Occurrence, Distribution and Transport of 

Pesticides, Trace Elements and Selected 

Inorganic Constituents into the Salton Sea 

Basin, California, 2001−2002 

By Lawrence A. LeBlanc, Roy A. Schroeder, James L. Orlando, and Kathryn M. 

Kuivila

Prepared in cooperation with the California State Water Resources Control Board 

Scientific Investigations Report 2004-5117

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

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

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

LeBlanc, L.A., Schroeder, R.A., Orlando, J.L., and Kuivila, K.M., 2004, Occurrence, Distribution and Transport of Pesticides, Trace Elements and Selected Inorganic Constituents into the Salton Sea Basin, California, 2001-2002: U.S. 

Geological Survey Scientific Investigations Report 2004-5117, 40 p.
iii

Contents

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

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

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

Hydrogeologic Setting .............................................................................. 3

Previous Studies .................................................................................... 3

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

Pesticide-Use Database and GIS Methods ............................................................ 5

Sample Collection Methods .............................................................................. 6

Large-Volume Water Samples ..................................................................... 6

Isolation of Suspended Sediments ............................................................... 6

Collection of Samples for Dissolved Pesticide Analysis ....................................... 6

Ultrafiltration and Colloid Isolation ............................................................... 8

Ultrafilter Calibration .............................................................................. 8

Bed Sediment ....................................................................................... 9

Laboratory Methods .................................................................................... 9

Solid-Phase Extraction of Water Samples ...................................................... 9

Liquid-Liquid Extraction of Ultrafiltered Water: Retentate and Filtrate Fractions ......... 11

Dissolved Organic Carbon Analysis ............................................................... 11

Sediment Analysis ................................................................................. 12

Method Validation ........................................................................... 12

Quality Assurance/Quality Control ......................................................... 13

Instrumental Analysis .............................................................................. 13

Sediment Organic Carbon Analysis ............................................................... 16

Trace Elements and Selected Inorganic Constituents .......................................... 16

Analysis Results and Discussion ........................................................................ 16

Field Parameters, DOC, and Streamflows ...................................................... 16

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

Suspended-Sediment Concentration, Particle Size, and Organic Carbon Content ......... 19

Pesticides Associated With Suspended Sediments .......................................... 24

Organic Carbon and Pesticides Associated With Bed Sediments ........................... 24

Distribution of Pesticides between Water, Suspended Solids, and Bed Sediments ...... 30

Distribution of Pesticides between Colloidal and Truly Dissolved Fractions ............... 34

Metals and Other Inorganic Constituents on Suspended Solids .............................. 34

Summary and Conclusions .............................................................................. 38

References cited .......................................................................................... 38
iv

Figures

Figure 1. Map showing location of study area and of sampling sites in the 

Salton Sea Basin, California .................................................................. 2

Figure 2. Graphs showing daily mean discharge during three sampling periods and 

42-year daily mean discharge for the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California .................................................................. 4

Figure 3. Graphs showing dissolved concentrations of selected pesticides at the 

outlet sites of the New, Alamo, and Whitewater rivers during (A) Fall 2001, 

(B) Spring 2002, and (C) Fall 2002 ............................................................ 20

Figure 4. Graphs showing dissolved concentrations of representative current-use 

pesticides at three sampling sites along the (A) Alamo, (B) New, and (

C) Whitewater rivers in the Salton Sea Basin, California, Fall 2001 ..................... 21

Figure 5. Graphs showing pesticides concentrations on suspended sediment from the 

outlet, near-shore, and off-shore sampling sites in the Salton Sea Basin, in the 

(A) Alamo, (B) New, and (C) Whitewater rivers in fall 2001, spring 2002, and 

fall 2002 ....................................................................................... 22

Figure 6. Graphs showing pesticides concentrations on selected suspended sediments 

from the Alamo River outlet and near-shore sites in the Salton Sea Basin, 

California, in (A) fall 2001, (B) Spring 2002, and (C) fall 2002 and from the New River 

outlet and near-shore site .................................................................. 29

Tables

Table 1. Pesticides applied within the Salton Sea Basin, California, April 2000–October 2001, 

that were included in the water and sediment analyses ................................. 5

Table 2. Pesticide sampling sites in the Salton Sea Basin, California ........................... 7

Table 3. Retention and mass balance estimates of vitamin B12 in deionized water and 

dissolved organic carbon in samples from the New River in the Salton Sea Basin, 

California, September 2002 .................................................................. 9

Table 4. Current-use pesticide analytes and quantification and confirmation ions monitored 

during gas chromatography/mass spectrometry analysis of water samples from 

sampling sites in the Salton Sea Basin, California ....................................... 10

Table 5. Method detection limits for pesticides in water samples from sampling sites in the 

Salton Sea Basin, California, 2001 ......................................................... 11

Table 6. Method detection limits for pesticides and DDTs in sediments from sampling 

sites in the Salton Sea Basin, California ................................................... 13

Table 7. Average recovery of pesticides used in the Salton Sea Basin, California, spiked 

to bottom sediment from an agricultural drain along the Sacramento River in 

northern California ........................................................................... 14

Table 8. Organochlorine analytes, internal and surrogate standards, and corresponding 

ions monitored for quantification and confirmation purposes ........................... 15

Table 9. Run conditions for the Saturn 2000 gas chromatography/mass spectrometry 

(GC/MS) system .............................................................................. 15

Table 10. Water parameters measured at sampling sites in the Salton Sea Basin, California, 

during fall 2001, spring 2002, and fall 2002 ................................................ 17

Table 11. Pesticide concentrations in water samples collected from sampling sites 

at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, California, 

fall 2001 ....................................................................................... 17
v

Table 12. Pesticide concentrations in water samples collected from sampling sites 

at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, California, 

spring 2002 .................................................................................... 18

Table 13. Pesticide concentrations in water samples collected from sampling sites 

at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, California, 

fall 2002 ....................................................................................... 18

Table 14. Suspended-sediment concentrations and percentage of fine-grained materials 

in water from sampling sites at the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California, during fall 2001, spring 2002, and fall 2002 ............... 23

Table 15. Percentage of organic carbon in suspended sediments and bed sediments 

from sampling sites at the Alamo, New, and Whitewater rivers in the Salton Sea 

Basin, California, during fall 2001, spring 2002, and fall 2002 ........................... 25

Table 16. Pesticide concentrations in suspended and bed sediments collected from 

sampling sites at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, 

California, fall 2001 ........................................................................... 26

Table 17. Pesticide concentrations in suspended and bed sediments collected from 

sampling sites at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, 

California, spring 2002 ........................................................................ 27

Table 18. Pesticide concentrations in suspended and bed sediments collected from the Alamo, 

New, and Whitewater rivers in the Salton Sea Basin, California, fall 2002 ............ 28

Table 19. Pesticides detected in water, suspended sediments, and bed sediments in the 

Salton Sea Basin, California, and key physical-chemical descriptors ............... 31

Table 20. Definition of terms used in equilibrium portioning calculations ........................ 32

Table 21. Example calculation of an aqueous concentration of chlorpyrifos, based on 

equilibrium partitioning and measured values of suspended sediment organic 

carbon and chlorpyrifos concentration from the Alamo River outlet site in the 

Salton Sea Basin, California, fall 2001 ...................................................... 32

Table 22. Predicted and measured aqueous concentrations in suspended sediments and 

bed sediments from the outlet and near-shore sites of the Alamo, New, and 

Whitewater rivers in the Salton Sea Basin, California, fall 2001 ........................ 33

Table 23. Percent distribution of pesticides between colloidal (retentate) and dissolved 

(filtrate) fractions of ultrafiltered water from the New River outlet and off-shore 

sites, Salton Sea Basin, California, September 2002 .................................... 35

Table 24. Pesticide concentrations in the combined fractions of ultrafiltered water 

extracted by liquid-liquid partitioning (40L) to solid-phase extraction extracted 

water (1L), New River outlet and off-shore sites in the Salton Sea Basin, 

California, fall 2002 ........................................................................... 35

Table 25. Element concentrations in suspended sediment from samples from the Alamo, 

New, and Whitewater rivers in the Salton Sea Basin, California, fall 2001 ............ 36

Table 26. Element enrichment ratios for suspended sediment from the Salton Sea Basin, 

California, fall 2001 ........................................................................... 37
vi

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

square kilometer (km2) 247.1 acre

Volume

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 61.02 cubic inch (in3) 

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram

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

°C=(°F-32)/1.8

DATUM

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD27)

Conversion Factors, Datum, and Abbreviations
vii

Abbreviations

CDPR California Department of Pesticide Regulation

GC gas chromatography

GC/MS gas chromatography/mass spectrometry

GIS Geographic Information System

GPC/HPLC gel permeation/high pressure liquid chromatography

MASE microwave-assisted solvent extraction

m/z mass/charge ratio

NWIS National Water Information System

NWQL National Water Quality Laboratory

psi pounds per square inch

rpm revolutions per minute

RWQCB Regional Water Quality Control Board

SIM selected ion monitoring

SIS selected ion storage

SPE solid phase extraction

TMDL total maximum daily load

UC University of California

USGS U.S. Geological Survey

BHC benzenehexachloride

C carbon

CO2 carbon dioxide

DDE dichlorodiphenyldichloroethylene

DDD dichlorodiphenyldichloroethane

DDT dichlorodiphenyltrichloroethane

DOC dissolved organic carbon

EPTC eptam

N nitrogen

N2 nitrogen gas

NPOC non-purgeable organic carbon

OC organic carbon

PAH polyclic aromatic hydrocarbon

TOC total organic carbon

UV/Vis ultraviolet/visible

cc cubic centimeter

dpm degrees per minute
viii

ev electronvolts

L liter

L/min liter per minute

mL milliliter

mL/min milliliter per minute

mg milligram

mg/kg milligram per kilogram

mg/L milligram per liter

mm millimeter

ng nanogram

ng/g nanogram per gram

ng/L nanogram per liter

ng/μL nanogram per microliter

nm nanometer

μamps microamperes

μL microliter

μm micrometer

μS/cm (μS cm-1) microsiemens per centimeter
Occurrence, Distribution, and Transport of Pesticides, 

Trace Elements, and Selected Inorganic Constituents into 

the Salton Sea Basin, California, 2001−2002 

By Lawrence A. LeBlanc, Roy A. Schroeder, James L. Orlando, and Kathryn M. Kuivila 

Abstract

A study of pesticide distribution and transport within the 

Salton Sea Basin, California, was conducted from September 

2001 to October 2002. Sampling for the study was done along 

transects for the three major rivers that flow into the Salton 

Sea Basin: the New and Alamo Rivers at the southern end 

of the basin and the Whitewater River at the northern end. 

Three stations were established on each river: an outlet station approximately 1 mile upstream of the river discharge, a 

near-shore station in the river delta, and off-shore station in the 

Salton Sea. Water and suspended and bed sediments were collected at each station in October 2001, March−April 2002, and 

September 2002, coinciding with peak pesticide applications 

in the fall and spring. 

Fourteen current-use pesticides were detected in the 

water column. Concentrations of dissolved pesticides typically decreased from the outlets to the sea in all three rivers, 

consistent with the off-shore transport of pesticides from the 

rivers to the sea. Dissolved concentrations ranged from the 

limits of detection to 151 nanograms per liter (ng/L); however, diazinon, eptam (EPTC), and malathion were detected at 

much higher concentrations (940–3,830 ng/L) at the New and 

Alamo River outlet and near-shore stations. Concentrations of 

carbaryl, dacthal, diazinon, and eptam were higher during the 

two fall sampling periods, whereas concentrations of atrazine, 

carbofuran, and trifluralin were higher during the spring. 

Current-use pesticides also were detected on suspended and 

bed sediments in concentrations ranging from method detection limits to 106 ng/g (nanograms per gram). Chlorpyrifos, 

dacthal, eptam, trifluralin, and DDE were the most frequently 

detected pesticides on sediments from all three rivers. The 

number and concentrations of pesticides associated with 

suspended sediments frequently were similar for the river 

outlet and near-shore sites, consistent with the downstream 

transport of sediment-associated pesticides out of the rivers. Seasonal trends in pesticide concentration were similar 

to those for dissolved concentrations in fall 2001 and spring 

2002, but not in fall 2002. Generally, the pesticides detected in 

the suspended sediments were the same pesticides detected in 

the bed sediments, and concentrations were similar, especially 

at the Alamo River outlet site in spring 2002 and fall 2002. 

Pesticides generally were not detected in sediments from the 

off-shore sites; however, the samples from these sites also 

had greater incidences of matrix interference during analysis. 

Sediment-associated pesticide concentrations were above 

equilibrium in water, suggesting a bound fraction of sedimentassociated pesticides that are resistant to desorption. Concentrations of trace elements and other inorganic constituents in 

suspended sediments collected during the fall 2001 followed 

expected trends with dilution of river-derived minerals owing 

to highly organic autochthonous production within the Salton 

Sea Basin. However, calculation of enrichment ratios provided 

evidence for the bioconcentration of several trace elements, 

notably selenium in the off-shore biota.

Introduction

Several studies by the U.S. Geological Survey (USGS) 

and others since the late 1960s identified elevated concentrations of agriculture-related chemicals and their metabolites in 

waters adjacent to the Salton Sea area (Irwin, 1971; Eccles, 

1979; Setmire, 1984; Spencer and others, 1985; Setmire and 

others, 1993, Crepeau and others, 2002). Elevated levels 

(parts per billion) of organochlorine and organophosphorous 

pesticides also were reported in these waters. Organochlorine pesticides were reported in bottom material collected 

from irrigation drainage canals (Setmire and others, 1993). 

More recently, Crepeau and others (2002) reported elevated 

concentrations of a number of current-use pesticides in water 

collected in 1996 and 1997 from the Alamo River and from 

the Salton Sea. The elevated pesticide concentrations in the 

Alamo River correspond with acute toxicity measured during 

standardized 96-hour aquatic toxicity tests done by deVlaming 

and others (2000). 
2 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Figure 1. Location of study area and of sampling sites in the Salton Sea Basin, California. 

2 

Recent requirements for the Regional Water Quality 

Control Board (RWQCB), Colorado River Region, to develop 

total maximum daily loads (TMDLs) for contaminants, 

including pesticides, pointed to a need for a more complete 

understanding of the occurrence and transport of pesticides 

within the Salton Sea Basin. In addition, proposed remedial 

actions, such as the Salton Sea Restoration Project (a federalstate-local endeavor), and proposed water transfers to the San 

Diego County Water Authority, which may influence pesticide 

concentrations in rivers and drains that discharge into the 

Salton Sea and potentially within the sea itself, also indicate a 

need for a better understanding of the occurrence and transport 

of pesticides within the basin. Water transfers may lead to a 

shrinking of the sea, exposing more of the shoreline which 

consists of potentially contaminated sediment. The RWQCB 

requested the USGS to conduct a study to examine the distribution of pesticides in water and on suspended sediments and 

bed sediments in the rivers that drain into the Salton Sea, as 

well as within the sea itself. As a part of the study, the USGS 

analyzed trace elements and other selected inorganic constituents in suspended sediments.

Purpose and Scope

The primary objective of this study was to determine 

the occurrence and transport of historical and current-use 

pesticides in the Salton Sea Basin. A major focus of the study 

was to examine the distribution of pesticides in water, on 

suspended sediments and bed sediments in the three rivers that 

drain into the Salton Sea, and in the sea itself. Three sampling 

sites were established on each river in a transect beginning 

at the river outlet and extending into the sea (fig. 1). Water, 

suspended sediments, and bed sediments were collected at 

each site.

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Introduction 3

This report presents the results for three sampling periods: October 2001, March and April 2002, and September 

2002. Included are descriptions of sample collection and laboratory analytical methods for pesticide analysis, along with 

concentrations of current-use and organochloride pesticides in 

water, suspended sediment, and bed sediment. Pesticide-use 

data for 2001 for the State of California (California Department of Pesticide Regulation, 2001) are also presented. 

