Descriptions and Comparisons of Nutrients,

Pesticides, and Volatile Organic Compounds

at Three Intensive Fixed Sites, 1996–98

Water-Resources Investigations Report 99–4155

U.S. Department of the Interior

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

U.S. Geological Survey

Water-Quality Assessment of South-Central Texas—

Descriptions and Comparisons of Nutrients,

Pesticides, and Volatile Organic Compounds

at Three Intensive Fixed Sites, 1996–98

By Patricia B. Ging

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99–4155

A contribution of the National Water-Quality Assessment Program

Austin, Texas

1999
ii

U.S. DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names is for

descriptive purposes only and does not imply

endorsement by the U.S. Government.

For additional information write to

District Chief

U.S. Geological Survey

8027 Exchange Dr.

Austin, TX 78754–4733

Email: dc_tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225–0286

Email: infoservices@usgs.gov
iii

FOREWORD

The mission of the U.S. Geological Survey

(USGS) is to assess the quantity and quality of the

earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound

decisions. Assessment of water-quality conditions and

trends is an important part of this overall mission.

One of the greatest challenges faced by waterresources scientists is acquiring reliable information

that will guide the use and protection of the Nation’s

water resources. That challenge is being addressed by

Federal, State, interstate, and local water-resource

agencies and by many academic institutions. These

organizations are collecting water-quality data for a

host of purposes that include: compliance with permits

and water-supply standards; development of remediation plans for specific contamination problems; operational decisions on industrial, wastewater, or watersupply facilities; and research on factors that affect

water quality. An additional need for water-quality

information is to provide a basis on which regionaland national-level policy decisions can be based. Wise

decisions must be based on sound information. As a

society we need to know whether certain types of

water-quality problems are isolated or ubiquitous,

whether there are significant differences in conditions

among regions, whether the conditions are changing

over time, and why these conditions change from

place to place and over time. The information can be

used to help determine the efficacy of existing waterquality policies and to help analysts determine the

need for and likely consequences of new policies.

To address these needs, the U.S. Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the

National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of

the program. The NAWQA Program builds upon an

existing base of water-quality studies of the USGS, as

well as those of other Federal, State, and local agencies.

The objectives of the NAWQA Program are to:

• Describe current water-quality conditions for a

large part of the Nation’s freshwater streams,

rivers, and aquifers.

• Describe how water quality is changing over

time.

• Improve understanding of the primary natural

and human factors that affect water-quality

conditions.

This information will help support the development

and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local

agencies to protect, use, and enhance water resources.

The goals of the NAWQA Program are being

achieved through ongoing and proposed investigations

of more than 50 of the Nation’s most important river

basins and aquifer systems, which are referred to as

study units. These study units are distributed throughout the Nation and cover a diversity of hydrogeologic

settings. More than two-thirds of the Nation’s freshwater use occurs within these study units and more

than two-thirds of the people served by public watersupply systems live within their boundaries.

National synthesis of data analysis, based on

aggregation of comparable information obtained from

the study units, is a major component of the program.

This effort focuses on selected water-quality topics

using nationally consistent information. Comparative

studies will explain differences and similarities in

observed water-quality conditions among study areas

and will identify changes and trends and their causes.

The first topics addressed by the national synthesis are

pesticides, nutrients, volatile organic compounds, and

aquatic biology. Discussions on these and other waterquality topics will be published in periodic summaries

of the quality of the Nation’s ground and surface water

as the information becomes available.

This report is an element of the comprehensive

body of information developed as part of the NAWQA

Program. The program depends heavily on the advice,

cooperation, and information from many Federal,

State, interstate, Tribal, and local agencies and the

public. The assistance and suggestions of all are

greatly appreciated.

Robert M. Hirsch

Chief Hydrologist
CONTENTS v

CONTENTS

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

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

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

Description of Study Unit ......................................................................................................................................... 2

Methods of Sample Collection ................................................................................................................................. 5

Nutrients ............................................................................................................................................................................... 7

Pesticides .............................................................................................................................................................................. 9

Volatile Organic Compounds ............................................................................................................................................... 12

Summary .............................................................................................................................................................................. 15

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

FIGURES

1–2. Maps showing:

1. Location of intensive fixed sites in the San Antonio region of the South-Central Texas study

unit ............................................................................................................................................................. 3

2. Land use/land cover in the San Antonio region of the South-Central Texas study unit ............................ 4

3. Hydrographs showing discharge and sample collection at intensive fixed sites in the San Antonio

region of the South-Central Texas study unit, April 1996–April 1998 ................................................................ 6

4. Storm hydrograph showing discharge and sample collection with autosampler at integrator site in

the San Antonio region of the South-Central Texas study unit ............................................................................ 7

5. Boxplots showing range and distribution of dissolved nitrite plus nitrate nitrogen and total phosphorus

concentrations at intensive fixed sites in the San Antonio region of the South-Central Texas study unit,

April 1996–April 1998 ......................................................................................................................................... 8

6–8. Graphs showing dissolved nitrite plus nitrate nitrogen concentrations:

6. At intensive fixed sites in the San Antonio region of the South-Central Texas study unit,

April 1996–April 1998 .............................................................................................................................. 10

7. With respect to discharge in base-flow samples at the urban indicator site in the San Antonio

region of the South-Central Texas study unit ............................................................................................ 10

8. With respect to discharge in base-flow samples at the integrator site in the San Antonio region

of the South-Central Texas study unit ........................................................................................................ 11

9–11. Graphs showing concentrations of selected pesticides in samples collected at the:

9. Agriculture indicator site in the San Antonio region of the South-Central Texas study unit,

January 1997–April 1998 .......................................................................................................................... 11

10. Urban indicator site in the San Antonio region of the South-Central Texas study unit,

January 1997–April 1998 .......................................................................................................................... 12

11. Integrator site in the San Antonio region of the South-Central Texas study unit,

January 1997–April 1998 .......................................................................................................................... 13

12. Boxplots showing range and distribution of selected pesticide concentrations at intensive fixed sites in

the San Antonio region of the South-Central Texas study unit, January 1997–April 1998 ................................. 14

TABLES

1–3. Number of samples collected each month at:

1. Agriculture indicator site, Medina River at LaCoste ................................................................................. 18

2. Urban indicator site, Salado Creek (lower station) at San Antonio ........................................................... 18

3. Integrator site, San Antonio River near Elmendorf ................................................................................... 18
vi

4. List of specific nutrients, pesticides, and volatile organic compounds analyzed in samples collected at

intensive fixed sites ............................................................................................................................................... 19

5–7. Concentrations of nutrients in samples collected at the:

5. Agriculture indicator site, Medina River at LaCoste ................................................................................. 20

6. Urban indicator site, Salado Creek (lower station) at San Antonio ........................................................... 21

7. Integrator site, San Antonio River near Elmendorf .................................................................................... 22

8–9. Number of detections and range in concentration of:

8. Pesticides detected in samples collected at intensive fixed sites ............................................................... 23

9. Volatile organic compounds detected in samples collected at intensive fixed sites ................................... 24
Abstract 1

Water-Quality Assessment of South-Central Texas—

Descriptions and Comparisons of Nutrients,

Pesticides, and Volatile Organic Compounds

at Three Intensive Fixed Sites, 1996–98

By Patricia B. Ging

Abstract

Water-quality samples were collected during

April 1996–April 1998 at three intensive fixed sites

in the San Antonio region of the South-Central

Texas study unit as part of the U.S. Geological

Survey National Water-Quality Assessment

Program. The sampling strategy for the intensive

fixed-site assessment is centered on obtaining

information about the occurrence and seasonal

patterns of selected constituents including nutrients, pesticides, and volatile organic compounds.

The three sites selected to determine the effects

of agriculture and urbanization on surface-water

quality in the study unit are Medina River at

LaCoste (agriculture indicator site), Salado Creek

(lower station) at San Antonio (urban indicator

site), and San Antonio River near Elmendorf

(integrator site).

Concentrations of two nutrients, dissolved

nitrite plus nitrate nitrogen and total phosphorus,

were largest at the integrator site, which is downstream of municipal wastewater treatment plants.

Nitrite plus nitrate nitrogen concentrations at this

site often exceeded the U.S. Environmental Protection Agency (EPA) maximum contaminant level

(MCL) for drinking water. All total phosphorus

concentrations at the site exceeded the EPA recommended maximum concentration for streams not

discharging directly into reservoirs. Nitrite plus

nitrate nitrogen concentrations at the integrator site

tended to be smaller, and total phosphorus concentrations at the urban site tended to be larger in

samples collected during stormflow than during

base flow. The most detections and largest concentrations of three pesticides (atrazine, diazinon, and

prometon) were in samples collected at the urban

site. Some pesticide concentrations at the agriculture site showed a seasonal pattern of increasing

concentrations during spring, the peak application

season. Four pesticides (atrazine, deethylatrazine,

diazinon, and prometon) were detected in at least

38 percent of samples collected at all three sites.

The concentrations of all detected pesticides that

have an MCL were less than the MCL at the three

sites. More volatile organic compounds (VOC)

were detected at the urban indicator site than at the

agriculture indicator site, mostly likely because

more sources are located in urbanized areas. The

most VOCs detected and the largest concentrations

of two VOCs (chloroform and tetrahydrofuran)

were in samples from the integrator site. More

VOCs were detected in samples collected at the

integrator site during stormflow than during base

flow. The concentrations of all detected VOCs that

have an MCL were less than the MCL at the three

sites.

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS)

implemented the National Water-Quality Assessment

(NAWQA) Program to describe the status and trends

in water quality of a large, representative part of the

Nation’s surface- and ground-water resources. This

program is based on a multidisciplinary approach using

standard protocols to collect data in more than 50 study

units (Hirsch and others, 1988; Leahy and others, 1990).

