Pesticides in Surface Water Measured at 

Select Sites in the Sacramento River Basin, 

California, 1996–1998

By Joseph Domagalski

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00-4203

National Water-Quality Assessment Program

Sacramento, California

2000

6230-27
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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. Geological Survey.

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

District Chief

U.S. Geological Survey

Placer Hall

6000 J Street

Sacramento, CA 95819-6129
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 60 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 the 60 study units and more than twothirds of the people served by public water-supply 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 Area ........................................................................................................................................... 2

Sampling Collection and Analysis......................................................................................................................................... 6

Selection of Sampling Sites......................................................................................................................................... 6

Collection Procedure ................................................................................................................................................... 11

Detection Frequency.............................................................................................................................................................. 15

Storm-Water Runoff .............................................................................................................................................................. 21

Comparison of Sampling Frequencies................................................................................................................................... 22

Summary and Conclusions .................................................................................................................................................... 23

References Cited.................................................................................................................................................................... 23

FIGURES

1. Map showing location of the Sacramento River Basin study unit, California....................................................... 3

2. Map showing physiographic provinces of the Sacramento River Basin, California.............................................. 4

3. Map showing land uses and land cover of the Sacramento River Basin, California ............................................. 5

4. Drainage basin boundary, land uses, and land cover of the Colusa Basin Drain drainage basin 

and location of the Colusa Basin Drain at Road 99E near Knights Landing site, 

Sacramento River Basin, California....................................................................................................................... 7

5. Drainage basin boundary, land uses, and land cover of the Arcade Creek drainage basin 

and location of the Arcade Creek near Del Paso Heights site, Sacramento River Basin, California..................... 8

6. Drainage basin boundary, land uses, and land cover of the Sacramento River at Freeport drainage basin 

and location of the Sacramento River at Freeport site, Sacramento River Basin, California ................................ 9

7. Drainage basin boundary, land uses, and land cover of the Yolo Bypass at Interstate 80 near West 

Sacramento drainage basin and location of the Yolo Bypass at Interstate 80 near West Sacramento site,

Sacramento River Basin, California....................................................................................................................... 10

8. Time series plot of molinate concentrations at the Colusa Basin Drain 

at Road 99E near Knights Landing site, Sacramento River Basin, California ...................................................... 20

9. Time series plot of diazinon concentrations at the Arcade Creek near Del Paso Heights site and the 

Colusa Basin Drain at Road 99E near Knights Landing site, Sacramento River Basin, California ...................... 21

TABLES

1. Pesticides analyzed and reporting limits in water samples, in the Sacramento River Basin, California .................. 12

2. Recoveries and standard deviations of pesticides measured by gas chromatography/mass spectrometry................ 13

3. Recoveries and standard deviations of pesticides measured by high performance liquid 

chromatography/ultraviolet light spectrometry......................................................................................................... 14

4. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by gas chromatography/mass spectrometry, in Colusa Basin Drain at Road 99E near Knights Landing, 

Sacramento River Basin, California.......................................................................................................................... 15

5. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by gas chromatography/mass spectrometry, in Arcade Creek near Del Paso Heights, 

Sacramento River Basin, California.......................................................................................................................... 16

6. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by gas chromatography/mass spectrometry, in Sacramento River at Freeport, 

Sacramento River Basin, California.......................................................................................................................... 17
VI Contents

7. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples 

analyzed by gas chromatography/mass spectrometry, in Yolo Bypass at Interstate 80 near West Sacramento, 

Sacramento River Basin, California ......................................................................................................................... 18

8. Maximum, minimum and median concentrations, and detection frequency of pesticides in water samples

analyzed by high performance liquid chromatography/ultraviolet light spectrometry, in Colusa Basin Drain

at Road 99E near Knights Landing, Sacramento River Basin, California................................................................ 18

9. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by high performance liquid chromatography/ultraviolet light spectrometry, in Arcade Creek 

near Del Paso Heights, Sacramento River Basin, California.................................................................................... 19

10. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by high performance liquid chromatography/ultraviolet light spectrometry, 

in Sacramento River at Freeport, Sacramento River Basin, California .................................................................... 19

11. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples

analyzed by high performance liquid chromatography/ultraviolet light spectrometry, 

in Yolo Bypass at Interstate 80 near West Sacramento, Sacramento River Basin, California .................................. 19

CONVERSION FACTORS, VERTICAL DATUM AND ABBREVIATIONS

Vertical Datum

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum 

of 1929—a geodetic datum derived from a general adjustment of the first-order level nets 

of the United States and Canada, formerly called Sea Level Datum of 1929.

Acronyms

GC/MS gas chromatography/mass spectrometry

GIRAS Geographic Retrieval and Analysis System

HPLC high performance liquid chromatography

NAWQA National Water-Quality Assessment

PERA probabilistic ecological risk assessment

TMDL total maximum daily load

UVD untraviolet light detection

μg/L microgram per liter

Multiply By To obtain

centimeter (cm) 0.3937 inch

cubic meter (m3

) 35.31 cubic foot

cubic meter per second (m3/s) 35.31 cubic foot per second

kilogram (kg) 2.205 pound (avoirdupois)

meter (m) 3.281 foot

square kilometer (km2

) 0.3861 square mile
Introduction 1

Pesticides in Surface Water Measured at 

Select Sites in the Sacramento River Basin, 

California, 1996–1998

By Joseph Domagalski

Abstract

Pesticides were measured in one urban 

stream, one agricultural stream, one site on the 

Sacramento River, and one large flood control 

channel over a period of 18 months during 

1996–1998. All sites were located within the 

Sacramento River Basin of California. 

Measurements were made on 83 pesticides or 

pesticide transformation products by either gas 

chromatography/mass spectrometry or by high 

performance liquid chromatography with 

ultraviolet light spectrometry. Some pesticides 

were detected frequently at the agricultural stream 

and downstream in the Sacramento River and at 

the flood control channel of the Sacramento River. 

These were pesticides related to rice farming 

(molinate, carbofuran, thiobencarb, and bentazon); 

herbicides used both agriculturally or for roadside 

maintenance (diuron, simazine, and metolachlor); 

or insecticides used on orchards and row corps 

(diazinon and chlorpyrifos). No pesticide concentrations above enforceable water quality criteria 

were measured at either the agricultural site or the 

Sacramento River sites. In contrast to the agricultural site, insecticides used for household, lawn, or 

garden maintenance were the most frequently 

detected pesticides at the urban site. Diazinon, an 

organophosphate insecticide, exceeded recommended criteria for the protection of aquatic life, 

and the diazinon levels were frequently above 

known toxic levels for certain zooplankton species 

at the urban site. Because of the low discharge of 

the urban stream, pesticide concentrations were 

greatly diluted upon mixing with Sacramento 

River water.

INTRODUCTION

To obtain a better understanding of the current 

status of pesticide concentrations in the rivers of the 

United States, the U.S. Geological Survey has been 

collecting data throughout the United States as part of 

the National Water-Quality Assessment (NAWQA) 

Program. The objectives of the NAWQA Program are to 

describe the status of, and trends in, the quality of a 

large, representative part of the nation’s surface- and 

ground-water resources and to provide a sound scientific understanding of the primary natural and human 

factors affecting the quality of these resources (Leahy 

and others, 1990). Gilliom and others (1995) detailed 

the general design of the NAWQA data collection 

program for pesticides. The program includes the collection of water samples for pesticides within relatively 

small watersheds, the land uses representing the major 

agricultural practices within a study unit or urban area, 

and the collection of water samples for pesticides at 

larger streams downstream of major agricultural and 

urban areas. In that way, information can be obtained on 

the effect of urban and agricultural runoff on receiving 

bodies of water of small to larger size. 

Pesticide residues in streams can have adverse 

effects on aquatic life or limit the beneficial uses of a 

stream, such as the use of stream water as a source of 

drinking water. Insecticide residues might affect 

aquatic insect populations, and herbicide residues 

might limit the growth of certain algal species. Fish 

populations can suffer indirect effects such as loss of a 

source of food when parts of an ecological system or 

food web are disrupted, as well as acute or chronic toxic 

effects from both insecticides and herbicides (Kuivila 

and Foe, 1995; Bennett and others, 1998). The use of a 
2 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

stream as a source of drinking water also can be limited 

when pesticide concentrations exceed regulatory 

criteria or when standard water treatment is unable to 

lower concentrations to meet regulatory criteria.