Suspended sediment concentration and the percentage of suspended sediment in the fine-grained fraction (less than 62 μm 

in diameter) were measured in all water samples. Dissolved 

organic carbon (DOC) was measured in water samples collected in October 2001. In September 2002, water from 

two sites on the New River were passed through an ultrafilter 

and both retentate (colloid) and filtrate fractions were analyzed 

for current-use pesticides. Organic carbon (OC) was measured 

in all suspended and bed sediments for all sampling periods. 

Trace elements and selected inorganic compounds were 

analyzed in suspended sediments collected during fall 2001.

Previous Studies

The earliest study of pesticides in the Salton Sea Basin 

include analysis of DDT in the Alamo and New Rivers in 

samples collected monthly between August 1969 and June 

1970 (Irwin, 1971). High concentrations were found in the 

river water because DDT was still being used in the United 

States and Mexico. DDT has not been detected in the water 

since 1986 (Setmire and others, 1990), 14 years after its use 

was banned in the United States and 3 years after its use was 

banned in Mexico. However, the metabolite DDE was still 

present in sediments from both rivers, and concentration 

gradients (decreasing concentration downstream) were similar 

to that of the aqueous DDT concentrations from 20 years prior 

(Schroeder, 1996). This was taken as strong evidence that 

DDT and (or) its metabolites persist in surrounding agricultural soils and continue to be transported to the agricultural 

drains and rivers through agricultural tailwater. Concentrations of DDE have been reported in sediments from rivers and 

drains in the Salton Sea Basin (Schroeder and others, 1988). 

Residues of DDE have also been reported recently in biota 

from this area (Schroeder and others, 1993; Setmire and 

others, 1993). 

Hydrogeologic Setting

The study area is in southeastern California; it includes 

the Salton Sea and the Coachella and Imperial Valleys adjacent to the north and south sides of the sea (fig. 1). This is an 

area of intensive agriculture; various crops are grown, including alfalfa, grapes, dates, cotton, and a variety of vegetables. 

The Salton Sea is a large (1,000 km2) shallow (mean 

depth = 8 m) saline lake; its shoreline is at an elevation of 

about 70 m below sea level. The sea was formed as a freshwater lake between 1905 and 1907 as the result of uncontrolled flooding from the Colorado River. The water level in 

the Salton Sea has been maintained largely by agricultural 

irrigation drainage, which presently is about 1.7 km3 per year 

(Schroeder and others, 2002). The largest rivers that flow into 

the sea, in order of their annual discharge, are the Alamo, New, 

and Whitewater Rivers. Forty-two year median daily flows 

ranged from 451 to 1,140 cubic feet per second (ft3/s) for the 

Alamo River, 456 to 758 ft3/s for the New River, and 81 to 120 

ft3/s for the Whitewater River (fig. 2) (U.S. Geological Survey, 

2001). The Alamo and New Rivers flow into the southern end 

of the basin and the Whitewater River flows into the northern 

end (fig. 1). The Salton Sea has no outlet and minimal groundwater interaction (Schroeder and others, 2002). Because it is 

located in an arid region (precipitation is about 7 cm per year) 

with high evapotranspiration (approximately 1.8 m per year) 

and because freshwater flows consist of a high percentage of 

agricultural return water (water high in dissolved salts), salinity has been steadily increasing over time. The present salinity 

in the Salton Sea measured as conductivity is approximately 

55,000 μS/cm; this is roughly equivalent to 45 parts per 

thousand (Setmire and Schroeder, 1998; Schroeder and others, 

2002), which is about 1−3 times that of ocean water.

The Alamo and New Rivers receive small amounts of 

treated sewage from towns in the Imperial Valley. The New 

River also receives a large amount of wastewater and potentially untreated sewage from the relatively large municipality of Mexicali before the river crosses over into the United 

States. In addition to the issues of bacteria and pesticides in 

water from treated and untreated sewage, other issues of concern in the Salton Sea Basin include elevated concentrations of 

metals and nutrients (Setmire and others, 1993, Schroeder and 

others, 2002).

At their outlets to the Salton Sea, the Alamo and New 

Rivers are approximately 1 m deep and suspended sediment 

concentrations are a few hundred milligrams per liter. Suspended-sediment data in the USGS National Water Information System (NWIS) consist of four measurements from 

1988−89 and ranged from 389 to 849 mg/L for the Alamo 

River and 351 to 613 mg/L for the New River (U.S. Geological Survey, 2001). Deposition of high loads of suspended 

sediment delivered by the rivers has resulted in the formation 

of broad shallow deltas, especially at the mouths of the New 

and Alamo Rivers. These shallow areas harbor large numbers 

of fish and birds and are adjacent to federal and state wildlife 

refuges.
4 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Figure 2. Daily mean discharge during three sampling periods and 42-year daily mean discharge for the Alamo, 

New, and Whitewater rivers in the Salton Sea Basin, California. 

4 

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Table 1. Pesticides applied within the Salton Sea Basin, 

California, April 2000–October 2001, that were included in the 

water and sediment analyses. 

[All values are in pounds (lbs.). Source: California Department of Pesticide 

Regulation (2001). Samples were also analyzed for the following pesticides, 

which had not been applied in the Salton Sea Basin during the study period: 

azinphos-methyl, butylate, diethylatyl-ethyl, fonofos, hexazinone, methidathion, monlinate, pebulate, sulfotep, and thiobencarb]

Pesticide

Pesticide application periods

November 

2000–October 

2001

November 

2000–March 

2001

April 2000–

October

2001

Alachlor 91 91 0

Atrazine 19,208 14,108 5,100

Bifenthrin 423 23 400

Carbaryl 3,337 432 2,905

Carbofuran 8,909 8,828 81

Chlorpyrifos 101,368 27,320 74,048

Cycloate 6,011 2,074 3,937

Cyfluthrin 4,690 2,352 2,338

Cypermethrin 1,734 1,091 643

Dacthal 42,799 5,361 37,438

Diazinon 39,280 8,392 30,888

EPTC (eptam) 106,569 41,560 65,009

Esfenvalerate 3,286 552 2,734

Ethafluralin 126 126 0

lambda Cyhalothrin 2,550 1,097 1,453

Malathion 122,730 91,950 30,780

Methyl parathion 15 0 15

Metolachlor 228 0 228

Napropamide 1,362 346 1,016

Oxyfluorfen 5,312 3,526 1,786

Pendimethalin 28,770 17,960 10,810

Permethrin 10,961 4,461 6,500

Phosmet 138 63 75

Prometryn 953 640 313

Simazine 126 124 2

Trifluralin 277,604 211,600 66,004

Numerous studies by the USGS during the last 30 years 

have documented the presence of organochlorine and currentuse pesticides in water. Bimonthly monitoring for a period 

of 1 year in 1977−78 by the USGS showed that the temporal 

pattern of concentrations for many pesticides in drains and 

rivers in the Imperial Valley matched the seasonal pattern of 

pesticide application in the valley, with the maximum concentrations occurring in the late winter and early spring and 

again in the early fall (Eccles, 1979). The concentrations of 

pesticides in samples collected by the USGS in March through 

April 1992 and again in 1995−96 exhibited the same temporal pattern (Roy Schroeder, U.S. Geological Survey, [retired] 

written commun., 2002). Monthly monitoring on the Alamo 

River in 1994–95 by the University of California (UC) at 

Davis confirmed the same temporal pattern and identified five 

commonly used pesticides (carbaryl, carbofuran, chlorpyrifos, 

diazinon, and malathion) as the cause of high mortality of 

Ceriodaphnia dubia during standardized aquatic toxicity 

tests (de Vlaming and others, 2000). A 1996–98 study by the 

USGS, in collaboration with the UC Davis Toxicology Laboratory, identified elevated levels (greater than 100 ng/L) of 

nine pesticides in water from the Alamo River 1 mi upstream 

from the discharge point at Garst Road, as well in water from 

Acknowledgments

The authors thank G. Edward Moon, Patricia von Puhl, 

and Patrick Nicholas for help with sampling and sample processing in the field, and Patrick Nicholas and Lupe Herrerra 

for help with sample preparation in the laboratory. The authors 

also thank Robert Gale and Bill Foreman for helpful discussions concerning the sediment multi-residue method and Bill 

Foreman for helpful advice concerning the gel permeation 

chromatography cleanup step of the sediment procedure. 

Finally, thank you to Charlie Pelizza and the staff of the Sonny 

Bono National Wildlife Refuge for the generous use of their 

facilities and equipment, including the 17-ft Boston Whaler 

used for sampling.

Pesticide-Use Database and GIS 

Methods

For the purpose of defining pesticide use in the Salton 

Sea watershed, data from the California Department of Pesticide Regulation’s (CDPR) Pesticide Use database (California 

Department of Pesticide Regulation, 2001) were compiled in a 

Geographic Information System (GIS). The Salton Sea watershed, derived from the 12-digit hydrologic unit boundaries 

for California, comprises all areas that drain naturally into the 

Salton Sea. Records of registered pesticide use for 2001 were 

extracted from the CDPR Pesticide-Use database (California 

Department of Pesticide Regulation, 2001) using the limits of 

the watershed boundary, which includes portions of Imperial 

and Riverside Counties. The data was reported as total 

pesticide use for 2001 (table 1).

Pesticide-Use Database and GIS Methods 5
6 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Sample Collection Methods

Transects consisting of three sampling sites each for the 

three major rivers in the Salton Sea Basin were established 

to examine the transport of pesticides (fig. 1). The outlet 

sites were established approximately 1.6 km upstream of the 

mouths of the New and Alamo Rivers and 6 km upstream of 

the mouth of the Whitewater River. The near-shore sites were 

established close to the river, and the off-shore sites were 

established in water 15-ft deep. Samples collected during fall 

2002 at the New River near-shore site were taken from above 

and below the halocline. The halocline, which was easily 

visible, is the sharp transition zone between the fresh, less 

dense river water flowing above the more dense, saltier Salton 

Sea water. The coordinates for each site are listed in table 2. 

Water temperature and conductivity were measured using a 

handheld digital thermometer (VWR Scientific, Westchester, 

Pa.) and an Orion conductivity meter (Orion Instruments, 

San Jose, Calif.), respectively. Fifty mL water samples were 

collected during the pumping of the large volume water 

samples (described in the section entitled “Large Volume 

Water Samples”) and sent to the organic chemistry laboratory 

at the USGS California District office in Sacramento for more 

accurate measurements of pH and conductivity. 

Large-Volume Water Samples

Large-volume water samples (for isolating suspended 

sediments) and bed sediment samples were collected at the 

nine sampling sites in the Salton Sea and adjacent rivers 

(fig. 1). Samples were collected using a large peristaltic pump 

powered by a portable generator and equipped with a stainless 

steel and Teflon inlet hose. Sample water was pumped directly 

into pre-cleaned 20 L stainless-steel containers for transport. 

The volume of the water samples varied for each sampling site 

but generally were greater than 300 L. One-liter aliquots were 

taken from the large-volume water samples for analysis of dissolved pesticides. The collection of samples from the off-shore 

sites required multiple boat trips, but sampling always was 

completed within a 24-hour period.

Samples from the three outlet sites were collected by 

pumping water samples into the stainless-steel containers 

from bridges and a culvert. The samples were collected from 

a single point at the center of the river channel using a pump 

inlet hose suspended at a depth of 0.5 m. The six near-shore 

and off-shore sites were accessed by boat and located using a 

handheld global positioning system device (Garmin GPS 12, 

Garmin International Inc., Olathe, Kans.). Samples from the 

three near-shore delta sites were collected at a depth representing the mixing zone between fresh river water and more saline 

sea water, which was determined at the time of collection 

using a handheld specific conductance meter. Samples from 

the deeper water off-shore sites were collected from a depth of 

approximately 0.5 m below the surface.

Additional samples were collected to measure suspendedsediment concentrations and percent fine-grained sediments. 

For these samples, water was pumped into pre-cleaned 

500-mL clear glass bottles during the large-volume sample 

collection.

Isolation of Suspended Sediments

Suspended sediments were isolated by pumping the 

water through a flow-through centrifuge (Westphalia model 

KA-2, Westfalia Corporation, Odele, Federal Republic of 

Germany). Water (300–900 L) was pumped at a flow rate of 

2 L/min through the centrifuge, which spun at 9,800 revolutions per minute (rpm) and operated at a centrifugal force of 

9,500 times the gravitational constant, g. A 2-L/min flow-rate 

has been shown to be an optimal flow rate for efficient capture 

of a wide variety of grain sizes (2 μm and larger) at a wide 

range of suspended sediment concentrations (Horowitz and 

others, 1989). Sediment was carefully removed from each 

of the concentric centrifuge bowls. Water remaining in the 

bowls was used to rinse the sediment off the surfaces of all 

the bowls. The resulting sediment slurry was stored refrigerated at 4°C in pre-cleaned glass bottles and transported to the 

organic chemistry laboratory at the USGS California District 

Office Sacramento for further dewatering. In the laboratory, 

the suspended material was futher dewatered by centrifuging 

in 200-mL stainless-steel centrifuge bottles for 20 minutes at 

10,000 rpm in a high-speed refrigerated centrifuge (Sorvall 

RC-5B centrifuge, Du Pont Company, Wilmington, Del.). The 

water was decanted and the remaining solid material scraped 

out of the centrifuge bottles and stored frozen in pre-cleaned 

glass screw-cap bottles until analysis.

Collection of Samples for Dissolved 

Pesticide Analysis

Samples for analysis of dissolved pesticides were collected in 1 L amber glass bottles from the outlet of the Westfalia flow-through centrifuge. The Westphalia centrifuge has 

been found to retain particles of 0.5 μm and greater (Horowitz 

and others, 1989). A similar centrifuge described by Rees and 

others (1991) retained particles of 0.3 μm and greater. Because 

of the large amount of water filtered for suspended sediments 

(300–900 L), the samples were collected during the beginning, 

middle, and end of the centrifuging process. There appeared 

to be no difference between these replicate samples in terms 

of the pesticides detected, or their relative concentrations 

(see discussion of replicate variability in the section entitled 

“Solid-Phase Extraction of Water Samples” in the “Methods” 

section). 
Table 2. Pesticide sampling sites in the Salton Sea Basin, California. 

[See figure 1 for location of sampling sites. Horizontal datum is North American Datum of 1927 (NAD27). USGS, U.S. Geological Survey. m, meter; km, kilometer]

Station name Latitude Longitude USGS station identifier USGS site name Description

Alamo River outlet 33°11′56′′ 115°35′46′′ 10254730 Alamo River near Niland Bridge at Garst RoadAlamo River near-shore 33°12′42′′ 115°37′14′′ 331242115371401 Salton Sea near mouth of Alamo River Within Salton Sea, approximately 0.25 km from 

river mouth

Alamo River off-shore 33°14′00′′ 115°38′00′′ 331400115380001 Salton Sea in Alamo River Delta Within Salton Sea approximately, 2.9 km from 

river mouth

New River outlet 33°05′59′′ 115°38′56′′ 33055911585601 New River at Lack Road near Calipatria Bridge at Lack RoadNew River near-shore 33°08′03′′ 115°41’40′′ 330803115414001 Salton Sea near mouth of New River Within Salton Sea, approximately 100 m from 

river mouth

New River off-shore 33°08′35′′ 115°43′45′′ 330835115434501 Salton Sea in New River Delta Within Salton Sea, approximately 3.5 km from 

river mouth

Whitewater River outlet 33°31′29′′ 116°04′36′′ 10259540 Whitewater River near Mecca Culvert at Lincoln RoadWhitewater River near-shore 33°30′06′′ 116°03′15′′ 333006116031501 Salton Sea near mouth of Whitewater River Within Salton Sea, approximately 500 m from 

river mouth

Whitewater River off-shore 33°29′58′′ 116°02′35′′ 332958116023501 Salton Sea in Whitewater River Delta Within Salton Sea, approximately 1.5 km from 

river mouth

Sample Sollection Methods 7
8 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Over the course of the study, a few samples were collected in a manner different from that described above. 

