GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC 

CHARACTERISTICS OF THE EDWARDS AQUIFER 

RECHARGE ZONE, BEXAR COUNTY, TEXAS

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 95-4030

Prepared in cooperation with the 

SAN ANTONIO WATER SYSTEM
GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC 

CHARACTERISTICS OF THE EDWARDS AQUIFER 

RECHARGE ZONE, BEXAR COUNTY, TEXAS

By William G. Stein and George B. Ozuna

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations 95-4030

Prepared in cooperation with the 

SAN ANTONIO WATER SYSTEM

Austin, Texas 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

by the U.S. Government.

For additional information write to:

District Chief 

U.S. Geological Survey 

8011 Cameron Rd. 

Austin, TX 78754-3898

Copies of this report can be purchased from:

U.S. Geological Survey 

Earth Science Information Center 

Open-File Reports Section 

Box 25286, Mail Stop 517 

Denver Federal Center 

Denver, CO 8.0225-0046
CONTENTS

Abstract .............................................................................................................................................................^ 1

Introduction ..........................................................................................................._^ 1

Methods of Investigation .......................................................................................................................................... 3

Acknowledgments .................................................................................................................................................... 3

Geologic Framework ............................................................................................................................................................ 4

General Features ....................................................................................................................................................... 4

Stratigraphy ............................................................................ 5

Hydrogeologic Characteristics ............................................................................................................................................. 6

General Features ....................................................................................................................................................... 4

Porosity and Permeability ......................................................................................................................................... 6

Summary .............................................................................................................................................................................. 7

References Cited................................................................................................................................................................... 8

PLATE

[Plate is in pocket]

1. Map showing hydrogeologic subdivisions of the Edwards aquifer recharge zone, Bexar County, Texas.

FIGURE

1. Map showing location of the study area ............................................................................................................... 2

TABLE

1. Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards

aquifer outcrop, Bexar County, Texas .................................................................................................................. 4

CONVERSION FACTORS AND VERTICAL DATUM

________________Multiply______________By______________To obtain___________________

acre-foot (acre-ft) 1,233 cubic meter

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

foot (ft) 0.3048 meter

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

mile (mi) 1.609 kilometer

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

CONTENTS ill
Geologic Framework and Hydrogeologic 

Characteristics of the Edwards Aquifer 

Recharge Zone, Bexar County, Texas

By William G. Stein and George B. Ozuna

Abstract

In Bexar County, residential and commercial development on the Edwards aquifer recharge 

zone is increasing. The aquifer possibly can be 

contaminated by spills, leakage of hazardous 

materials, or runoff from the rapidly developing 

urban areas that surround, or are built on, the 

intensely faulted and fractured, karstic limestone 

outcrops characteristic of the recharge zone. Furthermore, some of the hydrogeologic subdivisions 

that compose the Edwards aquifer have greater 

effective porosity than others. The areas where the 

most porous subdivisions crop out might provide 

efficient avenues for contaminants to enter the 

aquifer.

The Edwards aquifer recharge zone has relatively large permeability resulting, in part, from 

the development or redistribution of secondary 

porosity. Lithology, stratigraphy, diagenesis, and 

karstification account for the effective porosity 

and permeability in the Edwards aquifer outcrop. 

Karst features that greatly enhance effective 

porosity in the outcrop area include sinkholes and 

caves.

Hydrogeologic subdivision VI, the Kirschberg evaporite member, appears to be the most 

porous and permeable subdivision within the 

Kainer Formation. Hydrogeologic subdivision HI, 

the leached and collapsed members, undivided, is 

the most porous and permeable subdivision within 

the Person Formation. Hydrogeologic subdivision 

II, the cyclic and marine members, undivided, is 

moderately permeable, with both fabric- and notfabric-selective porosity.

The faults in northern Bexar County are part 

of the Balcones fault zone. Although most of the

faults in this area trend northeast, a smaller set of 

cross-faults trend northwest. Generally, the faults 

are en echelon and normal, with the downthrown 

blocks typically toward the coast.

INTRODUCTION

The Edwards aquifer, which comprises the 

Kainer and Person Formations of the Edwards Group 

(Rose, 1972) and the overlying Georgetown Formation 

in rocks of Lower Cretaceous age, is one of the most 

productive carbonate aquifers in the nation. The dissolution-modified, faulted limestone aquifer (Buszka 

and others, 1990) is the sole source of public-water 

supply for San Antonio (ninth largest city in the United 

States) and is the major source of water for Bexar 

County (fig. 1). In addition to providing public-water 

supply to more than 1 million people in south-central 

Texas, the Edwards aquifer supplies large quantities of 

water to agriculture, industry, and major springs. The 

major springs support recreational activities and businesses, provide flow to downstream users, and provide 

habitat for several threatened or endangered species.

