### Timothy J. Gish, John H. Prueger, C.S.T. Daughtry, William P. Kustas, Lynn McKee, Andrew Russ,

and Jerry L. Hatfield 

# 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

8Metolachlor 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

TIME AFTER PESTICIDE APPLICATION (hr) CUMULATIVE METOLACHLOR VAPOR LOST (%) 1998 1999 2000 2001 2002

9

# Herbicide 

# Volatilization Losses

(worse case scenario)

0 0 48

24 144 96 72 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

120 40 20 80 60 120 100 35,000 30,000
5,000 0 15,000 10,000 25,000 20,000 TIME (hours after application) CUMULATIVE METOLACHLOR VOLATILIZED (ng m 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%)

132005

0 600 1000

200 1200 1400
800 400 24 48 72 120 96 0 0 35 20 30
10 40 25 15 5 Soil surface dry, no linkage between peak soil temps and volatilization 45

0 35 20 30
10 40 25 15 5 50

24 48 72 120 96 0 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 1 st 

### 72

hrs after application)

15

# 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 1 st 

### 72

hrs after application)

16

# 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

18

# Comparison of Atrazine Volatilization and Surface Runoff

over 8 year

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 .....

20Time after Application, Hours Herbicide Volatilization Flux, μ g 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