U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report

Pesticides in Surface Waters of the

Upper Colorado River Basin, Colorado,

1996–98

By Nancy J. Bauch and Norman E. Spahr

Denver, Colorado

2000

00 – 4005
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

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

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225-0046
FOREWORD

The mission of the U.S. Geological Survey

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

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

decisions. Assessment of water-quality conditions and

trends is an important part of this overall mission.

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

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

water resources. That challenge is being addressed by

Federal, State, interstate, and local water-resource

agencies and by many academic institutions. These

organizations are collecting water-quality data for a

host of purposes that include: compliance with permits

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

water quality. An additional need for water-quality

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

decisions must be based on sound information. As a

society we need to know whether certain types of

water-quality problems are isolated or ubiquitous,

whether there are significant differences in conditions

among regions, whether the conditions are changing

over time, and why these conditions change from

place to place and over time. The information can be

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

need for and likely consequences of new policies.

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

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

the program. The NAWQA Program builds upon an

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

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

The objectives of the NAWQA Program are to:

• Describe current water-quality conditions for a

large part of the Nation’s freshwater streams,

rivers, and aquifers.

• Describe how water quality is changing over

time.

• Improve understanding of the primary natural

and human factors that affect water-quality

conditions.

This information will help support the development

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

agencies to protect, use, and enhance water resources.

The goals of the NAWQA Program are being

achieved through ongoing and proposed investigations

of 59 of the Nation’s most important river basins and

aquifer systems, which are referred to as study units.

These study units are distributed throughout the

Nation and cover a diversity of hydrogeologic settings.

More than two-thirds of the Nation’s freshwater use

occurs within the 59 study units and more than twothirds of the people served by public water-supply

systems live within their boundaries.

National synthesis of data analysis, based on

aggregation of comparable information obtained from

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

This effort focuses on selected water-quality topics

using nationally consistent information. Comparative

studies will explain differences and similarities in

observed water-quality conditions among study areas

and will identify changes and trends and their causes.

The first topics addressed by the national synthesis are

pesticides, nutrients, volatile organic compounds, and

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

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

as the information becomes available.

This report is an element of the comprehensive

body of information developed as part of the NAWQA

Program. The program depends heavily on the advice,

cooperation, and information from many Federal,

State, interstate, Tribal, and local agencies and the

public. The assistance and suggestions of all are

greatly appreciated.

Robert M. Hirsch

Chief Hydrologist

FOREWORD III
CONTENTS V

CONTENTS

Foreword ................................................................................................................................................................................ III

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

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

Purpose and Scope....................................................................................................................................................... 3

Description of the Upper Colorado River Basin Study Unit ....................................................................................... 3

Study Design................................................................................................................................................................ 6

Sampling and Laboratory Methods ............................................................................................................................. 8

Acknowledgments ....................................................................................................................................................... 13

Quality-Control Methods and Results ................................................................................................................................... 13

Hydrologic Conditions During Sampling.............................................................................................................................. 21

Occurrence and Distribution of Pesticides............................................................................................................................. 23

Pesticides at Fixed-Station Network............................................................................................................................ 26

Temporal Distribution........................................................................................................................................ 26

Spatial Distribution ............................................................................................................................................ 31

Pesticides at Synoptic Network ................................................................................................................................... 35

Comparison of Pesticides at Fixed-Station Network and Synoptic Network.............................................................. 40

Comparison With Results of Previous Studies ...................................................................................................................... 41

Summary and Conclusions .................................................................................................................................................... 43

References.............................................................................................................................................................................. 44

FIGURES

1–2. Maps showing:

1. Location of the Upper Colorado River Basin study unit, physiographic provinces, and

fixed-station sites for pesticide sampling .......................................................................................................... 2

2. Location of agricultural synoptic sites in the Grand Valley and Uncompahgre River Valley,

Upper Colorado River Basin ............................................................................................................................. 4

3. Graphs showing streamflow and pesticide sample-collection dates for fixed-station sites, Upper Colorado

River Basin, October 1996–May 1998...................................................................................................................... 22

4. (A) Graph showing frequency of pesticide detections and (B) boxplots showing concentrations

of detected pesticides for the fixed-station sites sampled in the Upper Colorado River Basin study unit,

October 1996–January 1998 ..................................................................................................................................... 29

5–8. Graphs showing:

5. Number of pesticide detections per sampling date, fixed-station sites, Upper Colorado River Basin,

October 1996–January 1998.............................................................................................................................. 30

6. Mean number of pesticide detections per sample by month, all fixed-station sites combined,

Upper Colorado River Basin, October 1996–January 1998.............................................................................. 31

7. Atrazine concentrations at Dry Creek near Begonia Road, Reed Wash near Mack, and Colorado River

near State Line, October 1996–January 1998 ................................................................................................... 32

8. Carbofuran and 2,4–D concentrations at Reed Wash near Mack, October 1996–January 1998....................... 33

9. (A) Graph showing frequency of pesticide detections and (B) boxplots showing concentrations of detected

pesticides for the agricultural synoptic sites sampled in the Upper Colorado River Basin, May 1998.................... 36

10–11. Graphs showing:

10. Number of pesticide detections per site in the Grand Valley and Uncompahgre River Valley, May 1998....... 39

11. Frequency of pesticide detections in the Grand Valley and Uncompahgre River Valley, May 1998................ 40
VI CONTENTS

TABLES

1. Harvested acreage for crops and percentage of harvested acreage irrigated in 10 counties in the

Upper Colorado River Basin study unit, Colorado, 1997 ....................................................................................... 5

2. Description of surface-water-sampling sites for pesticides in the Upper Colorado River Basin

fixed-station network............................................................................................................................................... 6

3. Agricultural synoptic surface-water-sampling sites, Grand Valley, Upper Colorado River Basin ......................... 7

4. Agricultural synoptic surface-water-sampling sites, Uncompahgre River Valley, Upper Colorado River Basin... 8

5. Pesticide target analytes, method reporting limits, drinking-water standards, and aquatic-life criteria ................. 9

6. Concentrations and relative percent differences for pesticides detected in replicate samples................................ 15

7. Mean and standard deviation of pesticide recovery in 30 field-spiked samples determined by gas

chromatography/mass spectrometry........................................................................................................................ 20

8. Mean and standard deviation of pesticide recovery in 30 field-spiked samples determined by

high-performance liquid chromatography............................................................................................................... 20

9. Mean and standard deviation of pesticide surrogate recovery ................................................................................ 21

10. Summary of pesticide occurrence and concentrations for all 44 surface-water sites sampled in the

Upper Colorado River Basin study unit, October 1996 through January 1998 and May 1998 .............................. 24

11. Statistical summary of concentrations of detected pesticides at fixed-station sites in the Upper Colorado

River Basin, October 1996–January 1998............................................................................................................... 27

12. Pesticide detections per land use and site type at fixed-station sites in the Upper Colorado River Basin,

October 1996–January 1998 ................................................................................................................................... 34

13. Statistical summary of concentrations of detected pesticides at 43 agricultural synoptic sites in the

Upper Colorado River Basin, May 1998................................................................................................................. 35

14. Pesticide concentrations exceeding freshwater criteria guidelines for the protection of aquatic life,

Grand Valley and Uncompahgre River Valley, Upper Colorado River Basin agricultural synoptic study,

May 1998 ................................................................................................................................................................ 38

15. Summary of selected pesticide occurrences and concentrations for all 1,058 surface-water sites sampled as

part of 20 National Water-Quality Assessment Program study units, 1992–96...................................................... 42

CONVERSION FACTORS AND ABBREVIATIONS

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

°C = 5/9 (°F – 32)

Multiply By To obtain

acre 4,047 square meter (m2)

acre-foot (acre-ft) 1,233 cubic meter (m3

)

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

inch 25.4 millimeter (mm)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

kilogram (kg) 2.2046 pound (lb)

liter (L) 0.2642 gallon (gal)

square mile (mi2

) 2.590 square kilometer (km2)
CONTENTS VII

ADDITIONAL ABBREVIATIONS

E estimated

GC/MS gas chromatography/mass spectrometry

HPLC high-performance liquid chromatography

μg/L micrograms per liter

μm micrometer

mm millimeter

MRL method reporting limit

NAWQA National Water-Quality Assessment

NWQL National Water Quality Laboratory

UCOL Upper Colorado River Basin study unit

USEPA U.S. Environmental Protection Agency

WY water year
Abstract 1

Pesticides in Surface Waters of the Upper

Colorado River Basin, Colorado, 1996–98

By Nancy J. Bauch and Norman E. Spahr

Abstract

Forty-four river, stream, and drain sites in

the Upper Colorado River Basin in Colorado were

sampled during 1996–98 to determine the occurrence and distribution of pesticides in the basin. In

a fixed-station study, 57 surface-water samples

were collected from October 1996 through

January 1998 at four sites. Each site was sampled

approximately monthly for up to a year, with

more frequent sampling during the spring and

summer growing season. In a synoptic study,

surface-water samples were collected at 43 sites

in the agricultural areas of the Grand Valley and

the Uncompahgre River Valley in May 1998.

Each site was sampled once.

Pesticide concentrations generally were low

and varied seasonally and across land uses.

Thirty-five pesticides were detected at least once

in the two studies, and 11 pesticides accounted for

almost 82 percent of the detections. Herbicides

were more commonly detected than insecticides,

and only the herbicides alachlor and atrazine were

detected in more than 50 percent of the samples.

Carbofuran was the most commonly detected

insecticide and was detected in 19 percent of the

samples. Pesticide detections increased and were

measured at higher concentrations in the summer

months and at the agriculture sites. All pesticide

concentrations were less than drinking-water

standards, and most complied with human-health

advisories and criteria for the protection of freshwater aquatic life. Pesticides in the Upper Colorado River Basin in Colorado were detected

slightly less frequently and generally at lower

concentrations than in 20 National Water-Quality

Assessment Program study-unit basins that

collected water-quality data from 1992 through

1996. Results from surface-water sampling

conducted during 1996–98 in the Upper Colorado

River Basin in Colorado indicate that beneficial

uses of water were not being impaired by the

presence of pesticides in surface waters in the

basin.

INTRODUCTION

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

began full implementation of the National WaterQuality Assessment (NAWQA) Program. The goals

of the NAWQA Program are to (1) describe current

water-quality conditions for a large part of the

Nation’s freshwater streams, rivers, and aquifers;

(2) describe how water quality is changing over time;

and (3) improve understanding of the primary natural

and human factors that affect water-quality conditions

(Leahy and others, 1990). Many of the Nation’s most

important river basins and aquifers, represented as

study units, are being investigated through the

NAWQA Program. The Upper Colorado River Basin

(UCOL) study unit (fig. 1), located primarily in

western Colorado, is one of the NAWQA study units.

As part of the NAWQA Program, investigations of the

hydrology and water quality of the UCOL began in

1994. The occurrence and distribution of pesticides in

surface water are two of the main topics of interest in

the NAWQA Program. Issues of concern include the

types and concentrations of pesticides in the Nation’s

rivers and streams, seasonal distribution of pesticides,

relation of pesticide occurrences and concentrations to

land use, and potential of pesticide concentrations to
2 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Gore Creek

Dry Creek

Reed Wash

COLORADO

PLATEAU

SOUTHERN

ROCKY

MOUNTAINS

N

Figure 1. Location of the Upper Colorado River Basin study unit, physiographic provinces, and fixed-station sites for

pesticide sampling.
INTRODUCTION 3

affect human health and aquatic ecosystems. In the

UCOL, water samples were collected at four sites

(fig. 1) during 1996–98 to determine temporal and

spatial variations in the occurrence and concentrations

of pesticides. A synoptic study of 43 sites (fig. 2) was

completed in 1998 to determine the spatial distribution

of pesticide concentrations within agricultural areas of

the UCOL.

Purpose and Scope

The purposes of this report are to (1) describe

pesticide concentrations in rivers, streams, and drains

in the UCOL study unit during 1996–98 and (2) determine the distribution of pesticide detections and

concentrations among different land uses, site types,

and agricultural subbasins in the study unit. Surfacewater sites in the UCOL basin were sampled, and

samples were analyzed for a series of pesticide

compounds to temporally and spatially describe pesticide concentrations. Four river and stream sites in the

study unit were sampled frequently for up to one year.

Forty-three river, stream, and drain sites in agricultural

areas of the study unit were sampled once during a

synoptic study.

Description of the Upper Colorado River

Basin study unit

The UCOL study unit has a drainage area of

about 17,800 mi2 (fig. 1), all located in western Colorado except for about 100 mi2 in eastern Utah (Driver,

1994). The study unit is divided almost equally into

two physiographic provinces—the Southern Rocky

Mountains in the east and the Colorado Plateau in the

west (fig. 1). The environmental setting of the UCOL

study unit and its physiographic provinces has been

described by Apodaca and others (1996).

Land uses in the UCOL study unit include

rangeland or forest, mining, urban, and agriculture.

Rangeland or forests make up about 85 percent of the

study unit area and are used for recreation, wildlife

habitat, livestock grazing, and logging. Tourism and

recreation are the major land-use activities in the

Southern Rocky Mountains province, and agriculture

is the predominant land-use activity in Delta, Mesa,

and Montrose Counties in the Colorado Plateau.

Tourism, recreation, and agriculture are all important

land-use activities in Gunnison County, located mostly

in the Southern Rocky Mountains physiographic province. Because of the semiarid climate (generally, less

than 10 in/yr of precipitation in the western valleys),

little agricultural production is possible without irrigation. Irrigation takes place mainly in river valleys and

in low-altitude areas of the Colorado Plateau. In the

Grand and Uncompahgre River Valleys (fig. 2), agricultural areas are supported by an extensive system of

canals and ditches. About 97 percent of the off-stream

water use in the study unit is for irrigation (Apodaca

and others, 1996).

The major crops produced in the study unit are

alfalfa and other hay, corn (grain and silage), dry

beans, grains, vegetables (primarily sweet corn and

onions), fruit (apples, tart cherries, peaches, and

pears), melons (cantaloupe and watermelon), and

grapes for wine production. Acreage and production

data for Colorado in 1997 are available by county for

alfalfa, other hay, corn (grain and silage), dry beans,

winter and spring wheat, barley, and oats (Colorado

Department of Agriculture, 1998) and are shown in

table 1 for 10 counties in the study unit. Alfalfa and

other hay together were the major crops produced in

each county in 1997 and were the only crops tallied for

Eagle, Grand, Gunnison, Ouray, Pitkin, and Summit

Counties. Dry beans and corn also were commonly

harvested in Delta and Montrose Counties, while corn

and winter wheat were commonly produced in Mesa

County in 1997. Most (97.6 percent) of the harvested

acreage was irrigated. Alfalfa, other hay, and winter

wheat were produced on the nonirrigated land.

Acreage and production data for individual counties

were not available for vegetables, fruit, melons, and

grapes.

Pesticide-use data for the 1996–98 sampling

period were not available for the study unit. Pesticide

use by crop type in Colorado was estimated by

Bohmont (1991, 1993) for seven regions in the State

for 1989 and statewide for 1992. In the 1989 survey,

four counties (Garfield, Mesa, Delta, and Montrose) of

the UCOL study unit were represented as the West

Central agricultural region. Ouray County was

included in the Four Corners region, and no pesticideuse data were reported for Eagle, Grand, Gunnison,

Pitkin, and Summit Counties. The six most common

pesticides applied to irrigated crops in the West

Central agricultural region in 1989 were (1) EPTC,

(2) alachlor, (3) 2,4-D, (4) parathion-ethyl [parathion],

(5) metolachlor, and (6) atrazine, as measured by
4 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

0

0 5

5 10 MILES

10 KILOMETERS

0

0

5

5

10 MILES

10 KILOMETERS

Figure 2. Location of agricultural synoptic sites in the Grand Valley and Uncompahgre River Valley, Upper Colorado River Basin.
INTRODUCTION

5

Table 1. Harvested acreage for crops and percentage of harvested acreage irrigated in 10 counties in the Upper Colorado River Basin study [Data from Colorado Department of Agriculture, 1998; --, no data]

Crop

County

Delta Eagle Garfield1 Grand Gunnison Mesa1 Montrose1 Ouray Pitkin 

Alfalfa 24,000 6,000 31,000 1,500 500 32,000 32,000 3,000 1,500 Hay2 5,500 5,500 10,000 41,000 22,500 12,000 9,000 8,000 4,000 

Corn—grain and silage 8,600 -- -- -- -- 10,000 11,800 -- -- Dry beans 3,500 -- -- -- -- 500 13,000 -- -- Winter wheat 500 -- 1,500 -- -- 4,000 2,000 -- -- Oats 900 -- 600 -- -- 500 1,000 -- -- Barley -- -- 200 -- -- 200 1,100 -- -- Spring wheat -- -- -- -- -- -- 500 -- -- Total harvested acres 43,000 11,500 43,300 42,500 23,000 59,200 70,400 11,000 5,500 Percentage harvested

acres irrigated

98.4 95.7 93.5 95.3 100 98.3 99.3 100 100 

1 Includes those areas of the county outside the boundaries of the Upper Colorado River Basin study unit. Actual values for the study unit may be less.2 Includes wild, millet, sudan, clover and timothy, grain, and other miscellaneous tame hays.
6 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

pounds active ingredient applied. These pesticides

were primarily applied to corn, dry beans, pasture, and

alfalfa. Newer pesticides, such as acetochlor, were not

included in the 1989 or 1992 pesticide-use surveys.

