U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3028

March 2006 Printed on recycled paper

Pesticides in the Nation’s Streams and Ground Water, 

1992–2001—A Summary

The use of pesticides to control weeds, insects, and other pests has resulted in a range of benefits, including 

increased food production and reduction of insect-borne disease, but also raises questions about possible adverse effects on the environment, including water quality. A decadal assessment by the National 

Water-Quality Assessment (NAWQA) Program of the United States Geological Survey (USGS) provides the 

most comprehensive national-scale analysis to date of pesticide occurrence and concentrations in streams 

and ground water, based on results from studies completed during 1992–2001. Among the major findings 

are that pesticides are frequently present in streams and ground water, are seldom at concentrations likely 

to affect humans, but occur in many streams at concentrations that may have effects on aquatic life or 

fish-eating wildlife. This Fact Sheet highlights national findings regarding pesticide occurrence, distribution 

and sources, and the potential for effects on humans, aquatic life, and fish-eating wildlife. It serves as a 

companion product to the USGS Circular entitled “The Quality of Our Nation’s Waters—Pesticides in the 

Nation’s Streams and Ground Water, 1992–2001.”

Pesticide Occurrence 

Pesticides and degradates are typically 

present throughout most of the year in 

streams draining watersheds with substantial agricultural or urban areas, but 

are less common in ground water. 

At least one pesticide was detected 

in water from all streams studied and, as 

shown in the graph below, pesticide compounds were detected throughout most of 

the year in water from streams with agricultural (97 percent of the time), urban 

(97 percent), or mixed-land-use watersheds (94 percent). In addition, organoPesticides in water

(most were used during the study period)

Organochlorine compounds in whole fish and 

bed sediment (most are no longer used)

Percentage of time (streams) or samples 

(ground water) with one or more detections 

Percentage of samples with

one or more detections

Shallow ground water

Shallow ground water

Stream water

Shallow ground water

Major aquifers

Agricultural areas

Urban areas

Undeveloped areas

Mixed land uses

Agricultural areas

Urban areas

Undeveloped areas

Mixed land uses

Stream water

Stream water

Stream water

61%

55%

 65%

29%

33%

97%

97%

94%

25 50 75 100

Bed sediment

Bed sediment

Fish tissue

Bed sediment

Bed sediment

Fish tissue

Fish tissue

Fish tissue

57%

80%

 57%

24%

60%

92%

94%

94%

0 0 25 50 75 100

chlorine pesticides (such as DDT) and 

their degradates and by-products were 

found in fish and bed-sediment samples 

from most streams in agricultural, urban, 

and mixed-land-use watersheds—and in 

more than half the fish from streams with 

predominantly undeveloped watersheds. 

Most of the organochlorine pesticides 

have not been used in the United States 

since before the NAWQA studies began, 

but their continued presence demonstrates 

their persistence in the environment. 

Pesticides were less common in 

ground water than in streams. They 

occurred most frequently in shallow 

ground water beneath agricultural and 

urban areas, where more than 50 percent 

of wells contained one or more pesticide compounds. About one-third of the 

deeper wells sampled, which tap major 

aquifers used for water supply, contained 

one or more pesticides or degradates.

 The findings show that streams are 

most vulnerable to pesticide contamination, but ground water also merits careful 

monitoring—especially in agricultural 

and urban areas. Shallow ground water in 

some of these areas is used for drinking 

water and ground-water contamination is 

difficult to reverse once it occurs.

 The occurrence of pesticides in 

streams and ground water does not 

necessarily cause adverse effects because 

detections were often at low concentrations. NAWQA’s assessment of potential 

effects is based on comparing measured 

concentrations to water-quality benchmarks, as described on the following 

page. Benchmarks are estimates of the 

concentrations above which pesticides 

may have adverse effects on humans, 

aquatic life, or fish-eating wildlife. For 

example, less than 10 percent of NAWQA 

stream sites and about 1 percent of wells 

had concentrations greater than a humanhealth benchmark.
Potential for Effects on 

Human Health

Concentrations of pesticides in streams 

and ground water were typically below 

water-quality benchmarks for human 

health.

