U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005–5094

Summary of Significant Results from Studies of 

Triazine Herbicides and Their Degradation Products 

in Surface Water, Ground Water, and Precipitation 

in the Midwestern United States During the 1990s

1990

0

10

20

30

40

50

60

Midwestern United States herbicide use, in million pounds per year

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Atrazine

Cyanazine

Simazine
Summary of Significant Results from 

Studies of Triazine Herbicides and Their

Degradation Products in Surface Water, 

Ground Water, and Precipitation in the 

Midwestern United States During the 1990s

By Elisabeth A. Scribner, E. M. Thurman, Donald A. Goolsby, Michael T. Meyer, 

William A. Battaglin, and Dana W. Kolpin

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005–5094
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

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

Box 25286, Denver Federal Center

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Scribner, E.A., Thurman, E.M., Goolsby, D.A., Meyer, M.T., Battaglin, W.A., and Kolpin, D.W., 2005, Summary of 

significant results from studies of triazine herbicides and their degradation products in surface water, ground water, 

and precipitation in the Midwestern United States during the 1990s: U.S. Geological Survey Scientific Investigations 

Report 2005–5094, 27 p.

Prepared by the U.S. Geological Survey in Lawrence, Kansas (http://ks.water.usgs.gov)
 iii

Contents

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

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

Triazine Herbicide Usage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Water-Quality Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Ground Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Chemistry and Transport of Triazine Herbicides in Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Persistence of Triazine Herbicides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figures

1. Map showing location of U.S. Geological Survey ToxicSubstances Hydrology Program study 

areas fortriazine herbicides in the Midwestern United States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2. Graph showing triazine herbicide use in Midwestern United States, 1990 to 2002. . . . . . . . . . . . . . . . . . . .4

3. Map showing location of study area, hydrologic units, and 147 stream sites sampled during 

1989–2002 in the Midwestern United States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Graphs showing concentrations in surface-water samples collected from 53 Midwestern 

streams during post-application runoff in 1989, 1990, 1994, 1995, 1998, and 2002 for atrazine 

andcyanazine,propazine and simazine, and cyanazine amide, deethylatrazine, and 

deisopropylatrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

5–8. Maps showing:

5. Location of 76 reservoirs sampled for herbicide analysis in the Midwestern United States 

during 1992 and 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

6. Three-dimensional computer images of atrazine concentrations in Perry Lake, 

northeasternKansas, during 1992 pre-application, first flush, summer, fall, and 1993 preapplicationsurveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

7. Location of wells in the U.S. Geological Survey Midwestern ground-water monitoring 

network sampled during 1991–94. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Location of wells sampled in Iowa during 1995–98 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9. Diagram showing degradation pathways for atrazine, cyanazine, propazine, and simazine 

todeethylatrazineand (or) deispropylatrazine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Diagram showing deethylatrazine-to-atrazine ratios for surface- and ground-water samples 

fromMidwesternUnited States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

11. Graph showing deethylatrazine-to-atrazine ratios for surface- and ground-water samples from 

MidwesternUnited States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

12–16. Diagrams showing:

12. Conceptual model and data for deethylatrazine-to-atrazine ratios in Perry Lake, 

northeasternKansas, during March 1992–March 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

13. Mixed-mode model of sorption for hydroxylated atrazine degradation products. . . . . . . . . . . . . 18

14. Proposed models for the mechanisms controlling concentrations of hydroxylated atrazine 

degradation products in streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
iv 

15. Degradation pathways for atrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

16. Degradation pathways forcyanazine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Tables

1. Methods of analysis for triazine herbicides and their degradation products . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Percentage of detections of triazine herbicides and degradation products in surface-water

samplescollected from selected streams in the Midwestern United States during 1989 and 1990 . . .6

3. Triazine herbicides and their degradation products analyzed in samples collected from 76 

Midwesternreservoir outflows, April 1992 through September 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 

4. Summary of analytical results for triazine herbicides and their degradation products in groundwater samples collected from wells in the Midwestern United States during 1991–93 . . . . . . . . . . . . . 10

5. Summary of analytical results for triazine herbicides and their degradation products in samples 

collected from 131 Iowa wells during 1995–98. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6. Summary of analytical results for triazine herbicides and their degradation product 

concentrationsmeasured in precipitation samples from 81 sites across the Midwestern and 

NortheasternUnited States from March 1990 through September 1991 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7. Physiochemical properties of triazine herbicides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Conversion Factors and Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Multiply By To obtain

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

microgram per liter (μg/L) 1 part per billion (ppb)

microgram per square meter per year 

[(μg/m2)/yr]

0.003276x10-6 ounce per billion (ppb)

mile (mi) l.609 kilometer (km)

milligram per liter (mg/L) 1 part per million (ppm)

milliliter per gram (mL/g) 0.007491 gallon per ounce (gal/oz)

millipascal (mPa) 0.000208854 pound-force per square foot (lbf/ft2)

mole per liter (mol/L) 1 gram molecular weight liter

pound (lb) 453.6 gram (g)

pound active ingredient per acre per 

year [(lb a.i./acre)/yr]

1.121 kilogram active ingredient per hectare 

per year [(kg a.i./ha)yr]

pound per acre (lb/acre) 1.121 kilogram per hectare (kg/ha)

square mile (mi2) 2.590 square kilometer (km2)
Summary of Significant Results from Studies of Triazine 

Herbicides and their Degradation Products in Surface 

Water, Ground Water, and Precipitation in the 

Midwestern United States During the 1990s

By Elisabeth A. Scribner, E.M. Thurman, Donald A. Goolsby, Michael T. Meyer, William A. Battaglin, and 

Dana W. Kolpin

Abstract

Nonpoint-source contamination of water resources from 

triazine herbicides has been a major water-quality issue during 

the 1990s in the United States. To address this issue, studies of 

surface water, ground water, and precipitation have been carried 

out by the U.S. Geological Survey in the Midwestern United 

States. 

Reconnaissance studies of 147 streams were conducted to 

determine the geographic and seasonal distribution of atrazine, 

cyanazine, propazine, and simazine. These studies showed that 

high concentrations of herbicides were flushed from cropland 

and transported through the stream system as pulses in response 

to spring and summer rainfall. The studies also revealed the 

persistence of herbicides and their degradation products in 

streams. 

An investigation of 76 reservoirs showed that the occurrence and temporal distribution of herbicides and their degradation products in reservoir outflow could be related to reservoir 

and drainage-basin characteristics, water and land use, herbicide use, and climate. Significant findings showed that concentrations of atrazine and its degradation products remained 

elevated all summer and into the fall and that recently applied 

atrazine mixed with atrazine applied the previous year as water 

moved through a reservoir. 

Reconnaissance studies of 303 ground-water wells were 

completed to determine hydrogeological and seasonal occurrence, concentration, and distribution of herbicides and their 

degradation products. Samples collected from across the Midwestern United States consistently revealed that triazine herbicide degradation products commonly were found more frequently than their parent herbicide and that ground-water age 

could be an important factor in explaining variations in herbicide contamination. 

A final study investigated precipitation in the Midwestern 

United States, northeast to the Atlantic Ocean, and northward to 

the Canadian border. It found that the highest herbicide concentrations in precipitation occurred following herbicide 

application to cropland. Atrazine was detected most often, followed by deethylatrazine, cyanazine, and deisoproplyatrazine. 

Mass deposition of herbicides by precipitation was greatest in 

areas where herbicide use was intense and decreased with distance from the Midwest.

Findings of the 1990s studies include an improved understanding of the occurrence, persistence, chemistry, and transport of triazine herbicides and their degradation products in the 

hydrologic environment. A significant increase in knowledge of 

triazine herbicides and development and improvement of analytical methods were accomplished in the past decade. The 

results produced are not only significant for the present (2005) 

but provide an important data set for future use. 

Introduction

The U.S. Geological Survey (USGS) Toxic Substances 

Hydrology (Toxics) Program provides unbiased scientific 

information on the behavior of toxic substances in the Nation’s 

hydrologic environments. The objectives of the program 

include development and quantification of methods to measure 

contaminants and their degradation products at levels low 

enough to understand the fate and transport in hydrologic systems. Investigations are focused on identifying persistent pesticides and their degradation products in surface water, ground 

water, and precipitation. The research involves developing 

methods to measure these pesticides in water samples at lowlevel concentrations. Results then are used by manufacturers, 

farmers, regulators, researchers, and the public (Buxton, 2000). 

Throughout the 1990s, a series of regional-scale waterquality studies were conducted by the Toxics Program in the 

Midwestern United States. The study area extended across parts 

of 12 States (fig. 1) and was selected because it is the largest and 

most intensive area of row-crop agriculture in the country. This 

area often is referred to as the Corn Belt (Thurman and others, 

1991; Infoplease, 2004). Most of the agricultural application of 

triazine herbicides in the United States is to crops in this region. 
2 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

The major Midwestern row crops include corn, sorghum, and 

soybeans, to which more than 52.8 million lb of triazine herbicides, such as atrazine, cyanazine, propazine, and simazine, 

were applied during 1997 (Gianessi and Marcelli, 2000). The 

regional-scale studies included reconnaissance of Midwestern 

streams, reservoirs, and ground water. Other studies were conducted to investigate herbicides dispersed by air currents, rainfall, snow, and dry deposition. 

Field-dissipation studies were conducted to examine the 

chemistry, fate, and transport of the triazine herbicides atrazine, 

cyanazine, propazine, simazine, and their degradation products 

in greater detail. These studies resulted in an improved understanding of the geochemical fate and transport of triazines and 

degradation products in hydrologic environments (Mills and 

Thurman, 1994a,b; Thurman and others, 1994). 

USGS has emphasized analytical methods development to 

compliment studies of pesticide fate and transport. The USGS 

Organic Geochemistry Research Laboratory was established in 

1987 in Lawrence, Kansas, to develop analytical methods and 

conduct research on the fate and transport of pesticides and their 

degradation products. Analytical methods developed for triazine compounds during the 1990s (table 1) included solid-phase 

extraction (SPE; Thurman and others, 1990; Mills and Thurman, 1992; Meyer and others, 1993; Thurman and Mills, 1998; 

Ferrer and others, 1999; Thurman and Snavely, 2000), gas chromatography/mass spectrometry (GC/MS; Zimmerman and 

Thurman, 1999; Kish and others, 2000), high-performance liquid chromatography (HPLC; Hostetler and Thurman, 2000), 

liquid chromatography/mass spectrometry (LC/MS; Ferrer and 

others, 2000; Thurman and others, 2001; Lee and others, 2002), 

and enzyme-linked immunosorbent assay (ELISA; Thurman 

and others, 1990; Aga and Thurman, 1993; Pomes and others, 

1996, 1998; Schraer and others, 2000). Analytical methods continue to be developed at the Organic Geochemistry Research 

Laboratory to measure concentrations of pesticides and their 

degradation products (Scribner and others, 2000b).

