Water Quality

in South-Central Texas

Texas, 1996–98

U.S. Department of the Interior

U.S. Geological Survey

Circular 1212
POINTS OF CONTACT AND ADDITIONAL INFORMATION

Front cover: Horsecollar Bend on the Frio River north of Leakey, Real County (photograph courtesy of TEXAS 

HIGHWAYS Magazine). 

Back cover: Left, Guadalupe River in the Hill Country; center, San Antonio Riverwalk (photographs courtesy 

of TEXAS HIGHWAYS Magazine); right, hay field near Utopia, Uvalde County (photograph by Clarence E. Ranzau, 

U.S. Geological Survey). 

The companion Web site for NAWQA summary reports:

http://water.usgs.gov/nawqa/

South-Central Texas contact and web site:

USGS State Representative

U.S. Geological Survey

Water Resources Division

8027 Exchange Dr.

Austin, TX 78754

e-mail: dc_tx@usgs.gov

http://tx.usgs.gov/sctx/

National NAWQA Program:

Chief, NAWQA Program

U.S. Geological Survey

Water Resources Division

12201 Sunrise Valley Drive, M.S. 413

Reston, VA 20192

http://water.usgs.gov/nawqa/

River Basin Assessments

Albemarle-Pamlico Drainage Basin (Circular 1157)

Allegheny and Monongahela River Basins (Circular 1202)

Apalachicola-Chattahoochee-Flint River Basin (Circular 1164)

Central Arizona Basins (Circular 1213)

Central Columbia Plateau (Circular 1144)

Central Nebraska Basins (Circular 1163)

Connecticut, Housatonic and Thames River Basins (Circular 1155)

Eastern Iowa Basins (Circular 1210)

Georgia-Florida Coastal Plain (Circular 1151)

Hudson River Basin (Circular 1165)

Kanawha-New River Basins (Circular 1204)

Lake Erie-Lake Saint Clair Drainages (Circular 1203)

Las Vegas Valley Area and the Carson and Truckee River Basins

(Circular 1170)

Lower Illinois River Basin (Circular 1209)

Long Island-New Jersey Coastal Drainages (Circular 1201)

Lower Susquehanna River Basin (Circular 1168)

Mississippi Embayment (Circular 1208)

Ozark Plateaus (Circular 1158)

Potomac River Basin (Circular 1166)

Puget Sound Basin (Circular 1216)

Other NAWQA summary reports

Red River of the North Basin (Circular 1169)

Rio Grande Valley (Circular 1162)

Sacramento River Basin (Circular 1215)

San Joaquin-Tulare Basins (Circular 1159)

Santee River Basin and Coastal Drainages (Circular 1206)

South Platte River Basin (Circular 1167)

Southern Florida (Circular 1207)

Trinity River Basin (Circular 1171)

Upper Colorado River Basin (Circular 1214) 

Upper Mississippi River Basin (Circular 1211)

Upper Snake River Basin (Circular 1160)

Upper Tennessee River Basin (Circular 1205)

Western Lake Michigan Drainages (Circular 1156)

White River Basin (Circular 1150)

Willamette Basin (Circular 1161)

National Assessments

The Quality of Our Nation‘s Waters—Nutrients and Pesticides (Circular 1225)
U.S. GEOLOGICAL SURVEY CIRCULAR

Water Quality 

in South-Central Texas, 

Texas, 1996–98

1212

By Peter W. Bush, Ann F. Ardis, Lynne Fahlquist, Patricia B. Ging, 

C. Evan Hornig, and Jennifer Lanning-Rush
Library of Congress Cataloging-in-Publications Data

The use of firm, trade, and brand names in this report is for identification purposes only and

does not constitute endorsement by the U.S. Government.

Free on application to the

U.S. Geological Survey

Information Services

Box 25286 Federal Center

Denver, CO 80225

Or call: 1-888-ASK-USGS

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

2000

Water quality in South-Central Texas, Texas, 1996–98 / by Peter W. Bush...[et al.].

p. cm. -- (U.S. Geological Survey Circular ; 1212)

Includes bibliographical references.

ISBN 0-607-95417-5 (alk. paper)

1. Water quality--Texas--Nueces River Watershed. 2. Water quality--Texas--San Antonio River 

Watershed. 3. Water quality--Texas--Guadalupe River Watershed. I. Bush, Peter W. II. Geological Survey 

(U.S.) III. Series.

TD224.T4 W379 2000

363.739’42’09764--dc21

00-049455
CONTENTS

Contents III

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM .............................................................. IV

SUMMARY OF MAJOR FINDINGS ................................................................................................... 1

INTRODUCTION TO THE UPPER PART OF THE SOUTH-CENTRAL TEXAS STUDY UNIT ......... 3

MAJOR FINDINGS ............................................................................................................................ 6

Stream Quality Is Affected Most by Urban Activities .................................................................... 6

NATIONAL PERSPECTIVE—Trends in Water Quality Associated With Urban Development, 

as Reflected in Bottom Sediment of Lakes and Reservoirs, Are Not Unique to San Antonio ... 12

NATIONAL PERSPECTIVE—Nationally and Locally, Biological Community Status Is 

Related to Watershed Development ......................................................................................... 13

Edwards Aquifer Water Quality Remains Excellent ..................................................................... 14

NATIONAL PERSPECTIVE—The Frequencies of Pesticide Detection in Ground Water 

Vary in Comparison With Those in Ground Water Nationally—Compounds Detected 

Were the Same ......................................................................................................................... 16

Trinity Aquifer Water Quality Is Mostly Unaffected by Human Activities ....................................... 20

STUDY UNIT DESIGN ...................................................................................................................... 22

GLOSSARY ....................................................................................................................................... 24

REFERENCES .................................................................................................................................. 25

APPENDIX A—WATER-QUALITY DATA FROM SOUTH-CENTRAL TEXAS IN A NATIONAL 

CONTEXT .......................................................................................................................................... 27
IV National Water-Quality Assessment Program 

THIS REPORT summarizes major findings about water quality in south-central Texas that emerged from an 

assessment conducted between 1996 and 1998 by the U.S. Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program. Water quality is discussed in terms of local and regional issues and compared to 

conditions found in all 36 NAWQA study areas, called Study Units, assessed to date. Findings also are explained in 

the context of selected national benchmarks, such as those for drinking-water quality and the protection of aquatic 

organisms. The NAWQA Program was not intended to assess the quality of the Nation’s drinking water, such as by 

monitoring water from household taps. Rather, the assessments focus on the quality of the resource itself, thereby 

complementing many ongoing Federal, State, and local drinking-water monitoring programs. The comparisons 

made in this report to drinking-water standards and guidelines are only in the context of the available untreated 

resource. Finally, this report includes information about the status of aquatic communities and the condition of 

instream habitats as elements of a complete water-quality assessment.

Many topics covered in this report reflect the concerns of officials of State and Federal agencies, water-resource 

managers, and members of stakeholder groups who provided advice and input during the assessment. Residents 

who wish to know more about water quality in the areas where they live will find this report informative as well. 

THE NAWQA PROGRAM seeks to improve scientific and public understanding of water quality in the Nation’s 

major river basins and ground-water systems. Better understanding facilitates effective resource management, 

accurate identification of water-quality priorities, and successful development of strategies that protect and restore 

water quality. Guided by a nationally consistent study design and shaped by ongoing communication with local, 

State, and Federal agencies, NAWQA assessments support the investigation of local issues and trends while 

providing a firm foundation for understanding water quality at regional and national scales. The ability to integrate 

local and national scales of data collection and analysis is a unique feature of the USGS NAWQA Program.

South-Central Texas is one of 51 water-quality assessments initiated since 1991, when the U.S. Congress 

appropriated funds for the USGS to begin the NAWQA Program. As indicated on the map, 36 assessments have 

been completed, and 15 more assessments will conclude in 2001. Collectively, these assessments cover about onehalf of the land area of the United States and include water resources that are available to more than 60 percent of 

the U.S. population.

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

1991–95

1994–98

1997–2001

Not yet scheduled

High Plains Regional

Ground-Water Study, 

1999–2004

NAWQA Study Units— 

Assessment schedule

South-Central Texas
Summary of Major Findings 1 

EXPLANATION

Boundary delineating the area of 

1. Edwards aquifer recharge zone

2. Edwards aquifer confined zone

3. Trinity aquifer

Balcones escarpment—Approximately

coincident with southern boundary of

Edwards aquifer recharge zone

Edwards aquifer

(recharge zone)

Confining

unit

Edwards aquifer

(confined zone)

Trinity aquifer

Streams lose flow to the

Edwards aquifer

A A'

0 25 50 KILOMETERS

0 25 50 MILES

EDWARDS PLATEAU

GULF

COASTAL

PLAIN

Nueces R

SAN

ANTONIO

Guadalupe R

1.

2.

3.

Sa n Antonio

R

A

A'

BEXAR

COUNTY

SAN

ANTONIO

100o 99o

98o

30o

29o

Stream and River Highlights

Numerous organic chemicals and trace 

elements were detected in streams and rivers 

in the upper part of the South-Central Texas 

Study Unit—in part because of high detection sensitivities (fractions of a part per billion) of laboratory analytical methods. Most 

concentrations of organic chemicals and 

trace elements were extremely low and many 

times less than levels of concern for human 

health or aquatic life. No concentrations of 

sampled chemical constituents except those 

of dissolved nitrite plus nitrate nitrogen 

(hereinafter, nitrate) downstream from 

wastewater treatment plants exceeded drinking-water standards or guidelines. Although 

surface water historically has not been a 

source of drinking water, streams and rivers 

are the major source of replenishment 

(recharge) to the Edwards aquifer, the principal water supply for much of the region. Surface water provides habitat for a wide variety 

of aquatic organisms and recreational opportunities for many people. 

• Nitrate concentrations downstream from 

wastewater treatment plants consistently 

were about 5 times greater than those 

downstream from agricultural land and 

about 20 times greater than those downstream from rangeland. All total phosphorus concentrations downstream from 

wastewater treatment plants exceeded a 

goal for controlling nuisance algae and 

aquatic plant growth.

• More pesticides and volatile organic compounds (VOC) 

at generally higher concentrations were detected in urban 

stream water than in agricultural stream water. Concentrations of each of six pesticides and one VOC exceeded 

aquatic-life guidelines in several (mostly urban) stream 

samples. The herbicide atrazine was detected in all urban 

and agricultural stream samples.

• Generally higher concentrations of organochlorine 

compounds and trace elements were detected in urban 

stream-sediment and fish-tissue samples than in agricultural and rangeland samples. Concentrations of several 

samples exceeded sediment guidelines for the protection 

of aquatic life or fish-tissue guidelines for the protection 

of fish-eating wildlife; most were from urban streams.

• Biological community status is related to watershed 

development. The most degraded algal, invertebrate, and 

fish communities were in urban streams, and the healthiest were in rangeland streams.

Major Influences on Streams and Rivers

• Wastewater-treatment plant effluent

• Runoff from urban areas

• Pesticide use in urban watersheds

• High flow caused by storms

Ground-Water Highlights

Ground water, primarily from the Edwards aquifer and 

also from the Trinity aquifer, is the principal source of 

water supply in the upper part of the Study Unit. In 

general, the quality of water in the Edwards and Trinity 

aquifers reflects little evidence of human activities—despite 

major urban development and agricultural land in places 

overlying the Edwards aquifer and increasing urban development in places overlying the Trinity aquifer. Numerous 

organic chemicals were detected in the Edwards aquifer, 

fewer in the Trinity aquifer, but most concentrations were 

extremely low. Concentrations of nutrients, organic chemicals, and trace elements generally were very low relative to 

The South-Central Texas Study Unit encompasses the Nueces, San Antonio, 

and Guadalupe River Basins. The 1996–98 assessment involved only the upper 

part of the Study Unit. Streams and rivers that originate in the rugged hills of the 

Edwards Plateau generally gain water as they flow southeastward toward the 

Edwards aquifer outcrop (recharge zone). As they flow across the highly 

permeable, faulted, and fractured rocks of the recharge zone, most lose 

substantial amounts of flow directly into the aquifer.

SUMMARY OF MAJOR FINDINGS
2 Water Quality in South-Central Texas

Not assessed

Urban

Pesticides3

Agricultural Rangeland

Total 

phosphorus2

Urban/

effluent

dominated

Percentage of samples with concentrations equal to

or greater than a health-related national guideline for

drinking water or aquatic life; or above a national goal 

for preventing excess algal growth

Percentage of samples with concentrations less than

a health-related national guideline for drinking water or 

aquatic life; or below a national goal for preventing 

excess algal growth

Percentage of samples with no detection

Trace

elements6

Organochlorines5

Volatile

organics4

Urban/effluent-dominated organochlorine and trace

element samples combined with urban samples

 Streams and rivers

Selected indicators of surface-water quality

Nitrate1

 Edwards aquifer

Urban

primarily

monitor

wells

Pesticides3

Radon

Recharge

zone 

Nonurban

primarily

domestic

wells

Confined

 zone

primarily

publicsupply

wells

Trinity aquifer

Upper and

middle zones 

primarily

domestic

wells

Nitrate1

Selected indicators of ground-water quality

Percentage of samples with no detection

Volatile

organics4

Percentage of samples with concentrations equal to

or greater than a health-related national guideline for

drinking water or aquatic life 

Percentage of samples with concentrations less than

a health-related national guideline for drinking water or 

aquatic life 

drinking-water standards and guidelines. Some radon concentrations exceeded a proposed drinking-water standard, 

but radon concentrations were among the lowest of 

NAWQA Study Units nationwide.

• The median nitrate concentration among primarily 

public-supply wells sampled in the confined zone of the 

Edwards aquifer was about six times less than the maximum contaminant level for nitrate in drinking water; 

nevertheless, the median was in the top 10 percent of 

median nitrate concentrations of major aquifers sampled 

by NAWQA nationwide. Median nitrate concentration in 

the Trinity aquifer was more than 10 times less than that 

in the Edwards aquifer. 

• The greatest frequencies of detection of pesticides and 

VOCs in the Edwards aquifer were in urban (northern San 

Antonio) recharge-zone samples. The outcropping fractured and faulted limestone of the recharge zone allows 

unrestricted downward movement of water, which can 

contain contaminants, into the ground-water-flow system.

• Four of the 5 most frequently detected pesticides in water 

from urban recharge-zone wells in the Edwards aquifer 

were the same as 4 of the 5 most frequently detected pesticides in surface water at urban sites in the San Antonio 

area. This finding illustrates the correlation between the 

quality of recently recharged ground water in an urban 

setting and the quality of urban stream and river water.

• The quality of water in the Trinity aquifer remains influenced primarily by the natural processes of water-rock 

interaction. Concentrations of dissolved solids, sulfate, 

iron, and strontium exceeded drinking-water standards or 

guidelines in some samples. Fewer pesticides and VOCs 

were detected in Trinity aquifer samples than in Edwards 

aquifer samples, possibly because of low permeability of 

the Trinity aquifer and little urban development.

Major Influences on Ground Water

• Urban development in the Edwards aquifer recharge zone

• The quality of stream and river water that recharges the 

Edwards aquifer

• The natural processes of water-rock interaction in the 

Trinity aquifer

1 Nitrate (as nitrogen), sampled in water.

2 Total phosphorus, sampled in water.

3 Insecticides, herbicides, and pesticide breakdown products, 

sampled in water.

4 Solvents, refrigerants, fumigants, and gasoline compounds, 

sampled in water.

5 Organochlorine compounds including DDT and PCBs, sampled in 

sediment.

6 Arsenic, mercury, and metals, sampled in sediment.
Introduction to the Upper Part of the South-Central Texas Study Unit 3 

100o 99o

98o

30o

29o

EXPLANATION

0 25 50 KILOMETERS

0 25 50 MILES

Urban

Balcones escarpment

Agricultural

Rangeland

Streamflow-gaging station—

Frio River at Concan

Edwards aquifer index well—

SAN J–17

ANTONIO

EDWARDS PLATEAU

GULF

COASTAL

PLAIN

Land use

Sampling site

The upper part of the SouthCentral Texas Study Unit (hereinafter, Study Unit) encompasses 

parts of the topographically rugged 

and picturesque Edwards Plateau 

and the comparatively flat, gently 

coastward-sloping Gulf Coastal 

Plain physiographic regions. An 

abrupt topographic break, the Balcones escarpment, separates the 

two landforms (fig. 1).

San Antonio is the principal 

urban area in the Study Unit. The 

agricultural areas of primary 

interest are west of San Antonio. 

Rangeland predominates in the 

Edwards Plateau.

The Study Unit contains the 

Edwards aquifer in the Balcones 

fault zone, a zone of northeastward-trending parallel faults that 

straddles the Balcones escarpment. 

The Study Unit also contains the 

Trinity aquifer (fig. 2) in the “Hill 

Country,” the local name for the 

eastern part of the Edwards Plateau 

in the region.