No streams draining undeveloped 

watersheds and only one stream with a 

mixed-land-use watershed had concentrations greater than a human-health benchmark. Annual mean concentrations of one 

or more pesticides exceeded a humanhealth benchmark in 8 of 83 agricultural 

streams and in 2 of 30 urban streams. 

Agricultural streams located in the Corn 

Belt (Illinois, Indiana, Iowa, Nebraska, 

Ohio, and parts of adjoining States) and 

the Mississippi River Valley accounted 

for most concentrations that exceeded 

benchmarks—all by atrazine, cyanazine 

(no longer in use by the end of the study), 

or dieldrin (no longer in use when the 

study began). The two urban streams 

where benchmarks were exceeded are in 

Texas (diazinon) and Hawaii (dieldrin). 

None of the NAWQA stream sites 

were located at drinking-water intakes. 

For perspective, however, the Nation’s 

1,679 public water-supply intakes on 

streams were evaluated in the context 

of NAWQA land-use classifications 

and pesticide findings. Eighty-seven 

percent of the water-supply intakes are 

on streams draining undeveloped and 

mixed-land-use watersheds and are 

therefore unlikely to withdraw water with 

concentrations that are greater than a 

human-health benchmark. The likelihood 

of pesticide concentrations exceeding 

a human-health benchmark is greatest 

for those streams draining agricultural 

or urban watersheds, which account for 

about 12 and 1 percent, respectively, of 

public water-supply intakes on streams.

Human-health benchmarks were 

seldom exceeded in ground water. One or 

more pesticides exceeded a benchmark 

in about 1 percent of the 2,356 domestic and 364 public-supply wells that 

were sampled. The greatest proportion 

of wells with a pesticide concentration 

greater than a benchmark was for those 

tapping shallow ground water beneath 

urban areas (4.8 percent). The urban 

wells with benchmark 

exceedances included 

1 public-supply, 3 

domestic, and 37 

observation wells. All 

concentrations greater 

than a benchmark 

were accounted for 

by dieldrin (72 wells), 

dinoseb (4), atrazine 

(4), lindane (2), and 

diazinon (1).

Screening-Level Assessment of Potential Effects

The potential for pesticides to adversely affect human health, aquatic life, or fisheating wildlife was evaluated by a screening-level assessment similar in concept 

to U.S. Environmental Protection Agency (USEPA) screening-level assessments. 

The screening-level assessment is primarily intended to identify and prioritize 

needs for further investigation and is not a substitute for comprehensive risk 

assessment, which includes additional factors. Concentrations of pesticides measured at NAWQA sampling sites were compared with water-quality benchmarks 

derived from standards and guidelines established by USEPA, toxicity values from 

USEPA pesticide risk assessments, and selected guidelines from other sources. 

Most of the benchmarks are estimates of no-effect levels, such that concentrations below the benchmarks are expected to have a low likelihood of adverse 

effects and concentrations above a benchmark may have adverse effects, with 

the likelihood increasing as the concentration increases. 

 Streams and wells with concentrations greater

 than human-health benchmarks for water

Mixed land uses

Stream water

Shallow ground water

Stream water

Shallow ground water

Stream water

Shallow ground water

Stream water

Major aquifers

none

none

9.6%

1.2%

6.7%

4.8%

1.5%

1.0%

Agricultural areas

Undeveloped areas

Urban areas

Percentage of stream sites or wells with

one or more pesticides exceeding 

a human-health benchmark

0 25 50 75 100

Potential for Effects on 

Aquatic Life and Wildlife

Concentrations of pesticides were greater 

than water-quality benchmarks for 

aquatic life and (or) fish-eating wildlife 

in more than half of the streams with 

substantial agricultural and urban areas 

in their watersheds.

