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 Buil ding) 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 estimat e 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 materia ls 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 sit uation 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 res ult, 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 eval uation, 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 applicatio n. 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

g rowing 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 s urface, 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 fi eld. J Hydrodyn 20:382

- 390.