Nationally, the most heavily used pesticides in agriculture in 1992 were the herbicides (1) atrazine, (2) metolachlor, (3) 2,4-D, (4) cyanazine, and (5) alachlor

(U.S. Geological Survey, 1998a).

Study Design

The surface-water sampling for this study was

designed to investigate the occurrence and temporal

and spatial distribution of pesticides in the rivers,

streams, and drains of the UCOL study unit. To

accomplish this, two sampling networks were developed—the fixed-station network and the synoptic

network.

The fixed-station network was developed out of

the larger surface-water-monitoring network of the

UCOL study unit (Spahr and others, 1996). Four

stations were designated for pesticide collection, one

in the Southern Rocky Mountains physiographic province and three in the Colorado Plateau physiographic

province (fig. 1, table 2). Three of the stations —Gore

Creek at mouth near Minturn, Dry Creek near Begonia

Road, near Delta, and Reed Wash near Mack—were

selected as “indicator” sites. These sites represent a

particular land use and in this study represented either

urban/forest or agriculture. The watershed for the Gore

Creek station includes Vail, Colorado, a resort town

with a permanent population of 4,353 in 1995 (Northwest Colorado Council of Governments, 1997) and

substantial increases in summer and winter because of

tourism. Most of the Gore Creek watershed is forest

land that is included in the White River National

Forest. Both Dry Creek near Begonia Road and Reed

Wash near Mack are natural watercourses in the agricultural areas of the Uncompahgre River Valley and

Grand Valley, respectively, and function as drains for

irrigation return flows during parts of the year. The

final station in the fixed-station network, the Colorado

River near Colorado-Utah State Line, was selected as

an “integrator” site. Located at the terminus of the

study unit, this station represented the integration of

water quality from all land uses in the Upper Colorado

River Basin in Colorado, including agriculture, forest,

mining, rangeland, and urban. Each station was

sampled approximately monthly for a 6-month to

1-year time period; sampling occurred from October

1996 through January 1998. Samples were collected

more frequently (weekly or biweekly) during the

spring and summer growing season. The total number

of samples collected at each site varied, depending on

expected pesticide use, and was greater for the Dry

Creek near Begonia Road and Reed Wash near Mack

agriculture sites.

Table 2. Description of surface-water-sampling sites for pesticides in the Upper Colorado River Basin fixed-station

network

[USGS, U.S. Geological Survey]

Site

number

(fig. 1)

Site name USGS site

number

Sampling

period

Number of

samples

(excluding

quality

assurance)

Site type Land use

Drainage

area

(square

miles)

FS1 Gore Creek at mouth near

Minturn

09066510 10/96–09/97 10 Indicator Urban/Forest 102

FS2 Dry Creek near Begonia

Road, near Delta

09149480 10/96–09/97 15 Indicator Agriculture 175

FS3 Reed Wash near Mack 09153290 10/96–09/97 24 Indicator Agriculture 16

FS4 Colorado River near

Colorado-Utah State Line

09163500 04/97–01/98 8 Integrator All1 17,843

1

Includes agriculture, forest, mining, rangeland, and urban.
INTRODUCTION 7

In the UCOL study unit, areas of intensive agriculture are located in the Grand Valley and the

Uncompahgre River Valley (fig. 2). Both areas have

been used for agriculture since the 1880’s. Extensive

systems of canals and ditches provide the means for

delivery of irrigation water, and many drains have

been developed to carry the irrigation return flows to

rivers and streams in the two areas. A synoptic study

investigating the water quality of agricultural drains

in the Grand and Uncompahgre River Valleys was

conducted in May 1998. Water samples collected from

a network of 43 river, stream, and drain sites were

analyzed for physical properties and chemical constituents, nutrients, major ions, pesticides, and suspended

sediment. This report focuses only on the pesticide

data. May was chosen as the time period for the

synoptic sampling because pesticides were frequently

detected in this month in the fixed-station sampling

during the previous year, and May is early in the

growing season. Site selection for the synoptic study

was based on the availability of historical pesticideoccurrence data (Butler and others, 1991, 1994)

and on the spatial distribution of streams and drains.

Twenty-three sites in the Grand Valley area (hereinafter Grand Valley), including two sites in the

fixed-station network pesticide-sampling scheme,

were sampled once in May 1998 (fig. 2, table 3).

Twenty sites in the Uncompahgre River Valley area

(hereinafter Uncompahgre River Valley), including

one fixed-station pesticide-sampling site, were

sampled once in May 1998 (fig. 2, table 4). The

Gunnison River near Grand Junction site (URV20),

located downstream from the Uncompahgre River

Valley, is included in table 4 with the Uncompahgre

River Valley sites because its streamflow includes all

of the streamflow of the Uncompahgre River Basin.

Table 3. Agricultural synoptic surface-water-sampling sites, Grand Valley, Upper Colorado River Basin

[USGS, U.S. Geological Survey]

Site

number

(fig. 2)

Site name USGS

site number

Sampling

date

GV1 Drain at 36 Road, East Orchard Mesa 390517108230501 05/05/98

GV2 Drain at C 1/2 and 33 3/4 Roads, Central Orchard Mesa 390322108253401 05/05/98

GV3 33 Road Drain at mouth near Clifton 390347108265800 05/05/98

GV4 Lewis Wash near Grand Junction 09106200 05/05/98

GV5 Drain at D and 29 3/4 Roads 390345108073301 05/06/98

GV6 Drain at C 1/2 and 28 1/2 Roads 390319108312501 05/06/98

GV7 Indian Wash at C 1/2 Road 390320108315901 05/06/98

GV8 Orchard Mesa Drain at Grand Junction 09152600 05/07/98

GV9 Leach Creek at Durham 09152650 05/06/98

GV10 Drain along Redlands Parkway, at mouth 390514108373201 05/07/98

GV11 Appleton Drain near mouth at G Road 390624108374900 05/07/98

GV12 Limekiln Gulch near mouth 390529108385401 05/07/98

GV13 Persigo Wash at River Road 390645108390101 05/11/98

GV14 Copeco Drain near mouth at Hwy 50 390723108400500 05/11/98

GV15 Hunter Wash at River Road 390717108400501 05/12/98

GV16 Goodwin Drain at Highway 50 390747108405600 05/12/98

GV17 Adobe Creek near Fruita 09152900 05/12/98

GV18 Little Salt Wash at Hwy 50 390938108443101 05/12/98

GV19 Big Salt Wash at Fruita 09153270 05/13/98

GV20 Reed Wash near Loma 09153300 05/13/98

GV21 Reed Wash near Mack 09153290 05/08/98

GV22 Salt Creek near Mack 09163490 05/13/98

GV23 Colorado River near Colorado-Utah State Line 09163500 05/20/98
8 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Sampling and Laboratory Methods

Water-quality samples for pesticide analysis

were collected by equal-width-increment sampling

and processed onsite using techniques described by

Shelton (1994). Pesticide samples were filtered in the

field through a 142-mm-diameter, 0.7-μm baked glassfiber filter and stored on ice for shipment or delivery to

the USGS National Water Quality Laboratory

(NWQL) in Arvada, Colorado.

The dissolved pesticide samples were extracted

and analyzed at the NWQL using methods described

by Zaugg and others (1995) and Werner and others

(1996). The concentrations of 83 pesticides and pesticide degradation products (hereinafter known as pesticides) were determined either by gas chromatography

with mass spectrometry (GC/MS) or high-performance liquid chromatography (HPLC) with ultraviolet

spectroscopy. Forty-seven pesticides were analyzed by

GC/MS and 39 by HPLC (table 5). Both methods

analyzed for carbaryl, carbofuran, and linuron. Water

samples from both the fixed-station network and the

synoptic network were analyzed for the same pesticides. A total of 155 (86 fixed-station, 69 synoptic)

water samples, including quality-assurance samples,

were analyzed for pesticides at the NWQL for this

UCOL pesticide study.

Pesticide concentrations are reported in terms of

method reporting limits (MRLs). The MRL is the

minimum concentration of a pesticide that can be reliably reported for an analytical method (Timme, 1995).

For this study, MRLs ranged from 0.001 to 1.2 μg/L

(table 5). Some MRLs for pesticides analyzed with the

HPLC method were adjusted upwards on December

15, 1997 (table 5). Pesticide concentrations less than

the MRL are reported with a “<” symbol. Concentrations for some pesticides are reported as estimated (E)

concentrations because of variable analytical performance, or a measured concentration was greater than

the maximum method calibration value for the particular pesticide. For a pesticide with an estimated

concentration, the pesticide has passed all qualitative

Table 4. Agricultural synoptic surface-water-sampling sites, Uncompahgre River Valley, Upper Colorado River

Basin

[USGS, U.S. Geological Survey]

Site

number

(fig. 2)

Site name USGS

site number

Sampling

date

URV1 Uncompahgre River at Montrose at Hwy 90 382831107530601 05/21/98

URV2 Happy Canyon Creek at Marine Road, near mouth 382928107541101 05/22/98

URV3 Cedar Creek near mouth 383041107544201 05/21/98

URV4 Spring Creek at Jay-Jay Road 383201107575301 05/20/98

URV5 Drain at Blossom Road, near Chipeta 383834108001701 05/19/98

URV6 Loutsenhizer Arroyo below Garnet Canal, at mouth 383953108001701 05/19/98

URV7 Dry Creek near Begonia Road, near Delta 09149480 05/21/98

URV8 Dry Creek at mouth, near Delta 384202108032001 05/21/98

URV9 Garnet Canal above Sweitzer Lake diversion 384247108012401 05/19/98

URV10 Uncompahgre River at Delta 09149500 05/14/98

URV11 Currant Creek near Read 09137050 05/15/98

URV12 Alfalfa Run at Austin 384649107570501 05/15/98

URV13 Peach Valley Arroyo near mouth 384604107570701 05/19/98

URV14 Unnamed drainage at Highway 92 near Read 384551107591901 05/14/98

URV15 Tongue Creek at Cory 09144200 05/18/98

URV16 Cummings Gulch at mouth 384448108070301 05/14/98

URV17 Seep Creek at G Road, near mouth 384408108091501 05/14/98

URV18 Roubideau Creek at mouth, near Delta 09150500 05/18/98

URV19 Gunnison River above Escalante Creek, near Delta 384527108152701 05/20/98

URV20 Gunnison River near Grand Junction 09152500 05/04/98
INTRODUCTION 9

Table 5. Pesticide target analytes, method reporting limits, drinking-water standards, and aquatic-life criteria

[μg/L, micrograms per liter; H, herbicide; I, insecticide; D, pesticide degradate; F, fungicide; --, no drinking-water standard, guideline, or criteria for

aquatic-life protection established; drinking-water standards are U.S. Environmental Protection Agency primary drinking-water standards; drinkingwater standards and guidelines are from U.S. Environmental Protection Agency (1999a), unless otherwise noted; criteria for aquatic-life protection are

chronic criteria from U.S. Environmental Protection Agency (1999b), unless otherwise noted]

Pesticide Trade or common name

Type of

pesticide

Chemical

Abstract

Service registry

number

Method

reporting

limit

(μg/L)

Drinkingwater

standard

or healthadvisory

guideline

(μg/L)

Freshwaterchronic

criterion for

protection

of aquatic

life

(μg/L)

Gas Chromatography/Mass Spectrometry analysis method

Acetochlor Guardian, Relay H 34256-82-1 0.002 -- --

Alachlor Lasso, Alanex H 15972-60-8 0.002 2 --

Atrazine Aatrex H 1912-24-9 0.001 3 11.8

Azinphos-methyl2 Guthion I 86-50-0 0.001 -- 0.01

Benfluralin Balan, Benefin H 1861-40-1 0.002 -- --

Butylate Sutan+, Genate Plus H 2008-41-5 0.002 3350 --

Carbaryl2, 4 Sevin, Carbatox I 63-25-2 0.003 3700 10.2

Carbofuran2, 4 Furadan I 1563-66-2 0.003 40 11.8

Chlorpyrifos Lorsdan, Dursban I 2921-88-2 0.004 320 0.041

Cyanazine Bladex H 21725-46-2 0.004 31 52

DCPA Dacthal H 1861-32-1 0.002 -- --

p,p'-DDE none D 72-55-9 0.006 60.1 --

Deethylatrazine2 none D 6190-65-4 0.002 -- --

Diazinon Spectracide, Basudin I 333-41-5 0.002 30.6 70.08

Dieldrin Panoram D-31, Octalox I 60-57-1 0.001 80.002 0.056

2,6-Diethylaniline none D 579-66-8 0.003 -- --

Disulfoton Di-Syston I 298-04-4 0.017 30.3 --

EPTC Eptam, Eradicane H 759-94-4 0.002 -- --

Ethalfluralin Sonalan H 55283-68-6 0.004 -- --

Ethoprop Mocap, Prophos I 13194-48-4 0.003 -- --

Fonofos Dyfonate I 944-22-9 0.003 310 --

alpha-HCH none D 319-84-6 0.002 60.006 --

gamma-HCH Lindane, Isotox I 58-89-9 0.004 0.2 70.01

Linuron4 Lorox, Linurex H 330-55-2 0.002 -- 97

Malathion Cythion, Malaspray I 121-75-5 0.005 3

200 0.1

Methyl parathion Penncap-M I 298-00-0 0.006 3

2 --

Metolachlor Dual, Bicep H 51218-45-2 0.002 3

70 57.8

Metribuzin Lexone, Sencor H 21087-64-9 0.004 3

100 51

Molinate Hydram, Ordram H 2212-67-1 0.004 -- --

Napropamide Devrinol H 15299-99-7 0.003 -- --

Parathion Alkron, Bladan I 56-38-2 0.004 -- 0.013

Pebulate Tillam H 1114-71-2 0.004 -- --

Pendimethalin Prowl, Stomp H 40487-42-1 0.004 -- --

cis-Permethrin none I 52341-33-00 0.005 -- --

Phorate Thimet, Rampart I 298-02-2 0.002 -- --

Prometon Pramitol, Gesafram H 1610-18-0 0.018 10100 --

Pronamide Kerb H 23950-58-5 0.003 3

50 --
10 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Gas Chromatography/Mass Spectrometry analysis method—Continued