An Unusual Physical System

Surface water and ground water 

in the Study Unit are uniquely 

interrelated. Rainfall reaches 

streams deeply incised into the 

marl, shale, and limestone of the 

Edwards Plateau as springflow and 

surface runoff. The streams generally gain water as they flow southsoutheastward from headwaters in 

the higher elevations of the plateau. 

As the major streams flow across 

the faulted and fractured carbonate 

rocks of the Edwards aquifer outcrop (recharge zone), they lose substantial amounts of flow directly 

into the highly permeable aquifer. 

Ground Water Predominates

Ground water accounts for 

nearly all of the water supply in 

the Study Unit (fig. 3), and the 

Edwards aquifer, one of the most 

productive aquifers in the world, is 

the principal source. Withdrawals 

from the Edwards aquifer meet 

the water-supply needs of more 

Rangeland streams near the Balcones 

escarpment often go dry.

Corn is among the top agricultural 

commodities in the region.

Upgrading roads goes hand in hand 

with increasing development north of 

San Antonio.

INTRODUCTION TO THE UPPER PART OF THE SOUTH-CENTRAL 

TEXAS STUDY UNIT

Figure 1. Land use in the Study Unit correlates with physiography. The rugged, thin-soiled terrain of the Edwards Plateau is 

largely undeveloped. The flatter, thicker-soiled terrain of the Gulf Coastal Plain is better suited to growing crops—primarily hay, 

sorghum, wheat, corn, and oats. (Land use from Landsat thematic mapper scenes acquired in 1991–93 [1].)
4 Water Quality in South-Central Texas

100o 99o

98o

30o

29o

Edwards aquifer outcrop 

(recharge zone)

EXPLANATION

0 25 50 KILOMETERS

0 25 50 MILES

Edwards aquifer subcrop 

(confined zone)—Downdip 

boundary from [2]

Trinity aquifer

Area not underlain by 

Edwards or Trinity aquifers

Balcones fault zone [3, fig. 2b]

CALDWELL

WILSON

MAVERICK KARNES DE WITT

COMAL

HAYS

KENDALL

KERR

REAL

ZAVALA FRIO

MEDINA

BEXAR

GUADALUPE

GONZALES

BANDERA

EDWARDS

BLANCO

KINNEY UVALDE

ATASCOSA

Comal 

Springs

San Marcos

Springs

Ground-water source TOTAL

Withdrawals are

from this area

145

290

435

580

(62)

Ground water

Surface water

(1,624) Population served,

in thousands

(1,624)

ESTIMATED WATER WITHDRAWALS IN 1995

0 0

25

50

WATER WITHDRAWALS,

IN MILLIONS OF GALLONS PER DAY

COMMERCIALDOMESTICSUPPLY

INDUSTRIAL

AND MINING

PERCENTAGE OF TOTAL

WATER WITHDRAWALS

PUBLICSUPPLY

IRRIGATIONAND LIVESTOCK

TOTAL

75

100

than 1.5 million people in the 

greater San Antonio region and 

support farming and ranching 

west of San Antonio. The aquifer 

sustains the flows of Comal and 

San Marcos Springs, which attract 

tourists to the region, yield base 

flow to the Guadalupe River to 

meet downstream water requirements, and provide habitat for several threatened and endangered 

species. The Edwards aquifer harbors diverse aquatic communities 

above and below land surface; at 

least 90 described species, one-half 

of which are subterranean, are 

unique to the region. 

The Trinity aquifer, although 

much less productive than the 

Edwards aquifer, is the most commonly used and reliable source of 

water supply in the Hill Country.

Development is Increasing

The watersheds of major 

streams in the Edwards Plateau 

that recharge the Edwards aquifer 

were largely undeveloped rangeland in the late 1990s—but that is 

changing. A common sight in the 

area is the construction of new 

subdivisions on land that for 

generations has been devoted to 

ranching. Edwards, Bandera, and 

Kendall Counties were among the 

top 10 statewide and the top 50 

nationally in percentage of population increase during 1990–98. 

Urban development in the Edwards 

aquifer recharge zone, particularly 

in Bexar County (San Antonio), 

also is increasing. Bexar County 

ranked fourth among counties 

statewide and 15th nationally in 

numeric population increase during 

1990–98. Continued development 

on a scale suggested by those U.S. 

Census Bureau statistics has the 

potential to affect surface- and 

ground-water quality in the Study 

Unit.

The Edwards Aquifer Is 

Particularly Vulnerable to 

Contamination

The highly permeable rocks that 

compose the Edwards aquifer 

recharge zone, and development in 

the watersheds of major streams in 

Figure 3. Nearly all the withdrawals from the Study Unit were ground water. About 

70 percent of the ground-water withdrawals were from the Edwards aquifer.

Figure 2. Areal extent of the Edwards and Trinity aquifers sampled during 1996–98 (modified from [4]). The outcrop of the 

Edwards aquifer essentially is the recharge zone; the subcrop essentially is the confined zone. The outcrop of the Trinity aquifer 

is approximately coincident with the Hill Country. The subcrop of the Trinity aquifer (not shown), which extends beneath the 

Edwards aquifer, is not a source of water supply in the region and was not sampled.
Introduction to the Upper Part of the South-Central Texas Study Unit 5 

620

630

640

650

660

670

680

690

1995 1996 1997 1998 1999

J MMJ SN J MMJ SN J MMJ SN J MMJ SNJ MMJ SN

Sampling

period

Mean monthly

water level, 1933–99

WATER LEVEL, IN FEET

ABOVE SEA LEVEL

5-day

water level

GROUND WATER

1995 1996 1997 1998 1999

10 J MMJ SN J MMJ SN J MMJ SN J MMJ SNJ MMJ SN

100

1,000

10,000

30,000

STREAMFLOW, IN CUBIC

FEET PER SECOND

Sampling period

Mean monthly

streamflow,

1923–99

Daily mean streamflow

SURFACE WATER the Edwards Plateau and in the 

recharge zone, make the Edwards 

aquifer particularly vulnerable to 

contamination. That vulnerability 

and the dependence of so many 

people on the aquifer combine to 

make the water quality of the 

Edwards aquifer and the streams 

that recharge it a critical issue for 

the future of the region.

Climate and Hydrologic 

Conditions Can Affect Water 

Quality

Hydrologic conditions in the 

Study Unit often are extreme. 

Months-long droughts that strain 

water supplies and produce widespread crop failure commonly are 

followed by wet periods that 

include torrential rains and flash 

floods. Such was the pattern during 

the 1996–98 period of intensive 

sampling (figs. 4, 5). 

Whether hydrologic conditions 

are wet or dry can affect water 

quality. For example, during wet 

conditions, proportionately more of 

the streamflow is surface runoff. 

Runoff that contains excessive 

nutrients, pesticides, or other 

contaminants washed off land surfaces can degrade stream quality. 

During dry conditions, stream 

quality is more strongly influenced 

by the quality of the base flow (and 

wastewater-treatment plant discharge, if present). 

The Study is a Benchmark 

for Changes to Come

This study is more of a benchmark from which to measure future 

water-quality changes than a documentation of problems. Population 

growth and development bring 

changes in land use, and land use 

can have a strong influence on 

water quality. Accordingly, part of 

the study design focuses on land 

use. Surface-water sampling was 

done to assess the effects of urban, 

agricultural, and rangeland use on 

stream quality during stormflows 

and during normal and low flows. 

Ground-water sampling was done 

to assess the overall quality of 

ground water in the Edwards and 

Trinity aquifers and to assess the 

relation between ground-water 

quality and urban land use in the 

recharge zone in San Antonio. 

Synoptic samples were collected 

primarily from domestic and monitor wells in the Edwards aquifer 

recharge zone, public-supply wells 

in the Edwards aquifer confined 

zone, and domestic wells in the 

upper and middle zones of the 

Trinity aquifer. (See “Study Unit 

Design,” p. 22.)

Figure 4. Daily mean streamflow of the Frio River at Concan, a basic site for 

sampling, reflects the regional droughts of 1996 and 1998 and the subsequent wet 

periods. Streamflow at the site can increase almost instantly by a factor of 100 or 

more for short periods in response to intense rainfall. The region near the Balcones 

escarpment is more prone to flash floods than anywhere else in the Nation [5, 

p. 63].

Figure 5. The water level in Edwards aquifer index well J–17 in Bexar County rose 

substantially during wet periods following the regional droughts of 1996 and 1998; 

and reflects the effects of pumpage at San Antonio as well as natural fluctuations in 

recharge and discharge. Although pumpage at San Antonio has increased fivefold 

since the 1930s, Edwards aquifer water levels show no long-term declines because 

the aquifer readily accepts recharge when rainfall is plentiful. 
6 Water Quality in South-Central Texas

MAJOR FINDINGS

West Nueces River

NU

E

CES RIV

E

R

Frio River

Seco Cr

Hondo Cr Medina

River

Blanco River

Salado

L

Cr

eo n Cr

RIVER

San Marcos River

Canyon

Lake

Medina

Lake

Geronimo

SAN

ANTONIO

SAN

MARCOS

NEW

BRAUNFELS

Cr

Sabinal River

SAN ANTONIO R

0 25 50 MILES

0 25 50 KILOMETERS

1

100o 99o

98o

30o

29o

Cibolo Cr

GUADALUPE

KERRVILLE

UVALDE ELMENDORF

LA COSTE

Agricultural

Rangeland

Urban

EXPLANATION

Urban/effluent dominated

Key land-use setting

Sampled material

Water, bed sediment, and tissue

Bed sediment and tissue

No tissue sampling 1

Stream Quality Is Affected 

Most by Urban Activities

Chemical and biological indicators in monitored streams in key 

land-use settings in the Study Unit 

(fig. 6) show that stream-quality 

degradation tends to be associated 

with urban land to a greater degree 

than with agricultural land or 

rangeland. With a few exceptions, 

pesticides, VOCs, and trace elements were either not detected or 

detected at concentrations below 

levels of concern for human health 

or aquatic life in all monitored 

streams.

Nutrient Concentrations Increased 

as a Result of WastewaterTreatment Plant Discharges

Discharges from wastewater 

treatment plants had the greatest 

effect on nutrient concentrations 

of any identifiable source in the 

Study Unit [6]. Nitrate concentrations were appreciably higher in an 

urban stream downstream from 

four wastewater treatment plants 

(San Antonio River at Elmendorf) 

than in streams in other land-use 

settings (fig. 7). Nitrate concentrations in the San Antonio River at 

Elmendorf generally increased as 

the percentage of wastewatertreatment plant discharge (effluent) in streamflow increased 

(fig. 8). Although the San Antonio 

River historically has not been a 

source of drinking water, the 

median nitrate concentration at 

Elmendorf (11 milligrams per 

liter [mg/L]) exceeded the U.S. 

Environmental Protection Agency 

(USEPA) maximum contaminant 

level (MCL) for drinking water 

(10 mg/L). No aquatic-life 

guideline for nitrate has been established. The flow-weighted mean 

nitrate concentration at Elmendorf, 

5.8 mg/L, was about the 90th percentile among 372 NAWQA 

stream sites nationwide.

Ammonia at sufficient concentrations, which vary with water 

temperature and acidity [7], 

can be toxic to fish. Median ammonia concentration at Elmendorf 

was 0.1 mg/L, the same as the 

Figure 6. Stream-water sampling in key land-use settings in the Study Unit was done at 9 sites, bed-sediment sampling at 15 

sites, and fish-tissue sampling at 14 sites. 

Nutrients in Water

 Nitrogen (primarily dissolved 

nitrate) and phosphorus (primarily 

dissolved phosphates and organic 

phosphorus in streams and orthophosphate in ground water) are 

essential to the health of plants and 

animals. Elevated concentrations of 

these nutrients in water, however, 

can cause problems. Too much 

nitrate in drinking water can reduce 

oxygen concentrations in the blood 

of infants to dangerously low levels. 

Excessive nitrate and phosphorus 

concentrations in surface water can 

cause nuisance algae and plant 

growth, which in turn can adversely 

affect aquatic ecosystems and 

water-related recreational activities. 

 Nitrogen and phosphorus occur 

naturally in water, but natural concentrations are increased by the 

introduction of fertilizers, manure, 

or treated sewage effluent into the 

water. 
Major Findings 7 

0

3

6

9

12

15

18

Salado Creek

Medina River

A M J J A SOND J F M A M J J A S OND J F M A

1996 1997 1998

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

Frio River

San Antonio River

20 40 60 80

0

5

10

15

20

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

PERCENTAGE OF STREAMFLOW

THAT WAS WASTEWATER-TREATMENT PLANT DISCHARGE

Median

USEPA drinking-water standard

100

estimated national background 

concentration in streams essentially 

unaffected by human activities [8]. 

Modern treatment plants effectively remove ammonia from 

wastewater by converting it to 

nitrate.

Land use had less effect on 

nutrient concentrations than proximity to wastewater treatment 

plants. The median nitrate concentration in an urban stream not 

affected by wastewater-treatment 

plant discharge (Salado Creek at 

San Antonio) was 0.56 mg/L, 

about the same as the estimated 

national background nitrate concentration (0.6 mg/L) in streams 

essentially unaffected by human 

activities [8]. The median nitrate 

concentration in an agricultural 

(cropland) stream (Medina River 

at La Coste) was 2.0 mg/L, higher 

than that of Salado Creek and 

showing the effect of runoff 

containing nitrate from fertilizers. 

The median is typical of median 

nitrate concentrations in surface 

water draining agricultural areas in 

selected NAWQA Study Units 

nationwide [8]. Median nitrate concentrations in four rangeland 

streams (Frio, Sabinal, Guadalupe, 

and Blanco Rivers) were near 

background levels (0.34 to 0.77 

mg/L). 

Nitrate concentrations vary over 

time in response to hydrologic 

conditions, wastewater-treatment 

plant discharges, plant and algal 

uptake, and timing of fertilizer 

application. However, no seasonal 

pattern in concentrations associated 

with any land-use setting was 

evident (fig. 7).

Total phosphorus concentrations 

also were highest in the San 

Antonio River at Elmendorf. The 

median total phosphorus concentration was 1.6 mg/L, and all 

concentrations were greater than 

the USEPA goal of 0.1 mg/L to 

control nuisance algae and aquatic 

plant growth. The flow-weighted 

mean total phosphorus concentration at Elmendorf, 1.2 mg/L, was 

about the 98th percentile among 

372 NAWQA stream sites nationwide. The median total phosphorus concentration in Salado Creek 

at San Antonio, the urban stream 

not affected by wastewatertreatment plant discharge, was 

0.066 mg/L, and individual concentrations commonly exceeded 

the USEPA goal of 0.1 mg/L. 

The median total phosphorus concentration for the Medina River at 

Figure 7. Nutrient concentrations were consistently higher in streams influenced 

by urban wastewater-treatment plant discharge (San Antonio River) and agricultural 

runoff (Medina River) than in an urban stream (Salado Creek) or a rangeland 

stream (Frio River). Greater rainfall and streamflow during 1997 and early 1998 

diluted nitrate concentrations associated with wastewater-treatment plant discharge 

in the San Antonio River at Elmendorf.

Figure 8. Differences in rainfall-runoff relations with flow in the San Antonio River 

at Elmendorf account for variability in the dilution of wastewater-treatment plant 

discharge. Concentrations of nitrate in the river increased appreciably with the 

percentage of wastewater-treatment plant discharge in the streamflow.
8 Water Quality in South-Central Texas

La Coste, the agricultural stream, 

was 0.030 mg/L; and the median 

concentration for each of the four 

rangeland streams was less than 

0.010 mg/L, the minimum reporting level. Total phosphorus concentrations in all streams exceeded the 

USEPA goal during stormflows. 

Higher phosphorus concentrations 

during stormflows is consistent 

with the adherence of phosphorus 

to sediment particles and the 

increase in suspended sediment 

concentrations during stormflows.

More Pesticides—at Generally 

Higher Concentrations—Were

Associated with Urban Land Use

More pesticides and their 

breakdown products (hereinafter, 

pesticides) were detected in Salado 

Creek at San Antonio (25 of 83 

analyzed) than in the San Antonio 

River at Elmendorf downstream 

from the wastewater treatment 

plants (18 of 83) and the Medina 

River at La Coste (15 of 83) [6]. 

Samples from rangeland streams 

were not analyzed for pesticides. 

The maximum concentrations of 

7 of the 10 pesticides detected in 

all three streams occurred in Salado 

Creek at San Antonio. In all three 

streams, some pesticide concentrations increased in the spring, which 

is consistent with the time of year 

for application.

No concentration of a pesticide 

for which a drinking-water 

standard has been established (25 

of 31 pesticides detected) exceeded 

that standard in any of the three 

streams. However, aquatic-life 

guidelines (established for 17 of 

31 pesticides detected) were 

exceeded in at least one sample 

for 5 pesticides in Salado Creek at 

San Antonio, 3 insecticides in the 

San Antonio River at Elmendorf, 

and 1 insecticide in the Medina 

River at La Coste (table 1). 

