U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5122

In cooperation with the San Antonio Water System

Lithologic and Physicochemical Properties and Hydraulics 

of Flow in and near the Freshwater/Saline-Water Transition 

Zone, San Antonio Segment of the Edwards Aquifer, 

South-Central Texas, Based on Water-Level and Borehole 

Geophysical Log Data, 1999–2007

500

600

700

800

900

1,000

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine

members

Leached and

collapsed

members

Regional dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone
Front cover:

Top, Graph showing daily mean equivalent freshwater head and borehole geophysical data in 

Tri-County 2 well (KX–68–21–403), San Antonio segment of the Edwards aquifer, south-central Texas, 

2003–07.

Bottom, San Antonio Water System drilling equipment used to drill Pitluk well, Bexar County, Texas 

(photograph by David Mahula, San Antonio Water System).

Back cover:

Top, U.S. Geological Survey pump hoist rig and logging truck collecting pumped and ambient 

flowmeter data from Kyle 2 well (LR–67–02–104), July 14, 2010 (photograph by R.B. Lambert, U.S. 

Geological Survey).

Middle, Monitor well C1 (AY–68–37–524) near the freshwater/saline-water transition zone in central 

Bexar County, Texas, being purged prior to sample collection, July 12, 2002 (photograph by R.B. Lambert, 

U.S. Geological Survey). 

Bottom, Closeup of U.S. Geological Survey pump hoist rig and Kyle 2 well (LR–67–02–104) during 

pumped flowmeter test, July 14, 2010 (photograph by R.B. Lambert, U.S. Geological Survey). 
Lithologic and Physicochemical Properties 

and Hydraulics of Flow in and near the 

Freshwater/Saline-Water Transition Zone, 

San Antonio Segment of the Edwards 

Aquifer, South-Central Texas, Based on 

Water-Level and Borehole Geophysical 

Log Data, 1999–2007 

By Rebecca B. Lambert, Andrew G. Hunt, Gregory P. Stanton, and 

Michael B. Nyman

In cooperation with the San Antonio Water System

Scientific Investigations Report 2010–5122

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

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

1-888-ASK-USGS

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

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

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

hydraulics of flow in and near the freshwater/saline-water transition zone, San Antonio segment of the Edwards 

aquifer, south-central Texas, based on water-level and borehole geophysical log data, 1999–2007: U.S. Geological 

Survey Scientific Investigations Report 2010–5122, 69 p. (Appendixes available online at http://pubs.usgs.gov/

sir/2010/5122/.)
iii

Acknowledgments

The authors gratefully acknowledge John Waugh, San Antonio Water System, who 

for many years has provided leadership, vision, and opportunities that have resulted in a 

better understanding of the Edwards aquifer and particularly the transition zone. The authors 

also acknowledge Denise Renaghan, formerly San Antonio Water System, for providing 

drilling information and water-level data, and David Mahula, San Antonio Water System, for 

providing water-level data and for helping with logistics to ensure that the monitoring wells 

were accessible for borehole geophysical logging. Alvin Schultz is acknowledged for his 

geologic insight and assistance in providing useful geologic information for constructing the 

hydrogeologic sections of the transects. Finally, the authors thank the property owners and 

ranchers who graciously provided access to their property and assistance in data collection 

during nearly a decade.
Blank Page
v

Contents

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

Introduction ....................................................................................................................................................2

Purpose and Scope .............................................................................................................................2

Previous Studies ..................................................................................................................................4

Hydrogeologic Setting ........................................................................................................................5

Description of Transects and Monitoring Wells .............................................................................8

Methods of Analysis .....................................................................................................................................8

Water-Level Data—Computation of Equivalent Freshwater Heads .........................................14

Borehole Geophysical Log Data ......................................................................................................16

Lithologic and Physicochemical Properties ...........................................................................................20

Lithologic Properties .........................................................................................................................20

East Uvalde Transect ................................................................................................................20

Tri-County Transect ..................................................................................................................20

Fish Hatchery Transect ............................................................................................................24

Kyle Transect .............................................................................................................................27

Physicochemical Properties ............................................................................................................29

Fluid Conductivity ......................................................................................................................35

East Uvalde Transect .......................................................................................................35

Tri-County Transect .........................................................................................................35

Fish Hatchery Transect ...................................................................................................36

Kyle Transect ....................................................................................................................37

Temperature ...............................................................................................................................37

East Uvalde Transect .......................................................................................................37

Tri-County Transect .........................................................................................................38

Fish Hatchery Transect ...................................................................................................38

Kyle Transect ....................................................................................................................38

Hydraulics of Flow ......................................................................................................................................39

Lateral Head Gradients at the Interface ........................................................................................39

East Uvalde Transect ................................................................................................................39

Tri-County Transect ..................................................................................................................40

Kyle Transect .............................................................................................................................41

Vertical Flow in the Boreholes .........................................................................................................45

East Uvalde Transect ................................................................................................................45

Tri-County Transect ..................................................................................................................48

Fish Hatchery Transect ............................................................................................................49

Kyle Transect .............................................................................................................................49

Relative Groundwater Flow Based on Temperature ....................................................................53

Relation Between the Transition Zone and the Freshwater Zone .............................................61

Summary .......................................................................................................................................................64

References ...................................................................................................................................................67
vi

Appendixes (available online at http://pubs.usgs.gov/sir/2010/5122/)

1.1. Data for computation of equivalent freshwater head in East Uvalde 1 

(YP–69–52–202), 2000–2007 

1.2. Data for computation of equivalent freshwater head in East Uvalde 2 

(YP–69–44–902), 1999–2007 

1.3. Data for computation of equivalent freshwater head in East Uvalde 3 

(YP–69–51–606), 2000–2007 

1.4. Data for computation of equivalent freshwater head in East Uvalde 4 

(YP–69–52–404), 2000–02 

1.5. Data for computation of equivalent freshwater head in Tri-County 1 

(KX–68–30–314), 2000–2007 

1.6. Data for computation of equivalent freshwater head in Tri-County 2 

(KX–68–31–403), 2000–2007 

1.7. Data for computation of equivalent freshwater head in Tri-County 3 

(KX–68–31–511), 2000–02 

1.8. Data for computation of equivalent freshwater head in Tri-County 4 

(KX–68–31–808), 2002–07 

1.9. Data for computation of equivalent freshwater head in Tri-County 5 

(KX–68–30–315), 2001–07 

1.10. Data for computation of equivalent freshwater head in Fish Hatchery 1 

(LR–67–09–113), 2001–07 

1.11. Data for computation of equivalent freshwater head in Fish Hatchery 2 

(LR–67–09–401), 2001–07 

1.12. Data for computation of equivalent freshwater head in Kyle 1 (LR–67–01–311), 

2000–2007 

1.13. Data for computation of equivalent freshwater head in Kyle 2 (LR–67–02–104), 

2000–2007 

1.14. Data for computation of equivalent freshwater head in Kyle 3 (LR–67–02–106), 

1999–2007 

1.15. Data for computation of equivalent freshwater head in Kyle 4 (LR–67–02–105), 

1999–2007 

Figures

1. Map showing areal extent of the freshwater/saline-water transition zone of the 

San Antonio segment of the Edwards aquifer, south-central Texas, and location of 

monitoring wells in and near the transition zone from which data were collected 

for this report ................................................................................................................................3

2. Map showing structural elements and depositional provinces associated with the 

San Antonio segment of the Edwards aquifer, south-central Texas ...................................6

3. Chart showing summary of Cretaceous-age stratigraphic units and associated 

hydrogeologic units, and relative permeabilities, of the San Antonio segment of the 

Edwards aquifer, south-central Texas ......................................................................................7

4. Map showing major storage and flow units and regional flow patterns in the San 

Antonio segment of the Edwards aquifer, south-central Texas ...........................................9

5–8. Hydrogeologic section of the:

5. East Uvalde transect (A–A’ ), San Antonio segment of the Edwards aquifer, 

south-central Texas ..........................................................................................................10

6. Tri-County transect (B–B’ ), San Antonio segment of the Edwards aquifer, 

south-central Texas ..........................................................................................................11
vii

7. San Marcos Fish Hatchery transect (C–C’ ), San Antonio segment of the 

Edwards aquifer, south-central Texas ...........................................................................12

8. Kyle transect (D–D’ ), San Antonio segment of the Edwards aquifer, southcentral Texas .....................................................................................................................13

9–26. Graphs showing: 

9. Monthly departure from normal (1971–2000) rainfall, National Weather 

Service station 417945/12921, San Antonio International Airport, Texas, 

1999–2007 ...........................................................................................................................15

10. Daily mean equivalent freshwater head and borehole geophysical data in 

East Uvalde 1 well (YP–69–52–202), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2002–07 ...........................................................................21

11. Daily mean equivalent freshwater head and borehole geophysical data in 

East Uvalde 2 well (YP–69–44–902), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2002–07 ...........................................................................22

12. Daily mean equivalent freshwater head and borehole geophysical data in 

East Uvalde 3 well (YP–69–51–606), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2002–07 ...........................................................................23

13. Borehole geophysical data in East Uvalde 4 well (YP–69–52–404), San 

Antonio segment of the Edwards aquifer, south-central Texas, 2002–07 ...............24

14. Daily mean equivalent freshwater head and borehole geophysical data in 

Tri-County 1 well (KX–68–30–314), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2003–07 ...........................................................................25

15. Daily mean equivalent freshwater head and borehole geophysical data in 

Tri-County 2 well (KX–68–31–403), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2003–07 ...........................................................................26

16. Borehole geophysical data in Tri-County 3 well (KX–68–31–511), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2003–06 ...............................27

17. Daily mean equivalent freshwater head and borehole geophysical data in 

Tri-County 4 well (KX–68–31–808), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2003–07 ...........................................................................28

18. Daily mean equivalent freshwater head and borehole geophysical data in 

Fish Hatchery 1 well (LR–67–09–113), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2002–07 ...........................................................................29

19. Daily mean equivalent freshwater head and borehole geophysical data in 

Fish Hatchery 2 well (LR–67–09–401), San Antonio segment of the Edwards 

aquifer, south-central Texas, 2003–07 ...........................................................................30

20. Daily mean equivalent freshwater head and borehole geophysical data in 

Kyle 1 well (LR–67–01–311), San Antonio segment of the Edwards aquifer, 

south-central Texas, 2002–07 .........................................................................................31

21. Daily mean equivalent freshwater head and borehole geophysical data in 

Kyle 2 well (LR–67–02–104), San Antonio segment of the Edwards aquifer, 

south-central Texas, 2003–07 .........................................................................................32

22. Daily mean equivalent freshwater head and borehole geophysical data in 

Kyle 3 well (LR–67–02–106), San Antonio segment of the Edwards aquifer, 

south-central Texas, 2002–07 .........................................................................................33

23. Daily mean equivalent freshwater head and borehole geophysical data in 

Kyle 4 well (LR–67–02–105), San Antonio segment of the Edwards aquifer, 

south-central Texas, 2002–07 .........................................................................................34

24. Daily mean equivalent freshwater head in monitoring wells of the East 

Uvalde transect, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1999–2007 ...............................................................................................................40
viii

25. Daily mean equivalent freshwater head in monitoring wells of the Tri-County 

transect, San Antonio segment of the Edwards aquifer, south-central Texas, 

2000–2007 ...........................................................................................................................42

26. Daily mean equivalent freshwater head in monitoring wells of the Kyle 

transect, San Antonio segment of the Edwards aquifer, south-central Texas, 

1999–2007 ...........................................................................................................................42

27. Conceptual diagram showing vertical flow in monitoring wells in the East Uvalde 

transect (A–A’ ), San Antonio segment of the Edwards aquifer, south-central Texas, 

2005–07 ........................................................................................................................................46

28. Conceptual diagram showing vertical flow in monitoring wells in the Tri-County 

transect (B–B’ ), San Antonio segment of the Edwards aquifer, south-central 

Texas, 2005–07 ............................................................................................................................50

29. Graph showing daily mean equivalent freshwater head in monitoring wells of the 

Fish Hatchery transect, San Antonio segment of the Edwards aquifer, south-central 

Texas, 2001–07 ............................................................................................................................52

30. Conceptual diagram showing vertical flow in monitoring wells in the Fish Hatchery 

transect (C–C’ ), San Antonio segment of the Edwards aquifer, south-central Texas, 

2005–07 ........................................................................................................................................54

31. Conceptual diagram showing vertical flow in monitoring wells in the Kyle 

transect (D–D’ ), San Antonio segment of the Edwards aquifer, south-central 

Texas, 2005–07 ............................................................................................................................56

32. Graph showing daily mean equivalent freshwater head in monitoring wells of the 

East Uvalde, Tri-County, Fish Hatchery, and Kyle transects, and index wells of 

Uvalde County, Medina County, and Bexar County in the freshwater zone and 

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

Edwards aquifer, south-central Texas, 1999–2007 ...............................................................62

33. Graphs showing specific conductance and equivalent freshwater head over time 

in wells (A) Tri-County 1 (2000–2007) and (B) Kyle 2 (2003–07) ...........................................63

Tables

1. Descriptive information for monitoring wells in and near the freshwater/salinewater transition zone of the San Antonio segment of the Edwards aquifer, southcentral Texas, from which data were collected for this report, 1999–2007 .......................4

2. Data from 15 monitoring wells transecting the freshwater/saline-water interface, 

San Antonio segment of the Edwards aquifer, south-central Texas, 2000–2007, 

used to compute average equivalent freshwater head ......................................................17

3. Summary of average daily mean equivalent freshwater heads and directions of 

lateral head gradients for coincident periods of record for East Uvalde monitoring 

wells transecting the freshwater/saline-water interface, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2000–2007 .........................................................41

4. Summary of average daily mean equivalent freshwater heads and directions of 

lateral head gradients for coincident periods of record for Tri-County monitoring 

wells transecting the freshwater/saline-water interface, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2000–2007 .........................................................43

5. Summary of average daily mean equivalent freshwater heads and directions of 

lateral head gradients for coincident periods of record for Kyle monitoring wells 

transecting the freshwater/saline-water interface, San Antonio segment of the 

Edwards aquifer, south-central Texas, 2000–07 ...................................................................44
ix

6. Temperature gradients in monitoring wells transecting the freshwater/salinewater transition zone, San Antonio segment of the Edwards aquifer, south-central 

Texas, 2002–07 ............................................................................................................................58

Conversion Factors, Datums, and Water-Quality Units

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

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

Hydraulic gradient

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

SI to Inch/Pound

Multiply By To obtain

Density

gram per cubic centimeter (g/cm3) 62.4220 pound per cubic foot (lb/ft3)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Datums

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

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27) 

and North American Datum of 1983 (NAD 83).

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

Water-Quality Units

Concentrations are reported in metric units. Chemical concentrations are reported in milligrams 

per liter (mg/L), units expressing the concentration of chemical constituents in solution as 

weight of solute (milligrams) per unit volume (liter) of water. 

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

The freshwater zone of the San Antonio segment of 

the Edwards aquifer in south-central Texas (hereinafter, the 

Edwards aquifer) is bounded to the south and southeast by a 

zone of transition from freshwater to saline water (hereinafter, 

the transition zone). The boundary between the two zones is 

the freshwater/saline-water interface (hereinafter, the interface), defined as the 1,000-milligrams per liter dissolved solids 

concentration threshold. This report presents the findings of a 

study, done by the U.S. Geological Survey in cooperation with 

the San Antonio Water System, to obtain lithologic properties 

(rock properties associated with known stratigraphic units) and 

physicochemical properties (fluid conductivity and temperature) and to analyze the hydraulics of flow in and near the transition zone of the Edwards aquifer on the basis of water-level 

and borehole geophysical log data collected from 15 monitoring wells in four transects during 1999–2007. No identifiable 

relation between conductivity values from geophysical logs 

in monitoring wells in all transects and equivalent freshwater 

heads in the wells at the times the logs were run is evident; 

and no identifiable relation between conductivity values and 

vertical flow in the boreholes concurrent with the times the 

logs were run is evident. The direction of the lateral equivalent 

freshwater head gradient and thus the potential lateral flow at 

the interface in the vicinity of the East Uvalde transect fluctuates between into and out of the freshwater zone, depending on 

recharge and withdrawals. Whether the prevailing direction on 

average is into or out of the freshwater zone is not clearly indicated. Equivalent freshwater head data do not indicate a prevailing direction of the lateral gradient at the interface in the 

vicinity of the Tri-County transect. The prevailing direction on 

average of the lateral gradient and thus potential lateral flow at 

the interface in the vicinity of the Kyle transect likely is from 

the transition zone into the freshwater zone. The hypothesis 

regarding the vertical gradient at the East Uvalde transect, and 

thus the potential for vertical flow near an interface conceptualized as a surface sloping upward in the direction of the 

dip of the stratigraphic units, is that the potential for vertical 

flow fluctuates between into and out of the freshwater zone, 

depending on recharge and withdrawals. At the Tri-County 

transect, a downward gradient on the freshwater side of the 

interface and an upward gradient on the saline-water side are 

evidence of opposing potentials that appear to have stabilized 

the position of the interface over the range of hydrologic 

conditions that occurred at the times the logs were run. At the 

Fish Hatchery transect, an upward gradient on the saline-water 

side of the interface, coupled with the assumption of a sloping 

interface, implies a vertical gradient from the transition zone 

into the freshwater zone. This potential for vertical movement 

of the interface apparently was opposed by the potential (head) 

on the freshwater side of the interface that kept the interface 

relatively stable over the range of hydrologic conditions during 

which the logs were run. The five flow logs for Kyle transect 

freshwater well KY1 all indicate upward flow that originates 

from the Glen Rose Limestone, the uppermost unit of the 

Trinity aquifer; and one log for well KY2 shows upward flow 

entering the borehole from the Trinity aquifer. These flow data 

constitute evidence of the potential for flow from the Trinity 

aquifer into the Edwards aquifer in the vicinity of the Kyle 

transect. Subsurface temperature data indicate that flow on 

average is more active, or vigorous, on the freshwater side of 

the interface than on the saline-water side. A hydraulic connection between the transition zone and the freshwater zone is 

indicated by similar patterns in the hydrographs of the 15 transect monitoring wells in and near the transition zone and three 

county index wells in the freshwater zone during 1999–2007. 

The data for this report in part support a conceptualization of 

regional flow in and near the transition zone in which water 

enters the transition zone from the freshwater zone in the western part of the Edwards aquifer, flows toward the northeast, 

and discharges to the freshwater zone in the eastern part of the 

aquifer. The data for this report support the hypothesis that the 

interface is likely to remain stable laterally and vertically over 

time. The most direct evidence in support of that hypothesis 

is the conductivity data from Tri-County transect well TC2 

Lithologic and Physicochemical Properties and Hydraulics 

of Flow in and near the Freshwater/Saline-Water 

Transition Zone, San Antonio Segment of the Edwards 

Aquifer, South-Central Texas, Based on Water-Level and 

Borehole Geophysical Log Data, 1999–2007

By Rebecca B. Lambert, Andrew G. Hunt, Gregory P. Stanton, and Michael B. Nyman
2 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

and Kyle transect well KY2 that show an essentially stable 

vertical interface position (depth in the borehole) over a range 

of hydrologic conditions. No single line of evidence confirms 

the hypothesis that the interface in the San Antonio segment 

of the Edwards aquifer is relatively stable over time, and thus 

the potential for irreversible movement of saline water into the 

freshwater zone is small; but the cumulative evidence from the 

water-level (head) and borehole geophysical log data supports 

that hypothesis.

Introduction

The San Antonio segment of the Edwards aquifer (hereinafter, the Edwards aquifer), is the primary source of water 

supply in south-central Texas. In addition to withdrawals for 

public supply, the Edwards aquifer provides water for irrigation, military, industrial, commercial, and domestic uses in San 

Antonio, Tex., and other communities in the region. The freshwater zone of the aquifer is bounded to the south and southeast 

by a zone of transition from freshwater to saline water (fig. 1). 

For this report, freshwater is defined as that containing less 

than 1,000 milligrams per liter (mg/L) dissolved solids concentration; slightly saline water, that containing 1,000 to 3,000 

mg/L dissolved solids concentration; moderately saline water, 

that containing 3,000 to 10,000 mg/L dissolved solids concentration; and very saline water, that containing 10,000 to 35,000 

mg/L dissolved solids concentration (Winslow and Kister, 

1956). The freshwater/saline-water interface (hereinafter, the 

interface) is the 1,000-mg/L dissolved solids concentration 

threshold. The freshwater/saline-water transition zone of the 

Edwards aquifer (hereinafter, the transition zone) is defined as 

the region of the aquifer with dissolved solids concentrations 

ranging from 1,000 to 10,000 mg/L (Schultz, 1994). The transition zone primarily contains slightly and moderately saline 

water, although very saline water is present in a few locations. 

In 1985, the U.S. Geological Survey (USGS), the San 

Antonio Water System (SAWS), and other Federal, State, and 

local agencies began a series of studies to learn more about the 

interaction between the freshwater and transition zones of the 

Edwards aquifer, and in particular, the potential for irreversible movement of saline water into the freshwater zone of 

the aquifer. The primary objective of the study reported here, 

done by the USGS in cooperation with SAWS, was to obtain 

lithologic properties (rock properties associated with known 

stratigraphic units) and physicochemical properties (fluid conductivity and temperature in this report) of the aquifer and to 

analyze the hydraulics of flow in and near the transition zone 

using water-level and borehole geophysical log data collected 

during 1999–2007 and, on the basis of findings, to assess the 

potential for irreversible movement of saline water into the 

freshwater zone of the aquifer. The study included drilling of 

additional monitoring wells by SAWS in transects across the 

interface. Monitoring wells from which data were collected for 

this report are in and near the transition zone in Uvalde County 

(East Uvalde transect, four wells), in Comal and Guadalupe 

Counties (Tri-County transect, five wells), and in Hays County 

(Fish Hatchery transect [two wells] and Kyle transect [four 

wells]) (fig. 1). Water-level, borehole geophysical log, and 

water-quality data were collected from selected monitoring 

wells in the four transects during various periods from October 

1999 through December 2007 (table 1).

Purpose and Scope

This report presents the findings of a study to obtain 

lithologic properties (rock properties associated with known 

stratigraphic units) and physicochemical properties (fluid 

conductivity and temperature) and to analyze the hydraulics 

of flow in and near the transition zone of the Edwards aquifer 

on the basis of water-level and borehole geophysical log data 

collected from 15 monitoring wells in four transects during 

1999–2007; and on the basis of findings, assesses the potential 

for irreversible movement of saline water into the freshwater 

zone of the Edwards aquifer. Water-level data were collected 

to obtain head distribution over time as an indicator of hydrologic conditions. Daily mean water levels from continuously 

measured hydraulic heads in monitoring wells of the East 

Uvalde transect, Tri-County transect, Fish Hatchery transect, 

and Kyle transect were converted to equivalent freshwater 

heads to account for differences in salinity of water in some 

wells. Conversion to equivalent freshwater heads was done so 

that accurate lateral head gradients between the transition zone 

and the freshwater zone could be computed. 

Geophysical log data were collected using conventional 

methods that include caliper, gamma, resistivity, induction, 

fluid conductivity, and fluid temperature, and using advanced 

methods that include borehole televiewer imaging tools and 

electromagnetic and heat-pulse vertical flowmeters. These 

tools were used to obtain lithologic properties from which 

stratigraphic units of the aquifer could be identified (the tops 

of the stratigraphic units penetrated were picked from drillers’ logs but were refined by the geophysical logs) and to 

obtain vertical flow (magnitude and direction under ambient 

conditions) relative to the stratigraphic units in each well. The 

directions of vertical flow indicated the directions of vertical 

gradients at each well. Spatial and temporal variation in fluid 

conductivity and temperature as measured from borehole geophysical logs is described with regard to hydrologic conditions 

where applicable. 

Water-quality data (discrete samples and field-measured 

properties) were collected from monitoring wells in the 

transition zone to characterize selected chemical properties 

and constituents of the water and changes in some of those 

properties and constituents over time. All the data for this 

report (except some data for computation of equivalent freshwater heads, which are contained in appendixes of this report) 

are in Lambert and others (2009), which also summarizes 

data-collection techniques, water-quality sampling methods, 

analytical methods, and quality assurance. 
Introduction

3

Figure 1. Areal extent of the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, and location of monitoring wells in and near the transition zone from which data were collected for this report (modified from Lambert and others, 2009, fig. 1). 

3,000 mg/L

10,000 mg/L

10,000 mg/L

3,000 mg/L

1,000 mg/L

D

D

Brackettville

Kyle

100°30'

100°00'

99°30'

99°00'

98°30'

98°00'

97°30'

28°30'

29°00'

29°30'

30°00'

30°30'

BEXAR

ATASCOSA

COUNTY

WILSON COUNTY MEDINA COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY BANDERA COUNTY

COMAL COUNTY KENDALL COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO COUNTY

KINNEY COUNTY GUADALUPE COUNTY

CALDWELL

COUNTY

MAVERICK COUNTY

60 MILES 0 30 15 45

Comal Springs

San Marcos Springs

Western storage unit

Western 

Medina

storage unit

Eastern Medina

storage unit

Eastern storage unit

South-central flow unit North-central flow unit

Eastern flow unit

Western-southern flow unit

East Uvalde transect

Tri-County  transect

Fish Hatchery transect

Kyle transect

Bexar Co. Index

Uvalde Co.

Index

Medina Co. Index

Recharge zone (Ashworth and Hopkins, 1995)Freshwater zoneTransition zoneSaline-water zoneGroundwater divide (modified from Maclay and Land, 1988, fig. 1; Maclay, 1995, plate 1)Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994)Slightly to moderately saline interface (3,000 mg/L dissolved solids concentration) (Schultz, 1994)Moderately to very saline interface (10,000 mg/L dissolved solids concentration) (Schultz, 1994)Boundary separating storage and flow unitsDirection of groundwater flowSpringWell and identifier

EXPLANATION

Bexar Co. Index

D

Base modified from U.S. Geological Survey digital data, 1:250,000Universal Transverse Mercator projection, Zone 14 Datum North American Datum of 1927
4 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Hydrogeologic sections along transects are used to 

illustrate vertical flow in the boreholes relative to stratigraphic units at each well and to hydrologic conditions in the 

aquifer at the times the flow logs were run during 2005–07. 

Hydrographs of equivalent freshwater heads for 2005–07 were 

superimposed on the sections to indicate hydrologic conditions 

at the times the flow logs were run. Temperature data were 

used to interpret relative rates of groundwater flow in the transition zone and in the freshwater zone near the transition zone. 

Finally, the relation between the transition zone and the 

freshwater zone is discussed in light of the findings presented, 

with a focus on the potential for irreversible movement of 

saline water into the freshwater zone of the aquifer. Average 

lateral head gradients computed for periods during 1999–2007 

indicate whether potential lateral flow at the interface along 

each transect was into, out of, or of mixed direction with 

regard to the freshwater zone. Vertical head gradients indicated 

by measured vertical flow in boreholes offer insight into the 

potential for upward flow from deeper, generally more-saline 

parts of the flow system toward shallower, generally fresher 

parts of the flow system.

Previous Studies

Previous studies were done that characterized the 

transition zone and contributed insight into the potential for 

movement of saline water into the freshwater zone of the 

aquifer. Published results of a series of studies begun in 1985 

by the USGS and other agencies are in Pavlicek and others 

(1987), William F. Guyton and Associates, Inc. (1986, 1988), 

Poteet and others (1992), Groschen (1994), and Groschen and 

Buszka (1997). William F. Guyton and Associates, Inc. (1988) 

described an aquifer test conducted near another monitoringwell transect, the San Antonio transect (Lambert and others, 2009, fig. 1), and reports calculated transmissivities and 

storage coefficients for each of the monitoring wells in that 

transect and nearby public-supply wells. Groschen (1994) 

concluded that the flow system in the saline-water zone probably is controlled to some extent by barrier faults that tend to 

restrict southeastward flow and that water movement in the 

saline zone generally is northeastward, parallel to flow in the 

freshwater zone. Groschen and Buzka (1997) used isotopic 

and other geochemical data from 17 wells completed in the 

saline-water zone to hypothesize that the saline-water zone is 

composed of at least two distinct hydrologic and geochemical 

regimes.

Other previous studies addressed aspects of the transition 

zone, although they did not specifically focus on areas near 

the monitoring transects. Maclay (1995), in a regional analysis 

of the Edwards aquifer, compared the variation in mineralogy, rock texture, and diagenetic processes in the freshwater 

zone with those in the transition zone and saline-water zone. 

Table 1. Descriptive information for monitoring wells in and near the freshwater/saline-water transition zone of the San Antonio 

segment of the Edwards aquifer, south-central Texas, from which data were collected for this report, 1999–2007.