Samples were collected at the New River near-shore site in 

fall 2002; the samples were collected from above and below 

the halocline to compare pesticide abundances from rivertransported water (above the halocline) with those in Salton 

Sea water (below the halocline). Normally, the near-shore 

water samples were collected at the freshwater/saltwater interface, where the pump hose continuously moved up and down 

through the halocline with the motion of the boat.

Ultrafiltration and Colloid Isolation

During the fall 2002 sampling period, water from the 

New River near-shore and off-shore sites was passed through 

an ultrafiltration apparatus consisting of an Amicon spiralwound cartridge filter (regenerated cellulose fiber with an 

area of 3.05 m2 and an effective molecular weight cutoff 

of 1,000 daltons) and an Amicon DC-10 centrifugal pump 

(Amicon Corporation, Beverly, Mass.). The water, which 

was passed through the flow-through centrifuge to remove 

suspended sediment, was separated into retentate and filtrate 

portions to determine colloidally bound (retentate) and ‘truly 

dissolved’ (filtrate) pesticide pools. Approximately 40 L of 

effluent from the flow-through centrifuge was pumped 

through the ultrafilter to yield approximately 2 to 4 L of 

retentate (the colloid fraction). The retentate (2–4 L) and a 

subsample of the filtrate (approximately 4 L) were preserved 

using 150 to 250 mL of methylene chloride and transported 

to the USGS California District organic chemistry laboratory 

in Sacramento. There, each sample was extracted three times 

with methylene chloride and the extracts were reduced and 

analyzed using gas chromatography/mass spectrometry 

(GC/MS) for current-use pesticides, as described below and 

in Crepeau and others (2000). 

Three samples were processed through the ultrafilter 

for analysis of pesticides in filtrate and retentate fractions. 

One sample was from the New River outlet site (basically 

fresh water) and one sample was from the considerably more 

saline New River off-shore site. To aid in the comparison of 

the outlet and the off-shore samples, sea salts (Instant Ocean, 

Aquarium Systems, Inc., Mentor, Ohio) were added to a second outlet sample so that the resulting conductivity matched 

the conductivity measured in the off-shore sample prior to 

ultrafiltration. This was done to examine how differences in 

salinity would effect the partitioning of pesticides between 

the colloidal and retentate fractions at the same site and with 

identical pesticide concentrations. Differences observed in 

partitioning between the river outlet and the off-shore samples greater than those observed between the salted and the 

unsalted outlet samples may be attributed to factors other than 

salinity. 

Ultrafilter Calibration

The Amicon filter was calibrated by determining the 

retention efficiency of a marker compound of a known size 

and shape. Vitamin B12, with a molecular size of 1,355 daltons, was added to 20 L of water and passed through the ultrafilter. Because the nominal cutoff given by the manufacturer 

is 1,000 daltons, it was expected that a large percentage of 

vitamin B12 would be retained by the filter if it was working 

properly. The retention efficiency of the filter was determined 

for deionized water with vitamin B12 and for deionized 

water with vitamin B12 and sea salts added. Concentrations 

of vitamin B12 were determined in the retentate and filtrate 

using ultraviolet/visible (UV/Vis) spectroscopy (Cary UV/Vis 

spectrophotometer, Varian Corporation, Walnut Creek, Calif.) 

at a wavelength of 360 nm. Samples of filtrate and retentate 

were monitored during the ultrafiltration. A mass balance was 

calculated by summing filtrate and retentate concentrations at 

the end of the filtration and comparing the mass balance value 

with the initial concentration.

Similar measurements and calculations were made for the 

three samples (New River outlet sample, New River off-shore 

sample, and the salt enhanced New River outlet sample) using 

dissolved organic carbon as a marker. DOC contains a spectrum of compounds, with a variety of molecular weights and 

sizes (some smaller than 1,000 daltons), and so would not be 

expected to be retained to the same degree as the vitamin B12. 

However, measuring DOC allowed us to confirm that a DOCrich (colloidal) fraction was successfully isolated, and massbalance calculations verified that no unexplained losses of 

DOC occurred as a result of the ultrafiltration. DOC concentrations were measured in the retentate and filtrate fractions of 

each sample as described in the next section. Ultrafiltrate and 

retentate DOC concentrations were summed and compared 

with DOC concentrations in whole water for mass balance 

calculations. Retention efficiciency was calculated by dividing 

the total mass of DOC in the retentate by the sum of DOC in 

retentate plus filtrate. 

Calibration of the Amicon filter resulted in 75 percent or 

better retention of vitamin B12, using both deionized water 

as well as deionized water with sea salts added (table 3). The 

mass balance between the filtrate and retentate was excellent, 

giving us further confidence that we could efficiently isolate 

dissolved organic material in the molecular weight range of 

1,000 daltons or larger. 
Laboratory Methods 9

Table 3. Retention and mass balance estimates of vitamin B12 

in deionized water and dissolved organic carbon in samples from 

the New River in the Salton Sea Basin, California, September 

2002. 

[DI, deionized. mg/L, milligram per liter]

Sample Retention

(percent)

Mass 

balance

(percent)

Concentration 

factor

Vitamin B12

Amicon filter, DI water 76 105 4.8

Amicon filter, DI water1 84 111 7.9

Amicon filter, DI water + sea salts 84 93 3.2

Amicon filter, DI water + sea salts1 75 87 6.4

Dissolved organic carbon (mg/L)

New River outlet 52 75 8.2

New River outlet + sea salts 22 120 15.2

New River off-shore 37 82 17.4

1

Duplicate sample.

Retention efficiency and mass balance estimates for DOC 

in the two water samples from the New River outlet and the 

one sample from the off-shore site are also shown in table 3. 

There was a difference in percent retention of organic carbon 

between the highly saline samples (New River outlet sample 

with salts added and the New River off-shore sample) and the 

outlet sample, which may be due to a salinity effect not replicated by the use of sea salts. Also, higher concentration factors 

used for the salt-enhanced outlet sample and the off-shore 

sample may have resulted in a decrease in retention efficiency 

of the dissolved organic material. The retention efficiency of 

low molecular weight DOC can be affected by higher concentrations factors (Guo and Santschi, 1996; Guo and others, 

2000). 

Adding salt to a second New River outlet sample, to 

approximate the salinity of the New River off-shore sample, 

did not provide useful information for interpreting the difference in partitioning between the two types of samples (highly 

saline off-shore sample and less saline river sample). Upon 

addition of the salt, a great deal of flocculation began to occur, 

which quite possibly removed both dissolved organic material 

and pesticides from the water. This was most likely due to the 

fact that several divalent ions were at near saturation concentrations in river water, a fact discussed by Schroeder and 

others (2002). 

Bed Sediment

Bed sediment samples were collected concurrently with 

the water samples at each of the nine sampling sites. Samples 

taken at the outlet sites were collected using either a 9-inch 

Ekman grab sampler or a 2-inch diameter, Teflon-barreled 

hand corer. Multiple grabs were required to obtain approximately 0.5 L for each site. Samples were collected only from 

the top 2 cm of undisturbed sediment collected during each 

grab. At the river sites, multiple core samples (5−7) were 

composited to make a sample. At the near-shore and off-shore 

sites, composites of 2 to 3 grabs were taken using the Eckman 

dredge. Sediment was scooped into cleaned, 0.5-L glass jars 

using a stainless-steel spoon. 

Laboratory Methods

Solid-Phase Extraction of Water Samples

Pesticides were extracted from water samples using solidphase extraction (SPE) cartridges. This method is described in 

detail in Crepeau and others (2000). Prior to sample extraction, the volume of water to be extracted was determined and 

100 μL of a 2 ng/μL surrogate (terbuthylazine) was spiked into 

the sample. The water then was pumped through a pre-cleaned 

and conditioned C8 SPE cartridge (Varian Bond-Elut, 500 mg, 

300 cubic centimeter size barrel, Varian Analytical Corporation, Walnut Creek, Calif.) using 12-volt ceramic-piston metering pumps at a rate of 20 to 25 mL/min. To condition the SPE 

cartridge, 3 mL of methanol followed by 3 mL of organic-free 

water was passed through the cartridge. The cartridge was 

then kept wet using water until the extraction step. After the 

extraction, excess water was removed from the cartridge by 

passing air through it using a luer-lock syringe. Cartridges 

were labeled and kept refrigerated; within 3 days of collection they were sent to the USGS California District organic 

chemistry laboratory in Sacramento for storage. Upon receipt 

of the samples in Sacramento, the cartridges were further dried 

using carbon dioxide (CO2) for at least 1 hour. Cartridges were 

stored frozen until analysis, which did not exceed the established holding time of 2 months (Crepeau and others, 2000).

Samples were eluted from the SPE cartridges using 9 mL 

of ethyl acetate, reduced in volume to 500 μL by evaporation 

using a stream of nitrogen gas (N-evap, Organomation Associates, Kansas City, Mo.) and appropriate concentrations of 

internal standards added. Deuterated polyclic aromatic hydrocarbon (PAH) compounds were used as internal standards and 

included d10-acenaphthene, d10-phenanthrene and d10-pyrene. 

The d subscript designation (for example, d10) refers to the 

number of labeled deuterium atoms incorporated into the 

compound. Extracts were brought to a final volume of 200 μL 

and analyzed using GC/MS as described in the “Instrumental 

Analysis” section. Full details of the extraction method are 

provided in Crepeau and others (2000).

Several quality-control procedures were implemented 

during this study. Samples processed for quality-control purposes included field and laboratory blanks, national laboratory 

sample replicates, and matrix spikes. 
10 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif. 10 

Table 4. Current-use pesticide analytes and quantification and 

confirmation ions monitored during gas chromatography/mass 

spectrometry analysis of water samples from sampling sites in 

the Salton Sea Basin, California. 

[See table 8 for internal and surrogate standards]

Pesticides Quantification

ions

Confirmation

ions

Alachlor 160 188, 146

Atrazine 200 202, 215

Azinphos-methyl 132 181, 160

Bifenthrin 181 166, 165

Butylate 146 156, 90

Carbaryl 144 115, 116

Carbofuran 164 149, 131

Chlorpyrifos 314 314, 197

Cyanazine 212 198, 172

Cycloate 154 155, 186

Dacthal 301 299, 303

Diazinon 179 137, 199

Diethylatyl-ethyl 188 160, 262

EPTC (eptam) 128 132, 127

Esfenvalerate 225 125, 167

Ethafluralin 276 316, 292

Fonofos 109 137, 110

Hexazinone 171 128, 172

lambda-Cyhalothrin 181 197, 141

Malathion 173 125, 173

Methidathion 145 93, 125

Methyl parathion 263 247, 246

Metolachlor 162 238, 240

Molinate 126 98, 127

Napropamide 100 125, 257

Oxyflurofen 300 300, 302

Pebulate 128 132, 127

Pendimethalin 252 191, 162

Permethrin 183 127, 163

Phosmet 160 133, 206

Piperonyl Butoxide 176 177, 149

Prometryn 184 241, 226

Simazine 201 186, 173

Sulfotep 322 238, 210

Thiobencarb 100 125, 257

Trifluralin 264 306, 248

Field equipment blanks consisted of 1-L samples of 

organic-free deionized water (as determined by the method of 

Bird and others [2003]) from the water-purification system at 

the California District organic chemistry laboratory in Sacramento. This water was passed through the centrifuge and 

collected in 1-L amber bottles and extracted using C8 SPE 

cartridges. In addition, a 1-L bottle of deionized water was 

left uncapped for several days to assess background pesticide 

concentrations in the field. For laboratory water blanks, 1 L 

of deionized water was extracted using a C-8 SPE cartridge. 

The sample was subsequently eluted and then analyzed using 

GC/MS. 

For a select number of sites (three per sampling trip), 

replicate water samples were collected, extracted using C18-

SPE cartridges and the same procedures as those used for the 

C8-SPE cartridges, and mailed to the USGS National Water 

Quality Laboratory (NWQL) in Denver, Colo., for analysis. 

This served as an independent check of our water analysis 

procedures. Sample handling and holding-time requirements 

of the NWQL were strictly adhered to.

For matrix spikes, a solution containing 400 ng of each 

of the analytes listed in table 4 was spiked to 1-L bottles of 

deionized water and processed using the SPE water extraction 

method. Three matrix spike samples were processed for each 

sampling period, for a total of nine matrix spike samples. 

Detection limits for dissolved pesticides were in the low 

nanogram per liter range (table 5). The average recoveries for 

pesticides spiked to the water matrix spike samples were 

85 ± 20 percent but generally were greater than 80 percent. 

Results of the analysis of replicate samples for fall 2001 

showed excellent agreement, within 10 percent (coefficient 

of variation). A comparison of pesticide concentrations in 

the nine replicate samples analyzed in Sacramento, Calif., 

and Denver, Colo., laboratories also showed excellent agreement, with concentrations often agreeing within 10 percent, 

and never exceeding 25 percent. Field or equipment blanks 

generally showed low detections of pesticides or no detection. However, low concentrations (1−2 ng/L) of diazinon and 

dacthal were discovered in two field blanks. The presence 

of background concentrations of a few pesticides was demonstrated during the fall 2002 sampling period by leaving a 

bottle of deionized water uncovered for several days during 

sample processing. Background concentrations of chlorpyrifos 

and dacthal (4 ng/L) and somewhat higher concentrations of 

EPTC, also known as eptam (30 ng/L), were detected in this 

sample. No pesticides were detected in the laboratory blanks 

processed at the USGS California District laboratory. 
Laboratory Methods 11

Table 5. Method detection limits for pesticides in water 

samples from sampling sites in the Salton Sea Basin, California, 

2001. 

Pesticides Method detection limit, 

in nanograms per liter

Alachlor1 2.1

Atrazine 4.2

Azinphos-methyl1 11.1

Butylate1 1.8

Carbaryl 4.2

Carbofuran 3.3

Chlorpyrifos 4.2

Cyanazine1 3.0

Cycloate 1.5

Dacthal 1.2

Diazinon 3.6

Diethatyl-ethyl1 3.6

EPTC (eptam) 4.5

Ethalfluralin1 2.4

Fonfos1 2.4

Hexazinone1 5.7

Malathion 2.1

Methidathion1 5.4

Methyl parathion1 4.2

Metolachlor1 3.3

Molinate1 2.7

Napropamide 7.2

Oxyfluorfen 4.2

Pebulate1 .6

Pendimethalin 2.4

Phosmet1 4.2

Piperonyl butoxide1 3.3

Simazine 6.9

Sulfotep1 1.2

Thiobencarb1 3.9

Trifluralin 3.0

1

Pesticide not detected in any sediment samples during any of the sampling 

periods.

Liquid-Liquid Extraction of Ultrafiltered Water: 

Retentate and Filtrate Fractions

Pesticides were isolated and concentrated from retentate 

and filtrate fractions of the ultrafiltered water by liquid-liquid 

extraction, using methylene chloride. The samples, which contained 200 to 250 mL methylene chloride, were poured into 

separatory funnels, and the terbuthylazine surrogate standard 

was added. Additional methylene chloride was added to the 

separatory funnels to bring the methylene chloride volume 

to 10 percent of the sample water volume; the samples were 

shaken for approximately 3 minutes. The methylene chloride 

was drawn out of the bottom of the separatory funnel and the 

process repeated twice more. The extracts were combined and 

dried over sodium sulfate. The volumes of the extracts then 

were reduced by rotary evaporation (Yamato Rotory Evaporator, Yamato Scientific Co, Japan) to approximately 10-mL, 

transferred to 10 mL condensor tubes and further reduced in 

volume to 500 μL by nitrogen evaporation. Deuterated PAH 

internal standards were added, and the samples were reduced 

to a final volume of 200 μL prior to injection into the GC/MS.

