Diazinon and Chlorpyrifos Loads in Precipitation 

and Urban and Agricultural Storm Runoff during 

January and February 2001 in the San Joaquin 

River Basin, California

By Celia Zamora, Charles R. Kratzer, Michael S. Majewski, and Donna L. Knifong

Water-Resources Investigations Report 03-4091

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with

CALIFORNIA DEPARTMENT OF PESTICIDE REGULATION 6211-19

Sacramento, California

2003
For additional information write to:

U.S. Geological Survey

Water Resources

Placer Hall, Suite 2012

6000 J Street

Sacramento, California 95819-6129

http://ca.water.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.

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

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

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

Study Area............................................................................................................................................................ 4

Application of Diazinon and Chlorpyrifos........................................................................................................... 8

Sampling Design and Methodology..................................................................................................................... 12

Sampling and Analytical Methods ....................................................................................................................... 24

Sampling Methods ...................................................................................................................................... 24

Analytical Methods ..................................................................................................................................... 25

 Quality Control Samples ..................................................................................................................................... 30

Field Quality Control Samples.................................................................................................................... 34

Laboratory Quality Control......................................................................................................................... 34

Hydrology During the Study Period..................................................................................................................... 35

Concentrations of Diazinon and Chlorpyrifos ..................................................................................................... 37

Diazinon ...................................................................................................................................................... 37

Chlorpyrifos ................................................................................................................................................ 43

Loads of Diazinon and Chlorpyrifos.................................................................................................................... 44

Diazinon ...................................................................................................................................................... 50

Chlorpyrifos ................................................................................................................................................ 52

Summary and Conclusions................................................................................................................................... 53

References Cited .................................................................................................................................................. 54
iv Figures

FIGURES

Figure 1. Map showing locations of data sites and drainage basins in the San Joaquin River Basin, 

California. A. Data sites. B. Drainage basins .................................................................................... 5

Figure 2. Map showing locations of precipitation sites and land use in the San Joaquin Valley, California ... 6

Figure 3. Map showing diazinon application in the San Joaquin Valley part of the San Joaquin River 

Basin, California. A. In major drainage basins. B. During dry period 1 (December 1, 2000, to 

January 26, 2001). C. During dry period 2 (January 27, 2001, to February 24, 2001)...................... 10

Figure 4. Map showing chlorpyrifos application in the San Joaquin Valley part of the San Joaquin River 

Basin, California. A. In major drainage basins. B. During dry period 1 (December 1, 2000, to 

January 26, 2001). C. During dry period 2 (January 27, 2001, to February 24, 2001)...................... 11

Figure 5. Graphs showing Daily precipitation at Modesto, California, and pesticide applications in the 

San Joaquin River Basin, California, December 2000 through February 2001. A. Diazinon. 

B. Chlorpyrifos .................................................................................................................................. 13

Figure 6. Graphs showing daily precipitation, and streamflow and sample collection times, at river sites 

in the San Joaquin River Basin, California. A. Daily precipitation at Modesto, California. 

B–H. Streamflow and sample collection times. I. Streamflow and sample collection times, and 

hourly precipitation at the McHenry storm drain site in Modesto, California................................... 14

Figure 7. Graphs showing streamflow and diazinon concentrations in the San Joaquin River Basin, 

California. A–G. River sites. H. McHenry storm drain site in Modesto, California ......................... 38

 Figure 8. Graphs showing streamflow and chlorpyrifos concentrations in the San Joaquin River Basin, 

California. A–G. River sites. H. McHenry storm drain site in Modesto, California ......................... 40

Figure 9. Graphs showing daily precipitation at Modesto, California, December 2000–February 2001, 

and concentrations of composite precipitation samples collected in urban and agricultural areas 

in the San Joaquin River Basin, California. A. Diazinon. B. Chlorpyrifos ....................................... 42

Figure 10. Graphs showing streamflow and diazinon loads in the San Joaquin Rive Basin, California. 

A—G. River sites. H, McHenry storm drain site in Modesto, California ......................................... 45

Figure 11. Graph showing streamflow and chlorpyrifos loads in the San Joaquin River Basin, California. 

A–G. River sites. H. McHenry storm drain site in Modesto, California ........................................... 47

Figure 12. Graphs showing daily precipitation at Modesto, California, December 2000–February 2001, 

and loads of composite precipitation samples collected in urban and agricultural areas in the 

San Joaquin River Basin, California. A. Diazinon. B. Chlorpyrifos ................................................. 51
Tables v

TABLES

Table 1. Names, locations, and types of data available for sites in the San Joaquin River Basin, 

California, January and February, 2001............................................................................................. 7

Table 2. Basin areas, almond orchard areas, and diazinon and chlorpyrifos appplication amounts for 

drainage basins in the San Joaquin River Basin, California .............................................................. 9

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous 

loading rates for water quality sampling sites in the San Joaquin River Basin, California............... 16

Table 4. Comparison of diazinon and chlorpyrifos concentrations using laboratory schedules 1319 and 

2001 for selected sites in the San Joaquin River Basin, California ................................................... 27

Table 5. Summary of quality control data on diazinon and chlorpyrifos concentrations for sites in the 

San Joaquin River Basin, California.................................................................................................. 31

Table 6. Comparison of diazinon and chlorypyrifos concentrations using Equal Width Increment and 

Integrated Grab collection methods for selected sites in the San Joaquin River Basin, 

California ........................................................................................................................................... 33

Table 7. Precipitation accumulations and summary of diazinon and chlorpyrifos concentrations and 

loads for composite precipitation samples collected at selected sites in the San Joaquin River 

Basin, California ................................................................................................................................ 36

Table 8. Diazinon and chlorpyrifos loading rates, loads, and yields in relation to agricultural applications 

for sites in the San Joaquin River Basin, California, January and February 2001 ............................ 49
vi Conversion Factors and Vertical Datum

CONVERSION FACTORS

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

°F=1.8 °C+32.

VERTICAL 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).

ABBREVIATIONS

lb a.i., pound active ingredient

lb a.i., pound active ingredient per day

mg/L, milligram per liter

μg/L, microgram per liter

μg/m2, microgram per square meter

CDPR, California Department of Pesticide Regulation

CMC, criterion maximum concentration

CCC, criterion continuous concentration

GC/FPD, gas chromatograph with flame photometric detection

GC/MS, gas chromatograph/mass spectrometer

LRL, laboratory reporting level

MDL, method detection limit

NWQL, National Water Quality Laboratory

PCO, pest control operator

QC, quality control

RPD, relative percent difference

RWQCB, Regional Water Quality Control Board

SPE, solid-phase extraction

TMDL, Total Maximum Daily Load

Toxics, Toxic Substances Hydrology (Program)

USEPA, U.S. Environmental Protection Agency

USGS, U.S. Geological Survey

Multiply By To obtain

inch (in.) 2.54 centimeter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

square mile (mi2) 2.590 square kilometer
Abstract 1

Diazinon and Chlorpyrifos Loads in Precipitation and 

Urban and Agricultural Storm Runoff during January and 

February 2001 in the San Joaquin River Basin, California

By Celia Zamora, Charles R. Kratzer, Michael S. Majewski, and Donna L. Knifong

ABSTRACT

The application of diazinon and chlorpyrifos 

on dormant orchards in 2001 in the San Joaquin 

River Basin was 24 percent less and 3.2 times

more than applications in 2000, respectively. A 

total of 16 sites were sampled during January and 

February 2001 storm events: 7 river sites, 8 

precipitation sites, and 1 urban storm drain. The 

seven river sites were sampled weekly during 

nonstorm periods and more frequently during 

storm runoff from a total of four storms. The 

monitoring of storm runoff at a city storm drain in 

Modesto, California, occurred simultaneously 

with the collection of precipitation samples from 

eight sites during a January 2001 storm event. The 

highest concentrations of diazinon occurred during 

the storm periods for all 16 sites, and the highest 

concentrations of chlorpyrifos occurred during 

weekly nonstorm sampling for the river sites and 

during the January storm period for the urban 

storm drain and precipitation sites. A total of 60 

samples (41 from river sites, 10 from precipitation 

sites, and 9 from the storm drain site) had diazinon 

concentrations greater than 0.08 μg/L, the 

concentration being considered by the California 

Department of Fish and Game as its criterion 

maximum concentration for the protection of 

aquatic habitats. A total of 18 samples (2 from 

river sites, 9 from precipitation sites, and 7 from 

the storm drain site) exceeded the equivalent 

California Department of Fish and Game guideline 

of 0.02 μg/L for chlorpyrifos. The total diazinon 

load in the San Joaquin River near Vernalis during 

January and February 2001 was 23.8 pounds 

active ingredient; of this amount, 16.9 pounds 

active ingredient were transported by four storms, 

1.06 pounds active ingredient were transported by 

nonstorm events, and 5.82 pounds active 

ingredient were considered to be baseline loads. 

The total chlorpyrifos load in the San Joaquin 

River near Vernalis during January and February 

2001 was 2.17 pounds active ingredient; of this 

amount, 0.702 pound active ingredient was 

transported during the four storms, and 1.47 

pounds active ingredient were considered as 

baseline load. The total January and February 

diazinon load in the San Joaquin River near 

Vernalis was 0.27 percent of dormant application; 

the total January and February chlorpyrifos load 

was 0.02 percent of dormant application. The 

precipitation samples collected during the January 

2001 storm event were analyzed for pesticides to 

evaluate their potential contribution to pesticide 

loads in the study area. When the average 

concentrations of diazinon and chlorpyrifos in the 

precipitation samples were compared with 

concentrations in urban storm runoff samples, 

68 percent of the diazinon concentration in the 

runoff could be accounted for in the precipitation. 

Chlorpyrifos, however, had average precipitation 

concentrations that were 2.5 times higher than 

what was detected in the runoff. Although no firm 

conclusions can be made from one storm event, 

preliminary results indicate that pesticides in 

precipitation can significantly contribute to 

pesticide loads in storm runoff.
2 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

INTRODUCTION

The organophosphate pesticides, diazinon and 

chlorpyrifos, are applied to control wood-boring 

insects in almond orchards and other dormant stone 

fruit orchards in December through February. The 

agricultural application of diazinon and chlorpyrifos 

includes aerial spraying or near-ground spraying from 

a tractor. Diazinon and chlorpyrifos have also been 

used in many urban applications, although this use is 

being phased out. The U.S. Environmental Protection 

Agency (USEPA) has taken steps to phase out and 

eliminate both diazinon and chlorpyrifos from 

household and nonagricultural use. Diazinon is one of 

the most commonly found pesticides in air, rain, and 

fog, and it is also commonly found in surface water in 

urban areas as a result of runoff from residential use 

(U.S. Environmental Protection Agency, 2000). The 

production and formulation of diazinon is scheduled to 

phase out and end completely during 2003. Effective 

December 31, 2003, diazinon will no longer be 

available for use by homeowners for lawn and garden 

pest control or for indoor pest control (U.S. 

Environmental Protection Agency, 2001). As of 

December 31, 2001 the USEPA has stopped the retail 

sale of chlorpyrifos to homeowners, limiting the use to 

certified, professional, or agricultural applicators (U.S. 

Environmental Protection Agency, 2001). 

Diazinon persists for 10 to 12 weeks in most 

soils when applied at recommended rates (Howard, 

1991). In water, it has a solubility of 68.8 mg/L (at 

20°C) and may sorb moderately to sediments, but it 

should not bioconcentrate in aquatic organisms 

(Howard, 1991). Hydrolysis and biodegradation may 

be significant fate processes in natural waters. 

Hydrolysis half-life (at 20°C) is 185 days at pH 7.4 

(Howard, 1991). Volatilization of diazinon from water 

can also be a very important transport process. If 

diazinon is released to the atmosphere, it will be 

expected to exist in both the vapor phase and 

particulate phase based upon its vapor pressure. The 

half-life for the vapor phase reaction of diazinon with 

photochemically produced hydroxl radicals is 

estimated to be 4.1 hours in an atmosphere under 

nonsmog conditions (Howard, 1991). 

Chlorpyrifos has usually been found to persist 

for about 9 to 17 weeks in soils, although this 

persistence can vary greatly depending on soil type, 

climate, and other factors (Howard, 1991). 

Chlorpyrifos has a lower solubility than diazinon 

(1.12 mg/L at 24°C) and a much greater tendency to 

partition from the water column to the sediments. 

Unlike diazinon, chlorpyrifos has significant potential 

to bioconcentrate in aquatic organisms. Hydrolysis and 

adsorption to aquatic sediments are probably the most 

significant fate processes for chlorpyrifos in natural 

waters. The hydrolysis half-life of chlorpyrifos (at 

20°C) is about 44 to 117 days near pH 7 (Howard, 

1991). Biodegradation and volatilization are probably 

less significant fate processes, especially in the 

presence of significant levels of suspended sediment 

(Howard, 1991). When released into the atmosphere, 

the half-life of the vapor phase of chlorpyrifos is 6.34 

hours when reacting with photochemically produced 

hydroxyls (Howard, 1991). 

Water is one of the primary transport 

mechanisms by which pesticides are carried from their 

target areas to other parts of the environment resulting 

in movement into and through all components of the 

hydrological cycle (Majewski and Capel, 1995). The 

primary transport mechanism of diazinon and 

chlorpyrifos in the San Joaquin River Basin during 

January and February is winter storm runoff following 

dormant spray application by growers. Previous studies 

have examined this transport of diazinon and 

chlorpyrifos from dormant spray application in the San 

Joaquin River Basin (Domagalski and others, 1997; 

Kratzer, 1998). Studies that have monitored the basin 

outlet at Vernalis during winter storm runoff include 

Kuivila and Foe (1995) and MacCoy and others (1995). 

Some studies have monitored only one subbasin 

upstream of Vernalis (Ganapathy and others, 1997; 

Poletika and Robb, 1998), whereas others included the 

basin outlet at Vernalis and one to three subbasins 

upstream of Vernalis (Domagalski and others, 1997; 

Kratzer, 1998; Bennett and others, 1998; Panshin and 

others, 1998). Studies monitoring Vernalis and more 

than three subbasins upstream of Vernalis include Ross 

and others (1996), Kratzer (1999), and Kratzer and 

others (2002).
Introduction 3

Urban use of pesticides includes homeowner 

application and application by licensed pest control 

operators (PCO). It is an important component to 

consider when monitoring the movement of pesticides 

in agricultural areas that are in proximity to urban 

areas. These pesticides become mobilized during storm 

events and move from the sites of application along 

impervious surfaces (such as driveways, sidewalks, and 

street gutters) to city storm drains, ending up in the 

nearest body of water. A study by Kratzer (1998) 

examined the transport of pesticides in storm runoff 

from urban and agricultural areas in the Tuolumne 

River Basin in the vicinity of Modesto, California.

The atmosphere is also an important component 

of the hydrological cycle to consider in assessing the 

effect of pesticides in the environment (Majewski and 

Capel, 1995). Pesticides have been recognized as 

potential air pollutants since 1946 (Daines, 1952). 

Three recent studies have specifically examined 

pesticide loading from the atmosphere in the Central 

Valley. One study included the collection of wet and 

dry deposition samples using an automatic wet–dry 

sensing collector to assess atmospheric contributions of 

insecticides to the San Joaquin River Basin as part of 

an overall examination of distribution and mass loading 

of insecticides in the San Joaquin River (Ross and 

others, 1996). A second study found high 

concentrations of diazinon and chlorpyrifos in rain and 

fog samples collected in the Central Valley as part of an 

overall study that examined the toxicity of diazinon and 

chlorpyrifos in samples collected from urban streams 

during the precipitation season (Howard and others, 

2000). A third study examined atmospheric transport of 

pesticides from agricultural application areas to the 

Sacramento, California, metropolitan area (Majewski 

and Baston, 2002). 

Studies have also been done on the toxicity of 

diazinon and chlorpyrifos in the San Joaquin River 

Basin (Bennett and others, 1998; Kuivila and Foe, 

1995). The most commonly used guidelines in 

California for short-term exposure (the criterion 

maximum concentration or CMC) in terms of 

concentrations are 0.08 μg/L for diazinon and 0.02 

μg/L for chlorpyrifos (Siepmann and Finlayson, 2000). 

The corresponding guidelines for long-term exposure 

(the criterion continuous concentration or CCC) are 

0.05 μg /L for diazinon and 0.014 μg /L for 

chlorypyrifos.

