UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

September 24, 2013 

MEMORANDUM 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 125401 

DP Barcode: 408878 

SUBJECT: Clomazone: Drinking Water Exposure Assessment for Proposed New Uses for 

Brassica (Head and Stem Subgroup SA), Southern Peas, and Rhubarb 

FROM: ·James Lin, Environmental Engineer ~ ~~r------ ERB2/EFED (7507P) \ 

THROUGH: Greg Orrick, Environmental Scientist 

Kristina Garber, Senior Biologist 

Brian Anderson, Branch Chief 

ERB2/EFED (7507P) 

TO: Sidney Jackson, Risk Manager Reviewer 

Barbara Madden, Risk Manager, RM 05. 

RSBIRD (7504P) 

Bonnie Cropp-Kohlligian 

RAB4/HED (7509P) 

0~ &f-)'1-13 

z. ,,, 

1(t¢[J 

This drinking water exposure assessment (DW A) memo updates an earlier memo with the same 

subject (DP barcode 408878; dated July 23, 2013). The maximum estimated drinking water 

concentration (EDWC) for use in dietary risk assessment is 550 Jlg/L rather than the previously 

reported value of 458 J..lg/L. The updated EDWC reflects a higher-end exposure scenario in rice 

than the previous value. This is the only change in the exposure assessment. Both this and the 

previous DW A rely on rice exposure modeling that was described in an ecological risk 

assessment (DP 363564, dated January 11, 2012) and that may be used for drinking water 

exposure assessment. The following is a summary of the exposure modeling approach. 

In a memo dated October 2, 2000 (DP 268905), HED Il).etabolism Assessment Review 

Committee (MARC) concluded that the parent compound and its major environmental degradate 

N-[(2-chlorophenyl)methyl]-3-hydroxy-2,2-dimethyl propanamide (FMC 65317) need to be 

included in drinking water assessments. The following exposure modeling is inclusive of the 

residues of concern ( clomazone plus FMC 65317) by way of producing a peak parent-only 

EDWC at the moment when permanent flood conditions start that is greater than any following 

residue-of-concern concentrations that may include FMC 65317. FMC 65317 is formed in 

aquatic conditions, especially in anaerobic conditions in which it reached 49-55% of the parent 

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compound 120 to 365 days after application. Therefore, it will not have formed at the time of 

flooding, which EDWCs in the memo represent, but may slowly form during flooding as 

clomazone concentrations decline. 

The Tier 1 Rice model1 calculates the water-column exposure expected on the day of application 

to a flooded rice paddy, accounting only for sorption to sediment as a dissipation process. No 

other degradation processes, such as hydrolysis, photolysis, or metabolism are considered. 

Clomazone used in dry-seeded rice culture is subject to dissipation (volatilization and aerobic 

metabolism) before flooding. According to the label, clomazone may be applied 14 days before 

rice is planted, up to 7 days after planting. To represent the possible exposure range, two 

modeling scenarios based on the application dates were used: (1) 14 days before rice planting 

and (2) 7 days after rice planting. 

The earliest that permanent flood would occur is 3 weeks after planting (Sunil Ratnayke, 

Biological Analysis Branch, 11/06/2008 Use of Clomazone in Rice Production (DP# 336342). 

To calculate EDWCs, this assessment incorporates dissipation of clomazone via volatilization 

and degradation (aerobic soil) and then calculates the flooding of the rice field. The soil 

metabolism half-life of clomazone is 80 days (this is the upper 90th percentile of three values). 

The approach used to estimate the volatilization flux is based on Woodrow eta!. (1997) equation 

as suggested by SAP. 

The flux equation based on the mean values for Woodrow et a!. equation_is shown below: 

Where: 

ln X= 19.35 + 1.0533 * ln [ VP x CF X ARI (Koc x Sw)] 

X- flux (!Jg/m2 hr); 

VP- vapor pressure (Pa); 

CF- conversion factor= 133.32 Paltorr; 

.AR- application rate (kg/ha) 

Koc- soil organic carbon-water partition coefficient (mL/g); and 

Sw- water solubility (mg/L) 

For clomazone the values are 1.44 x 104 torr (VP), 1100 mg/L (Sw), and 300 mL/g (Koc). 

The application rate (AR) is 0.896 kg/ha (0.8 lb/acre), the flux (X) will be 5.40 !Jg/m2 hr. 

The flux rate can also be expressed as 1.296 g/ha-day, which is about 0.14% ofthe application 

rate. Assuming the first order kinetics, this flux rate corresponds to a rate constant of0.0014/day 

and a half-life of 494.8 days. 

With the volatilization flux of about 0.14% of the application rate for the first day following the 

application, the remaining amount is 0. 799 lb ai/acre, which is calculated as 0.8 x (1 00%-

0.14%). Considering the only process is aerobic soil metabolism, the decay rate is ln(2)/80 = 

0.0087/day for clomazone applied to the rice field. 

(1) For 14 days before the planting, which implies flooding 35 days later, C = (0.799 lb ailacre) x 

ec-o.oos7 x 35) = 0.589 lbs ai/acre. Feeding this remaining load into the Tier 1 Rice Model, the 

initial concentration in the rice paddy is estimated to be 458 !Jg/L. 

1 Available online at: http://www.epa.gov/oppefedl/models/water/index.htm#rice 

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(2) For 7 days after the planting, the total duration before floodiney is 14 days. The 

corresponding concentration-is C = (0.799 lb ai!acre) x e<-0

·

0087 x 14 = 0.707lbs ai/acre. Feeding 

this remaining load into the Tier 1 Rice Model, the initial concentration in the rice paddy is 

estimated to be 550 j..lg/L. This is the maximum screening EDWC recommended for use in 

dietary risk assessment. 

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