In cooperation with the San Antonio Water System 

Geologic, Hydrologic, and Geochemical 

Identification of Flow Paths in the 

Edwards Aquifer, Northeastern Bexar 

and Southern Comal Counties, Texas 

Scientific Investigations Report 2007–5285 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover:  U.S. Geological Survey hydrologic technicians preparing to collect samples on Landa Lake, New

Braunfels, Texas.
Geologic, Hydrologic, and Geochemical 

Identification of Flow Paths in the 

Edwards Aquifer, Northeastern Bexar 

and Southern Comal Counties, Texas

By Cassi L. Otero

In cooperation with the San Antonio Water System

Scientific Investigations Report 2007–5285

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Otero, C.L., 2007, Geologic, hydrologic, and geochemical identification of flow paths in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas: U.S. Geological Survey Scientific Investigations Report 2007–

5285, 48 p. 
iii

Contents

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

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

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

Description of the Edwards Aquifer and Study Area .....................................................................3

Acknowledgments ...............................................................................................................................6

Methods of Investigation .............................................................................................................................6

Data Collection and Sample Analysis ..............................................................................................6

Quality Control and Quality Assurance of Geochemical Samples ............................................12

Methods of Data Analysis .........................................................................................................................12

Hydrologic Data ..................................................................................................................................12

Geochemical Data .............................................................................................................................12

Geologic and Hydrologic Identification of Flow Paths .........................................................................13

Southern Comal Flow Path ...............................................................................................................13

Central Comal Flow Path ..................................................................................................................17

Northern Comal Flow Path ...............................................................................................................17

Correlation of Water Levels .............................................................................................................18

Analysis of Hydrograph Recession Curves ...................................................................................18

Geochemical Identification of Flow Paths ..............................................................................................21

Major-Ion Chemistry ..........................................................................................................................21

Stable Isotopes ...................................................................................................................................21

Ground-Water Ages ..........................................................................................................................25

Summary .......................................................................................................................................................28

References ...................................................................................................................................................29

Appendix 1—Water-Level (2004–06) and Chemical Data (2003–06) ...................................................31

1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 

2004–06 ...................................................................................................................................33

1.2. Daily mean depth to water at well DX–68–23–502, Comal County, Texas, 2004–06 .....36

1.3. Chemical and isotope data in ground-water samples from wells and springs 

(by flow path) collected for this study, northeastern Bexar and southern Comal 

Counties, Texas, 2003–06 .....................................................................................................39

Figures

1–3. Maps showing: 

1. San Antonio segment of the Edwards aquifer, location of study area, and 

relation between Edwards and Trinity aquifers, south-central Texas .......................2

2. Study area and relation between recharge zone and confined zone of the 

Edwards aquifer and the Trinity aquifer, northeastern Bexar and southern 

Comal Counties, Texas .......................................................................................................4

3. Locations of major springs in the Comal Springs complex, southern Comal 

County, Texas .......................................................................................................................5
4. Idealized block diagram of the Edwards aquifer between Comal Springs fault and 

Hueco Springs fault, southern Comal County, Texas .............................................................7

5–7. Maps showing: 

5. Locations of wells and spring orifices from which water-quality samples 

were collected and locations of continuous water-level observation wells, 

northeastern Bexar and southern Comal Counties, Texas ..........................................8

6. Locations of hypothesized ground-water flow paths, northeastern Bexar 

and southern Comal Counties, Texas ............................................................................14

7. Potentiometric surface of Edwards aquifer in northeastern Bexar and 

southern Comal Counties, Texas, fall 2000 ...................................................................15

8. Hydrographs of water levels from six wells and discharge for Comal Springs and 

Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 

2004–September 2006 ...............................................................................................................16

9. Graph showing correlations between water levels for six wells and discharge for 

Comal Springs and Hueco Springs, northeastern Bexar and southern Comal 

Counties, Texas, March 2004–September 2006 ....................................................................19

10. Hydrographs of discharge at Comal Springs and Hueco Springs, southern Comal 

County, Texas, March 2004–September 2006, showing recession coefficients for 

recession curves .......................................................................................................................20

11. Trilinear diagrams showing composition of ground water in the Edwards aquifer 

in the southern, central, and northern Comal flow paths, northeastern Bexar and 

southern Comal Counties, Texas .............................................................................................22

12–14. Graphs showing: 

12. Relation of calcium concentration to magnesium/calcium ratio in samples 

collected from wells and springs in the Edwards aquifer and wells in the 

Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas...............23

13. Relation between delta oxygen-18 and delta deuterium for selected wells 

and springs, northeastern Bexar and southern Comal Counties, Texas, 

1996–2006, with insert showing same plot with axes extended to range of 

rainfall samples .................................................................................................................24

14. Apparent ages of ground water in 2003 samples from selected wells and 

springs in the Edwards aquifer, northeastern Bexar and southern Comal 

Counties, Texas, based on sulfur hexafluoride concentration ..................................26

15. Maps showing apparent age of water in selected wells in the Edwards 

aquifer, northeastern Bexar and southern Comal Counties, Texas, based on 

tritium/helium-3 ratio .................................................................................................................27

Table

1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and 

rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which 

data were collected and compiled for this report ..................................................................9

iv
v

Datums and Abbreviations

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

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

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

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

or micrograms per liter (μg/L).

Abstract

The U.S. Geological Survey, in cooperation with the 

San Antonio Water System, conducted a 4-year study during 2002–06 to identify major flow paths in the Edwards 

aquifer in northeastern Bexar and southern Comal Counties 

(study area). In the study area, faulting directs ground water 

into three hypothesized flow paths that move water, generally, from the southwest to the northeast. These flow paths 

are identified as the southern Comal flow path, the central 

Comal flow path, and the northern Comal flow path. Statistical correlations between water levels for six observation wells 

and between the water levels and discharges from Comal 

Springs and Hueco Springs yielded evidence for the hypothesized flow paths. Strong linear correlations were evident 

between the datasets from wells and springs within the same 

flow path and the datasets from wells in areas where flow 

between flow paths was suspected. Geochemical data (major 

ions, stable isotopes, sulfur hexafluoride, and tritium and 

helium) were used in graphical analyses to obtain evidence 

of the flow path from which wells or springs derive water. 

Major-ion geochemistry in samples from selected wells and 

springs showed relatively little variation. Samples from the 

southern Comal flow path were characterized by relatively 

high sulfate and chloride concentrations, possibly indicating 

that the water in the flow path was mixing with small amounts 

of saline water from the freshwater/saline-water transition 

zone. Samples from the central Comal flow path yielded the 

most varied major-ion geochemistry of the three hypothesized 

flow paths. Central Comal flow path samples were characterized, in general, by high calcium concentrations and low 

magnesium concentrations. Samples from the northern Comal 

flow path were characterized by relatively low sulfate and 

chloride concentrations and high magnesium concentrations. 

The high magnesium concentrations characteristic of northern 

Comal flow path samples from the recharge zone in Comal 

County might indicate that water from the Trinity aquifer is 

entering the Edwards aquifer in the subsurface. A graph of the 

relation between the stable isotopes deuterium and delta-18 

oxygen showed that, except for samples collected following an 

unusually intense rain storm, there was not much variation in 

stable isotope values among the flow paths. In the study area 

deuterium ranged from -36.00 to -20.89 per mil and delta-18 

oxygen ranged from -6.03 to -3.70 per mil. Excluding samples 

collected following the intense rain storm, the deuterium range 

in the study area was -33.00 to -20.89 per mil and the delta-18 

oxygen range was -4.60 to -3.70 per mil. Two ground-water 

age-dating techniques, sulfur hexafluoride concentrations and 

tritium/helium-3 isotope ratios, were used to compute apparent 

ages (time since recharge occurred) of water samples collected 

in the study area. In general, the apparent ages computed by 

the two methods do not seem to indicate direction of flow. 

Apparent ages computed for water samples in northeastern 

Bexar and southern Comal Counties do not vary greatly except 

for some very young water in the recharge zone in central 

Comal County.

Introduction

The Edwards aquifer is the main source of public water 

supply for the city of San Antonio, Texas, and the surrounding area and provides nearly all of the water for industrial, 

military, and irrigation use in the region (fig.1). Withdrawals 

from the aquifer to meet San Antonio’s increasing water-supply needs might be a threat to minimum mandated sustained 

flows at Comal Springs (Edwards Aquifer Authority, 2007), 

the largest spring in the Southwest. The springs supply water 

to downstream users, sustain federally-listed endangered species, and support local economies through tourism. Increased 

knowledge of the complex hydrologic processes that control 

water availability in the Edwards aquifer in the vicinity of 

Comal Springs is imperative for optimal resource management. The U.S. Geological Survey (USGS), in cooperation 

with the San Antonio Water System (SAWS), conducted a 

4-year study during 2002–06 in northeastern Bexar County 

and southern Comal County to identify flow paths in the 

Edwards aquifer. The study area (fig. 1) includes small parts 

Geologic, Hydrologic, and Geochemical 

Identification of Flow Paths in the 

Edwards Aquifer, Northeastern Bexar 

and Southern Comal Counties, Texas

By Cassi L. Otero
Bracken gap

! Alamo Heights horst 

!

GULF COASTAL PLAIN FRIO COUNTY ZAVALA COUNT

Y

ATASCOSA

COUNTY

WILSON COUNTY

UVALDE COUNT

Y

MEDINA COUNTY

BEXAR COUNTY

GUADALUPE

COUNTY

BANDER

A

COUNT

Y

COMAL COUNTY CALDWELL COUNTY

REAL

COUNT

Y KENDALL

COUNTY

KERR COUNT

Y HAYS

COUNTY

GILLESPIE COUNTY

BLANC

O

COUNTY

TRAVIS

COUNTY

KINNEY COUNT

Y

Boerne

San Antonio

Braunfels

Bulverde

New 100°30'

99°45'

99°0'

98°15' 97°30'

29°15'

30°0'

EXPLAN

ATION

Trinity aquiferEdwards aquifer recharge zone (outcrop)Edwards aquifer confined zoneLine of 1,000 milligrams per liter dissolved solids concentration (Schultz, 1994)Study area boundarySpring

0 30 45 60 MILES 15

Balcones 

fault 

zone

San Marcos

Springs

Hueco Springs

Comal Springs

San Antonio Springs San Pedro

Springs

0 140 210 280 MILES 70

Study area

TEXAS

SAN MARCOS

PLATFORM

DEVILS RIVER

TREND

MAVERICK

BASIN

Depositional provinces modifiedfrom Rose (1972, fig. 1)

Base modified from U.S. Geological Survey digital data Scale 1:250,000Universal Transverse Mercator projection, Datum NAD 83Zone 14

Figure 1. San Antonio segment of the Edwards aquifer, location of study area, and relation between Edwards and Trinity aquifers, south-central Texas. 

 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
of Guadalupe and Hays Counties in addition to northeastern 

Bexar County and southern Comal County.

Purpose and Scope

The purpose of this report is to describe major groundwater flow paths in the Edwards aquifer in northeastern 

Bexar and southern Comal Counties identified on the basis 

of geologic, hydrologic, and geochemical data. Hydrogeology and geologic structure based on work done by Maclay 

and Small (1984), Small (1986), Small and Hanson (1994), 

Maclay (1995), and Stein and Ozuna (1994) and flow path 

work done by Maclay and Land (1988) and Groschen (1996) 

provided a basis for the initial selection of hypothesized flow 

paths in the study area. Flow paths were further defined using 

hydrologic data collected from water-level observation wells 

and springflow monitoring sites in the study area. Altitudes 

of the potentiometric surface within each flow path provided 

evidence of flow directions. Comparisons of water-level and 

spring-discharge hydrographs and statistical correlation of 

water levels and spring discharges were used to show relations 

between water levels at wells and spring discharges within 

flow paths. Continuous water-level data were collected at six 

observation wells from mid-March 2004 through September 

2006. Discharge data were obtained from two springflow sites 

for the same time period. Ground-water chemistry and isotope 

data were compiled from samples collected from 76 wells 

and nine springs (and spring orifices of major springs) during 

1996–2006. Isotope data also were obtained from samples collected at four rainfall sites during 1998–2003.

Description of the Edwards Aquifer and Study 

Area

The San Antonio segment of the Edwards aquifer (hereinafter, Edwards aquifer) comprises Lower Cretaceous-age 

rocks of the Edwards Group (Rose, 1972) and the Georgetown 

Formation. The Edwards Group in the study area comprises 

two stratigraphic units, the basal Kainer Formation and the 

upper Person Formation. Each of those units comprises several 

informal members. The basal member of the Person Formation is a laterally extensive marine deposit consisting of poorly 

permeable, dense, carbonate mudstone known as the regional 

dense member (Maclay, 1995).

Most recharge to the Edwards aquifer occurs in the 

recharge zone (aquifer outcrop) west of Bexar County (fig. 1), 

where streams originating north of the aquifer flow across and 

lose most or all of their flows into highly faulted and fractured 

limestone. Additional recharge enters the aquifer through the 

recharge zone in Bexar, Comal, and Hays Counties. After 

the water enters the aquifer, it moves generally in an easterly 

direction to discharge points in Bexar County, mainly municipal water-supply wells. Water not discharged to wells then 

continues generally toward the northeast along and parallel 

to northeast-trending faults in the study area to discharge 

points in Comal and Hays Counties, primarily Comal Springs 

in Comal County and San Marcos Springs in Hays County 

(fig. 1). 

The study area is in an extensively faulted section of 

Cretaceous strata known as the Balcones fault zone (fig. 1). 

The fault zone developed as a result of extensional faulting 

and is characterized by a network of en-echelon, high-angle, 

mostly down-to-the-coast normal faults along the northwestern margin of the Gulf Coastal Plain (Maclay and Small, 

1984; Maclay, 1995). The Cretaceous strata were vertically 

displaced, intensively fractured, and differentially rotated 

within a series of southwest-to-northeast trending fault blocks 

(Barker and Ardis, 1996). The fault blocks, and their subsequent erosion and dissolution, are major factors affecting flow 

in the aquifer.

Maclay and Land (1988, fig. 22) defined four major 

flow units in the Edwards aquifer. The flow units originate in 

areas referred to as storage units in the recharge zone and are 

regions of confined flow that generally move water initially to 

the southwest and then to the east and northeast to discharge at 

major springs. The eastern flow unit described by Maclay and 

Land (1988) originates in the study area. The western-southern, south-central, and north-central flow units coalesce in 

northeastern Bexar County and southern Comal County in the 

vicinity of Cibolo Creek and Interstate Highway 35. 

Maclay and Small (1984, p. 50) estimated transmissivities 

for the Edwards aquifer to range from 200,000 to 2,000,000 

feet squared per day. Maclay and Small (1984) describe one of 

the most transmissive areas in the Edwards aquifer as occurring within a narrow, northeast-trending band downgradient 

from the area of coalescence of the three southernmost flow 

units. This band of high transmissivity (fig. 2) (labeled “R” in 

Maclay and Small [1984, fig. 20]) is bounded on the northwest and southeast by faults where less-permeable rocks of 

the upper confining unit of the aquifer are juxtaposed against 

rocks of the Edwards aquifer. 

Recharge to and flow within the Edwards aquifer in 

northeastern Bexar and southern Comal Counties are complicated by the structure and stratigraphy of the rocks. The 

study area (fig. 1) is on the structural high known as the San 

Marcos Platform (Rose, 1972). The San Marcos Platform 

is extensively faulted in the study area. The thickness of the 

Edwards aquifer in the study area is about 450 feet (Small and 

Hanson, 1994, p. 5; Stein and Ozuna, 1994, p. 5). Three major 

faults, the Comal Springs fault and the Hueco Springs fault in 

southern Comal County and the Northern Bexar fault (Maclay 

and Land, 1988) in northeastern Bexar County, are potentially 

effective barriers to flow in the Edwards aquifer in the study 

area (fig. 2). 

The Comal Springs complex (fig. 3) issues from the 

Comal Springs fault, which has as much as 500 feet of offset 

(Maclay and Land, 1988, p. A42) and is juxtaposed against the 

younger and less permeable upper confining unit. The springs 

developed because a roughly north-south trending transverse 

fault east of New Braunfels completely offsets the Edwards 

aquifer in the downthrown block of the Comal Springs fault, 

Introduction 
Bear

Salado Creek

Guadalupe River

Leon Creek

Cibolo Creek

Canyon Lake

BLANCO COUNTY

KENDALL COUNTY

COMAL COUNTY

HAYS 

COUNTY

BEXAR 

COUNTY

GUADALUPE

COUNTY

EXPLANATION

Trinity aquifer

Edwards aquifer recharge zone (outcrop)

Edwards aquifer confined zone

Line of 1,000 milligrams per liter dissolved solids

 concentration (Schultz, 1994)

Study area boundary

Fault

Spring

0 3.5 7 10.5 14 MILES

98°40'

98°30'

98°20'

98°10'

98°0'

29°30'

30°0'

Comal Springs fault

Springs fault

Hueco

fault

Cave Bat

fault

Northern Bexar

fault Creek

fault

HiddenValley

I–35

I–35

Band of high

transmissivity

reported by

 Maclay and Small

(1984, fig. 20)

AREA ENLARGED

IN FIGURE 3

San Pedro Springs

San Antonio Springs

Hueco Springs

Comal Springs

creating a barrier to northeastward flow and forcing water 

upward along the Comal Springs fault (Klemt and others, 

1979, fig. 8). About one-fourth of the springflow from the 

Comal Springs complex discharges from three large spring 

orifices (Comal 1, Comal 2, and Comal 3 on the west side of 

the complex) that are sourced in the upthrown block of the 

Comal Springs fault (Ogden and others, 1985; LBG-Guyton 

Associates, 2004). The remaining springflow is discharged 

from numerous spring orifices and seeps that are within and 

near the banks of Landa Lake; these springs and seeps are 

sourced in the downthrown block of the Comal Springs 

fault. 

