NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03–4075

Water-Quality Assessment of the Eastern Iowa

Basins: Selected Pesticides and Pesticide

Degradates in Streams, 1996–98
2

ACKNOWLEDGMENTS

Data Collection, Site Construction, Data Entry or Verification

James Cerveny, Hydrologic Technician, U.S. Geological Survey, Iowa City, Iowa

Eric Sadorf, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Debra Sneck-Fahrer, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Matthew Bobier, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Jennifer Tobias, Student Trainee, U.S. Geological Survey, Iowa City, Iowa

Joel Galloway, Hydrologic Technician, U.S. Geological Survey, Iowa City, Iowa

Jon Nania, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

James Klaus, Student Trainee, U.S. Geological Survey, Iowa City, Iowa

Denise Montgomery, U.S. Geological Survey, Iowa City, Iowa

Jake Freier, Student Trainee, U.S. Geological Survey, Iowa City, Iowa

Erik Smith, Student Trainee, U.S. Geological Survey, Iowa City, Iowa

Robert Einhellig, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Jeffery Harms, Hydrologic Technician, U.S. Geological Survey, Fort Dodge, Iowa

David Conell, Hydrologic Technician, U.S. Geological Survey, Fort Dodge, Iowa

Joseph Gorman, Supervisory Hydrologic Technician, U.S. Geological Survey, Council Bluffs, Iowa

Alvin Conkling, Lead Hydrologic Technician, U.S. Geological Survey, Fort Dodge, Iowa

James Sondag, Hydrologic Technician, U.S. Geological Survey, Council Bluffs, Iowa

Patrick Lustgraaf, Hydrologic Technician, U.S. Geological Survey, Council Bluffs, Iowa

Michael Turco, Hydrologist, U.S. Geological Survey, Council Bluffs, Iowa

Patrick Sweeney, Hydrologic Technician, U.S. Geological Survey, Council Bluffs, Iowa

Daron Tanko, Hydrologic Technician, U.S. Geological Survey, Council Bluffs, Iowa

Technical Advice and support

Dana Kolpin, Hydrologist, U.S. Geological Survey, Iowa City, Iowa 

Craig Harvey, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Technical Review

Robert Buchmiller, Supervisory Hydrologist, U.S. Geological Survey, Iowa City, Iowa

William Battaglin, Hydrologist, U.S. Geological Survey, Lakewood, Colorado

Daniel Sullivan, Hydrologist, U.S. Geological Survey, Middleton, Wisconsin

Report preparation

Kimberlee K. Barnes, Hydrologist, U.S. Geological Survey, Iowa City, Iowa

Mary Kidd, Editor, U.S. Geological Survey, Lakewood, Colorado

Robert Olmstead, Visual Information Specialist, U.S. Geological Survey, Lakewood, Colorado

Alene Brogan, Editorial Assistant, U.S. Geological Survey, Lakewood, Colorado

Approving Official

Keith J. Lucey, Central Region Reports Specialist, U.S. Geological Survey, Lakewood, Colorado

Cover photographs:

Top, photograph of a corn field in late spring from a farm near Riverside, Iowa

(photograph by Doug Schnoebelen, U.S. Geological Survey

Middle, aerial photograph of farms in east-central Iowa

(photograph by Steve Kalkhoff, U.S. Geological Survey)

Bottom, aerial photograph of the Cedar River as it flows through Cedar Rapids, Iowa 

(photograph by Steve Kalkhoff, U.S. Geological Survey)
The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management 

of water, biological, energy, and mineral resources. 

(http://www.usgs.gov/). Information on the quality 

of the Nation’s water resources is of critical interest to 

the USGS because it is so integrally linked to the longterm availability of water that is clean and safe for 

drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for 

the multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

critical to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

conditions changing over time? How do natural features and human activities affect the quality of streams 

and ground water, and where are those effects most 

pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide 

science-based insights for current and emerging water 

issues and priorities. NAWQA results can contribute to 

informed decisions that result in practical and effective 

water-resource management and strategies that protect 

and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. (http://water.usgs.gov/

nawqa/nawqamap.html). Collectively, these Study 

Units account for more than 60 percent of the overall 

water use and population served by public water 

supply, and are representative of the Nation’s major 

hydrologic landscapes, priority ecological resources, 

and agricultural, urban, and natural sources of 

contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding 

of how and why water quality varies regionally and 

nationally. The consistent, multi-scale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive, and allows direct comparisons of 

how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the communication and dissemination of credible, timely, and 

relevant science so that the most recent and available 

knowledge about water resources can be applied in 

management and policy decisions. We hope this 

NAWQA publication will provide you the needed 

insights and information to meet your needs, and 

thereby foster increased awareness and involvement 

in the protection and restoration of our Nation’s 

waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated 

understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation’s water resources. The Program, therefore, 

depends extensively on the advice, cooperation, 

and information from other Federal, State, interstate, 

Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD III

FOREWORD
Water-Quality Assessment of the Eastern 

Iowa Basins: Selected Pesticides and 

Pesticide Degradates in Streams,

1996–98

By Douglas J. Schnoebelen, Stephen J. Kalkhoff, Kent D. Becher,

and E.M. Thurman

Iowa City, Iowa

2003

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03–4075

Prepared as part of the

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046

For additional information write to:

District Chief

U.S. Geological Survey

Federal Building, Room 269

400 South Clinton

Iowa City, IA 52244

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, 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.

Copies of this report can be purchased 

from:

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225
CONTENTS V

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

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

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

Description of the Eastern Iowa Basins................................................................................................................................. 3

Geomorphology ........................................................................................................................................................... 3

Climate......................................................................................................................................................................... 6

Streamflow................................................................................................................................................................... 6

Land Use...................................................................................................................................................................... 6

Pesticide Use and Properties........................................................................................................................................ 9

Study Design and Methods of Study ..................................................................................................................................... 11

Sampling Site Selection............................................................................................................................................... 11

Sampling Methods ....................................................................................................................................................... 12

Analytical Methods...................................................................................................................................................... 14

Quality Assurance/Quality Control ............................................................................................................................. 15

Data Analysis............................................................................................................................................................... 16

Statistical Analysis of Pesticide and Pesticide Degradates................................................................................ 17

Ancillary Data.................................................................................................................................................... 17

Pesticides and Pesticide Degradates in Streams .................................................................................................................... 18

Occurrence and Distribution........................................................................................................................................ 18

Seasonal Variability ........................................................................................................................................... 21

Spatial Variability .............................................................................................................................................. 27

Relevance of Pesticides in Streams........................................................................................................................................ 32

Human Health.............................................................................................................................................................. 32

Aquatic Life ................................................................................................................................................................. 33

Summary and Conclusions .................................................................................................................................................... 33

References.............................................................................................................................................................................. 35

Appendix................................................................................................................................................................................ 41

1. Herbicide and insecticide target analytes, method detection limits, and drinking water standards .................. 43

2. Statistical summary of pesticides and pesticide degradaates at the basic-fixed sampling sites in the 

Eastern Iowa Basins, 1996–98 .......................................................................................................................... 46

FIGURES

1. Map showing landform regions and surface-water-quality sampling sites in the Eastern Iowa Basins study 

unit............................................................................................................................................................................. 4

2. Block diagrams showing physiography of landform regions in the Eastern Iowa Basins study unit ....................... 5

3. Graphs showing precipitation and discharge at selected sites in the Eastern Iowa Basins study unit, 1996–98,

and map showing site locations................................................................................................................................. 7

4. Map showing land cover in the Eastern Iowa Basins study unit, 1996–98............................................................... 8

5–17. Graphs showing:

5. Use of selected herbicides in Iowa, 1990–98 .................................................................................................... 11

6. Occurrence of pesticide compounds at basic-fixed sampling sites in the Eastern Iowa Basins study unit, 

1996–98............................................................................................................................................................. 18

7. Frequency distribution of selected herbicide concentrations at basic-fixed sampling sites in the Eastern 

Iowa Basins study unit, 1996–98....................................................................................................................... 19

8. Occurrence of pesticide and pesticide degradates at basic-fixed sampling sites using a common reporting 

limit ................................................................................................................................................................... 20

9. Detection of multiple compounds in samples from streams at basic-fixed sampling sites in Eastern Iowa 

Basins study unit, 1996–98 ............................................................................................................................... 21

10. Monthly concentrations of selected triazine herbicides (atrazine, cyanazine, simazine) and degradates 

at basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98 ................................................. 22

CONTENTS
VI CONTENTS

11. Monthly concentrations of selected chloroacetanilide herbicides (acetochlor, alachlor, metolachlor) 

and their degradates at basic-fixed sampling sites in streams in the Eastern Iowa Basins study unit, 

1996–98............................................................................................................................................................. 23

12. Seasonal variability of pesticides and pesticide degradates near the headwaters and the mouth of the 

Iowa River, 1996–98 ......................................................................................................................................... 24

13. Detection rate of herbicides and herbicide degradates during low and high base-flow conditions at 

synoptic sampling sites on streams that drain agricultural watersheds in the Eastern Iowa Basins 

study unit........................................................................................................................................................... 25

14. Concentrations of selected herbicides during low and high base-flow conditions in streams that drain 

agricultural watersheds in the Eastern Iowa Basins study unit ......................................................................... 26

15. Monthly detection rate of selected insecticides in streams in the Eastern Iowa Basins study unit................... 27

16. Concentrations of selected triazine herbicides (atrazine, cyanazine, simazine) and degradates at the 

basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98..................................................... 28

17. Concentrations of selected chloroacetanilide herbicides (acetochlor, alachlor, metolachlor) and their 

degradates at the basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98 ......................... 29

18. Map showing atrazine management areas in the Eastern Iowa Basins study unit .................................................... 30

TABLES

1. Common agricultural pesticides used in Iowa during 1996 and analyzed as part of the Eastern Iowa Basins 

study, 1996–98 ........................................................................................................................................................ 10

2. Description of surface-water-quality sampling sites in the Eastern Iowa Basins study unit................................... 13

3. Summary of percent recovery data for commonly detected pesticides spiked in the field for the Eastern 

Iowa Basins study unit, 1996–98 ............................................................................................................................ 16

4. Summary of replicate sample data for commonly detected parent pesticide and degradate pesticide

compounds .............................................................................................................................................................. 17

5. Pesticide compounds not detected in surface-water samples in the Eastern Iowa Basins study unit, 

1996–98................................................................................................................................................................... 19

6. Triazine and chloroacetanilide pesticide and pesticide degradates during low (August 1997) and high 

(May 1998) base flow at the synoptic sampling sites in the Eastern Iowa Basins study unit................................. 31

CONVERSION FACTORS AND ABBREVIATIONS

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

°F = (1.8 × °C) + 32

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

Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. For 

example, the water year ending September 30, 1996, is called water year 1996.

Multiply By To obtain

Length

centimeter (cm) 0.03937 inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

Area

cubic hectometer (hm3) 810.7 acre-foot 

hectare 2.471 acre

square kilometer (km2) 0.3861 square mile

Flow

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

Concentration

micrograms per liter (μg/L) 1.0 parts per billion
Abstract 1

Water-Quality Assessment of the Eastern Iowa 

Basins: Selected Pesticides and Pesticide 

Degradates in Streams, 1996–98

By Douglas J. Schnoebelen, Stephen J. Kalkhoff, Kent D. Becher, and E. Michael Thurman

Abstract

Water samples were collected in streams of 

the Eastern Iowa Basins study unit from 1996 to 

1998 as part of the U.S. Geological Survey’s 

National Water-Quality Assessment (NAWQA) 

Program. More than 350 samples were collected 

to document the occurrence, distribution, and 

transport of pesticides and pesticide degradates. 

The Eastern Iowa Basins study unit encompasses 

about 50,500 square kilometers (19,500 square 

miles) in eastern Iowa and southern Minnesota 

and is drained by four major rivers—the Wapsipinicon, Cedar, Iowa, and Skunk—which flow 

into the Mississippi River at the eastern border of 

Iowa. 

The most commonly detected pesticides—

acetochlor, alachlor, atrazine, cyanazine, and 

metolachlor—were those most heavily used on 

crops during the study. Atrazine and metolachlor 

were detected in 100 percent, and acetochlor, 

alachlor and cyanazine were detected in more 

than 70 percent of all surface-water samples. 

Four pesticide degradates—metolachlor ethane 

sulfonic acid, alachlor ethane sulfonic acid, 

metolachlor oxanilic acid, and acetochlor ethane 

sulfonic acid were detected in more than 

75 percent of the samples. Only one nonagricultural herbicide, prometon, was detected in more 

than 80 percent of the samples. Carbofuran, the 

most commonly detected insecticide, was found 

in 16 percent of all samples. 

 Mixtures of pesticide compounds 

commonly occurred in the samples. Five or more 

parent pesticide compounds were detected in 

50 percent of the samples. Four or more pesticide 

degradates were detected in 68 percent and seven 

or more pesticide degradates were detected in 

17 percent of the samples. 

Acetochlor, alachlor, atrazine, cyanazine, 

and metolachlor were generally present at low 

concentrations; median concentrations ranged 

from 0.01 to 0.22 microgram per liter. However, 

median concentrations for the pesticide degradates, 0.07 to 3.7 micrograms per liter, were 

larger than their parent compounds. Acetochlor, 

alachlor, atrazine, cyanazine, and metolachlor 

pesticide compounds were detected at an order of 

magnitude or higher in the late spring and 

summer than at other times of the year.

Pesticide concentrations generally peak 

following application in May and June and 

decrease during the growing season. A small 

secondary peak of atrazine, acetochlor, alachlor, 

cyanazine, and metolachlor concentrations 

occurred in late winter at all sites. The seasonal 

patterns for the triazine (atrazine and cyanazine) 

degradates were similar to the parent compounds 

(increasing in the spring), but the triazine degradates often had higher median concentrations than 

their parent compounds in the fall and winter. The 

chloroacetanilide (acetochlor, alachlor, and 

metolachlor) degradates did not follow a strong 

seasonal pattern like their parent compounds. In 

general, the chloroacetanilide degradates had 

constant and higher median concentrations when 

compared to their parent compounds throughout 

the year. The median concentrations for the 

chloroacetanilide pesticide degradates were often 

an order of magnitude higher than their parent 

compounds. 
2 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Concentrations of pesticides varied by landform region. Atrazine and cyanazine and their 

degradates were present in significantly greater 

concentrations in streams of the Southern Iowa 

Drift Plain than streams of either the Des Moines 

Lobe or the Iowan Surface. 

INTRODUCTION

The occurrence, fate, and transport of pesticide 

and pesticide degradate compounds in surface water 

are an important component of water quality in the 

Eastern Iowa Basins (EIWA) study unit of the 

U.S. Geological Survey (USGS) National WaterQuality Assessment Program (NAWQA). Pesticides 

used to control weeds, insects, and other pests often 

receive widespread public attention because of their 

potential impacts on humans or the environment. 

Pesticides and their breakdown transformation products (degradates) are transported primarily by water 

from areas where they are applied to nearby rivers and 

streams. However, one of the largest gaps in Iowa’s 

surface-water-quality data base is a lack of historical 

data on concentrations of pesticides in Iowa surface 

water (Iowa Department of Natural Resources, 1997, 

section 2, p. 47). Pesticides including herbicides, 

fungicides, and insecticides are used extensively in 

agricultural and urban settings in the EIWA study unit. 

The EIWA study is intended to provide much needed 

information on (1) the spatial and seasonal variability 

of pesticide and pesticide degradate concentrations 

and (2) natural and anthropogenic factors that may 

affect the fate and transport of pesticides in Iowa and 

in similar agricultural settings across the Midwest.

In 1991, the USGS began implementation of the 

NAWQA Program. The long-term goals of the 

NAWQA Program are to describe the current waterquality conditions and trends of the Nation’s water 

resources and to link assessment with an understanding of the natural and human factors that affect 

the quality of water (Gilliom and others, 1995). The 

EIWA study unit encompasses about 50,500 km2

(19,500 square miles) and is one of 15 NAWQA study 

units (Kalkhoff, 1994) that were selected nationally to 

begin assessment in 1994.

 In Iowa, and across much of the Midwest, the 

occurrence, distribution, transport, and fate of pesticides in surface water are water-quality topics of 

concern (Goolsby and others, 1991; Goolsby and Battaglin, 1993; Kalkhoff, 1993; Iowa Department of 

Natural Resources, 1994a; Hallberg and others, 1996). 

Common herbicides used in Iowa, such as atrazine, 

alachlor, and cynazine, are potential carcinogens and 

have Maximum Contaminant Levels (MCLs) established by the U.S. Environmental Protection Agency 

(USEPA) for drinking water (U.S. Environmental 

Protection Agency, 1995). Several pesticide degradates have been detected in surface waters of Iowa and 

the Midwest and can persist much longer than the 

parent compounds (Goolsby and Battaglin, 1993; 

Kalkhoff and others, 1998). Little is known about the 

occurrence of many pesticide degradates, and even 

less is known about their effects on human health and 

aquatic life (U.S. Geological Survey, 1999, p. 77). 

Several studies have raised additional concerns about 

potential effects of manmade chemicals in surface 

water on the endrocrine systems of aquatic and terrestrial organisms (Colborn and Clement, 1992; Colborn 

and others, 1993; Goodbred and others, 1997). 

Concentrations of pesticides are typically highest in 

streams draining agricultural regions and water 

samples from these streams almost always contain at 

least one pesticide compound (Gilliom and others, 

1999).

Surface waters are particularly vulnerable to 

contamination by pesticides because most agricultural 

and urban areas drain into surface-water systems. In 

particular, pesticides are commonly used on agricultural land and some agriculturally related practices 

such as tile drainage and cropping close to streambanks can cause increased susceptibility to pesticide 

transport. Long-term monitoring of pesticides and 

pesticide degradates is needed to better understand the 

occurrence, transport, and fate of pesticides and pesticide degradates in surface water. 

Purpose and Scope

This report summarizes results from selected 

pesticide and pesticide degradate analyses from 

samples collected from streams and rivers in the EIWA 

study unit for 1996–98. Results include (1) seasonal 

and spatial variability of pesticide and pesticide degradate concentrations and loads and (2) natural and 

human factors that may affect the fate and transport of 

pesticides in the Eastern Iowa Basins. Twelve sites on 

streams draining 320 to 32,400 km2 were sampled for 
DESCRIPTION OF THE EASTERN IOWA BASINS 3

pesticides and pesticide degradates monthly during the 

study. In addition, 25 sites on streams draining 

310–1,500 km2 watersheds were sampled for selected 

pesticides and pesticide degradates during base-flow 

conditions in August 1997 and May 1998 to better 

define spatial variability in the study unit. 

DESCRIPTION OF THE EASTERN IOWA 

BASINS

The EIWA study unit encompasses the Wapsipinicon, Cedar, Iowa, and Skunk River Basins, which 

cover about 50,500 km2 (19,500 mi2) of eastern Iowa 

and southeastern Minnesota (fig. 1). The four major 

rivers have their headwaters in the northwestern part of 

the study unit, flow southeastward, and discharge to 

the Mississippi River. Important components in the 

interpretation of the pesticide and pesticide degradate 

data include the geomorphology, climate, streamflow, 

land use, and types of pesticides used in the EIWA 

study unit. 

Geomorphology

The EIWA study unit consists of three major 

landform regions (Des Moines Lobe, Iowan Surface, 

Southern Iowa Drift Plain) (Prior, 1991) and one 

subregion (Iowan Karst, a subregion of the Iowan 

Surface) that have distinct spatial differences in topography, geology, and soils (fig. 1). These regions also 

broadly coincide with ecoregions and subecoregions 

of Iowa (Griffith and others, 1994). 

The Des Moines Lobe, in the western part of the 

study unit, is one of the youngest landforms in Iowa 

and is characterized by low local relief (15–30 m) and 

small slope (fig. 2). The Des Moines Lobe was formed 

approximately 12,000 to 14,000 years ago (Wisconsinan age) during the last glaciation in Iowa and has 

been only slightly altered since that time (Prior, 1991). 

The sediments of the Des Moines Lobe have not been 

exposed to erosion and weathering processes for as 

long a period of time when compared to sediments of 

the other older landform regions in the EIWA study 

unit. The topography consists of predominantly flat 

and slightly rolling land broken by curved bands of 

“knob and kettle” terrain (Buchmiller and others, 

1985). Originally, ponds and wetlands were characteristic of the Des Moines Lobe. Extensive ditching and 

tiling of fields beginning in the early 1900’s has 

increased the surface drainage in this area. The potential natural vegetation is bluestem prairie (Griffith and 

others, 1994), although corn and soybean production 

presently dominates. Stream development is poor with 

many small, low-gradient streams that drain into relatively few large rivers. Surficial material consists of 

loamy till that has an average thickness of approximately 30 m and alluvium in association with large 

streams. Surficial loess is absent. 

