Changes in Rice Pesticide Use and Surface 

Water Concentrations in the Sacramento 

River Watershed, California

By James L. Orlando and Kathryn M. Kuivila

In cooperation with the

Central Valley Regional Water 

Quality Control Board

Scientific Investigations Report 2004-5097

U.S. Department of the Interior

U.S. Geological Survey
U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225-0286

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

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

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Orlando, J.L., and Kuivila, K.M., 2004, Changes in rice pesticide use and surface water concentrations in the Sacramento 

River watershed, California: U.S. Geological Survey Scientific Investigations Report 2004–5097, 28 p.
iii

Contents

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

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

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Pesticide Use Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Project Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Study Design and Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Selection of Sampling Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Sample Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Surface Water 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Suspended Sediment 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Surface Water 2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Sample Processing and Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Dissolved Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Quality Assurance and Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Pesticides Associated with Suspended Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Analysis of Suspended Sediments and Dissolved Organic Carbon . . . . . . . . . . . . . . . . . . . . 12

Analytical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Pesticide Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Suspended Sediment and Dissolved Organic Carbon Concentrations . . . . . . . . . . . . . . . . . 13

Sampling Method Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Comparison of Pesticide Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Dissolved Pesticide Concentrations 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Dissolved Pesticide Concentrations 2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Performance Goals and MCLs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Historical Peak Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Analysis of Lambda-Cyhalothrin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Matrix Recoveries in 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Additional Method Testing in 2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Dissolved Organic Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Pesticide Partitioning Between Water and Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Sediment-Associated Pesticide Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Equilibrium Partitioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
iv

Figures

Figure 1. Map showing locations of sampling sites and rice growing areas within the 

Sacramento Valley of California. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 2. Graph showing amounts of pesticides (pounds active ingredient) applied to

rice annually in the Sacramento River watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3. Map showing drainage basins within the area under study . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 4. Graphs showing dissolved pesticide concentrations at CBD1, 

Sacramento Slough, and Tower in 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 5. Graphs showing dissolved pesticide concentrations at Butte Creek 

and CBD5 in 2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 6. Graphs showing pesticide application within CBD5 basin, peak pesticide 

concentrations at CBD5, and performance goals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 7. Graphs showing days that performance goals were exceeded each year at 

CBD5 and the pesticide use during 1992 through 2002 in the basin. . . . . . . . . . . . . . 23

Figure 8. Graph showing recovery of lambda-cyhalothrin in samples collected in 

2002 versus the dissolved organic carbon concentration (DOC) . . . . . . . . . . . . . . . . 24

Figure 9. Boxplot showing dissolved organic carbon (DOC) concentrations at the 

five sites sampled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
v

Tables

Table 1. Amounts of lambda-cyhalothrin, molinate, and thiobencarb applied 

during May–July 2002 in the watersheds of each site sampled in 2002 . . . . . . . . . . 6

Table 2. Rice pesticide sampling sites, Sacramento Valley, California . . . . . . . . . . . . . . . . . . . . . . 6

Table 3. Site profiles of in-stream water parameters measured in 2002 . . . . . . . . . . . . . . . . . . . . . 8

Table 4. Pesticide, dissolved organic carbon, and suspended sediment 

concentrations detected in water samples collected during 2002 . . . . . . . . . . . . . . 14

Table 5. Pesticide and dissolved organic carbon concentrations detected

in water samples collected during 2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 6. Suspended sediment-associated pesticides: observed concentrations

and predicted equilibrium-partitioning concentrations . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 7. Pesticide concentrations determined in quality assurance sample pairs 

collected in 2002 and the relative percent difference between pairs . . . . . . . . . . . . 17
vi

Conversion Factors, Datum, and Abbreviations

CONVERSION FACTORS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = 1.8 °C + 32.

Specific conductance is given microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L). 

DATUM

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

ABBREVIATIONS

C carbon

CBD Colusa Basin Drain

cc cubic centimeter

CO2 carbon dioxide

DOC dissolved organic carbon

GC/MS gas chromatography-mass spectrometry

GIS geographic information system

GPC/HPLC gel permeation chromatography/high pressure liquid 

 chromatography 

HCl hydrochloric acid

in. inch

km kilometer

L liter

μL microliter

lb pound

m meter

μm micrometer

Multiply By To obtain

feet (ft) 0.3048 meter

inch (in.) 2.54 centimeter

mile (mi) 1.609 kilometer

square mile (mi2) 12.590 square kilometer

pound, avoirdupois (lb) 0.4536 kilogram 
vii

MASE microwave-assisted solvent extraction

MDL method detection limit

mg milligram

μg/L microgram per liter

mi2 square mile

mL milliliter

N nitrogen

N2 nitrogen gas

ng nanogram

ng/g nanogram per gram dry weight

NPOC nonpurgeable organic carbon

Pt platinum

RPD relative percent difference

SPE solid-phase extraction

SSC suspended sediment concentration

μS/cm microsiemens per centimeter

v/v volume-to-volume ratio

Organizations

CDFG California Department of Fish and Game

CDPR California Department of Pesticide Regulation

CRWQCB California Regional Water Quality Control Board

CVRWQCB Central Valley Regional Water Quality Control Board

USGS U.S. Geological Survey
Changes in Rice Pesticide Use and Surface Water 

Concentrations in the Sacramento River Watershed, 

California

By James L. Orlando and Kathryn M. Kuivila 

Abstract 

Pesticides applied to rice fields in California are 

transported into the Sacramento River watershed by the 

release of rice field water. Despite monitoring and mitigation 

programs, concentrations of two rice pesticides, molinate and 

thiobencarb, continue to exceed the surface-water 

concentration performance goals established by the Central 

Valley Regional Water Quality Control Board. There have 

been major changes in pesticide use over the past decade, and 

the total amount of pesticides applied remains high. Molinate 

use has declined by nearly half, while thiobencarb use has 

more than doubled; carbofuran has been eliminated and 

partially replaced by the pyrethroid pesticide lambdacyhalothrin.

A study was conducted in 2002 and 2003 by the U.S. 

Geological Survey to determine if the changes in pesticide 

use on rice resulted in corresponding changes in pesticide 

concentrations in surface waters. During the rice growing 

season (May–July), water samples, collected weekly at three 

sites in 2002 and two sites in 2003, were analyzed for 

pesticides using both solid-phase and liquid-liquid extraction 

in combination with gas chromatography/mass spectrometry. 

Analytes included lambda-cyhalothrin, molinate, 

thiobencarb, and two degradation products of molinate: 

2-keto-molinate and 4-keto-molinate.

Molinate, thiobencarb, and 4-keto-molinate were 

detected in all samples, 2-keto-molinate was detected in less 

than half of the samples, and lambda-cyhalothrin was not 

detected in any samples. At two of the sites sampled in 2002 

(Colusa Basin Drain 1 and Sacramento Slough), 

concentrations of molinate were similar, but thiobencarb 

concentrations differed by a factor of five. Although 

concentrations cannot be estimated directly from application 

amounts in different watersheds, the ratio of molinate to 

thiobencarb concentrations can be compared with the ratio of 

molinate to thiobencarb use in the basins. The higher 

concentration ratio in the Sacramento Slough Basin, 

compared with the ratio in the basin area feeding the Colusa 

Basin Drain 1, is consistent with the higher use ratio, 

suggesting that differences in application amounts can 

explain the observed concentration differences. The samples 

from the downstream site (Tower) sampled in 2002 had the 

lowest concentrations of pesticides. Performance goals were 

exceeded for either molinate or thiobencarb in six samples 

from the upstream sites, but not in any samples from the 

downstream Tower site. In 2003, concentrations at upstream 

sites were much lower than the previous year with only one 

sample containing thiobencarb at a concentration above the 

performance goal. Lower concentrations could be partially 

due to delays in rice planting and pesticide application owing 

to spring rainstorms. 

Historical data is available on peak concentrations of 

molinate and thiobencarb measured at Colusa Basin Drain 5 

(one of our sites in 2003) since 1981. Implementing holding 

times for pesticide-treated rice field water in the early 1980s 

succeeded in decreasing concentrations in surface waters. 

Detailed pesticide use data is available since 1991 and 

changing use patterns for molinate and thiobencarb can 

explain some, but not all, of the trends in peak pesticide 

concentrations. A stronger relationship is seen between the 

lengths of time that performance goals were exceeded and the 

amount of a pesticide applied within a basin.

Different extraction and analytical techniques were used 

to improve the recovery and lower the method detection limit 

for lambda-cyhalothrin. Recoveries of lambda-cyhalothrin 

from solid-phase extraction cartridges typically vary, so 

subsamples were processed by liquid-liquid extraction. The 

advantage of using a larger sample volume (3 L instead of 

1 L) to lower detection limits was offset by poor recovery 

during the cleanup step using an activated carbon column. 

Results suggest that as the concentrations of dissolved 

organic carbon in the sample increase, the recovery goes 

down. Limiting the sample size to 1 L and eliminating the 

activated carbon cleanup step improved the analysis for 

lambda-cyhalothrin.
2 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Suspended sediment samples were collected at two sites 

in July 2002 for analysis of molinate, thiobencarb, and lambdacyhalothrin. Molinate was detected in both samples, 

thiobencarb was detected and quantified in one sample, but 

could not be quantified in the other, and lambda-cyhalothrin 

was not detected in either sample. Sediment concentrations of 

both molinate and thiobencarb were higher than predicted by 

calculated equilibrium partitioning using concentrations in the 

surrounding aqueous phase.