The Sacramento River Basin of California 

(fig. 1) was chosen as a NAWQA study unit, and data 

collection activities began in 1995. Previous studies of 

pesticides in this river basin have demonstrated that 

organophosphate insecticides such as diazinon can be 

present in concentrations sufficiently high to result in 

toxicity to aquatic insects, such as zooplankton 

(Ceriodaphnia dubia), and that residues of these 

organophosphate insecticides are present either 

seasonally after application on select crops or 

throughout the year, such as in urban streams (Kuivila 

and Foe, 1995; MacCoy and others, 1995; Domagalski, 

1996; Cooke and Connor, 1998). Other pesticides 

detected in previous studies include organo-nitrogen 

herbicides, such as simazine and molinate, and 

insecticides, such as carbofuran and chlorpyrifos 

(Cornacchia and others, 1984; MacCoy and others, 

1995; Domagalski, 1996; Cooke and Connor, 1998).

Diazinon and chlorpyrifos are used in orchards to 

control insects and in urban areas for household and 

garden or lawn pest control. The heaviest agricultural 

applications on orchards typically occur during winter, 

the season of highest rainfall. The organophosphate 

insecticides are applied to orchards during the winter to 

control the insect populations present in the subsequent 

spring. Runoff, following rainfall, may transport 

residues of diazinon and chlorpyrifos to receiving water 

bodies such as the Sacramento River (Domagalski, 

1996). 

Rice cultivation is the major agricultural land use 

of the Sacramento Valley. Rice fields are flooded during 

the May through September growing season, and 

controlled drainage of rice field water is a major source 

of herbicides and insecticides in agricultural streams 

(Domagalski and Kuivila, 1991). Herbicides are 

applied to rice fields during May to control aquatic 

grasses and other weeds, and insecticides are applied 

during May and June to prevent damage to the rice crop 

from aquatic organisms. 

Purpose and Scope

The purpose of this report is to present the results 

of pesticide monitoring for four select streams of the 

Sacramento River Basin: one urban stream, one 

agricultural stream, one site on the Sacramento River, 

and a flood control channel of the Sacramento River 

Basin. The study spans the period from November 1996 

to April 1998. The principal land use within the 

watershed of one stream, Arcade Creek near Del Paso 

Heights, was urban, whereas that of another stream, the 

Colusa Basin Drain at Road 99E near Knights Landing, 

was agricultural. The Sacramento River at Freeport, 

located near the mouth of the Sacramento River Basin, 

and another site on a flood control channel, the Yolo 

Bypass at Interstate 80 near West Sacramento, also 

were sampled for pesticides. The sites on the 

Sacramento River and the flood control channel 

represent an integration of all upstream land uses. 

Forest and rangeland cover about 80 percent of the 

basin upstream from the lower Sacramento River 

sampling site, agriculture is about 17 percent, and 

urban is about 1 percent.

Description of Study Area

The Sacramento River Basin (fig. 1) occupies 

nearly 70,000 km2 in the north central part of California 

(Domagalski and Brown, 1994). The Sacramento River 

is the largest river in California, with an average annual 

runoff of 27,100,000,000 m3 (California Department of 

Water Resources, 1993). The basin includes six 

physiographic provinces: the Great Basin, the Middle 

Cascade Mountains, the Sierra Nevada, the Klamath 

Mountains, the Central Valley, and the Coast Ranges 

(fig. 2). The northern part of the Central Valley, the 

Sacramento Valley, is the low-lying part of the basin; all 

other physiographic provinces are mountainous. Land 

cover or land use of the mountainous parts of the study 

unit is principally forest, although forest and rangeland 

are mixed in parts of the Coast Ranges and the Great 

Basin (fig. 3). Domagalski and others (1998) provide 

more information on the physiographic provinces of the 

Sacramento River Basin. The Sacramento Valley, which 

has the greatest population of the study unit, also is the 

region where the greatest effects, or potential effects, to 

surface water and ground water from agricultural and 

urban land use likely will occur. The Sacramento Valley 

also is the location of the greatest water use of the basin. 

Land uses and land cover are shown in figure 3.

The Sacramento Valley supports a diverse 

agricultural economy, much of which depends on the 

availability of irrigation water. More than 8,100 km2

 are 

irrigated. The major crops are rice, fruits and nuts, 

tomatoes, sugar beets, corn, alfalfa, and wheat. Dairy 

products also are important agricultural commodities. 

The soils of the Sacramento Valley are mostly clay with 

very slow to slow infiltration rates (Soil Conservation 

Service, 1993). Because of the widespread presence of 

clay soils and associated slow infiltration rates, and the 

availability of sufficient irrigation water from the 

Sacramento River, rice farming is possible. Rice 

production includes the seasonal creation of temporary 

wetlands. 

The largest cities of the basin are in the 

Sacramento Valley, including Chico, Red Bluff, 
Introduction 3

Figure 1. Map showing location of the Sacramento River Basin study unit, California.

Colusa

Red Bluff

Redding

80

Ar cade

Creek

Butte Creek

Feather River

Mill Creek

Colusa Basin

Drain

Sacramento

River

Cache Creek

Stony Creek

Deer Creek

Elder Creek

Thomes Creek

Spring

Creek

Keswick Reservoir 

Cottonwood Creek

123

41

122

121

40

39

McCloud River

Pit River

Big Ch ico Creek

Clear

Lake

Lake

Oroville

Shasta

Lake

Sacramento

River Basin

study unit

Sacramento

Valley

San

Francisco

Bay

Sutter

Buttes

CALIFORNIA

0

0 20 40 KILOMETERS

20 40 MILES

Sampling Site

 1 Colusa Basin Drain at Road 99E

 near Knights Landing

 2 Arcade Creek near Del Paso Heights

 3 Yolo Bypass at Interstate 80 

 near West Sacramento

4 Sacramento River at Freeport 

Study unit boundary

EXPLANATION

Chico

1

2

4 3

Sacramento

Slough Folsom Lake

Bear River

Yuba River

American

River

 Marysville

Del Paso Heights

Freeport

Yolo Bypass

Sacramento
4 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

Figure 2. Map showing physiographic provinces of the Sacramento River Basin, California.

Great Basin

Middle Cascade Mountains

Sierra Nevada

Klamath Mountains

Central Valley

Coast Ranges

Study unit boundary

Physiographic Provinces

EXPLANATION

0 20 40 MILES

0 20 40 KILOMETERS

123°

41°

122°

121°

40°

39°
Introduction 5

Figure 3. Map showing land uses and land cover of the Sacramento River Basin, California.

Barren land

Cropland and pasture

Forest land

Orchards

Perennial snow and ice

Range land

Tundra

Water

Wetland

Urban

Study unit boundary

EXPLANATION

123

41

122

121

40

39

0

0

20

20 40 Miles

40 Kilometers
6 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

Redding, and Sacramento. The Sacramento 

metropolitan area is home to more than a million 

people, which is nearly one half of the total population 

of 2,208,900 people in the Sacramento Valley (U.S. 

Department of Commerce, 1992). Urban runoff is 

another potential source of pesticides to receiving 

bodies of waters. Because the Sacramento metropolitan 

area is the largest urban area in the study unit, a stream 

was chosen in that location to determine the effects of 

the urban land use on water quality.

The average annual precipitation for the 

Sacramento River Basin is 91.4 cm, most of which 

occurs as rain or snow during November through 

March. Irrigation water is required for agriculture 

during the summer growing season because there is 

little or no rain. Two important hydrological events or 

climatic conditions occurred during this period of 

study. The first was a major flood that started on 

January 1, 1997, and affected a major part of the 

Sacramento River below Shasta Lake and tributaries. 

Rainfall was especially heavy within the Feather and 

Yuba river basins. The second was the El Niño winter 

of 1997–1998. The term El Niño refers to a weather 

phenomenon that occurs when masses of warmer than 

normal water build over much of the North and South 

American coasts during winter. The frequent result of 

an El Niño winter is higher than normal precipitation in 

northern California because of changes in the location 

of the jet stream (Philander, 1990). 