The purpose of this report is to provide a summary of triazine herbicide usage and water-quality effects, persistence, 

chemistry, and transport in hydrologic environments of the 

Midwestern United States using significant results from USGS 

studies conducted during the 1990s. The report also includes 

an extensive bibliography of publications resulting from 

these studies.

Triazine Herbicide Usage

Modern agricultural practices in the United States often 

involve widespread use of herbicides for production of crops. 

Because of concern about water contamination, several manufacturers voluntarily reduced the recommended application 

rates or, in other cases, removed herbicides from the market 

during the 1990s. Monitoring the effects of these usage changes 

is important in understanding the occurrence, persistence, 

chemistry, and transport of these herbicides and their degradation products.

The historical use of atrazine, cyanazine, and simazine in 

the Midwestern United States is shown in figure 2. In 1990, the 

Playa

Study area

lake

study

C o r n B e l t

0 500 KILOMETERS

0 500 MILES

CANADA

MEXICO

Figure 1. Location of U.S. Geological Survey Toxic Substances Hydrology Program study areas for triazine 

herbicides in the Midwestern United States.
Triazine Herbicide Usage 3

Table 1. Methods of analysis for triazine herbicides and their degradation products. 

[SPE, solid-phase extraction; GC/MS, gas chromatography/mass spectrometry; HPLC/DAD, high-performance liquid chromatography with diode array

detection; LC/MS, liquid chromatography/mass spectrometry; ELISA, enzyme-linked immunosorbent assay; x, method used; NA, not applicable]

Triazine compounds 

Method

SPE GC/MS HPLC/DAD LC/MS ELISA

Parent compounds

Atrazine x x NA x x

Cyanazine x x NA x x

Propazine x x NA x x

Simazine x x NA x NA

Degradation products

Deethylatrazine x x NA x NA

Deisopropylatrazine x x NA x NA

Deethylhydroxyatrazine x NA x NA NA

Deisopropylhydroxyatrazine x NA x NA NA

Hydroxyatrazine x NA x x NA

Cyanazine acid x NA NA x NA

Cyanazine amide x x NA x NA

Deethylcyanazine x x NA x NA

Deethylcyanazine acid x NA NA x NA

Deethylcyanazine amide x x NA NA NA

Hydroxysimazine x NA NA x NA

manufacturer of atrazine voluntarily reduced the maximum recommended application rate on corn and sorghum (U.S. Environmental Protection Agency, USEPA, written commun., 

January 23, 1990). In 1992, the maximum recommended application rate of atrazine on corn and sorghum was reduced again 

by the manufacturer to a range of 1.6 to 2.5 (lb a.i./acre)/yr 

(pounds active ingredient per acre per year) depending on soil 

organic residue and erosion potential. As much as 0.50 (lb 

a.i./acre)/yr could be used in subsequent applications (USEPA, 

written commun., March 8, 1993), with the total of all applications not to exceed 2.5 (a.i./acre)/yr (Scribner and others, 

2000a).

As seen in figure 2, the use of atrazine decreased from 

about 47 million lb in 1991 to 39 million lb in 1995, fluctuating 

to 52 million lb in 2001, then declining sharply to 42 million lb 

in 2002. Besides voluntary reductions in application rates, herbicide use fluctuates in response to annual changes in planted 

corn and soybean acreage and the introduction of new herbicide 

products (U.S. Department of Agriculture, 1991–2003). 

Cyanazine was historically one of the most intensively

used herbicides in the country since its introduction in 1972. 

However, in 1996, the manufacturer of cyanazine voluntarily

began reducing the maximum recommended application rate 

from 6.5 to 5 (lb a.i./acre)/yr to 3 (lb a.i./acre)/yr by January 1, 

1999. The manufacture of cyanazine was discontinued on 

December 31, 1999, with distribution of existing supplies 

terminated in September 2002 (Tony Catka, Dupont, oral 

commun., September 14, 2000). As shown in figure 2, approximately 20 million lb of cyanazine were used in the Midwestern 

United States in 1990, peaking at 26 million lb in 1994, then 

steadily declining to about 0.05 million lb usage by 2002 

(U.S. Department of Agriculture, 1991–2003). 

Simazine is applied to corn, primarily in Illinois, Indiana, 

Kentucky, Ohio, and on to the eastern seaboard. As can be seen 

in figure 2, simazine usage in the Midwestern United States 

increased from 0.80 million lb in 1990 to approximately 1 million lb in 2002 (U.S. Department of Agriculture, 1991–2003). 

Propazine was voluntarily removed from the market by the 

manufacturer before 1990. However, there were many sorghum 

farmers who confirmed that propazine was still a necessary

product in their region, and the manufacturer agreed to continue 

marketing the product to States that applied for U.S. Environmental Protection Agency (USEPA) Section 18 Emergency

Exemption. For the 1993–97 growing seasons, USEPA granted 

Section 18 Emergency Exemptions for the use of propazine on 

sorghum to one or more of the following States: Colorado, Kansas, New Mexico, Oklahoma, and Texas (U.S. Environmental 

Protection Agency, 1998). As a result of USEPA Section 18 

Emergency Exemption grants, propazine usage during 1993 

was approximately 100,000 lb, increasing to approximately

360,000 lb in 1994, with a sharp decline in 1997 to 40,000 lb 

(Jim Bone, Griffin, L.L.C, written commun., 2005).
4 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

Water-Quality Studies

Surface Water

Since 1989, USGS has conducted periodic reconnaissance 

studies of streams located in the Midwestern United States to 

determine the occurrence, distribution, and concentrations of 

herbicides. The studies also included measuring concentrations 

of herbicide degradation products in surface-water samples to 

gain information about the transport and fate of herbicides in the 

environment (Battaglin, 2002). Sampling sites were selected at 

147 USGS streamflow-gaging stations during 1989 to ensure 

geographic and seasonal distribution and regional-scale interpretation of the data (fig. 3). The streams were sampled before 

application of herbicides, during the first major runoff period 

after application, and during low-flow conditions in the fall 

when most of the streamflow was derived from ground water. 

A follow-up sampling at 53 sites of the 147 sites (fig. 3) 

was conducted in 1990 because of increased concern about the 

findings of high post-application concentrations of herbicides in 

1989. The distribution of major herbicide concentrations 

detected in these streams was essentially the same in 1989 and 

1990 for both the pre- and post-application samples. Subsequent studies in 1994, 1995, 1998, and 2002 indicated that the 

flush of herbicides following application is an annual occurrence (Goolsby and others, 1991a, 1991b; Thurman and others, 

1991, 1992; Scribner and others, 1993, 1994, 1998, 2000a, 

2003; Goolsby and Battaglin, 1995; Battaglin and others, 2003, 

2005).

The most striking feature of these reconnaissance data was 

that high concentrations of herbicides and their degradation 

products are mobilized by rainfall, then transported to streams 

with runoff. The majority of this transport takes place during the 

first rainfall or runoff after the application of herbicides. Subsequent runoff tends to produce lesser peaks in concentration. 

Because of these flushes, the detections of herbicides in larger 

Midwestern streams tends to be seasonal with higher percentages of detections in spring and early summer and lower percentages in fall and winter (table 2). Measurable amounts of 

atrazine, the most frequently detected herbicide, occurred in 

91 percent of the pre-application samples and 76 percent of the 

harvest samples, providing an indication of its persistence in 

surface water. Also, deethylatrazine, a triazine degradation 

product, occurred more frequently in pre-application, postapplication, and harvest samples than did deisopropylatrazine, 

also a triazine degradation product (table 2). These findings are 

significant because they indicate that some of the parent herbicides persist from year to year in soil and water, and that degradation products, such as deethylatrazine, both persist and are 

mobile. Results of studies of water samples collected from 

streams in eastern Iowa during the 1990s further substantiate 

these findings (Kalkhoff and others, 2003; Schnoebelen and 

others, 2003). 

Herbicide concentrations in Midwestern streams were 

expected to change between 1989 and 2002 as a result of change 

in herbicide use. In 1994, 1995, 1998, and 2002 postapplication runoff samples were collected at approximately

50 of the 147 sites sampled in 1989 (fig. 3). These samples were 

collected to assist in determining if changes in the application 

rates recommended by the manufacturer of atrazine had 

resulted in a decrease in atrazine concentrations in postapplication runoff (Scribner and others, 1998, 2000a). 

The percentage of samples with herbicides detected at or 

above the analytical reporting limit of 0.05 μg/L was greater in 

1995 than in 1989 for cyanazine and propazine, but less in 1995 

than in 1989 for simazine. The detection frequency for atrazine 

was 100 percent for every year. The frequency of detection of 

deethylatrazine was the same in 1989 and 1995, but the frequency of detection of deisopropylatrazine was greater in 1995 

than in 1989 (Battaglin and Goolsby, 1999; Scribner and others, 

2000a).

The distribution of concentrations for surface-water samples collected during post-application runoff in 1989, 1990, 

1994, 1995, 1998, and 2002 is shown in figure 4. Nondetections 

are plotted at the reporting limit of 0.05 μg/L. The use of atrazine and cyanazine was approximately the same in 1989 and 

1995; however, the median concentrations of both herbicides 

were higher in 1989 than in 1995 or 2002. Although herbicide 

concentrations in Midwestern streams are quite variable during 

post-application runoff, the data suggest that changes in herbicide use do affect herbicide concentrations in streams (Battaglin 

and Goolsby, 1999; Scribner and others, 2000a). Studies during 

1998 and 2002 of surface water in the Midwestern United States 

confirmed that herbicide degradation products were found to 

occur as frequently or more frequently and at concentrations 

1990

0

10

20

30

40

50

60

Herbicide use, in million pounds per year

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Atrazine

Cyanazine

Simazine

Figure 2. Triazine herbicide use in the Midwestern United States, 1990 

to 2002 (data from U.S. Department of Agriculture, 1991–2003).
Water-Quality Studies 5

EXPLANATION

Drainage area sampled in 1989

Drainage area of site sampled in 1990, 1994, 1995, 1998, 2002

Stream sampling site

Base from U.S. Geological Survey digital data,1:2,000,000, 1972

Albers Equal-Area Conic Projection

Standard Parallels 39°30’ and 43°30’, central median -90°30’

40°

45°

95° 85°

0 100 200 MILES

0 100 200 KILOMETERS

Figure 3. Location of study area, hydrologic units, and 147 sites sampled during 1989–2002 in the Midwestern United States.

that were frequently higher than the parent herbicides (Battaglin 

and others, 2003, 2005; Scribner and others, 2003).