The Iowan Surface is characterized by gently 

rolling topography with long slopes and low local 

relief (15–30 m) and small slope (fig. 2). Drainage is 

well developed although streams generally have small 

gradients. Surficial material consists of pre-Illinoianage (500,000–700,000 years old) loamy till covered by 

a thin veneer of windblown loess on the ridges and 

alluvium near the streams (Prior, 1991). Potential 

natural vegetation is bluestem prairie and oak-hickory 

forest, although corn and soybean production presently dominates (Prior, 1991; Griffith and others, 

1994).

The Iowan Karst is a subregion of the Iowan 

Surface where dissolution of soluble limestone and 

dolomite under cover of glacial drift has caused localized collapse of the land surface that resulted in a karst 

topography with numerous sinkholes. The surface 

drainage is well developed, and direct flow into 

bedrock units is common. This area is extensively used 

for agriculture and some fields are drained through 

agricultural drainage wells, which are a form of 

gravity-operated injection well. Field tile lines are 

typically connected to these drainage wells. The 

drainage wells and sinkholes can provide a conduit for 

surface runoff and field drainage to the underlying 

bedrock aquifer. 

The Southern Iowa Drift Plain is characterized 

by steeply rolling terrain with moderate local relief 

(30–90 m) separated by flat, tabular divides (fig. 2). 

Surficial material consists of pre-Illinoian-age glacial 

deposits mantled by loess. Soils on the lower slopes 

commonly are derived from till, whereas soils on the 

higher slopes and upland flats are derived from loess. 

The steeper slopes and loess of the Southern Iowa 

Drift Plain are conducive to erosion and increased 

sediment transport to streams. Alluvium is present in 

association with streams that form a well-developed 

drainage pattern. Potential natural vegetation is 

bluestem prairie and oak-hickory forest (Griffith and 

others, 1994), although corn and soybean production 

presently predominates.
4 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

MINNESOTA

IOWA

93°

43°

42°

41°

91°

92°

Marshalltown

Waterloo

Marion

Cedar

Rapids

Coralville

Iowa City

Mason

City

Albert

Lea Austin

Cedar

Falls

Ames

Landform regions (modified 

 from Prior, 1991)

Des Moines Lobe

Iowan Karst (subregion)

Iowan Surface

Southern Iowa Drift Plain

Major city

Major lakes

State boundary

County boundary

Study unit boundary

Major river

Basic-fixed site and map reference number (table 2)

Synoptic site and map reference number or letter (table 2)

8

8

s r

r

EXPLANATION

q

n

g

p f

o

e

k

h

b

c

d

m

i

a

j

l

12

11

2

10

10a

6

5

4

3 1

9

7

Base from U.S. Geological Survey

Digital data, 1:250,000, 1967

Albers Equal-Area Conic projection

Standard parallels 41°30' and 44°30',

Central meridian –92°

IOWA

MINNESOTA

Eastern

Iowa

Basins

Mississippi River

Index Map

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Figure 1. Landform regions and surface-water-quality sampling sites in the Eastern Iowa Basins study unit.
DESCRIPTION OF THE EASTERN IOWA BASINS 5

Leached glacial till

Leached glacial till

Calcareous glacial till

Des Moines Lobe

Iowan Surface

Southern Iowa

Drift Plain

EXPLANATION

SLOPE—Numbers indicate ranges

 in slope, in percent 9-30

Loess

Paleosol

Glacial till

Alluvium

NOT TO SCALE

NOT TO SCALE

NOT TO SCALE

Calcareous glacial till

Sand

wedges

Glacial

outwash

Figure 2. Physiography of landform regions in the Eastern Iowa Basins study unit (modified from 

Oschwald and others, 1965).
6 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Climate

The climate in the study unit is continental, with 

large differences in seasonal temperatures that result in 

distinct winter and summer seasons. Primary climatic 

effects in the EIWA study unit are warm, moist air 

from the Gulf of Mexico and surges of cold, dry air 

from Canada, which predominate in the summer and 

winter, respectively (U.S. Department of Commerce, 

1959). Mean monthly temperatures range from –16°C 

for the lows in January to 28°C for the highs in July 

(Wendland and others, 1992). The growing season 

(generally April–September) lasts about 127 days and 

is characterized by mean temperatures of 19°C in the 

southern part of the EIWA study unit to 16°C in the 

northern part.

Precipitation occurs mostly as rain associated 

with thunderstorms that occur from April through 

September. About 71 percent of the annual rainfall 

occurs during this period (Harry Hillaker, Iowa 

Department of Agriculture and Land Stewardship, oral 

commun., 2000). The mean annual precipitation 

ranges from 76 cm in the northern part to 94 cm in the 

southeastern part of the study unit (Wendland and 

others, 1992). Peak precipitation occurs in June when 

crop-moisture demands are at their greatest and diminishes sharply during the fall harvesting season. Precipitation during the cooler months of the year generally 

is of long duration and of moderate or low intensity, 

whereas precipitation during the late spring and 

summer tends to be of shorter duration and of higher 

intensity. Large runoff events generally occur during 

the spring. Snow can remain on the ground through the 

winter (December through February) though there are 

typically days of “winter thaw” with some runoff of 

meltwater over partially frozen ground.

The seasonal timing of precipitation is an 

important factor in stream-water quality, especially 

streams affected by nonpoint sources (such as agricultural runoff). For example, heavy precipitation in May 

or June following application of pesticides will allow 

more pesticides to be transported to streams than if 

this same amount of precipitation occurred in the fall 

or winter seasons. In addition, during wet periods, 

ground-water levels can rise and sustain tile-drain 

flow, potentially causing transport of chemical constituents from the subsurface to streams. 

Streamflow

Overland flow (direct surface runoff) and 

ground-water discharge are the major sources of 

streamflow in the Eastern Iowa Basins. Another source 

of flow to streams is interflow. Interflow is that part of 

the subsurface flow that moves at shallow depths and 

reaches the surface channels in a relatively short 

period of time and therefore is commonly considered 

part of overland flow. During a storm period, interflow 

slowly increases until the end of the storm period, then 

gradually decreases (Viessman and others, 1989, 

p. 171). Field tile drains can enhance the subsurface 

drainage component of flow to streams. In the study 

unit, the mean annual runoff (overland flow, groundwater discharge, and interflow) to streams (water years 

1951–80) averages about 25 percent of the annual 

precipitation and increases from less than 15 cm in the 

western part of the study unit to more than 23 cm in 

the southeastern part (Gebert and others, 1987). Total 

mean annual streamflow from the study unit (based on 

the entire period of record available) averages about 

11,340 hm3 (9.2 million acre-ft) (Kalkhoff, 1994). The 

mean annual streamflow from the Wapsipinicon River 

Basin, the combined Iowa and Cedar River Basins, and 

the Skunk River Basin averages about 1,360, 7,770, 

and 2,220 hm3

 (1.1, 6.3, and 1.8 million acre-ft) 

respectively.

Precipitation and, in particular, spring rains in 

the Eastern Iowa Basins varied during the period of 

study. In 1996, the months of May and June were 

wetter than normal and the median streamflow was 

slightly higher than the historical middle 50 percent 

of discharge (fig. 3). In contrast, median streamflow 

in June 1997 was generally lower than the middle

50 percent (25th to 75th percentile) of historical 

streamflow reflecting fewer large rain events during 

1997 (fig. 3). The median streamflow for June 1998 

was higher than the middle 50 percent of historical 

streamflow and remained high for most of 1998

(fig. 3).

Land Use

Abundant rainfall, flat topography, and fertile 

soils in the Eastern Iowa Basins produce conditions 

ideal for agricultural production of corn and soybeans. 

Agriculture (row crops, pasture, hay, and small grain) 

accounts for 92.9 percent of the land use in the study 

unit (fig. 4). Other land uses are forests (4.0 percent), 
DESCRIPTION OF THE EASTERN IOWA BASINS 7

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Base from U.S. Geological Survey digital data, 1:250,000, 1967

Albers Equal-Area Conic projection

Standard parallels 41°30' and 44°30', central meridian –92°

Precipitation data from National Oceanic and Atmospheric 

Administration (1996, 1997, 1998)

93°

43°

42°

41°

92°

91°

EXPLANATION

Basic-fixed site and map reference number (table 2)

Site sampled in August 1997

 and May 1998 and map reference number or letter

Blue shading in stream hydrographs is the 25th to 75th

 percentile of long-term streamflow

South Skunk River

North Skunk River

English River

Iowa River Iowa River

West Fork Cedar River

Cedar River

Skunk River

Wapsipinicon R

Cedar River

Shell Rock R

MINNESOTA

IOWA

12

11

10a

10

2

6

7

5

9

4

3

7

1

8

q

r

e

n

g

d

f

m

b

c

a

j

i

e

k

o

p

h

l

0

15

5

10

MONTHLY

PRECIPITATION,

IN INCHES

1997 1998 0

2,000

500

1,000

1,500

MEAN MONTHLY

STREAMFLOW, IN CUBIC

FEET PER SECOND

Mason City, Iowa

Iowa River near Rowan, Iowa

1996

0

15

5

10

MONTHLY

PRECIPITATION,

IN INCHES

1997 1998 0

20,000

5,000

10,000

15,000

MEAN MONTHLY 

STREAMFLOW, IN CUBIC 

FEET PER SECOND

Skunk River at Augusta, Iowa

Burlington, Iowa

1996

0

15

5

10

MONTHLY

PRECIPITATION,

IN INCHES

1997 1998 0

40,000

10,000

20,000

30,000

MEAN MONTHLY 

STREAMFLOW, IN CUBIC 

FEET PER SECOND

Iowa River at Wapello, Iowa

Wapello, Iowa

1996

s

Figure 3. Precipitation and discharge at selected sites in the Eastern Iowa Basins study unit, 1996–98, and map showing site

locations.
8 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

EXPLANATION

Land cover (based on satellite imagery from

U.S. Geological Survey, EROS Data Center, 1994)

Row crops

Urban

Forest

Grassland, pasture, hay

Water

Wetlands

State boundary

County boundary

Study unit boundary

Synoptic site and map reference number or letter

Basic-fixed site and map reference number

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Figure 4. Land cover in the Eastern Iowa Basins study unit, 1996–98.
DESCRIPTION OF THE EASTERN IOWA BASINS 9

urban (1.8 percent), and other purposes (1.3 percent). 

Industry, education, and research in the largest cities 

(Cedar Rapids, Waterloo/Cedar Falls, and Iowa 

City/Coralville) contribute significantly to the 

economy. The principal crops are corn, soybeans, oats, 

hay, and pasture on unirrigated land. Iowa ranked 

number one in the Nation for corn for grain and 

soybean production in 1997 (Sands and Holden, 

1998). 

Pesticide Use and Properties

Pesticides (herbicides and insecticides) are 

commonly used to control weeds, insects, and other 

pests in agricultural and urban areas of the Eastern 

Iowa Basins (table 1). Total pesticide use and the 

number of different pesticides used has increased 

nationally since the 1960’s (Larson and others, 1997, 

p. 3). In agricultural settings, pesticides are commonly 

applied by aerial spraying or ground application from 

tractors. During the last 20 years in Iowa, 95–99 

percent of the corn and soybean crops have been 

treated with pesticides (Mayerfield and others, 1996 

p. 18). Typically, large amounts (thousands of kilograms per year) of the common herbicides are applied 

annually in Iowa (fig. 5) and in the EIWA study unit. 

The amounts applied of the most commonly 

used pesticides such as acetochlor, atrazine, cyanazine, 

and metolachlor are almost an order of magnitude 

greater than of other pesticides such as bentazone, 

2,4–D, metribuzin, and trifluralin. In 1996, the pesticides atrazine, acetochlor, metolachlor, and cyanazine 

accounted for approximately 76 percent of the total 

agricultural pesticides applied in Iowa. The use of 

acetochlor, registered in 1994, has increased greatly, 

and the use of alachlor has declined (fig. 5)

Agricultural pesticides are typically applied 

during relatively short seasonal periods. For example, 

pre-emergent herbicides are applied after the seed bed 

has been prepared in the early spring and post-emergent herbicides are applied a few weeks after the crop 

germinates. The major application of herbicides in 

the EIWA study unit starts in late April to mid-May. 

Insecticides may be applied periodically throughout 

the growing season to control specific pests. 

The physical and chemical properties of pesticides will affect their fate and transport. A detailed 

discussion of pesticide chemical properties is beyond 

the scope of this report, but some generalizations of 

different pesticide groups or classes may prove 

helpful. One important characteristic for understanding the potential for environmental transport is 

the solubility of the pesticide in water. In this report, 

pesticides with water solubilities greater than a few 

parts per million are considered water soluble. Watersoluble pesticides applied to soil tend to be more 

mobile than nonsoluble types and can enter aquatic 

systems in the dissolved state. Environmental variables such as pH, hardness of water, naturally occurring organic substances (humic and fulvic acids), and 

biodegradation can affect the solubility of pesticides in 

water. In general, pesticides that are more water 

soluble are more likely to desorb from soils and are 

less likely to volatilize from water (Verschueren, 1983, 

p. 8). 

Two classes of pesticides that are most heavily 

used in the EIWA study unit and throughout the 

Midwest are the triazine and chloroacetanilide herbicides. The most common triazines—atrazine, cyanazine, and simazine—are used primarily on corn. Less 

commonly used triazine pesticides include metribuzin 

and prometon. Metribuzin is used on soybeans and 

prometon is primarily used for total vegetation control 

in noncrop areas of farms and around outbuildings and 

road rights-of-way (Larson and others, 1997; Capel 

and others, 1999). The most commonly used chloroacetanilides (acetochlor, alachlor, and metolachlor) are 

used on both corn and soybeans. Both the triazines and 

chloroacetanilide herbicide groups have moderate to 

high water solubility and relatively low soil-sorption 

coefficients and therefore can be relatively persistent 

in soil (Wauchope and others, 1992). As a result, they 

have moderate to strong potential for transport from 

fields through surface runoff, primarily in the 

dissolved phase (Goss, 1992). In addition, most are 

chemically stable in water and are unlikely to volatilize from water. The triazines are somewhat more 

resistant to biodegradation than are the chloroacetanilides (Muir, 1991).

Pesticide degradates are formed when a parent 

pesticide compound breaks down or degrades. Pesticide degradates often have been detected at higher 

concentrations than their parent compound (Kolpin 

and others, 1998; Scribner and others, 1998); however, 

their toxicity is relatively unknown (U.S. Geological 

Survey, 1999, p. 77). Few, if any, Federal water-quality 

criteria have been set for pesticide degradates. In 

general, pesticide degradates are more stable than the 

parent compounds, though laboratory data on degra
10 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Table 1. Common agricultural pesticides used in Iowa during 1996 and analyzed as part of the Eastern Iowa Basins study, 

1996–98 

[CAS number, Chemical Abstract Service registry number]

Pesticide

name

Chemical

class

CAS

number Use

Corn applications

2,4–D Chlorophenoxy acid 94−75−7 Post-emergence control of annual broadleaf weeds.

Acetochlor Chloroacetanilide 34256−82−1 Pre-emergence, early post-emergence, or pre-plant incorporated control of most 

annual grasses and broadleaf weeds.

Alachlor Chloroacetanilide 15972−60−8 Pre- and post-emergence control of most annual grasses and many broadleaf 

weeds.

Atrazine Triazine 1912−24−9 Pre- and post-emergence control of most annual grasses and broadleaf weeds. 

Used in combination with many other herbicides.

Bentazon Benzothiadiazole 25057−89−0 Post-emergence control of broadleaf weeds.

Bromoxynil Nitrile 1689–84–5 Post-emergence control of broadleaf weeds.

Carbofuran Carbamate 1563−66−2 Insecticide used on corn and soybeans. Control of soil-dwelling and foliar-feeding 

insects.

Chlorpyrifos Organophosphorus; 

pyridine

2921−88−2 Insecticide used on cropland and in households. Control of soil insects as well as 

household pests (ants, cockroaches, flies).

Cyanazine Triazine 21725−46−2 Control of annual grasses and broadleaf weeds. Used with many other herbicides.

Dicamba Benzoic acid 1918−00−9 Control of annual grasses and broadleaf weeds. Used with many other herbicides.

EPTC Carbamate 759–94–4 Control of annual grasses and some broadleaf weeds.

Fonofos Organophosphorus 944−22−9 Soil insecticide used on cropland to control soil insects.

Metolachlor Chloroacetanilide 51218–45–2 Post-emergence control of broadleaf weeds and some grasses.

Metribuzin Triazine 21087−64−9 Pre- and post-emergence control of broadleaf weeds and grasses.

Pendimethalin Nitro compound 40487–42–1 Control of most annual grasses and many broadleaf weeds.

Terbufos Cyanamid 13071–79–9 Insecticide used to control soil insects.

Soybean applications

2,4–D Chlorophenoxy acid 94−75−7 Post-emergence control of annual broadleaf weeds.

Acifluorfen Trifluoromethyl 62476–59–9 Pre- and post-emergence of many broadleaf weeds.

Alachlor Chloroacetanilide 15972–60–8 Pre- and post-emergence control of most annual grasses and many broadleaf 

weeds.

Bentazon Benzothiadiazole 25057−89−0 Post-emergence control of annual broadleaf weeds.

Metolachlor Chloracetanalide 51218−45−2 Post-emergence control of broadleaf weeds and some grasses.

Metribuzin Triazine 21087−64−9 Pre- and post-emergence control of broadleaf weeds and grasses.

Pendimethalin Nitro compound 40487–42–1 Control of most annual grasses and many broadleaf weeds.

Thiofensulfuron Sulfonylurea 79277−27−3 Post-emergence control of annual broadleaf weeds.

Trifluralin Trifluoromethyl;

dinitroaniline

1582−09−8 Pre-emergence control of many annual grasses and broadleaf weeds.

Other selected pesticides

Dieldrin Organochlorine 60–57–1 Insecticide for locusts, termites and other public health pests.

Diuron Urea 330–54–1 Total control of weeds on noncropland.

Prometon Triazine 1610−18−0 Total control of weeds on noncropland. Used around outbuildings and road rightsof-way.

Simazine Triazine 122−34−9 Herbicide for control of annual grasses and broadleaf weeds.

Tebuthiuron Urea 34014–18–1 Total control of weeds on noncropland.
STUDY DESIGN AND METHODS OF STUDY 11

date solubilities and organic-carbon partitions are few. 

Some degradates can form from the degradation of 

more than one compound. Deisopropylatrazine can 

form from the degradation of either atrazine, cyanazine, or simazine (Barbash and others, 1999, p. 6). 

Other degradates such as deethylatrazine, hydroxyatrazine, cyanazine amide, acetochlor ethane sulfonic 

acid (acetochlor ESA), acetochlor oxanilic acid 

(acetochlor OA), alachlor ethane sulfonic acid 

(alachlor ESA), alachlor oxanilic acid (alachlor OA), 

metolachlor ethane sulfonic acid (metolachlor ESA), 

and metolachlor oxanilic acid (metolachlor OA) 

appear to be specific to their respective parent pesticide compounds (Barbash and others, 1999, p. 6). 

STUDY DESIGN AND METHODS OF 

STUDY

The EIWA study unit design integrates waterquality information for both local and national assessment. The surface-water sampling design includes 

fixed-interval sampling (weekly, biweekly, and 

monthly), synoptic sampling (one-time sampling), and 

extreme-flow sampling (flood events). Surface-water 

sampling focuses on the physical and chemical characteristics of the streams, which include pH, dissolved 

oxygen, water temperature, specific conductance, 

90 9 91 92 93 94 95 96 97 8

0

10,000

2,000

4,000

6,000

8,000

 HERBICIDE USE, IN THOUSANDS 

OF KILOGRAMS

YEAR

EXPLANATION

Data from U.S. Department of

Agriculture, 2000

Acetochlor

Alachlor

Metolachlor

Atrazine

Cyanazine

Figure 5. Use of selected herbicides in Iowa, 1990–98.

suspended sediment, dissolved solids, major ions, 

nutrients, organic carbon, pathogenic bacteria, 

dissolved pesticides, and selected pesticide degradates. 

Only the pesticide and pesticide degradate data will be 

discussed in this report. All data have been previously 

published (Akers and others, 1999 and 2000). 

 Low analytical detection limits for pesticides 

(often less than 0.01 μg/L) were used as part of the 

study design to better define temporal trends and to 

document concentration trends. Low detection limits 

allow researchers to discern correlations between 

pesticide exposure and human health or aquatic life 

health. In addition, low detection limits can increase 

the likelihood that pesticides not detected in analyses 

are truly absent from waters sampled (Phillips and 

others, 1998). 

The target pesticides selected for analysis in 

surface-water samples for the NAWQA Program 

(Appendix 1) were selected from nearly 400 of the 

most commonly used pesticides in the United States 

(Gianessi and Anderson, 1996). The pesticides were 

prioritized according to three factors: national use 

(typically more than 3,600 kilograms of active ingredient applied annually), potential environmental 

significance (toxicity and leachability), and chemical 

properties (the ability to trap and extract the analyte 

from the appropriate solid-phase media) (Gilliom and 

others, 1995). In addition, 10 pesticide degradates 

(acetochlor ESA, acetochlor OA, alachlor ESA, 

alachlor OA, cyanazine amide, deethylatrazine, deisopropylatrazine, hydroxyatrazine, metolachlor ESA, 

and metolachlor OA) were added to the NAWQA 

target analyte list specifically for the EIWA study 

(Appendix 1).