Introduction

Background 

The Sacramento Valley of California is the nation's 

second largest producer of rice with nearly 2 million tons 

harvested from approximately 507,000 acres in 2003 (fig. 1) 

(U.S. Department of Agriculture, National Agricultural 

Statistics Service, 2004). Rice is a pesticide-intensive crop 

with over 5.9 million lb of pesticide active ingredients applied 

to rice in the Sacramento Valley in 2002 (California 

Department of Pesticide Regulation, 2002). Most of these 

pesticides are applied either directly to the soil prior to planting 

and flooding of the fields, or a few weeks after flooding, to 

control noxious weeds and pests. When the rice field water is 

released, it flows into local drains and creeks, and ultimately 

into the Sacramento River. 

Rice pesticide contamination in surface waters of the 

Sacramento Valley was first studied in the early 1980s by the 

California Department of Fish and Game (CDFG) in response 

to reported fish kills in agricultural drains. The study 

concluded that the fish kills were caused by exposure to high 

concentrations of molinate, an herbicide used on rice 

(Finlayson and others, 1982). This study also implicated 

another rice herbicide, thiobencarb, as the cause of taste and 

smell problems in city of Sacramento drinking water in May 

and June of 1981 and 1982. Concentrations as high as 

700 μg/L for molinate and 170 μg/L for thiobencarb were 

detected in 1982 in surface waters of the Sacramento Valley 

(California Department of Pesticide Regulation, 1992). 

The high concentrations of rice pesticides and related 

deleterious effects prompted the Central Valley Regional 

Water Quality Control Board (CVRWQCB) to establish 

performance goals for molinate and thiobencarb in surface 

water. In 1990, these goals were established at 30 μg/L for 

molinate and 1.5 μg/L for thiobencarb. The performance goal 

for molinate was subsequently reduced to 20 μg/L in 1991 and 

10 μg/L (the current goal) in 1992 (California Regional Water 

Quality Control Board, 1989). These performance goals are 

daily maximum concentrations and were established on the 

basis of the lowest observed effect concentrations for a number 

of aquatic species divided by an uncertainty factor of 10. In 

addition, California state law established primary maximum 

contaminant levels (MCLs) in drinking water for molinate at 

20 μg/L and thiobencarb at 70 μg/L, and a secondary MCL for 

thiobencarb at 1 μg/L (California Code of Regulations, 2003). 

Management practices were modified to help meet the 

performance goals. Mandatory holding times for rice field 

water after pesticide application were established to allow for 

the natural degradation and dissipation of pesticides prior to 

release into local surface waters. Current rice water holding 

times vary with the type of pesticide and local hydrologic 

conditions, but are generally on the order of 28 to 30 days. This 

practice has significantly reduced the levels of rice pesticides 

in surface waters. Other changes in pesticide management 

practices include recirculating field water systems, seepage 

control programs, and drift-reducing pesticide formulations 

and application procedures.

In 1984, the California Department of Food and 

Agriculture established the Rice Herbicides Control Program 

to coordinate efforts among multiple state agencies working on 

this issue. Management of this program was later passed to the 

California Department of Pesticide Regulation (CDPR), which 

continues a yearly monitoring program for rice pesticides in 

cooperation with CDFG, CVRWQCB, the California Rice 

Commission, and others. This program produces annual 

reports describing its protocols and findings. In addition, the 

U.S. Geological Survey (USGS) has conducted studies of the 

occurrence and fate of rice pesticides in both surface waters 

and ground waters of the Sacramento Valley (Domagalski and 

Kuivila, 1991; Bergamaschi and others, 1997; Crepeau and 

Kuivila, 2000; Domagalski, 2000; Dawson, 2001).

Pesticide Use Patterns

Since 1990, the CDPR’s full use reporting system has 

required pesticide applicators to provide detailed information 

on pesticide use. This data is extremely valuable in assessing 

trends in pesticide use, changes in application patterns, and 

potential for environmental contamination.

During the past decade, major changes have occurred in 

the types and amounts of pesticides applied to rice in the 

Sacramento Valley (fig. 2). Use of one herbicide, molinate, 

peaked in 1994 at nearly 1.5 million lb, but has declined 

significantly since then and is scheduled to be phased out by 

2009. In contrast, another herbicide, thiobencarb, has 

gradually increased in use from less than 75,000 lb in 1991 to 

over 800,000 lb in 2002. The most dramatic change has been 

the increase in application of propanil, which increased from 

less than 10,000 lb applied in 1991 to nearly 1.4 million lb in 

2002, becoming the most heavily used rice herbicide in 2002 

(California Department of Pesticide Regulation, 1992–2002).
Introduction 3

Butte Creek

CBD5

CBD1 Sacramento

Slough

Tower

Sutter

Buttes

Butte Creek

Colusa

Basin Drain

Feather River

Sutter Bypass

Sacramento

River

Sacramento

Durham

0 10 20 5 Miles

0 10 20 5 Kilometers

EXPLANATION

Sacramento Valley

Rice fields

2002 Sampling site

2003 Sampling site

City

CALIFORNIA

San

Francisco

Figure 1. Locations of sampling sites and rice growing areas within the Sacramento Valley of California.
4 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

1992 1993 1994 1995 1996 1997

Year

1998 1999 2000 2001 2002

Herbicides applied to rice, in pounds of active ingredient

Insecticides applied to rice, in pounds of active ingredient

Molinate

Propanil

Thiobencarb

Herbicides

Carbaryl

Carbofuran

Lambda-cyhalothrin

Insecticides

0

30,000

60,000

90,000

120,000

150,000

0

300,000

600,000

900,000

1,200,000

1,500,000

EXPLANATION

Figure 2. Amounts of pesticides (pounds active ingredient) applied to rice annually in the Sacramento River watershed.
Study Design and Sampling 5

There have been similar changes in the use of 

insecticides on rice. Carbofuran use has been phased out, 

declining from about 60,000 lb in 1992 to no use by 2001. 

The use of another insecticide, carbaryl, has varied 

considerably over the past decade. In contrast, lambdacyhalothrin, a pyrethroid insecticide, was first used in 1999 

and its use has since increased to nearly 2,400 lb in 2002 

(California Department of Pesticide Regulation, 2002). The 

application amount of lambda-cyhalothrin is much lower than 

the other insecticides because of its higher toxicity to target 

organisms. 

Current pesticide use data (2002) was incorporated into 

a geographic information system (GIS) coverage of the study 

area, which allowed for spatial and temporal mapping of 

pesticide use during the first year of the study. Through this 

process, it was also possible to determine the application 

amounts of various pesticides within selected subbasins of 

the Sacramento River watershed (table 1). This information 

was extremely useful during project planning and data 

interpretation. Pesticide use data covering the second year of 

the study (2003) was not yet available from CDPR.

Project Design

This project was designed to measure the concentrations 

of three rice pesticides and two rice-pesticide degradates in 

surface waters of the Sacramento Valley and to determine the 

influence of pesticide use changes over the past decade on 

pesticide concentrations in surface water. Specific pesticides 

measured in this study were molinate and thiobencarb 

(thiocarbamate herbicides), lambda-cyhalothrin (a pyrethroid 

insecticide), and 2-keto-molinate and 4-keto-molinate 

(degradates of molinate). Sample processing and analysis 

techniques focused on detecting lambda-cyhalothrin at the 

lowest concentration possible, owing to its toxicity to aquatic 

organisms at low levels. 

Water samples were collected weekly from May to July 

at three sites in 2002 and two sites in 2003 in the rice-growing 

region of the Sacramento Valley (fig. 1, table 2). Samples 

were collected from tributary creeks and agricultural drains 

that receive water directly from rice fields, and from the 

Sacramento River at a site downstream of most of the ricegrowing areas. Large-volume water samples to isolate 

suspended sediments were also collected on one occasion at 

two sites in 2002. All samples were analyzed for pesticides at 

the USGS’s Organic Chemistry Laboratory in Sacramento.

Purpose and Scope 

This report describes the methods used for collection 

and analysis of water and sediment samples collected as part 

of a two-year study of concentrations of rice pesticides in 

select streams in the Sacramento Valley of California. 

Pesticide concentration and ancillary data are presented. This 

report provides an interpretive analysis by evaluating 

measured concentrations in terms of current pesticide use 

patterns, comparing pesticide concentration data collected 

during the study with historical concentration data, and 

analyzing those factors that influence trends and patterns seen 

in the data. 

Acknowledgments

The authors gratefully acknowledge Lisa Jacobson of 

the USGS for assistance with sample collection and logistical 

support during this project. The authors also wish to 

acknowledge Patricia vonPhul, Lupe Hererra, Patrick 

Nicholas, and Theresa Pedersen of the USGS for their many 

hours of laboratory work and pesticide analyses. This project 

was funded by the State Water Resources Control Board 

(SWRCB No. 01-193-250-0), the USGS Federal/State 

Cooperative Program, and the USGS Toxic Substances 

Hydrology Program.

Study Design and Sampling

Study Area

The watershed of the Sacramento River above the city of 

Sacramento occupies over 23,000 mi2 within northern 

California and includes various land use types, from heavily 

forested uplands to intensely agricultural and urbanized areas 

within the Sacramento Valley. The specific area in this study 

falls within the boundary of the Sacramento Valley and 

extends from the city of Sacramento northwards, roughly 

81 mi, to the town of Durham (fig. 1). The topography of this 

area is predominantly low relief, with the exception of the 

Sutter Buttes, an extinct volcano, which rises abruptly from 

the center of the valley floor. 
6 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Table 1. Amounts of lambda-cyhalothrin, molinate, and thiobencarb applied during May–July 2002 in the watersheds of each site sampled in 2002

[Amounts are reported as pounds of active ingredient applied; see figure 3 for basin locations; lb, pound]

Watershed Lambda-cyhalothrin 

(lb)

Molinate 

(lb)

Thiobencarb 

(lb)

Colusa Basin Drain 1 (CBD1) 759 289,071 367,983

Sacramento Slough 970 392,031 334,645

Tower (excluding CBD1 and Sacramento Slough) 648 167,053 101,634

Tower 2,377 848,155 804,262

Table 2. Rice pesticide sampling sites, Sacramento Valley, California

[Horizontal datum, NAD 1927; mi2, square mile]

Official site name

(short site name)

USGS site 

identification No.