The most frequently detected 

pesticides in all three streams 

were atrazine, deethylatrazine, and 

prometon (fig. 9). Atrazine was 

detected in all of the samples from 

each of the three streams. 

The number of pesticides and 

the most frequently detected 

pesticides in each stream change 

when frequency of detection is 

based on a common concentration 

of 0.05 microgram per liter (μg/L). 

(See box, p. 9.) In Salado Creek 

at San Antonio, 13 pesticides 

were detected on the basis of the 

Pesticides in Streams—How Toxic to Aquatic Life?

According to the Extension Toxicology Network [9], the insecticide carbaryl, 

a general-use pesticide, is moderately toxic to aquatic organisms. The 

insecticide diazinon, some formulations of which are classified as restricted 

use (use by certified applicators only), is highly toxic to fish. The insecticide 

gamma-HCH, some formulations of which are classified as restricted use, 

is highly to very highly toxic to aquatic organisms. The insecticide malathion, 

a general-use pesticide, has a wide range of toxicities to aquatic organisms. 

DDE is a breakdown product of the insecticide DDT, which was banned for use 

in the United States in 1972. DDE has similar chemical and physical properties 

to DDT, which is very highly toxic to fish and to many aquatic invertebrate 

species. The herbicide tebuthiuron, a general-use pesticide, is slightly toxic to 

practically non-toxic to aquatic organisms. 

Atrazine is a restricted-use pesticide (home use exempted from restrictions) 

commonly used to kill broadleaf weeds. Atrazine is the most widely used herbicide in Texas corn and grain sorghum production [10] and the most widely used 

pesticide in the United States [11]. The Extension Toxicology Network [9] classifies atrazine as slightly toxic to aquatic organisms. No aquatic-life guideline 

has been established for the atrazine breakdown product deethylatrazine. 

The herbicide prometon, a general-use pesticide, has been shown in toxicological studies on selected fish species to have very low toxicity [12]. The herbicide simazine, also a general-use pesticide, is slightly toxic to practically nontoxic to aquatic organisms [9]; and the herbicide diuron, another general-use 

pesticide, is moderately toxic to fish and highly toxic to aquatic invertebrates 

[9].

Table 1. Pesticides and VOCs in stream water for which at least one sample 

exceeded an aquatic-life guideline 

[I, insecticide; H, herbicide; VOC, volatile organic compound] 

Compound Type

Number of exceedances / samples

Salado Creek

urban

San Antonio River

urban/effluent

dominated

Medina River

agricultural

Carbaryl I 1 / 35 2 / 26 0 / 32

Diazinon I 11 / 35 3 / 26 0 / 32

Gamma-HCH I 0 / 35 12 / 26 0 / 32

Malathion I 1 / 35 0 / 26 0 / 32

p,p'-DDE I 4 / 35 0 / 26 2 / 32

Tebuthiuron H 1 / 35 0 / 26 0 / 32

Trichloromethane VOC 0 / 22 8 / 13 0 / 7
Major Findings 9 

Simazine

Tebuthiuron

Deethylatrazine

Prometon

Atrazine

0 25 50 75 100

Carbon disulfide

Acetone

Methylbenzene

Methyl tert-butyl ether

cis-1,2-Dichloroethene

35 samples

22 samples

Salado Creek

urban

Chlorpyrifos

Diazinon

Prometon

Deethylatrazine

Atrazine

0 25 50 75 100

1,3- and 1,4-Dimethylbenzene

Tribromomethane

p-Isopropyltoluene

Trichloromethane

Carbon disulfide

32 samples

7 samples

Medina River

agricultural

Diazinon

Simazine

Deethylatrazine

Prometon

Atrazine

0 25 50 75 100

Tribromomethane

1,4-Dichlorobenzene

Bromodichloromethane

Trichloromethane

Chlorodibromomethane

26 samples

13 samples

San Antonio River

urban/effluent dominated

FREQUENCY OF DETECTION, IN PERCENT

PESTICIDES VOLATILE ORGANIC COMPOUNDS

common concentration (compared 

with 25 detected without regard to 

a common concentration). The 

five most frequently detected 

pesticides and their detection frequencies based on the common 

concentration were 

In the San Antonio River at 

Elmendorf, only 5 pesticides were 

detected on the basis of the common concentration (compared with 

18). The five detected and their 

detection frequencies were 

Tebuthiuron 91 percent

Atrazine 46 percent

Diazinon 46 percent

Diuron 37 percent

Carbaryl 31 percent.

Atrazine 46 percent

Diazinon 27 percent

Carbaryl 19 percent

Tebuthiuron 19 percent

Diuron 12 percent.

The number of pesticides 

detected and, in general, the 

frequencies of detection based 

on a common concentration of 

0.05 μg/L are substantially less 

than those not based on a common 

concentration. This result underscores the fact that the majority of 

pesticide detections in stream water 

represent extremely low concentrations. Less than 0.05 μg/L means 

less than 1 part contaminant per 

20 billion parts water, which is 

about the same concentration as 

an aspirin tablet dissolved in an 

olympic-size swimming pool.

All of the surface-water samples 

contained more than one pesticide, 

which is consistent with national 

NAWQA findings that show 

pesticides commonly occur in mixtures of several compounds [13]. 

Drinking-water standards for combinations of pesticides have not 

been established, and very little is 

known about the effects of mixtures of pesticides on aquatic life.

Most Volatile Organic Compound 

Concentrations Were Low Relative 

to Standards and Guidelines

The most VOCs (33 of 86 

analyzed) were detected in the 

Figure 9. Pesticides and VOCs were detected more frequently in urban streams 

than agricultural streams. The five pesticides and VOCs most frequently detected in 

stream water are shown. More herbicides than insecticides (shaded brown) were 

detected. 

About Frequencies of Detection

The minimum reporting level (MRL) is the smallest measured concentration 

of a constituent that may be reliably reported using a given analytical method; 

the MRL varies among constituents and analytical methods.Thus, the frequency at which a constituent is detected depends in part on its MRL. In 

general, the frequency of detection of a constituent increases as the MRL 

decreases. For example, atrazine was detected in 100 percent of the surfacewater samples in this study on the basis of an MRL of 0.001 μg/L. The 

detection frequency would have been 38 percent if the MRL for atrazine was 

0.05 μg/L.

Detection frequencies for pesticides and VOCs in water are reported in two 

ways in this report. The first is based on the MRLs of the methods used by the 

laboratory. The second is based on a common concentration for each group of 

constituents, which eliminates the effect of different MRLs for different constituents. The common concentration was selected as 0.05 μg/L for pesticides and 

0.1 μg/L for VOCs. 

In the Medina River at La Coste, 

5 pesticides also were detected 

on the basis of the common concentration (compared with 15). 

Detection frequencies of all 5 

pesticides based on the common 

concentration were less than 10 

percent.
10 Water Quality in South-Central Texas

San Antonio River at Elmendorf. 

Twenty-one VOCs were detected 

in Salado Creek at San Antonio. 

As was the case with pesticide 

detections, the fewest VOC detections (15) were in the Medina River 

at La Coste [6]. The maximum 

concentrations of 7 of the 10 VOCs 

detected in all three streams were 

from the San Antonio River at 

Elmendorf. No samples from 

rangeland streams were analyzed 

for VOCs. Of the 20 VOCs 

detected for which drinking-water 

standards and (or) aquatic-life 

guidelines have been established, 

one VOC—trichloromethane—

exceeded the Canadian waterquality guideline for the protection 

of aquatic life [14] in 8 of 13 samples in the San Antonio River at 

Elmendorf (table 1). Trichloromethane is moderately toxic to 

aquatic life [15], but it is not 

persistent in surface water; onehalf of it will have evaporated after 

several days. No other VOC concentrations in any of the three 

streams approached the levels of 

concern for the protection of 

human health or aquatic life. 

Four of the 5 VOCs most 

frequently detected in the San 

Antonio River at Elmendorf 

(bromodichloromethane, chlorodibromomethane, trichloromethane, and tribromomethane) 

(fig. 9), are by-products of water 

chlorination. The presence of 

these trihalomethane compounds 

is consistent with the fact that a 

major part of the flow of the river 

was wastewater-treatment plant 

discharge. On the basis of a common concentration of 0.1 μg/L, 

only 5 VOCs were detected (compared with 33 detected without 

regard to a common concentration). The same four trihalomethanes were detected at essentially 

the same frequencies as those 

detected without regard to a common concentration (fig. 9). The 

fifth, methylbenzene, was detected 

in 31 percent of the samples.

Four of the 5 VOCs most frequently detected in Salado Creek at 

San Antonio (cis-1,2-dichloroethene, methylbenzene, acetone, 

and carbon disulfide) (fig. 9) are 

industrial chemicals with a variety 

of uses. They probably entered the 

stream in urban runoff. The fifth 

VOC most frequently detected 

(methyl tert-butyl ether [MTBE]) 

was 1 of 3 VOCs detected on the 

basis of a common concentration 

of 0.1 μg/L. MTBE was the only 

one of the three with a detection 

frequency (50 percent) greater than 

10 percent. MTBE is a gasoline 

additive used primarily to reduce 

air pollution and has received 

recent publicity because of its 

potential to contaminate ground 

water. Although the use of MTBE 

in gasoline in the area has not been 

mandatory, it likely has been in 

some of the gasoline supplied to 

service stations and subsequently 

entered the environment during 

refueling at service stations or from 

engine exhaust, leaking storage 

tanks, or spills. 

The five VOCs most frequently 

detected in the Medina River at 

La Coste (fig. 9) are hard to associate with a specific agricultural use. 

None of the five were detected on 

the basis of a common concentration of 0.1 μg/L. Two other VOCs, 

benzene and methylbenzene, were 

detected on the basis of the common concentration (compared with 

15). The frequency of detection for 

both was 14 percent.

As with pesticides in stream 

water, the substantially smaller 

number of VOCs detected and, in 

general, the reduced frequencies 

of detection based on a common 

concentration show that the majority of VOC concentrations were 

extremely low—less than 1 part 

contaminant per 10 billion parts 

water.

Persistent Contaminants That 

Accumulate in Sediment and Fish 

Tissue Were Most Prevalent in 

Urban Streams

The occurrence of environmentally persistent organochlorine 

compounds (organochlorine 

insecticides and polychlorinated 

biphenols [PCB]), semivolatile 

organic compounds (SVOC) 

including polycyclic aromatic 

Fish for tissue analysis and community status assessment were collected by 

seining (shown above in the Blanco River near Wimberley) and by electrofishing.
Major Findings 11 

hydrocarbons (PAH), and trace 

elements (primarily metals) was 

assessed by measuring their 

concentrations in stream sediment 

and fish and clam tissue in key 

land-use settings. (Urban and 

urban/effluent dominated samples 

were grouped together as urban.) 

Sediment samples from 15 sites 

(fig. 6) were analyzed for 32 organochlorine pesticides, total PCBs, 

and 63 SVOCs. The greatest number of contaminants detected, and 

generally the highest concentrations (table 2), occurred in urban 

stream sediment. An average of 

26 contaminants per site were 

found at 6 urban sites. An average 

of 10 contaminants per site were 

found at 3 agricultural sites; and an 

average of 8 contaminants per site 

were found at 6 rangeland sites.

The total organochlorine 

pesticide concentrations (sum of 

concentrations of all pesticides 

detected at each site) in some 

urban sediment samples were 

among the highest of 836 NAWQA 

stream sites nationwide. Total 

concentrations at 3 of the 6 urban 

sites ranked in the top 12 percent. 

In contrast, no pesticides were 

detected in sediment from 2 of 

the 3 agricultural streams, and 

none were detected in sediment 

from any of the 6 rangeland sites. 

PCBs were not detected in any 

stream-sediment samples.

The stream-sediment samples 

also were analyzed for 44 trace 

elements. Although the frequencies 

of detection were similar among 

urban, agricultural, and rangeland 

sites (23 to 28 per site), the concentrations generally were highest in 

urban sediment (table 2).

For the 27 organochlorine 

pesticides and total PCBs analyzed 

in wholebody and carcass (wholebody without liver) fish tissue 

from nine sites, the greatest 

number of contaminants, and 

generally the highest concentrations (table 2), also were found 

in urban samples. An average of 

5 contaminants per sample were 

found in 7 urban fish-tissue 

samples; an average of 2 contaminants per sample were found in 

10 agricultural fish-tissue samples; 

and an average of 2 contaminants 

per sample were found in 4 rangeland fish-tissue samples. The fishtissue samples were composites 

from 3 to 9 fish of the same 

species, typically common carp 

(Cyprinus carpio).

The total organochlorine 

pesticide concentrations in urban 

wholebody and carcass fish-tissue 

samples ranked in the middle-tohigh range among concentrations 

in wholebody tissue from 505 

NAWQA sites nationwide. Three 

of the 7 urban sample concentrations were in the top 25 percent. 

As with total organochlorine pesticide concentrations in sediment, 

concentrations in agricultural and 

rangeland fish-tissue samples generally were lower than those in 

urban samples. Only 2 of the 

10 agricultural samples and none 

of the 4 rangeland samples contained total organochlorine pesticide concentrations in the upper 

50 percent of concentrations 

nationwide.

Total PCB concentrations in 

wholebody and carcass fish-tissue 

samples generally followed the 

same pattern. Concentrations were 

in the top 15 percent of wholebody 

PCB concentrations nationwide in 

4 of the 7 urban samples, 1 of the 

10 agricultural samples, and none 

of the 4 rangeland samples. 

Table 2. Concentrations of contaminants in urban, agricultural, and 

rangeland stream sediment and fish 

and clam tissue 

[Concentrations are average of samples from 

number of streams listed] 

Selected organochlorine pesticides and 

PAHs in sediment

[Concentrations in micrograms per kilogram, 

dry weight; <, less than]

Urban

6 sites

Agricultural

3 sites

Rangeland

6 sites

Chlordane 7.1 <1.0 <1.0

Total DDT 6.3 .64 <1.0

Total PAHs 1,327 150 90

Selected trace elements in sediment

[Concentrations in micrograms per gram, dry 

weight]

Urban

6 sites

Agricultural

3 sites

Rangeland

6 sites

Arsenic 6.1 4.0 1.7

Cadmium .5 .2 .3

Chromium 49.5 40.3 17.5

Copper 17.7 9.3 4.5

Lead 40.8 15.7 8.2

Mercury .1 .01 .1

Selenium .6 .8 .9

Zinc 93.5 60.3 74.2

Selected organochlorine compounds in 

wholebody and carcass fish tissue

[Concentrations in micrograms per kilogram, 

wet weight]

Urban

4 sites

Agricultural

3 sites

Rangeland

2 sites

Chlordane 43 7 4

Total DDT 110 26 28

Total PCBs 286 60 16

Selected trace elements in clam tissue

[Concentrations in micrograms per gram, wet 

weight]

Urban

4 sites

Agricultural

3 sites

Rangeland

5 sites

Arsenic 5.9 5.5 4.4

Cadmium 3.8 .5 .4

Chromium 3.9 2.6 1.8

Copper 60 31.0 21.8

Lead 3.3 1.7 .5

Mercury .2 .03 .1

Selenium 5.4 7.3 6.7

Zinc 277 143 164
12 Water Quality in South-Central Texas

Lorence Creek Lake

1

3

5

7

Lead

1

3

5

7

9

Total DDT

0

2

4

6

Total PCBs

NORMALIZED1 CONCENTRATION

1940 1950 1960 1970 1980 1990 2000

0

1

2

3

Total PAHs

Locations of the five lakes in addition to Lorence Creek 

Lake for which water-quality trends from sediment cores 

are graphed.

Trends in Water Quality Associated With Urban Development, as 

Reflected in Bottom Sediment of Lakes and Reservoirs, Are Not 

Unique to San Antonio

1 The graphed contaminant concentrations are “normalized” values. That is, the concentrations down each core were divided by the mean 

concentration of the top three samples in each core. This allows very different concentrations to be graphed together to show similarity in 

trends.

Urban development in watersheds is leaving similar marks on 

water quality nationwide. Changes in water quality over time as 

watersheds become more urban are recorded in the successive layers of bottom sediment in lakes and reservoirs (hereinafter, lakes). 