Figure 2. Study area and relation between recharge zone and confined zone of the Edwards aquifer and the Trinity aquifer, 

northeastern Bexar and southern Comal Counties, Texas. 

 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Figure 3. Locations of major springs in the Comal Springs complex, southern Comal County, Texas. 

Introduction 

Elizabeth

Torrey

California

Union

Lakeview

Klingemann

Edgewater

Cole

Acorn

Allen

Austin

Liberty

Houston

Chicago

Booneville

Marymont

Hayselton

Marsh

Dallas

Oakwood

Fredericksburg

Landa Park

Baden

Bell

Pl

a

yground

Mulberry

Course

Oak Tree

Mount Joy

Monument

Texas

Country Club

Dallas

Liberty

Torrey

Mulberry

Landa Lake

Golf

Comal 1

Comal 3

Comal 2

Comal 5

Comal 7

Comal—Spring Island

98°8'15" 98°8' 98°7'45"

29°42'45"

29°43'

29°43'15"

0 0.05 0.1 0.15 0.2 MILE

EXPLANATION

Edwards aquifer recharge zone (outcrop)

Edwards aquifer confined zone

Comal Springs complex

Line of 1,000 milligrams per liter dissolved

 solids concentration (Schultz, 1994)

Spring

Base from StreetMap USA, 2004
Hueco Springs issues from the Hueco Springs fault 

(fig. 2), which has 380 to 400 feet of offset in the vicinity of 

the springs (William F. Guyton and Associates, 1979, p. 25) 

and exposes much of the less-permeable upper confining 

unit on the downthrown block. The series of fault blocks 

immediately adjacent to and southeast of the Hueco Springs 

fault in southern Comal County are oriented mainly down-tothe-northwest (fig. 4). Consequently, this section of Edwards 

aquifer between the Hueco Springs fault and the Comal 

Springs fault is in a graben that is tilted opposite to the prevailing structure in the area. The fault blocks near Hueco Springs 

fault generally contain the entire thickness of the Edwards 

aquifer and parts of the upper confining unit, whereas the fault 

blocks near Comal Springs fault generally contain incomplete, 

unconfined sections of the Edwards aquifer exposed at the 

surface. The fault blocks contain numerous transverse faults 

that possibly impede ground-water flow. Northwest of Hueco 

Springs fault are faults that probably are not effective barriers 

to flow, including Bat Cave, Bear Creek, and Hidden Valley 

faults (fig. 2).

Acknowledgments

Thanks are extended to SAWS and other well owners who granted permission to have their wells sampled for 

water quality and measured for water levels. Special thanks 

are extended to Hanson Aggregates, Inc., and New Braunfels 

Utility (NBU) for access to their wells for use as real-time, 

continuous water-level observation sites for this study.

Methods of Investigation

Data Collection and Sample Analysis

Six wells were used for the collection of continuous 

water-level data (fig. 5; table 1). Two real-time, continuous 

water-level data-collection sites—DX–68–22–913 (Hanson) 

and DX–68–23–502 (Solms)—were established to collect 

hourly ground-water-level data for this study (appendix 1.1, 

1.2). Water-level data also were compiled on an hourly basis 

from four existing USGS real-time, continuous ground-waterlevel observation wells—AY–68–29–103 (HCV), AY–68–37–

203 (Bexar), DX–68–23–304 (NBU–LCRA), and DX–68–30–

208 (Bracken). Collection of continuous water-level data at 

the observation wells began in mid-March 2004 and continued 

through September 2005 at wells HCV and Bracken and 

through September 2006 at the four other wells. USGS-computed springflow data for Comal Springs and Hueco Springs 

also were compiled for mid-March 2004 through September 

2006. All water-level and spring-flow data are in the USGS 

National Water Information System for Texas (U.S. Geological Survey, 2006a). 

Ground-water-chemistry and isotope data were collected 

and compiled from 76 wells and nine springs (and spring orifices from major springs) during 1996–2006 (fig. 5; table 1). 

Data were collected for this study during 2003 (and during 

2006 at selected sites). Additional data were collected during 

1996–2006 as part of the National Water Quality Assessment (NAWQA) program (U.S. Geological Survey, 2006c). 

Two wells and two springs provided data from both sampling 

periods. Sixty-three wells and two springs provided NAWQA 

data only. Eleven wells and five springs provided data only for 

this study.

Samples were collected for dissolved gases (methane, 

carbon dioxide, oxygen, nitrogen, and argon), sulfur hexafluoride (SF6

), and tritium (3H) and helium-3 (3He) concentrations 

from 13 wells and seven springs during 2003 (table 1; appendix 1.3). 3

H/3He ratios were computed and used to estimate an 

apparent age (year sampled minus recharge year) of the water. 

Four wells (DX–68–22–810, DX–68–22–905, DX–68–23–

304, and DX–68–23–710) and six springs (Comal 1, Comal 3, 

Comal 5, Comal 7, Comal-Spring Island, and Hueco A) were 

resampled in 2006 for 3H/3He because samples collected in 

2003 resulted in either inconclusive or questionable age dates 

for these wells and springs. The 2006 sample for well DX–68–

22–810 resulted in an inconclusive age date.

Isotope data were collected and compiled from four 

rainfall sites (fig. 5, table 1). Two sites provided only data collected in 1998 as part of the NAWQA program (U.S. Geological Survey, 2006c). The remaining two sites provided both 

NAWQA data and 2003 data. 

Water-chemistry data included field properties (water 

temperature, pH, specific conductance, and dissolved oxygen), 

major ions, trace elements, nutrients, and alkalinity. Water 

samples were collected, processed, and preserved using standard USGS protocols as described in Wilde and others (1999, 

2003, and 2004). The concentrations of major ions, trace 

elements, and nutrients in the water samples were measured 

by the USGS National Water Quality Laboratory in Denver, 

Colo., using approved methods (Fishman and Friedman, 1989; 

Patton and Truitt, 1992, 2000; Faires, 1993; Fishman, 1993; 

American Public Health Association, 1998; Garbarino and 

others, 2006). 

The ratios of naturally occurring, stable isotopes of 

hydrogen (2/1H) and oxygen (18/16O) were measured by the 

USGS Stable Isotope Laboratory in Reston, Va., using 

approved methods (U.S. Geological Survey, 2005). Results for 

stable isotope analysis are reported as delta deuterium (δD) 

and delta 18-oxygen (δ18O), which represent the relative difference in parts per thousand (per mil) between the sample isotope ratio and the isotope ratio of a known standard (Kendall 

and McDonnell, 1998).

Dissolved gases and SF6 samples were analyzed by the 

USGS Chlorofluorocarbon Laboratory in Reston, Va., using 

approved methods (U.S. Geological Survey, 2006b). 3H/3He 

samples collected in 2003 were analyzed by personnel at the 

Nobel Gas Laboratory of Lamont-Doherty Earth Observatory 

in Palisades, N.Y., and samples collected in 2006 were 

 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Figure 4. Idealized block diagram of the Edwards aquifer between Comal Springs fault and Hueco Springs fault, southern Comal County, Texas. 

Upper confining unit

Edwards aquiferLower confining unit

EXPLANATION

Hueco Springs

Comal Springs

Comal Springs fault

Hueco Springs fault

Northwestern trough Southeastern trough

Bracken gap

SOUTHWEST

NORTHEAST

NOT TO SCALE

Methods of Investigation 
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Salado Creek

Blanco

River

Leon

Creek

SanAntonio

River

Guadalupe

River Canyon Lake

Cibolo Creek

San Antonio

New Braunfels

Bulverde

Rainfall site 2 Rainfall site 1

DX–68–15–901

DX–68–15–913

AY–68–21–806

DX–68–23–502

DX–68–22–913

DX–68–30–208

AY–68–37–203

AY–68–36–613

AY–68–36–614

DX–68–07–718

DX–68–08–701

DX–68–13– 206

DX–68–13–402 DX–68–15–605

DX–68–13–809

DX–68–16–708

DX–68–23–104 DX–68–21–305

DX–68–24–122 DX–68–21–210 DX–68–23–227

DX–68–23–620

DX–68–23–501

DX–68–23–504 DX–68–23–710 DX–68–22–810

DX–68–22–905

DX–68–22–901

DX–68–22–911

DX–68–30–215 DX–68–30–111

AY–68–30–109

AY–68–30–520

AY–68–29–610

AY–68–29–913

AY–68–29–929 AY–68–28–904

AY–68–29–714 AY–68–30–718 AY–68–28–807 AY–68–29–920 AY–68–29–811 AY–68–28–913 AY–68–29–914 AY–68–37–105

AY–68–37–124

AY–68–37–304

AY–68–37–601

AY–68–37–426

Hueco Springs

San Antonio Springs

San Pedro Springs

BEXAR COUNTY

KENDALL COUNTY

COMAL COUNTY

HAYS

COUNTY

08168000

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

Comal River

DX–68–23–326

DX–68–23–325

DX–68–23–301

DX–68–23–324

DX–68–23–303 DX–68–23–323

DX–68–23–304

Rainfall site 4Rainfall site 3 DX–68–23–601

DX–68–23–602

Comal

Springs

!08168710

!( !( !( !( !(

!( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !( !(

!( !(

Salado Creek

Leon

Creek

AY–68–29–216 AY–68–28–313

AY–68–29–217 AY–68–28–315 AY–68–29–112 AY–68–29–306

AY–68–29–215 AY–68–28–314 AY–68–29–113 AY–68–28–210 AY–68–29–114 AY–68–29–103 AY–68–29–214 AY–68–28–203 AY–68–28–113 AY–68–28–211 AY–68–29–109

AY–68–27–307 AY–68–28–205 AY–68–29–213 AY–68–28–609 AY–68–29–418 AY–68–28–601

AY–68–28–518 AY–68–28–517 AY–68–27–611

AY–68–28–407 AY–68–28–519

AY–68–29–414 AY–68–28–406 AY–68–28–515

AY–68–28–516 San Antonio

0 5 7.5 10 MILES 2.5

EXPLANATION

Trinity aquifer

Edwards aquifer recharge zone (outcrop)Edwards aquifer confined zoneStudy area boundaryLine of 1,000 milligrams per liter dissolved solids concentration (Schultz, 1994)Spring

98°30'

98°15'

98°0'

29°30'

30°0'

Monitoring site

!!!!!

s

Ground-water level and identifier

Ground-water quality and identifier

Ground-water level and quality and

Rainfall isotopes and identifier

Spring discharge and identifier

AY–68–29–103AY–68–28–315

DX–68–23–304

Rainfall site 4

08168710

 identifier

Figure 5. Locations of wells and spring orifices from which water-quality samples were collected and locations of continuous water-level observation wells, northeastern Bexar and southern Comal Counties, Texas. 

 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data 

were collected and compiled for this report—Continued.

USGS site

number

State well

number 

(fig. 5) 

USGS name Aquifer

Total 

depth 

(feet)

Open interval 

(feet)

Completion 

type

Data type Period of record

Ground-water sites (wells, springs, and spring orifices)

293746098265401 AY–68–21–806 NA Edwards 290 225–290 P QW, I 1998–2006293508098375101 AY–68–27–307 NA Edwards 270 190–270 P QW, I 1998–2006293429098373801 AY–68–27–611 NA Edwards 260 190–260 P QW, I 1998–2006293516098362801 AY–68–28–113 NA Edwards 320 250–320 P QW, I 1998–2006293518098332601 AY–68–28–203 NA Edwards 435 162–435 X QW, I 2005–06293504098332601 AY–68–28–205 NA Edwards 485 273–485 X QW, I 1996293530098343401 AY–68–28–210 NA Edwards 281 201–281 P QW, I 1998–2006293516098325501 AY–68–28–211 NA Edwards 300 220–300 P QW, I 1998–2006293635098302301 AY–68–28–313 NA Edwards 300 197–297 P QW, I 1998–2006293535098304101 AY–68–28–314 NA Edwards 241 170–240 P QW, I 1998–2006293611098311901 AY–68–28–315 NA Edwards 280 190–280 P QW, I 1998–2006293350098355801 AY–68–28–406 NA Edwards 310 224–304 P QW, I 1998–2006293425098350801 AY–68–28–407 NA Edwards 310 240–310 P QW, I 1998–2006293348098334101 AY–68–28–515 NA Edwards 305 245–305 P QW, I 1998–2006293340098344701 AY–68–28–516 NA Edwards 302 202–302 P QW, I 1998–2006293436098343001 AY–68–28–517 NA Edwards 261 181–261 P QW, I 1998–2006293439098324101 AY–68–28–518 NA Edwards 261 181–261 P QW, I 1998–2006293408098331301 AY–68–28–519 NA Edwards 280 179–280 P QW, I 1998–2006293451098313201 AY–68–28–601 NA Edwards 500 40–500 X QW 2005293459098321401 AY–68–28–609 NA Edwards 260 200–260 P QW, I 1998–2006293111098340901 AY–68–28–807 NA Edwards 685 420–685 X QW 2005293133098303201 AY–68–28–904 NA Edwards 640 528–640 X QW, I 1997293042098305201 AY–68–28–913 NA Edwards 787 448–787 X QW 2005293522098291201 AY–68–29–103 HCV Edwards 547 90–547 X WLC 2004–05293512098291701 AY–68–29–109 NA Edwards 600 197–590 X QW, I 1996–2006293559098284801 AY–68–29–112 NA Edwards 260 160–260 P QW, I 1998–2006293534098282801 AY–68–29–113 NA Edwards 241 160–240 P QW, I 1998–2006293528098274301 AY–68–29–114 NA Edwards 201 120–201 P QW, I 1998–2006293504098270901 AY–68–29–213 NA Edwards 222 142–222 P QW, I 1998–2006293520098254101 AY–68–29–214 NA Edwards 222 101–221 P QW, I 1998–2006293537098262401 AY–68–29–215 NA Edwards 180 120–180 P QW, I 1998–2006293643098264001 AY–68–29–216 NA Edwards 261 180–260 P QW, I 1998–2006293615098262301 AY–68–29–217 NA Edwards 257 188–257 P QW, I 1998–2006

Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report.[Datum for open interval is land surface; USGS, U.S. Geological Survey; NA, not applicable; P, perforated or slotted; QW, water chemistry; I, deuterium and oxygen isotopes; X, open hole; WLC, continuous water level; DG, dissolved gas; SF, sulfur hexafluoride; THE, tritium and helium isotopes; W, walled; S, screen; --, unknown; R, wire-wound; DIS, discharge]

Methods of Investigation 
Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data 

were collected and compiled for this report—Continued.

USGS site

number

State well

number 

(fig. 5) 

USGS name Aquifer

Total 

depth 

(feet)

Open interval 

(feet)

Completion 

type

Data type Period of record

Ground-water sites (wells, springs, and spring orifices)—Continued

293551098244801 AY–68–29–306 NA Edwards 295 56–295

X QW, I 1996–2006

293359098290301 AY–68–29–414 NA Edwards 710 380–660

X QW 2004

293456098280201 AY–68–29–418 NA Edwards 181 111–181

P QW, I 1998–2006

293358098231101 AY–68–29–610 NA Edwards 811 359–811

X QW, I, DG, SF, THE 2003–04

293120098285801 AY–68–29–714 NA Edwards 848 408–848

X QW 2004

293055098264301 AY–68–29–811 NA Edwards 612 209–612

X QW, I, DG, SF, THE 2003

293149098224201 AY–68–29–913 NA Edwards 784 360–784

X QW, I, DG, SF, THE 2003

293026098241401 AY–68–29–914 NA Edwards 845 423–845

X QW, I, DG, SF, THE 2003

293100098225401 AY–68–29–920 NA Edwards 655 493–655

X QW, I 1997

293145098224201 AY–68–29–929 NA Edwards 870 409–870

X QW 2004

293525098213701 AY–68–30–109 NA Edwards 710 314–710

X QW, I 1997

293440098195201 AY–68–30–520 NA Edwards 777 427–777

X QW, I, DG, SF, THE 2003

293119098211201 AY–68–30–718 NA Edwards 877 530–877

X QW 2004

292648098303701 AY–68–36–613 NA1 Edwards NA NA NA QW, I 1997292648098303401 AY–68–36–614 NA1 Edwards NA NA NA QW, I 1997292944098292301 AY–68–37–105 NA Edwards 1050 476–1,050

X QW 2004

292931098274601 AY–68–37–124 NA Edwards 557 390–557

X QW 2005

292845098255401 AY–68–37–203 Bexar Edwards 874 491–874

X WLC 2004–06

292808098230101 AY–68–37–304 NA Edwards 685 600–685

X QW, I 1997

292522098291901 AY–68–37–426 NA Edwards 1114 774–1,114

X QW 2004

292643098241801 AY–68–37–601 NA Edwards 1150 1012–1,150

X QW 2004

295458098143001 DX–68–07–718 NA Trinity 180 59–180

W QW, I 1996

295352098071201 DX–68–08–701 NA Edwards 240 239–240 S QW, I 1996–2006295013098255201 DX–68–13–206 NA Trinity 252 123–252

X QW, I 1996–2002

294743098291801 DX–68–13–402 NA Trinity 500 40–500

X QW, I 1996

294650098265801 DX–68–13–809 NA Trinity 580 100–580

W QW, I 1996

294739098075301 DX–68–15–605 NA Edwards 375 220–375

W QW, I 1996–2006

294533098082301 DX–68–15–901 Hueco A1 Edwards NA NA NA QW, I, DG, SF, THE 2003–06294533098082401 DX–68–15–913 Hueco B1 Edwards NA NA NA QW, I, DG, SF, THE 2003294604098060801 DX–68–16–708 NA Edwards 400 200–400

W QW 2005

294344098253801 DX–68–21–210 NA Trinity 464 -- -- QW, I 1996294437098235201 DX–68–21–305 NA Trinity 500 160–500

X QW, I 1996

293937098182501 DX–68–22–810 NA Edwards 405 -- -- QW, I, DG, SF, THE 2003–06293807098155301 DX–68–22–901 NA Edwards 255 148–255

X QW 2005

293823098153801 DX–68–22–905 NA Edwards 204 -- -- QW, I, DG, SF, THE 2003–06

10 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data 

were collected and compiled for this report—Continued.