Latitude 

(degree/minute/

second)

Longitude 

(degree/minute/

second)

Year 

sampled

Basin size 

(mi2)

Colusa Basin Drain at Road 99E near Knights 

Landing (CBD1)

11390890 38°48'45" 121°46'23" 2002 1,622

Sacramento Slough near Verona (Sacramento 

Slough)

384649121381101 38°46'49" 121°38'11" 2002 1,250

Sacramento River at Sacramento (Tower) 11447500 38°35'12" 121°30'16" 2002 23,508

Butte Creek at Gridley Road near Butte Sink NWR 

(Butte Creek)

392143121532901 39°21'43" 121°53'29" 2003 593

Colusa Trough at Colusa NWR (CBD5) 391100122030101 39°11'00" 122°03'01" 2003 1,019
Study Design and Sampling 7

The Sacramento River watershed contains two main 

river systems, the Sacramento and Feather Rivers, which, 

with other tributaries, drain the western slopes of the southern 

Cascade mountain range and the northern Sierra Nevada and 

the eastern foothills of the Coast Range. On the Sacramento 

Valley floor, these rivers are joined by numerous smaller 

creeks and agricultural drains. One major agricultural drain 

within the study area is the Colusa Basin Drain (CBD), which 

receives water from over 1 million acres of farmland along 

the western side of the Sacramento Valley. Other important 

hydrologic features of the valley are the Yolo and Sutter 

Bypasses, which are used in the spring and summer to grow 

rice. 

The primary land use in the Sacramento Valley is 

agriculture, with over 2 million acres of farmland (California 

Department of Water Resources, 1990, 1994a, 1994b, 1995a, 

1995b, 1995c, 1995d, 2000). This region accounts for over 

90 percent of the rice production in California. The 

Sacramento Valley also supports a large and rapidly 

expanding urban population of over 2.1 million people (U.S. 

Census Bureau, 2000).

Most precipitation in the Sacramento Valley occurs 

between November and March. The mean annual 

precipitation in the study area ranges from 15 in. per year on 

the valley floor to 30 in. per year in the Sierras. Typically, 

little or no precipitation falls during the rice planting and 

growing season (April–October). However, in 2002, a 

significant storm (1.81 in. at Sacramento) in mid-May forced 

some early releases of rice field water (National Oceanic and 

Atmospheric Administration, 2002). Rice planting was also 

delayed significantly in 2003 by heavy rains (3.89 in. at 

Durham) in mid- and late April (California Irrigation 

Management Information System, 2003). 

Selection of Sampling Sites

Sampling sites were selected to represent watersheds 

that have a significant amount of rice-growing acreage and 

pesticide use. Other criteria for site selection included 

proximity to rice fields, previous surface water detections of 

dissolved pesticides, proximity to current and historical rice 

pesticide monitoring sites, and the safety of field personnel 

during sampling.

In 2002, three sites were sampled: two sites located on 

large agricultural drains—Colusa Basin Drain at Road 99E 

near Knights Landing (CBD1) and Sacramento Slough near 

Verona (Sacramento Slough), and one site downstream of all 

rice fields Sacramento River at Sacramento (Tower) (fig. 1, 

table 2). Prior to sampling, water parameters (temperature, 

specific conductance, and pH) were measured in width and 

depth profile at each site to determine flow characteristics 

(table 3). These measurements indicated that flow was well 

mixed at both CBD1 and Sacramento Slough, but there was 

some lateral stratification at Tower as seen in the specific 

conductance readings. CBD1 and Sacramento Slough were 

used as historical monitoring sites for rice pesticides by 

CDPR, and CBD1 has been sampled previously by the 

USGS.

Lambda-cyhalothrin was not detected in water samples 

collected in 2002, so it was decided to relocate the sampling 

sites for the following year. The two new sites sampled in 

2003 were Butte Creek at Gridley Road near Butte Sink 

NWR (Butte Creek) and Colusa Trough at Colusa NWR 

(CBD5) (fig. 1, table 2). These sites were located farther 

upstream in the watershed, closer to areas of current (2003) 

rice production (fig. 1), to increase the likelihood of detecting 

lambda-cyhalothrin. CBD5 has been used as a monitoring site 

extensively over the past two decades by CDPR. 

The Sacramento Valley watershed can be divided into 

five subbasin watersheds (fig. 3), which are representative of 

the five sampling sites. The Sacramento Slough watershed 

and its subbasin, Butte Creek, consist of agricultural and 

urban lands within the Sacramento Valley as well as large 

areas of nonagricultural land in the Sierra Nevada. In 

contrast, the CBD1 watershed and its subbasin, CBD5, are 

located on the valley floor and consist predominantly of 

agricultural lands with some areas of flooded wetlands. The 

Tower watershed encompasses all four subbasins, additional 

agricultural lands, and also receives runoff from the city of 

Sacramento and the American River watershed. For this 

report, subbasins were created by modifying larger 

preexisting watershed boundaries on the basis of the location 

of the sites sampled in the study. Where required, subbasin 

boundaries were delineated using USGS 1:24,000 scale 

digital topographic maps.
8 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Table 3. Site profiles of in-stream water parameters measured in 2002

[ft, feet; °C, degrees Celsius; μS/cm, microsiemens per centimeter]

Site Date 

(mm/dd/yy) Time

Distance from 

right bank 

(ft)

Depth

(ft)

Temperature

(°C)

Specific 

conductance 

(μS/cm)

pH

CBD1 05/16/02 1000 40 1 22.3 582 7.8

CBD1 05/16/02 1002 40 6 22.2 581 7.8

CBD1 05/16/02 1005 60 1 22.3 581 7.8

CBD1 05/16/02 1007 60 5 22.2 580 7.8

CBD1 05/16/02 1010 60 7 22.2 580 7.8

CBD1 05/16/02 1015 90 1 22.3 580 7.8

CBD1 05/16/02 1017 90 5 22.2 579 7.8

CBD1 05/16/02 1020 90 8 22.2 579 7.8

CBD1 05/16/02 1024 120 1 22.3 579 7.8

CBD1 05/16/02 1026 120 5 22.2 579 7.8

Sacramento Slough 05/15/02 1010 10 1 21.2 399 7.7

Sacramento Slough 05/15/02 1012 10 10 21.3 399 7.8

Sacramento Slough 05/15/02 1015 20 1 21.3 398 7.8

Sacramento Slough 05/15/02 1017 20 10 21.3 400 7.8

Sacramento Slough 05/15/02 1020 30 1 21.0 401 7.8

Sacramento Slough 05/15/02 1022 30 10 21.3 399 7.8

Tower 05/15/02 1245 50 1 20.1 182 7.9

Tower 05/15/02 1248 50 10 20.1 180 7.9

Tower 05/15/02 1250 50 15 19.5 171 7.9

Tower 05/15/02 1255 120 1 20.0 185 7.9

Tower 05/15/02 1300 120 10 19.5 172 7.9

Tower 05/15/02 1302 120 20 19.2 161 7.9

Tower 05/15/02 1308 220 1 19.5 168 7.9

Tower 05/15/02 1310 220 10 19.5 169 7.9

Tower 05/15/02 1312 220 20 19.0 152 7.8

Tower 05/15/02 1315 320 1 19.5 167 7.9

Tower 05/15/02 1317 320 10 19.2 159 7.9

Tower 05/15/02 1320 320 20 19.0 152 7.8

Tower 05/15/02 1325 440 1 19.0 152 7.8

Tower 05/15/02 1327 440 10 19.0 153 7.8
Study Design and Sampling 9

A

B

C

D

E

0 30 60 15 Miles

0 40 20 Kilomete 80 rs

Sacramento

EXPLANATION

A

B

C

D

E

Basins

City

Sacramento River (Tower)

Sacramento Slough

Colusa Basin Drain 1 (CBD1)

Butte Creek

Colusa Basin Drain 5 (CBD5)

Figure 3. Drainage basins within the area under study.
10 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Sample Collection 

Surface Water 2002

Water samples were collected at three sites following 

pesticide application and the controlled release of water from 

Sacramento Valley area rice fields. Sampling occurred 

weekly from May 15 to July 24 at CBD1, Sacramento 

Slough, and Tower. Sampling at Sacramento Slough and 

Tower was conducted by boat, while sampling at CBD1 took 

place from a bridge. Water samples were collected using a 

5-L, Teflon-lined Niskin bottle suspended horizontally 1 m 

below the water surface in the center of flow. At each site, 

two samples were collected using the Niskin for a total 

sample volume of approximately 10 L. The water sample was 

poured into a solvent-washed, 20-L stainless steel milk can 

that had been rinsed three times using water collected in the 

first Niskin grab. The milk can was then placed on ice and 

transported to the USGS’s Organic Chemistry Laboratory in 

Sacramento. The Niskin bottle was rinsed thoroughly using 

deionized water and (or) native site water when changing 

sampling locations. 

In the laboratory, each sample was split into 10 1-L 

fractions using a precleaned Teflon cone splitter. The milk 

can containing the sample water was first shaken for 

approximately one minute to thoroughly mix the sample and 

then poured at a consistent rate through the cone splitter. Care 

was taken to ensure that the splitter reservoir was kept filled 

at a constant level with sample water during the procedure. 

Split samples were collected for dissolved pesticides and 

dissolved organic carbon (DOC) in 1-L amber glass bottles 

and for suspended sediment concentration (SSC) analysis in 

1-L plastic bottles.

