science for 

USGS

a changing world

In cooperation with the San Antonio Water System

Vulnerability of Ground Water 

to Contamination, Northern 

Bexar County, Texas

Water-Resources Investigations Report 03-4072

U.S. Department of the Interior 

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

U.S. Geological Survey

Vulnerability of Ground Water 

to Contamination, Northern 

Bexar County, Texas

By Amy R. Clark

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03-4072

In cooperation with the San Antonio Water System

Austin, Texas 

2003
U.S. DEPARTMENT OF THE INTERIOR

Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, 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 

8027 Exchange Dr. 

Austin, TX 78754-4733 

E-mail: dc tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225-0286

E-mail: infoservices@usgs.gov
CONTENTS

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

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

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

Previous Work .......................................................................................................................................................... 3

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

Hydrogeologic Setting ......................................................................................................................................................... 6

Method of Assessing Ground-Water Vulnerability .............................................................................................................. 8

Hydrogeologic Units ................................................................................................................................................ 8

Faults ...................................................................... 10

Caves and (or) Sinkholes .......................................................................................................................................... 11

Slopes ...............................................................................................................................................................^ 11

Soils ...................................................................... 12

Ground-Water Vulnerability Map ........................................................................................................................................ 12

Considerations and Limitations ........................................................................................................................................... 13

Summary .......................................................................................................................................................................^ 15

References ................................................................................. 16

PLATE

[Plate is in pocket]

1. Map showing vulnerability of ground water to contamination, northern Bexar County, Texas

FIGURES

1. Map showing locations of study area andBalcones fault zone ............................................................................ 2

2. Generalized section summarizing the hydrogeologic framework of south-central Texas .................................... 4

3. Diagram showing an example of how vulnerability ratings for the hy drogeologic units and other natural

features relate to composite ratings ...................................................................................................................... 9

4. Diagram showing cumulative relative frequency of composite vulnerability ratings computed for

selected categories of natural features and for the entire study area .................................................................... 14

TABLES

1. Summary of the geologic characteristics and hydrologic properties of the hydrogeologic units that crop

out in the study area of northern Bexar County, Texas ......................................................................................... 5

2. Composite vulnerability ratings for the hydrogeologic units and other natural features in the study

area of northern Bexar County, Texas .................................................................................................................. 10

CONTENTS
Vulnerability of Ground Water 

to Contamination, Northern 

Bexar County, Texas

By Amy R. Clark

Abstract

The Trinity aquifer, composed of Lower 

Cretaceous carbonate rocks, largely controls the 

ground-water hydrology in the study area of 

northern Bexar County, Texas. Discharge from 

the Trinity aquifer recharges the downgradient, 

hydraulically connected Edwards aquifer one of 

the most permeable and productive aquifers in the 

Nation and the sole source of water for more than a 

million people in south-central Texas. The unconfined, karstic outcrop of the Edwards aquifer makes 

it particularly vulnerable to contamination resulting from urbanization that is spreading rapidly 

northward across an "environmentally sensitive" 

recharge zone of the Edwards aquifer and its upgradient "catchment area," composed mostly of the 

less permeable Trinity aquifer.

A better understanding of the Trinity aquifer 

is needed to evaluate water-management decisions 

affecting the quality of water in both the Trinity and 

Edwards aquifers. A study was made, therefore, in 

cooperation with the San Antonio Water System to 

assess northern Bexar County's vulnerability to 

ground-water contamination. The vulnerability of 

ground water to contamination in this area varies 

with the effects of five categories of natural features (hydrogeologic units, faults, caves and (or) 

sinkholes, slopes, and soils) that occur on the outcrop and in the shallow subcrop of the Glen Rose 

Limestone.

Where faults affect the rates of recharge or 

discharge or the patterns of ground-water flow in 

the Glen Rose Limestone, they likewise affect the 

risk of water-quality degradation. Caves and sinkholes generally increase the vulnerability of ground

water to contamination, especially where their 

occurrences are concentrated. The slope of land 

surface can affect the vulnerability of ground water 

by controlling where and how long a potential contaminant remains on the surface. Disregarding the 

exception of steep slopes which are assumed to 

have no soil cover the greater the slope, the less 

the risk of ground-water contamination. Because 

most soils in the study area are uniformly thin, they 

have only minimal effect on the vulnerability of 

ground water to contamination.

The results of hydrogeologic mapping 

during the present study divide the outcrop of 

the Glen Rose Limestone into five mappable 

intervals, labeled (youngest to oldest) A through E. 

Of these intervals, only the middle (C) and the 

lowermost (E) generally provide appreciable 

permeability.

The vulnerability assessment provided 

herein was determined by combining the presumed 

effects of selected natural features (with individual 

vulnerability ratings ranging from 0 through 35) 

using a grid-based, multilayer system of digital 

datasets and geographic information system analysis. The resulting vulnerability map comprises 

composite vulnerability ratings that range from 26 

through 104. The relatively less vulnerable areas 

those containing no faults, sinkholes, or caves 

occupy about 92 percent of the study area. The 

most vulnerable areas are those containing both a 

fault and one or more caves. The distribution of the 

most vulnerable areas which trend from southwest to northeast, roughly parallel to the Balcones 

fault zone occur mainly where faults intersect 

caves.

Abstract
Study area 29°45

98°45'

29°30'

0 50 100 150 200 MILES 

i____i____i____i____i

29°15'

10 15 20 MILES

Figure 1 . Locations of study area and Balcones fault zone.

No part of the vulnerability assessment 

offered herein should preclude or minimize the 

importance of detailed, on-site inspections by qualified hydrogeologists. The evolving understanding 

of the vulnerability of ground water to contamination in northern Bexar County will improve as 

additional data are collected and analyzed.

INTRODUCTION

The ground-water hydrology of the study area in 

northern Bexar County, Texas (fig. 1), is controlled 

largely by the Trinity aquifer, a subsurface reservoir 

composed of relatively flat-lying carbonate strata of 

Early Cretaceous age. Discharge from the Trinity aquifer recharges the downgradient Edwards aquifer a

Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
steeply dipping, extensively faulted, and locally karstic 

sequence of mostly dolostone and limestone. One of the 

most permeable and productive aquifers in the Nation 

(providing large quantities of water to agricultural, 

industrial, and recreational interests in south-central 

Texas), the Edwards aquifer is the sole-source water 

supply for more than a million people in the greater 

San Antonio area of central Bexar County. The U.S. 

Environmental Protection Agency designation of the 

Edwards aquifer as a sole-source aquifer (van der 

Leeden and others, 1990) officially confirms that contamination of this aquifer would threaten the health and 

well being of residents, business and industry, as well as 

the environment of Bexar County.

