more resistive

Depth, in feet below land surface

Resistivity, in ohm-meters

Increasing resistivity

Increasing depth

Depth, in feet below land surface

Resistivity, in 

Increasin

Increasing depth

CondCondResisResisCondResis

A B C
Hydrogeologic Framework 17

followed by a conductor, which was followed by two or three 

layers progressively becoming more resistive. The pair of 

scenarios, which was equivalent to the layered-earth electrical 

scenario found at AMT11, had five or six alternating resistive 

and conductive layers; the five-layer scenario had a resistor 

at surface (AMT01, AMT03, and AMT09) and the six-layer 

scenario had a conductor at surface (AMT02).

The general electrical responses of the Trinity Group 

were interpreted using the picks made from the borehole 

geophysical logs and comparing those depths with the picks 

made from the AMT soundings. The Trinity Group was 

moderately resistive (between 15-160 ohm-meters) with a 

calculated log-mean of 50 ohm-meters. The most resistive 

part of the Trinity Group layer was found in the southeastern 

part of the study area, and the least resistive part of the 

Trinity Group layer was found near the northern edge of the 

study area. Because borehole geophysical logs provided high 

vertical resolution in the subsurface, the picks made from the 

borehole geophysical logs were preferentially chosen as the 

final picks over nearby TDEM and AMT soundings.

After the geophysical logs and soundings were 

compiled and interpreted and hydrostratigraphic picks were 

determined, grids were created for each surface using kriging 

interpolation techniques. Geosoft, Inc. (2012) contains a 

complete description of the kriging methods used for grid 

interpolation. Preliminary grids were used to identify outliers 

and areas requiring review. To aid in identifying outliers, 

the residual was calculated as the difference between the 

hydrostratigraphic pick at each geophysical log or AMT 

sounding to the interpolated grid value. All locations with a 

residual greater than an absolute value of 15 ft were evaluated 

through a correlation process to determine data-point 

uncertainty. The correlation process involved the comparison 

of the pick at a given site to the picks made at nearby sites 

to determine if it “correlated” with the nearby well picks. 

If the pick varied by more than 15 ft from the nearby picks 

and seemed to not coincide with overall hydrostratigraphy of 

the area, it was removed from the final grid. Throughout the 

process, all stratigraphic picks were reviewed and revised as 

needed to provide a better understanding of the stratigraphic 

unit.

Structural Interpretations

Changes in altitude of the tops and bases of aquifer units, 

unit thicknesses, and normal-fault zones of the EdwardsTrinity aquifer were interpreted using the surfaces created 

from the interpolation of the hydrostratigraphic picks. 

Fault zones were delineated based on the interpretation of 

cross sections of the interpolated top and base surfaces of 

the Edwards-Trinity aquifer units and are similar to faults 

delineated previously for the underlying Rustler aquifer 

(INTERRA Incorporated, 2011). Displacement along fault 

zones is included in the final interpretation of the tops, bases, 

and thicknesses of the aquifer units.

Changes in altitude of the top of the Edwards-Trinity 

aquifer, which is, in general, the top of the Edwards part of 

the aquifer (upper Cretaceous), closely matched those of the 

land-surface altitudes throughout most of the study area. The 

altitude of the top surface of the Edwards-Trinity aquifer was 

highest in the southern part of the study area near the Glass 

Mountains (about 4,315 ft; fig. 11). For comparison purposes, 

the altitude ramps in figures 11, 12, and 13 are the same 

and include the minimum altitude in the base surface of the 

Edwards-Trinity aquifer and the maximum altitude in the top 

surface of the Edwards-Trinity aquifer. The altitude decreased 

to the northeast and the lowest altitude near the northeastern 

edge of the study area at the Pecos River was about 2,250 ft. 

The Edwards-Trinity aquifer dipped more sharply than the 

slope of the land surface in two locations. The first location 

was near the north-central boundary of the study area in the 

Monument Draw trough (fig. 2) where the altitude of the top 

of the Edwards-Trinity aquifer drops to about 2,020 ft, which 

was the lowest interpolated altitude for the top of the EdwardsTrinity aquifer. The other location where the altitude of the 

top of the Edwards-Trinity aquifer dropped substantially was 

in the northwestern part of the study area in the Pecos trough 

(fig. 2) where the altitude was about 2,450 ft. 

Changes in altitude of the top of the Trinity Group 

(fig. 12) closely matched those of the top of the EdwardsTrinity aquifer (fig. 11) in most of the study area. The altitude 

of the top surface of the Trinity Group was highest in the 

southern part of the study area near the Glass Mountains 

(about 4,230 ft; fig. 12). The altitude decreased to the northeast 

similar to the top of the Edwards-Trinity aquifer and the 

lowest altitude near the northeastern edge of the study area at 

the Pecos River was about 2,250 ft. The lowest altitude of the 

top of the Trinity Group was near the north-central boundary 

of the study area in the Monument Draw trough (fig. 2) where 

the altitude was about 1,960 ft.

The base surface of the Edwards-Trinity aquifer (fig. 13) 

had similar spatial trends as the top of the Trinity Group 

(fig. 12) and the top of the Edwards-Trinity aquifer (fig. 11). 

The highest altitude of the base of the Edwards-Trinity aquifer 

was in the southern part of the study area near the Glass 

Mountains (about 4,110 ft). Similar to the top of the Trinity 

Group of the Edwards-Trinity aquifer, the altitude of the 

base of the Trinity Group decreased to the northeast, which 

is consistent with findings by Barker and Ardis (1992). The 

lowest altitude for the base of the Edwards-Trinity aquifer 

(about 1,555 ft) was in the north-central part of the study area 

in the Monument Draw trough (fig. 2). 

The thickness of the Edwards-Trinity aquifer in the study 

area was calculated as the difference in altitudes between its 

top and base (fig. 14). About 50 percent of the aquifer was 

between 234 and 362 ft thick, about 25 percent was less than 

234 ft thick, and about 25 percent was more than 362 ft thick. 

The minimum thickness was 5 ft and the maximum thickness 

was about 797 ft. The thickness of the Edwards part of the 

aquifer in the study area was calculated as the difference in 
Figure 11. The altitude of the top surface of the Edwards-Trinity aquifer estimated by interpolating the tops and bases of hydrostratigraphic subdivisregion study area, Texas.

PECO

COUN

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPTON 

COUNT

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Note: For comparison purposes, the altitude ramps in figures 11, 12, and 13 are the same and include the minimum altitude in the base surface of the Edwards-Trinity aquifer and the maximum altitude in the top surface 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashw

Top surface altitude of the Edwards-Trinity aquifer, in feet above North American Vertical Datum of 1988 High : 4,315

Low : 1,555

Edwards-Trinity aquifer (subcropEdwards-Trinity aquifer (outcropStudy area boundaryFault zone—Represents numerou

EXPLANATION

£¤285

£¤285

¦§ ̈20

¦§ ̈10
Figure 12. The altitude of the top surface of the Trinity part of the Edwards-Trinity aquifer estimated by interpolating the tops and bases of hydrostratPecos County region study area, Texas.

PECO

COUN

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPTON 

COUNT

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Note: For comparison purposes, the altitude ramps in figures 11, 12, and 13 are the same and include the minimum altitude in the base surface of the Edwards-Trinity aquifer and the maximum altitude in the top surface 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashw

£¤285

£¤285

¦§ ̈20

¦§ ̈10

Top surface altitude of the Trinity part of Edwards-Trinity aquifer, in feet above North American Vertical Datum of 1988 High : 4,315

Low : 1,555

Edwards-Trinity aquifer (subcroEdwards-Trinity aquifer (outcroStudy area boundaryFault zone—Represents numer

EXPLANATION
Figure 13. The altitude of the base surface of the Edwards-Trinity aquifer estimated by interpolating the tops and bases of hydrostratigraphic subdivregion study area, Texas.

PECO

COUN

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPTON 

COUNT

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Note: For comparison purposes, the altitude ramps in figures 11, 12, and 13 are the same and include the minimum altitude in the base surface of the Edwards-Trinity aquifer and the maximum altitude in the top surface 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashw

Base surface altitude of the Edwards-Trinity aquifer, in feet above North American Vertical Datum of 1988 High : 4,315

Low : 1,555

Edwards-Trinity aquifer (subcropEdwards-Trinity aquifer (outcropStudy area boundaryFault zone—Represents numerou

EXPLANATION

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£¤285

¦§ ̈20

¦§ ̈10
Hydrogeologic Framework 21

altitudes between the top of the Edwards-Trinity aquifer and 

the top of the Trinity Group (fig. 15). About 50 percent of 

the Edwards part of the aquifer was between 44 and 169 ft 

thick, about 25 percent was less than 44 ft thick, and about 25 

percent was more than 169 ft thick. The minimum thickness 

was 0 ft and the maximum thickness was about 723 ft. The 

thickness of the Trinity Group in the study area was calculated 

as the difference in altitudes between the top of the Trinity 

Group and the bottom of the Edwards-Trinity aquifer (fig. 16). 

About 50 percent of the Trinity Group was between 144 and 

224 ft thick, about 25 percent was less than 144 ft thick, and 

about 25 percent was more than 224 ft thick. The minimum 

thickness was 3 ft and the maximum thickness was about 

543 ft.

Some of the thinnest sections of the Edwards-Trinity 

aquifer were 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. It was determined 

that the aquifer was often thickest in the central part of the 

study area in the Monument Draw trough and at the western 

edge of the study area in the Pecos trough, which is consistent 

with findings by Barker and Ardis (1992). There were three 

areas of the Edwards-Trinity aquifer where substantial 

thickness variations were identified; in two areas, the thickness 

of the aquifer increased appreciably per unit of horizontal 

length (fig. 14). In the areas west of Fort Stockton and north of 

the Glass Mountains, the base of the Edwards-Trinity aquifer 

dips at a higher angle to the north than does the top of the 

Edwards-Trinity aquifer. In the third location, a valley exists 

in the top of the Edwards-Trinity aquifer between the Glass 

Mountains and the Davis Mountains that did not appear in 

the base of the Edwards-Trinity aquifer, which resulted in a 

thinner section of the aquifer.

Some of the thinnest sections of the Edwards part of the 

aquifer (fig. 15) were in the eastern part of the study area, 

near the northwestern slope of the Glass Mountains, and 

in the down-dip part of the Pecos trough (fig. 2). Based on 

interpretation of the data, it is likely that many of the thinnest 

sections of the Edwards part of the aquifer are associated 

with active or paleo-erosional features. Some of the thickest 

sections of the Edwards part of the aquifer were generally 

in the southern and central parts of the study area in the 

Monument Draw trough (fig. 2).

Some of the thinnest sections of the Trinity Group of the 

Edwards-Trinity aquifer were in the eastern part of the study 

area and in the ridge that separates the Pecos and Monument 

Draw troughs (fig. 2). Some of the thickest sections of the 

Trinity Group were in the western part of the study area in the 

Pecos trough and in the central part of the study area in the 

Monument Draw trough.

Faults in the study area likely 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 sediments collapsed into the voids 

created by the dissolution of Permian-age evaporite deposits. 

Figures 17 and 18 show examples of the roughly west-east 

(A–A′) and south-north (B–B′) orientated cross sections that 

were used to delineate the fault zones. The dips of these fault 

zones were not evaluated for this study. Also, although the 

Cretaceous (Edwards-Trinity) and Triassic (Dockum and 

underlying units) aquifers are shown in figures 17 and 18, 

the actual thicknesses of the Triassic units were not estimated 

for this study and are shown for reference only. Each fault 

zone represents a series of parallel and transverse faults 

that result in an overall displacement between two adjacent 

fault blocks. Fault zones (figs. 11–18) delineate domains 

in the hydrogeologic framework that generally align with 

previously identified structural features such as the Pecos and 

Monument Draw troughs (fig. 2). Some fault blocks, which 

are fault-bounded areas, shown in the cross sections (figs. 17 

and 18), particularly those in the Pecos and Monument Draw 

trough areas, are not represented in the surface fault-zone 

delineations (figs. 17–18) because of the coarse resolution of 

the surface delineation. Also, there is likely extensive faulting 

at a relatively higher resolution not shown in each of the fault 

blocks in the cross sections. 

The displacement along the delineated fault zones 

was calculated as the average displacement of combined 

faults within a given delineated fault block (figs. 11–16). 

For example, the displacement of the graben (an elongate 

trough or basin bounded by high-angle normal faults that dip 

toward one another [Neuendorf and others, 2005, p. 277]) that 

contains the Monument Draw trough (FB03) in cross section 

A–A′ (fig. 17) relative to the adjacent horst (an elongate block 

bounded by normal faults that dip away from one another 

[Neuendorf and others, 2005, p. 307]) that includes deposits 

(FB04; fig. 17) that approximately overly the Capitan Reef 

aquifer (fig. 4) was calculated as the difference between the 

average basal altitudes of the two fault blocks in the graben 

and the average basal altitudes of the two fault blocks in the 

horst. Displacement along the trend of a fault zone varied 

depending on the basal altitudes of the adjacent fault blocks at 

a given location. The maximum interpreted displacement at a 

location along a delineated fault zone was about 1,025 ft and 

is located along the FB01 and FB07 fault zone (figs. 17 and 

18) in the Barilla Mountains (fig. 2). The minimum interpreted 

displacement at a location along a delineated fault zone was 

about 1 ft and is located along the FB03 and FB04 fault zone 

(figs. 17 and 18) near Belding, Tex. (fig. 1).

Cross section A–A′ originates at the western boundary 

of the study area in the Pecos trough (fig. 2) and extends to 

the eastern boundary of the study area (fig. 17). This section 

shows 11 horizontal horst, graben, and stairstep fault blocks 

(faults expressed as numerous small fractures, breccia, or 

fault gouges [Neuendorf and others, 2005, p. 231]). Each of 

these fault blocks likely contains a series of faults that result 

in a cumulative displacement along the fault zones. The Pecos 

trough (fig. 2) near the western boundary is in a graben (FB01; 

fig. 17) created by the dissolution of Permian-age evaporite 

deposits and the subsequent collapse of overlying units. It is 

possible that Edwards-Trinity units are not connected between 

fault blocks within the Pecos trough and, thus, create a barrier 
Figure 14. The thickness of the Edwards-Trinity aquifer in the Pecos County region study area, Texas, calculated as the difference between the altitusurfaces of the Edwards-Trinity aquifer.

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A

Edwards-Trinity aquifer (subcrop)Edwards-Trinity aquifer (outcrop) Study area boundaryFault zone—Represents numerou

EXPLANATION

Edwards-Trinity aquifer thickness, in feet 5 to less than 86 86 to less than 234 234 to 362 Greater than 362 to 510 Greater than 510 to 797

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Figure 15. The thickness of the Edwards part of the Edwards-Trinity aquifer in the Pecos County region study area, Texas, calculated as the differenthe top and base surfaces of the Edwards part of the Edwards-Trinity aquifer.

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A

Edwards-Trinity aquifer (subcrop)Edwards-Trinity aquifer (outcrop) Study area boundaryFault zone—Represents numerou

EXPLANATION

Edwards part of Edwards-Trinity aquifer thickness, in feet 0 to less than 34 34 to less than 44 44 to 169 Greater than 169 to 314 Greater than 314 to 723

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Figure 16. The thickness of the Trinity part of the Edwards-Trinity aquifer in the Pecos County region study area, Texas, calculated as the difference top and base surfaces of the Trinity part of the Edwards-Trinity aquifer.

PE

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY

Fort Stockton

Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A

Edwards-Trinity aquifer (subcrop)Edwards-Trinity aquifer (outcrop) Study area boundaryFault zone—Represents numerou

EXPLANATION

Trinity part of Edwards-Trinity aquifer thickness, in feet 3 to less than 51 51 to less than 144 144 to 224 Greater than 224 to 317 Greater than 317 to 543

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Hydrogeologic Framework 25

Figure 17. Generalized map and detailed cross section A–A′ in the Pecos County region study area, Texas.

Aquifer modified from Ashworth and Hopkins, 1995

Lithologic and geophysical data from

Daniel B. Stephens and Associates, 2010;

Texas Water Development Board, 2011;

University of Texas Lands, 2010;

Meyer and others, 2011;

Texas Railroad Commission, 2010;

and U.S. Geological Survey, 2011

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

Fort Stockton

WARD COUNTY CRANE

COUNTY

UPTON

COUNTY

PECOS

COUNTY

TERRELL

COUNTY

BREWSTER

COUNTY

FRED DAVIS

COUNTY

REEVES

COUNTY

PRESIDIO

COUNTY

CROCKETT

COUNTY

§ ̈¦20

§ ̈¦10

§ ̈¦10

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B

B'

A

A'

FB01

FB02

FB03

FB04

FB05

FB06

FB08 FB09

FB10

FB07

0 10 20 MILES

0 10 20 KILOMETERS N

FB01 FB02 FB03 FB04 FB06 FB09

?

?

?

?

?

?

?

?

?

? ?

Leon–Belding

area WEST EAST

Distance, in feet

025,00050,00075,000100,000125,000150,000175,000

200,000225,000250,000275,000300,000325,000350,000375,000400,000425,000450,000

VERTICAL SCALE GREATLY EXAGGERATED

£¤285 § ̈¦10

BRACS-0210

BRACS-0769 BRACS-1355 BRACS-1532

BRACS-2026 BRACS-2208

BRACS-2212 BRACS-2661 BRACS-2663

B BRACS-2664 RACS-2923 DBSA-059

DBSA-104 DBSA-071

DBSA-120

DBSA-130

DBSA-131

DBSA-320 A A'

1,500

0

1,500

Distance,

 in feet

Leon–Belding area

Note: Faulting in the study area was mapped as fault zones; individual fault traces were not determined

B–B' (south to north, in part)

N

EXPLANATION

Edwards–Trinity aquifer (outcrop)

Edwards–Trinity aquifer (subcrop)

Aquifer units in cross section

 Units overlying Edwards-Trinity

 aquifer

 Edwards part of the

 Edwards-Trinity aquifer

 Trinity part of the Edwards-Trinity

 aquifer (Trinity Group)

 Dockum and underlying units

 (actual thickness

 not determined)

Line of section

Study area boundary

Leon–Belding area boundary

Fault zone—Represents numerous

 faults. Might not align with faults

 shown in example cross section

Identified top of Edwards part of the

 Edwards-Trinity aquifer

Identified top of the Trinity Group

Identified base of the Trinity Group

Fault (in section)—Arrow shows

 relative movement

Well (on section)

Well (on location map)

Well and identifier (on strip map)

Fault block identifier (bounded

 by fault zones)

DBSA-071

?

FB01

A A'

BRACS-0769

BRACS-2208BRACS-2212

DBSA-320

DBSA-071

DBSA-120

DBSA-130

DBSA-104

DBSA-059

BRACS-0210

BRACS-2923

BRACS-2663

BRACS-2664 BRACS-1355

BRACS-1532

BRACS-2026

DBSA-131

SECTION

B–B'

Interstate 10

State Highway 285

Interstate 10

A A'

BRACS-2661

1,500

2,000

2,500

3,000

3,500

Altitude, in feet

above North American Vertical Datum of 1988
26 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Figure 18. Generalized map and detailed cross section B–B′ in the Pecos County region study area, Texas.

?

?

?

?

? ?

?

? ?

Mountains

Glass

SECTION

A–A'

Interstate 10

Highway 285

DBSA-051

DBSA-104

DBSA-124

BRACS-2513

BRACS-2943

DBSA-212

BRACS-2222

Leon–Belding area

B B'

VERTICAL SCALE GREATLY EXAGGERATED

SOUTH NORTH

0

25,00050,00075,000100,000125,000150,000175,000200,000225,000250,000

Distance, in feet

FB08 FB03 FB04

Altitude, in feet

above North American Vertical Datum of 1988

2,000

2,500

3,000

3,500

4,000

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

Aquifer modified from Ashworth and Hopkins, 1995

Lithologic and geophysical data from

Daniel B. Stephens and Associates, 2010;

Texas Water Development Board, 2011;

University of Texas Lands, 2010;

Meyer and others, 2011;

Texas Railroad Commission, 2010;

and U.S. Geological Survey, 2011

Fort Stockton

WARD COUNTY CRANE

COUNTY

UPTON

COUNTY

PECOS

COUNTY

TERRELL

COUNTY

BREWSTER

COUNTY

FRED DAVIS

COUNTY

REEVES

COUNTY

PRESIDIO

COUNTY

CROCKETT

COUNTY

§ ̈¦20

§ ̈¦10

§ ̈¦10

£¤285

£¤285

B

B'

A

A'

FB01

FB02

FB03

FB04 FB05

FB06

FB08 FB09

FB10

FB07

0 10 20 MILES

0 10 20 KILOMETERS N

DBSA-051

FB08

EXPLANATION

?

B B'

Edwards–Trinity aquifer (outcrop)

Edwards–Trinity aquifer (subcrop)

Aquifer units in cross section

 Units overlying Edwards-Trinity

 aquifer

 Edwards part of the

 Edwards-Trinity aquifer

 Trinity part of the Edwards-Trinity

 aquifer (Trinity Group) 

 Dockum and underlying units

 (actual thickness

 not determined)

Line of section

Study area boundary

Leon–Belding area boundary

Fault zone—Represents numerous

 faults. Might not align with faults

 shown in example cross section

Identified top of Edwards part of the

 Edwards-Trinity aquifer

Identified top of the Trinity Group

Identified base of the Trinity Group

Fault (in section)—Arrow shows

 relative movement

Well (on section)

Well (on location map)

Well and identifier (on strip map)

Fault block identifier (bounded

 by fault zones)

BRACS-2222

BRACS-2513

BRACS-2943 DBSA-051 DBSA-104

DBSA-124 DBSA-212

1

§ ̈ £¤285 ¦10

B B'

1,500

0

1,500

Distance,

 in feet

Leon–Belding area

Note: Faulting in the study area was mapped as fault zones; individual fault traces were not determined

A–A' (west to east, in part) N
Geochemistry 27

to flow, but the disconnection at this location is not confirmed 

and, if it does exist at this location, may not be laterally 

continuous. Faults east of, and adjacent to, FB01 (fig. 17) 

stairstep up to a horst (FB02; fig. 17) formed from deposits 

that overlie more resistant rock units and form a ridge between 

the Pecos and Monument Draw troughs (fig. 2). Stairstep 

faults east of the ridge, also created by the dissolution of 

Permian-age evaporite deposits, form the graben (FB03; 

fig. 17) that is part of the Monument Draw trough (fig. 2). 

The next series of stairstep faults form a horst (FB04; fig. 17) 

to the east of the Monument Draw trough (fig. 2) and are in 

Cretaceous rocks that overlie the Capitan Reef aquifer (fig. 4). 

Cross section A–A′ is completed by a graben (FB06; fig. 17) 

and horst (FB09; fig. 17) that formed east of the Capitan Reef 

aquifer (fig. 4) (FB04; fig. 17).

Cross-section B–B′ originates in the Glass Mountains 

(fig. 2) along the southern boundary of the study area and 

extends north to the down-dip extent of the Edwards-Trinity 

aquifer (fig. 18). There are a series of stairstep fault zones, 

horsts, and grabens that separate the downthrown trough fault 

blocks (FB08 and FB03; fig. 18) in the south from the deposits 

overlying the Capitan Reef aquifer (fig. 4) (FB04; fig. 18) to 

the north. The intrablock faults correlate closely with thickness 

variations in the Edwards-Trinity aquifer (figs. 14–16). Also, 

the fault zones shown in the cross section are likely a fraction 

of those that formed as growth faults along the margins of the 

Glass Mountains and resistant Permian-age deposits along 

the Capitan Reef aquifer and as collapse features from the 

dissolution of Permian-age evaporite deposits. 