To verify the suitability of using a point-sample 

approach in this study, additional samples were collected for 

comparison using a cross-section approach. Once a month, 

water samples were collected using a U.S. D-77 depthintegrating sampler with a 3-L Teflon bottle in three verticals, 

equally spaced over the width of the channel at each site. A 

total volume of two liters was collected at each site. The 

sample was then split into 1-liter fractions by manually 

shaking the Teflon bottle and pouring the sample into a 1-L 

baked amber bottle and 1-L plastic bottle for pesticide and 

SSC analyses, respectively.

Suspended Sediment 2002

Suspended sediments for pesticide analysis were 

collected by sampling large volumes of water at CBD1 and 

Sacramento Slough on July 10 and 11, respectively. 

Approximately 300 L of water were collected at each site 

using a large volume peristaltic pump with a single stainless 

steel and Teflon inlet hose suspended at approximately 0.5 m 

below the water surface in the center of flow. Sample water 

was pumped directly into solvent-rinsed 20-L stainless steel 

soda kegs for transport to the USGS’s Sacramento Field 

Office.

Suspended sediment samples were concentrated using a 

Westphalia continuous flow centrifuge with a flow rate of 

2 L per minute. The resulting slurry was further centrifuged 

in the laboratory using a high-speed floor centrifuge to 

remove any remaining free water. The samples were then 

stored frozen for 18 months until analysis.

Surface Water 2003

Water samples were collected weekly from June 3 to 

July 22 at Butte Creek and CBD5. Sampling began 

approximately one month later than the previous year 

primarily because of late spring rains that delayed rice 

planting in the region. All samples were collected as midchannel grabs from bridges using a weighted 3-L Teflon 

bottle sampler. At each site, a precleaned Telfon bottle was 

used after having been rinsed three times using native water. 

Samples were collected at a depth of approximately 0.5 m. 

Following collection, the sample was split into subsamples by 

manually shaking the Teflon bottle and pouring the sample 

into two 1-L baked amber glass bottles for pesticide analysis 

and one 125-mL amber glass bottle for DOC analysis.

Sample Processing and Analytical 

Methods

Dissolved Pesticides

In 2002, replicate water samples for pesticide analysis 

were processed by two different methods: solid-phase 

extraction (SPE) and liquid-liquid extraction. For SPE 

extraction, a 1-L water sample was filtered through baked, 

0.7-μm glass fiber filters within 24 hours of sampling, and a 

surrogate compound, terbuthylazine, was added to provide 

quantitative data on extraction efficiency. The sample was 

then extracted onto C8 solid-phase extraction cartridges. The 

cartridge was then dried using compressed carbon dioxide, 

frozen, and stored for up to six months. For analysis, the 

cartridges were thawed and then eluted with 9 mL ethylacetate. The resulting extracts were then amended with 

internal standards, reduced to 200 μL, and analyzed for two 

rice pesticides (molinate and thiobencarb) and two degradates 

of molinate (2-keto-molinate and 4-keto-molinate) using a 

Varian Saturn GC/MS (gas chromatography-mass 

spectrometry) with ion-trap detection. Details of the 

analytical method are described in Crepeau and others 

(2000). 
Sample Processing and Analytical Methods 11

Liquid-liquid extraction was used for analysis of 

lambda-cyhalothrin because recovery of this pesticide is 

variable using SPE. During this process, a 3-L sample was 

filtered through a 0.7-μm glass fiber filter, transferred to a 

separatory funnel, and the terbuthylazine surrogate was 

added. The water was extracted three times with 

300 mL of methylene chloride each time. Excess water was 

removed from the methylene chloride by passing it through 

sodium sulfate. The extract was reduced using a rotary 

evaporator to approximately 1 mL volume, passed through a 

250-mg SPE activated carbon column (3 cc size barrel, 

Restek Corporation, Bellefonte, Pennsylvania), and eluted 

with an additional 10 mL of methylene chloride. The 

resulting extracts were further evaporated to 

0.5 mL, solvent exchanged into ethyl acetate, amended with 

internal standards, and reduced to 200 μL. The extracts were 

analyzed for lambda-cyhalothrin using a Varian Saturn 

GC/MS with ion-trap detection. The method detection limit 

for lambda-cyhalothrin was calculated at 0.039 μg/L, 

following the procedure described in Crepeau and others, 

2000. 

All pesticide samples collected in 2003 were processed 

by liquid-liquid extraction using a sample size of 1 L. Water 

samples were filtered through a 0.7-μm glass fiber filter, 

transferred to a separatory funnel, and terbuthylazine was 

added as a surrogate. The sample was extracted three times 

with 100 mL of methylene chloride each time. The methylene 

chloride extract was dried using sodium sulfate, reduced to 

0.5 mL, and solvent-exchanged into ethyl acetate. These 

samples were not cleaned up with the carbon column. The 

extracts were amended with internal standards, reduced to 

200 μL, and analyzed for three rice pesticides (lambdacyhalothrin, molinate, and thiobencarb) and two degradates 

of molinate (2-keto-molinate and 4-keto-molinate) using a 

Varian Saturn GC/MS with ion-trap detection. Details of the 

analytical method are described in Crepeau and others (2000) 

and method detection limits are listed in tables 4 and 5 later 

in this report.

Quality Assurance and Quality Control

Four types of quality control data were routinely 

collected: replicate samples, surrogate recovery, field 

equipment blanks, and matrix spikes. Replicate samples 

constituted 11 percent of the total samples collected, and 

differences between replicates were less than 25 percent for 

all of the pesticides detected.

Recovery of the surrogate, terbuthylazine, was used to 

assess the efficiency of each extraction. The average 

percentage recovery and standard deviation for the surrogate 

compound terbuthylazine was calculated for each year. 

Sample data were excluded if the recovery of terbuthylazine 

was outside the control limit of the annual mean plus or minus 

one and a half standard deviations (Kathryn M. Kuivila, U.S. 

Geological Survey, unpub. data, 2004). This initial screening 

resulted in the elimination of three field samples collected in 

2002 (two collected at CBD1 and one at Sacramento Slough), 

one 2002 field equipment blank, one 2002 matrix spike 

sample, and one 2003 matrix spike sample.

Five field equipment blanks (8 percent of samples) were 

collected over the course of the study with one taken after 

sampling at each site to assess potential sample 

contamination on a site by site basis. In 2002, there were three 

field equipment blanks, but one was excluded because the 

surrogate recovery was outside the control limit. Of the other 

two samples, one blank had no detections of any pesticides, 

while the other had a thiobencarb detection of 0.0107 μg/L. 

Closer examination of the sequence of samples and standards 

run on the GC/MS suggests that thiobencarb was carried over 

in the instrument from the previous injection of a much 

higher concentration standard. In 2003, two field equipment 

blanks were collected. One of these samples contained 

detectable concentrations of molinate (0.0165 μg/L) and 

thiobencarb (0.0053 μg/L). Contamination of this blank 

sample was likely the result of insufficient cleaning of the 

sampling equipment prior to collection of the blank sample. 

These values are more than an order of magnitude lower than 

any values measured in 2003 samples.

In 2002, there were three SPE matrix spike samples, but 

one was excluded owing to low surrogate recovery. One of 

the other samples had concentrations of molinate and 4-ketomolinate that were approximately 20 times that of the added 

spike concentration, making it difficult to accurately quantify 

the spike recovery. The recoveries of molinate, thiobencarb, 

2-keto-molinate, and 4-keto-molinate were very good, 

ranging from 77 to 93 percent. Lambda-cyhalothrin did not 

work as well on SPE cartridges, which had recoveries from 

29 to 46 percent.

Liquid–liquid extractions of samples were modified 

both years to improve the recovery and detection limits of 

lambda-cyhalothrin in the water samples. These results are 

discussed in detail later in this report.
12 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Pesticides Associated with Suspended Sediments

Suspended sediment samples were analyzed for 

pesticides by microwave-assisted solvent extraction (MASE) 

and GC/MS. Wet sediments were extracted to avoid any 

artifacts associated with the drying of sediment and for 

efficient extraction (Jayaraman and others, 2001). 

Approximately 5 g of sediment (by dry weight) were spiked 

with a surrogate solution containing 400 ng each of 

13C-labeled αBHC, chlorpyrifos, permethrin (cis/trans

mixture), and simazine (Cambridge Isotope Laboratories, 

Inc., Andover, Massachusetts). The sediments were extracted 

three times by MASE with a mixture of methylene chloride 

and acetone (50:50 v/v) using an MSP 1000 (CEM 

Corporation, Matthews, North Carolina). The resulting 

solution was back-extracted against water, dried using 

sodium sulfate, and reduced to 1 mL. The extracts were 

cleaned using activated carbon SPE, solvent-exchanged into 

ethyl acetate, and further purified using a gel permeation/high 

pressure liquid chromatography (GPC/HPLC) system. The 

sample was reduced to approximately 500 μL, internal 

standards were added, and the sample was brought to a final 

volume of 200 μL using N2 evaporation for GC/MS analysis. 

The details of the method and method detection limits are 

described in Lawrence A. LeBlanc, U.S. Geological Survey, 

unpub. data, 2004. Samples were analyzed for three rice 

pesticides: molinate, thiobencarb, and lambda-cyhalothrin. 

The method detection limits for molinate and thiobencarb are 

2.0 and 4.4 ng/g dry weight, respectively (Lawrence A. 

LeBlanc, U.S. Geological Survey, unpub. data, 2004). The 

method is not suitable for the two molinate degradates, 

2-keto-molinate and 4-keto-molinate. 