The population of Bexar County has increased an 

average of nearly 20 percent per decade since 1960, 

amounting to more than a 100-percent increase through 

2000 (U.S. Census Bureau, 2002). The majority of 

Bexar County's nearly 1.5 million population resides in 

San Antonio, one of the 10 largest cities in the United 

States. Most of the population growth and associated 

urban sprawl (urbanization) is occurring northward 

across an "environmentally sensitive" outcrop, or 

recharge zone, of the Edwards aquifer and into the 

"catchment area" of runoff from the comparatively less 

permeable Trinity aquifer (fig. 2). The karstic nature of 

the Edwards aquifer makes it particularly vulnerable to 

contamination associated with the rapidly increasing 

urbanization atop not only its recharge zone but also its 

upgradient catchment area.

The vulnerability of ground water to contamination in the study area depends on the distribution and 

characteristics of hydrogeologic units; the presence or 

absence of faults, caves, and sinkholes; and on the 

specific slopes and soils on the surface and (or) in the 

shallow subcrop of the Trinity aquifer. Because the individual effects of these conditions vary spatially, the 

composite (net) vulnerability varies accordingly.

The outcrop of the Trinity aquifer in northern 

Bexar County consists of the upper member and the 

uppermost part of the lower member of the Glen Rose 

Limestone (table 1). Because most water that recharges 

the Edwards aquifer must first discharge from the 

upgradient Trinity aquifer, the contamination of ground 

water in the Glen Rose Limestone affects the potential 

for contamination of the Edwards aquifer.

Relatively impervious streambeds of Glen Rose 

Limestone that drain toward the pervious recharge zone 

of the highly permeable Edwards aquifer make the 

Edwards aquifer highly susceptible to contamination

originating atop the Trinity aquifer. That susceptibility 

and a growing dependence on the Edwards aquifer make 

the quality of water that recharges this aquifer a critical 

concern for public officials who are expected to make 

informed, scientifically defensible water-management 

decisions regarding the future of this water resource 

(Focazio and others, 2002). A better understanding of 

the vulnerability of the Trinity aquifer to contamination 

in northern Bexar County is prerequisite to adequate 

evaluation of alternative water-management decisions 

affecting the quality of water in the downgradient 

Edwards aquifer.

Purpose and Scope

The present study was undertaken by the U.S. 

Geological Survey in cooperation with the San Antonio 

Water System to quantify and describe the vulnerability 

of ground water to contamination in the study area 

(180 square miles) of northern Bexar County. "Vulnerability to contamination," as applied herein depends on 

the distribution and characteristics of the hydrogeologic 

units and other (selected) natural features of the local 

environment all of which are intrinsic, or independent 

of human activity (National Research Council, 1993, 

p. 17). These characteristics include the: (1) relative 

permeability of the outcropping hydrogeologic units 

(composed predominantly of Glen Rose Limestone); 

(2) presence or absence of faults, caves, and sinkholes; 

and (3) influence of land-surface slope and soil type. 

In addition to summarizing the distribution and characteristics of the hydrogeologic units and other natural 

features in the study area, this report presents a map and 

an explanation of the area's vulnerability to groundwater contamination.

Previous Work

Much of the background information and methodology used for the present study results from work by 

Stein and Ozuna (1995) and A.K. Clark (2000, 2003). 

Interested readers are referred to the first two of these 

reports for explanations of the geologic framework, 

hydrogeologic characteristics, and ground-water 

vulnerability of the Edwards aquifer recharge zone in 

northern Bexar County.

Clark's (2000) use of five categories of natural 

features to assess the vulnerability of the Edwards aquifer on the basis of rating systems developed by Aller 

and others (1987) and the Texas Natural Resource Conservation Commission (1995) was adapted for the

INTRODUCTION
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EDWARDS PLATEAU

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HILL COUNTRY

Kendall County

Catchment area - GULF COASTAL PLAIN Bexar County

EXPLANATION

Edwards aquifer 

recharge zone

Navarro-Del Rio confining unit

Edwards aquifer

Unconfined

Confined

Trinity aquifer 

Edwards-Trinity aquifer 

Paleozoic basement rocks

Fault Arrows indicate direction 

of movement

Figure 2. Generalized section summarizing the hydrogeologic framework of south-central Texas (adapted from Barker and Ardis, 1996).
Table 1 . Summary of the geologic characteristics and hydrologic properties of the hydrogeologic units that crop out 

in the study area of northern Bexar County, Texas

[Groups, formations, and members modified from Rose (1972), Ashworth (1983), Barker and Ardis (1996); lithology modified from 

Dunham (1962); field identification and cave development adapted from A.K. Clark (2003); and porosity type modified from Choquette and 

Pray (1970). AQ, aquifer; CU, confining unit]

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Member (interval)

Dolomitic member

Basal nodular

member

Upper

Glen 

Rose

member

Interval A

Interval B

Interval C

Interval D

Interval E

Lower

Glen

Rose

member

Hydrologic 

function

AQ

AQ karst;

CUnot

karst

AQ associated with

fractures

and karst;

CUifnot

CU; AQ

associated

with fractures and

karst

AQ

AQ associated with

bioherms;

CU above

and below

bioherms

AQ

AQ mainly

in bioherms and

evaporite

bed, also

associated

with karst

and fractures; CU

elsewhere

Thickness 

(feet)

110-130

50-60

120+

120-150

18-26

135-180

6-16

320-350

Lithology

Mudstone to grainstone; crystalline limestone; chert

Shaly, nodular limestone; mudstone and

miliolid grainstone

Alternating and interfingering medium

bedded mudstone,

wackestone, and

packstone

Alternating and interfingering mudstone,

clay, wackestone,

packstone, and occasional grainstone

Highly altered crystalline limestone; chalky

mudstone

Thin bedded mudstone; thin to medium

bedded wackestone,

packstone, and local

thick bedded rudist

biostromes

Highly altered crystalline limestone; chalky

mudstone

Thick bedded mudstone; thin to medium

bedded mudstone,

wackestone, packstone, and clay.