Of the 178 streams sampled nationwide that have watersheds dominated by 

agricultural, urban, or mixed land uses, 

56 percent had one or more pesticides in 

water that exceeded at least one aquaticlife benchmark. Urban streams had 

concentrations that exceeded one or more 

benchmarks at 83 percent of sites—

mostly by the insecticides diazinon, 

chlorpyrifos, and malathion—although 

frequencies of exceedance declined 

during the study period. Concentrations 

exceeded benchmarks in 95 percent of 

urban streams sampled during 1993–1997 

and in 64 percent of streams during 

1998–2000. Agricultural streams had 

concentrations that exceeded one or more 

benchmarks at 57 percent of sites—most 

frequently by chlorpyrifos, azinphosmethyl, atrazine, p,p'-DDE, and alachlor. 

As the use of alachlor declined through 

the study period, its benchmark exceedances also declined, with none during the 

last 3 years of study.

 Aquatic-life benchmarks for 

organochlorine pesticide compounds 

in bed sediment also were frequently 

exceeded in urban areas (70 percent of 

urban stream sites). Most compounds 

that exceeded aquatic-life benchmarks 
for sediment were derived from organochlorine pesticides that have not been 

used since before the study began, such 

as DDT, chlordane, aldrin and dieldrin. In 

agricultural streams, aquatic-life benchmarks were exceeded at 31 percent of 

sites—most often by DDT compounds 

and dieldrin.

Comparisons of concentrations of 

organochlorine compounds in whole fish 

with wildlife benchmarks indicate a wide 

range of potential for effects on fish-eating wildlife. The ranges of percentages 

shown on the graph below reflect the 

uncertainties in benchmarks for fish-eating wildlife due to the lack of consensus 

on national-scale benchmarks or toxicity 

values. Similar to bed sediment, benchmarks for fish were exceeded most often 

by compounds related to DDT, dieldrin, 

and chlordane in urban streams, and by 

DDT compounds, dieldrin, and toxaphene in agricultural streams in areas 

where historical use on crops was most 

intense.

 Assessment and management of the 

potential effects of pesticides on aquatic 

life and wildlife are complicated by the 

combined presence in streams of (1) currently used pesticides and degradates, and 

(2) organochlorine pesticide compounds 

derived from pesticides that were largely 

banned prior to 1990. The widespread 

potential for adverse effects shown by the 

screening-level assessment—combined 

with the uncertainty due to the preliminary nature of the assessment and the 

complexity of pesticide exposure—indicate a continuing need to study the effects 

of pesticides on aquatic life and wildlife 

under the conditions of pesticide exposure that occur in the environment.

Frequently Detected 

Pesticides and Relations to 

Land Use and Pesticide Use

Pesticides detected most frequently in 

streams and ground water are among 

those used most heavily during the study 

or in the past. Their occurrence follows 

patterns in land use and use intensity, 

with additional influence—especially for 

ground water—by natural factors and 

management practices. 

The most frequently detected herbicides that are used mainly for agriculture—atrazine, metolachlor, cyanazine, 

alachlor, and acetochlor—generally were 

detected most often and at the highest 

concentrations in water samples from 

streams in agricultural areas with their 

greatest use, particularly in the Corn Belt. 

Five herbicides commonly used in urban 

areas—simazine, prometon, tebuthiuron, 

2,4-D, and diuron—and three commonly 

used insecticides—diazinon, chlorpyrifos, and carbaryl—were most frequently 

detected in urban streams throughout the 

Nation, often at higher concentrations 

than in agricultural streams. Total DDT 

was measured at some of the highest 

concentrations in bed sediment and fish 

in parts of the Southeast and in parts of 

California, Oregon, and Washington, 

where DDT was historically used on cotton, tobacco, orchards or other crops. 

Land use and pesticide use are not 

the only factors influencing the occurrence of pesticides. Natural features and 

land-management practices also affect 

their distribution, particularly in ground 

water. Ground water is most vulnerable 

to contamination in areas with highly 

permeable soil and aquifer materials and 

where drainage practices do not divert 

recharge to streams and other surface 

water.