Propachlor Ramrod, Bexton H 1918-16-7 0.007 390 --

Propanil Stam H 709-98-8 0.004 -- --

Propargite Comite, Omite I 2312-35-8 0.013 -- --

Simazine Aquazine, Princep H 122-34-9 0.005 4 110

Tebuthiuron Spike, Graslan H 34014-18-1 0.01 3500 51.6

Terbacil2 Sinbar, Herbicide 732 H 5902-51-2 0.007 390 --

Terbufos Counter, Contraven I 13071-79-9 0.013 3

0.9 --

Thiobencarb Bolero, Saturn H 28249-77-6 0.002 -- --

Triallate Far-Go, Avadex BW H 2303-17-5 0.001 -- 50.24

Trifluralin Treflan, Trim H 1582-09-8 0.002 35 1

0.2

High-Performance Liquid Chromatography analysis method

2,4-D Weed-Broom, Lawn-Keep H 94-75-7 0.035

110.15

70 14

2,4-DB Embutone, Venceweed H 94-82-6 0.035

110.24

-- --

2,4,5-T Brushtox, Dacamine H 93-76-5 0.035 370 --

2,4,5-TP Silvex, Fenoprop H 93-72-1 0.021 50 121.4

3-Hydroxycarbofurannone D 16655-82-6 0.014 -- --

Acifluorfen Blazer, Tackle H 50594-66-6 0.035 81 --

Aldicarb13 Temik I 116-06-3 0.016

110.55

147 151

Aldicarb sulfone13 Temik sulfone, Standak I, D 1646-88-4 0.016

110.10

147 151

Aldicarb sulfoxide13 Temik sulfoxide D 1646-87-3 0.021 147 151

Bentazon Basagran H 25057-89-0 0.014 3200 --

Bromacil Hyvar, Borea H 314-40-9 0.035 390 5

5

Bromoxynil Buctril, Brominal H 1689-84-5 0.035 -- 165

Carbaryl4 Sevin, Carbatox I 63-25-2 0.008 3700 10.2

Carbofuran4 Furadan I 1563-66-2 0.028

110.12

40 11.8

Chloramben17 Amiben, Vegiben H 133-90-4 0.011

110.42

3

100 --

Chlorothalonil18 Bravo, Forturf F 1897-45-6 0.035

110.48

81.5 50.18

Clopyralid Lontrel, Lontril T H 1702-17-6 0.05

110.23

-- --

Dacthal, mono-acid DCPA, mono-acid H 887-54-7 0.017 -- --

Table 5. Pesticide target analytes, method reporting limits, drinking-water standards, and aquatic-life criteria—Continued

[μg/L, micrograms per liter; H, herbicide; I, insecticide; D, pesticide degradate; F, fungicide; --, no drinking-water standard, guideline, or criteria for

aquatic-life protection established; drinking-water standards are U.S. Environmental Protection Agency primary drinking-water standards; drinkingwater standards and guidelines are from U.S. Environmental Protection Agency (1999a), unless otherwise noted; criteria for aquatic-life protection are

chronic criteria from U.S. Environmental Protection Agency (1999b), unless otherwise noted]

Pesticide Trade or common name

Type of

pesticide

Chemical

Abstract

Service registry

number

Method

reporting

limit

(μg/L)

Drinkingwater

standard

or healthadvisory

guideline

(μg/L)

Freshwaterchronic

criterion for

protection

of aquatic

life

(μg/L)
INTRODUCTION 11

High-Performance Liquid Chromatography analysis method—Continued

Dicamba Banvel, Banex H 1918-00-9 0.035 3200 510

Dichlobenil18 Casoron, Prefix D H 1194-65-6 0.02

111.2

-- 1237

Dichlorprop 2,4-DP, Kildip H 120-36-5 0.032 -- --

Dinoseb DNBP, Dinitro H 88-85-7 0.035 7 10.05

Diuron Karmex, Cekiuron H 330-54-1 0.02 3

10 121.6

DNOC18 Sinox, Elgetol 30 H, I 534-52-1 0.035

110.42

-- --

Fenuron Beet-Kleen H 101-42-8 0.013 -- --

Fluometuron Cotoran, Lanex H 2164-17-2 0.035 3

90 --

Linuron4 Lorox, Linurex H 330-55-2 0.018 -- 9

7

MCPA Metaxon, Kilsem H 94-74-6 0.05

110.17

3

10 192.6

MCPB Bexane, Trifolex H 94-81-5 0.035

110.14

-- --

Methiocarb Mesurol, Draza I 2032-65-7 0.026 -- --

Methomyl Lannate, Nudrin I 16752-77-5 0.017 3

200 --

Neburon Neburex, Kloben H 555-37-3 0.015 -- --

Norflurazon Evital, Solicam H 27314-13-2 0.024 -- --

Oryzalin Surflan, Ryzelan H 19044-88-3 0.019

110.31

-- --

Oxamyl Vydate I 23135-22-0 0.018 200 --

Picloram Tordon, Grazon PC H 1918-02-1 0.05 500 529

Propham Chem-Hoe, IFC H 122-42-9 0.035 3

100 --

Propoxur Baygon I 114-26-1 0.035 -- --

Triclopyr Garlon, Grazon H 55335-06-3 0.05

110.25

-- --

Table 5. Pesticide target analytes, method reporting limits, drinking-water standards, and aquatic-life criteria—Continued

[μg/L, micrograms per liter; H, herbicide; I, insecticide; D, pesticide degradate; F, fungicide; --, no drinking-water standard, guideline, or criteria for

aquatic-life protection established; drinking-water standards are U.S. Environmental Protection Agency primary drinking-water standards; drinkingwater standards and guidelines are from U.S. Environmental Protection Agency (1999a), unless otherwise noted; criteria for aquatic-life protection are

chronic criteria from U.S. Environmental Protection Agency (1999b), unless otherwise noted]

Pesticide Trade or common name

Type of

pesticide

Chemical

Abstract

Service registry

number

Method

reporting

limit

(μg/L)

Drinkingwater

standard

or healthadvisory

guideline

(μg/L)

Freshwaterchronic

criterion for

protection

of aquatic

life

(μg/L)
12 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

1 Guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999).

2 Concentrations for these pesticides are qualitatively identified and reported with an E code (estimated value) because of problems with gas

chromatography or extraction (Zaugg and others, 1995). 3

 U.S. Environmental Protection Agency lifetime-health advisory for a 70-kilogram adult (about 150 pounds).

4

 Analyzed by both gas chromatography/mass spectrometry and high-performance liquid chromatography methods.

5 Interim guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999). 6 U.S. Environmental Protection Agency risk-specific health advisory associated with a cancer risk of 10–6 (U.S. Environmental Protection

Agency, 1999c). 7 Guidelines for the protection of freshwater aquatic life are from International Joint Commission Canada and United States (1978). 8

 U.S. Environmental Protection Agency risk-specific health advisory associated with a cancer risk of 10–6. 9 Interim guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999).

Value applies to linuron, including linuron and its transformation products. 10 U.S. Environmental Protection Agency lifetime-health advisory for a 70-kilogram adult (about 150 pounds). Value is under review. 11 New reporting level for constituent, effective 12/15/97, based on analysis of performance data for schedule 2050 from the Organic Blind

Sample Program of the NWQL’s Quality Assurance Unit and from routine laboratory reagent spike recovery data (National Water Quality Laboratory, 1998). No change in historical results reported for detections is needed since all reported detections and concentrations have been and still are

considered reliable. 12 Criteria for the protection of aquatic life are recommended maximum concentrations in freshwater by National Academy of Sciences and

National Academy of Engineering, from Nowell and Resek (1994). 13 Concentrations for these pesticides are qualitatively identified and reported with an E code because of (1) low and variable recoveries of

aldicarb and aldicarb sulfone and (2) post-collection conversion of aldicarb to aldicarb sulfoxide, resulting in variable high bias in aldicarb

sulfoxide (National Water Quality Laboratory, 1998). 14 U.S. Environmental Protection Agency draft drinking-water standards. 15 Interim guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999).

Value is sum of aldicarb, aldicarb sulfone, and aldicarb sulfoxide. 16 Guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999). Value

applies to total bromoxynil, including the phenol, octanoate, and heptanoate forms. 17 Compound with variable analytical performance, for which concentrations are reported as estimated values.

18 Concentrations for these pesticides are qualitatively identified and reported with an E code because of variable recovery performance

during solid-phase extraction and HPLC analysis (Werner and others, 1996; National Water Quality Laboratory, 1998). 19 Interim guidelines for the protection of freshwater aquatic life are Canadian Water Quality Guidelines from Environment Canada (1999).

Value applies to all forms of MCPA and all transformation products.

Table 5. Pesticide target analytes, method reporting limits, drinking-water standards, and aquatic-life criteria—Continued

[μg/L, micrograms per liter; H, herbicide; I, insecticide; D, pesticide degradate; F, fungicide; --, no drinking-water standard, guideline, or criteria for

aquatic-life protection established; drinking-water standards are U.S. Environmental Protection Agency primary drinking-water standards; drinkingwater standards and guidelines are from U.S. Environmental Protection Agency (1999a), unless otherwise noted; criteria for aquatic-life protection are

chronic criteria from U.S. Environmental Protection Agency (1999b), unless otherwise noted]

Pesticide Trade or common name

Type of

pesticide

Chemical

Abstract

Service registry

number

Method

reporting

limit

(μg/L)

Drinkingwater

standard

or healthadvisory

guideline

(μg/L)

Freshwaterchronic

criterion for

protection

of aquatic

life

(μg/L)
QUALITY-CONTROL METHODS AND RESULTS 13

criteria during analysis, and only the concentration is

estimated, not the presence (National Water Quality

Laboratory, 1996). Throughout this report, estimated

concentrations above the MRL were treated as normal

concentrations, whereas estimated concentrations

below the MRL were treated as “less than” concentrations.

Acknowledgments

We thank Bob Boulger, Jeff Foster, Dave Hartle,

Dennis Smits, and Rick Szmajter, USGS, for their

efforts in collecting the water samples for the data

analyzed here. Dave Butler, USGS, provided valuable

insight into site selection for the synoptic study of

agricultural drains in the Grand and Uncompahgre

River Valleys. Thanks also are extended to Joe Domagalski and Bob Kimbrough for technical review of the

manuscript and to Mary Kidd for editorial review,

John Evans for design, and Alene Brogan for manuscript and layout.

QUALITY-CONTROL METHODS AND

RESULTS

Quality-control (QC) samples were used to

assess the bias and variability of water-quality data

that may be introduced by sample collection,

processing, shipping, and analysis. Without QC

samples, the quality of the data collected cannot be

determined. Field-blank, replicate, and field-matrixspike samples were collected and processed as QC

samples using the same equipment and procedures as

the stream-water or environmental samples in order to

evaluate the bias and variability of the pesticide data.

Pesticide surrogate solutions were added to each environmental and quality-assurance sample in the laboratory as an additional QC mechanism. The same QC

procedures were used in the fixed-station and synoptic

sampling. Quality-control data for both sampling

programs were analyzed together. Further information

on QC in the NAWQA Program and QC samples is

described in Shelton (1994).

Field-blank samples were used to test for bias,

the systematic error inherent in sampling and analytical methods (Spahr and Boulger, 1997). Blank

samples were organic-free water that did not contain

detectable concentrations of the pesticides of interest.

The blank water, Baker Analyzed HPLC (J.T. Baker

Company, Phillipsburg, New York), was processed

through the sampling equipment and handled as if it

were a typical stream sample. Because pesticides in a

water sample can be detected at low concentrations in

the laboratory, field blanks are important in determining possible contamination of a sample from

collection, processing, cleaning, shipping, and analysis procedures.

Replicate samples were two or four samples

collected simultaneously or in sequence in order to

yield samples of nearly identical composition. Replicates provide information on the precision of concentration values and the consistency in identifying

targeted pesticides. As such, replicate samples test for

variability, the degree of random error in independent

measurements of the same quantity (Spahr and

Boulger, 1997). Replicate analysis is based on the

comparison of the analytical results of the environmental sample with the analytical results of the QC

sample or samples. Replicates were analyzed by the

relative percent difference between replicate samples,

calculated by dividing the difference in concentration

between two replicate samples by the mean concentration of the two replicate samples and multiplying the

result by 100. Where more than two samples were

collected at a site, a range of relative percent differences was calculated.

The field-matrix-spike sample was an environmental sample that had been spiked with a solution

containing targeted pesticides and then analyzed as a

typical water-quality sample. Specifically, one liter of

each environmental sample to be analyzed by GC/MS

was spiked with the targeted pesticides at a concentration of 0.1 μg/L, while another liter, for HPLC analysis, was spiked with targeted pesticides at a

concentration of 1 μg/L. The spiked samples were

used to identify the recoverability of pesticides in the

environmental samples. Pesticide recovery can be

affected by pesticide degradation, interference from

other organic substances or matter in the sample, and

laboratory analysis. Spike solutions were provided by

the USGS NWQL.

Pesticide-spike-recovery data were calculated in

three steps. First, the expected or theoretical concentration of the spiked sample was determined by multiplying the concentration of the spike solution with the

amount of spike added to the environmental sample

and then dividing the result by the sample volume.
14 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Next, the concentration of the pesticide in the environmental sample (zero if reported less than or estimated

below the MRL) was subtracted from the concentration in the spiked sample. Finally, this result was

multiplied by 100 and then divided by the expected or

theoretical concentration of the spiked sample to

determine the pesticide recovery percentage.

Pesticide surrogates were used to assess analytical recovery and precision for each analysis. Surrogate solutions containing known concentrations of

organic compounds were added in the laboratory to

each of the 155 environmental and quality-assurance

samples prior to analysis. The surrogates were not

expected to be found in the environmental samples but

were expected to have similar chemical properties as

the pesticides of interest. The surrogates diazinon-d10,

terbuthylazine, and alpha-HCH-d6 were added to the

samples analyzed by GC/MS, and the surrogate

BDMC was added to the samples analyzed by HPLC.

Surrogates were reported as percent surrogate recovered and analyzed by mean surrogate recovery.

No pesticides were detected in the seven fieldblank samples. For four of the blank samples, the sites

sampled just prior to the blank processing had various

pesticides detected above the MRLs. Field-cleaning

procedures were, therefore, effective in preventing

contamination from one sample to another during

equipment use. The potential for contamination of

samples during sample collection, processing,

cleaning, shipping, and analysis procedures was

minimal.

As a result of the replicate sampling, 88 replicate groups for 20 pesticides were studied (table 6), a

replicate group here having at least one detected

concentration equal to or greater than the MRL for

the respective pesticide. Relative percent differences

were determined for these replicate groups, and the

differences generally were small (table 6). Differences

ranged from 0.0 to 100 percent, and most were less

than 20 percent. The only pesticide with a relative

percent difference above 41 percent was 2,4-D. As

such, variability in concentrations for the pesticide

sampling generally was low. Forty-seven of the

88 replicate groups consisted of a single pair of replicates, and 18 of these had a relative percent difference

of 0.0 percent. Of the 41 replicate groups with

multiple pairs of replicates, 22 contained at least one

pair with no relative percent difference. Of the

88 replicate groups studied, only 7 contained concentrations that were reported both above and below the

MRL for the respective pesticides and, thus, did not

have a relative percent difference computed. Except

for bromoxynil, these concentrations were less than

0.008 μg/L. Overall, there was good consistency in

identifying the targeted pesticides, and only at very

low concentrations were pesticides apt to have concentrations detected above and below the detection limit

for the same replicate group.

Mean pesticide recoveries in the 30 samples

spiked in the field for analysis by GC/MS ranged

from 34 to 227 percent with a median of 98 percent

(table 7). Deethylatrazine, p,p'-DDE, disulfoton,

cis-permethrin, and phorate all had mean recoveries

below 70 percent, with deethylatrazine and

cis-permethrin having the lowest recoveries, below

40 percent. Azinphos-methyl, carbaryl, and carbofuran

had high and variable recovery percentages. During

testing of the GC/MS analytical method, Zaugg and

others (1995) found azinphos-methyl, carbaryl,

carbofuran, deethylatrazine, and terbacil to have low

or highly variable spike recoveries. Because of this,

analytical results for these five pesticides are reported

by the NWQL as estimated (E) concentrations. There

is increased uncertainty in analytical precision for

estimated concentrations, but there is no increase in

uncertainty of analytical detection (Rinella and Janet,

1998).

Mean recoveries for the spiked samples

analyzed by the HPLC method were lower than those

for the GC/MS method; the recoveries ranged from 13

to 90 percent, and the median was 77 percent (table 8).

Low and/or variable recoveries were reported for aldicarb, aldicarb sulfone, carbofuran, chlorothalonil,

clopyralid, dicamba, dichlobenil, methomyl, oryzalin,

and picloram. Based on low or variable recovery

performance, the NWQL reports concentrations of

aldicarb, aldicarb sulfone, aldicarb sulfoxide,

chloramben, chlorothalonil, dichlobenil, and DNOC as

estimated values (Werner and others, 1996; National

Water Quality Laboratory, 1998). As with the estimated concentrations for the GC/MS analysis method,

the analytical results for the pesticides with estimated

concentrations as determined by the HPLC method are

reliable detections with greater than average uncertainty for numerical precision.