The physical and water-quality characteristics of 

streams within the study unit are interpreted within the 

framework of hydrologic conditions, land use, and 

landforms. In particular, sampling design for pesticides and pesticide degradates collected in the EIWA 

study unit depended on coordinated sampling of 

varying intensity and scope to maximize data collection over a range of hydrologic conditions and 

geomorphic regions. 

Sampling Site Selection

 Pesticides and pesticide degradates in EIWA 

study unit streams and rivers were assessed by 

sampling selected sites at two different scales and time 
12 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

periods (fig. 1). Six sites were selected on streams that 

were indicators of those draining different landforms 

and land-use patterns. Indicator sites generally represent smaller watersheds (320 to 1,080 km2) with relatively homogeneous land use and geology. An 

additional six sites were selected at sites on streams 

that integrate runoff from large watersheds containing 

multiple landforms and land uses. Integrator sites are 

large-river sites that represent large watersheds (6,050 

to 32,400 km2) and are influenced by combinations of 

land use and point sources. Integrator sites are typically located at the mouths of the largest rivers or 

downstream from indicator sites. Indicator and integrator sites are referred to as basic-fixed sites in subsequent sections of this report.

Natural factors such as soils, topography, and 

the hydrologic system may produce differences in the 

concentrations and timing of pesticides in streams 

even though the amount of crops grown in the watershed are similar. Four indicator sites (fig. 1, table 2) 

were selected to investigate water-quality differences 

in streams draining the three major landforms—

Des Moines Lobe (Iowa River near Rowan, site 3), 

Iowan Surface (Wolf Creek near Dysart, site 9), 

Southern Iowa Drift Plain (Old Mans Creek near Iowa 

City, site 6), and Iowan Karst subarea (Flood Creek 

near Powersville, site 7). 

Land use was the primary factor in selection of 

two additional indicator sites. Parts of the Wapsipinicon River bottomland have remained relatively unaltered by human activities, and thus the stream is 

unchannelized with larger amounts of wetlands and 

undisturbed flood plain than other streams in the study 

unit. Although the uplands are used for row-crop agriculture, the Wapsipinicon River near Tripoli (site 1) 

was selected as a defined “reference” site to investigate the effects of the natural flood plain on streamwater quality. Since the late 1980’s and early 1990’s, 

animal feeding operations have moved from a large 

number of smaller facilities to a smaller number of 

large-scale confinement facilities. These confinement 

facilities generate a substantial amount of animal 

waste that, if not properly applied, may be washed into 

streams or infiltrate into the ground water. An indicator site, South Fork Iowa River near New Providence (site 4) was selected to investigate the effect of 

the large-scale hog-confinement facilities on stream 

water quality. 

Sites near the mouth of the Wapsipinicon, Iowa, 

and Skunk Rivers (fig. 1, table 2) were selected to 

assess broadscale seasonal and spatial changes in 

pesticide concentrations in the larger rivers within the 

study unit. Data collection at these sites also was 

intended to provide information to estimate the 

amount of pesticides and pesticide degradates transported to the Mississippi River. The site on the Cedar 

River near Conesville (site 10) was selected to document the quality of the largest tributary to the Iowa 

River. Water-quality sampling was moved about 13 km 

upstream to a site (site 10a) near Nichols in October 

1996 because of bridge construction. Water-quality 

data collected from the Cedar River near Nichols was 

considered to be representative of the Cedar River near 

Conesville since there are no major tributaries or urban 

areas on the Cedar River between the sites. Data 

collected at both sites will be discussed as originating 

from the Cedar River near Conesville in the remainder 

of this report. 

The Iowa River at Marengo (site 5, fig.1, 

table 2) was selected as a site representative of the 

Iowa River upstream from the Coralville Reservoir. 

Previous studies have shown that certain pesticide 

concentrations are altered as water flows through the 

Coralville Reservoir (Schnoor, 1981); thus, a site 

upstream from the reservoir was necessary to assess 

the quality of Iowa River unaffected by reservoirs. In 

1996, the Cedar River at Gilbertville (site 8) was 

sampled to evaluate the effects of an urban area 

(Waterloo, Iowa) on water quality. 

Streams on 25 medium-sized (from about 

310 km2 to about 1,500 km2) watersheds were 

sampled during base-flow conditions in August 1997 

and May 1998 as part of a regional synoptic study to 

characterize the chemical, biological, and habitat 

conditions in Midwest streams (Sorenson and others, 

1999). Sites were selected to represent small to 

medium agricultural watersheds and to include moderately well drained to poorly drained soils (Sorenson 

and others, 1999). Agricultural land use among all 

sites in the synoptic study exceeded 90 percent. Six of 

the synoptic sites were existing EIWA indicator sites. 

Sampling Methods

Water-quality sample collection and streamflow 

measurements were made monthly at the basic-fixed 

sites to assess the seasonal and spatial character of 

selected constituents. In addition, three basic-fixed 

sites (fig. 1, table 2)—Iowa River near Rowan (site 3), 
STUDY DESIGN AND METHODS OF STUDY 13

Table 2. Description of surface-water-quality sampling sites in the Eastern Iowa Basins study unit

[bfs, basic-fixed site sampled monthly; ind, indicator site representing small to medium sized drainage basins of uniform land use; int, integrator site representing large drainage basins of mixed land use; intens, intensive site sampled weekly in spring and early summer 1997 and biweekly at end of summer 1997; 

syn, synoptic site; <, less than; -- data not available]

Map

reference 

number or 

letter

(fig. 1)

Site 

number Site name

Drainage 

area

(square

kilometers)

Site type

 Land use in contributing drainage area 

(percent)1

Agriculture Urban Forested Wetland

Other

(barren/

water)

1 05420680 Wapsipinicon River near Tripoli, IA 900 bfs, ind, syn 88.5 1.8 5.1 4.2 0.3

2 05422000 Wapsipinicon River near DeWitt, IA 6,050 bfs, int 87.4 2.1 7.2 2.7 .6

3 05449500 Iowa River near Rowan, IA 1,080 bfs, intens, 

ind, syn

94.6 1.8 1.6 1.5 .6

4 05451210 South Fork Iowa River near New 

Providence, IA

580 bfs, ind, syn 95.1 1.5 2.6 0.7 < .1

5 05453100 Iowa River at Marengo, IA 7,240 bfs, int 91.0 2.5 3.9 2.1 .6

6 05455100 Old Mans Creek near Iowa City, IA 520 bfs, ind, syn 91.9 2.6 4.4 1.0 .1

7 05461390 Flood Creek near Powersville, IA 320 bfs, ind, syn 95.3 1.1 2.8 .8 < .1

8 05464020 Cedar River at Gilbertville, IA2 13,600 bfs, int 90.7 3.0 3.5 1.9 .9

9 05464220 Wolf Creek near Dysart, IA 770 bfs, intens, 

ind, syn

95.6 1.8 1.9 .6 .1

10a 05464935 Cedar River near Nichols, IA3 19,570 bfs, int -- -- -- -- --

10 05465000 Cedar River near Conesville, IA 20,200 bfs, int 89.5 3.3 4.4 1.9 .9

11 05465500 Iowa River at Wapello, IA 32,400 bfs, intens, 

int

89.0 3.1 4.9 2.1 .9

12 05474000 Skunk River at Augusta, IA 11,200 bfs, int 87.1 2.9 7.7 1.8 .5

a 05420720 East Fork Wapsipinicon River near 

Tripoli, IA

370 syn 90.4 1.3 5.0 3.0 .2

b 05420900 Little Wapsipinicon River at Littleton, 

IA

380 syn 89.0 2.3 7.4 1.1 .2

c 05421700 Buffalo Creek near Stone City, IA 600 syn 90.1 2.2 6.6 1.6 .2

d 05421870 Mud Creek near Donahue, IA 310 syn 94.7 2.2 2.8 .2 < .1

e 05449200 East Branch Iowa River at Belmond, IA 500 syn 95.4 1.8 1.5 .9 0.3

f 05452020 Salt Creek at Belle Plaine, IA 560 syn 93.0 2.4 3.6 1.0 0.1

g 05455500 English River near Kalona, IA 1,500 syn 91.2 2.5 4.7 1.3 0.2

h 05456510 Turtle Creek at Austin, MN 400 syn 90.7 1.9 3.4 1.7 2.2

i 05457950 Little Cedar River near Floyd, IA 610 syn 93.4 1.3 3.4 1.5 0.2

j 05458870 Maynes Creek near Kelsey, IA 350 syn 95.0 1.1 3.0 0.7 0.1

k 05459300 Winnebago River near Fertile, IA 760 syn 91.4 2.0 2.6 3.3 0.7

l 05462770 Beaver Creek near Parkersburg, IA 370 syn 95.5 1.4 2.3 0.7 0.1

m 05463510 Black Hawk Creek at Waterloo, IA 850 syn 95.4 2.0 1.4 1.0 0.1

n 05465310 Long Creek near Columbus Junction, 

IA

400 syn 90.8 2.3 6.3 0.6 0.1

o 05469980 South Skunk River near Story City, IA 570 syn 94.0 3.1 1.9 0.6 0.4

p 05471120 East Branch Indian Creek near Iowa 

Center, IA

330 syn 93.6 2.2 3.3 0.7 0.2

q 05473060 Crooked Creek at Coppock, IA 740 syn 90.5 2.4 6.2 0.7 0.1

r 05473400 Cedar Creek near Oakland Mills, IA 1,400 syn 85.4 2.2 10.0 2.2 0.2

s 05473550 Big Creek near Lowell, IA 420 syn 87.2 3.3 8.2 1.2 0.1

1Land-use data from Hitt, 1994. 

2

Cedar River at Gilbertville was discontinued as a basic-fixed site in March 1997.

3

Sampling was moved upstream to site near Nichols (site 10a) in October 1996 because of bridge construction at the Conesville site (site 10).
14 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Wolf Creek near Dysart (site 9), and Iowa River at 

Wapello (site 11)—were sampled intensively (weekly 

and biweekly) during 1997 to define short-term variability. 

The 12 basic-fixed sites were sampled monthly 

starting in March 1996 through March 1997 for pesticides and pesticide degradates. Sampling at the Cedar 

River at Gilbertville (site 8) was discontinued in April 

1997. During April 1997 through March 1997 the 

collection of water samples for pesticide and pesticide 

degradates continued at only three basic-fixed sites. 

The three sites were sampled weekly starting in April 

through June 1997, biweekly from July through 

November 1997, and monthly from December 1997 

through March 1998. In April 1998, monthly sampling 

for pesticides and pesticide degradates resumed at all 

the basic-fixed sites (except Gilbertville) and 

continued through September 1998. 

 Samples were collected over the complete 

range of flow conditions including peak river stage 

after significant rainfall events. A total of 353 samples 

were collected for pesticide analysis at the basic-fixed 

sites, and 77 samples were collected for quality assurance/quality control (blanks, spikes, and replicates). 

Representative samples were obtained by 

collecting depth-integrated subsamples at equally 

spaced vertical sections across the stream (Ward and 

Harr, 1990) to account for any instream variability. A 

minimum of 10 equally spaced verticals and depthintegrated water samples were collected using a cablemounted or hand-held sampler (Shelton, 1994). 

Subsamples from five equally spaced verticals were 

collected from streams that were less than about 4 m 

wide. Samples were collected in Teflon bottles using 

Teflon nozzles. During ice conditions, samples were 

generally collected from a minimum of three equalwidth vertical sections. 

Water temperature, specific conductance, pH, 

and dissolved oxygen were measured using a multiparameter meter. Measurements were made at equalwidth increments across the stream section, and the 

median value was recorded as the value for the field 

parameter. Streamflow was obtained from instantaneous streamflow measurements or from streamflow 

records obtained from continuous gaging stations 

located at each of the sites. In addition to pesticides, 

water samples were collected for analysis of major 

ions, dissolved and suspended organic carbon, nutrients, and suspended sediment (Akers and others, 1999 

and 2000).

All equipment used to collect and process pesticide samples was precleaned with a 0.1-percent nonphosphate detergent, triple rinsed with tap water, triple 

rinsed with deionized water, and rinsed with methanol 

that was certified by the manufacturer to be free of 

pesticides. Once cleaned, equipment was air dried, 

wrapped in aluminum foil, and stored in a dust-free 

environment (Shelton, 1994) to ensure clean equipment for the next sample. 

 Water samples for pesticide and pesticide 

degradates were filtered through a 142-millimeterdiameter, 0.7-micrometer (μm) pore size, borosilicate 

glass-fiber filter placed in a stainless steel filter unit 

(Sandstrom, 1995). The filtered water was collected in 

amber-colored bottles and chilled for later analysis of 

pesticides and pesticide degradates. Both the glassfiber filters and amber-colored bottles had been 

precleaned by baking at 450o

C in a muffle furnace for 

at least 2 hours. Filtered water for pesticide analysis 

using gas chromatograph/mass spectrometer (GC/MS) 

techniques were extracted in the field laboratory at the 

sampling site by pumping filtered water through solidphase extraction (SPE) cartridges. Detailed descriptions of the SPE method are given in Shelton (1994) 

and also by Sandstrom and others (1992). The SPE 

cartridges were stored in amber vials, chilled, and sent 

by overnight air express to the USGS National Quality 

Laboratory (NWQL) for analysis. Filtered water 

samples for pesticide degradate analysis also were 

kept on ice and sent to the USGS Organic Geochemistry Research Laboratory (OGRL) in Lawrence, 

Kansas. 

Analytical Methods

Water samples were analyzed by the NWQL for 

two separate suites of pesticide compounds. Fortyeight pesticides and pesticide degradates (Appendix 1) 

were analyzed by capillary-column (GC/MS) with 

selected-ion monitoring (Zaugg and others, 1995). 

Additionally, high performance liquid chromatography (HPLC) techniques are used to analyze 44 pesticide compounds (Appendix 1) that are not readily 

amenable to gas chromatography or other high temperature analytical techniques (Werner and others, 1996). 

By using these laboratory methods it is sometimes 

possible to detect pesticide concentrations that are 

below the method detection limit (MDL) (Pritt, 1994). 

The MDL is defined as the minimum concentration of 
STUDY DESIGN AND METHODS OF STUDY 15

a substance that can be identified, measured, and 

reported with 99-percent confidence that the 

compound concentration is greater than zero 

(Wershaw and others, 1987). At times when concentrations are below the MDL, the compound has been 

positively identified, but the numerical value of the 

concentration has a larger variability than those values 

that are above the method detection limit. Therefore, a 

concentration reported below the MDL is identified 

with an “E” (estimated) code beside the value. 

The OGRL analyzed 10 pesticide degradates in 

water samples following solid-phase extraction (SPE) 

on octadecyl (C-18 cartridges) (Thurman and others, 

1990; Meyer and others, 1993). Analyses for six chloroacetanilide herbicide degradates—acetochlor ESA 

(ethane sulfonic acid), acetochlor OA (oxanilic acid), 

alachlor ESA, alachlor OA, metolachlor ESA, and 

metolachlor OA—and one atrazine degradate, 

hydroxyatrazine, were analyzed by HPLC following 

solid-phase extraction in the field (Meyer and others, 

1993; Ferrer and others, 1997). The other three pesticide degradates—deethylatrazine, deisopropylatrazine, and cyanazine-amide—are determined by 

GC/MS operated in selected ion-monitoring (SIM) 

mode following solid phase extraction (Zimmerman 

and Thurman, 1999). The method detection limit for 

deethylatrazine, deisopropylatrazine, and cyanazine 

amide was 0.05 μg/L and for all the other degradate 

compounds was 0.2 μg/L.

Quality Assurance/Quality Control

To ensure that data are reliable and accurate and 

to properly interpret water-quality data, information is 

needed to estimate the bias and variability that can 

result from sample collection, sample processing, 

sample shipment, and laboratory analysis. Bias is 

systematic error that can “skew” results in either a 

positive or negative direction. The most common 

source of positive bias in water-quality studies is 

contamination of samples from airborne gases and 

particulates or inadequately cleaned sampling equipment. Variability is the degree of random error of independent measurements of the sample quantity. 

Variability may be the result of errors in laboratory 

analytical procedures or in collection of samples in the 

field. Quality assurance/quality control (QA/QC) 

procedures are required to ensure that the data 

collected meet standards of reliability and accuracy.

The QA/QC design for the NAWQA surfacewater sampling is described in detail by Mueller and 

others (1997). The EIWA study unit followed the 

design guidelines of Mueller and others (1997). About 

15 to 20 percent of the total samples collected for the 

EIWA study unit were analyzed for quality control and 

included equipment blanks, field blanks, field spikes, 

and replicates. Generally, blanks and spikes are used to 

estimate sample bias while replicates are used to estimate sample variability.

A blank is a water sample that is intended to be 

free of the analytes of interest. Equipment blank 

samples of deionized water guaranteed by the manufacturer to be free of organic compounds and deionized water guaranteed by the manufacturer to be free 

of inorganic compounds were passed through all 

sampling equipment at the beginning of the field 

season. Equipment blank samples are collected in a 

“clean” environment such as the laboratory to examine 

the cleanliness of the equipment prior to sampling. A 

field blank is a specific type of blank sample collected 

in the field and used to demonstrate that: (1) equipment has been adequately cleaned to remove contamination introduced by samples obtained at the previous 

site; (2) sample collection and processing have not 

resulted in contamination; and (3) sample handling, 

transport, and laboratory analysis have not introduced 

contamination (Mueller and others, 1997). Field blank 

samples of the same deionized organic-free water that 

was used with equipment blank samples were 

collected by passing the deionized, organic-free water 

through all pumps, filter plates, and filters to verify 

cleanliness of sampling equipment and technique. 

Field-blank sample concentrations for pesticide 

constituents were typically equivalent to the MDL. In 

other words, the blanks were “clean” and did not indicate any pesticide contamination from the equipment 

or sample processing methods. There was no crosscontamination of samples from sampling equipment 

between sample collection. 

A spike is an environmental sample that is 

injected with a known mass of a specific analyte. A 

field spike is a specific type of spiked sample that is 

injected in the field prior to shipping. Field spikes are 

used to test for bias resulting from matrix interference 

or analyte degradation. Matrix interference is the 

effect that the matrix of the water sample itself has on 

the measurement of individual analytes within that 

sample. Bias is estimated from field spikes by calculating the percentage of the added analyte that is 
16 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

measured (recovered) in the sample. Spike recovery 

calculations are described by Mueller and others 

(1997, p. 5). The recovery of the spike can either be 

greater or less than 100 percent, so the bias can be 

either positive or negative. Typically, matrix interference and analyte degradation generally result in negative bias for spike recovery (Mueller and others, 1997).

Table 3 summarizes the percent recovery data 

for commonly detected pesticides from 19 water 

samples that were spiked in the field. The mean 

spike recoveries ranged from 83 to 108 percent and 

the median spike recoveries ranged from 83 to 

106 percent.

In addition, a “high spike” recovery experiment 

was performed to check the recovery of selected 

pesticide compounds at higher spike concentrations 

(3 and 6 μg/L) in sample water. Three water samples 

collected at Old Mans Creek near Iowa City (site 6) 

were spiked at low (0.1 μg/L), medium (3 μg/L), and 

high (6 μg/L) concentrations. The spike recoveries at 

the higher concentrations (3 and 6 μg/L) were not 

significantly different than those done at the lower 

(0.1 μg/L) concentrations.

Replicates are two or more samples collected or 

processed so that the samples are considered to be 

essentially identical in composition. Split replicates 

are prepared by dividing a single volume of sampled 

water into multiple samples. All replicates collected 

for the EIWA study unit were split replicates. Each 

replicate sample is an aliquot of the native water 

sample that is processed and prepared in the same way 

as the environmental sample. A replicate sample set 

consists of two samples—a regular environmental

Table 3. Summary of percent recovery data for commonly 

detected pesticides spiked in the field for the Eastern Iowa 

Basins study unit, 1996–98

Constituent Spike recovery, in percent

Minimum Maximum Mean Median

Acetochlor 78 142 105 101

Alachlor 84 139 108 106

Atrazine 83 132 107 105

Cyanazine 35 162 90 91

Metolachlor 19 163 104 102

Metribuzin 57 110 83 83

Prometon 23 118 96 99

Simazine 84 124 98 98

Trifluralin 67 116 88 84

sample and the replicate environmental sample. 

For the purposes of this report, the terms “regular environmental sample” and “replicate environmental 

sample” are used to identify the particular samples in a 

replicate pair.