Soil and debris, and any attached contaminants, carried by runoff to 

the lakes settle to the bottom. By analyzing short sections of sediment cores (vertical tubes of mud) extracted from lake bottoms 

across the country, the USGS has been documenting changes in 

water quality as the watersheds become more developed. Lorence 

Creek Lake, a small suburban lake in a rapidly urbanizing area in 

northern San Antonio, fits the pattern observed nationally. Trends in 

selected contaminants from Lorence Creek Lake sediment [16] generally match those from other lakes in watersheds in different parts 

of the country that have undergone rapid urban development in 

recent years. Lead concentrations generally peaked in the mid1970s and declined appreciably as unleaded gasoline and lower 

lead content in other products became widespread. Total DDT concentrations generally followed the historical use of DDT in the United 

States, which peaked from the late 1950s to the mid-1960s and then 

declined substantially. DDT was banned in 1972. Total PCB concentrations generally peaked in the mid-1960s and declined in the 

1970s, reflecting peak production and subsequent regulation of 

PCBs in the United States. Total PAH concentrations generally have 

increased substantially since about 1970. PAHs result from the burning of hydrocarbons and other organic material. The lower concentrations in Lorence Creek Lake sediment in the 1980s are thought to 

be due to washed-in soil from subsurface layers (older and thus low 

in PAH concentration) associated with major highway construction 

adjacent to the lake.

The NAWQA Program began a national study of 

water-quality trends using age-dated bottomsediment cores in 1992. Ages of core sediment are 

dated using a combination of physical and chemical 

markers, specifically the amount of cesium-137 (a 

radioactive by-product of nuclear weapons testing) 

at selected depths in the core.
Major Findings 13 

Sabinal River Rangeland

Frio River Rangeland

Blanco River Rangeland

Guadalupe River (upper) Mixed (rangeland)

Medina River Agricultural

Geronimo Creek4 Agricultural

Guadalupe River (lower) Mixed (agricultural)

Salado Creek Urban

San Antonio River Mixed (urban)

Fish

status

Invertebrate

status

Algal

status

Selected biological indicators1 of water quality—comparison of 

upper South-Central Texas Study Unit sites with NAWQA sites2

nationwide

Watershed

land use Stream 3

 Lowest 25 percent nationally (least degraded)

 Middle 50 percent nationally

 Highest 25 percent nationally (most degraded)

1 See Glossary.

4 Not one of the NAWQA national sites.

3 Watershed land use was categorized on the basis of national and

local (study-unit) criteria. Where national and study-unit criteria yielded 

different land-use settings, the study-unit land-use setting is in parentheses.

2 Represents 140 sites in the NAWQA national basic- and intensivesite network that have algal, invertebrate, and fish data.

In clam (Corbicula) tissue from 

12 sites, the concentrations of 

trace elements generally were 

highest in the samples from urban 

sites (table 2). In fish-liver tissue 

from nine sites, however, no clear 

association between trace element 

concentration and watershed land 

use was evident. 

At some sites, concentrations 

of some sediment contaminants 

exceeded the Canadian sediment 

guidelines for the protection of 

aquatic life [17]. The guidelines 

are termed “probable effects 

levels” (PEL). The PEL is the concentration above which adverse 

effects on aquatic life are predicted 

to occur frequently. PELs have 

been established for 18 of the 43 

organochlorine compounds and 

SVOCs and 8 of the 28 trace elements detected in sediment. In one 

each of 4 urban sediment samples, 

DDT, DDE, chlordane, and lead 

concentrations exceeded the 

respective PEL; in 1 rangeland sediment sample, the mercury concentration exceeded the PEL.

Guidelines to protect fish-eating 

wildlife (for the State of New York) 

[18] have been established for 8 of 

the 10 organochlorine compounds 

detected in wholebody and carcass 

fish tissue but not for the 19 trace 

elements detected in clam and fishliver tissue. Total PCB concentrations in 6 of the 7 urban fish-tissue 

samples and 2 of the 10 agricultural 

Nationally and Locally, Biological Community Status Is Related to 

Watershed Development

Selected biological community status 

indicators of stream quality nationwide 

show that stream quality is more likely to 

be degraded in watersheds dominated by 

urban and agricultural activities than those 

that are predominantly undeveloped. Biological community status indicators in the 

Study Unit generally were consistent with 

the national results. The algal status at the 

urban site on the San Antonio River was 

the highest of all NAWQA biological sites 

and is a reflection of the high nitrate and 

phosphorus conditions at the site. The fish 

status at the site was among the highest 

25 percent of NAWQA biological sites. 

Degraded habitat conditions (mostly 

unstable sands and muds) likely contributed to the degraded condition of algal and 

fish communities. These results are consistent with the finding that the quality of 

water, bed sediment, and fish tissue in the 

San Antonio River has been affected by 

proximity to development. 

The agricultural streams Medina River, 

Geronimo Creek, and lower Guadalupe 

River generally had lower indicator scores 

than the urban streams, except those for 

algal status.

The rangeland Sabinal and Frio Rivers 

had national scores among the lowest 

25 percent of NAWQA biological sites. 

Among biological sites in the Study Unit, 

these two streams are least affected by 

urban or agricultural development. 
14 Water Quality in South-Central Texas

SAN

ANTONIO

100o 99o

98o

30o

29o

SAN ANTONIO

SAN

MARCOS

NEW

BRAUNFELS

KERRVILLE

UVALDE

0 25 50 KILOMETERS

0 25 50 MILES

Pesticides detected

VOCs detected

Both detected

Neither detected

Edwards aquifer 

recharge zone

Edwards aquifer 

confined zone

Trinity aquifer 

EXPLANATION

Ground-water 

source sampled

Water-quality

analysis result

fish-tissue samples exceeded the 

guideline for PCBs. The guideline 

for total DDT was exceeded in 1 of 

the 7 urban fish-tissue samples. 

Floodflows, Non-Native Species 

Can Affect Biological Community 

Status

 Intense rainstorms on terrain 

conducive to rapid runoff result in 

frequent floodflows in the Study 

Unit. In June 1997, about a month 

before biological samples were 

collected, severe flooding on the 

Frio and Sabinal Rivers affected 

the biological communities, particularly the invertebrates. Estimated 

recurrence intervals for the peak 

floodflows at the sampling sites on 

the Frio and Sabinal Rivers were 

15 and 90 years, respectively [19]. 

For both rivers, the invertebrate 

communities were more degraded 

in 1997 than in 1996 and 1998 

during considerably drier conditions. Changes in the percentages 

of certain algae reflected less 

siltation and reduced nutrient concentrations after scouring floodflows. The fish communities were 

not noticeably affected by the 

flooding.

More than 30 non-native aquatic 

species, many of tropical origin, 

are known to exist in the Study 

Unit [20, table 5]. Non-native 

aquatic species are considered a 

threat to native species [21]. However, the findings of this study 

show that native fish species 

remain dominant at the locations 

sampled. The redbreast sunfish 

(Lepomis auritus), originally introduced in Texas as a game fish, was 

the most common non-native fish 

species, particularly in the Blanco 

River where clear-flowing water 

favors this species.

Edwards Aquifer Water 

Quality Remains Excellent

The quality of water in the 

Edwards aquifer is “excellent”

according to the Edwards Aquifer 

Authority (EAA), the State agency 

charged with managing, conserving, preserving, and protecting 

the aquifer [22]. Comprehensive 

analyses of water samples from 

88 wells (fig. 10) (one sample 

per well) in the Edwards aquifer 

in urban, agricultural, and rangeland areas of the recharge and 

confined zones support that characterization. However, the fact that 

water samples contained detectable 

concentrations of pesticides and 

VOCs, even though the levels 

were well below allowable maximums in drinking water, shows that 

human activities can affect the 

aquifer. 

Figure 10. Fifty-eight of the Edwards aquifer wells sampled were in the recharge zone, and 30 were in the confined zone. Most 

of the recharge-zone wells were for domestic supply or monitoring, and nearly all of the confined-zone wells were for public 

supply. Almost all of the urban recharge-zone samples contained both pesticides and VOCs. Most of the 31 sampled wells from 

the upper and middle zones of the Trinity aquifer were for domestic supply. About one-third of the Trinity aquifer samples 

contained no pesticides or VOCs, and only three samples contained both.
Major Findings 15 

The occurrence of contaminants 

in the Edwards aquifer is influenced by hydrogeology and land 

use. The faulted and fractured 

limestone of the Edwards aquifer 

recharge zone allows unrestricted 

downward movement of water 

containing contaminants into the 

ground-water-flow system, 

whereas the confined zone has a 

buffer (confining unit) between 

land surface and the aquifer that 

restricts the downward movement 

of water and contaminants. (See 

box above.) Thus, within the 

recharge zone, land use noticeably 

influences water quality; but in the 

confined zone, land use has much 

less effect.

Nutrient Concentrations Were Low

Nitrate, which dissolves readily 

in water, is widespread in the 

Edwards aquifer. It was detected in 

all but 1 of 88 samples. The median 

nitrate concentration was 1.4 mg/L 

in the recharge zone and 1.7 mg/L 

in the confined zone.

Primarily public-supply wells 

were sampled in the confined 

zone. The median concentration of 

1.7 mg/L in that zone, although 

well below the USEPA MCL for 

drinking water (10 mg/L), was in 

the top 10 percent of median 

nitrate concentrations of major 

aquifers sampled by NAWQA 

nationwide. The highly permeable, 

faulted and fractured rocks of the 

recharge zone readily allow infiltration of water that contains nitrate 

and dissolved oxygen. Nitrate is 

more stable under aerobic conditions. Nitrate commonly migrates 

large distances from recharge areas 

in fractured-rock aquifers that 

contain considerable dissolved 

oxygen [26]. The median dissolved 

oxygen concentration in the confined zone was 6.0 mg/L.

Orthophosphate, which 

accounts for nearly all the dissolved phosphorus, was less 

prevalent than nitrate. Orthophosphate was detected in 49 of 88 

well-water samples. Concentrations throughout the aquifer were 

low; the median concentration was 

0.015 mg/L.

Pesticide Concentrations Were 

Substantially Less Than DrinkingWater Standards and Guidelines, 

and Detections Were Most 

Frequent in Urban Recharge-Zone 

Wells

Seventeen of the 83 pesticides 

analyzed in recharge-zone samples 

and 18 of the 47 pesticides analyzed in confined-zone samples 

were detected (fig. 10); 17 of 18 

were the same in both zones. At 

least one-half of the water samples 

with a pesticide detection contained two or more pesticides. 

The concentration of each of the 

Hydrogeology and LandUse Distribution—Good for 

Edwards Aquifer Water 

Quality

A fortuitous combination of hydrogeology and land-use distribution 

has helped to maintain the quality of 

water in the Edwards aquifer. Nearly 

all agricultural areas and much of 

San Antonio, the only urban area 

large enough to affect water quality 

regionally, overlie the confined zone 

rather than the recharge zone. In the 

confined zone, the Navarro-Del Rio 

confining unit [23], which comprises 

hundreds of feet of low-permeability 

rocks, overlies the Edwards aquifer 

and forms a regional barrier to 

vertical ground-water flow. The confining unit insulates to a large extent 

the part of the Edwards aquifer 

beneath it from the effects of landuse activities.

Additionally, the streams that provide much of the recharge to the 

aquifer originate in and flow through 

what is now mostly undeveloped 

rangeland before reaching the 

recharge zone of the aquifer; thus, 

the streams are not carrying urban or 

agricultural runoff in the recharge 

water.

Finally, the quality of Edwards 

aquifer water generally is enhanced 

by the dynamic nature of the groundwater-flow system. Since the mid1930s, the annual flow through the 

system has averaged about 680,000 

acre-feet [24, table 1]. One estimate 

of regional ground-water velocity in 

the aquifer is 27 feet/day [25, p. 82]. 

Under those conditions, contaminants tend to be diluted and have 

less chance of remaining stationary 

and accumulating than they would in 

a less dynamic system.

Modified from [23, pl. 3]. 

Trinity

aquifer

A A'

A

A'

SAN ANTONIO

BEXAR

COUNTY

Confining unit

Edwards aquifer

(recharge zone)

1,000

sea

level

1,000

2,000

FEET

Edwards aquifer

(confined zone)
16 Water Quality in South-Central Texas

7

5

14 4

13

14

Herbicides in shallow ground water in urban areas Insecticides in shallow ground water in urban areas

8

Herbicides in major aquifers Insecticides in major aquifers

The maps are based on data from 25 urban ground-water land-use studies and 63 major aquifer surveys 

from NAWQA Study Units across the country. Most samples for land-use studies were collected from 

monitor wells. Most samples for major aquifer surveys were collected from domestic and public-supply wells.

Highest 30 percent (greater than 10.70 

percent detection)

Middle 45 percent (2.60–10.70 percent detection)

Lowest 25 percent 

(less than 2.60 percent detection–no detections)

Insecticide detection frequency

Drinking-water standards or guidelines—

Bold outline indicates exceedance by 

one or more pesticide concentrations. 

Number is percentage of well-water sample

concentrations that exceeded a standard 

or guideline.

EXPLANATION

7

Herbicide detection frequency

Highest 25 percent (greater than 62.10 

percent detection)

Middle 50 percent (19.40–62.10 percent detection)

Lowest 25 percent 

(less than 19.40 percent detection–no detections)

Study-unit boundary

9

4

1

5

7

1

The Frequencies of Pesticide Detection in Ground Water Vary in 

Comparison With Those in Ground Water Nationally—

Compounds Detected Were the Same

The frequencies of detection of herbicides and insecticides in recently 

recharged urban ground water in northern San Antonio (Edwards aquifer 

recharge zone) ranked among the highest 25 and 30 percent, respectively, of frequencies of detection in urban shallow ground water across 

the country. The same four pesticides—the herbicides atrazine, deethylatrazine, prometon, and simazine—were the most frequently detected in 

both northern San Antonio ground water and urban shallow ground water 

nationwide.

 The frequency of detection of herbicides in the Edwards aquifer was 

in the middle 50 percent of major aquifers nationally, and the frequency of 

detection of herbicides in the Trinity aquifer was in the lowest 25 percent. 

For insecticides, Edwards and Trinity aquifer water ranked in the middle 

45 percent of frequencies of detection in major aquifers nationally. 

Atrazine, deethylatrazine, prometon, and p,p'-DDE were the pesticides 

most frequently detected in major aquifers both locally and nationally.

Locally, herbicides were detected at a much greater frequency than 

insecticides. As the map explanation indicates, national ground-water 

data show the same finding. 
Major Findings 17 

13 pesticides detected for which 

drinking-water standards or guidelines have been established was 

substantially less than the respective allowable maximum [27]. 

However, standards for combinations of pesticides have not been 

established, and very little is 

known about the effects of 

mixtures of pesticides on human 

health.

More pesticides were detected 

and frequencies of detection were 

greater in urban recharge-zone 

samples than in nonurban rechargezone or confined-zone samples 

(fig. 11). Pesticide usage in urban 

areas likely is higher than in 

nonurban areas in the recharge 

zone. Nonurban areas in the 

recharge zone primarily are rangeland where usage is nonexistent or 

very low. Little if any direct (downward) recharge occurs in the confined zone compared with the 

recharge zone. Although pesticide 

usage in urban and agricultural 

areas overlying the confined zone 

could be greater, fewer pesticides 

will reach the aquifer because of 

the lack of direct recharge. 

As was the case for stream 

water, the pesticide detection 

picture changes considerably 

when detection frequency is based 

on a common concentration of 

0.05 μg/L. Among urban rechargezone samples, only five pesticides 

were detected (compared with 

17 without regard to a common 

concentration). Atrazine was the 

most frequently detected but only 

in 5.6 percent of the samples. On 

the basis of the common concentration, no pesticides were detected in 

any nonurban recharge-zone or 

confined-zone samples (compared 

with 5 and 6, respectively, without 

regard to a common concentration). As in stream water, the 

majority of pesticide detections 

thus represent concentrations of 

less than 1 part contaminant per 20 

billion parts water.