Dissolved Organic Carbon Analysis

Water samples were filtered using 2.7 μm (Whatman 

GF/A, Whatman Scientific, Maidstone, England) and 0.7 μm 

(Whatman GF/F) glass fiber filters and a 0.45 μm polypropylene filter (Pall Gelman GH Polypro Membrane, Gelman 

Sciences, Inc., Ann Arbor, Mich.). After filtration, the samples 

were stored in amber glass bottles at 4°C until analyzed. 

DOC measurements were made using a Shimadzu 

TOC-5000A total organic carbon analyzer (Shimadzu Scientific Instruments, Columbia, Md.). The instrument was calibrated using potassium hydrogen phthalate standards prepared 

in organic-free water, with standard concentrations bracketing the concentration of the samples. Aliquots of a sample 

(4.5 mL) were acidified using 30 μL of 2-normal hydrochloric 

acid and sparged using N2 for 3 minutes to remove inorganic 

carbon as CO2. The non-purgeable organic carbon (NPOC) 

was analyzed by direct injection of a liquid sample into a hightemperature (680oC) combustion tube packed with platinum 

catalyst. The CO2 produced by oxidation of the NPOC was 

detected using a non-dispersive infrared photometric cell. 

Each sample was injected three times and the resultant mean 

concentration for the three injections was calculated. Additional details about the instrumental method can be found in 

Bird and others (2003).
12 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Sediment Analysis

Extractions from suspended sediment and bottom 

sediment samples were performed using either soxhlet or 

microwave-assisted solvent extraction (MASE). The MASE 

system was an MSP 1000 (CEM Corporation, Matthews, 

N.C.). Soxhlet extraction was used on the samples collected 

from the deep-water areas because pressure buildup inside the 

teflon extraction vessels during the MASE extraction process 

occurred quite rapidly, sometimes leading to a containment 

failure and a loss of the sample. For both types of extraction 

methods, wet sediments were extracted to avoid any artifacts 

associated with the drying of sediment, and because water 

is needed in the sample for efficient extraction when using 

MASE (Jayaraman and others, 2001). Approximately 5 grams 

(dry weight) of sediment was extracted for each sample. Percent water in each sample was determined gravimetrically by 

weighing the wet sediments and then drying and reweighing to 

a constant dry weight. A wet weight equivalent to 5 grams dry 

weight was calculated for each sample after determining the 

percent water. 

After the wet sediments were weighed into the extraction vessels (teflon MASE vessels or pre-cleaned cellulose 

soxhlet extraction thimbles), a surrogate solution containing 400 ng each of 13C6

-α−benzenehexachloride (α-BHC), 13C6

-permethrin (cis/trans mixture), 13C3-simazine, and d10-

chlorpyrifos (Cambridge Isotope Laboratories, Inc, Andover, 

Mass.) was spiked into each sample. The sediments were 

extracted using a mixture of methylene chloride and acetone 

(50:50 volume/volume). The soxhlet extraction took 16 hours 

and used 150 mL of solvent whereas the MASE extraction, 

which was done twice, took only 20 minutes and used only 

30 mL of solvent. The samples in the MASE extractor reached 

temperatures of 120°C and pressures of 150 pounds per square 

inch (psi). Comparisons of MASE and soxhlet extraction techniques for the extraction of a wide variety of hydrocarbons, 

including current-use pesticides on soils and sediments, have 

yielded similar results (Pastor and others, 1997; Blanco and 

others, 2000 and Camel, 2000).

Extracts were decanted into separatory funnels containing 100 mL (for MASE) or 250 mL (for soxhlet) of deionized, 

organic-free water. The mixture was shaken for approximately 

1 minute and the methylene chloride removed. The aqueous phase was extracted twice more using two volumes of 

methylene chloride (2 × 30 mL for MASE and 2 × 100 mL for 

soxhlet). The combined extracts were then dried over sodium 

sulfate and reduced to approximately 1 mL by rotary evaporation. These extracts, many of which were darkly colored, were 

cleaned by passing them through an SPE column consisting 

of 500 mg of activated carbon (6-cubic centimeter size barrel, 

Restek Corporation, Bellefonte, Pa.) followed by elution using 

10 mL of methylene chloride.

Samples were then solvent-exchanged to ethyl acetate, 

reduced in volume to 0.5 mL by nitrogen evaporation and further purified using a gel permeation/high pressure liquid chromatography system (GPC/HPLC). The sample was injected 

into a Perkin Elmer HPLC consisting of a PE 410 4-stage 

pump and a LC-95 UV fixed wavelength detector (Perkin 

Elmer Corporation, Norwalk, Conn.). The analytical column 

was a gel permeation column from Polymer Laboratories 

(Amherst, Mass.). Column dimensions were 300 × 7.5 mm 

and the packing was polydivylbenzene (10 mm, 50-angstrom 

pore size). Ethyl acetate (which was used as the mobile phase 

in the procedure) was pumped through the column at a rate 

of 1 mL/min. The size of the collection window was verified 

daily using pesticide standards and monitoring at a wavelength 

of 254 nm. GPC/HPLC provided additional matrix cleanup, 

especially with the elimination of elemental sulfur, which is a 

major co-extractant that interferes with GC/MS instrumental 

analysis (Goerlitz and Law, 1971; Jensen and others, 1977). 

After the GPC/HPLC cleanup step, the sample extract, 

at a volume of 9 mL, was reduced to approximately 500 μL. 

For GC/MS analysis, internal standards (the same as for the 

water samples) were added to the sample and the sample was 

brought to a final volume of 500 μL by N2 evaporation. In the 

case of extremely colored extracts, the sample was brought to 

a volume of 1 mL prior to the GC/MS analysis.

Method Validation

Significant modifications to the multi-residue pesticide 

method for sediments (Bergamaschi and others, 1999) were 

made during this investigation. The resulting method included 

several additional analytes and improved method detection 

limits. Method detection limit concentrations ranged from 0.6 

to 7.6 ng/g (table 6). Pesticides having detection limits greater 

than 2 ng/g generally showed greater variability in calibration standards and in spiked sediment extracts, and were the 

first compounds to begin tailing as the GC column began to 

age. The sediment matrix spikes for bottom sediment from 

an agricultural drain along the Sacramento River in northern California showed reasonable recoveries for most of the 

analytes (table 7). Some analytes, however, such as disulfoton, 

cyanazine, butylate, eptam, pebulate, and phorate had recoveries of 60 percent or less. Of these compounds, only eptam was 

detected in significant quantities. Eptam most likely was lost 

during extract concentration (rotary evaporation and N2 evaporation steps) due to its low vapor pressure and high volatility 

(Mackay and others, 1997). 
Laboratory Methods 13

Table 6. Method detection limits for pesticides and DDTs 

in sediments from sampling sites in the Salton Sea Basin, 

California. 

[DDT refers to DDT and the metabolites DDD and DDE]

Pesticide Method detection limit, 

in nanograms per liter

Alachlor 1.1

Atrazine1 .6

Azinphos-methyl1 7.6

Bifenthrin .9

Butylate1 .5

Carbary1 1.2

Carbofuran 3.0

Chlorpyrifos 1.5

Cyanazine1 1.8

Cycloate1 1.8

Cyfluthrin1 7.9

Cypermethrin1 5.6

Dacthal .6

Diazinon 1.5

Diethatyl-ethyl1 1.2

Endosulfan I1 5.5

EPTC (eptam) .7

Esfenvalerate 1.4

Ethalfluralin1 1.9

Fenamiphos1 .8

Fonofos 2.5

Hexazinone1 3.2

lambda-Cyhalothrin .5

Malathion1 1.5

Methidathion 3.4

Methyl parathion1 1.6

Metolachlor 1.0

Molinate1 2.0

Napropamide1 1.6

Oxyfluorfen 6.1

Pebulate1 .8

Pendimethalin 4.0

Permethrin 1.4

Phosmet1 .8

Piperonyl butoxide1 1.2

Prometryn 1.8

Simazine1 2.1

Sulfotep1 1.1

Thiobencarb1 4.4

Trifluralin 1.4

p,p′-DDD 2.9

p,p′-DDE 3.7

p,p′-DDT 3.9

1

Pesticide not detected in any sediment samples during any of the sampling 

periods.

Quality Assurance/Quality Control 

Sediment matrix spike and laboratory blank samples were 

processed for quality-control purposes. For matrix spikes, 

a mixture containing 400 ng each of the pesticides listed in 

tables 4 and 8, was spiked into suspended and bed sediments 

(five samples) obtained from the Whitewater River outlet and 

near-shore sites collected during fall 2001. The sediments 

were extracted and analyzed as described in the “Sediment 

Analysis” section. In addition to matrix spikes, percent 

recoveries were calculated for the four surrogate compounds 

described previously in the “Sediment Analysis” section. 

Method detection limits were determined by spiking 

seven replicate sediment samples using a pesticide mix containing 50 ng each of the pesticides listed in table 4. Sediment 

from the agricultural drain in northern California also was 

used for matrix spike samples. Method detection limits were 

calculated by multiplying the standard deviation of the mean 

concentration of each analyte by 3.143, the corresponding 

value of Student’s t for p = 0.01 and 6 degrees of freedom 

(from Sokal and Rohlf, 1983). 

Matrix interference negatively affected both the surrogate and the matrix spike recoveries in the Salton Sea sediments. This effect was most prononounced in the off-shore 

suspended-solids samples, which consisted primarily of algal 

detritus. The mean recoveries of d6-α-HCH, d3-simazine, 

and d10-chlorpyrifos for bed sediments and suspended sediments were 66 ± 16 percent, 91 ± 35 percent, 78 ± 19 percent, respectively. Recovery of the d6

-permethrin surrogate 

was extremely variable (92 ± 77 percent), which made the 

quantitation of permethrin, cypermethrin, and cyfluthrin in 

the sediments suspect. Therefore the qualifier “D” was used 

when permethrin or similar compounds (such as cyfluthrin and 

cypermethrin) were detected to signify that the compound was 

detected but that quantitation is highly uncertain. 

Average recoveries for five sediment matrix spikes performed using suspended sediment and bed sediment from the 

Whitewater River off-shore sites were between 51and 109 percent for all compounds, with an overall mean average recovery 

of 72 ± 22 percent for the pesticides that were detected in 

suspended and bed sediments. 

Instrumental Analysis

Water and sediment extracts were analyzed for pesticides 

using a Saturn 2000 GC/MS ion trap system (Varian, Inc., 

Walnut Creek, Calif.). Run conditions are listed in table 9. 

The instrument was operated in full scan mode during most of 

the run; however, later in the run, selected ion storage (SIS), 

which is equivalent to selected ion monitoring (SIM) in a 

quadrupole instrument, was used to optimize instrument sensitivity to select analytes. 
14 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 7. Average recovery of pesticides used in the Salton Sea Basin, California, spiked to bottom sediment from an agricultural 

drain along the Sacramento River in northern California. 

Pesticides

Mean recovery ±

standard deviation

(n = 3)

(percent)

Pesticides

Mean recovery ±

standard deviation

(n = 3)

(percent)

Triazines/Triazones Organophosphates

Atrazine 83 ± 14 Azinphos-methyl 110 ± 11

Cyanazine 56 ± 11 Chlorpyrifos 69 ± 8.6

Hexazinone 110 ± 12 Diazinon 82 ± 12

Prometryn 70 ± 9.0 Dimethoate 89 ± 13

Simazine 83 ± 14 Disulfoton 41 ± 4.7

Terbuthylazine 73 ± 11 Fenamiphos 91 ± 9.0

Anilines Fonofos 82 ± 8.2

Ethafluralin 71 ± 7.3 Malathion 65 ± 9.0

Pendimethalin 79 ± 11 Methidathion 78 ± 9.5

Trifluralin 74 ± 9.0 Methyl parathion 80 ± 11

Chloracetanilides Phorate 59 ± 3.2

Alachlor 77 ± 9.3 Phosmet 89 ± 11

Metolachlor 68 ± 9.8 Profenfos 61 ± 8.8

Carbamates Sulfotep 65 ± 5.6

Carbaryl 100 ± 12 Pyrethroids

Carbofuran 78 ± 11 Bifenthrin 62 ± 8.5

Thiocarbamates Cyfluthrin 99 ± 8.8

Butylate 55 ± 2.2 Cypermethrin 97 ± 9.4

Cycloate 69 ± 3.7 Esfenvalerate 92 ± 12

EPTC (eptam) 54 ± 1.9 lambda-Cyhalothrin 74 ± 9.5

Molinate 79 ± 2.4 Permethrin 82 ± 12

Pebulate 58 ± 3.0 Miscellaneous

Thiobencarb 160 ± 15 Dacthal 71 ± 9.6

Diethatyl-ethyl 70 ± 9.7

Endosulfan 74 ± 8.1

Napropamide 81 ± 9.9

Oxyflurofen 88 ± 12

Propargite 79 ± 12
Laboratory Methods 15

Table 8. Organochlorine analytes, internal and surrogate 

standards, and corresponding ions monitored for 

quantification and confirmation purposes. 

[BHC, benzenehexachloride; alpha, beta, delta, and gamma are positional 

isomers. Quantification ion is the ion from the mass spectra used to quantify the compound. Confirmation ions are ions that, with the quantification ion, are used to identify the compound. d is a notation used to signify 

the number of deuterium atoms in a deuterium-labeled compound. d10

signifies the presence of 10 deuterium atoms]

Compounds Quantification

ions

Confirmation

ions

Organochlorine analytes

Aldrin 263 293, 265

BHC, alpha 181 183, 219

BHC, beta 183 181, 219

BHC, delta 181 183, 219

BHC, gamma 181 183, 219

Heptachlor 272 274, 270

Endosulfan I 241 195, 243

DDD, 4,4′ 235 237, 165

DDE, 4,4′ 318 246, 316

DDT, 4,4′ 235 237, 165

Dieldrin 277 279, 263

Endosulfan II 243 195, 207

Endrin 263 243, 245

Methoxychlor 227 228, 212

Internal standards

d10-Acenaphthene 162 164, 160

d12-Chrysene 240 241, 236

d10-Phenanthrene 188 187, 189

d10-Pyrene 212 211, 213

Terbuthylazine 214 173, 216

Surrogate standards

13C6

-alpha-BHC 189 187, 225

d10-Chlorpyrifos 324 260, 326

13C6

-Permethrin (cis/trans) 189 163, 127

13C3

-Simazine 204 189, 176

Table 9. Run conditions for the Saturn 2000 gas 

chromatography/mass spectrometry (GC/MS) system. 

[psi, pounds per square inch; dpm, degrees per minute; SIS, selected ion 

storage; m/z, mass/charge ratio; μamps, microamperes, a unit of charge; ev, 

electronvolts, a voltage measurement that describes the gain on the electron 

multiplier of the mass spectrometer]

Injection conditions: Splitless injection, pressure pulse of 50 

psi for 1.5 minutes

Injection temperature: 275°C

Oven program: 80°C, hold for 0.5 min, 80 to 120°C @ 

10 dpm

120 to 200°C @ 3 dpm, hold for 5 

minutes

200 to 219°C, hold for 5 minutes

219 to 300°C @ 10 dpm, hold for 10 

minutes

GC/MS conditions for most 

of the run:

For permethrin, 

cypermethrin, cyfluthrin, 

esfenvalerate, lambda 

cyhalothrin and azinphosmethyl:

Range SIS, collecting 90 to 450 m/z, 

emission current = 15 μamps

True SIS – only quantification and 

confirmation ions collected, electron 

multiplier + 300 ev offset, emission 

current 45 μamps

Analytical column: CPSIL 8-MS (Varian Corporation), 

30 m × 0.25 mm, 0.5 μm phase 

thickness
16 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Calibration of instrument response to each pesticide was 

made using an eight-point standard curve that spanned the 

range of sample concentrations. Standards were purchased 

from Supelco Inc. (Bellefonte, Pa.). In addition to the standard 

curve, a mid-level standard was run every six injections to 

verify that the response was within 10 percent of the standard 

curve. If compounds in the mid-level standard fell outside 

of this response window, the samples were reinjected. Each 

sample was analyzed twice, and replicate injections that had a 

greater than 25 percent variability were reanalyzed.