Suspended sediments were analyzed for organic carbon 

content using a Perkin Elmer CHNS/O analyzer (Perkin 

Elmer Corporation, Norwalk, Connecticut). Sediments were 

combusted at 925°C in silver boats, after being exposed to 

concentrated hydrochloric acid fumes in a dessicator for 

24 hours to remove carbonate minerals. Sediments were dried 

to a constant weight before CHN analysis. Acetanilimide was 

used for instrument calibration of elemental carbon and 

nitrogen. Purified products of high temperature combustion 

(CO2 for C, and N2 for N) were detected via a thermal 

conductivity detector.

Analysis of Suspended Sediments and Dissolved 

Organic Carbon

Unfiltered water samples were analyzed for SSC at the 

USGS’s Sediment Laboratory in Marina, California. Details 

of the analytical method can be found in Guy, 1969.

Water samples collected for DOC analysis were filtered 

in series through a 2.7-μm (Whatman GF/A) glass fiber filter, 

a 0.7-μm (Whatman GF/F) glass fiber filter, and a 0.45-μm 

polypropylene (Pall Gelman GH Polypro Membrane) filter 

within 24 hours of sample collection. After being filtered, the 

samples were stored in baked amber glass bottles at 4°C for 

no longer than 1 week until analysis. DOC concentrations 

were analyzed using a Shimadzu TOC-5000A total organic 

carbon analyzer. The instrument was calibrated using 

potassium hydrogen phthalate standards prepared in organicfree water, with standard concentrations bracketing the 

concentrations of the samples. Aliquots of filtered water 

sample (4.5 mL) were acidified using 30 μL of 2N HCl and 

sparged using N2 for 3 minutes to remove inorganic carbon as 

CO2. The nonpurgeable organic carbon (NPOC) was 

analyzed by direct injection of liquid sample into a high 

temperature (680°C) combustion tube packed with Pt 

catalyst. The CO2 produced by oxidation of the NPOC was 

detected using a nondispersive infrared photometric cell. 

Each value reported represents the mean of three injections of 

the same sample. Details of the analytical method are 

described in Bird and others, 2003.
Analytical Results 13

Analytical Results

Pesticide Concentrations

Dissolved rice pesticides and degradates were analyzed 

in 30 samples in 2002 (table 4). All samples contained 

detectable concentrations of three pesticides (molinate, 

4-keto-molinate, and thiobencarb), whereas twelve samples 

also contained 2-keto-molinate. Lambda-cyhalothrin was not 

detected in any of the samples. Molinate was detected at the 

highest concentration with a peak value of 11.7 μg/L 

(Sacramento Slough on May 30), and thiobencarb was 

detected at a maximum concentration of 7.16 μg/L (CBD1 on 

May 23). One of the molinate degradates, 4-keto-molinate, 

was also detected at high concentrations, with a peak 

concentration of 4.28 μg/L (CBD1 on June 12). Although the 

other molinate degradate, 2-keto-molinate, was detected in 

12 of the samples, the concentrations were much lower than 

either 4-keto-molinate or its parent. 

In 2003, 16 water samples were analyzed for dissolved 

rice pesticides and degradates (table 5). As in the previous 

year, molinate, 4-keto-molinate, and thiobencarb were 

detected in all of the samples, and lambda-cyhalothrin was 

not detected in any of the samples. Measurable 

concentrations of 2-keto-molinate were found only in two 

samples. Overall, all pesticide concentrations were two to 

three times lower than the previous year. Molinate was again 

detected at the highest concentration, but at a much lower 

value of 5.37 μg/L (CBD5 on June 10). The same sample 

contained the highest concentration of 4-keto-molinate 

(1.32 μg/L). In contrast, the maximum detected concentration 

of thiobencarb (3.81 μg/L) was detected in Butte Creek on 

June 3.

Two suspended sediment samples collected in 2002 

were analyzed for three pesticides: molinate, thiobencarb, 

and lambda-cyhalothrin (table 6). Both molinate and 

thiobencarb were detected in the Sacramento Slough sample 

at concentrations of 43.4 and 108 ng/g, dry weight, 

respectively. The sample from CBD1 had a similar molinate 

concentration with a value of 38.8 ng/g dry weight, but 

thiobencarb could not be quantified in this sample because of 

a co-eluting compound. The weight percentage of organic 

carbon in the sediments was 3.04 percent and 1.95 percent for 

CBD1 and Sacramento Slough sediments, respectively.

Suspended Sediment and Dissolved Organic Carbon 

Concentrations

Suspended sediment concentrations (SCC) were 

determined for 32 water samples collected at three sites in 

2002 (table 4), which ranged from 10 mg/L to 93 mg/L. 

Samples collected at CBD1 had the highest overall 

concentration (93 mg/L) and the highest average 

concentration (68 mg/L) of all the sites sampled in 2002. 

Slightly lower concentrations were seen at Sacramento 

Slough (maximum of 79 mg/L and average of 63 mg/L). 

Samples collected from Tower, the farthest site downstream, 

had the lowest SSC value (10 mg/L) as well as the lowest 

average concentration (19 mg/L). No temporal trends are 

apparent in SSC values for any of the sites sampled that can 

be determined at the sampling interval of this study.

Dissolved organic carbon concentrations were 

determined in 28 and 16 water samples collected in 2002 and 

2003, respectively (tables 4 and 5). Samples collected in 2002 

ranged in concentration from 1.16 to 7.79 mg/L. Similar to 

SSC, DOC concentrations were generally highest at CBD1 

and lowest at Tower. In 2003, DOC concentrations ranged 

from 2.65 to 8.68 mg/L. Water samples collected at CBD5 

had consistently higher DOC concentrations than samples 

collected at Butte Creek. 

Sampling Method Comparison

Samples were collected during 2002 to assess whether 

collecting mid-channel point samples was appropriate as 

compared with depth-integrated cross section samples for 

sampling dissolved pesticides and suspended sediments. A 

comparison of sample pairs collected using both an 

isokenetic depth-integrating sampler (U.S. D-77) and a point 

sampler (Niskin bottle) indicates that the two methods 

collected dissolved pesticides equally, but did not collect 

suspended sediments equally. 
14 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Table 4. Pesticide, dissolved organic carbon, and suspended sediment concentrations detected in water samples collected during 2002

[Pesticide concentrations are reported as micrograms per liter; lambda-cyhalothrin was not detected in any samples; DOC, dissolved organic carbon, SSC, 

suspended sediment concentration, MDL, method detection limit, ( ), pesticide concentrations detected below the method detection limit, nd, nondetection, NA, 

not measured; μg/L, micrograms per liter; mg/L, milligrams per liter]

Sampling Site Date 

(mm/dd/yy) Time

2-keto-molinate 

(μg/L) 

MDL=0.0076 

μg/L

4-ketomolinate

(μg/L) 

MDL=0.0117 

μg/L

Molinate\ 

(μg/L) 

MDL=0.0080 

μg/L

Thiobencarb 

(μg/L)

MDL=0.0060

μg/L

DOC 

Concentration 

(mg/L)

SSC

(mg/L)

CBD1 05/16/02 1030 0.0290 1.40 11.5 5.08 NA 71

CBD1 05/23/02 1000 .0529 2.16 11.4 7.16 4.85 NA

CBD1 05/29/02 1115 .0353 1.91 11.6 4.84 4.14 56

CBD1 06/05/02 0715 .0230 3.68 9.65 2.20 7.79 84

CBD1 06/12/02 0700 nd 4.28 4.92 1.36 7.32 40

CBD1 06/20/02 1050 nd 1.60 1.67 .813 6.42 93

CBD1 06/27/02 0750 nd 1.17 1.01 .600 5.81 83

CBD1 07/02/02 0640 nd 1.70 .764 .403 NA 85

CBD1 07/10/02 1300 nd .746 .443 .268 5.33 45

CBD1 07/18/02 1330 nd .546 .326 .177 4.74 55

CBD1 07/24/02 0715 NA NA NA NA 4.95 65

Sacramento Slough 05/15/02 1030 .0096 .367 2.81 .233 NA 66

Sacramento Slough 05/22/02 1015 .0439 .932 10.5 1.21 2.66 74

Sacramento Slough 05/30/02 0930 .0485 1.45 11.7 1.13 2.72 49

Sacramento Slough 06/06/02 0850 .0288 1.21 7.32 .635 3.02 63

Sacramento Slough 06/13/02 1000 .0111 1.41 5.42 .250 2.91 79

Sacramento Slough 06/19/02 1000 nd .770 2.63 .158 2.99 61

Sacramento Slough 06/26/02 0845 (.0071) 1.12 1.10 .143 3.33 74

Sacramento Slough 07/03/02 0840 nd .575 .908 .155 1.67 53

Sacramento Slough 07/11/02 0900 (.0053) .594 .769 .102 3.38 75

Sacramento Slough 07/17/02 1020 nd .537 .427 .0474 3.62 48

Sacramento Slough 07/23/02 0915 NA NA NA NA 3.25 55

Tower 05/22/02 1330 .0137 .217 2.68 .411 2.26 53

Tower 05/30/02 1100 nd .230 1.59 .237 1.58 15

Tower 06/06/02 1000 nd .273 .906 .181 1.86 18

Tower 06/13/02 1115 NA NA NA NA NA 10

Tower 06/19/02 1115 nd .0749 .156 .0465 1.23 17

Tower 06/26/02 1015 nd .0836 .0720 .0252 1.55 17

Tower 07/03/02 1045 nd .0597 .0388 .0154 4.17 19

Tower 07/11/02 1045 nd .0446 .0326 .0124 1.39 15

Tower 07/17/02 1150 nd .0343 .0797 .0271 1.16 23

Tower 07/23/02 1130 nd .0506 .0301 .0118 1.48 17

Equipment blank 05/30/02 1340 nd nd nd .0107 NA NA

Equipment blank 07/17/02 1200 nd nd nd nd NA NA
Analytical Results 15

Table 5. Pesticide and dissolved organic carbon concentrations detected in water samples collected during 2003