In the summer of 1993, a persistent wet-weather pattern 

resulted in flooding in the upper Mississippi River Basin. Concentrations of selected herbicides were measured at several 

locations on the Mississippi River during this flood and in adjacent shallow wells after the floodwaters receded. These studies 

determined that record flooding did not dilute the concentrations of herbicides as expected. Instead, concentrations of 

herbicides such as atrazine were similar to those seen during 

normal spring and summer flows (Goolsby and others, 1993; 

Kolpin and Thurman, 1995). 

Surface-water reservoirs in the Midwestern United States 

were also the focus of USGS studies as they are an integral 

component of the supply, management, and quality of water 

resources in the United States. Many reservoirs are designed to 

catch and retain spring and early summer runoff for recreation, 

water supply, flood control, or irrigation use. Regional studies 

have shown that this water contains elevated concentrations of 

herbicides and their degradation products for several months of 

the year (Leonard, 1988; Baker and Richards, 1989; Pereira and 

Rostadt, 1990; Thurman and others, 1991, 1992; Squillace and 

Thurman, 1992; Goolsby and others, 1996; Battaglin and 

Goolsby, 1998). USGS designed a monitoring network of 

76 hydrologically representative reservoirs (fig. 5) in the Midwestern United States described in Scribner and others (1996). 

Reservoir outflows were sampled eight times from April 1992 

through September 1993 for a total of 608 samples. The timing 
6 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

and frequency of sampling made it possible to determine 

approximately when maximum and minimum concentrations of 

herbicides occurred in the reservoir outflows (Stamer and others, 1998a).

One of the most significant findings from this study is the 

abundance and persistence of triazine herbicides and their degradation products in reservoirs. In many reservoirs, concentrations of atrazine and its degradation products remained elevated 

all summer and into the fall (Stamer and Zelt, 1992; Thurman 

and Fallon, 1996; Stamer and others, 1998a). As shown in 

table 3, atrazine occurred at the highest frequency (82.1 percent) and in the highest detectable concentration with a mean of 

1.36 μg/L, whereas cyanazine was detected in only 47.2 percent 

of samples with a mean detectable concentration of 0.61 μg/L 

(Stamer and others, 1998a). 

As reported by Goolsby and others (1996), atrazine, deethylatrazine, and deisopropylatrazine were detected in more than 

one-half of the reservoirs during the fall sampling, whereas only

atrazine was detected in more than one-half of the streams sampled in the fall of 1989. Another notable difference between the 

occurrence of herbicides in reservoirs and streams was the 

much greater frequency of detection of cyanazine and deisopropylatrazine in reservoirs. Neither cyanazine nor deisopropylatrazine was detected in streams during the fall because these 

compounds were no longer present in significant amounts on 

the agricultural fields where they were applied. 

A detailed study of Perry Lake in northeast Kansas (fig. 5) 

was completed during 1992–93 by Fallon and others (2002). 

Water samples were collected monthly from four locations 

upstream and downstream from the reservoir, as well as within 

the reservoir throughout the year and from runoff during April 

to August (Thurman and Fallon, 1996; Fallon and others, 2002). 

Three-dimensional images of the distribution of atrazine 

concentrations in Perry Lake during 1992 showed that recently

applied atrazine mixed with atrazine applied the previous year 

as water moved through the reservoir (fig. 6). Relations 

between atrazine and the degradation product, deethylatrazine, 

indicated whether the atrazine in the reservoir had been flushed 

off the fields immediately prior to sampling or from the preceding year. Changes in atrazine concentrations in the reservoir 

resulted from several factors, including herbicide application, 

which fueled and reset the system, and precipitation, which 

drove the system by flushing atrazine into the reservoir and 

determining the residence time of water in the reservoir. Concentrations varied between the main inflow and the public water 

supply intakes located at the opposite ends of the reservoir. The 

concentration range in the outflow varied much less than concentrations in the upstream parts of the reservoir because of 

mixing (Fallon and others, 2002).

The frequency of occurrence, concentrations, both annual 

average and peak, and loads of pesticides in streams or reservoir 

outflows is related to the magnitude of the herbicide use in the 

upstream drainage areas. In most cases, the annual loss of herbicides to streams represents less than 5 percent of the amount 

applied. Pesticide losses are also affected by soils, climate, land 

cover, other basin characteristics, and management or application techniques. These studies demonstrate the strength of relations between the use of herbicides and their occurrence in 

streams and reservoirs (Battaglin and Goolsby, 1995, 1997, 

1998; Clark and others, 1999; Clark and Goolsby, 2000). 

Ground Water

During the 1990s, the Midwestern United States was also 

the focus of research on agricultural chemical contamination in 

ground water. A series of studies by USGS began in 1991 to 

investigate the occurrence of agricultural herbicides in nearsurface aquifers of the Midwestern United States. To improve 

understanding of the movement and distribution of agricultural 

chemicals in ground water, USGS designed (Kolpin and 

Burkhart, 1991) a reconnaissance network that was geographically and hydrogeologically representative of near-surface 

aquifers in the corn- and soybean-producing region of the Midwestern United States (Burkart and Kolpin, 1993; Kolpin and 

others, 1994, 1996b; Thurman and others, 1998). 

The objectives of the study were to determine the hydrogeological and seasonal distribution of herbicides in nearsurface aquifers, determine the statistical relations of 

agricultural herbicides to natural factors, and obtain data on 

agricultural herbicide concentrations in ground water from geographic areas where few data existed (Burkart and Kolpin, 

1993; Kolpin and others, 1994, 1996a). The report that sets the 

stage for these studies is Kolpin and others (1993a), which 

describes locations, detailed land use, and local features surrounding 303 wells sampled in the Midwestern United States 

during 1991 (fig. 7). During the summer of 1992, additional 

water samples were collected from 101 of the 303 wells to 

Table 2. Percentage of detections of triazine herbicides and 

degradation products in surface-water samples collected from 

selected streams in the Midwestern United States during 1989 

and 1990 (from Thurman and others, 1991).

[n= total number of samples analyzed; <, less than]

 Triazine compound

Percentage of detections

Preapplication 

samples

(n=53)

Postapplication 

samples 

(n=132)

Harvest 

samples

(n=145)

Parent compound

Atrazine 91 98 76

Cyanazine 5 63 0

Propazine 0 40 <1

Simazine 7 55 3

Degradation product

Deethylatrazine 54 86 47

Deisopropylatrazine 9 54 0
Water-Quality Studies 7

198919901994199519982002198919901994199519982002

0.01

0.10

1.0

10

100

1,000

Atrazine Cyanazine

198919901994199519982002198919901994199519982002

0.01

0.10

1.0

10

100

Propazine Simazine

1994199519982002198919901994199519982002198919901994199519982002

0.01

0.10

1.0

10

100

Cyanazine amide Deethylatrazine Deisopropylatrazine

A. Atrazine and cyanazine

B. Propazine and simazine

C. Cyanazine amide, deethylatrazine, and deisopropylatrazine

Concentration, in micrograms per liter

EXPLANATION

Outlier more

 than 95th

 percentile

Outlier less

 than 5th

 percentile

95th percentile

75th percentile

Median

25th percentile

5th percentile

Figure 4. Concentrations in surface-water

samples collected from 53 Midwestern 

streams during post-application runoff in 

1989, 1990, 1994, 1995, 1998, and 2002 for

(A) atrazine and cyanazine, (B) propazine 

and simazine, and (C) cyanazine amide, 

deethylatrazine, and deisopropylatrazine 

(data for 1989–98 from Scribner and others, 2000a; data for 2002 from Battaglin and 

others, 2003).

examine the occurrence of triazine herbicides and their degradation products not analyzed in 1991 (Kolpin and others, 1993b, 

1995). Herbicides and their degradation products were detected 

in water samples from 28.4 percent of the 303 wells sampled 

during 1991. During 1992, herbicides and their degradation 

products were detected in 62 percent of the 101 wells sampled. 

The greater frequency of detection during 1992 was due primarily to the increased number of herbicide degradation products 

examined and a more sensitive analytical method that was available (Kolpin and others, 1993b, 1995, 1996b). 

Although numerous studies were conducted during the 

early 1990s to investigate the occurrence of herbicides in 

ground water, few had considered the degradation products of 

these herbicides. Information on degradation products is essential to fully understand the fate and occurrence of the parent 

herbicides and to determine the complete consequences of herbicide use on human health and the environment (Kolpin and 

others, 1995). Triazine degradation products, deethylatrazine 

and deisopropylatrazine, commonly were found in shallow 

aquifers across the Midwestern United States. As shown in 

table 4, during 1991, deethylatrazine (21.4 percent) was 

detected more frequently than its parent compound atrazine 

(20.8 percent) even though deethylatrazine had a higher reporting limit than atrazine, whereas deisopropylatrazine was
8 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

Table 3. Triazine herbicides and their degradation products analyzed in samples collected from 76 Midwestern 

reservoir outflows, April 1992 through September 1993 (from Stamer and others, 1998a).

[μg/L, micrograms per liter; n, number of samples; <, less than]

Herbicide or degradation 

products

Reporting

limit 

(μg/L)

Percentage of

samples with 

detections

Median

concentrations 

(μg/L)

Mean of samples 

with

detectable

concentrations

(μg/L)

Herbicides (n=608)

Atrazine 0.05 82.1 0.43 1.36

Cyanazine .05 47.2 <.05 .61

Propazine .05 4.1 <.05 .10

Simazine .05 19.7 <.05 .21

Herbicide degradation products (n=511)

Cyanazine amide .05 49.3 <.05 .43

Deethylatrazine .05 71.6 .17 .39

Deisopropylatrazine .05 61.8 .08 .26

Deethylcyanazine .05 25.2 <.05 .14

Deethylcyanazine amide .05 2.0 <.05 .62

0 100

0 100 200 KILOMETERS

Base from U.S. Geological Survey digital data 1:2,000,000, 1972 200 MILES Albers Equal Area Conic Projection

Standard Parallels 39°30' and 43°30', central meridian -90°30'

Iowa

North Dakota

South

Dakota

Nebraska

Kansas

Perry

Lake

site Missouri

Minnesota

CANADA

Michigan

Michigan

Ohio

Indiana

Illinois

37°

43°

49°

100° 94°

88°

82°

Wisconsin

Figure 5. Location of 76 reservoirs 

sampled for herbicide analysis in the 

Midwestern United States during 1992 

and 1993 (from Scribner and others, 

1996).

detected in 7.6 percent of the samples. In 1992 (table 4), deethylatrazine was detected in 31 percent, atrazine in 43 percent, 

and deisopropylatrazine in 18 percent of the samples collected. 