Pesticide concentrations in stream 

water also vary by season, with lengthy 

periods of low concentrations punctuated 

by seasonal pulses of much higher concentrations. For example, in streams that 

drain farmland throughout most of the 

Corn Belt, concentrations of herbicides 

were generally highest during spring 

runoff following pesticide applications. 

Similarly, concentrations of diazinon 

were highest during the winter in parts of 

the San Joaquin Valley, California, when 

applications to dormant almond orchards 

were followed by rainfall.

Understanding the correlations of 

pesticide occurrence with the amounts 

and characteristics of pesticides used can 

help managers to anticipate and prioritize 

High benchmark

Low benchmark

Streams with concentrations greater than wildlife

benchmarks for whole fish

87%

88%

88%

 Fish tissue

 Fish tissue

 Fish tissue

 Fish tissue

51%

88%

Percentage of stream sites with 

one or more organochlorine pesticide 

compounds exceeding a wildlife benchmark

4.8%

25%

34%

17%

Streams with concentrations greater than

aquatic-life benchmarks for water and bed sediment

Agricultural areas Agricultural areas

Urban areas

Undeveloped areas Undeveloped areas

Mixed land uses Mixed land uses

 Stream water 57%

 Bed sediment 31%

 Stream water

 Bed sediment

 Stream water

 Bed sediment

 Stream water

 Bed sediment

83%

70%

13%

8.4%

42%

36%

0 25 50 75 100 0 25 50 75 100

Percentage of stream sites with

one or more pesticide compounds

exceeding an aquatic-life benchmark

Urban areas
the pesticides most likely to affect water 

quality in different land-use settings. 

Some hydrologic settings where ground 

water is least vulnerable to contamination are those where streams are most 

vulnerable, and vice versa. The entire 

hydrologic system and its complexities, 

therefore, need to be considered in evaluating the potential for pesticide contamination of streams and ground water. In 

addition, seasonal patterns in pesticide 

concentrations are important to consider, 

both in managing the quality of drinking 

water withdrawn from streams in agricultural and urban settings and in evaluating the potential for adverse effects on 

aquatic life. Effective management of 

streams may require increased monitoring—including high-frequency sampling 

during seasons when intense pesticide use 

coincides with periods of high runoff—so 

that periods with elevated pesticide concentrations are adequately characterized. 

Mixtures and Degradates

Pesticides most commonly occur as mixtures of multiple compounds, rather than 

individually, including degradates resulting from the transformation of pesticides 

in the environment. 

Streams in agricultural and urban 

areas almost always contained complex 

mixtures of pesticides and degradates. 

More than 90 percent of the time, water 

samples from streams with agricultural, 

urban, or mixed-land-use watersheds contained 2 or more pesticides or degradates, 

and about 20 percent of the time they had 

10 or more. Mixtures were less common 

in ground water. Nevertheless, about half 

of the shallow wells in agricultural areas 

and about a third of shallow wells in 

urban areas contained 2 or more pesticides and degradates—less than 1 percent 

had 10 or more. The herbicides atrazine 

(and its degradate, deethylatrazine), simazine, metolachlor, and prometon were 

common in mixtures found in streams 

and ground water in agricultural areas. 

The insecticides diazinon, chlorpyrifos, 

carbaryl, and malathion were common in 

mixtures found in urban streams.

Degradates are often as common 

in streams and ground water as their 

parent pesticides. For example, atrazine, 

the most heavily used herbicide in the 

Nation during the study period, was 

found together with one of its several 

degradates, deethylatrazine, in about 

75 percent of stream samples and about 

40 percent of ground-water samples 

collected in agricultural areas across 

the Nation. Degradates are particularly 

important in ground water, which moves 

relatively slowly through soils and 

aquifers, providing the extended time and 

conditions favorable for transformation of 

pesticides. Most degradates are less toxic 

than their parent pesticide, but some have 

similar or greater toxicities. 