As mentioned previously, carbaryl and carbofuran were analyzed by both the GC/MS and the

HPLC methods. The mean recoveries for carbaryl for

the two methods were 196 and 75 percent, respectively, whereas the mean recoveries for carbofuran
QUALITY-CONTROL METHODS AND RESULTS 15

Table 6. Concentrations and relative percent differences for pesticides detected in replicate samples[μg/L, microgram per liter; <, less than; nc, not computed; GC/MS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography]

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Conc

in re

(

Acetochlor 0.008

0.007

13 Alachlor

(continued)

0.0310.034

9.2 Atrazine

(continued)

0000

0.0170.0150.0140.018

0.1020.009

3.0

5.7 - 25 0

0

0.0110.0110.0120.011

0.0 - 8.7 0

000

Alachlor 0.003

0.002

40 

0.0220.020

9.5 Atrazine 0.003

0.004

29 0

000

0.0310.0300.0310.029

0.0760.068

11

0.0 - 6.7 0

0

0.1050.1060.1050.104

0.1100.0860.0830.092

0.0 - 1.9

3.6 - 28 Benfluralin 0

000

0.0040.004

0.0 
16 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98 Benfluralin (continued) 0.004 0.004 0.0 Carbofuran (GC/MS) (continued) 0.007 0.007 0.007 0.007 Carbofuran (HPLC) (continued) 0

000

0.0 

0.004 0.004 0.0 0.982 0.978 0.986 0.953 0.41 - 3.4 Chlorpyrifos 0

000

Bromoxynil <0.035 0.070 0.040 <0.035 

nc 0.784 0.809 0.770 0.806 0.37 - 4.9 0

0

Carbaryl (GC/MS) 0.006 0.007 15 0.009 0.008 12 0

0

0.003 0.003 0.0 0

000

Carbofuran (HPLC) 0.470 0.310 41 

Carbofuran(GC/MS)

0.0090.009

0.0 0.620

0.6000.5800.630

1.6 - 8.3 Cyanazine 0

000

0.4800.485

1.0 

Table 6. Concentrations and relative percent differences for pesticides detected in replicate samples—Continued

[μg/L, microgram per liter; <, less than; nc, not computed; GC/MS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography]

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Conc

in re

(
QUALITY-CONTROL METHODS AND RESULTS 17

Cyanazine(continued)

0.0070.007

0.0 2,4-D

(continued)

0.4900.370

28 DCPA

(continued)

0

<0

<0.004

0.0070.006

<0.004

0.8700.8600.8000.880

00

nc 1.1 - 9.5

00

0.0160.0140.0130.015

0.2100.0700.1500.140

6.5 - 21 6.9 - 100 0

0

0.0230.024

4.3 0

0

2,4-DB 0.830

1.00

19

0.0080.009

12 0

0

DCPA 0.015

0.0140.0140.015

2,4-D 0.390

0.360

8.0 0.0 - 6.9 Deethylatrazine 0

0

0.6200.5600.6600.550

0.0100.0120.0110.011

0000

1.8 - 18 0.0 - 18 

Table 6. Concentrations and relative percent differences for pesticides detected in replicate samples—Continued

[μg/L, microgram per liter; <, less than; nc, not computed; GC/MS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography]

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Conc

in re

(
18 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98 Deethylatrazine (continued) 0.009 0.007 25 Diazinon (continued) <0.002 0.002 <0.002 <0.002 EPTC (continued) 0

000

nc 

0.007 0.009 0.010 0.008 11 - 35 Dicamba 0.150 0.150 0.0 Ethalfluralin 0

000

0.006 0.005 0.005 0.006 0.0 - 18 

EPTC 0.003 0.003 0.0 0.004 0.004 0.0 Metolachlor 0

0

0.002 0.003 40 0.003 <0.002 <0.002 <0.002 0

0

nc 0.199 0.202 1.5 <0

<0

0

<0

0.0030.003

0.0 0.061

0.0460.0460.059

0.0 - 28

0000

Diazinon 0.004

0.005

22 0.003

0.003

0.0 

Table 6. Concentrations and relative percent differences for pesticides detected in replicate samples—Continued

[μg/L, microgram per liter; <, less than; nc, not computed; GC/MS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography]

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Conc

in re

(
QUALITY-CONTROL METHODS AND RESULTS 19

Metolachlor(continued)

0.0160.017

6.1 Terbufos 0.017

0.0170.0170.015

Trifluralin(continued)

0000

0.0 - 12 

0.1300.137

5.2

0000

0.0650.0680.0620.066

Trifluralin 0.005

0.0050.0050.006

3.0 - 6.2 0.0 - 18

00

<0

0

0.0190.024

23 

Table 6. Concentrations and relative percent differences for pesticides detected in replicate samples—Continued

[μg/L, microgram per liter; <, less than; nc, not computed; GC/MS, gas chromatography/mass spectrometry; HPLC, high-performance liquid chromatography]

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Concentration

in replicates

(μg/L)

Relative

percent

difference

Pesticide

Conc

in re

(
20 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Table 7. Mean and standard deviation of pesticide recovery in 30 field-spiked samples determined by gas chromatography/

mass spectrometry (GC/MS)

Pesticide Mean recovery

(percent)

Standard deviation

(percent) Pesticide Mean recovery(percent)

Standard deviation

(percent)

Acetochlor 107 7.8 Malathion 89 17

Alachlor 108 9.0 Methyl parathion 96 16

Atrazine 102 7.2 Metolachlor 112 13

Azinphos-methyl 154 62 Metribuzin 74 9.7

Benfluralin 77 12 Molinate 97 4.8

Butylate 105 8.0 Napropamide 102 10

Carbaryl 196 130 Parathion 96 16

Carbofuran 227 310 Pebulate 100 6.9

Chlorpyrifos 89 8.2 Pendimethalin 85 18

Cyanazine 100 15 cis-Permethrin 39 11

DCPA 110 16 Phorate 67 14

p,p'-DDE 60 4.8 Prometon 97 8.0

Deethylatrazine 34 7.6 Pronamide 96 10

Diazinon 97 13 Propachlor 114 12

Dieldrin 99 12 Propanil 110 15

2,6-Diethylaniline 92 6.9 Propargite 100 27

Disulfoton 69 14 Simazine 98 6.9

EPTC 107 31 Tebuthiuron 114 27

Ethalfluralin 89 19 Terbacil 75 16

Ethoprop 106 14 Terbufos 77 5.5

Table 8. Mean and standard deviation of pesticide recovery in 30 field-spiked samples determined by high-performance liquid

chromatography (HPLC)

Pesticide Mean recovery

(percent)

Standard deviation

(percent) Pesticide Mean recovery(percent)

Standard deviation

(percent)

Acifluorfen 85 13 DNOC 78 8.8

Aldicarb 41 11 Fenuron 81 7.4

Aldicarb sulfone 20 16 Fluometuron 85 5.0

Aldicarb sulfoxide 66 13 3-Hydroxy-carbofuran 75 11

Bentazon 78 8.2 Linuron 78 7.0

Bromacil 71 5.9 MCPA 77 7.6

Bromoxynil 85 4.6 MCPB 70 6.4

Carbaryl 75 6.1 Methiocarb 67 4.8

Carbofuran 90 39 Methomyl 77 23

Chloramben1 75 8.3 Neburon 77 8.8

Chlorothalonil 54 17 Norflurazon 82 6.5

Clopyralid 13 13 Oryzalin 78 30

2,4-D 76 15 Oxamyl 63 8.6

2,4-DB 75 8.9 Picloram 51 26

Dacthal, mono-acid 73 10 Propham 71 9.2

Dicamba 58 27 Propoxur 79 16

Dichlobenil 46 16 Silvex 87 10

Dichlorprop 79 7.9 2,4,5-T 85 2.8

Dinoseb 85 11 Triclopyr 79 6.6

Diuron 81 6.2

1 Mean recovery and standard deviation are based on seven May 1998 samples.
HYDROLOGIC CONDITIONS DURING SAMPLING 21

were 227 and 90 percent, respectively (tables 7 and 8).

Because the recovery rates for the HPLC method were

closer to 100 percent than the recovery rates for the

GC/MS method, only the carbaryl and carbofuran

occurrences and concentrations resulting from the

HPLC method will be discussed in later sections of the

report. With recovery rates below 100 percent, the

carbaryl and carbofuran results from the HPLC

method may be biased slightly low; however, the

results from the GC/MS method would have a very

high bias because of the recovery rates being around

200 percent.

For the pesticides with low recovery percentages, the detection frequencies and concentration

ranges may be biased low. False negatives occur when

a pesticide is present in a sample at a concentration

greater than the MRL but is not detected because of

analytical problems. In the UCOL study, for example,

the mean recovery for dicamba was 58 percent (table

8). Because of this low recovery, dicamba may not

have been detected in a sample even though it was

actually present above the MRL. The probability of a

false positive (the detection of a pesticide at a concentration greater than the MRL when the actual concentration is below the reporting limit) is much lower than

a false negative when the data are censored at the

MRL (National Water Quality Laboratory, 1994).

Mean recoveries for the pesticide surrogates

terbuthylazine, diazinon-d10, alpha-HCH-d6, and

BDMC were 112, 100, 99.9, and 84.0 percent, respectively (table 9). As with the spiked samples, the mean

surrogate recovery for the BDMC samples analyzed

by HPLC was lower than the mean recoveries for the

terbuthylazine, diazinon-d10, alpha-HCH-d6 samples

analyzed by GC/MS. Recoveries for all samples but

one ranged between 64 and 141 percent, and most

recoveries fell between 80 and 120 percent. A

recovery of 0 percent was reported for one BDMC

sample; this surrogate may have accidentally been

omitted from the sample. These results of the surrogate recoveries show that no significant bias occurred

in the pesticide data, and variabilities in recovery, as

shown by the standard deviations in table 9, were

similar for the four surrogates.

HYDROLOGIC CONDITIONS DURING

SAMPLING

Streamflow and runoff were higher than

normal in the study unit during water year (WY) 1997

(October 1996 through September 1997). The annual

mean streamflow and total runoff for the Colorado

River near the Colorado-Utah State Line (encompassing streamflow from the entire study unit) for

WY 1997 were 9,826 ft3

/s and 7,114,000 acre-ft,

respectively (U.S. Geological Survey, 1998b). In

comparison, the long-term (WY 1970–97) mean

annual streamflow and runoff amounts were

6,847 ft3/s and 4,960,000 acre-ft, respectively. The

annual mean streamflow and runoff amounts for the

Gunnison River near Grand Junction for WY 1997

also were above the long-term (WY 1970–97) averages for this site. For calender year 1997, precipitation

was 35 percent above normal at Grand Junction,

mainly in April, May, August, and September. At

Montrose, precipitation in 1997 was 21 percent above

normal, mainly in April, May, June, and September

(National Oceanic and Atmospheric Administration,

1998a).

Streamflow for the Colorado River near State

Line and Gore Creek at mouth was dependent on

snowmelt runoff; streamflow was generally highest in

June during snowmelt (fig. 3). In contrast, streamflow

in Reed Wash was caused primarily by irrigation and

irrigation-return flow, with high streamflow from April

into November. Streamflow for Dry Creek near

Begonia Road was primarily dependent on irrigation

and irrigation-return flow but also included snowmelt

runoff from the Uncompahgre Plateau. For all four

fixed-station sites, water-quality samples were

collected over a wide range of streamflow conditions

(fig. 3). In figure 3, samples were positioned based on

instantaneous streamflow at the time of collection and,

thus, may not plot on the line representing daily mean

streamflow.

Table 9. Mean and standard deviation of pesticide

surrogate recovery

Pesticide surrogate

Mean

recovery

(percent)

Standard deviation

(percent)

Gas Chromatography/Mass Spectrometry (GC/MS) analysis

alpha-HCH-d6 99.9 13

diazinon-d10 100 11

terbuthylazine 112 10

High-Performance Liquid Chromatography (HPLC) analysis

BDMC 84.0 11
22 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

0

10,000

20,000

30,000

40,000

200

400

600

800

1,000

1,200

1,400

1,600

0

0

100

200

300

0

10

20

30

40

50

60

70

80

90

100

October January April July October January April

1996 1997 1997 1998

STREAMFLOW, IN CUBIC FEET PER SECOND

FIXED-STATION SAMPLE

DAILY MEAN STREAMFLOW

SYNOPTIC SAMPLE

COLORADO RIVER NEAR STATE LINE

REED WASH NEAR MACK

DRY CREEK NEAR BEGONIA ROAD

GORE CREEK AT MOUTH

Figure 3. Streamflow and pesticide sample-collection dates for fixed-station sites, Upper Colorado River Basin,

October 1996–May 1998. (Streamflow at sampling time is instantaneous measurement; graphs have different vertical scales.)
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 23

During the synoptic study in May 1998, the

monthly mean streamflows for the Colorado River

near State Line and Gunnison River near Grand Junction were 23 and 16 percent, respectively, above the

long-term (WY 1970–98) May mean monthly streamflows for the two sites. Above average streamflow at

each site may have been due to a greater snowpack or

more snowmelt in each basin. As irrigation return

flow, streamflow in the drains of the agricultural areas

was affected by irrigation practices, which in turn

were influenced by precipitation and temperature.

Precipitation in the Grand Valley and Uncompahgre

River Valley was slightly (less than 1 inch) below

normal during the synoptic study, as measured at

Grand Junction and Montrose, while the temperature

was slightly (less than 1°F) above normal at the two

sites (National Oceanic and Atmospheric Administration, 1998b). Precipitation and temperature values

greatly different from normal would affect irrigation

practices and growing conditions, pesticide applications, and, therefore, pesticides in streams and drains.

OCCURRENCE AND DISTRIBUTION OF

PESTICIDES

The occurrence and distribution of pesticides in

surface water and whether pesticides are detected in a

water sample or not depend on many factors, such as

the time, rate, and location of pesticide application,

crop type, precipitation or irrigation events, physical

and chemical characteristics of pesticides, and atmospheric transport and deposition. The time of pesticide

application affects when pesticides are detected in

streams. In surface water, pesticide detections typically occur after the first precipitation/irrigation event

following pesticide application. Also, small amounts

of pesticides applied per acre are less likely to be

detected than pesticides applied in large amounts. The

spatial distribution of pesticides detected in a stream

depends on the spatial distribution of pesticide application and crop type or agricultural practice. For

example, atrazine is a very common herbicide used for

corn and is commonly detected in streams near corn

fields, whereas it is not used and not commonly

detected in fruit-growing areas (U.S. Geological

Survey, 1998a; Gianessi and Puffer, 1990). Pesticides

applied just before or during a rainstorm may be transported more quickly in surface runoff to streams and

drains than pesticides applied during dry conditions.

The potential of pesticides to be transported from an

agricultural field into runoff water differs among pesticides and depends on the physical and chemical characteristics of each pesticide. Factors such as water

solubility, persistence, and acid/base, ionic, and sorption properties determine the runoff potential of a

pesticide (Larson and others, 1997). Pesticides with

large runoff potentials, such as atrazine, carbofuran,

and pendimethalin, are more likely to be transported

out of an agricultural field into surface water through

runoff than a pesticide that has a small runoff potential, such as malathion. Malathion has low soil persistence due to rapid degradation and, thus, is not readily

present or available to be included in runoff. Atrazine,

carbofuran, and pendimethalin, in contrast, are moderately to highly persistent in soil and are readily available for inclusion in runoff. Depending on the

characteristics of individual pesticides, storm runoff

can be an important mechanism in the transport of

pesticides to receiving waters. Finally, some pesticide

detections may not be due to local use of the pesticide

at all. It is possible that very low levels of detected

pesticides may be related to atmospheric transport and

deposition. As Majewski and Capel (1995) reported,

pesticides have been detected in the atmosphere

throughout the Nation, and pesticides applied in one

area may be transported and deposited in another area.

In the UCOL study unit, 35 pesticides were

detected at least once in 82 of the 100 samples

collected during the fixed-station and synoptic

sampling periods of October 1996 through January

1998 and May 1998 (table 10), respectively, a detection for this report being defined as a concentration of

a pesticide equal to or greater than the MRL of the

pesticide. Almost 93 percent (76 of 82) of these

samples contained two or more pesticide detections.