USGS site

number

State well

number 

(fig. 5) 

USGS name Aquifer

Total 

depth 

(feet)

Open interval 

(feet)

Completion 

type

Data type Period of record

Ground-water sites (wells, springs, and spring orifices)—Continued

293745098162001 DX–68–22–911 NA Edwards 450 400–450 X QW, I 1996–2006293838098161601 DX–68–22–913 Hanson Edwards 248 188–248 X WLC 2004–06294440098132701 DX–68–23–104 NA Edwards 450 -- -- QW, I, DG, SF, THE 2003294323098115101 DX–68–23–227 NA Edwards -- -- -- QW, I 1996–2006294300098080001 DX–68–23–301 Comal 11 Edwards NA NA NA QW, I, DG, SF, THE 1997–2006294249098080301 DX–68–23–303 NA Edwards 1045 586–1,045 X QW, I 1997294239098081401 DX–68–23–304 NBU–LCRA Edwards 1061 406–1,061 X WLC, QW, I, DG, SF, THE 1998–2006294248098081201 DX–68–23–323 Comal 31 Edwards NA NA NA QW, I, DG, SF, THE 2003–06294255098080501 DX–68–23–324 Comal 71 Edwards NA NA NA QW, I, DG, SF, THE 2003–06294304098075501 DX–68–23–325 Comal-Spring Island1 Edwards NA NA NA QW, I, DG, SF, THE 2003–06294314098074101 DX–68–23–326 Comal 51 Edwards NA NA NA QW, I, DG, SF, THE 2003–06294054098104601 DX–68–23–501 NA Edwards 210 25–210 X QW, I, DG, SF, THE 2003294054098103501 DX–68–23–502 Solms Edwards 448 46–448 X WLC 2004–06294019098114701 DX–68–23–504 NA Edwards 215 119–215 X QW 2005294225098080301 DX–68–23–601 NA Edwards 365 92–365 X QW 2004294206098090101 DX–68–23–602 NA Edwards 790 149–239 P

239–790 X QW, I, DG, SF, THE 2003

294137098093201 DX–68–23–620 NA Edwards -- -- -- QW, I 1996–2006293949098141001 DX–68–23–710 NA Edwards 217 -- -- QW, I, DG, SF, THE 2003–06294428098063701 DX–68–24–122 NA Edwards -- -- -- QW, I 1996–2006293653098201501 DX–68–30–111 NA Edwards 230 -- -- QW, I, DG, SF, THE 2003293636098190901 DX–68–30–208 Bracken Edwards 292 220–240 R WLC 2004–05

272–292 P

293729098173101 DX–68–30–215 NA Edwards 660 185–660 X QW 2005

Springflow sites

08168710 NA Comal Springs at New Braunfels, Tex.1 NA NA NA NA DIS 2004–0608168000 NA Hueco Springs near New Braunfels, Tex.1 NA NA NA NA DIS 2004–06

Rainfall sites

295025098243200 NA Rainfall site 1 NA NA NA NA I 1998295104098171900 NA Rainfall site 2 NA NA NA NA I 1998294230098093300 NA Rainfall site 3 NA NA NA NA I 1998–2003294233098093300 NA Rainfall site 4 NA NA NA NA I 1998–2003

1 Spring or spring orifice. 

Methods of Investigation 11
analyzed by USGS personnel at the USGS Noble Gas Laboratory in Denver, Colo., using approved methods (U.S. Geological Survey, 2006b).

Quality Control and Quality Assurance of 

Geochemical Samples

Duplicate samples for major ions and trace elements were 

collected from one of the 13 wells (DX–68–23–710) sampled 

for this study (appendix 1.3). Duplicate samples for SF6 were 

collected from all 13 wells sampled for this study. Duplicate 

samples for major ions, trace elements, and nutrients collected 

during 1996–2006 as part of the NAWQA program (U.S. Geological Survey, 2006c) were available for seven of the 65 wells 

supplying existing data for this study. Duplicate samples for 

major ions, trace elements, and nutrients were collected at one 

of the seven springs (DX–68–23–325) sampled for this study 

(appendix 1.3). Duplicate samples for SF6 were collected from 

all seven springs sampled for this study. 

Duplicate samples were used to evaluate the methods 

used by field and laboratory personnel to collect and 

analyze a given sample with consistent results. The nonzero relative percent differences between environmental and 

duplicate samples collected for major ions, trace elements, and 

nutrients for this study ranged from 0.41 percent for nickel 

in the spring sample to 54.5 percent for molybdenum in the 

spring sample with a median value of 3.1 percent. The greatest 

relative percent difference between the environmental and 

duplicate samples collected for SF6 for this study was 10.3 

percent in well DX–68–23–304.

Methods of Data Analysis

Hydrologic Data

Water levels measured in wells in the Edwards aquifer 

during October 30–November 3, 2000, in a multi-agency effort 

headed by the Edwards Aquifer Authority (Hamilton and 

Schindel, 2006) were used to construct a potentiometric-surface map for the study area. Water-level contours within each 

flow path were examined to determine flow directions in the 

Edwards aquifer.

Hydrographs of water levels at the six observation wells 

and discharge at Comal Springs and Hueco Springs were 

compared. Similarities in the water-level hydrographs might 

indicate either that the wells are responding to a wide-spread 

recharge event or that they are in the same flow path. Similarities in hydrographs for the spring discharge and water levels in 

wells could indicate that the spring and wells share the same 

flow path. 

Statistical correlations between datasets of water levels for the six observation wells and discharge for Comal 

Springs and Hueco Springs were analyzed using Pearson’s r 

(Helsel and Hirsch, 1995). Pearson’s r (or linear correlation 

coefficient) is a measure of the linear association between 

two variables. Pearson’s r was calculated for each of the 28 

combinations of datasets to indicate the strength of the linear 

association (correlations) between datasets. The assumption 

was that a strong linear correlation indicates a higher probability of a shared or common flow path than a nonlinear correlation or no correlation.

Hydrograph recession-curve analysis was done on 

spring-discharge hydrographs to identify the number and type 

of flow regimes (diffuse, fracture, and conduit) that characterize the ground-water flow path contributing to the spring. 

Hydrographs of spring discharge at Comal Springs and Hueco 

Springs were graphed, and recession curves (sections of the 

hydrograph where the discharge is falling after a sudden 

rise) spanning the study period were examined. Methods in 

Milanovich (1981), Bonacci (1993), Padilla and others (1994), 

Shevenell (1996), and Baedke and Krothe (2001) were used 

to examine the recession curves for breaks in the recession 

slope, which are indicative of a change from one flow regime 

to another within the karst continuum. A recession coefficient (α) was calculated for each part of the karst continuum. 

The value of α relates to the rate of release of water from the 

aquifer. In general, higher values of α indicate a steeper slope 

in the recession curve, and therefore, a release of water from 

conduit-type features in the aquifer, whereas lower values of 

α indicate a gentler recession slope and release of water from 

diffuse (matrix) features of the aquifer.

Geochemical Data

Geochemical data collected and compiled from selected 

wells and springs in the Edwards and Trinity aquifers in the 

study area were used to assess major-ion chemistry and apparent ground-water age (table 1). Ground-water flow paths were 

analyzed using geochemical and isotopic data. Additional 

18/16O and 2/1H isotopic concentrations were estimated from 

rainfall samples to determine the local meteoric water line. 

Dissolved gases and SF6 were used to determine apparent age (year sampled minus recharge year) of ground-water 

samples. Apparent age is determined by comparing the concentration of SF6

 in the water sample to an established annual 

atmospheric concentration (Busenberg and Plummer, 2000). 

SF6 is a trace gas in the atmosphere that accumulates in rainfall that eventually becomes recharge to the ground-water system. Mainly an anthropogenic compound, SF6

 also can occur 

naturally in fluid inclusions in some minerals and igneous 

rocks and in some volcanic and igneous fluids. Apparent age 

derived using this method does not take into account the mixing of young and old waters. The addition of excess air into 

the ground-water system, which can occur when air bubbles 

are dissolved during a rapid rise of the water table, increases 

the SF6 concentration in the ground water to levels greater than 

the air-water equilibrium concentration. If the existence of 

12  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
excess air is not considered in the computation of the recharge 

year, then the apparent age will be too young.

Apparent ages also were obtained using a method that 

involves measuring the relative abundance of tritium (3H) and 

helium-3 (3He) in a water sample. Tritium is a radioactive isotope of hydrogen that decays to 3

He with a half-life of 12.43 

years. Tritium was introduced into the ground-water system 

in a series of peaks beginning in 1952, caused by atmospheric 

testing of nuclear devices, and reached a maximum concentration during 1963–64 (Schlosser and others, 1988). Although 

tritium concentrations in rainfall generally have decreased 

since this mid-1960s peak, an annual atmospheric concentration remains. Using a helium isotope mass balance, the 

amount of 3He from the decay of 3H (tritiogenic 3He denoted 

as 3He*) is measured with the remaining 3H in the water 

sample (Plummer and others, 2003). The values are added 

to determine the amount of 3H that was present in the water 

sample at the time of recharge to the ground-water system. 

This calculated recharge concentration of 3H then is compared 

to the established annual atmospheric concentration to determine an apparent age or recharge year for the water sample. 

This method of age-dating a water sample has proved to be 

effective for waters recharged within about the past 30 years 

and takes into account the mixing of young and old waters. 

Problems arise with this method when large amounts of terrigenic helium (derived from crustal or mantle sources) are 

present in a sample, such as in aquifers with host rock containing uranium or thallium, or in samples where young water has 

mixed with relatively old water containing terrigenic helium 

sources. In these cases, the ratio of 3He/4He for the terrigenic 

helium must be known within about 1 percent to determine 

an apparent age. If this ratio cannot be defined within the 

1-percent limit, a range in age can be evaluated for a range in 

the terrigenic helium ratio (Schlosser and others, 1988).

Major-ion and stable isotope data were graphically 

evaluated to determine relations among constituents that could 

distinguish differences between flow paths in the study area. 

Piper trilinear diagrams were used to visually categorize the 

principal water compositions for each flow path (Hem, 1992). 

Scatterplots of constituents and ratios of constituents were 

constructed to graphically indicate variations in water samples 

collected from wells from different flow paths.

Geologic and Hydrologic Identification 

of Flow Paths 

Southern Comal Flow Path

The southern Comal flow path (SCFP) (fig. 6) is bounded 

on the northwest by the Comal Springs fault and on the 

southeast by the freshwater/saline-water interface (threshold of 

1,000-milligrams per liter (mg/L) dissolved solids concentration [Schultz, 1994]) in the Edwards aquifer (fig. 1). Although 

the interface is not an actual physical barrier to flow, previous 

studies (Maclay and Small, 1984; Groschen, 1994; Lindgren 

and others, 2004) have indicated that flow in the transition 

zone (zone in which dissolved solids concentration ranges 

from 1,000 to 10,000 mg/L) is considerably more sluggish 

than flow in the immediately adjacent freshwater zone, likely 

because of relatively low permeability and transmissivity in 

the transition zone. The 1,000-mg/L threshold was selected as 

the boundary because that concentration historically has been 

considered the separation between the freshwater and salinewater zones of the aquifer and is a well-documented marker 

within the aquifer. 

The potentiometric-surface map constructed from data 

collected in fall 2000 in a multi-agency effort organized by 

the Edwards Aquifer Authority indicates that water within the 

SCFP flows from southwest to northeast from areas of higher 

water-level altitude to areas of lower water-level altitude 

(fig. 7). Recharge to the SCFP is primarily regional, occurring 

in the Edwards aquifer recharge zone west of the study area. 

The Comal Springs fault fails to offset the entire thickness 

of the Edwards aquifer near the area where the boundaries of 

Bexar, Comal, and Guadalupe Counties come into contact. 

Maclay and Land (1988) referred to this area as the Bracken 

gap (fig. 6). Subsurface inflow to the SCFP can occur as 

ground water spills over from north of Comal Springs fault 

across the Bracken gap. The permeable sections of the aquifer 

in the SCFP, although faulted, are juxtaposed in a manner that 

promotes the flow of ground water in the study area from the 

center of Bexar County through southern Comal County. The 

ground water then encounters a transverse fault northeast of 

New Braunfels (fig. 6) that forms a barrier to ground-water 

flow and is forced up along the Comal Springs fault and 

through the overlying gravels to form a majority of the springs 

and seeps in the Comal Springs complex. The SCFP gradually narrows as it nears Comal Springs because saline water 

encroaches as the freshwater is discharged at the springs. The 

transmissivity in the freshwater zone of the Edwards aquifer southeast of the Comal Springs fault in southern Comal 

County is greater than in any other part of the aquifer (Maclay 

and Small, 1984). 

Water-level hydrographs for wells Bexar, Bracken, 

Solms, and NBU–LCRA and the discharge hydrograph for 

Comal Springs (fig. 8) provided evidence of patterns of 

ground-water flow in the SCFP. The hydrographs for the four 

wells and Comal Springs fluctuated in a similar manner, possibly indicating a common flow path for ground water flowing 

past the wells and discharging at Comal Springs. Wells Bexar 

and Bracken showed larger magnitudes of fluctuation than did 

wells Solms and NBU–LCRA. This indicates that amplitude 

of the pressure wave moving through the confined section of 

the aquifer likely is decreased by the substantial increase in 

horizontal hydraulic conductivity of the aquifer in the SCFP 

near Comal Springs. The relatively flat potentiometric surface 

near Comal Springs (fig. 7) also provides evidence of higher 

horizontal hydraulic conductivity at the northeastern end of the 

SCFP.

Geologic and Hydrologic Identification of Flow Paths 13
Figure 6. Locations of hypothesized ground-water flow paths, northeastern Bexar and southern Comal Counties, Texas. 

14  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

Canyon

Lake COMAL COUNTY

BLANCO COUNTY

KENDALL COUNTY

BEXAR COUNTY

HAYS

COUNTY

Bracken gap

Alamo Heights horst

0 5 10 15 20 MILES

S

Northern Comal flow path and inferred direction of flow

Central Comal flow path and inferred direction of flow

Southern Comal flow path and inferred direction of flow

Edwards aquifer recharge zone (outcrop)

Study area boundary

Line of 1,000 milligrams per liter dissolved solids 

 concentration (Schultz, 1994)

Inferred flow path barrier

Inferred normal fault

Normal fault

Unspecified fault

Spring and identifier

Water-quality site and identifier

EXPLANATION

Comal Springs

S S S

S

S

S

S

S

S

S S

S

S

S S

S

S

S

S

S

S

S

S

S S

S

S

S

S

S

S

S

S S

98°30'

98°15'

98°0'

29°30'

30°0'

Cibolo Creek

San Antonio River

Leon Creek

Salado Creek

Blanc R r

 

Guadalupe River

New

 Braunfels

San Pedro Springs

San Antonio Springs

Hueco Springs

Comal Springs

S S

S S

S

S

S

S

S

S

S

S

S

S S

S

S

S

S

S

S

S S

S

S o C

S

S

S

S

!(

!(

!(

!(

!(

!(!(

!(

AY–68–29–103 (HCV)

DX–68–23–502 (Solms)

AY–68–37–203 (Bexar)

DX–68–22–913 (Hanson)

DX–68–30–208 (Bracken)

DX–68–23–304 (NBU-LCRA)

Hueco Springs

 near New Braunfels

Comal Springs at New Braunfels

AY–68–29–103 (HCV) !(

Comal Springs fault

Springs fault

Hueco

fault Cave

Bat

fault

Northern Bexar

fault

Creek

fault

HiddenValley

Bear
BEXAR 

COUNTY

COMAL 

COUNTY

98°45'

98°30'

98°15'

98°0'

29°30'

29°15'

29°45'

30°0'

0 10 15 20 MILES 5

EXPLANATION

Water-level altitude, in feet above NAVD 88

596 to 630630 to 664664 to 698698 to 732732 to 766766 to 800800 to 834834 to 868868 to 902902 to 936

Northern Comal flow path boundar

y

Central Comal flow path boundar

y

Southern Comal flow path boundar

y

! 745

Inferred flow path internal barrierLine of 1,000 milligrams per liter  dissolved solids concentration  (Shultz, 1994)Study area boundaryGround-water-level site and water- level altitude, in feet above NAVD 88Spring

!!

!

!

!

!

!!

!

!

!

!

!!

!

!

!

!

!

!

!

!!

!!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!!!

!

!

!

!

!!

!!!

!

!

!

708

696

692

684

684

683

693

693

698

683 708 711

697695 694 698

694

697

697

700

700

706

745

692

694

707 740

771

772

724 762

770

731

706

721

783

749

781 819

762 743

800

785

768

789

843

828 823

795

834 861 927

872

778 782

689

625

680

679 671

629

654

667

651

659

637

630

635

651

662

665

697

686

631 604

705

684

650

646 641

644 625

626

622

733

725

792 848

792

697

693

684

Comal Springs

Figure 7. Potentiometric surface of Edwards aquifer in northeastern Bexar and southern Comal Counties, Texas, fall 2000. 