[Pesticide concentrations are reported as micrograms per liter; lambda-cyhalothrin was not detected in any samples; DOC, dissolved organic carbon; MDL, 

method detection limit; ( ), pesticide concentrations detected below the method detection limit; nd, nondetection; NA, not measured; μg/L, micrograms per liter; 

mg/L, milligrams per liter]

Sampling site Date 

(mm/dd/yy) Time

2-keto-molinate 

(μg/L) 

MDL=0.0076 

μg/L

4-keto-molinate 

(μg/L) 

MDL=0.0117 

μg/L

Molinate 

(μg/L) 

MDL=0.0080 

μg/L

Thiobencarb 

(μg/L) 

MDL=0.0060 

μg/L

DOC 

concentration 

(mg/L)

Butte Creek 06/03/03 1015 nd 0.0676 0.712 3.81 2.65

Butte Creek 06/10/03 0930 0.0177 .329 3.13 .603 3.40

Butte Creek 06/17/03 1051 nd .236 1.03 .507 3.16

Butte Creek 06/24/03 0800 nd .246 .522 .149 3.57

Butte Creek 07/01/03 0915 nd .315 .308 .100 3.04

Butte Creek 07/08/03 1100 nd .423 .266 .104 3.55

Butte Creek 07/15/03 0730 nd .961 .392 .129 3.31

Butte Creek 07/22/03 1100 nd .889 .359 .110 4.86

CBD5 06/03/03 1100 nd .610 3.38 .591 6.73

CBD5 06/10/03 1030 .0187 1.32 5.37 .513 8.20

CBD5 06/17/03 1145 nd .906 1.31 .543 8.68

CBD5 06/24/03 0900 nd .930 .894 .274 7.71

CBD5 07/01/03 1020 nd 1.06 .705 .331 6.45

CBD5 07/08/03 1140 nd 1.25 .588 .241 6.58

CBD5 07/15/03 0815 nd .762 .351 .185 5.24

CBD5 07/22/03 1145 nd .618 .263 .141 6.35

Equipment blank 06/17/03 1400 nd nd .0165 (.0053) NA

Equipment blank 07/08/03 1700 nd nd nd nd NA

Table 6. Suspended sediment-associated pesticides: observed concentrations and predicted equilibrium-partitioning concentrations

[Lambda-cyhalothrin was not detected in any samples; nd, nondetection; NC, not calculated; ng/g, nanograms per gram dry weight; foc, fraction of organic carbon 

in sediment; Koc, organic carbon: normalized sediment–water distribution coefficient; μg/L, micrograms per liter]

1Data from Mackay and others, 1997.

Sampling site Date 

(mm/dd/yy) Time

Sediment 

concentration 

(ng/g)

foc

(sediment) Koc1

Predicted water 

concentration

(μg/L)

Measured 

water 

concentration 

(μg/L)

Molinate CBD1 7/10/2002 1330 38.8 0.0304 83.2 15.3 0.443

Sacramento Slough 7/11/2002 0930 43.4 .0195 83.2 26.8 .769

Thiobencarb CBD1 7/10/2002 1330 nd .0304 891 NC .268

Sacramento Slough 7/11/2002 0930 108 .0195 891 6.23 .102
16 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

When comparing pesticide concentrations between 

samples collected at the same time, the relative percent 

difference (RPD) between pesticides collected by the two 

methods was less than 25 percent (table 7). This is within the 

analytical variability of pesticide replicates (Kathryn M. 

Kuivila, U.S. Geological Survey, unpub. data, 2004), 

suggesting that the point sample was representative of the 

cross-section for dissolved pesticides at the sites sampled in 

2002. For suspended sediment concentrations, the analytical 

method variability (Gray and others, 2000) ranges from a 

standard deviation of 2.6 mg/L at a concentration of 10 mg/L

(26 percent) to 5.3 mg/L at 100 mg/L (5.3 percent). In three 

of the eight sample pairs collected, the RPD for the two 

methods was greater than the analytical variability. Results 

also seem to be biased, with the highest suspended sediment 

concentration per pair collected by the D-77 sampler in 

75 percent of the sample sets. Previous studies have also 

shown discrepancies and bias in SSC values in comparing 

these two sampling methods (Martin and others, 1992). A 

further complicating factor in evaluating SSC is the limited 

range of flow velocity and depth within which the 

D-77 sampler operates isokinetically. It is likely that these 

limitations were a factor at CBD1 where flow velocity was 

low and at Tower because of channel depth. The 

D-77 sampler was phased out of use by the USGS at the end 

of 2002 after recognition of these limitations (USGS 

Technical Memorandum 2002.09, 2002). 

Comparison of Pesticide 

Concentrations

Dissolved Pesticide Concentrations 2002

Concentrations of molinate were similar at CBD1 and 

Sacramento Slough (figs. 4A and 4B). In contrast, 

thiobencarb concentrations were approximately five times 

higher at CBD1 than Sacramento Slough (figs. 4A and 4B). 

This difference is most likely due to differences in use; 

however, concentrations cannot be calculated directly from 

application amounts because the factors that control 

concentrations can vary considerably between basins. But, if 

those factors influence the transport of both molinate and 

thiobencarb in an individual basin in a similar manner, then 

ratios of molinate to thiobencarb concentration can be 

compared with ratios of their application amounts. The 

average ratio of molinate to thiobencarb concentration was 

11.1 for Sacramento Slough and 2.3 for CBD1. A comparison 

of the ratio of molinate to thiobencarb application in each 

basin shows a similar pattern with Sacramento Slough having 

the highest ratio (1.2) and CBD1 the lowest (0.8).

The peak concentrations of 4-keto-molinate at CBD1 

were double the concentrations at Sacramento Slough despite 

similar molinate concentrations in the two basins. The 

concentrations of 2-keto-molinate were much lower at all 

sites with respect to the other pesticides.

In all instances, concentrations of each pesticide 

detected were least in samples collected at Tower. Though 

Tower is representative of the largest basin in the study and, 

therefore, receives the greatest volume of pesticide exposed 

water, it is likely that dilution by pesticide-free runoff from 

the upper reaches of the watershed results in lower overall 

pesticide concentrations. 

At each of the sites, the maximum concentrations of 

molinate, thiobencarb, and 4-keto-molinate were detected at 

approximately the same time, May 22 through May 30, 2002 

(figs. 4A, 4B, and 4C). These detections took place just after 

an unseasonable storm (May 19–21) dropped over an inch 

and a half of rain throughout much of the Sacramento Valley. 

Minor flooding caused by this storm resulted in the release of 

water from rice fields just north of Sacramento that had 

recently been treated with thiobencarb, and which had not 

undergone the required holding period (Newhart, 2002). This 

water would have entered the Sacramento River and was 

likely present during the May 22 sampling at the Tower site. 

It is likely that this storm caused other uncontrolled releases 

of pesticide-treated water within the Sacramento Valley, 

contributing to elevated concentrations of pesticides at CBD1 

and Sacramento Slough.

Dissolved Pesticide Concentrations 2003

Maximum concentrations of each of the pesticides 

detected were generally less than half of the maximum 

concentrations detected during the previous year. It is likely 

that multiple factors contributed to the decline in pesticide 

concentrations detected in 2003. Locating the sampling sites 

farther upstream in the watershed certainly reduced the 

source acreage planted in rice and the total amount of 

pesticides applied that could potentially be detected at each of 

the sites. Another factor that may have contributed to lower 

pesticide concentrations was delay of both rice planting and 

pesticide applications caused by severe spring 2003 

rainstorms in the region. In response to this change in timing, 

water sampling for 2003 was delayed by two weeks relative 

to the previous year. Spring storms also contributed to a 

roughly 5 percent decrease in the total rice acreage from 2002 

to 2003, which also may have reduced the total amounts of 

pesticides applied in the study area (U.S. Department of 

Agriculture, 2004).
Comparison of Pesticide Concentrations 17

Table 7. Pesticide concentrations determined in quality assurance sample pairs collected in 2002 and the relative percent difference between pairs

[Values are reported as micrograms per liter; lambda-cyhalothrin was not detected in any samples; *, samples collected with depth-integrating sampler; QA, 

quality assurance; RPD, relative percent difference; NA, not applicable; nd, nondetection; μg/L, micrograms per liter]

Sampling site Date 

(mm/dd/yy) Time Sample type

2-ketomolinate 

(μg/L)

RPD 

(percent)

4-ketomolinate 

(μg/L)

RPD

(percent)

Molinate 

(μg/L)

RPD

(percent)

Thiobencarb

(μg/L)

RPD 

(percent)

CBD1* 05/23/02 0945 QA 0.0528 0.2 2.02 7.0 11.9 4.4 7.36 2.8

CBD1 05/23/02 1000 Regular .0529 2.16 11.4 7.16

CBD1* 07/18/02 1315 QA nd NA .455 18.2 .355 8.5 .184 3.9

CBD1 07/18/02 1330 Regular nd .546 .326 .177

Sacramento Slough* 05/22/02 1015 QA .0401 9.0 .849 9.3 9.58 9.2 1.16 4.5

Sacramento Slough 05/22/02 1015 Regular .0439 .932 10.5 1.21

Sacramento Slough* 06/19/02 1000 QA nd NA .933 19.1 2.36 10.5 .159 0.6

Sacramento Slough 06/19/02 1000 Regular nd .770 2.63 .158

Sacramento Slough* 07/17/02 1020 QA nd NA .440 19.8 .505 16.7 .0574 19.1

Sacramento Slough 07/17/02 1020 Regular nd .537 .427 .0474

Tower* 05/22/02 1330 QA nd NA .202 7.1 2.47 8.3 .437 6.1

Tower 05/22/02 1330 Regular .0137 .217 2.68 .411

Tower* 06/19/02 1115 QA nd NA .0822 9.3 .169 8.0 .0426 8.7

Tower 06/19/02 1115 Regular nd .0749 .156 .0465

Tower* 07/17/02 1200 QA nd NA .0424 21.1 .0981 20.7 .0302 10.8

Tower 07/17/02 1150 Regular nd .0343 .0797 .0271
18 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California Dissolved pesticide concentration, in micrograms per liter