Evaporite bed at top

composed of clay and

grainstone (corbula

bed)

Field 

identification

Massively bedded,

light gray; abundant

Toucasia

Massive, nodular and

mottled; Exogyra

texana

Near contact with

Edwards Group; stairstep topography; local

evaporite beds;

devoid of fossils

Relatively devoid

of fossils; those

few found include

Protocardia texana

and Tylostoma; stairstep topography

Boxwork voids, collapse breccias; forms

broad, gentle slopes

Profuse Orbitulina

texana; generally low

relief; stair-step

topography not as

well defined

Boxwork voids, collapse breccias; forms

broad, gentle slopes

Located below corbula bed; massive

mudstone; massive

bioherms near top

Cave 

development

Caves related to fault

structures or bedding

planes

Large lateral caves at

surface; a few caves

near Cibolo Creek

Some caves below contact with Edwards

Group; caves associated mostly with fractures and bedding

planes

Few caves

No known caves

Caves related to fractures and bedding

planes in massive

limestone unit

No known caves

Caves in bioherms;

very large sinkholes in

bioherms; the few

caves below bioherm

primarily are associated with solutionally

enlarged fractures

Types of porosity 

and permeability

Some bedding plane porosity and associated permeable zones

Stratigraphically controlled

porosity; large conduit

permeability near surface,

with little to no permeability

in subsurface

Near-surface porosity; high

probablity of permeability

near contact with Edwards

Group decreases with depth

Interparticle and fracture

porosity with some cavernous porosity near middle

of interval; little permeability away from caves and

fractures

Primarily vug porosity;

permeability associated

with solutionally enlarged

fractures

High probability of fracture,

cavern, and moldic porosity

in biostrome; little porosity

near base of member

Primarily vug porosity,

providing appreciable permeability in places

Fracture, cavern, and moldic

porosity in bioherms; mudstone and marl contain primarily fracture porosity,

with little permeability;

high probability of permeability in bioherms

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present study. The northern limit of the vulnerability 

map resulting from Clark's (2000, pi. 1) earlier work 

(pertaining to the Edwards aquifer) abuts the southern 

limit of the vulnerability map presented herein (Trinity 

aquifer).

Clark (2003) describes the results of a study at 

the Camp Bullis Training Site (U.S. Army) in northern 

Bexar County with the purpose of better understanding 

the conditions of recharge to the Glen Rose Limestone

INTRODUCTION
at that site and the consequential potential for contamination of the downgradient Edwards aquifer.

Acknowledgments

The author wishes to thank Brian Murphy and 

Chris Beal (Camp Stanley, U.S. Army), Dusty Bruns 

and Jerry Thomson (Camp Bullis, U.S. Army), and 

Deirdre Hisler (Government Canyon State Natural 

Area) for their valuable assistance during this study. 

Thanks are extended to Dr. George Veni (George Veni 

& Associates) and the Texas Speleological Survey for 

their assistance with the provisional release of cave 

locations and for aiding in the assessment of cave zones. 

Thanks also are owed to David Amsbury and William 

Ward (University of Texas-San Antonio), Alvin Shultz 

(independent consultant), and William Feathergail 

Wilson (Strata Geological Services, Inc.).

The author appreciates special contributions of 

U.S. Geological Survey colleagues, notably those of 

Diana Pedraza (for her considerable help with report 

preparation) and Allan Clark and Ted Small (retired) for 

their guidance and generous sharing of geologic expertise. The author also wishes to acknowledge the considerable benefit afforded by all the property owners who 

granted access to their property.

HYDROGEOLOGIC SETTING

The hydrogeologic framework of south-central 

Texas comprises (from northwest to southeast) the 

Edwards Plateau, Hill Country, and Gulf Coastal Plain 

(fig. 2). The Edwards Plateau is an upland region of 

thin, rocky soils that veneer an expansive mass of 

layered, relatively flat-lying sedimentary rocks. The 

Hill Country is a comparatively rugged, scenic region 

punctuated by a dendritic drainage system of deeply 

entrenched, steep-sided canyons that contain spring-fed 

streams, many of which originate near the southeastern 

margin of the Edwards Plateau. The Gulf Coastal Plain 

consists of a rolling to virtually flat prairie land atop a 

gulfward-thickening, sharply dipping wedge of mostly 

fine-grained sediments that confine the underlying 

Edwards aquifer.

The Balcones fault zone is a southwest-tonortheast trending system of mostly high-angle, downto-the-gulf, en echelon faults along the northwestern 

margin of the Gulf Coastal Plain. These faults profoundly affect flow patterns in the Edwards aquifer, one 

of the most productive subsurface reservoirs of potable 

water in the world. The Balcones escarpment, which

separates the Hill Country from the Gulf Coastal Plain, 

is characterized by a prominent break-in-relief along the 

updip edge of the Balcones fault zone.

Removed by erosion from most of the study area 

as well as from the rest of the Hill Country, the Edwards 

Group comprises the Edwards aquifer in the Balcones 

fault zone. In the study area as well as throughout the 

rest of the Hill Country, erosional remnants of the 

Edwards Group cap many of the highest hills that are 

otherwise composed of Glen Rose Limestone of the 

Trinity Group. The Kainer Formation of the Edwards 

Group, which in the study area consists only of its basal 

nodular and dolomitic members (table 1), is formed 

mostly of nodular limestone, dolomitic limestone, and 

mudstone. As the result of downfaulting in the Balcones 

fault zone, the younger Kainer Formation is juxtaposed 

alongside the older Glen Rose Limestone near the 

southern boundary of the study area (fig. 1).

The Glen Rose Limestone, which crops out 

over about 90 percent of the study area, is composed 

of (informal) lower and upper members. Whereas the 

upper member is characteristically thin-bedded and 

composed mostly of alternating beds of limestone 

and marl, the lower member of the Glen Rose Limestone typically is composed of relatively massive, 

fossiliferous limestone. The "stair-step," terraced-hill 

topography that dominates the Hill Country results 

largely from contrasting rates of erosion between the 

beds of relatively resistant limestone and comparatively 

soft marl in the upper Glen Rose. The Glen Rose Limestone is uncharacteristically permeable in its outcrop 

and shallow subsurface of northern Bexar County (Veni, 

1994), where caves and (or) sinkholes (surface expressions of structural collapse above a solutional cavity) 

have developed in elevated areas north of the Balcones 

escarpment.

Composed of faulted, fractured, and locally 

karstic carbonate rocks, the highly permeable Edwards 

aquifer is recharged primarily by runoff from drainage 

basins extending across the Hill Country from the 

Edwards Plateau. Known locally as the "catchment 

area" for the Edwards aquifer, these basins receive 

precipitation that averages about 29 inches per year 

over the Hill Country (Owenby and Ezell, 1992; 

Lanning-Rush, 2000). Much of the infiltrated precipitation discharges from upper parts of the Trinity aquifer 

as springflow or base flow into streams that convey 

this water to the outcrop (recharge zone) of the downgradient Edwards aquifer. Streams draining the catchment area flow generally southward atop the relatively

Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
impervious Glen Rose Limestone and lose all their 

base flow and most of their flood flow when and where 

they reach the pervious outcrop of the Edwards aquifer. 

Water entering the Edwards aquifer in Bexar County 

flows mostly southward to points of discharge (mainly 

municipal wells) and then northeastward parallel 

or nearly parallel to faults composing the northeasttrending Balcones fault zone into Comal and Hays 

Counties, where most of the remaining water discharges 

through Comal and San Marcos Springs.