The purpose of this report is determine the loads 

of diazinon and chlorpyrifos in the San Joaquin River 

Basin following application of dormant spray in the 

surrounding basins during January and February 2001. 

The study design was to sample two storms with the 

greatest potential for transporting diazinon and 

chlorpyrifos. Concentrations of diazinon and 

chlorpyrifos in the San Joaquin River are of special 

concern to the Regional Water Quality Control Board 

(RWQCB) during storm events following dormant 

spray application. The analysis in this report will be 

used by the RWQCB to assist in the development of a 

Total Maximum Daily Load (TMDL) for the San 

Joaquin River Basin. A TMDL is a process used to 

address water pollution problems caused by both point 

source and nonpoint source pollution. The data 

presented includes the calculation of diazinon and 

chlorpyrifos loads and the relation of these loads to 

application, runoff, atmospheric sources, and land use 

in each subbasin. The format and analysis used in this 

report is similar to a 2000 U.S. Geological Survey 

(USGS) study in the same area (Kratzer and others, 

2002). Several comparisons will be made in this report 

to the diazinon and chlorpyrifos loads reported in 2000. 

The authors to thank Emile Reyes, Emily 

Alejandrino, Nate Martin, Eric Oppenheimer, Tim 

Tadlock, Debbie Daniels, Gene Davis, and Matt 

McCarthy of the RWQCB. We especially thank 

Shakoora Azimi of the RWQCB for enlisting and 

organizing the storm sampling assistance from the 

forenamed individuals. We also thank Bill Ketscher, 

Dave Bakker, and Steve Carlson of the Modesto 

Irrigation District, and Greg Remsing and Blair 

Bradley of the City of Modesto, for their cooperation 

and help with the deployment and collection of the 

precipitation samplers. Special thanks to Willy Kinsey, 

Mark Johnson, Larry Shelton (Retired), Bill Templin, 

Henry Miyashita, and Jason May of the USGS for their 

assistance and for managing field crew assignments 

during the storm sampling.
4 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

STUDY AREA

The perennial San Joaquin River Basin drains 

7,345 mi2, of which 4,299 mi2 are in the Sierra Nevada, 

2,244 mi2 in the San Joaquin Valley, and 802 mi2 in the 

Coast Ranges (fig. 1). On the basis of USGS 

streamflow data for 1951–1990, 67 percent of the 

average streamflow in the San Joaquin River originates 

from the three main basins that drain from the east: the 

Merced (15 percent), Tuolumne (30 percent), and 

Stanislaus (22 percent) Basins. The remaining 

streamflow comes from the following sources: (1) Bear 

Creek Basin; (2) Mud and Salt Sloughs, and ephemeral 

creeks that drain from the west; (3) drainage canals that 

flow directly to the San Joaquin River; and (4) 

occasionally, the upper San Joaquin River above Bear 

Creek during high flow events. 

The water quality, streamflow, and precipitation 

sites mentioned in this report are identified in figures

1A and 2 and in table 1. A total of seven river sites were 

sampled 19–64 times each in January and February 

2001 for this study. The river sites include three sites 

on the main stem of the San Joaquin River (fig. 1A: 

sites 1, 6, and 13), three major east-side tributaries 

(fig 1A: sites 4, 9, and 12), and one west-side creek 

(fig. 1A: site 5). The water quality sampling sites on the 

Merced, Tuolumne, and Stanislaus Rivers were 

sampled downstream of USGS gaging stations (sites 3, 

9, and 11 respectively). Streamflow at the water quality 

sampling sites was estimated using traveltimes in the 

tributaries from the gaging stations (Kratzer and 

Biagtan, 1997). 

Additionally, urban storm-water runoff samples 

were collected at the McHenry storm drain (fig. 1: site 

8) during the January 23–26, 2001, storm period. 

McHenry storm drain is located in Modesto, and land 

use in its associated basin is 100 percent urban: 

approximately 70 percent residential and 30 percent 

commercial (Kratzer, 1998). The Modesto urban area 

accounts for about 62 percent of the population in 

Stanislaus County (Gronberg and others, 1998). Land 

use in the valley part of the Tuolumne River at Modesto 

drainage basin is about 53 percent agricultural, 13 

percent urban, and 34 percent other (Kratzer, 1998). 

The precipitation during the 2001 water year (October 

1, 2000, through September 30, 2001) in Modesto was 

13.10 in., with 6.54 in. falling between December 2000 

and March 2001. In the urban area, approximately 

40 percent of storm-water discharges to surface water, 

and 60 percent discharges to ground water by way of 

dry wells (Lisa Burns, City of Modesto, written 

commun., July 2002). 

Four of the eight precipitation collection sites 

were placed in urban land use areas (fig. 2: sites B, C, 

E, and G) and four in agricultural land use areas (fig. 2: 

sites A, D, F, and H) outside of Modesto. The urban 

sites were placed throughout Modesto to give good 

spatial representation of the urban area. The location of 

the urban sites included the downtown area (site E), an 

industrial area (site G), an established residential area 

(site B), and a new residential development (site C). 

Agricultural site locations were chosen to give good 

spatial representation of almond and stone-bearing fruit 

orchards surrounding Modesto. The locations of the 

agricultural sites were north, south, east, and west of 

Modesto (sites H, A, F, D, respectively). These sites 

were positioned near predominant orchard land use 

areas rather than directly within an orchard to minimize 

any direct influence of spraying operations. 
Study Area 5

San

Francisco

Los

Angeles

CALIFORNIA

San Joaquin

River Basin

Valley floor

Water quality sites

Streamflow sites

Water quality

and streamflow sites

San Joaquin River

County boundary

Valley floor

0

0 10

10 20 MILES

20 KILOMETERS

A

B

C

D

E

F

G

H

J I

San Joaquin River at Patterson

(site 6) Basin (A, B, C, D, E)

Tuolumne River (site 10) Basin (F)

McHenry storm drain (site 8) Basin (G)

Stanislaus River (site 12) Basin (H)

Other east side drainage areas (I)

Other west side drainage areas (J)

San Joaquin River near Stevinson

(site 1) Basin (A)

Mud and Salt Sloughs (B)

Merced River (site 4) Basin (C)

Orestimba Creek (site 5) Basin (D)

EXPLANATION

EXPLANATION

Fresno

County

Stanislaus

County

Merced

County

2

120

120

121

121

38

38

37

37

0

0 10

10 20 MILES

20 KILOMETERS

Modesto

Ripon

Stevinson

Be

Merced ar Creek

Merced River

Tuolumne Ri v er

Stanislaus Ri ver

Orestimba C reek

Dry Creek

Mariposa

County

Tuolumne

County

Calaveras

County

San Joaquin County

Madera

County

San

Benito

County

Alpine

County

1

3

4

5

6

8

9 10

11

13 12

7

Turlock

Vernalis

New Melones

Reservoir

New Don Pedro

Reservoir

Lake McClure San Joaquin

River

A

B

Figure 1. Locations of data sites and drainage basins in the San Joaquin River Basin, California. A. Data sites. B. Drainage basins. 

Basin names are slightly abbreviated; full versions are listed in table 2. Adapted from Kratzer and others, 2002.
6 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

A

B C D E F

G

H

Stevinson

0 10 MILES 5

0 10 KILOMETERS 5

Precipitation site

Land Use

Orchard

Other Agriculture

Urban

Native vegetation

EXPLANATION

Merced

Turlock

Modesto

Ripon

Vernalis

Merced River

Stanislaus River

Tuolomne River

San

Joaquin

River

121

121 30

37

30

37

Figure 2. Locations of precipitation sites and land use in the San Joaquin Valley, California.
Study Area 7

1River miles from Vernalis.2River miles from Merced River, San Joaquin River, and Vernalis, respectively.3Department of Water Resources site number.4River miles from Tuolumne, San Joaquin River, and Vernalis, respectively.

Table 1. Names, locations, and types of data available for sites in the San Joaquin River Basin, California, January and February, 2001[See figures 1A and 2 for site locations. Site location: unless otherwise noted, double entries for distances are in river miles from the San Joaquin River and Vernalis, rese

x, data available; —, no data available] 

Site 

number/

letter

Site name Site identification number Site location

Precipit

1 San Joaquin River near Stevinson 11260815 160.5 — 

2 Dry Creek near Snelling 11271320 217.8; 49.5; 95.2 — 

3 Merced River near Stevinson 11272500 4.8; 50.5 — 4 Merced River at River Road Bridge near Newman 11273500 1.1; 46.8 — 5 Orestimba Creek at River Road near Crows Landing 11274538 1.0; 37.7 — 6 San Joaquin River at Patterson Bridge near Patterson 11274570 126.5 — 

7 Dry Creek near Modesto 3BO-4130 45.5; 21.9; 33.1 — 

8 McHenry storm drain at Bodem Street at Modesto 373847120590801 NA — 9 Tuolumne River at Modesto 11290000 16.2; 27.4 — 

10 Tuolumne River at Shiloh Road Bridge near Grayson 11290200 3.6; 14.8 — 11 Stanislaus River at Ripon 11303000 15.7; 18.2 — 12 Stanislaus River at Caswell State Park near Ripon 374209121103800 8.5; 11.0 — 13 San Joaquin River near Vernalis 11303500 2 0.0 — 

A Precipitation Gage 4, TID lateral 3 Barnhardt Road near Turlock 373228120551201 NA x 

B Precipitation Gage 6, Wastewater Treatment Plant Rooftop at Modesto 373637121004601 NA x C Precipitation Gage 7, Cadoni Road Lift Station at Modesto 373725120543701 NA x 

D Precipitation Gage 3, MID Lateral 4 near Modesto 373750121092601 NA x 

E Wet/Dry Sampler 1 at MID Rooftop at Modesto 373834121000601 NA x 

F Wet/Dry Sampler 2 at MID Gage, Albers Road near Turlock 373841120504801 NA x 

G Precipitation Gage 8, Bowen and Aloha Street at Modesto 374028120594301 NA x H Precipitation Gage 5, Tully Road near Modesto 374351121004701 NA x 
8 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

APPLICATION OF DIAZINON AND 

CHLORPYRIFOS

The application of diazinon and chlorpyrifos 

during December through February to dormant almond 

and other stone-bearing fruit trees in the San Joaquin 

River Basin coincides with the rainy season for the 

region. To control the wood-boring insects, pesticides 

are most effective when applied during dry periods. 

Both diazinon and chlorpyrifos are intensely used 

during this time period (table 2). The distribution of 

chlorpyrifos and diazinon application in the study area 

for the December 2000 through February 2001 

dormant spray season is shown in figures 3 and 4, 

respectively. This period generally has the most 

intensive agricultural application of diazinon in the San 

Joaquin River Basin (Kratzer and others, 2002). In 

contrast, the agricultural application of chlorpyrifos in 

the San Joaquin River Basin is somewhat dispersed, 

with intensive application on alfalfa in March and 

intensive in-season application on almonds and walnuts 

during May through July (Panshin and others, 1998). 

Diazinon and chlorpyrifos are also applied in urban 

settings for household lawn and garden pest control as 

well as for structural pest control. 

The application data presented in this report 

were acquired from records maintained by the 

California Department of Pesticide Regulation 

(CDPR). The agricultural application data are reported 

by day of use, the crop on which used, and for area of 

use to the square mile. The agricultural application of 

diazinon and chlorpyrifos is presented (figs. 3 and 4,

respectively) for the two dry periods (December 1, 

2000, to January 26, 2001; and January 27 to February 

24, 2001) that preceded the two storm periods sampled 

in this study. The agricultural data are plotted at the 

geographic level of a section (1 mi2 within a township 

and range). The data are presented as three application 

categories of active ingredient (a.i.) per square mile 

(mi2) representing relatively low (less than 33 

lb a.i./mi2 for diazinon and less than 31 lb a.i./mi2 for 

chlorpyrifos), medium (33–70 lb a.i./mi2 for diazinon 

and 31–102 lb a.i./mi2 for chlorpyrifos), and relatively 

high (greater than 70 lb a.i./mi2 for diazinon and 

greater than 102 lb a.i./mi2 for chlorpyrifos) 

application areas. The only urban applications reported 

by CDPR are those by licensed PCO’s by county. 

Household use, daily use, and place of use (except for 

county) are not reported. Most of the urban areas 

within the study area are in Stanislaus and Merced 

Counties (figs. 1 and 2). Urban applications reported in 

this report are only for those two counties and are only 

reported by month. The December 2000 and January 

2001 urban application data are included in dry period 

1, and the February 2001 application data are included 

in dry period 2 (table 2). The overall urban application 

of diazinon for Stanislaus and Merced Counties during 

the 2001 dormant spray period was three times greater 

than the reported agricultural application, whereas the 

overall urban application of chlorpyrifos was three 

times less than the reported agricultural application for 

the same counties (table 2). 

The agricultural applications of diazinon and 

chlorpyrifos during dry periods 1 and 2 generally were 

dispersed unevenly across the study area. Diazinon 

application for dry period 1 occurred mostly in the 

northern half of the study area, with approximately 

39 percent of that application occurring in the San 

Joaquin River at Patterson Basins (fig. 3: basins A–E). 

The diazinon application for dry period 2 was less than 

that of dry period 1, with approximately 74 percent of 

the application occurring in the San Joaquin River at 

Patterson Basins. The application of chlorpyrifos for 

dry period 1 was also dispersed unevenly, with 

approximately 69 percent of the application occurring 

in the Merced River Basin (fig. 4: basin C) and 

58 percent occurring in the San Joaquin River near 

Stevinson Basin (fig. 4: basin A) during dry period 2.

The amount of diazinon for agricultural application 

during December 2000 through February 2001 

(table 2) was 24 percent less than the application

during the same period in the previous year, while the 

chlorpyrifos application was 3.2 times greater (Kratzer 

and others, 2002; California Department of Pesticide 

Regulation, 2001, 2002). During December 2000 

through February 2001, about 8,900 lb a.i. diazinon 

was applied to agricultural areas in the San Joaquin 

River Basin. This total compares with approximately 

11,700 lb a.i. during the same period in the previous 

year, and about 83,000 lb a.i. in 1992–1993 and 

56,100 lb a.i. in 1993–1994 (Panshin and others, 1998; 

Kratzer, 1999; Kratzer and others, 2002). During 

December 2000 through February 2001, about 

11,200 lb a.i. chlorpyrifos was applied to agricultural 

areas in the San Joaquin River Basin compared with 

approximately 3,500 lb a.i. during the same period in 

1999–2000 and 27,000 lb a.i. during the same period in 

1992–1993 (Panshin and others, 1998; Kratzer and 

others, 2002).
Application of Diazinon and Chlorpyrifos 9

1County application by licensed pest control operators is reported by month only (December 2000 and January 2001 is shown for dry period 1).2County application by licensed pest control operators is reported by month only (February 2001 data is shown for dry period 2).

Table 2. Basin areas, almond orchard areas, and diazinon and chlorpyrifos appplication amounts for drainage basins in the San Joaquin River Basin, California[See figure 1B for basin areas and locations. Dry period 1: December 1, 2000 to January 26, 2001; Dry period 2: January 24, 2001 to February 24, 2001; mi2, square mile

Basin

area

Site name Total basin area (mi2) Valley basin area (mi2) Almond orchard area (mi2)

Application (pound

Diazinon,

dry period 1

Diazinon,

dry period 2

A San Joaquin River near Stevinson 866 218 46 781 578 

E San Joaquin River at Patterson Bridge near Patterson 3,770 1,614 157 2,584 1,708 

G McHenry storm drain at Bodem Street at Modesto 1.3 1.3 0.0 0.0 0.0 

C Merced River at River Road Bridge near Newman 1,383 245 72.9 1,193 701 

F Tuolumne River at Shiloh Road Bridge near Grayson 1,862 149 20.8 237 39 

H Stanislaus River at Caswell State Park near Ripon 1,144 116 17.9 1,106 24 D Orestimba Creek at River Road near Crows Landing 195 33 5.5 0.0 0.0 

I San Joaquin River (other east side areas) 298 282 74.3 2,591 506 

J San Joaquin River (other west side areas) 323 112 8.8 49 22.5 

B Precipitation Gage 6, Wastewater Treatment Plant 

Rooftop at Modesto

492 484 1.9 49 0.0 

K San Joaquin River near Vernalis 7,395 2,273 279 6,566 2,300 

Urban Application Merced County 1,971 1,381 114 14,089 22,018 Stanislaus County 1,514 851 127 114,903 27,305 
10 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

B

A

C

0

<33

33 – 70

>70

EXPLANATION

Diazinon applied

(Pounds active ingredient

per square mile)

0 10 20 KILOMETERS

0 10 20 MILES

Valley

Floor

A

B

K

C

C

D

E

F G

H

I J

D

San Joaquin

River Basin

Figure 3. Diazinon application in the San Joaquin Valley part of the San Joaquin River Basin, California. A. In major drainage basins. B. During dry period 

1 (December 1, 2000, to January 26, 2001). C. During dry period 2 (January 27, 2001, to February 24, 2001). 