Most recharge to the Edwards aquifer is from 

direct infiltration of precipitation and streamflow loss 

in the recharge zone. Recharge to the Edwards aquifer 

averaged 683,000 acre-ft/yr during 1934-92 (Bader 

and others, 1993, table 4.1). After entering the aquifer, 

the water generally moves from west to east through 

Bexar County, where it is discharged by wells and 

springs. Springflow has varied over a wide range during historical time. Annual well withdrawals from the 

aquifer have gradually increased from 101,900 acre-ft 

in 1934 to 542,400 acre-ft in 1989 (Bader and others, 

1993, table 5.1). During the 10 years from 1983 to 

1992, well discharge exceeded springflow by 22 percent (Bader and others, 1993, table 5.1).

In Bexar County, residential and commercial 

development on the Edwards aquifer recharge zone is

Abstract
99>45' 28P45'

98°30-

29*30'

29°15'

Base modified from

University of Texas

Bureau of Economic Geology, 1974

EXPLANATION 

STUDY AREA 

EDWARDS AQUIFER OUTCROP

10 MILES

5 10 KILOMETERS

Figure 1. Location of the study area.

2 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Recharge Zone, Bexar County, Texas
increasing. According to Kipp and others (1993), a substantial increase in development activities over the 

Edwards recharge zone has been observed over the past 

year, particularly in Bexar County. Kipp and others 

(1993) further state that a greater threat to contamination of the Edwards aquifer comes with increased 

development.The aquifer possibly can be contaminated 

by spills, leakage of hazardous materials, or runoff 

from the rapidly developing urban areas that surround, 

or are built on, the intensely faulted and fractured, 

karstic limestone outcrops characteristic of the 

recharge zone. Furthermore, some of the hydrogeologic subdivisions that compose the Edwards aquifer 

have greater effective porosity than others, and in areas 

where they crop out, might provide efficient avenues 

for contaminants to enter the aquifer. According to 

Buszka (1987, p. 2), "carbonate aquifers, such as the 

Edwards, are readily susceptible to ground-water contamination where the presence of pollutants coincides 

with the outcrop of the aquifer." In Bexar County, the 

Edwards aquifer probably is most vulnerable to surface 

contamination in the rapidly urbanizing areas on the 

Edwards aquifer outcrop.

The U.S. Geological Survey, in cooperation with 

the San Antonio Water System, mapped the Edwards 

aquifer outcrop and described its hydrogeologic characteristics (porosity and permeability) to document 

conditions pertinent to movement and contamination 

of ground water.

This report describes the geologic framework 

and the hydrogeologic characteristics of the Edwards 

aquifer recharge zone in Bexar County and delineates 

the surface extent of the hydrogeologic subdivisions 

described for the confined zone of the Edwards aquifer 

by Maclay and Small (1976). This information will 

help to provide a better understanding of the processes 

controlling the spatial distribution of recharge and the 

flow of water into the aquifer. The information also will 

help determine the areas of the recharge zone that are 

most susceptible to potential contamination from landuse practices.

Methods of Investigation

The Kainer and Person Formations of the 

Edwards Group were divided into seven informal 

members by Rose (1972). Rose's (1972) categorization 

was further modified by Maclay and Small (1976), who 

added an eighth hydrogeologic subdivision, the overlying Georgetown Formation.

The outcrop of the Edwards aquifer recharge 

zone was mapped using the hydrogeologic subdivisions (table 1) modified from Maclay and Small 

(1976). The stratigraphic nomenclature and descriptions of Rose (1972) were used in mapping the geologic units for the San Marcos platform (fig. 1). The 

hydrogeologic subdivisions of the Edwards aquifer 

recharge zone in Bexar County are shown on plate 1.

Well logs and geologic map data were compiled 

and used in mapping the hydrogeologic subdivisions of 

the Edwards aquifer in the study area. The lower confining unit of the Edwards aquifer, the upper member of 

the Glen Rose Limestone of Lower Cretaceous age 

(table 1), was mapped where it is adjacent to the 

recharge zone. The Del Rio Clay, Buda Limestone, and 

Eagle Ford Group, constituting the upper confining 

units, were mapped along the southern boundary of the 

recharge zone.