The reporting limits for atrazine and deethylatrazine were lower 

in 1992 than in 1991. The fact that deethylatrazine was detected 

more frequently than deisopropylatrazine supports conclusions 

from field-dissipation studies during the 1990s that deethylation 

is the more stable, biotic degradation pathway (Adams and 

Thurman, 1991; Kolpin and others, 1994). 

Cyanazine was used extensively during the early 1990s but 

was detected in only 1.3 percent of the water samples in 1991 

and 3.0 percent of the water samples in 1992 (table 4). A 

method for cyanazine amide, a degradation product of cyanazine, was used to analyze water samples collected during 1992. 
Water-Quality Studies 9

(A) Pre-application survey,

March 30 to April 10, 1992

(B) First flush survey,

June 16 to 25, 1992

(C) Summer survey,

August 3 and 4, 1992

(E) Pre-application survey,

March 25 to 29, 1993

(D) Fall survey,

September 16 to 22, 1992

Greater than 15

10 to 15

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Less than 0.5

Triazine concentration,

in micrograms per liter

EXPLANATION

Figure 6. Three-dimensional computer images of atrazine concentrations in Perry Lake, northeastern Kansas, during 

(A) 1992 pre-application, (B) first flush, (C) summer, (D) fall, and (E) 1993 pre-application surveys (from Fallon and others, 

2002).
10 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

Table 4. Summary of analytical results for triazine herbicides and their degradation products in ground-water samples collected from 

wells in the Midwestern United States during 1991–93 (from Kolpin and others, 1993b, 1994; Kolpin and Thurman, 1995).

[μg/L, micrograms per liter; NA, not applicable]

Triazine compound

1991 1992 1993

Percentage 

of 

detection

Maximum 

concentration

(μg/L)

Reporting

limit

(μg/L)

Percentage 

of 

detection

Maximum 

concentration

(μg/L)

Reporting

limit

(μg/L)

Percentage 

of 

detection

Maximum 

concentration

(μg/L)

Reporting

limit

(μg/L)

Parent compound

Atrazine 20.8 2.1 0.05 43.0 1.0 0.005 23.6 1.8 0.05

Cyanazine 1.3 .68 .05 3.0 .02 .008 3.6 .88 .05

Simazine 1.6 .27 .05 13.0 .07 .005 NA NA NA

Degradation product

Cyanazine amide NA NA NA 9.3 .55 .05 NA NA NA

Deethylatrazine 21.4 2.3 .05 31.0 1.8 .015 23.6 1.7 .05

Deisopropylatrazine 7.6 1.2 .05 18.0 .28 .05 13.6 .48 .05

Base from U.S. Geological Survey digital data 1:2,000,000, 1972

Albers Equal Area Conic Projection

Standard Parallels 39°30' and 43°30', central meridian -90°30'

Iowa

North Dakota

South Dakota

Nebraska

Kansas

Missouri

Minnesota

CANADA

Wisconsin

Michigan

Michigan

Ohio

Indiana

Illinois

37°

43°

49°

0 100

0 100 200 KILOMETERS

200 MILES

100°

94°

88°

82°

Figure 7. Location of wells in the 

U.S. Geological Survey Midwestern 

ground-water monitoring network 

sampled during 1991–94 (from Kolpin and 

others, 1995).

As shown in table 4, cyanazine amide was detected in 

9.3 percent of the samples with a maximum concentration of 

0.55 μg/L (Kolpin and others, 1995). This greater frequency of 

detection suggests an increase in degradation product mobility

to ground water after transformation from cyanazine. Cyanazine may degrade to deisopropylatrazine by the loss of a 

cyanoisopropyl group (Meyer, 1994; Thurman and others, 

1994; Meyer and others, 2001). Simazine was detected in 

1.6 percent of the samples in 1991 and 13 percent in 1992 

(table 4). A study by Mills and Thurman (1994a) showed that 

atrazine, cyanazine, and simazine also can degrade to deisopropylatrazine. The dealkylation of simazine to deisopropylatrazine contributes to the amount of deisopropylatrazine as it may

degrade by two pathways to deisopropylatrazine (Mills and 

Thurman, 1992).

Water samples were collected during September and October from 110 wells during the historic 1993 flooding in the 

Mississippi River Basin (Goolsby and others, 1993; Kolpin and 

others, 1996b). There was a direct relation between increases in 

total herbicide concentration (between pre-flood and post-flood 
Water-Quality Studies 11

samples) and the occurrence of stream flooding near wells 

(Kolpin and Thurman, 1995; Kolpin and others, 1996b). More 

than 60 percent of the wells that had at least a 20-percent 

increase in total herbicide concentration in water samples were 

located in areas that also were affected extensively by stream 

flooding. Water in shallow wells more quickly reflected 

changes in water quality in response to changes in recharge 

from the 1993 flooding than did deeper wells (Kolpin and others, 1996b). Of the 110 wells sampled, 30 were reported to be 

unaffected by flooding or intense rainfall, 45 were affected by

stream flooding, and another 35 were affected by standing water 

from rainfall. 

Results from these samples were compared with those 

samples obtained during 1991 and 1992 from the same wells. 

As shown in table 4, there was no statistically significant difference in either the frequency of herbicide detection or total herbicide concentration between the previous samples and those 

collected for the 1993 study (Kolpin and Thurman, 1995). An 

additional study during July to August 1994 of samples collected from 38 wells showed that herbicides and their degradation products were detected in water from 76 percent of the 

wells (Kolpin and others, 1996b). 

An investigation of samples collected from 131 municipal 

wells completed in all the major aquifer types in Iowa (fig. 8) 

from 1995 to 1998 was made to compare the occurrence of herbicide degradation products in ground water with that of their 

parent compounds. An important finding of this study was the 

high frequency with which degradation products were detected 

in ground water (Kolpin and others, 1997, 1998, 2000). During 

1995, more than one herbicide compound was detected in samples from 44 of the wells, with as many as nine compounds 

being detected in a single sample (Kolpin and others, 1997). 

Table 5 summarizes analytical results from 1995 through 1998. 

Atrazine is the only herbicide for which the parent compound 

was found more often than any of its degradation products. This 

could be because of the greater environmental stability of atrazine compared to the other parent compounds under investigation. Although herbicides commonly were detected, their 

frequencies of detection and concentrations varied significantly

among the major aquifer types samples. These differences were 

much more pronounced when herbicide degradation products 

were included (Kolpin and others, 2000). Aquifer types presumed to have the most rapid recharge rates were those most 

likely to contain detectable concentrations of herbicides, indicating that ground-water age could be an important factor in 

explaining these variations in herbicide contamination (Kolpin 

and others, 2000). This study demonstrated that obtaining data 

on herbicide degradation products is critical for understanding 

the fate of herbicides in the hydrologic system (Kolpin and 

others, 2000).

A large-scale investigation of the occurrence of cyanazine 

and its degradation products, cyanazine acid, cyanazine amide, 

deethylcyanazine, and deethylcyanazine acid in samples from 

64 Iowa wells was conducted during 1999. Cyanazine was 

detected in only 3.1 percent of wells sampled. but its degradation products were commonly detected, deethylcyanazine acid 

(32.8 percent), followed by cyanazine acid (29.7 percent), 

cyanazine amide (17.2 percent), and deethylcyanazine (3.1 percent) (Kolpin and others, 2001). The more frequent presence of 

cyanazine acid and cyanazine amide compared to cyanazine is 

in agreement with the reported greater stability of these degradation products after transformation from cyanazine (Ferrer and 

others, 2000; Kolpin and others, 2001). Thus, the results of the 

study emphasized that data on the cyanazine degradation products are critical for understanding its fate and transport in the 

hydrologic system (Kolpin and others, 2001). 

As a continuation of the Iowa study, ground-water wells 

were sampled and analyzed for 21 herbicides and 24 herbicide 

degradation products during 2001. The frequency of detection 

increased from 17 percent when only herbicide parent 

96°

43°

42°

41°

95° 94° 93° 92°

91°

0 50

0 50 100 KILOMETERS

100 MILES

Iowa

EXPLANATION

Well sampled during 1995−98

Figure 8. Location of wells sampled in Iowa during 1995–98 

(from Kolpin and others, 2000).

Table 5. Summary of analytical results for triazine herbicides and 

their degradation products in samples collected from 131 Iowa 

wells during 1995–98 (from Kolpin and others, 2000).

[μg/L, micrograms per liter]

Triazine compound Percentage 

detection

Maximum 

concentration 

(μg/L)

Reporting 

limit

(μg/L)

Parent compound

Atrazine 37.4 2.1 0.05

Cyanazine 6.1 .51 .05

Simazine 0 0 .05

Propazine 0 0 .05

Degradation product

Cyanazine amide 19.8 .64 .05

Deethylatrazine 32.1 .59 .05

Deisopropylatrazine 21.4 1.1 .05

Hydroxyatrazine 11.4 1.3 .20
12 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

compounds were considered to 53 percent when both herbicide 

parent compounds and degradation products were considered. 

A significant difference in results among the major aquifer 

types was apparent when both herbicide parent compounds and 

their degradation products were considered (Kolpin and others, 

2004).

The ground-water samples collected across the Midwestern United States and in Iowa from 1991 through 2001 consistently revealed that triazine herbicide degradation products 

commonly were found more frequently than their parent herbicide, the frequency of herbicide detection was, in part, affected 

by a more sensitive analytical method, and ground-water age 

could be an important factor in explaining variations in herbicide contamination (Kolpin and others, 1994, 1996a, 2000, 

2004).

Precipitation

During the late spring and summer of 1990 and 1991, 

another USGS study focused on herbicides transported in the 

atmosphere by various processes. In the atmosphere, these compounds can be dispersed by air currents and redeposited on the 

land surface, lakes, and streams by rainfall, snow, and dry deposition, often at considerable distances from their source areas. 