Adapted from Kratzer and others, 2002. “K” represents the valley floor and includes all hydrologic basins contained within.
Application of Diazinon and Chlorpyrifos 11

A

0

<31

31 – 102

>102

EXPLANATION

Chlorpyrifos applied

(Pounds active ingredient

per square mile)

San Joaquin

River Basin

0 10 20 KILOMETERS

0 10 20 MILES

B C

A

B

C

D

E

F G

H

I J C

D

Valley

Floor

K

Figure 4. Chlorpyrifos application in the San Joaquin Valley part of the San Joaquin River Basin, California. A. In major drainage basins. B. During dry period 

1 (December 1, 2000, to January 26, 2001). C. During dry period 2 (January 27, 2001, to February 24, 2001). 

Adapted from Kratzer and others, 2002.
12 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

SAMPLING DESIGN AND METHODOLOGY

The primary transport mechanism for diazinon 

and chlorpyrifos to streams in the San Joaquin River 

Basin during January and February is runoff from 

winter storms (Kratzer and others, 2002). Figure 5

illustrates the daily precipitation totals as well as the 

diazinon and chlorpyrifos application amounts for the 

December 2000–February 2001 dormant sprayapplication period. The pesticides are applied to crops 

during dry periods to reduce the risk of “washing off” 

by precipitation. The sampling plan for this study was 

designed around two of these dry periods, identified as 

dry period 1 and dry period 2 (fig. 5). The definition of 

“dry period” includes application periods that occur 

preceding a storm period and the storm period itself 

(Kratzer and others, 2002). The storm period is 

included in the dry period because it is the runoff from 

the application that occurred preceding the storm that is 

sampled. The California Department of Pesticide 

Regulation (CDPR) records the amount of pesticide 

application during the dormant spray season. However, 

these totals are not available until the following year; 

therefore, any sampling design relies on information 

from county agriculture advisors with respect to 

applications in real-time.

The sampling design for this study was based on 

sampling the two storms during the January and 

February dormant season because this period had the 

greatest potential to transport diazinon and 

chlorpyrifos. These two storm periods are defined as 

“target” storm periods and occurred on January 23–26, 

2001, and February 18–24, 2001, which had 

precipitation totals of 0.87 in. and 1.81 in., respectively. 

The defined dry periods preceding the storms were 

December 1, 2000, to January 26, 2001 (dry period 1), 

and January 27, 2001, to February 24, 2001 (dry period 

2). Seven river sites throughout the San Joaquin River 

Basin (table 1) were sampled during the two target 

storm periods. Weekly samples also were collected 

from these river sites to assess the nonstorm transport 

of diazinon and chlorpyrifos during the dormant spray 

season. Additionally, storm runoff in an urban storm 

drain, McHenry storm drain (fig. 1: site 8), was 

sampled during the January 23–26, 2001 storm. 

Precipitation also was sampled for the following three 

storm events: January 8–12, 2001, January 23–26, 

2001, and February 9–11, 2001, in both agricultural 

and urban land-use areas (fig. 2: sites A–H). 

The sampling strategy for the river sites during 

the target storm periods was to begin sample collection 

and to collect several samples at each site throughout 

the storm runoff hydrograph. The river sites that were 

sampled in this study have either real-time streamflow 

data at the sampling locations or upstream from the 

sampling locations; therefore, it was relatively easy to 

discern the runoff as the storm progressed. Figure 6

illustrates the storm runoff hydrographs and sample 

collection times for the river sites and the McHenry 

storm drain. The timing of sample collection relative to 

storm runoff hydrographs at the river sites generally 

provided good coverage of the hydrographs, especially 

during the rising limb when higher concentrations are 

expected (fig. 6). 

Although only two target storms were 

extensively sampled during the January and February 

dormant spray season, a total of four storm events 

occurred. These additional two storms, defined as 

“nontarget” storm periods, occurred on January 8–12, 

2001, and February 9–11, 2001, and preceded each of 

the target storms. The weekly sampling date of January 

11 coincided with the first nontarget storm, but the 

weekly sampling date of February 15 did not coincide 

with the second nontarget storm. For each of these 

defined nontarget storms, samples were collected once 

at all river sites during the weekly sampling dates of 

January 11 and February 15 with the exception of the 

most downstream site, San Joaquin River at Vernalis, 

(fig. 1: site 13) where samples were collected more 

frequently during both nontarget storm events (table 3) 

to ensure that enough samples were collected to allow 

for adequate load calculations. In addition to the river 

samples collected during storm and nonstorm periods 

at the San Joaquin River near Vernalis site for this 

study, the USGS’s Toxic Substances Hydrology 

(Toxics) Program collected samples at this site from 

January through April 2001. The Toxics Program 

collected daily samples throughout the storm 

hydrographs for both target and nontarget storms, with 

more frequent sampling during critical periods, such as 

high application and runoff. The diazinon and 

chlorpyrifos concentrations of samples collected by the 

Toxics Program were included in this study for the San 

Joaquin River near Vernalis site (table 3). The addition 

of this sampling data ensures better coverage of the 

storm hydrographs throughout the dormant spray 

season at this farthest downstream sampling point.
Sampling Design and Methodology 13

0

100

200

300

400

500

600

700

800

900

1,000

0

200

400

600

800

1,000

1,200

1,400

Application, in pounds active ingredient per day

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

dry period 1 dry period 2

dry period 1 dry period 2

December 2000 January 2001 February 2001

1 9 17 25 2 10 18 26 3 11 19 27

Rainfall, in inches per day

Rainfall

Diazinon application

Chlorpyrifos application

Rainfall

A

B

Figure 5. Daily precipitation at Modesto, California, and pesticide applications in the San Joaquin River Basin, California, December 2000 through 

February 2001. A. Diazinon. B. Chlorpyrifos.
14 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

0

0.2

0.4

0.6

0.8

1

1.2

1.4

dry period 1 dry period 2

daily precipitation

streamflow

sample collection time

Precipitation, in inches per day

January 2001 February 2001 March

2001

A

B

C

D

E

1 5 9 13 17 21 25 29 2 6 10 14 18 22 26 2

100

100

200

300

400

500

0

0

0

200

300

400

500

600

Streamflow, in cubic feet per second

San Joaquin River near Stevinson

San Joaquin River at Patterson Bridge near Patterson

Orestimba Creek at River Road near Crows Landing

streamflow at Dry Creek near Snelling

Merced River at River Road Bridge near Newman

400

800

1,200

1,600

2,000

0

100

200

300

400

500

1 8 15 22 5 5 29 12 26 19

Figure 6. Daily precipitation, and streamflow and sample collection times, at river sites in the San Joaquin River Basin, California. A. Daily precipitation 

at Modesto, California. B–H. Streamflow and sample collection times. I. Streamflow and sample collection times, and hourly precipitation at the McHenry 

storm drain site in Modesto, California.
Sampling Design and Methodology 15

0

0.2

0.4

0.6

0.8

1

1.2

1.4

dry period 1 dry period 2

daily precipitation

hourly precipitation

Precipitation, in inches per day

Precipitation, in inches per day

January 2001 February 2001 March

2001

Date and time

A

F

G

H

I

1 5 9 13 17 21 25 29 2 6 10 14 18 22 26 2

0

0

Streamflow, in cubic feet per second

Tuolumne River at Shiloh Road Bridge near Grayson

sample collection time by USGS Toxics Project

San Joaquin River near Vernalis

McHenry storm drain at Bodem Street at Modesto

Stanislaus River at Caswell State Park near Ripon

streamflow

streamflow at Dry Creek at Modesto

sample collection time

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1,000

2,000

3,000

4,000

5,000

100

0

200

300

400

500

600

700

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

1 8 15 22 5 5 29 12 26 19

0

20

40

60

80

100

1/25/01 16:00 1/25/01 20:00 1/26/01 00:00 1/26/01 04:00

Figure 6.—Continued.
16 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San Joaqui

Continued Site number Site name Site identification number Date and time Collec- tion method Stream- flow (ft3/s) Diazinon concen- tration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da

1 San Joaquin River near Stevinson 11260815 1/4/01 1100 IG 29 <0.005 <0.00

1/11/01 0950 IG 71 0.009 0.001/18/01 1240 IG 43 0.131 0.031/26/01 1230 IG 208 0.289 0.321/26/01 2130 IG 256 0.205 0.281/27/01 0340 IG 270 0.128 0.181/27/01 1210 IG 293 0.156 0.241/28/01 0000 IG 299 0.150 0.241/28/01 1120 IG 262 0.140 0.19

2/1/01 1140 IG 110 0.056 0.032/8/01 1200 IG 41 0.024 0.00

2/15/01 1210 IG 130 0.083 0.052/22/01 0940 IG 49 0.014 0.002/25/01 0420 IG 228 0.020 0.022/25/01 0900 IG 247 0.018 0.022/25/01 1300 IG 278 0.022 0.032/25/01 1740 IG 336 0.039 0.072/25/01 2120 IG 391 0.094 0.192/26/01 0040 IG 446 0.130 0.312/26/01 0440 IG 498 0.115 0.302/26/01 1400 IG 554 0.070 0.20

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San Joaqui[See figure 1A for site locations. Collection method: a “t” following the collection method indicates that the sample was collected by the Toxics Substances Hydrology P

California District laboratory. Streamflow is in cubic feet per second. IG, integrated grab; MG, midpoint grab; BG, bank grab; P, pumped; EWI, equal-width increment (AS, autosampler; E, estimate; lb a.i./day, pound active ingredient per day; μg/L, microgram per liter; <, less than]
Sampling Design and Methodology 17

4 Merced River at River Road Bridge near Newman 11273500 1/4/01 1150 IG E258 <0.005 <0.00

1/11/01 1020 IG E285 0.021 0.031/18/01 1200 IG E273 0.016 0.021/26/01 0710 IG E370 0.013 0.021/26/01 1050 IG E378 0.014 0.021/26/01 1850 IG E380 0.050 0.101/26/01 2340 IG E370 0.095 0.181/27/01 0200 IG E366 0.044 0.081/27/01 0610 IG E368 0.047 0.091/27/01 1000 IG E374 0.276 0.551/27/01 1250 IG E380 0.413 0.841/27/01 2300 IG E374 0.435 0.87

2/1/01 1100 IG E280 0.042 0.062/8/01 1120 IG E254 E0.004 E0.00

2/15/01 1140 IG E316 0.068 0.112/22/01 0850 IG E263 E0.004 E0.002/25/01 0700 IG E285 E0.004 E0.002/25/01 1100 IG E287 E0.004 E0.002/25/01 1510 IG E292 0.006 0.002/25/01 1910 IG E298 0.016 0.022/25/01 2310 IG E306 0.044 0.072/26/01 0300 IG E334 0.037 0.062/26/01 1300 IG E389 0.025 0.05

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued

Site

number Site name

Site

identification

number

Date and time

Collection

method

Streamflow

(ft3/s)

Diazinon

concentration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
18 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California 5 Orestimba Creek at River Road near Crows Landing 11274538 1/4/01 1220 MG 7.9 0.023 0.00

1/11/01 1050 IG 33 0.038 0.001/18/01 1120 MG 43 0.009 0.001/25/01 1450 IG 56 E0.004 E0.001/25/01 1800 MG 59 E0.005 E0.001/25/01 2000 MG 64 0.015 0.001/25/01 2100 MG 60 0.019 0.001/25/01 2200 MG 57 0.020 0.001/26/01 0000 MG 59 0.012 0.001/26/01 0200 MG 59 0.010 0.001/26/01 0700 MG 56 0.008 0.00

2/1/01 1030 IG 18 0.062 0.002/8/01 1100 MG 6.6 0.022 <0.00

2/15/01 1110 IG 61 0.041 0.012/22/01 0830 IG 23 0.024 0.002/24/01 1430 MG 75 0.020 0.002/24/01 1630 MG 73 0.018 0.002/24/01 1930 MG 84 0.017 0.002/24/01 2120 MG 78 0.022 0.002/25/01 0030 IG 167 0.032 0.022/25/01 0330 IG 405 0.015 0.032/25/01 1030 IG 334 0.015 0.03

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued Site number Site name Site identification number Date and time Collec- tion method Stream- flow (ft3/s) Diazinon concen- tration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
Sampling Design and Methodology 19

6 San Joaquin River at Patterson Bridge near Patterson 11274570 1/4/01 1300 IG 950 0.005 0.02

1/11/01 1140 IG 1290 0.023 0.161/18/01 1050 IG 1340 0.020 0.14

1/26/01 830 IG 1280 0.028 0.19

1/26/01 1310 IG 1310 0.024 0.161/26/01 2020 IG 1350 0.020 0.141/27/01 1100 IG 1460 0.057 0.44

2/1/01 1000 IG 1230 0.039 0.252/8/01 1030 IG 1130 0.010 0.06

2/15/01 1040 IG 1530 0.050 0.41

2/22/01 800 IG 1230 0.011 0.07

2/24/01 2350 IG 1370 0.010 0.072/25/01 0300 IG 1410 0.011 0.082/25/01 0800 IG 1440 0.014 0.102/25/01 1200 IG 1520 0.016 0.132/25/01 1620 IG 1650 0.012 0.102/25/01 2000 IG 1710 0.011 0.102/25/01 2350 IG 1720 0.012 0.112/26/01 0340 IG 1710 0.012 0.11

8 McHenry Storm drain at Bodem Street at Modesto 373847120590801 1/25/01 1800 AS 1.56 0.506 0.00

1/25/01 1900 AS 72.9 0.947 0.371/25/01 2000 AS 44.8 0.922 0.221/25/01 2100 AS 23.3 0.708 0.081/25/01 2200 AS 7.25 0.691 0.021/25/01 2300 AS 4.35 0.727 0.011/26/01 0000 AS 2.56 0.756 0.011/26/01 0100 AS 1.89 0.761 0.001/26/01 0200 AS 1.21 0.823 0.00

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued

Site

number Site name

Site

identification

number

Date and time

Collection

method

Streamflow

(ft3/s)

Diazinon

concentration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
20 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California 9 Tuolumne River at Shiloh Road Bridge near Grayson 11290200 1/4/01 1350 IG E430 E0.005 E0.01

1/8/01 1630 IG E595 0.039 0.12

1/11/01 1240 IG E602 0.135 0.431/18/01 1340 IG E482 E0.005 E0.011/26/01 0500 IG E519 0.035 0.091/26/01 0910 IG E603 0.201 0.651/26/01 1500 IG E592 0.108 0.341/26/01 1820 IG E631 0.123 0.411/27/01 0200 IG E646 0.026 0.091/27/01 0930 IG E650 0.022 0.071/27/01 1330 IG E688 0.022 0.081/27/01 2100 IG E756 0.035 0.141/28/01 0900 IG E657 0.027 0.09

2/1/01 1240 IG E458 0.008 0.022/8/01 0950 IG E433 <0.005 <0.00

2/15/01 0950 IG E1040 E0.005 E0.022/21/01 1800 IG E1300 0.007 0.042/22/01 1040 IG E1640 E0.004 E0.032/24/01 2200 IG E3510 0.006 0.112/25/01 0200 IG E3420 E0.005 E0.092/25/01 0700 IG E3400 0.006 0.112/25/01 1100 IG E3460 E0.004 E0.072/25/01 1510 IG E3550 E0.004 E0.072/25/01 1900 IG E3590 E0.004 E0.072/25/01 2310 IG E3700 0.006 0.122/26/01 0300 IG E3810 E0.005 E0.102/26/01 1100 IG E3790 E0.003 E0.06

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued Site number Site name Site identification number Date and time Collec- tion method Stream- flow (ft3/s) Diazinon concen- tration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
Sampling Design and Methodology 21