All of the major rivers of the basin—the 

Sacramento, the Feather, the American, and the Yuba—

are impounded just above the margin of the Sacramento 

Valley (fig. 1). The reservoirs are managed to collect 

snowmelt and to provide flood protection during the 

winter and irrigation water during the summer. The 

upper Sacramento, the McCloud, and the Pit rivers 

supply water to Shasta Lake, which has a capacity of 

5,614,809,153 m3

. Lake Oroville, on the Feather River, 

has a capacity of 4,363,565,216 m3

. The reservoirs are 

managed for flood protection during the winter and 

provide water for irrigation to farms during the spring 

and summer and for cities throughout the year. The 

water entering most of the reservoirs tends to be of high 

quality because it originates as melting snow.

The rocks of the Sacramento River Basin are of a 

diverse assemblage. The Coast Ranges consist of ocean 

sediments and volcanics. The Klamath Mountains 

include accreted terrains, oceanic crust, and subduction 

zone complexes. The Cascade Mountains and the Great 

Basin consist predominantly of volcanics. The Sierra 

Nevada assemblage includes granitic plutons, volcanic, 

sedimentary, and metamorphic rocks. The Sacramento 

Valley is mainly composed of a thick assemblage of 

sediments derived from these adjacent highlands. The 

uppermost layer of Sacramento Valley sediments tends 

to produce clay soils over much of the valley. The 

coarser-grained soils found near the river channels are 

where most of the orchards are located (Domagalski 

and others, 1998).

SAMPLING COLLECTION AND ANALYSIS

Selection of Sampling Sites

Four sampling sites were chosen for this study: 

Arcade Creek near Del Paso Heights, located in an 

urban basin; Colusa Basin Drain at Road 99E near 

Knights Landing, located downstream of primarily 

agricultural land uses; Sacramento River at Freeport, 

located on the Sacramento River, integrated urban, 

agricultural, and other land uses; and Yolo Bypass at 

Interstate 80 near West Sacramento, a flood control 

channel used to control flooding on the lower 

Sacramento River system. Water is diverted to the Yolo 

Bypass when flow on the Sacramento River is projected 

to exceed channel capacity. Site locations and drainage 

basin boundaries associated with these sites are shown 

in figures 4 through 7. The drainage basin sizes of 

Arcade Creek, Colusa Basin Drain, Sacramento River 

at Freeport, and Yolo Bypass are 88; 4,274; 59,570; and 

59,388 km2

, respectively. 

Different land-use settings were investigated as 

part of this study. Agricultural land use within the 

Colusa Basin Drain is mainly rice production with 

lesser amounts of land planted in row crops or orchards. 

Variable amounts of land are placed in rice, depending 

on water availability for irrigation and crop rotation 

management. Orchard crops include walnuts, almonds, 

prunes, and other fruits. Water samples were collected 

at the Colusa Basin Drain at Road 99E near Knights 

Landing monthly from November 1996–March 1997 

and August 1997–April 1998, and twice per month 

from April 1997–July 1997. Water samples were 

collected at the Sacramento River at Freeport, a large 

river site where water quality is affected by different 

land uses, monthly from November 1996–March 1997 

and from August 1997–February 1998, and twice per 

month from April 1997–July 1997. For both the Colusa 

Basin Drain at Road 99E near Knights Landing and the 

Sacramento River at Freeport sites, the twice per month 

sampling was designed to coincide with the most 

intense use of pesticides in rice production. The Arcade 

Creek drainage basin is mainly urban land use. Water 

samples were collected monthly at the Arcade Creek 

near Del Paso Heights from November–December 

1996 and December 1997–April 1998, and twice per 

month from January–November 1997. Water was 

collected at the Yolo Bypass at Interstate 80 near West 
Sampling Collection and Analysis 7

Figure 4. Drainage basin boundary, land uses, and land cover of the Colusa Basin Drain drainage basin and location of the Colusa Basin 

Drain at Road 99E near Knights Landing site, Sacramento River Basin, California. GIRAS, Geographic Retrieval and Analysis System.

1% Urban

65% Agricultural

18%

Rangeland

16% Forest land

10 Miles

10 Kilometers

0

0

39

39

30

122 122

 30

5

5

<1% Water, wetland

 & barren land

100

50

0

150

200

250

300

350

400

450

Barren land

Cropland and pasture

Forest land

Orchards

Range land

Water

Wetland

Urban

Sampling site

41

122 121

40

Discharge at Road 99E, near Knights Landing

Discharge at Highway 20

Samples collected

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

1995 1996 1997 1998

Daily mean discharge,

in cubic meters per second

EXPLANATION

GIRAS land use/landcover
8 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

79% Urban

19% Ag

2% Barren land

EXPLANATION

GIRAS land use/landcover

Barren land

Cropland and pasture

Urban

Sampling site

Mean annual discharge (0.48 cubic meters per second)

Daily mean discharge

Samples collected

0 5 Kilometers

0 2.5 5 Miles

2.5

121 15

121 22 30

38 15

38 15

41

122 121

40

60

50

40

30

20

10

0

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

Daily mean discharge,

in cubic meters per second

1995 1996 1997 1998

Figure 5. Drainage basin boundary, land uses, and land cover of the Arcade Creek drainage basin and location of the Arcade Creek near Del

Paso Heights site, Sacramento River Basin, California. GIRAS, Geographic Retrieval and Analysis System.
Sampling Collection and Analysis 9

Urban 

Croplands and pasture

Orchards, etc.

Rangeland

Forest land

Water

Wetlands

Barren land

Tundra

Perennial snow or ice

Sampling site

16% Agricultural

63% 17% Rangeland

Forest 

land

1% Water

1% Barren land

1% Wetland

39

41

122 121

0 20 40 Kilometers

2% Urban

41

121

40

Mean annual discharge (666 cubic meters per second)

Daily mean discharge

Samples collected

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

1995 1996 1997 1998

3,500

3,000

2,500

2,000

1,500

1,000

500

0

Daily mean discharge,

in cubic meters per second

EXPLANATION

GIRAS land use/landcover

0 20 40 Miles

122

Figure 6. Drainage basin boundary, land uses, and land cover of the Sacramento River at Freeport drainage basin and location of the 

Sacramento River at Freeport site, Sacramento River Basin, California. GIRAS, Geographic Retrieval and Analysis System.
10 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

1% Urban

17% Agricultural

18%

Rangeland

61%

Forest

land

1% Water

1% Wetlands

1% Barren land

121

122 o

o

39 o

41 o

0

0

20 40 Miles

20 40 Kilometers

Daily mean discharge

Samples collected

12,000

10,000

8,000

6,000

4,000

2,000

0

Daily mean discharge,

in cubic meters per second

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

Oct.

Dec.

Feb.

April

June

Aug.

1995 1996 1997 1998

41

121

40

122

Urban 

Croplands and pasture

Orchards, etc.

Rangeland

Forest land

Water

Wetlands

Barren land

Tundra

Perennial snow or ice

Sampling site

EXPLANATION

GIRAS land use/landcover

Figure 7. Drainage basin boundary, land uses, and land cover of the Yolo Bypass at Interstate 80 near West Sacramento drainage basin, and

location of the Yolo Bypass at Interstate 80 near West Sacramento site, Sacramento River Basin, California. GIRAS, Geographic Retrieval 

and Analysis System.
Sampling Collection and Analysis 11

Sacramento on a monthly basis whenever water was 

flowing.

Sampling site selection was based on known 

water quality impairments of these streams with respect 

to pesticides. Several streams within the Sacramento 

River Basin are on the recent 303(d) (U.S. Environmental Protection Agency, accessed October 1, 1999) 

listing of impaired water bodies with the impairment 

attributed to pesticides. The Clean Water Act of the 

United States requires states to list rivers with known 

water quality impairments and to rank the impairment 

according to severity. The most recent listing available 

for the Sacramento River Basin is 1998. Both Colusa 

Basin Drain and Arcade Creek are listed as impaired by 

pesticides. The Colusa Basin Drain impairment is 

attributed to carbofuran, organochlorine pesticides, 

malathion, and methyl parathion. The impairment of 

Arcade Creek is attributed to chlorpyrifos and diazinon. 