A list of current-use organochlorine and pesticide 

analytes are presented in tables 4 and 8. Compounds were 

quantified using internal standards (see table 8). Recovery of 

the terbuthylazine surrogate was used to assess the efficiency 

of each water extraction. Recoveries of the surrogate standards 

(

13C-labeled α-BHC, simazine, permethrin, and

d10-chlorpyrifos) were monitored to check for matrix interference and procedural recoveries in the sediment method. 

Sediment Organic Carbon Analysis

Sediments were analyzed for organic carbon content 

using a Perkin Elmer CHNS/O analyzer (Perkin Elmer 

Corporation, Norwalk, Conn.). Sediments were weighed into 

5 × 9 mm silver boats (Costech Analytical Technologies, 

Valencia, Calif.), exposed to concentrated hydrochloric acid 

fumes in a dessicator for 24 hours (to remove carbonate minerals), dried to constant weight, and then combusted at 925°C. 

Acetanilimide was used for instrument calibration of elemental carbon and nitrogen. Purified products of high temperature 

combustion (CO2 for carbon [C]) and N2, (the gaseous form 

of the element nitrogen [N]) were detected using a thermal 

conductivity detector.

Trace Elements and Selected 

Inorganic Constituents

Suspended sediments collected during the fall 2001 

sampling period were analyzed by the USGS National Water 

Quality Laboratory in Denver, Colo., using methods described 

in Arbogast (1996) and Briggs (1999). Instrumental analysis 

was performed by inductively coupled argon-plasma emission 

spectrometry.

Analysis Results and Discussion

Field Parameters, DOC, and Streamflows

Specific conductance and temperature were measured in 

the field at all sites and times during sampling (table 10). Samples also were collected at the time of sampling, and specific 

conductance and pH were measured at the USGS California 

District Laboratory in Sacramento. The measurements indicate 

a slight trend of increasing pH from the outlet sites to the offshore sites, from approximately 7.3 pH units to 8.3. The gradient of conductance (measured as a surrogate for salinity) was 

extremely high and varied by a factor of more than 10 between 

the rivers and the Salton Sea Basin. Salinity in the Salton Sea 

Basin is discussed in depth by Schroeder and others (2002); 

Schroeder and others (1993); and Setmire and others, (1993). 

DOC was measured at all sites during the fall 2001 sampling period. Concentrations were elevated at all the sites and 

differed by a factor of 10 between the outlet and the off-shore 

sites. Values ranged from a low of 5.2 mg/L at the Whitewater River outlet site to 61 mg/L at the New River off-shore 

site. Values for the near-shore sites were either intermediate 

between these two extremes, as in the case of the Alamo and 

New Rivers, or similar in concentration to the off-shore sites, 

as in the case of the Whitewater River. DOC concentrations 

in samples from the off-shore sites were fairly uniform. DOC 

concentrations at the New River outlet and the off-shore sites 

measured in fall 2002 were slightly higher than those in 2001, 

although were within an order of magnitude.

Daily mean streamflows during the 2001 and 2002 sampling events were similar to flows for the 42-year period of 

record for the New and Alamo Rivers (fig. 2) (U.S. Geological 

Survey, 2001). For the Whitewater River, streamflows were 

approximately 40 percent lower than the 42-year average.

Dissolved Pesticide Concentrations

Dissolved pesticide concentrations ranged from below 

detection limit (1.7 ng/L) to 3,800 ng/L, with the highest 

concentrations detected at the outlet and the near-shore sites 

(tables 11–13). Generally, the compounds with the greatest 

concentrations and frequency were those pesticides applied 

in the largest quantities, and include atrazine, chlorpyrifos, 

dacthal, diazinon, eptam, malathion, pendimethalin, and 

trifluralin (table 1). 
Analysis Results and Discussion 17

Table 10. Water parameters measured at sampling sites in the Salton Sea Basin, California, during fall 2001, spring 2002, and fall 

2002. 

[°C, degrees Celsius, μS/cm, microsiemens per centimeter at 25°C. DOC, dissolved organic carbon concentration in mg/L as carbon. mg/L, milligram per liter. 

pH, specific conductance, and DOC were measured in the laboratory; temperature was measured in the field. , no data]

Parameter

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Fall 2001

Temp (°C) 28.9 28.0 29.8 26.1 26.4 29.2 22.0 25.3 31.9

pH (standard units) 7.4 7.8 8.1 7.8 7.6 8.1 7.7 7.8 7.8

Specific conductance 

(μS/cm)

3,080 11,500 53,300 3,800 34,500 53,800 1,900 53,200 53,200

DOC (mg/L) 6.3 11.6 50 6.6 33.6 53 5.2 52 52

Spring 2002

Temp (°C) 14.6  16.7     16.4 

pH (standard units) 7.3 7.8 6.9 7.6 8.0 7.8 7.4

Specific conductance 

(μS/cm)

26,000 39,600 57,100 4,100 20,900 37,700 2,160 53,900 52,500

DOC (mg/L)         

Fall 2002

Temp (°C) 28.9 28.0 29.8 26.1 26.4 29.2 22.0 25.3 31.9

pH (standard units) 7.4 7.9 8.4 7.5 8.1 8.3 7.4 8.1 8.3

Specific conductance 

(μS/cm)

3,300 9,210 32,600 3,930 16,340 32,700 2,140 9,810 32,300

DOC (mg/L)    9.1 61   

Table 11. Pesticide concentrations in water samples collected from sampling sites at the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California, fall 2001.

[Values are reported as nanograms per liter. Concentrations are the mean of duplicate injections. Water samples also were analyzed for alachlor, azinphosmethyl, butylate, cyanazine, diethylatyl-ethyl, ethafluralin, fonofos, hexazinone, methidathion, methyl parathion, metolachlor, molinate, napropamide, oxyfluorfen, pebulate, phosmet, piperonyl butoxide, sulfotep, and thiobencarb, which were not detected during this sampling period. nd, not detected]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Atrazine nd 14.9 165 8.4 78.6 145 nd 151 144

Carbaryl 56.4 44.4 nd nd nd nd nd nd nd

Carbofuran nd nd nd 34.8 nd nd nd nd nd

Chlorpyrifos 161 87.7 nd 44.3 15.0 nd nd nd nd

Cycloate 9.3 34.1 nd nd nd nd 9.6 nd nd

Dacthal 94.3 328 9.4 26.1 30.6 5.6 38.1 9.8 9.0

Diazinon 789 936 9.1 213.4 34.8 nd 32.4 8.5 9.3

EPTC (eptam) 418 3,830 84.9 3,490 1,300 17.9 102 51.0 46.2

Malathion 340 36.5 nd 585 nd nd 43.2 nd nd

Pendimethalin 59.5 nd nd nd nd nd nd nd nd

Simazine nd nd 21.0 nd nd nd nd nd nd

Trifluralin 37.4 37.6 nd 35.0 27.3 nd 22.5 nd nd
18 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 12. Pesticide concentrations in water samples collected from sampling sites at the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California, spring 2002. 

[Values are reported as nanograms per liter. Water samples were analyzed for alachlor, azinphos-methyl, butylate, carbaryl, cyanazine, cycloate, diethatyl-ethyl, 

ethalfluralin, fonofos, hexazinone, methidathion, methyl parathion, metolachlor, molinate, napropamide, pebulate, phosmet, piperonyl butoxide, simazine, sulfotep, and thiobencarb, which were not detected during this sampling period. nd, not detected; ( ), concentration below detection limit]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Atrazine 630 128 128 434 66.4 124 (2.1) 128 119

Carbofuran 530 18.9 nd nd nd nd nd nd 56.2

Chlorpyrifos 73.1 nd nd 20.7 9.5 nd nd nd nd

Dacthal 47.0 8.9 nd 31.7 15.2 6.9 8.5 nd 6.1

Diazinon (3.4) nd nd 22.7 13.8 nd 8.53 nd nd

EPTC (eptam) 50.6 53.8 50.2 nd nd nd 34.0 30.1 35.7

Malathion 1,100 nd nd 253 90.0 nd nd nd nd

Oxyflurofen 30.3 nd nd nd nd nd nd nd nd

Pendimethalin 156 nd nd 65.3 27.3 nd 20.0 nd nd

Trifluralin 600 15.8 nd 215 80.3 nd 10.5 nd nd

Table 13. Pesticide concentrations in water samples collected from sampling sites at the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California, fall 2002. 

[Values are reported as nanograms per liter. Concentrations are the mean of duplicate injections. Water samples were analyzed for alachlor, azinphos-methyl, 

butylate, cyanazine, diethatyl-ethyl, ethalfluralin, fonofos, hexazinone, malathion, methidathion, methyl parathion, metolachlor, molinate, pebulate, phosmet, 

piperonyl buto w detection limit]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Atrazine 19.7 160 108 nd 110 110 14.3 88.0 110

Carbaryl (2.2) (2.7) nd 19.3 nd nd nd nd nd

Carbofuran 9.7 7.8 25.7 nd 37.9 nd (2.2) 40.2 31.7

Chlorpyrifos 41.7 19.0 nd 7.0 nd nd (1.7) nd nd

Cycloate 6.1 10.4 nd nd nd nd nd nd nd

Dacthal 60.3 94.0 nd 6.7 3.2 3.2 110 9.0 3.6

Diazinon 970 48.0 nd 22.6 8.8 (3.0) 12.8 6.4 nd

EPTC (eptam) 52.9 24.9 13.0 22.3 24.5 22.5 100 24.5 10.7

Napropamide nd nd nd 35.9 nd nd nd nd nd

Oxyfluorfen 5.5 (2.0) nd nd nd nd nd nd nd

Pendimethalin 6.1 7.2 nd nd nd nd 61.9 nd nd

Simazine nd nd nd nd nd nd (6.5) nd nd

Trifluralin 4.6 5.5 nd 12.4 nd nd (1.5) nd nd
Analysis Results and Discussion 19

A total of 14 current-use pesticides were detected in 

water. Eptam was detected at the highest concentrations of 

all the pesticides (3,490 and 3,830 ng/L) at the New River 

outlet site and at the Alamo River near-shore site, respectively, 

during fall 2001 (table 11). The concentration of malathion 

was elevated at the Alamo River outlet site, especially during 

spring 2002, with a concentration of 1,100 ng/L 

(table 12). At the Alamo River outlet and near-shore sites, 

diazinon concentrations were elevated in both fall 2001 and 

fall 2002 (tables 11 and 13). Diazinon was implicated as 

a major cause of toxicity to test organisms in standardized 

aquatic toxicity tests of water collected from the Alamo River 

(deVlaming and others, 2000). Concentrations of pesticides in 

the Alamo River were within an order of magnitude of those 

reported in Crepeau and others (2000), who sampled monthly 

during 1996. However, concentrations of eptam, which in 

1996 were reported as high as 13,000 ng/L and often exceeded 

1,000 ng/L, were comparable (3,800 ng/L) only with the fall 

2001 concentrations. During the spring and fall of 2002, concentrations were only 13 to 100 ng/L. It should be noted, however, that sampling by Crepeau and others (2000) was done 

much more fre-quently (monthly) so sampling of a maximum 

or a “peak” concentration was more likely.

Although there was little difference in the concentrations of the compounds detected between spring and fall, 

dissolved concentrations of these pesticides varied between 

seasons. Concentrations of diazinon were elevated in fall 2001 

and fall 2002 relative to concentrations in spring 2002. Other 

pesticides that were higher in the fall compared with those 

in the spring were dacthal and eptam (fig. 3). Pesticides that 

were higher in the spring included atrazine, carbofuran, and 

trifluralin. Similar seasonal trends were observed by Crepeau and others (2000). These seasonal patterns in dissolved 

concentration also are reflected in the pesticide-use patterns 

for 2001 in table 1, in which the amounts of pesticides applied 

are separated into two groups representing application periods 

prior to and during sampling: winter/early spring (November–

March) and late spring/fall (April–October, table 1). Because 

the 2002 pesticide-use data from the California Department of 

Pesticide Regulation was not yet available during the writing 

of this report, the only direct comparison between pesticide 

use and dissolved concentration that could be made was for 

fall 2001. The most abundantly detected pesticides shown 

in fig. 3A (chlorpyrifos, dacthal, diazinon, and eptam) were 

among the most abundantly used pesticides during April 2000 

to October 2001 (table 1). Futhermore, aqueous concentrations 

of these compounds were higher during the fall 2001 sampling 

period than during the spring 2002 sampling period, which 

parallels pesticide applications for the previous winter/early 

spring 2001 application period (table 1) compared with those 

for the late spring/fall application period (table 1). Atrazine, 

carbofuran, and trifluralin, which were detected in high concentrations in spring (March) 2002 (fig. 3B), were used more 

heavily prior to and during the previous spring application 

period when compared with use during late spring/fall 2001 

(table 1). 

Dissolved pesticide concentrations in the Alamo River 

were higher during the fall 2001 sampling period relative to 

those during the fall 2002 sampling period (with the exception 

of diazinon which had similar concentrations). Pesticide concentrations in the New and the Alamo Rivers generally were 

higher than concentrations in the Whitewater River during fall 

2001 and spring 2002 (fig. 3). 

In fall 2002, concentrations of dacthal, eptam, and 

pendimethalin were higher at the Whitewater River outlet site 

compared with concentrations at the outlet sites of the other 

two rivers (fig. 3). Diazinon concentrations were highest at 

all three sites for the Alamo River (table 13). More pesticides were detected in the Alamo and New Rivers than were 

detected in the Whitewater River. Concentrations of most of 

the dissolved pesticides were highest at the outlet sites of all 

three rivers during the fall 2001 sampling period, consistent 

with an upstream source (fig. 4). However, concentrations of 

some compounds were elevated at the nearshore sites, such 

as those for carbaryl in the Alamo River (fig. 4A) and dacthal 

and trifluralin in the New River (fig. 4B). Concentrations in 

2001 also were elevated at the off-shore sites for atrazine in 

the Alamo River and diazinon in the New River. Similar trends 

were seen for some pesticides in spring 2002 and fall 2002 

(tables 12 and 13). Since the source of the pesticides presumably is not constant, but rather more episodic and tied to 

application times, high concentration pulses move through the 

watershed over time. With only a single timepoint sampling, 

it is difficult to explain why concentration patterns along the 

transects differ among pesticides, but it may be due to different use patterns for different pesticides. 

Suspended-Sediment Concentration, Particle 

Size, and Organic Carbon Content

Concentrations of suspended sediments were higher 

in the Alamo and New Rivers relative to concentrations in 

the much smaller Whitewater River (fig. 5, table 14), and 

decreased in all three rivers from the river outlet to the offshore sites (fig. 5). The percentage of fine-grained material 

(less than 62 μm) did not change appreciably from outlet 

sites to the off-shore sites. These trends are in agreement with 

historical trends described in Setmire and others (1990, 1993) 

and Schroeder and others (1993).
20 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Figure 3. Dissolved concentrations of selected pesticides at the outlet sites of the New, 

Alamo, and Whitewater rivers during (A) Fall 2001, (B) Spring 2002, and (C) Fall 2002. 

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Figure 4. Dissolved concentrations of representative current-use pesticides at three sampling 

sites along the (A) Alamo, (B) New, and (C) Whitewater rivers in the Salton Sea Basin, California, 

Fall 2001. 

Analysis Results and Discussion 21

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Carbaryl 

Chlorpyrifos 

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22 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif. .

Figure 5. Pesticides concentrations on suspended sediment from the outlet, near-shore, 

and off-shore sampling sites in the Salton Sea Basin, in the (A) Alamo, (B) New, and 

(C) Whitewater rivers in fall 2001, spring 2002, and fall 2002. 

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Table 14. Suspended-sediment concentrations and percentage of fine-grained materials in water from sampling sites at the Alamo, 

New, and Whitewater rivers in the Salton Sea Basin, California, during fall 2001, spring 2002, and fall 2002.

