Herbicides in the Pecatonica and Yahara Rivers in Southwestern

Wisconsin, May 1996–July 1996

by David J. Graczyk1 and James P. Vanden Brook2

U.S. Department of the Interior—U.S. Geological Survey Fact Sheet FS-175-97

1 U.S. Geological Survey

2 Wisconsin Department of Agriculture, Trade, and Consumer Protection

Figure 1. Location of sampling sites for herbicides.

Introduction

Herbicides, particularly those applied to corn, can be found in surface water

and ground water in Wisconsin (Sullivan and Richards, 1996; Matzen and

Saad, 1996). Wisconsin farmers applied 7.6 million pounds of corn herbicides

during 1996. Because of public concern about the amount of herbicides

applied to Wisconsin farm fields and the impact on surrounding watersheds,

a study was conducted to measure herbicide runoff in sections of two rivers in

southwestern Wisconsin. This is a cooperative project between the U.S.

Geological Survey (USGS) and the Wisconsin Department of Agriculture,

Trade and Consumer Protection (DATCP).

Prior to this study, surface-water samples had not been collected frequently

enough to calculate herbicide loads for entire watersheds in southwestern and

south-central Wisconsin. This fact sheet summarizes herbicide concentrations

in samples collected from the Pecatonica River at Martintown and the Yahara

River near Fulton from May 15 to July 15, 1996. Herbicide loads and unit area

loads are calculated for these two watersheds.

This study monitored commonly applied corn herbicides: alachlor, atrazine, metolachlor, cyanazine, and acetochlor. Acetochlor was introduced to

the market in 1994 as a replacement for corn herbicides such as alachlor.

Data Collection

Water samples were collected from the Pecatonica River at Martintown and

from the Yahara River near Fulton (fig. 1). These sites were selected because

of the extensive agricultural activity within the watersheds and because they

both are long-term USGS streamflow stations. Additionally, DATCP estimated that applications of atrazine to cornfields in the two watersheds were

significantly different. Atrazine use was prohibited on 94 percent of the

Yahara watershed whereas only 4 percent of the Pecatonica River watershed

was under atrazine use prohibition. It was expected that the effect of atrazine

use prohibitions would be reflected in the unit area loads of atrazine to surface

waters. Farmers apply most of the herbicides used in the watersheds to corn

and soybeans. The drainage area and land uses for each watershed are shown

in table 1. The amounts of pesticide active ingredient applied in each watershed are shown in table 2.

Samples were collected at each site weekly from May 15 to July 15, 1996,

without regard to streamflow conditions at the sampling site. When the

instantaneous discharge of the stream increased by at least 50 ft3

/s (cubic feet

per second) more than the mean daily discharge of the previous day, additional

daily storm runoff samples were collected until the streamflow decreased.

Samples were collected with a US D-77 water-quality sampler using the

Equal-Width-Increment (EWI) method (Edwards and Glysson, 1988). The

sample was transferred to a teflon-lined sample splitter and then split into two

pre-cleaned, one-liter brown glass bottles. The samples were analyzed for

commonly used herbicides by the DATCP water-quality laboratory. The

sampler and churn splitter were cleaned by methods outlined in Shelton

(1994). For quality assurance (QA), samples of deionized (MilliRO and

MilliQ) water were processed through the sampler and splitting equipment and

then analyzed for herbicides by the DATCP laboratory. These QA samples,

collected and processed near the beginning and at the end of the sampling

periods, contained no detectable concentrations of the herbicides analyzed in

this study.

Results

At the Pecatonica site, atrazine was the most frequently detected herbicide

(88 percent) in the 17 samples collected (fig. 2). Metolachlor was the next most

frequently detected herbicide (76 percent). Acetochlor and cyanazine were

detected in 59 percent of the samples. Alachlor and alachlor ESA were

detected in only 18 percent and 12 percent, respectively, of samples collected.

Data for the Yahara River show that atrazine and acetochlor were the most

frequently detected herbicides. Fifty-seven percent of the samples had concentrations of these herbicides above the laboratory analytical detection limit (fig.