Other streams listed as impaired by pesticides are the 

lower American River (organochlorine pesticides), the 

lower Feather River (diazinon), and the lower 

Sacramento River (diazinon). The impairment priority 

for both the lower Feather and Sacramento rivers is 

listed as high (U.S. Environmental Protection Agency, 

accessed October 1, 1999), which indicates that action 

should be taken to establish a control plan for pesticides, including the establishment of a total maximum 

daily load (TMDL). Sites listed as a medium to low 

priority will be given a TMDL plan only after all those 

for high priority sites are completed. The severity of 

impairments for the Colusa Basin Drain and Arcade 

Creek are listed as medium. The stream segments listed 

have been known to have concentrations of pesticides, 

periodically, in excess of amounts known to be toxic, or 

have been positively linked to toxicity (Cooke and 

Connor, 1998). 

Collection Procedures

Water samples were collected by either the equal 

width increment method or the equal discharge 

increment method (Edwards and Glysson, 1999). A 

modification of the method was necessary for the 

Sacramento River at the Freeport site. The method of 

Edwards and Glysson (1999) is only applicable to 

depths of approximately 5 m. Although the depth of 

water at the Sacramento River at the Freeport site was 

more than 5 m, only the upper 5 m were sampled. All 

sampling material was constructed of Teflon to 

minimize contamination or sampling artifacts. The 

sampling methods conformed to guidelines specified 

for the NAWQA Program (Shelton, 1994). After 

collection, the water sample was split for analyses of 

pesticides and other water quality constituents using a 

Teflon cone splitter (Capel and others, 1995).

Water samples for pesticide analyses were 

filtered shortly after collection using 0.7-micrometer 

glass fiber filters in an aluminum filter support and 

pumped through Teflon tubing. Following filtration, 

water samples were spiked with surrogate pesticide 

compounds. The surrogate compounds were either 

isotopically labeled pesticides that could be distinguished from pesticides used agriculturally or were of a 

similar chemical structure to agriculturally used 

pesticides. The purpose of the surrogate was to assess 

the overall efficiency of the analytical procedure. 

Pesticides were then extracted from water onto C-18 

and Carbopak-B cartridges. The cartridges were then 

shipped to the U.S. Geological Survey Laboratory in 

Denver, Colo., where the pesticides were eluted from 

the cartridges using appropriate solvents and analyzed 

according to the methods of Zaugg and others (1995) 

and Werner and others (1996). The method of Zaugg 

and others (1995) utilizes gas chromatography/mass 

spectrometry (GC/MS) and that of Werner and others 

(1996) utilizes high performance liquid chromatography (HPLC) with ultraviolet light spectrometry. 

Pesticides analyzed and their reporting limit by GC/MS 

or HPLC with ultraviolet light spectrometry are listed 

in table 1.

Quality control procedures included the 

collection of blank, replicate, and spiked samples. The 

procedure for blank samples was to place pesticide-free 

water through all of the sampling, filtration, and extraction steps to determine whether any artifacts interfered 

with the analysis. The results of the blanks indicated 

that no problems were present. Only one detection, 

EPTC, was noted at an estimated concentration of 

0.0011 μg/L (microgram per liter). EPTC was 

measured in environmental samples, but usually at 

higher levels than 0.0011 μg/L. Because only one 

detection of EPTC occurred out of six blank analyses, 

the environmental data for that compound were not 

censured. Replicates are designed to show the reproducibility of the method. The interpretation of replicate 

samples depends on the presence of pesticides in the 

environmental samples. For those compounds present, 

the replicate analyses were within the precision specified by Zaugg and others (1995) and Werner and others 

(1996). Spiked samples provide a better representation 

of analytical performance as known amounts of 

selected analytes are added to a water sample. Samples 

are equally split and pesticides are measured in both 

unspiked and spiked samples. The overall recovery of 

pesticides can then be calculated and compared to the 

published precision of the analytical method for each 

pesticide. The recoveries of pesticides spiked into water 

and analyzed by GC/MS and high performance liquid 

chromatography/ultraviolet light spectrometry are 

shown in tables 2 and 3, respectively. Not all pesticides 
12 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

Table 1. Pesticides analyzed and reporting limits in water samples, in the Sacramento River Basin, California 

[*, transformation product; number in parentheses indicates method reporting limit, in micrograms per liter] 

1

 Solid-phase extraction and gas chromatography/mass spectrometry (GC/MS) correspond to the method reporting limit.

2

 Solid-phase extraction and high performance liquid chromatography (HPLC) with ultraviolet light spectrometry correspond to the

method reporting limit.

Compound Compound Compound

Amides

Alachlor (0.002)1 Napropamide (0.003)1 Propanil (0.004)1

Metolachlor (0.002)1 Propachlor (0.007)1 Propyzamide (0.003)1

Carbamates

Aldicarb (0.016)2 Carbofuran, 3-Hydroxy* (0.014)2 Pebulate (0.004)1

Aldicarb sulfone* (0.016)2 EPTC (0.002)1 Propham (0.035)2

Aldicarb sulfoxide* (0.021)2 Methiocarb (0.026)2 Propoxur (0.035)2

Butylate (0.002)1 Methomyl (0.017)2 Thiobencarb (0.002)1

Carbaryl (0.003)1, (0.008)2 Molinate (0.004)1 Triallate (0.001)1

Carbofuran (0.003)1, (0.028)2 Oxamyl (0.018)2

Chlorophenoxy herbicides

2,4,5-T (0.035)2 Dacthal, mono-acid* (0.017)2 MCPB (0.035)2

2,4-D (acid) (0.035)2 Dichlorprop (2,4-DP) (0.032)2 Silvex (2,4,5-TP) (0.021)2

2,4-DB (0.035)2 MCPA (0.05)2 Triclopyr (0.05)2

Dinitroanilines

Benfluralin (0.002)1 Trifluralin (0.002)1 Pendimethalin (0.004)1

Ethafluralin (0.004)1 Oryzalin (0.019)2

Organochlorines

Chlorothalonil (0.035)2 Dichlobenil (0.02)2 alpha-HCH* (0.002)1

Dacthal (DCPA) (0.002)1 Dieldrin (0.001)1 GAMMA -HCH (0.004)1

p,p′−DDE* (0.006)1

Organophosphates

Azinphos-methyl (0.001)1 Ethoprop (0.003)1 Parathion (0.004)1

Chlorpyrifos (0.004)1 Fonofos (0.003)1 Phorate (0.002)1

Diazinon (0.002)1 Malathion (0.005)1 Terbufos (0.013)1

Disulfoton (0.017)1 Methyl parathion (0.006)1

Triazine herbicides:

Atrazine (0.001)1 Cyanazine (0.004)1 Prometon (0.018)1

Atrazine, desethyl* (0.002)1 Metribuzin (0.004)1 Simazine (0.005)1

Uracils

Bromacil (0.035)2 Terbacil (0.007)1

Ureas

Fenuron (0.013)2 Fluometuron (0.035)2 Neburon (0.015)2

Diuron (0.020)2 Linuron (0.002)1, (0.018)2 Tebuthiuron (0.010)1

Miscellaneous:

2,6-Diethylanaline* (0.003)1 Chloramben (0.011)2 DNOC (0.035)2

Acetochlor (0.002)1 cis-Permethrin (0.05)1 Norflurazon (0.024)2

Acifluorfen (0.035)2 Clopyralid (0.05)2 Picloram (0.05)2

Bentazon (0.014)2 Dicamba (0.035)2 Propargite (0.013)1

Bromoxynil (0.035)2 Dinoseb (0.035)2
Sampling Collection and Analysis 13

Table 2. Recoveries and standard deviations of pesticides measured by gas chromatography/mass spectrometry 

[All recoveries and standard deviation are expressed as percentages. The medians and standard deviations are calculated from six trials. na, 

not available because of poor analytical results; GC/MS, gas chromatography/mass spectrometry]