[LSD, land-surface datum; NAVD 88, North American Vertical Datum of 1988; Well descriptor: Freshwater, dissolved solids concentration less than 1,000 milligrams per liter; Saline water, dissolved solids concentration greater than 1,000 milligrams per liter; Interface, freshwater and saline water in stratified lenses]

U.S. Geological 

Survey site number Well name

Well 

identifier 

(fig. 1)

State well 

number

Year 

drilled

Well depth 

(feet below 

LSD)

Altitude 

of LSD 

(feet above 

NAVD 88)

Open interval

(feet below 

LSD)

Well 

descriptor 

based on 

water type

291443099325801 East Uvalde 1 EU1 YP–69–52–202 1998 1,500 874.02 985 to 1,500 Freshwater

291612099302001 East Uvalde 2 EU2 YP–69–44–902 1999 1,560 899.91 1,072 to 1,560 Freshwater

291136099375801 East Uvalde 3 EU3 YP–69–51–606 1999 1,400 877.55 768 to 1,400 Saline water

291133099363801 East Uvalde 4 EU4 YP–69–52–404 1999 1,463 867.02 950 to 1,463 Saline water

293610098152701 Tri-County 1 TC1 KX–68–30–314 1999 920 871.01 385 to 920 Freshwater

293424098134701 Tri-County 2 TC2 KX–68–31–403 1999 1,050 709.08 486 to 1,050 Interface

293245098121001 Tri-County 3 TC3 KX–68–31–511 1999 1,222 674.00 656 to 1,222 Saline water

293058098110501 Tri-County 4 TC4 KX–68–31–808 2000 1,562 648.92 1,000 to 1,562 Saline water

293632098172401 Tri-County 5 TC5 DX–68–30–315 2000 975 782.22 553 to 975 Freshwater

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 2000 280 714.73 216 to 280 Freshwater

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 2001 1,030 642.51 510 to 1,030 Saline water

295853097532901 Kyle 1 KY1 LR–67–01–311 1997 810 770.52 307 to 810 Freshwater

295858097521801 Kyle 2 KY2 LR–67–02–104 1998 975 674.32 427 to 975 Interface

295829097512601 Kyle 3 KY3 LR–67–02–106 1998 1,100 678.28 600 to 1,100 Saline water

295730097503201 Kyle 4 KY4 LR–67–02–105 1998 970 646.70 562 to 970 Saline water
Introduction 5

Schultz (1992–94) used water-quality data and borehole 

geophysical logs to better define the extent of the transition 

zone. Previous publications had shown only the estimated 

position of the interface on maps. Schultz’s work resulted in a 

more precise delineation of the interface and also the delineation of 3,000- and 10,000-mg/L dissolved solids concentration 

lines on maps. Hovorka and others (1998) explained how the 

structural framework and distribution of porosity and permeability influence the distribution and degree of salinity in the 

transition zone.

A number of previous studies have focused on water 

chemistry and the possible origin of salinity in the transition 

zone. An early geochemical study describing the regional 

variation in hydrocarbons in the rocks of the Edwards aquifer was done by Moredock and Van Siclen (1964). Clement 

(1989) and Oetting (1995) described the chemistry of the 

transition zone using geochemical methods. Oetting (1995) 

studied the evolution of freshwater and saline water in the 

Edwards aquifer and focused on the geochemical and isotopic 

constraints on fluid-rock processes and fluid mixing. Schultz 

and Halty (1997) discussed the dissolution of anhydrite by 

freshwater movement as a principal source of high sulfate 

concentrations in the Edwards aquifer on the basis of geophysical log analysis. More recently, a statistical analysis 

of historical major ion and trace element data from the San 

Antonio transect (Lambert and others, 2009, fig. 1) was done 

by Mahler (2008). The statistical analyses indicate, among 

other things, that although the transition-zone wells are less 

well connected to surficial hydrologic conditions than the 

freshwater-zone wells, there is some connection but the 

response time is longer.

Hydrogeologic Setting

The San Antonio segment of the Edwards aquifer (the 

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

western groundwater divide near Brackettville in Kinney 

County to the eastern groundwater divide near Kyle in Hays 

County (fig. 1). Northeast of the eastern groundwater divide 

is the Barton Springs segment of the Edwards aquifer. From 

its outcrop (recharge zone), the Edwards aquifer dips to the 

southeast at about 300 to 400 feet per mile and becomes 

buried and confined toward the present Gulf of Mexico coast 

(Maclay, 1995). From its outcrop immediately north of the 

Edwards aquifer recharge zone, the Trinity aquifer dips to 

the southeast beneath the Edwards aquifer, thus forming the 

northern lateral boundary and the underlying boundary of the 

Edwards aquifer. Depth below land surface to the top of the 

Edwards aquifer in the transition zone ranges from about 200 

feet in the northeastern part of the study area in Hays County 

to more than 2,600 feet in southern Medina County. The average thickness of the Edwards aquifer in the transition zone is 

about 500 feet; depths and thickness are based on data from 

available drillers’ logs from the transect monitoring wells 

(John Waugh, San Antonio Water System, written commun., 

2003). 

The present-day Edwards aquifer formed along a crustal 

zone of weakness known as the Ouachita structural belt 

(Maclay, 1995) and encompasses three depositional provinces 

(fig. 2). Structurally, the transition zone is included in the 

Balcones fault zone across much of the region and is bounded 

to the southeast by the Luling fault zone in Guadalupe and 

Caldwell Counties.

The Edwards aquifer comprises Cretaceous-age carbonate rocks of varying lithologies that were deposited in three 

depositional environments, or depositional provinces (fig. 3). 

These depositional environments in part influence the hydraulic conductivity and storage properties of the aquifer. In the 

westernmost part of the study area, the rocks of the Maverick 

Basin depositional province include the basinal facies of the 

West Nueces Formation, the McKnight Formation, and the 

Salmon Peak Formation (Lozo and Smith, 1964). Dividing 

the Maverick Basin from the Devils River Trend depositional province is the Uvalde salient (fig. 2). The Uvalde 

salient is a complex structural high in Uvalde County, with 

numerous faults at the margins where the Edwards Limestone 

has been raised to the land surface (Maclay, 1995, fig. 2) 

along with local volcanic rocks and igneous intrusives. To 

the east in eastern Uvalde and Medina Counties, the Edwards 

aquifer is composed of reefal facies of the Devils River 

Limestone that were deposited in the Devils River Trend 

depositional province (Lozo and Smith, 1964). The Devils 

River Limestone grades into shallow-water carbonate platform facies and backreef facies of the San Marcos Platform 

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

depositional province includes the Edwards Group (Person 

and Kainer Formations) and the Georgetown Formation. The 

Edwards aquifer regionally is confined by the overlying Del 

Rio Clay and the underlying Glen Rose Limestone, both of 

which have, for the most part, relatively low permeability 

(Maclay, 1995).

Recharge to the Edwards aquifer primarily results from 

channel losses along streams that cross the outcrop (recharge 

zone) and from direct infiltration of rainfall in the recharge 

zone (Maclay, 1995). The direction of groundwater flow is 

controlled in part by regional faulting (Maclay and Land, 

1988). Once in the aquifer, groundwater generally moves 

downdip and then is directed by faults to the east and northeast 

toward Comal Springs and San Marcos Springs, major springs 

in the northeastern part of the aquifer (fig. 1) (Groschen, 1994; 

Maclay, 1995). 

An additional source of recharge to the Edwards aquifer 

might be groundwater inflow from the Glen Rose Limestone, 

the uppermost unit of the Trinity aquifer (fig. 3). From its outcrop immediately north of the Edwards aquifer recharge zone, 

the Trinity aquifer dips to the southeast beneath the Edwards 

aquifer, thus forming the northern lateral boundary and the 

underlying boundary of the Edwards aquifer. Maclay (1995) 

indicated that Edwards aquifer model simulations showed 

two areas of possible inflow from the Trinity aquifer to the 

Edwards aquifer—one in northeastern Medina County and the 

other in Comal County. 
6 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 2. Structural elements and depositional provinces associated with the San Antonio segment of the Edwards aquifer, south-central Texas.

ANCESTRAL GULF OF MEXICO

PRESENT–DAY

GULF OF MEXICO

STUART CITY REEF TEXAS

0 50 100 150 200 MILES

OUACHITA STRUCTURAL BELT

Modified from Barker and Ardis, 1996, fig. 6

LOCATION MAP

MAVERICK

BASIN

DEVILS RIVER TREND

BALCONES 

FAULT ZONE SAN MARCOS 

PLATFORM

100°30'

100°

99°30'

99°

98°30'

98°

97°30'

28°30'

29°

29°30'

30°

60 MILES 0 30 15 45

MAVERICK MA

 BASIN 9 30

DEVILS RIVER

 TREND

SAN MARCOS

 PLATFORM

BALCONES FAULT ZONE

LULING FAULT ZONE

Uvalde Co.

Index

Medina Co.

Index

Tri-County 

transect

Kyle transect

East Uvalde 

transect

Fish Hatchery 

transect

Bexar Co. Index

Bexar Co. Index

Transition zoneRecharge zone boundary (Ashworth and Hopkins, 1995)Groundwater divide (modified from Maclay and Land, 1988, fig. 1;

Maclay, 1995, plate 1)Fault (Ewing, 1991)Fault (Collins and Hovorka, 1997)Contact between depositional provinces (Maclay, 1995, fig. 2)Well and identifierSpring 

EXPLANATION

D

BEXAR

ATASCOSA

COUNTY

WILSON

 COUNTY

MEDINA

COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY

BANDERA COUNTY

COMAL

 COUNTY KENDALL COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO

COUNTY

KINNEY COUNTY

GUADALUPE

COUNTY

CALDWELL

COUNTY

MAVERICK COUNTY

Trinity aquifer [updip of Edwards aquifer]

Base modified from U.S. Geological Survey digital data, 1:250,000Universal Transverse Mercator projection, Zone 14North American Datum of 1927

Balcones fault zone andUvalde salient fromMaclay (1995, fig. 2)

Barton Springs segment

of the Edwards aquifer

(northeast of the groundwater divide)

D

D

Comal Springs

San Marcos Springs
Introduction 7

Figure 3. Summary of Cretaceous-age stratigraphic units and associated hydrogeologic units, and relative permeabilities, of the San 

Antonio segment of the Edwards aquifer, south-central Texas (modified from Lindgren and others, 2004, fig. 4). 

MAVERICK

BASIN

DEVILS RIVER

TREND

SAN MARCOS

PLATFORM

HYDROGEOLOGIC

UNITS

ANACACHO

LIMESTONE

ANACACHO

LIMESTONE

ANACACHO

LIMESTONE

AUSTIN CHALK AUSTIN CHALK AUSTIN CHALK

EAGLE FORD

GROUP

EAGLE FORD

GROUP

EAGLE FORD

GROUP

BUDA

LIMESTONE

BUDA

LIMESTONE

BUDA

LIMESTONE

UPPER

CONFINING

UNIT

DEL RIO CLAY DEL RIO CLAY DEL RIO CLAY

UPPER CRETACEOUS

SALMON

PEAK

FORMATION

DEVILS

RIVER

LIMESTONE

GEORGETOWN

FORMATION

LOWER CRETACEOUS

Cyclic and

marine members

(undivided)

Leached

member

Collapsed

member

Regional dense member

Grainstone

member

Dolomitic

member

Basal nodular member

Very small

Kirschberg

evaporite

member

PERSON FORMATION KAINER FORMATION

EDWARDS AQUIFER

MCKNIGHT

FORMATION

WEST NUECES

FORMATION

GLEN ROSE

LIMESTONE

GLEN ROSE

LIMESTONE

GLEN ROSE

LIMESTONE

TRINITY

AQUIFER

UPPER MEMBER

OF THE GLEN

ROSE LIMESTONE

LOWER MEMBER OF THE

GLEN ROSE

LIMESTONE

2

2

2

2

3 3

Aquifer subdivision in the San Marcos platform area4

I

II

III

IV

V

VI

VII

VIII

3 Modified from Rose (1972).

Erosional hiatus 

Very small Very small Very small

Moderate Moderate Moderate

Very small Very small Very small

Small Small Small

Very small Very small Very small

Very small

Very small

Large Small to moderate

Large

Moderate to large

Moderate to large

Very small

Moderate to large

Moderate

Large

Moderate

Moderate

Very small

Very small

Moderate

Small

Small

STRATIGRAPHIC UNITS

Descriptors "very small, small, moderate, and large" refer to

relative permeability of stratigraphic units.

1

4 Maclay and Small (1984).

EDWARDS GROUP

SYSTEM

UPPER

ZONE

MIDDLE

ZONE

DEPOSITIONAL PROVINCE

Lozo and Smith (1964). 2

Location shown in figure 2. 1

1 1
8 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Maclay and Land (1988, fig. 2) delineated four major 

storage and flow units in the freshwater zone of the Edwards 

aquifer (fig. 4). A storage unit is a zone of storage in the 

recharge zone that is unconfined and thus contains a relatively 

large fraction of the water stored in the aquifer. A storage 

unit functions independently from the remaining parts of the 

aquifer, in part because of faulting, and contributes water to a 

connected flow unit (Maclay and Land, 1988). A flow unit is 

a part of the aquifer that includes a storage unit and a zone in 

which water is transmitted from the associated storage unit to 

major points of discharge. The transition zone is adjacent to 

the southernmost storage units and flow units of the freshwater zone. Groundwater flow through the southern part of the 

freshwater zone and the transition zone of the aquifer might 

be influenced by local structural features in the region and by 

variations in hydraulic conductivity associated with differences in stratigraphic units. 

Description of Transects and Monitoring Wells

The monitoring wells that provided data for this report 

(table 1) were drilled during 1997–2001 by SAWS. Most of 

the monitoring wells were constructed with 6-inch-diameter 

steel casing extending from land surface into the upper 20 feet 

of the Edwards aquifer. The remaining vertical extent of the 

borehole was completed as open hole. Where possible, the 

open-hole section of each well was drilled through the entire 

Edwards aquifer thickness, but some wells did not penetrate 

the entire thickness because of a depth limitation of the 

drill rig. The four transects (East Uvalde, Tri-County, Fish 

Hatchery, and Kyle) (fig. 1) consist of two to five wells per 

transect and were configured approximately perpendicular to 

and across the expected trace of the interface. A well descriptor is applied to each well on the basis of water type in the 

borehole (freshwater, saline water, or interface [freshwater 

atop saline water]). 

The East Uvalde transect is in the western part of the 

study area in southeastern Uvalde County. The four wells 

of the East Uvalde transect are completed in rocks of the 

Maverick Basin depositional province and the Devils River 

Trend depositional province (fig. 2). Two of the wells, East 

Uvalde 1 (EU1) and East Uvalde 2 (EU2), are freshwater 

wells completed in the Devils River Limestone (fig. 5). The 

remaining two wells, East Uvalde 3 (EU3) and East Uvalde 4 

(EU4), are saline-water wells completed in the West Nueces, 

McKnight, and Salmon Peak Formations. Schulz (1994) 

placed EU1 in the transition zone (fig. 5, inset map). However, 

EU1 was drilled in 1998, and water-quality samples collected 

from EU1 (Lambert and others, 2009) indicate that the well is 

completed in the freshwater zone of the Edwards aquifer. Thus 

the interface based on data for this report occurs between wells 

EU1 and EU4.

The Tri-County transect is northeast of San Antonio in 

Comal and Guadalupe Counties (fig. 1). The five wells of 

the transect are completed in rocks of the Edwards Group 

and the Georgetown Formation in the San Marcos Platform 

depositional province (figs. 2, 6). Tri-County 1 (TC1) and 

Tri-County 5 (TC5) are freshwater wells based on waterquality samples collected from these wells (Lambert and 

others, 2009). Tri-County 2 (TC2), designated an interface 

well because it intersects the interface, contains freshwater in 

the upper part of the well and saline water in the bottom part 

of the well. Thus the interface based on data for this report 

occurs at about the location of well TC2. Shultz (1994) located 

TC1 in the transition zone, before TC1 was drilled in 1999. 

Tri-County 3 (TC3) and Tri-County 4 (TC4) are both salinewater wells in the transition zone. Because of a blockage in the 

casing that occurred after drilling, data collection in TC5 was 

restricted to only water levels and a partial log of the upper 

section of the borehole for fluid conductivity and temperature 

collected in 2002. No flowmeter data were collected from 

TC5. 

The Fish Hatchery transect comprises two wells and is in 

the northeastern part of the Edwards aquifer in Hays County 

(fig. 1). Both wells are completed in rocks of the Edwards 

Group and the Georgetown Formation in the San Marcos 

Platform depositional province (fig. 2). Fish Hatchery 1 (FH1) 

is a freshwater well and Fish Hatchery 2 (FH2) is a salinewater well on the basis of water-quality samples (Lambert 

and others, 2009) (fig. 7). Thus the interface based on data for 

this report occurs between wells FH1 and FH2, but both were 

located in the transition zone by Schulz (1994) (fig. 7, inset 

map). The Fish Hatchery transect is about midway between 

Comal Springs and San Marcos Springs (fig. 1).

The Kyle transect comprises four wells and is at the 

northeastern end of the San Antonio segment of the Edward 

aquifer in Hays County, northeast of Comal Springs and 

San Marcos Springs (fig. 1). The Kyle transect wells are all 

completed in rocks of the Edwards Group and the Georgetown 

Formation of the San Marcos Platform depositional province 

(figs. 2, 8). Kyle 1 (KY1) is a freshwater well, and Kyle 2 

(KY2) is an interface well that intersects the interface and contains freshwater in the upper part of the well and saline water 

in the lower part. Thus the interface based on data for this 

report occurs about at the location of well KY2. Kyle 3 (KY3) 

and Kyle 4 (KY4) are saline-water wells on the basis of water 

samples (Lambert and others, 2009). Schultz (1994) located 

KY3 in the transition zone (fig. 8, inset map). 

Methods of Analysis

Water-level data and borehole geophysical data were 

analyzed for this report. Equivalent freshwater heads were 

computed from measured water-level data (daily mean depth 

to water, termed environmental-water head) collected by the 

USGS and SAWS from February 1999 through December 

2007 for 15 wells in the East Uvalde, Tri-County, Fish 

Hatchery, and Kyle transects to define hydraulic gradients in 

the horizontal direction. Because the boreholes in the wells 

were open to the entire length of the aquifer, the computed 

equivalent freshwater heads are composite heads that are the 
Methods of Analysis

9

3,000 mg/L

10,000 mg/L

10,000 mg/L

3,000 mg/L

1,000 mg/L

D

D

Brackettville

Kyle

100°30'

100°00'

99°30'

99°00'

98°30'

98°00'

97°30'

28°30'

29°00'

29°30'

30°00'

30°30'

BEXAR

ATASCOSA

COUNTY

WILSON COUNTY MEDINA COUNTY

ZAVALA COUNTY

UVALDE COUNTY

REAL

COUNTY BANDERA COUNTY

COMAL COUNTY KENDALL COUNTY

 COUNTY

HAYS

COUNTY

TRAVIS

COUNTY

FRIO COUNTY

BLANCO COUNTY

KINNEY COUNTY GUADALUPE COUNTY

CALDWELL

COUNTY

MAVERICK COUNTY

60 MILES 0 30 15 45

Comal Springs

San Marcos Springs

Western storage unit

Western 

Medina

storage unit

Eastern Medina

storage unit

Eastern storage unit

South-central flow unit North-central flow unit

Eastern flow unit

Western-southern flow unit

East Uvalde transect

Tri-County  transect

Fish Hatchery transect

Kyle transect

Bexar Co. Index

Uvalde Co.

Index

Medina Co. Index

Recharge zone (Ashworth and Hopkins, 1995)Freshwater zoneTransition zoneSaline-water zoneGroundwater divide (modified from Maclay and Land, 1988, fig. 1; Maclay, 1995, plate 1)Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994)Slightly to moderately saline interface (3,000 mg/L dissolved solids concentration) (Schultz, 1994)Moderately to very saline interface (10,000 mg/L dissolved solids concentration) (Schultz, 1994)Boundary separating storage and flow unitsDirection of groundwater flowSpringWell and identifier

EXPLANATION

Bexar Co. Index

D

Base modified from U.S. Geological Survey digital data, 1:250,000Universal Transverse Mercator projection, Zone 14 Datum North American Datum of 1927

Figure 4. Major storage and flow units and regional flow patterns in the San Antonio segment of the Edwards aquifer, south-central Texas (modified from Maclay and Land, 1988, fig. 22). 
10 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 5. Hydrogeologic section of the East Uvalde transect (A–A’ ), San Antonio segment of the Edwards aquifer, south-central Texas. 

A

A’

!

!

! ! 69–52–404

EU4

69–51–606EU3

69–44–902EU2

69–52–202EU1

99°30' 99°35'

29°15'

29°12'30"

0 6,250 12,500 FEET

0 1 2 MILES

Freshwater zoneTransition zoneFreshwater/saline-water interface (1,000 milligrams per liter dissolved solids concentration) (Schultz, 1994)Hydrogeologic section traceFault (Ewing, 1991)Well, identifier, and State wellnumber

EXPLANATION

69–44–902EU2!

Saline water

Freshwater

69–52–404

EU4

69–51–606

EU3

69–44–902

EU2

69–52–202

EU1

A A’

Land surface

1,0009008007006005004003002001000-100-200-300-400-500-600-700

-100-200-300-400-500-600-700

0

900800700600500400300200100

1,000

ALTITUDE, IN FEET RELATIVE TO NORTH AMERICAN VERTICAL DATUM OF 1988 Well Fault (Ewing, 1991) Formation contact

—Dashed where 

inferredFreshwater/saline-water interface

Edwards 

Upper 

aquifer

Trinity 

aquifer

confining 

unit

Upper Cretaceous

System

Stratigraphic unit of the

Maverick Basin

Hydrogeologic 

unit

Lower Cretaceous Salmon Peak Formation McKnight Formation West Nueces Formation Glen Rose Limestone

Edwards 

Upper 

aquifer

confining 

unit

Upper Cretaceous

System

Stratigraphic units of the

Devils River Trend Hydrogeologic unit

Lower Cretaceous Devils River Limestone

EXPLANATION

0 6,250 9,375 12,500 FEET 3,125

0 1 1.5 2 MILES 0.5

Vertical scale greatly

exaggerated
Methods of Analysis 11

B !

!

!

!

!

San Antonio

68–31–808

TC4

68–31–511TC3

68–31–403TC2

68–30–314

TC1

98°12'30" 98°15'

29°35'

29°32'30"

0 1 1.5 2 MILES 0.5

0 5,000 7,500 10,000 FEET 2,500

Freshwater zoneTransition zoneFreshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994)Slightly to moderately saline interface (3,000 mg/L dissolved solids concentration) (Schultz, 1994)Hydrogeologic section traceFault (Ewing, 1991; Collins and Hovorka, 1997)Well, identifier, and State well number

EXPLANATION

68–31–403TC2

68–30–315

TC5

B’

-1,000

-100-200-300-400-500-600-700-800-900

900800700600500400300200100

0

-100-200-300-400-500-600-700-800-900

-1,000

900800700600500400300200100

0

B

68–31–511

TC3

68–31–403

TC2

68–30–314

TC1

68–31–808

TC4

68–30–315

TC5 B’

ALTITUDE, IN FEET RELATIVE TO NORTH AMERICAN VERTICAL DATUM OF 1988

Freshwater

Saline water

Person 

Formation

Edwards aquifer

Upper 

confining 

unit

Trinity

aquifer

Upper 

Cretaceous

System Stratigraphic unit Hydrogeologic unit

Lower Cretaceous

Kainer 

Formation

Georgetown FormationCyclic and marine membersLeached and collapsed membersRegional dense memberGrainstone memberKirschberg evaporite memberDolomitic memberBasal nodular memberGlen Rose Limestone

WellFault (Ewing, 1991; Collins and Hovorka, 1997)Formation contact

—Dashed where inferred 

Freshwater/saline-water interface

EXPLANATION

Edwards Group BEXAR

COUNTY

GUADALUPE

COUNTY

0 1 1.5 2 MILES 0.5

0 5,000 7,500 10,000 FEET 2,500

Vertical scale greatly

exaggerated

Land surface

Figure 6. Hydrogeologic section of the Tri-County transect (B–B’ ), San Antonio segment of the Edwards aquifer, south-central Texas. 
12 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 7. Hydrogeologic section of the San Marcos Fish Hatchery transect (C–C’ ), San Antonio segment of the Edwards aquifer, 

south-central Texas. 

!(

!(

!

San Marcos

98°0' 97°57'30"

29°51'

29°49'30"

67–09–401

FH2

67–09–113

FH1

C

C’

EXPLANATION

67–09–401

FH2

Freshwater zone

Transition zone

Saline-water zone

Freshwater/saline-water interface (1,000 

milligrams per liter [mg/L] dissolved solids 

concentration) (Schultz, 1994)

Slightly to moderately saline interface (3,000 mg/L

dissolved solids concentration) (Schultz, 1994)

Moderately to very saline interface (10,000 mg/L 

dissolved solids concentration) (Schultz, 1994)

Hydrogeologic section trace

Fault (Texas Natural Resources Information 

System, 2009)

Well, identifier, and State well number

0 1,750 3,500 5,250 7,000 FEET

0 0.5 0.75 1 MILE 0.25

ALTITUDE, IN FEET RELATIVE TO NORTH AMERICAN VERTICAL DATUM OF 1988

Units

undifferentiated

Saline water

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

0

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

C

C’

67–09–401

FH2

67–09–113

FH1

 Freshwater

Person 

Formation

Edwards aquifer

Upper 

confining 

unit

Trinity

aquifer

Upper 

Cretaceous

System Stratigraphic unit Hydrogeologic unit Lower Cretaceous Kainer Formation

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

Well

Fault (Texas Natural Resources Information System, 2009)

Formation contact—Dashed where inferred 

Freshwater/saline-water interface

EXPLANATION

Edwards Group

Vertical scale greatly

exaggerated

Land surface

0 1,750 3,500 5,250 7,000 FEET

0 0.25 0.5 0.75 1 MILE
Methods of Analysis 13Figure 8. Hydrogeologic section of the Kyle transect (D–D’ ), San Antonio segment of the Edwards aquifer, south-central Texas. 

D

D’

! !

!

!

67–02–105

KY4

67–02–106KY3

67–02–104KY2

67–01–311

KY1

Kyle

97°51' 97°52'30" 97°54'

29°59'

29°58'

0 3,500 5,250 7,000 FEET 1,750

0 0.5 0.75 1 MILE 0.25

Freshwater zoneTransition zoneSaline-water zoneGroundwater divide (Maclay and Land, 1998, fig. 1)Freshwater/saline-water interface (1,000 milligrams per liter [mg/L] dissolved solids concentration) (Schultz, 1994)Slightly to moderately saline interface (3,000 mg/L dissolved solids concentration) (Schultz, 1994)

EXPLANATION

Moderately to very saline interface (10,000 mg/Ldissolved solids concentration) (Schultz, 1994)Hydrogeologic section traceFault (Ewing, 1991; Collins and Hovorka, 1997)Well, identifier, and State well number 67–02–104

KY2

Freshwater

Saline water

0 0

-100-200-300-400-500

800700600500400300200100

800700600500400300200100

-100-200-300-400-500

67–02–105

KY4

67–02–106

KY3

67–02–104

KY2

67–01–311

D

KY1 D’

ALTITUDE, IN FEET RELATIVE TO NORTH AMERICAN VERTICAL DATUM OF 1988

Vertical scale greatly

exaggerated

Person 

Formation

Edwards aquifer

Upper 

confining 

unit

Trinity

aquifer

Upper 

Cretaceous

System Stratigraphic unit Hydrogeologic unit

Lower Cretaceous

Kainer 

Formation

Georgetown FormationCyclic and marine membersLeached and collapsed membersRegional dense memberGrainstone memberKirschberg evaporite memberDolomitic memberBasal nodular memberGlen Rose Limestone

WellFault (Ewing, 1991; Collins and Hovorka, 1997)Formation contact

—Dashed where inferred 

Freshwater/saline-water interface

EXPLANATION

Edwards Group

0 3,500 5,250 7,000 FEET 1,750

0 0.5 0.75 1 MILE 0.25

Land surface
14 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

transmissivity-weighted average of multiple flow zones in a 

single borehole. The measured water-level data used in the 

computation of equivalent freshwater heads are in appendixes 1.1–1.15 of this report and in Lambert and others (2009). 

Borehole geophysical logging techniques can be useful 

for providing information on the heterogeneity in an aquifer 

as well as providing multiple, independent measurements that 

can be used to separate such formation properties as lithology, 

mineralogy, porosity, fracture distribution, and water salinity 

(Paillet, 1994). Borehole geophysical logs provide measurements that are collected from rocks saturated with water under 

natural stress conditions where measurements can be collected 

without missing or disturbing the samples (Paillet, 1994). For 

this report, borehole geophysical logs were used to identify 

stratigraphic units penetrated by the boreholes and graphically 

relate stratigraphic units, fluid conductivity, fluid temperature, 

and vertical flow over the lengths of the boreholes of each of 

the monitoring wells at several different times and hydrologic 

conditions. Here, “identify stratigraphic units penetrated” 

means that the tops of the stratigraphic units penetrated were 

picked from drillers’ logs but were refined by the geophysical 

logs. The stratigraphic units penetrated and water in the boreholes were characterized using an integrated analysis of the 

natural gamma, caliper, acoustic-televiewer, fluid conductivity (recorded as fluid resistivity), temperature, and flowmeter 

logs. The individual logs are presented in the form of composite graphs showing the change in each of the logs with depth 

relative to the stratigraphic units penetrated. 

Water-Level Data—Computation of Equivalent 

Freshwater Heads

Equivalent freshwater heads define hydraulic gradients in 

the horizontal direction, and environmental-water heads define 

hydraulic gradients in the vertical direction (Lusczynski, 

1961). Thus, equivalent freshwater heads are needed to 

compute lateral (horizontal) head gradients along transects to 

indicate whether there is a potential for lateral flow of saline 

water into the freshwater zone of the aquifer. A necessary 

assumption made in using lateral head gradients based on 

equivalent freshwater heads to infer flow direction is that the 

slope of the Edwards aquifer is small enough for the aquifer to 

be considered horizontal. Using equivalent freshwater heads 

to infer groundwater flow in structurally sloping aquifers 

containing water of variable density can lead to errors (Davies, 

1987). In the case of the Edwards aquifer, normal faulting has 

resulted in fault blocks that are offset, which might result in 

slopes large enough to call into question the assumption of a 

horizontal aquifer; nevertheless, the judgment was made that 

equivalent freshwater heads provide the best data from which 

to compute lateral gradients for this report. 