Caves and other karst features were located during mapping using information from Veni (1988). 

Aerial photographs were used to locate the most recent 

rock exposures so that relatively fresh outcrops could 

be examined. Original land-surface topography of 

excavated quarries was interpolated from exposed outcrops and 7-1/2-minute quadrangles. Edwards aquifer 

outcrop mapping also was interpolated through areas 

that are covered by alluvial deposits.

Determination of fault displacement generally is 

difficult in carbonate rocks. Fault traces commonly are 

obscure and difficult to identify in the field. Fault displacements were postulated and estimated by abrupt 

lithologic or stratigraphic dissimilarities and at least 

one of the following: fault scarps, fault breccia, long 

linear travertine or sparry calcite deposits, or steeply 

dipping strata thought to represent fault-bend folds. 

Fault-bend folds are bedding deformation associated 

with fault-block movement (Suppe, 1985, p. 343). The 

strike of these features was measured with a compass to 

determine the orientation of the faults. The lengths of 

many of the faults were projected by lineaments visible 

on land surface or in aerial photographs. Faults were 

inferred by the location of lineaments on photographs 

in areas where only slight stratigraphic dissimilarities 

were indicated, or where the faults extended beyond 

the mapped area.

Acknowledgments

Special thanks are extended to George Veni, a 

local consulting karst hydrologist, for his assistance in

INTRODUCTION
Table 1 . Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards 

aquifer outcrop, Bexar County, Texas

[Hydrogeologic subdivisions modified from Maclay and Small (1976); groups, formations, and members modified from Rose (1972); 

lithology modified from Dunham (1962); and porosity type modified from Choquette and Pray (1970). CU, confining unit; AQ, aquifer]

Hydrogeologic 

subdivision

Upper Cretaceous

Cretaceous Lower 

Upper 

confining 

units

I

II

m

IV

V

VI

vn

vni

Edwards aquifer

Lower 

confining 

unit

Group, 

formation, 

or member

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown 

Formation

1

u.

Kainer Formation

Cyclic and 

marine 

members, 

undivided

Leached and 

collapsed 

members, 

undivided

Regional 

dense 

member

Grainstone 

member

Kirschberg 

evaporite 

member

Dolomitic 

member

Basal nodular 

member

Upper member of the 

Glen Rose Limestone

Hydrologic 

function

CU

CU

CU

KarstAQ; 

notkarst 

CU

AQ

AQ

CU

AQ

AQ

AQ

Karst 

AQ; 

notkarst 

CU

CU; 

evaporite 

bedsAQ

Thickness 

(feet)

30-50

40-50

40-50

2-20

80-90

70-90

20-24

50-60

50-60

110-130

50-60

350-500

Uthology

Brown, flaggy shale and 

argillaceous limestone

Buff, light gray, 

dense mudstone

Blue-green to 

yellow-brown clay

Reddish-brown, gray to 

light tan marly 

limestone

Mudstone to packstone; 

miliolid grainstone; 

chert

Crystalline limestone; 

mudstone to grainstone; 

chert; collapsed breccia

Dense, argillaceous 

mudstone

Miliolid grainstone; 

mudstone to 

wackestone; chert

Highly altered 

crystalline limestone; 

chalky mudstone; chert

Mudstone to grainstone; 

crystalline limestone; 

chert

Shaly, nodular 

limestone; mudstone 

and miliolid grainstone

Yellowish tan, thinly 

bedded limestone 

and marl

Field 

identification

Thin flagstones; 

petroliferous

Porcelaneous limestone 

with calcite-filled 

veins

Fossiliferous; 

Ilymatogyra arietina

Marker fossil; 

Waconella 

wacoensis

Thin graded cycles; 

massive beds to 

relatively thin beds; 

crossbeds

Bioturbated ironstained beds separated 

by massive limestone 

beds; stromatolitic 

limestone

Wispy iron-oxide 

stains

White crossbedded 

grainstone

Box work voids, with 

neospar and travertine 

frame

Massively bedded 

light gray, Toucasia 

abundant

Massive, nodular and 

mottled, Exogyra 

texana

Stair-step topography; 

alternating limestone 

and marl

Cavern 

development

None

Minor surface karst

None

None

Many subsurface; 

