Timothy J. Gish, John H. Prueger, C.S.T. Daughtry,

William P. Kustas, Lynn McKee, Andrew Russ, 

and Jerry L. Hatfield

Volatilization

Ground Water Leaching

Primary Off-Site Transport Pathways for Herbicides

Agricultural Field

Surface Runoff

Volatilization

Ground Water Leaching

Primary Off-Site Transport Pathways for Herbicides

Agricultural Field

Surface Runoff

Herbicide Volatilization Trumps 

Runoff Losses:

A Multi-year Investigation

1
Pesticides Monitored

Atrazine

Low water solubility: 33 mg/L at 20 C

Low vapor pressure: 3.0 x10-7 mm Hg at 20° C

Soil half life: 60 to 100+ days

Moderate to high mobility in soils, especially if soil are 

low in clay and/or organic matter.

U.S. herbicide use on crops: 74,500,407 lbs 

a.i. applied in 1997*

Metolachlor

Slightly soluble: 530 mg/L at 20° C

Low vapor pressure: 3.14 x10-5 mm Hg at 25° C

Soil half-life: 15 to 70 days

Moderately mobile, especially if soil are low in clay and/or 

organic matter.

U.S. herbicide use on crops: 67,336,211 lbs 

a.i. applied in 1997*

* Gianessi, L.P. and M.B. Marcelli. 2000. Pesticide Use in 

U.S. Crop Production: 1997. National Center for Food 

and Agricultural Policy, Washington D.C. 

2
Philosophy and Approach

1. Multi-year vapor flux investigation (13 years so 

far, started in 1998).

2. Compute field-scale volatilization fluxes from 

turbulent fluxes of heat, momentum, and water 

vapor as determined from eddy covariance and 

pesticide profile measurements. Continuous day 

and night monitoring for first 5 days after 

application as a surface broadcast spray.

3
Manifold Used to Control Airflow

4
Philosophy and Approach (Cont.)

3. Monitor surface runoff from March (about 2 

months before herbicide application) through 

November (several months after crop 

senescence). Runoff monitoring started in 1999.

4. During the first 5 years factors influencing fieldscale herbicide volatilization and runoff were 

determined. During the second 5 years factors 

influencing off-site transport were evaluated.

5
Philosophy and Approach (cont.)

5. Starting in 2009, Herbicide volatilization 

monitoring was increased from 5 to 10 days.

6. Then in 2010, surface soil moisture arrays were 

installed to better quantify moisture impacts 

6
Meteorological Impacts on 

Herbicide Volatilization

(First 5 years)

7
4:30 am Soil Sampling

Soil Moisture and Pesticide Residue

8
Metolachlor volatilization ranged from 5-25%, even 

though management was identical (bare soil, tilled, 

corn production, etc...), same pesticide formulations 

applied as a surface broadcast spray, and texture and 

organic matter unchanged. Major difference was soil 

moisture and surface meteorological conditions.

Prueger, J.H., Gish, T.J., McConnell, L.L., McKee, L.G., 

Hatfield, J.L., and Kustas, W.P. Solar radiation, relative 

humidity, and soil water effects on metolachlor 

volatilization. Environ. Sci. Technol. 39:5219-5226. 

2005.

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100 110 120

CUMULATIVE METOLACHLOR VAPOR LOST (%)

TIME AFTER PESTICIDE APPLICATION (hr)

1998 1999 2000 2001 2002

9
Herbicide Volatilization Losses

(worse case scenario)

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6

TIME (Day afyer application)

METOLACHLOR VOLATILIZED (Percentage of applied)

Metolachlor, site 1

Atrazine, site 1

0

0

24 48 72 96 144

10

120

30

20

50

40

70

60

TIME (hours after application)

CUMULATIVE PESTICIDE VOLATIZATION 

(Percentage of applied)

Metolachlor

Atrazine 

Although the 2003 pesticide application was not representative 

of what a farmer would do (too wet) it does give extreme values 

for volatilization losses. Early morning soil samples (4:30 am 

dataset) indicated that 120 hours after application, 29% of the 

applied atrazine was gone from the top 3-cm of soil while 65% of 

the applied metolachlor was absent. The vapor flux data 

suggests that about 41% of the atrazine lost was through 

volatilization compared to 95% of the metolachlor lost was 

through volatilization! 10
GPR-Identified Subsurface Flow 

Pathways Impact on Surface Soil 

Moisture

Surface Topography (digital elevation map)