Pesticides measured by 

GC/MS Median recovery Lowest recovery Highest recovery Standard deviation

Acetochlor 97 92 106 5

Alachlor 100 88 118 10

Alpha-BHC 96 84 102 7.8

Atrazine 94 79 111 11.5

Atrazine, desethyl na na na na

Azinphos-methyl na na na na

Benfluralin 89 79 95 7.9

Butylate 98 90 118 9.7

Carbaryl na na na na

Carbofuran na na na na

Chlorpyrifos 85 63 104 15.3

Cyanazine 110 88 131 18.6

Dacthal 99 76 122 15.1

p,p'-DDE 48.5 37 82 16.3

Diazinon 90.5 67 101 24.3

Dieldrin 94 86 107 7.9

2,6-Diethylaniline 85.5 77 91 5.4

Disulfoton 59 42 84 16.5

EPTC 96.5 85 106 7.9

Ethalfluralin 103 96 129 13.2

Ethoprop 98 81 106 8.3

Fonofos 86.5 80 99 7.4

Lindane 92 81 105 9.7

Linuron 78.5 67 104 21.9

Malathion 98.5 71 108 18.4

Metolachlor 106 90 124 11.7

Metribuzin 100 82 127 16

Molinate 101.5 90 109 7.6

Napropamide 96 79 112 13.1

Parathion 116.5 96 139 14.7

Parathion, methyl 104 91 179 37.9

Pebulate 99 89 107 6.6

Pendimethalin 94.5 66 103 13.9

Permethrin 28.5 16 59 15.5

Phorate 64.5 40 81 16.9

Prometon 94 22 119 35.7

Propachlor 109 93 129 13.6

Propanil 106.5 90 113 9.3

Propargite 82 60 91 11.7

Pronamide 101.5 93 113 7.1

Simazine 91 62 114 19.2

Tebuthiuron 118.5 36 158 40.3

Terbacil na na na na

Terbufos 83 45 110 21.6

Thiobencarb 95.5 84 101 7.3

Triallate 91.5 80 102 7.3

Trifluralin 97 82 123 13.5
14 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

Table 3. Recoveries and standard deviations of pesticides measured by high performance liquid chromatography/ultraviolet light 

spectrometry 

[All recoveries and standard deviation are expressed as percentages. The medians and standard deviations are calculated from five trials. 

na, not available because of poor analytical results; HPLC/UVS, high performance liquid chromatography/ultraviolet light spectrometry] 

Pesticides measured by 

HPLC/UVS Median recovery Lowest recovery Highest recovery Standard deviation

2,4,5-T 73 43 92 21.3

2,4-D na na na na

2,4-DB 61.5 41 83 20.8

Silvex 89 51 100 21.1

3-Hydroxycarbofuran 71 28 100 28.5

DNOC na na na na

Acifluorfen 49 0 94 43.9

Aldicarb na na na na

Aldicarb sulfone na na na na

Aldicarb sulfoxide na na na na

Bentazon na na na na

Bromacil 74 58 79 11.2

Bromoxynil 84 39 86 22

Carbaryl 73 66 88 10.5

Carbofuran 69 47 92 17.2

Chloramben na na na na

Chlorothalonil na na na na

Clopyralid 0 0 25 11.2

Dacthal monoacid 48.5 0 79 33

Dicamba na na na na

Dichlobenil na na na na

Dichlorprop 73 25 87 28

Dinoseb 84 51 103 20.9

Diuron 84.5 51 103 20.9

Fenuron 83 47 97 20.2

Fluometuron 80 66 93 12.4

Linuron 81 65 86 10.1

MCPA 75 0 78 35.9

MCPB na na na na

Methiocarb 82 70 89 10.6

Methomyl 88 51 133 33.3

Neburon 78 67 79 6.8

Norfluorazon 87 63 91 14.1

Oryzalin 64 0 68 28.9

Oxamyl 48 0 80 34

Picloram 0 0 60 30

Propham na na na na

Propoxur 41 38 86 26.9

Triclopyr 80 35 87 21.5
Detection Frequency 15

were recovered in acceptable amounts. Poor 

performance was noted for azinphos-methyl, carbaryl, 

carbofuran, atrazine desethyl, and terbacil. Those five 

pesticides were analyzed by GC/MS. Poor analytical 

performance was noted for several compounds 

analyzed by high performance liquid 

chromatography/ultraviolet light spectrometry, 

especially 2,4-D, DNOC, aciflurofen, aldicarb and its 

metabolites, bentazon, chloramben, chlorothalonil, 

dicamba, dichlobenil, MCPB, picloram, and propham. 

Other compounds analyzed by high performance liquid 

chromatography/ultraviolet light spectrometry also had 

less than ideal performance as indicated by the 

calculated recovery. Few pesticides analyzed by high 

performance liquid chromatography/ultraviolet light 

spectrometry were detected in the water samples, 

although the lack of detections of some compounds 

may be related to the poor analytical performance.

DETECTION FREQUENCY

Detection frequency of pesticides analyzed by 

GC/MS is listed in tables 4 through 7. Detection 

frequency of pesticides analyzed by high performance 

liquid chromatography/ultraviolet light spectrometry is 

listed in tables 8 through 11. The pesticides are listed in 

order of detection frequency. 

The Colusa Basin Drain at Road 99E near 

Knights Landing site had the greatest number of 

Table 4. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by gas 

chromatography/mass spectrometry, in Colusa Basin Drain at Road 99E near Knights Landing, Sacramento River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. D, degradation product; H, herbicide; 

I, insecticide; e, estimated concentration; <, less than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency

percent

Propanil H 0.045 0.045 <0.004 4.8

Propargite I 0.052 0.052 <0.013 4.8

Alachlor H 0.012 0.011 <0.002 9.5

Atrazine, desethyl D e0.004 e0.003 <0.002 9.5

Pebulate H 0.011 0.011 <0.004 9.5

Tebuthiuron H 0.013 e0.009 <0.010 9.5

Prometon H e0.01 e0.005 <0.018 14.3

Atrazine H e0.003 0.005 <0.001 19

Metribuzin H 0.031 0.013 <0.004 19

Chlorpyrifos I 0.016 0.007 <0.004 28.6

Malathion I 0.054 0.0055 <0.005 33.3

Napropamide H 0.43 e0.004 <0.003 33.3

Pronamide H 0.035 0.0094 <0.003 38.1

Carbaryl I e0.1 e0.009 <0.003 42.9

Cyanazine H 0.44 0.005 <0.004 42.9

Dacthal H 0.0086 e0.001 e0.001 52.4

Trifluralin H 0.016 e0.002 e0.002 52.4

EPTC H 0.72 e0.003 e0.003 57.1

Diazinon I 0.098 e0.002 0.014 71.4

Metolachlor H 0.39 e0.004 0.024 90.5

Thiobencarb H 4.4 0.014 0.026 90.5

Simazine H 0.15 e0.003 0.014 95.2

Carbofuran I e0.4 e0.01 e0.03 100

Molinate H 19 0.009 0.1 100
16 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

pesticides detected by GC/MS (table 4). A total of 24 

out of 47 pesticides analyzed were detected at that site. 

In contrast, only 20 of these 47 pesticides were detected 

at the Arcade Creek near Del Paso Heights site 

(table 5), 16 of 47 at the Sacramento River at Freeport 

site (table 6), and 13 of 47 at the Yolo Bypass at 

Interstate 80 near West Sacramento site (table 7). 

Nine pesticides were detected at a frequency of 

50 percent or higher at the agricultural site (Colusa 

Basin Drain at Road 99E near Knights Landing), and 

eight pesticides were detected at a frequency of 

50 percent or higher at the urban site (Arcade Creek 

near Del Paso Heights). There were some differences in 

which pesticides were detected at this frequency, and 

those differences are related to either pesticide use or 

land use. For example, thiobencarb, carbofuran, and 

molinate were detected in 90.5 to 100 percent of the 

samples at the Colusa Basin Drain at Road 99E near 

Knights Landing site (table 4), but were detected at 

much lower frequency at Arcade Creek near Del Paso 

Heights site (table 5), which is the urban site. In the 

Sacramento Valley, thiobencarb and molinate are only 

used on rice. Carborfuran also is used on rice, but has 

uses on other crops, such as alfalfa. 