The Trinity aquifer is composed of three wateryielding zones, the upper two of which (table 1) crop out 

in the study area. The upper zone is composed of the 

upper member of the Glen Rose Limestone and the 

uppermost part of the middle zone is composed of the 

lower member of the Glen Rose Limestone. The downward movement of water typically is impeded by highly 

indurated, predominantly mudstone interbeds that are 

interspersed throughout the Glen Rose Limestone but 

are most common in the upper member of this formation (Barker and Ardis, 1996). Ground water flows laterally atop these hydraulically tight mudstones more 

readily than it flows vertically. Consequently, much of 

the infiltrated precipitation emerges from seeps and 

springs along the tops of these interbeds where they are 

breached by the rugged topography of the Hill Country. 

Notable differences in hydraulic head result from these 

vertically staggered interbeds of relatively impermeable 

strata (Ashworth, 1983).

The upper member of the Glen Rose Limestone is 

divided for this report into five mappable intervals, A 

through E (youngest to oldest), as first applied by A.K. 

Clark (2003) to the Camp Bullis area of northern Bexar 

County (pi. 1). Characteristics of these intervals are 

summarized below:

Interval A (greater than 120 feet thick) has an 

abundance of caves indicative of its 

generally well developed fracture, channel, 

and cavern porosity that in places provides 

appreciable permeability.

Interval B (120 to 150 feet thick) is similar to 

Interval A but with less cave development 

and permeability.

Interval C (18 to 26 feet thick), a partly to mostly 

dissolved zone of soluble carbonate rock, is 

characterized by breccia porosity, with 

collapse structures and boxwork permeability 

that typically intercept the downward 

percolation of ground water and divert it 

laterally.

Interval D (135 to 180 feet thick) generally has 

little porosity and permeability, but some 

local exceptions most notably, the 

biostrome near the top of the interval tend 

to have greater porosity and permeability.

Interval E (6 to 16 feet thick) is a partly to mostly 

dissolved sequence of evaporites similar to 

that in Interval C, with similar hydrogeologic 

properties and tendency to divert ground 

water laterally.

Faults in the Trinity aquifer of the Hill Country do 

not affect ground-water flow as much as they do in the 

Edwards aquifer of the Balcones fault zone (Maclay and 

Small, 1986; Barker and Ardis, 1996). Faults are much 

less common and have considerably less displacement 

north of the Balcones escarpment (fig. 2). Where faults 

occur in the study area, however, they can affect the 

rates of recharge and discharge as well as the patterns of 

ground-water flow and, therefore, the vulnerability of 

the aquifer to contamination. Because barrier faults 

impede ground-water flow, they typically increase 

retention times and, therefore, the risk of ground-water 

contamination. Where faults have produced fractures 

that increase the permeability of the aquifer, they might 

increase the vulnerability of the aquifer to water-quality 

degradation.

Where caves or sinkholes affect local drainage 

patterns, they generally increase the vulnerability of the 

aquifer to ground-water contamination. Most of the 

increased vulnerability results from increased infiltration rates owing not only to the obvious enhancement 

they provide to the local permeability but also to the 

internal drainage associated with the closed depressions 

that typically result from the formation of these features. In some places, these features provide passageways for water to directly enter the Trinity aquifer.

Although the Trinity aquifer discharges to most 

Hill Country streams, parts of this aquifer are recharged 

by some reaches of some streams (Reeves and Lee, 

1962). Cibolo Creek, which forms the northern boundary of the study area, crosses into the recharge zone of 

the Edwards aquifer and traverses the Balcones fault

HYDROGEOLOGIC SETTING
zone east of San Antonio. The upper part of the Cibolo 

Creek watershed includes a karstic reach characterized 

by caves and sinkholes. Although most reaches of 

Cibolo Creek are dry much of the time, they generally 

contain considerable runoff after heavy or prolonged 

rainstorms. Because much runoff in Cibolo Creek is 

diverted underground through sinkholes (Ashworth, 

1983, p. 47), these sinkholes might contribute recharge 

to the Edwards aquifer.

METHOD OF ASSESSING GROUNDWATER VULNERABILITY

The present assessment of northern Bexar 

County's vulnerability to ground-water contamination 

was accomplished in four basic steps. First, information 

gleaned from the hydrogeologic literature and well 

records was combined with the recent results of field 

mapping (A.R. Clark, 2003) to compile a comprehensive hydrogeologic map of the study area. Second, existing methods of assessing ground-water vulnerability 

were reviewed and modified as necessary to satisfy the 

particular objectives of the present study. Third, the 

hydrogeologic units and other natural features believed 

to most affect northern Bexar County's vulnerability to 

ground-water contamination were identified and 

assigned vulnerability ratings on a feature-by-feature 

basis. These vulnerability ratings (associated with each 

feature) were then integrated into composite ratings 

and, finally, a vulnerability map of the entire study area. 

The existing methods, individual vulnerability ratings, 

and resulting vulnerability map of composite ratings are 

discussed in greater detail below.

Two existing methods of assessing conditions of 

ground-water contamination were considered before 

combining aspects of both to obtain the most appropriate approach for the present study. The first method 

considered originates from Title 30 of the Texas Administrative Code. Chapter 213 of this code mandates that 

geologic assessments be performed and development 

plans be approved before land plats are completed on 

the Edwards aquifer recharge zone (John Mauser, 

Texas Natural Resource Conservation Commission, 

written commun., 1995). To assist water planners, managers, regulators, and land developers in complying 

with the associated regulations, the Texas Natural 

Resource Conservation Commission (as of September 

2002, the Texas Commission on Environmental Quality) in 1995 drafted a set of instructions to geologists 

making the required assessments. The current version

of these instructions (Texas Natural Resource Conservation Commission, 2002) identifies specific hydrogeologic features (including stratigraphic, structural, and 

karstic characteristics) and anthropogenic conditions 

(for example, sewer lines and other man-made objects) 

in the study area to help evaluate the potential for 

contamination.

The second method considered by the present 

study is known as DRASTIC (Depth to water, [net] 

Recharge, Aquifer media, Soil media, Topography 

[slope], Impact of the vadose [unsaturated] zone media, 

and Conductivity [hydraulic] of the aquifer). DRASTIC 

was developed by the U.S. Environmental Protection 

Agency to evaluate, using existing information, the pollution potential of any hydrogeologic setting in the 

United States (Aller and others, 1987). According to 

Aller and others (1987, p. 1), "Pollution potential is a 

combination of hydrogeologic factors, anthropogenic 

influences and sources of contamination in any given 

area."