Hydrologic conditions, as indicated by departure from 

normal (1971–2000) rainfall at the National Weather Service 

Station at the San Antonio International Airport (fig. 9), varied 

substantially during the study period (National Climatic Data 

Center, 2008). On average, conditions for the 9-year period 

1999–2007 were essentially normal, and similar variability 

relative to normal conditions likely characterized water levels 

in the aquifer during the period. Average annual rainfall 

for 1999–2007 was 32.68 inches per year, slightly less than 

the 1971–2000 average of 32.92 inches per year (National 

Weather Service, 2010). The years 1999 (16.41 inches), 2003 

(28.45 inches), 2005 (16.54 inches), and 2006 (21.34) were 

drier than normal; and the years 2000 (35.85 inches), 2001 

(36.72 inches), 2002 (46.27 inches), 2004 (45.32 inches), and 

2007 (47.25 inches) were wetter than normal. 

Pressure transducers installed in monitoring wells measured environmental-water head, which is the actual head in 

each well. Environmental-water heads differ from equivalent 

freshwater heads by amounts corresponding to the difference 

in salinity between the well water and freshwater (Lusczynski, 

1961). Because saline water is slightly denser than freshwater, 

the higher the salinity of the environmental water the greater 

the difference between the environmental-water head relative 

to the equivalent freshwater head. 

The conversion of measured environmental-water heads 

to equivalent freshwater heads can be done by applying the 

equation

p = ρgl, (1)

(Cooper and others, 1964, p. C28) where p is the pressure at 

the bottom of the well, ρ is the density of the water in the well, 

g is the acceleration of gravity, and l is the length of the water 

column in the well. Equating the right-side term of equation 1 

for environmental-water and freshwater columns and solving 

for the length of the freshwater column yields

l

f

 = (ρs/ρf)(ls ), (2)

where

l

f = length of equivalent freshwater column, in feet,

ρs = density of environmental water, in milligrams per 

cubic centimeter,

ρf = density of freshwater, in milligrams per cubic 

centimeter, 

ρs/ρf = density correction factor, and 

l

s = length of environmental-water column (total depth 

of the well minus the depth from land surface to 

the water level in the well), in feet.

The density of the environmental water for each well for a 

given temperature and salinity (as indicated by fluid conductivity) was obtained using a Web-based JavaScript calculator 

from the Johns-Hopkins University Applied Physics Laboratory (2005) to compute the United Nations Educational 

Scientific and Cultural Organization (UNESCO) International 

Equation of State as described in Fofonoff (1985) (hereinafter, 

density calculator). Values of length of the environmentalwater column, fluid conductivity in the water column, and 

temperature in the water column were entered into the calculator to compute the density of environmental water. The values 

of those three variables for each well used in the density 
Methods of Analysis 15

calculator were averages of the length of the environmentalwater column, depth-averaged conductivity, and depthaveraged temperature from all logs for the given well that 

were collected during 2002–07. Although values for length 

of the environmental-water column are available for each 

day of water-level record, depth-averaged conductivity and 

depth-averaged temperature values are available only for the 

several days when logs were run. The decision was made to 

average the depth-averaged conductivity and depth-averaged 

temperature values from all logs from each well, and for 

consistency the associated length of the environmental-water 

column values, and use those average values in the density 

calculator to obtain a density of environmental water for each 

well assumed to be representative of the period 1999–2007. 

The density of freshwater for each well, also assumed to be 

representative of the period 1999–2007, was interpolated from 

a table of density variation with temperature (van der Leeden 

and others, 1990, table 11–1) using the average temperature 

used in the density calculator. The density correction factor 

for each well needed to compute the length of the equivalent 

freshwater column was thus computed as the average density 

of environmental water divided by the average density of 

freshwater.

After the length of the equivalent freshwater column was 

computed using equation 2, the equivalent freshwater head 

was computed as

hf = hs + (lf-ls), (3)

where hf is freshwater head and hs is the environmental-water 

head. Environmental-water head was computed as altitude 

of land-surface datum minus depth to water measured by the 

Figure 9. Monthly departure from normal (1971–2000) rainfall, National Weather Service station 417945/12921, San Antonio 

International Airport, Texas, 1999–2007. 

1999 2000 2001 2002 2003 2004 2005 2006 2007

YEAR

ANNUAL RAINFALL RELATIVE TO NORMAL

RAINFALL DEPARTURE FROM NORMAL, IN INCHES

-5

0

5

10

15

Drier—Annual average is negative departure

from normal

Wetter—Annual average is positive departure

from normal
16 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

pressure transducer. Heads were reported as daily means in 

feet above North American Vertical Datum of 1988 (NAVD 

88). Data for the 15 monitoring wells for the dates of geophysical log data collection used to compute equivalent freshwater 

heads, including the average values of the variables used to 

obtain the density correction factor for each well, and the 

resultant density correction factor are in table 2. Daily mean 

water-level and associated data used to compute equivalent 

freshwater head and the resultant equivalent freshwater heads 

for the 15 monitoring wells for the respective periods of waterlevel record are in appendixes 1.1–1.15. 

Borehole Geophysical Log Data

The borehole geophysical methods discussed in this 

report were used to log wells in the East Uvalde, Tri-County, 

Fish Hatchery, and Kyle transects at various times (dates) over 

a range in hydraulic head conditions during 2002–07. Fluid 

conductivity and fluid temperature were collected using two 

methods. The first method, used in 2002, recorded the fluid 

conductivity and the second method, used during 2005–07, 

recorded fluid resistivity that was then converted to fluid conductivity. In addition to fluid conductivity and fluid temperature, other conventional geophysical logging methods were 

used to collect information from the monitoring wells. These 

methods included using the natural gamma, caliper, long and 

short normal resistivity, lateral resistivity, and induction conductivity tools. Advanced logging methods such as acoustic 

televiewer (ATV), heat pulse flowmeter, and electromagnetic 

(EM) flowmeter also were used. More in-depth descriptions of 

each of these logging tools and their applications are in Keys 

(1997) and Stanton and others (2007). 

Fluid conductivity is the ability of a fluid to conduct 

an electrical current and is the inverse of fluid resistivity 

(Keys, 1997). Fluid conductivity logs were run in each of the 

wells in 2002 and are displayed in units of microsiemens per 

centimeter at the temperature encountered in the borehole 

(Lambert and others, 2009). Fluid resistivity logs run during 

2005–07 in each of the wells were converted from fluid resistivity logs to fluid conductivity logs and also are displayed 

in units of microsiemens per centimeter at the temperature 

encountered in the borehole to allow for comparison to the 

2002 fluid conductivity logs. A depth-averaged fluid conductivity and a depth-averaged fluid temperature in the borehole 

were computed for each logging run. The depth-averaged 

conductivity and temperature values from each logging run 

then were averaged to obtain a fluid conductivity value and 

a temperature value assumed to be representative for each 

well for 1999–2007 for computation of equivalent freshwater 

heads, as described in the previous section. The conversion of 

fluid resistivity logs to fluid conductivity logs allows for the 

qualitative comparison of the borehole data to other datasets 

including water-quality monitor data and discrete waterchemistry data but does not imply that these measurements 

are the equivalent of high-precision specific conductance 

measurements. 

The different instrumentation and methods that were 

used during 2002–07 to measure or to calculate the borehole 

fluid conductivity in the monitoring wells constrained the 

accuracy of the calculated fluid conductivities. The limited 

accuracy of the borehole geophysical instrumentation to 

record very low fluid resistivities in high-salinity water 

limited the precision of the fluid conductivity datasets at 

relatively high salinities. Because of the differences in the 

magnitude of the borehole readings for each logging run 

and the limitations associated with using fluid resistivity 

logs to calculate the fluid conductivity logs at relatively high 

salinities, the computed fluid conductivity values for each 

logging run in each well are assumed to be representative 

of similar ranges in conductivity. The scales of the fluid 

conductivity and temperature logs were adjusted to normalize 

the log responses; that is, rather than common scales for 

conductivity and temperature logs, respectively, the scales 

were adjusted to overlay as many of the logs as possible to allow clearer visualization of differences in log 

response in relation to zones of possible inflow and outflow. 

Temperature logs were run concurrently with the fluid conductivity logs; the temperature logs are displayed in degrees 

Celsius. 

Flow logs were collected in wells of all four transects 

under ambient conditions to evaluate the differences in 

hydraulic head of the various transmissive zones tapped by 

the wells. Heat pulse and EM flowmeter data were obtained 

in a stationary mode in which vertical flow in the borehole 

was recorded with the probe held stationary at a specified 

depth. In most cases, a trolling EM flowmeter log was used 

with caliper and fluid conductivity data to detect zones of 

optimum flow and best borehole conditions for isolation of 

flow zones.

Ambient flow stations were measured over a nearly 

3-year period (2005–07) under differing hydraulic conditions 

to determine how vertical flow in the borehole changed with 

time and in relation to changes in the equivalent freshwater 

head in each well. Flow stations were chosen on the basis of 

previous logging runs to identify optimum flow zones and 

where there was a fairly competent section, or solid rock, in 

the borehole wall that allowed for a tight seal to make a measurement. Measured flows of less than 0.1 gallon per minute 

were considered estimated and within the error of the EM 

flowmeter instrumentation. By convention, positive low 

values on the logs indicate upward flow, and negative values 

indicate downward flow. Hydrographs of equivalent freshwater head for 2005–07, where available, are superimposed 

on diagrams showing vertical flow in wells to illustrate the 

relation between head in a particular well time and changes 

in fluid conductivity, temperature, and vertical flow in the 

borehole over time. 

The 15 wells were logged at various times during 

2002–07 by USGS personnel. All logs collected during 

2002–07 were recorded digitally and archived in the Log 

ASCII Standard (LAS) version 2 format, which is the USGS 

standard for borehole geophysical data storage. The borehole 
Methods of Analysis

17

Table 2. Data from 15 monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards aquifer, south-central Texas, 2000–2007, used to compute average equivalent freshwater head—Continued.

U.S. Geological 

Survey site number Well name

Well 

identifier 

(fig. 

1)

State well 

number

Data 

collection 

date

Altitude 

of LSD 

(ft 

above 

NAVD 

88)

Well 

depth 

(ft 

below 

LSD)

Daily 

mean 

depth to 

water

(ft below 

LSD)1

h

s

(ft 

above 

NAVD 

88)

l

s

(ft)

T

(°C)2

C

(μS/cm)2

ρs

(g/cm3)3

ρf

(g/cm3)4

Density 

correction 

factor 

(ρs /ρf)

lf

(ft)5

lf – ls

(ft)

hf

(ft above 

NAVD 

88)6

291443099325801 East Uvalde 1 EU1 YP–69–52–202 11/6/2002 874.02 1,500 67.84 806.18 1,432.16 21.93 418 0.999946 0.997786 1.0022 1,435.26 3.10 809.28

291443099325801 East Uvalde 1 EU1 YP–69–52–202 4/25/2005 874.02 1,500 50.90 823.12 1,449.10 33.17 356 .996725 .994639 1.0021 1,452.14 3.04 826.16

291443099325801 East Uvalde 1 EU1 YP–69–52–202 4/12/2006 874.02 1,500 100.55 773.47 1,399.45 30.34 146 .997505 .995542 1.0020 1,402.21 2.76 776.23

291443099325801 East Uvalde 1 EU1 YP–69–52–202 9/26/2006 874.02 1,500 122.46 751.56 1,377.54 28.58 394 .998088 .996065 1.0020 1,380.34 2.80 754.36

291443099325801 East Uvalde 1 EU1 YP–69–52–202 10/17/2007 874.02 1,500 48.85 825.17 1,451.15 28.23 633 .998373 .996166 1.0022 1,454.37 3.22 828.39

Average -- 1,421.88 28.45 389 .998185 .996102 1.0021 -- -- --

291612099302001 East Uvalde 2 EU2 YP–69–44–902 11/6/2002 899.91 1,560 90.55 809.36 1,469.45 21.98 424 .999988 .997774 1.0022 1,472.71 3.26 812.62

291612099302001 East Uvalde 2 EU2 YP–69–44–902 4/11/2006 899.91 1,560 124.51 775.40 1,435.49 24.77 104 .999129 .997102 1.0020 1,438.41 2.92 778.32291612099302001 East Uvalde 2 EU2 YP–69–44–902 9/28/2006 899.91 1,560 148.36 751.55 1,411.64 20.61 315 1.00018 .998074 1.0021 1,414.62 2.98 754.53291612099302001 East Uvalde 2 EU2 YP–69–44–902 10/16/2007 899.91 1,560 73.15 826.76 1,486.85 22.91 365 .999765 .997559 1.0022 1,490.14 3.29 830.05

Average -- 1,450.86 22.57 302 .999774 .997638 1.0021 -- -- --

291136099375801 East Uvalde 3 EU3 YP–69–51–606 11/7/2002 877.55 1,400 83.35 794.20 1,316.65 27.41 5,239 1.00020 .996397 1.0038 1,321.68 5.03 799.23291136099375801 East Uvalde 3 EU3 YP–69–51–606 4/28/2005 877.55 1,400 46.06 831.49 1,353.94 34.00 1,652 .996723 .994371 1.0024 1,357.14 3.20 834.69291136099375801 East Uvalde 3 EU3 YP–69–51–606 4/14/2006 877.55 1,400 92.01 785.54 1,307.99 30.96 4,975 .998886 .995352 1.0036 1,312.63 4.64 790.18291136099375801 East Uvalde 3 EU3 YP–69–51–606 9/25/2006 877.55 1,400 114.98 762.57 1,285.02 31.58 2,398 .997710 .995157 1.0026 1,288.32 3.30 765.87

291136099375801 East Uvalde 3 EU3 YP–69–51–606 10/23/2007 877.55 1,400 52.01 825.54 1,347.99 29.26 3,426 .998935 .995867 1.0031 1,352.14 4.15 829.69

Average -- 1,322.32 30.64 3,538 .998484 .995450 1.0030 -- -- --

291133099363801 East Uvalde 4 EU4 YP–69–52–404 11/7/2002 867.02 1,463 71.85 795.17 1,391.15 29.73 3,559 .998889 .995726 1.0032 1,395.57 4.42 799.59

291133099363801 East Uvalde 4 EU4 YP–69–52–404 3/3/2005 867.02 1,463 e32.70 834.32 1,430.30 32.37 1,335 .997279 .994905 1.0024 1,433.71 3.41 837.73291133099363801 East Uvalde 4 EU4 YP–69–52–404 4/28/2005 867.02 1,463 35.81 831.21 1,427.19 33.75 4,348 .997821 .994452 1.0034 1,432.03 4.84 836.05291133099363801 East Uvalde 4 EU4 YP–69–52–404 4/13/2006 867.02 1,463 81.08 785.94 1,381.92 32.53 741 .996967 .994854 1.0021 1,384.86 2.94 788.88

291133099363801 East Uvalde 4 EU4 YP–69–52–404 9/25/2006 867.02 1,463 104.18 762.84 1,358.82 32.24 1,939 .997429 .994947 1.0025 1,362.21 3.39 766.23

291133099363801 East Uvalde 4 EU4 YP–69–52–404 10/18/2007 867.02 1,463 40.27 826.75 1,422.73 30.35 3,070 .998546 .995539 1.0030 1,427.03 4.30 831.05

Average -- 1,402.02 31.83 2,499 .997819 .995079 1.0028 -- -- --

293610098152701 Tri-County 1 TC1 KX–68–30–314 1/27/2003 871.01 920 191.45 679.56 728.55 23.40 908 .998806 .997442 1.0014 729.55 1.00 680.56

293610098152701 Tri-County 1 TC1 KX–68–30–314 4/18/2005 871.01 920 189.77 681.24 730.23 25.92 439 .997972 .996804 1.0012 731.09 0.86 682.10

293610098152701 Tri-County 1 TC1 KX–68–30–314 4/21/2005 871.01 920 190.81 680.20 729.19 24.67 1,614 .998757 .997128 1.0016 730.38 1.19 681.39

293610098152701 Tri-County 1 TC1 KX–68–30–314 10/26/2005 871.01 920 e205.3 665.71 665.71 26.34 333 .997731 .996691 1.0010 666.40 0.69 666.40

293610098152701 Tri-County 1 TC1 KX–68–30–314 6/23/2006 871.01 920 221.43 649.58 698.57 24.51 751 .998419 .997168 1.0013 699.45 0.88 650.46

293610098152701 Tri-County 1 TC1 KX–68–30–314 11/6/2006 871.01 920 221.08 649.93 698.92 22.74 608 .998810 .997598 1.0012 699.77 0.85 650.78

293610098152701 Tri-County 1 TC1 KX–68–30–314 10/29/2007 871.01 920 e192.76 678.25 678.25 24.89 550 .998216 .997072 1.0011 679.03 0.78 679.03

Average -- 704.20 24.64 743 .998390 .997135 1.0013 -- -- --

Table 2. Data from 15 monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards aquifer, south-central Texas, 2000–2007, used to compute average equivalent freshwater head.[LSD, land-surface datum; ft, feet; NAVD 88, North American Vertical Datum of 1988; hs, environmental-water head; ls, length of environmental-water column; T, depth-averaged water temperature; oC, degrees 

Celsius; C, depth-averaged conductivity; μS/cm, microsiemens per centimeter at 25 °C; ρs, environmental-water density; g/cm3, grams per cubic centimeter; ρf, freshwater density; lf, length of equivalent freshwater column; hf, equivalent freshwater head; --, not applicable; e, estimated]
18 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone Table 2. Data from 15 monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards aquifer, south-central Texas, 2000–2007, used to compute average equivalent freshwater head—Continued. U.S. Geological Survey site number Well name Well iden- tifier (fig. 1) State well number Data collection date Altitude of LSD (ft above NAVD 88) Well depth (ft below LSD) Daily mean depth to water (ft below LSD)1

hs

(ft 

above 

NAVD 

88)

l

s

(ft)

T

(°C)2

C

(μS/cm)2

ρs

(g/cm3

)

3

ρf

(g/cm3

)

4

Density 

correction 

factor 

(ρs /ρf

)

l

f

(ft)5

l

f – ls

(ft)

hf

(ft above 

NAVD 

88)6

293424098134701 Tri-County 2 TC2 KX–68–31–403 1/30/2003 709.08 1,050 20.98 688.10 1,029.02 23.88 5,506 1.00104 .997327 1.0037 1,032.85 3.83 691.93

293424098134701 Tri-County 2 TC2 KX–68–31–403 5/17/2005 709.08 1,050 e26.07 683.01 1,023.93 28.22 2,928 .998638 .996169 1.0025 1,026.47 2.54 685.55

293424098134701 Tri-County 2 TC2 KX–68–31–403 10/20/2005 709.08 1,050 e37.48 671.60 1,012.52 27.01 1,846 .998577 .996509 1.0021 1,014.62 2.10 673.70

293424098134701 Tri-County 2 TC2 KX–68–31–403 11/2/2005 709.08 1,050 e37.55 671.53 1,012.45 28.23 2,928 .998619 .996166 1.0025 1,014.94 2.49 674.02

293424098134701 Tri-County 2 TC2 KX–68–31–403 4/20/2006 709.08 1,050 49.31 659.77 1,000.69 27.03 1,512 .998428 .996504 1.0019 1,002.62 1.93 661.70

293424098134701 Tri-County 2 TC2 KX–68–31–403 11/21/2006 709.08 1,050 52.40 656.68 997.60 23.59 3,135 1.00005 .997397 1.0027 1,000.25 2.65 659.33

293424098134701 Tri-County 2 TC2 KX–68–31–403 10/30/2007 709.08 1,050 24.45 684.63 1,025.55 25.42 1,381 .998869 .996934 1.0019 1,027.54 1.99 686.62

Average -- 1,014.54 26.20 2,748 .999166 .996729 1.0024 -- -- --

293245098121001 Tri-County 3 TC3 KX–68–31–511 1/28/2003 674 1,222 2.62 671.38 1,219.38 25.32 13,019 1.00419 .996960 1.0073 1,228.22 8.84 680.22

293245098121001 Tri-County 3 TC3 KX–68–31–511 10/26/2005 674 1,222 e8.3 665.70 1,213.70 28.42 3,494 .999049 .996111 1.0029 1,217.28 3.58 669.28

293245098121001 Tri-County 3 TC3 KX–68–31–511 4/18/2006 674 1,222 16.28 657.72 1,205.72 28.05 2,508 .998778 .996218 1.0026 1,208.82 3.10 660.82

293245098121001 Tri-County 3 TC3 KX–68–31–511 10/25/2006 674 1,222 24.72 649.28 1,197.28 25.53 6,546 1.00120 .996906 1.0043 1,202.44 5.16 654.44

Average -- 1,209.02 26.83 6,392 1.00072 .996558 1.0042 -- -- --

293058098110501 Tri-County 4 TC4 KX–68–31–808 1/30/2003 648.92 1,562 2.30 646.62 1,559.70 27.99 15,742 1.00476 .996235 1.0086 1,573.05 13.35 659.97

293058098110501 Tri-County 4 TC4 KX–68–31–808 10/21/2005 648.92 1,562 e-6.01 654.93 1,568.01 31.70 3,899 .998583 .995120 1.0035 1,573.47 5.46 660.39

293058098110501 Tri-County 4 TC4 KX–68–31–808 4/19/2006 648.92 1,562 -1.95 650.87 1,563.95 30.93 2,835 .998459 .995361 1.0031 1,568.82 4.87 655.74293058098110501 Tri-County 4 TC4 KX–68–31–808 10/24/2006 648.92 1,562 6.62 642.30 1,555.38 29.34 7,085 1.00060 .995878 1.0047 1,562.75 7.37 649.67

Average -- 1,561.76 29.99 7,390 1.00050 .995646 1.0049 -- -- --

293632098172401 Tri-County 5 TC5 KX–68–30–315 1/30/2003 782.22 975 103.02 679.20 871.98 21.60 438 .999252 .997859 1.0014 873.20 1.22 680.42

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 11/8/2002 714.73 280 112.20 602.53 167.80 21.37 545 .998366 .997910 1.0005 167.88 .08 602.61

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 4/20/2005 714.73 280 116.76 597.97 163.24 23.62 294 .997730 .997389 1.0003 163.30 .06 598.03

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 4/27/2006 714.73 280 125.99 588.74 154.01 24.14 155 .997538 .997262 1.0003 154.05 .04 588.78

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 9/21/2006 714.73 280 127.40 587.33 152.60 19.74 429 .998651 .998256 1.0004 152.66 .06 587.39

295019097592701 Fish Hatchery 1 FH1 LR–67–09–113 10/24/2007 714.73 280 117.13 597.60 162.87 22.28 464 .998117 .997705 1.0004 162.94 .07 597.67

Average -- 160.10 22.23 377 .998091 .997717 1.0004 -- -- --

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 11/8/2002 642.51 1,030 46.59 595.92 983.41 24.97 15,236 1.00504 .997052 1.0080 991.29 7.88 603.80

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 4/19/2005 642.51 1,030 46.32 596.19 983.68 27.63 5,778 .999898 .996336 1.0036 987.20 3.52 599.71

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 5/2/2006 642.51 1,030 52.07 590.44 977.93 27.57 3,057 .998827 .996352 1.0025 980.36 2.43 592.87

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 9/20/2006 642.51 1,030 53.38 589.13 976.62 24.64 7,879 1.00178 .997135 1.0047 981.17 4.55 593.68

294946097574501 Fish Hatchery 2 FH2 LR–67–09–401 10/25/2007 642.51 1,030 46.88 595.63 983.12 25.87 15,663 1.00486 .996817 1.0081 991.05 7.93 603.56

Average -- 980.95 26.14 9,523 1.00199 .996745 1.0053 -- -- --
Methods of Analysis

19

Table 2. Data from 15 monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards aquifer, south-central Texas, 2000–2007, used to compute average equivalent freshwater head—Continued.

U.S. Geological 

Survey site number Well name

Well 

identifier 

(fig. 

1)

State well 

number

Data 

collection 

date

Altitude 

of LSD 

(ft 

above 

NAVD 

88)

Well 

depth 

(ft 

below 

LSD)

Daily 

mean 

depth to 

water

(ft below 

LSD)1

h

s

(ft 

above 

NAVD 

88)

l

s

(ft)

T

(°C)2

C

(μS/cm)2

ρs

(g/cm3)3

ρf

(g/cm3)4

Density 

correction 

factor 

(ρs /ρf)

lf

(ft)5

lf – ls

(ft)

hf

(ft above 

NAVD 

88)6

295853097532901 Kyle 1 KY1 LR–67–01–311 11/5/2002 770.52 810 174.87 595.65 635.13 23.77 1,111 .998664 .997353 1.0013 635.96 0.83 596.48

295853097532901 Kyle 1 KY1 LR–67–01–311 3/9/2005 770.52 810 182.27 588.25 627.73 26.49 500 .997697 .996650 1.0011 628.39 .66 588.91

295853097532901 Kyle 1 KY1 LR–67–01–311 3/1/2006 770.52 810 211.00 559.52 599.00 28.22 348 .997115 .996169 1.0009 599.57 .57 560.09

295853097532901 Kyle 1 KY1 LR–67–01–311 5/3/2006 770.52 810 e226.6 543.92 583.40 26.87 260 .997447 .996547 1.0009 583.93 .53 544.45

295853097532901 Kyle 1 KY1 LR–67–01–311 6/15/2006 770.52 810 227.79 542.73 582.21 24.76 1,556 .998507 .997105 1.0014 583.03 .82 543.55

295853097532901 Kyle 1 KY1 LR–67–01–311 9/6/2006 770.52 810 236.22 534.30 573.78 23.65 311 .998298 .997382 1.0009 574.31 .53 534.83

295853097532901 Kyle 1 KY1 LR–67–01–311 11/5/2007 770.52 810 191.63 578.89 618.37 25.33 308 .997929 .996958 1.0010 618.97 .60 579.49

Average -- 602.80 25.58 628 .997957 .996893 1.0011 -- -- --

295858097521801 Kyle 2 KY2 LR–67–02–104 1/31/2003 674.32 975 76.12 598.20 898.88 23.13 6,361 1.00147 .997507 1.0040 902.45 3.57 601.77

295858097521801 Kyle 2 KY2 LR–67–02–104 7/28/2003 674.32 975 110.65 563.67 864.35 24.00 8,648 1.00219 .997298 1.0049 868.59 4.24 567.91

295858097521801 Kyle 2 KY2 LR–67–02–104 1/10/2005 674.32 975 e101.7 572.62 873.30 25.74 9,825 1.00212 .996851 1.0053 877.92 4.62 577.24

295858097521801 Kyle 2 KY2 LR–67–02–104 6/13/2006 674.32 975 128.13 546.19 846.87 22.53 4,841 1.00089 .997647 1.0033 849.62 2.75 548.94

295858097521801 Kyle 2 KY2 LR–67–02–104 9/12/2006 674.32 975 134.82 539.50 840.18 22.57 8,861 1.00273 .997638 1.0051 844.47 4.29 543.79

295858097521801 Kyle 2 KY2 LR–67–02–104 11/8/2007 674.32 975 98.20 576.12 876.80 24.68 2,841 .999456 .997125 1.0023 878.85 2.05 578.17

Average -- 866.73 23.78 6,896 1.00147 .997351 1.0041 -- -- --

295829097512601 Kyle 3 KY3 LR–67–02–106 11/4/2002 678.28 1,100 89.60 588.68 1,010.40 23.86 14,324 1.00510 .997332 1.0078 1,018.27 7.87 596.55

295829097512601 Kyle 3 KY3 LR–67–02–106 5/18/2005 678.28 1,100 e98.43 579.85 1,001.57 27.94 11,290 1.00211 .996249 1.0059 1,007.46 5.89 585.74

295829097512601 Kyle 3 KY3 LR–67–02–106 6/19/2006 678.28 1,100 117.63 560.65 982.37 23.17 14,932 1.00564 .997497 1.0082 990.39 8.02 568.67

295829097512601 Kyle 3 KY3 LR–67–02–106 9/13/2006 678.28 1,100 121.13 557.15 978.87 22.56 14,571 1.00570 .997640 1.0081 986.78 7.91 565.06

295829097512601 Kyle 3 KY3 LR–67–02–106 11/7/2007 678.28 1,100 100.86 577.42 999.14 25.22 13,485 1.00401 .996987 1.0070 1,006.18 7.04 584.46

Average -- 994.47 24.55 13,720 1.00452 .997158 1.0074 -- -- --

295730097503201 Kyle 4 KY4 LR–67–02–105 11/8/2002 646.7 970 63.65 583.05 906.35 24.11 29,075 1.01221 .997270 1.0150 919.93 13.58 596.63295730097503201 Kyle 4 KY4 LR–67–02–105 5/19/2005 646.7 970 e66.08 580.62 903.92 26.05 15,008 1.00438 .996769 1.0076 910.82 6.90 587.52

295730097503201 Kyle 4 KY4 LR–67–02–105 6/20/2006 646.7 970 74.91 571.79 895.09 26.35 14,233 1.00389 .996688 1.0072 901.56 6.47 578.26

295730097503201 Kyle 4 KY4 LR–67–02–105 9/13/2006 646.7 970 76.66 570.04 893.34 23.39 13,842 1.00490 .997444 1.0075 900.02 6.68 576.72

295730097503201 Kyle 4 KY4 LR–67–02–105 11/6/2007 646.7 970 68.00 578.70 902.00 25.52 12,507 1.00345 .996908 1.0066 907.92 5.92 584.62

Average -- 900.14 25.08 16,933 1.00569 .997023 1.0087 -- -- --

1 Depth-averaged value from pressure transducer just deep enough in water column to remain submerged.2 Depth-averaged value from fluid-profile data collected for each well.3 Computed using the John-Hopkins equation of state calculator (Johns-Hopkins University Applied Physics Laboratory, 2005) with ls, T, and C.4 Interpolated from table of pure water density variation with temperature (van der Leeden and others, 1990, table 11–1).5 lf = (ρs/ρf) (ls) from Cooper and others (1964, p. C28).6 hf = hs + (lf –ls).
20 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

geophysical logs collected during 2005–07 were collected 

according to the following American Society for Testing and 

Materials (ATSM) standards: (1) ASTM standard D 6726–01 

for borehole EM induction logging (American Society for 

Testing and Materials, 2001), (2) ASTM standard D 6274–98 

for gamma logging (American Society for Testing and 

Materials, 2004a), and (3) ASTM standard D 6167–97 for 

caliper logging (American Society for Testing and Materials, 

2004b). 