12 Stanislaus River at Caswell State Park near Ripon 374209121103800 1/4/01 1520 BG E360 0.005 0.01

1/11/01 1330 BG E445 0.051 0.121/18/01 1430 BG E366 0.018 0.031/26/01 1010 BG E407 0.067 0.141/26/01 1930 BG E413 0.054 0.12

1/27/01 000 BG E439 0.072 0.17

1/27/01 0300 BG E466 0.053 0.131/27/01 0700 BG E479 0.081 0.201/27/01 1020 BG E475 0.083 0.211/27/01 2000 BG E437 0.058 0.13

2/1/01 1320 BG E348 0.006 0.012/8/01 0850 BG E343 <0.005 <0.00

2/15/01 0900 BG E362 0.007 0.012/22/01 1120 BG E367 0.007 0.012/24/01 2310 BG E407 0.026 0.052/25/01 0300 BG E415 0.018 0.042/25/01 0800 BG E428 0.017 0.032/25/01 1200 BG E491 0.026 0.062/25/01 1600 BG E581 0.014 0.042/26/01 0010 BG E577 0.027 0.082/26/01 1000 BG E501 0.010 0.02

13 San Joaquin River near Vernalis 11303500 1/4/01 1440 IG 2020 E0.003 E0.03

1/8/01 1720 IG 2120 E0.004 E0.041/9/01 0900 IG 2290 0.024 0.291/9/01 1450 IG 2290 0.041 0.50

1/10/01 0915 IG-t 2280 0.014 0.171/11/01 1300 IG 2620 0.031 0.431/12/01 1105 IG-t 2840 0.044 0.671/13/01 1005 IG-t 2970 0.039 0.62

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued

Site

number Site name

Site

identification

number

Date and time

Collection

method

Streamflow

(ft3/s)

Diazinon

concentration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
22 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California 13 San Joaquin River near Vernalis—continued 11303500 1/14/01 1130 IG-t 3050 0.029 0.47

1/15/01 1030 IG-t 3030 0.020 0.321/16/01 1545 IG-t 2940 0.021 0.331/18/01 0940 IG 2560 0.019 0.261/19/01 1330 IG-t 2480 0.007 0.091/23/01 1515 IG-t 2260 0.017 0.201/26/01 0710 IG 2390 0.020 0.251/26/01 1100 IG 2440 0.028 0.361/26/01 1345 IG-t 2460 0.043 0.571/26/01 1710 IG 2490 0.104 1.401/26/01 2030 IG 2520 0.073 0.991/27/01 0040 IG 2540 0.168 2.301/27/01 0400 IG 2570 0.172 2.381/27/01 0800 IG 2610 0.201 2.831/27/01 1710 P-t 2750 0.141 2.081/27/01 1900 IG 2780 0.206 3.081/28/01 0200 IG 2830 0.235 3.581/28/01 0950 IG 2840 0.200 3.061/28/01 1650 P-t 2800 0.170 2.561/28/01 2230 IG 2770 0.220 3.281/29/01 0900 IG 2740 0.224 3.301/29/01 2020 IG 2680 0.185 2.671/30/01 0900 IG 2600 0.141 1.981/30/01 1800 IG 2550 0.126 1.731/31/01 0800 IG-t 2480 0.058 0.77

2/1/01 0900 IG 2400 0.035 0.452/4/01 1200 IG-t 2180 0.025 0.29

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued Site number Site name Site identification number Date and time Collec- tion method Stream- flow (ft3/s) Diazinon concen- tration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
Sampling Design and Methodology 23

13 San Joaquin River near Vernalis—continued 11303500 2/6/01 1005 IG-t 2140 0.024 0.27

2/8/01 1300 IG 2110 0.008 0.09

2/11/01 1315 IG-t 2270 0.025 0.302/12/01 0845 IG-t 2440 0.026 0.342/12/01 1730 IG 2500 0.028 0.372/13/01 1230 IG-t 2760 <0.004 <0.032/13/01 1710 IG 2850 0.117 1.802/14/01 0845 IG-t 2900 0.024 0.372/14/01 1700 IG 2890 0.019 0.292/15/01 0945 IG-t 2890 0.024 0.372/15/01 1320 IG 2890 0.024 0.372/16/01 0900 IG-t 2980 0.014 0.222/17/01 0920 IG-t 2940 0.006 0.092/20/01 0930 IG-t 2790 0.007 0.102/21/01 1700 IG 2700 0.012 0.17

2/22/01 600 IG 2960 0.012 0.19

2/22/01 1150 IG 3080 0.010 0.162/25/01 0900 IG 5380 0.013 0.372/25/01 1300 IG 5430 0.010 0.292/25/01 1730 IG 5580 0.011 0.332/25/01 2110 IG 5720 0.010 0.302/26/01 0110 IG 5860 0.007 0.222/26/01 0500 IG 5970 0.009 0.282/26/01 1200 IG 6040 0.013 0.422/26/01 1800 IG 6000 0.010 0.322/27/01 0700 IG 5810 0.006 0.182/27/01 1740 IG 5680 0.007 0.212/28/01 0730 IG 5450 0.010 0.292/28/01 1830 IG 5320 0.012 0.34

Table 3. Environmental data collected on diazinon and chlorpyrifos concentrations and instantaneous loading rates for water quality sampling sites in the San JoaquiContinued

Site

number Site name

Site

identification

number

Date and time

Collection

method

Streamflow

(ft3/s)

Diazinon

concentration

(μg/L)

Diazino

instant

neous

loadin

rates

(lb a.i/da
24 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

Urban storm runoff samples were collected from 

a city storm drain in Modesto on an hourly basis 

beginning on January 25 at 6 p.m. and ending on 

January 26 at 2 a.m. using an autosampler outfitted 

with a pressure transducer that measured water depth 

(fig. 6I). The storm sampling at this location was 

designed to determine the occurrences and 

concentrations of pesticides in urban storm-water 

runoff and to compare the results with those of 

agricultural sites and precipitation.

The purpose of collecting precipitation samples 

was to encompass an area that included both urban and 

agricultural land use so that the occurrence and 

concentrations of diazinon and chlorpyrifos in the 

atmosphere could be compared. Precipitation samples 

were collected at eight locations (fig. 2: sites A–H). 

Agricultural site locations (sites A, D, F, and H) were 

chosen to give good spatial representation of almond 

and stone-bearing fruit orchards outside of Modesto. 

The urban sites (sites B, C, E, G) were placed 

throughout Modesto to give good spatial representation 

of the urban area. Sites E and F were unlike the other 

precipitation collecting sites because they had wet–dry 

autosamplers. These samplers have a sensory unit that 

detects both wet and dry conditions. During dry 

periods, a container remains open to collect dry 

deposition from the atmosphere and, at the onset of 

precipitation, the apparatus closes the dry deposition 

collection container and opens a container for 

collecting precipitation. Two of these samplers were 

used in this study—one in an agricultural land-use area 

and one in an urban land-use area. 

The sampling strategy for collecting 

precipitation was to set out the collecting apparatus at 

the locations before a forecasted storm event. The 

accumulated precipitation was then retrieved after the 

event. The total precipitation for time intervals of 

interest during storm periods was taken from a 

precipitation gage that collected hourly precipitation 

data located atop the Modesto Irrigation District office 

roof at 1231 Eleventh Street in downtown Modesto. 

Precipitation accumulations at each of the eight sites 

were calculated on the basis of the total volume of 

precipitation collected and the surface area of the 

collection unit. Of the three storms that were sampled 

for the precipitation segment of the study, only the 

January 23–26 storm was sampled at all eight sites. 

SAMPLING AND ANALYTICAL METHODS

Sampling Methods

Samples from the January 23–26, 2001, and 

February 18–24, 2001, target storm periods at the river 

sites were collected in five different manners 

depending on the site and streamflow conditions. The 

five methods are (1) width- and depth-integrated using 

a D-77 isokinetic sampler with a Teflon nozzle bottle 

(Webb and others, 1999); (2) equally spaced, threepoint integrated grab using a 3-L Teflon bottle strapped 

into a metal cage suspended by a rope; (3) midpoint 

grab using the same sampler; (4) grab sample from the 

bank using a 3-L Teflon bottle; and (5) directly pumped 

into a 1-L amber glass bottles using a peristaltic pump 

equipped with a stainless steel and Teflon inlet hose 

suspended in mid-channel. Most samples were 

collected as integrated grabs or midpoint grabs. In the 

first storm, samples were collected using both widthand depth-integrated and three-point integrated 

methods at the San Joaquin River near Vernalis and the 

Merced River at River Road sites. Samples were 

collected by both methods at these two sites within ten 

minutes of one another to check for sampling method 

variability. Samples from the Stanislaus River at 

Caswell State Park were collected from the right bank 

because this site does not have a bridge. The sample 

collection method for this site involved strapping a 3-L 

Teflon bottle into the cage sampler and tossing the 

sampler from the right bank into the main channel. All 

samples collected in a 3-L Teflon bottle from various 

methods described above were then directly poured 

into three separate sterile 1-L amber glass bottles, 

capped, and stored on ice. 
Sampling and Analytical Methods 25

The McHenry storm drain site was sampled 

using an autosampler equipped with Teflon tubing and 

a rack with twenty-four 1-L glass bottles. Discharge 

was determined from water-depth data that was 

collected using a pressure transducer attached to the 

autosampler, then converted to discharge using a depthdischarge relation provided by the City of Modesto 

staff. 

Except for sites E and F (fig. 2), precipitation 

samples were collected using a 4-L amber glass bottle 

below a 31-cm diameter funnel enclosed in a 12-in. by 

36-in. segment of plastic pipe. The collection apparatus 

was set out before a precipitation event; at the end of a 

precipitation event, the samples were capped, 

collected, and stored on ice. The sample volume was 

measured using a graduated cylinder at the USGS 

California District Laboratory in Sacramento, 

California (hereinafter referred to as the “USGS 

laboratory”). For samples collected by the wet–dry 

automatic samplers, the 27-cm diameter collection 

containers were connected to Teflon tubing and drained 

into a 4-L glass amber bottle that was collected and 

measured in the same manner as the other precipitation 

samples.

The 3-L Teflon sample-collection bottle was 

cleaned after use at each site using the following 

protocol. After sampling a site, the sample bottle was 

rinsed with deionized water. Approximately 30 mL of 

dilute Liquinox solution was then placed into the 

sample bottle, and it was capped and shaken. The 

Liquinox solution was then completely rinsed with 

deionized water followed by adding approximately 

30 mL of methanol to the sample bottle, and it was 

capped and shaken. The methanol rinse was emptied 

into a waste container. The final step in the cleaning 

procedure was to rinse the sample bottle thoroughly 

with deionized water. At the next site, the sample bottle 

was rinsed three times with native water before 

collecting the sample. The autosampler used to collect 

storm-water runoff at McHenry storm drain was 

cleaned with Liquinox solution and rinsed with 

deionized water by the City of Modesto staff prior to 

deployment. The precipitation collection equipment—

the 4-liter amber glass bottles, funnels, collection 

buckets, and Teflon tubing—was cleaned after each 

storm event at the USGS laboratory using the same 

cleaning protocol that was used for the 3-L Teflon 

sample collection bottles.

Analytical Methods

The samples collected for this study were stored 

on ice and shipped overnight for analysis to the 

National Water Quality Laboratory (NWQL) in 

Denver, Colorado, within three days of collection. 

Samples collected by the Toxics Program were 

analyzed at the USGS laboratory. Samples sent to the 

NWQL were analyzed for two suites of pesticides: 

USGS laboratory schedule 2001 and USGS laboratory 

schedule 1319. The samples for schedule 2001 are 

filtered prior to analysis, whereas the samples for 

schedule 1319 are not. A total of 220 environmental 

samples collected at the river sites were analyzed at 

NWQL. Of this total, 198 were analyzed for schedule 

2001, whereas 22 samples at selected river sites were 

analyzed for both schedules 2001 and 1319. These 

22 samples were generally selected on the rising limb 

of the hydrographs for the selected sites.

 The samples analyzed for schedule 2001 were 

filtered, extracted, and analyzed at NWQL for 45 

dissolved pesticides. These samples were filtered 

through a baked 0.7-micron glass-fiber filter. The 

dissolved pesticides were extracted by a solid-phase 

extraction (SPE) cartridge containing porous silica 

coated with a C-18 phase and preconditioned with 

methanol. The absorbed pesticides and degradates were 

removed from the cartridges by elution with ethyl 

acetate. Extracts of the effluent were analyzed by a 

capillary-column gas chromatograph/mass 

spectrometer (GC/MS) operated in the selected-ion 

monitoring mode (Zaugg and others, 1995). 

The samples selected for laboratory schedule 

1319 were analyzed for 11 pesticides at NWQL. This 

laboratory schedule involves using hexane to extract 

the pesticides from whole-water (unfiltered) with a 

separatory funnel. The extracts are then concentrated 

using a Kuderna-Danish apparatus and nitrogen gas 

(Wershaw and others, 1987). Following solvent 

reduction, the pesticides are analyzed by dual 

capillary-column gas chromatography with flame 

photometric detection (GC/FPD). Samples analyzed by 

the USGS laboratory were also filtered through a baked 

0.7 micron glass-fiber filter. However, the USGS 

laboratory uses SPE cartridges with a C-8 bonded 

phase, not preconditioned, to isolate the pesticide from 

water. The cartridges are eluted with 9 mL of ethyl 

acetate and analyzed using a capillary-column GC/MS 

operated in full-scan mode (Orlando and others, 2003). 
26 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

Table 4 lists the concentrations of diazinon and 

chlorpyrifos for sites that were chosen for analysis 

using laboratory schedules 2001 and 1319, as well as 

the relative percent difference (RPD) between the two 

analytical methods. The relative percent difference is 

used to describe the variability between the two 

analytical methods. An RPD could not be calculated 

for diazinon and chlorpyrifos values less than the 

laboratory reporting level. Theoretically, the wholewater samples (schedule 1319) should have higher 

concentrations than the filtered samples (schedule 

2001) if the pesticides were partially associated with 

suspended sediments. This would particularly be 

expected in the case of chlorpyrifos as it readily sorbs 

to sediments. Comparison of reported concentrations 

from each method must take into account the 

differences in both reporting level and method 

performance. The diazinon and chlorpyrifos median 

recoveries from laboratory reagent water spike samples 

in schedule 1319 are 66 percent and 70 percent, 

compared with 93 percent and 84 percent for 

laboratory schedule 2001, respectively. The relative 

percent differences for these median recoveries are 

34 percent for diazinon and 18 percent for chlorpyrifos. 

The chlorpyrifos concentrations reported by schedule 

2001 for most samples listed in table 4 are below the 

higher schedule 1319 reporting level. For two samples 

with data reported from both methods, the chlorpyrifos 

data are comparable (within the expected method 

performance differences and variability). This is also 

the case for most of the diazinon concentrations 

reported by both methods. For several samples, the 

differences in diazinon concentrations reported by the 

two methods are substantially greater (>80 percent 

RPD) than expected on the basis of laboratory spike 

recovery differences. Interestingly, diazinon 

concentrations in schedule 2001 are all greater than 

schedule 1319, suggesting the amount of pesticide 

associated with suspended sediment in the whole water 

samples (schedule 1319) likely was small.

The method detection limit (MDL) is defined as 

the minimum concentration of a substance that can be 

identified, measured, and reported with 99 percent 

confidence that the analyte concentration is greater 

than zero. The MDL for laboratory schedule 2001 for 

diazinon and chlorpyrifos at NWQL is 0.003 μg /L 

with a 95 percent confidence level range of 0.0020 to 

0.0036 for water year 2001. The MDL for laboratory 

schedule 1319 for water year 2001 is 0.010 μg /L for 

diazinon (95 percent confidence level range of 0.0098 

to 0.0183) and 0.007 μg /L for chlorpyrifos (95 percent 

confidence level range of 0.0070 to 0.0131). Although 

the risk of reporting a false positive using the MDL is 

less than 1 percent, the probability of reporting a false 

negative is as much as 50 percent (that is, the analyte is 

reported as not present when it is present at the MDL) 

(Childress and others, 1999). To correct for the risk of 

reporting a false positive, NWQL uses a long-term 

method detection level for the MDL, which is based on 

a modification of the USEPA MDL procedure. NWQL 

sets a laboratory reporting level (LRL) at twice the 

MDL to reduce the probability of false negatives to less 

than one percent (Childress and others, 1999). These 

LRL values are reevaluated each year on the basis of 

the most recent quality control data and, consequently, 

may change from year to year. The LRL for both 

diazinon and chlorpyrifos for water year 2001 is 

0.005 μg/L. Sample concentrations with a “<” (less 

than) qualifier are reported as less than the LRL 

(<0.005 μg/L) in table 3. Data reported as less than the 

LRL are those that were either not detected, failed the 

identification criteria, or were detected at a 

concentration less than the MDL when the risk of false 

positive exceeds 1 percent (Childress and others, 

1999). However many of the concentrations reported as 

less than the LRL may actually be low-level detections 

near the MDL. For this study, half the MDL value 

(0.0015 μg/L) for both diazinon and chlorpyrifos was 

used for calculating instantaneous loads to have 

maximum information available for calculating loads 

and to be consistent with the method for calculating 

loads in a similar study in the same study area during 

water year 2000 (Kratzer and others, 2002). 