[mg/L, milligram per liter. Fines, percent particles less than 62 micrometer. *, not analyzed]

Sampling site

Fall 2001 Spring 2002 Fall 2002

Concentration 

(mg/L) Fines 

Concentration 

(mg/L) Fines 

Concentration 

(mg/L) Fines 

Alamo River outlet 418 80 379 91 226 84

Alamo River near-shore 255 88 145 95 47 86

Alamo River off-shore 35 76 15 96 15 81

New River outlet 245 97 397 77 338 80

New River near-shore (near surface) 145 48 241 84 80 84

New River near-shore (near bottom) 89 70 * * 36 65

New River off-shore 43 86 23 91 25 83

Whitewater River outlet 72 82 61 66 51 48

Whitewater River near-shore 39 66 11 77 17 57

Whitewater River off-shore 21 79 7 67 26 91

Analysis Results and Discussion 23
24 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Pesticides Associated With 

Suspended Sediments

Pesticide concentrations in suspended sediments 

ranged from the limits of detection (1–3 ng/g) to 106 ng/g 

(tables 16–18). The most frequently detected pesticides were 

those used in the highest amounts; they included chlorpyrifos 

and trifluralin. These compounds also were detected frequently in water (tables 11–13) as were dacthal and eptam 

(table 1). Other pesticides, such as carbaryl, carbofuran, 

diazinon, oxyflurofen, and pendimethalin were detected in 

only a few of the suspended-sediment samples. The only 

organochlorine pesticide consistently detected on suspended 

sediments was p,p′-DDE, a metabolite of DDT. 

Differences in the concentration and occurrence of pesticides by season were not as prononounced as the differences 

in the water samples, and were similar to those for dissolved 

concentrations. Diazinon, which had the highest dissolved 

concentrations in the fall, also was detected in suspended 

sediments in the fall. The maximum dissolved concentrations 

of dacthal and eptam were measured in fall 2001; dacthal and 

eptam concentrations also were high in suspended sediment 

in fall 2001 (fig. 6). Eptam was not detected during the spring 

and the fall 2002 sampling periods, which probably is due, in 

part, to the higher detection limits for eptam in suspended sediments (0.7 ng/g, or parts per million) compared with the limits for water (4 ng/L, or parts per trillion). Similarly, trifluralin 

concentrations were highest in water samples collected during 

the spring sampling period, but also were high in in suspended 

sediments from the New and Alamo Rivers (fig. 6). Suspended 

sediments are likely a combination of recent input from 

agricultural drainage and older resuspended bottom sediment, 

which may partly obscure concentration trends in suspended 

sediment associated with ongoing pesticide use. 

The Alamo River had more detections and higher concentrations of pesticides associated with suspended solids than 

the New and Whitewater Rivers. In some instances pesticide 

concentrations were higher at the outlet sites than at the nearshore sites (for example, trifluralin was higher at the outlet site 

of the Alamo River during spring 2002). In some instances 

pesticide concentrations at the near-shore sites were similar or 

exceeded pesticide concentrations at the outlet sites, which is 

consistent with the downstream transport of pesticides on suspended particles into the near-shore environment (fig. 6). The 

fact that pesticides typically were not detected in suspended 

sediment collected from the off-shore sites may have been due 

to dilution of river-derived suspended sediment by algal material formed in the Salton Sea. An equally important and 

related factor is that the high concentrations of algal material 

in these samples caused matrix interference that prevented 

detection of pesticides and caused low recoveries of spiked 

surrogates, as discussed in the “Quality Assurance/Quality 

Control” subsection of the “Sediment Analysis” section. 

Organic carbon concentrations in suspended sediment 

collected from the river (outlet) sites ranged from 0.93 to 

5.4 percent (table 15); the Whitewater River outlet site consistently had the highest organic carbon content. Suspended sediment at all the off-shore sites consisted of a high percentage of 

algal-derived detritus, which is reflected in extrordinarily high 

percent organic carbon concentrations. The Whitewater River 

near-shore site also had extremely high organic carbon concentrations (28.8 to 42.7 percent), unlike concentrations at the 

near-shore sites of the other two rivers. This is consistent with 

the lower outflow from the Whitewater River, which results in 

a smaller riverine influence at the near-shore site. 

Molar ratios of carbon to nitrogen in off-shore samples 

also suggest a biogenic origin for the suspended material. 

Ratio values ranged from 1.1 to 8.0 in off-shore suspended 

solids, which overlaps the traditional Redfield ratio for organic 

matter of 105:15, or about 7:1 (Chester, 1990). Although this 

number has been revised and refined for specific environments 

over the years, the fairly high percent nitrogen in these samples (5.1 to 6.7 percent) suggests that the suspended sediment 

is primarily of organic and autocthonous origin, as opposed to 

clastic (mineral) material deposited from the rivers. Finally, 

visual observations of intense algal blooms (red and green) 

during sample collection and the appearance (and smell) of the 

dewatered material support the assertion that suspended sediment in the sea consisted of highly organic material of algal 

origin.

Organic Carbon and Pesticides Associated 

With Bed Sediments

Organic carbon (OC) concentrations in bed sediments 

collected from the river outlet sites ranged from 0.25 to 

1.17 percent and were much lower than OC concentrations in 

suspended sediments (table 15). OC concentrations in offshore sediments were as high as 6.23 percent, although there 

was considerable variability between sites owing, in part, 

to differences in bottom sediment and the presence of algal 

detritus. The highest OC concentrations found were in bottom 

sediments collected from the off-shore site of the Whitewater 

River.

Eighteen pesticides were detected in bed sediments, 

primarily at the outlet sites (tables 16–18). In general, the 

pesticides detected were the same pesticides detected in the 

suspended sediments. These pesticides were applied in large 

amounts to fields in the study area; they include pesticides 

such as chlorpyrifos, dacthal, diazinon, and eptam (table 1). 

The greatest amount of pesticide detections and the highest concentrations were at the Alamo River outlet site in 

spring 2002. Concentrations of the same pesticides also were 

measured in spring 2002 at the Alamo River near-shore site 

and at the New River outlet and near-shore sites. In fall 2001, 

pesticide detections were more sporadic and concentrations 

were lower than those in spring 2002. More compounds were 

detected at the Alamo River outlet site in fall 2002 than in fall 
Analysis Results and Discussion 25

Table 15. Percentage of organic carbon in suspended sediments and bed sediments from 

sampling sites at the Alamo, New, and Whitewater rivers in the Salton Sea Basin, California, 

during fall 2001, spring 2002, and fall 2002. 

[*, not analyzed]

Sampling site Organic carbon

Fall 2001 Spring 2002 Fall 2002

Suspended sediments

Alamo River outlet 0.93 1.55 1.15

Alamo River near-shore 1.20 11.1 3.49

Alamo River off-shore 28.4 35.9 27.1

New River outlet 1.74 1.90 1.33

New River near-shore 2.71 4.41 5.31

New River off-shore 40.2 * 35.1

Whitewater River outlet 3.99 5.45 4.78

Whitewater River near-shore 42.7 28.8 33.6

Whitewater River off-shore 43.7 36.6 41.9

Bed sediments

Alamo River outlet 0.56 1.17 0.49

Alamo River near-shore 1.04 1.26 1.39

Alamo River off-shore .38 .36 .60

New River outlet .37 .47 .25

New River near-shore .94 .25 1.83

New River off-shore .78 1.39 .83

Whitewater River outlet .46 1.13 .46

Whitewater River near-shore * 1.65 .94

Whitewater River off-shore 4.25 3.25 6.23

2001 but concentrations were lower than those in spring. For 

the Whitewater River, more pesticides were detected at the 

outlet site during the two fall sampling periods than those that 

were detected during the spring sampling period. 

In several instances, concentrations of pesticides (for 

example, dacthal, eptam, and trifluralin) in the Alamo River in 

spring and fall 2002 were higher in the suspended sediments 

and lower or not detected in the bed sediments (tables 16–18). 

In other instances, they were similar, suggesting that pesticides 

in the suspended solids pool may come from more than one 

source (for example, from agricultural drainage and resuspension of bottom material). Bottom sediments from near-shore 

sites in the New and Alamo Rivers for fall 2001 had pesticide 

concentrations that were higher than those at the outlet sites, 

consistent with the settling of river-derived particles at the 

near-shore sites (table 16). Detections of pesticides in bottom 

sediment from the Whitewater River were much lower than 

detections for the New and Alamo Rivers. 

The metabolite p,p′-DDE was consistently detected in bed 

sediments from the outlet and the near-shore sites of all three 

rivers, with concentrations as much as 64 ng/g at the outlet 

sites. p,p′-DDD also was detected, although not as frequently. 

Concentrations of DDE and DDD at the outlet sites of all three 

rivers were similar to those reported in Setmire and others 

(1990).
26 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 16. Pesticide concentrations in suspended and bed sediments collected from sampling sites at the Alamo, New, and 

Whitewater rivers in the Salton Sea Basin, California, fall 2001. 

[Values are reported as nanograms per gram. nd, not detected; D, detected but quantitation uncertain; ( ), concentration below detection limit. Sediments were 

also analyzed for alachlor, atrazine, azinphos-methyl, cyanazine, cycloate, cypermethrin, endosulfan, esfenvalerate, ethalfluralin, fonofos, hexazinone, malathion, 

methidathion, methyl parathion, metolachlor, molinate, napropamide, oxyflurofen, pebulate, pendimethalin, phosmet, piperonyl butoxide, prometryn, simazine, 

sulfotep and thiobencarb, which were not detected during this sampling period]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Suspended sediments

Chlorpyrifos 50.0 63.9 nd 30.4 nd nd nd nd nd

Dacthal 17.7 38.2 nd nd nd nd nd nd nd

Diazinon 11.8 12.5 nd nd nd nd nd nd nd

EPTC (eptam) 6.6 9.2 nd 4.9 nd nd nd nd nd

Permethrin 18.7 8.7 nd D nd nd nd nd nd

Trifluralin 9.7 12.2 nd 4.6 6.8 nd (0.4) nd nd

p,p′-DDD 6.8 nd nd nd nd nd 3.8 nd nd

p,p′-DDE 84.8 72.7 nd 22.6 25.4 nd 62.5 nd nd

p,p′-DDT 9.5 9.8 nd nd nd nd nd nd nd

Bed sediments

Bifenthrin nd nd nd nd nd nd 7.5 nd nd

Carbaryl (0.8) nd nd nd nd nd nd nd nd

Carbofuran (2.3) nd nd nd (1.9) nd nd nd nd

Chlorpyrifos nd 10.5 nd 10.2 27.7 nd nd nd nd

Dacthal nd 7.9 nd nd nd nd 3.7 nd nd

Diazinon nd 4.9 nd nd nd nd nd 1.9 nd

Diethylatyl-ethyl nd nd nd nd nd nd 1.85 nd nd

EPTC (eptam) nd (2.3) nd (3.9) nd nd Nd nd nd

lambda-Cyhalothrin nd nd nd nd nd nd 11.2 nd nd

Permethrin nd 8.5 nd 7.9 D nd Nd nd nd

Prometryn nd nd nd nd nd nd 4.2 nd nd

Trifluralin nd (.7) nd (1.2) (1.9) nd (.6) (.5) nd

p,p′-DDD nd nd nd nd 93.9 nd Nd nd nd

p,p′-DDE 14.4 31.5 nd 17.6 23.6 nd 6.2 nd nd

p,p′-DDT nd nd nd nd nd nd nd nd nd
Table 17. Pesticide concentrations in suspended and bed sediments collected from sampling sites at the Alamo, New, and 

Whitewater rivers in the Salton Sea Basin, California, spring 2002. 

[Values are reported as nanograms per gram. nd, not detected; D, detected but quantiitation uncertain; (), concentration below detection limit. Sediments were 

analyzed for alachlor, atrazine, azinphos-methyl, bifenthrin, butylate, cyanazine, cycloate, cypermethrin, diethylatyl-ethyl, endosulfan, ethalfluralin, hexazinone, 

lambda-cyhalothrin, malathion, methyl parathion, metolachlor, molinate, napropamide, pebulate, phosmet, piperonyl butoxide, prometryn, simazine, sulfotep and 

thiobencarb, which were not detected during this sampling period]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Suspended sediments

Carbaryl nd nd 43.6 nd nd nd nd nd nd

Carbofuran nd nd nd nd nd nd 5.1 nd nd

Dacthal 7.1 nd nd nd nd nd 2.2 nd nd

Diazinon nd nd nd nd nd nd 2.0 nd nd

EPTC (eptam) (3.7) nd nd 5.6 nd nd nd nd nd

Esfenvalerate nd 2.9 nd nd nd nd nd nd nd

Fonofos nd nd nd nd nd nd (2.4) nd nd

Methidathion nd nd nd 34.3 nd nd nd nd nd

Oxyfluorfen 19.7 nd nd nd nd nd nd nd nd

Pendimethalin 42.4 nd nd nd nd nd nd nd nd

Permethrin nd D nd nd D nd D nd nd

Trifluralin 106.4 19.2 nd 33.3 62.5 nd (.6) nd nd

p,p′-DDD 4.9 nd nd nd nd nd nd nd nd

p,p′-DDE 101 nd nd 58.2 14.9 nd 31.7 nd nd

p,p′-DDT nd nd nd nd nd nd nd nd nd

Bed sediments

Chlorpyrifos 30.3 16.9 nd 5.5 2.9 nd nd nd nd

Dacthal 10.5 9.1 nd 1.9 nd nd nd nd nd

Diazinon 2.1 nd nd nd nd nd nd nd nd

EPTC (eptam) 6.4 6.7 nd 2.5 nd nd nd nd nd

Oxyfluorfen 89.5 nd nd nd nd nd nd nd nd

Pendimethalin 77.5 nd nd nd nd nd nd nd nd

Permethrin 12.0 nd nd 4.2 6.8 nd nd nd nd

Trifluralin 37.2 7.1 2.5 5.0 12.8 2.0 nd nd nd

p,p′-DDD 8.0 7.1 nd nd nd nd 3.5 12.8 nd

p,p′-DDE 63.8 55.8 2.1 14.0 9.3 3.7 14.9 8.3 nd

p,p′-DDT 5.8 nd nd nd nd nd nd nd nd

Analysis Results and Discussion 27
28 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 18. Pesticide concentrations in suspended and bed sediments collected from the Alamo, New, and Whitewater rivers in the 

Salton Sea Basin, California, fall 2002. 