Lithologic and Physicochemical 

Properties

Lithologic properties in each well were obtained from 

geophysical log data. These data were used to identify the 

stratigraphic units that contribute flow to the borehole and to 

determine vertical flow (magnitude and direction under ambient conditions) relative to the stratigraphic units in each well. 

Physicochemical properties (fluid conductivity and temperature) were obtained (and flow zones characterized) using an 

integrated analysis of the natural gamma, caliper, ATV, fluid 

conductivity (recorded as fluid resistivity), temperature, and 

EM flowmeter logs. Lithologic and physiochemical properties 

are discussed by transect in the following sections.

Lithologic Properties

East Uvalde Transect

For the freshwater wells EU1 and EU2, the natural 

gamma logs indicate carbonate rocks with a lack of contrast 

in the upper sections of the boreholes and increasing clayey 

material in the lower sections of the boreholes, as indicated 

by the greater variability in the gamma counts (figs. 10, 11). 

These wells are completed in the reefal facies of the Devils 

River Trend depositional province. In contrast, the gamma 

logs for saline-water wells EU3 and EU4 (figs. 12, 13) show 

greater variation in lithology down the boreholes than that 

in either wells EU1 or EU2, indicating that more clayey 

intervals are interbedded with carbonate rocks. The clayey 

intervals were more common in the bottom part of wells 

EU3 and EU4 corresponding to the McKnight Formation 

and the West Nueces Formation of the Maverick Basin 

depositional province. Wells EU3 and EU4 are completed 

in the basinal facies of the Maverick Basin depositional 

province. 

The caliper logs for all the East Uvalde transect 

wells confirm that the boreholes are about 6 inches in 

diameter along most of the lengths, except some intervals 

where the holes were enlarged beyond 6 inches to at least 

7–9 inches in diameter. In EU1 and EU2, the larger-diameter 

intervals correspond to vuggy sections in the Devils River 

Limestone (figs. 10, 11). In EU3 and EU4, the diameters 

of the boreholes were enlarged in the upper sections of the 

boreholes that correspond to the Salmon Peak and upper 

McKnight Formations (figs. 12, 13). The caliper logs as well 

as the gamma logs indicate that most of the porosity and permeability in the East Uvalde wells are associated with cleaner 

(less clayey) limestone sections and are the result of secondary 

porosity development.

The ATV log for EU1 shows that the borehole was 

competent in the upper sections and more vuggy and fractured in the lower sections, with the greatest number of 

vugs occurring at the bottom of the well (fig. 10). In EU2, 

the ATV image shows secondary porosity development in 

the form of bedding-plane fractures and vugs in the Devils 

River Limestone (fig. 11). The ATV images for EU3 and 

EU4 show numerous vugs and bedding-plane fractures in 

the borehole associated with enlarged intervals shown on 

the caliper logs. The highest concentrations of these vugs 

and bedding-plane fractures was in the Salmon Peak 

Formation in EU3 and in the Salmon Peak and West Nueces 

Formations in EU4 (figs. 12, 13). The striping effects on the 

ATV images were an artifact of poor centralization, or nonuniform borehole roundness, and not necessarily an indicator of 

vugs. 

Tri-County Transect

The natural gamma logs for Tri-County freshwater 

well TC1, interface well TC2, and saline-water wells TC3 

and TC4 (figs. 14–17) indicate that Edwards aquifer rocks 

are composed predominantly of limestone with a few minor 

clayey sections without any major contrast in lithology, 

despite the layered structure in the San Marcos Platform 

depositional province reflected by the members (hydrogeologic subdivisions) of the Person and Kainer Formations 

(fig. 3). The caliper logs for these wells show numerous 

enlargements of the borehole beyond its 6-inch diameter to 

nearly 10 inches in diameter. The enlargements have a high 

roughness factor and are especially prevalent in the upper 

sections of the borehole that correspond to the Georgetown 

Formation, cyclic and marine members, and leached and 

collapsed members, and in the lower sections that correspond to the grainstone, Kirschberg evaporite, and dolomitic 

members. 

The ATV logs of the Tri-County wells show fractures and 

vuggy intervals that correspond to the enlarged intervals measured by the caliper logs. The fractures and vuggy intervals are 

most common in the upper sections of the boreholes open to 

the Georgetown Formation, leached and collapsed members, 

cyclic and marine members, Kirschberg evaporite member, 

and dolomitic member. Small bedding-plane fractures and 

vugs or enlarged pores are interspersed throughout the remaining sections of the boreholes. The striping effect on the ATV 

image for well TC2 near about 600 to 620 feet is an artifact of 

poor centralization, or nonuniform borehole roundness, and 

not necessarily an indicator of vugs. 
Lithologic and Physicochemical Properties 21

Figure 10. Daily mean equivalent freshwater head and borehole geophysical data in East Uvalde 1 well (YP–69–52–202), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2002–07. 

1,000

1,100

1,200

1,300

1,400

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Devils

River

Limestone

Del Rio Clay

Estimated flow less than

0.1 gallon per minute (gpm)

0.5

0.5

0.5

0.5

Corrected flowstation 04/25/2005

Corrected flowstation 04/12/2006

Corrected flowstation 09/28/2006

Corrected flowstation 10/17/2007

gpm

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-0.5

-0.5

COND 11/06/2002 Stratigraphic unit

415 495

TEMP 11/06/2002

20.4 24.4

TEMP 04/25/2005

31.4 35.4

COND 04/25/2005

335 415

TEMP 04/12/2006

31.3 35.3

COND 04/12/2006

165 245

TEMP 09/26/2006

30.6 34.6

COND 09/26/2006

465 545

COND 10/17/2007

760 840

TEMP 10/17/2007

29.4 33.4

deg C

deg C

deg C

deg C

deg C

-0.5

-0.5

μS/cm

μS/cm

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

740

750

760

770

780

790

800

810

820

830

840

850

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

April 2006

Geophysical data collected 

September 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System
22 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 11. Daily mean equivalent freshwater head and borehole geophysical data in East Uvalde 2 well (YP–69–44–902), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2002–07. 

Devils

River

Limestone

1,100

1,200

1,300

1,400

1,500

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Estimated flow less than

0.1 gallon per minute (gpm)

1

Corrected flowstation 04/26/2005

Corrected flowstation 04/11/2006

Corrected flowstation 09/28/2006

Corrected flowstation 10/16/2007

gpm

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-1

-1 1

-1 1

-1 1

COND 11/06/2002 Stratigraphic unit

390 490

TEMP 11/06/2002

21 24

COND 04/26/2005

155 255

TEMP 04/11/2006

19.85 22.85

COND 04/11/2006

58 158

TEMP 09/28/2006

22.88 25.88

COND 09/28/2006

240 340

COND 10/16/2007

415 515

TEMP 10/16/2007

19.9 22.9

deg C

deg C

deg C

deg C

μS/cm

μS/cm

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

740

750

760

770

780

790

800

810

820

830

840

850

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

April 2006

Geophysical data collected 

September 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System
Lithologic and Physicochemical Properties 23

Figure 12. Daily mean equivalent freshwater head and borehole geophysical data in East Uvalde 3 well (YP–69–51–606), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2002–07. 

Del Rio Clay

Salmon Peak

Formation

McKnight

Formation

West Nueces

Formation

Glen Rose

Limestone

1,000

1,100

1,200

800

900

1,300

1,400

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Estimated flow less than

0.1 gallon per minute (gpm)

1

1

1

Corrected flowstation 04/28/2005

Corrected flowstation 09/26/2006

Corrected flowstation 10/23/2007

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-1

COND 11/07/2002 Stratigraphic unit

5200 6200

TEMP 11/07/2002

29 33

TEMP 04/28/2005

32 36

COND 04/28/2005

1350 2350

TEMP 04/14/2006

29.8 33.8

COND 04/14/2006

4400 5400

TEMP 09/25/2006

31.8 35.8

COND 09/25/2006

2150 3150

COND 10/23/2007

3400 4400

TEMP 10/23/2007

30 34

deg C

deg C

deg C

deg C

deg C

-1

-1

0 1

μS/cm

μS/cm

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

720

730

740

750

760

770

780

790

800

810

820

830

840

850

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

April 2006

Geophysical data collected 

September 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided by

the U.S. Geological Survey
24 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Fish Hatchery Transect

The natural gamma logs for freshwater well FH1 and 

saline-water well FH2 indicate that the borehole sections are 

composed of fairly clean limestone interbedded with clayey 

lenses (figs. 18, 19). These wells are completed in rocks of the 

San Marcos Platform depositional province (figs. 2, 3). Well 

FH1 is shallow compared to the other monitoring wells, which 

does not allow for in-depth analysis of the natural gamma log 

(fig. 18). The caliper log for well FH1 shows enlargements 

along the length of the nominal 6-inch-diameter borehole, 

with some enlargements increasing the diameter of the hole 

to at least 8–10 inches. These enlargements correspond to 

bedding-plane fractures and vugs in the cyclic and marine 

members and also appear on the ATV log (fig. 18). For well 

FH2, the natural gamma log indicates clayey intervals that 

correspond to the leached and collapsed members and parts 

of the Kirschberg evaporite, dolomitic, and basal nodular 

members. The caliper and ATV logs show enlargements of 

the hole diameter with rugose intervals in the lower part of the 

Georgetown Formation, cyclic and marine, leached and collapsed, grainstone, and Kirschberg evaporite members as well 

as small bedding-plane fractures in the Kirschberg evaporite 

and dolomitic members (fig. 19). 

Figure 13. Borehole geophysical data in East Uvalde 4 well [YP–69–52–404], San Antonio segment of the Edwards aquifer, southcentral Texas, 2002–07. (Daily mean equivalent freshwater head data not available for East Uvalde 4 well (YP–69–52–404), 2002–07.) 

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

1,000

1,100

1,200

1,300

1,400

1,050

950

1,150

1,250

1,350

1,450

Estimated flow less than

0.1 gallon per minute (gpm)

0.5

Corrected flowstation 04/28/2005

Corrected flowstation 04/13/2006

Corrected flowstation 09/25/2006

Corrected flowstation 10/18/2007

gpm

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-0.5

-0.5 0.5

-0.5 0.5

-0.5 0.5

Stratigraphic unit

COND 11/07/2002

4550 5350

TEMP 11/07/2002

33.8 39.8

COND 03/03/2005

COND 04/28/2005

TEMP 04/13/2006

TEMP 04/28/2005

33.6 39.6

COND 04/13/2006

580 1380

TEMP 09/25/2006

33.9 39.9

COND 09/25/2006

1900 2700

COND 10/18/2007

3230 4030

TEMP 10/16/2007

31.7 37.7

deg C

deg C

deg C

deg C

deg C

μS/cm

4050 μS/cm 4850

1200 μS/cm 2000

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps) 31 37

TEMP 03/03/2005

deg C

31 deg C 37

Salmon Peak

Formation

McKnight

Formation

West Nueces

Formation
Lithologic and Physicochemical Properties 25

Figure 14. Daily mean equivalent freshwater head and borehole geophysical data in Tri-County 1 well (KX–68–30–314), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2003–07. 

400

500

600

700

800

900

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine

members

Leached and

collapsed

members

Regional dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

2

Corrected flowstation 04/21/2005

Corrected flowstation 06/23/2006

Corrected flowstation 11/06/2006

Corrected flowstation 10/29/2007

gpm

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-2

-2 2

-2 2

-2 2

Stratigraphic unit

COND 04/18/2005

395 595

22.5 27.5

COND 04/21/2005

COND 10/26/2005

TEMP 06/23/2006

TEMP 10/26/2005

22.2 27.2

COND 06/23/2006

675 875

TEMP 11/06/2006

21 26

COND 11/06/2006

625 825

COND 10/29/2007

575 775

TEMP 10/29/2007

22.8 27.8

deg C

deg C

deg C

deg C

deg C

μS/cm

298 μS/cm 498

1525 μS/cm 1725

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

TEMP 04/21/2005

24.5 29.5

COND 01/27/2003

965 μS/cm 1165

TEMP 04/18/2005

24 29

TEMP 01/07/2003

21.5 deg C 26.5

deg C

660

670

680

690

640

650

660

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

April 2005

Geophysical data collected 

October 2005

Geophysical data collected 

June 2006

Geophysical data collected 

November 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided by

the U.S. Geological Survey
26 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 15. Daily mean equivalent freshwater head and borehole geophysical data in Tri-County 2 well (KX–68–31–403), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2003–07. 

500

600

700

800

900

1,000

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine

members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

2

Corrected flowstation 11/02/2005

Corrected flowstation 04/20/2006

Corrected flowstation 11/21/2006

Corrected flowstation 10/30/2007

gpm

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer Image

0 cps 100 0° 180°90° 270° 0°

GAM(NAT)

2 inch 12

CALIPER

-2

2

Corrected flowstation 05/17/2005

-2 gpm

-2 2

-2 2

-2 2

Stratigraphic unit

COND 05/17/2005

620 5620

26 32

COND 10/20/2005

COND 11/02/2005

TEMP 04/20/2006

TEMP 11/02/2005

26 32

COND 04/20/2006

40 5040

TEMP 11/21/2006

22.65 28.65

COND 11/21/2006

1465 6465

COND 10/30/2007

0 5000

TEMP 10/30/2007

24.25 30.25

deg C

deg C

deg C

deg C

deg C

μS/cm

575 μS/cm 5575

35 μS/cm 5035

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

TEMP 10/20/2005

25.4 31.4

COND 01/30/2003

1000 μS/cm 16000

TEMP 05/17/2005

25.5 31.5

TEMP 01/30/2003

22.55 deg C 28.55

deg C

640

650

660

670

680

690

700

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

May 2005

Geophysical data collected 

October 2005

Geophysical data collected 

November 2005

Geophysical data collected 

April 2006

Geophysical data collected 

November 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System and the U.S.

Geological Survey
Lithologic and Physicochemical Properties 27

Kyle Transect

The natural gamma logs for freshwater well KY1, interface well KY2, and saline-water wells KY3 and KY4 indicate 

that the wells are open to clean limestone with some clayey 

intervals in the cyclic and marine members and the leached 

and collapsed members in the upper part of the Edwards 

aquifer, and in the dolomitic and basal nodular members in the 

lower part of the Edwards aquifer (figs. 20–23). Wells KY1, 

KY2, and KY3 also are open to the Glen Rose Limestone 

(Trinity aquifer) at the base of the wells. The gamma logs for 

these wells indicate that the Glen Rose Limestone has greater 

clay content than the formations of the overlying Edwards 

aquifer (figs. 20–22).

The caliper logs from wells KY1, KY2, and KY3 indicate 

borehole enlargement to diameters greater than the nominal 

6 inches in the upper part of the borehole that corresponds to 

the cyclic and marine members and the leached and collapsed 

members, and to the grainstone, Kirschberg evaporite, and 

dolomitic members in the lower part of the borehole (figs. 

20–22). In well KY4, the caliper log indicates that the borehole is enlarged along most of its length from the cyclic and 

marine members to the upper part of the dolomitic member 

(fig. 23). 

The ATV logs from the Kyle wells confirm the enlarged 

diameter areas that were recorded by the caliper log. Some 

of the enlarged intervals are relatively more porous and 

vuggy, and other enlarged intervals show small zones of 

700

800

900

1,000

1,100

1,200

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Estimated flow less than

0.1 gallon per minute (gpm)

Corrected flowstation 04/18/2006

Corrected flowstation 10/25/2006

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer Image

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER -0.5 0.5

-0.5 0.5

Stratigraphic unit

COND 01/28/2003

TEMP 10/26/2005

TEMP 01/28/2003

24.9 34.9

COND 10/26/2005

2610 4610

TEMP 04/18/2006

24 34

COND 04/18/2006

1580 3580

COND 10/25/2006

5750 7750

TEMP 10/25/2006

22.4 32.4

deg C

deg C

deg C

deg C

13970 μS/cm 15970

μS/cm

μS/cm

μS/cm

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps)

21.4 31.4

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine

members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Figure 16. Borehole geophysical data in Tri-County 3 well (KX–68–31–511), San Antonio segment of the Edwards aquifer, south-central 

Texas, 2003–06. (Daily mean equivalent freshwater head data not available for Tri-County 3 well [KX–68–31–511], 2003–06.) 
28 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 17. Daily mean equivalent freshwater head and borehole geophysical data in Tri-County 4 well (KX–68–31–808), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2003–07. 

1,000

1,100

1,200

1,300

1,400

1,500

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine

members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

1

Corrected flowstation 01/13/2005

Corrected flowstation 04/19/2006

Corrected flowstation 10/24/2006

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 90° 270° 180° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER -1

-1 1

-1 1

Stratigraphic unit

28.7 34.7

COND 01/30/2003

TEMP 10/21/2005

TEMP 01/12/2005

32.8 38.8

COND 10/21/2005

2800 5500

TEMP 04/19/2006

31.5 37.5

COND 04/19/2006

1650 4350

COND 10/24/2006

6450 9150

TEMP 10/24/2006

31.6 37.6

deg C

deg C

deg C

deg C

deg C

16700 μS/cm 19400

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps) TEMP 01/30/2003

32 38

660

665

670

675

645

650

655

EXPLANATION

Geophysical data collected 

January 2005

Geophysical data collected 

October 2005

Geophysical data collected 

April 2006

Geophysical data collected 

October 2006

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided by

the U.S. Geological Survey

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988
Lithologic and Physicochemical Properties 29

bedding-plane fractures and vugs. The striping effects on the 

ATV image for well KY4 were artifacts of poor centralization 

of the logging instrument, nonuniform borehole roundness, or 

large diameter, and were not necessarily an indicator of vugs 

(fig. 23). 

Physicochemical Properties

Fluid conductivity and temperature are discussed by transect in the following sections. The discussions might involve 

vertical borehole flow where it is considered relevant to the 

Figure 18. Daily mean equivalent freshwater head and borehole geophysical data in Fish Hatchery 1 well (LR–67–09–113), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2002–07. 

Del Rio Clay

Georgetown

Formation

Person Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Estimated flow less than

0.1 gallon per minute (gpm)

Corrected flowstation 04/20/2005

Corrected flowstation 09/21/2006

Corrected flowstation 10/24/2007

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER -1 1

-1 1

-1 1

Stratigraphic unit

18 28

COND 04/20/2005

TEMP 04/27/2006

TEMP 04/20/2005

20.5 30.5

COND 04/27/2006

-200 400

TEMP 09/21/2006

16.3 26.3

COND 09/21/2006

100 700

COND 10/24/2007

100 700

TEMP 10/24/2007

18.8 28.8

deg C

deg C

deg C

deg C

deg C

-50 μS/cm 550

250 μS/cm 850

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps)

20 30

COND 11/08/2002 TEMP 11/08/2002

200

250

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

595

600

605

585

590

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

April 2006

Geophysical data collected 

September 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System
30 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

500

600

700

800

900

1000

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

2

Flowstation 04/19/2005

Corrected flowstation 05/02/2006

Corrected flowstation 09/20/2006

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER

-2

-2 2

-2 2

Corrected flowstation 10/25/2007

-2 gpm 2

Stratigraphic unit

22 29

COND 01/30/2003

TEMP 05/02/2006

TEMP 04/19/2005

25 32

COND 05/02/2006

1160 4160

TEMP 09/20/2006

23 30

COND 09/20/2006

6350 9350

COND 10/25/2007

16330 19330

TEMP 10/25/2007

23 30

deg C

deg C

deg C

deg C

deg C

15460 μS/cm 18460

μS/cm

μS/cm

μS/cm

Γενεραλιζεδ χαλιπερ προφιλε

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps) TEMP 11/08/2002

μS/cm 25 32

COND 04/19/2005

3200 6200

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

April 2005

Geophysical data collected 

May 2006

Geophysical data collected 

September 2006

Geophysical data collected 

October 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System 590

595

600

605

Figure 19. Daily mean equivalent freshwater head and borehole geophysical data in Fish Hatchery 2 well (LR–67–09–401), San Antonio 

segment of the Edwards aquifer, south-central Texas, 2003–07. 
Lithologic and Physicochemical Properties 31

Figure 20. Daily mean equivalent freshwater head and borehole geophysical data in Kyle 1 well (LR–67–01–311), San Antonio segment 

of the Edwards aquifer, south-central Texas, 2002–07. 

300

400

500

600

700

800

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

4

Flowstation 05/03/2006

Corrected flowstation 06/15/2006

Corrected flowstation 09/06/2006

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER

-4

4

Flowstation 03/09/2005

-4 gpm

-4 4

-4 4

Corrected flowstation 11/05/2007

-4 gpm 4

Stratigraphic unit

22 29

COND 03/01/2006

TEMP 06/15/2006

TEMP 05/03/2006

25 32

COND 06/15/2006

1160 4160

TEMP 09/06/2006

23 30

COND 09/06/2006

6350 9350

COND 11/05/2007

16330 19330

TEMP 11/05/2007

23 30

deg C

deg C

deg C

deg C

deg C

15460 μS/cm 18460

μS/cm

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps) TEMP 03/01/2006

μS/cm 25 32

COND 05/03/2006

3200 6200

22 29

COND 11/05/2002

TEMP 03/09/2005

deg C

deg C

15460 μS/cm 18460

TEMP 11/05/2002

μS/cm 25 32

COND 03/09/2005

3200 6200

530

540

550

560

570

580

590

600

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Geophysical data collected 

March 2005

Geophysical data collected 

March 2006

Geophysical data collected 

May 2006

Geophysical data collected 

June 2006

Geophysical data collected 

September 2006

Geophysical data collected 

November 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System

EXPLANATION
32 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 21. Daily mean equivalent freshwater head and borehole geophysical data in Kyle 2 well (LR–67–02–104), San Antonio segment 

of the Edwards aquifer, south-central Texas, 2003–07. 

500

600

700

800

900

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

Corrected flowstation 06/13/2006

Corrected flowstation 09/12/2006

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER -0.5 0.5

-0.5 0.5

Corrected flowstation 11/08/2007

-0.5 gpm 0.5

Stratigraphic unit

24.25 30.25

COND 01/10/2005

TEMP 06/13/2006

TEMP 09/12/2006

21 27

COND 09/12/2006

0 39500

COND 11/08/2007

0 39500

TEMP 11/08/2007

23.25 29.25

deg C

deg C

deg C

deg C

0 μS/cm 39500

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

degrees Celsius (deg C)

Natural gamma (GAM(NAT)), in

counts per second (cps)

TEMP 01/10/2005

μS/cm 21 27

COND 06/13/2006

0 39500

22 28

COND 01/31/2003

TEMP 07/28/2003

deg C

deg C

1000 μS/cm 40500

TEMP 01/31/2003

μS/cm 22 28

COND 07/28/2003

0 39500

540

550

560

570

580

590

600

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

January 2005

Geophysical data collected 

June 2006

Geophysical data collected 

September 2006

Geophysical data collected 

November 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the U.S. Geological

Survey
Lithologic and Physicochemical Properties 33

Figure 22. Daily mean equivalent freshwater head and borehole geophysical data in Kyle 3 well (LR–67–02–106), San Antonio segment 

of the Edwards aquifer, south-central Texas, 2002–07. 

600

700

800

900

1000

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Del Rio Clay

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

Glen Rose

Limestone

Estimated flow less than

0.1 gallon per minute (gpm)

1

Corrected flowstation 05/18/2005

Corrected flowstation 06/19/2006

Corrected flowstation 09/13/2006

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER

-1

-1 1

-1 1

Corrected flowstation 11/07/2007

-1 gpm 1

Stratigraphic unit

24.8 31.8

COND 05/18/2005

TEMP 06/19/2006

TEMP 09/13/2006

20.5 27.5

COND 09/13/2006

4000 34000

COND 11/07/2007

5000 35000

TEMP 11/07/2007

23.5 30.5

deg C

deg C

deg C

deg C

0 μS/cm 30000

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps)

TEMP 05/18/2005

μS/cm 22 29

COND 06/19/2006

5000 35000

22 29

COND 11/04/2002

18000 μS/cm 48000 deg C

TEMP 11/04/2002

560

570

580

590

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

DATE

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

May 2005

Geophysical data collected 

June 2006

Geophysical data collected 

September 2006

Geophysical data collected 

November 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System
34 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Estimated flow less than

0.1 gallon per minute (gpm)

1

Corrected flowstation 05/19/2005

Corrected flowstation 06/20/2006

Corrected flowstation 09/14/2006

gpm

gpm

gpm

Zone of inflow or outflow

Depth1 inch:100 feet

Acoustic televiewer

0° 180°90° 270° 0°

0 cps 100

GAM(NAT)

2 inch 12

CALIPER

-1

-1 1

-1 1

Corrected flowstation 11/06/2007

-1 gpm 1

Stratigraphic unit

24.2 29.2

COND 05/19/2005

TEMP 06/20/2006

TEMP 09/13/2006

23 28

COND 09/13/2006

10000 35000

COND 11/06/2007

7000 32000

TEMP 11/06/2007

25 30

deg C

deg C

deg C

deg C

10000 μS/cm 35000

μS/cm

μS/cm

Generalized caliper profile

Conductance (COND), in

microsiemens

per centimeter (μS/cm)

Temperature (TEMP), in

Natural gamma (GAM(NAT)), in degrees Celsius (deg C)

counts per second (cps)

TEMP 05/19/2005

μS/cm 23.8 28.8

COND 06/20/2006

10000 35000

23.7 28.7

COND 11/08/2002

25000 μS/cm 50000 deg C

TEMP 11/08/2002

600

700

800

900

Note: “Corrected” indicates that EM flowmeter data sets were corrected by zeroing out values measured in intervals

where no flow was expected, such as in the casing, and by subtracting those values from flowmeter measurements made

in open borehole; this adjustment allowed for more consistent flowmeter data sets and well hydraulic interpretations.

Georgetown

Formation

Person Formation Kainer Formation

Cyclic and

marine members

Leached and

collapsed

members

Regional

dense member

Grainstone

member

Kirschberg

evaporite

member

Dolomitic

member

Basal nodular

member

570

575

580

585

590

595

600

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT

NOV

DEC

2005 2006 2007

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

Geophysical data collected 

May 2005

Geophysical data collected 

June 2006

Geophysical data collected 

September 2006

Geophysical data collected 

November 2007

Daily mean equivalent 

freshwater head, in feet 

above North American

Vertical Datum of 1988.

Water-level data provided

by the San Antonio Water

System

DATE

Figure 23. Daily mean equivalent freshwater head and borehole geophysical data in Kyle 4 well (LR–67–02–105), San Antonio segment 

of the Edwards aquifer, south-central Texas, 2002–07. 
Lithologic and Physicochemical Properties 35

description of conductivity or temperature, but the principal 

discussion of vertical borehole flow is in the “Hydraulics of 

Flow” section. 

Water in the boreholes is classified as fresh or saline on 

the basis of dissolved solids concentration. So that conductivity can be associated with the salinity descriptors based on 

dissolved solids concentration as noted in the “Introduction” 

section, specific conductance and dissolved solids concentration from monitoring well data (Lambert and others, 2009) 

were related by regression (not included here) to yield the following threshold values of conductivity corresponding to the 

threshold values of dissolved solids concentration that describe 

freshwater and saline water

Descriptor

Dissolved solids 

concentration 

(milligrams per liter)

Conductivity

(microsiemens per 

centimeter at 25 

degrees Celsius)

Freshwater 1,000 1,520

Slightly saline water 3,000 4,560

Moderately saline 

water 10,000 15,200

Very saline water 35,000 53,200

Fluid Conductivity

East Uvalde Transect

Well EU1 is close to the interface of the aquifer, and 

fluid conductivities recorded during 2002–07 in EU1 indicate 

freshwater down the entire length of the borehole (fig. 10). At 

about 1,280-foot depth, the October 2007 log (curve) shows a 

sharp increase in fluid conductivity, and the September 2006 

log shows the beginning of a more subtle increase; no visible increase is evident on the three other logs. The increases 

do not appear to be related to heads measured at the time of 

the log runs, because the head in October 2007, when a sharp 

increase in conductivity at about 1,280 feet occurred, was near 

the head in April 2005, when no increase in conductivity at 

about 1,280 feet occurred. Minimal vertical flows (less than 

0.1 gallon per minute) do not appear to affect conductivity in a 

consistent manner.