The intensive fixed-site assessment is a component of

the surface-water study design for the high intensity

phase of sampling (Gilliom and others, 1995). The purpose of the intensive fixed-site assessment is to increase

the sampling frequency and the number (relative to
2 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

basic fixed-site sampling) of constituents analyzed at

selected sites in the study unit. Dissolved pesticides and

volatile organic compounds (VOC) were added to the

list of constituents analyzed that already included major

anions and cations, nutrients, suspended sediment, and

suspended and dissolved organic carbon. The site selection and sampling strategy for the intensive fixed sites is

centered on specific land use activities. The premise of

the sampling strategy is that relatively frequent sampling at a few carefully chosen sites during key periods

yields superior information about occurrence and seasonal patterns of constituents. This sampling strategy

can provide information on seasonal and short-term

temporal variability of general water quality and constituent transport and determine the occurrence and seasonal patterns of selected constituents such as nutrients,

pesticides, and VOCs (Gilliom and others, 1995).

The major water-quality issue in the SouthCentral Texas (SCTX) NAWQA study unit (fig. 1) is

the potential for contamination of the Edwards aquifer.

The Edwards aquifer is the main aquifer in the study

unit and is the sole source of water for the greater San

Antonio area and for ranchers and farmers in the region.

The intensive fixed-site assessment was designed to

determine the effects of agriculture and urbanization on

surface-water quality within the SCTX study unit that

ultimately could affect the water quality of the Edwards

aquifer. The Medina River watershed just west of San

Antonio was used as an agricultural land use indicator,

and the Salado Creek watershed within the San Antonio

city limits was used as an urban land use indicator. The

San Antonio River downstream of the San Antonio

city limits, which is downstream of the inflows of the

Medina River and Salado Creek, was used as an integrator of agricultural and urban land uses.

Purpose and Scope

The purpose of this report is to describe and

compare concentrations of nutrients, pesticides, and

VOCs at three sites affected by different upstream land

use in the San Antonio region of the study unit. Three

intensive fixed sites with different upstream land use

were selected for sampling on a weekly to monthly basis

during April 1996–April 1998. Descriptions and comparisons of concentrations and temporal patterns of

nutrients, pesticides, and VOCs at the three sites are

based on graphs, boxplots, and tables.

Description of Study Unit

The SCTX study unit in the San Antonio region

comprises mainly the Edwards aquifer and its catchment area. The entire study unit extends beyond the

San Antonio region to the Gulf of Mexico to include

the complete watersheds of three major rivers (Nueces,

San Antonio, and Guadalupe). Salado Creek and

Medina River are in the San Antonio River system. For

this report, the study area is defined as the San Antonio

region of the SCTX study unit (fig. 1).

The Edwards aquifer is the sole source of

water for about 1.3 million people in the San Antonio

region. Water from the aquifer provides habitat for

threatened and endangered species associated with

major springs in the region. The Edwards aquifer is a

dipping sequence of extensively faulted, fractured, and

dissolutioned limestone and dolostone that yields large

quantities of water to wells and springs. The aquifer

crops out and is unconfined in the recharge zone (fig. 1).

The aquifer is confined (artesian zone) beneath much

less permeable rocks downdip from the recharge zone.

Further downdip, where the rocks are virtually impermeable, they contain moderately saline to very saline

water (saline-water zone) (U.S. Geological Survey,

1994).

The major process that can affect the quality of

water from the Edwards aquifer in the study area is

urban development in the greater San Antonio area.

Land use in the city of San Antonio and surrounding

area (fig. 2) is predominantly commercial and residential, including several large military installations and

manufacturing and tourism facilities. Land use in the

rest of the study area is mainly forest and rangeland with

some agricultural land and small urban areas.

Three intensive fixed sites (fig. 1) were selected

to determine the effects of agriculture and urbanization

on surface-water quality in the study area. These sites

are Medina River at LaCoste (08180640), Salado Creek

(lower station) at San Antonio (08178800), and San

Antonio River near Elmendorf (08181800). The drainage area of the Medina River watershed upstream of

the sampling site, Medina River at LaCoste, is about

805 square miles (mi2

) of which 634 mi2 is upstream

of the dam forming Medina Lake west of San Antonio.

Streamflow at this site is controlled by the dam at

Medina Lake (Gandara and others, 1997). The predominant land use within the Medina watershed is forest and

rangeland with small urban areas. Land use in about

10 percent of the watershed is agricultural. The Medina
INTRODUCTION 3Figure 1. Location of intensive fixed sites in the San Antonio region of the South-Central Texas study unit.

NORTH

FORK

SOUTH

FORK

Johnson

Creek BLANCO

RIVER

RIVER

BLANCO

LITTLE

Carpers

Creek

Cypress

Creek

GUADALUPE

RIVER

MEDINA

Cibolo

Creek

WEST

NUECES

NUECES

RIVER

RIVER

DRY FRIO

FRIO

RIVER

Blanco

Creek

RIVER

RIVER

LEONA

RIVER

Seco

Hondo

Verde

Creek

Creek

Creek

Creek H no od

RIVER

MEDINA

RIVER

RIVER

Medio Creek

Leon

Creek

Creek

SAN

AN TONIO

Salado

CANYON  LAKE

MEDINA LAKE

CALAVERAS LAKE

BRAUNIG LAKE

SABINAL

0 10 20 30 40 MILES

98o

99o

100o

30o

29o

BLANCO

HAYS KENDALL

COMAL

EDWARDS REAL

UVALDE KINNEY

MAVERICK ZAVALA

KERR

BANDERA

GILLESPIE

BEXAR

MEDINA

FRIO

ATASCOSA

GUADALUPE

Edwards aquifer catchment area

Edwards aquifer outcrop (recharge zone)—

Modified from Ashworth and Hopkins (1995)

Edwards aquifer saline-water zone

08181800 Intensive fixed site and

 U.S. Geological Survey streamflow/water-quality station number

EXPLANATION

Edwards aquifer artesian zone—Modified from Ashworth and Hopkins (1995); lower boundary from Schultz (1994)

Digital base from U.S. Geological SurveyScale 1:250,000 quadranglesAlbers equal-area projection based onstandard parallels 45.5 and 29.5 degrees

08181800

08180640

08178800

SAN ANTONIO

TEXAS

Study area of this report

South-Central Texas study unit

Guadalupe watershed

San Antonio watershed

Nueces watershed
4 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Figure 2. Land use/land cover in the San Antonio region of the South-Central Texas study unit.

NORTH

FORK

SOUTH

FORK

Johnson

Creek BLANCO

RIVER

RIVER

BLANCO

LITTLE

Carpers

Creek

Cypress

Creek

GUADALUPE

RIVER

MEDINA

Cibolo

Creek

WEST

NUECES

NUECES

RIVER

RIVER

DRY FRIO

FRIO

RIVER

Blanco

Creek

RIVER

RIVER

LEONA

RIVER

Seco

Hondo

Verde

Creek

Creek

Creek

Creek H no od

RIVER

MEDINA

RIVER

RIVER

Medio Creek

Leon

Creek

Creek

SAN

AN TONIO

Salado

SABINAL

0 10 20 30 40 MILES

0 10 20 30 40 MILES

98o

99o

100o

30o

29o

BLANCO

HAYS KENDALL

COMAL

EDWARDS REAL

UVALDE KINNEY

MAVERICK ZAVALA

KERR

BANDERA

GILLESPIE

BEXAR

MEDINA

FRIO

ATASCOSA

GUADALUPE

Urban land

Agricultural landRangeland

Forest land

Water

Barren land

EXPLANATION

Land use/land cover modified from U.S. Geological Survey, 1990, and Hitt, 1994

Land use/land cover

08181800 Intensive fixed site and

 U.S. Geological Survey streamflow/water-quality station number

Digital base from U.S. Geological SurveyScale 1:250,000 quadranglesAlbers equal-area projection based onstandard parallels 45.5 and 29.5 degrees

08181800

08180640

08178800
INTRODUCTION 5

River sampling site was selected as an agriculture indicator site because of the proximity of agricultural activity. The predominant crops grown in the watershed are

cotton, corn, and sorghum. Most of the cultivated fields

are irrigated with Edwards aquifer water. Water quality

at this site is affected by runoff from the fields into the

river.

The drainage area of the Salado Creek watershed

upstream of the sampling site, Salado Creek (lower

station) at San Antonio, is about 189 mi2 and is mostly

within the San Antonio city limits. Land use in this

watershed is 80 percent urban and 20 percent agricultural and rangeland. Because of predominantly urban

land use upstream, the Salado Creek site is considered

an urban indicator site. The urban land use is predominantly commercial and residential with only a small percentage of industrial. Streamflow at this site might be

affected during storm events by 11 floodwater-retarding

structures in the upper part of the watershed (Gandara

and others, 1997). Closer to the site, streamflow is

maintained by precipitation and municipal wastewater

discharges.

The drainage area of the San Antonio River

watershed upstream of the sampling site, San Antonio

River near Elmendorf, is about 1,740 mi2. Land use in

this watershed is 14 percent urban, 22 percent agricultural, 61 percent rangeland and forest, and 3 percent

water and barren land. Because of mixed land use

upstream, the San Antonio River site is considered an

integrator site. The city of San Antonio discharges

wastewater effluent into the San Antonio River from

three treatment plants located within the watershed

(Gandara and others, 1997). The sampling site is downstream of Medina River and Salado Creek inflows to the

San Antonio River.

Methods of Sample Collection

The intensive fixed-site sampling strategy is to

collect detailed data on the occurrence of contaminants

such as pesticides and VOCs at fixed intervals and

during high flows for approximately 1 year. The emphasis of the intensive sampling strategy is to collect

samples when pesticides and VOCs are most likely

to be detected. Most samples were collected monthly

although pesticide sampling was more frequent during

the spring when pesticides usually are applied, and

VOC sampling was more frequent during the winter

because VOC detections tend to be greatest during cold

months (Lopes and Price, 1997). Pesticide and VOC

samples also were collected during high-flow events.

Tables 1–3 (at end of report) show when samples were

collected at the three intensive fixed sites. Stream

hydrographs for the three intensive fixed sites show discharge and sample collection (fig. 3).

Specific nutrients, pesticides, and VOCs analyzed

in the samples are listed in table 4 (at end of report).