The procedures adapted by the present study to 

assess ground-water vulnerability are consistent with 

the decision by A.K. Clark (2000) to consider the effects 

of hydrogeologic units, faults, caves and (or) sinkholes, 

slopes, and soils. Data specific to each of these features 

were translated from the hydrogeologic literature or 

obtained through field observation. Using geographic 

informations system (GIS) technology, the relevant data 

were entered into vector-type digital datasets for display 

and analysis. These datasets were distinguished by 

point, line, or polygon attributes, depending on the spatial aspects of each feature (for example, faults were 

stored as line data; caves as point data; and soils as polygon data). After vulnerability ratings were related to 

each feature on the basis of discernible subcategories, 

the datasets were converted to a grid-based system of 

five data layers (fig. 3). GIS algorithms were then used 

to integrate the feature-based individual ratings into the 

composite ratings reflected in the vulnerability map 

(pi. 1) of the entire study area. Finally, using documentation software, metadata were developed to aid in the 

maintenance and future use of key data. The five categories of natural features that are represented in the 

present assessment of vulnerability are discussed under 

separate headings below.

Hydrogeologic Units

A hydrogeologic unit is defined as a mappable 

rock sequence with common geologic characteristics

8 Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
Composite vulnerability 

(plate 1)

Hydrogeologic 

unit layer 20 25 25 25

Fault layer

Cave or sinkhole 

layer

Slope layer

Soil layer

15 25 25 20

I 30 meters20 20 20 20

0

I 30 meters30 0

0 35 o /&

-30 meters- y

10 10

10

«$>

-30 meters10 10 10 15

10 / 10 15 10

I 30 meters |x

Figure 3. An example of how vulnerability ratings for the hydrogeologic units and other natural features relate to 

composite ratings (modified from A.K. Clark, 2000).

METHOD OF ASSESSING GROUND-WATER VULNERABILITY 9
and hydrologic properties and, as a consequence, common vulnerability to contamination by introduced pollutants (Aller and others, 1987). Because the effective 

porosity and permeability differs from one hydrogeologic unit to another, some provide more opportunity 

for pollutants to penetrate their upper surfaces and 

potentially contaminate underlying aquifers. Table 1 

summarizes the geologic characteristics and hydrologic 

properties of the hydrogeologic subdivisions of the 

Edwards and Trinity Groups in northern Bexar County.

The hydrogeologic units in the study area were 

mapped on and digitized from 7.5-minute topographic 

maps. Field identification of the Kainer Formation and 

Glen Rose Limestone subdivisions were based on the 

identification of index fossils and on observable differences in lithology. Thicknesses of the evaporite beds 

in the upper member of the Glen Rose Limestone were 

determined on the basis of average thicknesses as 

inferred from well logs.

The hydrogeologic units were assigned vulnerability ratings (table 2) on the basis of previous hydrogeologic investigations (A.K. Clark, 2000) and more 

recent field observations (A.K. Clark, 2003). The most 

porous and permeable units, such as the dolomitic member of the Kainer Formation and intervals C and E 

(evaporite beds) of the upper member of the Glen Rose 

Limestone were assigned ratings of 25. The least porous 

and permeable (clay-rich) units in the Glen Rose Limestone were assigned ratings of 15.

Isolated outliers of the Edwards Group for 

example, the hilltop remnants in the Hill Country 

(fig. 2) were rated and included in the vulnerability 

map owing to an apparent hydraulic connection to the 

underlying Trinity aquifer. Where such remnants cap 

hills or ridges that are otherwise underlain by the Trinity 

Group, the Trinity aquifer might be vulnerable to the 

downward migration of contaminated ground water.

Faults

Faults were mapped in the field from the observation of abrupt lithologic and (or) stratigraphic dissimilarities and at least one of the following: fault scarp, 

fault breccia, long linear travertine or sparry calcite 

deposit, or steeply dipping strata thought to represent a 

fault-bend (drape) fold. The extent of many faults, or 

their extension into mapped areas, was inferred from 

lineaments visible on the land surface or in aerial 

photographs. All recognized faults were plotted on

Table 2. Composite vulnerability ratings for the 

hydrogeologic units and other natural features in the 

study area of northern Bexar County, Texas

[Modified from A.K. Clark (2000)]

Natural 

feature

Hydrogeologic unit

Fault

Cave or

sinkhole

Slope (with 

soil)

Soil

Subdivision of hydrogeologic 

unit or other natural feature

Kainer Formation:

Dolomitic member

Basal nodular member

Glen Rose Limestone:

Upper member

Interval A

Interval B

Interval C

Interval D

Interval E

Lower member

Fault present

No fault present

One cave present

More than one cave present 

(cave zone)

Sinkhole and closed depressions present

No cave or sinkhole present

Greater than 18 percent

Greater than 12 to 18 percent

Greater than 6 to 12 percent

Greater than 2 to 6 percent

Less than or equal to 2 percent

No soil present

Moderately permeable

Slightly permeable

Rating

25

15

15

15

25

15

25

20

20

0

30

35

20

0

1

3

5

9

10

20

15

10

7.5-minute topographic maps, from which they were 

later digitized into a vector dataset.

Faults are not as numerous or as well developed 

in the Glen Rose Limestone as they are in the Edwards 

Group and are less apt to enhance infiltration from land 

surface owing to their occurrence in less permeable 

strata, generally smaller vertical displacement, and less 

tendency to be associated with shattered zones. The 

density of fractures tends to increase near faults, providing the potential for additional ground-water flowpaths.

Because the dominant network of southwestto-northeast-trending faults in the Balcones fault zone

10 Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
(fig. 1) tends to enhance northeastern flow and impede 

southeastern flow in the Edwards aquifer (A.K. Clark, 

2000), faults might potentially control the direction and 

velocity of contaminant transport in parts of the study 

area. In areas near Cibolo Creek, where the combination 

of faulting and karstification has caused unusually permeable zones within the Glen Rose Limestone to resemble subsurface conduits, downgradient areas of the 

Edwards aquifer might be inherently more susceptible 

to a direct connection with potentially contaminated 

runoff from the Glen Rose Limestone.

For the present study, faults were assumed to 

increase the vulnerability of ground water to contamination owing to their known capacity to enhance recharge 

opportunities. Accordingly, all mapped faults were 

assigned a vulnerability rating of 20. This rating is consistent with the updated instructions to geologists 

assessing ground-water vulnerability in the recharge 

and transition zones of the Edwards aquifer (Texas 

Natural Resource Conservation Commission, 2002). 

Areas of no recognized faults were assigned a zero 

rating.

Caves and (or) Sinkholes

Bates and Jackson (1984) define a cave as a natural cavity, recess, chamber, or series of chambers and 

galleries beneath the surface of the earth, large enough 

for a person to enter. To qualify as a cave for the purposes of this report, the chamber must be large enough 

to accept an average-sized adult. Presently, 62 caves are 

located within the study area of northern Bexar County.