2). Cyanazine and metolachlor were the next most detected herbicides; 43

percent of the samples were above the laboratory analytical detection limit.

0 5 10 MILES

0 5 10 KILOMETERS

WISCONSIN

ILLINOIS

89° 25'

89°35'

43°10'

42°55'

90°15' 43°

42°

90°

Yahara River

near Fulton

Pecatonica River

near Martintown

0534500

05430175

EXPLANATION

Sampling site with 

identification number

Atrazine prohibition area

Watershed boundary

05430175

 

      







 



Pecatonica River

Watershed

Yahara River

Watershed
Figure 2. Herbicide concentration, minimum detection concentration, number of samples

above the minimum detection concentration, and number of samples collected at the

Pecatonica and Yahara Rivers.

Table 1. Land use characteristics for the two herbicide sampling watersheds (mi2, square miles; land use data from DATCP)

Station Name

USGS Station

Number

Drainage Area

(mi2

) Percent land use or land cover

Atrazine

Other Prohibition Area 2 Agricultural land1

Pecatonica River at Martintown 05434500 1034 96 4 4

Yahara River near Fulton 05430175 517 75 25 94

1 Includes row crops and pasture 2 Includes forest, lakes, wetlands and urban areas

Alachlor and alachlor ESA were detected in 29 percent and 14 percent,

respectively, of 14 samples collected.

The maximum herbicide concentration for the Pecatonica River was 8.07

μg/L (micrograms per liter) for cyanazine on June 17, 1996 (fig. 2 and fig. 3).

The next highest concentration detected was for atrazine (with metabolites)—

5.8 μg/L on May 29 (fig. 2 and fig. 3). The median concentrations of herbicides

in the Pecatonica River samples were 0.90 μg/L atrazine, 0.86 μg/L metolachlor,

0.61 μg/L cyanazine, and 0.27 μg/L acetochlor. Median concentrations are

reported here only for herbicides for which less than 50 percent of the sample

concentrations were below the detection limit.

In comparison, the median concentration of atrazine for three predominantly agricultural sites in eastern Wisconsin—Duck Creek, North Branch

Milwaukee River, and the Milwaukee River—ranged from approximately

0.04 μg/L to 0.30 μg/L (Sullivan and Richards, 1996). Median cyanazine and

metolachlor concentrations at the Pecatonica River were also higher than at

these three sites.

At the Yahara River, atrazine (with metabolites) had the highest concentration—9.93 μg/L on June 19 (fig. 2 and fig. 3). Cyanazine had the next highest

concentration—7.43 μg/L on June 17. The maximum atrazine concentration

detected in samples from the Yahara River was greater than that detected in

eastern Wisconsin streams, but less than that for streams monitored in Indiana

and Nebraska (Sullivan and Richards, 1996). The median concentrations of

herbicides in the Yahara River samples were 0.18 μg/L for atrazine, and 0.17

μg/L for acetochlor. The median atrazine concentrations for the Yahara River

samples were within the range of median atrazine concentrations for the three

streams in eastern Wisconsin mentioned above.

Herbicide concentrations in the Pecatonica and Yahara

Rivers differed from those in other streams in Wisconsin and

streams in other midwestern states. The maximum cyanazine

concentration for the Pecatonica and the Yahara Rivers was

greater than that detected in streams in eastern Wisconsin,

Indiana, and Nebraska. The median atrazine concentrations for

both the Pecatonica and Yahara Rivers were less than the

median concentrations for watersheds in Indiana and Nebraska

that drain extensive corn and soybean agricultural areas. Watersheds in Minnesota and Missouri, where grazing and small

grain farming are the major agricultural activities, had lower

median atrazine concentrations than either the Yahara or

Pecatonica Rivers. The median cyanazine concentration for the

Pecatonica River was greater than that detected in streams in

Indiana, Nebraska, Minnesota, and Missouri. The Pecatonica

had lower median metolachlor concentrations however, compared to a Nebraska watershed (Sullivan and Richards, 1996).