One objective of the replicate samples was to 

estimate the precision of concentration values from 

sample processing and analysis. Analyses of organic 

constituents are generally more variable than analyses 

of inorganic constituents. Replicate samples were 

compared by using relative percent differences. Relative percent difference (RPD) between replicate 

samples was calculated by equation:

RPD= |S1–S2|/(S1+S2/2) × 100 (1)

where:

S1 is equal to the concentration in the regular 

environmental sample, in micrograms per 

liter; and 

S2 is equal to the concentration in the replicate 

sample, in micrograms per liter.

Typically, if the relative percent difference is 

large it can indicate greater variability in those 

samples. In general, the replicate pesticide degradate 

samples had more variability than the replicate parent 

pesticide compound samples. The median RPD for the 

degradates ranged from 0 to 13.6 percent (table 4), and 

the median RPD for the parent pesticides ranged from 

0.1 to 5.3 percent.

Surrogates are added to all environmental and 

quality-control samples for pesticide analysis before 

sample preparation. A surrogate is similar to the physical and chemical properties of the analytes of interest 

but is not normally found in environmental samples. 

Typically, the number of surrogates varies from one to 

four compounds. Surrogates provide quality control 

by monitoring matrix effects and gross processing 

errors (Wershaw and others, 1987). Surrogate recoveries are expressed in percent and typically range from 

80 to 120 percent. 

Data Analysis

Summary statistics and graphical analysis were 

used to examine the seasonal and spatial distribution 

of pesticide and pesticide degradate concentration 

data. Statistical analysis and ancillary data sets were 

important parts of the data analysis. Tables and 
STUDY DESIGN AND METHODS OF STUDY 17

specific graphs of selected, most commonly detected 

pesticides were used to support relevant descriptions 

and discussion. A complete listing of all the pesticide 

concentration data collected for the EIWA study unit 

for 1996–98 can be found in Akers and others (1999 

and 2000). 

Statistical Analysis of Pesticide and Pesticide 

Degradates

Pesticide and pesticide-degradate data were 

graphically represented using boxplots to show the 

median and variability of the data. In this plot, a box is 

drawn from the 25th to the 75th quartile (interquartile 

range), and the median is shown as a horizontal line 

in the box. Lines are drawn from the ends of the box 

to the most extreme data values within or equal to 

1.5 times the interquartile range outside the quartile. 

Table 4. Summary of replicate sample data for commonly 

detected parent pesticide and degradate pesticide 

compounds

[ESA, ethane sulfonic acid; OA, oxanilic acid]

Constituent

Number of 

replicate 

samples

Median 

relative 

percent

difference

Number of 

replicate 

samples with 

greater than 

10 percent 

relative 

percent

difference

Parent pesticide compounds

Acetochlor 18 1.6 2

Alachlor 18 0.1 3

Atrazine 18 1.8 2

Cyanazine 18 5.1 6

Metolachlor 18 1.5 1

Prometon 18 5.3 5

Degradate pesticide compounds

Acetochlor ESA 20 12 11

Acetochlor OA 19 0 2

Alachlor ESA 20 7 8

Alachlor OA 19 0 1

Hydroxyatrazine 19 0 5

Deethylatrazine 17 0 4

Deisopropylatrazine 17 5.7 9

Cyanazine amide 16 0 4

Metolachlor ESA 20 2.5 3

Metolachlor OA 19 13.6 9

Outlier values can be shown above or below the lines 

on the boxplot with an asterisk or circle. Boxplots are 

useful for visually displaying variability when 

comparing data between sites.

The Statistical Analysis System (SAS Institute 

Inc., 1989) was used to calculate summary statistics 

such as mean, median, minimum, and maximum 

concentrations for chemical data. The Wilcoxon ranksum and Kruskal-Wallis nonparametric statistical tests 

(Helsel and Hirsch, 1992) were used to evaluate the 

statistical significance of concentration differences 

between groups of data. The probability (p) values 

generated by these statistical tests are used to quantify 

that concentration differences between groups of data 

are not the result of chance occurrence. A p-value of 

0.05 indicates a 95-percent confidence that the 

observed differences are not the result of chance 

occurrence. In this report, differences between groups 

with p-values of 0.05 or less were considered significant.

Ancillary Data 

Several ancillary data sets were developed in an 

effort to better understand the relation of pesticide 

concentrations in streams and rivers to natural and 

anthropogenic factors. Data sets with information on 

ground water, streamflow, surface-water runoff, basin 

characteristics, soil characteristics, and agricultural 

statistics (amounts of pesticides applied, planting data, 

and harvest data) were compiled and summarized in an 

effort to better understand the fate and transport of 

pesticide and pesticide degradates. 

Ground-water discharge and surface-water 

runoff that constituted the streamflow at the basicfixed sampling sites were estimated by separating 

streamflow hydrographs into their base-flow and 

surface-runoff components using the Base Flow Indicator (BFI) program (Wahl and Wahl, 1995). 

Computer separation of the streamflow hydrograph 

mimics the traditional manual methods (Rorabaugh, 

1963; Linsley and others, 1982) and at the same time 

removes many of the inconsistencies of the manual 

methods. 
18 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

PESTICIDES AND PESTICIDE 

DEGRADATES IN STREAMS

The pesticide and pesticide degradate data were 

evaluated in terms of occurrence (what pesticide 

compounds were found and how many), seasonal variability (when the pesticides compounds were found) 

and spatial variability (where the pesticides were 

found). Water-quality criteria and guidelines are used 

to evaluate the potential effects of the pesticides on 

humans (Appendix 1). However, potential risks to 

humans and aquatic life only can be partially 

addressed by comparison to established criteria and 

guidelines. Often the range of potential effects of 

pesticides is based on controlled experiments for a 

single contaminant over a small range of concentrations. Historically, data on the toxicity of mixtures of 

pesticides and of many of the pesticide degradates are 

unknown (U.S. Geological Survey, 1999, p. 76–77). 

Evaluating any potential effects of pesticides and what 

effects may occur from varying types and duration of 

exposure is beyond the scope of this report. Documentation on the occurrence, composition, and patterns of 

pesticides and pesticide degradates is important in 

developing a better understanding of the fate and 

transport of these compounds and their possible effects 

on the environment. 

Occurrence and Distribution

The most commonly applied pesticides in 

Iowa—acetochlor, atrazine, cyanazine, and metolachlor—are among those most often detected in 

streams in the EIWA study unit (fig. 6). Less 

frequently detected pesticides were generally those 

with lower use. Atrazine and metolachlor, the two 

most commonly used herbicides in Iowa for row-crop 

agriculture during 1996–98, were detected in all 

stream samples (fig. 6). Acetochlor, alachlor, and 

cyanazine were detected in more than 70 percent of the 

samples. Less frequently detected pesticides were 

2,4–D, EPTC, trifluralin, metribuzin, dicamba, and 

carbofuran, which were applied in Iowa at a rate of 

only 0.6 to 30 percent of the rate of application for 

atrazine (Sands and Holden, 1998). Alachlor was 

heavily applied in the 1980’s, but its use has rapidly 

decreased in the last 5 years as it is being replaced by 

acetochlor (fig. 5). Alachlor was commonly detected 

at low concentrations in rivers and streams in the 

EIWA study unit (fig. 6).

Forty-seven pesticides (table 5) that were 

analyzed for were not detected in streams in eastern 

Iowa and southern Minnesota. These compounds 

generally were not used or were applied in small 

amounts in the study unit.

Acetochlor, alachlor, atrazine, cyanazine, and 

metolachlor were generally detected at low concentrations, with median concentrations ranging from 0.01 

to 0.50 μg/L (Appendix 2). However, the maximum 

concentrations of these compounds were as large as 

48.1 μg/L, but alachlor had a maximum concentration 

of only 0.559 μg/L. Most atrazine concentrations 

(76 percent) exceeded 0.1 μg/L with almost 60 percent Agricultural herbicides

Carbaryl

p,pí-DDE

Lindane

Malathion

Dieldrin

Diazinon

Fonofos

Chlorpyrifos

Carbofuran

2,6-Diethylaniline

Alachlor OA

Cyanazine Amide

Acetochlor OA

Hydroxy-Atrazine

Deisopropylatrazine

Acetochlor ESA

Deethylatrazine

Metolachlor OA

Alachlor ESA

Metolachlor ESA

Triclopyr

Tebuthiuron

Prometon

Napropamide

Propyzamide

Molinate

Bromoxynil

Butylate

Dacthal

Acifluorfen

Pendimethalin

Diuron

Dicamba

Metribuzin

Trifluralin

EPTC

2,4-D

Simazine

Bentazon

Alachlor

Cyanazine

Acetochlor

Metolachlor

Atrazine

20 40 60 80 100

DETECTION RATE, IN PERCENT

Non-agricultural

herdicides

Herbicide

degradates

Insecticides

0

Figure 6. Occurrence of pesticide compounds at basicfixed sampling sites in the Eastern Iowa Basins study 

unit, 1996–98.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 19

of the samples in the 0.1 to 1.0 μg/L range (fig. 7). 

About 10 percent of the samples exceeded the MCL 

for atrazine of 3.0 μg/L (U.S. Environmental Protection Agency, 1995). Almost half of the samples had 

metolachlor concentrations in the range from 0.1 to 

1.0 μg/L (fig. 7).

Acetochlor was frequently detected, but in 

most samples (75 percent) at concentrations less than 

0.1 μg/L. Acetochlor concentrations did not exceed 

the USEPA 2.0-μg/L annual mean concentration registration requirement (U.S. Environmental Protection 

Agency, 1994a) at any site but did exceed this concentration in about 3 percent of the individual samples. 

The maximum concentration (10.6 μg/L) measured 

during the study exceeded the concentration that 

would trigger requirements for biweekly sampling for 

water-supply systems (U.S. Environmental Protection 

Agency, 1994a). 

Table 5. Pesticide compounds not detected in surface-water samples in the Eastern Iowa Basins study unit, 1996–98

[μg/L; micrograms per liter]

Constituent Method

detection limit

(μg/L)

Constituent Method

detection limit

(μg/L)

2–(2,4,5-trichlorophenoxy) propionic acid 0.06 Linuron 0.002

2,4,5–T .040 Methomyl .017

2,4–DB .10 MCPA .17

4,6-Dinitro-2-methylphenol .42 MCPB .13

Aldicarb .21 Methiocarb .026

Aldicarb sulfone .10 Neburon .07

Aldicarb sulfoxide .021 Norflurazon .042

Azinphos, methyl- .001 Oryzalin .31

Benfluralin .002 Oxamyl .018

Bromacil .06 Parathion .004

Carbofuran, 3-hydroxy- .29 Parathion, methyl- .006

Chloramben (Amiben) .14 Pebulate .004

Chlorothalonil .48 Permethrin, cis- .004

Clopyralid .23 Phorate .002

Dacthal, mono-acid .039 Picloram .05

Dichlobenil .07 Propham .035

Dichlorprop (2,4–DP) .032 Propachlor .007

Dinoseb (DNBP) .06 Propanil .004

Disulfoton .017 Propargite .013

Ethylfluralin .004 Terbacil .007

Ethoprophos .003 Terbufos .013

Fenuron .07 Thiobencarb .002

Fluometuron .06 Triallate .001

HCH, alpha- .002

0.001 100 0.01 0.1 1 10

CONCENTRATION, IN

MICROGRAMS PER LITER 

0

100

20

40

60

80

PERCENTAGE OF SAMPLES

WITH CONCENTRATION

EQUAL TO OR GREATER

THAN INDICATED 

CONCENTRATION

EXPLANATION

Atrazine

Metolachlor

Acetochlor

Figure 7. Frequency distribution of selected herbicide 

concentrations at basic-fixed sampling sites in the Eastern

Iowa Basins study unit, 1996–98.
20 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Many pesticides are used in both agricultural 

and urban settings, but three herbicides used almost 

exclusively in non-row-crop agriculture and urban 

settings (prometon, tebuthiuron, and triclopyr) also 

were present in streams in the EIWA study unit from 

1996 through 1998 (fig. 6). Prometon was one of the 

10 most frequently detected pesticides in the study 

unit and was present at very low concentrations (less 

than 0.1 μg/L) in more than 80 percent of the samples 

even though it is not used as a herbicide for crop areas. 

Median concentrations of prometon were low at all 

sites—typically less than 0.15 μg/L. The occurrence of 

prometon at all sites in small but fairly consistent 

concentrations is consistent with its application in the 

study unit and relatively long environmental life. 

Unlike most agricultural herbicides, prometon typically has a continuous, albeit low-level, presence in 

surface waters because there are no efficient loss 

mechanisms for removing prometon from the hydrologic system (Capel and others, 1999). Tebuthiuron, 

used on road rights-of-way and industrial sites, was 

detected in 7 percent of the samples. Triclopyr, used 

on road rights-of-way, industrial sites, and turf grass, 

was detected in 1 percent of the samples.

Several insecticides that have been identified as 

posing a high risk to aquatic invertebrates were 

detected in streams (fig. 6). Carbofuran was the most 

frequently detected insecticide (16 percent of all 

samples from streams). Although detected in only 

16 percent of all samples, carbofuran was detected in 

68 percent of the samples collected in June. When 

present, carbofuran concentrations generally were less 

than 0.80 μg/L. Chlorpyrifos was detected in about 

7 percent of the samples. As with most other insecticides, chlorpyrifos was detected most frequently in 

June (about 30 percent of the samples). The maximum 

concentration was 0.067 μg/L. Malathion was detected 

in three samples in spring and early summer at 

concentrations that ranged from 0.023 to 0.078 μg/L. 

Lower use relative to herbicides, short persistence, and 

application during periods of reduced runoff probably 

account for the overall low detection rate and low 

concentrations of insecticides in rivers and streams.

The pesticide-degradate compounds were some 

of the most frequently detected pesticide compounds 

in streams (fig. 6) and on average constituted the 

majority of the pesticide mass in water samples. Metolachlor OA, alachlor ESA, metolachlor ESA, deethylatrazine, and acetochlor ESA were detected in 

more than 75 percent of the samples. The frequent 

occurrence of the degradates is even more striking 

when comparing the parent pesticides and their degradates at a higher but common method detection level 

of 0.2 μg/L (the MDL for the degradates) (fig. 8). The 

degradates were detected much more frequently than 

their parent compounds with the exception of two 

degradates of atrazine—deethylatrazine and deisopropylatrazine (fig. 8). The degradates metolachlor 

ESA, alachlor ESA, metolachlor OA, and acetochlor 

ESA were detected in 75 to 100 percent of the samples 

compared to 4 to 55 percent for their respective parent 

compounds (fig. 8). In particular, the difference 

between detection frequencies of alachlor and alachlor 

ESA (using the 0.2-μg/L MDL) is large. Alachlor ESA 

is detected in almost all samples, and alachlor is 

detected in only about 4 percent of the samples (fig. 8). 

Even though alachlor usage has strikingly declined in 

the last 10 years (fig. 5), its degradate, alachlor ESA, is 

still one of the most frequently detected compounds in 

streams in the EIWA study unit.

In addition to being detected more frequently, 

concentrations of degradates were larger than their 

parent compounds. Median concentrations for the 

pesticide-degradate compounds ranged from 0.07 to 

3.7 μg/L (Appendix 2), which were larger than their 

parent pesticide compounds (median concentrations 

0.01 to 0.22 μg/L). However, the pesticide degradates 

typically had lower maximum concentrations when 

compared to the parent compounds. Maximum

Metolachlor ESAAlachlor ESAMetolachlor OAAcetochlor ESAMetolachlorHydroxy-AtrazineAtrazineAcetochlor OACyanazine AmideDeethylatrazineAlachlor OAAcetochlorDeisopropylatrazineCyanazineAlachlor

0

100

20

40

60

80

DETECTION FREQUENCY, 

IN PERCENT

Pesticide and pesticide degradate concentrations

 greater than or equal to 0.2 micrograms per liter

EXPLANATION

Figure 8. Occurrence of pesticide and pesticide degradates 

basic-fixed sampling sites using a common reporting limit.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 21

concentrations for the degradates ranged from 0.7 to 

12.4 μg/L (Appendix 2).

On average, approximately 83 percent of the 

total pesticide mass (parent compounds and degradates) in each sample can be accounted for by 

10 common degradates of acetochlor, alachlor, atrazine, cyanazine, and metolachlor. Concentrations of 

acetochlor ESA, alachlor ESA, and metolachlor ESA 

commonly were more than 10 times larger than their 

parent compounds. 

The use of a wide variety of pesticides in the 

EIWA study unit is reflected by the presence of 

multiple pesticide compounds in streams (fig. 9). 

Two or more compounds were detected in every 

sample, and five or more compounds were detected in 

50 percent of the stream samples (fig. 9). As many as 

16 pesticide compounds were detected in a single 

stream sample. Two pesticide compounds, atrazine 

and metolachlor, were detected in 100 percent of the 

samples analyzed (fig. 6). Mixtures of several pesticide degradates also were commonly found. At least 

one pesticide degradate, metolachlor ESA, was present 

in each water sample analyzed (fig. 9). Approximately 

68 percent of all the samples collected contain four 

or more pesticide degradates and approximately 

17 percent of all samples contain seven or more 

pesticide degradates (fig. 9). 

Seasonal Variability

Pesticide concentrations vary throughout 

theyear in samples collected at the basic-fixed sites

0 2345 6789 1

NUMBER OF DETECTED COMPOUNDS

0

100

20

40

60

80

FREQUENCY OF SAMPLES EXCEEDING

INDICATED NUMBER OF DETECTED 

COMPOUNDS, IN PERCENT

Parent compounds

Degradates

10 11

Figure 9. Detection of multiple compounds in samples from

streams at basic-fixed sampling sites in Eastern Iowa Basins 

study unit, 1996–98.

on Eastern Iowa Basins streams with the greatest 

concentrations generally occurring during rainfallrunoff periods soon after application in late spring and 

early summer. Concentrations of almost all pesticides 

peak in May and June following application and then 

decrease during the growing season (figs. 10 and 11). 

The exceptions were prometon, dicamba, and 2,4–D. 

In addition, there is a secondary peak in concentration 

of triazine and chloroacetanilide herbicides that often 

occurs during late winter (figs. 10 and 11). The 

secondary peak during late winter may be due to 

periods of winter thaw that can release pesticides from 

the surface and shallow subsurface to streams. Meltwater can rapidly transport herbicides through 

partially frozen soils to tile lines and streams in late 

winter (Gentry and others, 2000). Spring snowmelt has 

been shown to transport herbicides in runoff in field 

studies (Steinheimer and Scoggin, 2000). The daily 

changes in the frostline depth together with residual 

legacy of pesticide residues from previous year’s 

application may make pesticides available for transport even during the late winter. 

Several pesticide compounds—EPTC, 

metribuzin, and trifluralin—were typically only 

detected in May and June, but dicamba, 2,4–D, and 

prometon concentration patterns did not necessarily 

have the largest concentration peaks in the spring. 

Dicamba is applied to broadleaf weeds later in the 

season, which would probably explain its highest 

detection rate in July. The compound 2,4–D did not 

show a seasonal trend and commonly was not 

detected. The pesticide 2,4–D is used on broadleaf 

weeds often throughout the growing season and in 

road rights-of-way which may indicate why it did not 

display a strong seasonal pattern. Prometon, a triazine 

herbicide, had relatively constant concentrations from 

June through October. The half-life of prometon is on 

the order of hundreds to thousands of days (almost an 

order of magnitude longer than atrazine). The nearly 

constant concentration of prometon is consistent given 

its long half-life and mode of application. 

Most pesticides begin to break down or degrade 

by chemical and biological processes as soon as they 

are used, and the amount of the resulting degradates 

would be expected to follow a similar seasonal pattern 

as the parent pesticide compounds. Pesticide concentrations peak in May and June soon after application 

during periods of increased streamflow. Increased 

streamflow at this time of the year originates as runoff 

from rainfall. Parent pesticide concentrations decrease 
22 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

EXPLANATION

Simazine

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(17) Number of observations Minimum reporting level for atrazine and cyanazine

3 and more than 1.5 times the

Atrazine compounds

0.001

50

0.01

0.1

1

10

(17) (39) (31) (15) (35) (27) (15) (49) (54) (32) (21) (18)

Atrazine MCL

0.001

50

0.01

0.1

1

10

0.001

50

0.01

0.1

1

10

CONCENTRATION, IN MICROGRAMS PER LITER

 Cyanazine MCL

Simazine MCL

Cyanazine compounds

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

Cyanazine Cyanazine amide

Minimum reporting level for

 deethylatrazine, deisopropylatrazine

 and cyanazine amide

Maximum contaminant level

Atrazine Deethylatrazine Deisopropylatrazine

MCL

Figure 10. Monthly concentrations of selected triazine herbicides (atrazine, cyanazine, simazine) and 

degradates at basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 23

Acetochlor Acetochlor ESA Acetochlor OA

Alachlor Alachlor ESA Alachlor OA

Metolachlor Metolachlor ESA Metolachlor OA

(17) (39) (31) (15) (35) (27) (15) (49) (54) (32) (21) (18)

0.001

20

0.01

0.1

1

10

0.001

20

0.01

0.1

1

10

0.001

20

0.01

0.1

1

10

EXPLANATION

Acetochlor compounds

CONCENTRATION, IN MICROGRAMS PER LITER

Alachlor compounds

Metolachlor compounds

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(17) Number of observations Minimum reporting level for acetochlor, alachlor, and metolachlor

3 and more than 1.5 times the

Minimum reporting level for ESA and OA

 degradate compounds

Maximum contaminant level

Alachlor MCL

MCL

Figure 11. Monthly concentrations of selected chloroacetanilide herbicides (acetochlor, alachlor,

metolachlor) and their degradates at basic-fixed sampling sites in streams in the Eastern Iowa Basins 

study unit, 1996–98.
24 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

in the Iowa River through the summer as summer 

rains decrease and vegetative cover increases. The 

seasonal pattern for the pesticide degradates typically 

mirrored that of the parent compounds, but degradates 

commonly remained at higher concentrations 

throughout the year (fig. 12). During late summer and 

early winter, concentrations of the pesticide degradates 

in the Iowa River generally were an order of magnitude higher than the parent compounds both in the 

upper part of the watershed near Rowan and near the 

mouth at Wapello. 