Geologic and Hydrologic Identification of Flow Paths 15
760

740720700680660640620

3/14/2004

6/22/2004

9/30/2004

1/8/2005

4/18/2005

7/27/2005

11/4/2005

2/12/2006

5/23/2006

8/31/2006

DATE

WATER LEVEL, IN FEET ABOVE NGVD 29

6005004003002001000

D SPRING DISCHARGE, IN CUBIC FEET PER SECON

HCV (AY–68–29–103)

Hanson (DX–68–22–913)

Bexar (AY–68–37–203)

Bracken (DX–68–30–208)

Solms (DX–68–23–502)

NBU–LCRA (DX–68–23–304)

Comal Springs (08168710)

Hueco Springs (08168000)

Figure 8. Hydrographs of water levels from six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 2004–September 2006. 

16  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Central Comal Flow Path

The central Comal flow path (CCFP) (fig. 6) is bounded 

on the northwest by the Hueco Springs fault and on the 

southeast by the Comal Springs fault. The potentiometricsurface map indicates that water within the CCFP flows from 

southwest to northeast (fig. 7). As water moves into central 

Bexar County from the west it is bifurcated into two flow 

paths by the Alamo Heights horst (Maclay and Land, 1988) 

(fig. 6) in the subsurface southwest of the recharge zone in 

central Bexar County. Water that flows north of the horst 

becomes constrained northeast of the Comal Springs fault, 

which completely offsets the Edwards aquifer east of the 

horst, and becomes flow within the CCFP, whereas water that 

flows south of the horst becomes flow within the SCFP. Local 

recharge to the CCFP occurs in northeastern Bexar County as 

water infiltrates into the Edwards aquifer outcrop northwest 

of the Northern Bexar fault (fig. 6). The water then moves 

through the subsurface to the southeast until it encounters 

the Northern Bexar fault. The Northern Bexar fault acts as a 

barrier to flow at its northeastern end (Small, 1986, fig. 8), 

causing water to move to the southwest along the fault trace. 

The displacement along the Northern Bexar fault decreases 

southwestward along its trace until it is no longer a barrier to 

flow (Small, 1986, fig. 9), which allows water to enter the confined section of the aquifer. The local recharge remains north 

of Comal Springs fault as it merges with the northeastwardmoving water of the horst-induced CCFP. Additional inflow 

to the CCFP might occur in the subsurface when water from 

north of the Hueco Springs fault in Comal County crosses over 

the Hueco Springs fault in areas where the fault fails to fully 

offset the Edwards aquifer (Small, 1986, figs. 4–5).

 As the water in the CCFP continues flowing toward the 

northeast, structural features in the Edwards aquifer affect the 

patterns of flow. In eastern Bexar County, the Bracken gap 

conveys water from the CCFP into the SCFP (fig. 6). Beyond 

the Bracken gap, the aquifer containing the CCFP is heavily 

faulted and partially unconfined in the area directly northwest 

of Comal Springs. In the confined section of the CCFP in 

this area, the flow is funneled into two hydraulically connected troughs that follow roughly parallel grabens formed 

in the fault blocks between Comal Springs fault and Hueco 

Springs fault (fig. 4). The southeastern trough is unconfined 

at its northeastern end and receives recharge from direct 

infiltration of rainfall and streamflow losses. The northwestern trough receives subsurface inflow across sections of the 

Hueco Springs fault that fail to offset the entire thickness 

of the Edwards aquifer. Although flow within this section 

of the CCFP is complicated by the complexity of the faulting, in general, much of the water in the northwestern trough 

likely discharges at Hueco Springs and most of the water in 

the southeastern trough and in the unconfined sections of the 

aquifer in the CCFP discharges at springs Comal 1, Comal 2, 

and Comal 3 at the Comal Springs complex. Water that is not 

discharged at either Hueco or Comal Springs continues northeastward toward the San Marcos Springs (fig. 1).

Comparison of the water-level hydrographs for wells 

HCV and Hanson and discharge hydrographs for Comal 

Springs and Hueco Springs (fig. 8) provide insight into the 

movement of ground water in the CCFP. The hydrograph for 

well HCV shows little, if any, similarity to the hydrographs 

for well Hanson or either of the springs, indicating a lack of 

shared flow paths between well HCV and the other sites in the 

CCFP. Well HCV is in a heavily faulted section of the CCFP 

that is most likely locally isolated from the rest of the flow 

path. 

Well Hanson is in the unconfined section of the southeastern trough of the CCFP. In general, the hydrograph for 

well Hanson followed the fluctuation patterns of the discharge 

hydrographs of Comal Springs and Hueco Springs in number 

and relative vertical displacement for water levels greater than 

about 700 feet above NGVD 29. In this area, the regional 

dense member of the Person Formation within the Edwards 

aquifer acts as a confining unit. Well Hanson is completed 

below the regional dense member (approximate altitude 700 

feet above NGVD 29). When water levels in well Hanson 

rose above the regional dense member, the large magnitude of 

fluctuations in the hydrograph reflected confined properties 

similar to those at Comal Springs and Hueco Springs. When 

water levels in well Hanson fell to levels near or below the 

regional dense member, the Hanson well hydrograph tended to 

lose much of the vertical displacement and actually resembled 

the hydrographs of wells Solms and NBU–LCRA. As water 

levels fell in well Hanson and thus in that section of the CCFP, 

conditions changed from confined to unconfined. Furthermore, 

confined flow from the SCFP, under extreme pressure, might 

have pushed upward across Bracken gap into the CCFP to 

locally influence water levels in the CCFP. 

The Hueco Springs hydrograph (fig. 8) provided evidence 

that the CCFP might not be the only source of water to the 

Hueco Springs. An increase in discharge at Hueco Springs in 

August 2005 had no corresponding increase at Comal Springs, 

indicating that the pulse of water causing the increased discharge at Hueco Springs either was not sourced in the CCFP 

or did not reach Comal Springs. 

Northern Comal Flow Path

The northern Comal flow path (NCFP) (fig. 6) is in the 

Edwards aquifer recharge zone north of Hueco Springs fault 

and south of Bat Cave fault in Comal County. The potentiometric-surface map indicates that water in the NCFP flows 

from southwest to northeast (fig. 7). Recharge to the NCFP 

occurs from direct infiltration of rainfall and streamflow losses 

to the Edwards aquifer exposed at the surface in the NCFP. 

Additional inflow likely comes from the Trinity aquifer in the 

subsurface where the Bat Cave fault juxtaposes the Trinity 

aquifer against the Edwards aquifer. An undetermined amount 

of water from the NCFP might flow into the confined sections of the CCFP across Hueco Springs fault in areas where 

the fault does not completely offset the Edwards aquifer. The 

Geologic and Hydrologic Identification of Flow Paths 17
water remaining north of the Hueco Springs fault flows toward 

San Marcos Springs.

Correlation of Water Levels

Statistical correlation between water levels for the six 

observation wells and spring discharges for Comal Springs 

and Hueco Springs provides additional evidence for the 

hypothesized flow paths. Figure 9 graphically demonstrates 

the relations involving each of the 28 combinations of paired 

datasets. The Pearson’s r correlation coefficient indicates the 

strength of the linear correlation between dataset pairs and, 

thus, between water levels or spring discharges, or both. The 

larger the absolute value of r between -1 and 1, the stronger 

the linear correlation. The strongest correlations (r-values 

ranging from .991 to .999) were observed between the datasets 

for wells NBU–LCRA and Solms (SCFP) and well Hanson 

(CCFP), providing evidence for the hydraulic connection 

between the SCFP and the CCFP across the Bracken gap. 

The dataset for well Bracken, the well closest to the Bracken 

gap, is not as strongly correlated with datasets for wells 

NBU-LCRA, Solms, and Hanson. The well Bracken might 

be in a less transmissive part of the aquifer than the other 

three wells, allowing for greater fluctuations in water levels 

in the well Bracken dataset, which could account for weaker 

correlations with datasets from those wells. The dataset for 

well HCV displayed the weakest correlations with the other 

datasets, indicating a lack of shared flow paths with water at 

the other wells, Comal Springs, or Hueco Springs. The dataset 

for Comal Springs was strongly correlated with those for wells 

Bexar, NBU–LCRA, Hanson, and Solms (r-values ranging 

from .961 to .983) and fairly strongly correlated with the dataset from well Bracken (r-value of .954). The dataset for Hueco 

Springs did not correlate linearly with any of the datasets from 

the other wells or Comal Springs but did have monotonic relations (an increase in one variable corresponds to an increase 

in the other) with the datasets from wells Bexar, NBU–LCRA, 

Hanson, and Solms and Comal Springs. One reason for the 

lack of linear correlation between Hueco Springs and the other 

Edwards aquifer sites could be the influence of inflow from 

the Trinity aquifer. In addition, the northwestern trough of 

the CCFP, which provides most of the flow from the Edwards 

aquifer to Hueco Springs, could be less hydraulically connected to the rest of the CCFP during periods of low spring 

discharge than during periods of high spring discharge.

Analysis of Hydrograph Recession Curves

Hydrographs of discharge from Comal Springs and 

Hueco Springs were analyzed. Recession curves representing 

a range of flows were selected from each hydrograph for use 

in the analyses of hydrograph recession curves. The recession curves were examined for breaks (inflection points) in 

the recession slope, which are indicative of a change from one 

flow regime to another within the karst continuum. A recession coefficient (α) was calculated for each section of hydrograph between inflection points using Milanovich’s (1981) 

equation,

Q = Q e t 0

t-t α( ) 0

,

where Q0 is the initial discharge at time t0, the beginning of 

each recession slope, and Qt is the discharge at time t. Solving 

for α yields the equation

α = ln (Qt/Q0)

 ______________________ 

 (t-t0) .

For the hydrographs of this study (fig. 9), an α of about 0.18, 

corresponding to a steep (nearly vertical) slope, indicates conduit-driven drainage of a karst aquifer. An α of about 0.008, 

corresponding to a more horizontal slope, indicates the diffuse 

drainage of the primary porosity in the matrix of the aquifer. 

An α of about 0.02–0.09, corresponding to an intermediate 

slope, can indicate either drainage of the fractures in the aquifer or a mixture of conduit and diffuse drainage.

Figure 10 illustrates discharge magnitude and variability 

for Comal Springs and Hueco Springs from mid-March 2004 

through August 2006. Comal Springs discharge was consistently greater and more variable than discharge at Hueco 

Springs. In general, increases in discharge at Hueco Springs 

were smaller and shorter in duration than increases at Comal 

Springs.

Discharge-recession curves for Comal Springs were 

used to identify the flow regimes in the aquifer contributing 

to the springs. Flow varied from 202 to 509 cubic feet per 

second (ft3/s) during 2004–06 with the highest discharges 

occurring during November 21–25, 2004. The recession curve 

for the highest-discharge period indicates that fracture flow, 

or a mixture of conduit flow and diffuse flow, first dominated 

the ground-water system supplying water to the springs, 

followed by conduit flow and then fracture flow, or a mixture of conduit and diffuse flow, again. This pattern indicates 

that, during periods of high-discharge recession at the Comal 

Springs complex, the ground-water system supplying the 

springs might include upper and lower layers of the aquifer 

dominated by fractures or a mixture of conduits and aquifer 

matrix, with a middle section of the aquifer dominated by 

conduits. The lowest discharges occurred near the end of the 

study period (August 2006). During the lowest-discharge 

period, July 2005–August 2006, the recession curves for 

Comal Springs discharge reflected mostly an α of about 0.09, 

indicating fracture flow or a mixture of conduit and diffuse 

flow. The majority of the recession slopes for the Comal 

Springs hydrograph for the 2-year period had an α of about 

0.09, indicating that the overall flow through the aquifer to 

Comal Springs is fracture flow or a mixture of conduit and 

diffuse flow. During the 2-year period, α values less than 0.02 

were not observed for Comal Springs, indicating that diffuse flow never dominated the flow system supplying Comal 

Springs.

18  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Figure 9. Correlations between water levels for six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 

2004–September 2006. 

AY–68–37–203 Bexar

DX–68–23–304 NBU–LCRA

DX–68–22–913 Hanson

DX–68–23–502 Solms

AY–68–29–103 HCV

DX–68–30–208 Bracken

 08168710Comal Springs

 08168000Hueco Springs

SITE NUMBER AND NAME

Pearson's r equal to or greater than .99

Pearson's r equal to or greater than .96 but less than .99Pearson's r equal to or greater than .90 but less than .96

Pearson's r less than .90

EXPLANATION

Geologic and Hydrologic Identification of Flow Paths 19
DATE

DISCHARGE, IN CUBIC FEET PER SECOND

Hueco Springs (08168000)

Comal Springs (08168710)

α = 0.02, fracture flow or mixture of conduit 

α 

 flow and diffuse flow (with diffuse dominant)

= 0.18, conduit flo

w

α = 0.008, diffuse flo

w

α = 0.09, fracture flow or mixture of conduit flow  and diffuse flow (with conduit dominant)

3/14/2004 6/22/2004 9/30/2004 1/8/2005 4/18/2005 7/27/2005 11/4/2005 2/12/2006 5/23/2006 8/31/2006

0

500400300200100

November 21–25, 2004

November 2004

July 2005–August 2006

Figure 10. Hydrographs of discharge at Comal Springs and Hueco Springs, southern Comal County, Texas, March 2004–September 2006, showing recession coefficients for recession curves. 

20 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Discharge-recession curves for Hueco Springs were analyzed to identify the flow regimes in the aquifer contributing 

to the springs and to attempt to obtain additional evidence of 

contribution from the Trinity aquifer. Flow at Hueco Springs 

varied from 5.3 to 148 ft3/s during 2004–06 with the highest 

discharges occurring during November 2004. The recession 

curve for the highest-discharge period indicates that fracture 

flow or a mixture of conduit and diffuse flow first dominated 

the ground-water system supplying water to the springs. Following this period of fracture or mixed drainage, conduit flow 

controlled the release of water to the springs, followed by 

another period of fracture or mixed flow. The second period of 

fracture or mixed drainage was followed by periods in which 

diffuse flow became increasingly dominant and eventually 

prevailed. The lowest discharges occurred near the end of 

the study period (August 2006). During the lowest-discharge 

period, July 2005–August 2006, the increases in discharge at 

Hueco Springs were characterized by sudden high increases 

with recession curves that were dominated by conduit flow 

followed by fracture or conduit/diffuse mixed flow and ending 

with diffuse flow. During 2004–06, the recession slopes with 

an α of about 0.18 represented the greatest number of slopes 

for the Hueco Springs hydrograph, indicating that initial drainage of the aquifer to Hueco Springs following a recharge event 

is dominated by a fast-draining conduit system. Following the 

drainage of the conduits, the system typically is characterized 

by either fracture flow or a mixture of conduit and diffuse 

flow, or diffuse flow. 

Although the Comal Springs complex and Hueco Springs 

are only about 3 miles apart, the hydrograph recession slope 

analysis for the two springs reinforces the fact that the springs 

are sourced from a complex karst system. The initial recession 

slopes for Comal Springs predominately represent a mixture 

of conduit and fracture flow, or a mixture of conduit and 

diffuse flow, whereas the initial recession slopes for Hueco 

Springs predominately represent conduit flow. The initial 

mixed flow from Comal Springs likely represents the conduit 

flow contribution from the SCFP, which provides most of 

the flow from the springs, and the fracture flow contribution 

from the CCFP. The initial conduit flow from Hueco Springs 

provides evidence that the northwestern trough within the 

CCFP in Comal County might be highly transmissive, allowing for rapid movement of water through the aquifer to Hueco 

Springs. 

Geochemical Identification of Flow 

Paths

Major-Ion Chemistry

Trilinear diagrams constructed using major-ion chemistry from samples collected at wells in and springs emerging 

from the SCFP, CCFP, and NCFP (fig. 11) indicate that all 

three flow paths are dominated by calcium-bicarbonate type 

water. Relatively little variation in major-ion chemistry was 

observed. The water samples from wells and springs in the 

CCFP showed more variability in major-ion chemistry than 

those from the two other flow paths. The samples from wells 

in the NCFP consistently had higher percentages of bicarbonate and lower percentages of sulfate and chloride than the 

samples collected from sites in the SCFP. The comparatively 

high sulfate and chloride concentrations in the SCFP samples 

might indicate that the water in the flow path was mixing with 

small amounts of saline water from the freshwater/saline-water 

transition zone. 

A graph of the relation between calcium concentration and the ratio of magnesium to calcium concentrations 

provided further evidence for the hypothesized flow paths 

(fig. 12). Samples from wells in the SCFP generally yielded 

low calcium concentrations coupled with moderate Mg/Ca 

ratios (thus moderate magnesium concentrations). The highest 

calcium concentrations and lowest Mg/Ca ratios (contributed 

to by low magnesium concentrations) were in samples collected from the CCFP. The samples from the CCFP yielded 

data points that plotted in a grouping overlapping the data 

points for the SCFP. The high concentrations of magnesium in 

the samples collected from the recharge zone in Comal County 

(NCFP) might be evidence that water from the Trinity aquifer 

is entering the Edwards aquifer in the subsurface. The calcium 

concentration of water from Hueco Springs was lower when 

the spring discharge was low than when the spring discharge 

was high, indicating that during periods of low discharge 

Hueco Springs might receive a larger contribution from the 

Trinity aquifer than during periods of high discharge. 