For the 100 samples, 8,248 individual pesticide analyses were performed, and there were 476 detections

(5.8 percent of the possible total). Fifty-two additional

analyses were unreported because of difficulties in

the laboratory determination of concentration. Almost

82 percent (390 of 476) of the detections were for

the 11 most frequently occurring pesticides. Of these

11 pesticides, 9 were herbicides, 1 (carbofuran) was an

insecticide, and 1 (deethylatrazine) was a degradation

product of atrazine. Atrazine and alachlor, used on

corn and dry beans, were the most commonly detected

herbicides, whereas carbofuran, used on pests in

alfalfa, corn, and grains, was the most commonly

detected insecticide. Pesticide concentrations in the
24 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98 Table 10. Summary of pesticide occurrence and concentrations for all 44 surface-water sites sampled in the UCOL study unit, October 19

and May 1998

[≥, greater than or equal to; μg/L, micrograms per liter; MRL, method reporting limit, <, less than; E, estimated; --, method reporting limit greater than detection threshold

chromatography]

Pesticide

Number

of

samples

Number of

detections

Percentage

detected

Percentage

detected

≥0.01

(μg/L)

Percentage

detected

≥0.05

(μg/L)

MRL

(μg/L)

Concentration at indicated percentile

 (μg/L) 

10th 25th 50th 75th 

Atrazine 100 69 69 35 12 0.001 <0.001 <0.001 0.005 0.019 Alachlor 100 57 57 33 13 0.002 <0.002 <0.002 0.003 0.014 Metolachlor 100 46 46 23 10 0.002 <0.002 <0.002 <0.002 0.009 Deethylatrazine 100 37 37 5.0 0.0 0.002 <0.002 <0.002 <0.002 0.004 2,4-D 100 32 32 -- -- 0.035 10.15 <0.035 <0.035 <0.15 0.170 

Cyanazine 100 30 30 14 4.0 0.004 <0.004 <0.004 <0.004 0.006 DCPA 100 30 30 8.0 1.0 0.002 <0.002 <0.002 <0.002 0.002 Trifluralin 100 30 30 9.0 1.0 0.002 <0.002 <0.002 <0.002 0.003 EPTC 100 21 21 10 5.0 0.002 <0.002 <0.002 <0.002 <0.002 Acetochlor 100 19 19 15 7.0 0.002 <0.002 <0.002 <0.002 <0.002 Carbofuran (HPLC) 98 19 19 -- -- 0.028 10.12 <0.028 <0.028 <0.12 <0.12 Chlorpyrifos 100 13 13 2.0 0.0 0.004 <0.004 <0.004 <0.004 <0.004 Benfluralin 100 10 10 0.0 0.0 0.002 <0.002 <0.002 <0.002 <0.002 Diazinon 100 8 8.0 3.0 1.0 0.002 <0.002 <0.002 <0.002 <0.002 Pendimethalin 100 8 8.0 6.0 1.0 0.004 <0.004 <0.004 <0.004 <0.004 Propargite 100 6 6.0 -- 3.0 0.013 <0.013 <0.013 <0.013 <0.013 Simazine 100 5 5.0 5.0 1.0 0.005 <0.005 <0.005 <0.005 <0.005 Carbaryl (HPLC) 98 3 3.1 3.1 3.1 0.008 <0.008 <0.008 <0.008 <0.008 Diuron 98 3 3.1 -- 1.0 0.02 <0.02 <0.02 <0.02 <0.02 Azinphos-methyl 100 3 3.0 1.0 1.0 0.001 <0.001 <0.001 <0.001 <0.001 Dicamba 99 3 3.0 -- 3.0 0.035 <0.035 <0.035 <0.035 <0.035 Ethalfluralin 100 3 3.0 0.0 0.0 0.004 <0.004 <0.004 <0.004 <0.004 Terbufos 100 3 2.0 -- 0.0 0.013 <0.013 <0.013 <0.013 <0.013 2,4-DB 100 2 2.0 -- -- 0.035 <0.035 <0.035 <0.035 <0.24 Dieldrin 100 2 2.0 0.0 0.0 0.001 <0.001 <0.001 <0.001 <0.001 gamma-HCH 100 2 2.0 1.0 0.0 0.004 <0.004 <0.004 <0.004 <0.004 Malathion 100 2 2.0 2.0 0.0 0.005 <0.005 <0.005 <0.005 <0.005 Metribuzin 100 2 2.0 1.0 0.0 0.004 <0.004 <0.004 <0.004 <0.004 
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 25

Prometon 100 2 2.0 -- 1.0 0.018 <0.018 <0.018 <0.018 <0.018 Bentazon 98 1 1.0 1.0 1.0 0.014 <0.014 <0.014 <0.014 <0.014 Bromoxynil 99 1 1.0 1.0 1.0 0.035 <0.035 <0.035 <0.035 <0.035 MCPB 99 1 1.0 1.0 1.0 0.035

10.14

<0.035 <0.035 <0.035 <0.14 

Oryzalin 98 1 1.0 1.0 0.0 0.019

10.31

<0.019 <0.019 <0.019 <0.31 

Phorate 100 1 1.0 0.0 0.0 0.002 <0.002 <0.002 <0.002 <0.002 Pronamide 100 1 1.0 1.0 0.0 0.003 <0.003 <0.003 <0.003 <0.003 

1 New reporting level for constituent, effective 12/15/97 (National Water Quality Laboratory, 1998).

Table 10. Summary of pesticide occurrence and concentrations for all 44 surface-water sites sampled in the UCOL study unit, October 19

and May 1998—Continued

[≥, greater than or equal to; μg/L, micrograms per liter; MRL, method reporting limit, <, less than; E, estimated; --, method reporting limit greater than detection threshold

chromatography]

Pesticide

Number

of

samples

Number ofdetections

Percentage

detected

Percentage

detected

≥0.01

(μg/L)

Percentage

detected

≥0.05

(μg/L)

MRL

(μg/L)

Concentration at indicated percentile

 (μg/L) 

10th 25th 50th 75th 
26 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

UCOL study unit generally were low. Including all

samples and analyses, only atrazine and alachlor had

median concentrations (50th percentile in table 10)

greater than their MRLs, and the median concentrations were only slightly greater than the respective

MRLs (table 10). Carbofuran and 2,4-D were the only

pesticides having detections at consistently higher

concentrations than the other pesticides studied

(table 10). Carbofuran concentrations exceeded

0.447 μg/L in 10 percent of the samples. Concentrations of 2,4-D exceeded 0.17 μg/L in 25 percent of the

samples and 0.385 μg/L in 10 percent of the samples.

Pesticides were detected at 41 of the 44 pesticidesampling sites in the UCOL, all 4 sites of the fixedstation network, and 40 of the 43 synoptic sites.

Pesticides at Fixed-Station Network

Fifty-seven water samples collected from

October 1996 through January 1998 from the four

fixed-station network sites in the UCOL study unit

were analyzed for 83 pesticides. Twenty-three pesticides were detected at least once (table 11) in

42 samples. Forty samples contained two or more

pesticides. For the fixed-station samples, 4,687 individual pesticide analyses were performed. Detections

occurred in 253 analyses, or 5.4 percent of the total.

The 10 most frequently occurring pesticides accounted

for almost 85 percent of the 253 detections. The

frequency of detection for all 23 pesticides and their

measured detected concentrations are shown in

figure 4, in descending order of frequency. Sixteen

herbicides and seven insecticides were detected, with

only the herbicides atrazine (70 percent) and alachlor

(54 percent) being detected in more than 50 percent of

the samples. Carbofuran and chlorpyrifos, detected in

18 percent of the samples, were the most frequently

detected insecticides. Concentrations of the detected

pesticides in the fixed-station network generally were

low, ranging from 0.001 μg/L to 2.60 μg/L. More than

36 percent of the detections were greater than or equal

to 0.01 μg/L, and almost 22 percent were greater than

or equal to 0.05 μg/L. Carbofuran and 2,4-D

accounted for 47 percent of the detections that were

greater than or equal to 0.05 μg/L. Statistical summaries of the detected pesticide concentrations for each

of the four fixed-station sites individually and

combined are in table 11.

For the samples collected at the fixed-station

sites, no pesticide concentrations exceeded U.S. Environmental Protection Agency (USEPA) drinkingwater standards or health-advisory guidelines;

maximum pesticide concentrations were much lower

than these water-quality criteria. The criterion for the

protection of freshwater aquatic life, however, was

exceeded for one pesticide at Reed Wash on May 21,

1997. The estimated azinphos-methyl concentration of

0.062 μg/L was greater than the USEPA freshwaterchronic criterion for aquatic-life protection of 0.01

μg/L (table 5).

Interpretation of water-quality data based on

standards and guidelines has to be done with qualifications. The description of water quality in an area has to

be tempered by the knowledge that water-quality

criteria have not been set for many pesticides, pesticide mixtures and degradation products have not been

evaluated, and not all pesticide effects have been

studied.

Temporal Distribution

For the fixed-station sampling, pesticides were

detected throughout most of the October 1996 through

January 1998 sampling period (fig. 5). The mean

number of pesticide detections per sample by month

was greatest for April through August 1997

(fig. 6). Detections of herbicides were more common

than insecticides. Herbicides were detected in every

month that a detection occurred, while insecticides

were detected only from April through August 1997

(fig. 6). Atrazine was the only herbicide detected every

month in which a detection occurred, and it was

present in 40 of the 42 samples with detections. Atrazine is used as a season-long weed control in corn, so

its detection during much of the year is common.

Carbofuran and chlorpyrifos, the most commonly

detected insecticides, were found in 10 of the

42 samples with detections. All 10 detections occurred

in the spring and summer months, April through

August 1997. Because of their use as insecticides,

carbofuran and chlorpyrifos would not be expected to

be detected year round but would be more common

during the times of insect activity.

The concentrations of detected pesticides in the

fixed-station study were elevated from April through

August or September 1997, which corresponded to the

time of higher streamflows because of irrigation and

irrigation return flows and also snowmelt in May and
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 27

Table 11. Statistical summary of concentrations of detected pesticides at fixed-station sites in the Upper Colorado River Basin,

October 1996–January 1998

[μg/L, micrograms per liter; --, not computed; E, estimated; HPLC, high-performance liquid chromatography]

Pesticide

Number

of

samples

Number of

detections

Percentage

detected

Method

reporting limit

(μg/L)

Statistics for detections

Minimum

(μg/L)

Median

(μg/L)

Maximum

(μg/L)

Gore Creek at mouth near Minturn (urban/forest site)

Atrazine 10 1 10 0.001 -- -- 0.014

Benfluralin 10 1 10 0.002 -- -- E 0.002

Cyanazine 10 1 10 0.004 -- -- 0.006

Metolachlor 10 1 10 0.002 -- -- 0.007

Dry Creek near Begonia Road, near Delta (agriculture site)

Atrazine 15 15 100 0.001 E 0.003 0.021 0.105

Deethylatrazine 15 14 93 0.002 E 0.003 E 0.009 E 0.028

Alachlor 15 9 60 0.002 E0.003 0.011 0.031

DCPA 15 6 40 0.002 E 0.002 E 0.004 0.016

Cyanazine 15 5 33 0.004 0.004 0.008 0.013

Benfluralin 15 4 27 0.002 E 0.003 E 0.004 0.004

EPTC 15 3 20 0.002 E 0.002 E 0.002 E 0.003

Metolachlor 15 3 20 0.002 E 0.002 E 0.003 E 0.003

Diazinon 15 2 13 0.002 0.004 0.006 0.008

Ethalfluralin 15 2 13 0.004 0.004 0.006 0.009

Trifluralin 15 2 13 0.002 E 0.003 E 0.004 0.006

Oryzalin 1

14 1 7.1 0.019 -- -- E 0.030

Acetochlor 15 1 6.7 0.002 -- -- 0.008

Azinphos-methyl 15 1 6.7 0.001 -- -- E 0.003

2,4-D 15 1 6.7 0.035 -- -- E 0.620

Propargite 15 1 6.7 0.013 -- -- 0.046

Reed Wash near Mack (agriculture site)

Atrazine 24 18 75 0.001 E 0.001 0.009 0.127

Metolachlor 24 17 71 0.002 E 0.003 0.043 0.281

Alachlor 24 16 67 0.002 E 0.002 0.008 0.513

Trifluralin 24 14 58 0.002 E 0.002 0.006 0.021

2,4-D 24 13 54 0.035 0.120 0.270 E 0.980

Carbofuran (HPLC) 24 10 42 0.028 0.040 0.200 E 1.60

Chlorpyrifos 24 10 42 0.004 0.005 0.006 0.013

DCPA 24 9 38 0.002 E 0.002 E 0.003 0.026

EPTC 24 9 38 0.002 E 0.003 0.025 0.853

Cyanazine 24 8 33 0.004 0.006 0.020 0.159

Deethylatrazine 24 7 29 0.002 E 0.002 E 0.004 E 0.006

Propargite 24 4 17 0.013 0.043 0.168 0.230

2,4-DB 24 2 8.3 0.035 0.050 0.440 0.830

Dicamba 24 2 8.3 0.035 E 0.090 E 0.120 E 0.150

Terbufos 24 2 8.3 0.013 0.017 0.017 0.017

Azinphos-methyl 24 1 4.2 0.001 -- -- E 0.062

Benfluralin 24 1 4.2 0.002 -- -- E 0.003

Bentazon 24 1 4.2 0.014 -- -- E 2.60

Malathion 24 1 4.2 0.005 -- -- 0.026
28 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Colorado River near Colorado-Utah State Line (integrator site)

Alachlor 8 6 75 0.002 E 0.003 0.006 0.015

Atrazine 8 6 75 0.001 E 0.004 0.005 0.011

Metolachlor 8 6 75 0.002 E 0.003 0.005 0.007

DCPA 8 5 62 0.002 E 0.002 E 0.003 0.005

2,4-D 8 3 38 0.035 0.080 0.100 0.160

Cyanazine 8 2 25 0.004 0.006 0.007 0.008

Deethylatrazine 8 2 25 0.002 E 0.003 E 0.003 E 0.004

EPTC 8 2 25 0.002 E 0.002 E 0.002 E 0.003

Propargite 8 1 12 0.013 -- -- 0.014

Trifluralin 8 1 12 0.002 -- -- 0.004

All fixed-station sites combined

Atrazine 57 40 70 0.001 E 0.001 0.010 0.127

Alachlor 57 31 54 0.002 E 0.002 0.008 0.513

Metolachlor 57 27 47 0.002 E 0.002 0.016 0.281

Deethylatrazine 57 23 40 0.002 E 0.002 E 0.006 E 0.028

DCPA 57 20 35 0.002 E 0.002 E 0.003 0.026

2,4-D 57 17 30 0.035 0.080 0.270 E 0.980

Trifluralin 57 17 30 0.002 E 0.002 0.006 0.021

Cyanazine 57 16 28 0.004 0.004 0.008 0.159

EPTC 57 14 25 0.002 E 0.002 0.014 0.853

Carbofuran (HPLC) 1

55 10 18 0.028 0.040 0.200 E 1.60

Chlorpyrifos 57 10 18 0.004 0.005 0.006 0.013

Benfluralin 57 6 11 0.002 E 0.002 0.004 0.004

Propargite 57 6 11 0.013 0.014 0.077 0.230

Azinphos-methyl 57 2 3.5 0.001 E 0.003 E 0.032 E 0.062

Diazinon 57 2 3.5 0.002 0.004 0.006 0.008

2,4-DB 57 2 3.5 0.035 0.050 0.440 0.830

Dicamba 57 2 3.5 0.035 E 0.090 E 0.120 E 0.150

Ethalfluralin 57 2 3.5 0.004 0.004 0.006 0.009

Terbufos 57 2 3.5 0.013 0.017 0.017 0.017

Acetochlor 57 1 1.8 0.002 -- -- 0.008

Bentazon 57 1 1.8 0.014 -- -- E 2.60

Malathion 57 1 1.8 0.005 -- -- 0.026

Oryzalin 155 1 1.8 0.019 -- -- E 0.030

1 Number of samples is different because the results for one or more samples are unreported due to difficulties in the laboratory determination of

concentration.