Additionally, several diazinon and chlorpyrifos 

concentrations are reported as estimated, denoted by an 

“E” code in table 3. These samples have confirmed 

detections of the pesticides, but the concentrations are 

estimated because they are between the MDL and LRL 

or have some interference in the sample matrix 

(Childress and others, 1999). Laboratory schedule 

2001 is considered an information-rich analytical 

method by NWQL, resulting in better qualitative 

identification. As a result, information-rich methods are 

not restricted to censoring all measurements below the 

MDL and, therefore, positively identified analytes 

below the MDL are reported as estimated (“E” remark 

code) if other quality control criteria are met (Childress 

and others, 1999). Several concentrations are reported 

with an “E” code below the MDL for chlorpyrifos in 

table 3.
Sampling and Analytical Methods 27

Table 4. Comparison of diazinon and chlorpyrifos concentrations using laboratory schedules 1319 and 2001 for selected sites in the San Joaquin River Basin, Californ

Site

identification

number

Station name Date and time Laboratory schedule Diazinon concentration (μg/L) RPD Ch

co

11260815 San Joaquin River near Stevinson 1/26/01 1230 1319 0.212 30.7 

2001 0.289 

1/26/01 2130 1319 0.140 37.7 

2001 0.205 

2/25/01 1740 1319 0.031 22.8 

2001 0.039 

11273500 Merced River at River Road Bridge near Newman 2/25/01 0700 1319 <0.020 NA 

2001 E0.004 

2/25/01 1510 1319 E0.004 40 

2001 0.006 

11274538 Orestimba Creek at River Rd near Crows Landing 1/25/01 2200 1319 E0.010 66.7 

2001 0.020 

1/26/01 0200 1319 E0.008 22.2 

2001 0.010 

2/24/01 1430 1319 E0.003 150 

2001 0.021 

2/24/01 1930 1319 E0.012 34.4 

2001 0.017 

Table 4. Comparison of diazinon and chlorpyrifos concentrations using laboratory schedules 1319 and 2001 for selected sites in the San Joaquin River Basin, Californi[E, estimate; NA, not applicable; RPD, relative percent difference; μg/L, microgram per liter; <, less than]
28 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California 11274570 San Joaquin River at Patterson Bridge near Patterson 1/26/01 2020 1319 E0.013 42.4 

2001 0.020 

2/25/01 0800 1319 E0.006 80 

2001 0.014 

2/25/01 1620 1319 E0.005 82.3 

2001 0.012 

11290200 Tuolumne River at Shiloh Road Bridge near Grayson 1/26/01 0910 1319 0.160 22.7 

2001 0.201 

1/26/01 1820 1319 0.086 35.4 

2001 0.123 

2/25/01 0700 1319 E0.006 0 

2001 0.006 

2/25/01 1510 1319 <0.020 NA 

2001 E0.004 

11303500 San Joaquin River near Vernalis 1/27/01 0400 1319 0.130 27.8 

2001 0.172 

2/25/01 900 1319 E0.01 26.1 

2001 0.013 

2/25/01 1730 1319 E0.007 44.4 

2001 0.011 

Table 4. Comparison of diazinon and chlorpyrifos concentrations using laboratory schedules 1319 and 2001 for selected sites in the San Joaquin River Basin, Californ

Site identification number Station name Date and time Laboratory schedule Diazinon concentration

(μg/L)

RPD

Ch

co
Sampling and Analytical Methods 29

374209121103800 Stanislaus River at Caswell State Park near Ripon 1/26/01 1010 1319 0.049 31 

2001 0.067 

2/25/01 0800 1319 E0.012 34.4 

2001 0.017 

2/25/01 1600 1319 E0.010 33.3 

2001 0.014 

Table 4. Comparison of diazinon and chlorpyrifos concentrations using laboratory schedules 1319 and 2001 for selected sites in the San Joaquin River Basin, Californ

Site

identification

number

Station name Date and time Laboratory schedule Diazinon concentration (μg/L) RPD Ch

co
30 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

The USGS laboratory method for determination 

of the MDL is based on the USEPA procedures (U.S. 

Environmental Protection Agency, 1997) and is 

described as the minimum concentration of a substance 

that can be measured and reported with 99 percent 

confidence that the analyte concentration is greater 

than zero. MDLs are compound, matrix, and method 

dependent; therefore, at the USGS laboratory, MDLs 

are estimated in the same matrices as most of the 

samples. In 2001 and 2002, the most common sample 

matrices were obtained from the Sacramento and San 

Joaquin Rivers; thus, MDLs are calculated in three 

different matrices (water from the Sacramento and San 

Joaquin Rivers and organic free water). For each MDL 

calculation, seven or eight replicate samples were 

spiked at a concentration of 10 ng/L and analyzed with 

an unspiked matrix sample. For San Joaquin River 

water, the MDL for diazinon and chlorpyrifos at the 

USGS laboratory was 0.0036 μg/L and 0.0042 μg/L, 

respectively, in water year 2001 (Kathryn Kuivila, U.S. 

Geological Survey, written communication, 2002). 

In a linear regression using 21 replicate splits 

between NWQL and the USGS laboratory, NWQL 

values for diazinon were higher by 10 percent for the 

best fit line with an R2 value of 0.97 (Kathryn Kuivila, 

U.S. Geological Survey, written communication, 

2002). Therefore, diazinon concentrations for samples 

collected by the USGS Toxics Program at the San 

Joaquin River at Vernalis site during January and 

February 2001 were multiplied by 1.1 for use in this 

paper. A similar relation was found using 25 splits in a 

study by Kratzer in 1994 (Kratzer and Biagtan, 1997) 

where NWQL values were higher by 26 percent with a 

R2 value of 0.98. The USGS laboratory MDL for 

diazinon (0.0036 μg/L) was also multiplied by 1.1, and 

half this value (0.0018 μg/L) was used in calculating 

loads for diazinon concentrations reported as less than 

the MDL.

A similar relationship could not be produced for 

chlorpyrifos because analytical replicate splits with 

detections above the USGS laboratory’s MDL and 

NWQL’s LRL were variable. In water years 2001 and 

2002, a total of 39 replicate splits were analyzed for 

organophosphorus pesticides at both the USGS 

laboratory and NWQL. The analytical results of 

chlorpyrifos included four detections at both 

laboratories, five detections at only NWQL, and one 

detection at only the USGS laboratory. Although the 

detected concentrations for chlorpyrifos varied 

between the two laboratories, there was good 

agreement on nondetections in 29 of the 39 replicate 

splits. As a result, only those with chlorpyrifos 

concentrations below the USGS laboratory’s MDL are 

used in this report. Table 3 lists several chlorpyrifos 

concentrations as less than the MDL (<0.004 μg/L) of 

the USGS laboratory. Half this value (0.002 μg/L) was 

used for the calculation of chlorpyrifos loads at the San 

Joaquin River near Vernalis site. Samples with no 

chlorpyrifos concentration values listed in table 3 are 

those whose values were above the MDL at the USGS 

laboratory and are not used in this report.

 QUALITY CONTROL SAMPLES

Collection of quality control (QC) samples is 

necessary to evaluate the quality of the data. QC 

samples are collected, usually at the field site, in order 

to identify, quantify, and document bias and variability 

in data resulting from the collection, processing, 

shipping, and handling of samples by field and 

laboratory personnel (Wilde and others, 1998, p. 91). 

During the weekly sampling and the storm sampling 

events in January and February, 22 QC samples were 

collected out of a total of 251 (environmental and QC 

samples): 16 samples for standard QC sample testing 

(table 5) and 6 samples for comparison sampling 

methods (table 6). Of these 22 QC samples, 18 were 

field QC samples and 4 were laboratory QC samples. 

Four different types of field QC samples were collected 

in this study: field blanks, equipment blanks, split 

replicates, and sequential replicate samples that 

compared collection methods. The laboratory QC 

samples were laboratory-spiked environmental 

samples. All QC samples collected and reported in this 

study were analyzed at NWQL.
Quality Control Samples 31

Table 5. Summary of quality control data on diazinon and chlorpyrifos concentrations for sites in the San Joaquin River Basin, California.—Continued

Site identification number Site name Date and time Diazinon (μg/L)

Relative 

percent 

difference or 

percent 

recovery 

(diazinon)

REPLICATES

11273500 Merced River at River Road Bridge near Newman 2/25/01 1100 E0.004 0 

2/25/01 1101 E0.004 

11303500 San Joaquin River near Vernalis 1/26/01 1100 0.028 25 

1/26/01 1101 0.036 1/26/01 1710 0.104 12.2 1/26/01 1711 0.092 2/15/01 1320 0.024 8.0 2/15/01 1321 0.026 2/25/01 1300 0.010 5.3 2/25/01 1301 0.009 

FIELD BLANKS

11274538 Orestimba Creek at River Road near Crows Landing 1/18/01 1128 <0.005 NA 11260815 San Joaquin River near Stevinson 2/25/01 428 <0.005 NA 11273500 Merced River at River Road Bridge near Newman 2/25/01 2318 <0.005 NA 

Table 5. Summary of quality control data on diazinon and chlorpyrifos concentrations for sites in the San Joaquin River Basin, California[For replicates and spikes, first sample in each pair is the environmental sample, second sample is the quality control sample. E, estimate; NA, not applicable, cannot be c

concentration; μg/L, microgram per liter; —, no data available; <, less than]
32 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California 373847120590801 McHenry storm drain at Bodem Street at Modesto 1/25/01 2108 <0.005 NA 

374209121103800 Stanislaus River at Caswell State Park near Ripon 2/26/01 018 <0.005 NA 

EQUIPMENT BLANKS 373847120590801 McHenry storm drain at Bodem Street at Modesto 1/23/01 2358 <0.005 NA 

1/25/01 1308 <0.005 NA 

SPIKES 11303500 San Joaquin River near Vernalis 1/18/01 940 0.019 110.4 

1/18/01 943 0.135 

11290200 Tuolumne River at Shiloh Road Bridge near Grayson 2/25/01 200 E0.005 81.2 

2/25/01 203 0.092 

2/26/01 1100 E0.003 103.4 2/26/01 1103 0.120 

374209121103800 Stanislaus River at Caswell State Park, near Ripon 1/27/01 700 0.081 120.9 

1/27/01 703 0.208 

Table 5. Summary of quality control data on diazinon and chlorpyrifos concentrations for sites in the San Joaquin River Basin, California.—Continued Site identification number Site name Date and time Diazinon 

(μg/L)

Relative 

percent 

difference or 

percent 

recovery 

(diazinon)
Quality Control Samples 33

Table 6. Comparison of diazinon and chlorypyrifos concentrations using Equal Width Increment and Integrated Grab collection methods for selected sites in the San [E, estimate; EWI, equal width increment; IG, integrated grab; NA, not applicable; RPD, relative percent difference, μg/L, microgram per liter; <, less than.

Site identification 

number Site name Date and time Collection method Diazinon concentration (μg/L) RPD 

11273500 Merced River at River Road Bridge near Newman 1/11/2001 1020 IG 0.021 9.1 

1/11/2001 1030 EWI 0.023 

1/26/01 0710 IG 0.013 7.4 1/26/01 0700 EWI 0.014 1/26/01 1050 IG 0.014 25 1/26/01 1100 EWI 0.018 

2/8/01 1120 IG E0.004 NA 2/8/01 1130 EWI <0.005 

11303500 San Joaquin River near Vernalis 1/11/01 1300 IG 0.028 16.3 

1/11/01 1310 EWI 0.033 1/27/01 0800 IG 0.201 0.5 1/27/01 0810 EWI 0.202 0.5 
34 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

Field Quality Control Samples

Five field blank samples and two equipment 

blanks were processed throughout this study. Organicfree water was used as the blank solution to assess the 

degree of contamination introduced during field 

processing and handling, as well as laboratory handling 

of samples and equipment. The 3-L sample collection 

bottle was rinsed three times with organic-free water 

(instead of the native water rinse) after the standard 

cleaning procedure described in the sample processing 

section. The sample collection bottle was then filled 

with organic-free water and poured into a 1-L glass 

sample bottle and sent to NWQL with the 

environmental samples. There were no detections of 

diazinon and chlorpyrifos in any of the field and 

equipment blanks. The two equipment blanks were 

processed in the same manner as described above; 

however, the cleaning of the sampling equipment was 

done at the City of Modesto laboratory for the 

autosampler used at the McHenry storm drain site. 

Equipment blanks were completed prior to actual use 

of the equipment in the storm sampling to confirm the 

thoroughness of the cleaning procedure and to reduce 

the possibility of equipment contamination of field 

samples. 

 Five replicate samples were split from 3-L 

sample collection bottles at various sites to characterize 

the reproducibility of the complete sample processing 

and analytical process. The RPD between the 

environmental sample and the replicate is used to 

describe the variability in replicates (Kratzer and 

others, 2002). Four out of the five replicate samples had 

diazinon values above laboratory reporting limits with 

RPDs ranging from 0 to 25 percent and a median RPD 

of 5.3 percent. Only two of the five replicate samples 

had chlorpyrifos values above laboratory reporting 

limits with RPDs from 0 to 11.7 percent. 

In the first target storm (January 23–26, 2001), 

samples were collected using both width- and depthintegrated and three-point integrated collection 

methods at the San Joaquin River near Vernalis and 

Merced River at River Road sites (table 6). These 

samples were collected as a QC check on the 

variability introduced by the different sampling 

methods used in this study. Four samples at the Merced 

River at River Road and two samples at the San 

Joaquin River near Vernalis were collected by both 

methods within ten minutes of one another. The RPD 

for diazinon concentrations ranged from 0.5 to 

25 percent. The RPD for chlorpyrifos concentrations 

ranged from 15.4 to 54.5 percent. Thus, the RPDs for 

diazinon were within the range for replicate splits, 

while the RPDs for chlorpyrifos were outside the range 

for replicate splits. However, all the chlorpyrifos 

concentrations used for calculating the RPDs were very 

close to the detection level at which a small absolute 

difference in concentrations can result in a relatively 

high RPD (table 6). Generally, if streamflow at a site is 

well mixed, concentrations of dissolved constituents 

collected by different methods should not be 

significantly different (Martin and others, 1992). 

Laboratory Quality Control

Four laboratory-spiked environmental samples 

were used to evaluate the recovery of diazinon and 

chlorpyrifos in the sample matrix. The four samples 

were collected in the same manner as replicates in the 

field. The samples were labeled as a spike with 

instructions to NWQL to add their spike mixture after 

filtering. Spike recoveries are calculated by subtracting 

the measured concentrations in the environmental 

sample from the measured concentrations in the spiked 

sample and dividing by the theoretical concentration 

added to the spiked sample. Recoveries in the four 

laboratory spiked samples ranged from 81.2 percent to 

120.9 percent for diazinon and 78 percent to 

104.7 percent for chlorpyrifos (table 5). In addition to 

the laboratory-spiked samples, NWQL measures a 

laboratory control spike in each analytical set of 

environmental samples. This laboratory control spike 

has the target pesticides spiked into pesticide-grade 

water (inorganic blank water) at the laboratory and 

extracted, processed, and analyzed in the same manner 

as environmental samples. During December 2000 

through May 2001, NWQL ran 175 laboratory control 

spikes for laboratory schedule 2001. The median 

recovery for diazinon was 93 percent, with a 95 percent 

confidence interval of 74 to 118 percent. The median 

recovery for chlorpyrifos was 84 percent, with a 

95 percent confidence interval of 59 to 111 percent 

(Bruce Darnell, National Water Quality Laboratory, 

written commun., 2002).
Hydrology During the Study Period 35

HYDROLOGY DURING THE STUDY PERIOD

During the dormant season, diazinon and 

chlorpyrifos transport in the San Joaquin River Basin is 

primarily a function of the amount and timing of 

application and storm runoff from application areas 

(Kratzer and others, 2002). The weekly sampling 

during nonstorm periods usually occurred during stable 

and relatively low streamflows (fig. 6). These weekly 

nonstorm sampling events occurred on January 4, 11, 

and 18, and on February 1, 8, 15, and 22. However, 

three of the seven weekly sampling dates (January 11, 

and February 15 and 22) occurred during or shortly 

after nontarget or target storms. 