Most caves (or caverns) are formed by the dissolution of limestone, caused typically by meteoric or 

undersaturated ground water. Caves are primary contributors to the threat of contamination not only because 

of the unusually large volumes of water they can admit, 

but also because they commonly are point sources of 

contamination with the capacity to convey pollutants 

directly to area aquifers.

Sinkholes are closed, roughly circular depressions with subterranean drainage systems in karstic 

areas. Sinkholes are formed by the massive dissolution 

of carbonate minerals at or near land surface and (or) by 

the collapse of underlying caves. Sinkholes are commonly funnel-shaped, large-scale features with size 

measured in tens or hundreds of feet. In some cases a 

swallow hole, or swallet, might be present at the bottom 

of the sinkhole where surface water can disappear into 

underground fractures or solution cavities.

Potential cave and sinkhole locations were compiled for the study area from reports by private citizens, 

non-government organizations, and government agencies. Most caves and sinkholes were field-checked and 

pinpointed on 7.5-minute topographic maps with the aid 

of a global positioning system (GPS) device. Other 

locations such as those identified by caving organizations were field-checked where possible. All confirmed caves and sinkholes were digitized into the GIS 

database as points.

These point features were then consolidated 

within buffered areas which, in turn, were used to establish what are herein called "cave zones." Cave zones 

were established from criteria selected in collaboration 

with Dr. George Veni, a local karst hydrogeologist and 

2002 Texas Speleological Survey (TSS) president. As 

delineated, the cave zones depict irregular, polygonal 

patterns with geologic boundaries and (or) lithologic 

horizons where more than one cave exists or is likely to 

exist. Information regarding the location of caves was 

provided provisionally by the TSS with the understanding that cave locations would not appear as points on the 

final vulnerability map. This was believed necessary to 

protect not only the exact location of caves but also the 

rights of property owners.

Areas with no caves or sinkholes were assigned 

vulnerability ratings of zero. Sinkholes (including 

closed depressions) were rated as 20. Whereas a single 

cave was rated as 30, a cave zone (more than one cave) 

was rated 35. In addition to being assigned on the basis 

of advice from local karst specialists, these ratings are 

consistent with the current instructions to geologists 

making geologic assessments of the Edwards aquifer 

recharge and transition zones (Texas Natural Resource 

Conservation Commission, 2002).

The presently recognized locations and effects of 

caves and sinkholes in the study area depend most 

importantly on the observations of cave explorers and 

their efforts to map specific caves. The knowledge of 

cave and sinkhole locations should improve as access to 

uncharted areas is obtained and more caves are discovered and mapped. The appropriateness of cave zone 

analysis, likewise, should improve as the understanding 

of this concept evolves.

Slopes

The land-surface altitudes for determining slope 

in the study area were adopted from U.S. Geological 

Survey 7.5-minute digital elevation models (DEMs).

METHOD OF ASSESSING GROUND-WATER VULNERABILITY 11
OEMs are datasets that contain three-dimensional (x, y, 

and z) location and elevation data for each cell, or 30- 

by 30-meter (900-square meter) subdivision of the study 

area. The z direction (also measured in meters) represents the altitude of land surface at the center of each 

cell. The percent slope (calculated as the percent change 

in altitude over a specific map distance) was used to 

assign vulnerability ratings to all cells in the slope layer 

(GIS dataset).

The rating system used is based on the modification by A.K. Clark (2000) of the DRASTIC ratings 

for slope (Aller and others, 1987). Slopes greater than 

18 percent were rated as 1; slopes greater than 12 and as 

much as 18 percent were rated as 3; slopes greater than 

6 and as much as 12 percent were rated as 5; slopes 

greater than 2 and as much as 6 percent were rated as 9; 

and slopes less than or equal to 2 percent were rated as 

10. The assigned ratings are based on the assumption 

that 0- to 2-percent slopes provide the greatest opportunity for pollutants to infiltrate the surface because neither a pollutant nor precipitation is likely to exit the area 

as direct (surface-water) runoff. On the opposite end of 

the scale are slopes of greater than 18 percent providing a much larger likelihood of direct runoff and a much 

smaller opportunity for infiltration and, therefore, a relatively small risk of ground-water contamination.

As higher resolution DEMs become available, the 

slope dataset will improve accordingly. The resulting 

refinement in the slope data should provide more appropriate assessments of vulnerability, especially in areas 

where the effect of slope is not diminished by the effects 

of other natural features.

Soils

Soils can have an appreciable effect on whether 

or not, or how quickly, a fluid will infiltrate the 

land surface and perhaps percolate to the water table. 

Because different soils have different characteristics of 

permeability and hence, different infiltration capacities they present a variation of contamination risks. 

Soils can reduce the potential for ground-water 

contamination by trapping pollutants within its matrix 

and, thus, impede the further migration of a potential 

contaminant in the subsurface. The presence of 

fine-textured materials, such as silt or clay, can 

likewise decrease soil permeability and the threat of 

contamination.

The soil dataset used for the present study was 

adapted from a soil survey by the Natural Resources

Conservation Service. This dataset provides the areal 

distribution of soils, by name and slope, in northern 

Bexar County. The survey results include qualitative 

descriptors of the "degree of limitation for use as a 

sewage-disposal field" (Natural Resources Conservation Service, 1995, table 4) and relative permeability 

for each soil. With a cell resolution of 30 by 30 meters, 

the present soil dataset contains vulnerability ratings 

(table 2) assigned on the basis of the qualitative descriptors and relative permeability categories suggested by 

the Natural Resources Conservation Service (1995).

Cells with land-surface slopes of less than 30 percent were assigned ratings of 15 or 10, depending on 

the relative permeability of the soil type. For example, 

Tarrant association soils which cover 27 percent of the 

study area and are classified as moderately permeable 

were assigned a vulnerability rating of 15. Krum complex and Patrick soils which cover 12 and 2 percent of 

the study area, respectively, and are classified as slightly 

permeable were assigned a rating of 10. Other land 

areas which include ponds, pits, and quarries and 

account for 0.12 percent of the area were assigned a 

vulnerability rating of 20 on the basis of Taylor and 

others (1966).

Taylor and others (1966, p. 103) additionally indicate that, owing to erosion, steep areas characteristically 

do not support appreciable thicknesses of soil. Consequently, steep areas (slopes of 30 percent or greater, as 

interpreted for northern Bexar County) were assigned a 

vulnerability rating of 20 on the assumption that with 

little or no soil to impede infiltration such areas are 

relatively vulnerable to contamination.