[Values are reported as nanograms per gram. nd, not detected; D, detected but quantitation uncertain; ( ), concentration below detection limit, New River nearshore site, sample collected from above the halocline. Sediments were analyzed for the following pesticides that were not detected during this sampling period: 

atrazine, azinphos-methyl, bifenthrin, butylate, cyanazine, cycloate, cypermethrin, endosulfan, esfenvalerate, ethalfluralin, fonofos, hexazinone, malathion, 

methidathion, methylparation, molinate, napropamide, oxyflurofen, pebulate, permethrin, phosmet, piperonyl butoxide, prometryn, simazine, sulfotep and 

thiobencarb]

Pesticide

Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Suspended sediments

Carbaryl nd nd nd nd nd 89.5 nd nd nd

Carbofuran nd nd nd nd nd nd nd nd nd

Chlorpyrifos 12.3 nd nd nd nd nd nd nd nd

Dacthal 7.2 8.9 nd nd 3.9 nd nd nd nd

Diazinon 11.4 nd nd nd nd nd nd nd nd

Diethylatyl-ethyl 2.7 nd nd nd nd nd nd nd nd

EPTC (eptam) 4.8 nd nd nd nd nd nd nd nd

lambda-Cyhalothrin 16.8 nd nd nd nd nd nd nd nd

Metolachlor 7.8 nd nd nd nd nd nd nd nd

Pendimethalin nd nd nd nd nd nd 36.2 nd nd

Trifluralin 3.1 8.6 nd 18.2 (.4) nd nd nd nd

p,p′-DDD nd nd nd nd nd nd nd nd nd

p,p′-DDE 54.2 D nd 40.3 5.8 nd 29.2 nd nd

p,p′-DDT nd nd nd nd nd nd nd nd nd

Bed sediments

Alachlor 7.9 nd nd nd nd nd 4.4 nd nd

Carbofuran 8.2 nd nd nd nd nd nd nd nd

Chlorpyrifos 11.1 nd nd nd nd nd nd nd nd

Dacthal 3.9 nd nd 4.5 nd nd 3.7 nd nd

Diethylatyl-ethyl 9.0 nd nd nd 15.1 nd 1.8 nd nd

Metolachlor 7.9 nd nd nd 11.9 nd 5.2 nd nd

Trifluralin 2.6 nd nd 3.5 nd nd nd nd nd

p,p′-DDD 13.6 nd nd nd nd nd nd nd nd

p,p′-DDE 48.0 31.7 nd 11.5 4.9 nd 6.2 (2.3) nd

p,p′-DDT nd nd nd nd nd nd nd nd nd
Figure 6. Pesticides concentrations on selected suspended sediments from the Alamo River outlet and near-shore sites in the Salton 

Sea Basin, California, in (A) fall 2001, (B) Spring 2002, and (C) fall 2002 and from the New River outlet and near-shore sites in (D) fall 2001, 

(E) spring 2002, and (F) fall 2002.

Analysis Results and Discussion 29

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30 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Kd

Cs

CW

= -------

Distribution of Pesticides between Water, 

Suspended Solids, and Bed Sediments

There was a high amount of overlap among water, suspended solids, and bed sediments matrices in terms of the pesticides detected. Carbaryl, carbofuran, chlorpyrifos, dacthal, 

diazinon, eptam, and trifluralin, among the pesticides applied 

most heavily in the Salton Sea watershed, were often found 

in all three matrices, especially at the outlet and near-shore 

sampling sites. Pesticides found only in sediments included 

lambda-cyhalothrin, metolachlor, and permethrin. Those 

detected only in water included atrazine, cycloate, malathion, 

napropamide, and simazine (table 19).

The physical-chemical properties of the pesticides 

detected in water and sediments are listed in table 19. If the 

physical-chemical properties of pesticide compounds are the 

main control on the distribution and partitioning of pesticides 

between sediments and water, one would not expect compounds with high water solubility and low log Koc values (the 

logarithm of the organic carbon:water partition coefficient) to 

become associated with sediments, and vice versa. 

To examine the influence of the physical-chemical properties of pesticides on the distribution of compounds between 

water and sediments, predictions of dissolved concentrations 

based on equilibrium partitioning were made. If the pesticide 

concentrations in sediment are in equilibrium with the surrounding water, one should be able to predict aqueous concentrations using the measured sediment concentration and the 

solid:solution partitioning coefficient, Kd (from Schwarzenbach and others, 1993). Kd is defined as the concentration ratio 

of sediment: water

where 

CS is the concentration in sediment (mg/kg)

CW is the concentration in water (mg/L).

Organic chemicals are believed to associate most strongly with 

the organic carbon (OC) portion of sediment aggregates, and 

so the distribution of organic compounds between sediments 

and water can be modeled using an organic carbon to water 

partition coefficient Koc, which is defined as follows: 

where 

CS is the concentration of a compound in the organic carbon 

sediment pool (mg/kg of OC) 

CW is the concentration of a compound in water 

(mg/L).

Equations for calculating equilibrium partitioning are derived 

from first principles in Schwarzenbach and others (1993). 

There are several ways of determining Koc, both theoretical 

(through relations established between Koc and other physicalchemical constants) and experimental. These values have 

been compiled in several references, including Mackay and 

others (1997). The equations for Kd and Koc can be related by 

defining CS (concentration of a compound in sediment) as the 

product of the fraction of organic material in the sediment 

(f

OC) multiplied by the concentration of the pesticide in the 

sediment organic carbon pool (Coc): 

Kd

fOC COC •

CW

------------------------- fOC KOC = = •

CW

CS

Kd

------ fOC COC •

Kd

= = -------------------------

The terms described in the above model and their units are 

presented in table 20, and an example calculation is presented 

in table 21. 

Measured aqueous pesticide concentrations in samples 

from the Alamo, New, and Whitewater River sampling sites 

for fall 2001 and predicted concentrations based on suspended and bed sediment pesticide concentrations are given 

in table 22. Predicted aqueous values, which were determined 

using suspended-sediment concentrations generally were 

much higher than the measured aqueous values illustrating that 

suspended-sediment concentrations are above equilibrium with 

the surrounding water. One exception to this trend was for 

eptam. Predicted aqueous concentrations were higher than the 

measured aqueous concentrations at the Alamo River outlet 

site, similar at the Alamo River near-shore site, and lower at 

the New River outlet site. Predicted aqueous concentrations 

of trifluralin were similar to the measured aqueous concentrations at the Whitewater River outlet site, but they were higher 

than the measured concentrations at four other sites (table 22). 

Despite the above inconsistencies, it is clear that partitioning alone does not control pesticide distribution between 

water and suspended sediments. Bergamaschi and others 

(2001) noted this same phenomenon in the Sacramento and 

San Joaquin Rivers in northern California. They postulated 

that elevated concentrations of pesticides associated with suspended sediments in rivers are due to fresh sediments entering 

the rivers from surrounding agricultural fields during storm 

runoff. Domalgalski and Kuivila (1993) also noted this phenomenon for diazinon associated with suspended sediments in 

San Francisco Bay.

CS fOC COC = •

KOC

COC mg Kg –1

OC

 • 

CW(mg L –1

WATER

 •

= --------------------------------------------------- ,

The equation for Kd now becomes

,

Solving for Cw is now possible if one knows the value of Koc

for the specific compound, the concentration of the pesticide 

in the sediment, and the organic carbon (OC) concentration in 

the sediment: 

,

,
Table 19. Pesticides detected in water, suspended sediments, and bed sediments in the Salton Sea Basin, California, and key physical-chemical descriptors. 

[Matrix designations: W, water; BS, bed sediment; SS, suspended sediment. mg/L, milligram per liter. na, not available. Koc, the organic carbon-water partition 

coefficient. Data are from Mackay and others (1997)]

Pesticides Matrix Molecular weight Water solubility (mg/L) Log Koc

Atrazine W 215.68 30 2.2

Bifenthrin BS 422.9 .1 5.11−5.48

Butylate SS, BS 217.38 45 2.73

Carbaryl W, SS, BS 201.22 104 2.5

Carbofuran W, SS, BS 221.3 1

350,700 2.1

Cycloate W 215.4 75-95 2.6

Chlorpyrifos W, SS, BS 350.6 .3, 1.1 3.78

Cyfluthrin SS, BS 434.3 0.0012−0.003, 2 4 −5.13

Dacthal W, SS, BS 332.0 .50 3.78

Diazinon W, SS, BS 304.36 40 2.76

Diethylatyl-ethyl SS, BS 311.8 105 3.15

EPTC (eptam) W, SS, BS 189.31 365 2.38

Esfenvalerate SS 419.9 .002 3.72

lambda−Cyhalothrin BS 449.9 5.00 × 10-3 na

Malathion W 330.36 145 3.25

Oxyflurofen W, SS, BS 361.7 .1 2.63−2.80

Napropamide W 271.4 73 2.48−2.85

Pendimethalin W, SS, BS 281.3 .3 1.48−3.70

Permethrin SS, BS 391.3 1.53 × 10-5 4.8

Simazine W 201.67 5.0 2.14

Trifluralin W, SS, BS 335.5 .3, variable 3.63

1

Multiple values are reported in the literature and listed here.

Analysis Results and Discussion 31

Another factor contributing to the disequilibrium between 

water and sediment pesticide concentrations is that, over time, 

pesticides associated with sediment can become increasingly 

resistant to desorption into water. Because in many cases pesticides are applied directly to agricultural soil, a portion of the 

pesticides can become incorporated into the soil matrix and 

form associations (including chemical bonds) that can cause 

the pesticides to become resistant to desorption when the soil 

enters the aqueous environment. This desorption-resistant 

fraction, called the bound residue fraction in soils, has been 

described by several researchers (Calderbank, 1989; Führ and 

Ophoff, 1996; Lerch and others, 1997). 

For bed sediments, a comparison of sediment and aqueous concentrations did not show a consistent trend. For many 

pesticides, predicted aqueous concentrations were higher than 

measured concentrations by a factor of only two or less and 

often were below measured concentrations. This was consistently true for eptam and trifluralin, which differ markedly 

in aqueous solubility and log Koc. Bed sediments presumably 

have a longer residence time in the watershed compared with 

suspended sediments, and so have had a longer time to come 

into equilibrium with the surrounding water. Degradative 

processes can also confound predictions based on the partitioning model. Finally, it should be realized that there is much 

variability in reported values of Koc. Although the use of Koc

as a predictor of sorption for hydrophobic chemicals assumes 

an independence of the structure of the sorbing compound 

(as long as it is non-polar and hydrophobic) and the type of 

organic carbon present in the system (Boethling and Mackay, 

2000), literature values can vary for some compounds as much 

as 50 percent. This can be a function of different methods of 

determining Koc, as well as different sorbing properties of the 

test soil carbon. An in depth discussion of the limitations of 

the use of Koc can be found in Boethling and Mackay (2000) as 

well as in Swarzenbach and others (1993). 
32 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 20. Definition of terms used in equilibrium portioning calculations. 

[mg/kg, milligrams per kilogram; mg/L, milligram per liter; g OC/g, grams of organic carbon per gram]

Term Definition Units

COC Concentration of pesticide in sediment organic carbon mg/kgOC

CS Concentration of compound in sediment mg/kg dry weight

CW Concentration of compound in water mg/L

f

OC Fraction of organic carbon in sediment g OC/g sediment dry weight

Kd Sediment to water partition coefficient (mg/kgsediment)/(mg/Lwater)

KOC Sediment to organic carbon partition coefficient (mg/kgOC)/(mg/Lwater)

Table 21. Example calculation of an aqueous concentration of chlorpyrifos, based on equilibrium partitioning and measured values 

of suspended sediment organic carbon and chlorpyrifos concentration from the Alamo River outlet site in the Salton Sea Basin, 

California, fall 2001.

[CHN, carbon hydrogen, nitrogen]

Example calculation: CW = (fOC • COC)/Kd

Alamo River suspended sediment, fall 2001

Compound Log KOC

1 KOC2 f

OC

3

(g OC/g sed)

Kd

4 Cs5

(ng/g dry 

weight)

COC

6

(ng/g OC)

COC

7

(mg/kg OC)

CW

8

 

(mg/L)

CW

9

 

(ng/L)

CHLORPYRIFOS 3.78 6025.6 0.0093 56.0 50 5376 5.38 0.0009 892

1

Organic carbon:water partition coefficient (Mackay and others, 1997).

2

KOC = 10(LogK

OC).

3

f

OC = fraction of sediment organic carbon (measured by CHN analysis).

4

Kd = sediment:water partition coefficient for chlorpyrifos = f

OC × KOC.

5

Cs = Sediment chlorpyrifos concentration (in ng/g sediment dry weight).

6

COC = Sediment concentration of chlorpyrifos in sediment organic carbon: ng/g OC – (ng/g sediment dry weight)/(f

OC).

7

Units conversion: mg/kg = (ng/g)/1,000.

8

Cw = Predicted aqueous concentration (mg/L).

9

Units conversion: ng/L = (mg/L)*1,000,000.
Table 22. Predicted and measured aqueous concentrations in suspended sediments and bed sediments from the outlet and nearshore sites of the Alamo, New, and Whitewater rivers in the Salton Sea Basin, California, fall 2001. 

[ng/L, nanogram per liter. nd, not detected]

Pesticides

Suspended sediment Bed sediment

Predicted

(ng/L)

Measured

(ng/L)

Predicted

(ng/L)

Measured

(ng/L)

Alamo River outlet

Carbaryl nd nd 452 56.4

Chlorpyrifos 892 161 nd nd

EPTC (eptam) 2,958 418 nd nd

Trifluralin 245 37.4 nd nd

Alamo River near-shore

Chlorpyrifos 884 87.7 168 87.7

Dacthal 528 328 126 328

Diazinon 1,810 936 819 936

EPTC (eptam) 3,196 3,830 922 3,830

Trifluralin 238 37.6 16 37.6

New River outlet

Chlorpyrifos 290 44.3 458 44.3

EPTC (eptam) 1,174 3,490 4,394 3,490

Trifluralin 62.0 35.0 76.0 35.0

New River near-shore

Carbaryl nd nd 1,850 <4

Chlorpyrifos nd nd 489 15.0

EPTC (eptam) nd nd 1,774 1,300

Trifluralin 58.8 27.3 47.4 27.3

Whitewater River outlet

Dacthal nd nd 133 38.1

Trifluralin 17.6 22.5 31 22.5

Whitewater River near-shore

Diazinon nd nd 200 8.5

Trifluralin nd nd 7.1 <3

Analysis Results and Discussion 33

Although the distribution of pesticides between water and 

sediments was not quantitatively predictable based on physical-chemical properties, the presence or absence of pesticides 

in sediments, along with concentration patterns, appeared to 

be due to a combination of the physical-chemical properties 

of pesticides and pesticide-use patterns. Trifluralin, which 

has a relatively low solubility (0.3 mg/L) and a relatively high 

log Koc (3.6) compared with the other pesticides in table 19, 

was detected in sediment samples from all three sampling 

periods. Use of trifluralin was high in the watershed in 2001 

(277,604 lbs); concentration patterns matched the pesticideuse patterns, with higher suspended-sediment concentrations 

during the spring 2002 sampling period relative to those during the fall 2001 and fall 2002 sampling periods, and higher 

pesticide use during the winter/early spring 2001 (November 

2000 through March 2001) application period compared 

with use during the late spring/fall 2001 (April 2000 through 

October 2001) application period (tables 1, 17, 16, and 18, 

respectively). Chlorpyrifos, which has a low solubility 

(1.2 – 3 × 10-3 mg/L) and high log Koc (3.78), had high use 

during the fall 2001 application period (74,048 lbs); however, it was detected in sediment samples only during the 

fall. Use of chlopyrifos, although high, was half that for 

trifluralin between November 2000 and October 2001 (total 

of 101,368 lbs). Dacthal, which had an overall lower use 

(42,799 lbs) than chlorpyrifos and similar physical-chemical properties, was detected consistently and at the highest 

concentrations during both fall sampling periods (also the time 

of highest use) in the Alamo River. Permethrin, despite being 

used in far lower quantities (10,961 lbs) than chlorpyrifos, 

dacthal, and trifluralin, was detected in sediments but not in 

water during fall 2001 and spring 2001. This was not surprising given its extremely low water solubility (1 × 10-5 mg/L) 

and high log Koc value (4.8). Variable recoveries of the pyrethroid compounds from SPE cartridges cannot be discounted 

as a reason that none of these compounds were detected in 

water.
34 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Compounds having high aqueous solubility and a low 

tendency to partition onto sediments (for example, compounds 

with a low log Koc value) were detected occasionally in the 

sediment samples. Eptam, with a high aqueous solubility 

(365 mg/L), a low log Koc value (2.38), and extremely high 

use (106,569 lbs) was detected frequently in the sediment samples, although at low concentrations (below detection limits to 

9.2 ng/g). Atrazine, with a lower aqueous solubility 

(30 mg/L) but a similar log Koc (2.2), was detected only 

in water (table 19). The amount of atrazine applied in the 

watershed (19,208 lbs) was much lower than that for eptam. 