Subsurface Topography 

(GPR)

Spatial Dynamics of Cascading Pools of Subsurface Water

11
First attempts to quantifying the impacts 

of soil moisture on herbicide off-site 

transport

12
0

5000

10000

15000

20000

25000

30000

35000

00 202 0 404 0 606 0 808 0 100100 120 120

35,000

30,000

5,000

0

15,000

10,000

25,000

20,000

CUMULATIVE METOLACHLOR VOLATILIZED (ng m

TIME (hours after application)

2

)

2004

2005

2006

Wet location

Dry location

Gish, T.J., Prueger, J.H., Kustas, W.P., Hatfield, J.L., 

McKee, L.G. and Russ, A.L. 2009. Soil moisture and 

metolachlor volatilization observations over three years. 

J. Environ. Quality 38:1785-1795. 

Comparison of Metolachlor Volatilization

Between Site V1 (dry) and Site V2 (wet)

Field-averaged soil moisture = about 16%

(Theta g est. from Theta v: V1= 0.14; V2 =28)

Field-averaged soil moisture = about 13%

(Theta g: V1=11%, V2 20%) 

Field-averaged soil moisture = about 10%

(Theta g: V1 = 9% and V2 = 15%)

13
0

200

400

600

800

1000

1200

1400

0 24 48 72 96 120

Time after Application (hr)

Metolachlor Flux (ug/m^2)

0

5

10

15

20

25

30

35

40

T-surface (Deg C)

Metolachlor Vapor Loss, V2

Surface Soil Temperature, V2

2005

0

600

1000

200

1200

1400

800

400

0 24 48 72 96 120

0

35

20

30

10

40

25

15

5

0

100

200

300

400

500

600

700

800

900

1000

0 24 48 72 96 120

Time after Application (hr)

Metolachlor Flux (ug/m^2)

0

5

10

15

20

25

30

35

40

45

50

T-surface (Deg C)

Metolachlor Vapor Los, V1

Surface Soil Temperature

Soil surface dry, no linkage between peak soil temps and volatilization 

45

0

35

20

30

10

40

25

15

5

50

0 24 48 72 96 120

0

600

1000

200

800

400

SOIL TEMPERATURE (Degree C)

METOLACHLOR VOLATILIZATION MASS FLUX RATES (μg m-2 2h-1

)

TIME (hours after pesticide application) 

Surface moist but soil cool, linkage lost

Soil surface moist, linkage between peak soil temps and volatilization 

14
Metolachlor Volatilization 

vs Average Soil Moisture 

(Relationship Between Surface Soil Moisture 

(4:30 am samples) and Metolachlor Volatilization 

Losses (using V1 only) and a focus on the 1st 72 

hrs after application) 

X Data

0 5 10 15 20 25

Y Data

0

10000

20000

30000

40000

50000

60000

Average Surface Gravimetric Soil Water Content (%)

Cumulative Metolachlor Volatilization, First 72 hrs (ug)

METOLACHLOR

2007

2006 2005

2002

2000

2004

2001

2003

X Data

0 5 10 15 20 25

Y Data

0

10000

20000

30000

40000

50000

60000

Average Surface Gravimetric Soil Water Content (%)

Cumulative Metolachlor Volatilization, First 72 hrs (ug)

METOLACHLOR

2007

2006 2005

2002

2000

2004

2001

2003

15
X Data

0 5 10 15 20 25

Y Data

0

2000

4000

6000

8000

10000

Average Surface Gravimetric Soil Water Content (%)

Cumulative Atrazine Volatilization, First 72 hrs (ug)

ATRAZINE

2007

2006 2005

2002

2000

2004

2001

2003

X Data

0 5 10 15 20 25

Y Data

0

2000

4000

6000

8000

10000

Average Surface Gravimetric Soil Water Content (%)

Cumulative Atrazine Volatilization, First 72 hrs (ug)

ATRAZINE

2007

2006 2005

2002

2000

2004

2001

2003

Atrazine Volatilization vs 

Average Soil Moisture 

(Relationship Between Surface Soil Moisture 

(4:30 am samples) and Metolachlor Volatilization 

Losses (using V1 only) and a focus on the 1st 72 

hrs after application) 