Aquifer Tests and Hydraulic Properties

Aquifer-test data collected at 47 groundwater wells were 

compiled from other reports or agencies such as the TWDB 

Groundwater Database (Texas Water Development Board, 

2011), Thornhill Group, Inc., (2008, 2009), and Meyers (1969) 

and used in this study. One aquifer test was completed with 

ambient and pumping borehole flowmeter measurements by 

the USGS as part of this study. 

Transmissivity is an aquifer hydraulic property that is 

defined by the rate water is transmitted horizontally through 

a unit width of an aquifer under a unit hydraulic gradient. It 

equals the hydraulic conductivity of an aquifer multiplied by 

the aquifer thickness (or saturated thickness if unconfined) 

(Fetter, 1988). Higher transmissivity values correlate with 

higher yields and less drawdown in a well. Previously reported 

transmissivity values were calculated using one or more of 

the following methods: the Thiem equilibrium formula 

(Thiem, 1906), the Theis nonequilibrium formula (Theis, 

1935), the Jacob modified formula (Cooper and Jacob, 1946), 

or methods documented in Ferris and others (1962) and 

Wenzel (1942). 

Most historical aquifer-test data included pump rate or 

yield in gallons per minute and drawdown, in feet. These 

data were used to estimate the hydraulic properties (specific 

capacity and transmissivity) in the study area. Specific 

capacity can be used to provide the maximum yield for a 

well and to estimate the transmissivity of an aquifer. Specific 

capacity is obtained by dividing the pump rate or yield by the 

total drawdown. Using the Jacob modified formula (Cooper 

and Jacob, 1946), Driscoll (1986) developed an approximating 

formula for estimating transmissivity from specific capacity 

in confined and unconfined aquifers. Transmissivity was 

estimated from specific capacity using Driscoll’s equation for 

an unconfined aquifer if previously published transmissivity 

values were not available (Driscoll, 1986, appendix 16D). 

Driscoll’s equation is:

T = 1,500 X Q/s (for an unconfined aquifer) (1)

where

T is transmissivity, in gallons per day per foot of 

drawdown;

Q is well yield or pumping rate, in gallons per 

minute;

s is drawdown at any point in the vicinity of a 

well discharging at a constant rate, in feet; 

and

Q/s is specific capacity, in gallons per minute per 

foot of drawdown.

The calculated and estimated transmissivity values 

ranged from 1,500 to 1,216,000 gallons per day per foot of 

drawdown [(gal/d/ft)]. The transmissivity value and data 

source for each well used in this dataset are shown in table 4, 

at end of report.

Figure 19 shows the distribution of the log of transmissivity values measured in the Edwards-Trinity aquifer 

in the study area. Although the tests were done at different 

pumping rates and durations, which limits direct comparison 

of the results, the map provides a general understanding of 

transmissivity distributions across the study area. The highest 

transmissivity values were measured in the Monument Draw 

trough (fig. 2), which is also one of the thickest parts of the 

Edwards-Trinity aquifer, and the lowest values were measured 

in the eastern part of the study, near some of the thinnest 

parts of the aquifer. Hydraulic conductivity values, which 

were calculated using the transmissivity values and estimated 

saturated thickness of the aquifer (Ritzema, 1994), generally 

showed the same patterns as the transmissivity values 

(table 4). Faulting within the FB03 fault block (figs. 17–18) 

in the Leon-Belding area of the Monument Draw trough 

(figs. 1–2) likely contributed to higher transmissivity values in 

that area.

Geochemistry

Variations in the chemistry of groundwater are caused 

by constituents dissolved in water, reactions among these 

constituents, reactions between these constituents and minerals 

in the rocks through which the water flows (water-rock 
Figure 19. The log of transmissivity values calculated and estimated using aquifer-test data from wells completed in the Edwards-Trinity aquifer in thstudy area, Texas.

P

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

Fort Stockton

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

£¤285

£¤285

¦§ ̈20

¦§ ̈10

T1

T3 T2

T4

T5 T6

T7

T8

T9

T10

T11

T13

T14

T15

T16

T17

T18

T19

T20

T26

T28 T27

T29

T30

T31 T32

T33

T34

T35 T36

T37

T38

T39 T40

T41

T42

T45

T46

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

EXPLANATION

Edwards-Trinity aquifer (outcrop)Edwards-Trinity aquifer (subcrop)Study area boundaryFault zone—Represents numerous faults

Aquifer-test sites and site identification number (table 4)—L transmissivity, in gallons per day per foot of drawdown

7.31–8.80 8.81–10.28 10.29–11.77 11.78–13.25 13.

T46 T14 T35 T36 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A
Geochemistry 29

interaction), and mixing of water from different sources 

(Hem, 1992; Small and Ozuna, 1993). Interpreting chemical 

variability in an aquifer provides insight into the quality of 

the groundwater, hydrologic flow paths, potential recharge 

sources, extent of water-rock interaction, and groundwater 

mixing pathways. In order to assess the groundwater 

chemistry (geochemistry) of the Pecos County region, 

geochemical samples were collected in the study area in 

2010 and 2011 from 38 wells completed in the Pecos Valley, 

Igneous, Edwards-Trinity, Rustler, Dockum, and Capitan Reef 

aquifers and four springs as part of the first phase of this study 

(fig. 20, table 5 at end of report). Pearson and others (2012) 

describe the methods used for collection, laboratory analysis, 

and quality control of the data that were collected or compiled 

for the study area and used in this report. An important 

revision and update in this report is aquifer codes for several 

wells contained in the geodatabase, which was developed by 

Pearson and others (2012), were changed when the sampling 

interval of the wells were compared to the hydrogeologic 

framework developed in this phase of the study (table 6 at end 

of report). 

Analysis of the geochemical samples provided 

insight into the chemical characteristics of water from 

different sources and different aquifers. Distinct chemical 

characteristics of these water samples were used to 

qualitatively evaluate aquifer interaction, groundwater-flow 

paths, water-rock interaction, mixing of water from different 

sources, and to identify likely source waters and geochemical 

endmembers.

Although the Edwards-Trinity aquifer was the focus of 

the geochemical sampling, samples were collected from other 

aquifers in the groundwater system in an effort to understand 

how water from these sources might interact with and affect 

the Edwards-Trinity aquifer. Only those analytical results most 

relevant to the understanding of flow paths, potential recharge 

sources, and mixing pathways of the groundwater system are 

discussed in this report. Geochemical results are available 

from the U.S. Geological Survey National Water Information 

System (NWIS) (U.S. Geological Survey, 2011) and from 

Pearson and others (2012).

Overview of Geochemical Data

Geochemical physical properties and constituents used to 

evaluate aquifer interaction, groundwater-flow paths, waterrock interaction, and mixing of water in the groundwater 

system include specific conductance, hydrochemical facies, 

sulfate and chloride concentrations, silica concentrations, 

stable isotopes of oxygen and hydrogen, strontium isotopes, 

environmental tracers, and concentrations of organic compounds and nutrients (tables 7 and 8 at end of report). These 

results were used in combination to identify the chemical 

characteristics of water from different sources and to determine how water from different sources might be mixing in 

the groundwater system. The qualitative geochemical analysis 

does not include a quantitative evaluation of residence times 

in the aquifer nor does it include geochemical flow-path 

modeling of the groundwater system.

Specific Conductance

Specific conductance measures the ability of water 

to conduct an electrical current and is related to the ion 

concentration; typically, there is a monotonic relation between 

specific conductance and the dissolved-solids concentration 

(Hem, 1992). The specific-conductance values measured in 

waters in the study area (table 7) are comparable to previously 

published data (Small and Ozuna, 1993; Uliana and Sharp, 

2001; Thornhill Group, Inc., 2008; Texas Water Development 

Board, 2011). Specific-conductance values were generally 

quite variable within and between different aquifers. Higher 

specific conductance values reflecting more saline water were 

associated with relatively high concentrations of selected 

major ions, including chloride (Cl), sulfate (SO4), sodium 

(Na), calcium (Ca), and magnesium (Mg). Rainwater has 

very low specific conductance values (specific-conductance 

values usually less than 25 microsiemens per centimeter at 25 

degrees[μS/cm]) (National Atmospheric Deposition Program, 

1997). Compared to rainwater, groundwater typically has 

higher specific conductance resulting from the dissolution of 

subsurface minerals and rock matrix. Specific conductance 

in geochemical samples collected in the study area ranged 

from 273 to 7,260 μS/cm (table 7). The higher specific 

conductance values in the study area might be caused by 

the groundwater mixing with more saline groundwater or 

dissolution of evaporites. In the absence of other sources of 

salinity, specific conductance values would likely increase 

along groundwater-flow paths as a result of progressive waterrock interaction.

Specific conductance of water in the four Pecos Valley 

aquifer wells ranged from 362 to 1,690 μS/cm with an 

average value of 1,020 μS/cm and median value of 1,020 

μS/cm (fig. 20, table 7). Samples collected from sites Q7 

and Q15, which are wells completed in the Igneous aquifer, 

had specific conductance values of 331 and 408 μS/cm, 

respectively. The average specific conductance value measured in samples from the Igneous aquifer (370 μS/cm) was 

the lowest of the sampled hydrostratigraphic units. Specific 

conductance values measured in the samples collected from 

20 wells completed in the Edwards-Trinity aquifer ranged 

from 273 to 7,260 μS/cm with an average value of 2,340 

μS/cm and median value of 1,830 μS/cm. The large range 

of specific conductance values in samples collected from 

the Edwards-Trinity aquifer indicates that processes such 

as mixing with recent, local recharge, mixing with more 

saline water sources, and progressive water-rock interaction 

along regional groundwater-flow paths might influence 

groundwater compositions. Higher specific conductance 

values in the Edwards-Trinity aquifer (relative to the median) 

generally were measured in samples collected from wells 

completed in the Monument Draw trough (sites Q14, Q16, 

Q18, Q22, Q23, Q24, Q26, and Q37); from site Q35, which 
Figure 20. Locations of groundwater-well and spring sites for geochemical data collection in the Pecos County region study area, Texas.

P

CO

BREWSTER 

COUNTY

PRESIDIO 

COUNTY

REEVES 

COUNTY

JEFF DAVIS 

COUNTY

UPT

COU

WARD 

COUNTY

CRANE 

COUNTY Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

Fort Stockton

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

£¤285

£¤285

¦§ ̈20

¦§ ̈10

Q1

Q2

Q4

Q7

Q8

Q9

Q10

Q11 Q12 Q13 Q14

Q15

Q16

Q17

Q19

Q20

Q21

Q22 Q23

Q24 Q25

Q28 Q30 Q29 Q31 

Q33

Q34

Q35

Q36 Q37

Q38

Q39

Q40

Q41

Q43 Q44

Q6

Q18

Q26

Q27

Q42

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A

EXPLANATION

Edwards-Trinity aquifer (outcrop)Edwards-Trinity aquifer (subcrop)Study area boundaryFault zone—Represents numerous faults

Well, DoWell, RuWell, Ca

Geochemistry site and site identification numb by site type and contributing aquifer Spring Well, Pecos Valley aquifer Well, Igneous aquifer Well, Edwards-Trinity aquifer

Q1

Q36Q15

Q8

Q21

Q10Q20
Geochemistry 31

is located near the down-dip extent of the aquifer; and from 

site Q30, which is near the western boundary of the study 

area. Specific conductance values in samples collected from 

the five wells completed in the Dockum aquifer ranged from 

736 to 3,200 μS/cm with an average value of 1,680 μS/cm 

and median value of 1,560 μS/cm. Specific conductance 

values in samples collected from the five wells completed 

in the Rustler aquifer ranged from 553 to 3,980 μS/cm with 

an average value of 2,030 μS/cm and median value of 2,050 

μS/cm. Samples collected from sites Q20 and Q41, which 

are wells completed in the Capitan Reef aquifer, had specific 

conductance values of 2,290 and 4,160 μS/cm, respectively. 

The average specific-conductance value measured in samples 

from the Capitan Reef aquifer (3,230 μS/cm) was the 

highest of the sampled hydrostratigraphic units. The specific 

conductance measured in samples collected from the four 

springs in the study area (sites Q1, Q2, Q4, and Q6) ranged 

from 2,430 to 6,730 μS/cm with an average value of 4,710 μS/

cm and median value of 4,840 μS/cm. The average specificconductance value measured in samples from the spring 

sites was higher than the samples collected from any of the 

hydrostratigraphic units (fig. 20, table 7).

Hydrochemical Facies

The composition of groundwater principally is controlled 

by the composition of recharge water, water-rock interaction, 

and the mixing of water from different sources. The term 

“facies” refers to a classification scheme used to describe 

water in terms of the major cation and anion milliequivalents 

composition (figs. 20 and 21, table 7). A trilinear (“Piper”) 

diagram (Piper, 1953) is a useful tool for evaluating the 

relative abundance of major cations and anions and classifying 

facies or water types. Hydrochemical facies were complex 

and varied between and within the aquifers in the study 

area. Variations in facies were predominantly associated 

with different locations in the study area and likely result 

from localized groundwater processes and progressive 

mineralization along groundwater-flow paths. Facies ranged 

from simple, such as Na-HCO3 type water, to complex, such 

as Na-Ca-Mg-SO4-Cl-HCO3 type water.

Sulfate and Chloride Concentrations

Variations in SO4 and Cl concentrations in groundwater 

and their relations with other major and trace elements can 

be used to qualitatively assess water-rock interaction with 

evaporite deposits, such as anhydrite (CaSO4), gypsum 

(CaSO4∙2H2O), and halite (NaCl). The groundwater SO4 and 

Cl concentrations measured in the study area (figs. 20 and 22, 

table 7) are comparable to previously published data (Small 

and Ozuna, 1993; Uliana and Sharp, 2001; Thornhill Group, 

Inc., 2008; Texas Water Development Board, 2011) and the 

variability in ranges generally corresponded with specific 

conductance. SO4 concentrations in the Pecos Valley aquifer 

ranged from 11.8 to 3,020 milligrams per liter (mg/L) with an 

average value of 1,440 mg/L and median value of 1,360 mg/L, 

and Cl concentrations ranged from 7.16 to 4,770 mg/L with an 

average of 2,190 mg/L and median value of 2,000 mg/L. SO4

concentrations were 12.9 and 16.0 mg/L and Cl concentrations 

were 8.39 and 8.81 mg/L for samples from wells completed 

in the Igneous aquifer (sites Q7 and Q15, respectively). SO4

concentrations in the Edwards-Trinity aquifer ranged from 

12.2 to 2,010 mg/L with an average value of 537 mg/L and 

median value of 358 mg/L, and Cl concentrations ranged 

from 5.16 to 1,370 mg/L with an average value of 363 

mg/L and median value of 282 mg/L. Relatively higher 

concentrations of both SO4 and Cl were measured in samples 

collected from Edwards-Trinity aquifer sites Q14, Q16, Q18, 

Q22, Q23, Q24, Q26, Q30, Q35, and Q37, which are the 

same wells with relatively high specific conductance. SO4

concentrations in the Dockum aquifer ranged from 91.8 to 

586 mg/L with an average value of 296 mg/L and median 

value of 335 mg/L, and Cl concentrations ranged from 

58.2 to 675 mg/L with an average value of 265 mg/L and 

median value of 195 mg/L. SO4 concentrations in the Rustler 

aquifer ranged from 52.5 to 2,270 mg/L with an average 

value of 748 mg/L and median value of 357 mg/L, and Cl 

concentrations ranged from 23.5 to 332 mg/L with an average 

value of 194 mg/L and median value of 179 mg/L. SO4

concentrations in samples collected from wells completed 

in the Capitan Reef aquifer (sites Q20 and Q41) were 421 

and 2,320 mg/L, and Cl concentrations were 370 and 354 

mg/L, respectively. The SO4 concentration relative to the Cl 

concentration measured in samples collected from two sites 

(site Q29, completed in the Rustler aquifer and site Q41, 

completed in the Capitan Reef aquifer), deviated from the 

generally linear pattern of the rest of the measured values 

(fig. 22). The relatively higher concentration of SO4 at these 

sites indicates groundwater interaction with a higher SO4

source compared to the other sites, possibly from evaporite 

deposits of anhydrite and gypsum. SO4 concentrations measured in spring water (sites Q1, Q2, Q4, and Q6) ranged from 

437 to 1,890 mg/L with an average value of 1,220 mg/L and 

median value of 1,270 mg/L, and Cl concentrations ranged 

from 431 to 1,220 mg/L with an average value of 849 mg/L 

and median value of 873 mg/L. 

Silica Concentrations

Variations in silica (Si) concentrations in groundwater 

can be used to qualitatively assess water-rock interaction 

with rocks composed of silicate minerals (Hem, 1992). The 

groundwater Si concentrations measured in the study area 

(fig. 20, table 7) are comparable to previously published data 

(Texas Water Development Board, 2011). Si concentrations 

in the Pecos Valley aquifer ranged from 22.6 to 53.5 mg/L 

with an average value of 34.0 mg/L and median value of 

29.8 mg/L. Si concentrations were 43.6 and 43.4 mg/L in 

samples collected from wells completed in the Igneous aquifer 

(sites Q7 and Q15, respectively) with an average value of 

43.5 mg/L, which was the highest average value among the 
32 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

aquifers. Si concentrations in the Edwards-Trinity aquifer 

ranged from 10.4 to 52.0 mg/L with an average value of 

21.6 mg/L and median value of 20.8 mg/L. Si concentrations 

in the Dockum aquifer ranged from 11.1 to 30.9 mg/L with 

an average value of 19.8 mg/L and median value of 16.2 mg/L. 

Si concentrations in the Rustler aquifer ranged from 0.479 

to 19.6 mg/L with an average value of 13.4 mg/L and median 

value of 14.0 mg/L. Si concentrations in samples collected 

from wells completed in the Capitan Reef aquifer (sites 

Q20 and Q41) were 21.0 and 13.9 mg/L. Si concentrations 

measured in spring water ranged from 22.8 to 35.4 mg/L 

with an average value of 28.9 mg/L and median value of 

28.7 mg/L.

Stable Isotopes

Citing the work of Faure (1986), Uliana and others (2007, 

p. 338) noted that oxygen and hydrogen isotopes of the water 

molecule are indicators of conditions present at the time and 

place of groundwater recharge. The ratios of oxygen-18/

oxygen-16 isotopes (δ18O in per mil) and hydrogen-2/

hydrogen-1 (δD in per mil) are compared to the global 

meteoric water line (MWL) of Craig (1961), which represents 

the composition of rainfall around the globe. Stable isotope 

values for groundwater and spring water in the study area also 

were compared to annual weighted mean precipitation values 

(-6.50 per mil and -44.0 per mil for δ18O and δD, respectively, 

Figure 21. Trilinear diagram showing relations between major cations and anions in groundwater and spring water collected in the 

Pecos County region study area, Texas.

80

20

40

60

80

20

40

60

80

Bicarbonate

Sulfate + chloride

Calcium + magnesium

20

40

60

80

20

40

60

80

Calcium

Sodium

Magnesium

80

60

40

20

20

40

60 Chloride

Sulfate

80

60

40

20

80

60

40

20

80

60

40

20

80

60

40

20

100 100

100 100

0 100

0

0

0

0

0

0

0

100

100

0

100

100

0

100

Milliequivalents, in percent

EXPLANATION

Site, by contributing aquifer

 or site type (table 7)

 Pecos Valley aquifer

 Igneous aquifer

 Edwards-Trinity aquifer

 Dockum aquifer

 Rustler aquifer

 Capitan Reef aquifer

 Spring
Figure 22. Relation between sulfate concentrations and chloride concentrations for samples collected from groundwater and spring sites in the Pecarea, Texas.

1

10

100

1,000

10,000

Sulfate, in milligrams per liter

1 10 100 1,000 

Chloride, in milligrams per liter

EXPLANATION

Site, by contributi or site type (tab Pecos Valley aq Igneous aquifer Edwards-Trinity  Dockum aquifer Rustler aquifer Capitan Reef aq Spring

Q29 Q41
34 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

1962–1988) from the nearest Global Network of Isotopes 

in Precipitation (GNIP) station (site 7622500 at Chihuahua, 

Mexico, about 300 miles southwest of Fort Stockton, Tex., 

at an altitude of about 4,670 ft [International Atomic Energy 

Agency, 2011]) and to estimates of stable isotope values in 

precipitation for the Trans-Pecos region made by Uliana and 

others (2007). Stable isotope results of samples collected for 

this study were compared to those of Uliana and others (2007) 

to help determine if the isotopic compositions measured were 

consistent with recent, local recharge (recharge during the 

Holocene and in the study area) or if the isotopic composition 

was indicative of other hydrologic processes (fig. 23, table 7). 

Uliana and others (2007) estimated that precipitation presently 

occurring to the south and west of the study area would have 

δ18O values that range from -7.50 to -5.70 per mil. Stable 

isotope values that plot along the MWL near the present 

precipitation estimates likely reflect a dominant component 

of water that recharged under recent, local climatic conditions 

(hereinafter referred to as young water). Values that plot along 

the MWL and lower than the present precipitation estimates 

are consistent with a component of water that recharged during 

the wetter, cooler climate of the late Pleistocene (hereinafter 

referred to as old water) (Uliana and others, 2007). For the 

purposes of this study, samples with δ18O values from -8.34 

per mil (the lowest measured value, at site Q30) to -7.50 

per mil are considered to be dominated by old water; values 

greater than -7.50 per mil are considered to be young water. 

Values that substantially deviate from the MWL indicate that 

the water has been affected by processes such as evaporation 

or extensive water-rock interaction. 

The δ18O and δD values for samples collected in the 

Pecos County region study area are comparable to those 

reported by Uliana and others (2007). Groundwater in the 

Pecos Valley aquifer is predominately consistent with recent, 

local recharge (figs. 20, 23, and 24, table 7). The values 

measured in samples collected from sites Q42 and Q43 in the 

Pecos Valley aquifer substantially deviate from the MWL and 

indicate some evaporation of the water likely occurred prior 

to recharge (fig. 23). δ18O and δD values for samples from the 

Igneous aquifer had the highest values along the MWL; these 

samples likely represent recent, local recharge water. δ18O and 

δD values for samples from the Edwards-Trinity aquifer lie 

along the MWL but cover a broad range from -8.34 to -6.42 

per mil in δ18O values; this range, with values higher and 

lower than -7.50, indicates that the Edwards-Trinity aquifer 

groundwater samples include varying mixtures of recent, local 

recharge and water that recharged during different climatic 

conditions. The lowest Edwards-Trinity aquifer δ18O value was 

measured at site Q30, which is interpreted to be dominated 

by older (Pleistocene) recharge; the highest value, which 

was similar to values measured for the Igneous aquifer, was 

measured in samples from site Q9, which is consistent with 

recent, local recharge. δ18O and δD values for samples from 

the Dockum aquifer are consistent with recent, local recharge. 