Fluid conductivities recorded in well EU2 indicate freshwater down the entire length of the borehole (fig. 11). The 

presence of freshwater in the borehole was expected, as EU2 

is in a part of the freshwater zone of the aquifer that transmits 

substantial quantities of freshwater (fig. 4). Fluid conductivity of all the logs is relatively constant down the borehole, 

although slight variability at the top of the borehole decreases 

with depth, to about 1,500 feet where conductivity in all the 

logs decreases. As with well EU1, changes in conductivity do 

not appear to be related to heads measured at the time of the 

log runs. Also as with well EU1, vertical flow in the borehole 

for all log runs was less than 0.1 gallon per minute, and no 

consistent relation between conductivity and flow appears evident. An inflow/outflow zone is coincident with the inflection 

points in the conductivity curves at about 1,500 feet, where 

water in the borehole becomes fresher; the fresher water might 

be related to the coincident inflow/outflow zone. The inflection is more subdued in the April 2005 and April 2006 curves, 

which are associated with slight downward flow; whereas the 

inflection is more prominent in the October 2006 and October 

2007 logs, which are associated with slight upward flow. 

The logs run in well EU3 indicate saline water in the 

borehole during 2005–07 (fig. 12). The most salient feature 

of the fluid conductivity logs for well EU3 is a sharp increase 

in conductivity at about 1,240 feet visible in each of the logs. 

The increases for April 2006 and especially October 2007 are 

substantially larger than the increases for the other months. As 

with conductivity in wells EU1 and EU2, conductivity in EU3 

does not appear to be related to head measured at the time 

of the log runs—the head when the largest sharp increase at 

about 1,240 feet occurred (October 2007) was about the same 

as the head when one of the smallest sharp increases at about 

1,240 feet occurred (April 2005). Vertical flow in the borehole 

of well EU3 during the log runs was relatively large for the 

April 2005 log compared to that of the logs for September 

2006 and October 2007 and mostly upward. Conductivity 

throughout the borehole was more consistent in April 2005, 

when flow was relatively large and upward, than in October 

2007, when flow was minimal and mostly downward; but the 

conductivity log for September 2006, when flow also was 

minimal and mostly downward, is similar to the log of April 

2005. So on the basis of these mixed results, the relation 

between vertical flow in the borehole and conductivity appears 

problematic. One other notable feature of the conductivity logs 

is the apparent coincidence between the sharp increases in 

conductivity at about 1,240 feet and an inflow/outflow zone in 

the lower part of the McKnight Formation.

Well EU4 contained saline water during four of six 

log runs, but for one of the runs (September 2006), salinity 

was entirely in the freshwater range and for another (March 

2005) was in the fresh and slightly saline ranges (fig. 13). 

Except for the run in November 2002, the conductivity logs 

show consistency, or minimal variation, throughout the 

length of the borehole. No concurrent head data are available for comparison. Vertical flow in the borehole during the 

log runs was mostly less than 0.1 gallon per minute, and any 

salient effects of vertical flow on conductivity values are not 

apparent. 

Tri-County Transect

The fluid conductivity logs for well TC1 indicate that the 

borehole fluid column contained freshwater with higher conductivities measured in the upper part of the borehole, above 

about 530 feet, associated with the cyclic and marine members 

and the leached and collapsed members (fig. 14). The fluid 

conductivity in the middle part of the borehole from about 

530 to 660 feet was nearly constant with increasing depth. 

The lack of change in conductivity with depth is attributed to 

downward flow in the borehole, which appears to have entered 
36 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

the borehole from the cyclic and marine members and the 

leached and collapsed members, increased in magnitude down 

the borehole, moved past the regional dense member, and 

then exited into the grainstone member, Kirschberg evaporite 

member, and upper part of the dolomitic member. From about 

660 feet to the bottom of the well, fluid conductivity increased 

with depth as downward flow in the borehole decreased. 

Relative differences between conductivity values in individual 

logs do not relate in a consistent manner to head measured at 

the time of the log runs. In other words, larger relative differences are not consistently related to relatively high head or 

relatively low head. 

On the basis of fluid conductivity, well TC2 intersected 

the interface at a depth of about 800 feet, at or slightly above 

the contact between the Kirschberg evaporite member and the 

dolomitic member (fig. 15). Above about 800 feet, freshwater 

was in the borehole during all log runs, and below about 800 

feet, saline water was in the borehole. At about 600 feet, 

there was an increase in fluid conductivity on all logs coincident with an inflow/outflow zone at the contact between the 

cyclic and marine members and the leached and collapsed 

members; downward flow in the borehole appears on the 

flow logs (except the April 2006 log) to begin at or near this 

inflow/outflow zone, increase at an inflow/outflow zone in 

the leached and collapsed members, and cease at about 

740 feet as water appears to have entered the grainstone 

member at an inflow/outflow zone. From about 600 feet to 

the interface at about 800 feet, fluid conductivity indicated 

by all the conductivity logs was essentially constant with 

depth. Although the interface position in the borehole was 

about 60 feet below the zone at which borehole flow ceased, 

the relative increase in conductivity was least for the April 

2006 log, the log for which no borehole flow was indicated. 

Whether or not borehole flow was occurring did not affect 

the position of the interface, which is essentially the same 

on all logs. Similarly, changes in head between the times 

the logs were run did not affect the position of the interface. 

If changes in borehole flow or changes in head affected relative increases in the conductivity values in the saline section 

of the borehole, it was not in a consistent or identifiable 

manner. 

The fluid conductivity logs for well TC3 indicate saline 

water along the entire length of the borehole for all log runs 

(fig. 16). The shape of three of the four logs (2005 and 2006 

logs) from top to bottom was consistent and nearly vertical, 

although the ranges of values reflected among the logs varied 

widely (slightly saline to very saline). No head data concurrent with the log runs are available. Whether mostly downward borehole flow (April 2006) or mostly upward borehole 

flow (October 2006) was occurring did not appear to affect 

the shape of the curves or the conductivity values. The 

January 2003 log, for which conductivity values are substantially larger than those of the 2005 and 2006 logs, shows 

increasing conductivity with depth beginning at about 1,100 

feet, coincident with an inflow/outflow zone in the middle 

of the dolomitic member. But no borehole flow data are 

available concurrent with that log run, so whether borehole 

flow influenced conductivity changes cannot be assessed. 

The fluid conductivity logs of well TC4 (fig. 17) indicate 

saline water down the entire length of the borehole and are 

generally similar in shape—consistent and nearly vertical—to 

corresponding logs of well TC3 (fig. 16); and as with well 

TC3, the ranges of values reflected among the logs varied 

widely (slightly saline to very saline). Conductivity values 

reflected in the three logs for which concurrent head data are 

available (October 2005, April 2006, and October 2006) do 

not relate in a consistent manner to head measured at the time 

of the log runs. In other words, relatively high conductivity 

values are not consistently related to relatively high head or 

relatively low head. There are two conductivity logs for which 

concurrent flow measurements are available, April 2006 and 

October 2006. Conductivity values reflected in the April 2006 

log, when no borehole flow was measured, were about 3,000 

to 4,000 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm) less than conductivity values reflected in the October 

2006 log, when upward borehole flow between 0.1 and 1 

gallon per minute was measured throughout nearly all of the 

borehole (dolomitic member to Georgetown Formation). Head 

in the well during the April 2006 log run with no borehole 

flow was about was about 7–8 feet higher than head in the well 

during the October 2006 log run with upward borehole flow.

Fish Hatchery Transect

The fluid conductivity logs for well FH1 all indicate 

freshwater in the borehole (fig. 18). The logs were consistent 

in shape (vertical) and conductivity value down the short 

length of the borehole, except for the October 2007 log, which 

showed a sharp increase in conductivity (still well within the 

freshwater range) at about 225 feet. Other than the October 

2007 logs, heads measured at the time of the log runs and vertical flow in the borehole, or lack of, did not affect the shape 

of conductivity logs or values in identifiable ways.

The fluid conductivity logs for well FH2 indicated 

saline water, ranging from slightly saline to very saline 

(fig. 19). The fluid conductivity curves from November 2002 

and October 2007 show changes in slope to more vertical in 

the upper part of the borehole from about 575 to 580 feet; 

conductivity values for the two logs throughout the borehole were in the very saline range. The three other logs (April 

2005, May 2006, and September 2006) do not show a similar 

change in slope, and measured conductivity values generally 

were lower, indicating slightly to moderately saline water. 

No identifiable relation between conductivities and heads at 

the times the logs were run is apparent. Concurrent flow logs 

are available for all the conductivity logs except November 

2002. Flow among the logs, essentially all upward from the 

basal nodular member to the cyclic and marine members, 

ranges from negligible (October 2007, very saline water) to 

as much as about 1.1 gallons per minute (September 2006, 

moderately saline water). The relation between conductivity 

and flow among the logs is not consistent. 
Lithologic and Physicochemical Properties 37

Kyle Transect

Fluid conductivity logs for well KY1 indicate freshwater 

throughout the borehole during all log runs (fig. 20). In the 

upper part of the borehole from about 350 to 460 feet, most 

of the fluid conductivity curves show a series of “steps” of 

increasing fluid conductivity with depth from the top of the 

borehole (Georgetown Formation) to about 400 feet (near the 

base of the leached and collapsed members). Fresher water 

near the bottom of the borehole at about 750 feet is indicated 

on each of the logs. This fresher water appears to be in the 

Glen Rose Limestone (Trinity aquifer). Upward flow from 

the Glen Rose Limestone to the base of the leached and collapsed members is indicated on all available flow logs, ranging 

from negligible (May 2006) to nearly 4 gallons per minute 

(September 2006). The consistent and similar shapes of the 

fluid conductivity curves down the length of the borehole from 

about 400 to about 750 feet and the nearly vertical slopes of 

the curves are attributed to the substantial (relative to flows in 

boreholes of wells in other transects) upward flow from the 

Trinity aquifer that exits the borehole into numerous inflow/

outflow zones of the Edwards aquifer. 

Well KY2 is the second of two wells of this report 

that intersects the interface. Fluid conductivity logs show 

that the interface intersected the borehole in the lower part 

at a depth of about 650 to 750 feet, corresponding to inflow/

outflow zones in the Kirschberg evaporite member and 

upper part of the dolomitic member (fig. 21). The range in 

salinity of water between the upper and lower sections of 

the borehole indicated by most of the logs is large—from 

fresh to very saline. Unlike in interface well TC2, the position 

of the interface in the borehole changed about 50 feet over 

time. The position of the interface in the borehole during the 

June 2006 conductivity log (about 700 feet) was close to that 

of the September 2006 log, and the heads at the times of the 

two logs are similar (540–550 feet above NAVD 88); the 

position of the interface in the borehole during the January 

2005 log (about 750 feet) was close to that of the November 

2007 log, and the heads at the times of those two logs are 

similar (570–580 feet above NAVD 88). Although the position of the interface bears some relation to head at the time 

the logs were run, the conductivity values below the interface 

do not. The lowest conductivity values below the interface 

among the logs (November 2007) occurred when head was 

relatively high; and the highest conductivity values below the 

interface among the logs (January 2005) also occurred when 

head was relatively high, about the same as that in November 

2007. 

Fluid conductivity logs run in well KY3 indicate that 

water in the borehole at the times of the log runs was moderately to very saline (fig. 22). The shapes of the conductivity 

curves are mostly consistent among the logs and show little 

variation with depth. No identifiable relation between conductivity values and head at the times the logs were run is evident; 

and no identifiable relation between conductivity values and 

the mostly upward flow in the borehole concurrent with the 

times the logs were run is evident.

The fluid conductivity logs run in well KY4 indicate 

moderately to very saline water, with little variation in fluid 

conductivity down the length of the borehole (fig. 23). In the 

November 2002 log, a spike in the fluid conductivity in the 

upper part of the cyclic and marine members is associated 

with a fracture at about 580 feet and identified as an inflow/

outflow zone. At the bottom of the borehole at about 980 feet, 

a small spike in the fluid conductivity is associated with an 

inflow/outflow zone in the basal nodular member. This spike 

in conductivity might have been higher-salinity water that 

entered the borehole from a fracture or a vug. No identifiable 

relation between conductivity values and head at the times the 

logs were run is evident; and no identifiable relation between 

conductivity values and flow in the borehole (mixed among 

the logs between upward and downward) concurrent with the 

times the logs were run is evident.

Temperature

East Uvalde Transect

The temperature logs run in well EU1 indicate a gradual 

increase in the temperature down the borehole for all logging 

runs and slight changes in slope of the temperature curves 

occurring at about 1,250 feet (fig. 10). Most temperatures 

measured were between about 29 and 33 degrees Celsius (°C), 

although temperatures indicated by the November 2002 log 

were about 22 °C. The greatest range in temperature among 

the temperature curves is at the top and bottom of the borehole. The changes in slope of the temperature curves at about 

1,250 feet coincide with an inflow/outflow zone in the middle 

part of the Devils River Limestone. Below the 1,250-foot 

inflow/outflow zone, the borehole temperature increases correlate with increases in the fluid conductivity that occur at about 

1,280 feet in the September 2006 and October 2007 logs. 

The temperature logs run in well EU2 show a pattern 

similar to the pattern of temperature logs run in well EU1, 

indicating increasing borehole temperature with increasing depth (fig. 11). The temperatures measured in well EU2 

logging runs ranged from about 20 to 24 °C, generally cooler 

than the temperatures measured in well EU1. In the lowermost 

section of the borehole, the temperature curves are consistent 

with the fluid conductivity curves in that they reflect change 

to slightly warmer temperatures and slightly fresher water, 

respectively.

For well EU3, the range in temperature in the borehole 

was from about 29 to 36 °C for all logging runs (fig. 12). 

Three distinct segments of the temperature curves are indicated down the length of the borehole. The first section is in 

the upper part of the borehole (from about 790 to 1,000 feet) 

associated with the Salmon Peak Formation. The concaveupward configuration of the temperature curves might be 

related to downward flow that entered the borehole from an 

inflow/outflow zone in the Salmon Peak Formation at about 

790–825 feet and upward flow that entered the borehole 
38 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

from an inflow/outflow zone in the McKnight Formation at 

about 1,090 feet, both of which appear to have converged to 

exit the borehole in an inflow/outflow zone in the Salmon 

Peak Formation at about 900–920 feet. The second distinct 

temperature section occurred from about 1,070 to 1,240 feet 

and is associated with the McKnight Formation. The temperature logs in this depth range show more-vertical slopes, 

which do not appear to be associated with borehole flow 

because flow directions were mixed among the similarly 

shaped temperature logs. The third and lowermost segment 

of the temperature logs from about 1,240 feet to the bottom 

of the borehole, associated with the West Nueces Formation, 

shows temperatures increasing with depth at a greater rate 

than in the overlying borehole segment. Temperatures increasing with depth at a greater rate are consistent with increases 

in fluid conductivity and minimal flows (estimated to be 

less than 0.1 gallon per minute) in this lowermost borehole 

segment.

The fluid temperatures in well EU4 were the warmest 

of any of the wells of this report, with a range in temperature 

from about 32 to nearly 40 °C (fig. 13). All the temperature 

logs indicate a steady increase in temperature with depth until 

the lower part of the McKnight Formation, where the rate of 

increase lessens. The curves reflect relatively undisturbed 

water in the borehole, which is consistent with the lack of 

substantial variation in all but one of the conductivity logs and 

minimal flows in the borehole.

Tri-County Transect

The fluid temperature logs for well TC1 show increasing temperature with depth and range from about 21 to 26 °C 

(fig. 14). The logs are similar in shape, each indicating subtle 

points of inflection (changes in slope) at about 560 feet, 

coincident with an inflow/outflow zone in the lower part of 

the leached and collapsed members, and at about 680 feet, 

coincident with the contact between the grainstone member 

and the Kirschberg evaporite member. The zone between the 

points of inflection thus described is generally that of maximum vertical slope of the temperature logs and maximum 

downward flow in the borehole, as indicated on all flow logs. 

The relatively greater rates of increase in temperature with 

depth below about 680 feet are consistent with relatively 

greater rates of increase in conductivity in generally the same 

section of the borehole.

The fluid temperature logs for well TC2 show generally 

consistent and gradual increases in temperature with depth in 

the upper part of the borehole from about 500 to about 800 

feet (fig. 15); temperatures range from about 23 to 31 °C. At 

about 800 feet, where the interface intersects the borehole, a 

“shoulder” in the temperature curves coincides with a similar 

shoulder in the fluid conductivity curves. In the saline-water 

zone below the interface, temperature increased with depth at 

a slightly greater rate than in the freshwater zone above the 

interface. As with conductivity values, temperatures do not 

appear to have been affected by downward flow in the borehole (except on the April 2006 flow log) that originated in the 

leached and collapsed members and ceased in the grainstone 

member. 

The fluid temperature logs in well TC3 show increasing 

temperature with depth and range from about 24 to 33 °C 

(fig. 16). Slight changes in slope (to more vertical) of the 

temperature curves occur in the upper part of the borehole 

at about 690 feet, and more pronounced changes in slope 

(to less vertical) occur near the bottom of the well at about 

1,100 feet. The changes in slopes at about 690 and 1,100 

feet correspond to inflow/outflow zones near the top of the 

cyclic and marine members and in the middle of the dolomitic 

member, respectively. Mostly upward flow from the dolomitic 

member to the cyclic and marine members occurred during 

the October 2006 temperature log, and mostly downward 

flow from and to the same units occurred during the April 

2006 log; relatively vertical temperature-log slopes with 

negligible variation might be associated with borehole flow in 

either direction.

As with other temperature logs, temperature logs for 

well TC4 show increasing temperatures with depth (fig. 17); 

temperatures range from about 29 to 39 °C. Slight changes 

in slope to more vertical occurred in the temperature logs at 

about 1,100 feet, coincident with a relatively large inflow/

outflow zone at the contact between the cyclic and marine 

members and the leached and collapsed members. The slope 

of the October 2006 temperature log over the length of the 

borehole, associated with upward flow in the borehole over 

most of its length, is slightly less than the slope of the April 

2006 temperature log, associated with negligible flow in the 

borehole over most of its length. 

Fish Hatchery Transect

The temperature logs for well FH1 were consistent down 

the length of the borehole and ranged from about 18 to 24 °C 

(fig. 18); slopes were essentially vertical, and variation was 

minimal. Vertical flow in the borehole, or lack of, did not 

affect the temperature curves in identifiable ways.

For well FH2, the temperature curves show changes in 

slope of the curves to more vertical at about 575–580 feet 

(fig. 19); temperatures range from about 23 to 32 °C. The temperature from about 580 to about 980 feet increased uniformly 

with depth for each log. Upward flow in the borehole from the 

basal nodular member to the leached and collapsed members 

(to depths coincident with the changes in slope of the temperature curves to more vertical) during the temperature logs 

ranged from negligible (October 2007) to about 1 gallon per 

minute (September 2006); because shapes of the temperature 

curves were about the same whether borehole flow was negligible or measurable, the effect of flow on fluid temperature is 

unclear.

Kyle Transect

The borehole temperature logs for well KY 1 show a 

step-like pattern of increasing temperature in the upper part 

of the aquifer from about 350 to 460 feet, similar to but more 
Hydraulics of Flow 39

subdued than the step-like pattern of the conductivity logs for 

the same section of the borehole (fig. 20); temperatures range 

from about 20 to 29 °C. From about 460 to 750 feet, the temperature curves indicated near-vertical slopes with negligible 

variation in temperature values. The temperatures in this section of the borehole might have been influenced by substantial 

(relative to flows in boreholes of wells in other transects) 

upward flow from the Trinity aquifer that exits the borehole 

into numerous inflow/outflow zones of the Edwards aquifer. 

The temperature logs for well KY2, the second of 

two wells that intersect the interface, show consistency and 

increasing temperature with depth down the borehole to about 

650 feet (fig. 21); temperatures range from about 21 to 27 °C. 

Changes in slope of the temperature curves to less vertical 

(temperatures increasing with depth at a greater rate than in 

the overlying, upper borehole segment) occurred at about 650 

feet. The points of inflection are coincident with the uppermost depth of the interface recorded on a fluid conductivity 

log (January 2003 log). Vertical flows in the borehole concurrent with temperature logs were less than 0.5 gallon per 

minute and mixed with regard to direction, so evaluating the 

influence of borehole flow on temperature in this interface 

well is problematic.

The temperature logs for well KY3 also show increasing temperature with depth, with temperatures ranging from 

about 21 to 28 °C (fig. 22). The temperature logs show little 

variation in slope with depth, except for the September 2006 

log, which indicates minor variability at about 630 feet and 

an inflection point at about 850 feet that begins a generally greater rate of increase in temperature with depth to 

the bottom of the borehole. Flow in the borehole during the 

September 2006 temperature log was less than 0.05 gallon 

per minute, mostly downward, and somewhat greater in the 

segment below about 850 feet coincident with the greater rate 

of increase in temperature with depth to the bottom of the 

borehole. 

The temperature logs in well KY4, as with the logs in 

other wells, show increasing temperature with depth (fig. 23); 

temperatures range from about 23 to 30 °C. As with temperature logs for well KY3, the most variability is evident in the 

September 2006 log. Also like the September 2006 log for 

well KY3, the rate of increase in temperature with depth in 

the lower segment of the borehole is generally greater than 

that in the upper segment of the borehole; the same applies 

to the June 2006 temperature log, although to a lesser degree. 

Vertical borehole flow is mixed among the logs with regard 

to direction and did not affect temperatures in an identifiable 

way.

Hydraulics of Flow

Hydraulics of flow in and near the freshwater/salinewater transition zone of the aquifer in this report focuses on 

two topics: (1) lateral head gradients computed from equivalent freshwater heads to evaluate the potential for lateral flow 

at the interface; and (2) measurements of vertical flow in 

boreholes to indicate the directions of vertical gradients in 

the aquifer and thus the potential for upward movement of 

water. The interface is conceptualized as a surface sloping 

upward toward the direction of dip of the stratigraphic units as 

indicated in the hydrogeologic sections in figures 5–8, which 

implies horizontal and vertical components of head gradient 

across the interface. 

Lateral Head Gradients at the Interface

Lateral head gradients, computed from equivalent 

freshwater heads for periods of coincident water-level data 

for 2000–2007, are used to indicate whether potential lateral 

flow at the interface in the East Uvalde, Tri-County, and Kyle 

transects was into the freshwater zone, out of the freshwater 

zone, or mixed with regard to direction (head higher or lower 

at the interface than on either side) for the respective periods 

of record. Lateral gradients were not computed for the Fish 

Hatchery transect because of the relatively large difference in 

altitude between the open-hole sections of wells FH1 and FH2, 

caused by fault offset (fig. 7) and the relative shallowness of 

well FH1; in other words, the difference in vertical altitude 

between the open-hole sections of the two monitoring wells 

was considered too large for accurate equivalent freshwater 

heads to be computed. The assumption of a horizontal aquifer, 

necessary for computation of accurate equivalent heads, was 

judged not applicable for the Fish Hatchery transect. 

East Uvalde Transect

Daily mean equivalent freshwater heads for the four East 

Uvalde transect monitoring wells for coincident periods of 

record during the study period generally were higher from 

November through May and lower from June through October 

for most years. Heads in all monitoring wells followed a similar pattern except when irrigation withdrawals in the Uvalde 

County area affected heads (caused drawdowns) in wells EU3 

and EU4, which occurred for the most part in the summer 

(June–September) of each year (fig. 24). Heads in wells EU1 

and EU2 were very similar, indicating that a hydraulic connection between these two wells is likely. Heads in wells EU3 

and EU4 were similar, indicating that a hydraulic connection 

between the two saline-water wells also is likely (fig. 24). 

The directions of lateral head gradient at the interface in 

the transect for 19 coincident periods of record for 2000–2007 

indicate a nearly even split between into and out of the freshwater zone (table 3). The direction of the lateral head gradient 

along the East Uvalde transect was out of the freshwater zone 

into the transition zone during a majority of coincident periods 

of record (10 of 13) during January 2000–November 2005 

(table 3). Annual rainfall at San Antonio (fig. 9) was above 

normal for 4 of the 6 years of 2000–2005, and the lateral head 

gradients computed for January 2000–November 2005 likely 

reflect, on average, a period of above-normal (wet) hydrologic 
40 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

conditions, implying relatively high recharge. Despite likely 

above-normal conditions on average, the hydrographs in 

figure 24 indicate relatively substantial influence of irrigation 

withdrawals on the heads in wells EU3 and EU4; without the 

effects of irrigation withdrawals, the lateral gradients during 

much of the 2000–2005 period of record might have been 

reversed. 

During December 2005–December 2007, the direction 

of the lateral head gradient was from the transition zone into 

the freshwater zone (six of six coincident periods of record). 

Annual rainfall at San Antonio was below normal for 2006 

and above normal for 2007 (fig. 9); and rainfall during 2005 

was only 50 percent of normal. The lateral head gradients 

computed for coincident periods during December 2005–

December 2007 likely reflect, on average, a period of belownormal (dry) hydrologic conditions, implying relatively low 

recharge. However, the hydrographs in figure 24 indicate relatively less influence of irrigation withdrawals on the heads in 

wells EU3 and EU4 during 2005–07 than during 2000–2005. 

Relatively less irrigation withdrawals during dry conditions is 

counterintuitive: It might be that the effect of irrigation withdrawals on heads is correlated more closely with antecedent 

hydrologic conditions (antecedent recharge) than concurrent 

conditions. Regardless, the directions of the gradient at the 

interface along the East Uvalde transect during all 19 coincident periods of record are affected to a greater or lesser degree 

by hydrologic conditions (antecedent and concurrent recharge) 

and relatively greater and smaller withdrawals. Based on 

these findings, albeit for a relatively short composite period of 

record, the direction of the head gradient and thus the potential 

lateral flow at the interface in the vicinity of the East Uvalde 

transect fluctuates between into and out of the freshwater 

zone, depending on recharge and withdrawals. Whether the 

prevailing direction on average is into or out of the freshwater 

zone is not clearly indicated; previously, Maclay (1995, p. 37) 

reported on the basis of “limited” data that the prevailing 

hydraulic gradient in Uvalde County (and Bexar County) was 

out of the freshwater zone, and that “most of the flow from 

the freshwater zone of the aquifer to the saline-water [transition] zone is in southeastern Uvalde and southwestern Medina 

Counties.”

Tri-County Transect

Equivalent freshwater heads in all five monitoring wells 

in the Tri-County transect for coincident periods of record 

from April 2000 through December 2007 show similar patterns of response (fig. 25). The lowest equivalent freshwater 

heads generally were during the summer (June–September) of 

each year and drier-than-normal periods. Equivalent freshwater heads in freshwater wells TC1 and TC5 were consistently lower than those in saline-water well TC3 and interface 

well TC2 and generally higher relative to altitudes of equivalent freshwater heads in saline-water well TC4. The greatest 

Figure 24. Daily mean equivalent freshwater head in monitoring wells of the East Uvalde transect, San Antonio segment of the 

Edwards aquifer, south-central Texas, 1999–2007. 

291443099325801 (EU1)

291612099302001 (EU2)

291136099375801 (EU3)

291133099363801 (EU4)

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

DATE

1999 2000 2001 2002 2003 2004 2005 2006 2007

850

800

750

700

650

600

550

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Graph enlarged on figure 27
Hydraulics of Flow 41

variation in equivalent freshwater head was in the freshwater 

and interface wells (TC5, TC1, and TC2), although head in 

saline-water well TC3 was similar to that in the three wells 

during 2000–2002. 

The directions of the lateral head gradient in the transect are indicated for 30 coincident periods of record for 

2000–2007 (table 4). The direction of the lateral head gradient 

in all but one of the coincident periods was either mixed (25) 

or uncertain (four) (table 4). Equivalent freshwater heads at 

the interface (well TC2) were higher on average than heads on 

the freshwater side of the interface (wells TC1 and TC5) and 

on the saline-water side of the interface (wells TC3 and TC4), 

regardless of whether hydrologic conditions were wet or dry. 

Why equivalent freshwater heads generally were higher at the 

interface than on either side of it cannot be explained with 

available data. There does not appear to have been a prevailing 

direction of the lateral gradient at the interface in the vicinity 

of the Tri-County transect. 

A factor that might adversely affect the accuracy of 

equivalent freshwater heads, and thus computed gradients 

along the transect, is that altitudes of the open intervals of the 

wells are substantially different because of faulting (fig. 6), 

which might make the assumption of a horizontal aquifer 

(necessary for computation of accurate equivalent freshwater 

heads) not valid. Other factors also add uncertainty to the computations: for example, structural complexity associated with 

faulting could result in compartmentalization of the aquifer 

into zones of differing head; the simplifications necessary in 

the computation of fluid density (depth-averaging of fluid conductivity and temperature); the assumption that a single fluid 

density correction factor is applicable for each well, despite 

variable hydrologic conditions, as discussed in the section 

“Water-Level Data—Computation of Equivalent Freshwater 

Heads.” 

Kyle Transect

Equivalent freshwater heads during coincident periods for 

freshwater well KY1 and interface well KY2 responded similarly to varying hydrologic conditions and withdrawals from a 

nearby public-supply well (fig. 26). In general, when heads in 

the two wells peaked (reflecting relatively wet conditions), the 

Table 3. Summary of average daily mean equivalent freshwater heads and directions of lateral head gradients for coincident periods 

of record for East Uvalde monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards 

aquifer, south-central Texas, 2000–2007.