Stable Isotopes

The relation between δD and δ18O in water samples from 

the Edwards aquifer, by flow path, is shown in figure 13. The 

graph shows that, except for samples collected from wells 

in the Edwards aquifer recharge zone in northeastern Bexar 

County (CCFP) following an unusually intense rain storm 

produced by a tropical system in October 1998, there was not 

much variation in stable isotope values among the flow paths. 

In the study area δD in ground water ranged from -36.00 per 

mil (well AY–68–28–313 in the CCFP) to -20.89 per mil 

(well DX–68–23–620 in the SCFP) and δ18O ranged from 

-6.03 per mil (well AY–68–28–313 in the CCFP) to -3.70 per 

mil (spring Hueco A in the CCFP). Excluding ground-water 

samples collected from wells in the recharge zone of northeastern Bexar County following the October 1998 storm, the 

δD range in the study area was -33.00 to -20.89 per mil and 

the δ18O range was -4.60 to -3.70 per mil. 

The local meteoric water line (LMWL; δD=8.8032 

δ18O+17.825) was calculated using rainfall isotope data collected in the study area. All ground-water samples, except one, 

plotted below the LMWL. The plotting positions indicate that 

the water evaporated, to varying degrees, before entering the 

aquifer as recharge. 

Geochemical Identification of Flow Paths  21
Figure 11. Trilinear diagrams showing composition of ground water in the Edwards aquifer in the southern, central, and northern 

Comal flow paths, northeastern Bexar and southern Comal Counties, Texas. 

PERCENT MILLIEQUIVALENTS

SOUTHERN COMAL

 FLOW PATH

CALCIUM

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

MAGNESIUM

 0 

 20 

 40 

 60 

 80

 100 

SODIUM PLUS POTASSIUM CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

 0 

 20 

 40

 60 

 80 

 100 

100 

 80 

 60 

 40 

 20 

 0 

CARBONATE PLUS BICARBONATE

 100

 80 

 60 

 40 

 20 

 0 

SULFATE

 0 

 20

 40 

 60 

 80 

 100 

SULFATE PLU S CHLORIDE

 0 

 20 

 40 

 60 

 80

 100 

CALCIUM PLUS MAGNESIUM

 100

 80 

 60

 40 

 20

 0 

 100 

 80 

 60 

 40 

 20

 0 

 100 

 0 

 20 

 40 

MAGN

PERCENT MILLIEQUIVALENTS

CENTRAL COMAL

 FLOW PATH

CALCIUM

 80

 60

 40

 20

 0 

 60 

 80 

 100 

ESIUM

 0 

 20 

 40 

 60 

 80 

 100

SODIUM PLUS POTASSIUM

CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80

 60

 40

 20

 0 

CARBONATE PLUS BICARBONATE

 100 

 80 

 60 

 40 

 20 

 0 

SULFATE

 0 

 20 

 40 

 60 

 80 

 100 

SULFATE PLU S CHLORIDE

 0 

 20

 40

 60

 80

 100 

CALCIUM PLUS MAGNESIUM

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80

 60

 40

 20

 0 

CALCIUM

100

 80

 60

 40

 20 

 0 

 0 

 20 

 40

 60

 80

 100 

MAGNESIUM

 0 

 20 

 40 

 60 

 80 

 100 

SODIUM PLUS POTASSIUM CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80

 60

 40

 20 

 0 

CARBONATE PLUS BICARBONATE

 100 

 80 

 60

 40

 20

 0 

SULFATE

 0 

 20 

 40 

 60 

 80 

 100 

SULFATE PLU S CHLORIDE

 0 

 20

 40 

 60 

 80

 100 

CALCIUM PLUS MAGNESIUM

 100 

 80 

 60 

 40 

 20 

 0 

 100

 80

 60 

 40

 20

 0 NORTHERN COMAL

 FLOW PATH

PERCENT MILLIEQUIVALENTS

22  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
0

0.70.60.50.40.30.20.1

60 70 80 90 100 110 120 130 140 150

CALCIUM, IN MILLIGRAMS PER LITER

MAGNESIUM/CALCIUM

Sample, by sourc

Southern Comal flow pathCentral Comal flow pathNorthern Comal flow pathHueco Springs during high dischargeHueco Springs during low dischargeTrinity aquifer

e

Figure 12. Relation of calcium concentration to magnesium/calcium ratio in samples collected from wells and springs in the Edwards aquifer and wells in the Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas. 

Geochemical Identification of Flow Paths  23
-10.00

-20.00-30.00-40.00-50.00-60.00

0

-10 -9 -8 -7 -6 -5 -4 -3

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL Southern Comal flow path Central Comal flow path Northern Comal flow path

Trinity aquiferRainfalGlobal meteoric water line (Craig, 1961)

l

Sample, by sourc

Local meteoric water line

e

-35.0-40.00

0

-30.00-25.00

-6.1 -5.6 -5.1 -4.6 -4.1 -3.6

DELTA OXYGEN-18, IN PER MIL

L DELTA DEUTERIUM, IN PER MI

-20.00

Area of graphshown below

Figure 13. Relation between delta oxygen-18 and delta deuterium for selected wells and springs, northeastern Bexar and southern Comal Counties, Texas, 1996–2006, with insert showing same plot with axes extended to range of rainfall samples. 

24  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Samples collected from wells and springs in the SCFP 

plotted in a relatively small area on the graph (fig. 13) with δD 

values ranging from -24.40 to -20.89 per mil and δ18O values ranging from -4.28 to -3.92 per mil. The largest range in 

isotopic composition was observed in samples collected from 

the CCFP, with δD values ranging from -36.00 to -21.39 per 

mil and δ18O values ranging from -6.03 to -3.70 per mil. CCFP 

samples with δD values greater than about -4.3 per mil overlap 

the grouping of SCFP samples. The samples from wells in the 

NCFP plot in a narrow area that spans the area occupied by the 

SCFP samples with δD values ranging from -24.40 to -22.44 

per mil and δ18O values ranging values ranging from -4.48 to 

-3.86 per mil. 

Ground-Water Ages

Apparent ages of ground water in samples collected 

in 2003 derived from SF6 analysis (fig. 14) ranged from 

about 21 years in well DX–68–23–104 in the NCFP to 

present day (less than 0.5 year or recharged in 2003) at 

spring Comal 7 in the SCFP. Ages ranged from about 17 years 

(well AY–68–29–914) to present day (spring Comal 7) in 

the SCFP, with the oldest water in the upgradient section of 

the SCFP and the youngest water from a spring at the downgradient end of the SCFP. The young apparent age of the 

Comal 7 spring might be evidence that young water from the 

upthrown block of Comal Springs fault is supplying additional 

recharge to the spring. The SF6 method of age dating water 

does not take into account the mixing of young and old waters 

and, therefore, reflects the age of the young water. In the 

CCFP, ages ranged from about 16 years (well DX–68–22–810) 

to about 1 year (well DX–68–30–111), with both the oldest 

and youngest water in the upgradient section of the flow path. 

Water in the NCFP likely receives inflow from the subsurface 

as a result of inter-aquifer hydraulic connection with the Trinity aquifer, which has substantially lower transmissivity (thus, 

possibly water of older apparent age) than does the Edwards 

aquifer. The apparent age of the water in well DX–68–23–104 

might reflect the influence of the Trinity aquifer on the NCFP.

The apparent ages of the samples collected in 2003 

from the various orifices of the Comal Springs complex and 

Hueco Springs, computed using SF6 analysis, provide evidence 

of the complex nature of the karst aquifer in which the springs 

are sourced. The samples from the orifices at the Comal 

Springs complex range in age from about 6 years to present 

day (less than 0.5 year or recharged in 2003). The samples 

from Hueco Springs provided ages that range from about 7 to 

8 years. The recent ages observed from the orifices at Comal 

Springs indicate an inflow of water near the springs complex, 

possibly from the unconfined section of the Edwards aquifer 

at the northeastern end of the southeastern trough of the CCFP 

in Comaal County. The relatively old water at Hueco Springs 

indicates a possible inflow of water into the ground-water 

system supplying the springs from a source of water yielding 

relatively older water than the Edwards aquifer, possibly the 

Trinity aquifer.

Apparent ages of ground-water samples collected in 

2003 derived from 3H/3He analysis (fig. 15) ranged from 

greater than 45 years in well DX–68–23–104 in the NCFP 

to present day (less than 0.5 year or recharged in 2003) in 

well DX–68–22–810 in the CCFP. Ages ranged from 21.3 

to 8.8 years in the SCFP. In general, the samples from wells 

in Bexar County (upgradient section of the SCFP) showed 

younger ages than wells and springs in the downgradient 

section of the SCFP. The samples from a spring in the 

Comal Spring complex, Spring Island, indicated ages similar 

to those for samples from the wells in Bexar County. A 

possible explanation for the anomalously young age of water 

in the Comal Springs complex is that there is a local source 

for the spring. Well DX–68–23–602 in the SCFP had a 

younger age than the wells in Bexar County. This well is on 

the escarpment formed along the Comal Springs fault and 

might be receiving inflow from the unconfined CCFP on the 

upthrown side of the fault. In the CCFP, ages ranged from 

17.3 years to present day (less than 0.5 year or recharged in 

2003) with the oldest water in well AY–68–30–520 near 

the Bexar-Comal County line in the central section of the 

flow path and the youngest water in the central section of 

the flow path where the Edwards aquifer is exposed, allowing 

recharge from infiltration of rainfall and streamflow losses 

to occur. The data from samples collected at Hueco Springs 

indicate very young water (apparent age less than 0.5 year) 

mixed with very old water. The oldest water was in well 

DX–68–23–104 in the NCFP. Water in this well is tritium 

depleted, indicating that the water has undergone complete 

tritium decay. The apparent age of the water is not attainable 

by the 3H/3He method, but based on the tritium decay rate the 

water is estimated to have been recharged more than 45 years 

ago. A possible source for this old water could be inflow from 

the underlying Trinity aquifer. 

Apparent ages for ground-water samples collected in 

2006 ranged from 41.1 years at spring Comal 7 in the SCFP to 

present day (less than 0.5 year or recharged in 2006) in wells 

DX–68–23–710 and DX–68–22–905 in the CCFP (fig. 15). In 

general, samples collected in 2003 yielded younger apparent 

ages than samples collected during 2006. Water levels in the 

Edwards aquifer were higher during the sampling in 2003 than 

they were during the sampling period in 2006, likely resulting in faster flow rates in the aquifer because of increased 

hydraulic gradients. Many of the samples contained a mixture 

of young and old water, especially those collected from areas 

near the downgradient ends of the flow paths. The samples 

collected from Hueco Springs in 2006, like those collected in 

2003, showed a mixture of very young water (apparent age 

less than 0.5 year) with very old water (possibly sourced from 

the Trinity aquifer). 

Geochemical Identification of Flow Paths  25
Figure 14. Apparent ages of ground water in 2003 samples from selected wells and springs in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, 

based on sulfur hexafluoride concentration (modified from Busenberg and Plummer, 2000, fig. 6A). 

26  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas DX–68–23–104 AY–68–29–914 DX–68–22–810 DX–68–22–905 DX–68–23–710 DX–68–23–602 DX–68–15–901 Hueco 

A

AY–68–29–913

DX–68–15–913Hueco B

AY–68–29–610

DX–68–23–323Comal 3

DX–68–23–325Comal-Spring Island

DX–68–23–304

DX–68–23–301Comal 1

DX–68–23–501 DX–68–23–326

Comal 5

DX–68–30–111

DX–68–23–324Comal 7

Well or spring and identifier

—Refer to table 1

Southern Comal flow pathCentral Comal flow pathNorthern Comal flow path

AY–68–29–914

TION, SULFUR HEXAFLUORIDE CONCENTRA

ARTS PER TRILLION BY VOLUME IN P

6543210

1980 1985 1990 1995 2000 2005

YEAR

20 15 10 5 0

APPARENT AGE
!(

!(

!(

!(!(!(!(!(

!(

!(

!(

!(

!(

!(!(

!(

!(

AY–68–29–914

AY–68–29–913

AY–68–29–610

AY–68–22–520

DX–68–22–810

DX –68–22–905

DX–68–23–710

DX–68–23–501

DX–68–23–602

DX–68–15–901 (Hueco A)

DX–68–15–913 (Hueco B)

DX–68–23–326 (Comal 5)

DX–68–23–325 (Spring Island)

DX–68–23–324 (Comal 7)

DX–68–23–323 (Comal 3)

DX–68–23–304 (NBU)

DX–68–23–301 (Comal 1)

DX–68–23–104

<0.5

<0.5

28.2

30.3

41.1

29.1

33.8 <0.5

29.7

0 6 3 9 12 MILES

Sample year 2006

98°30'

98°15'

98°0'

29°45'

30°0'

!(

!(

!(

!(!(!(!(!(

!(

!(

!(

!(

!(

!(!(

!(

!(

AY–68–29–914

AY–68–29–913

AY–68–29–610

AY–68–22–520

DX–68–22–810

DX –68–22–905

DX–68–23–710

DX–68–23–501

DX–68–23–602

DX–68–15–901 (Hueco A)

DX–68–15–913 (Hueco B)

DX–68–23–326 (Comal 5)

DX–68–23–325 (Spring Island)

DX–68–23–324 (Comal 7)

DX–68–23–323 (Comal 3))

DX–68–23–304 (NBU)

DX–68–23–301 (Comal 1)

DX–68–23–104

>45

8.8

<0.5

<0.5

17.3

11.71

12.08

16.06

17.39

<0.5

10.75

14.55

21.35

19.82

98°30'

98°15'

98°0'

29°45'

30°0'

Sample year 2003

0 6 3 9 12 MILES

AY–68–29–913

16.06

DX–68–23–501

11.71

DX–68–23–104

>45

>

<

EXPLANATION

Study area boundary

!(

Southern Comal flow path well, 

!(

!(

identifier, and age of water in years

Central Comal flow path well, 

identifier, and age of water in years

Northern Comal flow path well,

identifier, and age of water in years

= greater than

= less than

BE

COUNT

XAR 

Y

COMAL COUNTY

BE

COUNT

XAR 

Y

COMAL COUNTY

Figure 15. Apparent age of water in selected wells in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, 

based on tritium/helium-3 ratio. 

Geochemical Identification of Flow Paths  27
Summary

The U.S. Geological Survey (USGS), in cooperation with 

the San Antonio Water System, conducted a 4-year study during 2002–06 to identify major flow paths in the Edwards aquifer in northeastern Bexar and southern Comal Counties (study 

area). Geologic structure, surface- and ground-water data, and 

geochemistry were used to identify flow paths. Knowledge of 

geologic structure and previous flow path analyses conducted 

by the USGS provided a basis for the initial selection of 

hypothesized flow paths in the study area. 

Historical and new data were used in analyses. Continuous water-level data were collected at six observation wells 

from mid-March 2004 through September 2006. Discharge 

data were obtained from two springflow sites for the same 

time period. Ground-water chemistry and isotope data were 

compiled from samples collected from 76 wells and nine 

springs (and spring orifices of major springs) during 1996–

2006. Isotope data also were obtained from samples collected 

at four rainfall-sampling sites during 1998–2003.

The southern Comal flow path (SCFP) is bounded on the 

northwest by the Comal Springs fault and on the southeast 

by the freshwater/saline-water transition zone in the Edwards 

aquifer. Most of the water in this flow path enters the aquifer 

west of the study area. Additional inflows occur as spillover 

from north of the Comal Springs fault at the Bracken gap, an 

area near the Bexar-Comal-Guadalupe County line intersection where the fault has not completely offset the entire aquifer 

thickness. The SCFP gradually narrows as it nears Comal 

Springs because saline water encroaches as the freshwater is 

discharged at the springs.

The central Comal flow path (CCFP) is bounded on the 

northwest by the Hueco Springs fault and on the southeast 

by the Comal Springs fault. Water that is diverted north by 

the Alamo Heights horst in central Bexar County flows north 

of Comal Springs fault, accounting for the regional source 

to this flow path. Local recharge from infiltration of rainfall 

and streamflow leakage is supplied to the flow path from the 

Edwards aquifer recharge zone in northeastern Bexar County. 

These mixed waters then flow northeast through a highly 

fractured and faulted section of the aquifer. The complex faulting produces a hydraulically connected, two-trough system in 

the aquifer that is confined in some areas and unconfined in 

others. The troughs channel flow to discharge points at both 

Comal Springs and Hueco Springs. Water that does not discharge at either of the springs continues flowing northeastward 

toward San Marcos Springs. 

The northern Comal flow path (NCFP) is in the Edwards 

aquifer recharge zone in Comal County and is bounded on 

the north by the Bat Cave fault and on the south by the Hueco 

Springs fault. Recharge to this flow path occurs from the 

Edwards aquifer recharge zone northwest of Bat Cave fault in 

Comal County from direct infiltration of rainfall and streamflow losses to the Edwards aquifer exposed at the surface. 

Additional inflow likely comes from the Trinity aquifer in the 

subsurface where the Bat Cave fault juxtaposes the Trinity 

aquifer against the Edwards aquifer. Flow also might occur 

between the CCFP and the NCFP in central Comal County 

through a section of the Hueco Springs fault that does not 

completely offset the entire thickness of the Edwards aquifer.

A potentiometric-surface map derived from synoptic 

water-level measurements made in fall 2000 was used to identify the generally southwest to northeast flow directions within 

the flow paths. Statistical correlations between water levels for 

six observation wells and discharges from Comal Springs and 

Hueco Springs (28 combinations of paired datasets) yielded 

additional evidence for the hypothesized flow paths. Strong 

linear correlations were evident between the datasets from 

wells and springs within the same hypothesized flow path 

and the datasets from wells in areas where flow between flow 

paths was suspected. 