δ18O and δD values measured for samples from the Rustler 

aquifer in the Monument Draw trough (sites Q13, Q19, and 

Q29), from the Capitan Reef aquifer, and from San Solomon 

and Comanche springs (sites Q1 and Q4, respectively) indicate 

these water samples are dominated by older recharge. The 

values for samples from the remaining two Rustler wells (sites 

Q10 and Q38) indicate the groundwater reflects recent, local 

recharge. δ18O and δD values for samples from Santa Rosa 

and Diamond Y springs (sites Q2 and Q6, respectively) are 

consistent with recent, local recharge, but also fall slightly 

below the MWL. Because a discharge orifice could not be 

located at either of these springs, pools created by the springs 

were sampled (Pearson and others, 2012). Evaporation could 

have occurred from the pool and surface runoff from land 

adjacent to the spring pools could also have flowed into these 

pools. The extent of these processes is unknown but might 

account for stable isotope values slightly below the MWL.

Strontium Isotopes

Strontium (Sr) commonly substitutes for Ca in mineral 

complexes in low-temperature aqueous geochemical 

environments and is a common trace element in carbonate 

rocks (Musgrove and others, 2009; Banner, 2004). As a result, 

Sr isotopes have been used to evaluate sources of dissolved 

constituents to groundwater and water-rock interaction 

processes and to gain insights into regional groundwater-flow 

paths and groundwater mixing (Musgrove and Banner, 1993; 

Banner and others, 1994; Uliana and others, 2007; Musgrove 

and others, 2009). The Sr isotopic ratio (87Sr/86Sr) often 

provides a useful diagnostic signal of the source of dissolved 

constituents to a fluid because 87Sr/86Sr undergoes negligible 

fractionation during mineral-solution reactions (Banner, 2004). 

This lack of fractionation makes them particularly useful 

in tracing regional groundwater-flow paths and identifying 

mixing relations in regional flow systems (Banner and 

Kaufman, 1994). Uliana and others (2007) detail the origins of 

many 87Sr/86Sr signatures in the Trans-Pecos region.

Because of water-rock interaction processes and 

negligible fractionation, groundwater that is in equilibrium 

with the strontium-bearing minerals in aquifer rocks is 

expected to have 87Sr/86Sr values that reflect the isotopic ratio 

of the minerals (Banner and Kaufman, 1994; Uliana and 

others, 2007). The average 87Sr/86Sr value reported by Cameron 

and others (1996) for igneous rocks in the Davis Mountains 

was 0.7067 and Uliana and others (2007) used a range of 

values from 0.7030 to 0.7080. Permian and Cretaceous-age 

carbonate and evaporite rocks should range from 0.7068 to 

0.7084 (Burke and others, 1982; Brookins 1988; Denison 

and others, 1998). In general, siliciclastic rocks often have 

higher 87Sr/86Sr values than carbonate rocks because many 

siliciclastics contain rubidium-87 (87Rb)-bearing minerals 

(Faure, 1986). 87Rb is radioactive and decays to 87Sr, which 

increases the 87Sr/86Sr ratios in siliciclastic rocks. Therefore, 

the siliciclastic rocks that compose the Pecos Valley and 

Dockum aquifers, as well as siliciclastic rocks that are 

components of the predominately carbonate Permian and 

Cretaceous-age units (table 1), likely have higher 87Sr/86Sr 
Figure 23. Relation between oxygen-18/oxygen-16 and hydrogen-2/hydrogen-1 isotopic ratios for samples collected from groundwater and spring siregion study area, Texas, and the global meteoric water line (Craig, 1961).

-9 -8 -7 -6 -5 -4 -3 

Oxygen-18/oxygen-16 isotopic ratio, in per mil

[MWL, global meteroric water line; GNIP, Global Network of Isotopes in Precipitation station 7622500 at Chihuahua, Mexico] 

-60-50-40-30-20-10

0

Hydrogen-2/hydrogen-1 isotopic ratio, in per mil EXPLANAT

Site, by contribu or site type (t Pecos Valley a Igneous aquife Edwards-Trinit Dockum aquife Rustler aquifer Capitan Reef a Spring GNIP referenc

MWL
Figure 24. Oxygen-18/oxygen-16 isotopic ratios for samples collected from groundwater and spring sites in the Pecos County region study area, Tex

PEC

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COUNT

WARD 

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COUNTY Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

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-6.56

-6.23

-7.16

-6.42

-6.66

-7.29

-7.47

-7.84

-7.89

-6.12

-7.82

-6.97

-7.7

-7.92

-7.14

-7.78

-7.18

-7.47 -7.47

-6.72

-8.34 -7.61

-6.87

-6.75 -7.5 -7.81

-7.24

-6.85

-6.85

-6.83 -6.81

-7.33

-7.74

-2.21

-7.21

-8.3

-7.45

-6.83

-7.89

-7.06

-6.84

-4.86

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashw

EXPLANATION

Edwards-Trinity aquifer (outcrop)Edwards-Trinity aquifer (subcrop)Study area boundaryFault zone—Represents numerous faults

Geochemistry site (table 7), by site type and contr

 represents oxygen-18/oxygen-16 isotopic ratio  Spring Well, Pecos Valley aquifer Well, Igneous aquifer Well, Edwards-Trinity aquifer

Well, DocWell, RustWell, Capi

-7.29

-7.84

-6.12 -7.92

-6.85

-7.45 -6.87
Geochemistry 37

values than the carbonate rocks in the groundwater system. 

For example, although outside the Pecos County region study 

area, Langman and Ellis (2010) reported a 87Sr/86Sr value of 

0.7091 for a rock sample collected from the Dockum Group in 

the southern High Plains region of Texas.

Ratios of 87Sr/86Sr measured in water samples collected in 

the study area (figs. 20, 25, and 26, table 7) are comparable to 

those reported by Uliana and others (2007). Ratios of 87Sr/86Sr 

in samples collected from the Pecos Valley aquifer ranged 

from 0.70799 to 0.70839. 87Sr/86Sr values in samples collected 

from the Igneous aquifer were 0.70727 and 0.70757 (sites 

Q7 and Q15, respectively). These values were low relative 

to the other aquifers, reflecting interaction with low 87Sr/86Sr 

igneous rocks. 87Sr/86Sr values in samples collected from the 

Edwards-Trinity aquifer covered a broad range from 0.70788 

to 0.70979; the higher 87Sr/86Sr values in this range (greater 

than 0.70900) were measured in samples from nearly all of the 

wells completed in the Monument Draw trough: site Q25, in 

the eastern part of the study area, site Q11 in the southern part 

of the study area, and site Q30 near the western boundary of 

the study area (fig. 20). Also, because 14 of the 20 EdwardsTrinity aquifer 87Sr/86Sr values were higher than the expected 

range of values for groundwater interacting with Cretaceousage carbonate rocks (0.7068 to 0.7084; Burke and others, 

1982), it is expected that there are alternative or additional 

sources (siliciclastics) of Sr within the interpreted complex 

stratigraphy and structural geology of the Pecos County region 

(fig. 2, table 1). 87Sr/86Sr values in samples collected from 

the Dockum aquifer ranged from 0.70843 to 0.70975; values 

greater than 0.70900 were measured at site Q34, which is a 

well completed in Monument Draw trough, and at site Q21, 

which is near the eastern boundary of the study area. 87Sr/86Sr 

values in samples collected from the Rustler aquifer ranged 

from 0.70758 to 0.70977 and 87Sr/86Sr values in samples 

collected from the Capitan Reef aquifer sites Q20 and Q41 

were 0.70969 and 0.70751, respectively. Like Cretaceousage carbonate rocks, the expected range of 87Sr/86Sr values 

for water interacting with the Permian-age carbonate and 

evaporite rocks of the Rustler and Capitan Reef aquifers is 

from 0.7068 to 0.7084 (Brookins 1988; Denison and others, 

1998), and values higher than this range indicate an alternative 

or additional source for Sr in the groundwater. 87Sr/86Sr ratios 

in samples collected from the four springs ranged from 

0.70898 to 0.70991 and were generally high relative to other 

samples and higher than the values expected for groundwater 

interacting with carbonate and evaporite rocks of Permian and 

Cretaceous age.

Environmental Tracers

Among their many uses, environmental tracers are useful 

for understanding when groundwater recharge occurred, 

isolating the water from the atmosphere. Samples were 

collected at select sites in the study area and analyzed for 

tritium (3H) and (or) helium-3 (3He) to gain insights regarding 

the age of the groundwater. 3H is a radioactive isotope of 

hydrogen with a half-life of 12.32 years and commonly is 

measured in picocuries per liter (pCi/L) or tritium units (TU), 

in which 3.22 pCi/L equals 1 TU (Lucas and Unterweger, 

2000). 3H in rainfall has two sources, natural cosmogenic 3H 

or that produced by the widespread atmospheric testing of 

nuclear weapons that began around 1950 and continued until 

about 1970. Given its relatively short half-life, the presence of 

3

H in groundwater is indicative of groundwater recharge that 

occurred in the last 60 years (that is “post-bomb” recharge; 

Clark and Fritz, 1997). The input of 3H to the atmosphere as a 

result of nuclear testing elevated 3H concentrations in rainfall 

compared to pre-bomb concentrations for more than 50 years 

(beginning in the 1950s), with concentrations peaking in the 

1960s. 3H concentrations in rainfall since 2006 have globally 

decreased to approximately pre-bomb background levels of 

about 2 to 10 TU (Clark and Fritz, 1997; Phillips and Castro, 

2003).

Interpreting groundwater ages with 3H is qualitative in 

that it can provide insight into distinguishing between prebomb and post-bomb water but not determine apparent ages. 

The 3H/3He method, which is based on the radioactive decay 

of 3H to 3He, can be used to estimate groundwater ages at a 

higher resolution than using only 3H concentrations (Schlosser 

and others, 1988; Schlosser and others, 1989; Solomon and 

Cook, 1999). Samples were collected for 3H-3He analysis, 

but these results were not available at the time this report was 

prepared (March 2012). 3H decay is not the only possible 

source of 3He, and groundwater-age determination required 

an assessment of other possible 3He sources. That assessment 

typically includes measurement of helium-4 (4He) to aid in 

distinguishing between He sources (Thatcher and others, 

1977).

Sites with 3H concentrations greater than 1 TU were 

considered to potentially represent water with a sufficient 

component of post-bomb water for 4He screening and potential 3

H/3He dating (fig. 20, table 8). Eight of the 23 samples had 3

H concentrations greater than 1 TU (sites Q4, Q7, Q15, Q17, 

Q23, Q27, Q28, and Q43). 4He screening results indicated 

that the samples from all of these sites are potentially datable 

using the 3H/3He method. Thus, based on the 3H and 4He 

concentrations, at least some component of the groundwater 

collected at these sites is likely post-bomb recharge. The 

low 3H concentrations measured in the majority of collected 

samples (15 out of 23) indicate that groundwater in the study 

area likely is predominantly pre-bomb recharge.

Organic Compounds and Nutrients

Organic compound (pesticide and herbicide) and nutrient 

concentrations in groundwater in the study area are potential 

indicators of surface-water infiltration and anthropogenic 

influences on groundwater. Detectable concentrations of 

organic compounds in a groundwater sample likely result 

from infiltration of rain or irrigation water following the 

surface application of pesticides and herbicides. Nutrient 

concentrations in groundwater (predominantly in the form 
Figure 25. Relation between strontrium-87/strontium-86 isotopic ratio and strontium concentration for groundwater and spring samples collected in study area, Texas.

0

5.0•103

1.0•104

1.5•104

Strontium, in micrograms per liter

0.7075 0.7080 0.7085 0.7090 0.7095

Strontium-87/strontium-86 isotopic ratio

EXPLANATION

Site, by contributing aquifer or site type (table 7) Pecos Valley aquifer Igneous aquifer Edwards-Trinity aquifer Dockum aquifer Rustler aquifer Capitan Reef aquifer Spring
Figure 26. Strontium-87/strontium-86 isotopic ratios for samples collected from groundwater and spring sites in the Pecos County region study area

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COUNT

WARD 

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COUNTY Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

Fort Stockton

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0.70991

0.70925

0.70898

0.70727

0.70816

0.70823

0.70829

0.70954 0.70788 0.70977 0.70893

0.70757

0.70975

0.7083 0.70969 0.70901

0.70906

0.70974

0.70963 0.70959

0.70904

0.70807

0.70758

0.70843 

0.70961 0.70975

0.70824

0.70839

0.70969

0.70763

0.70854

0.70877

0.70813

0.70882

0.70970

0.70867

0.7094

0.70799

0.70979

0.70751

0.70813

EXPLANATION

Edwards-Trinity aquifer (outcrop)Edwards-Trinity aquifer (subcrop)Study area boundaryFault zone—Represents numerous faults

Geochemistry site (table 7), by site type and contr

 represents strontium-87/strontium-86 isotopic r Spring Well, Pecos Valley aquifer Well, Igneous aquifer Well, Edwards-Trinity aquifer

WelWelWel

0.70991

0.70977

0.70757 0.70969

0.70843

0.70813

0.70979

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashw
40 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

of nitrate and nitrite) that are higher than the prevailing 

background concentration might result in agricultural or 

municipal areas as a result of input from livestock manure, 

septic systems, municipal treatment systems, or fertilizers. 

National background concentrations of nitrate in groundwater 

have been estimated to be 1.0 mg/L (Dubrovsky and others, 

2010). Land-cover data obtained from the National Land 

Cover Dataset (NLCD) 2006 (Fry and others, 2011) show the 

distribution of land cover in the study area (fig. 27). Because 

of the small amount of developed areas and the presence of 

agricultural areas (shown as cultivated crops), the combined 

concentrations of nitrate and nitrite (NO3+NO2) were used 

to evaluate the potential presence of excess nutrients from 

irrigation, specifically cultivated crops, in the groundwater 

system. 

Eight groundwater samples from the Igneous, EdwardsTrinity, and Dockum aquifers and two spring samples were 

analyzed for organic compounds, specifically pesticides 

and herbicides (table 7). Atrazine, a widely used herbicide, 

was detected in one sample collected from the EdwardsTrinity aquifer (site Q26) and one spring (site Q4). CIAT 

(deethylatrazine), an atrazine degradate, was detected in 

samples from three wells completed in the Edwards-Trinity 

aquifer (sites Q18, Q24, and Q26) and two springs (sites Q4 

and Q6). The four deethylatrazine detections were qualified 

as estimated (“E”) by the USGS National Water Quality 

Laboratory under laboratory reporting conventions used 

during the study period (Zaugg and others, 1995); a remark 

code of “E” indicates that a compound was identified at 

an estimated concentration, but concentration could not be 

accurately quantified. As explained in Oden and others (2011, 

p. 9), a constituent concentration is considered estimated 

by the laboratory when results are greater than the longterm method detection level and less than the laboratory 

reporting level. Deethylatrazine is permanently coded “E” 

because of poor laboratory recoveries. Sites with detectable 

concentrations (fig. 20) are in the Leon-Belding and Fort 

Stockton areas (fig. 1).

NO3+NO2 concentrations (as N) ranged from not detected 

(< 0.02) to 6.67 mg/L. Concentrations greater than 1 mg/L 

(table 7) were measured in samples collected from two wells 

completed in the Pecos Valley aquifer (sites Q27 and Q43). 

Concentrations of NO3+NO2 in samples collected from the 

two wells completed in the Igneous aquifer (sites Q7 and 

Q15) were less than 1 mg/L. NO3+NO2 concentrations were 

greater than 1 mg/L in 8 out of 20 samples collected from 

wells completed in the Edwards-Trinity aquifer. These sites 

include Q18, Q24, and Q26, which were also sites for which 

the samples contained detectable atrazine or deethylatrazine. 

NO3+NO2 concentrations were greater than 1 mg/L in 

three of the five samples collected from wells completed 

in the Dockum aquifer (sites Q21, Q39, and Q44) (fig. 20). 

NO3+NO2 concentrations were greater than 1 mg/L in only 

one of the five samples collected from wells completed in the 

Rustler aquifer (site Q10) and in none of the samples collected 

from wells completed in the Capitan Reef aquifer. NO3+NO2 

concentrations were greater than 1 mg/L in samples collected 

from Santa Rosa, Comanche, and Diamond Y springs (sites 

Q2, Q4, and Q6, respectively). It is possible that the elevated 

NO3+NO2 concentrations measured in samples collected from 

Santa Rosa and Diamond Y springs (2.95 and 6.67 mg/L, 

respectively) reflect groundwater discharge, but it is also 

possible that edge-of-field surface runoff from land adjacent 

to the pools that occur where each spring discharges is the 

source of the elevated concentrations. Additionally, NO3+NO2

concentrations might also be affected by processes within the 

aquifers. While most groundwater samples were oxic (table 7), 

denitrification might be influencing NO3+NO2 concentrations 

in samples with low (less than 0.5 mg/L) dissolved oxygen 

concentrations.

Geochemical Endmembers

Geochemical endmembers in the groundwater system 

were selected based on variations in geochemical composition 

in order to qualitatively evaluate groundwater-flow paths and 

mixing processes. Selected endmembers were: (1) regional 

groundwater flow in the Edwards-Trinity aquifer that 

originated as recharge northwest of the study area and enters 

the study area near the western corner, (2) recharge entering 

the groundwater system along the southern boundary of the 

study area from the Barilla and Davis Mountains, (3) recharge 

entering the groundwater system at the southern studyarea boundary from the Glass Mountains, and (4) regional 

groundwater flow in the Edwards-Trinity aquifer east of 

the Monument Draw trough. Although not denoted for 

the purposes of this report as geochemical endmembers, 

groundwater from the Dockum and Rustler aquifers likely mix 

with and affect groundwater compositions of the EdwardsTrinity aquifer.

The composition of the sample collected from site 

Q30 (fig. 20, tables 7 and 8), which is a well completed in 

the Edwards-Trinity aquifer in the western corner of the 

study area, was used to represent endmember 1: regional 

groundwater flow in the Edwards-Trinity aquifer entering the 

study area from the northwest. Endmember 1 was a Na-CaCl-SO4

 type water; specific conductance was 2,710 μS/cm; 

SO4 concentration was 498 mg/L; Cl concentration was 473 

mg/L; δ18O and δD values were -8.34 and -57.7 per mil, 

respectively; 87Sr/86Sr ratio was 0.70979; tritium concentration 

was 0.25 TU; no organic compounds were detected; and 

NO3+NO2 concentration was 0.135 mg/L. In summary, 

endmember 1 represents relatively mineralized, old water that 

likely recharged northwest of the study area under different 

climatic conditions and is flowing through the Edwards-Trinity 

aquifer along regional groundwater-flow paths.

Recharge along the southern boundary of the study area 

occurs as runoff from the mountains and moves vertically 

downward through underlying rocks and into the gravels 

along the slopes of the mountains (fig. 2) (Armstrong and 

McMillion, 1961; Small and Ozuna, 1993; Bochici, 1997; 

Uliana and Sharp, 2001; Uliana and others, 2007). There are 
Figure 27. National Land Cover Database 2006 land-cover categories in the Pecos County region study area, Texas.

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COUNTY Pecos River

Pecos River

104°00' 103°30' 103°00' 102°30'

31°00'30°30'

Fort Stockton

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Q1

Q2

Q4

Q7

Q8

Q9

Q10

Q11 Q12 Q13 Q14

Q15

Q16

Q17

Q19

Q20

Q21

Q22 Q23

Q24 Q25

Q28

Q30 Q29

Q31

Q Q3

33

Q34

Q35

Q36 Q37

Q38

Q39

Q40

Q41

Q43

Q44

Q6

Q18

Q26

Q27

Q42

0 5 10 15 20 MILES0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from Ashwo

Q1

Land cover category

Open waterDeveloped, open spaceDeveloped, low intensityDeveloped, medium intensityDeveloped, high intensity

Barren landDeciduous forestEvergreen forestMixed forestShrubland

Grassland/herbaceousCultivated cropsWoody wetlandsEmergent herbaceous wetlands

Study are

EdwardsFault zon faultsCultivateGeochem identif

EXPLANATION
42 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

two distinct mountain-recharge endmembers because the 

Barilla and Davis Mountains are primarily igneous rocks 

composed of silicate minerals (endmember 2) and the Glass 

Mountains are sedimentary rocks composed primarily of 

carbonate minerals (endmember 3). Water-rock interaction 

with these different rock types results in different recharge 

compositions. The composition of the sample collected from 

site Q7 (fig. 20, tables 7 and 8), which is a well completed 

in the Igneous aquifer southwest of the study area, was used 

to represent endmember 2. Endmember 2 was a Ca-NaHCO3

 type water; specific conductance was 408 μS/cm; SO4

concentration was 12.9 mg/L; Cl concentration was 8.39 

mg/L; δ18O and δD values were -6.23 and -41.6 per mil, 

respectively; 87Sr/86Sr ratio was 0.70727, which was the lowest 

value measured in this study; the tritium concentration was 

1.2 TU; organic compounds were not analyzed; and NO3+NO2

concentration was 0.632 mg/L. Si concentrations (table 7) also 

provided insight into this endmember. Si concentrations would 

likely be higher in groundwater interacting with igneous 

rocks of the Barilla and Davis Mountains. Relatively higher 

Si concentrations for samples collected from the Igneous 

aquifer and wells completed in proximity to the Barilla and 

Davis Mountains are consistent with this hypothesis. The 

Si concentration at site Q7 was 43.6 mg/L. In summary, 

endmember 2 represents relatively dilute, recent recharge with 

a relatively high concentration of Si and a low 87Sr/86Sr value 

that are indicative of interaction with low 87Sr/86Sr igneous 

rocks. Finally, the composition of endmember 2 is consistent 

with values reported by Uliana and others (2007) for samples 

collected in the Barilla and Davis Mountains, which were 

represented by their endmember B.

The composition of the sample collected from site 

Q8 (fig. 20 and table 7), which is a well completed in 

the Edwards-Trinity aquifer near the southwest boundary 

of the study area, was used to represent endmember 3; 

that is, recharge from the Glass Mountains. Endmember 3 

was a type Na-HCO3 water; specific conductance was 587 

μS/cm; SO4 concentration was 56.0 mg/L; Cl concentrations 

was 15.9 mg/L; δ18O was -7.16 per mil and δD was -49.0 

per mil, (predominantly young water); 87Sr/86Sr was 

0.70816; environmental tracers and organic compounds 

were not analyzed; and NO3+NO2 was not detected. The Si 

concentration was 14.3 mg/L, which is lower than the Si 

concentration of endmember 2 as well as the average value 

of 21.6 mg/L for Edwards-Trinity aquifer water. From the 

geochemical assessment, endmember 3 represents relatively 

dilute, recent recharge with a composition indicative of 

interaction with carbonate rocks.

The composition of the sample collected from site Q25 

(fig. 20, tables 7 and 8), which is a well completed in the 

Edwards-Trinity aquifer in the eastern part of the study 

area, was used to represent endmember 4; that is, regional 

groundwater flow in the Edwards-Trinity aquifer east of 

the Monument Draw trough. Endmember 4 was a type NaCa-Mg-SO4

-Cl-HCO3 water; specific conductance was 

1,520 μS/cm; SO4 concentration was 277 mg/L; Cl 

concentration was 203 mg/L; δ18O was -7.47 per mil and 

δD was -49.6 per mil; 87Sr/86Sr ratio was 0.70959; tritium 

concentration was 0.09 TU; and organic compounds and 

NO3+NO2 were not detected. Endmember 4 represents 

relatively mineralized water that is likely a mixture of 

recharge under current and different climatic conditions and 

is flowing through the Edwards-Trinity aquifer along regional 

groundwater-flow paths.