[NAVD 88, North American Vertical Datum of 1988; grayshade represents interface location; --, no data]

Period 

number

Period start 

date

Period end 

date

Length 

of 

period 

(days)

Average daily mean equivalent freshwater head for period

(feet above NAVD 88)

Direction of lateral head 

gradient across freshwater/

saline-water interface 

relative to freshwater zone

East Uvalde transect monitoring wells1

EU2 EU1 EU4 EU3

1 1/20/2000 4/19/2000 91 761.68 760.35 775.11 775.18 Into

2 6/13/2000 7/3/2000 21 745.96 745.12 745.34 746.82 Into

3 7/20/2000 8/25/2000 37 737.73 735.92 637.91 657.65 Out of

4 10/25/2000 11/30/2000 37 753.57 753.05 -- 734.88 Out of

5 12/1/2000 2/6/2001 68 771.51 771.23 772.68 773.41 Into

6 9/11/2001 11/27/2001 78 773.31 772.49 757.96 760.76 Out of

7 1/11/2002 3/20/2002 69 780.75 779.35 763.22 766.93 Out of

8 3/23/2002 6/9/2002 79 770.40 769.11 -- 749.94 Out of

9 7/24/2002 10/14/2002 77 791.59 790.51 -- 740.14 Out of

10 1/8/2003 4/12/2003 95 811.05 809.92 -- 808.79 Out of

11 7/20/2003 4/26/2004 272 794.97 794.03 -- 787.46 Out of

12 9/15/2004 1/18/2005 125 828.25 826.74 -- 826.57 Out of

13 5/20/2005 11/14/2005 179 809.19 807.50 -- 804.47 Out of

14 12/2/2005 2/7/2006 68 799.05 797.54 -- 807.71 Into

15 2/22/2006 3/14/2006 21 786.84 785.11 -- 800.60 Into

16 8/10/2006 9/24/2006 46 751.20 750.39 -- 761.95 Into

17 10/4/2006 1/31/2007 120 755.52 754.99 -- 769.48 Into

18 3/20/2007 5/9/2007 51 762.86 762.69 -- 775.32 Into

19 10/24/2007 12/31/2007 69 825.72 824.60 -- 830.14 Into

1

 Listed in order of relative position in transect, from freshwater zone to transition zone.
42 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

DATE

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

1999 2000 2001 2002 2003 2004 2005 2006 2007

530

540

550

560

570

580

590

600

610

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

295853097532901 (KY1)

295858097521801 (KY2)

295829097512601 (KY3)

295730097503201 (KY4)

Graph enlarged on figure 31

Figure 26. Daily mean equivalent freshwater head in monitoring wells of the Kyle transect, San Antonio segment of the Edwards 

aquifer, south-central Texas, 1999–2007. 

Figure 25. Daily mean equivalent freshwater head in monitoring wells of the Tri-County transect, San Antonio segment of the Edwards 

aquifer, south-central Texas, 2000–2007. 

293610098152701 (TC1)

293424098134701 (TC2)

293245098121001 (TC3)

293058098110501 (TC4)

293632098172401 (TC5)

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

DATE

1999 2000 2001 2002 2003 2004 2005 2006 2007

620

630

640

650

660

670

680

690

700

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Graph enlarged on figure 28

(Wells TC1, TC2, TC4)
Hydraulics of Flow 43

head in well KY2 at the interface was lower than the head at 

KY1 on the freshwater side of the interface; and when heads 

in the two wells were at low points (reflecting relatively dry 

conditions), the head in KY2 was higher than the head at KY1. 

Equivalent freshwater heads from saline-water wells KY3 and 

KY4 in the transition zone were, for the most part, consistently higher than those of wells KY1 and KY2 and fluctuated 

considerably less than the heads in KY1 and KY2. 

The directions of lateral head gradient at the interface are 

indicated for 42 coincident periods of record (table 5). The 

direction of equivalent freshwater head gradients for coincident periods in the Kyle transect was into the freshwater zone 

for about one-half the periods (22), mixed for one-third of the 

periods (14), out of the freshwater zone for three periods, and 

uncertain for three. The longest continuous period of gradient 

into the freshwater zone (May 2005–December 2006) coincided with the middle and latter parts of the longest sustained 

period of below-normal hydrologic conditions during the study 

period (figs. 9, 26). The January 2000–November 2000 continuous period of gradient into the freshwater zone followed 

Table 4. Summary of average daily mean equivalent freshwater heads and directions of lateral head gradients for coincident periods 

of record for Tri-County monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards 

aquifer, south-central Texas, 2000–2007. 

[NAVD 88, North American Vertical Datum of 1988; grayshade represents interface location; --, no data; ?, uncertain (cannot be determined with available data)]

Period 

number

Period 

start date

Period end 

date

Length 

of period 

(days)

Average daily mean equivalent freshwater head for period

(feet above NAVD 88)

Direction of lateral head 

gradient across freshwater/

saline-water interface 

relative to freshwater zone

Tri-County transect monitoring wells1

TC5 TC1 TC2 TC3 TC4

1 4/22/2000 12/11/2000 234 -- 643.38 649.42 648.35 -- Mixed

2 12/14/2000 1/23/2001 41 -- 660.71 670.37 664.62 -- Mixed

3 2/10/2001 4/3/2001 53 -- 665.88 675.00 669.51 -- Mixed

4 5/12/2001 6/5/2001 25 665.67 666.06 674.65 670.70 -- Mixed

5 6/13/2001 8/29/2001 78 -- 654.28 659.83 659.76 -- Mixed

6 10/12/2001 11/17/2001 37 663.16 661.77 670.94 666.24 -- Mixed

7 1/9/2002 3/5/2002 46 668.04 668.62 676.82 672.48 -- Mixed

8 4/19/2002 5/21/2002 33 660.95 663.36 670.08 667.91 -- Mixed

9 5/22/2002 9/6/2002 108 662.64 660.85 670.00 666.42 656.05 Mixed

10 9/7/2002 9/29/2002 23 671.58 668.01 679.50 673.94 -- Mixed

11 10/3/2002 11/15/2002 44 676.06 672.81 684.51 -- -- ?

12 11/16/2002 2/23/2003 99 678.70 678.54 688.73 -- 663.55 Mixed

13 2/27/2003 3/27/2003 29 677.67 678.28 687.92 -- 664.40 Mixed

14 4/2/2003 7/2/2003 92 669.26 670.70 678.64 -- 662.64 Mixed

15 8/20/2003 1/27/2004 161 667.43 667.62 675.65 -- 660.65 Mixed

16 3/1/2004 6/8/2004 100 668.93 668.87 677.59 -- 661.53 Mixed

17 7/19/2004 10/6/2004 80 671.08 671.32 679.84 -- 664.16 Mixed

18 10/22/2004 11/24/2004 34 677.71 674.21 683.44 -- 663.93 Mixed

19 11/25/2004 12/16/2004 22 687.51 682.03 -- -- 665.59 ?

20 4/22/2005 8/22/2005 123 672.57 673.70 680.97 -- 667.57 Mixed

21 9/1/2005 10/18/2005 48 666.43 666.75 673.96 -- 663.06 Mixed

22 10/27/2005 12/9/2005 44 665.79 666.30 -- -- 662.29 ?

23 12/10/2005 4/16/2006 129 662.36 663.97 670.49 -- 661.17 Mixed

24 8/2/2006 8/28/2006 27 642.61 -- 648.34 -- 649.26 Into 

25 11/23/2006 1/3/2007 42 652.26 -- 659.66 -- 651.61 Mixed

26 1/23/2007 2/12/2007 21 657.98 656.87 -- -- 653.65 ?

27 3/22/2007 4/15/2007 25 661.64 658.98 667.94 -- 654.50 Mixed

28 4/20/2007 5/24/2007 35 664.60 663.28 672.19 -- 657.06 Mixed

29 6/13/2007 7/2/2007 20 665.58 664.36 672.81 -- 658.96 Mixed

30 11/7/2007 12/31/2007 55 676.80 677.43 685.45 -- 667.32 Mixed

1

 Listed in order of relative position in transect, from freshwater zone to transition zone.
44 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Table 5. Summary of average daily mean equivalent freshwater heads and directions of lateral head gradients for coincident periods 

of record for Kyle monitoring wells transecting the freshwater/saline-water interface, San Antonio segment of the Edwards aquifer, 

south-central Texas, 2000–07. 

[NAVD 88, North American Vertical Datum of 1988; grayshade represents interface location; --, no data; ?, uncertain (cannot be determined with available data)]

Period 

number

Period 

start date

Period end 

date

Length 

of period 

(days)

Average daily mean equivalent freshwater head for period

(feet above NAVD 88)

Direction of lateral head 

gradient across freshwater/

saline-water interface 

relative to freshwater zone 

Kyle transect wells1

KY1 KY2 KY3 KY4

1 1/11/2000 4/11/2000 92 561.22 563.67 581.26 583.91 Into

2 4/14/2000 5/31/2000 48 555.63 559.43 579.62 583.24 Into

3 6/2/2000 9/10/2000 101 551.63 556.01 577.87 582.37 Into

4 10/1/2000 12/7/2000 68 557.86 558.32 576.23 -- Into

5 12/8/2000 5/4/2001 148 577.64 572.23 584.62 583.46 Mixed

6 5/9/2001 7/18/2001 71 573.36 572.35 588.68 586.69 Mixed

7 7/29/2001 8/28/2001 31 566.36 567.35 586.54 586.66 Into

8 10/18/2001 10/31/2001 14 565.89 565.78 584.99 586.17 Into

9 12/8/2001 1/22/2002 36 577.46 -- 589.11 586.69 Into (probably)

10 2/7/2002 3/2/2002 24 573.52 -- 582.12 585.25 Into (probably)

11 3/22/2002 7/25/2002 126 575.06 -- 588.24 587.65 Into (probably)

12 8/2/2002 11/3/2002 94 587.12 -- 590.69 588.16 ?

13 1/27/2003 2/10/2003 25 603.40 598.44 598.67 590.69 Out of (probably)

14 3/13/2003 4/2/2003 21 597.22 596.75 599.71 591.81 Mixed

15 5/28/2003 6/23/2003 27 566.33 568.88 591.63 590.91 Into

16 7/2/2003 9/14/2003 75 564.44 567.25 588.72 589.23 Into

17 10/7/2003 12/3/2003 58 568.44 570.61 579.47 587.22 Into

18 12/9/2003 3/8/2004 91 572.61 570.88 578.66 586.02 Mixed

19 3/30/2004 6/30/2004 93 589.49 585.78 581.93 585.86 Out of

20 7/15/2004 9/6/2004 54 577.41 576.91 582.77 585.36 Mixed

21 9/8/2004 9/21/2004 14 561.97 564.91 580.69 585.49 Into

22 10/27/2004 11/23/2004 28 567.98 566.47 579.00 584.74 Mixed

23 12/9/2004 1/9/2005 32 578.31 574.16 583.04 586.17 Mixed

24 1/20/2005 3/7/2005 47 584.25 580.65 585.59 587.57 Mixed

25 3/11/2005 3/30/2005 20 589.30 584.26 587.96 588.55 Mixed

26 4/2/2005 4/19/2005 18 584.25 583.46 588.01 588.17 Mixed

27 4/27/2005 5/16/2005 20 583.05 581.32 587.24 588.40 Mixed

28 5/21/2005 6/21/2005 32 576.46 576.73 586.47 588.48 Into

29 6/23/2005 7/25/2005 33 572.46 573.61 584.91 588.05 Into

30 8/11/2005 9/19/2005 40 565.91 569.19 581.96 587.08 Into

31 4/8/2006 5/2/2006 25 543.87 -- 570.14 580.96 Into

32 5/10/2006 6/12/2006 34 543.77 548.92 568.43 580.20 Into

33 7/7/2006 9/5/2006 61 542.28 549.76 567.22 578.75 Into

34 9/16/2006 10/14/2006 29 541.48 546.65 563.96 577.30 Into

35 11/10/2006 12/12/2006 33 560.95 563.76 567.88 576.74 Into

36 1/9/2007 4/7/2007 89 567.20 562.93 569.53 577.74 Mixed

37 4/8/2007 5/22/2007 45 -- 585.63 577.82 580.05 ?

38 6/18/2007 9/9/2007 84 -- 588.27 584.76 583.77 Out of (probably)

39 9/10/2007 9/30/2007 21 583.89 582.60 587.37 586.35 Mixed

40 10/1/2007 10/28/2007 28 574.35 574.49 585.41 -- Into

41 11/8/2007 11/22/2007 15 581.11 579.06 584.88 586.67 Mixed

42 11/26/2007 12/31/2007 36 -- 583.84 584.82 586.55 ?

1

 Listed in order of relative position in transect, from freshwater zone to transition zone.
Hydraulics of Flow 45

and partly coincided with a period of below-normal hydrologic 

conditions. The August 2001–July 2002 continuous period 

of gradient into the freshwater zone likely was a period of 

normal or slightly above-normal hydrologic conditions. Mixed 

gradients tended to prevail during above-normal conditions; 

and for periods of mixed conditions on average, equivalent 

freshwater heads on the transition-zone side of the interface 

(wells KY3 and KY4) usually were higher than heads on the 

freshwater side of the interface (well KY1). Gradients out of 

the freshwater zone occurred during what likely were periods 

of well-above-normal hydrologic conditions (winter 2003, 

spring 2004, summer 2007) (figs. 9, 26). The direction of the 

lateral gradient at the interface in the vicinity of the Kyle transect, like that in the vicinity of the East Uvalde transect, probably fluctuates between into and out of the freshwater zone, 

depending on recharge and withdrawals; but the prevailing 

direction on average of the lateral gradient and thus potential 

lateral flow at the interface in the vicinity of the Kyle transect 

likely is from the transition zone into the freshwater zone.

Vertical Flow in the Boreholes

The magnitude and direction of vertical flow in the 

boreholes is a reflection of head differences between zones, 

or stratigraphic units, in and adjacent to the Edwards aquifer 

that can change in response to changing hydrologic conditions over time. Vertical flows in the boreholes were measured 

under ambient conditions and differing hydraulic conditions to 

indicate the directions of vertical gradients in the aquifer and 

thus the potential for upward movement of water.

Equivalent freshwater heads are used in this section primarily to indicate hydrologic conditions (wet or dry) and for 

consistency with other sections of the report (one set of heads 

throughout). Although environmental-water heads (unadjusted 

field-measured heads), not equivalent freshwater heads, define 

hydraulic gradients in the vertical direction (Lusczynski, 

1961), the directions of vertical gradients here are indicated by 

the directions of measured flows.

East Uvalde Transect

The flowmeter logs for well EU1 indicate estimated 

flow less than 0.1 gallon per minute, with minor exceptions, 

in the borehole at the times logs were run during 2005–07 

(fig. 10). The exceptions involved slightly larger flows (0.1 to 

0.5 gallon per minute) at the bottom of the borehole during the 

April 2005 and September 2006 logs. The flow at the bottom 

of the borehole was upward in April 2005, when the equivalent freshwater head was high (about 824 feet above NAVD 

88), and downward in September 2006, when the equivalent 

freshwater head was low (about 753 feet above NAVD 88). 

However, the flow at the bottom of the borehole was negligible 

during the other two flow logs (April 2006 and October 2007), 

when the head in the well was relatively low (about 775 feet 

above NAVD 88) and high (about 828 feet above NAVD 88), 

respectively; a lack of consistency in magnitude or direction 

of vertical flow at the bottom of the borehole relative to head 

and minimal flow throughout the remainder of the borehole 

preclude identifying a relation between vertical flow and head 

at the time the log was run in well EU1.

The flowmeter logs for well EU2 indicate estimated flow 

less than 0.1 gallon per minute for the entire length of the 

borehole at the times logs were run during 2005–07 (fig. 11). 

The vertical direction of the minimal flows recorded was 

mixed, and no clear relation between vertical flow and head at 

the time the log was run in well EU2 is evident.

The flowmeter logs for well EU3 (fig. 12) indicate 

slightly greater magnitudes of flow (as much as 0.3 gallon 

per minute) than in the boreholes of wells EU1 and EU2 for 

the April 2005 log when head in the well was high (about 

838 feet above NAVD 88). The direction of flow for the 

April 2005 log was upward from an inflow/outflow zone 

near the base of the McKnight Formation to an inflow/outflow zone in the middle of the Salmon Peak Formation; and 

downward from an inflow/outflow zone in the upper part of 

the Salmon Peak Formation to the same inflow/outflow zone 

in the middle of the Salmon Peak Formation that was the 

sink for the upward flow. Flows indicated by the two other 

logs (September 2006 and October 2007) were almost all 

less than 0.1 gallon per minute and of mixed direction. The 

head in the well in September 2006 was low (about 765 feet 

above NAVD 88), and the head in October 2007 was high 

(about 825 feet above NAVD 88). No consistent relation 

between vertical flow and head at the time the log was run in 

well EU3 is evident.

The flowmeter logs for well EU4 (fig. 13) indicate mostly 

flows less than 0.1 gallon per minute. Although head data for 

well EU4 concurrent with the times of the log runs 

are not available, hydrographs of equivalent freshwater head 

for wells EU3 and EU4 for coincident periods of record 

during 2000–2002 match closely (fig. 24). Assuming heads 

in well EU4 were close to those in well EU3 at the times of 

log runs in well EU4, again as with the three other wells, no 

consistent relation between vertical flow and head at the time 

the log was run in well EU4 is evident. For example, near the 

bottom of the borehole, the April 2005 log during high head 

in well EU3 indicates flow of 0.3 to 0.4 gallon per minute 

into the West Nueces Formation; but the October 2007 log, 

also during high head in well EU3, shows flow of 0.1 to 0.2 

gallon per minute out of the same zone in the West Nueces 

Formation.

Rather than assessing vertical borehole flow by comparing flow logs for the same well at different times as in 

figures 10–13, another way is to compare flow logs for the 

different wells in the transect at the same times, as in figure 27. Figure 27 shows representations of the flow logs for 

wells EU1–EU4 superimposed on four hydrogeologic sections 

of the transect, one section for each of the four times a series 

of flow logs was run. Concurrent hydrographs of equivalent 

freshwater head for wells EU1–EU3 (no concurrent head data 

available for well EU4) also are shown to indicate relative 

hydrologic conditions for each of the four series of flow logs. 
46 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 27. Conceptual diagram showing vertical flow in monitoring wells in the East Uvalde transect (A–A’ ), San Antonio segment of the 

850

840

830

820

810

800

790

780

770

760

750

740

730

720

710

700

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

JAN 2005

FEB 2005

MAR 2005

APR 2005

MAY 2005

JUN 2005

JUL 2005

AUG 2005

SEP 2005

OCT 2005

NOV 2005

DEC 2005

JAN 2006

FEB 2006

MAR 2006

APR 2006

MAY 2006

JUN 2006

DATE

Saline water

Freshwater

69–52–404

EU4 69–51–606

EU3

69–44–902

 EU2

69–52–202

EU1

A A’

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

April 2006

NF

ND

NF

NF

NC

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Saline water

Freshwater

69–52–404

EU4 69–51–606

EU3

69–44–902

 EU2 69–52–202

 EU1

A A’

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

April 2005

NF NF

NF

NF

NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

0 6,250 12,500 FEET 3,125

0 1 2 MILES 0.5

Vertical scale greatly exaggerated

Land surface

Land surface
Hydraulics of Flow 47

JULY 2006

AUG 2006

SEP 2006

OCT 2006

NOV 2006

DEC 2006

JAN 2007

FEB 2007

MAR 2007

APR 2007

MAY 2007

JUN 2007

JUL 2007

AUG 2007

SEP 2007

OCT 2007

NOV 2007

DEC 2007

DATE

Saline water

Freshwater

69–52–404

EU4 69–51–606

EU3

69–44–902

EU2 69–52–202

EU1

A

A’

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

October 2007

NF

NF

NF

NC

NF NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Saline water

Freshwater

69–52–404

EU4 69–51–606

EU3

69–44–902

EU2 69–52–202

EU1

A A’

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1,000

September 2006

NF

NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

0 6,250 12,500 FEET 3,125

0 1 2 MILES 0.5

Vertical scale greatly exaggerated

Time period flow measurements

were made

Monitoring well

Fault (Ewing, 1991)

Formation contact—Dashed where 

inferred

69–52–202 (EU1)

69–44–902 (EU2)

69–51–606 (EU3)

Freshwater/saline-water interface

Equivalent freshwater head with

water-level trend from hydrograph

No change

No data

Upward flow in the borehole

Downward flow in the borehole

Magnitude of flow (gallon per

minute, gpm):

Less than 0.1 gpm (No measurable

flow)

0.1 to 0.5 gpm

 

NC

ND

Edwards 

aquifer Trinity aquifer Upper confining 

unit

Upper Cretaceous System

Stratigraphic unit

(Maverick Basin)

Hydrogeologic 

unit

Lower Cretaceous

Salmon Peak 

Formation

McKnight Formation

West Nueces 

Formation

Glen Rose

Limestone

Edwards 

aquifer

Upper 

confining 

unit

Upper Cretaceous System

Stratigraphic unit

(Devils River trend)

Hydrogeologic 

unit

Lower 

Cretaceous

Devils River 

Limestone

EXPLANATION

NF

Land surface

Land surface

Edwards aquifer, south-central Texas, 2005–07. 
48 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

The sections and hydrographs in figure 27 allow regional visualization of vertical flows relative to hydrologic conditions that 

is not apparent in the flow logs of figures 10–13.

Several observations can be made on the basis of the 

head and flow data in figure 27. In general, the flow data for 

three of the four times logs were run (excluding April 2006 

because of missing data) indicate more vertical flow (regardless of direction) on the saline-water side of the interface than 

on the freshwater side. The section diagrams indicate that the 

direction of vertical flow in all the wells with measurable 

flow was mostly upward, and magnitude of flow generally 

was greater when equivalent freshwater heads were high 

than when they were low. Equivalent freshwater heads at 

the times of each of the four flow logs, whether relatively 

high or relatively low, were higher in saline-zone well EU3 

than in freshwater wells EU1 and EU2. Although equivalent 

freshwater heads were about the same for the October 2007 

log as for the April 2005 log, vertical flows were larger in 

April 2005 than in October 2007 probably because antecedent heads were substantially higher during the months before 

April 2005 than during the months before October 2007 

(fig. 24). 

As indicated in the sections, the interface is assumed 

to be a sloping surface, which implies horizontal and vertical components of head gradient across it. The equivalent 

freshwater head data and the vertical flow data indicate that, 

under the hydraulic head conditions at the times the flow logs 

were run (equivalent freshwater heads in the transition zone 

higher than those in the freshwater zone), the directions of 

the lateral and vertical components of the head gradient at 

the interface were into the freshwater zone and upward (thus 

also into the freshwater zone), respectively. If vertical flow 

data were available for times (or antecedent conditions) when 

equivalent freshwater heads in the freshwater zone were higher 

than those in the transition zone, the direction of the gradient 

likely would have been downward, out of the freshwater zone. 

The hypothesis regarding the vertical gradient in the vicinity 

of the East Uvalde transect, and thus the potential for vertical 

flow near the interface, is that the potential for vertical flow 

(like the potential for lateral flow) fluctuates between into 

and out of the freshwater zone, depending on recharge and 

withdrawals. 

Tri-County Transect

The flowmeter logs for freshwater well TC1 show 

downward flow in the borehole for each of the four logs run 

(fig. 14). Flow entered the borehole from the Georgetown 

Formation and the formation members above the regional 

dense member and exited the borehole into the formation 

members below the regional dense member, except the basal 

nodular member into which no flow was recorded for any log. 

Flow was less than 1 gallon per minute and maximum as it 

passed the regional dense member during all logs; there is no 

identifiable relation between flow and head in well TC1 at the 

time the logs were run.

Three of the four flowmeter logs for interface well 

TC2 (fig. 15) show downward flow entering the borehole 

from the cyclic and marine members and the leached and 

collapsed members and most flow exiting the borehole 

immediately beneath the regional dense member into the 

grainstone member; no flow was recorded in the April 2006 

log. As was the case for well TC1, flow was less than 1 gallon 

per minute, maximum as it passed the regional dense member, and does not show an identifiable relation to head 

at the time the logs were run. The interface during all logs 

was about at the contact between the Kirschberg evaporite 

member and the underlying dolomitic member. No flow was 

recorded in any of the logs in the saline zone beneath the 

interface.

The two flowmeter logs for saline-water well TC3 

(fig. 16) show essentially no flow (less than 0.1 gallon per 

minute) for the April 2006 log and mixed-direction flow for 

the September 2006 log. Flow during the September 2006 

log entered the borehole from the grainstone member, moved 

upward past the regional dense member, and exited the borehole into the leached and collapsed members. Downward flow 

was recorded over a short vertical distance between inflow/

outflow zones within the Kirschberg evaporite member. 

Flowmeter logs were not run in well TC3 in April 2005 and 

October 2007, periods of relatively high heads, because the 

well was flowing. No concurrent head data are available for 

well TC3. 

Two of the three flowmeter logs for saline-water well 

TC4 (January 2005 and October 2006) show upward flow that 

entered the borehole from the formation members below the 

regional dense member (mostly excluding the basal nodular 

member), was maximum (about 0.8 gallon per minute) as it 

passed the regional dense member, and exited the borehole 

into the formation members above the regional dense member 

and the Georgetown Formation. The upward flow pattern in 

these two logs is roughly a mirror image of the downward 

flow pattern shown in the logs for freshwater well TC1. The 

April 2006 log, as with wells TC2 and TC3, shows essentially 

no flow. January 2005 and October 2006 flow logs generally 

are similar, although the equivalent freshwater head in well 

TC4 at the time of the January 2005 log was near the highest 

of the period of record and at the time of the October 2006 log 

was near the lowest of the period of record; thus no identifiable relation between head in well TC4 and flow at the time of 

the logs is evident. No flow data were available for well TC4 

for October 2007.

A more regional perspective is illustrated by representations of the flow logs for wells TC1–TC4 superimposed 

on hydrogeologic sections of the transect, one for each of the 

four times a flow log was run, and concurrent hydrographs 

of equivalent freshwater head for wells TC1, TC2, and TC4 

(no concurrent head data available for well TC3) (fig. 28). 

At all four times the flow logs were run, which encompassed 

a range of hydrologic conditions from wet (April 2005) to 

dry (September 2006) for the period of record (2000–2007) 

(fig. 25), the interface was essentially in the same position 
Hydraulics of Flow 49

in well TC2, below the regional dense member about at the 

contact between the grainstone member and the Kirschberg 

evaporite member; some combination of head gradient and 

permeability (each with lateral and vertical components) was 

functioning to keep the interface stable as heads changed. The 

permeability of the regional dense member is low enough to 

create a head gradient that drives water across it under the 

range of hydrologic conditions that occurred at the times the 

logs were run. The gradient was downward on the freshwater 

side of the interface and upward on the saline-water side 

of the interface at the times logs were run (except the April 

2006 flow log [relatively dry conditions] for saline-water well 

TC3 in which, for unknown reasons, essentially no flow was 

recorded). The downward gradient on the freshwater side 

of the interface and the upward gradient on the saline-water 

side (upward gradient at well TC3 indicated directly by the 

well flowing when heads were high) are evidence of opposing potentials that appear to have stabilized the position of the 

interface over the range of hydrologic conditions that occurred 

at the times the logs were run. Regionally, the data indicate 

a classic flow-system configuration with recharge conditions 

in the relatively high-altitude, updip parts of the aquifer and 

discharge conditions in the relatively low-altitude, downdip 

parts of the aquifer; in this case, the inflection point (inflection 

surface in three dimensions) probably is coincident with the 

interface. 

Fish Hatchery Transect

The three flow logs for relatively shallow freshwater well 

FH1 (fig. 18) show small vertical borehole flows within the 

leached and collapsed member of mixed direction: downward 

flow less than 0.5 gallon per minute in the April 2005 log 

when equivalent freshwater head was high (fig. 29), essentially 

no flow (less than 0.1 gallon per minute) in the September 

2006 log when equivalent freshwater head was low, and 

minimal upward flow (less than 0.2 gallon per minute) in the 

October 2007 log when equivalent freshwater head was once 

again high. Thus no identifiable relation between head in well 

FH1 and flow at the time of the logs is evident.

The four flow logs for saline-water well FH2 (fig. 19) 

show vertical borehole flows ranging from no flow to upward 

flow of about 1.1 gallons per minute. The largest upward flow, 

generally increasing from the basal nodular member to the 

maximum at the regional dense member and then decreasing to the cyclic and marine members, was recorded in the 

September 2006 log when equivalent freshwater head in the 

well was low. Smaller upward flow, again maximum (about 

0.5 gallon per minute) at the regional dense member, was 

recorded in the April 2005 log when equivalent freshwater 

head in the well was high; and upward flow recorded in the 

October 2007 log, when equivalent freshwater head in the well 

again was high, was about the same as that recorded in April 

2005. The potential for an inverse relation between upward 

flow and head in the well from the data of these three logs 

is negated by the data for the May 2006 log when head was 

relatively low: The early May 2006 log, like the April 2006 

logs for wells TC2–TC4, shows essentially no flow. As with 

the Tri-County transect, the permeability of the regional dense 

member appears low enough to create a head gradient that 

drives water across it under the range of hydrologic conditions 

that occurred at the times the logs were run.