Table 11. Statistical summary of concentrations of detected pesticides at fixed-station sites in the Upper Colorado River Basin,

October 1996–January 1998

[μg/L, micrograms per liter; --, not computed; E, estimated; HPLC, high-performance liquid chromatography]

Pesticide

Number

of

samples

Number of

detections

Percentage

detected

Method

reporting limit

(μg/L)

Statistics for detections

Minimum

(μg/L)

Median

(μg/L)

Maximum

(μg/L)
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 29

0.001

0.01

0.1

1

10

CONCENTRATION, IN MICROGRAMS PER LITER

Atrazine

Alachlor

Metolachlor

Deethylatrazine

DCPA

2,4-D

Trifluralin

Cyanazine

EPTC

Carbofuran

Chlorpyrifos

Benfluralin

Propargite

Azinphos-methyl

Diazinon

2,4-DB

Dicamba

Ethalfluralin

Terbufos

Acetochlor

Bentazon

Malathion

Oryzalin

B

PESTICIDE

0

20

40

60

80

100

FREQUENCY OF DETECTION, IN PERCENT

PESTICIDE

A

Atrazine

Alachlor

Metolachlor

Deethylatrazine

DCPA

2,4-D

Trifluralin

Cyanazine

EPTC

Carbofuran

Chlorpyrifos

Benfluralin

Propargite

Azinphos-methyl

Diazinon

2,4-DB

Dicamba

Ethalfluralin

Terbufos

Acetochlor

Bentazon

Malathion

Oryzalin

40

31

27

23 20

17

17

16

14

10

10

6

6

2

2

2

2

1

1

1

1

2 2

10

DATA POINT

DATA POINT

75th PERCENTILE

25th PERCENTILE

90th PERCENTILE

10th PERCENTILE

MEDIAN

NUMBER OF DETECTIONS

HERBICIDE

INSECTICIDE

Figure 4. (A) Frequency of pesticide detections and (B) concentrations of detected pesticides for the

fixed-station sites sampled in the Upper Colorado River Basin study unit, October 1996–January 1998.

(Individual data points were plotted when fewer than 10 data points were available.)
30 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Gore Creek at mouth

0

1

2

3

4

5

Sample with no detections

Insecticide

Herbicide

Dry Creek near Begonia Road

0

2

4

6

8

10

NUMBER OF DETECTIONS

Reed Wash near Mack

0

2

4

6

8

10

12

14

NUMBER OF DETECTIONS

Colorado River near State LIne

0

1

2

3

4

5

6

7

8

10/01/9611/01/9612/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/9711/01/9712/01/9701/01/9802/01/98

NUMBER OF DETECTIONS

DATE

NUMBER OF DETECTIONS

Figure 5. Number of pesticide detections per sampling date, fixed-station sites, Upper Colorado River Basin,

October 1996–January 1998. (Graphs have different vertical scales.)
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 31

1996 1997 1998

MEAN NUMBER OF DETECTIONS 

PER SAMPLE 

0

1

2

3

4

5

6

7

8

Number of samples

Insecticide

Herbicide

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

DATE

4

2

2 2

3

3

7

8

8

7

4

5

1 1

4

June (fig. 3). The highest pesticide concentrations

occurred primarily in May, June, and July. This can be

seen, for example, with atrazine concentrations at Dry

Creek, Reed Wash, and Colorado River near State

Line and carbofuran and 2,4-D concentrations at Reed

Wash (figs. 7 and 8). At Dry Creek, detectable concentrations of atrazine were present in most samples, with

the highest concentrations occurring in May through

August. At Reed Wash, atrazine concentrations were

highest in May through July and were below the detection limit prior to May. For Dry Creek and Reed Wash,

atrazine concentrations were greater in the irrigation

return flows and snowmelt of the spring and summer

months and were undetected or detected at lower

levels in the winter and early spring months when

streamflow was at a minimum (fig. 7). The atrazine

concentration at Colorado River near State Line was

highest during snowmelt (fig. 7). At Reed Wash,

carbofuran and 2,4-D were detected in the April

through August irrigation return flows and undetected

in the low-flow winter and early spring months (fig. 8).

For both Dry Creek and Reed Wash, irrigation return

flows and snowmelt were important mechanisms in the

transport of pesticides from agricultural fields to

drainage streams.

Although it can be assumed that the atrazine,

carbofuran, and 2,4-D detections at Dry Creek and

Reed Wash in the irrigation return flows of the summer

months were related to pesticide use during the same

time period, it is not possible to correlate some of the

pesticide detections directly to pesticide use without

recent (1996–98) pesticide-application data. A few of

the detections in the summer months were at much

higher concentrations than the rest of the summer

detections (figs. 7 and 8); however, without application

data, these spikes in the concentration data cannot be

correlated to recent pesticide applications. In addition,

it cannot be determined if some of the pesticides that

were detected infrequently were not used in the study

unit or if any of the pesticides that were used went

undetected.

Spatial Distribution

Pesticide detections varied among the different

land uses and site types in the UCOL study unit

(table 12). Detections were most common at the agriculture sites. One or more pesticides were detected in

100 percent and 83 percent of the samples at Dry

Creek and Reed Wash, respectively. Seventy-five

percent of the samples at the integrator site, Colorado

River near State Line, contained at least one pesticide

detection, while only 10 percent of the samples at

the urban/forest site of Gore Creek had one or more

detections. At each site, herbicides were detected more

often than insecticides (table 12). A list of the pesticides detected at each site, along with concentration

statistics, is in table 11. Only the herbicides atrazine,

cyanazine, and metolachlor were detected at all four

fixed-station sites. Twelve pesticides (six herbicides

and six insecticides) were detected only at the agriculture sites.

Figure 6. Mean number of pesticide detections per sample by month, all fixed-station sites combined,

Upper Colorado River Basin, October 1996–January 1998.
32 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

Atrazine, Dry Creek near Begonia Road

0.00

0.02

0.04

0.06

0.08

0.10

0.12 10/01/96

11/01/96

12/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/97

11/01/97

12/01/9701/01/9802/01/98

DATE

CONCENTRATION, IN MICROGRAM

PER LITER

Atrazine, Reed Wash near Mack

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

DATE

CONCENTRATION, IN MICROGRAM

PER LITER

Atrazine, Colorado River near State Line

0.000

0.002

0.004

0.006

0.008

0.010

0.012

DATE

CONCENTRATION, IN MICROGRAM 

PER LITER

MRL=0.001

MRL

MRL=0.001

MRL=0.001

PESTICIDE SAMPLE

METHOD REPORTING

LIMIT, IN MICROGRAM

PER LITER

10/01/96

11/01/96

12/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/97

11/01/97

12/01/9701/01/9802/01/98

10/01/96

11/01/96

12/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/97

11/01/97

12/01/9701/01/9802/01/98

Figure 7. Atrazine concentrations at Dry Creek near Begonia Road, Reed Wash near Mack, and Colorado

River near State Line, October 1996–January 1998. (Graphs have different vertical scales.)
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 33

2,4-D, Reed Wash near Mack

0.0

0.2

0.4

0.6

0.8

1.0

1.2

DATE

CONCENTRATION, IN 

 MICROGRAM PER LITER

Carbofuran, Reed Wash near Mack

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

10/01/9611/01/9612/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/9711/01/9712/01/9701/01/9802/01/98

DATE

CONCENTRATION, IN 

MICROGRAMS PER LITER

MRL=0.035

MRL=0.15

MRL=0.003

PESTICIDE SAMPLE

10/01/9611/01/9612/01/9601/01/9702/01/9703/01/9704/01/9705/01/9706/01/9707/01/9708/01/9709/01/9710/01/9711/01/9712/01/9701/01/9802/01/98

MRL METHOD REPORTING

LIMIT, IN MICROGRAMS

PER LITER

Among the land-use/site-type classifications, the

median concentrations (maximum concentrations for

single detections at Gore Creek) of atrazine, cyanazine, and metolachlor were greatest at the two agriculture sites (table 11). The median concentration for the

atrazine detections was highest at Dry Creek, and the

median concentrations of cyanazine and metolachlor

were highest at Reed Wash. The median concentrations for these three pesticides were 0.021, 0.020, and

0.043 μg/L, respectively (table 11). All three median

concentrations were much lower than water-quality

criteria—the drinking-water standard for atrazine of

3 μg/L and the lifetime-health advisories for cyanazine

and metolachlor of 1 μg/L and 70 μg/L, respectively

(table 5). Detecting the highest median concentrations

at the agriculture sites would be expected, as pesticide

use would be more concentrated in the agriculture

areas as compared to the urban/forest area or in the

study unit as a whole (integrator site). The remaining

median concentrations of atrazine, cyanazine, and

metolachlor for the different land-use/site-type classifications were low, 0.014 μg/L or less. In comparing

the urban/forest Gore Creek site to the integrator site

Colorado River near State Line, median atrazine and

Figure 8. Carbofuran and 2,4-D concentrations at Reed Wash near Mack, October 1996–January 1998.

(Graphs have different vertical scales.)
34 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

metolachlor concentrations were greater at the

urban/forest site, and the median cyanazine concentration was greater at the integrator site. These differences between the two sites, however, were low,

0.009 μg/L or less.

Eight other pesticides were detected in common

at sites representing two of the land-use/site-type classifications. Benfluralin was jointly detected at the

urban/forest site and the agriculture sites (table 11).

Alachlor, DCPA, 2,4-D, deethylatrazine, EPTC,

propargite, and trifluralin were all detected at the agriculture sites and the integrator site (table 11). For all

eight pesticides, the highest median concentration was

again at either of the two agriculture sites. Some of

these median concentrations at the agriculture sites

were low, only a few nanograms per liter or so above

the reporting limit for the respective pesticides. Only

EPTC, propargite, and 2,4-D had median concentrations substantially larger than their reporting limits, up

to a difference of 0.585 μg/L for the 2,4-D reporting

limit and detected concentration. The median concentrations for pesticides detected at the integrator and

urban/forest sites overall tended to be low, within a

few nanograms per liter of the reporting limit for most

pesticides (table 11).

In comparing the pesticide data for Dry Creek

and Reed Wash, there are some differences in detections even though both are agricultural areas. Metolachlor, trifluralin, 2,4-D, carbofuran, chlorpyrifos,

and deethylatrazine all had at least a 40-percent difference in percentage of samples with detected pesticides

at the two sites (table 11). The first three pesticides

had a detection rate of greater than 50 percent at Reed

Wash but 20 percent or less at Dry Creek. Carbofuran

and chlorpyrifos were each detected at Reed Wash in

42 percent of the samples but were not detected at Dry

Creek. These differences may be related to pesticide

use, especially with carbofuran and chlorpyrifos, but

also may be related to the sampling schedule. Reed

Wash was sampled weekly or biweekly for pesticides

from April through September 1997, while Dry Creek

was only sampled monthly from late April through

September 1997 (fig. 5). The detection percentages for

Dry Creek may have been higher with more samples.

Trifluralin, for example, was detected at Dry Creek in

13 percent (2 of 15) of the samples, with the detections

occurring monthly in July and August. Presuming that

trifluralin was present the entire 2-month period, the

trifluralin detection frequency would have been higher

if Dry Creek had also been sampled weekly or

biweekly during this time. Deethylatrazine was more

commonly detected in Dry Creek (93 percent) than

Reed Wash (29 percent), possibly because atrazine,

the parent compound, was more common in Dry Creek

and had a higher median concentration. Reed Wash

and Dry Creek also had differences in the number and

concentration of pesticides detected in more than 30

percent of the samples at each site. At Reed Wash, 10

pesticides were detected in more than 30 percent of the

samples, and five had median concentrations greater

than or equal to 0.020 μg/L: 2,4-D (0.270 μg/L),

carbofuran (0.200 μg/L), metolachlor (0.043 μg/L),

EPTC (0.025 μg/L), and cyanazine (0.020 μg/L). At

Dry Creek, five pesticides were detected in more than

30 percent of the samples, and only atrazine (0.021

μg/L) had a median concentration above 0.020 μg/L.

These differences between Reed Wash and Dry Creek

may, again, be related to pesticide use and sampling

schedule.

Table 12. Pesticide detections per land use and site type at fixed-station sites in the Upper Colorado River Basin,

October 1996–January 1998

Site name Land use Site type

Number of

samples

(excluding

quality

assurance)

Number of

samples

with

pesticide

detections

Number of

pesticides

detected

Number of

herbicides

detected

Number of

insecticides

detected

Dry Creek near Begonia Road,

near Delta

Agriculture Indicator 15 15 16 13 3

Reed Wash near Mack Agriculture Indicator 24 20 19 13 6

Colorado River near

Colorado-Utah State Line

All1 Integrator 8 6 10 9 1

Gore Creek at mouth near

Minturn

Urban/forest Indicator 10 1 4 4 0

1Includes agriculture, forest, mining, rangeland, and urban.
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 35

Pesticides at Synoptic Network

Forty-three sites in the Grand Valley and

Uncompahgre River Valley were sampled once for

pesticides in May 1998 as part of a synoptic study of

the water quality of rivers, streams, and drains in agricultural areas of the UCOL study unit. Thirty-one

pesticides were detected at least once (table 13) in

samples for 40 of the 43 sites. Of these 40 samples

with detections, 36 contained two or more detected

pesticides. For the synoptic samples, 3,561 individual

pesticide analyses were performed. Detections

occurred in 223 analyses, or 6.3 percent. Almost

75 percent (167 of 223) of the detections were for the

10 most frequently occurring pesticides. The

frequency of detection for all 31 pesticides and their

measured concentrations are shown in figure 9, in

descending order of frequency. Twenty-one herbicides

Table 13. Statistical summary of concentrations of detected pesticides at 43 agricultural synoptic sites in the Upper Colorado

River Basin, May 1998

[μg/L, micrograms per liter; E, estimated; HPLC, high-performance liquid chromatography; --, not computed]

Pesticide

Number

of

samples

Number of

detections

Percentage

detected

Method

reporting limit

(μg/L)

Statistics for detections

Minimum

(μg/L)

Median

(μg/L)

Maximum

(μg/L)

Atrazine 43 29 67 0.001 E 0.002 0.009 1.04

Alachlor 43 26 60 0.002 E 0.002 0.021 0.751

Metolachlor 43 19 44 0.002 E 0.002 0.009 0.442

Acetochlor 43 18 42 0.002 0.007 0.028 2.61

2,4-D 43 15 35 0.15 0.15 0.25 E 3.50

Cyanazine 43 14 33 0.004 0.004 0.009 0.618

Deethylatrazine 43 14 33 0.002 E 0.003 E 0.006 E 0.013

Trifluralin 43 13 30 0.002 E 0.003 0.006 0.051

DCPA 43 10 23 0.002 E 0.002 0.010 0.109

Carbofuran (HPLC) 43 9 21 0.12 0.22 0.53 E 1.80

Pendimethalin 43 8 19 0.004 0.006 0.021 0.099

EPTC 43 7 16 0.002 E 0.003 0.006 0.061

Diazinon 43 6 14 0.002 E 0.003 0.008 0.104

Simazine 43 5 12 0.005 0.012 0.014 0.065

Benfluralin 43 4 9.3 0.002 E 0.004 E 0.006 0.009

Carbaryl (HPLC) 43 3 7.0 0.008 0.050 0.230 0.780

Chlorpyrifos 43 3 7.0 0.004 0.006 0.007 0.029

Diuron 43 3 7.0 0.02 E 0.02 0.04 E 18.0

Dieldrin 43 2 4.7 0.001 E 0.003 E 0.003 E 0.004

gamma-HCH 43 2 4.7 0.004 0.007 0.009 0.012

Metribuzin 43 2 4.7 0.004 0.006 0.009 0.011

Prometon 43 2 4.7 0.018 0.031 0.215 0.399

Azinphos-methyl 43 1 2.3 0.001 -- -- E 0.006

Bromoxynil 1

42 1 2.3 0.035 -- -- 0.090

Dicamba 1

42 1 2.3 0.035 -- -- E 1.50

Ethalfluralin 43 1 2.3 0.004 -- -- 0.005

Malathion 43 1 2.3 0.005 -- -- 0.014

MCPB 43 1 2.3 0.14 -- -- 0.22

Phorate 43 1 2.3 0.002 -- -- E 0.003

Pronamide 43 1 2.3 0.003 -- -- 0.012

Terbufos 43 1 2.3 0.013 -- -- 0.018

1 Number of samples is different because the result for one sample is unreported due to difficulties in the laboratory determination of concentration.
36 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

0.001

0.01

0.1

1

10

CONCENTRATION, IN MICROGRAMS PER LITER

Atrazine

Alachlor

Metolachlor

Acetochlor

2,4-D

Cyanazine

Deethylatrazine

Trifluralin

DCPA

Carbofuran

Pendimethalin

EPTC

Diazinon

Simazine

Benfluralin

Carbaryl

Chlorpyrifos

Diuron

Dieldrin

gamma-HCH

Metribuzin

Prometon

Azinphos-methyl

Bromoxynil

Dicamba

Ethalfluralin

Malathion

MCPB

Phorate

Pronamide

Terbufos

0

10

20

30

40

50

60

70

80

90

100

Atrazine

Alachlor

Acetochlor

2,4-D

Cyanazine

Deethylatrazine

Trifluralin

DCPA

Carbofuran

Pendimethalin

EPTC

Diazinon

Simazine

Benfluralin

Carbaryl

Chlorpyrifos

Diuron

Dieldrin

gamma-HCH

Metribuzin

Prometon

Azinphos-methyl

Bromoxynil

Dicamba

Ethalfluralin

Malathion

MCPB

Phorate

Pronamide

Terbufos

PESTICIDE

PESTICIDE

FREQUENCY OF DETECTION, IN PERCENT

29

26

19

18

15

14

14

13

10

9

8

7

6

5

4

3

3

3

2 2

2

1

1

1

1

1

1

1

1

2

1

Metolachlor

HERBICIDE

INSECTICIDE

A

B 10 DATA POINT

DATA POINT

75th PERCENTILE

25th PERCENTILE

90th PERCENTILE

10th PERCENTILE

MEDIAN

NUMBER OF DETECTIONS

Figure 9. (A) Frequency of pesticide detections and (B) concentrations of detected pesticides for the

agricultural synoptic sites sampled in the Upper Colorado River Basin, May 1998. (Individual data points

were plotted when fewer than 10 data points were available.)
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 37

and 10 insecticides were detected in the synoptic

study. Herbicides accounted for almost 87 percent

(194 of 223) of the detections. Only two herbicides

were detected in more than 50 percent of the

samples—atrazine (67 percent) and alachlor

(60 percent). Carbofuran, with a detection rate of

21 percent, was the most common insecticide.