Molinate is the most heavily used pesticide on 

rice. It is applied during early May to control aquatic 

grasses prior to the planting of rice. During 1996, a total 

of 639,000 kg of molinate were applied to fields prior to 

rice planting. Most of that molinate was applied during 

May (California Department of Pesticide Regulation, 

1996). A time series plot of molinate concentrations at 

the Colusa Basin Drain at Road 99E near Knights 

Landing site is shown in figure 8. Consistent with the 

pesticide use, the highest concentrations of molinate are 

detected in May and June, but residues of molinate can 

be detected throughout the year. Molinate also is one of 

the most frequently detected pesticides downstream at 

the Sacramento River at Freeport site. Although the 

Colusa Basin Drain is one of the largest sources of 

agricultural pesticide residues to the Sacramento River 

(Foe and Connor, 1991; Bennett and others, 1998) the 

much greater discharge of the Sacramento River dilutes 

Table 5. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by gas 

chromatography/mass spectrometry, in Arcade Creek near Del Paso Heights, Sacramento River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. D, degradation product; H, herbicide; 

I, insecticide; e, estimated concentration; <, less than] 

Pesticide Type of pesticide Maximum 

concentration

Minimum 

concentration 

Median 

concentration

Detection frequency,

percent

2,6-Diethylaniline D e0.002 e0.002 <0.003 3.3

Molinate H 0.054 0.054 <0.004 3.3

Propanil H 0.091 0.091 <0.004 3.3

Thiobencarb H e0.004 e0.004 <0.002 3.3

Carbofuran I e0.05 e0.05 <0.003 6.7

Metribuzin H 0.18 0.052 <0.004 6.7

Benfluralin H 0.013 e0.0014 <0.002 10

EPTC H 0.014 e0.001 <0.002 13.3

Atrazine H 0.027 0.027 <0.001 16.7

Pendimethalin H 0.16 0.011 <0.004 16.7

Tebuthiuron H 0.078 0.013 <0.010 23.3

Trifluralin H 0.02 e0.002 <0.002 33.3

Malathion I 0.63 0.012 0.013 53.3

Chlorpyrifos I 0.045 0.0048 0.0076 73.3

Simazine H 0.19 0.008 0.019 73.3

Metolachlor H 0.67 e0.003 0.008 80

Dacthal H 0.019 e0.0004 0.0041 83.3

Prometon H 0.52 0.03 0.1 96.7

Carbaryl I e2 e0.02 e0.2 100

Diazinon I 1.4 0.081 0.28 100
Detection Frequency 17

agricultural pesticide concentrations. The highest 

concentrations of molinate in the Sacramento River 

also are measured during the May–June period. 

Thiobencarb, the other major rice herbicide, also 

was detected in water samples collected from the 

Sacramento River at Freeport site with the highest 

concentrations during the same period as that for 

molinate. The concentrations of molinate, carbofuran, 

and thiobencarb, measured in the Colusa Basin Drain 

or the Sacramento River, are below any existing water 

quality goals. However, the Colusa Basin Drain is listed 

on the 303(d) list as having a water quality impairment 

attributable to carbofuran (U.S. Environmental 

Protection Agency, accessed October 1, 1999). The 

impairment is listed as a medium priority. 

Other herbicides, including simazine, 

metolachlor, dacthal, and EPTC, also were detected in 

the water samples from either the Sacramento River or 

the Colusa Basin Drain, or other sites. 

The concentrations of molinate and other pesticides used in rice farming, as measured in this study, 

either in the Colusa Basin Drain or the Sacramento 

River, were greatly reduced over concentrations 

measured in past years and this represents a significant 

improvement. During the late 1970s, the levels of rice 

pesticides in the Colusa Basin Drain were periodically 

acutely toxic to fish (Bennet and others, 1998); acute 

toxicity to carp (Cyprinus carpio) was attributable to 

molinate. During the early 1980s, consumers of 

drinking water in the city of Sacramento reported an 

objectionable taste during May and June, which was 

attributable to thiobencarb. Concentrations of molinate, 

measured in the Colusa Basin Drain by various state 

agencies, were as high as 340 μg/L during the early 

1980s (Cornacchia and others, 1984). Concentrations 

of thiobencarb were as high as 110 μg/L in the Colusa 

Basin Drain in the early 1980s (Cornacchia and others, 

1984). The concentrations of molinate and thiobencarb 

were as high as 27 μg/L and 6 μg/L, respectively, in the 

lower Sacramento River during the same period 

(Cornacchia and others, 1984). A management program 

began in 1990 to reduce the levels of these pesticides in 

receiving bodies of water. The program consisted of 

requiring rice field water to be retained on the field for 

a specified period to allow pesticide concentrations in 

water to be reduced through some mechanism, such as 

volatilization, biological processes, or sunlight-induced 

degradation. Currently, water is required to be held on 

the field for about 1 month. Because of this requirement, the drainage water is no longer toxic to aquatic 

Table 6. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by gas 

chromatography/mass spectrometry, in Sacramento River at Freeport, Sacramento River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. D, degradation product; H, herbicide; 

I, insecticide; e, estimated concentration; <, less than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

Chlorpyrifos I e0.003 e0.003 <0.004 5.3

Dacthal H e0.002 e0.002 <0.002 5.3

Atrazine, desethyl D e0.001 e0.001 <0.002 5.3

Malathion I e0.004 e0.004 <0.005 5.3

Pebulate H 0.0056 0.0056 <0.004 5.3

Propanil H 0.029 0.029 <0.004 5.3

Cyanazine H 0.02 0.01 <0.004 10.5

Atrazine H e0.002 e0.001 <0.001 15.8

EPTC H 0.022 e0.001 <0.002 15.8

Carbofuran I e0.04 e0.01 <0.003 26.3

Carbaryl I e0.06 e0.03 <0.003 31.6

Diazinon I 0.046 e0.002 <0.002 42.1

Thiobencarb H 0.17 e0.004 <0.002 42.1

Simazine H 0.02 e0.003 0.006 68.4

Molinate H 1.6 e0.002 0.0077 73.7

Metolachlor H 0.026 e0.002 0.0041 78.9
18 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

organisms, and concentrations in the Sacramento River 

are extremely low due to dilution (Bennet and others, 

1998).

In contrast to the agricultural basin, insecticides 

tend to be detected at a higher frequency and at higher 

concentrations at the urban site, Arcade Creek near Del 

Paso Heights. Herbicides also were detected at a 

frequency of greater than 50 percent at the urban site. 

Chlorpyrifos was detected at a frequency of 73.3 

percent at the urban site (table 5), but at a frequency of 

only 28.6 percent at the agricultural site, Colusa Basin 

Drain at Road 99E (table 4). Both carbaryl and diazinon 

Table 7. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by gas 

chromatography/mass spectrometry, in Yolo Bypass at Interstate 80 near West Sacramento, Sacramento River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. H, herbicide; I, insecticide; e, estimated 

concentration; <, less than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

Dacthal H e0.0010 e0.0010 <0.002 25

EPTC H e0.0025 e0.0025 <0.002 25

Malathion I 0.015 0.015 <0.005 25

Metribuzin H 0.009 0.009 <0.004 25

Napropamide H 0.017 0.017 <0.003 25

Pronamide H e0.0031 e0.0031 <0.003 25

Trifluralin H e0.0031 e0.0031 <0.002 25

Carbaryl I e0.0089 e0.0035 <0.003 50

Thiobencarb H 0.0077 0.0065 <0.002 50

Diazinon I 0.053 0.017 0.017 75

Molinate H 0.0181 e0.0038 0.01 75

Metolachlor H 0.01 0.004 0.007 100

Simazine H 0.0491 0.021 0.0325 100

Table 8. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by high 

performance liquid chromatography/ultraviolet light spectrometry, in Colusa Basin Drain at Road 99E near Knights Landing, Sacramento 