The Most Frequently Detected 

Pesticides Were the Same in 

Ground Water and Surface Water 

Four of the 5 pesticides most frequently detected in ground water 

from urban recharge-zone wells—

the herbicides deethylatrazine, 

atrazine, simazine, and prometon 

(fig. 11)—were the same as 4 of 

the 5 pesticides most frequently 

detected in urban streams (Salado 

Creek at San Antonio and San 

Antonio River at Elmendorf) 

(fig. 9). Although both sites are 

Figure 11. Pesticides and VOCs in the Edwards aquifer were most frequently detected in urban recharge-zone wells. Shown 

are those pesticides and VOCs detected in more than 10 percent of samples. Only one insecticide, p,p'-DDE (shaded brown), a 

breakdown product of DDT, was detected in more than 10 percent of samples; the other pesticides shown here are herbicides. 

p,p’-DDE

Prometon

Simazine

Atrazine

Deethylatrazine Deethylatrazine

Prometon

Atrazine

Deethylatrazine

0 25 50 75 100

Methylbenzene

1,1,1-Trichloroethane

Tetrachloroethene

Trichloromethane

0 25 50 75 100

Carbon disulfide

Chloromethane

Trichloromethane

0 25 50 75 100

Bromodichloromethane

Chlorodibromomethane

Trichloromethane

36 samples 22 samples 29 samples

36 samples 22 samples 30 samples

Urban

recharge zone

Nonurban

recharge zone

Confined zone

FREQUENCY OF DETECTION, IN PERCENT

PESTICIDES

VOLATILE ORGANIC

COMPOUNDS

EDWARDS AQUIFER

0 25 50 75 100

Chloromethane

Carbon disulfide

31 samples

31 samples

Upper and

middle zones

TRINITY AQUIFER

Bromodichloromethane

Dichloromethane

2-Butanone

Tribromomethane Tetrachloroethene

Iodomethane Tetrachloroethene
18 Water Quality in South-Central Texas

The Edwards Aquifer Harbors Diverse Subterranean and Unique Spring-Dependent Aquatic 

Species

The Edwards aquifer contains one of the most diverse 

subterranean biological communities in the world. At least 

43 species of underground aquatic animals can be found 

[21], including two species of blind catfish (a predator and 

a forager) living several hundred feet below the surface at 

San Antonio [28]. The blind catfishes’ highly modified 

adaptations to cave life, including degree of eye reduction, 

are evidence that these animals are among the oldest of 

cave fish, having entered the ground-water system as long 

as 20 million years ago. Organic matter brought in from 

distant recharge areas would not be sufficient to support 

these fish and the other animals living in the deep parts of 

the aquifer. Rather, the aquifer is sustaining an ancient 

self-contained community, with hydrogen-sulfide-fixing 

bacteria possibly the primary food source.

An additional 47 surface-water species are found only 

in the aquifer’s associated springs and spring runs. With 

this high diversity of unique, geographically restricted 

organisms, it is not surprising that 13 of the subterranean 

and spring-dwelling species—5 salamanders, 4 fish, 

3 invertebrates, and 1 plant—are federally or State-listed 

as endangered or threatened. The spring-dwelling species 

are dependent on the consistent quantity and quality of 

flow from Comal and San Marcos Springs for their survival. 

The colorless, eyeless, and gilled Texas blind salamander 

(Tyhlomolge rathbuni) is an endangered species that lives 

in the subterranean waters below San Marcos. 

(Photograph by J.N. Fries, U.S. Fish and Wildlife Service.)

downstream from the recharge 

zone, the contaminants detected 

at the sites likely are typical of 

contaminants in urban runoff in 

northern San Antonio, which is in 

the recharge zone.

Atrazine and deethylatrazine 

were the most frequently detected 

pesticides in Edwards aquifer water 

and were among the top three most 

frequently detected pesticides in 

stream water. Although atrazine 

was detected in more than threefourths of urban recharge-zone 

wells, the maximum measured 

concentration was about 23 times 

less than the drinking-water MCL, 

3 μg/L. No drinking-water standard 

or guideline has been established 

for deethylatrazine. 

Two Volatile Organic Compounds 

Were Frequently Detected at Low 

Concentrations

Thirty-four of 86 VOCs analyzed were detected in samples 

from Edwards aquifer wells. 

Unlike pesticides, the fewest 

VOCs (12) were detected in 

urban recharge-zone samples. Sixteen were detected in nonurban 

recharge-zone samples, and 27 

were detected in confined-zone 

samples. In general, however, 

frequencies of detection were 

greatest in urban recharge-zone 

samples.

Trichloromethane, the most 

frequently detected VOC, was 

detected in three-fourths of the 

urban recharge-zone and confinedzone samples (fig. 11). Measured 

concentrations of trichloromethane 

were very low; the largest was 

about 80 times less than the 

drinking-water MCL, 100 μg/L. 

Tetrachloroethene also was 

frequently detected. It was detected 

in three-fourths of the urban 

recharge-zone samples. The 

largest measured concentration 

of tetrachloroethene was about 

12 times less than the drinkingwater MCL, 5 μg/L. 

When detection frequency is 

based on a common concentration 

of 0.1 μg/L, only 7 VOCs were 

detected—5 in urban rechargezone samples, 3 in nonurban 

recharge-zone samples, and 5 in 

confined-zone samples. Trichloromethane remained the most frequently detected VOC, but it was 

detected only in about 20 percent 

of urban and nonurban rechargezone samples. No other VOC was 

detected in more than 10 percent 

of recharge-zone or confined-zone 

samples.

MTBE, a gasoline additive of 

recent concern because of its 

potential to contaminate ground 

water, was detected in 2 samples, 

1 from the urban recharge zone and 

1 from the confined zone. Concentrations were more than 200 times 

less than the lifetime health advisory, 20 μg/L.
Major Findings 19 

0.001

0.01

0.1

10

100

1,000

10,000

1

CONCENTRATION, IN MILLIGRAMS PER LITER

RADON, IN PICOCURIES PER LITER

CONSTITUENT

Chloride

Dissolved solids

Fluoride

Iron

Nitrate

Orthophosphate

Radon

Strontium

Sulfate

Standard or guideline

Edwards aquifer

Trinity aquifer

Lead

Arsenic

1

10

100

10,000

100,000

1,000,000

10,000,000

1,000

CONCENTRATION, IN MICROGRAMS PER LITER

Analyses Show Very Low 

Concentrations of Arsenic—

Detections of Lead Could Be 

Related to Well Construction

Arsenic, ranked first on the 

Agency for Toxic Substances and 

Disease Registry (ATSDR) and 

USEPA 1999 list of priority hazardous substances [29], was 

detected at concentrations many 

times less than the current (2000) 

drinking-water MCL of 50 μg/L 

(fig. 12). The median concentration 

of this naturally occurring element 

was about 1 μg/L, still less than a 

proposed new standard of 5 μg/L 

that is being considered for adoption by the USEPA in 2001. 

Lead, ranked second on the 

ATSDR and USEPA 1999 list 

of priority hazardous substances, 

also was detected in Edwards aquifer samples. Lead concentrations 

ranged from 1 to 9 μg/L with a 

median of about 2 μg/L (fig. 12), 

well below the drinking-water 

action level for lead, 15 μg/L. 

Lead was detected only in previously existing domestic and publicsupply wells and not in any of 30 

PVC monitor wells. The monitor 

wells were constructed in the urban 

recharge zone in cooperation with 

the Edwards Aquifer Authority as a 

part of the 1996–98 assessment. 

This finding indicates that detections of lead could be related to 

metal parts of the wells or pumps.

Radon Was Prevalent in the 

Edwards Aquifer, But 

Concentrations Were Low 

Compared With Other Study Units

Radon is a colorless, odorless, 

radioactive gas that forms naturally 

from uranium in rocks. Ground 

water in contact with some rock 

types—for example, light-colored 

volcanic rocks, granites, and 

dark-colored shales—can contain 

elevated concentrations of radon 

[30].

Radon was detected in 41 of 

58 wells in the Edwards aquifer. 

(Radon was not analyzed in samples from the 30 monitor wells 

constructed in the urban recharge 

zone.) Concentrations ranged 

from 80 to 780 picocuries per 

liter (pCi/L), with a median concentration of 150 pCi/L. The 75thpercentile radon concentration in 

samples from wells completed in 

the Edwards and Trinity aquifers 

ranked 32 among 35 NAWQA 

Study Units nationwide.

Radon dissolved in water generally poses a smaller health risk than 

radon in indoor air, which has been 

linked to lung cancer in humans 

[31]. The USEPA has proposed an 

MCL for radon in drinking water of 

300 pCi/L and an alternative MCL 

of 4,000 pCi/L, the higher level 

applicable when accompanied by a 

Figure 12. A comparison of concentrations in samples from wells completed in the Edwards and Trinity aquifers shows that, in 

general, the Trinity aquifer contains higher concentrations of dissolved constituents than the Edwards aquifer.
20 Water Quality in South-Central Texas

mitigation program to address 

radon risks in indoor air. About 

25 percent of the sample concentrations from the Edwards aquifer 

exceeded the proposed MCL of 

300 pCi/L. 

Trinity Aquifer Water Quality 

Is Mostly Unaffected by 

Human Activities

As in the Edwards aquifer, the 

presence of pesticides and VOCs 

in ground water of the upper and 

middle zones of the Trinity aquifer 

is evidence that human activities 

can affect the aquifer; but as of 

the late 1990s, the effects of human 

activities were minimal. The concentrations of these contaminants 

were well below drinking-water 

standards and guidelines, and the 

water quality of the aquifer remains 

influenced primarily by the natural 

processes of water interacting 

with surrounding rock. Concentrations of some of the products of 

these natural processes—dissolved 

solids, sulfate, and iron—exceeded 

nonenforceable guidelines related 

to esthetic effects in drinking 

water in some samples; some 

concentrations of strontium and 

radon exceeded a lifetime health 

advisory (strontium) and a proposed drinking-water standard 

(radon).

In the largely undeveloped Hill 

Country, 28 of 31 mostly domestic 

wells sampled were in rangeland 

settings, 2 were in urban settings, 

and 1 was in an agricultural setting. 

Because of the predominance of 

rangeland, the hydrogeologic 

characteristics of the aquifer, and 

the depth to water—the median 

depth to water in sampled wells 

was 209 feet—land use probably 

has not been a major influence 

regionally on aquifer water quality.

Natural Water Chemistry Affects 

Water Quality

The 31 Trinity aquifer wells 

yielded hard water that generally 

was high (greater than 500 mg/L) 

in dissolved solids and rich in calcium, bicarbonate, magnesium, 

and sometimes sulfate. The concentrations of some common 

constituents (fig. 12) illustrate 

the chemical variability of Trinity 

aquifer water and, in large part, 

reflect the mineral composition 

of the rocks that compose the aquifer. Dissolved solids concentrations 

in 19 of 31 samples were greater 

than the USEPA nonenforceable 

Although Both the Trinity 

and Edwards Aquifers 

Predominantly are 

Limestone, Hydrogeologic 

Differences Contribute to 

Differences in Water 

Quality

Trinity aquifer water generally is 

more mineralized than Edwards 

aquifer water (fig. 12); Edwards 

aquifer water tends to contain higher 

nitrate concentrations and more 

pesticides and VOCs. 

Some of the hydrogeologic characteristics that result in natural mineralization of Trinity aquifer water—

low permeability and a sluggish flow 

system—tend to slow the downward 

movement of water that could contain contaminants. Trinity aquifer 

rocks are less permeable than those 

of the Edwards aquifer, with fewer 

faults and fractures to provide pathways for water movement. Water 

flows more slowly in the Trinity 

aquifer than in the Edwards aquifer, 

which allows more time for minerals 

in the Trinity aquifer rocks to dissolve. Tritium concentrations from 

more than one-half the Trinity aquifer water samples indicate that 

the water is older (at least 50 years) 

than most of the Edwards aquifer 

water. Most of the Edwards aquifer 

water samples had tritium concentrations that indicate recharge within 

the last decade. (Tritium is a radioactive isotope of hydrogen that is 

particularly suited for recharge 

studies because it enters the hydrologic cycle as part of the water molecules, and its concentrations in 

precipitation were increased substantially by atmospheric nuclear 

testing during the 1950s and early 

1960s.)

Sluggish flow associated with low 

permeability implies that the downward movement of water, and potentially the downward movement of 

contaminants, is much slower in the 

Trinity aquifer than in the Edwards 

aquifer recharge zone. Downward 

flow in the Trinity aquifer is further 

restricted, and diverted laterally to 

incised streams, by resistant beds of 

shale, marl, and limestone, particularly in the upper zone; similar 

impediments to downward flow are 

not present in the Edwards aquifer 

recharge zone. 

A typical Edwards aquifer sample—

more permeable than the Trinity 

aquifer, even without faults or 

fractures.

A typical Trinity aquifer sample—

conspicuously lacking the 

permeability of the Edwards aquifer.
Major Findings 21 

drinking-water guideline of 500 

mg/L. Five of 31 sulfate sample 

concentrations exceeded a similar 

nonenforceable guideline of 250 

mg/L. Iron was detected in 23 of 

31 samples, and concentrations 

exceeded the nonenforceable 

guideline of 0.3 mg/L in 7 of the 

samples. Strontium, which was 

detected in each of 29 samples, 

exceeded the USEPA lifetime 

health advisory level of 17 mg/L in 

2 of the 29 samples. 

Nutrient Concentrations Were Very 

Low

Nitrate was detected in 27 of the 

31 well-water samples but generally at very low concentrations. 

The median concentration was 

0.12 mg/L. Although the water that 

recharges the Trinity aquifer and 

much of the water that recharges 

the Edwards aquifer originate in 

the same region—the Edwards Plateau—the median nitrate concentration in Trinity aquifer samples 

was about 14 times less than that in 

Edwards aquifer recharge-zone 

samples. The difference largely is 

attributable to the ease with which 

water flows vertically to the subsurface in the Edwards aquifer 

recharge zone relative to that in the 

Trinity aquifer. 

Orthophosphate was detected in 

16 of the 31 well-water samples. 

As with nitrate, concentrations 

were very low. The median concentration was 0.015 mg/L. 

Few Pesticides Were Detected—

And Concentrations Were Very 

Low

Only 4 of 83 pesticides analyzed 

were detected in 7 of the 31 

Trinity aquifer well-water samples 

(fig. 10). Atrazine was detected in 

3 samples, prometon in 2 samples, 

and chlorpyrifos and diazinon in 

1 sample each. Unlike Edwards 

aquifer and national NAWQA findings that show that pesticides 

commonly occur in mixtures of 

several compounds [13], none of 

the samples contained more than 

one pesticide. 

The pesticide concentration in 

each sample was very low—at 

or near the minimum reporting 

level—and tens of times less than 

the applicable drinking-water 

standard or guideline. When detection frequency is based on a common concentration of 0.05 μg/L, no 

pesticides were detected in any 

Trinity aquifer water sample.

VOCs Were Detected More 

Frequently Than Pesticides—

Also at Very Low Concentrations

Analyses of water samples from 

31 Trinity aquifer wells detected 

16 VOCs of the 86 analyzed. Carbon disulfide was the VOC most 

frequently detected (fig. 11); it was 

detected in 11 of 31 samples.

None of the eight VOCs 

detected for which drinking-water 

standards or guidelines have been 

established had concentrations 

near those standards or guidelines. 

MTBE was not detected in any 

sample. On the basis of a common 

concentration of 0.1 μg/L, the number of VOCs detected dropped 

from 16 to 3, which reiterates the 

fact that VOC concentrations were 

very low. The three VOCs were 

detected in only one sample each.

Arsenic and Lead Concentrations 

Were Low; Radon Concentrations 

Were Higher Than in Edwards 

Aquifer

As in the Edwards aquifer, 

arsenic and lead were detected 

in the Trinity aquifer at low concentrations relative to current 

(2000) or proposed drinking-water 

standards. All arsenic concentrations were less than 2 μg/L 

(fig. 12); the median lead concentration was 2.1 μg/L. Whether lead 

is actually in the aquifer or was 

introduced by metal parts associated with the wells is unknown.

Radon was detected in 30 of 

31 Trinity aquifer water samples 

in concentrations throughout a 

range similar to that in Edwards 

aquifer samples (fig. 12). The 

median concentration, 295 pCi/L, 

was about twice that in Edwards 

aquifer samples and about the 

same as the USEPA-proposed 

MCL of 300 pCi/L. The median 

radon concentration of Trinity 

aquifer samples is greater than 

that of Edwards aquifer samples 

probably because granitic rocks 

north of the Hill Country, the likely 

source of the radon, are closer to 

the Trinity aquifer than to the 

Edwards aquifer.

NAWQA sampling protocols and 

procedures are detailed, exacting, 

and identical nationwide to reduce 

inconsistencies and enhance the 

quality of data for use in spatial and 

trend analysis. Here, a hydrologist is 

ensuring that the chemistry of the water

is stable and representative of the 

aquifer before actual sampling of a 

domestic well begins.
22 Water Quality in South-Central Texas

General water chemistry—

Basic sites

Pesticides and VOCs—

Intensive sites

Contaminants in bed sediment—

Bed-sediment sites

Contaminants in fish tissue—

Fish-tissue sites

EXPLANATION

Stream material sampled

East Central Texas Plains

Three reaches

Central Texas Plateau

Southern Texas Plains

Texas Blackland Prairies

Single reach

Ecoregions—Modified from [32]

Ecological sites

EXPLANATION

SAN ANTONIO

SAN

ANTONIO

San Antonio land-use study site—

Edwards aquifer recharge zone

(see inset map)

Trinity aquifer and upper- and 

middle-zone sampling site

Edwards aquifer confined zone

and sampling site

Edwards aquifer recharge zone

and sampling site

Ground-water source and

water-quality survey site

EXPLANATION

STUDY UNIT DESIGN

Stream Chemistry—Basic and intensive sites were selected primarily to assess the occurrence and distribution of dissolved 

compounds in stream water. Basic sites were sampled less frequently and for fewer compounds than intensive sites. Intensive 

sites were sampled to evaluate the seasonal effects of land use on water quality and to determine the occurrence of pesticides 

and VOCs. Sampling of streambed sediments and fish tissue was done to assess the occurrence and distribution of organic 

compounds and trace elements.