Soils have minimal effect on the variability of 

ground-water vulnerability in the study area because the 

outcrop of the Glen Rose Limestone typically is not 

conducive to thick accumulations of soil. Where soils 

are present, they tend to be thin and uniform in composition not the kind of thick, loamy soils with relatively 

high capacities for buffering an underlying aquifer from 

contamination.

GROUND-WATER VULNERABILITY MAP

Plate 1 shows the spatial distribution of groundwater vulnerability to contamination in the study area as 

composited from the individual vulnerability ratings 

assigned to the selected natural features (hydrogeologic 

units, faults, caves or sinkholes, slopes, and soils). Vulnerability is indicated by a series of discrete ranges from 

least vulnerable (shades of yellow) to intermediately

12 Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
vulnerable (shades of orange) to most vulnerable 

(shades of red). The possible composite ratings range 

from 26 to 110; actual ratings range from 26 to 104. 

Least-vulnerable areas (composite ratings of 26 to 55) 

account for about 92 percent of the study area; intermediately vulnerable areas (composite ratings of 56 to 75) 

account for about 7 percent; and most-vulnerable areas 

(composite ratings of 76 to 104) account for about 1 percent. The distribution of the most vulnerable areas 

trends from southwest to northeast, roughly parallel to 

the Balcones fault zone.

Table 2 and figure 4 show that if an area represented by a 30- by 30-meter cell on the vulnerability 

map does not contain a fault, sinkhole, or cave, then the 

composite vulnerability rating for that cell must fall 

between 26 and 55 (26 equals 15 for hydrogeologic unit 

[ignoring a few, minor outcrops of the dolomitic member] plus 1 for slope plus 10 for soil; 55 equals 25 for 

hydrogeologic unit plus 10 for slope plus 20 for soil). If 

a cell contains a fault (rated as 20) or sinkhole (20) or 

cave (30) or cave zone (35), then the composite rating 

for that cell would range from 46 (minimum of 26 plus 

20) to 90 (maximum of 55 plus 35). Finally, the composite rating for a cell containing both a fault and one or 

more caves would range from 76 to 110.

Figure 4 further shows that, in general, areas 

with no fault, cave, or sinkhole are the least vulnerable; 

areas with either a fault or sinkhole or one or more caves 

are relatively more vulnerable; and areas with both a 

fault and one or more caves are the most vulnerable. 

The relation between the presence of these features and 

increasing vulnerability is not absolute, however, as 

indicated by the overlap in the three ranges of composite 

ratings representing feature combinations overlain on 

the cumulative relative frequency distribution.

CONSIDERATIONS AND LIMITATIONS

The method used in this study to assess groundwater vulnerability to contamination is a subjective rating method known as an index method. The index 

method has been a popular approach for assessment 

of ground-water vulnerability because it is straightforward computationally and uses commonly available 

data. However, the method has three primary shortcomings to producing a scientifically defensible assessment 

(Focazio and others, 2002): (1) The ratings for individual factors, although relatively consistent and logical 

and based on best engineering judgment, are subjectively selected; (2) the compositing of the factors is

totally subjective and not a process-based method that 

is, the physics of the hydrologic system are not incorporated; and (3) the uncertainty or sensitivity of the computed results cannot be determined.

The results provided herein are relevant to the 

scales of the USGS 7.5-minute DBMS and the 30- by 

30-meter cells used to assess vulnerability over the 180- 

square-mile study area. The results (pi. 1) are likely to 

be less appropriate, therefore, for relatively small areas 

of less than about 10,000 square feet. Site-specific 

investigation is recommended to fill the gaps resulting 

from the shortcomings inherent in the scale of the 

present study.

Many assumptions went into the determination 

of the vulnerability ratings (table 2) assigned to the 

hydrogeologic units (table 1) and other natural features 

believed to most affect the vulnerability of ground water 

to contamination in the study area (pi. 1). Where any 

one or a combination of these ratings is flawed, the composite vulnerability assessment (pi. 1) is likely to be 

biased, accordingly. Owing to the subjectivity of the 

vulnerability-rating process, the relatively large scale of 

the present analysis, and the resulting potential for 

biased results, any photographic or digital enlargement 

of the information on plate 1 is not recommended.

It is unlikely that all faults, caves, and sinkholes 

in the study area have been accounted for, or that the 

effects of soil, slope, and hydrogeologic unit have been 

completely incorporated. Therefore, it should not be 

assumed that all features affecting vulnerability are 

entirely or accurately incorporated in the composite 

vulnerability patterns depicted on plate 1. Many features that are known to affect vulnerability are likely 

unmapped because their surface expressions are 

obscured by soil, colluvium, and (or) vegetation. For 

example, yet-undetected faults might include those with 

no noticeable disruption of the present topography, particularly if the rocks on both sides of such faults have 

experienced similar weathering effects or if the rates of 

displacement have been no different than the rates of 

erosion. Similarly, the portals of yet-unrecognized 

caves and sinkholes are possibly covered by trees, 

brush, and (or) associated debris.

Considering that the Edwards aquifer is recharged 

by runoff from the outcrop of the Trinity aquifer might 

indicate an inverse relation between the vulnerability of 

ground water to contamination in the Glen Rose Limestone and a resulting vulnerability in the downgradient 

Edwards aquifer. In other words, areas of Glen Rose 

Limestone that are most vulnerable to contamination

CONSIDERATIONS AND LIMITATIONS 13
0)

cr

f

a

o

o

c

o.

0)

ff

o 

o

I

ao'

DO 

(D

s

o 

o

 t

1.0

0.9

0.8

O 0.7

z

LU

O 

S

LL 

LU

0.6

<; 0.5

LJJ 

DC 

LJJ 

^ 0.4

3

z> 

I 0.3

0.2

0.1

FAULT, SINKHOLE, 

CAVE, Of) CAVE ZONE

NO FAULT, SINKHOLE, 

OF) CAVE

20

>»»«, 

60 

VULNERABILITY RATING

80 

Figure 4. Cumulative relative frequency of composite vulnerability ratings computed for selected categories of natural 

study area.
might offer the greatest protection against that contamination directly affecting the Edwards aquifer. By 

the same reasoning, the relatively impermeable upgradient areas (of relatively little local vulnerability) might 

pose the greatest threat to the pervious recharge zone 

of the downgradient Edwards aquifer. Although such 

an inverse relation might seem intuitive, the exact nature 

of such a relation has not been studied nor is it known 

whether or not or to what extent it might apply over 

the long term.

Although plate 1 is available now as another tool 

for water managers and regulators making land-use 

decisions, the vulnerability map will change as additional observations are made and more accurate 

analyses are completed. Less subjective, less biased 

delineations of vulnerability will evolve as additional 

data on hydrogeologic units, faults, caves, sinkholes, 

slopes, and soils are collected, analyzed, and factored 

into future analyses.