Four storm periods occurred during the dormant 

spray season: January 8–12 (2.86 in. at Modesto), 

January 23–26 (0.87 in. at Modesto), February 9–11 

(0.60 in. at Modesto) and February 18–24 (1.81 in. at 

Modesto). The January 23–26 and February 18–24 

storms were considered target storms and were 

sampled frequently throughout the storm hydrograph 

whereas the other two storm periods were considered 

nontarget storm periods. Though the storm that 

occurred during January 8–12 resulted in the highest 

precipitation during the study period, it was considered 

a nontarget storm because indications from the county 

agricultural advisors at the time suggested that most of 

the dormant spray had not been applied. 

The next storm (January 23–26, 0.87 in. at 

Modesto) was considered a target storm period because 

of the additional time for dormant spray application. 

However the nontarget storm produced more runoff 

than the first target storm at the Merced River at River 

Road, Stanislaus River at Caswell State Park, and San 

Joaquin River near Vernalis sites. A relatively small 

nontarget storm occurred on February 9–11 (0.60 in. at 

Modesto). A reservoir release of approximately 

600 ft3/s occurred in the Merced River Basin during 

this same period, which explains much of the 

hydrograph rise at the Merced River at River Road and 

San Joaquin River near Patterson sites (fig. 6). In 

addition, a reservoir release of approximately 800 ft3/s 

during February 14–18 in the Tuolumne River Basin 

accounts for much of the rise in the hydrographs for the 

Tuolumne River and San Joaquin River near Vernalis. 

Combined releases from the major reservoirs on the 

Merced, Tuolumne, and the Stanislaus Rivers increased 

from about 800 ft3/s before February 8 to 1,300 ft3/s on 

February 14, to 3,700 ft3/s on February 22 through the 

end of the month. The traveltimes from these reservoir 

releases to the sampling sites generally range from one 

to three days. Thus, large reservoir releases of Sierra 

Nevada water show up as a new baseflow at these 

sampling sites from about February 23 through the rest 

of the month. This baseflow of “clean” water 

originating above the pesticide application area, serves 

to greatly dilute pesticide concentrations in the rivers 

during this period. 

The storm water discharge at the McHenry storm 

drain (fig. 1: site 8) was sampled on an hourly basis 

beginning on January 25 at 6 p.m. and ending on 

January 26 at 2 a.m. A total of nine samples were 

collected during this period. The initial discharge of 

1.6 ft3/s at 6 p.m. rapidly increased to a discharge of 

73 ft3/s at 7 p.m. (fig. 6I). 

The precipitation samples collected in this study 

were composite samples collected during the following 

three storm periods: January 8–12, January 23–26, and 

February 9–11. The precipitation totals for each of 

these storm periods at the Modesto precipitation gage 

were 2.86 inches, 0.87 inches, and 0.60 inches, 

respectively. These precipitation amounts reflect the 

total amount of precipitation recorded by the 

precipitation gage located on the roof of the MID office 

building in downtown Modesto. The precipitation 

amounts collected by the composite samplers were 

calculated on the basis of the volume collected and the 

surface area of the collection apparatus (table 7). 
36 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

1Calculated from sample volume collected (in liters) and the depth of precipitation (in inches) per liter for the 1.02-ft diameter funnel sampler at sites A–D, G, H (0

diameter wet/dry automatic sampler at sites E and F (0.70 in./L). 2Sample volume collected corresponds to only 0.61 in. precipitation, whereas Modesto precipitation gage at site recorded 2.86 in. 3Recorded precipitation (2.86 in.) was adjusted to a calculated precipitation (2.09 in.) using the average calculated precipitation/recorded precipitation ratio from 1

(0.75 in.) events at site E.

4Sample volume collected corresponds to only 0.62 in. precipitation, whereas Modesto precipitation gage at site recorded 0.87 in. To be consistent with other sites

used to calculate loads instead of the recorded precipitation. 5Sample volume collected corresponds to only 0.45 in.precipitation, whereas Modesto precipitation gage at site recorded 0.60 in. To be consistent with other sites, th

to calculate loads instead of the recorded precipitation.

Table 7. Precipitation accumulations and summary of diazinon and chlorpyrifos concentrations and loads for composite precipitation samples collected at selected site

California

[in., inches; μg/L, microram per liter; μg/ft2, microgram per square foot; —, no sample taken]

Site 

letter

Site identification 

number Site name

Composite

precipitation

period

Calculated 

precipitation1

(inches)

Diazinon

concentration

(μg/L)

Diaz

loa

(μg/

A 373228120551201 Precipitation Gage 4, TID lateral 3 Barnhardt Road near 

Turlock

1/23/2001–1/26/2001 0.44 0.491 0.

B 373637121004601 Precipitation Gage 6, Wastewater Treatment Plant Rooftop at 

Modesto

1/23/2001–1/26/2001 0.48 0.908 0.

C 373725120543701 Precipitation Gage 7, Cadoni Road Lift Station at Modesto 1/23/2001–1/26/2001 0.44 0.472 0.

D 373750121092601 Precipitation Gage 3, MID Lateral 4 near Modesto 1/23/2001–1/26/2001 0.46 0.188 0.

E 373834121000601 Wet/Dry Sampler 1 at MID Rooftop at Modesto 1/8/2001–1/12/2001 2— 0.151 30.

1/23/2001–1/26/2001 40.62 0.544 0.

2/9/2001–2/12/2001 50.45 0.030 0.

F 373841120504801 Wet/Dry Sampler 2 at MID Gage, Albers Road near Turlock 1/23/2001–1/26/2001 0.64 0.870 1.

2/9/2001–2/12/2001 0.57 0.175 0.

G 374028120594301 Precipitation Gage 8, Bowen and Aloha Street at Modesto 1/23/2001–1/26/2001 0.45 0.486 0.H 374351121004701 Precipitation Gage 5, Tully Road near Modesto 1/23/2001–1/26/2001 0.46 0.675 0.
Concentrations of Diazinon and Chlorpyrifos 37

CONCENTRATIONS OF DIAZINON AND 

CHLORPYRIFOS

The concentrations of diazinon and chlorpyrifos 

in the samples collected during this study are presented 

in tabular form (table 3) and shown graphically (figs. 7,

8, and 9). Although the urban storm runoff site, 

McHenry storm drain, is presented with the river sites 

in table 3 and in figures 7 and 8, it is discussed 

separately. The concentrations of diazinon and 

chlorpyrifos for the composite precipitation samples 

also are discussed separately (table 7 and fig. 9). 

Diazinon

The highest concentrations of diazinon occurred 

during the rising limbs of storm runoff for both of the 

target storms that were sampled, January 23–26 and 

February 18–24. Of the 220 environmental river 

samples collected, 41 exceeded the proposed CMC of 

0.08 μg /L for diazinon. Thirty-four of these 

“exceedances” occurred during the first target storm 

(January 23–26), three occurred during the second 

target storm (February 18–24), and four occurred 

during nonstorm periods. All river sites had 

exceedances except for Orestimba Creek at River Road 

and San Joaquin River near Patterson. The following 

samples exceeded the CMC: sixteen samples at the San 

Joaquin River near Vernalis; thirteen samples at San 

Joaquin River near Stevinson; and four samples at the 

Tuolumne River at Shiloh Road, Merced River at River 

Road, and Stanislaus River at Caswell State Park. 

Higher concentrations occurred during the first storm at 

all sites, except Orestimba Creek. Most samples 

collected during the second storm were diluted 

considerably by reservoir releases.

The storm runoff samples collected at the urban 

storm runoff site, McHenry storm drain, had the 

highest diazinon concentrations found in this study 

(fig. 7H and table 3). All nine samples collected at the 

McHenry storm drain exceeded the CMC. The lowest 

concentration collected was more than 6 times greater 

than the CMC and the highest concentration collected 

was nearly 12 times the CMC. The site was sampled on 

an hourly basis beginning at 6 p.m. on January 25 and 

ending at 2 a.m. on January 26. The concentrations 

nearly doubled from 0.506 μg/L at 6 p.m. to a 

maximum of 0.947 μg/L at 7 p.m. After dropping to 

0.922 μg/L at 8 p.m., the diazinon concentrations on 

the falling limb of the storm hydrograph ranged from 

0.691 μg/L to 0.823 μg/L. Other than the sharp increase 

on the rising limb, diazinon concentrations were 

essentially independent of flow. This same relation was 

found for this site in a February 1995 storm event 

(Kratzer, 1998). 

During the January 23–26, 2001, storm event, 

composite precipitation samples were collected at four 

sites in the Modesto urban area and four sites in the 

agricultural areas surrounding Modesto (fig. 2, sites A–

H). All precipitation samples exceeded the CMC. The 

highest diazinon concentrations at the agricultural sites 

occurred at sites east and north of Modesto (sites F and 

H, respectively) (fig. 9A and table 7). These 

concentrations were 0.870 and 0.675 μg/L, 

respectively. These high concentrations may reflect 

more recent localized applications near these sampling 

sites than at the other two agricultural sites. Site D had 

the lowest concentrations of the agricultural sites. This 

site is located west of Modesto in an area of less dense 

orchard land use. Concentration values for an area 

(agricultural or urban) were calculated as an average 

rather than as a weighted value of the distance each 

sampling site was from source. The average was 

selected because the concentration gradient away from 

an orchard, in the weighted value selection, would 

depend on various factors that are beyond the scope of 

the project objectives. In the averages method, 

however, a standard deviation is given as an indication 

of the variability for the calculated averages. The 

average and standard deviation concentrations for 

diazinon at the agricultural sites are 0.556 and 

0.251 μg/L, respectively.

The concentrations in the precipitation at the 

urban sites were generally more uniform and slightly 

less than at the agricultural sites, with the exception of 

the sample at site B, the City of Modesto Wastewater 

Treatment Plant, which had the highest diazinon 

concentration (0.908 μg/L) at any of the eight 

precipitation sites. The average and standard deviation 

concentrations for diazinon at the urban sites are 0.603 

and 0.178 μg/L, respectively. When the concentration 

at the Wastewater Treatment Plant is removed from the 

calculations, the average and standard deviation 

concentrations at the urban sites are 0.501 and 

0.031 μg/L, respectively. 
38 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

100

0

200

300

400

500

600

Streamflow, in cubic feet per second

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Diazinon concentration, in micrograms per liter

A

B

C

D

streamflow

Diazinon concentration

Diazinon concentration below LRL

San Joaquin River near Stevinson

Merced River at River Road Bridge near Newman

San Joaquin River near Patterson Bridge near Patterson

Orestimba Creek at River Road near Crows Landing

0

100

200

300

400

500

0

0.1

0.2

0.3

0.4

0.5

0

400

800

1,200

1,600

2,000

0

0.02

0.04

0.06

0

100

200

300

400

500

0

0.02

0.04

0.06

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

Figure 7. Streamflow and diazinon concentrations in the San Joaquin River Basin, California. A–G. River sites. H. McHenry storm drain site in Modesto, 

California.

(LRL, laboratory reporting limit; NWQL, National Water Quality Laboratory; MDL, method detection limit)
Concentrations of Diazinon and Chlorpyrifos 39

0

1,000

2,000

3,000

4,000

5,000

0

0.05

0.1

0.15

0.2

0.25

streamflow

Diazinon concentration

Diazinon concentration below LRL

San Joaquin River near Stevinson

Stanislaus River at Caswell State Park near Ripon

San Joaquin River near Vernalis

McHenry storm drain at

Bodem Street at Modesto

Diazinon concentration

analyzed at Calif. district lab

Diazinon concentration

analyzed at Calif. district lab below MDL

E

F

G

H

0

200

400

600

Streamflow, in cubic feet per second

0

0.02

0.04

0.06

0.08

Diazinon concentration, in micrograms per liter

0

2,000

4,000

6,000

0.05

0

0.1

0.15

0.2

0.25

0

20

40

60

80

100

0

0.2

0.4

0.6

0.8

1

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

1/25/01 16:00 1/25/01 20:00 1/26/01 00:00 1/26/01 04:00

Date and time

Figure 7.—Continued.
40 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

100

0

200

300

400

500

600

0

0.002

0.004

0.006

0.008

A

B

C

D

Streamflow

Chlorpyrifos concentration

Chlorpyrifos concentration below LRL

San Joaquin River near Stevinson

Merced River at River Road

San Joaquin River at Patterson Bridge near Patterson

Orestimba Creek at River Road

0

100

200

300

400

500

Streamflow, in cubic feet per second

0

0.004

0.008

0.012

0.016

Chlorpyrifos concentration, in micrograms per liter

0

400

800

1,200

1,600

2,000

0

0.002

0.004

0.006

100

0

200

300

400

500

0

0.02

0.04

0.06

0.08

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

Figure 8. Streamflow and chlorpyrifos concentrations in the San Joaquin River Basin, California. A–G. River sites. H. McHenry storm drain site in Modesto, 

California. 

(LRL, laboratory reporting limit; NWQL, National Water Quality Laboratory; MDL, method detection limit)
Concentrations of Diazinon and Chlorpyrifos 41

0

1,000

2,000

3,000

4,000

5,000

0

0.004

0.008

0.012 E

F

G

H

Streamflow

Chlorpyrifos concentration

Chlorpyrifos concentration below LRL

Chlorpyrifos concentration below MDL at Calif. district lab

Tuolumne River at Shiloh Road Bridge near Grayson

Stanislaus River at Caswell State Park near Ripon

San Joaquin River near Vernalis

100

0

200

300

400

500

600

700

0

0.01

0.02

0.03

0.04

Chlorpyrifos concentration, in micrograms per liter

1,000

0

2,000

3,000

4,000

5,000

6,000

7,000

Streamflow, in cubic feet per second

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

McHenry storm drain at Bodem Street at Modesto

0

20

40

60

80

100

1/25/01 16:00 1/25/01 20:00 1/26/01 00:00 1/26/01 04:00

0.01

0

0.02

0.03

0.04

January 2001

Date and time

February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

Figure 8.—Continued.
42 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Precipitation, in inches per day

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Diazinon concentration, in micrograms per liter

Daily Precipitation

Composite diazinon

concentration for urban sites

Composite diazinon

concentration for agricultural

sites

15 9 13 17 21 25 29 2 6 10 14 18 22 26 30 3 7 11 15 19 23 27

December 2000 January 2001 February 2001

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Daily precipitation

Composite chlorpyrifos

concentration for urban sites

Composite chlorpyrifos

concentration for agricultural

sites

Chlorpyrifos concentration, in micrograms per liter

E

E

E

D

A

G

C

F

B

H

F

E

D

E

F

C

G

A E

B

F

H

Diazinon

Chlorpyrifos

A

B

Figure 9. Daily precipitation at Modesto, California, December 2000–February 2001, and concentrations of composite precipitation samples collected in 

urban and agricultural areas in the San Joaquin River Basin, California. A. Diazinon. B. Chlorpyrifos.
Concentrations of Diazinon and Chlorpyrifos 43

The average diazinon concentration from the two 

precipitation sites in the McHenry storm drain basin 

(fig. 2: sites E and G), accounted for 68 percent of the 

flow-weighted average diazinon concentration in 

McHenry storm drain. Diazinon concentrations at the 

downtown Modesto site (site E) were much lower in 

storms before and after the January 23–26 storm 

(fig. 9A). The diazinon concentration at the site in the 

agricultural area east of Modesto was much lower in 

the storm after the January 23–26 storm (fig. 9A: site 

D). Two of the three samples from other storms 

exceeded the diazinon CMC. The higher precipitation 

concentrations for the January 23–26 storm may be due 

to the timing and amount of application just prior to 

this storm relative to the other two storms (fig. 5A). 