Triazine herbicides were reported in precipitation in the Midwestern United States by Goolsby and others (1997), which 

highlighted the major findings of this study. The report that 

describes the location of the study area, background areas, and 

sampling sites is Goolsby and others (1995).

Herbicide concentrations exhibited distinct geographic 

and seasonal patterns. The highest concentrations occurred in 

the Midwestern United States following herbicide application 

to cropland. The highest concentrations occurred following herbicide application to cropland. Rainfall-weighted 

concentrations of 0.20 to 0.40 μg/L for atrazine were typical 

throughout the Midwest, and weighted concentrations as large 

as 0.60 to 0.90 μg/L occurred in precipitation at several sites. 

Concentrations of 1.0 to 3.0 μg/L were measured in a few individual samples. Atrazine was detected most often, followed by

deethylatrazine, cyanazine, and deisopropylatrazine. 

Table 6 contains a summary of the concentrations of triazine herbicides detected in 6,230 precipitation samples screened 

by enzyme-linked immunosorbent assay and then confirmed in 

2,085 of the samples using gas chromatography/mass spectrometry (Pomes and others, 1998). The most frequently detected 

herbicide was atrazine, which was present in 30.2 percent of the 

samples analyzed by gas chromatography/mass spectrometry, 

followed by deethylatrazine at 17.4 percent. Cyanazine was 

detected in 7.2 percent of the samples. Although herbicides 

were detected in a significant number of samples, concentrations were relatively small (Goolsby and others, 1997; Stamer 

and others, 1998b; Thurman and Cromwell, 2000). 

Because of the large temporal and spatial variation in the 

amount of rainfall, it is difficult to make meaningful comparisons of herbicide concentrations among sites or over time on the 

basis of individual samples. Instead, comparisons were made 

with rainfall-weighted concentrations. Overall, the spatial patterns of the weighted atrazine concentrations in 1990 and 1991 

were similar and generally reflected the intensity of the use of 

atrazine. The total amount of atrazine deposited annually in precipitation in the study area represented about 6 percent of the 

atrazine applied annually to crops in the study area (Goolsby

and others, 1997).

The study showed that the occurrence of atrazine in rainfall is highly seasonal. The detection frequency began to 

increase in mid-April following application of herbicides to 

cropland and peaked in late May or early June. Atrazine was 

detected in low concentrations at sites remote from cropland, 

such as Maine and Isle Royale in northern Lake Superior 

(Stamer and others, 1998b; Thurman and Cromwell, 2000). 

Table 6. Summary of analytical results for triazine herbicides and their degradation product concentrations measured in precipitation 

samples from 81 sites across the Midwestern and Northeastern United States from March 1990 through September 1991 (from Goolsby 

and others, 1997).

[ELISA, enzyme-linked immunosorbent assay; GC/MS, gas chromatography/mass spectrometry; n, total number of samples analyzed; μg/L, micrograms per liter; 

<, less than; --, not determined]

Triazine compound Percentage of 

detections 

Concentration (μg/L) for indicated percentiles1

1Reporting limits are 0.10 μg/L for triazine compounds analyzed by ELISA and 0.05 μg/L for all compounds analyzed by GC/MS.

75th 90th 95th 99th 100

(maximum)

 ELISA analysis (n=6,230)

Triazine compounds 25.5 0.10 0.24 0.42 1.3 16

GC/MS analysis (n=2,085)

Atrazine 30.2 .07 .23 .40 1.0 11

Cyanazine 7.2 -- <.05 .07 .27 2.0

Deethylatrazine 17.4 <.05 .11 .15 .39 .75

Deisopropylatrazine 2.6 -- -- <.05 .17 1.2

Simazine 1.5 -- -- <.05 .07 1.5
Chemistry and Transport of Triazine Herbicides in Water 13

Chemistry and Transport of Triazine 

Herbicides in Water

Degradation pathways may be followed from herbicide 

application on the soil to its entrance into an aquifer. These degradation pathways depend on the characteristics of the transport 

pathway (Adams and Thurman, 1991). Herbicides derived from 

point and nonpoint sources can be transported to streams by

runoff from agricultural and urban areas, discharge from reservoirs and aquifers, and precipitation (Goolsby and others, 

1993). The physiochemical properties of the triazine herbicides 

as well as the soil half-life and such factors as intensity of use, 

application methods, timing, intensity, duration of rainfall, and 

antecedent soil moisture conditions are also important in determining the amount of herbicides that can be transported into 

streams. Physiochemical properties shown in table 7 indicate 

that soluble and mobile herbicides, such as atrazine, cyanazine, 

propazine, and simazine are transported primarily in the dissolved phase. 

Water solubility determines how easily herbicides wash 

off soil and crop residues and leach through the soil. Herbicides 

with solubilities greater than about 30 mg/L are likely to wash 

off the soil, whereas those with solubilities less than 1 mg/L are 

likely to remain on the soil surface during storms (Becker and 

others, 1989). The soil sorption coefficient (Koc) is a measure of 

the tendency of a herbicide to sorb to soil particles. The larger 

the Koc value, the more strongly the herbicide will be adsorbed 

to soil particles. Those herbicides with Koc values less than 

about 500 mL/g tend to be transported primarily in the dissolved 

phase, whereas Koc values greater than 1,000 mL/g are transported primarily on suspended-sediment particles (Becker and 

others, 1989; Goolsby and others, 1993). Half-life in soil is the 

length of time required for herbicides to degrade in the soil to 

one-half of their previous concentration. The longer the soil 

half-life, the more persistent the herbicide is, and the longer it 

will be available to wash off in storm runoff. Once herbicides 

are in surface water, the half-life generally is much longer than 

in soil because surface water contains much less organic matter 

and fewer microorganisms to degrade the herbicides (Goolsby

and others, 1993). The water solubility of deethylatrazine is 

670 mg/L and for deisopropylatrazine is 3,200 mg/L. The water 

solubilities of triazine degradation products increase as carbon 

atoms are removed by dealklyation (Mills and Thurman, 

1994a). The result is that the degradation products are more soluble and mobile than the parent compound. 

A study of selected triazine herbicides and their degradation products was designed to define the relative rates and transport mechanisms of four parent triazine herbicides—atrazine, 

cyanazine, propazine, simazine—and two triazine degradation 

products—deethylatrazine and deisopropylatrazine—in the 

unsaturated zone and in surface runoff (Meyer, 1994; Mills and 

Thurman, 1994a; Thurman and others, 1994; Meyer and Thurman, 1996). This choice of parent herbicides was used because 

each degradation product had two parent herbicide sources, 

each source requiring the removal of either an ethyl or an 

isopropyl side chain. As shown by figure 9, atrazine and propazine degrade to deethylatrazine by deethylation and deisopropylation. Similarly, atrazine, cyanazine, and simazine can all 

dealkylate to deisopropylatrazine by removal of an isopropyl 

and ethyl side chain. Differences in the concentration of the 

dealkylated degradation product from the two different sources 

should indicate any preferential removal of ethyl versus isopropyl side chain. Furthermore, because deethylatrazine and deisopropylatrazine have different side chains remaining, their 

relative rate of removal should provide additional information 

on the stability of the ethyl side chain versus an isopropyl side 

chain (Thurman and others, 1994). 

The dealkylation field study found the preferential 

removal of an ethyl side chain from atrazine relative to the 

removal of an isopropyl side chain. The high ratio of deethylatrazine to atrazine, referred to as the DAR, in shallow soil is 

evidence that simazine may degrade more rapidly to deisopropylatrazine (ethyl side chain removal) than propazine to deethylatrazine (isopropyl side chain removal). Furthermore, 

deethylation rates of atrazine and simazine are comparable, and 

approximately two to three times more rapid than the rates of 

deisopropylation from atrazine and propazine. This indicates 

that the removal of an ethyl side chain is preferential over an 

isopropyl side chain, regardless of parent triazine compound. 

Continued dealkylation of the degradation products in the 

unsaturated zone, where the parent compound is removed, also 

shows a preferential removal of ethyl side chains over isopropyl 

side chains, as deisopropylation can occur to a significant 

Table 7. Physiochemical properties of triazine herbicides (from Goolsby and others, 1993).

[mg/L, milligrams per liter; mL/g, milliliters per gram; pKa, negative logarithm of the acid dissociation constant; mPa, millipascal]

Triazine herbicide Water solubility 

(mg/L)

Soil sorption 

coefficient 

(Koc, mL/g)

pKa Soil half-life 

(days)

Vapor pressure

(mPa)

Atrazine 33 100 1.7 60 0.04

Cyanazine 170 190 1.7 14 .0002

Propazine 5 154 1.7 45 .004

Simazine 6.2 138 1.6 75 .000810
14 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

N

N

N

NH

Cl

C2H5

NH

C2H5

Simazine

Cyanazine

N

N

N

NH

Cl

C2H5

NH

C

CH3

CH3 C

N

C

N

N

N

NH

Cl

H

NH

CH

CH3

CH3

2 5

Atrazine

N

N

N

NH

Cl

CH

NH

CH

CH3 CH3

CH3 CH3

Propazine

N

N

N

NH

Cl

C2H5

NH2

Deisopropylatrazine

(DIA)

N

N

N

NH2

Cl

NH

CH

CH3

CH3

Deethylatrazine

(DEA)

Figure 9. Degradation pathways for atrazine, cyanazine, propazine, and simazine to deethylatrazine and (or) deispropylatrazine (from Thurman and others, 1994).

degree in the unsaturated zone. Therefore, the small concentrations of deisopropylatrazine commonly reported in the environment do not result purely from a smaller production of the 

degradation product but also to the more rapid removal of the 

isopropyl group once produced. The substantial turnover rate of 

deisopropylatrazine in the environment is evidence for the presence of a didealkylated degradation product in the unsaturated 

zone. The data supporting these conclusions are presented in 

Mills and Thurman (1994a).

To understand the geochemistry of the deethylatrazine-toatrazine ratio (DAR) as an indicator of surface-water and 

ground-water interaction, it is valuable to compare the degradation product deethylatrazine to atrazine. The resulting ratio is 

called the DAR and is defined by Adams and Thurman (1991):

. (1)

The DAR may be a valid indicator of soil-mediated transport of atrazine to an aquifer. A DAR of unity (1.0) or greater 

could be an indicator of nonpoint-source contamination of an 

aquifer (Adams and Thurman, 1991), whereas a small DAR 

may be an indicator of point-source contamination of an aquifer. In clay- and silt-loam plots, the average concentrations of 

atrazine in the soil were about four times higher than the average concentrations of deethylatrazine. The concentrations of 

deethylatrazine, however, were significantly higher than the 

concentrations of atrazine in soil-water lysimeter samples collected at about the same time as the cores were collected 

(Adams and Thurman, 1991). These results indicate that atrazine is absorbed more strongly than deethylatrazine in clay- and 

silt-loam soils. Thus, the recharging ground water has a higher 

concentration of deethylatrazine than atrazine giving rise to 

DAR values greater than 1.0 (Adams and Thurman, 1991).