From a regional perspective, the head and flow data for 

the Fish Hatchery transect (fig. 30) yield some additional 

observations. Because well FH1 is freshwater and well FH2 is 

saline water, the interface, conceptualized as a sloping surface 

as indicated in the sections in figure 30, must occur between 

the two wells. Heads and possibly also structure (fault offset) 

combined to keep the interface beneath the bottom of well 

FH1 at the times the logs were run. Because well FH1 is open 

only to the uppermost part of the Edwards aquifer, the small, 

mixed-direction vertical flows over the short segment of borehole offer little information about vertical flow throughout the 

aquifer near the interface on the freshwater side. Vertical flows 

in the borehole of well FH2 were upward whether hydrologic 

conditions were wet or dry. There appears to have been an 

upward gradient on the saline-water side of the interface under 

a wide range of hydrologic conditions. This upward gradient, coupled with the assumption of a sloping interface as 

conceptualized on the sections in figure 30, implies a vertical 

gradient from the transition zone into the freshwater zone. 

This potential for vertical movement of saline water into the 

freshwater zone (vertical movement of the interface) apparently was opposed by the potential (head) on the freshwater 

side of the interface that kept the interface beneath the bottom 

of well FH1 over the range of hydrologic conditions during 

which the logs were run. It is possible that fault offset near the 

interface also helps to restrict the movement of saline water by 

reducing the area of contact between adjacent blocks of aquifer. Although the hydrographs of wells FH1 and FH2 (fig. 29) 

reflect the same general pattern, the hydrograph of well FH2 is 

attenuated compared to that of well FH1, indicating hydraulic 

communication between the two wells is somewhat restricted. 

Kyle Transect

The five flow logs for freshwater well KY1 (fig. 20) 

all indicate upward flow ranging from negligible (less than 

0.1 gallon per minute, May 2006) to nearly 4 gallons per 

minute (September 2006). The upward flow originates from 

the Glen Rose Limestone, the uppermost unit of the Trinity 

aquifer. These flow data constitute evidence of the potential 

for flow from the Trinity aquifer into the Edwards aquifer in 

the vicinity of well KY1. Upward flow in the borehole was 

largest adjacent to the Glen Rose Limestone, the basal nodular 

member, and the lower part of the dolomitic member; flow 

slowed as it likely exited the borehole into the units from the 

dolomitic member upward to the regional dense member and 

had essentially ceased by the time it reached the leached and 

collapsed members. The largest upward flow occurred during 

the September 2006 log when equivalent freshwater head in 

the well was about at its lowest level for the period of record 
50 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 28. Conceptual diagram showing vertical flow in monitoring wells in the Tri-County transect (B–B’ ), San Antonio segment of the 

DATE

JAN 2005

FEB 2005

MAR 2005

APR 2005

MAY 2005

JUN 2005

JUL 2005

AUG 2005

SEP 2005

OCT 2005

NOV 2005

DEC 2005

JAN 2006

FEB 2006

MAR 2006

APR 2006

MAY 2006

JUN 2006

730

720

710

700

690

680

670

660

650

640

630

620

610

600

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Saline water

Freshwater

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

B

B’ 68–31–511

TC3

68–31–403

TC2

68–30–314

TC1

68–31–808

TC4

April–June 2006

NF

NF

NF

NF

Saline water

Freshwater

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

B

B’ 68–31–511

TC3

(well flowing)

68–31–403

TC2

68–30–314

TC1

68–31–808

TC4

January–May 2005 NF

NF

NF NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

ND

Land surface

Land surface

Person 

Formation

Edwards aquifer Upper confining 

unit

Trinity

aquifer

Upper 

Cretaceous System

Stratigraphic unit

Hydrogeologic 

unit

Lower Cretaceous

Kainer 

Formation

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

EXPLANATION

0 0.5 1 2 MILES

0 5,000 10,000 FEET 2,500

Vertical scale greatly exaggerated

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988
Hydraulics of Flow 51

Edwards aquifer, south-central Texas, 2005–07. 

DATE

JUL 2006

AUG 2006

SEP 2006

OCT 2006

NOV 2006

DEC 2006

JAN 2007

FEB 2007

MAR 2007

APR 2007

MAY 2007

JUN 2007

JUL 2007

AUG 2007

SEP 2007

OCT 2007

NOV 2007

DEC 2007

Saline water

Freshwater

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

B

B’ 68–31–511

TC3

(well flowing)

68–31–403

TC2

68–30–314

TC1

68–31–808

TC4ND

NF

NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

ND

October 2007

Land surface

Saline water

Freshwater

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

-1,000

-900

-800

-700

-600

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

900

1,000

0

B

B’ 68–31–511

TC3

68–31–403

TC2

68–30–314

TC1

68–31–808

TC4

October–November 2006

NF NF

NF

NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Land surface

Time period flow measurements

were made

Monitoring well

Fault (Ewing, 1991; Collins and

Hovorka, 1997)

Formation contact—Dashed where inferred

68–30–314 (TC1)

68–31–403 (TC2)

68–31–808 (TC4)

Freshwater/saline-water interface

Equivalent freshwater head with

water-level trend from hydrograph

No change

No data

Upward flow in the borehole

Downward flow in the borehole

Magnitude of flow (gallon per

minute, gpm):

Less than 0.1 gpm (No measurable

flow)

0.1 to 0.5 gpm

0.5 to 1.0 gpm

 

NF

NC

ND

0 0.5 1 2 MILES

0 5,000 10,000 FEET 2,500

Vertical scale greatly exaggerated
(fig. 26). However, upward flow was nearly as large during the 

March 2005 log when equivalent freshwater head in the well 

was relatively high for the period of record. 

As shown by the associated conductivity logs, the 

relatively strong upward flow originating from the Glen Rose 

Limestone was fresher than the water in the borehole adjacent to the overlying members of the Edwards aquifer. The 

relatively fresh water is an indication that the source of the 

Trinity aquifer water entering the borehole likely was infiltration of streamflow or direct recharge on the outcrop of the 

Trinity aquifer (aided in entering the Trinity aquifer by faults 

or fractures) rather than upward leakage from deeper, likely 

more-saline parts of the Trinity aquifer.

The three flow logs for interface well KY2 (fig. 21) show 

differing vertical flow patterns. Two of the three logs (June 

2006 and November 2007) recorded essentially no flow and 

downward flow less than 0.1 gallon per minute, respectively. 

The September 2006 log shows upward flow of 0.3–0.4 

gallon per minute entering the borehole from the Glen Rose 

Limestone and exiting the borehole into the upper part of the 

dolomitic member and the overlying Kirschberg evaporite 

member, at about the same level as the interface, as indicated by the associated conductivity log. Above the interface, 

downward flow of about 0.4 gallon per minute occurred from 

the lower part of the leached and collapsed members, past 

the regional dense member, and probably into the Kirschberg 

evaporite member at about the same level as the interface; 

there was slight (less than 0.2 gallon per minute) upward flow 

from the leached and collapsed members toward the cyclic 

and marine members. The position of the interface in well 

KY2 was not quite as stable as that in well TC2, varying over 

several tens of feet among the times the logs were run.

The four flow logs for saline-water well KY3 (fig. 22) 

show mostly downward borehole flow at relatively low rates, 

exceeding 0.1 gallon per minute beginning at the Kirschberg 

evaporite member and increasing to as much as 0.4 gallon 

per minute at the basal nodular member. As in wells KY1 

and KY2, the largest flow, although in the opposite direction, 

was recorded during the September 2006 log when equivalent 

freshwater head in the well was about at its lowest level for 

the period of record (fig. 26). Flows recorded during the June 

2006 and November 2007 logs, when equivalent freshwater 

heads were relatively low and relatively high, respectively, 

were less than 0.1 gallon per minute throughout the length of 

the borehole.

The four logs for saline-water well KY4 (fig. 23) show 

mixed-direction flows in the 0–0.5 gallon per minute range. 

Flow during the May 2005 log, when equivalent freshwater 

head in the well was relatively high, was mostly downward 

throughout the borehole, except adjacent to the grainstone 

member and the basal nodular member; flow during the June 

and September 2006 logs was mostly upward throughout the 

borehole, the maximum occurring adjacent to the regional 

dense member. The largest upward flow occurred, as in wells 

KY1 and KY2, during the September 2006 log when equivalent freshwater head in the well was at about its lowest level 

for the period of record (fig. 26). There was essentially no 

vertical flow in the borehole during the November 2007 log. 

52 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP

DEC

DATE

1999 2000 2001 2002 2003 2004 2005 2006 2007

580

590

600

610

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

295019097592701 (FH1)

294946097574501 (FH2)

Figure 29. Daily mean equivalent freshwater head in monitoring wells of the Fish Hatchery transect, San Antonio segment of the 

Edwards aquifer, south-central Texas, 2001–07. 
Hydraulics of Flow 53

Considering the head and flow data for the four monitoring wells of the Kyle transect together from a regional 

perspective (fig. 31), the salient observation remains the 

upward borehole flow in freshwater well KY1, and to a lesser 

extent in interface well KY2, that originated from the Glen 

Rose Limestone of the Trinity aquifer. Upward flow of water 

occurred in well KY1 that was lower in conductivity than the 

overlying freshwater in the borehole under all head conditions at the times the logs were run and that was relatively 

strong (3–4 gallons per minute) during two of the logs (March 

2005 and September 2006). No clear relation between rate of 

upward borehole flow from the Trinity aquifer and heads in 

the transect monitoring wells is apparent, however. Equivalent 

freshwater head in well KY1 at the time of the March 2005 log 

was about the third highest of the 2000–2007 period of record 

(fig. 26); and heads in the three other Kyle wells were within a 

few feet of the head in KY1. In contrast, equivalent freshwater 

head in well KY1 at the time of the September 2006 log was 

the lowest of the period of record, and heads in wells KY3 and 

KY4 were about 30 and 45 feet higher, respectively, than that 

in well KY1. If the source of the Trinity aquifer water flowing into the borehole of well KY1 (and KY2) was infiltration 

of streamflow or direct recharge on the outcrop of the Trinity 

aquifer as hypothesized, then the head driving the upward flow 

into the boreholes might have been more closely related to the 

head in the outcrop of the Trinity aquifer rather than the head 

in the Trinity aquifer in the vicinity of the Kyle transect. 

The patterns of vertical flow in the vicinity of the Kyle 

transect might be the most complex of the patterns among the 

transects. On the freshwater side of the interface, the relatively strong upward borehole flow of water from the Trinity 

aquifer masked any downward vertical flow that might have 

occurred as a result of recharge conditions in the relatively 

high-altitude, updip parts of the Edwards aquifer; the transect 

is close to the recharge zone of the Edwards aquifer. On the 

saline-water side of the interface, the directions of vertical 

flow were mixed, or there was essentially no flow, under the 

hydrologic conditions during 2005–07, and no identifiable 

relation between flow direction and conditions. The structure 

of the aquifer, the fact of offset fault blocks (fig. 31), might 

contribute to the complexity by affecting hydraulic communication in the vicinity of the transect. In other words, the heads 

causing vertical flows in the different wells of the transect 

might reflect zones of the aquifer not strongly connected 

hydraulically. The fact that hydrographs of saline-water wells 

KY3 and KY4 are attenuated relative to those of freshwater 

well KY1 and interface well KY2 (figs. 26, 31), and less 

affected by withdrawals, indicates some degree of hydraulic 

“insulation” between the saline-water and freshwater sides of 

the interface. 

Relative Groundwater Flow Based on 

Temperature

Subsurface temperature data can provide information about groundwater flow rates (Anderson, 2003). Heat 

is transported upward by conduction throughout the Earth’s 

crust. In the absence of appreciable groundwater flow, 

conduction is the only heat transport mechanism and results 

in a conductive geothermal gradient, or simply “conductive 

thermal gradient.” In the case of a conductive thermal gradient, 

temperature increases linearly with depth at a rate dependent 

on regional heat flow from the Earth’s interior into the base of 

the crust, heat production in the crust, and the thermal conductivity of the material composing the crust. 

If groundwater flow is appreciable, heat is transported 

in the subsurface by convection as well as by conduction. 

Convective heat flow typically causes a subsurface temperature gradient to diverge from a conductive thermal gradient 

and become nonlinear. Conductive thermal gradients become 

disturbed by convective heat flow when groundwater flow 

rates are on the order of centimeters per year or greater 

(Ingebritsen and Sanford, 1998). Subsurface temperature data 

therefore can be used to indicate zones of negligible groundwater flow (temperature [thermal] gradients greater than or 

equal to the conductive thermal gradient) and zones of active 

groundwater flow (temperature gradients less than the conductive thermal gradient or the absence of a temperature gradient). 

The conductive thermal gradient can be defined as

dT/dz = (T2-T1)/∆z, (4)

where T1 and T2 are the temperatures at two points (depths) 

separated by a distance ∆z. The term dT/dz is expressed 

in units of temperature per unit distance (degrees Celsius 

per 100 feet [°C/100 ft] or degrees Celsius per 100 meters 

[°C/100 m]) and is a vector quantity that has both magnitude 

and direction (Beardsmore and Cull, 2001, p. 12). By convention, a positive gradient is in the direction of increasing 

temperature. Thermal gradients for the open intervals of 

wells were computed for each well (table 6) using geophysical log data collected for this report (Lambert and others, 

2009).

Subsurface temperature data collected by King and 

Simmons (1972, p. 138) indicate that the conductive thermal gradient in Uvalde County typically is 2.16 °C/100 m. 

Temperature data in Woodruff and Foley (1985, p. 138) 

collected at depth in the Balcones fault zone (below the 

zone of potential groundwater flow) indicate that conductive 

thermal gradients in the study area ranged from 2.05 to 2.46 

°C/100 m. On the basis of these studies, a conductive thermal 

gradient of 2.19 °C/100 m was assumed for the transition 

zone. 

The average temperature gradients computed from 

temperatures measured during 2002–07 for this report 

ranged from 0.67 to 2.11 °C/100 m for the freshwater wells 

and from 1.44 to 3.07 °C/100 m for the interface and salinewater wells (table 6). The temperature gradients in five of 

the six freshwater wells (no data for freshwater well TC5) 

were lower than the assumed conductive thermal gradient, 

indicating active groundwater flow on the freshwater side 

of the interface (freshwater zone). Temperature gradients 
54 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 30. Conceptual diagram showing vertical flow in monitoring wells in the Fish Hatchery transect (C–C’ ), San Antonio segment of 

610

600

590

580

570

560

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

DATE

JAN 2005

FEB 2005

MAR 2005

APR 2005

MAY 2005

JUN 2005

JUL 2005

AUG 2005

SEP 2005

OCT 2005

NOV 2005

DEC 2005

JAN 2006

FEB 2006

MAR 2006

APR 2006

MAY 2006

JUN 2006

Saline water -500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

0

C

C’

67–09–401

FH2

67–09–113

FH1

NF NF NF

NF

April 2005

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Units

undifferentiated

Saline water -500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

ND

NF

NF

NF

NF

C

C’

67–09–401

FH2

67–09–113

FH1 May 2006

NC

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Units

undifferentiated

0 1,750 3,500 7,000 FEET

0 0.5 1 MILE 0.25

Vertical scale greatly exaggerated

Land surface

Land surface
Hydraulics of Flow 55

the Edwards aquifer, south-central Texas, 2005–07. 

DATE

JUL 2006

AUG 2006

SEP 2006

OCT 2006

NOV 2006

DEC 2006

JAN 2007

FEB 2007

MAR 2007

APR 2007

MAY 2007

JUN 2007

JUL 2007

AUG 2007

SEP 2007

OCT 2007

NOV 2007

DEC 2007

Saline water -500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

0

C

C’

67–09–401

FH2

67–09–113

FH1

September 2006

N F

N F

NC NC

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Units

undifferentiated

Saline water

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

0

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

C

C’

67–09–401

FH2

67–09–113

FH1

October 2007

NF NF NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Units

undifferentiated

Land surface

Land surface

Person 

Formation

Edwards aquifer

Upper 

confining 

unit

Trinity

aquifer

Upper 

Cretaceous

System Stratigraphic unit Hydrogeologic 

unit

Lower Cretaceous

Kainer 

Formation

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

Time period flow measurements

were made

Monitoring well

Fault (Texas Natural Resources

Information System, 2009)

Formation contact—Dashed where 

inferred

67–09–113 (FH1)

67–09–401 (FH2)

Freshwater/saline-water interface

Equivalent freshwater head with

water-level trend from hydrograph

No change

No data

NC

ND

NF

Upward flow in the borehole

Downward flow in the borehole

Magnitude of flow (gallon per 

minute, gpm):

Less than 0.1 gpm (No measurable

flow)

0.1 to 0.5 gpm

0.5 to 1 gpm

1 to 2 gpm

 

EXPLANATION

0 1,750 3,500 7,000 FEET

0 0.5 0.25 1 MILE

Vertical scale greatly exaggerated
56 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 31. Conceptual diagram showing vertical flow in monitoring wells in the Kyle transect (D–D' ), San Antonio segment of the JAN 2005 FEB 2005 MAR 2005 APR 2005 MAY 2005 JUN 2005 JUL 2005 AUG 2005 SEP 2005 OCT 2005 NOV 2005 DEC 2005 JAN 2006 FEB 2006 MAR 2006 APR 2006 MAY 2006

JUN 2006

DATE

520

530

540

550

560

570

580

590

600

610

620

630

640

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Freshwater

Saline water

0 0

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

67–02–105 KY4

67–02–106 KY3

67–02–104 KY2

67–01–311 KY1

D

D’

May–June 2006NF NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Freshwater

Saline water

0 0

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

67–02–105 KY4

67–02–106 KY3

67–02–104 KY2

67–01–311 KY1

D

D’

March–May 2005

NC

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

ND

Person 

Formation

Edwards aquifer Upper confining 

unit

Trinity

aquifer

Upper 

Cretaceous System

Stratigraphic unit

Hydrogeologic 

unit

Lower Cretaceous

Kainer 

Formation

Georgetown Formation

Cyclic and marine members

Leached and collapsed members

Regional dense member

Grainstone member

Kirschberg evaporite member

Dolomitic member

Basal nodular member

Glen Rose Limestone

Time period flow measurements

were made

Monitoring well

Fault (Ewing, 1991; Collins and

Hovorka, 1997)

Formation contact—Dashed where 

inferred

67–01–311 (KY1)

67–02–104 (KY2)

67–02–106 (KY3)

67–02–105 (KY4)

Freshwater/saline-water interface

Equivalent freshwater head with

water-level trend from hydrograph

No change

No data

Upward flow in the borehole

Downward flow in the borehole

Magnitude of flow (gallon per 

minute, gpm):

Less than 0.1 gpm (No measurable

flow)

0.1 to 0.5 gpm

0.5 to 1 gpm

1 to 2 gpm

2 to 4 gpm

NF

NC

ND

EXPLANATION

0 3,500 7,000 FEET

0 0.5 1 MILE

Vertical scale greatly exaggerated

Land surface Land surface
Hydraulics of Flow 57

Edwards aquifer, south-central Texas, 2005–07. JUL 2006 AUG 2006 SEP 2006 OCT 2006 NOV 2006 DEC 2006

JAN 2007

FEB 2007

MAR 2007

APR 2007

MAY 2007

JUN 2007

JUL 2007

AUG 2007

SEP 2007

OCT 2007

NOV 2007

DEC 2007

DATE

Freshwater

Saline water

0 0

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

67–02–105 KY4

67–02–106 KY3

67–02–104 KY2

67–01–311 KY1

D

D’

November 2007

NC

NF

NF

NF

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

Freshwater

Saline water

0 0

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

-500

-400

-300

-200

-100

100

200

300

400

500

600

700

800

67–02–105 KY4

67–02–106 KY3

67–02–104 KY2

67–01–311 KY1

D

D’

September 2006

N F N F

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

0 3,500 7,000 FEET

0 0.5 1 MILE

Vertical scale greatly exaggerated

Land surface

Land surface
58 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Table 6. Temperature gradients in monitoring wells transecting the freshwater/saline-water transition zone, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2002–07—Continued.

Well 

identifier 

(fig. 1)

Date

Well descriptor based on 

water type

Depth 

of well 

(ft)

T1

(°C)

D1

(ft)

T2

(°C)

D2

(ft)

T2–T1

(°C)

D2–D1

(ft)

D2–D1

(m)

Temperature 

gradient

(°C/100 ft)

Temperature 

gradient

(°C/100 m)

EU1 11/6/2002 Freshwater 1,500 22.02 985 22.83 1,500 0.81 515 157 0.16 0.52

4/25/2005 Freshwater 1,500 32.29 985 33.90 1,493 1.61 508 155 .32 1.04

4/12/2006 Freshwater 1,500 32.28 985 34.14 1,491 1.86 506 154 .37 1.21

9/26/2006 Freshwater 1,500 31.99 985 33.95 1,491 1.96 506 154 .39 1.27

10/17/2007 Freshwater 1,500 30.30 985 32.25 1,491 1.95 506 154 .39 1.26

Average .32 1.06

EU2 11/6/2002 Freshwater 1,560 22.10 1,072 23.02 1,558 0.92 486 148 .19 .62

4/11/2006 Freshwater 1,560 24.89 1,072 25.79 1,554 0.90 482 147 .19 .61

9/28/2006 Freshwater 1,560 20.78 1,072 22.01 1,554 1.23 482 147 .26 .84

10/16/2007 Freshwater 1,560 23.03 1,072 23.92 1,554 0.89 482 147 .18 .61

Average .20 .67

EU3 11/7/2002 Saline water 1,400 29.71 768 30.64 1,045 0.93 277 84 .34 1.10

4/28/2005 Saline water 1,400 32.83 768 35.62 1,400 2.79 632 193 .44 1.45

4/14/2006 Saline water 1,400 30.50 768 31.25 945 0.75 177 54 .42 1.39

9/25/2006 Saline water 1,400 32.43 768 35.75 1,400 3.32 632 193 .53 1.72

10/23/2007 Saline water 1,400 30.54 768 33.51 1,400 2.97 632 193 .47 1.54

Average .44 1.44

EU4 11/7/2002 Saline water 1,463 31.78 950 35.00 1,460 3.22 510 155 .63 2.07

3/3/2005 Saline water 1,463 34.86 950 38.36 1,459 3.50 509 155 .69 2.26

4/28/2005 Saline water 1,463 32.16 950 35.42 1,440 3.26 490 149 .66 2.18

4/13/2006 Saline water 1,463 34.67 950 38.07 1,446 3.40 496 151 .69 2.25

9/25/2006 Saline water 1,463 34.60 950 39.56 1,445 4.96 495 151 1.00 3.29

10/18/2007 Saline water 1,463 32.64 950 36.13 1,445 3.49 495 151 .71 2.32

Average .73 2.39

TC1 1/27/2003 Freshwater 920 22.72 385 25.46 915 2.74 530 162 .52 1.69

4/18/2005 Freshwater 920 25.33 385 28.17 896 2.84 511 156 .56 1.82

4/21/2005 Freshwater 920 23.83 385 26.36 861 2.53 476 145 .53 1.74

10/26/2005 Freshwater 920 25.55 385 28.65 868 3.10 483 147 .64 2.10

6/23/2006 Freshwater 920 23.49 385 26.15 860 2.66 475 145 .56 1.84

11/6/2006 Freshwater 920 21.73 385 25.55 861 3.82 476 145 .80 2.63

10/29/2007 Freshwater 920 24.26 385 26.93 903 2.67 518 158 .52 1.69

Average .59 1.93

TC2 1/30/2003 Interface 1,050 23.90 486 27.49 1,036 3.59 550 168 .65 2.14

5/17/2005 Interface 1,050 26.51 486 30.28 1,035 3.77 549 167 .69 2.26

10/20/2005 Interface 1,050 26.86 486 31.03 1,030 4.17 544 166 .77 2.52

11/2/2005 Interface 1,050 26.51 486 30.28 1,034 3.77 548 167 .69 2.26

4/20/2006 Interface 1,050 26.75 486 30.42 1,038 3.67 552 168 .67 2.18

11/21/2006 Interface 1,050 23.35 486 28.43 1,026 5.08 540 164 .94 3.09

10/30/2007 Interface 1,050 25.48 486 28.93 1,038 3.45 552 168 .63 2.05

Average .72 2.36

Table 6. Temperature gradients in monitoring wells transecting the freshwater/saline-water transition zone, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2002–07.

[ft, feet; T1, temperature at first point; °C, degrees Celsius; D1, depth at first point; T2, temperature at second point; D2, depth at second point; m, meters] 
Hydraulics of Flow 59

Table 6. Temperature gradients in monitoring wells transecting the freshwater/saline-water transition zone, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2002–07—Continued.

Well 

identifier 

(fig. 1)

Date

Well descriptor based on 

water type

Depth 

of well 

(ft)

T1

(°C)

D1

(ft)

T2

(°C)

D2

(ft)

T2–T1

(°C)

D2–D1

(ft)

D2–D1

(m)

Temperature 

gradient

(°C/100 ft)

Temperature 

gradient

(°C/100 m)

TC2 - 

upper

1/30/2003 Interface 1,050 23.90 486 25.09 799 1.19 313 95 0.38 1.25

5/17/2003 Interface 1,050 26.51 486 28.18 797 1.67 311 95 .54 1.76

10/20/2005 Interface 1,050 26.86 486 28.82 800 1.96 314 96 .62 2.05

11/2/2005 Interface 1,050 26.51 486 28.17 797 1.66 311 95 .53 1.75

4/20/2006 Interface 1,050 26.75 486 28.75 801 2.00 315 96 .63 2.08

11/21/2006 Interface 1,050 23.35 486 25.71 803 2.36 317 97 .74 2.44

10/30/2007 Interface 1,050 25.48 486 26.84 804 1.36 318 97 .43 1.40

Average .55 1.82

TC2 - 

lower

1/30/2003 Interface 1,050 25.08 799 27.49 1,036 2.41 238 72 1.01 3.33

5/17/2003 Interface 1,050 28.18 797 30.28 1,035 2.10 237 72 .89 2.90

10/20/2005 Interface 1,050 28.82 800 31.03 1,030 2.21 229 70 .96 3.16

11/2/2005 Interface 1,050 28.26 798 30.28 1,034 2.02 236 72 .86 2.81

4/20/2006 Interface 1,050 28.75 801 30.42 1,038 1.67 237 72 .71 2.31

11/21/2006 Interface 1,050 25.71 803 28.43 1,026 2.72 223 68 1.22 4.01

10/20/2007 Interface 1,050 26.84 804 28.93 1,038 2.09 234 71 .89 2.93

Average .93 3.07

TC3 1/28/2003 Saline water 1,222 26.06 656 29.35 1,213 3.29 557 170 .59 1.94

10/26/2005 Saline water 1,222 29.10 656 33.19 1,169 4.09 513 156 .80 2.62

4/18/2006 Saline water 1,222 28.73 656 32.29 1,171 3.56 515 157 .69 2.27

10/25/2006 Saline water 1,222 26.31 656 31.15 1,170 4.84 514 157 .94 3.09

Average .76 2.48

TC4 1/30/2003 Saline water 1,562 30.05 1,000 33.69 1,559 3.64 559 170 .65 2.14

10/21/2005 Saline water 1,562 33.98 1,000 38.25 1,560 4.27 560 171 .76 2.50

4/19/2006 Saline water 1,562 32.98 1,000 36.68 1,558 3.70 558 170 .66 2.17

10/24/2006 Saline water 1,562 32.14 1,000 37.49 1,558 5.35 558 170 .96 3.15

Average .76 2.49

FH1 11/8/2002 Freshwater 280 21.61 216 21.72 277 0.11 61 19 .18 .59

4/20/2005 Freshwater 280 24.06 216 24.12 255 0.06 39 12 .16 .51

4/27/2006 Freshwater 280 24.30 216 24.43 267 .13 51 16 .25 .84

9/21/2006 Freshwater 280 19.93 216 20.12 267 .19 51 16 .37 1.22

10/24/2007 Freshwater 280 22.48 216 22.59 262 .11 46 14 .24 .78

Average .24 .79

FH2 11/8/2002 Saline water 1,030 24.61 510 28.34 1,025 3.73 515 157 .72 2.37

4/19/2005 Saline water 1,030 27.32 510 31.08 998 3.76 488 149 .77 2.53

5/2/2006 Saline water 1,030 26.93 510 31.14 988 4.21 478 146 .88 2.89

9/20/2006 Saline water 1,030 24.14 510 29.59 989 5.45 479 146 1.14 3.74

10/25/2007 Saline water 1,030 25.55 510 29.22 1,025 3.67 515 157 .71 2.34

Average .85 2.77
60 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Table 6. Temperature gradients in monitoring wells transecting the freshwater/saline-water transition zone, San Antonio segment of 

the Edwards aquifer, south-central Texas, 2002–07—Continued.