16
1 Table 1. Atrazine and Metolachlor Application Dates and Runoff Losses

Year Application 

 Date

 Number of 

Runoff Events†

 Runoff Losses

 Atrazine Metolachlor

 (percentage of applied)

2000 13 June 8 0.04 0.03

2001 20 June 8 2.94 2.45

2002 24 April 9 < 0.01 < 0.01

2003 14 July 5 0.91 0.42

2004 21 May 3 < 0.01 < 0.01

2005 10 May 4 0.18 0.11

2006 3 May 4 0.06 0.06

2007 8 May 0* Likely <0.01 Likely <0.01

†

2 Number of runoff events during the first 5 months after herbicide application. 

* Herbicide runoff was monitored for only the 1st 3 month after application. During this 

4 time no runoff was observed.

Herbicide Runoff

(over 8 years) 

Gish, T.J., J.H. Prueger, C.S.T. Daughtry, W.P. Kustas, 

L.G. McKee, A.L. Russ, and J.L. Hatfield. 2011. 

Comparison of field-scale herbicide runoff and 

volatilization losses: An eight year field investigation. J. 

Environ. Qual. 40:1432-1442. 17
Comparison of Metolachlor 

Volatilization and Surface Runoff

over 8 year

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

2000 2001 2002 2003 2004 2005 2006 2007

Year Metolachlor Off-Site Transport (Percentage of applied)

Volatilization Losses

Runoff 60

50

40

30

20

10

70

0 METOLACHLOR OF-SITE TRANSPORT (percentage of applied)

YEAR

2000 2001 2002 2003 2004 2005 2006 2007

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

2000 2001 2002 2003 2004 2005 2006 2007

Year Metolachlor Off-Site Transport (Percentage of applied)

Volatilization Losses

Runoff

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

2000 2001 2002 2003 2004 2005 2006 2007

Year Metolachlor Off-Site Transport (Percentage of applied)

Volatilization Losses

Runoff 60

50

40

30

20

10

70

0 METOLACHLOR OF-SITE TRANSPORT (percentage of applied)

YEAR

2000 2001 2002 2003 2004 2005 2006 2007

18
Comparison of Atrazine 

Volatilization and Surface Runoff

over 8 year

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

2000 2001 2002 2003 2004 2005 2006 2007

Year

Atrazine Off-Site Transport (Percentage of applied)

Volatilization Losses

Runoff Losses

12

10

8

6

4

2

14

0 ATRAZINE OFF-SITE TRANSPORT (Percentage of applied)

YEAR

2000 2001 2002 2003 2004 2005 2006 2007

19
Future Volatilization Work

at OPE3

1) Longer evaluation period (5 to 10 days)

2) Other compounds....

3) Development of new herbicide

Management strategies....

4) Understanding soil moisture

temperature interactions...

5) Model development and evaluation .....

20
Metolachlo Flux

0

200

400

600

800

1000

1200

0 24 48 72 96 120 144 168 192 216 240

Metolachlo Flux

Atrazine FLux

0

20

40

60

80

100

120

140

160

180

200

0 24 48 72 96 120 144 168 192 216 240

Atrazine FLux

Time after Application, Hours

Herbicide Volatilization Flux, 

μg m-2

Rainfall Events

(23 mm)

(23 mm)

(8 mm)

(8 mm)

(3 mm)

(3 mm)

Herbicide Volatilization Observations 

(extending from 5 to 10 days)

(11.1 %) (+ 5.2%)

(2.9 %) (+ 1.7 %)

21
Difficulties in Solving Temperature and 

Soil Moisture Interactions

22
Soil Moisture Arrays

(about 74 locations within 35 m of flux towers)

23
• Volatilization of two common herbicides 

increased dramatically with increasing 

surface soil water content. Data indicates 

that soil water content trumps organic 

matter content with regard to herbicide 

volatilization.

• Significant volatilization occurs each year, 

while herbicide runoff is highly dependent 

on a runoff event occurring within the first 

two weeks of application. When averaged 

over herbicide and time, herbicide 

volatilization was 25 times greater than 

runoff. 

• Surface soil temperature and herbicide 

vapor fluxes were coupled when surface 

soil were moist but as soils dried out or 

became cool this trend became 

decoupled.

• Volatilization is a critical loss pathway for 

metolachlor and atrazine even though 

they have low vapor pressures and are 

considered “non-volatile”.

A Few Conclusions

24
Thank you for listening

Any questions?

25