River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. H, herbicide; I, insecticide; e, estimated 

concentration; <, less than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

Bromoxynil H 0.06 0.06 <0.035 4.8

Linuron H e0.03 e0.03 <0.018 4.8

Oryzalin H e0.03 e0.03 <0.019 4.8

Norfluorazon H 0.06 e0.02 <0.024 9.5

Carbaryl I 0.04 e0.0010 <0.008 14.3

Carbofuran I 0.2 0.15 <0.028 14.3

2,4-D H 0.78 0.11 <0.035 19

Triclopyr H 1.1 0.22 <0.05 19

MCPA H 0.94 0.08 <0.05 33.3

Bentazon H 0.13 e0.05 <0.014 47.6

Diuron H 0.69 0.04 0.08 66.7
Detection Frequency 19

were detected at 100 percent frequency at the urban site 

(table 5) compared to 42.9 and 71.4 percent, respectively, at the agricultural site (table 4). Of greater 

significance are the higher concentrations of diazinon 

measured throughout the year at the urban site. A time 

series plot of diazinon for the urban and agricultural 

sites is shown in figure 9. The higher concentrations of 

diazinon at the agricultural site tend to be measured 

during the winter, the time of greatest agricultural use, 

but higher concentrations of diazinon can be measured 

throughout the year at the urban site. The measured 

concentrations of diazinon at the urban site are 

frequently above the levels toxic to Ceriodaphnia 

dubia, an aquatic invertebrate frequently used for 

Table 9. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by high 

performance liquid chromatography/ultraviolet light spectrometry, in Arcade Creek near Del Paso Heights, Sacramento River Basin, 

California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. H, herbicide; I, insecticide; e, estimated 

concentration; <, less than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

Linuron H e0.03 e0.03 <0.018 3.6

MCPA H 0.06 0.06 <0.05 3.6

Oryzalin H 1.5 0.08 <0.019 21.4

2,4-D H 1.4 e0.02 <0.035 28.6

Triclopyr H e3 0.09 <0.05 32.1

Carbaryl I e0.6 0.05 <0.008 42.9

Diuron H 1.4 0.12 0.5 85.7

Table 10. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by high 

performance liquid chromatography/ultraviolet light spectrometry, in Sacramento River at Freeport, Sacramento River Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. H, herbicide; e, estimated concentration; <, less 

than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

Bentazon H e0.002 e0.002 <0.014 5.6

Dichlobenil H e0.08 e0.08 <0.02 5.6

Triclopyr H e0.03 e0.03 <0.05 5.6

Diuron H 0.12 e0.004 e0.006 61.1

Table 11. Maximum, minimum, and median concentrations, and detection frequency of pesticides in water samples analyzed by high 

performance liquid chromatography/ultraviolet light spectrometry, in Yolo Bypass at Interstate 80 near West Sacramento, Sacramento River 

Basin, California

[The median concentrations denoted with a less than sign (<) indicate that the detection frequency was less than 50 percent and therefore, 

the median concentration is the detection limit. Concentrations are in micrograms per liter. H, herbicide; e, estimated concentration; <, less 

than] 

Pesticide Type of

pesticide

Maximum

concentration

Minimum

concentration

Median

concentration

Detection

frequency,

percent

MCPA H 0.06 <0.05 0.05 50

Oryzalin H e0.0200 <0.02 0.02 50

Diuron H 0.13 e0.002 0.07 100
20 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998 Nov. 7 Dec. 3 Jan. 14 Feb. 18 Dec. 17 Jan. 21 Feb. 26 March 18 March 11 April 9 April 24

April 15

May 9

May 23

June 17

July 10July 28

Aug. 26

Sept. 18

Oct. 30

Nov. 12

1996 1997 1998

20

18

16

14

12

10

8

6

4

2

0

Molinate, in micrograms per liter

toxicity testing (Cooke and Connor, 1998). The 

International Joint Commission for the Great Lakes 

suggested a more recent guideline of 0.080 μg/L 

(fig. 9). The greater frequency and higher concentration 

of insecticides in the Arcade Creek is consistent with 

the use of these compounds for household or garden 

pest control. Diazinon is a major pesticide used for that 

purpose. 

If Arcade Creek can be characterized as a typical 

urban stream for the Sacramento metropolitan area, 

then it is likely that toxic levels of diazinon also are 

present in other urban drainages. The Arcade Creek 

watershed is entirely contained within the Sacramento 

metropolitan area. The flow in the stream throughout 

most of the year is entirely urban runoff either from rain 

or lawn irrigation, with minor amounts of flow 

attributable to ground water. Flow in Arcade Creek is 

extremely low in November, the month in which lawn 

irrigation generally does not occur and before the rainy 

season. Other similar urban streams or drainage canals 

within the Sacramento metropolitan area probably have 

similar toxicity levels. In fact, insecticides such as 

diazinon, chlorpyrifos, and carbaryl have high detection 

frequency at many urban streams across the United 

States (Larson and others, 1999).

Relatively few pesticides analyzed by high 

performance liquid chromatography/ultraviolet light 

spectrometry were detected at the sampling sites. Three 

exceptions were bentazon, carbaryl, and diuron. The 

use of bentazon, an herbicide used on rice, was 

suspended in California in 1989 pending a review and 

formally banned in 1992 by the California Department 

of Pesticide Regulation (Miller-Maes and others, 1993) 

because of frequent detections in ground water 

samples. Bentazon was detected at a frequency of 

47.6 percent at the agricultural site (table 8) and a 

frequency of 5.6 percent at the Sacramento River at 

Freeport site (table 10). Despite not being in use for at 

least 6 years, bentazon can still be detected in water 

samples from both the agricultural stream and 

Figure 8. Time series plot of molinate concentrations at the Colusa Basin Drain at Road 99E near Knights Landing site, Sacramento River 

Basin, California.
Storm-Water Runoff 21

0 J

0.5

1.0

1.5

0.1

0.2

0.3

0.4

0.6

0.7

1.4

0.8

0.9

1.3

1.2

1.1

Arcade Creek near Del Paso Heights

 (urban stream)

Colusa Basin Drain at Road 99E near

 Knights Landing (agricultural stream)

Probable toxic effects to Ceriodaphnia dubia

Guideline proposed by the International Joint

 Commission for the Great Lakes

 Dec. 10

Jan. 13Jan. 29

Feb. 7

Feb. 21

March 7

March 21

April 10April 23

May 12May 23May 27

June 13June 26

July 11July 21

Aug. 8

Aug. 29

Sept. 5

Sept. 26

Oct. 8

Oct. 17

Nov. 7

Nov. 14

Dec. 19

Jan. 9

Feb. 17

March 9

Nov. 26

April 23

1996 1997 1998

Diazinon, in micrograms per liter

downstream in the Sacramento River. Carbaryl is an 

insecticide with widespread use, especially in the urban 

environment. Diuron is an herbicide used on various 

agricultural commodities, with the most use occurring 

during the winter and the lowest use during the 

summer. Other pesticides analyzed by high 

performance liquid chromatography/ultraviolet light 

spectrometry were the herbicides linuron, MCPA, 

oryzalin, and triclopyr.

STORM-WATER RUNOFF

Pesticides in storm-water runoff have been 

previously reported by Domagalski (1996) for 

agricultural streams, sites on large tributaries to the 

Sacramento River, and the Sacramento River. In that 

study, it was shown that diazinon concentrations on the 

lower Sacramento River increase with discharge, 

reaching a peak concentration just prior to peak river 

flow, with decreasing concentrations measured 

thereafter due to dilution. In the present study, storm 

water was not collected across a storm hydrograph, but 

rather samples were collected from the Yolo Bypass, a 

channel that only has water in response to flood 

management of the Sacramento River. When the 

channel capacity of the Sacramento River is expected to 

be exceeded, water can be diverted to the Yolo Bypass 

to prevent flooding in downstream areas, such as the 

city of Sacramento. Therefore, storm-water runoff is 

present in the Yolo Bypass as water is diverted out of the 

Sacramento River. 

The same suite of pesticides detected at either the 

agricultural site or the urban site is detected at the Yolo 

Bypass site (table 7). Most pesticides detected at the 

Yolo Bypass, with the exception of simazine, generally 

had lower concentrations than those measured at either 

the agricultural or urban site. Run-off from Arcade 

Creek, however, does not flow into the Yolo Bypass. 