Well 

identifier 

(fig. 1)

Date

Well descriptor based on 

water type

Depth 

of well 

(ft)

T1

(°C)

D1

(ft)

T2

(°C)

D2

(ft)

T2–T1

(°C)

D2–D1

(ft)

D2–D1

(m)

Temperature 

gradient

(°C/100 ft)

Temperature 

gradient

(°C/100 m)

KY1 11/5/2002 Freshwater 810 22.16 307 25.24 807 3.08 500 152 0.62 2.02

3/1/2005 Freshwater 810 25.89 307 29.37 808 3.48 501 153 .69 2.28

3/9/2005 Freshwater 810 24.78 307 27.07 810 2.29 503 153 .46 1.49

5/3/2006 Freshwater 810 24.91 307 28.03 801 3.12 494 151 .63 2.07

6/15/2006 Freshwater 810 22.66 307 25.66 789 3.00 482 147 .62 2.04

9/6/2006 Freshwater 810 20.83 307 25.21 810 4.38 503 153 .87 2.86

11/5/2007 Freshwater 810 23.58 307 26.67 810 3.09 503 153 .61 2.02

Average .64 2.11

KY2 1/31/2003 Interface 975 22.64 427 25.86 930 3.22 503 153 .64 2.10

7/28/2003 Interface 975 22.97 427 26.23 931 3.26 504 153 .65 2.12

1/10/2005 Interface 975 25.22 427 28.57 918 3.35 491 150 .68 2.24

6/13/2006 Interface 975 21.64 427 26.13 931 4.49 504 154 .89 2.92

9/12/2006 Interface 975 21.67 427 26.13 933 4.46 506 154 .88 2.89

11/8/2007 Interface 975 24.17 427 27.31 933 3.14 506 154 .62 2.04

Average .73 2.39

KY2 - 

upper

1/31/2003 Interface 975 22.64 427 24.13 651 1.49 224 68 .66 2.18

7/28/2003 Interface 975 22.97 427 24.82 704 1.85 277 84 .67 2.20

1/10/2005 Interface 975 25.22 427 26.77 670 1.55 243 74 .64 2.10

6/13/2006 Interface 975 21.64 427 24.11 707 2.47 280 85 .88 2.89

9/12/2006 Interface 975 21.67 427 24.17 712 2.50 285 87 .88 2.87

11/8/2007 Interface 975 24.17 427 26.20 745 2.03 318 97 .64 2.10

Average .73 2.39

KY2 - 

lower

1/31/2003 Interface 975 24.13 651 25.86 930 1.73 279 85 .62 2.03

7/28/2003 Interface 975 24.82 704 26.23 931 1.41 227 69 .62 2.04

1/10/2005 Interface 975 26.77 670 28.57 918 1.80 248 76 .73 2.38

6/13/2006 Interface 975 24.11 707 26.13 931 2.02 224 68 .90 2.97

9/12/2006 Interface 975 24.17 712 26.13 933 1.96 220 67 .89 2.92

11/8/2007 Interface 975 26.20 745 27.31 933 1.11 188 57 .59 1.94

Average .72 2.38

KY3 11/4/2002 Saline water 1,100 24.00 600 26.8 1,094 2.80 494 151 .57 1.86

5/18/2005 Saline water 1,100 26.20 600 29.39 1,067 3.19 467 142 .68 2.24

6/29/2006 Saline water 1,100 23.28 600 27.33 1,066 4.05 466 142 .87 2.85

9/13/2006 Saline water 1,100 22.24 600 27.24 1,059 5.00 459 140 1.09 3.57

11/7/2007 Saline water 1,100 25.48 600 28.19 1,061 2.71 461 141 .59 1.93

Average .76 2.49

KY4 11/8/2002 Saline water 970 24.55 562 27.18 967 2.63 405 123 .65 2.13

5/18/2005 Saline water 970 25.36 562 27.79 955 2.43 393 120 .62 2.03

6/20/2006 Saline water 970 24.32 562 27.91 966 3.59 404 123 .89 2.92

9/13/2006 Saline water 970 24.15 562 27.75 961 3.60 399 122 .90 2.96

11/6/2007 Saline water 970 25.99 562 28.56 965 2.57 403 123 .64 2.09

Average .74 2.43
Hydraulics of Flow 61

in the nine interface or saline-water wells, with one exception (well EU3), were greater than the conductive thermal 

gradient, indicating negligible groundwater flow on the 

saline-water side of the interface (transition zone) when the 

geophysical logs were run. If the freshwater (upper) and 

saline-water (lower) sections of the two interface wells (TC2 

and KY2) are considered separately, the average temperature 

gradient in the upper section of well TC2, 1.82 °C/100 m, 

indicates active flow in the freshwater zone, and the average temperature gradient in the lower section of well TC2, 

3.07 °C/100 m, indicates negligible flow in the transition 

zone. For well KY2, the average temperature gradient on 

both sides of the interface is essentially the same, 2.39 and 

2.38 °C/100 m, and in the range indicating negligible flow. 

The findings regarding relative groundwater flow based 

on temperature, that flow on average is more active, or 

vigorous, on the freshwater side of the interface than on 

the saline-water side, are consistent with previous studies 

(for example, Maclay and Land, 1988; Lindgren and others, 

2004) that indicate substantially more-sluggish flow (resulting from lower lateral permeability) in the transition zone 

than the freshwater zone.

Relation Between the Transition Zone and the 

Freshwater Zone

The freshwater zone and transition zone of the Edwards 

aquifer are connected by stratigraphy, structure, and hydraulics. Stratigraphic units, faults, and the groundwater flow system extend seamlessly at the interface. Thus, a hydraulic connection between the transition zone and the freshwater zone, 

as indicated by similar patterns of hydrographs of monitoring 

wells in the transition zone (transect interface and saline-water 

wells) and monitoring wells in the freshwater zone (transect 

freshwater wells and, for example, county index wells [fig. 2]) 

(fig. 32), would be expected. The hydrographs of the 15 transect monitoring wells and three county index wells generally 

show the same seasonal lows and seasonal highs in varying 

degrees in response to combinations of hydrologic stresses and 

withdrawals; for example, the low of summer 2000, the high 

of winter 2002–03, the high of winter 2004–spring 2005, the 

low of summer-fall 2006. The relative strength of the hydraulic connection among wells is assumed to be indicated by how 

closely hydrographs match one another (excluding hydrographs obviously influenced by withdrawals such as those for 

wells EU3 and EU4).

Except for the hydrograph of the Uvalde County index 

well (possibly because of structural complexity associated 

with the Uvalde salient [Clark, 2003]), the hydrographs of 

the monitoring well in each transect farthest from the interface into the transition zone (TC4, FH2, and KY4) are the 

most attenuated in response to hydrologic stresses (recharge, 

natural discharge, and withdrawals). The attenuated response 

to stresses is attributed to relatively low permeability in the 

transition zone (possibly aided by fault offset). This hypothesis 

of relatively low permeability in the transition zone relative 

to that in the freshwater zone is consistent with the findings 

regarding relative groundwater flow based on temperature, that 

flow on average is relatively vigorous on the freshwater side of 

the interface and relatively sluggish on the saline-water side.

The hydrographs of figure 32, showing equivalent freshwater heads decreasing from southwest to northeast, support 

the long-established understanding that regional flow in the 

freshwater zone is from southwest to northeast, generally 

parallel to the faulting (fig. 2) and the interface. Because of 

the indicated hydraulic connection between the transition zone 

and the freshwater zone, logic would suggest that regional 

flow in the transition zone, to the extent that it occurs assuming relatively low permeability, also is from southwest to 

northeast. 

Assuming potential regional flow in the transition zone 

(however small relative to that in the freshwater zone) is most 

likely from southwest to northeast, for this to occur on average, water would enter the transition zone from the freshwater 

zone in the western part of the Edwards aquifer, flow toward 

the northeast, and discharge to the freshwater zone in the 

eastern part of the aquifer. Maclay (1995, p. 32) hypothesizes 

on the basis of geologic and hydrochemical information that, 

“A small flux of fresh groundwater enters the saline-water 

[transition] zone in the western part of the study area [San 

Antonio and Barton Springs segments of the Edwards aquifer];” and “In the eastern part of the [study] area, small flows 

from the saline-water zone of the aquifer might be entering 

the freshwater zone of the aquifer . . . .” Maclay (1995) does 

not explicitly note regional flow toward the northeast in the 

transition zone; however, Lindgren and others (2004, plates 

5–7) graphically indicates simulated regional flow entering 

the transition zone from the freshwater zone in an area west 

of the East Uvalde transect, flowing toward the northeast in 

the transition zone, and converging on the interface in the area 

between Comal and San Marcos Springs (fig. 2) as the transition zone (in two dimensions) narrows greatly to funnel flow 

toward the area of the Kyle transect. 

The data for this report in part support the conceptualization of regional flow in and near the transition zone thus 

described. At the East Uvalde transect, the direction of the 

lateral head gradient at the interface likely fluctuates between 

into and out of the freshwater zone, depending on recharge and 

withdrawals, on the basis of the directions of the lateral gradient for coincident periods of head record during 2000–2007. 

The probable fluctuating gradient over time, or a gradient on 

average from the freshwater zone into the transition zone as 

Maclay (1995) and Lindgren and others (2004) have implied, 

could result in a net inflow of freshwater into the transition 

zone in the vicinity of the East Uvalde transect. At the TriCounty transect, equivalent freshwater heads generally were 

higher at the interface than on either side of it for coincident 

periods of head record during 2000–2007. Thus these data do 

not indicate a potential for lateral flow at the interface in the 

vicinity of the Tri-County transect, which is consistent with 

flow in the transition zone primarily parallel to the interface 
62 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

Figure 32. Daily mean equivalent freshwater head in monitoring wells of the East Uvalde, Tri-County, Fish Hatchery, and Kyle transects, and index wells of Uvalde County, Medina County, and Bexar County in the freshwater zone and the freshwater/saline-water transition zone of the San Antonio segment of the Edwards aquifer, south-central Texas, 1999–2007. 

DATE

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

MAR

JUN

SEP

DEC

1999 2000 2001 2002 2003 2004 2005 2006 2007

900

850800750700

650600550500

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

EXPLANATION

EU1 (YP–69–52–202)EU2 (YP–69–44–902)EU3 (YP–69–51–606)EU4 (YP–69–52–404)TC1 (KX–68–30–314)TC2 (KX–68–31–403)TC3 (KX–68–31–511)TC4 (KX–68–31–808)TC5 (DX–68–30–315)FH1 (LR–67–09–113)FH2 (LR–67–09–401)KY1 (LR–67–01–311)KY2 (LR–67–02–104)KY3 (LR–67–02–106)KY4 (LR–67–02–105)Uvalde County Index (YP–69–50–302)Medina County Index (TD–69–47–306)Bexar County Index (AY–68–37–203)

EU2

EU1

EU3

EU4

Uvalde County Index

Medina County Index

Bexar County Index

TC1

TC4

TC2

TC5

TC3

KY1 KY2

KY3 KY4

FH1

FH2
Hydraulics of Flow 63

Figure 33. Specific conductance and equivalent freshwater head over time in wells (A) Tri-County 1 (2000–2007) and (B) Kyle 2 

(2003–07). 

EXPLANATION

Daily mean equivalent freshwater head (appendixes 1.5, 1.13)

Daily mean specific conductance (Lambert and others, 2009, table 24)

0

5,000

10,000

15,000

20,000

25,000

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER 

AT 25 DEGREES CELSIUS

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER 

AT 25 DEGREES CELSIUS

540

550

560

570

580

590

600

630

640

650

660

670

680

ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988 ALTITUDE, IN FEET ABOVE NORTH AMERICAN VERTICAL DATUM OF 1988

690

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

OCT

JAN

APR

JUL

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APR

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APR

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OCTDEC

DATE

2000 2001 2002 2003 2004 2005 2006 2007

DATE

2003 2004 2005 2006 2007 JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAR MAY JUL

SEP

NOV

DEC

A

B
64 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

(toward the northeast). As explained at the beginning of the 

“Lateral Head Gradients at the Interface” section, lateral 

gradients were not computed for the Fish Hatchery transect 

because the assumption of a horizontal aquifer, necessary for 

computation of accurate equivalent heads, was judged not 

applicable for the Fish Hatchery transect. At the Kyle transect, 

the direction of potential lateral flow at the interface, like that 

in the vicinity of the East Uvalde transect, probably fluctuates 

between into and out of the freshwater zone, depending on 

recharge and withdrawals; but the prevailing direction at the 

interface on average in the vicinity of the Kyle transect, based 

on the directions of the lateral gradient for coincident periods 

of head record during 2000–2007, likely is from the transition 

zone into the freshwater zone. Another factor that might contribute to potential flow on average from the transition zone 

into the freshwater zone in the vicinity of the Kyle transect is 

the physical setting—the substantial narrowing of the transition zone. If flow from a relatively large volume (flow area) of 

aquifer is funneled into a relatively small volume of aquifer, 

then the potential for water to discharge from the small volume 

could become relatively greater. 

The data for this report support the hypothesis that the 

interface, conceptualized as a sloping surface as indicated in 

figures 5–8, is likely to remain stable laterally and vertically 

over time. The most direct evidence in support of that hypothesis is the conductivity data from interface wells TC2 and 

KY2 that show an essentially stable vertical interface position 

(depth in the borehole) over a range of hydrologic conditions. 

At well TC2, the interface position was essentially unchanged 

as equivalent freshwater heads differed by as much as about 25 

feet among the times geophysical logs were run (fig. 15). At 

well KY2, the interface position varied over a range of roughly 

50 feet as equivalent freshwater heads differed by as much as 

35 feet; 50 feet of variation is considered “stable” relative to 

a regional scale of miles. At the Fish Hatchery transect, the 

interface (again assuming a sloping configuration) was contained between wells FH1 and FH2, spaced 1.8 miles apart, 

throughout the range of hydrologic conditions that occurred at 

the times geophysical logs were run, despite an upward gradient (indicated by upward borehole flow) on the saline-water 

side of the interface (well FH2) during each log run. 

A stable interface over time does not necessarily mean 

no flow from the transition zone into the freshwater zone. 

Continuous specific conductance recorded by monitors 

installed at depths of 700–800 feet below land surface in wells 

TC1 and KY2 (Lambert and others, 2009, table 24) indicates 

variability over time (fig. 33). Although generally no clear 

relations between conductivity (or temperature or borehole 

flow) and equivalent freshwater head at the times geophysical logs were run could be identified, the graphs of specific 

conductance and equivalent freshwater head over time in 

wells TC1 (2000–2007) and KY2 (2003–07) show inverse 

relations. During the periods of record, specific conductance 

(microsiemens per centimeter at 25 °C) varied over a range of 

several hundred in freshwater well TC1 and several thousand 

in interface well KY2. (The conductance monitor was on the 

saline-water side of the interface in well KY2). Specific conductance generally increased when equivalent freshwater head 

decreased, indicating movement of more-saline water toward 

the freshwater zone, but then specific conductance generally 

decreased when heads rose again. The relation indicates that 

more-saline water that moved toward or entered the freshwater 

zone during dry conditions was diluted and “washed away” 

by freshwater flow when wet conditions returned. From a 

regional perspective considering the hypothesis of relatively 

low permeability in the transition zone relative to that in the 

freshwater zone, it follows that the relatively low-permeability 

rocks of the transition zone allow relatively little flow toward 

the freshwater zone, even if gradients toward the freshwater 

zone are relatively large and that the relatively high-permeability rocks of the freshwater zone allow relatively large 

flows that are easily capable of diluting and washing away 

any encroaching saline water, even if gradients in the freshwater zone are relatively small. Further, in the vicinity of the 

Kyle transect, any upward flow of freshwater from the Trinity 

aquifer, for which the potential is indicated by relatively strong 

upward borehole flows in well KY1 and to a lesser extent 

in well KY2, would tend to contribute to the dilution of any 

saline water leaking into the freshwater zone in that area. 

No single line of evidence confirms the hypothesis that the 

interface in the San Antonio segment of the Edwards aquifer is 

relatively stable over time, and thus the potential for irreversible movement of saline water into the freshwater zone is 

small; but the cumulative evidence from the water-level (head) 

and borehole geophysical log data supports that hypothesis. 

Summary

The freshwater zone of the San Antonio segment of 

the Edwards aquifer in south-central Texas (hereinafter, the 

Edwards aquifer) is bounded to the south and southeast by 

a zone of transition from freshwater to saline water (hereinafter, the transition zone). Freshwater is defined here as that 

containing less than 1,000 mg/L dissolved solids concentration; slightly saline water, that containing 1,000 to 3,000 mg/L 

dissolved solids concentration; moderately saline water, that 

containing 3,000 to 10,000 mg/L dissolved solids concentration; and very saline water, that containing 10,000 to 35,000 

mg/L dissolved solids concentration. The freshwater/salinewater interface (hereinafter, the interface) is the 1,000-mg/L 

dissolved solids concentration threshold. The transition zone is 

defined here as the region of the aquifer with dissolved solids 

concentrations ranging from 1,000 to 10,000 mg/L. 

This report presents the findings of a study, done by the 

U.S. Geological Survey in cooperation with the San Antonio 

Water System, to obtain lithologic properties (rock properties 

associated with known stratigraphic units) and physicochemical properties (fluid conductivity and temperature) and to analyze the hydraulics of flow in and near the transition zone of 

the Edwards aquifer on the basis of water-level and borehole 

geophysical log data collected from 15 monitoring wells in 
Summary 65

four transects during 1999–2007; and on the basis of findings, 

assesses the potential for irreversible movement of saline water 

into the freshwater zone of the Edwards aquifer. Monitoring 

wells from which data were collected for this report are in and 

near the transition zone in Uvalde County (East Uvalde [EU] 

transect, four wells), in Comal and Guadalupe Counties (TriCounty [TC] transect, five wells), and in Hays County (Fish 

Hatchery [FH] transect, two wells; and Kyle [KY] transect, 

four wells). A well descriptor is applied to each well on the 

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

interface [freshwater atop saline water]).

Water-level data were collected to obtain head distribution over time as an indicator of hydrologic conditions. Daily 

mean water levels from continuously measured hydraulic 

heads in monitoring wells of the transects were converted 

to equivalent freshwater heads so that accurate lateral head 

gradients between the transition zone and the freshwater zone 

could be computed. 

Geophysical log data were collected using conventional 

methods (caliper, gamma, resistivity, induction, fluid conductivity, and fluid temperature) and advanced methods (borehole 

televiewer imaging tools and electromagnetic and heatpulse vertical flowmeters). These tools were used to obtain 

lithologic properties from which stratigraphic units of the 

aquifer could be identified (the tops of the stratigraphic units 

penetrated were picked from drillers’ logs but were refined 

by the geophysical logs) and to obtain vertical flow (magnitude and direction under ambient conditions) relative to the 

stratigraphic units in each well. The directions of vertical flow 

indicated the directions of vertical gradients at each well.

The natural gamma logs for East Uvalde freshwater 

wells EU1 and EU2, completed in the reefal facies of the 

Devils River Trend depositional province, indicate carbonate rocks with a lack of contrast in the upper sections of the 

boreholes and increasing clayey material in the lower sections 

of the boreholes. In contrast, the gamma logs for saline-water 

wells EU3 and EU4 show greater variation in lithology down 

the boreholes than that in either wells EU1 or EU2, indicating that more clayey intervals corresponding to the McKnight 

Formation and the West Nueces Formation of the Maverick 

Basin depositional province are interbedded with carbonate 

rocks. The natural gamma logs for Tri-County freshwater 

well TC1, interface well TC2, and saline-water wells TC3 

and TC4 indicate that Edwards aquifer rocks are composed 

predominantly of limestone with a few minor clayey sections 

without any major contrast in lithology, despite the layered 

structure in the San Marcos Platform depositional province 

reflected by the members (hydrogeologic subdivisions) of 

the Person and Kainer Formations. The natural gamma logs 

for Fish Hatchery freshwater well FH1 and saline-water 

well FH2 (both wells completed in rocks of the San Marcos 

Platform depositional province) indicate that the borehole 

sections are composed of fairly clean limestone interbedded 

with clayey lenses. For well FH2 (deeper of the two), the 

natural gamma log indicates clayey intervals that correspond to the leached and collapsed members and parts of 

the Kirschberg evaporite, dolomitic, and basal nodular members. The natural gamma logs for Kyle transect freshwater 

well KY1, interface well KY2, and saline-water wells KY3 

and KY4 indicate that the wells are open to clean limestone 

with some clayey intervals in the cyclic and marine members 

and the leached and collapsed members in the upper part of 

the Edwards aquifer, and in the dolomitic and basal nodular 

members in the lower part of the Edwards aquifer. Wells KY1, 

KY2, and KY3 also are open to the Glen Rose Limestone 

(Trinity aquifer) at the base of the wells. The gamma logs for 

these wells indicate that the Glen Rose Limestone has greater 

clay content than the formations of the overlying Edwards 

aquifer.

Fluid conductivities recorded in East Uvalde transect 

wells EU1 and EU2 indicate freshwater down the entire 

lengths of the boreholes. Fluid conductivities recorded in well 

EU3 indicate saline water in the borehole. Well EU4 contained saline water during four of six log runs, but for one of 

the runs, salinity was entirely in the freshwater range and for 

another was in the fresh and slightly saline ranges. The seven 

conductivity logs for Tri-County transect well TC1 indicate 

freshwater. On the basis of fluid conductivity, well TC2 intersected the interface at or slightly above the contact between 

the Kirschberg evaporite member and the dolomitic member. 

Above that contact, freshwater was in the borehole during 

all log runs, and below it, saline water was in the borehole. 

Neither borehole flow nor changes in head between the times 

the logs were run affected the position of the interface, which 

is essentially the same on all logs. The fluid conductivity logs 

for wells TC3 and TC4 indicate saline water along the entire 

lengths of the boreholes for all log runs; the ranges of values 

reflected among the logs varied widely (slightly saline to 

very saline). The fluid conductivity logs run in Fish Hatchery 

well FH1 all indicate freshwater in the borehole. The fluid 

conductivity logs for well FH2 indicate saline water, ranging 

from slightly saline to very saline. Fluid conductivity logs for 

Kyle transect well KY1 indicate freshwater throughout the 

borehole during all log runs. Fresher water near the bottom 

of the borehole is indicated on each of the logs. This fresher 

water appears to be in the Glen Rose Limestone (Trinity aquifer). Well KY2 is the second of two wells of this report that 

intersects the interface. Fluid conductivity logs show that the 

interface intersected the borehole in the Kirschberg evaporite 

member and upper part of the dolomitic member. The range in 

salinity of water between the upper and lower sections of the 

KY2 borehole indicated by most of the logs is large—from 

fresh to very saline. Unlike in interface well TC2, the position 

of the interface in the KY2 borehole changed about 50 feet 

over time. Although the position of the interface bears some 

relation to head at the time the logs were run, the conductivity 

values below the interface do not. Fluid conductivity logs run 

in wells KY3 and KY4 indicate that water in the boreholes at 

the times of the log runs was moderately to very saline. No 

identifiable relation between conductivity values from logs in 

monitoring wells in all transects and heads in the wells at the 

times the logs were run is evident; and no identifiable relation 
66 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone

between conductivity values and vertical flow in the boreholes 

concurrent with the times the logs were run is evident.

The temperature logs for wells of the East Uvalde 

transect indicate temperatures ranged from about 20 °C (well 

EU2) to about 40 °C (well EU4); for the Tri-County transect, 

about 21 °C (well TC1) to about 39 °C (well TC4); for the 

Fish Hatchery transect, about 18 °C (well FH1) to about 32 °C 

(well FH2); and for the Kyle transect, about 20 °C (well KY1) 

to about 30 °C (well KY4). Temperatures increased with depth 

in all the logs. 

The directions of the lateral equivalent freshwater head 

gradient at the interface in the East Uvalde transect for 19 

coincident periods of head record for 2000–2007 indicate a 

nearly even split between into the freshwater zone (nine) and 

out of the freshwater zone (10). The directions of the lateral 

gradient at the interface are affected to a greater or lesser 

degree by hydrologic conditions (antecedent and concurrent 

recharge) and relatively greater and smaller withdrawals. The 

direction of the equivalent freshwater head gradient and thus 

the potential lateral flow at the interface in the vicinity of the 

East Uvalde transect fluctuates between into and out of the 

freshwater zone, depending on recharge and withdrawals. 

Whether the prevailing direction on average is into or out of 

the freshwater zone is not clearly indicated.

The directions of the lateral head gradient at the interface 

in the Tri-County transect were mixed (head higher at the 

interface than on either side) in 25 of 30 coincident periods of 

record for 2000–2007, into the freshwater zone in one period, 

and uncertain in four periods. Why equivalent freshwater 

heads generally were higher at the interface than on either side 

of it cannot be explained with available data. There does not 

appear to have been a prevailing direction of the lateral gradient at the interface in the vicinity of the Tri-County transect. 

The directions of lateral head gradient at the interface 

in the Kyle transect for 42 coincident periods of record for 

2000–2007 were into the freshwater zone for about one-half 

the periods (22), mixed for one-third of the periods (14), out of 

the freshwater zone for three periods, and uncertain for three. 

The direction of the lateral gradient at the interface in the 

vicinity of the Kyle transect, like that in the vicinity of the East 

Uvalde transect, probably fluctuates between into and out of 

the freshwater zone, depending on recharge and withdrawals; 

but the prevailing direction on average of the lateral gradient 

and thus potential lateral flow at the interface in the vicinity 

of the Kyle transect likely is from the transition zone into the 

freshwater zone.

The direction of vertical flow in all four East Uvalde transect wells with measurable flow was mostly upward, and magnitude of flow generally was greater when equivalent freshwater heads were high than when they were low. Equivalent 

freshwater heads at the times of each of the four flow logs, 

whether relatively high or relatively low, were higher on the 

saline-water side of the interface than on the freshwater side. 

The hypothesis regarding the vertical gradient in the vicinity 

of the East Uvalde transect, and thus the potential for vertical flow near an interface conceptualized as a surface sloping 

upward in the direction of the dip of the stratigraphic units, 

is that the potential for vertical flow fluctuates between into 

and out of the freshwater zone, depending on recharge and 

withdrawals.

At all four times the flow logs were run in Tri-County 

transect wells, which encompassed a range of hydrologic conditions from wet to dry for the period of record (2000–2007), 

the interface was essentially in the same position in well TC2, 

below the regional dense member about at the contact between 

the grainstone member and the Kirschberg evaporite member; 

some combination of head gradient and permeability (each 

with lateral and vertical components) was functioning to keep 

the interface stable as heads changed. A downward gradient on 

the freshwater side of the interface and an upward gradient on 

the saline-water side are evidence of opposing potentials that 

appear to have stabilized the position of the interface over the 

range of hydrologic conditions that occurred at the times the 

logs were run. 

Vertical flows in the borehole of Fish Hatchery transect 

well FH2 were upward whether hydrologic conditions were 

wet or dry. There appears to have been an upward gradient on 

the saline-water side of the interface under a wide range of 

hydrologic conditions. This upward gradient, coupled with the 

assumption of a sloping interface, implies a vertical gradient 

from the transition zone into freshwater zone. This potential 

for vertical movement of saline water into the freshwater zone 

(vertical movement of the interface) apparently was opposed 

by the potential (head) on the freshwater side of the interface 

that kept the interface beneath the bottom of well FH1 over 

the range of hydrologic conditions during which the logs were 

run.

The five flow logs for Kyle transect freshwater well 

KY1 all indicate upward flow ranging from less than 0.1 to 

nearly 4 gallons per minute that originates from the Glen Rose 

Limestone, the uppermost unit of the Trinity aquifer; and one 

log for well KY2 shows upward flow of 0.3–0.4 gallon per 

minute entering the borehole from the Trinity aquifer. These 

flow data constitute evidence of the potential for flow from the 

Trinity aquifer into the Edwards aquifer in the vicinity of the 

Kyle transect. 

Subsurface temperature data were used to indicate zones 

of negligible groundwater flow (temperature [thermal] gradients greater than or equal to the conductive thermal gradient) 

and zones of active groundwater flow (temperature gradients 

less than the conductive thermal gradient or the absence of a 

temperature gradient). The findings regarding relative groundwater flow based on temperature indicate that flow on average is more active, or vigorous, on the freshwater side of the 

interface than on the saline-water side.

The freshwater zone and transition zone of the Edwards 

aquifer are connected by stratigraphy, structure, and hydraulics. A hydraulic connection between the transition zone and 

the freshwater zone is indicated by similar patterns in the 

hydrographs of the 15 transect monitoring wells in and near 

the transition zone and three county index wells in the freshwater zone during 1999–2007. 
References 67

The data for this report in part support a conceptualization of regional flow in and near the transition zone in which 

water enters the transition zone from the freshwater zone in 

the western part of the Edwards aquifer, flows toward the 

northeast, and discharges to the freshwater zone in the eastern 

part of the aquifer. The probable fluctuating head gradient over 

time at the interface in the East Uvalde transect, or a gradient 

on average from the freshwater zone into the transition zone 

as previous studies have implied, could result in a net inflow 

of freshwater into the transition zone in the vicinity of the 

East Uvalde transect. Head data do not indicate a potential for 

lateral flow at the interface in the vicinity of the Tri-County 

transect, which is consistent with flow in the transition zone 

primarily parallel to the interface (toward the northeast). The 

head gradient at the interface in the Kyle transect also probably fluctuates between into and out of the freshwater zone, 

depending on recharge and withdrawals, but the prevailing 

direction at the interface on average likely is from the transition zone into the freshwater zone. 

The data for this report support the hypothesis that the 

interface is likely to remain stable laterally and vertically 

over time. The most direct evidence in support of that hypothesis is the conductivity data from interface wells TC2 and 

KY2 that show an essentially stable vertical interface position 

(depth in the borehole) over a range of hydrologic conditions. 

Continuous specific conductance and concurrent head data 

show that when more-saline water moved toward or entered 

the freshwater zone during dry conditions, it was diluted 

and “washed away” by freshwater flow when wet conditions 

returned. The hypothesis is that relatively low-permeability 

rocks of the transition zone allow relatively little flow toward 

the freshwater zone, even if gradients toward the freshwater 

zone are relatively large and that relatively high-permeability 

rocks of the freshwater zone allow relatively large flows that 

are easily capable of diluting and washing away any encroaching saline water, even if gradients in the freshwater zone 

are relatively small. No single line of evidence confirms the 

hypothesis that the interface in the San Antonio segment of 

the Edwards aquifer is relatively stable over time, and thus the 

potential for irreversible movement of saline water into the 

freshwater zone is small; but the cumulative evidence from the 

water-level (head) and borehole geophysical log data supports 

that hypothesis.

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Lambert, Hunt, Stanton, and Nyman—Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transition Zone—SIR 2010–5122

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