Molinate

Thiobencarb

4-Keto-molinate

5/15/02 5/29/02 6/12/02 6/26/02 7/10/02 7/24/02

Molinate

Thiobencarb

4-Keto-molinate

Molinate

Thiobencarb

4-Keto-molinate

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

Date

CBD1

Tower

Sacramento Slough

A

B

C

Figure 4. Dissolved pesticide concentrations at CBD1, Sacramento Slough, and Tower in 2002.
Historical Peak Concentrations 19

Concentrations of molinate and 4-keto-molinate were 

greater at CBD5 than at Butte Creek during the first six weeks 

of the study, but were greater in Butte Creek samples during 

the final two weeks. Concentrations of thiobencarb had the 

opposite relation. The ratio of molinate to thiobencarb 

concentration was higher at CBD5 (3.8) than at Butte Creek 

(2.9). Although pesticide use data for 2003 were not available 

from DPR, a general comparison can be made using the 2002 

data. The ratios of application amounts in 2002 in the two 

basins had a pattern similar to the concentration ratios: CBD5 

was higher (0.95) than Butte Creek (0.7).

The highest concentrations of molinate were detected on 

June 10 at both sites. Likewise, the highest concentrations of 

thiobencarb were seen on the same day (June 3) at both sites 

(fig. 5A and 5B). The fact that the highest concentrations of 

both molinate and thiobencarb were not observed on the same 

day indicates that this timing is likely pesticide dependent. It 

is likely that the timing of maximum concentrations is a 

function of the time the bulk of each pesticide was applied 

and, therefore, when pesticide treated waters were released 

after the respective holding times had past. Because detailed 

pesticide use data for 2003 is not yet available, this 

hypothesis could not be explored further.

Performance Goals and MCLs

In 2002, six water samples contained concentrations of 

either molinate or thiobencarb above established water 

quality goals. Molinate concentrations exceeded the 

performance goal of 10 μg/L in three consecutive weekly 

samples at CBD1 and in two samples at Sacramento Slough. 

In these samples, molinate concentrations were generally just 

above the performance goal. Also at CBD1, four consecutive 

weekly samples contained concentrations of thiobencarb that 

exceeded the performance goal of 1.5 μg/L. Three of these 

samples corresponded with the elevated molinate 

concentrations. The performance goal for thiobencarb was 

not exceeded in any samples from Sacramento Slough.

In addition, thiobencarb concentrations exceeded the 

secondary drinking water MCL of 1.0 μg/L in five 

consecutive weekly samples from CBD1 and two samples at 

Sacramento Slough. In these samples, the performance goal 

for thiobencarb was exceeded by up to 450 percent, and the 

secondary MCL by up to 700 percent. Water quality goals for 

molinate and thiobencarb were not exceeded in any samples 

collected at Tower.

A single water sample collected at Butte Creek during 

the 2003 sampling period contained thiobencarb at an 

elevated concentration of 3.81 μg/L. This concentration was 

more than double the established performance goal for 

thiobencarb, (1.5 μg/L) and exceeded the secondary drinking 

water MCL (1.0 μg/L) by nearly four times. In all other 

samples collected in 2003, both thiobencarb and molinate 

were present at concentrations well below established water 

quality goals.

Historical Peak Concentrations

Using data obtained by CDPR through the Rice 

Pesticide Monitoring Program, it is possible to compare peak 

molinate and thiobencarb concentrations observed at CBD5 

during this study with over two decades of similar data. The 

maximum concentrations of molinate and thiobencarb 

detected at CBD5 during the past 22 years are shown in 

figure 6. In the early 1980s, the maximum concentrations at 

CBD5 were very high, with concentrations of molinate and 

thiobencarb reaching 697 μg/L and 170 μg/L, respectively. 

Once mandatory holding times for pesticide-treated field 

water were established in 1984, peak concentrations 

decreased considerably. In the past decade, peak molinate 

concentrations have generally continued to decrease, while 

peak thiobencarb concentrations increased in the late 1990s 

and then declined again.

Besides the holding times, another factor influencing 

pesticide concentrations is the change in amounts of 

pesticides applied over the years. Detailed pesticide use data 

are available from 1991 to 2002 (California Department of 

Pesticide Regulation, 2002) and are shown in figure 6. 

Although maximum molinate concentrations at CBD5 show 

a general decreasing trend similar to the pattern of use, there 

is still much more variability than can be explained by 

changes in application amounts alone. Thiobencarb 

concentrations increase with increasing use until 1998, when 

concentrations began to decline despite a continuing increase 

in use (fig. 6). Some of the variability in peak concentrations 

may be explained by the intentional and unintentional early 

releases of rice field water (Crepeau and Kuivila, 2000).

Despite implementing holding times, the concentrations 

of both molinate and thiobencarb typically exceed the 

performance goals (fig. 6). Molinate concentrations 

measured at CBD5 were greater than the performance goal of 

10 μg/L every year until 2003. In contrast, thiobencarb 

concentrations did not exceed the performance goal of 

1.5 μg/L at CBD5 during a four-year period (1988–1991). 

But as use of thiobencarb began to increase, the 

concentrations exceeded the performance goal every year 

until 2003.
20 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Molinate

Thiobencarb

4-Keto-molinate

6/4/03 6/11/03 6/18/03 6/25/03

Date

7/2/03 7/9/03 7/16/03 7/23/03

Molinate

Thiobencarb

4-Keto-molinate

Dissolved pesticide concentration, in micrograms per liter

Butte Creek

CBD5

Dissolved pesticide concentration, in micrograms per liter

0

2

4

6

8

10

12

0

2

4

6

8

10

12

A

B

Figure 5. Dissolved pesticide concentrations at Butte Creek and CBD5 in 2003.
Historical Peak Concentrations 21

1981 1983 1985 1987 1989 1991 1993

Year

1995 1997 1999 2001 2003

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003

Thiobencarb application

Maximum Thiobencarb concentration

170 micrograms per liter

Performance

Goal

Performance

Goal

Holding times begin

0

100

200

300

400

500

Maximum Molinate concentration

Molinate application

Holding times begin

697 micrograms per liter

0

50,000

100,000

150,000

200,000

250,000

0

5

10

15

20

25

30

35

40

Thiobencarb applied in CBD5 basin, in pounds Molinate applied in CBD5 basin, in pounds

0

100,000

200,000

300,000

400,000

500,000

Maximum Thiobencarb concentration, in micrograms per liter Maximum Molinate concentration, in micrograms per lite

A r

B

Figure 6. Pesticide application within CBD5 basin, peak pesticide concentrations at CBD5, and performance goals. Data during 1981 through 2003.
Analysis of Lambda-Cyhalothrin 22

There appears to be no direct correlation between 

molinate and thiobencarb applications and observed maximum 

concentrations of these pesticides within the CBD5 basin. 

However, when pesticide use in the basin is plotted against the 

consecutive number of days during which the respective 

performance goals for these pesticides were exceeded, there is 

a very good relation (fig. 7). Examining the data in this manner 

indicates that the duration of potential exposure to elevated 

levels of molinate and thiobencarb is likely linked to the total 

use of these pesticides within the basin.

Analysis of Lambda-Cyhalothrin

Matrix Recoveries in 2002

Large-volume filtered water samples (3 L) were processed 

by liquid-liquid extraction, and analyzed to maximize recovery 

and lower the detection level for lambda-cyhalothrin for the 

2002 samples. The drawback to extracting 3 L of water, in 

contrast to 1-L samples, is that more organic compounds are 

extracted that interfere with the analysis and cause problems 

with quantification. The extract was cleaned up by passing it 

through an activated carbon column. Method validation 

included testing the recovery of lambda-cyhalothrin through 

this cleanup step. Methylene chloride spiked using lambdacyhalothrin and passed through the carbon column had very 

good recovery (107 percent). Addition of a sample matrix from 

the liquid-liquid extraction of 3 L of water from CBD1 

decreased the recovery to 75 ± 5 percent. But, during this study, 

matrix spike recoveries of lambda-cyhalothrin were even 

lower, ranging from 49 percent to 60 percent. 

One possibility is that elevated DOC concentrations 

decreased the efficiency of the pesticide recovery. The 

concentrations of DOC in these samples ranged from 2.72 to 

7.79 mg/L. There is a strong trend of decreasing recovery of 

lambda-cyhalothrin with increasing DOC concentration

(fig. 8). The apparent loss of the pesticide could have occurred 

during the sample processing, in either the extraction or carbon 

column cleanup steps or it could be an artifact of the 

instrumental analysis.

Additional Method Testing in 2003

Further method testing was added during the 2003 

sampling. To determine if lambda-cyhalothrin was being lost 

during sample processing, all samples included a matrix spike, 

the carbon cleanup step was eliminated, and the sample size 

was decreased to 1 L. DOC concentrations were similar to 2002 

values, ranging from 2.65 to 8.68 mg/L. Matrix spike 

recoveries were very good (106 percent ± 9.8 percent). This 

suggests that the liquid–liquid extraction efficiency was not the 

problem.

 Standard additions were done on all samples to 

determine if there was matrix interference in the GC/MS 

analysis. Before analysis, each eluted sample was measured 

precisely and a 100-μL subsample was taken. A matrix spike 

was added directly to the subsample as the standard addition 

sample. Both the unspiked and standard addition samples were 

analyzed by GC/MS. The amount of added matrix spike is 

compared with the calculated difference between the two 

samples. Any problems or inconsistencies with matrix 

interference can be identified by the standard addition method. 