Chlorpyrifos

Unlike diazinon, the highest concentrations of 

chlorpyrifos collected at the river sites did not occur 

during the two target storm events. Of the 220 

environmental river samples collected, only two 

exceeded the proposed CMC of 0.02 μg/L. A 

concentration of 0.038 μg/L occurred on January 4 

during the weekly sampling at the Stanislaus River at 

Caswell State Park site (fig. 8F and table 3), and a 

concentration of 0.068 μg/L occurred at the Orestimba 

Creek at River Road site during the weekly sampling 

on February 1 (fig. 8D and table 3). Though the 

remaining chlorpyrifos concentrations of samples 

collected were below the CMC, the concentrations at 

the river sites sampled on the rising limb of storm 

events were generally higher than those collected on 

the falling limb of a storm event or during routine 

weekly sampling.

The urban storm runoff site, McHenry storm 

drain, had a similar concentration pattern as for 

diazinon, but concentrations were more than one order 

of magnitude less (fig. 8H and table 3). Seven of the 

nine samples exceeded the CMC. The concentration 

was below the LRL in the first sample taken at 6 p.m. 

on January 25, and the maximum concentration 

(0.035 μg/L) occurred during the second peak flow at 

8 p.m. The concentrations after 8 p.m. ranged from 

0.018 to 0.033 μg/L. The concentration pattern was 

essentially independent of flow and was similar to the 

pattern for this site during a February 1995 storm event 

(Kratzer, 1998). However, the chlorpyrifos 

concentrations in the February 1995 storm event were 

about twice those found in this study.

The composite precipitation samples collected at 

the four sites in the Modesto urban area (fig. 2: sites B, 

C, E, and G) and the four sites in the agricultural area 

(sites A, D, F, and H) followed the same trend as the 

diazinon concentrations; that is, if the diazinon 

concentration at one site was high relative to the other 

sites, it was also high for the chlorpyrifos concentration 

relative to the other sites (fig. 9B and table 7). The 

highest precipitation concentration at the agricultural 

sites occurred at the sites east and north of Modesto 

(sites F and H, respectively). The average concentration 

for chlorpyrifos at these sites was 0.127 μg/L, and was 

more than twice the concentrations at the other two 

agricultural sites. These high concentrations may 

reflect more recent localized applications near these 

two sites than at the other two agricultural sites. Site D, 

had the lowest concentrations of all the agricultural 

sites. This site was located west of Modesto in an area 

of less dense orchard use. The average and standard 

deviation concentrations of chlorpyrifos at the 

agricultural sites are 0.085 and 0.045 μg/L, 

respectively. 

The concentrations of chlorpyrifos in the 

precipitation at the urban sites were more consistent 

and slightly less than at the agricultural sites, with the 

exception of the sample from site B, the City of 

Modesto Wastewater Treatment Plant. As with 

diazinon, this site had the highest chlorpyrifos 

concentration (0.086 μg/L). It is unclear why the 

concentrations for both diazinon and chlorpyrifos in 

the precipitation sample at this site were unusually 

high. One possible explanation could be that there was 

an application to the trees and other plants in the 

Wastewater Treatment Plant nursery area near the 

sampling site before the storm event sampling took 

place. In addition, there is a large apartment complex 

near this site. It is also possible that the diazinon and 

chlorpyrifos residues detected in the rain were due to 

volatilization from fumigation activities at the 

complex. The average and standard deviation 

concentrations for chlorpyrifos at the urban sites are 

0.069 and 0.011 μg/L, respectively. When the 

concentration at the Wastewater Treatment Plant is 

removed from the calculations, the average and 

standard deviation concentrations are 0.064 and 

0.006 μg/L, respectively. 
44 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

The average chlorpyrifos concentration from the 

two precipitation sites in the McHenry storm drain 

basin (fig. 2: sites E and G) had an average 

concentration in precipitation that was over 2.5 times 

higher than what was detected in the McHenry storm 

drain. As with diazinon, chlorpyrifos concentrations at 

the downtown Modesto site (site E) were much lower 

in the storm before and after the January 23–26 storm 

(fig. 9B). The chlorpyrifos concentration at the site in 

the agricultural area east of Modesto was much lower 

in the storm after the January 23–26 storm (fig. 9B: 

site D). One of three samples exceeded the CMC. 

LOADS OF DIAZINON AND CHLORPYRIFOS

Instantaneous loads of diazinon and chlorpyrifos 

were calculated for each sample collected in this study 

(table 3 and figs. 10–12). For samples with 

concentrations less than the MDL, the concentration 

was set to half of the MDL to calculate instantaneous 

loads. A simple substitution of half the MDL produces 

less bias in the summary statistics for concentration at a 

site than the alternatives of zero or the MDL (Helsel, 

1990). At the San Joaquin River near Vernalis, all 

instantaneous loads were connected and the following 

categories of loads were calculated from the area under 

the curves: target storms, nontarget storms, nonstorm, 

and baseline. Baseline loads were calculated for all 

sites. This was set to the loading rate on the true 

nonstorm, weekly sampling date of February 8 for all 

sites. This load was subtracted from the total load for 

the defined storm periods to calculate storm loads at 

each site. Loads for target storms were calculated for 

all sites except the San Joaquin River near Patterson. 

Insufficient samples were collected at this site to define 

the storm loads.

 Attempts were made to use more robust methods 

to calculate loads. These methods involved the use of 

the load estimating programs, ESTIMATOR and 

LOADEST2. ESTIMATOR is a log-linear multiple 

regression model of a constituent against measured 

environmental variables that calculates daily, monthly, 

and annual loads (Cohn and others, 1989; Cohn and 

others, 1992). ESTIMATOR can only be used when the 

concentration data file for a site contains at least 25 

observations per year for a minimum of a two-year 

period and at least 20 percent of the observations are 

above the detection limit. LOADEST2 uses a rating 

curve method to calculate loads and estimates the 

parameters of the rating curve by either the maximumlikelihood method or the linear attribution method 

(Crawford, 1996). Both methods assume that the 

rating-curve residual errors are normally distributed. It 

was not possible to use these two methods to calculate 

storm loads because the short-term nature of the data 

collected in this study does not meet the minimal data 

requirements of these programs. 

The storm load was defined in this study as the 

total load minus the baseline load. To calculate a storm 

load at a site, the connected instantaneous load line was 

brought down to the baseline load line, and the area 

enclosed was digitized. The dashed lines that complete 

the storm loads (figs. 10 and 11) are called estimated 

storm loads. At the beginning of the storm load, this 

line is based on the beginning of the storm runoff 

hydrograph. At the end of the storm load, this line 

represents the estimated end of the storm hydrograph. 

This line is estimated for sites on the basis of the trend 

in the instantaneous loads, the storm hydrograph from 

the valley application areas, and the traveltimes 

between the sites. An estimated traveltime from the 

farthest upstream site, San Joaquin River near 

Stevinson site to the farthest downstream site, San 

Joaquin River near Vernalis, was considered in 

extending the load line for San Joaquin River near 

Vernalis (Kratzer and Biagtan, 1997). 

Loads were divided by pesticide application and 

by drainage area to illustrate basins that have relatively 

higher or lower loading rates and yields (table 8). In 

this report, storm loads as related to application are 

based on the dry period preceding the storm. Thus, 

loads in the first target storm are related to the first dry 

period (December 1, 2000 to January 26, 2001), 

whereas loads in the second storm are related to the 

second dry period (January 27, 2001, to February 24, 

2001). The significance of loading during the different 

periods (target storms, nontarget storms, nonstorm, and 

baseline loads) is best illustrated by the loading rate in 

pounds active ingredient per day (lb a.i./d). Maximum 

loading rates for each target storm are presented for all 

sites except San Joaquin River at Patterson where only 

maximum instantaneous loads are presented.
Loads of Diazinon and Chlorpyrifos 45

B

A

C

D

100

200

300

400

500

Streamflow, in cubic feet per second

0

0.2

0.4

0.6

0.8

1

Diazinon load, in pounds active ingredient per day

400

0

0

800

1,200

1,600

2,000

0

0.1

0.2

0.3

0.4

0.5

100

0

200

300

400

500

0

0.01

0.02

0.03

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

100

0

200

300

400

500

600

0

0.1

0.2

0.3 Streamflow

Baseline load

Load with Diazinon concentration below LRL

Target storm load

Estimated storm load curve

San Joaquin River near Stevinson

Merced River at River Road Bridge near Newman

San Joaquin River at Patterson Bridge near Patterson

Orestimba Creek at River Road near Crows Landing

Figure 10. Streamflow and diazinon loads in the San Joaquin Rive Basin, California. A—G. River sites. H, McHenry storm drain site in Modesto, California.

(LRL, laboratory reporting limit,; NWQL, National Water Quality Laboratory, MDL, method detection limit)
46 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

0

1,000

2,000

3,000

4,000

5,000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

100

0

200

300

400

500

600

700

0

0.05

0.1

0.15

0.2

0.25

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Streamflow, in cubic feet per second

0

1

2

3

4

Diazinon load, in pounds active ingredient per day

0

20

40

60

80

100

1/25/01 16:00 1/25/01 20:00 1/26/01 00:00 1/26/01 04:00

0.1

0

0.2

0.3

0.4

Date and time

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

E

F

G

H

Streamflow

Baseline load

Diazinon load

Load with diazinon concentration below LRL

Target storm load

Estimated storm load curve

Tuolumne River at Shiloh Road Bridge near Grayson

Diazinon load using concentrations analyzed

at Calif. district lab

Diazinon load using concentrations below

MDL at Calif. district lab

Nontarget storm load

Nonstorm load

Target storm load curve

San Joaquin River at Vernalis

McHenry storm drain at Boden

Street at Modesto

Stanislaus River at Caswell State Park near Ripon

Figure 10.—Continued.
Loads of Diazinon and Chlorpyrifos 47

0

100

200

300

400

500

600

0

0.004

0.008

0.012

0

100

200

300

400

500

0

0.01

0.02

0.03

Chlorpyrifos load, in pounds active ingredient per day

0

400

800

1,200

1,600

2,000

Streamflow, in cubic feet per second

0

0.01

0.02

0.03

0.04

0

100

200

300

400

500

0

0.004

0.008

0.012

Streamflow

Baseline load

Load with chlorpyrifos concentration below LRL

Target storm load

Target storm load with concentration below LRL

Estimated storm load curve

San Joaquin River near Stevinson

Merced River at River Road Bridge near Newman

San Joaquin River near Patterson Bridge near Patterson

Orestimba Creek at River Road near Crows Landing D

C

B

A

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

Figure 11. Streamflow and chlorpyrifos loads in the San Joaquin River Basin, California. A–G. River sites. H. McHenry storm drain site in Modesto, California. 

(LRL, laboratory reporting limit; NWQL, National Water Quality Laboratory; MDL, method detection limit)
48 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

0

1,000

2,000

3,000

4,000

5,000

Streamflow, in cubic feet per second

0.0000

0.0050

0.0100

0.0150

0.0200

0.0250

0.0300

0.0350

0.0400

0

100

200

300

400

500

600

700

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0

2,000

4,000

6,000

0

0.04

0.08

0.12

0.16

0.2

Chlorpyrifos load, in pounds active ingredient per day

Streamflow

Baseline load

Chlorpyrifos load

Load with chlorpyrifos concentration below LRL

Target storm load

Target storm load with concentration below LRL

Estimated storm load curve

Chlorpyrifos load using concentration below MDL at Calif. district lab

Nontarget storm load

Target storm load

Tuolumne River at Shiloh Road Bridge near Grayson

Stanislaus River at Caswell State Park

San Joaquin River near Vernalis

McHenry storm drain at Bodem Street at Modesto

0

20

40

60

80

100

1/25/01 16:00 1/25/01 20:00 1/26/01 0:00 1/26/01 4:00

0

0.002

0.004

0.006

0.008

0.01

January 2001 February 2001 March

2001

1 8 15 22 5 5 29 12 26 19

Date and time

H

G

F

E

Figure 11.—Continued.
Loads of Diazinon and Chlorpyrifos 49

Table 8. Diazinon and chlorpyrifos loading rates, loads, and yields in relation to agricultural applications for sites in the San Joaquin River Basin, California, January a[See table 2 and figure 1B for basin names and locations. Value on top is diazinon, value in ( ) on bottom is chlorpyrifos. g/mi2, grams per square mile; mi2, square mile; 

ingredient; %, percent; NA, not applicable; <, less than]

Basin

Site 

number Site name

Basin 

valley 

area 

(mi2)

Dry 

Period 1, 

application (lb 

a.i.)

Baseline 

loading 

rate

(lb a.i.)

Storm 1 

load

(lb a.i.)

Storm 1 

load as % 

of Dry 

Period 1 

application

(percent)

Storm 1 

yield, as 

load per 

valley 

basin 

area)

(g/mi2)

Dry Period 2, application

(lb a.i.)

Storm 2 

load

(lb a.i.)

Storm 2 

load as % 

of Dry 

Period 2 

application

(percent)

St

yie

loa

v

bas

(g

A 1 San Joaquin River near 

Stevinson

218 78 

(751)

0.005 

(<0.001)

1.04 

(0.034)

0.133 

(0.004)

2.16 

(0.071)

578 

(2,539)

0.447 

(0.015)

0.077 

(<0.001)

(

C 4 Merced River at River Road 

Bridge near Newman

245 1,193 

(4,712)

0.005 

(0.002)

2.35 

(0.079)

0.197 

(0.001)

4.35 

(0.146)

701 

(62)

0.058 

(0.001)

0.008 

(0.002)

(

D 5 Orestimba Creek at River Road 

near Crows Landing

33 0 

(0)

0.001 

(<0.001)

0.004 

(<0.001)

inf 

(inf)

0.055 

(<0.014)

0 

(0)

0.038 

(0.013)

inf 

(inf)

(

E 6 San Joaquin River at Patterson 

Bridge near Patterson

1,614 2,58 

(5,538)

0.06 

(0.012)

1,691 

(2,545)

G 8 McHenry storm drain at Bodem 

Street at Modesto

1.3 NA 

NA

NA NA

0.03 

(0.001)

NA NA

10.4 

(<0.349)

NA NA

NA NA 

F 10 Tuolumne River at Shiloh Road 

Bridge near Grayson

149 237(74) <0.003 

(0.004)

0.37 

(0.044)

0.156 

(0.059)

1.13 

(0.134)

39.4 

(378)

0.218 

(0.067)

0.553 

(0.018)

(

H 12 Stanislaus River at Caswell 

State Park near Ripon

116 1,106 

(122)

<0.003 

(<0.003)

0.463 

(0.026)

0.042 0.021

1.81 

(0.102)

24 

(324)

0.080 

(0.008)

0.333 

(0.002)

(

K 13 San Joaquin River near Vernalis 2,273 6,566 

(6,808)

0.091 

(0.023)

11.6 

(0.301)

0.176 

(0.004)

2.31 

(0.060)

2,300 

(4,372)

0.988 

(0.109)

0.043 

(0.002)

(
50 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

 Loading from atmospheric sources is a 

potentially important component in the overall 

contributions of contaminants to the San Joaquin River 

Basin. The atmosphere is a major pathway by which 

pesticides and other organic and inorganic compounds 

are transported and deposited in areas sometimes far 

removed from their sources (Majewski and Capel, 

1995). Diazinon and chlorpyrifos loads from the 

precipitation collected at the urban and agricultural 

sites in the Modesto area were calculated on the basis 

of concentrations in the collected sample (μg/L) 

multiplied by the precipitation (inches) and a 

conversion factor (table 7). These loads are expressed 

in micrograms per square meter (μg/ft2). Figures 12A

and 12B illustrate the composite precipitation loads for 

diazinon and chlorpyrifos as well as the corresponding 

recorded precipitation totals at the downtown Modesto 

precipitation gage.

Diazinon

Instantaneous loads of diazinon at the river sites 

are closely related to streamflow (fig. 10). At all sites, 

the instantaneous diazinon storm loading rates (lb 

a.i./d) peaked during the rising limb of the storm 

hydrograph for both target storms (January 23–26 and 

February 18–24). The maximum storm loading rates at 

the river sites for the January target storm (not 

including the baseline loading rate) were 3.49 lb a.i./d 

at San Joaquin River near Vernalis, 0.871 lb a.i./d at 

Merced River near River Road, 0.650 lb a.i./d at 

Tuolumne River at Shiloh Road, 0.319 lb a.i./d at San 

Joaquin near Stevinson, 0.209 lb a.i./d at Stanislaus 

River at Caswell State Park, and less than 0.006 

lb a.i./d at Orestimba Creek near River Road. The 

instantaneous storm loading rates for the February 

target storm were all less than or equal to 0.081 lb a.i/d, 

except for San Joaquin River near Vernalis 

(0.332 lb a.i./d), San Joaquin River near Stevinson 

(0.307 lb a.i./d), and Tuolumne River at Shiloh Road 

(0.117 lb a.i./d). For the first nontarget storm period 

(January 8–12), samples were collected during the 

weekly sampling on January 11 at all river sites and 

daily at the farthest downstream site, San Joaquin River 

near Vernalis, for the duration of the storm hydrograph. 