Groundwater-Flow System 

Geochemical and groundwater-level 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. Pearson and others 

(2012) detail the methods used for collection, analysis, and 

quality control of the groundwater-level data used in this 

report. The geochemical characteristics and endmembers 

were used to qualitatively evaluate the mixing of waters from 

different sources and the flow-path evolution of the chemical 

characteristics of aquifer water. Groundwater-level data were 

used to create potentiometric-surface maps of the EdwardsTrinity aquifer, assess regional groundwater gradients, and 

compute vertical gradients between the Edwards-Trinity 

and underlying aquifers. Structural features such as bed 

orientation and thickness, mountains, troughs, and faults play 

a substantial role in the distribution of recharge, local and 

regional groundwater-flow paths, spring discharge, and aquifer 

interaction.

Groundwater-level altitudes and geologic data for the 

study area were compiled from NWIS (U.S. Geological 

Survey, 2011), the TWDB Groundwater Database (Texas 

Water Development Board, 2011), and Middle Pecos 

Groundwater Conservation District (Paul Weatherby, Middle 

Pecos Groundwater Conservation District, written commun., 

2011). A set of three criteria were used to determine which 

wells were completed in the Edwards-Trinity aquifer. Wells 

were considered completed in the Edwards-Trinity aquifer if: 

1. the total depth was deeper than the top of the Cretaceousage rocks,

2. the top of openings (screen, slots, open hole) in the well 

had a depth deeper than the top of the Cretaceous-age 

rocks, and

3. the total depth of the well was less than 50 ft below the 

base of the Cretaceous-age rocks. 

If there were no opening or total-depth data associated 

with a well, the agency-assigned geologic data were used 

to identify the aquifer in which the well was screened so 

that groundwater-level measurements could be related 

to the specific aquifer in which the well was completed. 

Wells completed in multiple aquifers were not used in the 

groundwater-level analysis. All groundwater-level data 
Groundwater-Flow System 43

collected from the Edwards-Trinity aquifer were separated 

into two seasons (summer and winter) for predevelopment 

years (before 1950) and for each year from 1950 to 2011. 

The summer season was defined as the months from May 

through October, and the winter season was defined as the 

months from November through April. These seasons were 

chosen as 6-month periods that coincide roughly with the 

start and the end of the agricultural irrigation season. The 

winter-season measurements were used for groundwaterflow analysis because groundwater-level data collected 

during these months are less affected by irrigation pumping. 

Figure 28 shows a potentiometric-surface map using the 

average of the groundwater-level data collected at each well 

completed in the Edwards-Trinity aquifer for the winter 

months during 1980–2010 (table 9 at end of report). The 

potentiometric-surface grid was generated using minimum 

curvature interpolation techniques. Geosoft, Inc. (2012) 

describes the minimum-curvature methods used for grid 

generation. 

Regional Groundwater Flow

Groundwater-level altitudes in the Edwards-Trinity 

aquifer generally decrease from southwest to northeast and 

regional groundwater flow is from the south and southwest 

to the north and northeast in the study area (fig. 28). The 

highest groundwater-level altitudes (more than 3,300 ft) 

were measured near the southwestern boundary of the study 

area. Some of the lowest groundwater-level altitudes (about 

2,400 ft) were measured in the down-dip extent of the 

Edwards-Trinity aquifer in the Monument Draw trough; a few 

groundwater altitudes of less than 2,300 ft were measured near 

the eastern part of the study area. 

Two groundwater divides were observed in the study 

area from the potentiometric surface of the Edwards-Trinity 

aquifer (fig. 28). A groundwater divide begins where the 

boundaries of Jeff Davis, Brewster, and Pecos Counties come 

together, then extends generally northwest to the ReevesPecos County boundary. Groundwater west of this divide 

flows from the Barilla and Davis Mountains to the down-dip 

sections of the Edwards-Trinity along the northwestern studyarea boundary into the Pecos trough (fig. 2). Groundwater 

east of this divide flows along the western margin of the 

Monument Draw trough. The other groundwater divide 

trends along the center of Monument Draw trough and is 

potentially related to the upwelling of water from deeper units. 

Groundwater west of the divide predominantly flows from 

the Glass Mountains to the down-dip section of Monument 

Draw trough, and groundwater east of the divide flows from 

the Glass Mountains into the down-dip section of Monument 

Draw trough and the eastern part of the study area. Because 

the potentiometric-surface map was developed using the 

average of the groundwater-level data collected in winter 

months, the locations of the divides likely vary during the 

irrigation season when groundwater pumping increases.

Recharge

Integrated results of this and previous studies indicate 

the Edwards-Trinity aquifer is recharged by: (1) regional 

groundwater flow in the Edwards-Trinity aquifer entering 

the study area from the northwest; (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. These sources of recharge were 

qualitatively evaluated for this report and are consistent with 

previously reported sources (Armstrong and McMillion, 

1961; Hiss, 1976; Small and Ozuna, 1993; Boghici, 1997; 

Uliana and Sharp, 2001; Uliana and others, 2007; Daniel B. 

Stephens and Associates, 2010). Neither the absolute nor the 

relative recharge proportion from each potential source was 

estimated. Also, although a component of the groundwater 

appears to have recharged under conditions similar to the 

recent, local climate, the only samples collected from the 

Edwards-Trinity aquifer that likely recharged during about 

the past 60 years (post-bomb) were collected from wells in 

mountain recharge areas and in areas receiving agricultural 

return flow. These results are consistent with local climate 

data because substantial, recent recharge is unlikely given 

the discrepancy between an average annual rainfall of 14 

inches (National Weather Service, 2011) and annual potential 

evapotranspiration rates of as much as 109 inches (Boghici, 

1997).

Regional groundwater flow in the Edwards-Trinity 

aquifer that originated as recharge northwest of the study 

area enters the study area near the western boundary (fig. 

28). This regional groundwater-flow path was previously 

documented by Uliana and Sharp (2001) and Uliana and 

others (2007). The potentiometric surface shows that the 

groundwater-level altitudes are highest in the western part 

of the study area (fig. 28), and regional groundwater flow is 

generally towards the north. The groundwater divide near the 

Pecos-Reeves County boundary likely limits the connection 

of this regional flow path with the rest of the study area. The 

groundwater composition is consistent with older (Pleistocene) 

mineralized groundwater with high 87Sr/86Sr values (site Q30; 

endmember 1).

Recharge to the Edwards-Trinity aquifer along the 

southern boundary of the study area occurs from the Barilla, 

Davis, and Glass Mountains (fig. 28) (Armstrong and 

McMillion, 1961; Small and Ozuna, 1993; Boghici, 1997; 

Uliana and Sharp, 2001; Uliana and others, 2007). The 

geochemical composition of this relatively dilute, recent 

recharge is characterized by low specific conductance and 

stable isotope values similar to local rainfall. Although the 

hydrochemical facies for recharge endmembers from the 

Barilla and Davis Mountains (site Q7; endmember 2) and 

the Glass Mountains (site Q8; endmember 3) are similar 

(Ca-Na-HCO3 and Na-HCO3, respectively) and low 87Sr/86Sr 

values (0.70727 and 0.70816, respectively) are consistent 
Figure 28. Potentiometric-surface map of the Edwards-Trinity aquifer developed using the average of the winter (November through April) groundweach site for 1980–2010 in the Pecos County region study area, Texas.

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EXPLANATION

Edwards-Trinity aquifer outcropEdwards-Trinity aquifer subcropStudy area boundaryFault zone—Represents numerous faultsCultivated cropland area boundary

Potentiometric contour, 1980 to 2010—Shows altitude at which water would have stood in tightly cased well. Dashed where approximately located. Interval 25 feet. Datum is North American Vertical Datum of 1988 Minor contour Major contour

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2500 L55

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Aquifer modified from A

Cultivated cropland boundary based on National 
Groundwater-Flow System 45

with interaction with igneous rocks or carbonate rocks and 

not with siliciclastic rocks with high 87Sr/86Sr values, Si 

concentrations (43.6 and 14.3 mg/L) indicate these waters are 

likely from different sources. The composition of the Igneous 

aquifer sample from site Q15, the Pecos Valley aquifer sample 

collected from site Q27, and the Edwards-Trinity aquifer 

sample collected from site Q28 (fig. 28, tables 7 and 8) are 

similar to that of the Igneous aquifer recharge endmember 2 

(high stable isotope values, low 87Sr/86Sr values, and high 

Si concentrations) and are west of the inferred groundwater 

divide. The high Si concentration and low 87Sr/86Sr values in 

groundwater samples from these wells are likely influenced 

by interaction with igneous rocks of the Barilla and Davis 

Mountains.

The results of this study indicate that return flows 

from irrigation likely contribute recharge to the EdwardsTrinity aquifer in agricultural areas (figs. 27 and 28). 

Detections of organic compounds (atrazine and its degradate 

deethylatrazine) and elevated NO3+NO2 in the Leon-Belding 

and down-gradient parts of the Edwards-Trinity aquifer, as 

well as near other agricultural areas, are consistent with a 

component of recharge from agricultural return flow (figs. 27 

and 28, table 7). Generally, detections of organic compounds 

and elevated NO3+NO2 concentrations were measured in the 

shallowest aquifer units. For example, organic compounds 

and elevated NO3+NO2 concentrations were measured 

in the Edwards-Trinity aquifer in the Leon-Belding and 

downgradient areas but not in the underlying Rustler and 

Capitan Reef aquifers.

Upwelling of groundwater from deeper aquifers to the 

Edwards-Trinity aquifer has been previously documented 

(Hiss, 1976; Small and Ozuna, 1993; Barker and others, 

1994). Upwelling is likely the result of groundwater flow 

from underlying aquifers along fault zones (Ashworth, 1990; 

Boghici, 1997) or, as hypothesized by Small and Ozuna 

(1993), it may occur in areas where a Triassic shale unit that 

separates the Trinity Group of the Edwards-Trinity aquifer 

from underlying units is absent. Upwelling groundwater 

contributes higher concentrations of salinity, including higher 

SO4 and Cl concentrations, to overlying units as a result of 

extensive interaction with Permian-age evaporite deposits. 

Faulted zones (figs. 2, 17, 18, and 28) in the Leon-Belding and 

Fort Stockton areas (fig. 1) are some areas where upwelling 

is likely occurring. The high-salinity water collected from 

the Edwards-Trinity aquifer (sites Q16, Q18, Q22, Q23, 

Q24, and Q26; specific conductance from 2,040 to 3,820 

μS/cm) (fig. 28, table 7) and Comanche Springs (site Q4; 

specific conductance 3,570 μS/cm), which discharges from 

the Edwards-Trinity aquifer, is compositionally similar to 

the high-salinity water collected from the underlying Rustler 

and Capitan Reef aquifers (sites Q19, Q20, and Q29; specific 

conductance 2,290 to 3,980 μS/cm). Groundwater-level data 

for the other aquifers in the groundwater system were limited. 

However, groundwater-level altitudes collected by the TWDB 

in 2001 from two wells completed in the Edwards-Trinity 

aquifer and in 2002 from two wells completed in the Rustler 

aquifer were used to evaluate vertical gradients in the LeonBelding area (fig. 2) (Edwards-Trinity site numbers L48 

and L45 [fig. 28, table 9]; and TWDB Rustler site numbers 

5216609 [site Q19; fig. 28, table 5] and 5216608 [less than 

0.5 miles northeast of L57]; Texas Water Development Board, 

2011). Groundwater altitudes of 3,060 and 3,076 ft in the 

Rustler wells (measured Jan. 2, 2002, at sites 5216609 and 

5216608, respectively) and 2,955 (measured May 9, 2001, at 

site L48) and 2,977 ft (measured April 2, 2001, at site L45) 

in the Edwards-Trinity wells show an upward gradient from 

the Rustler to the Edwards-Trinity aquifer of about 83 to 

121 ft, which is consistent with the geochemical results and 

a historical upward gradient reported by Small and Ozuna 

(1993).

Geochemical Mixing and Discharge

Results of this study are consistent with previous studies 

of the region and indicate that groundwater generally flows 

north into the down-dip extent of the Edwards-Trinity aquifer 

or northeast out of the study area (fig. 28) (Barker and others, 

1994; Anaya and Jones, 2009). Groundwater naturally 

discharges from the aquifer from springs, into overlying or 

underlying units at the down-dip extent of the aquifer, or to 

the Pecos River (Barker and others, 1994; Anaya and Jones, 

2009). The four springs sampled in the study area (table 5) are 

likely connected to groundwater discharge along fault zones 

(figs. 17–18 and 28).

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 down-dip 

extent of the Edwards-Trinity aquifer (Barker and others, 

1994). The composition of samples collected from San 

Solomon Spring (site Q1; fig. 28, tables 7 and 8), which 

discharge from the Edwards-Trinity aquifer, are similar to 

those of the regional groundwater flow characterized by 

endmember 1 (site Q30). Previous studies indicate there are 

also local influences on San Solomon Spring (LaFave and 

Sharp, 1987; Sharp and others, 1999; Uliana and others, 

2007).

Recharge from the Barilla and Davis Mountains also 

flows toward the Pecos trough and is thought to naturally 

discharge to other aquifers in the groundwater system (Barker 

and others, 1994). The Si concentrations in Edwards-Trinity 

aquifer samples from sites Q12 and Q17 (21.1 and 26.7 mg/L, 

respectively), which are intermediate between the mountain 

recharge endmembers 2 and 3 concentrations of 43.6 mg/L 

and 14.3 mg/L, indicated there might be mixing of recharge 

from the Barilla and Davis Mountains with recharge that 

has interacted with igneous rocks to a lesser degree than the 

igneous signature of endmember 2. The potentiometricsurface map (fig. 28) shows that recharge from the Glass 

Mountains primarily flows to the Monument Draw trough 

(fig. 2) and to the east. The Si concentration in the sample 

collected from site Q11 (28.4 mg/L) indicated that there could 
46 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

be some mixing of recharge from the Glass Mountains and 

groundwater that has been in contact with silicate-rich igneous 

or siliciclastic rocks. The relatively high 87Sr/86Sr value 

(0.70954) in the sample collected from site Q11, however, was 

more consistent with a siliciclastic source than igneous rocks 

of the Barilla and Davis Mountains.

Groundwater flow in the Edwards-Trinity aquifer in the 

Monument Draw trough originated as recharge in the Glass 

Mountains, agricultural return flow, or upwelling groundwater 

from lower units. Groundwater generally flows north and 

northeast in the Monument Draw trough and naturally 

discharges from springs or to other aquifers in the groundwater 

system (Barker and others, 1994). Based on low stable isotope 

ratios, groundwater in the Edwards-Trinity, Rustler, and 

Capitan Reef aquifers in the Monument Draw trough probably 

originated as recharge during the wetter, cooler climates of the 

Pleistocene. Recent recharge to the Edwards-Trinity aquifer 

near the Glass Mountains in the Monument Draw trough area 

is hydrologically possible, but low stable isotope values of 

groundwater from site Q14 of δ18O -7.89 per mil and δD -55.0 

per mil (fig. 28, table 7) and undetected 3H (table 8) indicate 

the Edwards-Trinity aquifer water in this area is predominantly 

old. Similarly, high specific conductance, SO4 concentrations, 

and Cl concentrations, and similar δ18O, δD, and 87Sr/86Sr 

values in groundwater samples collected from wells completed 

in the Monument Draw trough roughly south of Interstate 

Highway 10 (I-10) (fig. 28, tables 7 and 8) support earlier 

findings that groundwater from the Capitan Reef aquifer 

is mixing with groundwater flowing through Permian-age 

evaporite deposits in the Rustler aquifer (Small and Ozuna, 

1993). Groundwater discharge from Comanche Springs (Q4) 

has a similar composition to groundwater samples collected 

from wells in this area (fig. 28, tables 7 and 8), including 

the detection of organic compounds and elevated NO3+NO2

concentrations that were measured in samples from upgradient 

Edwards-Trinity aquifer sites. This compositional similarity, 

along with the absence of flow at the springs caused by the 

lowering of the water table from pumping during the irrigation 

season (Small and Ozuna, 1993), confirms the connection 

between the springs, upgradient Edwards-Trinity groundwater, 

and return flow from the Leon-Belding area.

Except in isolated areas, a clear connection between 

the Edwards-Trinity aquifer and the underlying Rustler is 

not apparent in the Monument Draw trough north of I-10 

(figs. 3 and 4). Upwelling in this area might be inhibited by 

the shale and siltstone confining units in the Lower Dockum 

Group (table 1; Barker and others, 1994), or these wells might 

not intersect faults or other structural features that act as 

preferential vertical flow paths. Samples collected from sites 

Q33 and Q40 (fig. 28, table 7), which are wells completed in 

the Edwards-Trinity aquifer where it overlies, or is near the 

edge of, the estimated extent of the Dockum aquifer, have 

somewhat different compositions than groundwater from 

samples collected from wells in the Monument Draw trough 

roughly south of I-10. However, samples from the well at site 

Q37 and Diamond Y and Santa Rosa Springs (sites Q6 and 

Q2) (fig. 28, tables 7 and 8), both of which discharge from 

the Edwards-Trinity aquifer, are compositionally similar to 

wells in the Monument Draw trough south of I-10. Site Q37 

is on the eastern side of Monument Draw trough, completed 

in a portion of the Edwards-Trinity aquifer that is likely not 

underlain by the Dockum aquifer; Diamond Y Spring are east 

of the estimated extent of the Dockum aquifer; and Santa Rosa 

Springs appear to be the result of a localized upwelling that is 

not affected by the presence of the Dockum aquifer. Diamond 

Y and Santa Rosa Springs are within fault zones (figs. 

17–18 and 28) (Baumgardner and others, 1982; Veni, 1991; 

Boghici, 1997) that create preferential flow paths for lower 

aquifers to discharge to the surface. Finally, groundwater 

likely discharges from the down-dip extent of the EdwardsTrinity aquifer in the Monument Draw trough to overlying or 

underlying aquifer units (Barker and others, 1994).

Groundwater in the eastern part of the study area likely 

originates 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 or to the Pecos River. Groundwater in the eastern 

part of the study area has little compositional similarity to 

the groundwater samples collected in the Monument Draw 

trough area. The composition of groundwater collected from 

Edwards-Trinity aquifer site Q25 (endmember 4) (fig. 28, 

tables 7 and 8) is representative of regional groundwater flow 

in the Edwards-Trinity aquifer east of the Monument Draw 

trough. This endmember and the groundwater collected from 

the Edwards-Trinity aquifer site Q32 are more compositionally 

similar in terms of salinity to the groundwater collected from 

Edwards-Trinity aquifer sites Q8 and Q11, which are west 

of the Monument Draw trough near the Glass Mountains, 

than to groundwater samples collected in the Monument 

Draw trough area even though the regional gradient appears 

to move groundwater through the trough into the east. 

Conversely, the 87Sr/86Sr and stable isotope values of 

groundwater collected from sites Q25 and Q32 are different. 

The 87Sr/86Sr value of groundwater collected from site Q25 

was 0.70959 and δ18O was -7.47 per mil, which are similar 

to values measured in groundwater samples collected 

in the Monument Draw trough area and are consistent 

with a mixture of young and old groundwater interacting 

with siliciclastic rocks. The 87Sr/86Sr and δ18O values in 

groundwater collected from site Q32 were 0.70853 and -6.75 

per mil, respectively, which were more similar to the values 

measured in groundwater collected from site Q35 (0.70824 

and -7.24) than site Q25 and are consistent with relatively 

young water interacting with Cretaceous-age carbonate rocks. 

Conversely, the groundwater collected from sites Q32 and 

Q35 are compositionally dissimilar in every other way. These 

compositional inconsistencies support the theory that there are 

localized influences on groundwater chemistry and regional 

groundwater flow.
Summary 47

Summary

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, 

conducted a comprehensive, integrated analysis of available 

hydrogeologic data in order to develop a conceptual model of 

the Edwards-Trinity and related aquifers in the Pecos County 

region study area in west Texas. The conceptual model of the 

hydrogeologic framework, geochemistry, and groundwaterflow system in the 4,700 square-mile study area was 

developed in an effort to better understand the groundwater 

system and establish a scientific foundation for resourcemanagement decisions. Development of the conceptual model 

is the second phase of a three-phase groundwater-availability 

study being conducted in the Pecos County region by the 

USGS and the cooperators. 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. Data compiled in the first phase 

of the study were used in this report to develop the conceptual 

model. The third phase of the study involves a numerical 

groundwater-flow model of the Edwards-Trinity aquifer in 

order to simulate groundwater conditions based on various 

groundwater-withdrawal scenarios.

Analysis of well, geophysical, geochemical, and 

hydrologic data contributed to the development of the 

conceptual model. Lithologic information obtained from 

well reports and geophysical data were used to describe the 

hydrostratigraphy and structural features of the groundwater 

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. 

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.

Subsurface data were obtained from well reports; natural 

gamma, electric, and electromagnetic induction borehole 

geophysical logs; and audio-magnetotelluric soundings. The 

subsurface data were analyzed in order to map the top and 

base of the Edwards-Trinity aquifer, 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.

The thickness of the Edwards-Trinity aquifer in the study 

area was calculated as the difference in altitudes between the 

top and the base of the Edwards-Trinity aquifer. About 50 

percent of the aquifer was between 234 and 362 ft thick with 

a minimum of about 5 ft and maximum of about 797 ft. Some 

of the thinnest sections of the Edwards-Trinity aquifer were 

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 based on interpretations of cross sections of the top and base surfaces of the 

Edwards-Trinity aquifer units. Faults 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 Permianage evaporite deposits. Fault zones delineate domains in the 

hydrogeologic framework that generally align with previously 

identified structural features such as the Pecos and Monument 

Draw troughs. 

Transmissivity values calculated and estimated from 

historical aquifer-test data ranged from 1,500 to 1,216,000 

gallons per day per foot. The highest transmissivity values 

were measured in the Monument Draw trough area, which is 

also one of the thickest parts of the Edwards-Trinity aquifer 

and is in a faulted area. The lowest values were measured in 

the eastern part of the study, near some of the thinnest parts of 

the aquifer. Hydraulic conductivity values generally showed 

the same trends as the transmissivity values.

Analysis of the geochemical samples provided insight 

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, groundwater-flow paths, 

water-rock interaction, and mixing of water from different 

sources and to identify likely source waters and geochemical 

endmembers. Useful geochemical properties included specific 

conductance, hydrochemical facies, sulfate and chloride 

concentrations, silica concentrations, oxygen and hydrogen 

stable isotopes, strontium isotopes, environmental tracers, 

and concentrations of organic compounds and nutrients. The 

qualitative geochemical analysis did not include a quantitative 

evaluation of residence times in the aquifer nor did it include 

geochemical flow-path modeling of the groundwater system.

Geochemical and isotopic results indicate 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 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 between and within aquifer units.

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 water that is likely a mixture of recharge 
48 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

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.