Hydrograph recession slope analysis for Comal and 

Hueco Springs reinforces the fact that the springs are sourced 

from a complex karst system. The initial recession slopes for 

Comal Springs predominately represent a mixture of conduit and fracture flow, or a mixture of conduit and diffuse 

flow, whereas the initial recession slopes for Hueco Springs 

predominately represent conduit flow. The initial mixed flow 

from Comal Springs likely represents the conduit flow contribution from the SCFP, which provides most of the flow from 

the springs, and the fracture flow contribution from the CCFP. 

The initial conduit flow from Hueco Springs provides evidence that the northwestern trough within the CCFP in Comal 

County might be highly transmissive, allowing for rapid movement of water through the aquifer to Hueco Springs. 

Geochemical data (major ions, stable isotopes, sulfur 

hexafluoride, and tritium and helium) were used in graphical 

analyses to obtain evidence of the flow path from which wells 

or springs derive water. Major-ion geochemistry in samples 

from selected wells and springs showed relatively little variation. Samples from the SCFP were characterized by relatively 

high sulfate and chloride concentrations possibly indicating 

that the water in the flow path was mixing with small amounts 

of saline water from the freshwater/saline-water transition 

zone. Samples from the CCFP yielded the most varied majorion geochemistry of the three hypothesized flow paths. CCFP 

samples were characterized, in general, by high calcium 

concentrations and low magnesium concentrations. Samples 

from the NCFP were characterized by relatively low sulfate 

and chloride concentrations and high magnesium concentrations. The high magnesium concentrations characteristic of 

NCFP samples from the recharge zone in Comal County might 

indicate water from the Trinity aquifer is entering the Edwards 

aquifer in the subsurface.

A graph of the relation between the stable isotopes δD 

and δ18O showed that, except for samples collected from wells 

in the Edwards aquifer recharge zone in northeastern Bexar 

County following an unusually intense rain storm produced 

by a tropical system, there was not much variation in stable 

isotope values among the flow paths. In the study area δD in 

ground water ranged from -36.00 to -20.89 per mil and δ18O 

28  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
ranged from -6.03 to -3.70 per mil. Excluding samples collected from wells in the recharge zone of northeastern Bexar 

County following the intense rain storm, the δD range in the 

study area was -33.00 to -20.89 per mil and the δ18O range was 

-4.60 to -3.70 per mil.

Two ground-water age-dating techniques, sulfur hexafluoride concentrations and tritium-helium-3 isotope ratios, 

were used to compute apparent ages (time since recharge 

occurred) of water samples collected from wells and springs in 

the study area. In general, the apparent ages computed by the 

two methods do not seem to indicate direction of flow. Apparent ages computed for water samples in northeastern Bexar 

and southern Comal Counties do not vary greatly except for 

some very young water in central Comal County. Additional 

recharge from direct infiltration of precipitation and streamflow losses into the exposed Edwards aquifer in central Comal 

County might account for younger age dates in the middle 

section of the CCFP.

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30 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix

Appendix 1—Water-Level (2004–06) and Chemical Data 

(2003–06)

Appendix 1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 2004–06—Continued.

Water year 2004 (Oct. 2003–Sept. 2004) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 --- --- --- --- --- --- 109.05 106.24 106.52 98.11 99.03 104.68

2 --- --- --- --- --- --- 109.10 106.11 106.84 97.78 --- 104.87

3 --- --- --- --- --- --- 109.06 105.97 107.44 97.75 --- 104.86

4 --- --- --- --- --- 109.37 108.94 105.95 107.85 97.66 --- 104.77

5 --- --- --- --- --- 109.32 108.75 106.01 107.97 97.68 --- 104.63

6 --- --- --- --- --- 109.30 108.64 106.05 108.07 97.85 --- 104.48

7 --- --- --- --- --- 109.28 108.58 105.96 108.20 98.24 --- 104.15

8 --- --- --- --- --- 109.23 108.51 105.86 108.34 98.50 --- 104.08

9 --- --- --- --- --- 109.30 108.42 105.70 108.32 98.53 --- 104.05

10 --- --- --- --- --- 109.36 108.38 105.53 106.48 98.58 --- 104.08

11 --- --- --- --- --- 109.40 108.24 105.44 102.91 98.63 --- 104.23

12 --- --- --- --- --- 109.42 107.99 105.42 100.11 98.73 --- 104.24

13 --- --- --- --- --- 109.35 107.93 105.52 99.56 98.85 103.36 104.27

14 --- --- --- --- --- 109.27 107.83 105.57 99.98 98.97 103.34 104.31

15 --- --- --- --- --- 109.11 107.75 105.40 100.60 99.13 103.33 104.23

16 --- --- --- --- --- 109.11 107.77 105.14 101.16 99.28 103.41 104.19

17 --- --- --- --- --- 109.11 107.74 104.97 101.65 99.33 103.57 104.28

18 --- --- --- --- --- 109.11 107.60 104.91 102.05 99.40 103.87 104.43

19 --- --- --- --- --- 109.10 107.49 104.99 102.48 99.58 104.18 104.41

20 --- --- --- --- --- 109.05 107.43 105.15 102.66 99.99 104.14 104.40

21 --- --- --- --- --- 109.05 107.38 105.15 102.81 100.31 104.23 104.50

22 --- --- --- --- --- 109.03 107.35 105.11 103.02 100.40 104.14 104.70

23 --- --- --- --- --- 108.99 107.38 105.06 103.22 --- 104.16 104.90

24 --- --- --- --- --- 108.94 107.35 105.12 103.39 100.20 104.22 105.06

25 --- --- --- --- --- 108.94 107.06 105.28 103.56 100.26 104.29 105.04

26 --- --- --- --- --- 108.96 106.73 105.55 103.41 99.98 104.33 104.92

27 --- --- --- --- --- 108.90 106.53 105.83 103.34 99.81 104.30 104.88

28 --- --- --- --- --- 108.83 106.38 106.12 102.89 99.66 104.24 105.01

29 --- --- --- --- --- 108.82 106.25 106.31 101.68 99.41 104.18 105.07

30 --- --- --- --- --- 108.88 106.19 106.32 99.86 99.21 104.22 105.10

31 --- --- --- --- --- 108.99 --- 106.37 --- 98.99 104.46 ---

Mean --- --- --- --- --- --- 107.79 105.62 103.88 --- --- 104.56

Maximum --- --- --- --- --- --- 109.10 106.37 108.34 --- --- 105.10

Minimum --- --- --- --- --- --- 106.19 104.91 99.56 --- --- 104.05

Appendix 1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 2004–06.

[In feet below land surface; ---, not collected or computed]

Appendix  33
Appendix 1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 2004–06—Continued. 

Water year 2005 (Oct. 2004–Sept. 2005) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 105.75 98.49 80.83 --- 89.78 89.41 88.23 92.37 95.10 --- --- 106.10

2 105.67 98.18 81.73 --- 89.85 89.34 88.13 92.71 94.87 --- --- 106.51

3 103.63 98.07 82.56 --- 89.93 89.47 88.10 93.38 94.77 --- --- 106.60

4 101.08 98.13 83.22 --- 89.96 89.41 88.10 93.80 94.75 --- --- 106.57

5 100.38 98.37 83.84 --- 89.86 89.31 88.14 94.04 94.78 --- --- 106.55

6 100.37 98.85 84.39 --- 89.75 88.86 88.35 93.76 94.83 --- --- 106.63

7 100.47 99.26 85.50 --- 89.67 88.32 88.82 93.63 94.95 --- --- 106.99

8 100.61 99.55 86.30 --- 89.42 87.65 89.05 93.63 95.12 --- --- 107.24

9 100.71 99.76 87.16 --- 89.27 87.28 89.18 93.55 95.27 --- --- 107.34

10 100.79 99.94 88.13 --- 89.14 87.05 89.06 93.60 95.40 --- 103.79 107.18

11 101.08 100.31 88.66 --- 88.99 87.02 89.13 93.91 95.46 --- 103.19 107.11

12 101.43 100.66 --- --- 88.86 87.15 89.23 94.06 95.56 --- 103.04 107.02

13 101.83 101.00 --- --- 88.84 87.30 89.39 93.92 95.71 --- 103.03 106.89

14 102.05 101.14 --- 88.48 88.85 --- 89.95 93.92 95.91 --- 103.07 106.79

15 102.10 100.83 --- --- 88.87 87.45 90.36 93.95 96.17 --- 103.17 106.98

16 102.35 100.61 --- --- 88.98 87.44 90.31 94.02 96.45 --- 103.33 106.94

17 102.57 99.19 --- --- 89.19 --- 90.28 94.15 96.68 --- 103.49 106.87

18 102.68 96.19 --- --- 89.49 --- 90.27 94.28 96.87 --- 103.59 106.84

19 102.66 94.01 --- --- 89.57 --- 90.30 94.38 97.10 --- 103.67 106.90

20 102.92 92.47 --- --- 89.34 --- 90.41 94.38 97.43 --- 103.88 107.01

21 103.07 91.56 --- --- 89.30 --- 90.60 94.40 98.19 --- 104.00 107.16

22 103.26 89.80 --- --- 89.30 86.86 90.79 94.47 98.49 --- 104.22 107.31

23 103.28 84.60 --- --- 89.27 87.01 90.99 94.58 98.48 --- 104.78 107.38

24 102.32 81.94 --- --- 89.37 87.31 91.11 94.78 98.67 --- 105.14 107.45

25 99.48 --- --- 89.69 89.50 87.57 91.09 95.04 --- --- 105.09 107.59

26 97.45 --- --- 90.12 89.57 87.51 91.24 95.31 --- --- 105.08 107.76

27 97.00 --- --- 89.97 89.53 87.54 91.58 95.55 --- --- 105.14 108.00

28 97.07 79.72 --- 89.71 89.47 87.57 92.15 95.90 --- --- 105.26 108.24

29 97.32 79.88 --- 89.65 --- 87.79 92.08 95.48 --- --- 105.40 108.54

30 97.95 80.35 --- 89.63 --- 87.99 92.22 95.26 --- --- 105.57 108.75

31 98.45 --- --- 89.70 --- 88.10 --- 95.22 --- --- 105.80 ---

Mean 101.22 --- --- --- 89.39 --- 89.95 94.24 --- --- --- 107.17

Maximum 105.75 --- --- --- 89.96 --- 92.22 95.90 --- --- --- 108.75

Minimum 97.00 --- --- --- 88.84 --- 88.10 92.37 --- --- --- 106.10

34  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 2004–06—Continued.

Water year 2006 (Oct. 2005–Sept. 2006) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 108.88 107.83 --- --- 110.85 112.18 114.66 117.79 120.01 123.79 125.97 128.54

2 109.00 107.79 --- --- 110.83 112.28 114.58 117.82 120.16 123.90 126.07 128.59

3 109.11 107.85 --- --- 110.87 112.37 114.57 117.80 120.27 124.01 126.17 128.64

4 109.10 108.12 --- --- 110.92 112.52 114.64 117.81 120.36 124.07 126.27 128.69

5 108.39 108.14 --- 109.49 110.94 112.75 114.71 117.81 120.49 123.98 126.37 128.73

6 107.78 108.11 --- 109.57 111.03 112.88 114.79 117.71 120.64 123.80 126.48 128.71

7 107.87 108.09 --- 109.74 111.07 113.01 114.94 117.50 120.77 123.71 126.61 128.70

8 107.86 108.12 --- 109.77 111.13 113.10 115.07 117.35 120.92 123.62 126.75 128.66

9 107.79 108.19 --- --- 111.25 113.20 115.15 117.28 121.08 123.55 126.85 128.59

10 107.74 108.28 --- --- 111.38 113.32 115.22 117.15 121.25 123.55 126.93 128.49

11 107.76 108.33 --- 109.97 111.49 113.42 115.35 117.09 121.39 123.57 127.05 128.39

12 107.74 108.34 --- 109.99 111.50 113.50 115.47 117.04 121.58 123.66 127.09 128.34

13 107.64 108.35 --- 110.08 111.49 113.61 115.60 117.03 121.79 123.74 127.15 128.18

14 107.53 108.34 --- 110.17 111.54 113.76 115.76 117.05 121.99 123.85 127.23 128.06

15 107.42 108.37 --- 110.20 111.66 113.91 115.85 117.11 122.21 123.98 127.29 127.95

16 107.34 108.45 --- 110.26 111.76 114.04 115.97 117.17 122.40 124.10 127.36 127.78

17 107.32 108.51 --- 110.42 111.87 114.17 116.11 117.30 122.56 124.22 127.44 127.59

18 107.30 108.73 --- --- 111.94 114.28 116.33 117.41 122.62 124.38 127.53 127.39

19 107.32 108.71 --- --- 111.92 114.37 116.58 117.55 122.65 124.55 127.63 127.22

20 107.39 108.67 --- --- 111.89 114.43 116.81 117.72 122.54 124.74 127.72 127.04

21 107.49 108.68 --- --- 111.88 114.48 116.95 117.91 122.48 124.88 127.82 126.85

22 107.54 --- --- --- 111.90 114.54 117.02 118.11 122.58 125.02 127.92 126.68

23 107.57 --- --- --- --- 114.63 117.07 118.33 122.64 125.13 128.03 126.53

24 107.62 109.07 --- --- 112.08 114.68 117.13 118.54 122.74 125.27 128.14 126.38

25 107.62 109.08 --- 110.68 112.09 114.70 117.23 118.79 122.83 125.35 128.21 126.28

26 107.66 --- --- 110.72 112.10 114.68 117.33 119.04 122.96 125.44 128.28 126.20

27 107.74 --- --- 110.79 112.07 114.67 117.41 119.25 123.13 125.52 128.35 126.10

28 107.83 108.99 --- 110.79 112.10 114.67 117.49 119.45 123.31 125.61 128.41 126.02

29 107.89 108.96 --- 110.74 --- 114.64 117.57 119.61 123.47 125.70 128.46 125.94

30 107.88 --- --- 110.79 --- 114.64 117.68 119.74 123.64 125.78 128.48 125.88

31 107.85 --- --- 110.84 --- 114.70 --- 119.86 --- 125.87 128.50 ---

Mean 107.84 --- --- --- --- 113.81 116.03 117.97 121.92 124.46 127.37 127.57

Maximum 109.11 --- --- --- --- 114.70 117.68 119.86 123.64 125.87 128.50 128.73

Minimum 107.30 --- --- --- --- 112.18 114.57 117.03 120.01 123.55 125.97 125.88

Appendix  35
Appendix 1.2. Daily mean depth to water at well DX-68-23-502, Comal County, Texas, 2004–06.

Water year 2004 (Oct. 2003–Sept. 2004) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 --- --- --- --- --- --- 32.40 30.44 31.13 27.62 27.93 29.71

2 --- --- --- --- --- --- 32.36 30.25 31.50 27.36 28.13 29.75

3 --- --- --- --- --- --- 32.14 30.29 31.75 27.04 28.38 29.76

4 --- --- --- --- --- --- 31.99 30.34 31.88 26.79 28.51 29.69

5 --- --- --- --- --- --- 31.97 30.32 31.77 26.70 28.64 29.43

6 --- --- --- --- --- --- 31.99 30.32 31.76 26.88 28.75 29.37

7 --- --- --- --- --- --- 31.92 30.30 32.00 26.96 28.57 29.43

8 --- --- --- --- --- --- 31.88 30.08 32.09 26.96 28.48 29.49

9 --- --- --- --- --- --- 31.80 29.94 31.63 26.99 28.58 29.52

10 --- --- --- --- --- --- 31.60 30.01 30.35 26.82 28.74 29.55

11 --- --- --- --- --- --- 31.38 30.08 30.02 26.69 28.84 29.59

12 --- --- --- --- --- --- 31.36 30.03 29.66 26.76 28.94 29.48

13 --- --- --- --- --- --- 31.43 29.97 29.31 26.91 29.04 29.60

14 --- --- --- --- --- --- 31.41 29.89 29.31 26.97 28.95 29.67

15 --- --- --- --- --- --- 31.36 29.67 29.35 27.09 28.89 29.58

16 --- --- --- --- --- --- 31.36 29.47 29.33 27.19 29.07 29.55

17 --- --- --- --- --- --- 31.19 29.54 29.34 27.14 29.29 29.59

18 --- --- --- --- --- 32.29 31.04 29.68 29.37 27.07 29.44 29.61

19 --- --- --- --- --- 32.29 31.14 29.70 29.27 27.26 29.48 29.53

20 --- --- --- --- --- 32.14 31.22 29.79 29.22 27.53 29.55 29.62

21 --- --- --- --- --- 32.01 31.25 29.91 29.38 27.75 29.56 29.75

22 --- --- --- --- --- 32.15 31.28 29.82 29.58 27.92 29.44 29.78

23 --- --- --- --- --- 32.28 31.29 29.78 29.60 27.95 29.51 29.82

24 --- --- --- --- --- 32.28 31.18 29.97 29.52 27.76 29.58 29.83

25 --- --- --- --- --- 32.27 30.96 30.18 29.46 27.67 29.57 29.76

26 --- --- --- --- --- 32.28 30.84 30.34 29.25 27.61 29.60 29.69

27 --- --- --- --- --- 32.12 30.85 30.50 29.03 27.80 29.65 29.79

28 --- --- --- --- --- 32.00 30.78 30.67 28.93 27.92 29.58 29.90

29 --- --- --- --- --- 32.16 30.72 30.68 28.73 27.99 29.35 30.00

30 --- --- --- --- --- 32.29 30.62 30.68 28.10 28.04 29.50 30.02

31 --- --- --- --- --- 32.37 --- 30.84 --- 27.97 29.66 ---

Mean --- --- --- --- --- --- 31.42 30.11 30.05 27.33 29.07 29.66

Maximum --- --- --- --- --- --- 32.40 30.84 32.09 28.04 29.66 30.02

Minimum --- --- --- --- --- --- 30.62 29.47 28.10 26.69 27.93 29.37

Appendix 1.2. Daily mean depth to water at well DX–68–23–502, Comal County, Texas, 2004–06.