Other constituents analyzed were major cations and

anions, suspended and dissolved organic carbon, and

suspended sediment, the concentrations of which are

consistent among sites (Gandara and others, 1997,

1998).

Discrete fixed-interval and some high-flow

samples were obtained for most constituents analyzed

by collecting depth-integrated subsamples at equalwidth increments (EWI) across the stream channel

using either a US DH–81 or a US D–77 sampler

(Edwards and Glysson, 1988; Shelton, 1994). Suspended and dissolved organic carbon samples were

collected separately in a baked amber glass bottle using

a weighted sampler at a single midstream vertical

(Shelton, 1994). VOC samples were collected with

a specially designed Wildco sampler, which has 40-

milliliter (mL) vials placed directly in the sampler to

collect a sample at a single point in the stream (Shelton,

1997). In addition, autosamplers were used to collect

storm composite samples at a single point in the stream

by programming the sampler to collect subsamples

throughout the rise and fall of stream discharge (fig. 4).

Autosamplers were used because of the short time

period for the rise and fall. Field measurements of

specific conductance, pH, water temperature, dissolved

oxygen, and alkalinity were collected with the samples.

Samples were processed in the field immediately

after collection to reduce the chance of chemical or

biological alteration. After collection, all samples were

composited in glass bottles using a cone splitter. Major

cation and anion samples were filtered through a

Gelman capsule filter of 0.45-micrometer (μm) pore

size, and pesticide samples were filtered through a glass

fiber filter of 0.45-μm pore size. Suspended and dissolved organic carbon samples were filtered through a

silver filter of 0.45-μm pore size (Shelton, 1994). Major

cation samples were preserved with nitric acid (HNO3),

and VOC samples were preserved with concentrated

hydrochloric acid (HCl).

Suspended and dissolved organic carbon filtering

equipment was cleaned with organic-free deionized

water. The pesticide filter assembly and VOC sampler

were cleaned with dilute phosphate-free detergent
6 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile Organic

Compounds at Three Intensive Fixed Sites, 1996–98

20,000

10,000

1,000

100

10

Agriculture indicator site

Medina River at LaCoste

(08180640)

Sample collection

10,000

1,000

100

10

1.0

0.01

0.1

Urban indicator site

Salado Creek at San Antonio

(08178800)

Sample collection

20,000

10,000

1,000

100

10

A M J J A S O N D J F M A MJ J A S O N D J F M A

1996 1997 1998

Integrator site

San Antonio River near Elmendorf

(08181800)

DISCHARGE, IN CUBIC FEET PER SECOND

Sample collection

Figure 3. Hydrographs showing discharge and sample collection at intensive fixed sites in the San Antonio region

of the South-Central Texas study unit, April 1996–April 1998.
NUTRIENTS 7

solution, deionized water, and methanol. All other

equipment used to collect and process samples was

cleaned with a dilute phosphate-free detergent solution,

rinsed with tap water, soaked in 5-percent HCl, rinsed

with deionized water, rinsed with methanol, and then air

dried (Shelton, 1994; 1997).

Quality-control samples comprised field blanks,

equipment blanks, replicates, trip blanks, and field

matrix spikes. These quality-control samples are

described in Mueller and others (1997). One field blank

and one replicate for major cations and anions, nutrients, suspended and dissolved organic carbon, and

suspended sediment were collected each month during

the study. In addition, four pesticide field blanks and

three VOC field blanks were collected during the study.

Two equipment blanks for trace elements and nutrients

were collected. Four sets of split-concurrent replicates

were collected during the study and analyzed for major

cations and anions, nutrients, suspended sediment,

and pesticides. One replicate and one trip blank were

collected for VOC analysis. One field matrix spike was

collected for pesticides and one for VOCs.

Quality-control samples were obtained for

autosamplers by collecting an EWI sample at the same

time that a discrete one-time sample was collected by

the autosampler. This procedure was to help determine

if there was bias in chemical concentrations from samples collected at a single point in the stream with an

autosampler compared to samples collected using the

EWI method, which might help determine a correction

factor for chemical concentrations of samples collected

with an autosampler. Field blank samples also were

collected with the autosamplers.

NUTRIENTS

Nutrients in surface water are essential for aquatic

plant and animal life, but large nutrient concentrations

can have adverse ecological effects. In general, sources

of nutrients include wastewater treatment plants, fertilizer, manure, plant decay, and atmospheric deposition

(Hem, 1989). Concentrations of nutrients analyzed in

samples collected during the study at the three intensive

fixed sites are shown in tables 5–7 (at end of report).

Concentrations of nutrients in equipment blank samples

were small compared to concentrations in environmental samples. Small concentrations of some nutrients

detected in field blanks could cause a positive bias in

small nutrient concentrations in environmental samples.

The small concentrations of nutrients in field blanks

probably were from residual contamination in the collection and processing equipment. The nutrients of most

concern are nitrite plus nitrate nitrogen and phosphorus.

Nitrite plus nitrate nitrogen is a concern for drinking water because excessive concentrations of nitrate

DISCHARGE, IN CUBIC FEET PER SECOND

3,000

2,500

1,500

1,000

500

0

2,000

0015

0815

1615

0015

0815

1615

0015

0815

1615

0015

TIME

Sample collection

San Antonio River near Elmendorf

08181800

Figure 4. Storm hydrograph showing discharge and sample collection with autosampler at integrator site in the

San Antonio region of the South-Central Texas study unit.
8 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

can cause methemoglobinemia (blue baby syndrome),

which restricts oxygen transport in the bloodstream of

infants (Hem, 1989). The maximum contaminant level

(MCL) for dissolved nitrite plus nitrate nitrogen in

U.S. Environmental Protection Agency (EPA) drinking

water regulations is 10 milligrams per liter (mg/L) for

water delivered to any user of a public water system

(U.S. Environmental Protection Agency, 1996). Boxplots of the distribution of dissolved nitrite plus nitrate

nitrogen concentrations for the three intensive fixed

sites are shown in figure 5. Median concentrations were

1.97 mg/L at the agriculture indicator site, 0.551 mg/L

3.0

2.5

2.0

1.5

1.0

0.5

0

TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER

18.0 3.5

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0

DISSOLVED NITRITE PLUS NITRATE NITROGEN 

CONCENTRATION, IN MILLIGRAMS PER LITER

AGRICULTURE

INDICATOR SITE

URBAN

INDICATOR

SITE

INTEGRATOR

SITE

Data value greater than 3.0 times

the IQR outside the box

Largest data value within 1.5 times

the IQR above the box

75th percentile

Median (50th percentile)

25th percentile

Smallest data value within 1.5 times

the IQR below the box

(42) Number of samples

EXPLANATION

Interquartile

range (IQR)

AGRICULTURE

INDICATOR SITE

URBAN

INDICATOR

SITE

INTEGRATOR

SITE

Maximum contaminant level for drinking

water (U.S. Environmental Protection

Agency, 1996)

(42)

(52)

(37)

(42)

(52)

(37)

Figure 5. Range and distribution of dissolved nitrite plus nitrate nitrogen and total phosphorus concentrations at

intensive fixed sites in the San Antonio region of the South-Central Texas study unit, April 1996–April 1998.
PESTICIDES 9

at the urban indicator site, and 10.6 mg/L at the integrator site.

Nitrite plus nitrate nitrogen concentrations were

graphed for the three intensive fixed sites to show any

concentration fluctuations over time or seasonal patterns (fig. 6). Nitrite plus nitrate nitrogen concentrations

at all three sites did not indicate seasonal patterns.

Nitrite plus nitrate nitrogen concentrations at the integrator site, which is downstream from three municipal

wastewater treatment plants, often exceeded the MCL

for drinking water.

Nitrite plus nitrate nitrogen concentrations were

smaller in samples collected during storm events (larger

discharge) than during base flow at the agriculture

indicator site (table 5). Nitrite plus nitrate nitrogen concentrations in storm-event samples at the urban indicator site did not vary from concentrations in base-flow

samples (table 6). Nitrite plus nitrate nitrogen concentrations also were smaller in samples collected during

storm events than during base flow at the integrator site

(table 7). Nitrite plus nitrate nitrogen concentrations in

base-flow samples at the agriculture indicator site did

not seem to vary with discharge, but nitrite plus nitrate

nitrogen concentrations in base-flow samples at the

urban indicator site generally increased with increasing

discharge (fig. 7). In contrast, nitrite plus nitrate nitrogen concentrations in base-flow samples at the integrator site generally decreased with increasing discharge

(fig. 8).

Phosphorus often is associated with eutrophication, which is the enrichment of a body of water with

nutrients that results in accelerated algal or plant production (Hem, 1989). Eutrophication can result in fish

kills, unpleasant odors, loss of recreational value, and

other water-quality related problems (Land and others,

1998). The EPA recommends that total phosphorus

concentrations not exceed 0.10 mg/L in streams not

discharging directly into reservoirs and not exceed

0.05 mg/L in streams discharging directly into reservoirs (U.S. Environmental Protection Agency, 1986).

Boxplots of total phosphorus concentrations for the

three intensive fixed sites are shown in figure 5. Total

phosphorus concentrations at the agriculture indicator

site exceeded 0.10 mg/L only in two samples collected

during storm events (table 5). Total phosphorus concentrations at the urban indicator site ranged from <0.010

mg/L to 0.545 mg/L and also were greater in samples

collected during storm events (table 6). Total phosphorus concentrations in all samples collected at the integrator site exceeded 0.10 mg/L (but were less than 3.0

mg/L) (table 7).

PESTICIDES

Pesticides (herbicides and insecticides) commonly are used in the study area to control unwanted

vegetation and insects and to improve crop production;

although useful, pesticides can adversely affect the

environment and human health. The greatest potential

for adverse effects of pesticides is through contamination of the hydrologic system (Larson and others, 1997).

Water is one of the primary pathways by which pesticides are transported from their application areas to

other parts of the environment.