Plate 1 provides only one of the many considerations that should contribute toward any assessment of the risks and effects of potential ground-water 

contamination in northern Bexar County. No part of the 

vulnerability assessment offered herein is intended to 

preclude or minimize the importance of detailed, onsite 

inspections by qualified hydrogeologists (Texas Natural 

Resource Conservation Commission, 2002).

SUMMARY

The ground-water hydrology of the 180-squaremile study area in northern Bexar County is controlled 

largely by the Trinity aquifer, a subsurface reservoir 

composed of Lower Cretaceous carbonate rocks. Discharge from the Trinity aquifer recharges the downgradient, hydraulically connected Edwards aquifer one of 

the most permeable and productive aquifers in the 

Nation and the sole source of water for more than a million people in south-central Texas. The unconfined, 

karstic nature of the Edwards aquifer makes it particularly vulnerable to contamination resulting from urbanization that is spreading rapidly northward across the 

"environmentally sensitive" recharge zone of the 

Edwards aquifer and its "catchment area" on the outcrop of the upgradient, comparatively less permeable 

Trinity aquifer.

A better understanding of the Trinity aquifer is 

needed to properly evaluate water-management decisions affecting the quality of water in the Trinity as well 

as in the Edwards aquifer. In addition to depending on

the distribution and characteristics of the study area's 

hydrogeologic units, the vulnerability of ground water 

to contamination depends on the presence or absence of 

faults, caves, and sinkholes and on the slopes and soils 

on the surface and in the shallow subcrop of the Trinity 

aquifer. The spatial distribution of the resulting vulnerability varies with the composite (net) effect of these 

natural features. The present study was undertaken in 

cooperation with the San Antonio Water System 

(SAWS) to assess the vulnerability of ground water to 

contamination in northern Bexar County.

The upper two (of three) water-yielding zones in 

the Trinity aquifer crop out over about 90 percent of the 

study area. The upper zone is composed of the upper 

member of the Glen Rose Limestone and the uppermost 

part of the middle zone is composed of the lower member of the Glen Rose Limestone. Whereas the upper 

member is characteristically thin-bedded and composed 

mostly of alternating beds of relatively resistant limestone and comparatively soft marl, the lower member 

typically is composed of relatively massive, fossiliferous limestone. The Glen Rose Limestone is uncharacteristically permeable in northern Bexar County, where 

caves and sinkholes have developed in elevated areas 

north of the Balcones fault zone.

For purposes of the present study, the upper member of the Glen Rose Limestone is divided into five 

mappable intervals, A through E (youngest to oldest): 

Interval A (greater than 120 feet thick) has an 

abundance of caves indicative of its 

generally well developed fracture, channel, 

and cavern porosity.

Interval B (120 to 150 feet thick) is similar to 

Interval A, but with less cave development 

and permeability.

Interval C (18 to 26 feet thick), a partly to 

mostly dissolved zone of soluble carbonate 

rock with collapse structures and boxwork 

permeability.

Interval D (135 to 180 feet thick) generally has 

little porosity and permeability, but some 

local exceptions.

Interval E (6 to 16 feet thick) is a partly to mostly 

dissolved sequence of evaporites similar to 

that in Interval C.

Although there are fewer and less pronounced 

faults in the Trinity aquifer than in the Edwards aquifer, 

where faults occur they can affect the rates of recharge

SUMMARY 15
and discharge as well as the patterns of ground-water 

flow and, therefore, the vulnerability to contamination. Barrier faults typically increase retention times 

and, therefore, the risk of ground-water contamination. 

Where faults and associated fractures increase the local 

permeability, they likewise can increase the risk of 

water-quality degradation.

Where caves or sinkholes affect local drainage 

patterns, they generally increase the vulnerability to 

ground-water contamination. In some places these features provide passageways for water to directly enter the 

Trinity aquifer. Most of the vulnerability resulting from 

caves and sinkholes is caused by increased infiltration 

rates owing to the enhancement of the local permeability and the internal drainage associated with the closed 

depressions that typically characterize such features.

The slope of land surface can affect the vulnerability of the underlying aquifer to contamination 

by controlling where and how long a potential contaminant remains on the surface. Disregarding the exception 

of steep slopes which are assumed to have no soil 

cover the greater the slope, the less the risk of groundwater contamination.

Because most soils in the study area are uniformly thin, they have only minimal effect on the 

vulnerability of ground water to contamination. 

Because Tarrant association soils are "moderately permeable," they are assumed least effective in reducing 

the vulnerability to ground-water contamination. Areas 

underlying the "slightly permeable" Krum complex and 

Patrick soils are assumed somewhat less vulnerable to 

contamination.

The vulnerability assessment provided herein 

was accomplished in four basic steps. First, information 

from the literature and well records was combined 

with the results of recent field mapping to compile a 

hydrogeologic map of the study area. Second, existing 

methods of assessing ground-water vulnerability were 

modified to best satisfy the present objectives. Third, 

the hydrogeologic units and other natural features 

believed to most affect the vulnerability of ground water 

to contamination were rated on a scale of 0 to 35. 

Fourth, vulnerability ratings assigned to each feature 

were incorporated into a grid-based, multilayer (GIS) 

database that allowed the integration of applicable data 

into a ground-water vulnerability map of the entire 

study area (where composite vulnerability ratings range 

from 26 through 104).

The relatively less vulnerable areas those with 

no faults, sinkholes, or caves have composite vulnerability ratings of 55 or less. Areas of relatively intermediate vulnerability those with either a fault or sinkhole 

or cave or cave zone (more than one cave) have composite ratings ranging from 46 to 90. The relatively 

more vulnerable areas those with both a fault and one 

or more caves have ratings greater than 75.

The relatively less vulnerable areas those with 

no faults, sinkholes, or caves occupy about 92 percent 

of the study area. The distribution of the most vulnerable areas which trend from southwest to northeast, 

roughly parallel to the Balcones fault zone occur 

mainly where faults intersect caves.

This report offers another tool for water managers and regulators who must make decisions regarding 

the risks and effects of potential ground-water contamination in the study area of northern Bexar County. The 

understanding of the vulnerability of ground water to 

contamination in the area is evolving as additional data 

on hydrogeologic units, faults, caves, sinkholes, slopes, 

and soils are collected and analyzed. No part of the vulnerability assessment offered herein is intended to preclude or minimize the importance of detailed, onsite 

inspections by qualified hydrogeologists.

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16 Vulnerability of Ground Water to Contamination, Northern Bexar County, Texas
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* U.S. GOVERNMENT PRINTING OFFICE: 2003 573-091! ' 52076 Re ion No. 8