The widespread and common occurrence of pesticide mixtures, particularly 

in streams, means that the total combined 

toxicity of pesticides in water and other 

media often may be greater than that of 

any single pesticide compound that is 

present. Continued research is needed on 

the potential toxicity of pesticide mixtures, including degradates, to humans, 

aquatic life, and wildlife. NAWQA data 

on the occurrence and characteristics of 

mixtures and degradates is helping to 

target and prioritize toxicity assessments. 

From Monitoring to Prediction

National-scale modeling and prediction 

of pesticide levels in unmonitored areas is 

increasingly possible for some pesticides, 

particularly for streams. 

NAWQA data are sufficiently extensive to support national statistical models 

that can be used to estimate the concentrations or occurrence of some pesticides 

in streams and ground water where they 

have not yet been measured. Such spatial 

extrapolation is fundamental to extending NAWQA’s findings on sources and 

factors that affect pesticide occurrence 

—such as pesticide use and land use, 

climate, and soil characteristics—to a 

more comprehensive national assessment 

that includes unmonitored areas. The 

NAWQA model for atrazine in streams, 

for example, can be used to predict the 

likelihood that the annual average concentration of atrazine in untreated stream 

water exceeds the USEPA drinking-water 

standard of 3 micrograms per liter in any 

stream in the Nation (see map below). 

Atrazine concentrations were predicted 

to be highest in the Corn Belt and parts 

of the southern Mississippi River Valley, 

where use is high and natural features 

favor the transport of pesticides by runoff 

to streams. Streams shown in red on the 

map are estimated to have more than a 5-

percent chance of having an annual mean 

concentration greater than the USEPA 

standard. Some of these streams may not 

be suitable as sources of drinking water 

without the use of strategies to lower 

concentrations. These types of analyses 

can be used to identify locations that have 

the greatest likelihood of water-quality 

problems and that are the highest priority 

for additional monitoring.

Expanding the capability for prediction and extrapolation is a critical step 

for national water-quality assessment, as 

well as for cost-effective management 

of water resources, because both require 

more information (pesticides, places, and 

times) than can be directly measured.

Less than 5%

Probability of exceeding 3 μg/L atrazine

5% or greater

1_10

Prediction of atrazine in streams
Trends in Pesticides

Some trends in pesticide concentrations 

are already evident, showing responses to 

regulatory actions and reduced use. 

NAWQA findings provide a framework for assessing whether pesticide 

levels in the Nation’s streams and ground 

water are increasing or decreasing over 

time. For many streams and aquifers, it is 

too early to tell whether levels are changing, because historical data are insufficient or too inconsistent. Some trends 

are evident, however, and a first look at 

trends shows examples of both decreasing 

and increasing levels of certain pesticides 

in streams and ground water. 

The most complete story of trends 

in response to regulatory action and 

reduced pesticide use is the decline in 

organochlorine pesticide concentrations 

that followed reductions in use during the 

1960s and bans on uses in the 1970s and 

1980s. Concentrations of total DDT in 

fish, for example, decreased rapidly from 

the 1960s through the 1970s, and then 

more slowly during the 1980s and 1990s, 

as documented by data from the U.S. Fish 

and Wildlife Service and NAWQA. Just 

as notable as the declines, however, is 

the finding that persistent organochlorine 

pesticide compounds still occur at levels 

greater than benchmarks for fish-eating 

wildlife in many urban and agricultural 

streams across the Nation.

In contrast, NAWQA findings show 

that concentrations of relatively mobile 

and short-lived pesticides in stream water 

respond more rapidly to changes in use 

than the less mobile and more persistent 

organochlorine insecticides. For example, 

increases in acetochlor and decreases 

in alachlor are evident in streams in the 

Corn Belt, where acetochlor partially 

replaced alachlor for control of weeds 

in corn beginning in 1994. The changes 

in use were reflected quickly in stream 

concentrations, generally within 1 to 2 

years. Similarly, concentrations of diazinon decreased significantly from 1998 to 

2004 in 5 of 7 urban and mixed-land-use 

streams sampled in the Northeast, consistent with the USEPA-mandated phase-out 

of nonagricultural uses of diazinon that 

began in 2002. 