Samples collected at the three intensive fixed sites

were analyzed for 84 pesticides and pesticide metabolites (table 4). The pesticides detected at the sites and

the range in concentrations are listed in table 8 (at end

of report). No pesticides were detected in field blanks.

The agriculture indicator site had the least number of

pesticides detected with 15 in the 32 samples collected.

The most commonly detected pesticides at the agriculture site were atrazine, deethylatrazine, and prometon,

each detected in at least 20 samples. Diazinon was

detected in 12 samples. Concentrations of some pesticides detected (for example, atrazine and diazinon)

increased in the spring during peak pesticide application

(fig. 9). Concentrations of all pesticides detected at the

agriculture site were less than 0.12 microgram per liter

(μg/L) (table 8).

The urban indicator site had the most pesticides

detected with 25 in the 35 samples collected (table 8).

The most commonly detected pesticides were atrazine,

deethylatrazine, diazinon, prometon, simazine, and

tebuthiuron, each detected in at least 27 samples.

Tebuthiuron, bromacil, and diuron had the largest concentrations among the pesticides detected at the urban

site, 2.8, 1.6, and 1.1 μg/L, respectively. Concentrations

of some of the pesticides detected at the urban site also

increased in spring (fig. 10).

Results from the integrator site showed 18 pesticides detected in the 26 samples collected. The most

commonly detected pesticides were atrazine, deethylatrazine, diazinon, prometon, and simazine, each

detected in at least 20 samples. Carbaryl, chlorpyrifos,

lindane, metolachlor, and tebuthiuron were detected in

at least 15 samples. Concentrations of all pesticides

detected at the integrator site were less than 0.44 μg/L

(table 8). As at the agriculture and urban indicator sites,
10 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Maximum contaminant level for

drinking water (U.S. Environmental

Protection Agency, 1996)

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0

DISSOLVED NITRITE PLUS NITRATE NITROGEN 

CONCENTRATION, IN MILLIGRAMS PER LITER

A M J J A S O N D J FMAM J A J S O N D J F M A

1996 1997 1998

Agriculture indicator site

Urban indicator site

Integrator site

Figure 6. Dissolved nitrite plus nitrate nitrogen concentrations at intensive fixed sites in the San Antonio region of

the South-Central Texas study unit, April 1996–April 1998.

2.00

1.80

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0

DISSOLVED NITRITE PLUS NITRATE NITROGEN 

CONCENTRATION, IN MILLIGRAMS PER LITER

0 5 10 15 20 25 30 35

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 7. Dissolved nitrite plus nitrate nitrogen concentrations with respect to discharge in base-flow samples at

the urban indicator site in the San Antonio region of the South-Central Texas study unit.
PESTICIDES 11

0.12

0.10

0.08

0.06

0.04

0.02

0

J F M A M J J ASON D J F M A

1997 1998

0.12

0.10

0.08

0.06

0.04

0.02

0

J F M A M J J A S O N D J F M A

1997 1998

Atrazine

J F M A M J J ASON D J F M A

1997 1998

Deethylatrazine

0.08

0.06

0.04

0.02

0

Diazinon

0.010

0.008

0.006

0.004

0.002

0

J F M A M J J A S O N D J F M A

1997 1998

CONCENTRATION, IN MICROGRAMS PER LITER

Prometon

Nondetection

Nondetection

Figure 9. Concentrations of selected pesticides in samples collected at the agriculture indicator site in the San

Antonio region of the South-Central Texas study unit, January 1997–April 1998.

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0

DISSOLVED NITRITE PLUS NITRATE NITROGEN

CONCENTRATION, IN MILLIGRAMS PER LITER

0 100 200 300 400 500

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 8. Dissolved nitrite plus nitrate nitrogen concentrations with respect to discharge in base-flow samples at

the integrator site in the San Antonio region of the South-Central Texas study unit.
12 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

concentrations of some of the pesticides detected at the

integrator site showed increases in spring (fig. 11).

Under provisions of the Safe Drinking Water Act,

the EPA has established MCLs for concentrations of

certain chemicals in drinking water (U.S. Environmental Protection Agency, 1996). These MCLs are healthbased standards and are a result of chronic toxicity tests

conducted with animals (Larson and others, 1997). Not

all pesticides analyzed in the samples collected at the

intensive fixed sites have an MCL listed in the drinking

water regulations. Concentrations of all pesticides that

were detected at the intensive fixed sites and have an

MCL were less than the MCL.

Four pesticides detected in a large percentage

(38 to 100 percent) of samples collected at the three

intensive fixed sites were atrazine, deethylatrazine,

diazinon, and prometon. Boxplots of these pesticide

concentrations show that median concentrations of atrazine at the three sites were similar (fig. 12). The largest

median concentration of deethylatrazine was at the agriculture indicator site. The largest median concentrations

of diazinon and prometon were at the urban indicator

site. The largest concentrations of three of the four pesticides shown were at the urban indicator site (atrazine,

0.75 μg/L; diazinon, 0.33 μg/L; prometon, 0.24 μg/L).

VOLATILE ORGANIC COMPOUNDS

VOCs are present in many natural and synthetic

materials and commonly are used as fuel additives,

solvents, and aerosols. Despite their high evaporation

rates, VOCs have been detected in natural waters

0.80

0.60

0.40

0.20

0

J F M A M J J A SO N D

1997 1998

J F M A

J F M A M J J A S O N D

1997 1998

J F M A

Nondetection

0.60

0.40

0.20

0

J F M A M J J A SO N D

1997 1998

J F M A

0.40

0.30

0.20

0.10

0

Nondetection

0.25

0.20

0.15

0.10

0.05

0

J F M A M J J A S O N D

1997 1998

J F M A

CONCENTRATION, IN MICROGRAMS PER LITER

Atrazine Deethylatrazine

Diazinon Prometon

Figure 10. Concentrations of selected pesticides in samples collected at the urban indicator site in the San

Antonio region of the South-Central Texas study unit, January 1997–April 1998.
VOLATILE ORGANIC COMPOUNDS 13

(Kotzias and Sparta, 1993). The likely sources of

VOCs in surface water are industrial and wastewater

discharges, accidental fuel and oil spills, and chlorinated municipal drinking water. However, all of the

water sources that sustain streamflow are susceptible to

contamination by VOCs. VOCs in rainfall originate

from vehicle and industrial emissions; direct runoff

from streets and paved surfaces is another source of

VOCs (Terracciano and O’Brien, 1997). Contamination

of water by VOCs is a concern because of the threat to

human health. Possible effects to humans ingesting

water containing VOCs include eye, nose, and throat

irritation; central nervous system responses such as dizziness, headaches, and loss of short-term memory; and

carcinogenesis (Wallace, 1993).

Samples collected for VOCs were analyzed for

86 compounds (table 4). The VOCs detected at the three

intensive fixed sites and the range in concentrations are

listed in table 9 (at end of report). Small concentrations

of carbon disulfide, toluene, and xylene were detected

in VOC field blanks. These same VOCs also were

detected in source water for VOC field blanks, indicating that the source of the VOCs in the field blanks likely

was not associated with sample collection and processing. The agriculture indicator site had the least number

of VOCs detected with 15 in the 7 samples collected.

The most commonly detected VOCs at the agriculture

site were bromoform, carbon disulfide, chloroform,

p-isopropyltoluene, and meta- and para-xylene, each

detected in at least three samples. Concentrations of all

VOCs detected at the agriculture site were less than

0.40

0.30

0.20

0.10

0

Atrazine

J F M A M J J A S O N D

1997 1998

J F M A

Deethylatrazine

Diazinon Prometon

0.03

0.02

0.01

0

J F M A M J J A S O N D

1997 1998

J F M A

0.20

0.15

0.10

0.05

0

CONCENTRATION, IN MICROGRAMS PER LITER

J F M A M J J A S O N D

1997 1998

J F M A

Nondetection

Nondetection

0.05

0.04

0.03

0.02

0.01

0

J F M A M J J A S O N D

1997 1998

J F M A

Figure 11. Concentrations of selected pesticides in samples collected at the integrator site in the San Antonio

region of the South-Central Texas study unit, January 1997–April 1998.
14 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Figure 12. Range and distribution of selected pesticide concentrations at intensive fixed sites in the San Antonio

region of the South-Central Texas study unit, January 1997–April 1998.

0.4

0.3

0.2

0.1

0

Atrazine

1.0

0.8

0.9

0.7

0.6

0.5

0.12

0.10

0.08

0.06

0.04

0.02

0

Deethylatrazine

Data value greater than 3.0 times

the IQR outside the box

Data value 1.5 to 3.0 times

the IQR outside the box

Largest data value within 1.5 times

the IQR above the box

75th percentile

Median (50th percentile)

25th percentile

Smallest data value within 1.5 times

the IQR below the box

(32) Number of samples

EXPLANATION

Interquartile

range (IQR)

AGRICULTURE

INDICATOR SITE

URBAN

INDICATOR

SITE

INTEGRATOR

SITE

AGRICULTURE

INDICATOR SITE

URBAN

INDICATOR

SITE

INTEGRATOR

SITE

Diazinon

0.40

0.30

0.20

0.10

0

CINCENTRATION, IN MICROGRAMS PER LITER

0.25

0.20

0.15

0.10

0.05

0

Prometon

(32)

(35)

(26)

(26)

(26)

(26)

(35)

(35)

(32)

(32)

(32)

(35)
SUMMARY 15

1.0 μg/L. More VOCs were detected at the site in

samples collected during storm events than during

base-flow conditions—six compounds (benzene,

ethylbenzene, naphthalene, 1,2,4-trimethylbenzene,

1,3,5-trimethylbenzene, and xylene) were detected only

in storm-event samples.

More VOCs were detected at the urban indicator

site (21 in 22 samples, table 9) than at the agriculture

site, probably because the sources of VOCs are more

likely to be in urban areas. The most commonly detected

VOCs at the urban site were cis-1,2-dichloroethene,

methyl tert-butyl ether (MTBE), and toluene—all

detected in at least 12 samples. Concentrations of all

VOCs detected were less than 1.0 μg/L except those for

acetone, which were less than 7.0 μg/L (table 9). Correlation between discharge and number of VOCs detected

was not apparent at the urban site.