Pesticides were still available for transport to 

streams during late winter and early spring at least 

9 months after application. A secondary peak in 

concentrations of both pesticide parent and degradate 

compounds occurred in the Iowa River during late 

February through March when snow was melting and 

the upper soil layer was thawing (fig. 12). 

Deethylatrazine concentrations generally follow 

the seasonal trend of the parent compound, but at 

slightly lower concentrations (fig. 10). However, in the 

late fall and winter, the median deethylatrazine 

1996 1997 1998

0.1

1

10

100

10

100

1,000

10,000

100,000

DAILY MEAN STREAMFLOW,

IN CUBIC FEET PER SECOND

Iowa River near Rowan, Iowa (Headwaters)

CONCENTRATION, IN MICROGRAMS PER LITER

1996 1997 1998

0.1

1

10

100

10

100

1,000

10,000

100,000

DAILY MEAN STREAMFLOW,

IN CUBIC FEET PER SECOND

Iowa River at Wapello, Iowa (Mouth)

Stream discharge

Total parent compounds

Total degradates

CONCENTRATION, IN MICROGRAMS PER LITER

J F MAM J J ASON D J F MAM J J A S ON D J F MAM J J ASON D

J F M AM J J ASON D J F MAM J J A SON D J F M AM J J A S ON D

EXPLANATION

Figure 12. Seasonal variability of pesticides and pesticide degradates near the headwaters and the 

mouth of the Iowa River, 1996–98.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 25

concentrations are higher than the median atrazine 

concentrations, but the difference is less than one-tenth 

of a microgram per liter (fig. 10). In contrast the chloroacetanilide herbicide degradates, metolachlor ESA, 

acetochlor ESA, and alachlor ESA, have significantly 

(p<0.05) greater median concentrations than their 

parent compounds and do not vary seasonally as much 

as their parent compounds (fig. 11). The median 

concentrations of the degradates of the chloroacetanilide herbicides are much higher year round than 

their parent compounds. Concentrations of chloroacetanilide herbicide degradates during these periods 

indicate these compounds are particularly persistent in 

the environment. The higher concentrations of many 

of the degradates in the fall and winter months also 

indicate contributions from interflow or shallow 

ground water. Other studies indicate that pesticide 

degradates commonly occur in the alluvial aquifers 

adjacent to streams (Sadorf and Linhart, 2000; Savoca 

and others, 2000). These data seem to indicate that 

alachlor ESA is persistent and stable. 

Seasonal variability in pesticides and pesticide 

degradates was documented in detail at 25 synoptic 

sites on streams that drained agricultural watersheds 

ranging in size from 310 to 1,500 km2 (table 2). 

Samples were collected from these sites in August 

1997 and May 1998 during base-flow conditions. The 

two most heavily applied chloroacetanilide herbicides 

(acetochlor and metolachlor) and their ethane sulfonic 

acid degradates were detected more frequently 

(fig. 13) and at greater concentrations during the 

spring (high base-flow conditions) than during the late 

summer (low base-flow conditions) (fig. 14). During 

late spring, pesticides from recent applications may be 

transported to the stream through short ground-water 

flow paths. Another potential source is from storage in 

streambanks. Rain events in late April and early May 

may result in high stream levels, which can produce a 

reverse gradient in adjacent alluvial aquifers transporting pesticides from the stream into the streambank. Water containing these pesticides subsequently 

drains back into the stream upon reestablishment of 

the normal hydrologic gradient during base-flow 

conditions (Squillace and others, 1993).

Differences between the detection rates and 

concentrations of triazine herbicides and their degradates were not significantly different between late 

spring (high) and late summer (low) base-flow conditions in streams (fig. 13 and 14). One exception was 

hydroxyatrazine. In addition to hydroxyatrazine,

acetochlor OA and alachlor OA were detected more 

frequently in the late summer than in late spring baseflow conditions. The structure of acetochlor OA and 

alachlor OA molecules are very similar and thus may 

behave in a similar manner in the environment. 

Hydroxyatrazine and alachlor OA were detected only 

in late summer. Chemical properties such as rate of 

formation and degradation may influence the timing of 

the occurrence of the oxanilic acid degradates. As 

discussed earlier, hydroxyatrazine may be formed in 

the soil, transported to the streambed on sediment, and 

remobilized by ground-water inflow primarily during 

summer. 

In contrast to the most commonly used herbicides, insecticides generally were present only during 

the summer. A few insecticides that have been identified as posing a potential risk to aquatic invertebrates 

were detected in streams from May through 

September (fig. 15), the months when most application 

normally occurs. Carbofuran was the most frequently 

detected insecticide (16 percent of all the samples). 

Although detected in less than 20 percent of all 

samples, carbofuran was detected in 68 percent of the 

samples collected in June (fig. 15). When present, 

carbofuran concentrations generally were less than 

0.80 μg/L. Chlorpyrifos was detected in about 

7 percent of the samples. As with the other insecticides, chlorpyrifos was detected most frequently in 

June (about 30 percent of the samples). The highest 

concentration was 0.067 μg/L. Malathion was detected 

Low flow

High flow

0

100

20

40

60

80

DETECTION RATE, 

IN PERCENT

AtrazineDeethylatrazineDeisopropylatrazineHydroxyatrazineCyanazineCyanazine amide

Metolachlor ESAAlachlor ESAMetolachlor OAAcetochlor ESA

MetolachlorAcetochlor OAAlachlor OAAcetochlorAlachlor

Figure 13. Detection rate of herbicides and herbicide 

degradates during low and high base-flow conditions at 

synoptic sampling sites on streams that drain agricultural

watersheds in the Eastern Iowa Basins study unit.
26 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

in only three samples in spring and early summer at 

concentrations that ranged from 0.023 to 0.078 μg/L. 

In addition, insecticides such as diazinon and dieldrin 

that have been typically detected in urban areas in 

other NAWQA studies throughout the Nation (U.S. 

Geological Survey, 1999) were rarely detected (less 

than a 2 percent detection frequency) in the EIWA 

study unit. Most likely this is because the amount of 

urban land where diazinon (a common urban insecticide) might be used in the EIWA study unit is small 

(1.8 percent). In general, the overall low detection rate 

and low concentrations of insecticides in streams of 

the EIWA study unit are probably due to their lower 

use (relative to herbicides), short half-life, and application during periods of reduced runoff.

Figure 14. Concentrations of selected herbicides during low and high base-flow conditions in streams that drain 

agricultural watersheds in the Eastern Iowa Basins study unit.

BFS SYN

0.01

10

0.02

0.05

0.1

0.2

0.5

1

2

5

BFS

BFS

SYN

SYN

0.01

10

0.02

0.05

0.1

0.2

0.5

1

2

5

CONCENTRATION, IN MICROGRAMS PER LITER

EXPLANATION

Method detection limit

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

and more than 1.5 times 

Outlier data value less than or equal to 3

interquartile range outside the quartile

Outlier data value more than 3 times the

BFS SYN

SITE TYPE

BFS SYN

Atrazine

Metolachlor Metolachlor ESA Metolachlor OA

BFS SYN BFS SYN

Deethylatrazine Deisopropylatrazine

Low base flow

High base flow

Basic-fixed sampling site

Synoptic sampling site

the
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 27

Figure 15. Monthly detection rate of selected insecticides in 

streams in the Eastern Iowa Basins study unit.

Spatial Variability

 Numerous factors such as soil type, pesticide 

soil-water partition coefficients, application rates, and 

pesticide use all affect the spatial variability of pesticide concentrations in streams in eastern Iowa. A 

detailed analysis of all variables that can affect this 

spatial variability was beyond the scope of this report. 

However, statistically significant differences for 

concentrations of the triazines and the chloroacetanilide compounds were shown between basicfixed sites (figs. 16 and 17) that may be related to 

topographic and geologic differences in landform 

regions. Median concentrations for atrazine and cyanazine were statistically larger for sites at the Skunk 

River at Augusta and Old Mans Creek near Iowa City 

than the other sites. The Skunk River at Augusta site 

has 74.7 percent of its drainage and the Old Mans 

Creek near Iowa City site has 100 percent of its 

drainage within the Southern Iowa Drift Plain. 

Metolachlor concentrations were significantly 

(p < 0.05) larger at the South Fork Iowa River near 

New Providence and Iowa River at Marengo sites than 

at the other sites (fig. 17). The median metolachlor 

concentration for the South Fork Iowa River site 

(100 percent drainage on the Des Moines Lobe) was 

0.50 μg/L and the median metolachlor concentration 

for the Iowa River at Marengo site (44.5 percent 

drainage on the Des Moines Lobe, 37.2 percent on the 

Iowan Surface, and 18.3 percent on the Southern Iowa 

Drift Plain) was 0.358 μg/L. Median concentrations of 

acetochlor were significantly (p < 0.05) larger at the 

South Fork Iowa River (0.036 μg/L), Iowa River at 

Marengo (0.036 μg/L), and Skunk River at Augusta 

(0.040 μg/L) sites than at the other sites (fig. 17). 

Based solely on agricultural land use, it is somewhat surprising that there are significant concentration 

differences among the sampling sites. One possible 

0

100

20

40

60

80

DETECTION RATE, IN 

PERCENT

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

Carbofuran

Chlorpyrifos

Fonofos

Diazinon

explanation for differences among sites is difference in 

herbicide use between landform regions. Stoltenberg 

and Pope (1990) reported that atrazine was applied to 

only 25 percent of the corn in north-central Iowa in 

contrast to 69 percent of the corn in southeastern Iowa 

during the 1980’s. Application rates also were less in 

north-central Iowa in areas with lower permeability till 

soils that contained greater organic carbon content 

because of farmers’ concern that "carryover" of atrazine may damage subsequent soybean crops. Also, in 

response to the detection of atrazine in ground water, 

atrazine management areas were established in northeastern Iowa in areas where bedrock aquifers are close 

to the land surface (Iowa Department of Agriculture 

and Land Stewardship, 1999). No more than 1.4 kg of 

atrazine may be applied per hectare per year (1.5 lb per 

acre per year) in these management areas. The atrazine 

management areas correspond closely with the Iowan 

Karst subregion (fig.18).

Two closely related factors, herbicide use and 

degradation properties of the herbicides, may account 

for the generally greater detection rate and larger 

concentrations of triazine compounds in streams 

draining soils developed on windblown loess 

(Southern Iowa Drift Plain) than in streams draining 

till soils (Des Moines Lobe and Iowan Surface). 

Because of soil properties, triazine pesticide-use rates 

are apparently less in areas with predominantly till 

soils (Stoltenberg and Pope, 1990). These soils have 

high organic carbon content with pH values near 7.0 

that decreases the degradation of the triazines and 

helps to retain (carry over) atrazine and cyanazine in 

the soil. Cyanazine sorption has been correlated with 

fine soil texture and greater organic carbon content 

(Reddy and others, 1997). Alachlor, metolachlor, and 

acetochlor probably were used to offset reduced 

triazine pesticide use in areas with till soils.

Median alachlor concentrations were low at all 

sites (generally less than 0.1 μg/L) and probably 

reflected the relatively low use in the study unit. 

However, the median alachlor concentrations were 

significantly higher (p < 0.05) at two sites—Wapsipinicon River at Tripoli and Wapsipinicon River at Dewitt 

(fig. 17) than at the other sites. Because the Wapsipinicon River drains part of the atrazine management 

area, greater alachlor concentrations may be due to its 

preferential use. 
28 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Atrazine compounds

Simazine

(53) (21) (22) (23) (12) (23) (54)

Atrazine

MCL

Simazine

MCL

(24) (52) (22) (26) (21)

0.001

50

0.01

0.1

1

10

0.001

50

0.01

0.1

1

10

0.001

50

0.01

0.1

1

10

Indicator sites

(smaller streams draining basins with one predominant landform or land use)

Integrator sites (large rivers draining basins with 

mixed landforms and land use)

CONCENTRATION, IN MICROGRAMS PER LITER

Cyanazine

MCL

Wapsipinicon R., Tripoli

 (site 1)

Iowa R., Rowan

 (site 3)

South Fork Iowa R., New 

Providence (site 4)

Old Mans Cr., Iowa City

 (site 6)

Flood Cr., Powersville

 (site 7)

Wolf Cr., Dysart

 (site 9)

Wapsipinicon R., DeWitt

 (site 2)

Iowa R., Marengo

 (site 5)

Cedar R., Gilbertville

 (site 8)

Cedar R., Conesville

 (site 10, 10a)

Iowa R., Wapello

 (site 11)

Skunk R., Augusta

 (site 12)

EXPLANATION

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(24) Number of observations Method detection limit for atrazine and cyanazine

3 and more than 1.5 times the

Method detection limit for deethylatrazine,

 deisopropylatrazine, and cyanazine amide

Maximum contaminant level

Atrazine Deethylatrazine Deisopropylatrazine

Cyanazine Cyanazine amide

MCL

Figure 16. Concentrations of selected triazine herbicides (atrazine, cyanazine, simazine) and degradates at 

the basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 29

(24) (52) (26) (54) (22) (53) (21) (12) (23) (22) (23) (21)

Acetochlor

compounds

Alachlor compounds Alachlor MCL

0.001

20

0.01

0.1

1

10

0.001

20

0.01

0.1

1

10

0.001

20

0.01

0.1

1

10

CONCENTRATION, IN MICROGRAMS PER LITER

Metolachlor compounds

Indicator sites

(smaller streams draining basins with one predominant landform or land use)

Integrator sites (large rivers draining basins with 

mixed landforms and land use)

Wapsipinicon R., Tripoli

 (site 1)

Iowa R., Rowan

 (site 3)

South Fork Iowa R., New 

Providence (site 4)

Old Mans Cr., Iowa City

 (site 6)

Flood Cr., Powersville

 (site 7)

Wolf Cr., Dysart

 (site 9)

Wapsipinicon R., DeWitt

 (site 2)

Iowa R., Marengo

 (site 5)

Cedar R., Gilbertville

 (site 8)

Cedar R., Conesville

 (site 10, 10a)

Iowa R., Wapello

 (site 11)

Skunk R., Augusta

 (site 12)

EXPLANATION

25th percentile

Median

75th percentile

interquartile range outside the quartile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 

interquartile range outside the quartile

Outlier data value more than 3 times the

(24) Number of observations Method detection limit for acetochlor, alachlor, and metolachlor

3 and more than 1.5 times the

Method detection limit for ESA and OA

 degradate compounds

Maximum contaminant level

Acetochlor Acetochlor ESA Acetochlor OA

Alachlor Alachlor ESA Alachlor OA

Metolachlor Metolachlor ESA Metolachlor OA

MCL

Figure 17. Concentrations of selected chloroacetanilide herbicides (acetochlor, alachlor, metolachlor) and 

their degradates at the basic-fixed sampling sites in the Eastern Iowa Basins study unit, 1996–98.
30 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

The occurrence of degradates and the ratio of 

degradate to parent compound were substantially 

different for the triazines than for the chloroacetanilide 

compounds. Concentrations of the triazine degradates 

tended to follow the pattern of the parent compounds

—highest concentrations during the early summer 

followed by decreasing concentrations during the late 

summer and fall (fig. 10). However, concentrations of 

the triazine degradates were lower than their parent 

compounds except in the fall and winter when they 

were slightly higher. In contrast, the ESA and OA 

degradates of alachlor, acetochlor, and metolachlor 

were present in higher concentrations than their parent 

compounds throughout the year (fig. 11).

Synoptic sampling during high and low baseflow conditions at 25 sites confirmed the spatial 

pattern seen at the monthly sampling (basic-fixed) 

sites. Much of the spatial variability during base-flow 

conditions was related to landform types and their 

geologic and hydrologic differences. Overall, pesticide 

detections and concentrations were different in the 

Southern Iowa Drift Plain than in the Des Moines 

Lobe or the Iowan Surface. The main difference, with 

two exceptions, is that the triazine herbicides and their 

0 40 20 60 80 MILES

0 40 20 60 80 KILOMETERS

EXPLANATION

Des Moines Lobe

Iowan Karst (subregion)

Iowan Surface

Southern Iowa Drift Plain

Atrazine management area (atrazine

management areas from Iowa

Department of Natural Resources,

1994b)

Landform regions (modified

 from Prior, 1991)

Basic-fixed sampling sites

Indicator

Integrator

Figure 18. Atrazine management areas in the Eastern Iowa Basins 

study unit.
PESTICIDES AND PESTICIDE DEGRADATES IN STREAMS 31

degradates were present in significantly greater 

concentrations in streams of the Southern Iowa Drift 

Plain than in streams of the two other physiographic 

areas (table 6). The exceptions were that acetochlor 

and acetochlor OA had significantly greater concentrations in the Southern Iowa Drift Plain streams during 

high base-flow conditions and low base-flow conditions, respectively. 

Results of sampling in the EIWA study unit 

from 1996 through 1998 concur with previous investi

Table 6. Triazine and chloroacetanilide pesticide and pesticide degradates during low (August 1997) and high (May 1998) 

base flow at the synoptic sampling sites in the Eastern Iowa Basins study unit

[μg/L, micrograms per liter; bold text indicates concentrations from sites in one or more landforms are significantly different; <, less than]

Constituent Base-flow type

Landforms (defined by Prior, 1991)

Des Moines Lobe

(7 sites)

Iowan Surface

(11 sites)

Southern Iowa Drift Plain

(7 sites)

Detection rate

(Percent)

Median

concentration

(μg/L)

Detection rate

(Percent)

Median

concentration

(μg/L)

Detection 

rate

(Percent)

Median

concentration

(μg/L)

Acetochlor Low 0 <0.05 9 <0.05 0 <0.05

High 57 .06 27 <.05 100 .12

Acetochlor ESA Low 86 .41 64 .27 86 .45

High 86 .63 91 .65 100 .76

Acetochlor OA Low 29 <.20 9 <.20 86 .53

High 14 <.20 9 <.20 0 <.20

Alachlor Low 0 <.05 0 <.05 0 <.05

High 0 <.05 0 <.05 0 <.05

Alachlor ESA Low 100 1.4 100 1.9 100 .90

High 100 1.1 100 1.6 100 1.0

Alachlor OA Low 29 <.20 27 <.20 43 <.20

High 0 <.20 27 <.20 0 <.20

Atrazine Low 100 .17 91 .13 100 .25

High 100 .15 100 .12 100 .50

Deisopropylatrazine Low 43 <.05 82 .06 100 .13

High 43 <.05 100 .08 100 .17

Deethylatrazine Low 57 .06 100 .12 100 .14

High 100 .07 100 .17 100 .21

Hydroxyatrazine Low 14 <.05 55 .27 100 1.0

High 0 <.05 0 <.05 0 <.05

Cyanazine Low 14 <.05 9 <.05 43 <.05

High 14 <.05 9 <.05 86 .16

Cyanazine amide Low 0 <.05 0 <.05 71 .07

High 14 <.05 0 <.05 57 .09

Metolachlor Low 57 .06 46 <.05 86 .07

High 100 .20 73 .07 100 .17

Metolachlor ESA Low 100 4.0 100 2.5 100 1.4

High 100 5.1 100 5.0 100 2.8

Metolachlor OA Low 100 .75 100 .45 100 .49

High 100 .78 100 .98 100 .81
32 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

gations that found seasonal patterns of concentrations 

and occurrences of pesticides in agricultural streams 

tend to repeat each year and correspond to patterns of 

application, streamflow, and ground-water flow 

(U.S. Geological Survey, 1999). The seasonal pattern 

of the commonly applied herbicides such as atrazine 

and alachlor in midwestern rivers is well documented 

(Larson and others 1997; Richards and Baker, 1991; 

Goolsby and Battaglin, 1993). Concentrations are 

generally low in the late fall and winter and peak in the 

spring and early summer. In general, increased 

concentrations of herbicides in streams draining agricultural areas can occur within a few days to a few 

weeks, depending on the timing and number of rainfall 

events and the size of the drainage basin (Larson and 

others, 1997). The first runoff-inducing rain event after 

the application of pesticides potentially can move 

substantial amounts of the pesticide to surface waters 

(Schottler and others, 1994; Larson and others, 1997; 

Goolsby and Battaglin, 1993).