The conversion of atrazine to deethylatrazine is primarily

the result of the metabolic activity of soil bacteria and fungi. In 

the case of nonpoint-source contamination, soil microorganisms can convert significant quantities of atrazine to deethylatrazine, thereby increasing the DAR. Conversely, point-source 

contamination resulting from direct entry of atrazine into an 

aquifer by a natural or artificial conduit would not involve 

prolonged contact of the applied atrazine with soil microorganisms. In such a point-source-contamination occurrence, soil 

microorganisms would have little opportunity to degrade to 

deethylatrazine. Therefore, in the case of point-source contamination, the DAR may be expected to be much less than unity; 

that is, the concentration of deethylatrazine would be less than 

that of atrazine in the aquifer (Adams and Thurman, 1991). 

The DAR hypothesis is predicated on the assumption that 

atrazine degrades slowly in an aquifer because of low organic 

carbon concentrations, small microbial populations, and anaerobic conditions. This is substantiated by Wehtje and others 

(1983) who determined that, under aquifer conditions, atrazine 

did not undergo deethylation or deisopropylation and only

slowly underwent abiotic degradation to deisopropylhydroxyatrazine.

Adams and Thurman (1991) found that atrazine transport 

through the unsaturated zone gave DAR values greater than 1.0, 

whereas atrazine transported off the field by surface runoff had 

DAR values much less than 1.0. Deisopropylatrazine was rapidly degraded in the unsaturated zone but was an important persistent degradation product in surface runoff from the fields. 

Thus, the DAR may be an indicator of ground-water recharge 

by atrazine-laden water. Figure 10 shows that the 

DAR [ ] deethylatrazine, mol/L

[ ] atrazine, mol/L = ----------------------------------------------------------
Chemistry and Transport of Triazine Herbicides in Water 15

Pre-application

DAR = 0.3

Deisopropylatrazine = 9 percent detections

Harves

Land surface

Land surface

Land surface

t

DAR = 0.4

Deisopropylatrazine = 0 percent detections 

Post-application

DAR = less than 0.1

Deisopropylatrazine = 54 percent detections 

SW input (small)

SW input (small)

SW inpu t (large)

GW input (large)

GW input (large)

GW

GW

GW

SW

Stream SW

Stream

Stream

EXPLANATION

Water table

Ground water

Surface water

Figure 10. Deethylatrazine-to-atrazine ratios (DAR) for surface- and ground-water samples from Midwestern 

United States (from Thurman and others, 1992).

pre-application samples had a high DAR of 0.3 but also contained a few detections of deisopropylatrazine (9 percent). 

These results suggest both a surface-water and a ground-water 

origin of herbicides during this sampling period. The postapplication samples collected during this period had a low DAR 

(less than 0.1) and the highest number of detections of deisopropylatrazine (54 percent), which indicate that surface runoff was 

the major contributor of herbicides. Finally, the post-harvest 

sampling during low streamflow had the highest DAR of 0.4 

and detections of deisopropylatrazine, which indicate that alluvial ground water was likely the major source of herbicide 

(Thurman and others, 1992). Following the surface-water 

reconnaissance of Midwestern streams, Thurman and others 

(1992) reported DAR values less than 0.1 in streams shortly

after herbicide application, whereas DAR values generally were 

0.3 or higher in the fall at harvest during base-flow conditions. 

As shown by figure 11, the median DAR values obtained from 

USGS ground-water reconnaissance for both sampling periods 

were about 0.7. The DAR ranged from about 0.1 to 8.4. These 

results indicate that most of the atrazine detected in ground 

water was derived from nonpoint sources and had infiltrated the 

soil slowly (Kolpin and others, 1994).

Further field-dissipation studies of atrazine and cyanazine 

demonstrated that about 6 percent of the atrazine degraded to 

deethylatrazine and about 3 percent degraded to deisopropylatrazine (Adams and Thurman, 1991; Mills and Thurman, 

1994a). Cyanazine degraded only to the common degradation 

product, deisopropylatrazine. Thus, the ratios of deisopropylatrazine to deethylatrazine (D2R) and atrazine to cyanazine 

(ACR) were used to differentiate the nonpoint source of deisopropylatrazine to surface water. Meyer and others (2001) 

successfully tested the D2R discrimination diagram in two 

basins with very different application rates of atrazine and 

cyanazine to check the validity of the D2R. The D2R also was 

used to show that cyanazine contributed a considerable amount 

(40 percent) of the deisopropylatrazine that was transported 

during flooding of the Mississippi River in 1993. The D2

R is a 

useful water-quality monitoring tool to measure nonpointsource contributions of deisopropylatrazine (Meyer and others, 

2001).

A regional study of nine streams in the Midwestern Corn 

Belt confirmed that deethylatrazine and deisopropylatrazine 

occur frequently in surface water that has received storm runoff 

containing two parent triazine herbicides, atrazine and cyanazine (Scribner and others, 1994; Pomes and others, 1996). The 

concentrations of deethylatrazine and deisopropylatrazine in 

surface water varied with the hydrologic conditions of the basin 

and the timing of runoff, with maximum concentrations
16 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

34

44

114

69

50

10

5.0

1.0

0.50

0.05

0.01

0.10

Pre-application

Post-application

Post-application

Harvest

Ground-water

reconnaissance

Surface-water

reconnaissance

Ratio of deethylatrazine to atrazine concentration

EXPLANATION

Number of samples

90th percentile

75th percentile

Median

25th percentile

10th percentile

34

Figure 11. Deethylatrazine-toatrazine ratios (DAR) for surfaceand ground-water samples from 

Midwestern United States (from 

Kolpin and others, 1994).

reaching 5 μg/L (deethylatrazine plus deisopropylatrazine). 

Early rainfall, followed by a dry summer, delayed the maximum 

concentrations, giving a second flush of triazine degradation 

products to surface water. Replicated field-dissipation studies 

of atrazine and cyanazine indicated that D2

Rs varied from 0.4 

+ 0.1, when atrazine was the major triazine compound present, 

to 0.6 + 0.1 when significant amounts of cyanazine were 

present. A comparison of transport time of deethylatrazine and 

deisopropylatrazine from field plots to their appearance in surface water indicated that storage and dilution were occurring in 

the alluvial aquifers of the nine stream basins (Thurman and 

others, 1994).

In a study by Thurman and Fallon (1996), a conceptual 

model was presented to explain how the DAR could be used to 

follow runoff through Perry Lake in northeast Kansas 

(fig. 12A). In the model, the reservoir already contained water 

with atrazine and deethylatrazine present from the previous 

year’s runoff. The arrows show the direction of the flow with 

the dam at the downstream end of the reservoir. Near the dam, 

the water has been in the reservoir longer and contains water 

and herbicides from the previous summer, whereas the 

upstream water contains herbicides that had entered the 

reservoir during the fall and winter. Therefore, the DAR values 

were higher at the upstream end of the reservoir. The actual 

DAR values for each time period are shown in figure 12B. The 

concept of the DAR proved to be a useful tool in the ability to 

follow water movement through a reservoir.

In the precipitation study described earlier (Goolsby and 

others, 1997), deethylatrazine and deisopropylatrazine were 

detected in 17.4 and 2.6 percent, respectively, of the samples 

analyzed. Deethylatrazine was present in more than one-half of 

the samples that contained atrazine and was the third most frequently detected compound. The median DAR value calculated 

for the 352 rainfall samples that contained both atrazine and 

deethylatrazine was 0.5. All of these DAR values were 

determined for rainfall samples collected from May through 

July of 1990 and 1991 when the median DAR values for 

streams in this area were less than 0.1 (Thurman and others, 

1992). Considerably more deethylatrazine than atrazine was 

present in rainfall compared to streams draining the Corn Belt. 

There are several possible explanations for this observation. 

One is that deethylatrazine evaporates from the soil into the 

atmosphere faster than atrazine, resulting in a higher DAR value 

in rainfall. Another is that the high DAR value in rainfall is from 

atrazine transformed to deethylatrazine in the atmosphere by

photochemical processes. Thus, it is possible that the high 

median DAR value of 0.5 in rainfall reflects the photocatalyzed 

degradation of atrazine in the atmosphere (Goolsby and 

others, 1997).

Finally, other important degradation products of atrazine 

that provide evidence of surface-water/ground-water interaction are hydroxylated atrazine degradation products (HADPs), 

which include hydroxyatrazine, deethylhydroxyatrazine, and 

deisopropylhydroxyatrazine. In general, HADPs are strongly

bound to soil by two sorption mechanisms, cation exchange and 

hydrophobic interaction, which occur simultaneously. This is 

referred to as a mixed-mode model of sorption as shown in figure 13. Evidence of this model is provided by chromatographic 

studies, using solid-phase extraction and reverse-phase liquid 

chromatography, as well as soil-extraction studies that show 
Chemistry and Transport of Triazine Herbicides in Water 17

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

North

Pre-herbicide application

(A) Conceptual model

(B) Data from Perry Lake

March 1992 pre-application

High DAR = 0.22 to 0.29

Low DAR = 0.18 to 0.21

June 1992 post-application

High DAR = not present

Low DAR = 0.09 to 0.17

Post-herbicide application

High DARs

Low DARs

Direction of flow

EXPLANATION

Dam

Dam Dam

Dam

Water with high DARs enters

reservoir in pre-application

inflow

Water with low DARs enters

reservoir in post-application

runoff, pushing water with

higher DARs farther 

downstream

Figure 12. Conceptual model and data for deethylatrazine-to-atrazine ratios (DARs) in Perry Lake, 

northeastern Kansas, during March 1992–March 1993 (modified from Thurman and Fallon, 1996, fig. 5).
18 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

the release of HADPs bound by mixed-mode sorption (Lerch 

and others, 1997).