[In feet below land surface; ---, not collected or computed]

36  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.2. Daily mean depth to water at well DX–68–23–502, Comal County, Texas, 2004–06—Continued.

Water year 2005 (Oct. 2004–Sept. 2005) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 30.04 26.48 17.83 19.59 21.61 21.43 20.70 23.46 24.96 28.97 29.80 31.46

2 29.71 26.35 17.81 19.65 21.57 21.38 20.61 23.63 24.86 29.10 29.93 31.52

3 29.11 26.22 17.83 19.72 21.59 21.40 20.57 23.78 24.81 29.08 30.07 31.46

4 28.98 26.15 17.67 19.85 21.65 21.42 20.74 23.93 24.78 29.21 30.25 31.33

5 28.93 26.09 17.55 20.02 21.46 21.24 20.87 24.06 24.68 29.54 30.26 31.31

6 28.85 25.95 17.69 20.09 21.32 20.92 21.02 24.20 24.75 29.80 30.16 31.45

7 28.80 25.77 17.83 20.20 21.42 20.76 21.11 24.23 24.90 29.98 30.13 31.51

8 --- 25.84 17.88 20.19 21.53 20.67 21.23 24.13 25.01 30.13 30.32 31.52

9 28.58 25.92 17.91 20.13 21.44 20.59 21.16 24.15 25.09 30.16 30.46 31.58

10 28.37 25.94 18.03 20.27 21.52 20.50 21.08 24.28 25.15 30.15 30.44 31.54

11 28.42 26.00 17.94 20.48 21.42 20.44 21.34 24.31 25.25 30.45 30.31 31.29

12 28.52 26.06 17.84 20.54 21.26 20.27 21.53 24.37 25.27 30.71 30.28 31.20

13 28.57 26.05 18.13 --- 21.04 20.10 21.65 24.36 25.42 30.87 30.21 31.21

14 28.51 25.86 18.38 --- 21.17 20.20 21.80 24.34 25.66 30.94 29.95 31.13

15 28.45 25.76 18.45 --- 21.26 20.30 21.93 24.23 25.85 31.02 30.04 31.03

16 28.38 25.76 18.55 --- 21.27 20.27 21.89 24.30 26.01 30.87 30.15 31.02

17 28.25 24.84 18.67 --- 21.33 20.20 21.86 24.41 26.16 30.43 30.13 31.03

18 28.31 24.15 18.62 --- 21.38 20.18 22.03 24.46 26.31 30.23 30.18 30.96

19 28.41 23.81 18.57 --- 21.22 20.06 22.22 24.52 26.36 30.17 30.25 31.17

20 28.46 23.40 18.67 --- 21.14 19.94 22.33 24.60 26.65 30.11 30.25 31.32

21 28.56 22.90 18.83 --- 21.30 20.01 22.39 24.65 26.93 30.03 30.21 31.39

22 28.58 21.49 18.99 --- 21.46 20.20 22.43 24.61 27.17 29.98 30.39 31.42

23 28.28 19.38 19.12 --- 21.47 20.27 22.49 24.80 27.38 29.89 30.55 31.46

24 27.95 18.83 19.04 --- 21.50 20.31 22.46 24.97 27.60 29.66 30.69 31.47

25 27.52 18.38 19.00 21.53 21.59 20.36 22.61 25.16 27.78 29.77 30.86 31.46

26 27.32 17.96 19.08 21.61 21.47 20.27 22.77 25.26 27.82 29.87 30.97 31.72

27 27.19 17.90 19.20 21.65 21.27 20.22 23.02 25.30 28.08 29.90 31.03 31.95

28 27.08 17.69 19.37 21.59 21.35 20.34 23.21 25.16 28.34 29.89 30.99 32.11

29 27.01 17.71 19.57 21.51 --- 20.47 23.36 24.91 28.53 29.83 31.13 32.19

30 26.89 17.83 19.71 21.39 --- 20.52 23.47 24.83 28.74 29.83 31.28 32.26

31 26.68 --- 19.65 21.47 --- 20.59 --- 24.94 --- 29.72 31.35 ---

Mean --- 23.42 18.50 --- 21.39 20.51 21.86 24.46 26.21 30.01 30.42 31.45

Maximum --- 26.48 19.71 --- 21.65 21.43 23.47 25.30 28.74 31.02 31.35 32.26

Minimum --- 17.69 17.55 --- 21.04 19.94 20.57 23.46 24.68 28.97 29.80 30.96

Appendix  37
Appendix 1.2. Daily mean depth to water at well DX–68–23–502, Comal County, Texas, 2004–06—Continued.

Water year 2006 (Oct. 2005–Sept. 2006) daily mean values

Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

1 32.28 31.04 32.73 32.62 33.55 34.87 36.45 38.99 41.46 45.04 46.20 47.72

2 32.26 31.09 32.79 32.71 33.59 34.97 36.38 --- 41.42 44.66 46.32 47.64

3 32.38 31.20 32.78 32.88 33.71 35.11 36.58 39.19 41.41 44.54 46.47 47.31

4 --- 31.28 32.78 33.04 33.80 35.21 36.69 39.23 41.39 44.41 46.66 47.30

5 33.48 31.23 32.87 33.17 33.80 35.24 36.84 39.07 41.63 43.74 46.61 47.42

6 31.33 31.20 32.79 33.25 33.86 35.49 36.92 38.67 41.97 43.51 46.34 47.03

7 30.42 31.42 32.78 33.23 33.96 35.62 37.02 38.20 42.18 43.38 46.32 46.82

8 30.30 31.56 32.71 33.12 34.03 35.73 37.11 38.03 42.40 43.19 46.45 46.72

9 30.28 31.64 32.80 33.25 34.09 35.78 37.08 37.97 --- 43.11 46.53 46.39

10 30.34 31.72 32.74 33.28 34.13 --- 37.29 37.97 --- 43.31 46.64 46.06

11 30.35 31.77 32.62 33.26 34.16 --- 37.50 38.04 --- 43.62 46.83 46.03

12 30.23 31.77 32.67 33.35 34.06 --- 37.70 38.08 --- 43.94 46.82 45.85

13 30.17 31.73 32.71 33.47 34.20 --- 37.85 38.18 --- 44.23 46.61 45.51

14 30.16 31.91 32.73 33.41 34.36 --- 38.01 38.19 --- 44.48 46.84 45.32

15 30.11 32.02 32.78 33.44 34.46 36.80 38.19 38.35 --- 44.47 47.10 45.19

16 30.05 32.11 32.72 33.49 34.49 36.79 38.21 38.59 --- 44.46 47.28 44.97

17 30.21 32.17 32.65 33.54 34.61 36.88 38.43 38.90 --- 44.78 47.41 44.72

18 30.28 32.20 32.53 33.54 34.66 36.89 38.71 39.19 --- 45.24 47.55 44.54

19 30.35 32.18 32.52 33.50 34.55 36.83 38.94 39.41 --- 45.47 47.43 44.39

20 30.41 32.10 32.60 --- 34.55 36.82 39.15 39.62 --- 45.61 47.30 44.27

21 30.51 32.22 32.63 33.54 34.60 36.83 39.08 39.74 43.68 45.70 47.41 44.15

22 30.56 32.40 32.67 33.38 34.61 36.80 38.83 40.12 43.78 45.68 47.60 44.12

23 30.52 32.50 32.51 33.42 34.67 36.81 38.74 40.47 43.96 45.41 47.60 44.01

24 30.71 32.43 32.44 33.60 34.73 36.80 38.92 40.76 44.00 --- 47.64 43.73

25 30.76 32.37 32.39 33.66 34.71 36.71 39.09 41.04 43.97 45.64 47.72 43.73

26 30.84 32.40 32.43 33.61 34.60 36.55 39.17 41.29 44.19 45.71 47.62 43.84

27 30.90 32.34 32.56 33.65 34.68 36.59 39.26 41.53 44.53 45.84 47.32 43.84

28 31.00 32.50 32.70 33.60 34.81 36.57 39.23 41.46 44.76 45.95 47.39 43.85

29 31.02 32.64 32.78 33.45 --- 36.48 39.13 41.25 44.99 45.92 47.63 43.89

30 30.86 32.69 32.71 33.43 --- 36.45 38.91 41.23 45.22 45.75 47.52 43.85

31 30.98 --- 32.63 33.53 --- 36.48 --- 41.39 --- 45.94 47.57 ---

Mean --- 31.93 32.67 --- 34.29 --- 38.05 --- --- --- 47.06 45.34

Maximum --- 32.69 32.87 --- 34.81 --- 39.26 --- --- --- 47.72 47.72

Minimum --- 31.04 32.39 --- 33.55 --- 36.38 --- --- --- 46.20 43.73

38  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued. 

Site 

type

State well

number

(USGS name)

Date Time

Temperature, 

water 

(°C)

pH 

(standard 

units)

Specific 

conductance 

(μS/cm)

Dissolved 

oxygen 

(mg/L)

Calcium 

(mg/L)

Magnesium 

(mg/L)

Southern Comal flow path

W AY–68–29–610 7/28/2003 1200 22.5 7 608 -- 100 13.5

7/28/2003 dup 1201 -- -- -- -- -- --

W AY–68–29–913 7/28/2003 1100 24.5 7.1 515 -- 73.2 15.9

7/28/2003 dup 1101 -- -- -- -- -- --

W AY–68–29–914 7/28/2003 1000 25 7.2 493 -- 68.2 16.1

7/28/2003 dup 1001 -- -- -- -- -- --

W DX–68–23–304 6/18/2003 1400 24 7.2 558 2.5 79.9 17

(NBU–LCRA) 6/18/2003 dup 1401 -- -- -- -- -- --

3/1/2006 1330 24.5 7.1 533 4.5 84.6 16.5

Sp DX–68–23–324 7/22/2003 1230 23.9 7.1 547 -- 84.7 17

(Comal 7) 7/22/2003 dup 1231 -- -- -- -- -- --

3/8/2006 1430 23.5 7.1 561 5.4 77.7 16.4

Sp DX–68–23–325 8/7/2003 1030 -- -- -- -- 79.4 17.8

(Comal-Spring Island) 8/7/2003 dup 1100 -- -- -- -- 98.7 18.5

3/8/2006 1400 23.5 7.1 562 5.1 82.5 15.6

Sp DX–68–23–326 7/22/2003 1330 24 7.1 548 -- 86.8 17.6

(Comal 5) 7/22/2003 dup 1331 -- -- -- -- -- --

3/8/2006 1330 23.5 7.1 565 4.9 77.5 16.1

W DX–68–23–602 7/24/2003 1100 23 7 544 -- 83.2 13.5

7/24/2003 dup 1101 -- -- -- -- -- --

Central Comal flow path

W AY–68–29–811 7/28/2003 0900 23.5 7 555 -- 82.9 17.4

7/28/2003 dup 0901 -- -- -- -- -- --

W AY–68–30–520 7/17/2003 1100 26 7 527 5.5 74.9 16.7

7/17/2003 dup 1101 -- -- -- -- -- --

Sp DX–68–15–901 3/5/2003 1400 20 6.7 613 6.7 109 9.81

(Hueco A) 3/5/2003 dup 1401 -- -- -- -- -- --

3/16/2006 1300 20.5 7.1 594 5.5 86 17.9

Sp DX–68–15–913 3/6/2003 1100 20 6.6 615 6.5 107 9.71

(Hueco B) 3/6/2003 dup 1101 -- -- -- -- -- --

Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06.

[USGS, U.S. Geological Survey; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °C; mg/L, milligrams per liter; W, well; --, not analyzed for or 

not detected; dup, duplicate; Sp, spring; R, Rainfall; CaCO3, calcium carbonate; <, less than; E, estimated; μg/L, micrograms per liter; pptv, parts per trillion by 

volume; δD, delta deuterium; per mil, parts per thousand; δ18O, delta oxygen-18; 3H, tritium; TU, tritium units; 3He, helium-3; M, presence of material verified 

but not quantified]

Appendix  39
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued. 

Site 

type

State well

number

(USGS name)

Date Time

Temperature, 

water 

(°C)

pH 

(standard 

units)

Specific 

conductance 

(μS/cm)

Dissolved 

oxygen 

(mg/L)

Calcium 

(mg/L)

Magnesium 

(mg/L)

Central Comal flow path—Continued

W DX–68–22–810 7/11/2003 1000 23 6.8 535 6.7 88.3 11.2

7/11/2003 dup 1001 -- -- -- -- -- --

3/14/2006 1300 22.8 7.1 544 5.8 98.6 6.56

W DX–68–22–905 7/15/2003 1100 22.5 6.9 476 6.9 81 10.5

7/15/2003 dup 1101 -- -- -- -- -- --

3/16/2006 1100 21.5 7.2 534 6.3 85.8 11

Sp DX–68–23–301 7/22/2003 1030 23.5 7 543 -- 85.3 16

(Comal 1) 7/22/2003 dup 1031 -- -- -- -- -- --

3/1/2006 1110 23 7.2 558 5.3 80.9 17.1

Sp DX–68–23–323 7/22/2003 1130 23.5 7 544 -- 87.1 16.5

(Comal 3) 7/22/2003 dup 1131 -- -- -- -- -- --

3/1/2006 1150 23 7.2 556 5.2 81.9 16.2

W DX–68–23–501 7/24/2003 1200 23.5 7 563 -- 83.9 14.2

7/24/2003 dup 1201 -- -- -- -- -- --

W DX–68–23–710 7/15/2003 1200 23 6.7 588 7.5 110 8.75

7/15/2003 dup 1201 -- -- -- -- -- --

3/14/2006 1200 22.3 7.1 591 6.4 109 7.99

3/14/2006 dup 1230 22.5 7.1 591 6.4 109 7.53

W DX–68–30–111 7/17/2003 1000 22 6.7 664 6.3 115 8.2

7/17/2003 dup 1001 -- -- -- -- -- --

Northern Comal flow path

W DX–68–23–104 7/9/2003 1300 23.5 7 527 4.3 62.4 32.6

7/9/2003 dup 1301 -- -- -- -- -- --

Rainfall sites

R Rainfall site 3 6/5/2003 900 -- -- -- -- -- --

7/8/2003 1400 -- -- -- -- -- --

R Rainfall site 4 6/5/2003 900 -- -- -- -- -- --

40 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Potassium 

(mg/L)

Sodium 

adsorption 

ratio

Sodium 

(mg/L)

Sodium 

(percent)

Alkalinity 

(mg/L as 

CaCO3)

Bicarbonate 

(mg/L)

Carbonate 

(mg/L)

Chloride 

(mg/L)

Fluoride 

(mg/L)

Silica 

(mg/L)

Southern Comal flow path

AY–68–29–610 2.26 0.3 10.3 7 257 313 0 15.7 0.19 12.3

 -- -- -- -- -- -- -- -- -- --

AY–68–29–913 1.24 .3 10.6 8 210 256 0 17.7 .2 12.8

 -- -- -- -- -- -- -- -- -- --

AY–68–29–914 1.16 .3 10.1 8 194 237 0 19.8 .2 12.6

 -- -- -- -- -- -- -- -- -- --

DX–68–23–304 1.42 .3 10.9 8 234 285 0 17.7 .26 12.3

(NBU–LCRA) -- -- -- -- -- -- -- -- -- --

1.41 .3 9.61 7 244 296 <1 15.4 .26 12.7

DX–68–23–324 1.28 .3 10.6 8 220 268 0 17.6 .23 12.9

(Comal 7) -- -- -- -- -- -- -- -- -- --

1.57 .3 10.4 8 242 295 <1 16.9 .27 12.5

DX–68–23–325 1.47 .3 9.78 7 228 278 0 15.6 .2 12.7

(Comal-Spring Island) 1.58 .2 9.48 6 226 276 0 16.9 .21 12.8

1.46 .3 9.53 7 250 305 <1 15.3 .25 12.2

DX–68–23–326 1.33 .3 10.9 8 220 268 0 18.2 .23 12.7

(Comal 5) -- -- -- -- -- -- -- -- -- --

1.46 .3 10.1 8 238 290 <1 16.6 .27 12.7

DX–68–23–602 1.18 .2 8.27 6 236 288 0 13.7 .22 12.7

 -- -- -- -- -- -- -- -- -- --

Central Comal flow path

AY–68–29–811 1.29 .3 10.6 8 225 275 0 16.2 .21 12.7

 -- -- -- -- -- -- -- -- -- --

AY–68–30–520 1.41 .3 10.8 8 196 239 0 20.9 .22 12.8

 -- -- -- -- -- -- -- -- -- --

DX–68–15–901 1.07 .2 7.98 5 268 327 0 12.4 .15 9.9

(Hueco A) -- -- -- -- -- -- -- -- -- --

1.41 .2 9.29 7 272 331 <1 14.1 .3 11

DX–68–15–913 1.07 .2 7.92 5 266 324 0 13.3 .15 9.9

(Hueco B) -- -- -- -- -- -- -- -- -- --

Appendix  41
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Potassium 

(mg/L)

Sodium 

adsorption 

ratio

Sodium 

(mg/L)

Sodium 

(percent)

Alkalinity 

(mg/L as 

CaCO3)

Bicarbonate 

(mg/L)

Carbonate 

(mg/L)

Chloride 

(mg/L)

Fluoride 

(mg/L)

Silica 

(mg/L)