The integrator site had the most VOCs detected,

33 in the 13 samples collected (table 9). The most

commonly detected VOCs at the integrator site were

bromoform, chlorodibromomethane, chloroform,

1,4-dichlorobenzene, and dichlorobromomethane, each

detected in at least 10 samples. Four of these VOCs

(bromoform, chlorodibromomethane, chloroform, and

dichlorobromomethane) commonly are referred to as

trihalomethanes (THMs), which are generated when

water is chlorinated at treatment plants. THMs are

present in chlorinated water worldwide in concentrations typically ranging from 10 to 30 μg/L (Thurman,

1986). Concentrations of THMs in samples collected

from the integrator site were less than the typical range.

More VOCs were detected in samples collected at the

integrator site during winter storm events than in other

samples. Two of the largest VOC concentrations from

the study were chloroform, 4.6 μg/L, and tetrahydrofuran, 3.9 μg/L, detected in samples collected at the

integrator site. The concentrations of some VOCs

detected at the integrator site were about equal to or

larger than the combined concentrations of the same

VOCs detected at the agriculture and urban indicator

sites upstream of the integrator site.

Not all VOCs analyzed in the samples collected at

the intensive fixed sites have an MCL listed in the EPA

drinking water regulations. Concentrations of all VOCs

that were detected at the intensive fixed sites and have

an MCL were less than the MCL.

SUMMARY

Three sites with different upstream land use in

the San Antonio region of the SCTX NAWQA study

unit (study area) were selected as intensive fixed sites.

The sampling strategy for intensive fixed sites is based

on obtaining information about the occurrence and

seasonal patterns of selected water-quality constituents,

which include nutrients, pesticides, and VOCs. The

three intensive fixed sites selected to determine the

effects of agriculture and urbanization on surface-water

quality in the study area are Medina River at LaCoste

(agriculture indicator site), Salado Creek (lower station)

at San Antonio (urban indicator site), and San Antonio

River near Elmendorf (integrator site). Surface-water

quality is a major issue in the SCTX study unit because

of the potential for contamination of the Edwards

aquifer.

Samples were collected at the three intensive

fixed sites during April 1996–April 1998. Sample

collection usually was done when pesticides and VOCs

were likely to be detected. Most samples were collected

monthly, although pesticide sampling was more

frequent during the spring, and VOC sampling was

more frequent during the winter. Some samples were

collected with autosamplers, which can collect composite samples during storms. Quality-control samples

comprised field blanks, equipment blanks, replicates,

trip blanks, and field matrix spikes.

Seasonal patterns in nitrite plus nitrate nitrogen

concentrations were not evident at all three intensive

fixed sites. Nitrite plus nitrate nitrogen concentrations

often exceeded the MCL (10 mg/L) for drinking water

at the integrator site, which is downstream of municipal

wastewater treatment plants. Nitrite plus nitrate nitrogen concentrations in samples collected during stormflow were smaller than concentrations in samples

collected during base flow at the agriculture and integrator sites.

Total phosphorus concentrations at the agriculture indicator site were mostly less than 0.10 mg/L,

the maximum concentration recommended by EPA for

streams not discharging directly into reservoirs, and the

majority of concentrations at the urban indicator site

also were less than 0.10 mg/L. All total phosphorus

concentrations at the integrator site were greater than

0.10 mg/L.

Samples collected at the three intensive fixed

sites were analyzed for 84 pesticides. Concentrations

of some pesticides detected tended to increase in the
16 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

spring during peak pesticide application. More pesticides were detected at the urban indicator site and

at the integrator site than at the agriculture indicator

site. Four pesticides (atrazine, deethylatrazine, diazinon, and prometon) were detected in at least 38 percent

of samples at all sites. The largest concentrations of

atrazine (0.75 μg/L), diazinon (0.33 μg/L), and prometon (0.24 μg/L) were at the urban indicator site. The

concentrations of detected pesticides that have an MCL

were less than that MCL at all three intensive fixed sites.

Samples collected for VOCs were analyzed for 86

compounds. More VOCs were detected at the urban

indicator site than at the agriculture indicator site, most

likely because more sources of VOCs are located in

urbanized areas. The most VOCs detected (33 in 13

samples) and the largest concentrations of two VOCs

(chloroform, 4.6 μg/L and tetrahydrofuran, 3.9 μg/L)

were in samples collected at the integrator site. Some of

the VOCs detected (THMs), especially at the integrator

site, could be associated with the chlorination process at

wastewater treatment plants in the watershed. More

VOCs were detected in samples collected at the integrator site during winter storm events than during base

flow. The concentrations of all detected VOCs that have

an MCL were less than the MCL at all three intensive

fixed sites.

Sampling at the three intensive fixed sites during

April 1996–April 1998 indicates that more detections

and larger concentrations of nutrients, pesticides,

and VOCs tend to be associated with an urban or

integrated urban setting. Continued sampling of these

constituents at the sites could provide additional information on changes in surface-water quality with urban

development.

REFERENCES

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

Texas: Texas Water Development Board Report 345,

69 p.

Edwards, T.K., and Glysson, D.G., 1988, Field methods

for measurement of fluvial sediment: U.S. Geological

Survey Open-File Report 86–531, 118 p.

Gandara, S.C., Gibbons, W.J., Andrews, F.L., Jones, R.E.,

and Barbie, D.L., 1997, Water resources data, Texas,

water year 1996—Volume 3: U.S. Geological Survey

Water-Data Report TX–96–3, 347 p.

______1998, Water resources data, Texas, water year 1997—

Volume 3: U.S. Geological Survey Water-Data Report

TX–97–3, 414 p.

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of

the National Water-Quality Assessment Program—

Occurrence and distribution of water-quality conditions:

U.S. Geological Survey Circular 1112, 33 p.

Hem, J.D., 1989, Study and interpretation of the chemical

characteristics of natural water: U.S. Geological Survey

Water-Supply Paper 2254, 263 p.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988,

Concepts for a National Water-Quality Assessment

Program: U.S. Geological Survey Circular 1021, 42 p.

Hitt, K.J., 1994, Refining 1979’s land-use data with 1990

population data to indicate new residential development:

U.S. Geological Survey Water-Resources Investigations

Report 94–4250, 15 p.

Kotzias, D. and Sparta, C., 1993, VOCs and water pollution,

in Bloemen, H.J.Th., and Burn, J., eds., Chemistry

and analysis of volatile organic compounds in the

environment: New York, Blackie Academic and

Professional, p. 175–201.

Land, L.F., Moring, J.B., Van Metre, P.C., Reutter, D.C.,

Mahler, B.J., Shipp, A.A., and Ulery, R.L., 1998, Water

quality in the Trinity River Basin, Texas, 1992–95:

U.S. Geological Survey Circular 1171, 39 p.

Larson, S.J., Capel, P.D., and Majewski, M.S., 1997,

Pesticides in surface waters—Distribution, trends, and

governing factors: Chelsea, Mich., Ann Arbor Press,

373 p.

Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990,

Implementation plan for the National Water-Quality

Assessment Program: U.S. Geological Survey

Open-File Report 90–174, 10 p.

Lopes, T.J., and Price, C.V., 1997, Study plan for urban

stream indicator sites of the National Water-Quality

Assessment Program: U.S. Geological Survey

Open-File Report 97–25, 15 p.

Mueller, D.K., Martin, J.D., and Lopes, T.J., 1997, Qualitycontrol design for surface-water sampling in the

National Water-Quality Assessment Program: U.S.

Geological Survey Open-File Report 97–223, 17 p.

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.

Shelton, L.R., 1994, Field guide for collecting and processing

stream-water samples for the National Water-Quality

Assessment Program: U.S. Geological Survey

Open-File Report 94–455, 42 p.

______1997, Field guide for collecting samples for analysis

of volatile organic compounds in stream water for the

National Water-Quality Assessment Program: U.S.

Geological Survey Open-File Report 97–401, 14 p.

Terracciano, S.A., and O’Brien, A.K., 1997, Occurrence of

volatile organic compounds in streams on Long Island,

New York, and New Jersey—Overview of available data
REFERENCES 17

and reconnaissance sampling: U.S. Geological Survey

Fact Sheet 97–063, 4 p.

Thurman, E.M., 1986, Organic geochemistry of natural

waters: Dordrecht, Netherlands, Martinus Nijhoff/

Dr. W. Junk Publishers, 497 p.

U.S. Environmental Protection Agency, 1986, Quality

criteria for water 1986: Washington, D.C., U.S. Environmental Protection Agency Report 440/5–86–001, 453 p.

______1996, Drinking water regulations and health advisories: Washington, D.C., U.S. Environmental Protection

Agency, 11 p.

U.S. Geological Survey, 1990, Land use and land cover

digital data from 1:250,000- and 1:1,000,000-scale

maps: U.S. Geological Survey Geodata Users Guide 4,

33 p.

______1994, National Water-Quality Assessment Program—

South-Central Texas: U.S. Geological Survey, National

Water-Quality Assessment Fact Sheet 94–048, 2 p.