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. The geochemical 

characteristics and endmembers were used to qualitatively 

evaluate the mixing of water from different sources and the 

flow-path evolution of the chemical characteristics of aquifer 

water. Historical and current groundwater-level data were 

used to create a potentiometric-surface map of the EdwardsTrinity aquifer, assess regional groundwater gradients, and 

compute vertical gradients between the Edwards-Trinity 

and underlying aquifers. Structural features such as bed 

orientation and thickness, mountains, troughs, and faults play 

a substantial role in the distribution of recharge, local and 

regional groundwater-flow paths, spring discharge, and aquifer 

interaction.

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 Edwards-Trinity 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 EdwardsTrinity aquifer that likely recharged during the last 60 years 

(post-bomb) were collected from wells in mountain recharge 

areas and in areas receiving agricultural return flow.

Groundwater generally flows north into the down-dip 

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 down-dip 

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 Edwards-Trinity 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 water 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 down-dip 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 down-dip extent or to the 

Pecos River.

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52 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 1. Hydrostratigraphic section in the Pecos County region study area, Texas (modified from Brand and DeFord, 1958; Rose, 1972; 

Rees and Buckner, 1980; Smith and Brown, 1983; and Ashworth and Hopkins, 1995).

[Water-yielding properties: yields (gallons per minute) - small less than 50, moderate 50 to 500, large is more than 500; classification of water dissolved-solids 

concentration (milligrams per liter) - fresh less than 1,000, slightly saline 1,000 to 3,000, moderately saline 3,000 to 10,000.]

Era Period Series 

or group

Stratigraphic 

unit

Approximate 

maximum thickness 

(feet)

Cenozoic

Quaternary 

and Tertiary Alluvium 1,150

Tertiary Volcanic Rocks, Undivided 1,000+

Mesozoic

Cretaceous

Gulfian 

Series

Terlingua 

Group Boquillas Formation

250

Western 

Pecos County

Eastern 

Pecos County

Comanchean Series

Washita Group

Western Pecos County Eastern Pecos County

100 200

Sixshooter Group

Buda Limestone

Boracho 

Formation

Edwards Group

Fort Lancaster 

Formation 410 350 University Mesa 

Marl

Burt Ranch 

Member

Fredericksburg 

Group Finlay Formation Fort Terrett Formation 165 200

Trinity Group Trinity Sands

Maxon Sands 300

Glen Rose Formation 200+

“Basal” Sand 100

Triassic Dockum Group Middle 600

Lower 70

Paleozoic

Permian

Ochoan Series

600

Southern 

Pecos County Northern Pecos County Southern Pecos County

Northern 

Pecos County

Tessey 

Limestone

Rustler Formation

1,050

450

Salado Formation 2,200

Castile Formation 2,300

Guadalupian 

Series

Whitehorse 

Group

Gilliam 

Limestone

Capitan 

Limestone

Guadalupian 

Formations; 

undivided

870 1,650 1,900

Lower Guadalupian Formations; undivided 2,000

Lower Permian Formations; undivided 10,000

Pennsylvanian Pennsylvanian Formations; undivided 6,000

Table 1
Table 1 53

Table 1. Hydrostratigraphic section in the Pecos County region study area, Texas (modified from Brand and DeFord, 1958; Rose, 1972; 

Rees and Buckner, 1980; Smith and Brown, 1983; and Ashworth and Hopkins, 1995).—Continued

[Water-yielding properties: yields (gallons per minute) - small less than 50, moderate 50 to 500, large is more than 500; classification of water dissolved-solids 

concentration (milligrams per liter) - fresh less than 1,000, slightly saline 1,000 to 3,000, moderately saline 3,000 to 10,000.]

Character

of rocks

Regional water 

yielding properties

Major 

and minor 

aquifers 

(figs. 3 and 4)

Unconsolidated silt, sand, gravel, clay, boulders, caliche, 

gypsum, and conglomerate

Yields range from small to large quantities of 

fresh to moderately saline water Pecos Valley

Lavas, pyroclastic tuffs, volcanic ash, tuff breccias, fragmental 

breccias, agglomerates; few thin beds of conglomerates, 

sandstones, and freshwater limestones

Yields small quantities of freshwater Igneous

Brown to red flaggy limestone interbedded with shale Not known to yield water

Soft nodular limestone, marl, and thin-bedded hard granular 

limesone

Does not yield water in most of the study 

area; however, may yield small quantities in 

Reeves County

EdwardsTrinity

Hard massive limestone, thin-bedded limestone, and soft 

nodular limestone with some clay Yields small quantities of water

Soft nodular limestone, marl, and hard massive ledge-forming 

limestone Yields small quantities of water

Massive ledge-forming limestone and soft nodular limestone Yields small quantities of fresh to moderately 

saline water

Crossbedded, fine- to coarse-grained, poorly to well-cemented 

quartz sand with some silt, shale, and limestone

Yields small to moderate quantities of fresh to 

slightly saline water

Reddish-brown to gray coarse-grained sandstone Yields small to moderate quantities of fresh to 

slightly saline water Dockum

Red shale and siltstone Not known to yield water

Sand, shale, gypsum, and anhydrite Not known to yield water

Southern Pecos County Northern Pecos County Southern Pecos 

County Northern Pecos County

Limestone and dolomite

Red shale, sandstone, anhydrite, 

dolomite, limestone, conglomerate, 

and halite

Not known to 

yield water

Yields small to large 

quantities of slightly to 

moderately saline water

Rustler

Mostly halite, with anhydrite and 

some dolomite Not known to yield water

Mostly calcareous anhydrite, with 

halite and associated salts and some 

limestone

Not known to yield water

Limestone, 

dolomite, 

and 

sandstone

Limestone, 

dolomite, 

and reef 

talusDolomite, limestone, anhydrite, shale, 

and sandstone

Yields freshwater 

to a few wells 

in the Glass 

Moutains

Yields moderate to large 

quantities of moderately 

saline water

Capitan Reef

Dolomite, dolomitic limestone, limestone, and siliceous shale Yields small to large quantities of moderately 

saline water

Shale, siliceous shale, limestone, dolomitic limestone, 

sandstone, and basal conglomerate Yields small quantities of water

Limestone, sand, sandstone, shale chert, and conglomerate Yields small quantities of water
54 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

302122102504501 30.35600 102.35600 3,113 3,091 3,046 2,957

302630102503801 30.44176 102.84396 4,354 4,234 4,234 3,982

303503102303601 30.58421 102.51007 3,413 3,228 3,228 2,903

303824102285001 30.64000 102.48052 3,264 3,129 3,129 2,974

303852102432901 30.64790 102.72471 3,548 3,398 3,328 2,998

303948103205801 30.66344 103.34970 3,545 3,483 3,450 3,350

304134102312601 30.69279 102.52399 3,170 3,153 3,035 2,902

304153103090501 30.69796 103.15140 3,384 3,285 3,167 2,992

304210102443201 30.70269 102.74228 3,480 3,380 3,220 3,071

304551102361201 30.76448 102.60387 2,929 2,929 2,849 2,766

304622102312401 30.77304 102.52379 3,333 3,257 3,119 3,033

304711103003301 30.78642 103.00932 3,239 3,196 3,078 2,939

304715103263501 30.78740 103.44343 3,447 -- 3,152 2,987

304728102304401 30.79098 102.51216 3,067 3,067 3,017 2,817

305042102595601 30.84509 102.99899 3,088 3,076 2,958 --

305055103110801 30.84864 103.18567 3,121 3,093 2,871 2,761

305548103161401 30.93771 103.26958 3,037 3,017 2,797 2,597

305604102581301 30.93455 102.97030 2,907 -- -- 2,605

305627103071901 30.94075 103.12200 2,991 -- 2,695 2,263

305706102095501 30.95175 102.16536 2,981 2,981 2,741 2,471

305740103110901 30.96120 103.18608 2,979 2,949 2,879 2,779

305835102134701 31.01156 102.25855 2,302 2,162 2,044 1,894

310041102152901 30.97650 102.22977 2,370 2,340 2,296 2,225

BRACS-0101 31.04111 103.38472 2,903 2,803 -- 2,753

BRACS-0109 30.97583 103.73055 3,181 2,902 2,595 2,402

BRACS-0111 31.22222 102.93222 2,530 2,517 -- 2,212

BRACS-0113 31.00083 103.09472 2,943 2,878 -- --

BRACS-0135 31.20805 102.93778 2,549 2,534 -- --

BRACS-0138 30.95333 102.83305 2,832 2,652 -- 2,363

BRACS-0157 31.24750 102.91972 2,500 2,492 -- 2,080

BRACS-0160 31.23889 102.91944 2,509 2,501 -- 2,082

BRACS-0192 31.06472 103.40722 2,884 2,804 -- 2,624

BRACS-0198 31.03972 102.81583 2,685 2,664 -- 2,355

BRACS-0209 30.96167 103.33000 3,040 2,965 -- 2,682

BRACS-0210 30.94611 103.41000 3,161 3,156 -- 2,780

BRACS-0299 31.04944 102.35222 2,333 2,298 2,264 2,218

BRACS-0308 30.93000 103.46000 3,145 3,095 -- 2,724

BRACS-0328 30.91333 103.17722 3,039 3,004 -- 2,647

BRACS-0341 31.14444 102.56722 2,388 2,364 -- 2,199

BRACS-0519 31.20830 103.07636 2,653 2,023 2,003 1,593

BRACS-0520 31.21101 103.05238 2,634 2,086 2,044 1,619

BRACS-0521 31.17668 103.02671 2,660 2,150 2,150 1,749

BRACS-0522 31.04934 103.02272 2,778 2,616 2,498 2,313

BRACS-0523 31.00094 103.09373 2,944 2,876 2,614 2,494

BRACS-0524 30.96626 103.20461 2,963 2,813 2,813 2,663

BRACS-0525 31.19085 102.83513 2,472 2,437 -- 2,107

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.

[--, not used]

Table 2
Table 2 55

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

BRACS-0526 30.99283 102.21311 2,304 2,054 2,039 1,844

BRACS-0542 31.10201 102.55492 2,395 2,355 2,293 2,175

BRACS-0544 30.98961 102.26984 2,338 2,176 2,058 1,938

BRACS-0627 31.02472 102.84555 2,870 2,870 -- 2,489

BRACS-0633 31.21417 103.02917 2,617 2,252 -- 1,742

BRACS-0730 30.93423 102.93300 2,925 2,845 2,845 2,695

BRACS-0731 30.94153 102.94986 2,903 2,813 2,813 2,663

BRACS-0732 31.22609 102.83262 2,442 2,402 2,402 2,183

BRACS-0733 30.94907 103.03827 3,035 3,035 2,965 2,615

BRACS-0734 30.96357 103.00668 2,958 2,848 2,748 2,568

BRACS-0735 31.22850 102.85153 2,454 2,354 2,354 2,154

BRACS-0736 31.15706 102.94037 2,582 2,547 2,252 2,097

BRACS-0737 31.04779 103.01047 2,777 2,677 2,617 2,437

BRACS-0738 31.24765 102.86736 2,457 2,422 2,422 2,197

BRACS-0739 30.98453 103.05897 2,920 2,810 2,710 2,535

BRACS-0741 30.80259 102.99008 3,193 3,143 3,023 2,833

BRACS-0744 31.19169 102.88496 2,519 2,469 2,369 2,147

BRACS-0746 31.01443 102.84010 2,851 2,826 2,661 2,401

BRACS-0747 31.19244 102.93365 2,550 2,500 -- --

BRACS-0749 30.82914 102.95679 3,104 3,044 2,989 --

BRACS-0753 31.20135 102.94071 2,559 2,509 2,374 2,246

BRACS-0755 31.13003 102.89910 2,610 2,540 2,480 2,280

BRACS-0757 30.80957 102.94955 3,187 3,047 -- --

BRACS-0762 31.25080 102.84100 2,430 2,335 2,335 2,140

BRACS-0764 30.91562 103.00171 2,990 -- -- 2,545

BRACS-0766 31.00553 102.94862 2,799 2,564 -- --

BRACS-0768 30.80934 102.93241 3,206 3,106 3,076 2,936

BRACS-0769 30.84665 102.94038 3,068 3,048 3,048 2,843

BRACS-0770 31.12670 102.94008 2,624 2,589 2,414 2,154

BRACS-0771 31.08683 102.91504 2,711 2,711 2,601 2,451

BRACS-0772 31.09040 102.92793 2,681 2,681 2,578 2,428

BRACS-0773 30.74136 102.93528 3,551 3,551 3,451 3,266

BRACS-0777 31.03613 102.93529 2,818 2,818 2,668 2,538

BRACS-0780 30.85778 102.93244 3,057 3,007 -- --

BRACS-0782 30.97206 102.89709 2,864 2,864 2,784 2,642

BRACS-0783 30.75097 103.14153 3,534 3,534 3,534 3,206

BRACS-0934 30.97444 103.27250 2,974 2,829 -- 2,548

BRACS-0935 30.93805 103.25944 3,030 3,002 -- 2,620

BRACS-0937 30.97417 103.64028 3,001 2,418 -- --

BRACS-0943 31.01306 103.52000 2,950 2,844 2,844 2,570

BRACS-0944 31.05306 103.46139 2,921 2,776 -- 2,597

BRACS-0963 31.07917 103.62333 2,894 2,304 -- 1,955

BRACS-0974 30.97194 102.17528 2,383 2,383 -- 2,021

BRACS-0990 30.99667 102.37555 2,487 2,487 -- --

BRACS-1028 31.06139 103.62000 2,911 2,326 -- 1,986

BRACS-1192 31.03447 103.47014 2,954 2,764 2,764 2,714
56 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

BRACS-1201 31.15030 103.03856 2,706 2,311 2,311 1,880

BRACS-1203 31.18958 102.87461 2,512 2,477 2,457 2,155

BRACS-1212 31.13737 102.51653 2,391 2,315 -- 2,161

BRACS-1213 31.07577 102.45276 2,333 2,303 2,283 2,103

BRACS-1214 31.07524 102.36937 2,289 2,239 2,239 2,069

BRACS-1345 31.01694 103.69972 3,084 2,564 2,334 2,184

BRACS-1346 31.04028 103.58972 2,882 2,697 2,662 2,402

BRACS-1347 31.01917 103.51778 2,945 -- -- 2,625

BRACS-1354 30.96944 103.55333 2,967 -- 2,484 2,334

BRACS-1355 30.97722 103.52889 2,980 2,980 -- --

BRACS-1478 30.95732 103.36377 3,066 3,066 -- 2,721

BRACS-1479 30.99617 103.28807 2,955 2,902 -- 2,655

BRACS-1481 31.14586 103.00910 2,682 2,332 -- 1,946

BRACS-1482 31.09035 103.03724 2,780 2,450 -- --

BRACS-1521 31.02076 103.52570 2,936 -- -- 2,606

BRACS-1522 30.93780 103.31012 3,075 3,025 -- 2,699

BRACS-1523 30.93539 103.26028 3,036 3,022 -- 2,635

BRACS-1525 30.86167 103.42722 3,306 3,306 -- 2,962

BRACS-1527 30.96861 103.39271 3,044 3,016 -- 2,734

BRACS-1530 31.00844 103.37838 2,945 2,945 -- 2,796

BRACS-1532 30.97841 103.53645 2,972 2,954 -- --

BRACS-1533 30.94464 103.57559 3,010 2,775 -- 2,233

BRACS-1538 30.90699 103.57838 3,138 2,963 -- 2,456

BRACS-1540 31.02707 103.55891 2,896 2,896 -- 2,583

BRACS-1541 31.05026 103.60564 2,895 2,415 -- 2,092

BRACS-1568 31.00827 103.20741 2,903 2,573 -- 2,320

BRACS-1586 30.93806 103.18667 3,016 3,016 -- 2,739

BRACS-1625 31.05021 102.36771 2,366 2,256 2,251 2,116

BRACS-1626 30.95847 102.90540 2,870 2,810 -- 2,605

BRACS-1627 31.00020 103.12570 2,917 2,867 2,715 2,603

BRACS-1628 30.98685 103.23485 2,923 2,733 -- 2,459

BRACS-1629 30.93716 103.12898 2,993 2,903 -- --

BRACS-1630 30.95826 103.13609 2,974 2,894 -- 2,349

BRACS-1639 30.91583 103.09555 3,038 2,943 -- 2,394

BRACS-1641 31.03361 102.39000 2,394 2,319 -- 2,099

BRACS-1642 31.04389 102.39694 2,368 2,318 2,238 2,095

BRACS-1647 31.12785 102.52808 2,386 2,325 -- 2,164

BRACS-1648 31.10293 102.58501 2,464 2,464 -- 2,283

BRACS-1650 31.00373 102.53770 2,536 2,471 -- 2,106

BRACS-1651 31.02515 102.51900 2,453 2,363 -- 2,062

BRACS-1652 30.99661 102.54653 2,570 2,505 -- 2,160

BRACS-1654 31.09593 102.94353 2,652 2,572 -- 2,332

BRACS-1655 30.96800 102.65588 2,726 2,726 -- 2,567

BRACS-1658 30.96743 102.57421 2,711 2,691 -- 2,491

BRACS-1659 31.04967 102.48284 2,371 2,286 -- 2,065

BRACS-1660 31.09867 102.44283 2,327 2,274 -- 2,075
Table 2 57

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

BRACS-1667 30.96458 102.28388 2,375 2,235 -- 1,893

BRACS-1669 30.98912 102.31238 2,486 2,476 2,326 2,221

BRACS-1681 31.06475 103.52950 2,876 2,876 2,651 2,456

BRACS-1704 30.99439 103.67245 3,049 2,611 2,124 2,083

BRACS-1705 30.98096 103.68008 3,065 2,567 -- 1,935

BRACS-1981 30.83688 102.69043 2,951 2,829 2,711 2,561

BRACS-1982 31.11386 102.53687 2,410 2,380 -- 2,209

BRACS-1984 30.93423 103.41634 3,317 3,317 -- 2,895

BRACS-1985 31.19859 102.95412 2,581 2,561 2,561 2,315

BRACS-1986 31.24079 102.97340 2,563 2,508 2,508 2,333

BRACS-1992 30.80612 102.65878 3,004 3,004 2,784 2,544

BRACS-1997 31.13050 102.85201 2,589 2,519 2,419 2,199

BRACS-1998 30.77326 103.51899 3,765 3,715 3,505 3,345

BRACS-2002 30.98461 103.21682 2,926 2,866 2,786 2,586

BRACS-2004 30.75005 103.33497 3,433 3,433 3,158 3,043

BRACS-2008 30.83654 103.45462 3,461 3,461 3,461 3,053

BRACS-2010 30.86716 103.27643 3,343 3,343 3,343 2,913

BRACS-2011 31.05707 102.40528 2,345 2,223 2,105 1,985

BRACS-2012 30.88377 103.23200 3,174 3,174 3,084 2,934

BRACS-2015 30.73693 103.17870 3,613 3,613 3,373 3,213

BRACS-2017 31.04011 102.43908 2,407 -- -- 2,097

BRACS-2019 31.03129 102.26479 2,291 2,021 1,961 1,841

BRACS-2020 31.22852 103.07432 2,630 2,032 2,032 1,600

BRACS-2026 30.96192 103.45549 3,085 2,995 2,995 2,875

BRACS-2033 31.02639 103.38380 2,925 2,813 2,813 2,740

BRACS-2037 30.98931 103.50246 3,005 2,953 2,835 2,665

BRACS-2145 30.82604 103.17534 3,216 3,164 3,046 2,966

BRACS-2147 31.06344 102.45971 2,341 2,211 2,211 2,041

BRACS-2150 30.70695 103.25022 3,380 3,365 3,345 3,200

BRACS-2151 30.81561 102.51381 3,245 -- -- 2,705

BRACS-2153 31.10459 102.96820 2,651 2,621 2,571 2,461

BRACS-2154 30.97626 102.94308 2,854 2,854 2,754 2,614

BRACS-2161 30.83718 102.51338 2,805 2,805 2,755 2,580

BRACS-2169 31.04222 102.43322 2,411 2,361 2,361 --

BRACS-2182 30.84668 103.01816 3,094 3,049 -- --

BRACS-2186 30.89022 102.56684 3,033 2,763 2,763 2,583

BRACS-2192 31.03419 102.96265 2,805 2,805 2,730 2,580

BRACS-2194 30.87545 102.59659 2,918 2,918 2,748 2,628

BRACS-2203 30.93063 103.35175 3,119 3,112 2,994 2,844

BRACS-2204 30.80490 103.53533 4,052 3,682 3,542 3,390

BRACS-2206 30.86788 102.57487 2,869 2,749 2,749 2,529

BRACS-2207 31.07271 102.36447 2,291 2,201 2,161 2,051

BRACS-2208 30.80653 102.76004 3,040 -- -- 2,715

BRACS-2210 31.17944 103.04391 2,663 2,143 -- 1,673

BRACS-2211 30.82964 102.77051 3,118 3,118 2,868 2,713

BRACS-2212 30.80104 102.75377 3,053 2,993 -- --
58 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