Pesticide concentrations in the synoptic study

ranged from 0.002 to 18.0 μg/L (table 13). Many

concentrations were low. The highest concentration of

18.0 μg/L (an estimated value) was very uncommon,

as the next highest concentration, also estimated, was

3.50 μg/L. More than 59 percent of the detections

were greater than or equal to 0.01 μg/L. More than

30 percent of the detections were greater than or equal

to 0.05 μg/L, with more than 35 percent of these

detections due to high carbofuran and 2,4-D concentrations.

Twenty-two pesticides were detected more than

once in the synoptic study. The maximum concentration of each of the 22 pesticides was detected among

10 sites having stream discharges between 1.45 and

185 ft3

/s. Almost 82 percent (18 of 22) of the

maximum concentrations were found in two distinct

discharge ranges. The maximum concentrations of

nine pesticides were detected in samples from streams

or drains having a discharge between 12.7 and 20 ft3/s,

and nine other pesticides had maximum concentrations in streams or drains with discharges between 75

and 185 ft3/s. Three pesticides had maximum concentrations at discharges of less than 4.5 ft3/s, and one

pesticide had a maximum concentration at a discharge

of 40 ft3

/s. It is possible that in areas with lower

discharges, there was not enough surface runoff or

subsurface percolation to transport pesticides from the

fields to streams and drains. In larger streams and

drains, in contrast, pesticide concentrations were

diluted because of the higher streamflows. In the

synoptic study, stream discharge ranged between 0.33

and 18,190 ft3

/s, with a median of 37.0 ft3/s.

Discharge values below 23 ft3

/s were common for

drains, such as the 33 Road Drain near Clifton

(17 ft3

/s), while discharge values above 429 ft3/s were

mainly found at the Colorado, Gunnison, and

Uncompahgre River sites. Ninety percent of the

discharge values were less than 561 ft3

/s, with other

discharge percentiles and discharge values being

70 percent, 145 ft3/s; 25 percent, 9.12 ft3/s; and

10 percent, 2.06 ft3

/s.

In both the Grand Valley and the Uncompahgre

River Valley, sampling sites were named as rivers,

creeks, washes and arroyos, drains, runs, gulches, and

canals (tables 3 and 4). During the synoptic sampling,

the nonriver sites all functioned as drains for irrigation

return flows in the agricultural areas, whether they

were natural water features or manmade ditches.

Some, such as the Dry Creek sites and Roubideau

Creek at mouth, also carried snowmelt runoff. But

because all sites functioned as drains, it was not

possible to determine if there were major differences

in pesticide detections and concentrations between the

natural and manmade watercourses. There were differences in pesticide detections and concentrations

among the river sites, though. The number of pesticide

detections per site for four river sites—Uncompahgre

River at Delta, Gunnison River above Escalante Creek,

Gunnison River near Grand Junction, and Colorado

River near State Line—was in the mid-range of detections for all synoptic sites. Only one pesticide was

detected at the site Uncompahgre River at Montrose,

upstream from the main agricultural areas in the

Uncompahgre River Valley. The Uncompahgre River

at Delta site had the most detections among the river

sites, and the concentrations tended to be higher than

at the other river sites. This site is just downstream

from most of the irrigated land in the Uncompahgre

River Valley and, thus, had water that was more

concentrated with pesticides than the river sites farther

downstream. In general, the concentration of a pesticide detected at the river sites was either below the

median concentration for the particular pesticide at all

synoptic sites or just above the median.

No pesticide concentrations from the synoptic

study were detected above USEPA drinking-water

standards; however, one pesticide had a detected

concentration above USEPA health-advisory guidelines. The diuron concentration of 18 μg/L for the

sample from Indian Wash was almost double the lifetime health advisory of 10 μg/L for a 70-kilogram

adult. This sample was collected during a rainstorm,

an event that may have contributed to the high concentration value in the stream. Runoff from the storm may

have been large enough to wash diuron from the soils

in the area and transport the pesticide to the receiving

stream. The diuron concentration of 18 μg/L was estimated because the concentration was above the HPLC

upper calibration standard of 1.4 μg/L for diuron

(R. Brenton, U.S. Geological Survey, oral commun.,

1999). Criteria for the protection of freshwater aquatic
38 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

life for four pesticides (carbaryl, diazinon, diuron, and

gamma-HCH) were exceeded in five analyses (table

14), of which concentrations of three (for carbaryl,

diazinon, and gamma-HCH) only slightly exceeded

criteria concentrations.

Pesticide detections in the synoptic study

differed somewhat between the Grand Valley and the

Uncompahgre River Valley. The number of herbicide

and insecticide detections for each synoptic site per

valley is shown in figure 10. The Colorado River near

State Line site, while located near the Grand Valley, is

shown separately because it is the integrator site and

drains both the Grand Valley and Uncompahgre River

Valley. The Colorado River near State Line is, thus,

not included in the calculations for the Grand Valley.

Only Drain at D and 29 3/4 Roads in the Grand Valley

and Tongue Creek and Currant Creek in the

Uncompahgre River Valley had no pesticide concentrations detected above the MRLs.

Pesticide detections were slightly more frequent

in the Uncompahgre River Valley than the Grand

Valley. Just over 7.1 percent of the analyses performed

on the samples for the Uncompahgre River Valley had

detected pesticides, whereas 5.4 percent of the analyses for the Grand Valley had detected pesticides. The

particular pesticides detected in each valley differed,

as did the frequency of detections (fig. 11). Of the

31 pesticides detected in the synoptic study, 21 pesticides (15 herbicides and 6 insecticides) were detected

in the Grand Valley and 21 pesticides (16 herbicides

and 5 insecticides) were detected in the Uncompahgre

River Valley. Nine pesticides detected in the Grand

Valley were not found in the Uncompahgre River

Valley: carbofuran, simazine, carbaryl, diuron,

metribuzin, prometon, azinphos-methyl, malathion,

and MCPB. Nine pesticides detected in the

Uncompahgre River Valley were not found in the

Grand Valley: EPTC, benfluralin, chlorpyrifos,

gamma-HCH, bromoxynil, dicamba, ethalfluralin,

pronamide, and terbufos. Twelve pesticides were

detected in both basins; three (2,4-D, metolachlor, and

diazinon) were more frequently detected in the Grand

Valley and seven (atrazine, alachlor, cyanazine, deethylatrazine, acetochlor, pendimethalin, and trifluralin)

were more frequently detected in the Uncompahgre

River Valley. DCPA and dieldrin were detected at

almost the same frequencies in both basins.

As stated previously, a pesticide detection

depends on many factors. The time, rate, and location

of pesticide application, crop type, precipitation or

irrigation events, physical and chemical characteristics

Table 14. Pesticide concentrations exceeding freshwater criteria guidelines for the protection of aquatic life, Grand Valley

and Uncompahgre River Valley, Upper Colorado River Basin agricultural synoptic study, May 1998

[μg/L, micrograms per liter; HPLC, high-performance liquid chromatography; GV, Grand Valley; URV, Uncompahgre River Valley;

NAS/NAE, National Academy of Sciences/National Academy of Engineering; E, estimated]

Pesticide

Detected

concentration

(μg/L)

Criterion

concentration

(μg/L)

Site name and number

(see fig. 2, tables 3 and 4)

Sampling

date Basin

Environment Canada (1999) guidelines for protection of freshwater aquatic life

Carbaryl (HPLC) 0.780 0.2 Indian Wash (GV7) 05/06/98 GV

0.230 0.2 Orchard Mesa Drain (GV8) 05/07/98 GV

International Joint Commission Canada and United States (1978) objectives for the protection of aquatic life

Diazinon 0.104 0.08 Indian Wash (GV7) 05/06/98 GV

gamma-HCH 0.012 0.01 Drain at Blossom Road (URV5) 05/19/98 URV

NAS/NAE freshwater criteria for protection of aquatic life (Nowell and Resek, 1994)

Diuron E 18.01 1.6 Indian Wash (GV7) 05/06/98 GV

1 Concentration was designated as estimated because the concentration value exceeded the HPLC upper calibration standard of 1.4 μg/L for

diuron (R. Brenton, U.S. Geological Survey, oral commun., 1999).
OCCURRENCE AND DISTRIBUTION OF PESTICIDES 39

Uncompahgre River Valley

0

2

4

6

8

10

12

14

SITE

NUMBER OF DETECTIONS

Insecticide

Herbicide

Grand Valley

0

2

4

6

8

10

12

14

SITE

NUMBER OF DETECTIONS

Insecticide

Herbicide

Adobe Creek (GV17)

Hunter Wash (GV15)

Copeco Drain (GV14)

Indian Wash (GV7)

Reed Wash near Loma (GV20)

Drain along Redlands Parkway (GV10)

Little Salt Wash (GV18)

Limekiln Gulch (GV12)

Reed Wash near Mack (GV21)

33 Road Drain (GV3)

Big Salt Wash (GV19)

Drain at C 1/2 and 33 3/4 Roads (GV2)

Appleton Drain (GV11)

Goodwin Drain (GV16)

Persigo Wash (GV13)

Drain at 36 Road (GV1)

Drain at C 1/2 and 28 1/2 Roads (GV6)

Leach Creek GV9)

Orchard Mesa Drain (GV8)

Salt Creek near Mack (GV22)

Drain at D and 29 3/4 Roads (GV5)

Lewis Wash (GV4)

Colorado River near State Line (GV23)

Seep Creek (URV17)

Cummings Gulch (URV16)

Loutsenhizer Arroyo (URV6)

Uncompahgre River at Delta (URV10)

Dry Creek at mouth (URV8)

Garnet Canal (URV9)

Unnamed Drainage at Highway 92 (URV14)

Alfalfa Run (URV12)

Dry Creek near Begonia Road (URV7)

Spring Creek (URV4)

Gunnison River above Escalante Creek (URV19) 

Drain at Blossom Road (URV5)

Gunnison River near Grand Junction URV20)

Happy Canyon Creek (URV2)

Cedar Creek (URV3)

Peach Valley Arroyo (URV13)

Roubideau Creek (URV18)

Uncompahgre River at Montrose (URV1)

Currant Creek (URV11)

Tongue Creek (URV15)

Figure 10. Number of pesticide detections per site in the Grand Valley and Uncompahgre River Valley, May 1998.

(Letters and numbers in parentheses refer to site locations described in figure 2 and tables 3 and 4.)
40 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

0

10

20

30

40

50

60

70

80

90

100

PESTICIDE

FREQUENCY OF DETECTION, IN PERCENT

Grand Valley

Uncompahgre River Valley

0

10

20

30

40

50

60

70

80

90

100

PESTICIDE

FREQUENCY OF DETECTION, IN PERCENT

Uncompahgre River Valley

Grand Valley

Metolachlor (H)

Atrazine (H)

2,4-D (H)

Carbofuran (I)

Alachlor (H)

Cyanazine (H)

DCPA (H)

Deethylatrazine (H)

Simazine (H)

Acetochlor (H)

Diazinon (I)

Carbaryl (I)

Diuron (H)

Pendimethalin (H)

Trifluralin (H)

Metribuzin (H)

Prometon (H)

Azinphos-methyl (I)

Dieldrin (I)

Malathion (I)

EPTC (H)

MCPB (H)

Benfluralin (H)

Chlorpyrifos (I)

gamma-HCH (I)

Bromoxynil (H)

Dicamba (H)

Ethalfluralin (H)

Pronamide (H)

Terbufos (I)

Metolachlor (H)

Atrazine (H)

2,4-D (H)

Carbofuran (I)

Alachlor (H)

Cyanazine (H)

DCPA (H)

Deethylatrazine (H)

Simazine (H)

Acetochlor (H)

Diazinon (I)

Carbaryl (I)

Diuron (H)

Pendimethalin (H)

Metribuzin (H)

Prometon (H)

Trifluralin (H)

Azinphos-methyl (I)

Dieldrin (I)

Malathion (I)

MCPB (H)

EPTC (H)

Benfluralin (H)

Chlorpyrifos (I)

gamma-HCH (I)

Bromoxynil (H)

Dicamba (H)

Ethalfluralin (H)

Pronamide (H)

Terbufos (I)

of pesticides, and atmospheric transport and deposition may affect whether or not a pesticide is detected.

The number of sites with the maximum concentrations for the 12 pesticides detected in both basins

were almost evenly split between the two basins.

Concentrations of five pesticides (atrazine, DCPA,

diazinon, metolachlor, and pendimethalin) were

highest at four sites (Adobe Creek, Copeco Drain,

Indian Wash, and Reed Wash near Loma) in the Grand

Valley. The maximum concentrations of seven pesticides (acetochlor, alachlor, cyanazine, 2,4-D, deethylatrazine, dieldrin, and trifluralin) were detected at

three sites (Loutsenhizer Arroyo, Seep Creek, and

Unnamed Drainage at Highway 92) in the

Uncompahgre River Valley.

Comparison of Pesticides at Fixed-Station

Network and Synoptic Network

Of the 35 pesticides detected in the UCOL,

23 pesticides (16 herbicides, 7 insecticides) were

detected in samples from the fixed-station network and

31 pesticides (21 herbicides, 10 insecticides) were

detected in samples from the synoptic network.

Nineteen pesticides were detected in common at least

once in both networks, and 13 pesticides were detected

in common at least twice in both networks. Of the

19 pesticides detected in common in the two networks,

10 pesticides had higher median concentrations

(maximum concentrations for single detections)

detected in the synoptic study, and 7 pesticides had

Figure 11. Frequency of pesticide detections in the Grand Valley and Uncompahgre River Valley, May 1998. Pesticides are

shown from highest to lowest frequency of detection for each valley (H, herbicide; I, insecticide).
COMPARISON WITH RESULTS OF PREVIOUS STUDIES 41

higher median concentrations detected in the fixedstation network. Some of the differences in the

detected median concentrations were large (0.33 μg/L

for carbofuran), while seven of the differences were

very small (0.002 μg/L or less). Two pesticides, deethylatrazine and trifluralin, had the same median

concentration detected in both networks. Of the 13

pesticides detected two or more times in each network,

the maximum detected concentrations of 11 were from

in the synoptic network, and only 2 were greatest in

the fixed-station network. As stated previously, more

than 59 percent of the pesticide detections in the

synoptic network had concentrations greater than or

equal to 0.01 μg/L, and more than 30 percent had

concentrations greater than or equal to 0.05 μg/L. This

compares to more than 36 percent and almost

22 percent, respectively, in the fixed-station network.