Wallace, L.A., 1993, VOCs and the environment and public

health—Exposure, in Bloemen, H.J.Th., and Burn, J.,

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Academic and Professional, p. 1–24.
18 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Table 3

Table 2

Table 1

Table 1. Number of samples collected each month at agriculture indicator site, Medina River at LaCoste[SOC/DOC, suspended and dissolved organic carbon; VOCs, volatile organic compounds]Table 2. Number of samples collected each month at urban indicator site, Salado Creek (lower station) at San Antonio[SOC/DOC, suspended and dissolved organic carbon; VOCs, volatile organic compounds]Table 3. Number of samples collected each month at integrator site, San Antonio River near Elmendorf[SOC/DOC, suspended and dissolved organic carbon; VOCs, volatile organic compounds]

Type of

sample

1996 1997 1998

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

Major ions 1 1 1 0 1 1 1 1 1 1 0 2 3 5 3 4 4 4 1 1 1 1 1 2 1Nutrients 1 1 1 0 1 1 1 1 1 1 0 2 3 5 3 4 4 4 1 1 1 1 1 2 1SOC/DOC 1 1 1 0 1 1 1 1 1 1 0 2 3 5 3 4 4 4 1 1 1 1 1 2 1Suspended sediment 1 1 1 0 1 1 1 1 1 1 0 2 3 5 3 4 4 4 1 1 1 1 1 2 1Pesticides 0 0 0 0 0 0 0 0 0 1 0 2 3 5 3 4 4 4 1 1 1 0 0 2 1VOCs 0 0 0 0 0 0 0 0 0 1 0 2 0 0 1 0 0 0 0 1 1 0 0 1 0

Type of

sample

1996 1997 1998

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

Major ions 1 1 1 1 0 1 1 1 1 1 2 2 4 5 2 4 4 4 2 2 3 4 2 2 1Nutrients 1 1 1 1 0 1 1 1 1 1 2 2 4 5 2 4 4 4 2 2 3 4 2 2 1SOC/DOC 1 1 1 1 0 1 1 1 1 1 2 2 4 5 2 4 4 4 2 2 3 4 2 2 1Suspended sediment 1 1 1 1 0 1 1 1 1 1 2 2 4 5 2 4 4 4 2 2 3 4 2 2 1Pesticides 0 0 0 0 0 0 0 0 0 1 1 1 4 5 2 4 4 4 1 1 2 2 1 1 1VOCs 0 0 0 0 0 0 0 0 0 1 2 2 2 1 0 2 1 1 2 2 3 3 0 0 0

Type of sample

1996 1997 1998

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

Major ions 1 1 1 0 1 1 1 1 1 1 0 2 3 2 2 2 2 2 2 1 2 3 2 2 1Nutrients 1 1 1 0 1 1 1 1 1 1 0 2 3 2 2 2 2 2 2 1 2 3 2 2 1SOC/DOC 1 1 1 0 1 1 1 1 1 1 0 2 3 2 2 2 2 2 2 1 2 3 2 2 1Suspended sediment 1 1 1 0 1 1 1 1 1 1 0 2 3 2 2 2 2 2 2 1 2 3 2 2 1Pesticides 0 0 0 0 0 0 0 0 0 1 0 2 3 2 2 2 2 2 2 1 2 2 1 1 1VOCs 0 0 0 0 0 0 0 0 0 1 0 2 1 1 1 1 0 1 1 1 1 2 0 0 0
Table 4 19

Table 4

Table 4. List of specific nutrients, pesticides, and volatile organic compounds analyzed in samples collected at intensive fixed sites

[MDL, method detection limit; mg/L, milligrams per liter; μg/L, micrograms per liter]

1 U.S. Geological Survey National Water-Quality Laboratory schedule 2702.

2 U.S. Geological Survey National Water-Quality Laboratory schedules 2001, 2050.

3 U.S. Geological Survey National Water-Quality Laboratory schedule 2020.

Nutrients1 MDL

(mg/L)

Pesticides2

(herbicides and insecticides)

MDL

(μg/L)

Volatile organic

compounds3 MDL(μg/L)

Nitrogen, ammonia, dissolved 0.015, .020 Acifluorfen 0.035 Acetone 2.452

Nitrogen, ammonia plus organic, dissolved .20, .10 Alachlor .002 Acrolein .716

Nitrogen, ammonia plus organic, total .20, .10 Aldicarb .016 Acrylonitrile .613

Nitrogen, nitrite, dissolved .010 Aldicarb sulfone .016 Benzene .016

Nitrogen, nitrite plus nitrate, dissolved .050 Aldicarb sulfoxide .021 Bromobenzene .018

Phosphorus, total .010 Atrazine .001 Bromochloromethane .022

Phosphorus, dissolved .010 Azinphos-methyl .001 Bromoform .052

Phosphorus, orthophosphate, dissolved .010 Benfluralin .002 Bromomethane .074

Bentazon .014 2-Butanone .825

Bromacil .035 n-Butylbenzene .093

Bromoxynil .035 sec-Butylbenzene .024

Butylate .002 tert-Butylbenzene .048

Carbaryl .003 Carbon disulfide .040

Carbofuran .003 Chlorobenzene .014

Chloramben .011 Chlorodibromomethane .091

Chlorothalonil .035 Chloroethane .060

Chlorpyrifos .004 Chloroform .026

Clopyralid .05 Chloromethane .127

Cyanazine .004 3-Chloropropene .098

2,4-D .035 2-Chlorotoluene .021

Dacthal .017 4-Chlorotoluene .028

2,4-DB .035 1,2-Dibromoethane .018

DCPA .002 Dibromomethane .025

p,p'-DDE .006 1,2-Dichlorobenzene .024

Deethylatrazine .002 1,3-Dichlorobenzene .027

Diazinon .002 1,4-Dichlorobenzene .025

Dicamba .035 Dichlorobromomethane .024

Dichlobenil .02 trans-1,4-Dichloro-2-butene .346

Dichlorprop (2,4-DP) .032 Dichlorodifluoromethane .048

Dieldrin .001 1,1-Dichloroethane .033

2,6-Diethylaniline .003 1,2-Dichloroethane .067

Dinoseb .035 1,1-Dichloroethene .022

Disulfoton .017 cis-1,2-Dichloroethene .019

Diuron .02 trans-1,2-Dichloroethene .016

DNOC .035 1,2-Dichloropropane .034

EPTC .002 1,3-Dichloropropane .058

Esfenvalerate .019 2,2-Dichloropropane .039

Ethalfluralin .004 1,1-Dichloropropene .013

Ethoprop .003 cis-1,3-Dichloropropene .046

Fenuron .013 trans-1,3-Dichloropropene .067

Fluometuron .035 Diethyl ether .085

Fonofos .003 Diisopropylether .049

alpha-HCH .002 Ethylbenzene .015

gamma-HCH (Lindane) .004 Ethyl tert-butyl ether (ETBE) .027

3-Hydroxycarbofuran .014 Ethyl methacrylate .139

Linuron .002 o-Ethyl toluene .050

Malathion .005 Hexachlorobutadiene .071

MCPA .05 Hexachloroethane .181

MCPB .035 2-Hexanone .373

Methiocarb .026 Isopropylbenzene .016

Methomyl .017 p-Isopropyltoluene .055

Methyl parathion .006 Methyl acrylate .306

Metolachlor .002 Methyl acrylonitrile .285

Metribuzin .004 Methyl tert-butyl ether (MTBE) .056

Molinate .004 Methylene chloride .191

1-Naphthol .007 Methyl iodide .038

Napropamide .003 Methyl methacrylate .175

Neburon .015 4-Methyl-2-pentanone (MIBK) .187

Norflurazon .024 Naphthalene .125

Oryzalin .019 tert-Pentyl methyl ether (TAME) .056

Oxamyl .018 n-Propylbenzene .021

Parathion .004 Styrene .021

Pebulate .004 1,1,1,2-Tetrachloroethane .022

Pendimethalin .004 1,1,2,2-Tetrachloroethane .066

cis-Permethrin .005 Tetrachloroethene .019

Phorate .002 Tetrachloromethane .044

Picloram .05 Tetrahydrofuran .574

Prometon .018 1,2,3,4-Tetramethylbenzene .115

Pronamide .003 1,2,3,5-Tetramethylbenzene .120

Propachlor .007 Toluene .019

Propanil .004 1,2,3-Trichlorobenzene .133

Propargite .013 1,2,4-Trichlorobenzene .094

Propham .035 1,1,1-Trichloroethane .016

Propoxur .035 1,1,2-Trichloroethane .032

Simazine .005 Trichloroethene .019

2,4,5-T .035 Trichlorofluoromethane (CFC 11) .046

Tebuthiuron .01 1,2,3-Trichloropropane .035

Terbacil .007 1,1,2-Trichlorotrifluoroethane .016

Terbufos .013 1,2,3-Trimethylbenzene .062

Thiobencarb .002 1,2,4-Trimethylbenzene .028

2,4,5-TP .021 1,3,5-Trimethylbenzene .022

Triallate .001 Vinyl acetate .631

Triclopyr .05 Vinyl bromide .050

Trifluralin .002 Vinyl chloride .056

meta- and para-Xylene .032

ortho-Xylene .032
20 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Table 5

Table 5. Concentrations of nutrients in samples collected at the agriculture indicator site, Medina River at LaCoste

[ft3/s, cubic feet per second; mg/L, milligrams per liter; <, less than; MDL, method detection limit]

Date

Discharge

(ft3/s)

Nitrogen,

ammonia,

dissolved

(mg/L)

Nitrogen,

nitrite,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

total

(mg/L)

Nitrogen,

nitrite plus

nitrate,

dissolved

(mg/L)

Phosphorus,

total

(mg/L)

Phosphorus,

dissolved

(mg/L)

Phosphorus,

orthophosphate,

dissolved

(mg/L)