BRACS-2222 30.76278 103.06633 3,312 3,042 -- 2,812

BRACS-2225 30.99741 102.96687 2,832 2,832 2,736 2,502

BRACS-2226 30.73992 103.06305 3,415 3,395 3,285 3,145

BRACS-2230 30.73716 103.08348 3,330 3,190 -- 2,980

BRACS-2237 30.92991 102.63428 3,113 3,113 -- 2,961

BRACS-2240 30.78615 103.46353 3,566 3,466 3,286 3,126

BRACS-2247 30.64390 103.25482 3,480 3,480 3,480 3,325

BRACS-2252 31.23881 103.07869 2,619 2,049 1,964 1,554

BRACS-2255 31.00500 102.21113 2,280 2,060 2,040 1,810

BRACS-2258 30.71379 103.28194 3,335 3,305 3,185 3,090

BRACS-2263 30.81347 102.82316 3,155 3,155 -- 2,855

BRACS-2264 31.06550 102.40337 2,331 2,226 2,226 1,966

BRACS-2265 31.07118 102.41585 2,353 2,248 2,248 1,998

BRACS-2268 30.76702 103.11723 3,451 3,451 3,321 3,096

BRACS-2295 30.99133 102.67243 2,636 -- 2,436 --

BRACS-2313 31.12583 102.89809 2,617 2,583 2,457 2,317

BRACS-2452 30.87898 102.47309 2,716 2,716 2,556 2,406

BRACS-2453 30.93657 102.58736 2,934 2,744 2,714 2,629

BRACS-2454 30.96603 102.59072 2,807 2,807 2,687 2,597

BRACS-2513 31.18653 102.97079 2,607 2,347 -- 2,107

BRACS-2659 30.93602 103.67832 3,204 -- -- 2,354

BRACS-2660 30.93044 103.57864 3,047 2,837 2,837 2,292

BRACS-2661 30.97555 103.53611 2,980 2,951 -- 2,434

BRACS-2663 30.97917 103.55417 2,952 2,946 -- 2,410

BRACS-2664 30.96880 103.51518 3,045 3,045 2,825 2,655

BRACS-2665 30.96689 103.64024 3,018 -- 2,426 2,028

BRACS-2666 30.98029 103.57663 2,943 -- 2,313 2,085

BRACS-2668 30.93604 103.74402 3,319 2,419 2,169 1,969

BRACS-2669 30.92550 103.79484 3,406 2,956 2,956 2,446

BRACS-2670 30.94163 103.66829 3,144 2,786 -- --

BRACS-2744 30.96465 102.28367 2,375 2,255 1,965 --

BRACS-2904 30.94875 103.55675 2,998 2,836 -- 2,271

BRACS-2909 30.99887 103.16815 2,926 2,840 -- --

BRACS-2910 31.07580 103.08670 2,810 2,320 -- --

BRACS-2911 31.08418 103.03573 2,779 2,422 -- 1,919

BRACS-2912 31.11792 103.11281 2,763 2,313 -- 1,903

BRACS-2913 31.14389 103.08611 2,715 2,048 -- 1,720

BRACS-2923 30.99842 103.64334 2,991 2,361 -- 1,925

BRACS-2924 30.98840 103.62820 2,980 2,385 -- 1,969

BRACS-2926 30.99106 103.64371 2,998 2,455 -- --

BRACS-2942 31.15243 103.01463 2,684 2,346 -- 1,955

BRACS-2943 31.22802 102.96004 2,554 2,534 -- 2,314

BRACS-2945 31.20861 102.99417 2,602 2,327 -- 2,072

BRACS-2946 31.20940 102.95644 2,564 -- -- --

BRACS-3264 30.98361 103.44205 3,033 2,963 -- 2,837

DBSA-036 30.81761 103.27782 3,252 3,213 2,928 2,857
Table 2 59

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

DBSA-039 30.75100 103.16760 3,495 3,494 3,293 3,138

DBSA-040 30.84284 103.02623 3,100 3,045 2,915 2,790

DBSA-041 30.85833 103.02736 3,076 3,042 -- 2,614

DBSA-043 30.75471 103.06810 3,345 3,078 -- 2,852

DBSA-044 30.76306 103.00694 3,272 3,242 3,043 2,714

DBSA-045 30.70377 103.24315 3,409 3,400 -- 3,206

DBSA-046 30.70718 103.25866 3,353 3,344 -- 3,170

DBSA-047 30.67946 103.21452 3,453 3,442 -- 3,163

DBSA-049 30.83185 103.19955 3,146 3,071 2,891 2,793

DBSA-051 30.74283 103.07800 3,334 -- -- 3,058

DBSA-052 30.74267 103.08117 3,320 3,151 -- --

DBSA-053 30.74524 103.00021 3,296 3,296 2,976 2,796

DBSA-054 30.68197 103.12469 3,490 3,190 3,090 2,715

DBSA-055 30.94478 102.94840 2,892 -- -- 2,686

DBSA-056 30.94498 102.93862 2,892 2,842 2,842 2,695

DBSA-057 30.92335 102.93428 2,968 2,963 2,748 2,698

DBSA-058 30.86957 103.00247 3,041 -- 2,729 --

DBSA-059 30.86610 103.04056 3,062 3,012 2,462 2,312

DBSA-061 30.83043 102.95392 3,101 3,051 -- 2,866

DBSA-062 30.87479 102.90796 3,055 3,017 -- 2,625

DBSA-063 30.85412 102.90950 3,049 3,024 2,829 2,695

DBSA-064 30.80139 102.99889 3,196 3,169 3,118 2,859

DBSA-065 30.85642 102.90196 3,033 2,998 2,848 2,661

DBSA-066 30.81876 102.88520 3,130 3,120 2,914 2,881

DBSA-067 30.75770 102.90531 3,442 3,442 -- 3,133

DBSA-068 30.74312 102.91567 3,634 3,634 3,448 3,311

DBSA-070 30.66601 102.90874 3,466 3,431 3,246 2,955

DBSA-071 30.85766 102.97734 3,054 3,030 -- 2,707

DBSA-073 30.62451 103.14530 3,690 3,640 -- 3,249

DBSA-074 30.63910 103.06406 3,749 3,679 3,509 3,179

DBSA-075 30.59485 103.02183 3,843 3,808 3,798 3,748

DBSA-076 30.58608 103.12334 3,958 3,913 3,691 3,616

DBSA-077 30.60114 102.97080 3,738 3,678 -- 3,518

DBSA-078 30.64638 102.96949 3,644 3,575 3,355 3,150

DBSA-079 30.64589 102.88489 3,549 3,494 3,277 3,164

DBSA-080 30.61599 102.86091 3,700 3,685 2,960 2,890

DBSA-081 30.79969 103.03890 3,181 -- -- 2,762

DBSA-082 30.81505 103.02453 3,160 3,110 2,793 2,650

DBSA-083 30.79855 103.01440 3,202 3,131 -- 2,804

DBSA-084 30.76344 103.04282 3,309 3,279 3,279 2,950

DBSA-085 30.74792 103.05654 3,543 3,353 3,123 3,078

DBSA-086 30.72105 103.15928 3,487 3,487 -- 3,235

DBSA-087 30.69572 103.02660 3,486 3,331 2,795 2,590

DBSA-088 30.67661 103.02219 3,534 3,455 2,837 2,657

DBSA-089 30.66518 103.04358 3,602 3,602 3,178 3,107

DBSA-090 30.81608 102.96619 3,154 3,089 2,964 2,677
60 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

DBSA-091 30.77362 102.94773 3,398 3,388 3,198 2,962

DBSA-092 30.69928 102.97718 3,479 3,479 3,094 2,706

DBSA-093 30.67625 102.95187 3,504 3,483 3,074 2,745

DBSA-094 30.69013 102.91888 3,396 3,201 2,809 2,636

DBSA-095 30.69090 102.83673 3,309 3,257 2,772 2,646

DBSA-096 30.66216 102.83509 3,409 3,239 2,939 2,774

DBSA-097 30.96886 103.14349 2,967 2,890 -- 2,462

DBSA-098 30.97212 103.13810 2,964 2,906 -- 2,483

DBSA-099 30.95497 103.13222 2,972 2,876 2,500 2,334

DBSA-100 30.92149 103.11474 3,008 2,918 -- 2,338

DBSA-101 30.92557 103.15312 3,015 2,985 -- 2,536

DBSA-102 30.88752 103.14824 3,078 3,002 2,603 2,565

DBSA-103 30.88246 103.13142 3,075 3,005 2,625 2,535

DBSA-104 30.87376 103.04722 3,045 2,975 2,466 2,335

DBSA-105 30.88916 103.08508 3,062 2,977 2,762 2,542

DBSA-107 30.96245 102.88053 2,879 2,795 2,683 2,557

DBSA-108 30.90077 102.98753 2,979 2,929 2,699 2,509

DBSA-109 30.90050 102.89341 2,980 2,900 -- 2,610

DBSA-110 30.91598 102.85998 2,919 2,859 2,751 2,630

DBSA-111 30.88715 103.02782 3,006 2,981 -- 2,484

DBSA-112 30.86597 103.00654 3,048 -- 2,822 --

DBSA-113 30.98443 103.02628 3,034 3,034 2,654 2,537

DBSA-114 30.91735 103.00551 2,999 2,999 -- 2,554

DBSA-115 30.84964 103.03246 3,090 3,040 2,842 2,707

DBSA-116 30.83180 103.02364 3,124 3,054 2,944 2,829

DBSA-117 30.88136 102.87716 2,941 2,941 2,681 2,625

DBSA-118 30.85433 102.88516 3,002 2,982 2,897 2,685

DBSA-119 30.86818 102.92472 3,086 3,086 2,806 2,704

DBSA-120 30.85843 102.99702 3,062 3,012 2,782 2,659

DBSA-121 30.84515 102.99178 3,083 3,058 -- 2,797

DBSA-124 30.91430 103.03516 3,206 3,206 2,996 2,486

DBSA-125 30.89546 103.02813 3,046 3,046 2,736 2,576

DBSA-126 30.85354 103.05756 3,095 3,040 2,940 2,530

DBSA-127 30.88081 103.01438 3,008 2,988 2,827 2,559

DBSA-128 30.87188 103.00596 3,039 2,979 -- 2,578

DBSA-130 30.86300 103.02164 3,064 2,990 -- 2,559

DBSA-131 30.86755 103.03802 3,061 2,987 -- --

DBSA-132 30.84495 103.01109 3,096 -- 2,872 --

DBSA-133 30.78701 103.05560 3,205 3,187 -- 2,869

DBSA-134 30.78707 103.05347 3,206 3,176 -- 2,855

DBSA-136 30.88515 102.79960 2,973 2,943 2,788 2,493

DBSA-137 30.80919 102.96216 3,183 3,113 3,006 2,773

DBSA-138 30.96679 102.85950 2,862 2,792 -- 2,517

DBSA-139 30.90253 102.87825 2,961 -- 2,683 2,586

DBSA-140 30.90572 102.93458 3,081 3,081 2,760 2,641

DBSA-141 30.89308 102.93134 3,122 -- -- 2,594
Table 2 61

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

DBSA-142 30.88351 102.92979 3,142 2,997 2,718 2,524

DBSA-143 30.88857 102.89481 3,004 2,939 2,718 2,591

DBSA-144 30.89213 102.88336 2,980 2,966 2,720 2,632

DBSA-145 30.90148 102.85575 2,921 2,886 2,813 2,646

DBSA-146 30.91087 102.82786 2,897 2,867 2,737 2,551

DBSA-147 30.88935 102.83192 2,971 2,811 2,561 2,461

DBSA-148 30.86527 102.88777 2,984 2,964 2,874 2,584

DBSA-149 30.86559 102.88877 2,989 2,971 -- 2,593

DBSA-150 30.91333 103.17722 3,039 3,004 -- 2,647

DBSA-151 30.68389 103.45111 3,722 3,672 3,292 3,122

DBSA-152 30.70000 103.41444 3,604 3,514 -- 3,099

DBSA-153 30.72667 103.33333 3,376 3,376 -- 3,060

DBSA-154 30.72722 103.33389 3,371 3,371 -- 3,053

DBSA-155 30.72750 103.33500 3,368 3,368 -- 3,047

DBSA-156 30.65444 103.35472 3,567 3,483 -- 3,323

DBSA-157 30.63556 103.30806 3,442 3,433 -- 3,282

DBSA-158 30.63194 103.16306 3,640 3,625 -- 3,264

DBSA-161 30.57083 103.20500 3,927 3,908 3,766 3,671

DBSA-163 30.97472 102.89028 2,873 2,859 -- 2,626

DBSA-164 30.95361 102.87889 2,877 2,860 2,753 2,627

DBSA-165 30.95806 102.89611 2,882 2,792 2,697 2,577

DBSA-166 30.87639 102.98972 3,030 2,950 2,830 2,556

DBSA-167 30.89806 102.93194 3,103 3,103 2,778 2,611

DBSA-168 30.88472 102.90972 3,061 3,019 -- 2,615

DBSA-169 30.87667 102.89028 3,000 2,920 2,800 2,556

DBSA-171 30.90500 102.89111 2,968 2,890 -- 2,585

DBSA-172 30.96194 102.84889 2,830 2,760 2,751 2,478

DBSA-173 30.90611 102.83806 2,914 2,784 2,744 2,495

DBSA-174 30.87278 102.90667 3,051 2,993 -- 2,606

DBSA-175 30.77306 102.90194 3,340 3,328 -- 3,043

DBSA-176 30.85667 102.77806 3,053 3,053 -- 2,666

DBSA-177 30.84517 102.99220 3,083 3,062 -- 2,803

DBSA-178 30.84512 102.99756 3,088 3,067 -- --

DBSA-179 30.83006 102.97460 3,100 3,025 2,884 2,735

DBSA-181 31.02009 103.26006 2,897 2,617 2,517 2,412

DBSA-182 31.00726 103.32115 2,952 2,837 2,667 2,507

DBSA-183 31.06246 103.31201 2,863 2,733 2,578 2,418

DBSA-184 30.98268 103.32626 3,004 2,794 2,674 2,554

DBSA-188 30.80170 103.03060 3,188 3,133 2,768 --

DBSA-189 30.80810 103.04360 3,159 2,967 2,697 2,583

DBSA-190 30.78920 103.02780 3,219 3,169 -- 2,837

DBSA-191 30.78780 103.02670 3,223 3,158 -- 2,829

DBSA-193 30.78833 103.00167 3,250 3,170 -- 2,874

DBSA-197 30.74310 103.21396 3,433 3,433 -- 3,091

DBSA-199 30.72045 103.16864 3,578 3,578 3,483 3,326

DBSA-200 30.71839 103.16225 3,493 3,493 -- 3,259
62 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

DBSA-201 30.71925 103.16655 3,540 3,540 -- 3,305

DBSA-203 30.75199 102.97328 3,400 3,360 3,211 2,870

DBSA-204 30.64366 103.16820 3,588 3,518 -- 3,159

DBSA-205 30.69429 103.06752 3,538 3,538 3,181 2,814

DBSA-208 30.77372 102.94765 3,395 3,385 3,195 2,955

DBSA-210 30.71609 103.16714 3,510 3,510 3,434 --

DBSA-211 30.72922 103.11029 3,322 3,237 -- 2,922

DBSA-212 30.65791 103.09612 3,715 3,715 3,487 3,030

DBSA-214 30.70229 103.06421 3,536 3,536 3,098 2,843

DBSA-216 30.73513 103.18741 3,574 3,574 3,452 3,189

DBSA-217 30.72077 103.20475 3,601 3,601 -- 3,237

DBSA-218 30.79551 102.96050 3,244 3,104 2,969 2,774

DBSA-220 30.70713 103.19519 3,591 3,591 3,301 3,181

DBSA-223 30.88088 103.40379 3,301 3,301 -- 3,076

DBSA-224 30.77515 103.42563 3,590 3,590 3,412 3,271

DBSA-228 30.98683 103.23491 2,923 2,734 -- 2,460

DBSA-229 30.89575 103.12843 3,056 3,042 2,620 2,551

DBSA-230 31.02881 102.90003 2,731 2,706 2,706 2,453

DBSA-231 31.04620 102.89913 2,782 2,782 2,782 2,517

DBSA-251 30.90939 102.87435 2,944 -- 2,729 --

DBSA-255 30.91918 102.86270 2,918 2,838 2,763 2,623

DBSA-256 30.91525 102.85698 2,919 2,859 2,779 2,626

DBSA-257 30.88119 102.85087 2,993 2,903 2,823 2,707

DBSA-258 30.88677 102.86808 2,929 -- 2,759 --

DBSA-259 30.88166 102.84805 2,996 2,906 2,806 --

DBSA-260 30.88135 102.84943 2,994 2,904 2,827 2,706

DBSA-261 30.88140 102.85122 2,992 2,902 -- 2,707

DBSA-265 30.90828 102.87469 2,947 -- 2,732 --

DBSA-310 31.09256 103.04454 2,784 -- 2,103 --

DBSA-311 31.08878 103.09438 2,787 2,512 2,274 --

DBSA-312 31.05870 102.80656 2,624 2,581 2,521 2,337

DBSA-313 30.91173 103.00093 2,995 2,945 2,715 2,525

DBSA-314 30.93779 103.43380 3,326 3,326 3,036 2,916

DBSA-315 30.93127 103.40476 3,261 3,261 2,941 2,821

DBSA-316 30.87068 103.26405 3,336 3,336 2,956 2,876

DBSA-318 30.85440 103.01811 3,074 -- 2,834 --

DBSA-320 30.81678 102.81948 3,164 3,164 -- --

DBSA-322 30.81955 102.65095 2,970 2,970 -- 2,537

DBSA-324 31.01231 103.33244 2,944 -- 2,804 2,684

DBSA-325 30.99298 103.30716 2,975 2,925 2,825 2,705

DBSA-326 30.98358 103.28544 2,970 2,886 2,777 2,642

DBSA-327 31.01290 103.26838 2,913 2,803 2,623 2,533

DBSA-328 31.02781 103.25969 2,886 2,626 2,516 2,416

DBSA-329 31.03164 103.24734 2,870 2,590 2,390 2,290

DBSA-330 30.77244 102.85862 3,281 3,231 3,063 2,931

DBSA-331 30.77541 102.80033 3,118 3,048 2,893 2,677
Table 2 63

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

DBSA-333 30.79551 102.77492 3,070 3,050 2,822 2,670

DBSA-334 30.85935 103.47376 3,421 3,421 2,961 2,896

DBSA-335 30.99862 103.16741 2,925 -- 2,564 2,425

RRC-4237130439 30.90436 102.92809 3,081 -- -- 2,631

RRC-4237130512 30.96859 102.25849 2,377 2,377 2,307 2,157

RRC-4237130938 30.67648 103.05709 3,520 -- 3,250 3,120

RRC-4237134760 30.86302 102.93169 3,059 -- -- 2,694

RRC-4237134898 30.90304 102.32704 2,527 2,492 -- 2,297

RRC-4237134909 30.89240 102.31876 2,648 2,628 2,553 --

RRC-4237135042 30.88697 102.31262 2,571 -- 2,441 2,301

RRC-4237136582 30.99401 102.41091 2,560 2,508 2,390 2,240

RRC-4237136860 30.53832 102.91787 4,056 3,964 3,846 3,666

RRC-4237137144 30.81913 102.39868 3,233 3,113 3,103 2,898

RRC-4237137145D1 30.97233 102.99202 2,905 2,853 2,735 2,585

RRC-4237137149 30.79113 102.36360 3,244 2,979 2,844 2,684

RRC-4237137184 30.98321 102.95896 2,827 -- -- 2,377

RRC-4237137192 30.81190 102.42367 2,866 2,866 2,606 2,451

RRC-4237137193 31.21404 102.93741 2,541 -- 2,321 2,171

RRC-4237137219 31.08091 102.78678 2,564 2,392 -- 2,104

RRC-4237137222 30.95380 102.41601 2,936 2,736 2,736 2,332

RRC-4237137226H1 31.02169 102.94414 2,821 2,769 2,651 2,501

RRC-4237137249 30.83260 102.46539 2,740 2,740 2,710 2,560

RRC-4237137308 31.02083 102.92737 2,854 -- 2,524 2,424

RRC-4237137343 31.00069 102.91066 2,792 2,792 2,682 2,532

RRC-4237137344 31.00180 102.93251 2,806 2,736 2,691 2,506

RRC-4237137345 30.98513 102.91801 2,813 2,813 2,703 2,553

RRC-4237137346 30.98643 102.92052 2,815 2,815 2,705 2,555

RRC-4237137348 30.98543 102.94906 2,838 2,696 2,578 2,388

RRC-4237137404 30.44705 102.91642 4,391 4,306 -- 4,111

RRC-4237137420 31.18497 103.01739 2,641 2,371 2,361 --

RRC-4237137425 30.43616 102.80653 4,203 4,161 4,043 3,823

RRC-4237137432 30.94433 102.40882 2,631 2,631 2,631 2,331

RRC-4237137440 31.20530 103.07624 2,656 2,026 2,026 1,636

RRC-4237137476 31.00547 102.93700 2,806 -- 2,696 --

RRC-4237137512 30.47826 102.80501 4,088 3,774 3,683 3,605

RRC-4237137513 30.97139 102.46972 2,633 2,633 2,443 2,273

RRC-4237137521 31.22954 102.95019 2,547 2,547 2,447 --

RRC-4237137546 31.08070 102.78273 2,560 -- -- 2,090

RRC-4237137549 30.79703 102.48402 2,986 2,986 2,926 2,694

RRC-4237137552 30.80645 102.49553 2,983 2,921 -- 2,613

RRC-4237137563 30.77833 102.47648 3,350 3,350 3,100 2,940

RRC-4237137659 31.05036 102.78674 2,615 2,450 2,335 2,180

RRC-4237137690 31.18991 102.83923 2,475 2,440 -- 2,145

RRC-4237137769 30.97606 102.47663 2,574 2,542 2,424 2,274

RRC-4237137896 30.93913 102.97209 2,899 2,837 2,719 --

RRC-4237137898 30.92596 102.92080 2,957 2,860 2,742 2,592
64 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 2. Sites contributing well reports, borehole geophysical logs, and surface geophysical soundings used to determine the lithologies, 

hydrostratigraphic units, and tops and bases of the hydrostratigraphic units in the Pecos County region study area, Texas.—Continued

[--, not used]

Site 

identifier 

(fig. 7)

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Land surface 

altitude 

(feet)

Altitude of the top 

of the EdwardsTrinity aquifer 

(feet)

Altitude of 

the top of the 

Trinity Group 

(feet)

Altitude of the base 

of the EdwardsTrinity aquifer 

(feet)

RRC-4237137899 30.95065 102.95198 2,884 2,844 2,844 2,674

RRC-4237137943 31.19705 103.00962 2,623 2,316 2,198 1,993

RRC-4237137947 31.04948 102.67692 2,550 2,435 2,435 2,285

RRC-4237138229 30.97959 102.89657 2,861 -- -- 2,623

RRC-4237138416 30.87378 102.48960 2,678 2,496 2,378 2,228

RRC-4237138566 30.99109 102.39304 2,559 -- -- 2,154

RRC-4237138567 31.00559 102.39790 2,543 2,447 -- 2,163

RRC-4237138635 31.00456 102.39528 2,520 2,428 -- --

RRC-4237138636 30.99320 102.39055 2,540 -- -- 2,170

USGS-AMT01 30.57745 103.28333 3,649 3,492 -- 3,266

USGS-AMT02 30.51932 103.30687 3,794 3,499 3,422 3,289

USGS-AMT03 30.71023 103.52157 3,725 3,485 3,314 3,157

USGS-AMT04 30.88350 103.38389 3,278 2,986 -- 2,884

USGS-AMT05 30.80659 103.48194 3,646 3,364 -- 3,200

USGS-AMT06 30.60335 102.78842 3,536 3,280 -- 2,706

USGS-AMT09 31.06002 103.13731 2,841 2,707 2,677 2,582

USGS-AMT10 30.94134 102.55057 2,888 2,623 -- 2,409

USGS-AMT13 30.86516 103.82792 3,659 3,560 3,553 3,409

UTL-4237100408 30.85729 102.33328 2,702 2,602 2,562 2,352

UTL-4237100624 30.91815 102.37499 2,491 2,476 2,476 2,261

UTL-4237100629 30.88577 102.30522 2,562 -- 2,482 2,302

UTL-4237100632 30.91466 102.40170 2,564 2,564 2,564 2,430

UTL-4237100760 30.91809 102.21097 2,963 -- 2,707 2,558

UTL-4237101084 30.96110 102.21109 2,495 2,495 2,375 2,225

UTL-4237101169 30.89871 102.32453 2,593 -- 2,483 --

UTL-4237101303 30.90291 102.43366 2,682 2,682 2,572 2,472

UTL-4237101376 31.00633 102.70556 2,627 2,480 2,377 2,307

UTL-4237101391 30.91191 102.41052 2,613 2,613 2,613 2,455

UTL-4237102196 30.93593 102.42189 2,626 2,626 2,596 2,446

UTL-4237104143 30.89737 102.67229 3,002 2,980 2,862 2,782

UTL-4237104725 30.93361 102.65531 3,113 3,093 3,093 2,943

UTL-4237110713 30.92267 102.63851 3,088 3,088 3,088 2,935

UTL-4237110836 30.83082 102.57477 2,808 2,808 -- 2,568

UTL-4237111117 30.92991 102.63428 3,113 3,113 3,113 2,961

UTL-4237111280 30.91496 102.67222 2,939 2,929 -- 2,756

UTL-4237111415 30.94856 102.62639 2,787 2,787 2,787 2,617

UTL-4237130857 30.80820 102.59066 2,857 2,857 2,832 2,675

UTL-4237132468 30.83847 102.65610 2,937 2,937 2,804 2,497

UTL-4237132640 30.86691 102.64927 2,858 2,728 2,668 2,468

UTL-4237132819 30.94366 102.35298 2,504 2,504 2,454 2,274

UTL-4237133557 30.77395 102.66017 3,104 2,944 2,769 2,594

UTL-4237133677 30.85197 102.65775 2,880 2,808 2,650 2,480

UTL-4237134344 30.86660 102.69992 3,075 2,804 2,804 2,582

UTL-4237134536 30.88957 102.70809 2,899 2,789 2,724 2,619

UTL-4237135055 30.88176 102.70735 2,906 2,786 2,711 2,604

UTL-4237136157 30.99542 102.69718 2,646 2,516 2,471 2,356
Table 3 65

Table 3. Location of geophysical soundings and the nearby borehole geophysical log the sounding was compared to in the Pecos 

County region study area, Texas.