RELEVANCE OF PESTICIDES IN 

STREAMS

Pesticides have been a very important tool in the 

control of unwanted vegetation in crops, road rightsof-way, lawns, and other urban areas and in controlling 

insects and other biological pests in both agricultural 

and urban areas. However, transport of excess pesticides to streams may have unintended effects on 

humans that use the rivers and streams as a source of 

water supply and to aquatic organisms living in the 

stream. A brief overview of the environmental significance of pesticides in eastern Iowa streams is 

discussed in this section. Pesticide relevance will be 

discussed in relation to drinking-water criteria for 

human-health effects and in relation to results from 

toxicological research on algae, benthic invertebrates, 

and fish. Additional discussion on the effects on 

human health and on aquatic life in streams can be 

found in Larson and others (1997), and national 

primary drinking-water regulations are available for 

individual pesticide compounds from the U.S. 

Environmental Protection Agency (1995).

Human Health

During the sampling period of this study, 1996–

98, the city of Iowa City utilized water directly from 

the Iowa River as part of their municipal water supply. 

A number of smaller communities use water from 

reservoirs as source of supply. During the 3 years of 

this study, no drinking-water criteria (on a yearly 

average) were exceeded, but MCL’s were exceeded in 

individual samples for several compounds. MCL 

concentrations in individual samples were most 

frequently exceeded for the triazine herbicides atrazine 

(10 percent of the samples) and cyanazine (3.8 percent 

of the samples). Although atrazine concentrations 

exceeded the MCL in only 10 percent of the samples, 

all exceedances occurred in 3 months: May, June, and 

July (fig. 10). Storage of spring runoff in reservoirs 

potentially can extend the seasonally high spring 

pesticide concentrations into the summer and lengthen 

the potential annual exposure to these compounds 

(Kalkhoff, 1993). Pesticide concentrations exceed 

1.0 μg/L in some Iowa reservoirs used for municipal 

water supplies (Miller and Kennedy, 1995) during the 

winter, 6 to 9 months after application.

Acetochlor, a herbicide that was conditionally 

registered for use beginning in 1994, was frequently 

detected, but in most samples (75 percent) at concentrations less than 0.1 μg/L. Acetochlor concentrations 

did not exceed the USEPA 2.0-μg/L annual mean 

concentration registration requirement at any site but 

did exceed this concentration in about 3 percent of the 

individual samples. The maximum concentration 

(10.6 μg/L) measured during the study exceeded the 

concentration that would trigger requirements for 

biweekly sampling for water-supply systems 

(U.S. Environmental Protection Agency, 1994a). 

Only limited research has been conducted on the 

acute and chronic human and environmental effects of 

pesticide degradates. In one study (Heydens and 

others, 2000), alachlor was found to break down into 

at least one less toxic compound, alachlor ESA. 

Natural infiltration of stream water may transport pesticides to shallow alluvial aquifers during the 

spring runoff during rising river stages (Squillace, 

1996). In addition, pumping of municipal wells 

located near rivers can induce additional infiltration of 

river water into the adjacent alluvial aquifers (Schulmeyer and Schnoebelen, 1998; Boyd, 2000 and 2001; 

Savoca and others, 2002). Many communities in Iowa 

use ground water that is pumped from alluvial aquifers 

located adjacent to rivers and streams because this is a 

dependable source of supply. With the exception of 

karst bedrock, total herbicide concentrations (sum of 

the concentrations of all compounds analyzed) were 
SUMMARY AND CONCLUSIONS 33

greatest from municipal wells pumping from alluvial 

aquifers in Iowa in 1995 (Kolpin and others, 1997). 

Aquatic Life 

Pesticide concentrations in eastern Iowa and 

southern Minnesota streams may occasionally reach 

levels that have been identified as affecting algae, 

macroinvertebrates, and fish. In contrast to humans 

who are generally exposed to pesticides through ingestion of water, aquatic organisms living in streams are 

continually exposed to any pesticides occurring in the 

stream. The effects of this exposure is not fully understood. A large amount of research has investigated the 

effects of pesticides on aquatic organisms; several of 

these studies are referenced in the following discussion. 

The direct effects of pesticides on algae were 

not investigated during this study but may be inferred 

on the basis of concentrations found to affect algae in 

other investigations. The concentration of most pesticides in sampled streams in the EIWA study unit probably was not high enough to be acutely toxic to algae 

during 1996–98. However, similar seasonally high 

concentrations have been shown to temporarily reduce 

photosynthetic activity or alter the community structure. In field mesocosm studies, phytoplankton and 

macrophytes were reduced after atrazine exposures 

greater than 20 μg/L, but populations were quickly 

reestablished (Solomon and others, 1996). Effects 

were ecologically important only at exposures of 

50 μg/L or greater (Solomon and others, 1996). The 

atrazine concentration was 20 μg/L or more in only 

five samples (1.2 percent) and approached 50 μg/L 

(Appendix 2) in only one sample during this study. 

Alachlor has been shown to affect algal biomass at 

concentrations of 10 μg/L or greater but did not significantly affect biomass at a concentrations of 1 μg/L 

(Spawn and others, 1997). The maximum alachlor 

concentration in samples described in this report was 

less than 1.0 μg/L.

Agricultural activity in a watershed was a factor 

accounting for differences in benthic invertebrate 

assemblages in EIWA streams (Brigham and Sadorf, 

2001). However, agriculture’s effect on habitat, 

nutrient concentrations, or pesticide concentrations as 

the primary factor in the differing community structure 

could not be determined with the available data. 

The possible effects of concentrations of pesticides determined for this report on fish communities is 

unclear. Physical characteristics (stream size) and 

nutrient enrichment (total phosphorus and organic 

carbon concentrations) were more significant determinants of fish community structure than pesticide 

concentrations (Sullivan, 2000), A subsequent study 

on the Cedar River (Phillips and Summerfelt, 2001) 

indicated that acetochlor, deethylatrazine, metolachlor, 

and parathion in water during April and May 1998 and 

1999 did not affect the cholinesterase (ChE) activity in 

larval walleye. No effect on ChE activity was noted 

even though the parathion concentration exceeded 

U.S. Environmental Protection Agency (1999) aquatic 

health guidelines in one sample in 1999 (Phillips and 

Summerfelt, 2001). However, another study has shown 

that exposures to insecticides that exceeded U.S.Environmental Protection Agency (1999) aquatic health 

guidelines over a several months have been linked to 

depressed ChE in common carp (Cyprinus carpio) 

(Gruber and Munn, 1998). 

Laboratory research has suggested that atrazine 

concentrations as small as 0.1 μg/L affected some 

frogs (Hayes and others, 2002). When exposed to 

increasing amounts of atrazine, as many as 20 percent 

of the frogs exposed during their early development 

produced multiple sex organs or had both male and 

female organs (Hayes and others, 2002). In this report, 

more than 50 percent of the water samples collected in 

April and more than 75 percent of the samples 

collected in May contained atrazine concentration 

greater than 0.1 μg/L (fig. 10), and maximum concentrations in May were nearly 50 μg/L. 

SUMMARY AND CONCLUSIONS 

 A water-quality assessment of streams in four 

major watersheds in eastern Iowa and parts of southern 

Minnesota was conducted from 1996 through 1998 as 

part of the U.S. Geological Survey’s NAWQA 

Program. Samples were collected in the EIWA study 

unit, an area that encompasses about 50,500 km2

(19,500 mi2

) and includes the Wapsipinicon, Cedar, 

Iowa, and Skunk watersheds. Twelve basic-fixed sites 

were sampled monthly during the first year of the 

study (March 1996 through March 1997) and 11 

basic-fixed sites were sampled after March 1997. The 

basic-fixed sites ranged from 320 to 32,400 km2

 (124 

to 12,500 mi2) in size. Three basic-fixed sites (Iowa 
34 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

River at Rowan, Wolf Creek near Dysart, and Iowa 

River at Wapello) were intensively sampled from April 

1997 through March 1997. Additional samples were 

collected during floods to define pesticide concentrations through the full range of streamflow that 

occurred during the study. Samples were analyzed for 

82 pesticide parent compounds and 10 pesticide degradate compounds. More than 350 samples were 

collected for pesticide analysis at the basic-fixed sites. 

To better define spatial variability, samples were 

collected from 25 sites during low base-flow conditions during August 1997 and during high base-flow 

conditions in May 1998. 

The most commonly detected herbicides were 

those most heavily used on crops. Atrazine and metolachlor were detected in all surface-water samples. 

Acetochlor, alachlor, and cyanazine were detected in 

more than 70 percent of the samples. Seventy-six 

percent of the atrazine samples exceeded 0.1 μg/L 

with almost 60 percent in the 0.1 to 1.0 μg/L range. 

About 10 percent of the samples exceeded the atrazine 

MCL for drinking water of 3.0 μg/L. Less frequently 

detected pesticides (detected in 30 percent or less of 

the samples) such as EPTC, trifluralin, metribuzin, 

carbofuran, dicamba, and 2,4–D were applied to crops 

in Iowa at a rate of only 0.6 to 30 percent of the 

amount applied for atrazine alone. 

 Many pesticides are used in both non-rowcrop agriculture and urban settings, but three herbicides— prometon, tebuthiuron, and triclopyr—used 

almost exclusively in non-row-crop and urban settings 

were detected. Prometon was detected in more than 

80 percent of the samples, but at very low concentrations (less than 0.1 μg/L). Prometon’s chemical persistence may explain its relatively high detection rate 

relative to its low use. Tebuthiuron was detected in 

7 percent and triclopyr was detected in 1 percent of the 

samples.

Insecticides were rarely detected, and when they 

were detected, concentrations were near the method 

detection limit. Carbofuran was the most commonly 

detected insecticide (16 percent of the samples). Other 

insecticides were detected in less than 10 percent of 

the samples. 

The pesticide degradates were some of the most 

frequently detected pesticide compounds in the study 

unit and on average constituted the majority of the 

pesticide residue in water samples. Metolachlor ESA, 

alachlor ESA, metolachlor OA, and acetochlor ESA 

were detected in more than 75 percent of the samples. 

On average, approximately 83 percent of the total 

pesticide mass (parent and degradate compounds) can 

be accounted for by the 10 common degradates of 

acetochlor, alachlor, atrazine, cyanazine, and metolachlor. Median concentrations for pesticide degradate 

compounds were higher than those of their parent 

compounds. Median concentrations for the degradates 

ranged from 0.07 to 3.7 μg/L compared to median 

concentrations for their parent compounds that ranged 

from 0.01 to 0.22 μg/L. Concentrations of metolachlor 

ESA, alachlor ESA, and acetochlor ESA were often 

more than 10 times greater their parent compounds.

Mixtures of pesticides were common. At least 

one pesticide degradate (metolachlor ESA) was 

present in each water sample analyzed. Approximately 

68 percent of all the samples collected contain four 

or more pesticide degradates, and approximately 

17 percent of all samples contain seven or more pesticide degradates. As in the case of the degradates, the 

parent pesticide compounds were commonly found 

together. Two parent pesticide compounds, atrazine 

and metolachlor, were detected in all of the samples 

analyzed. Two or more parent pesticide compounds 

were detected in all samples, and five or more 

compounds were detected in 50 percent of the 

samples.

Concentrations of pesticide compounds in 

streams follow strong seasonal patterns. The highest 

herbicide concentrations occurred in the spring (May 

and June) during spring runoff events. A secondary 

peak in pesticide concentrations occurred in late 

winter, typically in March, at all sites for the chloroacetanilides and triazines (with the exception of 

prometon). This secondary peak in pesticide concentrations during late winter may be due to early thaw 

that can release pesticides from the soil surface and 

shallow subsurface to surface waters. 

 The detection rates and concentrations of most 

triazine herbicides and their degradates were not 

significantly different between late spring and late 

summer base-flow conditions in streams. Exceptions 

were hydroxyatrazine, acetochlor OA, and alachlor 

OA. These three degradates were detected most 

frequently in late summer during base-flow conditions. Hydroxyatrazine and alachlor OA were detected 

only in late summer. Hydroxyatrazine may be formed 

in the soil, transported to the streambed on sediment, 

and remobilized by ground-water inflow primarily 

during summer. Chemical properties such as rate of 
REFERENCES 35

formation and degradation may influence the timing of 

the occurrence of the oxanilic acid degradates. 

The pattern of occurrence and concentration was 

found to differ between the degradates of triazine and 

chloroacetanilide herbicides. The triazine herbicide 

degradates tended to follow a monthly concentration 

pattern similar to their parent compounds, but the 

concentrations were lower than their parent compounds, except in the fall and winter. In contrast, the 

chloroacetanilide degradates were present in greater 

concentrations than their parent compounds 

throughout the year. 

Pesticide compounds varied between landform 

regions. The triazine herbicides and their degradates 

were present in greater concentrations (statistically 

significant) in streams in the Southern Iowa Drift Plain 

than in streams in the Des Moines Lobe or the Iowan 

Surface. Much of the variability may be related to the 

geologic and hydrologic differences between landforms. Because of differences in soil properties, 

triazine pesticide-use rates are less in areas with 

predominantly till soils such as the Des Moines Lobe 

and Iowan Surface compared to areas with windblown 

loess soils. The alternative use of other classes of 

herbicides may account for greater occurrence and 

concentration of some chloroacetanilide herbicides in 

the Des Moines Lobe and the Iowan Surface.

The presence of pesticides in streams may 

seasonally affect municipalities that use surface water 

in eastern Iowa and southern Minnesota. Although the 

annual average concentrations did not exceed the 

MCL, concentrations of atrazine and cyanazine in 

individual samples occasionally exceed the MCL in 

spring and early summer. Wells used for municipal 

supply located near streams may induce infiltration 

from the stream into the alluvial aquifers and subsequently into the water supply. Pesticides present in the 

stream may be transported through the alluvial aquifer 

to the water-supply wells. The possible effects of 

pesticides in streams to aquatic organisms are not 

known. Concentrations may occasionally reach levels 

that have been identified as affecting algae, fish, and 

amphibians. 

The results presented in this report provide an 

assessment of the presence of pesticide compounds 

that were commonly used during and possibly several 

years preceding the study period 1996–98. However, 

agricultural practices continue to evolve with emphasis 

both on the use of less pesticides and on the use of 

genetically modified crops that are resistant to insects 

and glyphosate herbicides. Commonly used herbicides, like metolachlor, are being reformulated to be 

more effective and require less product, and a number 

of products, for example cyanazine, will no longer be 

used. Because pesticides are transported to streams 

and because pesticides may affect drinking-water 

supplies and aquatic organisms, there is a need to 

continue monitoring currently used pesticides and to 

monitor new compounds that are applied in the environment. 

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APPENDIX
42 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Appendix 1. Herbicide and insecticide target analytes, method detection limits, and drinking-water standards

 [Concentrations in micrograms per liter, MCL, maximum contaminant level; HA, health advisory; HPLC, high performance 

liquid chromatography, GC/MS, gas chromatography/mass spectrometry; --, no regulation or guideline]

Compound Trade name(s) Analysis 

method

Method 

detection 

limit

(μg/L)

Regulation or 

guideline for 

drinking 

water

(MCL or HA)

(μg/L)

Herbicides

2, 4–D 2, 4–D and others HPLC 0.035 170

2,4–DB Butoxone HPLC 0.035 --

2,4,5–T Line Rider, Weedone HPLC 0.035 70

2,4,5–TP Silvex HPLC 0.021 150

2,6–Diethylaniline Degradate of alachlor GC/MS 0.003 --

Acetochlor Harness GC/MS 0.002 --

Acetochlor ESA (ethane sulfonic acid) Degradate of acetochlor HPLC 0.20 --

Acetochlor OA (oxanilic acid) Degradate of acetochlor HPLC 0.20 --

Acifluorfen Blazer, Tackle HPLC 0.035 --

Alachlor Lasso GC/MS 0.002 12

Alachlor ESA (ethane sulfonic acid) Degradate of alachlor HPLC 0.20 --

Alachlor OA (oxanilic acid) Degradate of alachlor HPLC 0.20 --

Atrazine Aatrex GC/MS 0.001 13

Benfluralin Balan, Benefin GC/MS 0.002 --

Bentazon Basagran HPLC 0.014 20

Bromacil Bromax 90, Urox B HPLC 0.035 90

Bromoxynil Buctril, Brominal HPLC 0.035 --

Butylate Sutan+ GC/MS 0.002 350

Chloramben Amiben HPLC 0.011 100

Clopyralid Lontrel HPLC 0.050 --

Cyanazine Bladex GC/MS 0.004 1

Cyanazine amide Degradate of cyanazine HPLC 0.20 --

Dacthal Dacthal GC/MS 0.002 4,000

Deethylatrazine Degradate of atrazine GS/MS 0.002 --

Deisopropylatrazine Degradate of atrazine GS/MS 0.20 --

Dicamba Banvel HPLC 0.035 200

Dichlobenil Casoron HPLC 0.020 --

Dichlorprop 2,4–DP HPLC 0.032 --

Dinoseb Basanite and others HPLC 0.035 17

Diuron Diurex and others HPLC 0.020 10

EPTC Eptam GC/MS 0.002 --

Ethalfluralin Sonalan GC/MS 0.004 --

Fenuron Beet-Kleen HPLC 0.013 --

Fluometuron Cotoran HPLC 0.035 90

Hydroxyatrazine Degradate of atrazine HPLC 0.20 --

Linuron Lorox GC/MS 0.002 --

MCPA MCPA and others HPLC 0.050 10

MCPB Thistrol HPLC 0.050 --

Metolachlor Dual GC/MS 0.002 100
 43

Herbicides—Continued

Metolachor ESA (ethanesulfonic acid) Degradate of metolachlor HPLC 0.20 --

Metolachlor OA (oxanilic acid) Degradate of metolachlor HPLC 0.20 --

Metribuzin Lexone, Sencor GC/MS 0.004 200

Molinate Ordram GC/MS 0.004 --

Napropamide Devrinol GC/MS 0.003 --

Neburon Neburex, Neburon HPLC 0.015 --

Norflurazon Evital, Zorial HPLC 0.024 --

Oryzalin Surflan HPLC 0.019 --

Pebulate Tillam GC/MS 0.004 --

Pendimethalin Prowl, Stomp GC/MS 0.004 --

Picloram Tordon HPLC 0.050 1500

Prometon Pramitol GC/MS 0.018 100

Pronamide Kerb GC/MS 0.003 50

Propachlor Ramrod GC/MS 0.007 90

Propanil Stampede GC/MS 0.004 --

Propham IPC HPLC 0.035 100

Simazine Princep, Aquazine GC/MS 0.005 14

Tebuthiuron Spike, Graslan GC/MS 0.010 500

Terbacil Sinbar GC/MS 0.007 90

Thiobencarb Bolero GC/MS 0.002 --

Triallate Far-Go GC/MS 0.001 --

Triclopyr Garlon HPLC 0.050 --

Trifluralin Treflan and others GC/MS 0.002 5

Insecticides

1-Naphthol Degradate of carbaryl HPLC 0.007 --

3-OH-carbofuran Degradate of carbofuran HPLC 0.014 --

Aldicarb Temik HPLC 0.016 13

Aldicarb sulfone Degradate of aldicarb HPLC 0.016 12

Aldicarb sulfoxide Degradate of aldicarb HPLC 0.021 14

Azinphos-methyl Guthion GC/MS 0.001 --

Carbaryl Sevin GC/MS 0.003 700

Carbofuran Furadan GC/MS 0.003 140

Chlorothalonil Bravo HPLC 0.035 --

Chlorpyrifos Dursban, Lorsban GS/MS 0.004 20

p,p’–DDE Degradate of DDT GC/MS 0.006 --

Diazinon Diazinon and others GC/MS 0.002 0.6

Dieldrin Panoram D-31 GCMS 0.001 --

Disulfoton Disyston and others GC/MS 0.017 0.3

Esfenvalerate Asana HPLC 0.019 --

Appendix 1. Herbicide and insecticide target analytes, method detection limits, and drinking-water standards

—Continued

 [Concentrations in micrograms per liter, MCL, maximum contaminant level; HA, health advisory; HPLC, high performance 

liquid chromatography, GC/MS, gas chromatography/mass spectrometry; --, no regulation or guideline]

Compound Trade name(s) Analysis 

method

Method 

detection 

limit

(μg/L)

Regulation or 

guideline for 

drinking 

water

(MCL or HA)

(μg/L)
44 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Ethoprop Mocap GC/MS 0.003 --

Insecticides—Continued

Fonofos Dyfonate GC/MS 0.003 10

alpha-HCH Lindane (impurity) GC/MS 0.002 --

gamma-HCH Lindane GC/MS 0.011 10.2

Malathion Malathion and others GC/MS 0.005 200

Methiocarb Mesurol HPLC 0.026 --

Methomyl Lannate and others HPLC 0.017 200

Methyl-parathion Penncap-M GC/MS 0.006 2

Oxamyl Vydate HPLC 0.018 1200

Parathion Alkron GC/MS 0.004 --

cis-Permethrin Ambush, Pounce GC/MS 0.005 --

Phorate Thinet and others GC/MS 0.002 --

Propargite Comite, Omite GS/MS 0.013 --

Propoxur Baygon HPLC 0.035 3

Terbufos Counter GC/MS 0.013 0.9

1U.S. Environmetal Protection Agency (USEPA) maximum contaminant level or health advisory level for drinking water (Nowell 

and Resek, 1994).