Stream Ecology—The primary objective of the stream ecology component was to assess surface-water quality by integrating 

physical, chemical, and biological factors. Therefore, the ecological sites were primarily the same as the basic sites. Ecological 

sites, some with one reach and some with three reaches, were distributed among different land uses and ecological regions.

Ground-Water Chemistry—Aquifer surveys were done to provide a broad assessment of water quality in the Edwards aquifer 

(recharge and confined zones) and the Trinity aquifer (upper and middle zones). The aquifer surveys involved sampling 

primarily existing domestic and public-supply wells. The primary objective of the land-use study was to characterize the effects 

of urban land use on the quality of recently recharged ground water in the Edwards aquifer. A second objective was to learn 

more about the human and natural factors that affect ground-water quality.
Study Unit Design 23 

SUMMARY OF DATA COLLECTION IN THE UPPER PART OF THE SOUTH-CENTRAL TEXAS STUDY UNIT, 1996–98

Study

component What data were collected and why Types of sites sampled Numberof sites

Sampling frequency

and period

Stream Chemistry

Basic sites—

General water 

chemistry

Streamflow, field parameters,1 major ions, 

nutrients, organic carbon, and suspended 

sediment to determine concentrations and 

seasonal variations

1

 Field parameters are water temperature, specific conductance, dissolved oxygen, alkalinity, and pH.

Streams draining basins ranging in size 

from 130 to 3,500 square miles and 

representing urban, agricultural, and 

rangeland use

9 Monthly plus storms; 

April 1996–June 1998

Intensive sites—

Pesticides and 

VOCs

Constituents for basic sites plus 83 pesticides and 

86 VOCs to determine concentrations and 

seasonal variations

A subset of basic sites draining urban 

and agricultural land-use areas 

3 Weekly to monthly plus 

storms; January 1997–

March 1998

Bed-sediment 

sites—

Contaminants in 

bed sediment

Total PCBs, 32 organochlorine pesticides, 63 

SVOCs, and 44 trace elements to determine 

occurrence and spatial distribution

Depositional zones of all basic and 

intensive sites, and six additional 

similar sites for increased spatial 

distribution

15 Once; August 1995,2

November 1996, 

November 1997, or 

February 1998

2 A few data were collected before the 1996–98 sampling period. 

Fish-tissue sites—

Contaminants in 

fish tissue

Total PCBs, 27 organochlorine pesticides, and 22 

trace elements in fish and clam tissue to 

determine occurrence 

All basic and intensive sites, and five 

additional similar sites for increased 

spatial distribution

14 Once at 8 sites, 2–3 times 

at 6 sites; selected 

months August 

19952

–February 1998

Stream Ecology

Single-reach 

assessment

Fish, macroinvertebrates, algae, and aquatic and 

riparian habitat surveys to assess ecological 

conditions at a single stream reach

Stream reaches collocated with basic 

sites draining urban, agricultural, and 

rangeland areas

9 Yearly for 3 years; July–

August 1996, 1997, 

1998

Multiple-reach 

assessment

Fish, macroinvertebrates, algae, and aquatic and 

riparian habitat surveys to assess ecological 

conditions at three stream reaches

Stream reaches collocated with a subset 

of basic sites draining urban, 

agricultural, and rangeland areas

4 Once; July–August 1996

Ground-Water Chemistry

Aquifer survey—

Edwards aquifer 

recharge zone

Field parameters,1 turbidity, major ions, nutrients, 

organic carbon, trace elements, 83 pesticides, 86 

VOCs, radon, and tritium to describe the spatial 

distribution of ground-water quality 

Primarily existing open-hole domestic 

wells less than 600 feet deep, selected 

using a statistically based random 

process

28 Once; June–August 1996

Aquifer survey— 

Edwards aquifer 

confined zone

Field parameters,1 turbidity, major ions, nutrients, 

organic carbon, trace elements, 47 pesticides, 86 

VOCs, radon, and tritium to describe the spatial 

distribution of ground-water quality 

Primarily existing open-hole publicsupply wells 400–2,700 feet deep, 

selected using a statistically based 

random process

30 Once; June–August 1997

Aquifer survey—

Trinity aquifer 

upper and middle 

zones 

(undifferentiated)

Field parameters,1 turbidity, major ions, nutrients, 

organic carbon, trace elements, 83 pesticides, 86 

VOCs, radon, and tritium to describe the spatial 

distribution of ground-water quality 

Primarily existing open-hole domestic 

wells less than 800 feet deep, selected 

using a statistically based random 

process

31 Once; June–August 1996

Land-use study—

Edwards aquifer 

recharge zone in 

San Antonio

Field parameters,1 turbidity, major ions, nutrients, 

organic carbon, trace elements, 83 pesticides, 86 

VOCs, and chlorofluorocarbons to assess effects 

of urban land use on the quality of recently 

recharged ground water

Monitor wells 180–320 feet deep, 

constructed in light-commercial and 

residential land-use areas in 

metropolitan San Antonio at sites 

selected using a statistically based 

random process 

30 Once; October–

December 1998

Special Studies

Paired watershed 

study in the 

Edwards aquifer 

recharge zone

Streamflow, specific conductance, alkalinity, pH, 

major ions, nutrients, suspended sediment, trace 

elements, and 83 pesticides to compare quality 

of stormwater runoff from an urbanizing 

watershed and a rangeland watershed

Two small (less than 2 square miles) 

watersheds in the Edwards aquifer 

recharge zone: one an urbanizing 

watershed in northern San Antonio 

and the other a rangeland watershed 

in Uvalde County

2 Eight storms; August 

1996–February 1998

Lorence Creek Lake 

bottom-sediment 

core study

Selected trace elements, organochlorine 

compounds, and PAHs to determine historical 

occurrence of contaminants in the watershed of a 

small (4 acres) lake in suburban San Antonio

Two cores collected at the center of the 

lake (deepest part) 

1 Once; August 1996

Guadalupe River 

Basin study

Streamflow, field parameters,1 major ions, 

nutrients, organic carbon, suspended sediment, 

trace elements, bacteria, and 83 pesticides to 

assess possible effects of recreation, 

urbanization, and agriculture in the basin

Selected reaches upstream and 

downstream from cities and 

tributaries

21 Twice; December 1996–

January 1997, and 

June 1997 (some 

sites) and June 1998 

(remaining sites)
24 Water Quality South-Central Texas

Action level—A concentration that, when reached, triggers 

public water systems to take treatment steps if the 

action level is exceeded in more than 10 percent of tapwater samples.

Algae—Chlorophyll-bearing nonvascular, primarily aquatic 

species that have no true roots, stems, or leaves; most 

algae are microscopic, but some species can be as large 

as vascular plants.

Aquatic-life guideline—Specific level of water quality 

which, if reached, may adversely affect aquatic life. 

Aquatic-life guidelines are nonenforceable and are 

issued by a governmental agency or other institution.

Base flow—Sustained, low flow in a stream; ground-water 

discharge is the source of base flow in most places.

Biological indicator—A quantitative measure of biological 

conditions that may reflect habitat disturbance, chemical contamination, or naturally harsh conditions. The 

status of algae, invertebrates (insects, worms, and 

clams), or fish provides a record of water quality and 

stream conditions that water-chemistry indicators might 

not reveal. Algal status focuses on changes in the percentage of certain algae in response to increasing siltation and higher nutrient concentrations in many 

regions. Invertebrate status averages 11 metrics that 

summarize changes in richness (number of taxa), tolerance, trophic conditions, and dominance associated 

with water-quality degradation. Fish status sums the 

scores of four metrics (percentages of tolerant, omnivorous, and non-native individuals, and percentage of 

individuals with anomalies) that increase in response to 

water-quality degradation. Indicator scores increase as 

habitat disturbance, chemical contamination, or harsh 

conditions increase.

Breakdown product—A compound derived by chemical, 

biological, or physical action upon a pesticide. The 

breakdown is a natural process which may result in a 

more toxic or a less toxic compound and a more persistent or less persistent compound.

Carbonate rocks—Rocks (such as limestone or dolostone) 

that are composed primarily of minerals (such as calcite 

and dolomite) containing the carbonate ion (CO3

2-). 

Community—In ecology, the species that interact in a common area.

Confined aquifer (artesian aquifer)—An aquifer that is 

completely filled with water under pressure and that is 

overlain by material that restricts the movement of 

water.

Confining unit—A layer of sediment or lithologic unit of 

low permeability that bounds an aquifer.

Constituent—A chemical or biological substance in water, 

sediment, or biota that can be measured by an analytical 

method.

Drinking-water standard or guideline—A threshold concentration in a public drinking-water supply, designed 

to protect human health. As defined here, standards are 

U.S. Environmental Protection Agency regulations that 

specify the maximum contamination levels for public 

water systems required to protect the public welfare; 

guidelines have no regulatory status and are issued in an 

advisory capacity.

Ecoregion—An area of similar climate, landform, soil, 

potential natural vegetation, hydrology, or other ecologically relevant variables.

Eutrophication—The process by which water becomes 

enriched with plant nutrients, most commonly nitrogen 

and phosphorus. 

Lifetime health advisory—An advisory guideline for 

drinking-water exposure over a 70-year lifetime, considering noncarcinogenic adverse health effects.

Maximum contaminant level (MCL)—Maximum permissible level of a contaminant in water that is delivered to 

any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental 

Protection Agency. 

Minimum reporting level—The smallest measured concentration of a constituent that may be reliably reported 

using a given analytical method. 

Monitor well—A well designed for measuring water levels 

and testing ground-water quality.

Outcrop—That part of a geologic formation that is exposed 

at land surface.

Organochlorine compound—Synthetic organic compounds containing chlorine. As generally used, term 

refers to compounds containing mostly or exclusively 

carbon, hydrogen, and chlorine. Examples include organochlorine insecticides, polychlorinated biphenyls, and 

some solvents containing chlorine.

Organochlorine insecticide—A class of organic insecticides containing a high percentage of chlorine. Includes 

dichlorodiphenylethanes (such as DDT), chlorinated 

cyclodienes (such as chlordane), and chlorinated benzenes (such as gamma-HCH). Most organochlorine 

insecticides were banned because of their carcinogenicity, tendency to bioaccumulate, and toxicity to wildlife.

Permeability—A measure of the relative ease of fluid flow 

in porous rocks.

Picocurie (pCi)—One trillionth (1 x 10-12) of the amount of 

a radioactive nuclide represented by a curie (Ci). A 

curie is the quantity of any radioactive nuclide that 

yields 3.7 x 1010 radioactive disintegrations per second 

(dps). A picocurie yields 0.037 dps, or 2.22 disintegrations per minute.

Polychlorinated biphenyls (PCB)—A mixture of chlorinated derivatives of biphenyl, marketed under the trade 

name Aroclor with a number designating the chlorine 

content (such as Aroclor 1260). PCBs were used in 

transformers and capacitors for insulating purposes and 

GLOSSARY
Glossary and References 25

in gas pipeline systems as a lubricant. Further sale for 

new use was banned by law in 1979.

Polycyclic aromatic hydrocarbon (PAH)—A class of 

organic compounds with a fused-ring aromatic structure. PAHs result from incomplete combustion of 

organic carbon (including wood), municipal solid 

waste, and fossil fuels, as well as from natural or 

anthropogenic introduction of uncombusted coal and 

oil. PAHs include benzo(a)pyrene, fluoranthene, and 

pyrene.

Recharge—Water that infiltrates the ground and reaches the 

saturated zone.

Semivolatile organic compound (SVOC)—Operationally 

defined as a group of synthetic organic compounds 

that are solvent-extractable and can be determined by 

gas chromatography/mass spectrometry. SVOCs 

include phenols, phthalates, and polycyclic aromatic 

hydrocarbons.

Species (taxa) richness—The number of species (taxa) 

present in a defined area or sampling unit.

Subcrop—That part of a geologic formation that is buried; 

that is, not exposed at land surface.

Tolerant species—Those species that are adaptable to (tolerant of) human alterations to the environment and often 

increase in number when human alterations occur.

Trace element—An element found in only minor amounts 

(concentrations less than 1.0 milligram per liter) in 

water or sediment; includes arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc.

Volatile organic compound (VOC)—An organic chemical 

that has a high vapor pressure relative to its water 

solubility. VOCs include components of gasoline, fuel 

oils, and lubricants, as well as organic solvents, fumigants, some inert ingredients in pesticides, and some 

by-products of chlorine disinfection.

Water-quality guideline—Specific level of water quality 

which, if reached, might adversely affect human health 

or aquatic life. Water-quality guidelines are nonenforceable and are issued by a governmental agency or other 

institution. 

Water-quality standard—State-adopted and U.S. Environmental Protection Agency-approved ambient standard 

for water bodies. Standards include the use of the water 

body and the water-quality criteria that must be met to 

protect the designated use or uses.

1. Vogelmann, J.E., Sohl, T., Campbell, P.V., and Shaw, 

D.M., 1998, Regional land cover characterization using 

Landsat thematic mapper data and ancillary data 

sources: Environmental Monitoring Assessment, v. 51, 

p. 415–428.

2. Schultz, A.L., 1994, 1994 review and update of the 

position of the Edwards aquifer freshwater/saline-water 

interface from Uvalde to Kyle, Texas: Edwards Underground Water District Report 94–05, 31 p.

3. Caran, S.C., Woodruff, C.M. Jr., and Thompson, E.J., 

1982, Lineament analysis and inference of geologic 

structure—Examples from the Balcones/Ouachita trend 

of Texas: Austin, University of Texas, Bureau of Economic Geology Geological Circular 82–1, 11 p.

4. Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of 

Texas: Texas Water Development Board Report 345, 

69 p.

5. Bomar, G.W., 1983, Texas weather: Austin, Texas, 

University of Texas Press, 265 p.

6. Ging, P.B., 1999, Water-quality assessment of southcentral Texas—Descriptions and comparisons of 

nutrients, pesticides, and volatile organic compounds 

at three intensive fixed sites, 1996–98: U.S. Geological 

Survey Water-Resources Investigations Report 

99–4155, 24 p.

7. Mueller, D.K., and Helsel, D.R., 1996, Nutrients in the 

Nation’s waters—Too much of a good thing?: U.S. 

Geological Survey Circular 1136, 24 p.

8. Mueller, D.K., Hamilton, P.A., Helsel, D.R., Hitt, K.J., 

and Ruddy, B.C., 1995, Nutrients in ground water and 

surface water of the United States—An analysis of data 

through 1992: U.S. Geological Survey Water-Resources 

Investigations Report 95–4031, 74 p.

9. Extension Toxicology Network (ETOXNET), 1996, 

Pesticide information profiles: Accessed January 19, 

2000, at URL 

http://ace.orst.edu/info/etoxnet/

10. Baumann, P.A., and Ketchersid, Mary, 1998, Some 

facts about atrazine: The Texas A&M University 

System, Texas Agricultural Extension Service, 

L–5240, 2 p.

11. Aspelin, A.L., and Grube, A.H., 1999, Pesticides 

industry sales and usage—1996 and 1997 market 

estimates: U.S. Environmental Protection Agency 

Office of Prevention, Pesticides and Toxic Substances, 

733–R–99–001, 46 p.

REFERENCES 
26 Water Quality South-Central Texas

12. Pesticide Management Education Program, 1999, 

Prometon (Pramitol) herbicide profile: Cornell 

University, accessed January 19, 2000, at URL 

http://pmep.cce.cornell.edu/profiles/herb-growthreg/

naa-rimsulfuron/prometon/herb-prof-prometon.html

13. U.S. Geological Survey, 1999, The quality of our 

Nation’s waters—Nutrients and pesticides: U.S. 

Geological Survey Circular 1225, 82 p.

14. Canadian Council of Ministers of the Environment, 

1999, Canadian water quality guidelines for the protection of aquatic life: Summary table in Canadian environmental quality guidelines, 1999, Canadian Council 

of Ministers of the Environment, Winnipeg.

15. earthsystems.org, 2000, Chloroform: Accessed 

February 25, 2000, at URL 

http://earthsystems.org/gopher/toxics/Chloroform

16. Ging, P.B., Van Metre, P.C., and Callender, Edward, 

1999, Bottom sediments of Lorence Creek Lake, San 

Antonio, Texas, reflect contaminant trends in an urbanizing watershed: U.S. Geological Survey Fact Sheet 

FS–149–99, 4 p.

17. Canadian Council of Ministers of the Environment, 

1999, Canadian sediment quality guidelines for the protection of aquatic life: Summary tables in Canadian 

environmental quality guidelines, 1999, Canadian 

Council of Ministers of the Environment, Winnipeg.