Malathion was used in larger quantities (122,730 lbs) than was 

eptam during November 2000 through October 2001. Malathion has a lower aqueous solubility and a higher log Koc value 

and therefore a greater tendency to partition onto sediments; 

but it was not detected in the sediment samples, which contradicts observations for the other pesticides. The distribution of 

pesticides between water and suspended and bed sediments 

appears to be a complicated function of pesticide use, 

physical-chemical properties, and the residence time of 

sediments in the watershed. 

Distribution of Pesticides between Colloidal 

and Truly Dissolved Fractions

The distribution of pesticides between the colloidal 

(retentate) and the “truly dissolved” (filtrate) fractions of the 

ultrafiltered water are presented in table 23. The term truly 

dissolved is used here to distinguish these concentrations from 

aqueous concentrations in water that has not been separated 

into colloidal, retentate, and filtrate fractions. For the sample 

collected from the New River outlet station in fall 2002, 4.7 

to 25 percent of the pesticides detected were associated with 

the retentate (colloidal) fraction. These pesticides included 

atrazine, carbofuran, dacthal, diazinon, eptam, molinate, and 

napropamide. All the atrazine in this sample appeared to 

be associated with the colloidal fraction, although the concentration was below the method detection limit (2 ng/μL), 

which makes this high percentage somewhat suspect. In the 

sample collected from New River off-shore site, between 4.7 

and 18.2 percent of the pesticides carbofuran, diazinon, and 

eptam were associated with the colloidal (retentate) fraction. 

Fonofos, which was completely found in the retentate fraction, had a very low concentration (4.8 ng/L) and, so again, the 

distribution is suspect. All the detectable atrazine (99 ng/L) 

is associated with the filtrate (truly dissolved) fraction in the 

New River off-shore sample. 

Pesticide concentrations in the ultrafiltered samples (with 

the filtrate and retentate fractions combined) and the filtered 

water extracted by SPE are shown in table 24. Although there 

are differences between the two samples, the concentrations of 

many of the pesticides are within a factor of two. 

A limited amount of samples were analyzed for this 

study. From this work, however, it can be noted that a measurable percentage of dissolved pesticides appear to be associated with colloids, which may have implications for the fate 

of dissolved pesticides (flocculation of colloid-associated 

pesticides at higher salinities) and pesticide bioavailability 

(lower bioavailability with greater colloidal complexation) and 

is worthy of further study.

Metals and Other Inorganic Constituents on 

Suspended Solids

Metals associated with continental crustal materials, such 

as iron (Fe), aluminum (Al), titanium (Ti), barium (Ba), chromium (Cr), and vanadium (V) were elevated in suspended sediments from the river outlet sites relative to those from the offshore sites owing to the dilution off-shore by large amounts of 

organic-rich particulate material (table 25). Cadmium (Cd), 

copper (Cu), and zinc (Zn) are more soluble and have greater 

potential for release from sediments and redistribution in the 

environment (Boehm and others, 1987; Allen, 1995), including bioaccumulation into organisms. The concentrations of 

these metals were more evenly distributed between the river 

outlet sites and the off-shore sites in the Alamo and New River 

transects. Although the metals were higher in concentration in 

suspended sediments at the outlet sites, the difference in concentrations between the river outlet sites and the off-shore sites 

differed by a factor of 2 or less. Differences in concentrations 

in metals derived from the continental crust were factors of 10 

or greater between the river outlet sites and the off-shore sites. 

Differences in concentrations between the river outlet sites 

and the off-shore sites for the Whitewater River were a often 

a factor of 10 or greater for both continental crust-derived elements and the more biologically active trace metals. Selenium 

(Se), which also can become remobilized from sediments, 

was elevated in the off-shore suspended sediments relative to 

suspended sediments in the river outlet sediments by factors of 

10 or greater for all three transects.
Table 23. Percent distribution of pesticides between colloidal (retentate) and dissolved (filtrate) fractions of ultrafiltered water from 

the New River outlet and off-shore sites, Salton Sea Basin, California, September 2002. 

[All values are in percent. nd, not detected]

Analyte

New River outlet New River off-shore

Filtrate Retentate Filtrate Retentate

Atrazine nd 100 100 nd

Butylate 100 nd nd nd

Carbaryl 93 7 nd nd

Carbofuran 88 12 82 18

Dacthal 82 18 nd nd

Diazinon 74 26 92 8

Disulfoton nd nd nd nd

EPTC (eptam) 87 13 95 5

Fenamiphos nd nd nd nd

Fonofos 100 nd nd 100

Methyl parathion 100 nd nd nd

Metolachlor 100 nd nd nd

Molinate 86 14 nd nd

Napropamide 89 11 nd nd

Pebulate nd nd nd nd

Table 24. Pesticide concentrations in the combined fractions of ultrafiltered water extracted by liquid-liquid partitioning (40L) to 

solid-phase extraction extracted water (1L), New River outlet and off-shore sites in the Salton Sea Basin, California, fall 2002. 

[Values in parenthesis are below detection limit. SPE, solid-phase extraction. ng/L, nanogram per liter; nd, not detected]

Pesticides

New River outlet New River off-shore

Ultrafiltered water

(ng/L)

SPE-extracted water

(ng/L)

Ultrafiltered water

(ng/L)

SPE-extracted water

(ng/L)

Atrazine 2.0 12.6 99.4 99.0

Bifenthrin nd 6.9 nd nd

Carbaryl 10.0 19.3 nd nd

Carbofuran 16.7 5.4 52.4 nd

Chlorpyrifos nd 7.02 nd nd

Cyfluthrin 29.2 14.9 nd nd

Dacthal 1.1 6.7 nd 2.6

Diazinon 12.2 22.6 8.5 (3.0)

Diethylatyl-ethyl nd nd 5.8 nd

EPTC(eptam) 8.5 22.3 12.6 15.0

Fonofos nd nd 4.8 nd

Metolachlor 2.4 nd nd nd

Molinate 10.8 nd nd nd

Napropamide 20.6 35.8 nd nd

Piperonyl butoxide 1.5 6.1 nd nd

Trifluralin (.1) 12.4 nd nd

Analysis Results and Discussion 35
36 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Table 25. Element concentrations in suspended sediment from samples from the Alamo, New, and Whitewater rivers in the Salton 

Sea Basin, California, fall 2001. 

[Concentrations are in micrograms per gram dry sediment weight, except for boldface values, which are in percent. Eu, Au, Ho, Nd, S, Ta, Sn, Yb and total 

carbon were not analyzed. <, less than]

Element Alamo River New River Whitewater River

Outlet Near-shore Off-shore Outlet Near-shore Off-shore Outlet Near-shore Off-shore

Aluminum (Al) 7.8 8.4 0.46 6.7 6.8 0.14 7.9 0.13 0.11

Antimony (Sb) .71 .82 .2 .76 .62 .02 .62 .02 <.02

Arsenic (As) 8.3 9.2 17 9.1 9.1 30 11 27 27

Barium (Ba) 490 460 34 520 480 19 630 14 13

Beryllium (Be) 1.9 2.1 .18 2 1.8 .04 2 .03 .03

Bismuth (Bi) <.005 <.005 <.005 .17 .12 <.005 .84 .1 <.005

Cadmium (Cd) .4 .44 .18 .45 .4 .12 .36 .15 .09

Calcium (Ca) 5.5 5.8 1.2 5.1 4.8 .88 5.5 .51 .54

Cerium (Ce) 63 71 3.4 67 62 1.2 85 .7 .8

Chromium (Cr) 120 70 60 57 110 28 83 7.9 8.2

Cobalt (Co) 10 11 2.8 9.9 9.5 4.5 19 5.9 4.5

Copper (Cu) 28 34 16 37 31 18 69 7.9 8

Gallium (Ga) 15 17 .91 15 14 .27 21 .27 .27

Iron (Fe) 2.9 3.3 .2 3 2.6 .071 5.2 .056 .051

Lanthanum (La) 33 37 2.2 35 32 .7 45 .5 .5

Lead (Pb) 22 24 27 23 23 15 27 2.5 1.9

Lithium (Li) 44 51 22 46 43 15 44 12 15

Magnesium (Mg) 2 2.1 .42 2.1 2 .36 2.3 .4 .45

Manganese (Mn) 670 700 250 650 740 140 1,200 170 120

Mercury (Hg) .04 .04 .06 .07 .07 .05 .08 .03 .04

Molybdenum (Mo) 6.3 1.3 3 2.5 6.5 2.3 4.1 1.1 1

Nickel (Ni) 51 34 22 30 43 11 36 6 6

Niobium (Nb) 18 20 <2 17 12 <2 26 <2 <2

Phosphorous (P) .11 .13 .48 .2 .24 .54 .29 .6 .55

Potassium (K) 2 2.1 .42 2.1 2 .36 2.3 .4 .45

Scandium (Sc) 11 12 .9 11 9.5 <.3 16 <.3 <.3

Selenium (Se) .7 1 8.7 1 1.4 16 1 14 11

Silver (Ag) <3 <3 <3 <3 <3 <3 <3 <3 <3

Sodium (Na) .54 .52 6.4 .6 1.6 4.6 1.3 3.8 4.7

Strontium (Sr) 320 340 230 320 280 200 500 150 140

Thallium (Tl) 3,100 3,600 170 3,400 3,000 50 6,200 50 40

Thorium (Th) 11 12 .52 11 10 .18 15 .13 .09

Titanium (Ti) .62 .68 .01 .65 .61 <.003 .77 <.003 <.003

Uranium (U) 3 3.6 5.7 3.3 3 4.7 4.4 1.2 1.2

Vanadium (V) 78 90 16 80 69 23 110 17 16

Yttrium (Y) 23 25 1.6 27 30 .5 26 <.3 .3

Zinc (An) 86 100 92 120 100 62 190 27 27

Total Organic Carbon .9 1.2 30 1.7 2.7 39 3.6 39 38

Total Inorganic Carbon 7.8 8.4 .46 6.7 6.8 .14 1.1 .02 .02
Table 26. Element enrichment ratios for suspended sediment 

from the Salton Sea Basin, California, fall 2001. 

[*, value not calculated because too many concentrations were below the 

detection limit. ~, about]

Element Symbol Enrichment ratio

Antimony Sb 3

Arsenic As 120

Barium Ba 1.5

Beryllium Be 1.2

Bismuth Bi *

Cadmium Cd 14

Calcium Ca 6.2

Cerium Ce .8

Chromium Cr 11

Cobalt Co 15

Copper Cu 14

Gallium Ga .9

Iron Fe 1

Lanthanum La .9

Lead Pb 19

Lithium Li 15

Magnesium Mg 16

Manganese Mn 8

Mercury Hg 30

Molybdenum Mo 18

Nickel Ni 31

Niobium Nb *

Phosphorus P 140

Potassium K 8.1

Scandium Sc ~1.3

Selenium Se 550

Silver Ag *

Strontium Sr 21

Thallium Tl .3

Thorium Th .7

Titanium Ti .7

Uranium U 40

Vanadium V 8.8

Yttrium Y 1

Zinc Zn 18

Analysis Results and Discussion 37

Ratios were calculated to demonstrate the enrichment of 

certain metals in off-shore algae. This was done by normalizing metal concentrations to a single constituent associated with 

the riverine contribution of continental crust materials (in this 

case aluminum) and calculating the ratio of off-shore particulate concentrations to riverine particulate concentrations for 

each constituent. The resulting ratio is a measure of the degree 

to which metals and other constituents are concentrated in the 

off-shore suspended particulates and, by inference, enriched in 

algae. The use of enrichment ratios is described in more detail 

in Schroeder (1985, 1995) and Schroeder and others (1988). 

The enrichment ratio values for trace metals and other 

inorganic constituents are presented in table 26. Those elements associated with aluminosilicate materials, such as the 

rare earth elements (for example, lanthanum), had ratios of 

approximately 1 (no enrichment). A number of constituents 

that are important in biological metabolism, such as calcium 

and potassium, as well as magnesium and zinc, had ratios of 

6 to 18. Elements known as “heavy metals” such as copper, chromium, lead, nickel, and mercury have been shown 

to accumulate in phytoplankton (Fisher, 1986; Luoma and 

others, 1998) and had ratios between 10 and 30. Phosphorous, an essential nutrient, had a very high enrichment ratio 

(140), which is consistent with the observation that off-shore 

suspended sediment contained a high percentage of algal 

material. Selenium had a ratio value that was the same order 

of magnitude as phosphorous. Arsenic also had a very high 

enrichment value.
38 Occurrence, Distribution, Transport of Pesticides, Trace Elements, and Inorganic Constituents, Salton Sea Basin, Calif.

Summary and Conclusions

Current-use pesticides were detected in water, suspended 

sediments, and bed sediments collected from the Salton Sea 

Basin, California, during the fall 2001, spring 2002, and fall 

2002 sampling periods. Dissolved concentrations were similar 

to concentrations reported in previous studies, which were 

shown to cause toxicity to aquatic organisms. Differences in 

pesticide concentrations between spring and fall most likely 

reflect pesticide use during the application periods. Aqueous pesticide concentrations along the transects (river outlet, 

near-shore delta, and off-shore sites) for many pesticides were 

highest at the outlet sites compared with concentrations at the 

near-shore and off-shore sites, which reflects dilution of the 

river water by water from the Salton Sea. However, there were 

also instances where aqueous concentrations of some compounds were similar or higher at the near-shore and off-shore 

sites than at the outlet sites, which may have been due to the 

transport of concentration pulses into the Salton Sea. The 

number of pesticides detected and their concentrations generally were higher in samples from the New and Alamo Rivers 

than those in the samples from the Whitewater River. Overall 

concentrations were higher in fall 2001 than in spring 2002 

and fall 2002. A measurable fraction (10−25 percent) of pesticides were determined to be associated with colloidal material 

isolated by ultrafiltration. 

The concentration of current-use pesticides in sediments 

ranged from 1 to 106 ng/g. There were differences between 

rivers in terms of the numbers of pesticides detected and 

their concentrations; the New and Alamo Rivers had more 

compounds that had higher concentrations in suspended and 

bed sediments than did the Whitewater River for both suspended and bed sediments. An exception was DDE, which 

was found in similar concentrations in bed sediments from 

all three rivers. Seasonal trends for pesticides associated with 

suspended sediments were not as pronounced as those for the 

dissolved concentrations, although they were similar for a few 

pesticides, notably chlorpyrifos, which had higher sediment 

concentrations in fall 2001, and trifluralin, which had higher 

concentrations in spring 2002. In the Alamo River, suspended 

sediment pesticide concentrations were nearly as high or 

higher at the near-shore site compared with concentrations 

at the outlet site; this may have been due to the transport of a 

pulse of sediment from the river (outlet) site to the near-shore 

site. Elevated concentrations of pesticides in suspended sediments at the near-shore sites compared with concentrations 

at the outlet sites also were seen for fewer compounds in the 

Alamo and New Rivers in spring 2002 and fall 2002. There 

was no consistent detection of pesticides in suspended or bed 

sediments from the off-shore sites, which also had the highest 

degree of matrix interference. 

As with water, the pesticides detected in sediments were 

the pesticides most heavily used in the watershed; thus, the 

differences in concentration between seasons were at least 

partly related to pesticide-use patterns. Chlorpyrifos, dacthal, 

and eptam, used heavily during fall 2001, were detected in 

suspended sediments. Trifluralin, used heavily during spring 

2001, was detected at high concentrations in spring 2002. 

Concentrations of pesticides on suspended sediments generally were above equilibrium with the water; this may be due to 

the presence of a sediment-associated pesticide fraction that is 

resistant to desorption. Concentrations in bed sediment were 

closer to equilibrium, which partly may be due to the longer 

residence time of bed sediments in the watershed.

The concentrations of metals in suspended sediments 

followed expected trends, with continental crustal materials 

decreasing in concentration from the river to the off-shore 

sites. Enrichment ratios provided evidence of elevated concentrations of several metals, most notably selenium, in off-shore 

suspended sediments, which, based on visual and geochemical characterization, were composed primarily of algae. The 

high enrichment values for these constituents may represent 

concentrations of trace metals in the algae within the Salton 

Sea Basin.

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