Appendix 1. Herbicide and insecticide target analytes, method detection limits, and drinking-water standards

—Continued

 [Concentrations in micrograms per liter, MCL, maximum contaminant level; HA, health advisory; HPLC, high performance 

liquid chromatography, GC/MS, gas chromatography/mass spectrometry; --, no regulation or guideline]

Compound Trade name(s) Analysis 

method

Method 

detection 

limit

(μg/L)

Regulation or 

guideline for 

drinking 

water

(MCL or HA)

(μg/L)
 45

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum

Wapsipinicon River near Tripoli, Iowa (site 1; fig. 1, table 2)

2,6-Diethylaniline 24 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 24 86.4 0.0 0.15 .022 <.002 1.9

Acetochlor ESA 22 72.7 -- 1.2 .87 <.20 3.6

Acetochlor OA 22 54.5 -- .50 .24 <.20 2.4

Alachlor 24 90.9 0.0 .044 .025 <.002 .231

Alachlor ESA 22 100 -- 2.79 3.0 .35 4.67

Alachlor OA 22 68.2 -- .30 .27 <.20 .78

alpha-HCH 24 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 24 100 9.1 .83 .15 .078 8.6

Azinphos-methyl 24 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 24 0.0 -- <.002 <.002 <.002 <.002

Butylate 24 0.0 0.0 <.001 <.002 <.002 <.002

Carbaryl 24 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 24 22.7 0.0 .04 <.003 <.003 .35

Chlorpyrifos 24 9.1 0.0 <.004 <.004 <.004 .022

cis-Permethrin 24 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 24 81.8 0.0 .076 .015 <.004 .653

Cyanazine-amide 23 21 -- .10 <.05 <.05 .73

Dacthal 24 0.0 -- <.002 <.002 <.002 <.002

Deethylatrazine 23 100 -- .24 .18 .10 .93

Deisopropylatrazine 21 78.2 -- .09 .07 <.05 .50

Diazinon 24 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 24 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 24 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 24 22.7 -- .005 <.002 <.002 .088

Ethalfluralin 24 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 24 0.0 -- <.003 <.003 <.003 <.003

Fonofos 24 0.0 0.0 <.003 <.003 <.003 <.003

Hydroxyatrazine 22 31.8 -- .30 <.20 <.20 .75

Lindane 24 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 24 0.0 -- <.002 <.002 <.002 <.002

Malathion 24 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 24 100 0.0 .77 .22 .050 7.6

Metolachlor ESA 22 100 -- 5.2 4.7 1.6 12

Metolachlor OA 22 95.4 -- 1.3 1.1 <.20 3.2

Metribuzin 23 14.3 0.0 .004 <.004 <.004 .029

Molinate 24 0.0 -- <.004 <.004 <.004 <.004

Napropamide 24 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 24 0.0 0.0 <.006 <.006 <.006 <.006
46 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Parathion 24 0.0 -- <.004 <.004 <.004 <.004

Wapsipinicon River near Tripoli, Iowa (site 1; fig. 1, table 2)—Continued

Parathion-methyl 24 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 24 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 24 9.1 -- <.004 <.004 <.004 .038

Phorate 24 0.0 -- <.002 <.002 <.002 <.002

*Prometon 24 81.8 0.0 .012 <.009 <.009 .058

Propachlor 24 0.0 0.0 <.007 <.007 <.007 <.007

Propanil 24 0.0 -- <.004 <.004 <.004 <.004

Propargite 24 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 24 4.5 0.0 .004 <.003 <.003 .064

Simazine 24 40.9 0.0 .006 <.005 <.005 .032

Tebuthiuron 24 22.7 0.0 <.010 <.010 <.010 .016

Terbacil 24 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 24 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 24 0.0 -- <.002 <.002 <.002 <.002

Triallate 24 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 24 4.5 0.0 <.002 <.002 <.002 .005

Wapsipinicon River near DeWitt, Iowa (site 2, fig. 1, table 2)

2,6-Diethylaniline 22 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 22 86.4 0.0 .422 .031 <.002 2.3

Acetochlor ESA 22 72.7 -- 1.3 .98 <.20 3.9

Acetochlor OA 21 47.6 -- .88 <.20 <.20 4.9

Alachlor 22 86.4 0.0 .035 .016 <.002 .14

Alachlor ESA 22 100 -- 2.2 2.3 .62 3.6

Alachlor OA 21 23.8 -- .23 <.20 <.20 .46

alpha-HCH 22 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 22 100 22.7 1.7 .19 .11 8.2

Azinphos-methyl 22 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 22 0.0 -- <.002 <.002 <.002 <.002

Butylate 22 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 22 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 22 22.7 0.0 .063 <.003 <.003 .47

Chlorpyrifos 22 9.1 0.0 <.004 <.004 <.004 .029

cis-Permethrin 22 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 22 90.9 0.0 .178 .020 <.004 .96

Cyanazine-amide 20 35 -- .20 <.05 <.05 .79

Dacthal 22 4.5 -- <.002 <.002 <.002 <.002

Deethylatrazine 21 100 -- .34 .26 .11 .88

Deisopropylatrazine 21 66.7 -- .14 .08 <.05 .53

Diazinon 22 0.0 0.0 <.002 <.002 <.002 <.002

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 47

Dieldrin 22 0.0 0.0 <.001 <.001 <.001 <.001

Wapsipinicon River near DeWitt, Iowa (site 2, fig. 1, table 2)—Continued

Disulfoton 22 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 22 18.2 -- .004 <.002 <.002 .064

Ethalfluralin 22 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 22 0.0 -- <.003 <.003 <.003 <.003

Fonofos 22 9.1 0.0 <.003 <.003 <.003 .016

Hydroxy atrazine 21 66.7 -- .37 .30 <.20 .92

Lindane 22 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 22 0.0 -- <.002 <.002 <.002 <.002

Malathion 22 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 22 100 0.0 1.02 .297 .028 6.0

Metolachlor ESA 22 100 -- 4.0 4.3 1.9 6.1

Metolachlor OA 21 95.2 -- 1.1 .80 <.20 3.5

Metribuzin 22 9.1 0.0 <.004 <.004 <.004 0.02

Molinate 22 0.0 -- <.004 <.004 <.004 <.004

Napropamide 22 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 22 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 22 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 22 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 22 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 22 0.0 -- <.004 <.004 <.004 <.004

Phorate 22 0.0 -- <.002 <.002 <.002 <.002

*Prometon 22 81.8 0.0 .009 .009 <.009 .015

Propachlor 22 9.1 0.0 <.007 <.007 <.007 .015

Propanil 22 0.0 -- <.004 <.004 <.004 <.004

Propargite 22 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 22 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 22 40.9 0.0 .008 <.005 <.005 .027

Tebuthiuron 22 22.7 0.0 <.010 <.010 <.010 .021

Terbacil 22 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 22 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 22 0.0 -- <.002 <.002 <.002 <.002

Triallate 22 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 22 4.5 0.0 <.002 <.002 <.002 <.002

Iowa River near Rowan, Iowa (site 3, fig. 1, table 2)

2,6-Diethylaniline 52 5.9 -- <.003 <.003 <.003 <.003

Acetochlor 52 84.3 0.0 .074 .014 <.002 .921

Acetochlor ESA 50 70 -- .43 .28 <.20 1.5

Acetochlor OA 50 24 -- .29 <.20 <.20 1.3

Alachlor 52 78.4 0.0 .023 .006 <.002 .559

Alachlor ESA 50 100 -- 1.5 1.4 .46 2.8

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
48 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Alachlor OA 50 14 -- .22 <.20 <.20 .67

Iowa River near Rowan, Iowa(site 3, fig. 1, table 2)—Continued

alpha-HCH 52 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 52 100 5.9 .426 .069 .023 6.7

Azinphos-methyl 52 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 52 0.0 -- <.002 <.002 <.002 <.002

Butylate 52 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 52 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 52 9.8 0.0 .010 <.003 <.003 .30

Chlorpyrifos 52 2.0 0.0 <.004 <.004 <.004 <.004

cis-Permethrin 52 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 52 41.2 0.0 .015 <.004 <.004 .26

Cyanazine-amide 49 8.1 -- .05 <.05 <.05 .17

Dacthal 52 3.9 -- <.002 <.002 <.002 .007

Deethylatrazine 47 68.1 -- .106 .07 .025 .82

Deisopropylatrazine 47 38.3 -- .05 <.05 <.05 .39

Diazinon 52 2.0 0.0 <.002 <.002 <.002 .004

Dieldrin 52 2.0 0.0 <.001 <.001 <.001 .011

Disulfoton 52 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 52 13.7 -- <.002 <.002 <.002 .049

Ethalfluralin 52 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 52 0.0 -- <.003 <.003 <.003 <.003

Fonofos 52 0.0 0.0 <.003 <.003 <.003 <.003

Hydroxyatrazine 50 34 -- .31 <.20 <.20 1.7

Lindane 52 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 52 0.0 -- <.002 <.002 <.002 <.002

Malathion 52 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 52 100 0.0 .76 .20 .036 11.6

Metolachlor ESA 50 100 -- 5.0 5.0 2.2 10

Metolachlor OA 50 100 -- 1.0 .91 .41 3.0

Metribuzin 52 17.6 0.0 <.004 <.004 <.004 .026

Molinate 52 0.0 -- <.004 <.004 <.004 <.004

Napropamide 52 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 52 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 52 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 52 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 52 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 52 2.0 -- <.004 <.004 <.004 .103

Phorate 52 0.0 -- <.002 <.002 <.002 <.002

*Prometon 52 98.0 0.0 .013 .010 <.009 .063

Propachlor 52 3.9 0.0 <.007 <.007 <.007 .007

Propanil 52 0.0 -- <.004 <.004 <.004 <.004

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 49

Propargite 52 0.0 -- <.013 <.013 <.013 <.013

Iowa River near Rowan, Iowa(site 3, fig. 1, table 2)—Continued

Propyzamide 52 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 52 15.7 0.0 <.005 <.005 <.005 .016

Tebuthiuron 52 0.0 0.0 <.010 <.010 <.010 <.010

Terbacil 52 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 52 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 52 0.0 -- <.002 <.002 <.002 <.002

Triallate 52 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 52 31.4 0.0 <.002 <.002 <.002 .038

South Fork Iowa River near New Providence, Iowa (site 4, fig. 1, table 2)

2,6-Diethylaniline 22 4.5 -- .001 <.003 .001 <.003

Acetochlor 22 86.4 0.0 .133 .036 <.002 .915

Acetochlor ESA 21 66.7 -- .70 .35 <.20 1.9

Acetochlor OA 21 38.1 -- .47 <.20 <.20 2.6

Alachlor 22 72.7 0.0 .021 .007 <.002 .28

Alachlor ESA 21 100 -- .78 .77 .22 1.3

Alachlor OA 21 4.7 -- .20 <.20 <.20 .27

alpha-HCH 22 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 22 100 9.1 1.92 .186 .072 20

Azinphos-methyl 22 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 22 0.0 -- <.002 <.002 <.002 <.002

Butylate 22 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 22 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 22 18.2 0.0 .084 <.003 <.003 1.5

Chlorpyrifos 22 4.5 0.0 <.004 <.004 <.004 .013

cis-Permethrin 22 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 22 59.1 0.0 .014 0.005 <.004 0.10

Cyanazine-amide 20 5 -- .05 <.05 <.05 .09

Dacthal 22 4.5 -- <.002 <.002 <.002 .003

Deethylatrazine 19 100 -- .228 0.140 0.06 .99

Deisopropylatrazine 19 31.6 -- .08 <.05 <.05 .41

Diazinon 22 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 22 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 22 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 22 18.2 -- .002 <.002 <.002 .016

Ethalfluralin 22 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 22 0.0 -- <.003 <.003 <.003 <.003

Fonofos 22 0.0 0.0 <.003 <.003 <.003 <.003

Hydroxyatrazine 21 19 -- .29 <.20 <.20 1.3

Lindane 22 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 22 0.0 -- <.002 <.002 <.002 <.002

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
50 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Malathion 22 0.0 0.0 <.005 <.005 <.005 <.005

South Fork Iowa River near New Providence, Iowa (site 4, fig. 1, table 2)—Continued

Metolachlor 22 100 0.0 1.4 0.50 .079 11

Metolachlor ESA 21 100 -- 5.3 5.1 1.8 9.5

Metolachlor OA 21 100 -- 1.3 .96 .31 5.3

Metribuzin 22 13.6 0.0 .004 <.004 <.004 .019

Molinate 22 0.0 -- <.004 <.004 <.004 <.004

Napropamide 22 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 22 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 22 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 22 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 22 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 22 9.1 -- .004 <.004 <.004 .038

Phorate 22 0.0 -- <.002 <.002 <.002 <.002

*Prometon 22 77.3 0.0 .016 <.009 <.009 .099

Propachlor 22 0.0 0.0 <.007 <.007 <.007 <.007

Propanil 22 0.0 -- <.004 <.004 <.004 <.004

Propargite 22 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 22 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 22 50.0 0.0 .052 <.005 <.005 .691

Tebuthiuron 22 13.6 0.0 <.010 <.010 <.010 <.010

Terbacil 22 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 22 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 22 0.0 -- <.002 <.002 <.002 <.002

Triallate 22 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 22 22.7 0.0 .002 <.002 <.002 .015

Iowa River at Marengo, Iowa (site 5, fig. 1, table 2)

2,6-Diethylaniline 23 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 23 87.0 0.0 .190 .036 <.002 1.5

Acetochlor ESA 23 69.6 -- .56 .41 .14 1.6

Acetochlor OA 23 39.1 -- .39 <.20 <.20 1.4

Alachlor 23 73.9 0.0 .013 .005 <.002 .053

Alachlor ESA 23 100 -- .97 .31 .31 1.9

Alachlor OA 23 26.1 -- .21 <.20 <.20 .32

alpha-HCH 23 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 23 100 13.0 1.05 .156 .073 7.61

Azinphos-methyl 23 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 23 0.0 -- <.002 <.002 <.002 <.002

Butylate 23 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 23 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 23 13.0 0.0 .027 <.003 <.003 0.445

Chlorpyrifos 23 8.7 0.0 .004 <.004 <.004 .029

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 51

cis-Permethrin 23 0.0 -- <.005 <.005 <.005 <.005

Iowa River at Marengo, Iowa (site 5, fig. 1, table 2)—Continued

Cyanazine 23 95.7 4.3 .350 0.024 <.004 6.3

Cyanazine-amide 21 42.8 -- .15 <.05 <.05 .97

Dacthal 23 0.0 -- <.002 <.002 <.002 <.002

Deethylatrazine 21 100 -- .211 .13 .05 .88

Deisopropylatrazine 21 66.7 -- .10 .07 <.05 .41

Diazinon 23 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 23 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 23 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 23 8.7 -- <.002 <.002 <.002 .007

Ethalfluralin 23 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 23 0.0 -- <.003 <.003 <.003 <.003

Fonofos 23 8.7 0.0 <.003 <.003 <.003 .016

Hydroxy atrazine 23 39.1 -- .33 <.20 <.20 1.2

Lindane 23 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 23 0.0 -- <.002 <.002 <.002 <.002

Malathion 23 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 23 100 0.0 .847 .358 .046 3.59

Metolachlor ESA 23 100 -- 3.7 3.8 2.0 5.6

Metolachlor OA 23 -- .83 .69 <.20 1.9

Metribuzin 23 26.1 0.0 .006 <.004 <.004 .036

Molinate 23 0.0 -- <.004 <.004 <.004 <.004

Napropamide 23 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 23 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 23 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 23 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 23 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 23 8.7 -- .005 <.004 <.004 .039

Phorate 23 0.0 -- <.002 <.002 <.002 <.002

*Prometon 23 87.0 0.0 .013 .010 <.009 .044

Propachlor 23 4.3 0.0 <.007 <.007 <.007 .007

Propanil 23 0.0 -- <.004 <.004 <.004 <.004

Propargite 23 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 23 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 23 47.8 0.0 .009 <.005 <.005 .054

Tebuthiuron 23 0.0 0.0 <.010 <.010 <.010 <.010

Terbacil 23 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 23 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 23 0.0 -- <.002 <.002 <.002 <.002

Triallate 23 0.0 -- <.001 <.001 <.001 <.001

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
52 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Trifluralin 23 13.0 0.0 <.002 <.002 <.002 .008

Old Mans Creek nr Iowa City, Iowa (site 6, fig. 1, table 2)

2,6-Diethylaniline 26 3.8 -- <.003 <.003 <.003 .003

Acetochlor 26 88.5 0.0 .549 .028 <.002 5.7

Acetochlor ESA 26 80.8 -- .77 .54 <.20 2.0

Acetochlor OA 26 46.2 -- .47 <.20 <.20 2.1

Alachlor 26 57.7 0.0 .021 .005 <.002 .296

Alachlor ESA 26 96.2 -- 1.1 1.1 <.20 1.9

Alachlor OA 26 23.1 -- .23 <.20 <.20 .51

alpha-HCH 26 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 26 100 19.2 1.9 0.37 .078 20

Azinphos-methyl 26 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 26 0.0 -- <.002 <.002 <.002 <.002

Butylate 26 7.7 0.0 .002 <.002 <.002 .018

Carbaryl 26 3.8 0.0 .004 <.003 <.003 .057

Carbofuran 26 15.4 0.0 .008 <.003 <.003 .064

Chlorpyrifos 26 26.9 0.0 .007 <.004 <.004 .033

cis-Permethrin 26 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 26 96.2 11.5 .91 .102 <.004 9.9

Cyanazine-amide 25 68.0 -- .32 .14 <.05 2.8

Dacthal 26 3.8 -- <.002 <.002 <.002 .002

Deethylatrazine 25 92.0 -- .223 .16 <.05 .76

Deisopropylatrazine 25 80 -- .15 .13 <.05 .53

Diazinon 26 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 26 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 26 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 26 26.9 -- .004 <.002 <.002 .026

Ethalfluralin 26 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 26 0.0 -- <.003 <.003 <.003 <.003

Fonofos 26 26.9 0.0 .026 <.003 <.003 .42

Hydroxyatrazine 26 69.2 -- .43 .30 <.20 1.2

Lindane 26 3.8 0.0 <.004 <.004 <.004 .018

Linuron 26 0.0 -- <.002 <.002 <.002 <.002

Malathion 26 3.8 0.0 .005 <.005 <.005 .078

Metolachlor 26 100 0.0 .972 .114 .031 10

Metolachlor ESA 26 100 -- 1.9 1.7 .77 3.2

Metolachlor OA 26 88.5 -- .57 .49 <.20 1.4

Metribuzin 26 19.2 0.0 .015 <.004 <.004 .15

Molinate 26 0.0 -- <.004 <.004 <.004 <.004

Napropamide 26 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 26 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 26 0.0 -- <.004 <.004 <.004 <.004

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 53

Parathion-methyl 26 0.0 0.0 <.006 <.006 <.006 <.006

Old Mans Creek nr Iowa City, Iowa (site 6, fig. 1, table 2)—Continued

Pebulate 26 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 26 3.8 -- .002 <.004 <.004 .006

Phorate 26 0.0 -- <.002 <.002 <.002 <.002

*Prometon 26 26.9 0.0 <.009 <.009 <.009 .019

Propachlor 26 3.8 0.0 <.007 <.007 <.007 .004

Propanil 26 0.0 -- <.004 <.004 <.004 <.004

Propargite 26 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 26 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 26 19.2 0.0 .009 <.005 <.005 .088

Tebuthiuron 26 0.0 0.0 <.010 <.010 <.010 <.010

Terbacil 26 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 26 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 26 0.0 -- <.002 <.002 <.002 <.002

Triallate 26 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 26 23.1 0.0 .004 <.002 <.002 .007

Flood Creek near Powersville, Iowa (site 7, fig. 1, table 2)

2,6-Diethylaniline 21 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 21 57.1 0.0 .162 .005 <.002 2.56

Acetochlor ESA 18 55.6 -- .86 .40 <.20 2.1

Acetochlor OA 17 23.5 -- .45 <.20 <.20 1.7

Alachlor 21 38.1 0.0 .017 <.002 <.002 0.14

Alachlor ESA 18 100 -- 1.9 1.7 .91 3.3

Alachlor OA 17 5.9 -- .22 <.20 <.20 .45

alpha-HCH 21 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 21 100 0.0 .350 .152 .037 2.0

Azinphos-methyl 21 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 21 0.0 -- <.002 <.002 <.002 <.002