18. Newell, A.J., Johnson, D.W., and Allen, L.K., 1987, 

Niagara River biota contamination project—Fish flesh 

criteria for piscivorous wildlife: New York State 

Department of Environmental Conservation, Division 

of Fish and Wildlife, Bureau of Environmental Protection, Technical Report 87–3, 182 p.

19. Raines, T.H., Asquith, W.H., and Brown, D.S., 1998, 

Floods in south-central Texas, June 1997: U.S. Geological Survey Fact Sheet FS–053–98, 6 p.

20. Ourso, R.T., and Hornig, C.E., 1999, Stream and aquifer biology of south-central Texas—A literature review: 

U.S. Geological Survey Open-File Report 99–243, 

38 p.

21. Bowles, D.E., and Arsuffi, T.L., 1993, Karst aquatic 

ecosystems of the Edwards Plateau region of central 

Texas, USA—A consideration of their importance, 

threats to their existence, and efforts for their conservation: Aquatic Conservation, Marine and Freshwater 

Ecosystems, v. 3, p. 317–329.

22. Esquilin, Roberto, 1999, Edwards aquifer hydrogeologic report for 1998: Accessed February 11, 2000, 

at URL 

http://www.e-aquifer.com/hydro98/first.htm

23. Barker, R.A., and Ardis, A.F., 1996, Hydrogeologic 

framework of the Edwards-Trinity aquifer system, 

west-central Texas: U.S. Geological Survey Professional Paper 1421–B, 61 p.

24. Slattery, R.N., and Brown, D.S., 1999, Recharge to and 

discharge from the Edwards aquifer in the San Antonio 

area, Texas, 1998: Accessed March 2, 2000, at URL 

http://tx.usgs.gov/reports/dist/dist-1999-01

25. Maclay, R.W., and Small, T.A., 1986, Carbonate geology and hydrology of the Edwards aquifer in the San 

Antonio area, Texas: Texas Water Development Board 

Report 296, 90 p.

26. Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood Cliffs, N.J., Prentice-Hall, 604 p.

27. NAWQA Pesticide National Synthesis Project, 2000, 

Water-quality criteria: U.S. Geological Survey, 

accessed April 26, 2000, at URL 

http://water.wr.usgs.gov/pnsp/anstrat/index.html#hdr3

28. Langecker, T.G., and Longley, Glenn, 1993, Morphological adaptations of the Texas blind catfishes 

Trogloglanis pattersoni and Satan eurystomus

(Siluriformes: Ictaluridae) to their underground environment: Copeia, v. 93, no. 4, p. 976–986.

29. Agency for Toxic Substances and Disease Registry, 

2000, 1999 CERCLA list of priority hazardous 

substances: Accessed March 23, 2000, at URL 

http://www.atsdr.cdc.gov/99list.html

30. Otton, J.K., Gunderson, L.C.S., and Schumann, R.R., 

[1993], The geology of radon: U.S. Geological Survey 

General Interest Publication, 29 p.

31. U.S. Environmental Protection Agency, 1999, Proposed 

radon in drinking water rule: U.S. Environmental 

Protection Agency Report 815–F–99–006, accessed 

May 9, 2000, at URL 

http://www.epa.gov/OGWDW/radon/fact.html

32. Omernik, J.N., 1987, Ecoregions of the conterminous 

United States: Annals of the Association of American 

Geographers, v. 77, no. 1, p. 118–125.
Water-Quality Data in a National Context 27 

Streams in agricultural areas 

Streams in urban areas

Streams and rivers draining mixed land uses 

Shallow ground water in agricultural areas

Shallow ground water in urban areas 

Major aquifers 

Detected concentration in Study Unit

Frequencies of detection, in percent. Detection frequencies 

were not censored at any common reporting limit. The lefthand column is the study-unit frequency and the right-hand 

column is the national frequency 

Not measured or sample size less than two

Study-unit sample size. For ground water, the number of 

samples is equal to the number of wells sampled

National ranges of detected concentrations, by land use, in 36 

NAWQA Study Units, 1991–98—Ranges include only samples

in which a chemical was detected. Land-use areas categorized 

nationally as mixed were categorized locally as agricultural, urban, 

or rangeland

Drinking-water quality (applies to ground water and surface water)

Protection of aquatic life (applies to surface water only)

Prevention of eutrophication in streams not flowing directly into 

lakes or impoundments

No benchmark for drinking-water quality

No benchmark for protection of aquatic life

*

**

66 38

Other herbicides detected

Alachlor (Lasso, Bronco, Lariat, Bullet) **

Benfluralin (Balan, Benefin, Bonalan) * **

Bentazon (Basagran, Bentazone) **

Bromacil (Hyvar X, Urox B, Bromax) 

Cyanazine (Bladex, Fortrol) 

DCPA (Dacthal, chlorthal-dimethyl) * **

Dicamba (Banvel, Dianat, Scotts Proturf) 

Metribuzin (Lexone, Sencor) 

Oryzalin (Surflan, Dirimal) * **

Pronamide (Kerb, Propyzamid) **

Triclopyr (Garlon, Grandstand, Redeem, Remedy) * **

Trifluralin (Treflan, Gowan, Tri-4, Trific) 

CHEMICALS IN WATER

Concentrations and detection frequencies, South-Central Texas, 

1996–98—Detection sensitivity varies among chemicals and, thus, 

frequencies are not directly comparable among chemicals. Surfacewater graphs do not include data from storm composite samples 

collected in the South-Central Texas Study Unit

Lowest

25 percent

Middle

50 percent

Highest

25 percent

National water-quality benchmarks

National benchmarks include standards and guidelines related to 

drinking-water quality, criteria for protecting the health of aquatic life, and 

a goal for preventing stream eutrophication due to phosphorus. Sources 

include the U.S. Environmental Protection Agency and the Canadian 

Council of Ministers of the Environment

|

|

|

--

This appendix is a summary of chemical concentrations 

and biological indicators assessed in South-Central Texas. 

Selected results for this Study Unit are graphically 

compared to results from as many as 36 NAWQA Study 

Units investigated from 1991 to 1998 and to national 

water-quality benchmarks for human health, aquatic life, or 

fish-eating wildlife. The chemical and biological indicators 

shown were selected on the basis of frequent detection, 

detection at concentrations above a national benchmark, 

or regulatory or scientific importance. The graphs illustrate 

how conditions associated with each land use sampled in 

South-Central Texas compare to results from across the 

Nation and how conditions compare among the several 

land uses. Graphs for chemicals show only detected 

concentrations and, thus, care must be taken to evaluate 

detection frequencies in addition to concentrations when 

comparing study-unit and national results. For example, 

simazine concentrations for monitor-well samples of 

ground water in urban areas were similar to the national 

distribution, but the detection frequency was much higher 

(57 percent compared with 18 percent).

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Pesticides in water—Herbicides

Study-unit sample size

12

Atrazine (AAtrex, Atrex, Atred, Gesaprim) 

100 88 | | 34 100 86 | | 33 100 87 | | 22

-- 40 | 0 80 30 | 30 20 18 | 88

2,4-D (Aqua-Kleen, Lawn-Keep, Weed-B-Gone) 

3 15 | | 32 3 18 | | 31 0 11 | | 20

-- <1 | 0 3 1 | 29 0 <1 | 59

Deethylatrazine (Atrazine breakdown product) * ** 100 75 34 97 62 33 91 75 22

-- 39 0 97 28 30 27 19 88

Diuron (Crisuron, Karmex, Diurex) **

0 13 | 32 50 22 | 30 15 20 | 20

-- 4 | 0 3 3 | 29 0 2 | 59

Metolachlor (Dual, Pennant) 

18 81 | | 34 39 64 | | 33 59 83 | | 22

-- 18 | 0 0 9 | 30 3 5 | 88

Prometon (Pramitol, Princep) **

62 44 | 34 100 86 | 33 95 60 | 22

-- 12 | 0 30 21 | 30 11 5 | 88

Simazine (Princep, Caliber 90) 6 61 | | 34 79 77 | | 33 68 74 | | 22

-- 21 | 0 57 18 | 30 5 5 | 88

Tebuthiuron (Spike, Tebusan) 

0 22 | | 34 100 39 | | 33 55 32 | | 22

-- 3 | 0 7 7 | 30 0 3 | 88

APPENDIX—WATER-QUALITY DATA FROM SOUTH-CENTRAL TEXAS 

IN A NATIONAL CONTEXT

For a complete view of South-Central Texas data and for additional information about specific benchmarks used, visit our Web site at http://water.usgs.gov/nawqa/. 

Also visit the NAWQA Data Warehouse for access to NAWQA data sets at http://water.usgs.gov/nawqa/data. 
28 Water Quality in South-Central Texas

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Herbicides not detected

Acetochlor (Harness Plus, Surpass) * **

Acifluorfen (Blazer, Tackle 2S) **

Bromoxynil (Buctril, Brominal) * 

Butylate (Sutan +, Genate Plus, Butilate) **

Chloramben (Amiben, Amilon-WP, Vegiben) **

Clopyralid (Stinger, Lontrel, Transline) * **

2,4-DB (Butyrac, Butoxone, Embutox Plus, Embutone) * **

Dacthal mono-acid (Dacthal breakdown product) * **

Dichlorprop (2,4-DP, Seritox 50, Lentemul) * **

2,6-Diethylaniline (Alachlor breakdown product) * **

Dinoseb (Dinosebe) 

EPTC (Eptam, Farmarox, Alirox) * **

Ethalfluralin (Sonalan, Curbit) * **

Fenuron (Fenulon, Fenidim) * **

Fluometuron (Flo-Met, Cotoran) **

Linuron (Lorox, Linex, Sarclex, Linurex, Afalon) * 

MCPA (Rhomene, Rhonox, Chiptox) 

MCPB (Thistrol) * **

Molinate (Ordram) * **

Napropamide (Devrinol) * **

Neburon (Neburea, Neburyl, Noruben) * **

Norflurazon (Evital, Predict, Solicam, Zorial) * **

Pebulate (Tillam, PEBC) * **

Pendimethalin (Pre-M, Prowl, Stomp) * **

Picloram (Grazon, Tordon) 

Propachlor (Ramrod, Satecid) **

Propanil (Stam, Stampede, Wham) * **

Propham (Tuberite) **

2,4,5-T **

2,4,5-TP (Silvex, Fenoprop) **

Terbacil (Sinbar) **

Thiobencarb (Bolero, Saturn, Benthiocarb) * **

Triallate (Far-Go, Avadex BW, Tri-allate) * 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Pesticides in water—Insecticides

Study-unit sample size

Volatile organic compounds (VOCs) in ground water

These graphs represent data from 16 Study Units, sampled from 1996 to 1998

Study-unit frequency of detection, in percent

National frequency of detection in percent Study-unit sample size

CONCENTRATION, IN MICROGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 

Other insecticides detected 

Carbofuran (Furadan, Curaterr, Yaltox) 

Chlorpyrifos (Brodan, Dursban, Lorsban) 

Terbufos (Contraven, Counter, Pilarfox) **

Insecticides not detected

Aldicarb (Temik, Ambush, Pounce) 

Aldicarb sulfone (Standak, aldoxycarb) 

Aldicarb sulfoxide (Aldicarb breakdown product) 

Azinphos-methyl (Guthion, Gusathion M) * 

Dieldrin (Panoram D-31, Octalox, Compound 497) 

Disulfoton (Disyston, Di-Syston) **

Ethoprop (Mocap, Ethoprophos) * **

Fonofos (Dyfonate, Capfos, Cudgel, Tycap) **

alpha-HCH (alpha-BHC, alpha-lindane) **

3-Hydroxycarbofuran (Carbofuran breakdown product) * **

Methiocarb (Slug-Geta, Grandslam, Mesurol) * **

Methomyl (Lanox, Lannate, Acinate) **

Methyl parathion (Penncap-M, Folidol-M) **

Oxamyl (Vydate L, Pratt) **

Parathion (Roethyl-P, Alkron, Panthion, Phoskil) * 

cis-Permethrin (Ambush, Astro, Pounce) * **

Phorate (Thimet, Granutox, Geomet, Rampart) * **

Propargite (Comite, Omite, Ornamite) * **

Propoxur (Baygon, Blattanex, Unden, Proprotox) * **

Other VOCs detected

Benzene 

Bromobenzene (Phenyl bromide) * 

Bromodichloromethane (Dichlorobromomethane) 

2-Butanone (Methyl ethyl ketone (MEK)) * 

Carbon disulfide * 

Chlorobenzene (Monochlorobenzene) 

Chlorodibromomethane (Dibromochloromethane) 

Chloroethane (Ethyl chloride) * 

Chloromethane (Methyl chloride) 

Dibromomethane (Methylene dibromide) * 

1,3-Dichlorobenzene (m-Dichlorobenzene) 

1,4-Dichlorobenzene (p-Dichlorobenzene) 

Dichlorodifluoromethane (CFC 12, Freon 12) 

1,1-Dichloroethane (Ethylidene dichloride) * 

cis-1,2-Dichloroethene ((Z)-1,2-Dichloroethene) 

Dichloromethane (Methylene chloride) 

1,2-Dichloropropane (Propylene dichloride) 

1,2-Dimethylbenzene (o-Xylene) 

1,3 & 1,4-Dimethylbenzene (m-&p-Xylene) 

1-4-Epoxy butane (Tetrahydrofuran, Diethylene oxide) * 

Carbaryl (Carbamine, Denapon, Sevin) 

12 9 | | 34 52 46 | | 33 50 16 | | 22

-- <1 | 0 7 2 | 30 0 1 | 88

p,p'-DDE 6 8 | | 34 9 2 | | 33 0 4 | | 22

-- 4 | 0 17 2 | 30 5 2 | 88

Diazinon (Basudin, Diazatol, Neocidol, Knox Out) 38 16 | | 34 73 70 | | 33 68 39 | | 22

-- <1 | 0 10 2 | 30 1 2 | 88

gamma-HCH (Lindane, gamma-BHC) 

0 1 | | 34 0 1 | | 33 57 4 | | 21

-- <1 | 0

0 <1 | 88

Malathion (Malathion) 9 5 | | 34 30 21 | | 33 14 6 | | 22

-- <1 | 0 0 <1 | 30 0 <1 | 88

Methyl tert-butyl ether (MTBE) 

-- 4 | 0 3 16 | 30 1 6 | 88

Tetrachloroethene (Perchloroethene) 

-- 18 | 0 77 29 | 30 16 16 | 88

Trichloromethane (Chloroform) 

-- 35 | 0 77 51 | 30 42 30 | 88
Water-Quality Data in a National Context 29 

CONCENTRATION, IN MILLIGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 

CONCENTRATION, IN MILLIGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 

Nutrients in water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

CONCENTRATION, IN MILLIGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Dissolved solids in water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Ethyl tert-butyl ether (Ethyl-t-butyl ether (ETBE)) * 

Ethylbenzene (Phenylethane) 

Iodomethane (Methyl iodide) * 

Isopropylbenzene (Cumene) * 

4-Methyl-2-pentanone (Methyl isobutyl ketone (MIBK)) * 

Methylbenzene (Toluene) 

2-Propanone (Acetone) * 

Tribromomethane (Bromoform) 

1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113) * 

1,1,1-Trichloroethane (Methylchloroform) 

Trichloroethene (TCE) 

Trichlorofluoromethane (CFC 11, Freon 11) 

1,2,4-Trimethylbenzene (Pseudocumene) * 

VOCs not detected

tert-Amylmethylether (tert-amyl methyl ether (TAME)) * 

Bromochloromethane (Methylene chlorobromide) 

Bromoethene (Vinyl bromide) * 

Bromomethane (Methyl bromide) 

n-Butylbenzene (1-Phenylbutane) * 

sec-Butylbenzene * 

tert-Butylbenzene * 

3-Chloro-1-propene (3-Chloropropene) * 

1-Chloro-2-methylbenzene (o-Chlorotoluene) 

1-Chloro-4-methylbenzene (p-Chlorotoluene) 

Chloroethene (Vinyl chloride) 

1,2-Dibromo-3-chloropropane (DBCP, Nemagon) 

1,2-Dibromoethane (Ethylene dibromide, EDB) 

trans-1,4-Dichloro-2-butene ((Z)-1,4-Dichloro-2-butene) * 

1,2-Dichlorobenzene (o-Dichlorobenzene) 

1,2-Dichloroethane (Ethylene dichloride) 

1,1-Dichloroethene (Vinylidene chloride) 

trans-1,2-Dichloroethene ((E)-1,2-Dichlorothene) 

2,2-Dichloropropane * 

1,3-Dichloropropane (Trimethylene dichloride) * 

trans-1,3-Dichloropropene ((E)-1,3-Dichloropropene) 

cis-1,3-Dichloropropene ((Z)-1,3-Dichloropropene) 

1,1-Dichloropropene * 

Diethyl ether (Ethyl ether) * 

Diisopropyl ether (Diisopropylether (DIPE)) * 

Ethenylbenzene (Styrene) 