might be associated 

with earlier 

karst development

Extensive lateral 

development; large 

rooms

Very few; only 

vertical fracture 

enlargement

Few

Probably extensive 

cave development

Caves related to 

structure or 

bedding planes

Large lateral caves at 

surface; a few caves 

near Cibolo Creek

Some surface cave 

development

Porosity/ 

permeability type

Primary porosity lost/ 

low permeability

Low porosity/low 

permeability

None/primary upper 

confining unit

Low porosity/low 

permeability

Laterally extensive; both 

fabric and not 

fabric/water-yielding

Majority not fabric/one of 

the most permeable

Not fabric/low 

permeability; vertical 

barrier

Not fabric/ 

recrystallization reduces 

permeability

Majority fabric/one of the 

most permeable

Mostly not fabric; some 

bedding planefabric/water-yielding

Fabric; stratigraphically 

controlled/large conduit 

flow at surface; no 

permeability in 

subsurface

Some water production at 

evaporite 

beds/relatively 

impermeable

locating some of the caves while mapping in the field. 

The authors also express their thanks to all the property 

owners who granted permission to enter their property.

GEOLOGIC FRAMEWORK 

General Features

A regional dip of the rocks of the hydrogeologic 

subdivisions of 10 to 15 ft/mi to the southeast in Bexar

County was reported by Arnow (1959, p. 19), and the 

thicknesses of the rocks were reported by Rose (1972). 

The faults in northern Bexar County are part of the 

Balcones fault zone (fig. 1). Although most of the 

faults in the area trend southwest to northeast, a smaller 

set of cross-faults trend southeast to northwest. Generally, the faults are en echelon and normal, with the 

downthrown blocks typically toward the coast (southeast). Some of the faults might not result in topographic

4 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Recharge Zone, Bexar County, Texas
relief, partly because the rocks on both sides of the fault 

have similar weathering characteristics, and possibly 

because the rate of movement is no faster than the rate 

of erosion.

Maclay and Small (1984, p. 33) define flowbarrier faults as faults that have vertical displacement 

greater than 50 percent of the total thickness of the 

aquifer, sufficient to juxtapose permeable layers 

against relatively less permeable layers. The thickness 

of the Edwards aquifer in Bexar County is about 450 ft; 

50 percent of this is about 225 ft. Therefore, faults in 

the study area with a vertical displacement of about 225 

ft or greater were designated as flow-barrier faults, as 

indicated by their displacements on plate 1. A series of 

flow-barrier faults extends from the southwest part of 

the study area toward the northeast.

Some of the faults in Bexar County mark the 

trace of shatter zones, where the faults are not single, 

sharp breaks as shown by a single line placed on a map 

(Arnow, 1959, p. 20). Field observations of linear 

sparry travertine (a clear to translucent secondarily 

precipitated calcite) deposits within many of the fault 

shatter zones, and of water discharging from a fault 

exposed in a road cut northwest of the Loop 1604 and 

1-10 intersection shortly after local rainfall, support the 

hypothesis of ground-water flow within some faults.

Stratigraphy

In Bexar County, the Edwards aquifer consists 

of approximately 450 ft of limestone, with lesser quantities of chert, dolomite, and evaporite (primarily anhydrite/gypsum) (table 1). The major formal lithostratigraphic units of the Edwards aquifer are the Kainer, 

Person, and Georgetown Formations (Maclay and 

Small, 1976). The Kainer and Person Formations are 

subdivided into informal members by Rose (1972). The 

rocks of these members, or subdivisions, were deposited in shallow to very shallow marine waters (Rose, 

1972) and reflect depositional environments resulting 

from slight changes in water level, water chemistry, 

temperature, and circulation. All of these factors can 

cause subtle to not-so-subtle changes in the overall 

lithology of the various members and some changes 

within the individual members.

The Georgetown Formation is not known to 

yield water in Bexar County. Arnow (1959) noted that 

water-well drillers did not distinguish between the 

Georgetown Limestone and the underlying Edwards 

Limestone. Thus, according to Arnow (1959, p. 15),

"The Georgetown is part of the aquifer that comprises 

the Edwards and associated limestones...."

The thickness of the Kainer Formation (Rose, 

1972, p. 18) ranges from about 260 to 310 ft in Bexar 

County (table 1). The lithology of the Kainer Formation includes marine sediments consisting of fossiliferous (most commonly rudistids) mudstones and 

wackestones that grade upward into intertidal and 

supratidal dolomitic mudstones with evaporites, and 

terminate in a shallow marine miliolid grainstone. The 

lower part of the dolomitic member of the Kainer Formation in western Bexar County is distinctly more burrowed, similar to the facies equivalents of the burrowed 

member of the Fort Terrett Formation (Rose, 1972, p. 