The concentration of herbicides that are transported to a

stream is a function of the physiochemical properties, such as

water solubility, soil half-life, and soil sorption coefficient of

the herbicides; timing and rate of the herbicide application; and

overland runoff from rainfall and farming practices (Goolsby

and Pereira, 1995). Peak herbicide concentration in streams is

dependent upon the timing of the herbicide application and

when rainfall occurs in amounts great enough to generate

overland runoff. Concentrations of all herbicides were less than

the laboratory analytical detection limit in late June and continued to stay below the analytical detection limit until data

collection ended in July. The streamflow and the concentrations of six of the herbicides detected in samples collected at the

Pecatonica and the Yahara Rivers and the approximate period

of herbicide application are shown in figure 3.

At the Pecatonica River, the peak concentration of atrazine was detected in

late May to early June, whereas the peak cyanazine and metolachlor concentrations occurred during a major runoff period in the middle of June (fig. 3).

The monthly mean streamflow for the Pecatonica River at Martintown during

May was 22 percent greater than the mean monthly flow for the period of

record (1940–95) (Holmstrom and others, 1997). During June, streamflow

was 122 percent greater and during July, it was 127 percent greater than mean

monthly flow for the period of record.

At the Yahara River, atrazine, cyanazine, and metolachlor concentrations

peaked in the middle of June (fig. 3). The monthly mean streamflow at the

Yahara River during May was 20 percent greater than the mean monthly flow

for the period of record (1977–95); during June it was 224 percent greater and

during July it was 99 percent greater (Holmstrom and others, 1997).

The load for 5 of the 6 herbicides studied was determined for both rivers.

Herbicide load is the mass transport of the herbicide by the river and is a

function of water discharge and concentration of the herbicide. The integration

method was used to determine the load at each site (Porterfield, 1972). For

samples that contained herbicide concentrations below the laboratory analytical detection limit, the concentrations were set to one-half the analytical

detection limit for the purpose of load calculation. The daily loads were

summed for the period May 15 to July 15 (fig. 4). The calculated herbicide

loads at the Pecatonica River ranged from 47.2 lbs (pounds) of alachlor to 484

lbs for atrazine with metabolites. For the Yahara River, loads ranged from 36.1

lbs of alachlor to 289 lbs for atrazine with metabolites (fig. 4). The unit area

loads (load divided by drainage area) were greater in the Yahara River

watershed than in the Pecatonica River watershed for all the herbicides except

0.01

10

0.02

0.03

0.04

0.05

0.07

0.1

0.2

0.3

0.4

0.5

0.7

1

2

3

4

5

7

CONCENTRATION, IN MICROGRAMS PER LITER

15 8

17 14

10 6

17 14

10 8

17 14

 3 4

17 14

13 6

17 14

2 2

17 14

ATRAZINE CYANAZINE ACETOCHLOR ALACHLOR METOLACHLOR ALACHLOR ESA

EXPLANATION

Concentration for Pecatonica River

Concentration for Yahara River

Minimum detection concentration

 3 

 17 

Number of samples above 

detection concentration

Number of samples collected
Figure 3. Discharge hydrographs, herbicide concentration, and period of herbicide application for the Pecatonica and Yahara Rivers from May 15 to July 15,

1996. (Laboratory analytical detection limit for acetochlor is 0.10 μg/L; for atrazine and alachlor is 0.15 μg/L; for metolachlor is 0.25 μg/L; for cyanazine is 0.50

μg/L and for alachlor ESA is 1.0 μg/L.)

Table 2. Estimated pounds of pesticide active ingredient applied during the spring of 1996 (application data from DATCP)

Watershed Estimated pounds of pesticide active ingredient applied in the spring of 1996

Acetochlor Alachlor Atrazine Metolachlor Cyanazine

Pecatonica River 42,000 32,000 85,000 106,000 88,000

Yahara River 26,000 20,000 3,200 66,000 56,000

for metolachlor (fig. 4). The unit area load for the Pecatonica River ranged

from 0.05 lbs/mi2

 (pounds per square mile) for alachlor to 0.47 lbs/mi2 for

atrazine with metabolites. For the Yahara River, unit area loads ranged from

0.07 lbs/mi2 for alachlor to 0.55 lbs/mifor atrazine with metabolites (fig. 4).