The recoveries were very good (104 percent ± 9.3 percent), 

suggesting that matrix interference was not a problem in these 

samples.

The results from both years suggest that the loss of 

lambda-cyhalothrin likely occurred during the carbon cleanup 

step due to elevated DOC concentrations.

Dissolved Organic Carbon

The concentrations of DOC varied considerably between 

the five sites sampled, with CBD5 and CBD1 being greater 

than Sacramento Slough and Butte Creek and all sites being 

greater than Tower as shown in the box plots in 

figure 9. On the basis of a worldwide average DOC 

concentration in surface waters of approximately 5.8 mg/L (as 

reported by Boggs and others, 1985), it is possible to classify 

water samples collected at CBD5 as having high or above 

average DOC concentrations, samples from CBD1 as average 

to slightly below average, and those collected at Sacramento 

Slough, Butte Slough, and Tower as well below average.

The difference in DOC concentrations between the sites 

may be explained by the respective land-use practices within 

each watershed. Waters at CBD1 and CBD5 are almost entirely 

agricultural return water (mostly from flooded rice fields), 

whereas the Sacramento Slough and Butte Creek sites receive 

runoff from their respective upper watersheds where there is no 

agriculture in addition to rice field water. Besides runoff from 

both basins, the Tower site also receives runoff from the city of 

Sacramento and the American River watershed. The lower 

DOC values for the Sacramento Slough, Butte Creek, and 

Tower sites relative to the CBD1 and CBD5 sites are likely the 

result of dilution by low DOC water from the mountainous, 

upper reaches of the watersheds.

The results of this study suggest that dissolved organic 

matter, quantified in terms of DOC, influences the analysis of 

lambda-cyhalothrin. The concentration and type of dissolved 

organic matter can influence the toxicity of the contaminant by 

increasing or decreasing its bioavailability to aquatic species 

(Kukkonen and Oikari, 1987; Benson and Long, 1991; Day, 

1991; Kadlec and Benson, 1995). It is, therefore, valuable to 

measure DOC concentrations in surface water samples 

especially when those same water samples may contain 

measurable levels of organic pesticides. 
Analysis of Lambda-Cyhalothrin 23

1993 1994 1995 1996 1997 1998

Year

1999 2000 2001 2002

Consecutive days with performance goal exceeded

0

10

20

30

40

Thiobencarb applied in CBD5 basin, in pounds Molinate applied in CBD5 basin, in pounds

Consecutive Days Exceeded

Thiobencarb Application

Goal exceeded on 2nonconsecutive sampling days

Exceeded through last sample

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Consecutive days with performance goal exceeded

0

10

20

30

40

50

Consecutive Days Exceeded

Molinate Application

Goal exceeded on 3nonconsecutive sampling days

0

50,000

100,000

150,000

200,000

250,000

0

100,000

200,000

300,000

400,000

500,000

A

B

Figure 7. Days that performance goals were exceeded each year at CBD5 and the pesticide use during 1992 through 2002 in the basin.
24 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Dissolved organic carbon, in milligrams per liter

234567 8 9

Percentage recovery

of lambda-cyhalothrin

46

48

50

52

54

56

58

60

62

Figure 8. Recovery of lambda-cyhalothrin in samples collected in 2002 versus the dissolved organic carbon concentration (DOC). 
Analysis of Lambda-Cyhalothrin 25

CBD1CBD5

Sacramento

Slough

Butte Creek

Tower

Dissolved organic carbon, in milligrams per liter

90th percentile

75th percentile

50th percentile

25th percentile

10th percentile

(3) Number in parentheses

indicates number of samples

EXPLANATION

(9)

(8)

(10)

(8)

(9)

Worldwide average DOC concentration1

1 Boggs and others, 1985

0

2

4

6

8

10

Figure 9. Dissolved organic carbon (DOC) concentrations at the five sites sampled.
26 Changes in Rice Pesticide Use and Surface Water Concentrations in the Sacramento River Watershed, California

Pesticide Partitioning Between Water 

and Sediments

Sediment-Associated Pesticide Concentrations

Two suspended sediment samples collected at CBD1 and 

Sacramento Slough were analyzed for sediment-associated 

pesticides and contained detectable concentrations of molinate 

and thiobencarb but not lambda-cyhalothrin. A sample was not 

collected at Tower because of low suspended sediment and 

dissolved pesticide concentrations. 

The concentration of molinate (43.4 ng/g) was greater in 

the Sacramento Slough sample than in the CBD1 sample 

(38.8 ng/g), which is similar to the pattern of use (tables 1

and 6). Thiobencarb also was detected in the Sacramento 

Slough sample, but could not be quantified in the CBD1 

sample because of co-elution of interfering compounds. The 

concentration of thiobencarb was more than two times that of 

molinate in the Sacramento Slough sediment sample (table 6) 

despite the almost equal use of the two pesticides in the basin 

(table 1). This can be explained by thiobencarb having a higher 

organic carbon to normalized sediment–water distribution 

coefficient (Koc; table 6) than molinate. Lambda-cyhalothrin is 

more hydrophobic than molinate or thiobencarb and would be 

expected to preferentially sorb to sediments; however, the very 

low application amounts (table 1) may account for the lack of 

detection of lambda-cyhalothrin.

The suspended sediment sampling at CBD1 was 

conducted at a time corresponding to the second lowest SSC 

value (45 mg/L) recorded for that site during the study. In 

contrast, the sample collected at Sacramento Slough 

corresponded to the second highest SSC value (75 mg/L) at 

that site. 

Equilibrium Partitioning

If pesticides partition between water and sediments 

following equilibrium partitioning models (Swarzenbach and 

others, 1993), the aqueous concentration can be predicted from 

the sediment concentration. The equation becomes:

where 

Using the measured concentrations of molinate 

associated with suspended sediments, the dissolved 

concentration can be calculated for the two samples. For 

molinate, the model overestimates the dissolved 

concentrations by a factor of 34, suggesting the sediments are 

not in equilibrium with the surrounding aqueous phase. 

Although the measured concentrations of molinate varied by a 

factor of two between the samples, the ratio of predicted to 

measured concentrations agreed very well. The predicted 

concentration of dissolved thiobencarb in the Sacramento 

Slough was also overestimated, in this case by a factor of 61. 

These results agree with previous studies of current-use 

pesticides in the Sacramento and San Joaquin Rivers 

(Domagalski and Kuivila, 1993; Bergamaschi and others, 

1997, 2001) that showed sediment concentrations elevated 

above values predicted by equilibrium partitioning models.

Summary

Water samples collected during the rice growing season 

in 2002 and 2003 at a total of five surface water sites within the 

Sacramento Valley were analyzed for five currently used rice 

pesticides and pesticide degradates. Molinate, thiobencarb, 

and 4-keto molinate were detected in every sample. 2-ketomolinate was detected in less than half the samples and at much 

lower concentrations than the other pesticides present. 

Lambda-cyhalothrin was not detected in any of the samples.

 Concentrations of molinate were similar at CBD1 and 

Sacramento Slough in 2002, but thiobencarb concentrations 

were five times higher at CBD1. Maximum concentrations of 

all pesticides detected in 2003 were less than half of the 2002 

levels; this was likely due to the change in sampling locations 

and spring rains that delayed planting and pesticide application 

and may have decreased amounts of pesticides applied. 

Although concentrations cannot be estimated directly from 

application amounts in the different watersheds, the ratio of 

molinate to thiobencarb concentrations is comparable to the 

ratio of molinate to thiobencarb use in each basin during 2002.

Concentrations of molinate or thiobencarb were found to 

exceed established water quality performance goals of 10 

mg/L and 1.5 mg/L, respectively, in six samples in 2002, but 

only in a single sample in 2003 (thiobencarb at Butte Creek). 

Performance goals for these two rice pesticides were exceeded 

for up to four consecutive weeks in 2002. Over the past decade, 

changes in pesticide use appear to influence trends in historical 

peak concentrations, but do not explain all of the variability in 

concentrations. There is a better relation between pesticide use 

and the period of time that performance goals are exceeded in 

a basin. 

Cw is the predicted concentration in water (mg/L),

CSed is the concentration in sediment (ng/g dry weight),

foc is the fraction of sediment organic carbon, and

Koc is the organic carbon normalized sediment–water 

distribution coefficient for each pesticide.

Cw

CSed

focKoc

= --------------
References Cited 27

Sample processing and analytical methods were 

optimized to analyze for lambda-cyhalothrin in water. Results 

from cleanup steps using activated carbon SPE columns 

showed that recoveries decreased with increasing DOC 

concentrations in the water samples. Liquid-liquid extraction 

of 1-L samples using no cleanup appeared to yield the highest 

and most consistent recoveries, resulting in a method detection 

limit of 0.0039 μg/L and matrix spike recoveries of 106 ± 9.8 

percent. Despite the method optimization, lambda-cyhalothrin 

was not detected in any water samples. The amounts applied 

are low (less than 0.5 percent of the amount of molinate or 

thiobencarb applied) and because lambda-cyhalothrin is 

hydrophobic, it is expected to partition onto sediments. 

Two suspended sediment samples were collected in 

July 2002 at CBD1 and Sacramento Slough. Concentrations of 

molinate and thiobencarb associated with suspended sediments 

were above predicted levels when in equilibrium with 

dissolved pesticide concentrations. These results agree with 

previous studies in San Francisco Bay that have also detected 

elevated concentrations of molinate and thiobencarb associated 

with suspended sediments. Lambda-cyhalothrin was not 

detected in the two suspended sediment samples. 

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