30) northwest of the study area. Major collapsed features noted elsewhere by Rose (1972) in the Kirschberg 

evaporite member were not evident in Bexar County. 

This might indicate fewer massive gypsum deposits 

and more interbedded limestone that would have prevented major collapses after evaporite removal.

The Person Formation (Rose, 1972, p. 19) is 

about 170 ft thick in Bexar County (table 1). The 

regional dense member at the base of the Person Formation is a dense, argillaceous mudstone, which is 

easily recognized in cores and usually recognizable 

on geophysical logs (Small, 1985). Deposition of the 

Person Formation above the regional dense member 

continued with dolomitic biomicrite, which contains 

layers of collapsed breccias, burrowed mudstones, and 

stromatolitic limestone. The cyclic and marine members, undivided, consist of small upward-grading 

cycles of mudstones to grainstones that range from 

massive to thin beds and occasionally are crossbedded. 

Much of the cyclic and marine members, undivided, in 

Bexar County might have been removed by erosion 

prior to the deposition of the Georgetown Formation 

(Rose, 1972).

A depositional hiatus occurred before the open 

marine, biomicritic Georgetown Formation (Rose, 

1972) was deposited. Parts of the Person Formation 

were above sea level during this time and were substantially eroded along the ridge of the San Marcos platform (Rose, 1972). The Georgetown Formation 

generally is a marly limestone and usually contains the 

brachiopod Waconella wacoensis, formerly Kingena 

wacoensis (Roemer), pectens, and other pelecypods. 

Most exposures of the Georgetown Formation that 

were identified in the study area were from 2 to 5 ft 

thick, and consisted of dark reddish-brown weathered 

and friable material with some shaly limestone

GEOLOGIC FRAMEWORK
remnants. However, an outcrop in a quarry near 

Helotes Creek, east of FM 1560, contains an unweathered tannish-yellow biomicrite sequence of Georgetown Formation that is approximately 20 ft thick. This 

particular Georgetown Formation outcrop evidently 

has undergone a different diagenetic history than the 

other Georgetown Formation outcrops in Bexar 

County.

The Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford Group overlie the Georgetown 

Formation (pi. 1, table 1). The Del Rio Clay is a weathered, poorly compacted, friable, blue-green to yellowbrown clay, with some areas having accumulations of 

Ilymatogyra arietina, formerly Exogyra arietina (Roemer). The contact between the Del Rio Clay and the 

Buda Limestone is transitional to disconformable 

depending on locality (Rose, 1972, p. 27,43). The 

Buda Limestone outcrop is distinctly nodular with 

small, calcite-filled veins, and generally is buff to light 

gray, porcelaneous limestone (Sellards and others, 

1933, p. 397). The Eagle Ford Group unconformably 

overlies the Buda Limestone (Arnow, 1959). The Eagle 

Ford Group consists of thin flagstones of brown, flaggy 

shale and argillaceous limestone. Some of the freshly 

fractured flagstones emit a petroliferous odor, indicative of a high organic content.

Field identification of the various members in the 

Kainer and Person Formations was based on their characteristic lithologies and fossils (table 1). Red clay soil 

that resembles the "terra rossa" of Pleistocene age, 

described by Young (1986, p. 63) as a diagenetically 

altered paleosol, commonly is evident in outcrops of 

the Edwards Group, but rarely in the Glen Rose Limestone or in the clays or limestones of the upper confining units. According to Young (1986, p. 65), the red 

clay soil indicates that lithology was important in the 

development of central Texas terra rossa. Large accumulations of terra rosa relative to the volume of pore 

space in the limestone decrease the effective porosity 

and, thereby, decrease the permeability.

HYDROGEOLOGIC CHARACTERISTICS 

General Features

The Edwards aquifer has relatively large permeability resulting, in part, from the development or redistribution of secondary porosity (Maclay and Small, 

1976). Lithology, stratigraphy, diagenesis, and selective dissolution (karstification) account for the effective porosity and permeability in the Edwards aquifer 

outcrop. Karst features that can greatly enhance the 

effective porosity in the outcrop include sinkholes and 

caves. The dry-subhumid climate (Thornthwaite, 1952) 

is not favorable for rapid karst development. The presence of caves in the Edwards Group limestone in Bexar 

County is random and the morphology is controlled by 

the local stratigraphy.