For the most part, the unit area loads of herbicides in the Pecatonica and

Yahara Rivers were consistent with the application rates of herbicides for each

watershed except for atrazine (table 2, fig. 4). Alachlor and acetochlor were the

least-used herbicides and had the lowest unit area loads for both watersheds

(fig. 4). In the Yahara River watershed, metolachlor was applied at rates higher

than cyanazine but the unit area load for metolachlor was less than that for

cyanazine (fig. 4). In the Pecatonica River watershed, metolachlor was the

most heavily applied herbicide and had the second highest unit area load (fig.

4). In both watersheds, atrazine had the highest unit area load as compared to

the other herbicides but was the least-applied herbicide in the Yahara River

watershed and the third-most herbicide applied in the Pecatonica River

watershed (table 2, fig. 4).

Ninety-four percent of the Yahara River watershed is in the atrazine

prohibition area (fig. 1). Because much less atrazine is applied in this

watershed compared to the Pecatonica River watershed (table 2), the unit area

loads would be expected to be less. It is important to note, however, that the

May June July

1996

0

3,500

500

1,000

1,500

2,000

2,500

3,000

STREAMFLOW, IN CUBIC FEET PER SECOND

0

10

2

4

6

8

HERBICIDE CONCENTRATIONS, IN MICROGRAMS PER LITER

EXPLANATION

May June July

1996

500

3,500

1,000

1,500

2,000

2,500

3,000

STREAMFLOW, IN CUBIC FEET PER SECOND

0

10

2

4

6

8

HERBICIDE CONCENTRATIONS, IN MICROGRAMS PER LITER

Pecatonica River near Martintown, Wis. Yahara River near Fulton, Wis.

Pecatonica River near Martintown, Wis Yahara River near Fulton, Wis.

May June July

1996

500

3,500

1,000

1,500

2,000

2,500

3,000

STREAMFLOW, IN CUBIC FEET PER SECOND 

0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

HERBICIDE CONCENTRATIONS, IN MICROGRAMS PER LITER

May June July

1996

0

3.0

0.5

1.0

1.5

2.0

2.5

HERBICIDE CONCENTRATIONS, IN MICROGRAMS PER LITER

0

3,500

500

1,000

1,500

2,000

2,500

3,000

STREAMFLOW, IN CUBIC FEET PER SECOND

approximate herbicide 

application period

approximate herbicide 

application period

approximate herbicide 

application period approximate herbicide 

application period

Discharge

Atrazine

Cyanazine

Acetochlor

EXPLANATION

Discharge

Alachlor

Metolachlor

Alachlor ESA
For more information, please contact:

District Chief

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

(608) 828-9901 U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-175-97

Layout and illustrations: Michelle Greenwood

Text editing: Gail Moede

Banner graphic: Karen Lonsdorf

Figure 4. Herbicide loads and unit area loads for the Pecatonica and Yahara Rivers from May 15 to July 15, 1996.

area just upstream from the sampling site is not in the atrazine prohibition area

(fig. 1). Thus the high concentrations of atrazine detected in June—9.93 μg/

L and 6.25 μg/L—may be a result of herbicides applied near the sampling site,

and may not be representative of the whole watershed.

The median atrazine concentration for the Yahara River samples was less

than that for the Pecatonica River samples. The higher unit area load for the

Yahara River may be a result of the extreme streamflow in June (224 percent

greater than the long-term mean monthly flow for June). Seventy-nine percent

of the atrazine load for the study period was transported in 5 days, or 8 percent

of the days monitored at the Yahara River.

Conclusions

In general, this study showed that the most heavily applied agricultural

herbicides were detected more frequently and at higher concentrations in

surface waters than herbicides which were less commonly used or applied at

very low rates. Herbicide loadings to surface water for alachlor, acetochlor,

cyanazine and metolachlor roughly correspond to the total amount of the

specific herbicide applied within the watershed. Both watersheds showed a

brief (two- to four- week) period of herbicide detections and elevated concentrations. This result has been seen in other studies and is probably related to the

moderate to high solubility and relatively short half-life of the herbicides

found, as well as the lack of ground cover in the weeks after planting, which

maximizes surface-water runoff.