Central Comal flow path—Continued

DX–68–22–810 0.81 0.2 5.66 4 246 300 0 10.2 <0.17 12

 -- -- -- -- -- -- -- -- -- --

.75 .2 6.11 5 267 325 <1 10.3 .11 11.4

DX–68–22–905 .83 .2 5.57 5 204 248 0 9.78 <.17 10.7

 -- -- -- -- -- -- -- -- -- --

.89 .2 7.85 6 233 284 <1 11.5 .15 10.7

DX–68–23–301 1.24 .2 9.47 7 218 265 0 15.8 .22 12.6

(Comal 1) -- -- -- -- -- -- -- -- -- --

1.48 .3 10.5 8 237 288 <1 17.3 .3 12.6

DX–68–23–323 1.34 .3 9.82 7 223 272 0 15.9 .22 12.6

(Comal 3) -- -- -- -- -- -- -- -- -- --

1.39 .3 9.49 7 244 297 <1 15.7 .24 12.9

DX–68–23–501 1.24 .2 8.82 7 247 301 0 14.1 .21 12.7

 -- -- -- -- -- -- -- -- -- --

DX–68–23–710 .97 .1 4.49 3 283 345 0 8.76 <.17 12.9

 -- -- -- -- -- -- -- -- -- --

1 .1 4.27 3 299 365 <1 7.65 .14 12.6

1.03 .1 4.14 3 299 364 0 7.51 .13 12.5

DX–68–30–111 1.73 .3 13.3 8 242 295 0 22.8 <.17 12.7

 -- -- -- -- -- -- -- -- -- --

Northern Comal flow path

DX–68–23–104 1.21 .2 5.94 4 254 310 0 8.21 .45 12.9

 -- -- -- -- -- -- -- -- -- --

Rainfall sites

Rainfall site 3 -- -- -- -- -- -- -- -- -- --

 -- -- -- -- -- -- -- -- -- --

Rainfall site 4 -- -- -- -- -- -- -- -- -- --

42  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Sulfate 

(mg/L)

Residue 

(sum of 

constituents) 

(mg/L)

Dissolved 

ammonia 

plus organic 

nitrogen 

(mg/L)

Dissolved 

ammonia 

nitrogen 

(mg/L)

Dissolved 

nitrite plus 

nitrate 

nitrogen 

(mg/L)

Dissolved 

nitrite 

nitrogen 

(mg/L)

Dissolved 

orthophosphate 

phosphorus

(mg/L)

Dissolved 

phosphorus 

(mg/L)

Aluminum 

(μg/L)

Southern Comal flow path

AY–68–29–610 25.7 342 <0.10 <0.04 1.66 <0.008 E0.01 <0.04 E1.2

 -- -- -- -- -- -- -- -- --

AY–68–29–913 21.2 287 <.10 <.04 1.84 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

AY–68–29–914 17.1 270 <.10 <.04 1.91 <.008 <.02 <.04 E.9

 -- -- -- -- -- -- -- -- --

DX–68–23–304 24.4 312 <.10 <.04 1.81 .036 <.02 <.04 E1.1

(NBU–LCRA) -- -- -- -- -- -- -- -- --

22.9 309 -- -- -- -- -- -- 2.1

DX–68–23–324 23.5 308 <.10 <.04 1.91 <.008 <.02 <.04 <1.6

(Comal 7) -- -- -- -- -- -- -- -- --

25.3 306 -- -- -- -- -- -- <1.6

DX–68–23–325 22 304 <.10 <.04 1.78 <.008 <.02 <.04 E1.1

(Comal-Spring Island) 21.9 324 <.10 <.04 1.78 <.008 E.01 <.04 E.8

21.9 309 -- -- -- -- -- -- 2.8

DX–68–23–326 23.7 312 <.10 <.04 1.85 <.008 <.02 <.04 <1.6

(Comal 5) -- -- -- -- -- -- -- -- --

25 302 -- -- -- -- -- -- E1.4

DX–68–23–602 17.1 300 <.10 <.04 1.89 <.008 E.01 <.04 2.7

 -- -- -- -- -- -- -- -- --

Central Comal flow path

AY–68–29–811 29.2 314 <.10 <.04 1.76 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

AY–68–30–520 22.9 287 <.10 <.04 1.89 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

DX–68–15–901 17.6 333 E.06 <.04 .92 <.008 <.02 <.04 <1.6

(Hueco A) -- -- -- -- -- -- -- -- --

29.2 332 -- -- -- -- -- -- <1.6

DX–68–15–913 17.9 330 E.06 <.04 .91 <.008 <.02 <.04 <1.6

(Hueco B) -- -- -- -- -- -- -- -- --

Appendix  43
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Sulfate 

(mg/L)

Residue 

(sum of 

constituents) 

(mg/L)

Dissolved 

ammonia 

plus organic 

nitrogen 

(mg/L)

Dissolved 

ammonia 

nitrogen 

(mg/L)

Dissolved 

nitrite plus 

nitrate 

nitrogen 

(mg/L)

Dissolved 

nitrite 

nitrogen 

(mg/L)

Dissolved 

orthophosphate 

phosphorus

(mg/L)

Dissolved 

phosphorus 

(mg/L)

Aluminum 

(μg/L)

Central Comal flow path—Continued

DX–68–22–810 6.53 291 <0.10 <0.04 1.8 <0.008 <0.02 <0.04 <1.6

 -- -- -- -- -- -- -- -- --

8.97 303 -- -- -- -- -- -- E1.3

DX–68–22–905 9.09 261 <.10 <.04 2.55 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

27.9 295 -- -- -- -- -- -- E.8

DX–68–23–301 20.9 301 <.10 <.04 1.94 <.008 <.02 <.04 <1.6

(Comal 1) -- -- -- -- -- -- -- -- --

25.5 307 -- -- -- -- -- -- <1.6

DX–68–23–323 21.1 307 <.10 <.04 1.96 <.008 <.02 <.04 <1.6

(Comal 3) -- -- -- -- -- -- -- -- --

23.2 307 -- -- -- -- -- -- E1.0

DX–68–23–501 19.1 310 E.07 <.04 1.83 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

DX–68–23–710 4.65 328 <.10 <.04 1.72 <.008 <.02 <.04 <1.6

 -- -- -- -- -- -- -- -- --

5.13 327 -- -- -- -- -- -- <1.6

5.17 326 -- -- -- -- -- -- <1.6

DX–68–30–111 21.1 387 <.10 <.04 10.5 <.008 <.09 <.04 <1.6

 -- -- -- -- -- -- -- -- --

Northern Comal flow path

DX–68–23–104 9.76 293 <.10 <.04 1.59 <.008 <.02 <.04 E1.2

 -- -- -- -- -- -- -- -- --

Rainfall sites

Rainfall site 3 -- -- -- -- -- -- -- -- --

 -- -- -- -- -- -- -- -- --

Rainfall site 4 -- -- -- -- -- -- -- -- --

44  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Antimony 

(μg/L)

Barium 

(μg/L)

Beryllium 

(μg/L)

Cadmium 

(μg/L)

Chromium 

(μg/L)

Cobalt 

(μg/L)

Copper 

(μg/L)

Lead 

(μg/L)

Manganese 

(μg/L)

Molybdenum 

(μg/L)

Southern Comal flow path

AY–68–29–610 <0.30 35 <0.06 <0.04 <0.8 0.201 1.9 1.77 <0.2 0.6

 -- -- -- -- -- -- -- -- -- --

AY–68–29–913 <.30 43 <.06 <.04 <.8 .128 .8 1.02 <.2 .7

 -- -- -- -- -- -- -- -- -- --

AY–68–29–914 <.30 53 <.06 <.04 <.8 .118 1.4 1.39 E.1 .7

 -- -- -- -- -- -- -- -- -- --

DX–68–23–304 <.30 56 <.06 <.04 <.8 .189 .7 <.08 2.6 .8

(NBU–LCRA) -- -- -- -- -- -- -- -- -- --

<.20 50 <.06 <.04 .24 .118 .6 .54 .5 .7

DX–68–23–324 <.30 51 <.06 <.04 <.8 .174 .3 <.08 <.2 .6

(Comal 7) -- -- -- -- -- -- -- -- -- --

<.20 57 <.06 <.04 .35 .125 1.6 .2 E.1 .7

DX–68–23–325 <.30 45 <.06 <.04 <.8 .177 .5 <.08 E.2 .8

(Comal-Spring Island) <.30 45 <.06 <.04 <.8 .177 .5 <.08 E.2 1.4

<.20 52 <.06 <.04 .3 .12 E.3 E.08 <.2 .6

DX–68–23–326 <.30 53 <.06 <.04 <.8 .173 .4 <.08 <.2 .6

(Comal 5) -- -- -- -- -- -- -- -- -- --

<.20 55 <.06 <.04 .33 .118 1.8 .18 <.2 .7

DX–68–23–602 <.30 37 <.06 E.03 <.8 .252 1.8 1.45 <.2 .6

 -- -- -- -- -- -- -- -- -- --

Central Comal flow path

AY–68–29–811 <.30 33 <.06 <.04 1.3 .148 1.1 1.39 <.2 .7

 -- -- -- -- -- -- -- -- -- --

AY–68–30–520 <.30 53 <.06 <.04 <.8 .138 .8 .96 <.2 .7

 -- -- -- -- -- -- -- -- -- --

DX–68–15–901 <.30 34 <.06 E.03 <.8 .26 .6 E.07 .2 .5

(Hueco A) -- -- -- -- -- -- -- -- -- --

<.20 33 <.06 <.04 .08 .273 .7 .14 <.2 1

DX–68–15–913 <.30 34 <.06 <.04 <.8 .26 .7 .35 .4 .5

(Hueco B) -- -- -- -- -- -- -- -- -- --

Appendix  45
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Antimony 

(μg/L)

Barium 

(μg/L)

Beryllium 

(μg/L)

Cadmium 

(μg/L)

Chromium 

(μg/L)

Cobalt 

(μg/L)

Copper 

(μg/L)

Lead 

(μg/L)

Manganese 

(μg/L)

Molybdenum 

(μg/L)

Central Comal flow path—Continued

DX–68–22–810 <0.30 30 <0.06 0.05 <0.8 0.275 1.1 5.47 0.2 0.5

 -- -- -- -- -- -- -- -- -- --

<.20 33 <.06 .09 .06 .39 2.5 3.76 .6 E.2

DX–68–22–905 <.30 27 <.06 E.02 <.8 .196 .4 1.45 <.2 .4

 -- -- -- -- -- -- -- -- -- --

<.20 29 <.06 <.04 .08 .277 2.5 .7 .2 .6

DX–68–23–301 <.30 44 <.06 <.04 <.8 .179 .6 .11 <.2 .6

(Comal 1) -- -- -- -- -- -- -- -- -- --

<.20 55 <.06 <.04 .24 .12 .4 <.08 1.1 .7

DX–68–23–323 <.30 45 <.06 <.04 <.8 .179 .3 <.08 <.2 .6

(Comal 3) -- -- -- -- -- -- -- -- -- --

<.20 50 .09 <.04 .27 .113 .5 .32 <.2 .6

DX–68–23–501 <.30 38 <.06 <.04 E.4 .248 3.2 .79 <.2 .6

 -- -- -- -- -- -- -- -- -- --

DX–68–23–710 <.30 38 <.06 E.04 <.8 .247 1.4 1.3 <.2 E.2

 -- -- -- -- -- -- -- -- -- --

<.20 36 <.06 <.04 .11 .414 3.3 .31 <.2 <.4

<.20 37 <.06 <.04 .11 .437 3.2 .29 <.2 <.4

DX–68–30–111 <.30 37 <.06 E.02 <.8 .283 1.3 .38 3.8 .9

 -- -- -- -- -- -- -- -- -- --

Northern Comal flow path

DX–68–23–104 <.30 58 <.06 .07 <.8 .087 1.6 1.8 <.2 2.7

 -- -- -- -- -- -- -- -- -- --

Rainfall sites

Rainfall site 3 -- -- -- -- -- -- -- -- -- --

 -- -- -- -- -- -- -- -- -- --

Rainfall site 4 -- -- -- -- -- -- -- -- -- --

46  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Nickel 

(μg/L)

Silver 

(μg/L)

Zinc 

(μg/L)

Uranium 

(μg/L)

Sulfur 

hexafluoride 

(pptv)

δD

(per mil)

δ18O 

(per mil)

3

H

(TU)

3

He,

tritiogenic

(TU)

Southern Comal flow path

AY–68–29–610 3.01 <0.2 3.2 0.8 4.08 -23.09 -4.25 2.45 0

 -- -- -- -- 3.98 -- -- -- --

AY–68–29–913 2.07 <.2 2.3 .75 3.9 -22.83 -4.03 2.23 -23.53

 -- -- -- -- 3.52 -- -- -- --

AY–68–29–914 2.01 <.2 2.8 .69 1.57 -24.03 -4.22 1.99 -32.14

 -- -- -- -- 1.77 -- -- -- --

DX–68–23–304 1.47 <.2 6.8 .84 5.08 -24.14 -4.09 1.63 -57.05

(NBU–LCRA) -- -- -- -- 4.13 -- -- -- --

1.68 <.2 1.2 .78 -- -23.83 -4.14 1.22 5.04

DX–68–23–324 2.59 <.2 E.7 .77 5.36 -23.31 -4.08 1.85 -38.23

(Comal 7) -- -- -- -- 5.49 -- -- -- --

1.53 <.2 4.5 .77 -- -22.67 -4.12 1.35 12.24

DX–68–23–325 2.47 <.2 <1.0 .82 4.29 -23.02 -4.23 2.13 -26.43

(Comal-Spring Island) 2.46 <.2 <1.0 .81 4.18 -- -- -- --

1.52 <.2 2 .74 -- -24.27 -4.13 1.26 5.67

DX–68–23–326 2.64 <.2 E.5 .78 4.66 -23.27 -4.05 2.03 -61.35

(Comal 5) -- -- -- -- 4.51 -- -- -- --

1.6 <.2 3.9 .76 -- -23.52 -4.15 1.33 7.57

DX–68–23–602 2.1 <.2 4 .68 2.98 -23.16 -4.14 2.21 .40

 -- -- -- -- 3.82 -- -- -- --

Central Comal flow path

AY–68–29–811 2.35 <.2 3 .81 15.69 -20.89 -3.92 2.13 -3.38

 -- -- -- -- 7.69 -- -- -- --

AY–68–30–520 2.34 <.2 1.6 .78 6.33 -23.2 -4.12 1.93 -28.98

 -- -- -- -- 6.31 -- -- -- --

DX–68–15–901 5.22 <.2 2.1 .88 3.7 -25.87 -4.54 1.98 -6.06

(Hueco A) -- -- -- -- 3.47 -- -- -- --

2.83 <.2 2.7 .93 -- -22.95 -3.96 1.53 -3.93

DX–68–15–913 4.89 <.2 3.1 .88 3.81 -24.46 -4.46 2.14 -6.03

(Hueco B) -- -- -- -- 3.87 -- -- -- --

Appendix  47
Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, 

northeastern Bexar and southern Comal Counties, Texas, 2003–06—Continued.

State well

number

(USGS name)

Nickel 

(μg/L)

Silver 

(μg/L)

Zinc 

(μg/L)

Uranium 

(μg/L)

Sulfur 

hexafluoride 

(pptv)

δD

(per mil)

δ18O 

(per mil)

3

H

(TU)

3

He,

tritiogenic

(TU)

Central Comal flow path—Continued

DX–68–22–810 2.92 <0.2 642 0.65 1.96 -23.82 -4.28 2.30 -2.67

 -- -- -- -- 1.77 -- -- -- --

.28 <.2 796 .6 -- -23.2 -4.29 1.42 -2.64

DX–68–22–905 1.78 <.2 1.7 .63 2.88 -24.26 -4.18 2.21 .92

 -- -- -- -- 2.75 -- -- -- --

2.77 <.2 4.3 .69 -- -22.83 -4.33 1.60 -.08

DX–68–23–301 2.79 <.2 E.6 .74 4.33 -24.22 -4.13 2.03 -23.80

(Comal 1) -- -- -- -- 4.32 -- -- -- --

3.47 <.2 20.6 .84 -- -24.12 -4.16 1.25 4.84

DX–68–23–323 2.68 <.2 <1.0 .76 4.19 -23.36 -4.2 1.12 -49.90

(Comal 3) -- -- -- -- 4.19 -- -- -- --

1.42 <.2 1.5 .78 -- -23.22 -4.13 1.23 5.30

DX–68–23–501 2.07 <.2 10.6 .72 6.08 -22.66 -4.14 2.25 -14.22

 -- -- -- -- 4.45 -- -- -- --

DX–68–23–710 2.11 <.2 1.7 .71 2.42 -25.3 -4.5 .34 .73

 -- -- -- -- 2.07 -- -- -- --

3.95 <.2 5 .61 -- -25.14 -4.58 1.23 .02

2.98 <.2 4.9 .61 -- -- -- -- --

DX–68–30–111 4.4 <.2 375 .78 4.64 -24.63 -4.2 2.15 -.01

 -- -- -- -- 5.16 -- -- -- --

Northern Comal flow path

DX–68–23–104 1.51 <.2 21.4 1.02 1.16 -22.44 -3.86 .00 -15.28

 -- -- -- -- 1.14 -- -- -- --

Rainfall sites

Rainfall site 3 -- -- -- -- -- -35.76 -4.92 -- --

 -- -- -- -- -- -38.04 -6.02 -- --

Rainfall site 4 -- -- -- -- -- -41.7 -6.1 -- --

48  Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas
Prepared by the USGS Lafayette Publishing Service Center.

Information regarding water resources in Texas is available at 

http: //tx.usgs.gov/