Dow AgroSciences LLC

9330 Zionsville Road

Indianapolis, IN 46268-1054

13356993.1

October 10, 2011

Document Processing Desk 

Risk Management and Implementation Branch 5

Pesticide Re-evaluation Division 7508P

Office of Pesticide Programs

U. S. Environmental Protection Agency

One Potomac Yard (South Building)

2777 S. Crystal Drive

Arlington, VA 22202

Attention: Andrea Mojica

Re: CLARIFICATION ON THE UTILITY OF PRZM/EXAMS MODELING TO PREDICT 

CONCENTRATIONS OF 1,3-D IN SURFACE WATER

Dear Ms. Mojica:

This letter follows up on the Dow AgroSciences (DAS) 1,3-D applicant meeting with NMFS and 

EPA on September 19, 2011. Please forward a copy of the letter and its attachments to NMFS 

and confirm that a copy is placed in the docket.

At that meeting, the subject of the ability of PRZM/EXAMS modeling to estimate accurately 

concentrations of 1,3-D in salmonid habitat was considered. The EPA scientists present, Mark 

Corbin and Bill Eckel, both stated their professional opinions that the volatility process simulated 

in PRZM underpredicts losses of volatile materials such as fumigants to the atmosphere and that 

therefore the model overpredicts the amount of chemical entrained in runoff. Reference was 

made to the EPA Telone® California redlegged frog effects determination (ED)1

, where a 

detailed analysis of this situation is presented.

DAS directs NMFS’ attention to section 3.2.4.5 of the ED, in which PRZM model predictions 

are compared to data from a 1,3-D field runoff study. The model overpredicts chemical losses in 

runoff by a factor of 100. Because of this result, EPA states in section 6.1.2 of the ED that 

modeling results could not be used in the assessment. 

 1 Abdel-Saheb, I., Borges, S., Eckel, W.P. 2008. Risks of Telone use to federally threatened California Red-legged 

Frog, pesticide effects determination. Environmental Fate and Effects Division, Office of Pesticide Programs, 

Washington, DC.
September 28, 2011

Andrea Mojica

Page 2

13356993.1

The same conclusion applies to NMFS’ current evaluaton. However, DAS points out that the 

results from the field study, although relevant to model evaluation, should not be considered 

representative of 1,3-D use conditions in the Pacific salmonid ESUs. The storm event simulated 

in the study corresponds to an intense rainstorm with a 50-year return frequency that occurred 

relatively soon after application. Neither of these circumstances is typical of site conditions 

found in western irrigated agriculture, which is generally characterized by area-wide cyclonic 

storm systems during the rainy season and long periods when precipitation is absent during the 

growing season. Runoff from irrigation will not occur until after 1,3-D has dissipated and the 

crop is planted.

DAS also points out that generally accepted theory on soluble chemical extraction from the 

upper layer of the soil profile emphasizes the view that only a thin zone at the soil surface 

interacts with rainfall and overland flow. Several studies confirming this conclusion are 

attached.2,3,4 This zone is typically on the order of a few millimeters in thickness. For pesticides 

applied to the soil surface, the extraction zone can be important in controlling the amount of 

soluble chemical transported in runoff. However, the implication for fumigants such as 1,3-D, 

that are injected 12-18 inches into the soil profile or are carried into the profile by drip irrigation, 

is that only a very small fraction of the applied chemical mass, if any, will be available for 

extraction into runoff water. This consideration confirms that concentrations of 1,3-D carried by 

overland flow in California and the Pacific Northwest will be negligible. Further, any residues 

of 1,3-D dissolved in runoff water will rapidly be stripped from the liquid phase by 

volatilization, as is mentioned in the EPA ED.

Sincerely,

Nicholas N. Poletika, Ph.D.

Research Leader

Enclosures 

cc: Dr. Tony Hawkes

National Marine Fisheries Service

510 Desmond Drive SE, Suite 102

Lacy, WA 98503

 2 Wallach, R., Jury, W.A., Spencer, W.F. 1988. Transfer of chemicals from soil solution to surface runoff: a 

diffusion-based soil model. Soil Sci Soc Am J 52:612-618. 3 Ahuja,L.R. 1990. Modeling soluble chemical transfer to runoff with rainfall impact as a diffusion process. Soil Sci 

Soc Am J 54:312-321. 4 Tong, J., Yang. J. 2008. Analysis of soluble chemical transfer by runoff water in field. J Hydrodyn 20:382-390.