Porosity and Permeability

According to Choquette and Pray (1970, p. 212), 

porosity in sedimentary carbonates is either fabric 

selective or not fabric selective. Fabric-selective porosity is secondary porosity that preferentially developed 

along specific sedimentary structures, strata, or mineralogy. Not-fabric-selective porosity is secondary 

porosity that developed generally without influence of 

sedimentary structures as well as secondary porosity 

that developed preferentially along fractures or faults 

not associated with the original sedimentary or diagenetic processes. Effective, or drainable, porosity consists of pores that are well-connected by sufficiently 

large openings, generally greater than 0.1 micrometer 

in diameter. In the Edwards aquifer, effective porosity 

is more closely associated with large permeability than 

with total porosity, which includes unconnected or 

dead-end pores (Maclay and Small, 1976).

Permeability is the capacity of a porous rock to 

transmit water. According to Ford and Williams (1989, 

p. 130), permeability depends on the physical properties of the rock, particularly pore size, shape, and distribution. Ford and Williams (1989, p. 150) further state 

that, "As a consequence of the effects of fissuring and 

differential solution, permeability may be greater in 

some directions than in others as well as in certain preferred stratigraphic horizons."

Downdip from the recharge zone, the Edwards 

aquifer is confined above and below (table 1). The 

lower and upper confining units, the eight hydrogeologic subdivisions and the names of the corresponding 

members, and the type of porosity and permeability 

observed in the field within the subdivisions are discussed in ascending order.

The upper member of the Glen Rose Limestone 

is relatively impermeable and generally acts as a lower 

confining unit of the Edwards aquifer. However, in 

some areas, the upper member of the Glen Rose Limestone yields substantial quantities of water to wells.

6 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Recharge Zone, Bexar County, Texas
Hydrogeologic subdivision VIII (basal nodular 

member) has negligible porosity and permeability 

in the subsurface, and can be regarded as part of the 

lower confining unit (Maclay and Small, 1984). In the 

outcrop, this subdivision is extensively karstified and 

has secondary porosity in the form of large lateral 

caves. The lateral cave development might result from 

dissolution associated with perching of infiltrating 

meteoric water on the underlying relatively impermeable upper member of the Glen Rose Limestone (Kastning, 1986). The perching would allow time for 

dissolution to occur within this subdivision. Many 

seeps and springs discharge from the lower part of this 

hydrogeologic subdivision in Bexar County.

Hydrogeologic subdivision VII (dolomitic member) generally is porous and relatively permeable. 

Some of the evaporite beds within this subdivision are 

burrowed and dissolved to the extent of being honeycombed and, therefore, permeable. Many of the beds 

contain isolated molds, casts, and burrows with large 

secondary (fabric selective) porosity but little permeability because the openings rarely are connected. 

Therefore, the permeable layers are restricted to 

solution-enlarged bedding planes.

Hydrogeologic subdivision VI (Kirschberg 

evaporite member) appears to be the most porous and 

permeable subdivision of the Kainer Formation. The 

porosity, chiefly fabric selective, has been described as 

boxwork (Maclay and Small, 1976) because of the configuration of voids and the secondary neospar and travertine deposits. The boxwork porosity does not prevail 

throughout hydrogeologic subdivision VI, but is interbedded with massive limestone beds.

Hydrogeologic subdivision V (grainstone member) is widely recrystallized. The recrystallization 

greatly reduces the effective porosity and permeability 

of this subdivision; however, there is local interparticle 

and intraparticle porosity and local fracture porosity 

and permeability.

Hydrogeologic subdivision IV (regional dense 

member) is probably an effective vertical confining 

unit between the underlying Kainer Formation and 

the overlying members of the Person Formation. However, caves, faults, and fractures, primarily not-fabricselective porosity, and fracture-associated permeability, might greatly reduce the confining effects of this 

subdivision in some areas. Many of the caves in this 

subdivision are vertical shafts with major horizontal 

cavern development either above or below the subdivision.

Hydrogeologic subdivision III (leached and collapsed members, undivided) is the most porous and 

permeable subdivision within the Person Formation. 

The predominant type of porosity in this subdivision is 

not fabric selective where evaporite minerals have been 

dissolved. However, breccia porosity resulting from 

evaporite dissolution can be either fabric selective or 

not fabric selective (Choquette and Pray, 1970). Cave 

porosity and permeability associated with faulting and 

(or) evaporite dissolution also is common.