Unexpectedly, atrazine was found at similar concentrations and loadings in

the two watersheds despite the near total prohibition of its use in the Yahara

River watershed. Illegal use of atrazine in the Yahara River watershed was not

considered to be the reason for the unexpected atrazine unit area loads. A study

that determined farmers’ compliance with atrazine restrictions in Wisconsin,

including Atrazine Prohibition Areas, showed that farmers had a greater than

98 percent compliance rate with atrazine prohibition areas (Nowak and others,

1993). Unusually heavy rainfall (and subsequent runoff) in the Yahara River

watershed preceding and during the study period, and the presence of an area

just upstream of the sampling point in which atrazine use is allowed, may

account for the higher atrazine unit area loads for the Yahara River watershed

as compared to the Pecatonica River watershed. During the sampling period,

the Yahara River watershed had a similar amount of surface-water discharge

as the Pecatonica River watershed, despite having about 50 percent of the

drainage area. Although the atrazine use area upstream from the Yahara River

watershed sampling point comprises only four percent of the watershed, the

abnormally heavy rains falling on fields just recently treated with atrazine

could have resulted in very high rates of runoff, which in turn, accounted for

the high atrazine loadings in the watershed.

Both the Yahara and Pecatonica watersheds experienced much greater than

average surface-water discharge during the 1996 sampling period and this may

have skewed some of the study results, especially those for atrazine. To shed

further light on the issues brought up by this study, USGS and DATCP plan

to continue this study in 1997. To address the unexpected atrazine results in the

Yahara River watershed, an additional sampling point will be located in the

upper third of the watershed, within an atrazine prohibition area.

References Cited

Edwards, T. K., and Glysson, G. D., 1988, Field methods for measurement of

fluvial sediment: U. S. Geological Survey Open-File Report 86-531, 118 p.

Goolsby, D. A., and Pereira W.E., 1995, Pesticides in the Mississippi River in

Meade, R.A. editor: Contaminants in the Mississippi River, 1987–92: U.S.

Geological Survey Circular 1133, 140 p.

Holmstrom, B. K., Olson, D. L. and Ellefson, B. R., 1997, Water Resources Data,

Wisconsin, Water Year 1996: U. S. Geological Survey Water Data Report

WI-96-1, 464 p.

Matzen, A. M. and Saad, D. A., 1996, Pesticides in Groundwater in the Western

Lake Michigan Drainages, Wisconsin and Michigan 1983–1995: U. S.

Geological Fact Sheet FS-192-96, 4 p.

Nowak, Peter, Wolf S., Hartley, H., and McCallister, R., 1993, Final Report:

Assessment of 1992 Wisconsin Atrazine Rule: University of Wisconsin,

Madison. College of Agricultural and Life Sciences.

Porterfield,George, 1972, Computation of fluvial sediment discharge: U.S.

Geological Survey Techniques of Water-Resource Investigations, Book 3,

Chapter C3 66 p.

Shelton, L.R., 1994, Field guide for collecting and processing stream-water

samples for the National Water-quality Assessment Program: U.S. Geological Survey Open-File Report 94-455, 42 p.

Sullivan, D.J. and Richards, K.D., 1996, Pesticides in streams of the western Lake

Michigan drainages, Wisconsin and Michigan, 1993–95: U.S. Geological

Fact Sheet FS-107-96, 4 p.

Atrazine Acetochlor Alachlor Metolachlor Cyanazine

HERBICIDE

0

500

100

200

300

400

HERBICIDE LOAD, IN POUNDS,

FROM MAY 15 TO AUGUST 15, 1996

Atrazine Acetochlor Alachlor Metolachlor Cyanazine

HERBICIDE

0

0.7

0.1

0.2

0.3

0.4

0.5

0.6

0.7

UNIT AREA HERBICIDE LOAD, IN POUNDS PER SQUARE MILE,

FROM MAY15 TO AUGUST 15, 1996

Yahara River

Pecatonica River