04–16–96 23 0.0200 0.0100 <0.20 0.30 2.20 0.0300 0.0100 0.0100

05–23–96 22 .0400 .0100 <.20 .30 1.60 .0200 <.010 <.010

06–24–96 35 .0500 .0100 <.20 .30 1.50 <.010 <.010 <.010

08–06–96 29 .0200 .0100 <.20 .30 2.00 .0300 .0300 .0100

09–04–96 18 <.015 .0100 <.20 .30 2.10 .0300 <.010 <.010

10–07–96 29 <.015 <.010 <.20 <.20 3.40 <.010 <.010 <.010

11–08–96 32 .0300 .0200 <.20 .20 2.60 <.010 <.010 <.010

12–06–96 24 <.015 .0200 <.20 <.20 2.60 .0600 .0200 <.010

01–24–97 21 <.015 .0100 <.20 <.20 3.60 .0300 <.010 <.010

03–05–97 26 .0200 <.010 <.20 .20 2.40 <.010 <.010 <.010

03–21–97 25 <.015 <.010 <.20 <.20 2.40 .0400 <.010 <.010

04–08–97 33 .0200 .0100 <.20 <.20 1.80 .0100 <.010 <.010

04–16–97 32 <.015 <.010 <.20 .24 1.97 .0300 <.010 <.010

04–22–97 37 .0940 <.010 <.20 .26 1.97 .0130 <.010 <.010

05–02–97 33 <.015 <.010 .20 <.20 1.74 .0130 .0100 <.010

05–08–97 33 <.015 <.010 <.20 .21 1.71 .0240 <.010 <.010

05–14–97 36 .0210 .0130 <.20 .23 1.24 .0260 <.010 <.010

05–21–97 32 .0190 <.010 <.20 .23 1.49 .0320 <.010 <.010

05–27–97 33 .0190 <.010 <.20 .25 1.56 .0270 <.010 <.010

06–03–97 32 <.015 <.010 <.20 <.20 1.55 .0610 .0360 .0100

06–17–97 46 <.015 <.010 <.20 .28 1.20 .0260 .0190 .0290

06–22–97 2,760 .0630 <.010 .23 5.8 .577 1.34 .0160 .0160

07–03–97 1,050 .0150 <.010 .31 .30 .417 <.010 <.010 <.010

07–15–97 296 .0300 <.010 <.20 .23 .882 <.010 <.010 .0130

07–22–97 198 <.015 <.010 .26 .26 1.21 .0280 <.010 .0130

07–30–97 102 <.015 <.010 <.20 <.20 1.69 <.010 <.010 <.010

08–05–97 78 .0200 .0110 .24 <.20 2.48 <.010 <.010 .0120

08–12–97 59 <.015 .0100 <.20 <.20 2.99 <.010 <.010 .0150

08–19–97 54 <.015 .0100 <.20 <.20 3.04 <.010 <.010 <.010

08–27–97 56 <.015 .0130 <.20 <.20 2.64 <.010 <.010 <.010

09–03–97 54 <.015 <.010 <.20 <.20 2.42 <.010 .0160 <.010

09–10–97 52 <.015 <.010 <.20 <.20 2.51 <.010 <.010 <.010

09–16–97 49 <.015 <.010 <.20 <.20 2.48 <.010 <.010 <.010

09–24–97 54 <.015 <.010 <.20 <.20 2.52 .0110 <.010 <.010

10–23–97 51 <.015 <.010 <.20 <.20 2.70 <.010 <.010 <.010

Change in MDL for nitrogen, ammonia, dissolved; nitrogen, ammonia plus organic, dissolved;

and nitrogen, ammonia plus organic, total for the following analyses; see table 4

11–25–97 71 <.020 <.010 <.10 .11 1.93 <.010 <.010 .0190

12–22–97 94 <.020 <.010 .11 .15 2.59 <.010 .0130 .0110

01–20–98 57 <.020 <.010 <.10 .12 2.27 <.010 <.010 .0100

02–26–98 118 <.020 <.010 .12 .23 1.25 <.010 <.010 .0190

03–16–98 959 .0370 .0370 .40 1.9 1.38 .466 .0120 .0110

03–24–98 670 .0320 <.010 .11 .19 .295 <.010 <.010 <.010

04–24–98 63 .0330 .0110 .12 .12 2.44 .0130 .0160 <.010
Table 6 21

Table 6

Table 6. Concentrations of nutrients in samples collected at the urban indicator site, Salado Creek (lower station) at San Antonio

[ft3/s, cubic feet per second; mg/L, milligrams per liter; <, less than: MDL, method detection limit]

Date

Discharge

(ft3/s)

Nitrogen,

ammonia,

dissolved

(mg/L)

Nitrogen,

nitrite,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

total

(mg/L)

Nitrogen,

nitrite plus

nitrate,

dissolved

(mg/L)

Phosphorus,

total

(mg/L)

Phosphorus,

dissolved

(mg/L)

Phosphorus,

orthophosphate,

dissolved

(mg/L)

04–22–96 4 0.110 0.0100 0.30 0.50 0.310 0.100 0.0400 0.0300

05–21–96 1 .150 .0200 .30 .60 .300 .200 .0700 .0800

06–27–96 9 .100 .0500 .40 .70 .600 .150 .0700 .0600

07–29–96 1 .120 <.010 .40 .70 .200 .120 .0300 .0400

09–03–96 2 .0500 .0200 .30 .40 .390 .0700 .0400 .0600

10–15–96 3 .0700 .0200 .20 .30 .200 .0500 .0400 .0400

11–06–96 2 .0300 .0100 <.20 .20 .0900 .0300 .0400 .0300

12–05–96 11 .0300 .0100 <.20 .30 .380 .0800 .0600 .0400

01–23–97 4 <.015 .0300 <.20 .40 .250 .0600 .0200 <.010

02–07–97 4 .0200 .0100 .30 .40 .160 .0400 <.010 <.010

02–20–97 24 <.015 .0300 .30 .40 .500 .0400 .0100 .0200

03–06–97 6 .0800 .0200 .30 .40 .570 .0800 .0700 .0600

03–20–97 11 .0700 .0300 .30 .30 .500 .120 .110 .0400

04–09–97 12 .0870 .0370 .37 .51 .713 .102 .0650 .0750

04–16–97 7 .0360 .0220 .27 .37 .638 .0710 .0570 .0670

04–23–97 6 .0520 <.010 .23 .30 .427 .0810 .0770 .0520

04–26–97 834 .123 .0230 .35 1.7 .506 .528 .0960 .0990

05–02–97 21 .0470 .0210 .30 .23 .825 .0850 .0690 .0730

05–07–97 17 .0400 .0200 <.20 .32 .685 .0550 .0380 .0700

05–14–97 23 .0560 .0320 .24 .38 .367 .0920 .0650 .0640

05–22–97 21 .0470 .0190 .34 .48 .559 .150 .109 .0840

05–27–97 15 .0550 .0210 .37 .40 .551 .121 .109 .0840

06–02–97 10 .0200 .0100 .26 .30 .413 .126 .0650 .0780

06–17–97 24 .0290 .0150 .26 .42 .517 .111 .0730 .0710

07–03–97 68 <.015 .0110 .35 .24 .602 .0250 .0160 .0270

07–14–97 31 <.015 <.010 <.20 <.20 .975 .0130 <.010 .0270

07–22–97 27 .0260 <.010 .27 .29 .862 .0470 .0310 .0310

07–29–97 21 <.015 <.010 <.20 <.20 .742 .0190 .0420 .0260

08–06–97 5 .0230 .0110 <.20 <.20 .604 <.010 <.010 .0160

08–12–97 6 .0300 <.010 <.20 .34 .339 .0160 .0300 .0290

08–21–97 12 <.015 .0100 <.20 <.20 .548 .0570 .0290 .0250

08–27–97 13 .0150 <.010 <.20 <.20 .560 .0280 .0210 .0220

09–03–97 12 .0180 <.010 <.20 <.20 .433 .0160 .0250 .0160

09–09–97 10 <.015 <.010 <.20 .21 .651 .0160 .0150 .0240

09–17–97 15 <.015 <.010 .23 <.20 .836 .0220 .0230 .0300

09–25–97 25 .0170 <.010 <.20 .28 .784 .0930 .0270 .0370

10–22–97 21 <.015 <.010 <.20 <.20 1.16 .0340 .0270 .0270

Change in MDL for nitrogen, ammonia, dissolved; nitrogen, ammonia plus organic, dissolved;

and nitrogen, ammonia plus organic, total for the following analyses; see table 4

10–28–97 22 <.020 <.010 <.10 .13 1.12 .0350 .0100 .0230

11–06–97 18 <.020 .0390 <.10 .10 1.14 .0210 <.010 .0160

11–19–97 26 .362 <.010 .14 .16 1.11 .0540 .0290 .0420

12–10–97 21 <.020 <.010 <.10 <.10 .904 .0310 <.010 .0180

12–17–97 14 <.020 .0340 <.10 .14 .834 .0660 .0610 .0380

12–20–97 1,230 <.020 .0210 .26 1.4 .629 .456 .0850 .0720

01–06–98 1,180 <.020 <.010 .26 .44 .565 .114 .0410 .0640

01–12–98 25 <.020 <.010 .17 .25 1.05 .0790 .0580 .0530

01–22–98 22 <.020 .0150 <.10 .13 1.10 .0320 .0100 .0280

01–31–98 1,190 .0360 .0110 .35 1.4 .726 .371 .0570 .0520

02–19–98 18 <.020 <.010 .22 .33 .406 .0590 .0350 .0390

02–21–98 1,210 .0520 <.010 .27 1.4 .443 .436 .0750 .0690

03–16–98 1,070 .0360 .0560 .36 2.0 .459 .545 .0790 .0830

03–23–98 31 .0400 <.010 .19 .31 .517 .0400 .0230 .0270

04–27–98 10 .0330 <.010 .26 .26 <.050 .0380 .0260 <.010
22 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Table 7

Table 7. Concentrations of nutrients in samples collected at the integrator site, San Antonio River near Elmendorf

[ft3/s, cubic feet per second; mg/L, milligrams per liter; <, less than; MDL, method detection limit]

Date

Discharge

(ft3/s)

Nitrogen,

ammonia,

dissolved

(mg/L)

Nitrogen,

nitrite,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

dissolved

(mg/L)

Nitrogen,

ammonia

plus organic,

total

(mg/L)

Nitrogen,

nitrite plus

nitrate,

dissolved

(mg/L)

Phosphorus,

total

(mg/L)

Phosphorus,

dissolved

(mg/L)

Phosphorus,

orthophosphate,

dissolved

(mg/L)