[TDEM, time-domain electromagnetic sounding; AMT, audio-magnetotelluric sounding; USGS, U.S. Geological Survey]

Sounding

identifier

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

USGS station number 

for borehole log 

comparison

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

TDEM1 30.85286 102.89278 310221102534201 30.85286 102.89278

TDEM2 30.84514 102.99889 305042102595601 30.84509 102.99899

TDEM3 30.78642 103.00917 304711103003301 30.78642 103.00932

TDEM4 30.64000 102.48028 303824102285001 30.64000 102.48052

AMT07 30.64030 102.47209 303824102285001 30.64000 102.48052

AMT08 30.44190 102.84390 302630102503801 30.44176 102.84396

AMT11 30.77301 102.52359 304622102312401 30.77304 102.52379

AMT12 31.13473 103.29427 310806103171901 31.13502 103.28796

Table 3
66 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 4. Compiled transmissivity values using data from aquifer tests conducted at wells completed in the Edwards-Trinity aquifer in or 

near the Pecos County region study area, Texas.

[Trans., transmissivity; [(gal/d)/ft], gallons per day per foot; [(gal/d)/ft2], gallons per day per square foot; --, not available; JMF, Jacob modified formula; TNEF, 

Theis nonequilibrium formula; TRF, Theis recovery formula; DF, Driscoll’s formula; USGS, U.S. Geological Survey; TWDB, Texas Water Development Board]

Site 

identifier 

(fig. 19)

TWDB 

well 

number

Source 

site 

name

Year 

of data 

collection

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Trans.

[(gal/d)/ft]

Aquifer 

thickness 

(feet)

Hydraulic 

conductivity 

[(gal/d)/ft2]

Log of 

Trans. 

value

Method Source

T1 -- M-3 2009 30.76829 103.02616 1,078,000 1,202 8,900 13.891 JMF Thornhill Group, Inc., 2009

T2 -- M-9 2009 30.77201 103.02618 726,000 1,201 6,050 13.685 JMF Thornhill Group, Inc., 2009

T3 -- CITY 

No. 5

2009 30.77221 103.03584 755,000 2,202 3,400 13.891 JMF Thornhill Group, Inc., 2009

T4 -- S-4 2009 30.82763 103.04356 363,000 3,402 1,060 14.737 JMF Thornhill Group, Inc., 2009

T5 -- S-3 2009 30.82737 103.04354 389,000 3,301 1,200 14.166 JMF Thornhill Group, Inc., 2009

T6 -- S-10 2009 30.83067 103.02697 1,017,000 1,902 5,300 13.832 JMF Thornhill Group, Inc., 2009

T7 -- S-32 2009 30.85885 103.03509 637,000 3,602 1,800 12.934 JMF Thornhill Group, Inc., 2009

T8 -- S-26 2009 30.85883 103.03722 842,000 3,602 2,300 13.696 JMF Thornhill Group, Inc., 2009

T9 -- C-B2 2009 30.89105 103.03141 1,070,000 2,802 3,821 13.883 JMF Thornhill Group, Inc., 2009

T10 -- C-3 2009 30.89123 103.03452 1,216,000 2,702 4,500 14.011 JMF Thornhill Group, Inc., 2009

T11 -- S-33 2007 30.91737 102.61213 96,000 -- -- 11.472 JMF Thornhill Group, Inc., 2008

T13 -- -- 1957 30.88750 102.89194 18,100 2,803 65 9.804 TNEF Meyers, 1969, p. 408

T14 -- -- 1957 30.89250 102.90222 7,000 1,603 44 8.854 TRF Meyers, 1969, p. 408

T15 -- -- 1957 30.96944 102.88167 4,580 2,103 22 8.429 TNEF Meyers, 1969, p. 411

T16 -- -- 1957 30.95611 102.88556 5,640 2,203 26 8.638 JMF Meyers, 1969, p. 411

T17 -- -- 1957 30.96778 102.88472 2,800 1,203 23 7.937 TRF Meyers, 1969, p. 412

T18 -- -- 1957 30.96667 102.89444 7,150 1,803 40 8.875 TNEF Meyers, 1969, p. 412

T19 -- -- 1959 31.02028 103.55000 8,260 203 413 9.019 JMF Meyers, 1969, p. 427

T20 -- Fritz 2011 30.78740 103.44340 13,713 4,103 33 9.526 DF USGS

T26 4561604 -- 1969 31.05110 102.37780 7,800 1,603 49 8.962 DF TWDB, 2011

T27 4561605 -- 1969 31.05360 102.37690 4,350 1,503 29 8.378 DF TWDB, 2011

T28 4561608 -- 1969 31.05060 102.38360 4,350 1,703 26 8.378 DF TWDB, 2011

T29 4561610 -- 1969 31.04390 102.39690 3,300 1,603 21 8.102 DF TWDB, 2011

T30 4561901 -- 1968 31.03360 102.39000 1,800 1,803 10 7.496 DF TWDB, 2011

T31 4561902 -- 1968 31.03000 102.38940 2,850 1,703 17 7.955 DF TWDB, 2011

T32 4561903 -- 1968 31.02810 102.38670 4,350 1,703 26 8.378 DF TWDB, 2011

T33 4561904 -- 1968 31.02690 102.38830 4,650 1,703 27 8.445 DF TWDB, 2011

T34 4561905 -- 1968 31.02440 102.38940 2,700 1,703 16 7.901 DF TWDB, 2011

T35 5216302 -- 2007 30.85190 103.02560 73,500 2,603 283 11.205 DF TWDB, 2011

T36 5216308 -- 2007 30.84500 103.03440 192,000 2,403 800 12.165 DF TWDB, 2011

T37 5216309 -- 2007 30.86390 103.03250 609,000 5,303 1,149 13.320 DF TWDB, 2011

T38 5216603 -- 2007 30.83060 103.02610 265,500 2,203 1,207 12.489 DF TWDB, 2011

T39 5216618 -- 2007 30.83030 103.03030 436,500 2,503 1,746 12.987 DF TWDB, 2011

T40 5216619 -- 2007 30.79310 103.00610 87,000 1,203 725 11.374 DF TWDB, 2011

T41 5305301 -- 2001 30.99670 102.37560 6,600 1,703 39 8.795 DF TWDB, 2011

T42 5305903 -- 1973 30.89500 102.37690 9,375 1,003 94 9.146 DF TWDB, 2011

T45 5313203 -- 2000 30.83860 102.44970 1,920 503 38 7.560 DF TWDB, 2011

T46 5313209 -- 1999 30.84470 102.45560 1,500 603 25 7.313 DF TWDB, 2011

1

Unknown well completion depth; assumed 400 ft below land surface for site T2 and assumed 500 ft below land surface for site T5.

2

Thickness from Thornhill Group, Inc., 2009 report.

3

Thickness approximated from water level and interpreted base of aquifer (rounded to nearest 10 ft).

Table 4
Table 5 67

Table 5

Table 5. Data-collection sites providing data for the geochemical analysis in the Pecos County region study area, Texas.

[USGS, U.S. Geological Survey; --, not applicable; ET, Edwards-Trinity; PC QW, Pecos County water quality]

Site 

identifier 

(fig. 20)

USGS 

station name

State 

well number

USGS 

station number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Site 

type

Contributing 

aquifer

Q1 San Solomon Spring -- 08427500 30.94292 103.78824 Spring --

Q2 Santa Rosa Spring -- 08437000 31.26743 102.95828 Spring --

Q4 Comanche Springs -- 08444500 30.88628 102.87495 Spring --

Q6 Diamond Y Spring -- 08446600 31.00190 102.92358 Spring --

Q27 PS-52-02-404 PS-52-02-404 305502103504101 30.91737 103.84518 Well Pecos Valley

Q36 WD-46-62-201 WD-46-62-201 310625103175201 31.10685 103.29777 Well Pecos Valley

Q42 US-45-43-807 US-45-43-807 311602102400601 31.26942 102.67609 Well Pecos Valley

Q43 US-45-43-8xx (PA 1) US-45-43-8xx 311602102400901 31.26934 102.68214 Well Pecos Valley

Q15 PS-52-11-702 PS-52-11-702 304605103444601 30.77100 103.74800 Well Igneous

Q7 PS-52-34-303 -- 302955103451101 30.49860 103.75300 Well Igneous

Q11 US-52-22-8xx (Farm Well 3) US-52-22-8xx 303941103175001 30.66139 103.29720 Well Edwards-Trinity

Q12 PS-52-20-601 PS-52-20-601 304006103315601 30.66827 103.53216 Well Edwards-Trinity

Q14 US-53-17-501 US-53-17-501 304117102560101 30.68806 102.93361 Well Edwards-Trinity

Q16 US-52-16-910 US-52-16-910 304646103013401 30.77931 103.02615 Well Edwards-Trinity

Q17 US-52-13-801 US-52-13-801 304715103263501 30.78740 103.44343 Well Edwards-Trinity

Q18 US-52-16-611 US-52-16-611 304802103003901 30.80088 103.01110 Well Edwards-Trinity

Q22 US-52-16-3xx (S-21) US-52-16-3xx 305132103015701 30.85899 103.03244 Well Edwards-Trinity

Q23 US-53-09-306 US-53-09-306 305140102521101 30.87393 102.88229 Well Edwards-Trinity

Q24 US-52-08-909 US-52-08-909 305331103020501 30.89210 103.03516 Well Edwards-Trinity

Q25 US-53-03-9xx US-53-03-9xx 305354102373501 30.89825 102.62647 Well Edwards-Trinity

Q26 US-53-01-907 US-53-01-907 305419102545301 30.90560 102.91610 Well Edwards-Trinity

Q28 PS-52-02-4xx (Balmorhea) PS-52-02-4xx 305509103510101 30.91911 103.85027 Well Edwards-Trinity

Q30 WD-52-02-507 WD-52-02-507 305531103474201 30.92539 103.79511 Well Edwards-Trinity

Q32 US-53-07-105 US-53-07-105 305836102131701 30.97667 102.22139 Well Edwards-Trinity

Q33 US-53-01-210 US-53-01-210 305859102571001 30.98293 102.95271 Well Edwards-Trinity

Q35 US-45-60-903 US-45-60-903 310136102311601 31.02670 102.52102 Well Edwards-Trinity

Q37 US-45-58-2xx US-45-58-2xx 310718102484801 31.12162 102.81354 Well Edwards-Trinity

Q40 US-46-56-309 US-46-56-309 311235103000901 31.20974 103.00262 Well Edwards-Trinity

Q8 BK-52-29-8xx (Brewster County 

ET Well)

BK-52-29-8xx 303222103263701 30.53950 103.44346 Well Edwards-Trinity

Q9 US-52-07-502 US-52-07-502 303342103064001 30.93779 103.18711 Well Edwards-Trinity

Q21 US-53-13-208 US-53-13-208 305112102265901 30.85341 102.44965 Well Dockum

Q31 US-52-06-603 US-52-06-603 305559103154101 30.93305 103.26194 Well Dockum

Q34 US-53-01-208 US-53-01-208 305949102552301 30.99718 102.92291 Well Dockum

Q39 US-46-55-9xx (Weatherby Ranch) US-46-55-9xx 310949103090401 31.16341 103.15103 Well Dockum

Q44 US-46-48-701 US-46-48-701 311610103050901 31.26959 103.08683 Well Dockum

Q10 US-53-19-7xx (PC QW) US-53-19-7xx 303852102432902 30.64799 102.72470 Well Rustler

Q13 US-52-24-501 US-52-24-501 304020103025202 30.67295 103.05601 Well Rustler

Q19 US-52-16-609 US-52-16-609 304805103013301 30.80129 103.02618 Well Rustler

Q29 US-53-01-5xx (Apache 3) US-53-01-5xx 305529102560601 30.92470 102.93490 Well Rustler

Q38 WD-46-54-901 WD-46-54-901 310806103171901 31.13502 103.28796 Well Rustler

Q20 US-52-16-504 US-52-16-504 304807103025301 30.80241 103.04844 Well Capitan Reef

Q41 US-45-49-203 US-45-49-203 311422102555101 31.23974 102.93097 Well Capitan Reef
68 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 6

Table 6. Contributing-aquifer revisions for groundwater wells providing data for the geochemical analysis based on the comparison of 

well production depths with the hydrostratigraphy of hydrogeologic framework for the Pecos County region study area, Texas.

[USGS, U.S. Geological Survey]

Site identifier 

(fig. 20)

USGS 

station number

Previously identified contributing aquifer 

(Pearson and others, 2012)

Updated 

contributing aquifer

Q12 304006103315601 Igneous Edwards-Trinity

Q30 305531103474201 Cretaceous Undivided Edwards-Trinity

Q40 311235103000901 Pecos Valley Edwards-Trinity

Q21 305112102265901 Edwards-Trinity Dockum

Q31 305559103154101 Edwards-Trinity Dockum

Q34 305949102552301 Edwards-Trinity Dockum

Q39 310949103090401 Edwards-Trinity Dockum

Q10 303852102432902 Edwards-Trinity Rustler

Q13 304020103025202 Edwards-Trinity Rustler
Table 7 69

Table 7

Table 7. Summary of selected physical properties and constituents measured in groundwater and spring water samples and 

hydrochemical facies, Pecos County region study area, Texas.

[USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; δ18O, delta 

oxygen-18; δD, delta deuterium; 87Sr/86Sr, strontium-87/strontium-86; CIAT, Deethylatrazine; Na, sodium; Ca, calcium; Mg, magnesium; Cl, chloride; SO4, 

sulfate; HCO3, bicarbonate; --, not available; E, estimated; <, nondetection less than laboratory reporting level]

Site 

identifier 

(fig. 20)

USGS 

station number

Site 

type

Contributing 

aquifer

Sample 

date

Sample 

start 

time

Specific 

conductance, 

water, 

unfiltered 

(μS/cm)

Calcium, 

water, 

filtered 

(mg/L)

Magnesium, 

water, 

filtered 

(mg/L)

Sodium, 

water, 

filtered 

(mg/L)

Potassium, 

water, 

filtered 

(mg/L)

Bicarbonate, 

water, 

filtered, 

inflectionpoint titration 

method 

(incremental 

titration 

method), 

field

(mg/L)

Q1 08427500 Spring -- 8/18/2010 11:00 2,430 143 54.0 291 14.3 239

Q2 08437000 Spring -- 1/25/2011 14:00 6,100 462 180 696 21.6 283

Q4 08444500 Spring -- 1/25/2011 10:00 3,570 276 119 362 12.0 259

Q6 08446600 Spring -- 8/8/2010 13:00 6,730 504 249 837 26.2 395

Q27 305502103504101 Well Pecos Valley 8/15/2010 19:00 362 58.6 4.79 11.6 2.96 204

Q36 310625103175201 Well Pecos Valley 9/2/2010 15:00 783 76.0 17.0 68.3 4.79 232

Q42 311602102400601 Well Pecos Valley 8/17/2010 18:00 1,250 695 352 2,010 19.1 315

Q43 311602102400901 Well Pecos Valley 8/17/2010 21:00 1,690 856 384 2,700 36.3 190

Q15 304605103444601 Well Igneous 6/22/2011 14:00 331 45.4 5.93 16.7 3.60 182

Q7 302955103451101 Well Igneous 9/2/2010 11:00 408 54.0 6.92 22.2 3.15 225

Q11 303941103175001 Well Edwards-Trinity 8/25/2010 10:00 1,390 100 27.7 145 8.26 251

Q12 304006103315601 Well Edwards-Trinity 6/23/2011 11:00 1,310 27.0 4.38 265 7.43 409

Q14 304117102560101 Well Edwards-Trinity 8/25/2010 16:00 2,520 265 44.4 233 11.6 277

Q16 304646103013401 Well Edwards-Trinity 8/16/2010 14:00 2,040 138 48.4 227 11.5 280

Q17 304715103263501 Well Edwards-Trinity 8/28/2010 14:00 680 84.8 14.1 40.9 4.75 273

Q18 304802103003901 Well Edwards-Trinity 8/20/2010 10:00 2,640 206 60.8 260 11.0 263

Q22 305132103015701 Well Edwards-Trinity 8/16/2010 11:00 3,070 197 75.9 335 13.3 287

Q23 305140102521101 Well Edwards-Trinity 8/10/2010 17:00 3,150 257 91.1 309 18.3 288

Q24 305331103020501 Well Edwards-Trinity 8/17/2010 10:00 3,820 278 109 421 15.2 286

Q25 305354102373501 Well Edwards-Trinity 6/21/2011 10:00 1,520 114 46.6 138 11.5 269

Q26 305419102545301 Well Edwards-Trinity 8/6/2010 13:00 3,240 259 108 298 13.4 280

Q28 305509103510101 Well Edwards-Trinity 9/1/2010 16:00 273 50.5 4.43 10.5 2.80 177

Q30 305531103474201 Well Edwards-Trinity 6/22/2011 11:00 2,710 157 60.3 331 15.1 260

Q32 305836102131701 Well Edwards-Trinity 8/13/2010 11:00 1,610 126 51.7 133 5.49 290

Q33 305859102571001 Well Edwards-Trinity 8/5/2010 13:00 1,570 120 40.4 137 11.4 243

Q35 310136102311601 Well Edwards-Trinity 8/26/2010 15:00 4,730 515 113 449 11.2 237

Q37 310718102484801 Well Edwards-Trinity 8/27/2010 15:00 7,260 462 225 876 35.9 275

Q40 311235103000901 Well Edwards-Trinity 8/11/2010 9:00 1,530 138 38.8 126 9.514 204

Q8 303222103263701 Well Edwards-Trinity 9/2/2010 9:00 587 21.0 9.7 95.9 5.47 270

Q9 303342103064001 Well Edwards-Trinity 8/27/2010 11:00 1,150 108 41.0 80.5 5.10 286

Q21 305112102265901 Well Dockum 8/13/2010 15:00 3,200 255 98.6 328 11.6 270

Q31 305559103154101 Well Dockum 8/6/2010 17:00 736 81.8 16.3 43.6 4.69 241

Q34 305949102552301 Well Dockum 8/4/2010 12:00 2,080 146 49.1 202 13.0 256

Q39 310949103090401 Well Dockum 8/31/2010 13:00 809 87.3 18.7 54.5 4.68 238

Q44 311610103050901 Well Dockum 8/12/2010 12:00 1,560 149 33.5 126 7.38 201

Q10 303852102432902 Well Rustler 8/10/2010 13:00 553 61.1 19.6 22.6 2.18 222

Q13 304020103025202 Well Rustler 9/1/2010 10:00 2,050 135 45.0 226 11.1 281

Q19 304805103013301 Well Rustler 8/19/2010 17:00 2,510 251 53.1 220 10.4 262

Q29 305529102560601 Well Rustler 8/31/2010 17:00 3,980 589 213 177 17.3 244

Q38 310806103171901 Well Rustler 8/29/2010 20:00 1,080 67.6 30.5 100 13.1 32.2

Q20 304807103025301 Well Capitan Reef 8/19/2010 19:00 2,290 145 49.9 249 12.2 267

Q41 311422102555101 Well Capitan Reef 8/11/2010 14:00 4,160 675 201 234 12.1 182
70 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 7. Summary of selected physical properties and constituents measured in groundwater and spring water samples and 

hydrochemical facies, Pecos County region study area, Texas.—Continued

[USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; δ18O, delta 

oxygen-18; δD, delta deuterium; 87Sr/86Sr, strontium-87/strontium-86; CIAT, Deethylatrazine; Na, sodium; Ca, calcium; Mg, magnesium; Cl, chloride; SO4, 

sulfate; HCO3, bicarbonate; --, not available; E, estimated; <, nondetection less than laboratory reporting level]

Site 

identifier 

(fig. 20)

Sulfate,

water, 

filtered

(mg/L)

Chloride, 

water, 

filtered 

(mg/L)

Silica,

water,

filtered 

(mg/L as 

silicon 

dioxide)

Nitrate 

plus 

Nitrite, 

water, 

filtered 

(mg/L)

Dissolved 

oxygen

(mg/L)

Hydrochemical 

facies

δ18O 

(per mil) 

(fig. 23)

δD 

(per 

mil)

87Sr/86Sr 

(fig. 25)

Strontium, 

water, 

filtered 

(μg/L)

Atrazine 

(μg/L)

CIAT 

(μg/L)

Q1 437 431 24.6 0.788 3.47 Na-Ca-Cl-SO4 -8.30 -58.1 0.70991 2,340 -- --

Q2 1,550 1,180 32.8 2.95 4.19 Na-Ca-Mg-Cl-SO4 -6.56 -47.0 0.70925 9,060 -- --

Q4 987 565 22.8 5.74 2.10 Na-Ca-Mg-SO4-Cl -7.45 -50.5 0.70898 6,230 0.0064 E0.0141

Q6 1,890 1,220 35.4 6.67 6.35 Na-Ca-Mg-SO4-Cl -6.83 -49.5 0.70970 8,970 <0.007 E0.0213

Q27 11.8 7.16 53.5 1.22 6.14 Ca-HCO3 -6.84 -46.3 0.70799 240 -- --

Q36 115 70.4 33.8 0.964 5.83 Ca-Na-HCO3-SO4-Cl -6.85 -46.2 0.70839 1,300 -- --

Q42 2,600 3,930 25.8 <0.04 1.02 Na-Ca-Cl-SO4 -4.86 -39.9 0.70813 12,300 -- --

Q43 3,020 4,770 22.6 2.10 2.73 Na-Ca-Cl-SO4 -2.21 -26.0 0.70813 15,000 -- --

Q15 16.0 8.81 43.4 0.184 7.96 Ca-Na-HCO3 -6.12 -44.6 0.70757 257 <0.008 <0.006

Q7 12.9 8.39 43.6 0.632 4.44 Ca-Na-HCO3 -6.23 -41.6 0.70727 198 -- --

Q11 222 195 28.4 0.207 0.216 Na-Ca-Cl-SO4-HCO3 -7.29 -50.1 0.70954 1,730 -- --

Q12 271 58.6 21.1 <0.02 0.280 Na-HCO3-SO4 -7.47 -50.2 0.70788 1,030 <0.008 <0.006

Q14 661 340 20.8 <0.04 0.306 Ca-Na-SO4-Cl -7.89 -55.0 0.70893 3,610 -- --

Q16 341 317 20.2 0.309 0.897 Na-Ca-Cl-SO4-HCO3 -7.82 -54.1 0.70975 2,140 -- --

Q17 57.7 50.8 26.7 0.832 1.28 Ca-Na-HCO3 -6.97 -45.6 0.70830 754 -- --

Q18 546 438 19.9 3.53 E1.171 Na-Ca-Cl-SO4 -7.70 -54.2 0.70901 4,170 <0.007 E0.0065