Hydrogeologic subdivision n (cyclic and marine 

members, undivided) is moderately permeable, with 

both fabric- and not-fabric-selective porosity. Field 

observations indicate that this subdivision has only 

slightly less porosity and permeability than subdivision 

III.

Hydrogeologic subdivision I (Georgetown Formation) yields little water to wells. This subdivision 

has negligible porosity and permeability. Where 

severely weathered in the outcrop (and, thus, relatively 

more permeable), this subdivision is considered a part 

of the unsaturated zone of the Edwards aquifer.

The upper confining units consist of the Del Rio 

Clay, Buda Limestone, and the Eagle Ford Group 

(Rose, 1972). Because the Del Rio Clay consists 

mostly of clay and has negligible effective porosity and 

permeability, it forms the primary upper confining unit 

of the Edwards aquifer.

SUMMARY

The Edwards aquifer is one of the most productive carbonate aquifers in the nation. It is the sole 

source of public-water supply for San Antonio and for 

most of Bexar County. In addition, the Edwards aquifer 

provides large quantities of water to agriculture, industry, and major springs. The major springs support recreational activities and businesses, provide water to 

downstream users, and provide habitat for several 

threatened or endangered species.

The dissolution-modified, faulted limestone 

aquifer is recharged in its outcrop area in the Balcones 

fault zone. In Bexar County, residential and commercial development on the recharge zone of the Edwards 

aquifer is increasing. The aquifer possibly can be contaminated by spills, leakage of hazardous materials, or 

runoff from the rapidly developing urban areas that surround, or are built on, the intensely faulted and fractured, karstic limestone outcrop of the recharge zone. 

Furthermore, some of the hydrogeologic subdivisions

SUMMARY
that compose the Edwards aquifer have greater effective porosity than others, and might provide more efficient avenues for contaminants to enter the aquifer in 

areas where they crop out.

The Edwards aquifer comprises the Kainer and 

Person Formations of the Edwards Group and the overlying Georgetown Formation. The Kainer and Person 

Formations consist of seven informal members. These 

seven members, together with the overlying Georgetown Formation, compose the eight informal hydrogeologic subdivisions of the aquifer.

The Edwards aquifer has relatively large permeability resulting, in part, from the development or redistribution of secondary porosity. Lithology, stratigraphy, 

diagenesis, and karstification account for the effective 

porosity and permeability in the Edwards aquifer outcrop. Karst features that greatly enhance effective 

porosity and permeability include sinkholes and caves.

Hydrogeologic subdivision VI (Kirschberg 

evaporite member) appears to be the most porous and 

permeable subdivision in the Kainer Formation. 

Hydrogeologic subdivision III (leached and collapsed 

members, undivided) is the most permeable subdivision in the Person Formation. Hydrogeologic subdivision II (cyclic and marine members, undivided) is 

moderately porous and permeable.

REFERENCES CITED

Arnow, Ted, 1959, Ground-water geology of Bexar County, 

Texas: Texas Board of Water Engineers Bulletin 5911, 

62 p.

Bader, R.W., Walthour, S.D., and Waugh, J.R., 1993,

Edwards aquifer hydrogeologic status report for 1992: 

Edwards Underground Water District Report 93-05, 

71 p.

Buszka, P.M., 1987, Relation of water chemistry of the 

Edwards aquifer to hydrogeology and land use, San 

Antonio region, Texas: U.S. Geological Survey WaterResources Investigations Report 87-4116,100 p.

Buszka, P.M., Zaugg, S.D., and Werner, M.G., 1990, Determination of trace concentrations of volatile organic 

compounds in ground water using closed-loop stripping, Edwards aquifer, Texas: Bulletin of Environmental Contamination and Toxicology, p. 507-515.

Choquette, P.W., and Pray, L.C., 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: American Association of Petroleum 

Geologists Bulletin, v. 54, no. 2, p. 207-250.

Dunham, R.J., 1962, Classification of carbonate rocks 

according to depositional texture, in Classification of

Carbonate Rocks Symposium: American Association 

of Petroleum Geologists Memoir 1, p. 108-121.

Ford, D.C., and Williams, P.W., 1989, Karst geomorphology 

and hydrology: London, Chapman and Hall, 601 p.

Kastning, E.H., 1986, Cavern development in the New 

Braunfels area, central Texas, in Abbott, P.L., and 

Woodruff, C.M., Jr., eds., The Balcones escarpment 

geology, hydrology, ecology and social development in 

central Texas: Geological Society of America, p. 91- 

100.