As shown by figure 14A, during runoff conditions, a combination of dissolved-phase transport by surface runoff and 

desorption from suspended stream sediment is the source of 

HADPs, whereas during base-flow conditions, the primary

source of HADPs is desorption from bed sediments in the 

stream, as shown in figure 14B. Dissolved-phase transport of 

hydroxyatrazine by surface runoff has been measured directly

in end-of-field samples from a north-central Missouri field 

treated with atrazine in 1994. Surface runoff from precipitation 

6 to 7 days after atrazine application showed hydroxyatrazine 

concentrations of 3.3 to 5.4 μg/L in flow-weighted samples 

collected throughout the course of the precipitation. Thus, surface runoff has the ability to extract or desorb a fraction of the 

hydroxyatrazine present in the soil. In addition, continuous 

monitoring of hydroxyatrazine in Goodwater Creek, also 

located in Missouri, showed that hydroxyatrazine mass flux was 

directly related to streamflow, indicating that increased 

hydroxyatrazine mass flux during runoff resulted from dissolved-phase transport (Lerch and others, 1998). 

Persistence of Triazine Herbicides

Another important finding of USGS research during the 

1990s was the persistence of triazine herbicides in surface 

water. Once applied to soil and crops, triazine compounds begin 

to degrade through biotic and abiotic reactions. Much of the 

research on the fate of triazine compounds prior to the late1980s 

addressed only the parent herbicide because most of the triazine 

degradation products were assumed not to be effective herbicides and, therefore, were not of particular interest to the agronomists. During the 1990s, there was an increased focus on

Clay mineral

Organic matter coating

Hydrophobic interaction

Cation exchange

HN

N

HN

NH

O

NH

C−COO+

N

N

N

H

H NH

O

H2N

Si—O+

Figure 13. Mixed-mode model of sorption for

hydroxylated atrazine degradation products (from 

Lerch and others, 1997).

Stream water containing

dissolved-phase HA and

suspended sediment

Older bed

sediment

Surface runoff

Surface runoff

(A) Runoff conditions

Stream

Stream water containing

dissolved-phase HA

Older bed

sediment

Newer bed sediment

containing dissolvephase HA

(B) Base-flow conditions

Stream

Ground water Ground water

Figure 14. Proposed models for the 

mechanisms controlling concentrations of hydroxylated atrazine degradation products (HADPs) in streams. The 

figure depicts the cross-sectional view 

of a stream channel. (A) During runoff 

conditions, hydroxyatrazine (HA) is 

transported primarily from atrazinetreated fields to the stream in the dissolved phase. An additional source of 

HA in streams is suspended sediment 

derived from erosion of HA-contaminated soil. (B) During baseflow conditions (that is, ground water as the 

source of streamflow), HA is desorbed 

by ground water flowing through the 

HA-contaminated sediment deposited 

in the streambed (from Lerch and 

others, 1998).
Persistence of Triazine Herbicides 19

accounting for the ultimate fate of triazine degradation products 

(Adams and Thurman, 1991; Thurman and others, 1994).

These studies found that the frequency of detection, or 

apparent order of stability, of triazine herbicides and their degradation products was atrazine, deethylatrazine, deisopropylatrazine, and cyanazine. It was also found that temporal trends in 

the number of detections in both pre-application and postharvest samples, and in the range of the distribution during 

sampling periods, of various herbicides was significant. In a 

study by Thurman and others (1991, 1994), measurable 

amounts of atrazine, the most frequently detected herbicide, 

occurred in 91 percent of the pre-application samples, 

98 percent of the post-application samples, and 76 percent of 

the harvest samples. The atrazine degradation product, 

deethylatrazine, was found in many pre-application, postapplication, and harvest samples that contained atrazine.

Substantial evidence exists that another major biotic degradation process, dealkylation for atrazine by both bacteria and 

soil fungi, does occur. Several bacteria and fungi species have 

been shown to prefer deethylation, whereas others prefer deisopropylation. The presence of deethylatrazine in surface and 

ground water is ascribed almost exclusively to the degradation 

of atrazine by the removal of an ethyl group. Atrazine degradation pathways are shown in figure 15. Deethylatrazine is the 

most important degradation product found in water-quality

studies throughout the Midwestern United States. Deisopropylatrazine is formed by the removal of an isopropyl group and 

also may occur from the degradation of cyanazine and simazine (Mills and Thurman, 1994a; Meyer and others, 2001). It 

may be degraded further to didealkylatrazine by removal of an 

ethyl group (fig. 15). Both the production of deethylatrazine and 

the degradation of deisopropylatrazine proceed by removal of 

an ethyl group. Therefore, the higher concentrations of deethylatrazine and the smaller concentrations of deisopropylatrazine 

during atrazine dealkylation may be a reflection of the greater 

ease of deethylation versus deisopropylation. These studies also 

show that concentrations of deethylatrazine and deisopropylatrazine in surface water vary with hydrologic conditions in the 

basin and the timing of runoff, with maximum concentrations 

reaching 5 μg/L (deethylatrazine plus deisopropylatrazine) 

(Mills and Thurman, 1994a; Thurman and others, 1994, 1998).

Early studies of atrazine degradation products focused on 

the chlorinated atrazine degradation products because of their 

greater water solubility, lower soil adsorption compared to the 

parent compound, and the fact that they are more amenable to 

gas chromatography/mass spectrometry analysis than the 

hydroxy degradation products. However, hydroxylated atrazine 

degradation products, particularly hydroxyatrazine, are the 

major degradation products of atrazine in most soils. Hydroxylated atrazine degradation products (HADPs), which include 

hydroxyatrazine, deethylhydroxyatrazine, and deisopropylhydroxyatrazine, are a major class of atrazine degradation products that form in the environment through chemical, biological, 

and photochemical hydrolysis of atrazine or dealkylated atrazine degradation products, resulting in replacement of chlorine 

with a hydroxyl group at the two-position of the triazine ring. In 

soil or water, the rate of hydrolysis is enhanced by extremes in 

pH, dissolved organic matter, sorption to soil colloids, and the 

presence of photosensitizing compounds. HADPs have been 

shown to be more persistent in soil than atrazine and humic 

acids (Lerch and others, 1998).

Studies by Meyer (1994), Meyer and Thurman (1996), and 

Meyer and others (1996, 2001) suggest that cyanazine is more 

stable in the aquatic environment compared to the soil environment. The studies also showed that the half-life of cyanazine in 

the soil varied from 14 to 21 days (Meyer and Thurman, 1996). 

However, degradation products of cyanazine have been identified in the soil as long as 4 years after application (Meyer and 

Thurman, 1996). The cyanazine degradation pathway is shown 

in figure 16. The degradation process for herbicides generally

increases the water solubility and the polarity of the compound. 

The increase in solubility is caused by the loss of carbon, the 

incorporation of oxygen, and the addition of carboxylic-acid 

functional groups. For every carbon atom that is removed, the 

water solubility will increase from two to three times (Meyer 

and Thurman, 1996). 

A study by Meyer (1994) showed that atrazine and cyanazine were dissipated in the top 12 in. of the soil and they were 

not detected below 18 and 24 in. Deethylatrazine was the primary dealkylated compound produced from the degradation of 

atrazine. Stable production of deethylatrazine and deisopropylatrazine in the shallow soil was marked by relatively constant 

D2R of about 0.4 throughout the growing season. In the deeper 

soil where the parent compound, atrazine, was diminished, the 

D2R was generally less than 0.2 or nonexistent due to the 

absence of deisopropylatrazine. The systematic decrease in the 

concentrations of deisopropylatrazine relative to deethylatrazine with depth was determined to be from the preferential 

removal of the ethyl group from deisopropylatrazine and not 

from the preferential transport of deethylatrazine (Meyer, 1994; 

Meyer and others, 2001). 

It was found that cyanazine amide dissipated rapidly and 

was not readily transported through the shallow soil (fig. 16). 

The concentration of cyanazine amide often was greater than 

cyanazine in both soil and pore water, and by mid-season, concentrations indicated that cyanazine amide was more readily

transported through the unsaturated zone than cyanazine 

(Meyer, 1994; Meyer and others, 1996). The deethylated cyanazine degradation products, deethylcyanazine and deethylcyanazine amide, were identified only in the early season when 

the concentrations of cyanazine and cyanazine amide were 

high. They dissipated rapidly and were not transported at detectable levels deeper than 12 in. The primary degradation product 

of cyanazine that was measured with depth was deisopropylatrazine. It was detected as deep as 30 in. at concentrations 

greater than deethylatrazine (Meyer, 1994; Meyer and others, 

2001). 

The high DAR values showed that atrazine was not 

responsible for the deisopropylatrazine found in the soil-core 

samples in the cyanazine field plots. Also, deisopropylatrazine 

generally was detected later in the growing season and at deeper 
20 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

 

Cyanuric acid

Ring cleavage

CO2 + NH3 + H2O

Figure 15. Degradation pathways for atrazine.
Persistence of Triazine Herbicides 21

N

N

N

Cl

NH C C

H3C

CH3

NH2 N

Deethylcyanazine

N

N

N

Cl

NH C C

H3C

CH3

CH3 NH

CH2

O

OH

Cyanazine acid

N

N

N

Cl

NH C C

H3C

CH3

NH2

O

OH

Deethylcyanazine acid

N

N

N

Cl

CH3 NH

CH2

NH2

Deisopropylatrazine

N

N

N

Cl

NH

NH

C C

H3C 2

CH3

CH3 NH

CH2

O

Cyanazine amide

N

N

N

Cl

NH C C

H3C

CH3

CH3 NH

CH2

N

Cyanazine

N

N

N

Cl

NH C C

H3C

CH3

NH2

NH2

O

Deethylcyanazine amide

N

N

N

Cl

H N2 NH2

Didealkylatrazine

Figure 16. Degradation 

pathways for cyanazine.

levels than either cyanazine or cyanazine amide. Thus, the 

detection of deisopropylatrazine at depth is probably the result 

of the degradation of another cyanazine degradation product 

that is rapidly transported through the unsaturated zone, cyanazine acid. Cyanazine amide and cyanazine acid both have 

modified isopropyl moieties that may be removed by microbial 

degradation. The rapid dissipation of cyanazine amide suggests 

that cyanazine acid is formed rapidly. Furthermore, because 

cyanazine acid is ionic at normal pH, it may be preferentially

leached through the soil. As it is leached, it also may undergo 

deethylation and deisopropylation (Meyer and others, 1996, 

2001). 

As previously shown in figure 9, simazine also degrades to 

deisopropylatrazine. However, as already stated, about 15 to 
22 Summary of Significant Results From Studies of Triazine Herbicides and Their Degradation Products

20 times more atrazine than simazine was applied in the United 

States during the early 1990s (Gianessi, 1992), which is why the 

presence of deethylatrazine and deisopropylatrazine in surface 

and ground water is ascribed almost exclusively to the degradation of atrazine. 