Q22 578 556 22.7 1.58 0.921 Na-Ca-Cl-SO4 -7.78 -53.9 0.70974 3,710 -- --

Q23 807 502 22.8 4.51 2.97 Na-Ca-Mg-SO4-Cl -7.18 -47.5 0.70940 7,320 -- --

Q24 908 758 23.3 1.61 1.17 Na-Ca-Mg-Cl-SO4 -7.47 -52.3 0.70963 5,330 <0.007 E0.0075

Q25 277 203 10.6 <0.02 0.770 Na-Ca-Mg-SO4-ClHCO3-7.47 -49.6 0.70959 2,370 <0.008 <0.006

Q26 883 470 19.7 4.04 1.43 Na-Ca-Mg-SO4-Cl -7.06 -48.8 0.70904 5,300 0.017 E0.013

Q28 12.2 5.16 52.0 1.18 4.48 Ca-HCO3 -6.72 -45.3 0.70807 215 -- --

Q30 498 473 20.8 0.135 0.450 Na-Ca-Cl-SO4 -8.34 -57.7 0.70979 2,720 <0.008 <0.006

Q32 217 240 18.3 1.81 6.32 Ca-Na-Mg-Cl-HCO3-

SO4

-6.75 -44.5 0.70853 2,150 -- --

Q33 238 247 10.4 <0.04 0.180 Ca-Na-Mg-Cl-SO4-

HCO3

-7.50 -50.6 0.70961 2,250 -- --

Q35 1,510 783 12.2 <0.04 0.447 Ca-Na-SO4-Cl -7.24 -50.7 0.70824 8,790 -- --

Q37 2,010 1,370 28.3 0.391 2.99 Na-Ca-Mg-SO4-Cl -6.85 -48.6 0.70969 10,300 -- --

Q40 376 180 13.0 <0.04 E0.230 Ca-Na-SO4-Cl-HCO3 -7.33 -50.0 0.70877 3,080 -- --

Q8 56.0 15.9 14.3 <0.04 0.149 Na-HCO3 -7.16 -49.0 0.70816 575 -- --

Q9 276 55.9 25.5 5.74 3.93 Ca-Na-Mg-SO4-HCO3 -6.42 -44.6 0.70823 257 -- --

Q21 586 675 16.2 5.63 4.45 Na-Ca-Mg-Cl-SO4 -7.14 -48.1 0.70906 4,040 -- --

Q31 91.8 58.2 14.4 <0.04 0.140 Ca-Na-HCO3-SO4-Cl -6.87 -46.3 0.70843 1,000 -- --

Q34 335 331 11.1 <0.04 0.140 Na-Ca-Cl-SO4-HCO3 -7.81 -53.0 0.70975 2,610 -- --

Q39 119 68.1 26.3 1.09 7.32 Ca-Na-HCO3-SO4-Cl -6.81 -47.2 0.70854 1,350 -- --

Q44 346 195 30.9 2.43 4.41 Ca-Na-SO4-Cl-HCO3 -7.21 -48.5 0.70882 2,800 <0.007 <0.014

Q10 52.5 23.5 14.0 2.19 4.03 Ca-Mg-HCO3 -6.66 -45.1 0.70829 772 -- --

Q13 357 323 19.6 <0.04 0.0740 Na-Ca-Cl-SO4-HCO3 -7.84 -53.3 0.70977 2,200 -- --

Q19 704 332 18.7 <0.04 E0.170 Ca-Na-SO4-Cl -7.89 -53.2 0.70867 3,190 -- --

Q29 2,270 179 14.0 <0.04 6.57 Ca-Mg-SO4 -7.61 -52.7 0.70758 10,000 -- --

Q38 354 110 0.479 <0.04 0.129 Na-Ca-Mg-SO4-Cl -6.83 -47.6 0.70763 3,390 -- --

Q20 421 370 21.0 <0.04 E 0.252 Na-Ca-Cl-SO4 -7.92 -54.9 0.70969 2,650 -- --

Q41 2,320 354 13.9 <0.04 1.25 Ca-Mg-SO4 -7.74 -52.8 0.70751 10,200 -- --
Table 8 71

Table 8

Table 8. Tritium concentrations and helium-4 screen interpretations for groundwater samples collected in the Pecos County region 

study area, Texas.

[USGS, U.S. Geological Survey; pCi/L, picocuries per liter; TU, tritium units (3.22 pCi/L = 1 TU); --, not available; ssLc, sample specific critical level; 

R, radiochemical nondetect]

Site 

identifier 

(fig. 20)

USGS station 

number

Site 

type

Contributing 

aquifer

Sample 

date

Sample 

start 

time

Tritium, 

water, 

unfiltered

(pCi/L)

Tritium, 

water, 

unfiltered

(TU)

ssLc Analysis 

date

Preliminary 

assessment 

of viability of 

age information, based 

on helium-4 

measurement1

Q2 08437000 Spring -- 1/25/2011 14:00 1.8 0.55 0.31 4/19/2011 --

Q4 08444500 Spring -- 1/25/2011 10:00 6.5 2.0 0.56 4/19/2011 Dateable

Q6 08446600 Spring -- 8/8/2010 13:00 1.9 0.59 0.43 2/7/2011 --

Q27 305502103504101 Well Pecos Valley 8/15/2010 19:00 3.5 1.1 0.40 2/7/2011 Dateable

Q36 310625103175201 Well Pecos Valley 9/2/2010 15:00 R0.26 -- 0.37 1/31/2011 --

Q43 311602102400901 Well Pecos Valley 8/17/2010 21:00 4.2 1.3 0.41 1/31/2011 Dateable

Q15 304605103444601 Well Igneous 6/22/2011 14:00 9.9 3.1 0.54 12/1/2011 Dateable

Q7 302955103451101 Well Igneous 9/2/2010 11:00 3.8 1.2 0.39 1/31/2011 Dateable

Q11 303941103175001 Well EdwardsTrinity8/25/2010 10:00 0.34 0.11 0.36 1/31/2011 --

Q12 304006103315601 Well EdwardsTrinity6/23/2011 11:00 0.94 0.29 0.25 12/1/2011 --

Q14 304117102560101 Well EdwardsTrinity8/25/2010 16:00 R0.17 -- 0.35 1/31/2011 --

Q17 304715103263501 Well EdwardsTrinity8/28/2010 14:00 3.7 1.1 0.43 1/31/2011 Dateable

Q18 304802103003901 Well EdwardsTrinity8/20/2010 10:00 R0.0 -- 0.36 1/31/2011 --

Q23 305140102521101 Well EdwardsTrinity8/10/2010 17:00 4.3 1.3 0.40 2/7/2011 Dateable

Q24 305331103020501 Well EdwardsTrinity8/17/2010 10:00 1.3 0.40 0.36 2/7/2011 --

Q25 305354102373501 Well EdwardsTrinity6/21/2011 10:00 0.29 0.090 0.25 12/1/2011 --

Q26 305419102545301 Well EdwardsTrinity8/6/2010 13:00 2.7 0.85 0.42 2/7/2011 --

Q28 305509103510101 Well EdwardsTrinity9/1/2010 16:00 6.7 2.1 0.44 1/31/2011 Dateable

Q30 305531103474201 Well EdwardsTrinity6/22/2011 11:00 0.80 0.25 0.25 12/1/2011 --

Q9 303342103064001 Well EdwardsTrinity8/27/2010 11:00 0.13 0.040 0.35 1/31/2011 --

Q31 305559103154101 Well Dockum 8/6/2010 17:00 R0.0 -- 0.37 2/7/2011 --

Q44 311610103050901 Well Dockum 8/12/2010 12:00 0.44 0.14 0.41 2/7/2011 --

Q10 303852102432902 Well Rustler 8/10/2010 13:00 0.070 0.022 0.39 2/7/2011 --

1

Dateable refers to the potential for obtaining valid groundwater age information based on preliminary results.
72 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 9

Table 9. Average winter (November through April) groundwater-level data used for the 1980–2010 compiled potentiometric-surface 

map of the Edwards-Trinity aquifer in the Pecos County region study area, Texas.—Continued

[TWDB, Texas Water Development Board]

Site 

identifier 

(fig. 28)

TWDB 

well 

number

Source 

station number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Groundwater-level altitude data from 1980 to 2010

Number of 

water-level 

measurements for 

the site

Minimum 

water 

level 

(feet)

Maximum 

water 

level 

(feet)

Average water 

level (feet) (value 

used to generate 

water-level 

contour map fig. 28)

Data 

collection 

period

L1 5202704 5202704 30.90750 103.83694 1 3,454.23 3,454.23 3,454.23 1990

L2 5202511 5202511 30.94056 103.80361 2 3,323.25 3,326.25 3,324.75 1997–2001

L3 5202608 5202608 30.93861 103.75694 1 3,275.93 3,275.93 3,275.93 1997

L4 4659508 4659508 31.04583 103.70055 21 2,701.74 2,728.82 2,715.83 1988–2009

L5 4660101 4660101 31.11611 103.59639 19 2,594.25 2,651.02 2,629.25 1988–2006

L6 4652404 4652404 31.18833 103.58555 27 2,585.41 2,609.59 2,594.46 1980–2009

L7 5204210 5204210 30.97139 103.56389 2 2,721.15 2,732.85 2,727.00 1988–89

L8 4660807 4660807 31.02111 103.56250 16 2,625.34 2,661.67 2,650.11 1988–2004

L9 4660808 4660808 31.03111 103.56000 1 2,622.24 2,622.24 2,622.24 1988

L10 4660809 4660809 31.03083 103.55889 1 2,718.52 2,718.52 2,718.52 1988

L11 4660810 4660810 31.03028 103.55611 1 2,625.51 2,625.51 2,625.51 1988

L12 5237202 5237202 30.48555 103.44389 2 3,581.81 3,583.15 3,582.48 2002

L13 5213801 5213801 30.78740 103.44343 3 3,072.20 3,085.76 3,076.87 2006–10

L14 5205601 5205601 30.95611 103.41278 1 2,890.04 2,890.04 2,890.04 1987

L15 5213303 5213303 30.84778 103.40639 1 3,153.07 3,153.07 3,153.07 1987

L16 5221301 5221301 30.72805 103.40583 7 3,163.77 3,182.09 3,173.12 2002–8

L17 5230104 5230104 30.61055 103.36611 1 3,315.82 3,315.82 3,315.82 1987

L18 5230105 5230105 30.61222 103.36583 6 3,318.53 3,323.90 3,319.55 1984–92

L19 5230107 303717103214801 30.62144 103.36383 2 3,343.44 3,381.83 3,362.63 2006–10

L20 5230108 303718103214601 30.62182 103.36324 2 3,342.75 3,344.78 3,343.77 2006–10

L21 5206501 5206501 30.94167 103.32361 29 2,870.84 2,886.68 2,879.09 1980–2009

L22 4662801 310238103191701 31.04401 103.32139 151 2,753.34 2,755.31 2,754.33 2008–10

L23 5206504 5206504 30.93861 103.30750 1 2,843.31 2,843.31 2,843.31 1999

L24 5214801 5214801 30.75555 103.29500 1 3,201.43 3,201.43 3,201.43 1987

L25 5206303 5206303 30.97444 103.27250 1 2,748.55 2,748.55 2,748.55 1987

L26 5207401 5207401 30.91944 103.22055 1 2,809.52 2,809.52 2,809.52 1987

L27 5215501 5215501 30.81167 103.19722 1 2,956.76 2,956.76 2,956.76 1987

L28 5207502 303342103064001 30.93779 103.18711 1 2,869.84 2,869.84 2,869.84 2006

L29 5215201 5215201 30.85861 103.18500 1 2,979.84 2,979.84 2,979.84 1987

L30 5207801 5207801 30.91417 103.17833 1 2,913.58 2,913.58 2,913.58 1987

L31 4663902 4663902 31.04056 103.15389 15 2,692.49 2,703.99 2,699.75 1984–2001

L32 5215301 5215301 30.85250 103.15333 1 2,984.91 2,984.91 2,984.91 1987

L33 5207302 5207302 30.97780 103.14330 95 2,659.73 2,808.75 2,770.35 1984–2008

L34 4663302 310652103080601 31.11444 103.13500 1 2,499.90 2,499.90 2,499.90 1983

L35 5208402 5208402 30.94075 103.12200 1 2,813.57 2,813.57 2,813.57 2010

L36 5224102 5224102 30.72722 103.10972 1 3,064.50 3,064.50 3,064.50 1987

L37 5208801 5208801 30.87861 103.07083 27 2,885.60 2,973.93 2,945.57 1982–2009

L38 5216801 5216801 30.75556 103.07000 1 3,081.22 3,081.22 3,081.22 1988

L39 4664801 310039103035701 31.01095 103.06624 1 2,713.43 2,713.43 2,713.43 1984

L40 5208909 5208909 30.89210 103.03516 1 2,946.59 2,946.59 2,946.59 1988

Table 9. Average winter (November through April) groundwater-level data used for the 1980–2010 compiled potentiometric-surface 

map of the Edwards-Trinity aquifer in the Pecos County region study area, Texas.

[TWDB, Texas Water Development Board]
Table 9 73

Table 9. Average winter (November through April) groundwater-level data used for the 1980–2010 compiled potentiometric-surface 

map of the Edwards-Trinity aquifer in the Pecos County region study area, Texas.—Continued

[TWDB, Texas Water Development Board]

Site 

identifier 

(fig. 28)

TWDB 

well 

number

Source 

station number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Groundwater-level altitude data from 1980 to 2010

Number of 

water-level 

measurements for 

the site

Minimum 

water 

level 

(feet)

Maximum 

water 

level 

(feet)

Average water 

level (feet) (value 

used to generate 

water-level 

contour map fig. 28)

Data 

collection 

period

L41 5216907 5216907 30.78694 103.03083 1 3,003.25 3,003.25 3,003.25 1987

L42 53027 53027 30.77210 103.03080 1 2,993.26 2,993.26 2,993.26 2009

L43 5208905 5208905 30.88694 103.02972 1 2,942.07 2,942.07 2,942.07 1988

L44 5208906 5208906 30.89167 103.02889 1 2,945.58 2,945.58 2,945.58 1988

L45 5216605 5216605 30.80861 103.02694 2 2,970.33 2,977.41 2,973.87 1988–2001

L46 5216304 5216304 30.84528 103.01861 1 2,978.68 2,978.68 2,978.68 1988

L47 5216303 5216303 30.84556 103.01194 7 2,852.62 3,050.52 2,956.78 1980–89

L48 5216611 304802103003901 30.80088 103.01110 4 2,955.01 3,008.01 2,980.00 1988–2001

L49 4656309 311235103000901 31.20974 103.00262 182 2,405.23 2,415.07 2,410.37 2007–10

L50 5309101 5309101 30.85667 102.99972 1 2,924.91 2,924.91 2,924.91 1988

L51 5309105 5309105 30.84528 102.99917 22 2,944.41 2,983.42 2,967.17 1980–2008

L52 5309102 5309102 30.86833 102.98944 1 2,943.88 2,943.88 2,943.88 1988

L53 5301102 5301102 30.99222 102.96583 1 2,706.66 2,706.66 2,706.66 1993

L54 5301101 5301101 30.99556 102.96528 1 2,701.78 2,701.78 2,701.78 1993

L55 5325101 5325101 30.60417 102.96167 1 3,079.44 3,079.44 3,079.44 1987

L56 4557805 4557805 31.02083 102.95611 1 2,698.51 2,698.51 2,698.51 1993

L57 5301503 5301503 30.95426 102.95382 1 2,832.38 2,832.38 2,832.38 2010

L58 5301206 5301206 30.98305 102.94611 1 2,740.54 2,740.54 2,740.54 1993

L59 5301204 5301204 30.99778 102.94583 1 2,717.87 2,717.87 2,717.87 1993

L60 5301205 5301205 30.98222 102.94417 1 2,745.46 2,745.46 2,745.46 1993

L61 5301207 5301207 30.97444 102.93805 1 2,823.21 2,823.21 2,823.21 1993

L62 530105 530105 30.90111 102.93500 2 2,994.25 3,003.25 2,998.75 2006–8

L63 4557201 4557201 31.08417 102.92750 1 2,616.78 2,616.78 2,616.78 1987

L64 4549201 4549201 31.23139 102.92500 1 2,464.48 2,464.48 2,464.48 1984

L65 4557807 310002102551100 31.00071 102.92016 1 2,743.36 2,743.36 2,743.36 1987

L66 5301202 5301202 30.98389 102.92000 3 2,767.04 2,776.37 2,773.00 1991–93

L67 4557802 4557802 31.00778 102.91833 1 2,774.30 2,774.30 2,774.30 1993

L68 5301907 305419102545301 30.90560 102.91610 1 2,884.39 2,884.39 2,884.39 1981

L69 5301305 5301305 30.99056 102.91333 1 2,741.86 2,741.86 2,741.86 1993

L70 530103 530103 30.85472 102.90970 2 2,918.51 2,936.43 2,927.47 2006

L71 5301607 5301607 30.95583 102.90861 1 2,799.53 2,799.53 2,799.53 1993

L72 4557603 4557603 31.05667 102.90722 30 2,667.82 2,681.03 2,679.82 2007–9

L73 5309301 305110102533401 30.85286 102.89278 31 2,888.48 2,936.40 2,913.94 1980–2010

L74 5301304 5301304 30.99194 102.89250 1 2,751.44 2,751.44 2,751.44 1993

L75 4549301 4549301 31.20861 102.89139 24 2,472.26 2,481.34 2,477.28 1980–2005

L76 4549902 4549902 31.12555 102.88805 1 2,546.64 2,546.64 2,546.64 2004

L77 5301902 5301902 30.88972 102.88417 27 2,847.64 2,929.56 2,913.08 1980–2009

L78 5301908 5301908 30.88952 102.88397 1 2,903.70 2,903.70 2,903.70 2010

L79 530102 530102 30.87389 102.88250 2 2,901.83 2,928.13 2,914.98 2006–8

L80 5309306 5309306 30.87389 102.88222 1 2,891.41 2,891.41 2,891.41 2007
74 Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers

Table 9. Average winter (November through April) groundwater-level data used for the 1980–2010 compiled potentiometric-surface 

map of the Edwards-Trinity aquifer in the Pecos County region study area, Texas.—Continued

[TWDB, Texas Water Development Board]

Site 

identifier 

(fig. 28)

TWDB 

well 

number

Source 

station number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Groundwater-level altitude data from 1980 to 2010

Number of 

water-level 

measurements for 

the site

Minimum 

water 

level 

(feet)

Maximum 

water 

level 

(feet)

Average water 

level (feet) (value 

used to generate 

water-level 

contour map fig. 28)

Data 

collection 

period

L81 4557904 4557904 31.02000 102.87944 1 2,712.33 2,712.33 2,712.33 1993

L82 5301608 5301608 30.92694 102.87611 3 2,837.03 2,844.20 2,841.06 2006–8

L83 530201 530201 30.95111 102.86750 3 2,790.19 2,814.42 2,806.06 2006–8

L84 5302705 5302705 30.89389 102.86722 21 2,848.92 2,899.06 2,876.55 1983–2007

L85 5302102 5302102 30.97528 102.85944 11 2,744.10 2,774.99 2,766.63 1982–97

L86 5302710 305404102512701 30.90120 102.85770 56 2,864.80 2,895.68 2,876.53 2007–9

L87 5302403 305603102505901 30.93435 102.85013 3 2,789.68 2,803.68 2,796.83 1983–87

L88 5310103 5310103 30.84278 102.84639 1 2,890.53 2,890.53 2,890.53 1987

L89 5302708 305234102504301 30.87618 102.84521 29 2,832.81 2,890.13 2,869.55 1980–2010

L90 4558802 4558802 31.01889 102.82611 1 2,663.29 2,663.29 2,663.29 1987

L91 5302804 5302804 30.90500 102.81917 2 2,833.83 2,844.21 2,839.02 1999–2004

L92 5302802 5302802 30.90111 102.81583 9 2,830.43 2,846.33 2,840.14 1984–95

L93 5302803 5302803 30.89333 102.81555 1 2,823.93 2,823.93 2,823.93 1987

L94 520201 520201 30.89000 102.78860 1 2,814.12 2,814.12 2,814.12 2006

L95 521002 521002 30.78360 102.70500 3 2,825.52 2,826.35 2,825.91 2004–7

L96 5311501 5311501 30.83167 102.67444 1 2,762.29 2,762.29 2,762.29 1987

L97 521001 521001 30.77639 102.59420 2 2,753.47 2,753.47 2,753.47 2006–7

L98 5312205 5312205 30.86222 102.58000 1 2,742.53 2,742.53 2,742.53 1987

L99 5320201 5320201 30.73417 102.56111 1 2,749.16 2,749.16 2,749.16 1987

L100 5312203 5312203 30.85250 102.56083 3 2,726.72 2,735.76 2,729.99 1980–90

L101 5320901 5320901 30.66555 102.52389 1 2,642.44 2,642.44 2,642.44 1987

L102 4560303 4560303 31.10444 102.52083 1 2,317.40 2,317.40 2,317.40 1993

L103 4552901 4552901 31.13639 102.51778 1 2,331.01 2,331.01 2,331.01 1987

L104 45529 310810102310200 31.13626 102.51764 1 2,327.57 2,327.57 2,327.57 1987

L105 5320302 5320302 30.71861 102.50361 1 2,742.07 2,742.07 2,742.07 1987

L106 4561404 4561404 31.06611 102.48805 1 2,302.49 2,302.49 2,302.49 2002

L107 5313201 5313201 30.83611 102.45611 1 2,584.94 2,584.94 2,584.94 1987

L108 5313501 5313501 30.82805 102.45333 1 2,649.81 2,649.81 2,649.81 1987

L109 5306703 305415102222801 30.90460 102.37514 1 2,456.44 2,456.44 2,456.44 1987

L110 456101 456101 31.05667 102.37170 3 2,259.35 2,261.15 2,260.29 2006–07

L111 5306704 5306704 30.91389 102.37028 1 2,428.56 2,428.56 2,428.56 1987

L112 5306401 5306401 30.92444 102.35583 1 2,397.39 2,397.39 2,397.39 1987

L113 5306803 5306803 30.90458 102.31327 1 2,371.37 2,371.37 2,371.37 2010

L114 5314201 5314201 30.86139 102.30667 1 2,474.97 2,474.97 2,474.97 1987

L115 5306901 305357102172001 30.89923 102.28912 189 2,348.83 2,351.25 2,349.43 2007–10

L116 4562901 310041102152901 31.01156 102.25855 28 2,240.29 2,252.56 2,247.17 1980–2010

L117 5307106 5307106 30.97650 102.22977 1 2,229.27 2,229.27 2,229.27 2010

L118 5315201 5315201 30.86306 102.20667 1 2,678.63 2,678.63 2,678.63 1987

L119 530704 530704 30.97583 102.19440 3 2,201.58 2,222.86 2,215.27 2006–9
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Bumgarner and others—Hydrogeologic Framework, Geochemistry, and Groundwater-Flow System of the Edwards-Trinity and Related Aquifers—SIR 2012–5124

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I SBN 978- 1- 4113-3431- 1

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