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 NewUses 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 BrianAnderson, Branch Chief ERB2/EFED (7507P) 

### TO: Sidney Jackson, Risk Manager Reviewer BarbaraMadden, RiskManager, RM 05
. RSBIRD (7504P) Bonnie Cropp-Kohlligian RAB4/HED (7509P)

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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 thanthe previously reported value of458 

### J..lg/L. The updated EDWC reflects a higher-end exposure scenario in rice than the previous value. This is the only change inthe exposure assessment. Boththis andthe previous DW A rely onrice 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 ofthe exposure modeling approach. In a memo dated October 2, 2000 (DP 268905), HED Il).etabolism Assessment Review Committee (MARC) concludedthatthe 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 ofthe residues ofconcern (clomazone plus FMC 65317) by way ofproducing a peakparent-only EDWC at the momentwhenpermanent flood conditions startthat 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 inwhich it reached 49-55% ofthe parent
1compound 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 model 1 calculates the water-column exposure expected onthe day ofapplication to a flooded rice paddy, accounting only for sorptionto sediment as a dissipationprocess. No otherdegradation processes, such as hydrolysis, photolysis, or metabolism are considered. Clomazone used in dry-seeded rice culture is subjectto dissipation (volatilization and aerobic metabolism) before flooding. According to the label, clomazone maybe applied 14 days before rice is planted, up to 7 days after planting. To represent the possible exposure range, two modeling scenarios based onthe application dates were used: (1) 14 days before rice planting and (2) 7 days after rice planting. The earliestthatpermanent flood would occur is 3 weeks after planting (Sunil Ratnayke, Biological Analysis Branch, 11/06/2008 Use ofClomazone in Rice Production (DP# 336342). To calculate EDWCs, this assessment incorporates dissipation ofclomazone via volatilization and degradation (aerobic soil) and then calculates the flooding ofthe rice field. The soil metabolism half-life ofclomazone is 80 days (this is the upper 90thpercentile ofthree values). The approach usedto estimate the volatilization flux is based on Woodrow

### eta!. (1997) equation as suggested by SAP. The flux equation based onthe mean values for Woodrow 

### eta!. 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- vaporpressure (Pa); CF- conversion factor= 133.32 Paltorr;

.AR- application rate (kg/ha) 

### Koc- soil organic carbon-waterpartition 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 of494.8 days. With the volatilization flux ofabout 0.14% ofthe 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 (100%- 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.oos7x35) = 0.589 lbs ai/acre. Feedingthis remaining load into the Tier 1 Rice Model, the initial concentration inthe rice paddy is estimated to be 458 !Jg/L. 1Available online at: http://www.epa.gov/oppefedl/models/water/index.htm#rice
2(2) For 7 days afterthe 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 1Rice Model, the initial concentration inthe rice paddy is estimatedto be 550 

### j..lg/L. This is the maximum screening EDWC recommended for use in dietary risk assessment.



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