Therefore, it would be expected that urban-use 

pesticides would have low concentrations in the Yolo 

Bypass. The two most frequently detected compounds 

at the Yolo Bypass at Interstate 80 near West 

Sacramento site were the herbicides metolachlor and 

simazine, which are used on either orchards or for 

roadside and highway weed control. Molinate, the rice 

herbicide, was detected at a frequency of 75 percent. 

Diazinon also was detected at a frequency of 

Figure 9. Time series plot of diazinon concentrations at the Arcade Creek near Del Paso Heights site and the Colusa Basin Drain at Road 99E 

near Knights Landing site, Sacramento River Basin, California.
22 Pesticides in Surface Water Measured at Select Sites in the Sacramento River Basin, California, 1996–1998

75 percent. Although diazinon was detected at a high 

frequency, and the concentrations measured were 

above the National Academy Sciences guidelines of 

0.009 μg/L (Nowell and Resek, 1994) for the protection 

of aquatic health, the water was not acutely toxic to 

C. dubia with respect to diazinon. Acute toxicity for 

C. dubia occurs at a concentration of approximately 

0.35 μg/L (Amato and others, 1992). 

The high flows occurring during the flood of 

January 1999 could possibly have resulted in an initial 

pulse of pesticides to the Sacramento River or its 

tributaries, followed by a considerable amount of 

dilution. Rainfall amounts during the El Niño year were 

different in that no single high-flow event, comparable 

in magnitude to the January 1997 flood, occurred. 

Rainfall amounts were higher than normal throughout 

the Sacramento Valley, which would likely result in 

early pesticide runoff events, followed by relatively 

lower concentrations or detection frequency later in the 

rainy season.

COMPARISON OF SAMPLING FREQUENCIES

The sampling frequency of the NAWQA 

Program was limited mainly to monthly or twice per 

month sampling and, as a result, the actual exposures of 

aquatic organisms to pesticides are difficult to assess. 

However, other agencies or programs within the U.S. 

Geological Survey have completed a considerable 

amount of pesticide sampling within the Sacramento 

River Basin. The U.S. Geological Survey Toxic 

Substances Hydrology Program sampled the 

Sacramento River at Freeport three times a week for 

almost 3 years and also completed several storm 

samplings for pesticides (Kuivila and Foe, 1995; 

MacCoy and others, 1995). 

A high frequency of sampling allows for the 

completion of a probabilistic ecological risk 

assessment (PERA). A PERA is a statistically valid 

model of pesticide or other toxic contaminant exposure 

to aquatic organisms and allows water quality 

managers to determine what percentage of aquatic 

organisms will be adversely affected by pesticides 

within a stream system at given concentrations. A 

control program can then be designed that will allow 

managers to decide what level of protection is desired 

for a given stream. Therefore, the design of a protection 

program for a specific percentage of organisms on the 

basis of statistically relevant information is possible by 

development of a PERA. Completion of a PERA 

requires that toxicological information is available on 

the organisms of concern and the toxicants of concern, 

as well as adequate data on actual toxicant 

concentrations.

A PERA has been completed for the lower 

Sacramento River with respect to the exposure of 

aquatic organisms (fish and invertebrates) to levels of 

diazinon (Norvatis Crop Protection, 1997). The PERA 

considered the long-term viability of fish populations in 

the lower Sacramento River and the productivity of 

invertebrate communities. The study assessed the 

exposure to diazinon because it is the most frequently 

detected organophosphate insecticide in the 

Sacramento River Basin and it has been linked to 

toxicity to aquatic organisms, especially invertebrates 

(Foe, 1995; Kuivila and Foe, 1995; Domagalski, 1996). 

The usage or guidelines for establishing a PERA 

have been given by the U.S. Environmental Protection 

Agency (1992, 1996) and Environment Canada 

(1996a,b). An example of a pesticide PERA has been 

published by Solomon and others, 1996.

A PERA requires knowledge of the exposure 

effects on an organism from a chemical and detailed 

knowledge about the duration of exposure and the 

variation in chemical concentration during exposure. 

The PERA, with respect to pesticides, requires 

toxicological information and concentration data across 

a range of flow conditions so that a valid statistical 

evaluation of exposure can be completed. In practice, 

the toxicological information may be available, but 

sufficient data on exposure are lacking, especially for 

aquatic systems. Invertebrates were considered because 

they are important in the diet of fish and any chemical 

exposure, which limits their productivity, could have 

consequences for the predatory organisms. Nine fish 

species, including salmon (Oncorhynchus tshawytscha)

and striped bass (Morone saxatilis), were part of the 

assessment. The prey organisms for these fish include 

copepoda (Eurytemora, Cyclops, and Sinocalanus), 

mysids (Neomysis), amphipods (Corophium), and 

cladocera (Daphnia, Bosmina, and Diaphonosoma). 

The PERA showed that fish are not at risk from direct 

toxic levels of diazinon because the concentrations are 

well below toxic levels (Norvatis Crop Protection, 

1997). The assessment did show, however, that 

invertebrate populations are at greater risk, but mainly 

in agriculturally dominated streams or drainage 

channels throughout the Central Valley, and mainly 

during January and February (Norvatis Crop 

Protection, 1997). This PERA did not address urban 

risk due to insufficient data. However, it was pointed 

out that any ecological damage would be brief and 

limited to cladocerans (Norvatis Crop Protection, 

1997). Norvatis Crop Protection (1997) cited four lines 

of evidence to support this conclusion for fish and for 

invertebrates. The first is that the diazinon 

concentrations lethal to cladocerans (toxic levels are 

approximately 0.5 to 1.5 μg/L) are nontoxic to most 

other invertebrates and are orders of magnitude lower 
Summary and Conclusions 23

than those toxic to fish. The second line of evidence is 

that cladocerans, such as C. dubia, reproduce rapidly, 

and any short term toxic effects are rapidly offset by 

rapid reproduction. The third line of evidence is that 

none of the fish species of concern depend on 

cladocerans as the critical component of their diets and, 

therefore, would not be affected by sharp decreases in 

cladoceran biomass. A final line of evidence was that 

microcosm and mesocosm studies with diazinon have 

shown that severe reductions in cladoceran populations 

can occur with little or no measurable effect on the rest 

of the ecosystem, and that cladoceran populations do in 

fact recover rapidly following lethal exposures to 

diazinon. Most of the data used by Norvatis Crop 

Protection for the risk assessment was collected prior to 

the sampling of the current NAWQA investigation. 

Although data from the NAWQA Program was not 

used in the PERA developed by Norvatis, the exposure 

data collected by the NAWQA Program is consistent 

with the findings of that risk assessment. There were 

no detections of diazinon within the lower Sacramento 

River at levels in excess of 0.35 μg/L, which are toxic 

to C. dubia (Amato and others, 1992). However, 

concentrations in excess of 1 μg/L were measured at 

the urban stream, indicating that probable adverse 

effects may occur.

SUMMARY AND CONCLUSIONS

Detection of pesticides in surface waters of the 

Sacramento River Basin can be attributed to pesticide 

use. Consistent with the mainly agricultural land use in 

rice cultivation in the Colusa Basin Drain, pesticides 

used on rice were the most frequently detected at the 

Colusa Basin Drain at Road 99E near Knights Landing 

sampling site and at the downstream site on the 

Sacramento River, the Sacramento River at Freeport. 

There were no measured concentrations at either the 

agricultural site or the large river site, which exceeded 

water quality criteria for specific rice pesticides, nor 

were any concentrations sufficiently high to affect 

aquatic life. Other pesticides detected at the agricultural site and downstream at the Sacramento River site 

were herbicides and insecticides used either on 

orchards or for the control of weeds along roadways. 

In contrast to the agricultural stream, pesticides 

most frequently detected at the urban stream were 

insecticides used for the control of insects on lawns, 

gardens, or buildings. The concentrations of diazinon, 

an organophosphate insecticide that was detected in 

every water sample collected at the urban stream, were 

always above the recommended guidelines established 

by the International Joint Commission for the Great 

Lakes for the protection of aquatic health or were 

frequently at levels of acute toxicity to aquatic 

invertebrates such as C. dubia. Mixing with the 

Sacramento River dilutes the concentrations of 

diazinon and other insecticides to generally low to 

nondetectable levels.

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