Butylate 21 4.8 0.0 <.002 <.002 <.002 .002

Carbaryl 21 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 21 14.3 0.0 .025 <.003 <.003 .440

Chlorpyrifos 21 4.8 0.0 <.004 <.004 <.004 .004

cis-Permethrin 21 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 21 42.9 0.0 .016 <.004 <.004 .11

Cyanazine-amide 18 22.2 -- .06 <.05 <.05 .19

Dacthal 21 9.5 -- <.002 <.002 <.002 .002

Deethylatrazine 16 100 -- .209 .155 .07 .53

Deisopropylatrazine 16 37.5 -- .05 <.05 <.05 .17

Diazinon 21 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 21 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 21 0.0 0.0 <.017 <.017 <.017 <.017

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
54 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

EPTC 21 19.0 -- <.002 <.002 <.002 .007

Flood Creek near Powersville, Iowa (site 7, fig. 1, table 2)—Continued

Ethalfluralin 21 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 21 0.0 -- <.003 <.003 <.003 <.003

Fonofos 21 0.0 0.0 <.003 <.003 <.003 <.003

Hydroxyatrazine 17 5.9 -- .24 <.20 <.20 .90

Lindane 21 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 21 0.0 -- <.002 <.002 <.002 <.002

Malathion 21 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 21 100 0.0 .618 .096 .004 3.56

Metolachlor ESA 18 100 -- 6.1 4.7 1.7 12

Metolachlor OA 17 58.8 -- 1.1 .35 <.20 3.3

Metribuzin 20 10.0 0.0 <.004 <.004 <.004 .007

Molinate 21 0.0 -- <.004 <.004 <.004 <.004

Napropamide 21 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 21 4.8 0.0 <.006 <.006 <.006 .006

Parathion 21 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 21 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 21 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 21 0.0 -- <.004 <.004 <.004 <.004

Phorate 21 0.0 -- <.002 <.002 <.002 <.002

*Prometon 21 57.1 0.0 <.009 <.009 <.009 .010

Propachlor 21 0.0 0.0 <.007 <.007 <.007 <.007

Propanil 21 0.0 -- <.004 <.004 <.004 <.004

Propargite 21 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 21 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 21 23.8 0.0 <.005 <.005 <.005 .017

Tebuthiuron 21 0.0 0.0 <.010 <.010 <.010 <.010

Terbacil 21 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 21 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 21 0.0 -- <.002 <.002 <.002 <.002

Triallate 21 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 21 23.8 0.0 .002 <.002 <.002 .010

Cedar River at Gilbertville, Iowa (site 8, fig. 1, table 2)

2,6-Diethylaniline 12 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 12 66.7 0.0 .160 .012 <.002 1.66

Acetochlor ESA 11 27.3 -- .38 <.20 <.20 1.7

Acetochlor OA 11 18.2 -- .30 <.20 <.20 1.3

Alachlor 12 50.0 0.0 .006 .002 <.002 .03

Alachlor ESA 11 90.9 -- 1.8 2.0 <.20 2.6

Alachlor OA 11 27.3 -- .22 <.20 <.20 .32

alpha-HCH 12 0.0 0.0 <.002 <.002 <.002 <.002

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 55

Atrazine 12 100 0.0 .232 .096 .061 1.5

Cedar River at Gilbertville, Iowa (site 8, fig. 1, table 2—Continued

Azinphos-methyl 12 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 12 0.0 -- <.002 <.002 <.002 <.002

Butylate 12 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 12 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 12 8.3 0.0 .018 <.003 <.003 .20

Chlorpyrifos 12 8.3 0.0 <.004 <.004 <.004 .009

cis-Permethrin 12 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 12 66.7 0.0 .029 .007 <.004 .21

Cyanazine-amide 11 27.3 -- .07 <.05 <.05 .14

Dacthal 12 8.3 -- <.002 <.002 <.002 .002

Deethylatrazine 12 100 -- 0.14 .13 .07 .35

Deisopropylatrazine 12 66.7 -- .06 .05 <.05 .14

Diazinon 12 16.7 0.0 .002 <.002 <.002 .008

Dieldrin 12 0.0 0.0 <.001 <.001 <.001 <.001

Disulfoton 12 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 12 16.7 -- <.002 <.002 <.002 .007

Ethalfluralin 12 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 12 0.0 -- <.003 <.003 <.003 <.003

Fonofos 12 0.0 0.0 <.003 <.003 <.003 <.003

Hydroxyatrazine 11 27.3 -- .23 <.20 <.20 .38

Lindane 12 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 12 0.0 -- <.002 <.002 <.002 <.002

Malathion 12 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 12 100 0.0 .278 .176 .035 1.0

Metolachlor ESA 11 90.9 -- 3.2 3.4 <.20 5.4

Metolachlor OA 11 81.8 -- .62 .51 <.20 1.5

Metribuzin 12 8.3 0.0 <.004 <.004 <.004 0.018

Molinate 12 0.0 -- <.004 <.004 <.004 <.004

Napropamide 12 0.0 -- <.003 <.003 <.003 <.003

p,p'-DDE 12 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 12 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 12 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 12 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 12 0.0 -- <.004 <.004 <.004 <.004

Phorate 12 0.0 -- <.002 <.002 <.002 <.002

*Prometon 12 100 0.0 .013 .011 <.009 .025

Propachlor 12 0.0 0.0 <.007 <.007 <.007 <.007

Propanil 12 0.0 -- <.004 <.004 <.004 <.004

Propargite 12 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 12 0.0 0.0 <.003 <.003 <.003 <.003

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
56 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Simazine 12 25.0 0.0 <.005 <.005 <.005 .014

Cedar River at Gilbertville, Iowa (site 8, fig. 1, table 2—Continued

Tebuthiuron 12 25.0 0.0 <.010 <.010 <.010 .010

Terbacil 12 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 12 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 12 0.0 -- <.002 <.002 <.002 <.002

Triallate 12 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 12 0.0 0.0 <.002 <.002 <.002 <.002

Wolf Creek near Dysart, Iowa (site 9, fig. 1, table 2)

2,6-Diethylaniline 53 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 53 73.1 0.0 .117 .011 <.002 1.25

Acetochlor ESA 51 64.7 -- .57 .29 <.20 5.1

Acetochlor OA 51 21.6 -- .46 <.20 <.20 6.8

Alachlor 53 84.6 0.0 .019 .006 <.002 .26

Alachlor ESA 51 98.0 -- 1.2 1.2 <.20 2.8

Alachlor OA 51 15.7 -- .21 <.20 <.20 .51

alpha-HCH 53 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 53 100 11.5 1.18 .18 .048 20

Azinphos-methyl 53 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 53 0.0 -- <.002 <.002 <.002 <.002

Butylate 53 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 53 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 53 13.5 0.0 .031 <.003 <.003 .79

Chlorpyrifos 53 7.7 0.0 <.004 <.004 <.004 .067

cis-Permethrin 53 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 53 94.2 5.8 .139 .028 <.004 1.61

Cyanazine-amide 51 47.1 -- .15 <.05 <.05 1.4

Dacthal 53 1.9 -- <.002 <.002 <.002 .002

Deethylatrazine 46 97.8 -- .208 .140 <.002 .98

Deisopropylatrazine 46 84.8 -- .13 .10 <.05 .52

Diazinon 53 0.0 0.0 <.002 <.002 <.002 <.002

Dieldrin 53 5.8 0.0 <.001 <.001 <.001 .004

Disulfoton 53 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 53 21.2 -- <.002 <.002 <.002 .015

Ethalfluralin 53 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 53 0.0 -- <.003 <.003 <.003 <.003

Fonofos 53 1.9 0.0 <.003 <.003 <.003 .007

Hydroxyatrazine 51 51.0 -- .33 .20 <.20 1.1

Lindane 53 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 53 0.0 -- <.002 <.002 <.002 <.002

Malathion 53 1.9 0.0 <.005 <.005 <.005 .038

Metolachlor 53 100 0.0 .581 .15 .027 8.72

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 57

Metolachlor ESA 51 100 -- 4.6 4.5 2.3 8.1

Wolf Creek near Dysart, Iowa (site 9, fig. 1, table 2)—Continued

Metolachlor OA 51 96.1 -- .83 .53 <.20 6.8

Metribuzin 53 15.4 0.0 .012 <.004 <.004 .169

Molinate 53 1.9 -- <.004 <.004 <.004 .023

Napropamide 53 1.9 -- <.003 <.003 <.003 .004

p,p’-DDE 53 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 53 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 53 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 53 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 53 13.5 -- .008 <.004 <.004 .111

Phorate 53 0.0 -- <.002 <.002 <.002 <.002

*Prometon 53 80.8 0.0 <.009 <.009 <.009 .046

Propachlor 53 1.9 0.0 <.007 <.007 <.007 .006

Propanil 53 0.0 -- <.004 <.004 <.004 <.004

Propargite 53 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 53 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 53 53.8 0.0 <.005 <.005 <.0031 .046

Tebuthiuron 53 3.8 0.0 <.010 <.010 <.010 .006

Terbacil 53 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 53 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 53 0.0 -- <.002 <.002 <.002 <.002

Triallate 53 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 53 32.7 0.0 <.002 <.002 <.002 .030

Cedar River near Conesville, Iowa1 (site 10 fig. 1, table 2)

2,6-Diethylaniline 23 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 23 81.8 0.0 .408 .016 <.002 7.1

Acetochlor ESA 22 72.7 -- .68 .36 <.20 2.7

Acetochlor OA 22 27.2 -- .40 <.20 <.20 2.5

Alachlor 23 68.2 0.0 .020 .005 <.002 .24

Alachlor ESA 22 100 -- 1.6 1.5 .73 3.2

Alachlor OA 22 0.0 -- <.20 <.20 <.20 <.20

alpha-HCH 23 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 23 100 4.5 1.20 .140 .085 20

Azinphos-methyl 23 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 23 0.0 -- <.002 <.002 <.002 <.002

Butylate 23 0.0 0.0 <.002 <.002 <.002 <.002

Carbaryl 23 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 23 9.1 0.0 <.007 <.003 <.003 .029

Chlorpyrifos 23 0.0 0.0 <.004 <.004 <.004 <.004

cis-Permethrin 23 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 23 81.8 4.5 .300 .012 <.004 6.3

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
58 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Cyanazine-amide 22 32.7 -- .09 <.05 <.05 .55

Cedar River near Conesville, Iowa1 (site 10 fig. 1, table 2)—Continued

Dacthal 23 9.1 -- <.002 <.002 <.002 .004

Deethylatrazine 20 100 -- .191 .120 .100 .58

Deisopropylatrazine 20 95.0 -- .10 .08 <.05 .30

Diazinon 23 13.6 0.0 <.002 <.002 <.002 .018

Dieldrin 23 0.0 0.0 <.001 <.001 <.001 .01

Disulfoton 23 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 23 4.5 -- .005 <.002 <.002 .078

Ethalfluralin 23 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 23 0.0 -- <.003 <.003 <.003 <.003

Fonofos 23 4.5 0.0 .005 <.003 <.003 0.081

Hydroxyatrazine 22 36.4 -- .27 <.20 <.20 .79

Lindane 23 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 23 0.0 -- <.002 <.002 <.002 <.002

Malathion 23 4.5 0.0 <.005 <.005 <.005 0.023

Metolachlor 23 100 0.0 .84 .29 .040 10

Metolachlor ESA 22 100 -- 3.5 3.2 1.6 8.5

Metolachlor OA 22 100 -- .76 .62 .33 3.2

Metribuzin 23 9.1 0.0 .004 <.004 <.004 0.052

Molinate 23 0.0 -- <.004 <.004 <.004 <.004

Napropamide 23 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 23 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 23 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 23 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 23 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 23 0.0 -- <.004 <.004 <.004 <.004

Phorate 23 0.0 -- <.002 <.002 <.002 <.002

*Prometon 23 95.5 0.0 .012 .011 <.009 .078

Propachlor 23 4.5 0.0 <.007 <.007 <.007 .007

Propanil 23 0.0 -- <.004 <.004 <.004 <.004

Propargite 23 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 23 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 23 27.3 0.0 .011 <.005 <.005 .16

Tebuthiuron 23 4.5 0.0 <.010 <.010 <.010 .010

Terbacil 23 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 23 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 23 0.0 -- <.002 <.002 <.002 <.002

Triallate 23 0.0 -- <.001 <.001 <.001 <.001

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 59

Trifluralin 23 9.1 0.0 <.002 <.002 <.002 .009

Iowa River at Wapello, Iowa (site 11, fig. 1, table 2)

2,6-Diethylaniline 54 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 54 78.8 0.0 .163 .017 <.002 2.14

Acetochlor ESA 52 90.4 -- .68 .50 <.20 2.6

Acetochlor OA 50 48.0 -- .41 <.20 <.20 2.1

Alachlor 54 73.1 0.0 .018 .006 <.002 .235

Alachlor ESA 52 100 -- 1.2 1.1 .58 2.6

Alachlor OA 50 0.0 -- <.20 <.20 <.20 <.20

alpha-HCH 54 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 54 100 9.6 .858 .160 .073 6.32

Azinphos-methyl 54 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 54 0.0 -- <.002 <.002 <.002 <.002

Butylate 54 7.7 0.0 <.002 <.002 <.002 .022

Carbaryl 54 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 54 9.6 0.0 .005 <.003 <.003 .094

Chlorpyrifos 54 5.8 0.0 <.004 <.004 <.0029 .048

cis-Permethrin 54 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 54 86.5 3.8 .169 0.022 <.004 2.6

Cyanazine-amide 52 32.7 -- .18 <.05 <.05 1.9

Dacthal 54 7.7 -- <.002 <.002 <.002 .003

Deethylatrazine 49 98.0 -- .224 .150 <.002 .80

Deisopropylatrazine 49 85.7 -- .12 .08 <.05 .46

Diazinon 54 9.6 0.0 .002 <.002 <.002 .057

Dieldrin 54 3.8 0.0 <.001 <.001 <.001 .004

Disulfoton 54 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 54 17.3 -- <.002 <.002 <.002 0.014

Ethalfluralin 54 0.0 -- <.004 <.004 <.004 <.004

Ethoprophos 54 0.0 -- <.003 <.003 <.003 <.003

Fonofos 54 3.8 0.0 <.003 <.003 <.003 .021

Hydroxyatrazine 50 64.0 -- .38 .27 <.20 1.2

Lindane 54 0.0 0.0 <.004 <.004 <.004 <.004

Linuron 54 0.0 -- <.002 <.002 <.002 <.002

Malathion 54 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 54 100 0.0 0.65 0.28 0.044 6.1

Metolachlor ESA 52 100 -- 3.2 3.0 1.6 7.2

Metolachlor OA 50 100 -- .76 .70 .29 2.5

Metribuzin 54 13.5 0.0 .005 <.004 <.004 .055

Molinate 54 1.9 -- <.004 <.004 <.004 .004

Napropamide 54 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 54 1.9 0.0 <.006 <.006 <.006 .006

Parathion 54 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 54 0.0 0.0 <.006 <.006 <.006 <.006

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
60 Water-Quality Assessment of the Eastern Iowa Basins: Selected Pesticides and Pesticide Degradates in Streams, 1996–98

Pebulate 54 0.0 -- <.004 <.004 <.004 <.004

Iowa River at Wapello, Iowa (site 11, fig. 1, table 2)—Continued

Pendimethalin 54 7.7 -- .004 <.004 <.004 .085

Phorate 54 0.0 -- <.002 <.002 <.002 <.002

*Prometon 54 94.2 0.0 .011 <.009 <.09 .082

Propachlor 54 3.8 0.0 <.007 <.007 <.007 .007

Propanil 54 0.0 -- <.004 <.004 <.004 <.004

Propargite 54 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 54 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 54 65.4 0.0 .009 .006 <.005 .059

Tebuthiuron 54 0.0 0.0 <.010 <.010 <.010 <.010

Terbacil 54 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 54 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 54 0.0 -- <.002 <.002 <.002 <.002

Triallate 54 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 54 11.5 0.0 <.002 <.002 <.002 0.007

Skunk River at Augusta, Iowa (site 12, fig. 1, table 2)

2,6-Diethylaniline 21 0.0 -- <.003 <.003 <.003 <.003

Acetochlor 21 95.2 0.0 .877 .040 <.002 10.6

Acetochlor ESA 20 80.0 -- .72 .56 <.20 2.7

Acetochlor OA 19 47.4 -- .72 <.20 <.20 4.0

Alachlor 21 61.9 0.0 .042 .011 <.002 .53

Alachlor ESA 20 100 -- .53 .48 .21 .96

Alachlor OA 19 15.8 -- .22 <.20 <.20 .50

alpha-HCH 21 0.0 0.0 <.002 <.002 <.002 <.002

Atrazine 21 100 19.0 3.91 0.364 0.108 48.1

Azinphos-methyl 21 0.0 0.0 <.001 <.001 <.001 <.001

Benfluralin 21 0.0 -- <.002 <.002 <.002 <.002

Butylate 21 4.8 0.0 <.002 <.002 <.002 .005

Carbaryl 21 0.0 0.0 <.003 <.003 <.003 <.003

Carbofuran 21 23.8 0.0 .017 <.003 <.003 .122

Chlorpyrifos 21 4.8 0.0 .005 <.004 <.004 .059

cis-Permethrin 21 0.0 -- <.005 <.005 <.005 <.005

Cyanazine 21 100 14.3 1.32 .096 .019 14

Cyanazine-amide 20 55.0 -- .49 .10 <.05 4.5

Dacthal 21 0.0 -- <.002 <.002 <.002 <.002

Deethylatrazine 19 100 -- .320 .140 .06 1.4

Deisopropylatrazine 19 94.7 -- .20 .11 <.05 .70

Diazinon 21 4.8 0.0 <.002 <.002 <.002 .004

Dieldrin 21 9.5 0.0 <.001 <.001 <.001 .006

Disulfoton 21 0.0 0.0 <.017 <.017 <.017 <.017

EPTC 21 19.0 -- <.002 <.002 <.002 0.005

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum
 61

Ethalfluralin 21 0.0 -- <.004 <.004 <.004 <.004

Skunk River at Augusta, Iowa (site 12, fig. 1, table 2)—Continued

Ethoprophos 21 0.0 -- <.003 <.003 <.003 <.003

Fonofos 21 4.8 0.0 <.003 <.003 <.003 .003

Hydroxyatrazine 19 57.9 -- .42 .22 <.20 1.1

Lindane 21 4.8 0.0 <.004 <.004 <.004 .005

Linuron 21 0.0 -- <.002 <.002 <.002 <.002

Malathion 21 0.0 0.0 <.005 <.005 <.005 <.005

Metolachlor 21 100 0.0 1.27 0.214 0.057 9.61

Metolachlor ESA 20 100 -- 2.6 2.5 .81 4.7

Metolachlor OA 19 89.5 -- .74 .59 <.20 1.9

Metribuzin 21 28.6 0.0 .010 <.004 <.004 .071

Molinate 21 0.0 -- <.004 <.004 <.004 <.004

Napropamide 21 0.0 -- <.003 <.003 <.003 <.003

p,p’-DDE 21 0.0 0.0 <.006 <.006 <.006 <.006

Parathion 21 0.0 -- <.004 <.004 <.004 <.004

Parathion-methyl 21 0.0 0.0 <.006 <.006 <.006 <.006

Pebulate 21 0.0 -- <.004 <.004 <.004 <.004

Pendimethalin 21 14.3 -- .006 <.004 <.004 .045

Phorate 21 0.0 -- <.002 <.002 <.002 <.002

*Prometon 21 90.5 0.0 .015 .011 <.009 .068

Propachlor 21 4.8 0.0 <.007 <.007 <.007 .021

Propanil 21 0.0 -- <.004 <.004 <.004 <.004

Propargite 21 0.0 -- <.013 <.013 <.013 <.013

Propyzamide 21 0.0 0.0 <.003 <.003 <.003 <.003

Simazine 21 52.4 0.0 .013 .005 <.005 .079

Tebuthiuron 21 28.6 0.0 <.010 <.010 <.010 .035

Terbacil 21 0.0 0.0 <.007 <.007 <.007 <.007

Terbufos 21 0.0 0.0 <.013 <.013 <.013 <.013

Thiobencarb 21 0.0 -- <.002 <.002 <.002 <.002

Triallate 21 0.0 -- <.001 <.001 <.001 <.001

Trifluralin 21 9.5 0.0 <.002 <.002 <.002 0.006

Appendix 2. Statistical summary of pesticides and pesticide degradates at the basic-fixed sampling sites in the Eastern 

Iowa Basins, 1996–98—Continued

[<, less than indicated detection level; MCL, U.S. Environmental Protection Agency maximum contaminant level; --, MCL not established; * includes 

estimated concentrations less than the method reporting limit]

Constituent

Number 

of 

samples

Detection 

rate, in 

pecent

Percentage 

of samples 

exceeding 

MCL

Concentration, in micrograms per liter

Mean Median Minimum Maximum