Ground water responds more slowly 

than stream water to changes in pesticide 

use. A persistent pesticide or degradate 

can remain in ground water long after its 

use is discontinued because of the slow 

rates of ground-water flow and the resulting long residence time of water and 

pesticides in ground-water flow systems. 

Long-term and consistent data for 

assessing trends is essential for tracking water-quality response to changes in 

pesticide use and management practices, 

for providing early warning of unanticipated problems, and for updating and 

improving models. Long-term monitoring 

is particularly important for assessing the 

occurrence of pesticides in ground water 

and the occurrence of persistent compounds in streams because concentrations 

change slowly, sometimes taking decades 

to respond to changes in use.

Priorities for Filling 

Information Gaps for 

Pesticides

The NAWQA water-quality assessment serves as a foundation for 

improving water-resource assessment and management, but major 

gaps in critical information about 

pesticides still persist and continue 

to present challenges to scientists, 

managers, and policymakers. Some 

of the most important steps needed 

to fill gaps are listed below:

Improve tracking of pesticide use 

in agricultural and non-agricultural areas, including amounts, 

locations, and timing

Add assessments of pesticides 

not yet studied, including some 

already in use as well as new 

pesticides

Improve assessment and 

understanding of degradates, 

including their distribution and 

potential effects

Evaluate toxicities of mixtures 

and their potential to affect 

humans, aquatic life, and wildlife

Evaluate the performance of 

management practices and their 

effects on concentrations and 

transport of pesticides

Improve methods for prediction 

of pesticide levels in unmonitored areas

Sustain and expand long-term 

monitoring for trends

•

•

•

•

•

•

•

500

0

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1969 1974 1979 1984 1989 1994 1999

Total DDT concentration in whole fish, 

in micrograms per kilogram

Median

90th percentile

Declining DDT concentrations in whole fish
NAWQA Approach to Pesticide Assessment

NAWQA assessments of pesticides followed a nationally consistent approach in 

51 of the Nation’s major river basins and aquifer systems, referred to as “Study 

Units,” with 20 Study Units studied during 1992–1995; 16 during 1996–1998; and 15 

during 1998–2001. Nationally, water samples for pesticide analysis were collected 

from 186 stream sites, bed-sediment samples were collected from 1,052 stream 

sites, and fish samples were collected from 700 stream sites. Ground-water 

samples were collected from 5,047 wells.

Most water samples were analyzed for 75 pesticides and 8 degradates, including 

20 of the 25 most heavily used herbicides and 16 of the 25 most heavily used insecticides. In addition, 32 organochlorine pesticide compounds were analyzed in bed 

sediment and (or) fish tissue, including 19 pesticides and 13 degradates or manufacturing by-products. Most of the organochlorine pesticides are no longer used in 

the United States, but organochlorine compounds still persist in the environment.

Contacts for additional 

information or questions

Robert J. Gilliom, Chief, 

National Pesticide Synthesis

U.S. Geological Survey

Placer Hall—6000 J Street

Sacramento, CA 

95819-6129

Email: rgilliom@usgs.gov

(916)-278-3094

By Robert J. Gilliom and Pixie A. Hamilton

Layout and design by Phillip J. Redman

USGS promotes public 

access to water quality 

information 

This Fact Sheet and the Circular are free 

of charge and are on the World Wide Web 

at http://water.usgs.gov/nawqa.

Included at this web site are data used for 

national summaries of pesticides, maps 

of chemical use and pesticide occurrence, 

information on sampling designs and 

methodology, and background on data 

analysis. 

NAWQA Study Units

Initiated 1991

Initiated 1994

Initiated 1997

High Plains Regional Ground 

Water Study, initiated 1999