04–16–96 138 0.240 0.200 0.90 1.0 15.0 2.20 1.90 1.90

05–21–96 120 .0900 .0700 .70 .80 16.0 2.90 2.60 2.60

06–27–96 292 .110 .0400 .70 1.0 8.10 .850 .710 .720

08–05–96 95 .0400 .0300 .60 .70 12.0 2.20 2.10 2.00

09–03–96 241 .0600 .0300 .60 .80 12.0 2.00 1.90 1.80

10–11–96 148 .0500 .0300 .70 .70 15.0 2.00 2.00 1.10

11–04–96 209 .0300 .0400 .60 .80 14.0 2.30 2.10 1.10

12–05–96 302 .0900 .0200 .60 .80 12.0 1.90 1.90 1.10

01–23–97 147 .150 .170 1.0 1.0 15.0 2.10 2.20 2.10

03–06–97 201 .170 .0900 .90 1.1 14.0 1.80 1.70 1.60

03–20–97 260 .0900 .0700 .80 1.0 11.0 1.60 1.50 1.50

04–09–97 302 .0800 .0400 .60 .90 9.50 2.00 1.90 1.10

04–23–97 222 .229 .168 .81 .97 12.8 2.15 1.99 2.00

04–26–97 2,750 .138 .0300 .37 2.8 2.44 1.36 .370 .400

05–06–97 186 .125 .101 .67 .85 12.2 2.17 2.03 2.01

05–22–97 315 .0500 .0300 .52 .95 7.92 .885 .720 .720

06–02–97 197 .0320 .0780 .55 .74 9.48 1.11 1.08 1.00

06–21–97 4,850 .0540 .0590 .28 1.1 1.84 .536 .490 .270

07–14–97 856 .0440 .0260 <.20 .44 4.48 .522 .420 .420

07–30–97 365 .0830 .0630 .55 .62 7.63 1.23 1.13 .960

08–14–97 180 .0430 .0360 .56 .74 10.7 1.54 1.44 1.38

08–21–97 135 .0230 .0460 .51 .62 10.6 1.50 1.35 1.33

09–11–97 299 .0840 .151 .59 .81 10.5 1.60 1.51 1.34

09–17–97 142 .0310 .0490 .63 .70 11.4 1.92 1.79 1.66

10–08–97 1,110 .0300 .0350 .48 1.3 4.90 .935 .570 .540

10–22–97 324 .0840 .0740 .47 .53 9.77 1.70 1.62 1.62

Change in MDL for nitrogen, ammonia, dissolved; nitrogen, ammonia plus organic, dissolved;

and nitrogen, ammonia plus organic, total for the following analyses; see table 4

11–19–97 318 <.020 .0210 .55 .59 10.8 1.63 1.70 1.58

12–17–97 212 <.020 .0620 .65 .75 11.8 2.00 1.99 1.74

12–20–97 1,250 <.020 .0350 .37 3.7 3.04 1.50 .360 .350

01–06–98 1,050 .124 .0430 .57 .78 3.37 .497 .460 .450

01–22–98 265 .0210 .0360 .52 .62 10.8 1.54 1.47 1.40

01–31–98 1,270 .0300 .0130 .35 1.2 2.88 .608 .340 .370

02–19–98 462 .125 .0450 .67 .91 6.97 .888 .810 .770

02–21–98 1,080 .127 .0180 .35 1.7 3.00 .776 .380 .340

03–16–98 1,360 .129 .0690 .40 3.1 2.31 1.26 .270 .270

03–26–98 812 .0410 .0370 .24 .55 3.91 .492 .400 .380

04–28–98 218 .369 .108 .98 1.1 11.3 1.81 1.73 1.76
Table 8 23

Table 8 Table 8. Number of detections and range in concentration of pesticides detected in samples collected at intensive fixed sites

[μg/L, micrograms per liter]

1 Values below method detection limit estimated by U.S. Geological Survey National Water-Quality Laboratory.

Agriculture indicator site Urban indicator site Integrator site

Pesticide

Number ofdetections

(32 samples)

Range in

concentration

(μg/L)

Pesticide

Number ofdetections

(35 samples)

Range in

concentration

(μg/L)

Pesticide

Number ofdetections

(26 samples)

Range in

concentration

(μg/L)

Atrazine 32 0.0060–.11 Alachlor 3 0.0040–.018 Alachlor 4 0.0030–.020Benfluralin 3 .0026–.0047 Atrazine 35 .0040–.75 Atrazine 26 .0080–.38Carbaryl 4 .0044–.090 Benfluralin 2 .0036–.0047 Carbaryl 16 .0051–.36Chlorpyrifos 7 1.0033–.0075 Bentazon 1 .020 Carbofuran 5 .012–.042

2,4-D 1 .090 Bromacil 7 .060–1.6 Chlorpyrifos 15 .0028–.0094DCPA 1 1.0019 Carbaryl 21 .0071–.33 DCPA 9 1.0006–.016

p,p'-DDE 2 1.0015–.0030 Chlorpyrifos 16 1.0027–.011 Deethylatrazine 25 .0067–.029

Deethylatrazine 32 .0020–.11 Cyanazine 2 .0045–.011 Diazinon 20 .0060–.16Diazinon 12 1.0010–.059 2,4-D 1 .22 Diuron 3 .070–.24

Malathion 3 .0040–.0080 DCPA 6 .0006–.0031 alpha-HCH 1 .014Metolachlor 6 1.0010–.0060 p,p'-DDE 4 .0017–.0046 Lindane 16 .0050–.044Prometon 20 .0023–.0094 Deethylatrazine 34 1.0016–.054 Malathion 7 .0060–.036

Simazine 2 1.0018–.0026 Diazinon 27 .0040–.33 Metolachlor 18 .0030–.016

Terbufos 1 .0080 Diuron 15 .020–1.1 Prometon 26 1.0056–.044

Trifluralin 2 .0022–.0052 Malathion 13 .0050–.11 Propachlor 1 1.0009

MCPA 1 .080 Simazine 21 1.0025–.043Metolachlor 16 .0030–.018 Tebuthiuron 15 1.0073–.43Metribuzin 1 .029 Terbufos 1 1.011Oryzalin 1 .030Prometon 35 1.016–.24

Pronamide 1 .014Propanil 1 .0078Simazine 29 1.0039–.25Tebuthiuron 34 .041–2.8Triclopyr 2 .070–.81
24 Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, and Volatile

Organic Compounds at Three Intensive Fixed Sites, 1996–98

Table 9

Table 9. Number of detections and range in concentration of volatile organic compounds detected in samples collected at intensive fixed sites[μg/L, micrograms per liter]

1 Values below method detection limit estimated by U.S. Geological Survey National Water-Quality Laboratory.

Agriculture indicator site Urban indicator site Integrator site

Volatile organic

compound

Number ofdetections

(7 samples)

Range in

concentration

(μg/L)

Volatile organic

compound

Number ofdetections

(22 samples)

Range in

concentration

(μg/L)

Volatile organic

compound

Number ofdetections

(13 samples)

Range in

concentration

(μg/L)

Acetone 1 10.70 Acetone 7 11.0–6.1 Acetone 3 11.0–2.0

Benzene 2 .036–.10 Benzene 4 1.013–.028 Benzene 5 1.015–.082

Bromoform 3 1.020–.030 2-Butanone 1 .90 Bromochloromethane 1 1.020Carbon disulfide 4 1.0080–.030 Carbon disulfide 6 1.020–.025 Bromoform 10 1.050–.28Chlorodibromomethane 1 1.020 Chloromethane 4 1.010–.024 2-Butanone 3 1.30–1.2Chloroform 4 1.0080–.030 1,4-Dichlorobenzene 2 1.010 Carbon disulfide 7 1.010–.033Dichlorobromomethane 2 1.010 1,1-Dichloroethane 1 1.013 Chlorobenzene 1 1.0060Ethylbenzene 2 1.0090–.030 cis-1,2-Dichloroethene 18 1.010–.065 Chlorodibromomethane 13 .10–1.9

p-Isopropyltoluene 4 1.0040–.081 Ethylbenzene 2 1.010–.012 Chloroform 13 .40–4.6Naphthalene 1 1.020

p-Isopropyltoluene 3 1.0070–.010 Chloromethane 2 1.020–.024

Toluene 2 1.010–.14 Methyl tert-butyl ether (MTBE) 13 .074–.57 2-Chlorotoluene 1 .050

1,2,4-Trimethylbenzene 1 1.010 Methylene chloride 1 .80 Dibromomethane 1 1.010

1,3,5-Trimethylbenzene 1 1.0050 4-Methyl-2-pentanone (MIBK) 1 1.10 1,4-Dichlorobenzene 11 1.0080–.030meta- and para-Xylene 3 1.010–.040 Napthalene 1 1.083 Dichlorobromomethane 13 .19–2.3

ortho-Xylene 1 1.020 Styrene 2 1.0050–.020 cis-1,2-Dichloroethene 1 1.014

Tetrachloroethene 5 1.0020–.0070 Ethylbenzene 3 1.010–.036Toluene 12 1.020–.040 o-Ethyl toluene 1 1.0111,1,2-Trichlorotrifluoroethane 2 1.0050–.0060

p-Isopropyltoluene 3 1.010–.026

1,2,4-Trimethylbenzene 5 1.0080–.038 Methyl tert-butyl ether (MTBE) 1 .080

meta- and para-Xylene 5 1.015–.038 Methyl iodide 1 1.010

ortho-Xylene 3 1.011–.020 4-Methyl-2-pentanone (MIBK) 1 1.080

Naphthalene 2 1.081–.13n-Propylbenzene 1 1.0080Styrene 1 1.010Tetrachloroethene 4 1.0020–.0080Tetrahydrofuran 4 1.40–3.91,2,3,5-Tetramethylbenzene 1 1.026Toluene 7 .040–.211,2,3-Trimethylbenzene 2 1.022–.0231,2,4-Trimethylbenzene 2 .050–.0531,3,5-Trimethylbenzene 1 1.016meta- and para-Xylene 4 1.010–.056ortho-Xylene 2 1.029–.034
Ging— Water-Quality Assessment of South-Central Texas—Descriptions and Comparisons of Nutrients, Pesticides, —USGS WRIR 99–4155

and Volatile Organic Compounds at Three Intensive Fixed Sites, 1996–98

Austin, TX 78754–47338027 Exchange Dr.U.S. Geological SurveyDistrict Chief