Ethyl methacrylate * 

1-Ethyl-2-methylbenzene (2-Ethyltoluene) * 

Hexachlorobutadiene 

1,1,1,2,2,2-Hexachloroethane (Hexachloroethane) 

2-Hexanone (Methyl butyl ketone (MBK)) * 

p-Isopropyltoluene (p-Cymene) * 

Methyl acrylonitrile * 

Methyl-2-methacrylate (Methyl methacrylate) * 

Methyl-2-propenoate (Methyl acrylate) * 

Naphthalene 

2-Propenenitrile (Acrylonitrile) 

n-Propylbenzene (Isocumene) * 

1,1,2,2-Tetrachloroethane * 

1,1,1,2-Tetrachloroethane 

Tetrachloromethane (Carbon tetrachloride) 

1,2,3,4-Tetramethylbenzene (Prehnitene) * 

1,2,3,5-Tetramethylbenzene (Isodurene) * 

1,2,4-Trichlorobenzene 

1,2,3-Trichlorobenzene * 

1,1,2-Trichloroethane (Vinyl trichloride) 

1,2,3-Trichloropropane (Allyl trichloride) 

1,2,3-Trimethylbenzene (Hemimellitene) * 

1,3,5-Trimethylbenzene (Mesitylene) * 

Trace elements in ground water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Arsenic 

-- 58 | 0 13 36 | 30 4 37 | 89

Chromium 

-- 85 | 0 100 79 | 30 97 73 | 89

Zinc 

-- 28 | 0 3 29 | 30 90 66 | 89

CONCENTRATION, IN MICROGRAMS PER LITER

 0.01 0.1 1 10 100 1,000 10,000 100,000 

Ammonia, as N * **

45 84 44 67 86 49 68 75 99

-- 78 0 37 71 30 70 70 89

Dissolved ammonia plus organic nitrogen, as N * ** 27 78 44 61 74 49 41 62 99

-- 28 0 3 30 30 12 24 89

Dissolved nitrite plus nitrate, as N ** 100 95 | 44 98 97 | 49 100 91 | 99

-- 81 | 0 97 74 | 30 96 71 | 89

Orthophosphate, as P * **

34 79 44 94 72 49 64 74 99

-- 59 0 90 52 30 43 61 89

Total phosphorus, as P * ** 55 92 | 44 98 90 | 49 66 88 | 98

Dissolved solids * ** 100 100 44 100 100 49 100 100 99

-- 100 0 100 100 30 100 100 89
30 Water Quality in South-Central Texas

Fish tissue from streams in agricultural areas

Fish tissue from streams in urban areas

Fish tissue from streams draining mixed land uses

Sediment from streams in agricultural areas 

Sediment from streams in urban areas 

Sediment from streams draining mixed land uses

Protection of fish-eating wildlife (applies to fish tissue)

Protection of aquatic life (applies to bed sediment)

No benchmark for protection of fish-eating wildlife

No benchmark for protection of aquatic life

|

|

**

CHEMICALS IN FISH TISSUE

AND BED SEDIMENT

Concentrations and detection frequencies, South-Central Texas, 

1996–98—Detection sensitivity varies among chemicals and, thus, 

frequencies are not directly comparable among chemicals. Study-unit 

frequencies of detection are based on small sample sizes; the 

applicable sample size is specified in each graph

Lowest

25 percent

Middle

50 percent

Highest

25 percent

National benchmarks for fish tissue and bed sediment

National benchmarks include standards and guidelines related to 

criteria for protection of the health of fish-eating wildlife and aquatic 

organisms. Sources include the U.S. Environmental Protection Agency, 

other Federal and State agencies, and the Canadian Council of 

Ministers of the Environment

*

CONCENTRATION, IN MICROGRAMS PER KILOGRAM

(Fish tissue is wet weight; bed sediment is dry weight)

 0.1 1 10 100 1,000 10,000 100,000 

National ranges of concentrations detected, by land use, in 36 

NAWQA Study Units, 1991–98—Ranges include only samples

in which a chemical was detected. Land-use areas categorized 

nationally as mixed were categorized locally as agricultural, urban, 

or rangeland

Detected concentration in Study Unit

Frequencies of detection, in percent. Detection frequencies 

were not censored at any common reporting limit. The lefthand column is the study-unit frequency and the right-hand 

column is the national frequency

Not measured or sample size less than two

Study-unit sample size

66 38

--

12

Organochlorines in fish tissue (whole body)

and bed sediment

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

1 The national detection frequencies for total PCB in sediment are biased low because about 

30 percent of samples nationally had elevated detection levels compared to this Study Unit. 

See http://water.usgs.gov/nawqa/ for additional information.

Other trace elements detected

Lead 

Selenium 

Uranium 

Trace elements not detected

Cadmium

Radon-222 

-- 99 | 0 -- 100 | 0 80 97 | 89 

CONCENTRATION, IN PICOCURIES PER LITER

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

 0.01 0.1 1 10 100 1,000 10,000 100,000 

Total Chlordane (sum of 5 chlordanes) 

33 38 | 3 100 75 | 2 100 56 | 4

0 9 | 2 100 57 | 3 17 11 | 6

o,p'+p,p'-DDD (sum of o,p'-DDD and p,p'-DDD) * 

33 49 3 100 69 2 50 50 4

0 27 | 2 100 50 | 3 0 20 | 6

p,p'-DDE * ** 100 90 3 100 94 2 100 92 4

50 48 2 100 62 3 33 39 6

o,p'+p,p'-DDE (sum of o,p'-DDE and p,p'-DDE) * 100 90 3 100 94 2 100 92 4

50 48 | 2 100 62 | 3 33 39 | 6

o,p'+p,p'-DDT (sum of o,p'-DDT and p,p'-DDT) * 

0 31 3 50 53 2 0 29 4

0 19 | 2 100 38 | 3 0 11 | 6

Total DDT (sum of 6 DDTs) **

100 90 | 3 100 94 | 2 100 93 | 4

50 49 2 100 66 3 33 41 6

Dieldrin (Panoram D-31, Octalox) * 

0 53 3 50 42 2 25 38 4

0 13 | 2 0 30 | 3 17 9 | 6

Dieldrin+aldrin (sum of dieldrin and aldrin) ** 0 52 | 3 50 42 | 2 25 38 | 4

0 13 2 0 29 3 17 9 6

Total PCB 1

33 38 | 3 100 81 | 2 100 66 | 4

0 2 | 2 0 21 | 3 0 9 | 6

Pentachloroanisole (PCA) * **

0 6 3 50 14 2 0 8 4
Water-Quality Data in a National Context 31 

Other SVOCs detected

Acenaphthene 

Acenaphthylene 

Acridine **

Anthracene 

Benz[a]anthracene 

Benzo[a]pyrene 

Benzo[b]fluoranthene **

Benzo[ghi]perylene **

Benzo[k]fluoranthene **

Butylbenzylphthalate **

Chrysene 

p-Cresol **

Di-n-butylphthalate **

Di-n-octylphthalate **

Dibenz[a,h]anthracene 

Diethylphthalate **

Dimethylphthalate **

9H-Fluorene (Fluorene) 

Indeno[1,2,3-cd]pyrene **

Isoquinoline **

1-Methyl-9H-fluorene **

2-Methylanthracene **

4,5-Methylenephenanthrene **

1-Methylphenanthrene **

1-Methylpyrene **

Naphthalene 

Phenanthrene 

Pyrene 

SVOCs not detected

C8-Alkylphenol **

Azobenzene **

Benzo[c]cinnoline **

2,2-Biquinoline **

4-Bromophenyl-phenylether **

4-Chloro-3-methylphenol **

bis(2-Chloroethoxy)methane **

2-Chloronaphthalene **

2-Chlorophenol **

4-Chlorophenyl-phenylether **

1,2-Dichlorobenzene (o-Dichlorobenzene) **

1,3-Dichlorobenzene (m-Dichlorobenzene) **

1,4-Dichlorobenzene (p-Dichlorobenzene) **

1,2-Dimethylnaphthalene **

1,6-Dimethylnaphthalene **

3,5-Dimethylphenol **

2,4-Dinitrotoluene **

2-Ethylnaphthalene **

Isophorone **

Nitrobenzene **

N-Nitrosodi-n-propylamine **

N-Nitrosodiphenylamine **

Pentachloronitrobenzene **

Phenanthridine **

Quinoline **

1,2,4-Trichlorobenzene **

2,3,6-Trimethylnaphthalene **

Other organochlorines detected

Total-HCH (sum of alpha-HCH, beta-HCH, gamma-HCH, and delta-HCH) **

Heptachlor epoxide (Heptachlor breakdown product) * 

Heptachlor+heptachlor epoxide (sum of heptachlor and heptachlor epoxide) **

Mirex (Dechlorane) **

Organochlorines not detected

Chloroneb (Chloronebe, Demosan) * **

DCPA (Dacthal, chlorthal-dimethyl) * **

Endosulfan I (alpha-Endosulfan, Thiodan) * **

Endrin (Endrine) 

gamma-HCH (Lindane, gamma-BHC, Gammexane) * 

Hexachlorobenzene (HCB) **

Isodrin (Isodrine, Compound 711) * **

p,p'-Methoxychlor (Marlate, methoxychlore) * **

o,p'-Methoxychlor * **

cis-Permethrin (Ambush, Astro, Pounce) * **

trans-Permethrin (Ambush, Astro, Pounce) * **

Toxaphene (Camphechlor, Hercules 3956) * **

CONCENTRATION, IN MICROGRAMS PER KILOGRAM, DRY WEIGHT

 0.1 1 10 100 1,000 10,000 100,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Semivolatile organic compounds (SVOCs)

in bed sediment

Study-unit sample size

CONCENTRATION, IN MICROGRAMS PER GRAM

(Fish tissue is wet weight, bed sediment is dry weight)

 0.01 0.1 1 10 100 10,000 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Trace elements in fish tissue (livers) and 

bed sediment

Study-unit sample size

Anthraquinone **

0 21 2 100 83 3 33 39 6

9H-Carbazole **

0 19 2 100 76 3 17 33 6

Dibenzothiophene **

0 12 2 0 64 3 17 30 6

2,6-Dimethylnaphthalene **

100 65 2 67 74 3 83 77 6

bis(2-Ethylhexyl)phthalate **

100 91 2 100 99 3 100 95 6

Fluoranthene 

50 66 | 2 100 97 | 3 83 78 | 6

Phenol **

100 81 2 100 82 3 67 80 6

Arsenic * 

50 56 2 0 38 2 83 76 6

100 99 | 2 100 98 | 3 100 97 | 6
32 Water Quality in South-Central Texas

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Biological indicator value, South-Central Texas, by land use, 

1996–98—Land-use areas categorized nationally as mixed were 

categorized locally as agricultural, urban, or rangeland

Biological status assessed at a site

National ranges of biological indicators, in 16 NAWQA Study 

Units, 1994–98

Streams in undeveloped areas

Streams in agricultural areas

Streams in urban areas

Streams in mixed-land-use areas

75th percentile

25th percentile

BIOLOGICAL INDICATORS

Higher national scores suggest habitat disturbance, water-quality 

degradation, or naturally harsh conditions. The status of algae, 

invertebrates (insects, worms, and clams), and fish provide a 

record of water-quality and stream conditions that waterchemistry indicators may not reveal. Algal status focuses on the 

changes in the percentage of certain algae in response to 

increasing siltation, and it often correlates with higher nutrient 

concentrations in some regions. Invertebrate status averages 11 

metrics that summarize changes in richness, tolerance, trophic 

conditions, and dominance associated with water-quality 

degradation. Fish status sums the scores of four fish metrics 

(percent tolerant, omnivorous, non-native individuals, and percent 

individuals with external anomalies) that increase in association 

with water-quality degradation

 0 10 20 30 40 50 60 70 80 90 100

CONCENTRATION, IN MICROGRAMS PER GRAM

(Fish tissue is wet weight, bed sediment is dry weight)

 0.01 0.1 1 10 100 10,000 1,000 

Study-unit sample size

 0 5 10 15 20

 Algal status indicator

Undeveloped

Agricultural

Urban

Mixed

 Invertebrate status indicator

Undeveloped

Agricultural

Urban

Mixed

 Fish status indicator

Undeveloped

Agricultural

Urban

Mixed

Cadmium * 

100 77 2 50 72 2 67 95 6

100 98 | 2 100 100 | 3 83 98 | 6

Chromium * 50 62 2 100 72 2 67 54 6

100 100 | 2 100 99 | 3 100 100 | 6

Copper * 100 100 2 100 100 2 100 100 6

100 100 | 2 100 99 | 3 100 100 | 6

Lead * 

0 11 2 50 41 2 67 41 6

100 100 | 2 100 100 | 3 100 99 | 6

Mercury * 

50 71 2 0 59 2 50 80 6

50 82 | 2 100 97 | 3 67 93 | 6

Nickel * **

100 42 2 50 44 2 50 50 6

100 100 2 100 100 3 100 100 6

Selenium * 100 99 2 100 100 2 100 99 6

100 100 | 2 100 100 | 3 100 100 | 6

Zinc * 

100 100 2 100 100 2 100 100 6

100 100 | 2 100 99 | 3 100 100 | 6
A COORDINATED EFFORT

Coordination with agencies and organizations in the South-Central Texas Study Unit was integral to the success of 

this water-quality assessment. We thank those who served as members of our liaison committee and those who 

contributed in other ways. 

Federal Agencies

U.S. Department of Agriculture—Natural Resources 

Conservation Service, U.S. Environmental Protection 

Agency, U.S. Fish and Wildlife Service, U.S. Postal 

Service

State Agencies

Texas Natural Resource Conservation Commission, 

Texas Parks and Wildlife Department, Texas Water 

Development Board 

Local Agencies

Bexar-Medina-Atascosa Counties Water Control and 

Improvement District No. 1, Bexar Metropolitan Water 

District, Edwards Aquifer Authority, Guadalupe-Blanco 

River Authority, Headwaters Underground Water Conservation District, Medina County Groundwater Conservation District, Nueces River Authority, San Antonio 

River Authority, San Antonio Water System, Springhills 

Water Management District, Upper Guadalupe River 

Authority, Uvalde County Underground Water Conservation District

Universities

Texas Water Resources Institute—Texas A&M University, Bureau of Economic Geology—University of 

Texas at Austin, University of Texas at San Antonio, 

Edwards Aquifer Research and Data Center—

Southwest Texas State University

Other public and private organizations

City of San Antonio, City of Shavano Park, Encino 

Park Homeowners Association, Fort Clark Springs 

Association, Parkwood Maintenance Association, 

Texas Center for Policy Studies, The Club at Sonterra, 

The Nature Conservancy of Texas, Town of Hollywood 

Park 

In addition to the numerous landowners who allowed us to collect water-quality samples on their property, we thank 

the following individuals for contributing to this effort.

Michael Albach

Steve Ansley

Bruce Bernard

James Bisese

James Blair

Chris Braun

Luana Buckner

John Burch

Paul Buszka

Jerry Casile

Jay Cederberg

Art Clark

Mark Clementz

Bill Cofer

Cameron Cornett

Wendy Danchuk

Rod DeWeese

Greg Ellis

Jeff Eman

Rob Esquilin

Tommy Fisk

John Flager

T. James Fries

Mike Gonzales

Tom Gonzales

Elisa Graffy

Charles Hartmann

Vic Hildebran

Sue Hovorka

Ivan James

Marshall Jennings

Susan Kemper

Mary Kidd

Gayle Kipp

Glenn Longley

Chiquita Lopez

Mike Lopez

Cecilio Martinez

Mark Mathews

Vidal Mendoza

Buddy Miller 

Jesse Mireles

Keith Muhlestein

Venezia Muniz

Kirk Nixon

Phil Nordstrom

Mike Nyman

Jim O’Connor

Bob Olmstead

Dane Ohe

Cassi Otero

Michelle Othon

Robert Ourso

George Ozuna

Jody Palmer

Brian Petri

Stephen Porter

Phil Redman

Dave Reutter

Maurice Rimkus

John Rosendale

Lana Ruiseco

Valarie Schmidt

Larry Shelton

Scott Sherrill

Allison Shipp

Gail Sladek

Ted Small

Gloria Smith

Grant Snyder

Milton Sunvison

Ed Swibas

Lu Tan

Gene Taylor

Chad Thompson

John Tumlinson

Peter Van Metre

James Waclawczyk

John Waugh

Craig Weiss

George Wissmann

Jennifer Wilson

Phillip Wright
Bush and others— Water Quality in South-Central Texas

U.S. Geological Survey Circular 1212

National Water-Quality Assessment (NAWQA) Program

South-Central Texas

NAWQA

San Antonio

Nueces River

Guadalupe River

San Antonio River

Gulf of Mexico