Finally, propazine has not been sold since 1990, except to 

States that requested a USEPA Section 18 Emergency Exemption; however, it still occurred routinely in water because it was 

a contaminant present in atrazine at about 1 percent (Thurman 

and Fallon, 1996). Propazine is very persistent in the soil. It is 

broken down primarily by microbial action and, therefore, will 

persist longer in dry, cold conditions or in other conditions that 

inhibit microbial activity. In a study of playa lakes in the High 

Plains of Texas (fig. 1), atrazine and propazine were detected 

frequently (Thurman and others, 2000). Because propazine has 

a longer soil half-life (35 to 231 days) than atrazine (45 to 

60 days), it is thought that atrazine was responsible for the 

majority of the deethylatrazine. Commonly, propazine was 

detected at much smaller concentrations than atrazine, perhaps 

because of little use during the 1990s. However, it is apparent 

from these data that propazine was still being used in Texas during the 1990s. Figure 9 also shows the structures of propazine 

and its respective dealkylation reactions to deethylatrazine by

the removal of an isopropyl group (Thurman and others, 2000).

Summary

Triazine herbicides in surface water, ground water, and 

precipitation have been a major water-quality concern 

throughout the United States. The herbicides described in this 

report include atrazine, cyanazine, propazine, and simazine. 

Modern agricultural practices in the United States often require 

extensive use of these herbicides for crop production. Because 

of public concern about possible water contamination, several 

manufacturers have voluntarily reduced the recommended 

application rates and, in some cases, have removed herbicides 

from the market. 

During the 1990s, numerous studies were conducted in the 

Midwestern United States as part of the USGS Toxic Substances Hydrology Program to document the usage of triazine 

herbicides and formation of their degradation products in the 

hydrologic environment. Studies showed that changes in 

application rates affected the frequency of detection of some of 

the triazine herbicides and their degradation products and 

revealed that herbicide use also fluctuated in response to annual 

changes in planted corn and soybean acreage as well as introduction of new herbicide products to the market. Monitoring the 

effects of these changes are important to understanding the 

occurrence, fate, and transport of these herbicides and their 

degradation products in surface water, ground water, and 

precipitation.

The first major source studied was surface water. A reconnaissance study of 147 stream sampling sites was conducted 

during 1989 with subsequent studies during 1990, 1994, 1995, 

1998, and 2002 of about 50 of the 147 stream sampling sites to 

determine the geographic and seasonal distribution of herbicides. Results showed that large amounts of triazine herbicides 

and their degradation products were flushed into streams with 

runoff. Much of this transport took place during the first runoff 

period after application of herbicides. Subsequent runoff tended 

to produce lesser concentration peaks. Herbicide detections in 

larger Midwestern streams tended to be seasonal with higher 

percentages in spring and early summer and lower percentages 

in fall and winter.

The use of atrazine and cyanazine was approximately the 

same in 1989 and 1995; however, the median concentrations of 

both herbicides in stream samples were higher in 1989 than in 

1995 or 2002. Although herbicide concentrations in Midwestern streams were quite variable during post-application runoff, 

the data suggest that changes in herbicide use affected herbicide 

concentrations in streams. These studies also confirmed that 

herbicide degradation products were found to occur as frequently or more frequently and at concentrations that were often 

higher than the parent herbicides. In 1993, a study during flooding in the upper Mississippi River Basin determined that flooding did not dilute the concentrations of herbicides but rather 

concentrations were similar to those seen in normal spring and 

summer flows.

A study of 76 reservoirs in the Midwestern United States 

reported an abundance and persistence of triazine herbicides 

and their degradation products in reservoirs. Significant findings showed that concentrations of atrazine and its degradation 

products remained elevated all summer and into the fall. A further study of the distribution of atrazine in Perry Lake (Kansas) 

during 1992 showed that recently applied atrazine mixed with 

atrazine applied the previous year as water moved through the 

reservoir. The reservoir studies demonstrated the relation 

between herbicide use and their occurrence in streams and 

reservoirs.

The second major source studied was ground water. 

Reconnaissance studies of 303 ground-water wells in the Midwestern United States began in 1991 with additional water samples collected from 101 of the 303 ground-water wells in 1992 

to examine the occurrence of triazine herbicides and their 

degradation products. Triazine degradation products, deethylatrazine and deisopropylatrazine, commonly were found in 

ground water. During 1991, deethylatrazine was detected more 

frequently (21.4 percent of the samples) than its parent compound atrazine (20.8 percent of the samples) even though deethylatrazine had a higher reporting limit than atrazine, whereas 

deisopropylatrazine was detected in 7.6 percent of the samples. 

During 1992, the reporting limits for atrazine and deethylatrazine were lowered, and deethylatrazine was detected in 

31 percent of the water samples while atrazine was present in 

43 percent, and deisoproplyatrazine was found in 18 percent of 

the samples collected.

Samples collected from 110 wells during the historic 1993 

flooding in the Mississippi River Basin showed that there was a 

direct relation between increases in total herbicide concentration in ground water (between pre-flood and post-flood 
References Cited 23

samples) and the occurrence of stream flooding near wells. 

Results from these samples were compared to those samples 

obtained during 1991 and 1992 from the same wells. There 

were no statistically significant differences in either the frequency of herbicide detection or total herbicide concentration 

between the previous samples and those collected for the 1993 

flood study. However, water from about 63 percent of the wells 

that had at least a 20-percent increase in total herbicide concentration also were severely affected by stream flooding.

An important finding of an investigation of samples collected from 131 municipal wells in Iowa was the high frequency

with which degradation products were detected in ground 

water. Ground-water samples collected from across the Midwestern United States and in Iowa from 1991 to 2001 consistently revealed that triazine herbicide degradation products 

commonly were found more frequently than their parent herbicide, the frequency of herbicide detection was affected by a 

more sensitive analytical method, and ground-water age could 

be an important factor in explaining variations in herbicide 

contamination.

The third major source studied was precipitation. This 

study area included the upper Midwest (where the use of herbicides is most intense), the Northeast to the Atlantic Ocean, and 

northward to the Canadian border, a total of 26 States. The 

results identified the occurrence and temporal distribution of 

triazine herbicides and their degradation products in the Midwest. Because of the large temporal and spatial variation in the 

amount of rainfall, it is difficult to make meaningful comparisons of herbicide concentrations among sites or over time on the 

basis of individual weekly samples. Comparisons can best be 

made with rainfall-weighted concentrations. The highest concentrations occurred following herbicide application to cropland. Rainfall-weighted concentrations of 0.20 to 0.40 μg/L for 

atrazine were typical throughout the Midwest, and weighted 

concentrations as large as 0.60 to 0.90 μg/L occurred in precipitation at several sites. Concentrations of 1.0 to 3.0 μg/L were 

measured in a few individual samples. Atrazine was detected 

most often, followed by deethylatrazine, cyanazine, and deisopropylatrazine. The large ratio (about 0.50) of deethylatrazine 

to atrazine in precipitation was greatest in areas where herbicide 

use was intense and decreased with distance from the Midwest.

Other important findings of the 1990s USGS studies 

included the chemistry, transport, and persistence of triazine 

herbicides and their degradation products in surface water and 

ground water. A number of field-dissipation studies were carried out for atrazine, cyanazine, propazine, and simazine in the 

unsaturated zone and in surface runoff. These dissipation 

studies resulted in outlining the structure and transport of many

degradation products of triazine herbicides in the aquatic environment. A dealkylation field study found the preferential 

removal of an ethyl side chain from atrazine relative to the 

removal of an isopropyl side chain. The high ratio of deethylatrazine to atrazine (referred to as the DAR) in shallow soil was 

evidence that simazine degraded more rapidly to deisopropylatrazine than propazine to deethylatrazine. Furthermore, deethylation rates of atrazine and simazine were comparable. This 

indicates that the removal of an ethyl side chain was preferential 

over an isopropyl side chain, regardless of parent triazine compound. It was also found that atrazine transport through the 

unsaturated zone gave DAR values much greater than 1.0, 

whereas atrazine transported in runoff had DAR values much 

less than 1.0. Deisopropylatrazine was rapidly degraded in the 

unsaturated zone but was an important degradation product in 

surface runoff from the fields.

An additional finding of USGS research was the persistence of triazine herbicides and their degradation products in 

surface water. Studies found that the frequency of detection, or 

apparent order of stability, was atrazine, deethylatrazine, deisopropylatrazine, and cyanazine. The temporal trends in the number of detections of various herbicides were also significant for 

several reasons. First, they indicated that some compounds persisted from year to year in soil and water. Second, degradation 

products, such as deethylatrazine, were both persistent and 

mobile in water. Third, the data showed that the DAR, which 

has been proposed as an indicator of nonpoint-source contamination of ground water, might be used also as a tracer of groundwater discharge to streams. Once it was applied, atrazine began 

to degrade through the action of soil microbes and abiotic losses 

and chemical reactions.

Findings of the 1990s studies by the USGS include an 

improved understanding of the occurrence, persistence, chemistry, and transport of triazine herbicides and their degradation 

products in the hydrologic environment. A significant increase 

in knowledge of triazine herbicides and development and 

improvement of analytical methods were accomplished in the 

past decade. The results produced are not only significant for 

the present but provide an important data set for future use.

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Printed on recycled paper

Scribner and others—Summary of Significant Results from Studies of Triazine Herbicides and Their Degradation Products in Surface Water, Ground 

Water, and Precipitation in the Midwestern United States During the 1990s—US

GS/SIR 2005–5094

N

N

N

Cl

NH C C

H3C

CH 3

NH 2 N

Deethylcyanazine

N

N

N

Cl

NH C C

H3C

CH 3

CH 3 NH

CH 2

O

OH

Cyanazine acid

N

N

N

Cl

NH C C

H3C

CH 3

NH 2

O

OH

Deethylcyanazine acid

N

N

N

Cl

CH 3 NH

CH 2

NH 2

Deisopropylatrazine

N

N

N

Cl

NH

NH

C C

H3C 2

CH 3

CH 3 NH

CH 2

O

Cyanazine amide

N

N

N

Cl

NH C C

H3C

CH 3

CH 3 NH

CH 2

N

Cyanazine

N

N

N

Cl

NH C C

H3C

CH 3

NH 2

NH 2

O

Deethylcyanazine amide

N

N

N

Cl

H N2 NH 2

Didealkylatrazine