Standards and guidelines for the protection of human

and aquatic health were exceeded more frequently in

the synoptic sampling. The concentration of one

diuron analysis in the synoptic study was above the

lifetime human health advisory, and the concentrations

of four pesticides were detected above criteria for the

protection of freshwater aquatic life in five synoptic

analyses. In the fixed-station network, no human

health advisories were exceeded, and only one pesticide had a concentration detected above the freshwater

aquatic-life criterion.

Acetochlor and pendimethalin were the two

pesticides with the greatest differences in percentage

of detection between the two networks. Acetochlor

was detected much more frequently in the synoptic

sampling than in the fixed-station sampling, 42 percent

as compared to 1.8 percent, respectively. Pendimethalin was detected in 19 percent of the synoptic

samples and was undetected in the fixed-station

samples. Acetochlor is a preplant and preemergence

herbicide and thus would be more common early in

the growing season, such as the May synoptic time

period, rather than being detected year round, as in the

fixed-station sampling. A similar situation would

exist for pendimethalin, a preemergence and early

postemergence herbicide.

COMPARISON WITH RESULTS OF

PREVIOUS STUDIES

The results from the UCOL fixed-station and

synoptic sampling can be compared to the occurrence

and concentrations of pesticides from 20 NAWQA

study units that collected water-quality data from 1992

through 1996. Table 15 lists the study results for

selected pesticides from this NAWQA group—the 10

most commonly detected pesticides and four others

that are common in the UCOL study unit. Fifty-seven

percent (8 of 14) of the selected pesticides were

detected less frequently in the UCOL study unit than

in the larger NAWQA group, and 64 percent (9 of 14)

had lower concentrations at the 90th percentile in the

UCOL (tables 10 and 15). For example, atrazine, the

most commonly detected pesticide in both groups, was

detected in 69 percent of the samples in the UCOL

study unit and in 79 percent of the NAWQA group

samples. Twelve percent of all atrazine samples in the

UCOL study unit had detected concentrations greater

than or equal to 0.05 μg/L, whereas 41 percent of all

atrazine samples for the NAWQA group had detected

concentrations greater than or equal to 0.05 μg/L.

Considering only the samples with pesticide detections, almost 50 percent of the samples with detections

in the UCOL had concentrations greater than or equal

to 0.01 μg/L, and 20 percent were greater than or

equal to 0.05 μg/L. Nationally, again among only the

pesticide samples with detections, most of the

NAWQA group samples with detected concentrations

had concentration values greater than 0.01 μg/L, and

more than one-half were greater than or equal to

0.05 μg/L (U.S. Geological Survey, 1998a).

Some pesticides that were commonly detected

in the NAWQA group were rare or undetected in the

UCOL study unit (tables 10 and 15). Simazine,

prometon, diazinon, and tebuthiuron were detected in

25 percent or more of the samples in the NAWQA

group but only in 8 percent or less (undetected for

tebuthiuron) of the UCOL samples. All four pesticides

have substantial urban and suburban use nationally but

are probably used less frequently in the UCOL study

unit because of its predominantly rural and undeveloped character. Sites classified as urban in the

NAWQA group consisted of sites in or near major

metropolitan areas of the Nation, including Atlanta,

Georgia; Denver, Colorado; and Washington, D.C. In

contrast, the urban site in the UCOL, Gore Creek at

the mouth, is located downstream from the small

resort town of Vail, Colorado, and most of the Gore

Creek watershed is forest land in the White River

National Forest rather than urban land. For these

reasons, Gore Creek at the mouth is a very different
42 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

urban site than the sites classified as urban in the

NAWQA group. In comparing agricultural areas

between the two groups, alachlor and 2,4-D were

detected more frequently in the UCOL. This may be a

reflection of the amount of alfalfa, corn, dry beans, and

various grains grown in the UCOL as compared to the

NAWQA group. In studying agricultural land use, the

focus of the 1992–96 NAWQA study units was limited

to the most important agricultural settings within the

study units; thus, the results are not an unbiased representation of all streams in agricultural settings nationally (U.S. Geological Survey, 1998a). Some crops

grown and pesticides used in certain parts of the

country may not be represented by this NAWQA

group. In this comparison of the UCOL study unit and

the NAWQA group, carbofuran has been excluded

from the discussion because the carbofuran results for

the two groups have been reported for different analysis methods: HPLC for the UCOL study unit and

GC/MS for the NAWQA group.

In 1995–96, streambed sediment and wholebody fish were collected at selected sites in the UCOL

basin and analyzed for particular organochlorine

pesticides and PCB’s (Stephens and Deacon, 1998).

Nine sites were common to that study and the pesticide sampling described in this report: Gore Creek at

mouth, Dry Creek near Begonia Road, Tongue Creek

at Cory, Gunnison River near Grand Junction, 33 Road

Drain at mouth, Appleton Drain near mouth, Copeco

Drain near mouth, Reed Wash near Mack, and Colorado River near State Line. Six pesticides were

analyzed for in both studies: alpha-HCH, gammaHCH, DCPA, dieldrin, p,p'-DDE, and cis-permethrin.

Concentrations of DCPA, dieldrin, and/or p,p'-DDE

were detected in the streambed sediments and/or

whole-body fish at many of the nine sites. For the

surface-water sampling, DCPA was the only pesticide

detected at any of the nine sites in common. Dieldrin

was not detected at any common sites, and p,p'-DDE

was undetected in the surface-water sampling. Since

1974, the use of dieldrin has been restricted to termite

control and nonfood plant treatment, whereas DDT,

the parent compound of p,p'-DDE, has been banned

from use in the United States since 1972. The presence

of DCPA, dieldrin, and p,p'-DDE in streambed sediment and whole-body fish is not uncommon, though,

because organochlorine compounds are extremely

persistent in the environment and bind to sediment and

to fatty tissue in aquatic biota. As can be seen with the

nondetections of dieldrin and p,p'-DDE in the water

column and the detections in sediment and fish, the

absence of a pesticide in a water sample from a particTable 15. Summary of selected pesticide occurrences and concentrations for all 1,058 surface-water sites sampled as part

of 20 NAWQA study units, 1992–96 (U.S. Geological Survey, 1998a)

[Percentages are rounded; ≥, greater than or equal to; μg/L, micrograms per liter; MRL, method reporting limit; <, less than; E, estimated; --, method

reporting limit greater than detection threshold]

Pesticide

Number

of

samples

Percentage

detected

Percentage

detected

≥ 0.01

(μg/L)

Percentage

detected

≥ 0.05

(μg/L)

MRL

(μg/L)

Concentration at indicated

percentile (μg/L)

Maximum

concentration

(μg/L) 10th 50th 90th

Atrazine 5,196 79 66 41 0.001 < MRL 0.026 0.70 E 120

Metolachlor 5,191 69 52 28 0.002 < MRL 0.011 0.33 E 70

Simazine 5,198 66 54 25 0.005 < MRL 0.012 0.17 E 20

Deethylatrazine 5,197 57 38 18 0.002 < MRL 0.096 0.096 E 1.1

Prometon 5,198 49 40 14 0.018 < MRL < MRL 0.066 E 4.2

Diazinon 5,158 36 25 10 0.002 < MRL < MRL 0.051 3.8

Cyanazine 5,198 32 29 15 0.004 < MRL < MRL 0.11 E 160

Alachlor 5,198 31 23 9.9 0.002 < MRL < MRL 0.049 E 25

Tebuthiuron 5,198 25 17 2.8 0.01 < MRL < MRL 0.018 E 6.4

DCPA 5,196 24 7.1 2.4 0.002 < MRL < MRL 0.006 E 100

EPTC 5,195 18 11 4.0 0.002 < MRL < MRL 0.012 E 40

Trifluralin 5,196 15 5.8 0.8 0.002 < MRL < MRL 0.01 0.51

Acetochlor 1,987 12 9.4 4.4 0.002 < MRL < MRL 0.008 3.8

2,4-D 3,400 12 -- 10 0.035 < MRL < MRL 0.050 E 15
SUMMARY and conclusions 43

ular site does not necessarily mean that the pesticide is

not present at the site. The pesticide may have accumulated in the sediment and aquatic biota.

Butler and others (1991, 1994) collected water

samples for pesticide analysis in the Uncompahgre

River Valley in July 1988 and the Grand Valley in

June and August 1991 as part of a reconnaissance

investigation of irrigation-project areas. Two sites and

14 pesticides in the Uncompahgre River Valley reconnaissance study were also studied as part of the 1998

synoptic pesticide investigation in the UCOL study

unit. Three pesticides—malathion, methyl parathion,

and parathion—were detected at the Uncompahgre

River Valley reconnaissance sites in July 1988. For the

Grand Valley reconnaissance investigation in June

and August 1991, 9 sites from the June sampling and

20 insecticides were in common with the UCOL

synoptic pesticide sampling. Parathion and 3-hydroxycarbofuran were detected once above their respective

MRLs at two different sites in the Grand Valley reconnaissance, whereas diazinon was detected above the

reporting limit once at four sites. Of the five pesticides—3-hydroxycarbofuran, diazinon, malathion,

methyl parathion, and parathion—detected in the

reconnaissance investigation that were also analyzed

for as part of the UCOL study, only diazinon and

malathion were detected. This difference in pesticide

detections may depend on sampling time and sites

sampled, crops grown and insects or weeds common

to the particular crops, pesticide usage, and the

reporting limits for the various pesticides being

studied. The reconnaissance investigation of Butler

and others (1991, 1994) occurred later in the growing

season, from June through August, than the UCOL

pesticide synoptic sampling in May. Also, the UCOL

fixed-station sites sampled in June through August

were not the same sites sampled in the June through

August reconnaissance investigation. Pesticide application varies depending on the time of year; for

example, preemergence herbicides are applied before

or at the beginning of the growing season, while insecticides are commonly applied later in the growing

season. Different crops may have been grown in 1988

and 1991 as compared to 1997, resulting in different

pesticides being applied and, therefore, detected.

Finally, the MRLs for the pesticides studied in the

reconnaissance investigation were generally much

higher than those in the UCOL study; the MRL for

carbaryl was 0.5 μg/L in 1988 and 1991 and

0.008 μg/L (for HPLC) in 1997, for example.

SUMMARY AND CONCLUSIONS

Forty-four sites in the Upper Colorado River

Basin study unit were sampled for pesticides during

1996–98. Four fixed-station sites in the UCOL

surface-water network were sampled from October

1996 through January 1998. These sites were sampled

approximately monthly for up to 1 year, and samples

were collected more frequently during the spring and

summer growing season. Forty-three sites in the Grand

Valley and Uncompahgre River Valley, intensive agricultural areas in the UCOL, were each sampled once

in May 1998 as part of a water-quality study of rivers,

streams and drains in these agricultural areas. Pesticides were detected at 41 of the 44 UCOL sites.

Concentrations generally were low. For all samples

and analyses, only atrazine and alachlor had median

concentrations greater than their respective MRLs; in

both cases, only slightly above the MRLs. Carbofuran

and 2,4-D were the only pesticides to be detected

consistently at concentrations higher than the other

pesticides studied, with 90th percentile concentrations

of 0.447 and 0.385 μg/L, respectively. Increased pesticide detections and concentrations occurred after the

probable time of pesticide application and runoff from

storm events and in agricultural areas during the

growing season.

For the fixed-station sites, herbicides were

more commonly detected than insecticides. The herbicides atrazine and alachlor were the only pesticides

detected in more than 50 percent of the samples, with

atrazine being the most commonly detected pesticide

(70-percent detection rate). Carbofuran and chlorpyrifos were the most commonly detected insecticides,

each with a detection rate of 18 percent. Detected

concentrations of all pesticides in the fixed-station

network generally were low, ranging from 0.001 to

2.60 μg/L. Pesticide detections and concentrations in

the fixed-station study differed seasonally and across

land uses and site types. The mean number of pesticides detected and pesticide concentrations were

higher in the summer months and at the two agriculture sites. Only one detection, for azinphos-methyl at

the agriculture site of Reed Wash, exceeded the criterion for the protection of freshwater aquatic life.

In the synoptic study, as in the fixed-station

study, herbicides were more common than insecticides, and atrazine and alachlor were the only

pesticides detected in more than 50 percent of the

samples, 67 and 60 percent, respectively. Carbofuran
44 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98

(21 percent) was the most commonly detected insecticide. Concentrations for the pesticides detected in the

synoptic study ranged from 0.002 to 18.0 μg/L, and

many were low. The maximum concentration of

18.0 μg/L was an estimated concentration for diuron,

which exceeded the lifetime human-health advisory

for a 70-kilogram adult. This sample was collected

during a rainstorm, which may have contributed to the

high concentration. Four pesticides—carbaryl, diazinon, diuron, and gamma-HCH—had five detections

above criteria for the protection of freshwater aquatic

life. Pesticides were detected only slightly more

frequently in the Uncompahgre River Valley than in

the Grand Valley. Of the 12 pesticides detected in both

areas, the maximum concentrations of 5 pesticides

were detected at four sites in the Grand Valley, and the

maximum concentrations of 7 pesticides were detected

at three sites in the Uncompahgre River Valley.

Pesticides in the synoptic study were more

frequently detected above 0.01 and 0.05 μg/L than in

the fixed-station study—more than 59 and 30 percent,

respectively, for the synoptic study and more than

36 and almost 22 percent, respectively, for the fixedstation study. The synoptic study focused only on agriculture sites during the growing season when greater

pesticide use would be expected. The fixed-station

study included samples from urban/forest and integrator sites along with two agriculture sites for up to

one year. Human-health advisories and criteria for the

protection of freshwater aquatic life were exceeded

more frequently in the synoptic sampling.

Most pesticides detected in the UCOL study

unit during the fixed-station and synoptic sampling

periods were not detected at concentrations of

concern, and beneficial uses of water were not being

impaired by the presence of pesticides in surface

waters. No pesticides were detected above drinkingwater standards. Slightly more than one-half of the

pesticides in the UCOL were detected less frequently

than in the larger 1992–96 NAWQA group, and

concentrations generally were lower in the UCOL. In

the agricultural areas of the study unit, concentrations

of various pesticides were occasionally detected above

human-health guidelines and criteria for the protection

of freshwater aquatic life. The few pesticides that had

detected concentrations above water-quality guidelines may be of concern, but without more intensive

sampling of the agricultural synoptic sites, it cannot be

stated that the exceedances reflect a chronic problem

or are a rare occurrence. With regard to water-quality

guidelines and standards, though, it must be stated

again that not all pesticides have had standards and

criteria established, and for those with standards and

criteria, the values have been based on the effects of

the pesticides individually. Most samples for the

UCOL study unit had more than one pesticide detected

per sample, and the effects of combinations of pesticides on human and aquatic health are not known.

Also, it is not possible to know if pesticide detections

and concentrations in the UCOL study unit have

decreased or increased over time because of the short

(1996–98) sampling period. Only through long-term

sampling would it be possible to determine trends in

pesticide detections and concentrations in the study

unit. As was shown by the detection of pesticides in

streambed sediment and whole-body fish in the UCOL

during 1995–96 that were undetected in water samples

collected during 1996–98, pesticides may be present at

a particular site and not be detected in a stream-water

sample for that site. Many organochlorine pesticides

bind to sediment and fatty tissues of aquatic biota and

are extremely persistent in the environment. These

pesticides may be undetected in the stream-water

sample but still be present at the site in streambed

sediment and fish.

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N.E., 1996, Environmental setting and implications on

water quality, Upper Colorado River Basin, Colorado

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Investigations Report 95–4263, 33 p.

Bohmont, B.L., 1991, Colorado pesticide use survey,

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Bohmont, B.L., 1993, Colorado pesticide use survey,

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