The instantaneous loading rates were less than 0.119 

lb a.i./d, except for Tuolumne River at Shiloh Road 

(0.435 lb a.i./d) and San Joaquin River near Vernalis 

(0.347 lb a.i./d). For the second nontarget storm period 

(February 9–11), samples were collected during the 

weekly sampling on February 15 at all river sites and 

twice per day at the San Joaquin River near Vernalis for 

the duration of the storm hydrograph. The February 15 

weekly samples had instantaneous loading rates less 

than or equal to 0.111 lb a.i./d, except at San Joaquin 

River near Patterson (0.448 lb a.i./d) and San Joaquin 

River near Vernalis (0.283 lb a.i./d). The highest 

instantaneous loading rate for this nontarget storm 

period at San Joaquin River near Vernalis was 

1.71 lb a.i./d on February 13. 

The total January and February 2001 diazinon 

loads were calculated only for the San Joaquin River 

near Vernalis. The total diazinon load in the San 

Joaquin River near Vernalis during January and 

February 2001 was 23.8 lb a.i. (table 8). This total 

includes, 1.06 lb a.i of nonstorm load, 4.35 lb a.i. of 

nontarget storm load, 5.82 lb a.i. of baseline load, and 

12.6 lb a.i. of target storm load (fig. 10 and table 8). 

The first target storm (January 23–26, 2001) 

transported 11.6 lb a.i. of diazinon to Vernalis and the 

second target storm (February 18–24) transported 

0.988 lb a.i. The 12.6 lb a.i. of diazinon transported 

during the two storms compares with 27.4 lb a.i. 

transported during two storms in January and February 

1994 (Kratzer, 1999) and 8.17 lb a.i. in two storms in 

January and February 2000 (Kratzer and others, 2002). 
Loads of Diazinon and Chlorpyrifos 51

159 13 17 21 25 29 2 6 10 14 18 22 26 30 3 7 11 15 19 23 27

December 2000 January 2001 February 2001

Daily precipitation

Composite diazinon load for

urban sites

Composite diazinon load for

agricultural sites

Diazinon

Daily precipitation

Composite chlorpyrifos load for

urban sites

Composite chlorpyifos load for

agricultural sites

Chlorpyrifos

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0

0.05

0.1

0.15

0.2

0.25

Chlorpyrifos load, in micrograms per square feet

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Precipitation, in inches per day Precipitation, in inches per day

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Diazinon load, in micrograms per square feet

E

B

F

E

H

G

C

D

A

F

E

E

F

H

B

E

C

G

A

D

F

E

Figure 12. Daily precipitation at Modesto, California, December 2000–February 2001, and loads of composite precipitation samples collected in urban and 

agricultural areas in the San Joaquin River Basin, California. A. Diazinon. B. Chlorpyrifos.
52 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

 Approximately 36 percent of the 11.6 lb a.i. 

diazinon load transported during the first target storm 

to San Joaquin River near Vernalis came from four 

sites: Merced River at River Road (2.35 lb a.i.), San 

Joaquin River near Stevinson (1.04 lb a.i.), Stanislaus 

River near Caswell State Park (0.463 lb a.i.), and 

Tuolumne River at Shiloh Road (0.370 lb a.i.). 

Approximately 79 percent of the 0.988 lb a.i diazinon 

load transported to Vernalis during the second target 

storm came from four sites: San Joaquin River near 

Stevinson (0.447 lb a.i), Tuolumne River at Shiloh 

Road (0.218 lb a.i.), Stanislaus River at Caswell State 

Park (0.080 lb a.i.), and Orestimba Creek at River Road 

(0.038 lb a.i.). These loads during runoff from the two 

target storms do not include the baseline loads of 

1.20 lb a.i. at San Joaquin River near Vernalis, 

0.045 lb a.i. at Merced River at River Road, 0.021 lb 

a.i. at Tuolumne River at Shiloh Road, 0.055 lb a.i. at 

San Joaquin River near Stevinson, 0.211 a.i. at 

Stanislaus River at Caswell State Park, and less than 

0.003 lb a.i. at Orestimba Creek at River Road at River 

Road. The baseline loading rates in January through 

February 2001 were 30 to 77 percent less than in 

January through February 2000, except for San Joaquin 

River near Stevinson, which was 2.5 times more 

(table 8). The total load at San Joaquin River near 

Vernalis for January and February 2001 was 0.27 

percent of dormant application. This compares with 

0.17 percent during January through February 2000 at 

San Joaquin River near Vernalis (Kratzer and others, 

2002).

The McHenry storm drain instantaneous loading 

rates follow the storm hydrograph very closely 

(fig. 10H, table 3). The maximum instantaneous load of 

0.372 lb a.i./d occurred shortly before the peak of the 

storm hydrograph. The total load for the January 23–26 

storm was 0.03 lb a.i. This load compares to a load of 

0.024 lb a.i. during a February 1995 storm sampled at 

this site (Kratzer, 1998). 

Figure 12A illustrates the diazinon loads from 

precipitation collected at urban and agricultural sites in 

the Modesto area. Composite precipitation samples 

were collected during the January 23–26 storm at all 

eight sites. The resulting loads, in μg/ft2, are presented 

in table 7. The largest load for the urban locations was 

0.86 μg/ft2 at the Wastewater Treatment Plant (site B) 

and 0.80 μg/ft2 collected by the wet–dry autosampler in 

downtown Modesto (site E). The largest load for the 

agricultural locations was 1.3 μg/ft2 collected by the 

wet–dry auto sampler east of Modesto (site F) and 

0.76 μg/ft2 collected by the funnel sampler north of 

Modesto (site H). 

Chlorpyrifos

As with diazinon, the instantaneous loads of 

chlorpyrifos at the river sites were usually closely 

related to streamflow. Two exceptions were the weekly 

nonstorm samples on January 4 at the Stanislaus River 

at Caswell State Park (fig. 11F) and on February 1 at 

Orestimba Creek at River Road (fig. 11D). The 

instantaneous chlorpyrifos loading rates usually peaked 

during the rising limb of the storm hydrograph for both 

target storms (January 23–26 and February 18–24). 

The maximum instantaneous loading rates at the river 

sites for the January target storm (not including the 

baseline loading rate) were 0.153 lb a.i./d at San 

Joaquin River near Vernalis, 0.030 lb a.i./d at Merced 

River near River Road and at Tuolumne River at Shiloh 

Road, and less than or equal to 0.015 lb a.i./d at 

Stanislaus River at Caswell State Park, San Joaquin 

River near Stevinson, and Orestimba Creek at River 

Road at River Road sites. The maximum instantaneous 

loading rates for the February target storm were all less 

than or equal to 0.031 at all sites. For the first nontarget 

storm period (January 8–12), samples were collected 

during the weekly sampling on January 11 at all river 

sites and daily at the San Joaquin River near Vernalis 

throughout the storm hydrograph. The instantaneous 

loading rates were all less than or equal to 0.028 lb 

a.i./d. For the second nontarget storm period (February 

9–11), samples were collected on February 15 during 

the weekly sampling at all river sites and twice per day 

at the San Joaquin River near Vernalis throughout the 

storm hydrograph. Loading rates were less than or 

equal to 0.026 lb a.i./d at all sites with the exception of 

San Joaquin River at Patterson which had a 

instantaneous loading rate of 0.041 a.i./d. The highest 

instantaneous loading rate for this nontarget storm 

period at San Joaquin River near Vernalis was 0.161 

lb a.i./d on February 13. 
Summary and Conclusions 53

The total January and February 2001 

chlorpyrifos loads were calculated only for the San 

Joaquin River near Vernalis. The total chlorpyrifos load 

in the San Joaquin River near Vernalis during January 

and February 2001 was 2.17 lb a.i. (table 8). This total 

includes 1.47 lb a.i. of baseline load, 0.291 lb a.i 

nontarget storm load, and 0.410 lb a.i. target storm load 

(fig. 11G and table 8). A nonstorm category was not 

considered for chlorpyrifos load as samples were either 

at or below the baseline loading rate. 

The first target storm (January 23–26, 2001) 

transported 0.301 lb a.i. of chlorpyrifos to San Joaquin 

River near Vernalis and the second target storm 

(February 18–24, 2001) transported 0.109 lb a.i. The 

0.410 lb a.i. of chlorpyrifos transported during the two 

target storms is almost 20 percent of the 2.17 lb a.i. of 

chlorpyrifos transported in two storms in January and 

February 2000 (Kratzer and others, 2002). 

Approximately 61 percent of the 0.301 lb a.i. 

chlorpyrifos transported during the first target storm to 

San Joaquin near Vernalis came from four sites: 

Merced River at River Road (0.079 lb a.i.), Tuolumne 

River at Shiloh Road (0.044 lb a.i.), San Joaquin River 

near Stevinson (0.034 lb a.i.), and Stanislaus River at 

Caswell State Park (0.026 lb a.i.). Approximately 

61 percent of the 0.109 lb a.i. of chlorpyrifos 

transported to San Joaquin River near Vernalis during 

the second target storm came from the Tuolumne River 

at Shiloh Road (0.067 lb a.i.). Some of this chlorpyrifos 

in the Tuolumne River comes from the storm drains in 

the Modesto urban area. These loads from both target 

storms in January and February do not include the 

baseline loads of 0.303 lb a.i. at San Joaquin River near 

Vernalis, 0.028 lb a.i. at Tuolumne River at Shiloh 

Road, 0.021 lb a.i. at Stanislaus River at Caswell State 

Park, 0.018 lb a.i. at Merced River at River Road, 

0.011 lb a.i. at San Joaquin River near Stevinson, and 

less than 0.003 lb a.i. at Orestimba Creek at River 

Road. The total load at San Joaquin River near Vernalis 

for January and February 2001 was 0.02 percent of 

dormant application. This is much lower than the 

0.16 percent during January through February 2000 

(Kratzer and others, 2002)

 As with diazinon, the McHenry storm drain 

chlorpyrifos loading rates are closely related to the 

storm hydrograph (fig. 11H). The maximum 

instantaneous loading rate of 0.009 lb a.i. occurred 

shortly before the peak of the storm hydrograph, and 

instantaneous loading rates decreased to less than 

0.001 lb a.i. at the end of the nine-hour sampling 

period. The total load in the McHenry storm drain 

produced by the January 23–26 storm period was 

0.001 lb a.i. This load compares with a total load of 

less than 0.001 lb a.i. of chlorpyrifos during a February 

1995 storm sampled at this site (Kratzer, 1998). 

Figure 12B illustrates the chlorpyrifos loads 

from precipitation collected at urban and agricultural 

sites. Composite precipitation samples were collected 

during the January 23–26 storm at all eight sites. The 

resulting loads, in μg/ft2, are listed in table 7. The 

largest loads for the urban locations were 0.10 μg/ft2

collected by the funnel sampler at the Wastewater 

Treatment Plant (site B) and 0.09 μg/ft2 collected by 

the wet–dry auto sampler in downtown Modesto (site 

E). The largest loads for the agricultural locations were 

0.22 μg/ft2 collected by the wet–dry auto sampler east 

of Modesto (site F) and 0.12 μg/ft2 collected by the 

funnel sample north of Modesto (site H).

SUMMARY AND CONCLUSIONS

The agricultural application of diazinon and 

chlorpyrifos during December 2000 through February 

2001 in the San Joaquin River Basin was 24 percent 

less and 3.2 times more, respectively, than that applied 

during the same time periods in 1999–2000. A total of 

seven river sites were sampled from 19 to 64 times 

each during January and February 2001. Samples were 

collected weekly during nonstorm periods and several 

times during storm runoff from a total of four storms. 

Of these four storms, two were defined to be target 

storms. These target storms had the greatest potential to 

produce runoff that would transport applied diazinon 

and chlorpyrifos. The two target storm periods were 

January 23–26, 2001 (0.87 in. at Modesto), and 

February 18–24, 2001 (1.81 in. at Modesto). The 

highest concentrations of diazinon and chlorpyrifos at 

the seven river sites occurred during storm runoff for 

diazinon and during weekly sampling for chlorpyrifos. 

Of the 220 environmental samples collected at river 

sites, 41 exceeded the proposed critical CMC of 

0.08 μg/L for diazinon and only two exceeded the 

CMC of 0.02 μg/L for chlorpyrifos. 
54 Diazinon and Chlorpyrifos Loads in Precipitation and Storm Runoff during January and February 2001, San Joaquin River Basin, California

The total diazinon load in the San Joaquin River 

near Vernalis during the two target storms in January 

and February 2001 was 12.6 lb a.i. This compares with 

8.17 lb a.i. during two storms in January and February 

2000 and with 27.4 lb a.i. during two storms in January 

and February 1994. During the two target storms in 

January and February 2001, the main sources of 

diazinon in the San Joaquin River Basin were Merced 

River Basin (19.5 percent), San Joaquin River near 

Stevinson Basin (12.0 percent), Tuolumne River Basin 

(4.6 percent), and Stanislaus River Basin (4.3 percent). 

The total diazinon load in the San Joaquin River near 

Vernalis during January and February 2001 was 23.8 

lb a.i. and was 0.27 percent of the December 1, 2000, 

through February 24, 2001, application. This compares 

with a total diazinon load of 19.6 lb a.i. during January 

and February 2000, which was 0.17 percent of the 

December 1, 1999, through February 14, 2000, 

application. 

The total chlorpyrifos load in the San Joaquin 

River near Vernalis during the two target storms in 

January and February 2001 was 0.410 lb a.i. This 

compares with 2.17 lb a.i. transported in two storms in 

January and February 2000. The main sources of 

chlorpyrifos during the two target storms in 2001 were 

Tuolumne River Basin (27 percent), Merced River at 

River Road (20 percent), San Joaquin River near 

Stevinson (12 percent), Stanislaus River at Caswell 

State Park (8.3 percent), and Orestimba Creek at River 

Road (3.4 percent). The total chlorpyrifos load in the 

San Joaquin River near Vernalis during January and 

February 2001 was 2.17 lb a.i. and was 0.02 percent of 

the December 1, 2000, through the February 24, 2001, 

application. This compares with the total chlorpyrifos 

load of 5.68 lb a.i. during January and February 2000, 

which was 0.16 percent of the December 1999 through 

February 14, 2000, application. 

The McHenry storm drain in Modesto was 

sampled nine times (hourly) during the January 23–26, 

2001, storm. Composite precipitation samples were 

collected at eight sites during this same storm—four 

sites in the urban area and four sites in the outlying 

agricultural areas. Every sample collected from the 

McHenry storm drain during the January 23–26 storm 

event exceeded the CMC for diazinon; and seven of the 

nine samples exceeded the CMC for chlorpyrifos. All 

composite precipitation samples exceeded the CMC for 

both diazinon and chlorpyrifos. 

The average diazinon concentration from the two 

precipitation sites in the McHenry storm drain basin 

(sites E and G), accounted for 68 percent of the flowweighted average diazinon concentration in McHenry 

storm drain. Chlorpyrifos, however, had an average 

concentration in the precipitation that was over 2.5 

times higher than what was detected in the McHenry 

storm drain. The contribution of the diazinon and 

chlorpyrifos concentrations in the precipitation to the 

concentrations in McHenry storm drain seems to be 

dependent on the physical and chemical properties of 

each pesticide. Diazinon is much more soluble than 

chlorpyrifos, which was reflected in the higher 

concentrations in McHenry storm drain. Chlorpyrifos 

has a greater tendency to sorb to suspended material 

and surfaces. No firm conclusions can be made on the 

basis of one storm event, but it appears that pesticides 

in precipitation significantly contribute to pesticide 

loads observed in runoff. However, a more detailed 

analysis of pesticide use before a sampling event is 

needed. The sampling of atmospheric deposition 

continued during 2002–2003 at six sites in the San 

Joaquin River Basin, including sites E and F.

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