Kipp, O.K., Farrington, P.T., Albach, M.J., 1993, Urban 

development on the Edwards aquifer recharge zone: 

Edwards Underground Water District Report 93-09, 

40 p.

Maclay, R.W., and Small, T.A., 1976, Progress report on 

geology of the Edwards aquifer, San Antonio area, 

Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate rocks: U.S. 

Geological Survey Open-File Report 76-627, 65 p.

____1984, Carbonate geology and hydrology of the 

Edwards aquifer in the San Antonio area, Texas: U.S. 

Geological Survey Open-File Report 83-537,72 p.

Rose, PR., 1972, Edwards Group, surface and subsurface, 

central Texas: Austin, University of Texas, Bureau of 

Economic Geology Report of Investigations 74,198 p.

Sellards, E.H., Adkins, W.S., and Plummer, F.B., 1933, The 

geology of Texas, v. 1, Stratigraphy: Austin, University 

of Texas, Bureau of Economic Geology Bulletin 3232, 

1,007 p.

Small, T.A., 1985, Identification and tabulation of geological 

contacts in the Edwards aquifer, San Antonio area, 

Texas: Texas Department of Water Resources LP-199, 

54 p.

Suppe, J., 1985, Principles of structural geology: 

Englewood Cliffs, N. J., Prentice-Hall, 537 p.

University of Texas, Bureau of Economic Geology, 1974, 

Geologic atlas of Texas, San Antonio sheet: Scale 

1:250,000.

Thornthwaite, C.W., 1952, Evapotranspiration in the hydrologic cycle, in The physical and economic foundation 

of natural resources, v. n, The physical basis of water 

supply and its principal uses: U.S. Congress, House of 

Representatives, Committee on Interior and Insular 

Affairs, p. 25-35.

Veni, George, 1988, The caves of Bexar County (2d ed.): 

Austin, University of Texas, Texas Memorial Museum, 

Speleological Monographs 2, 300 p.

Young, Keith, 1986, The Pleistocene terra rossa of central 

Texas, in Abbott, PL., and Woodruff, C.M., Jr., eds., 

The Balcones escarpment geology, hydrology, ecology and social development in central Texas: Geological Society of America, p. 63-70.

8 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Recharge Zone, Bexar County, Texas
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

PREPARED IN COOPERATION WITH THE

SAN ANTONIO WATER SYSTEM

WATER-RESOURCES INVESTIGATIONS REPORT 95-4030

Hydrogeologic subdivisions of the Edwards aquifer recharge zone PLATE 1

Stein, W.G., and Ozuna, G.B., 1995, Geologic framework and hydrogeologic 

characteristics of the Edwards aquifer recharge zone, Bexar County, Texas

98°22'30"

26°35'

29°32'30" -

; San 

\ Antonio

LOCATION MAP

98°47'30" 

29°37'30"

EXPLANATION

EAGLE FORD GROUP, BUDA LIMESTONE, AND 

DEL RIO CLAY (UPPER CONFINING UNITS)

I GEORGETOWN FORMATION

II CYCLIC AND MARINE MEMBERS, UNDIVIDED

SCALE 1:75 000

LEACHED AND COLLAPSED MEMBERS, UNDIVIDED

IV REGIONAL DENSE MEMBER

 2S°42'30"

98°47'30"

V GRAINSTONE MEMBER

VI KIRSCHBERG EVAPORITE MEMBER

VII DOLOMITIC MEMBER

BASAL NODULAR MEMBER

UPPER MEMBER OF THE GLEN ROSE 

LIMESTONE (LOWER CONFINING UNIT)

LOWER CONFINING UNIT - unmapped area 

(includes Glen Rose Limestone and sections 

of Kainer Formation on tops of hills)

HYDROGEOLOGIC SUBDIVISIONS I - VIIIModified from Maclay and Small (1976)

PRIMARY FAULT--U, 

upthrown; D, down thrown

V CAVE

INFERRED FAULT

(».. SPRING

ESTIMATED VERTICAL DISPLACEMENT 

AT FAULT, IN FEET

Base modified irom u.b. Geological

Survey 1:24,000 quadrangles

Universal Transverse Mercator Projection, zone 14

MAP SHOWING HYDROGEOLOGIC SUBDIVISIONS OF THE EDWARDS AQUIFER RECHARGE ZONE, BEXAR COUNTY, TEXAS

By 

William G. Stein and George B. Ozuna

1995