UNITED STATES ENVIRONMENTAL PROTECTION AGE CY 

WASHINGTON, D. C. 20460 

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTlON 

ENVIRONMENTAL RISK BRANCH Ill 

ENVIRONMENTAL FATE & EFFECTS DIVISION 

OFFICE OF PESTICIDE PROGRAMS 

PC CODE 128968 

MEMORANDUM 

DP BARCODE: 396343 

DATE: OX/21/20 12 

SUBJECT: Drinking Water Assessment for New Use of Fluroxypyr in Rice (Paddies) 

TO: Seyed Tadayon, Risk Assessor 

Michael Metzger, Branch Chief 

Registration Action Branch 7 

Health Effects Division 

FROM: 

THRU: 

Bethany Benbow, Reviewer 

Kathryn Montague, Product Manager 23 

Herbicide Branch 

Registration Division (7505P) 

(\ l\ aoe?~ oC?.(")..\ :-1_ Ronald Parker, Ph.D. \!....-~, ~V'-

Senior Environmental Engineer 

Environmental Risk Branch 3 

Environmental Fate and Effects Division (7507P) 

Executive Summary. GF-2764 (fluroxypyr-MHE) is an herbicide intended for post-emergent 

selective control of broadleaf weeds. It is currently registered for use on whea1t, barley, oats, 

com, sorghum, fallow cropland, non-cropland (i.e., rangeland), and others. Tbis memorandum 

evaluates the proposed new fluroxypyr use on rice, and the potential resulting residues that may 

occur in drinking water resources. The proposed new use label for application to rice permits a 

single maximum application of 0.34 lb ai per acre per year and allows for two applications with a 

maximum total of 0.568 lb ai per acre per year with a minimum retreatment interval (RTI) of ten 

days. 

1 
2 

The fluroxypyr estimated drinking water concentration (EDWC) value is 540 μg/L (ppb) for both 

acute and chronic exposure. Details of the EFED Tier 1 Rice Model can be found at 

http://www.epa.gov/oppefed1/models/water/rice_tier_i.htm. 

A screening assessment for the impact of fluroxypyr on potential groundwater drinking water 

sources, using the Tier 1 groundwater model SCI-GROW, was also conducted. EDWC values 

based on use of the EFED SCI-GROW model are 0.055 μg/L (ppb) for both acute and chronic 

exposures. SCI-GROW model details can be found at: 

http://www.epa.gov/oppefed1/models/water/#scigrow. Tables A1 and A2 in Appendix A of this 

report provide a summary of the inputs of values used in the Tier 1 Rice model. Tables A3 and 

A4 in Appendix A provide a summary of the inputs of values used in the SCI-GROW model and 

a copy of the SCI-GROW output file, respectively. 

Previous Drinking Water Exposure Assessments. Results from this evaluation for use on 

paddy rice can be compared with previous drinking water assessments (DWA) – including uses 

for rangeland and permanent grass pasture (8/13/03, DP Barcode #292471), and for onion 

(5/30/06, DP Barcode #314020). Previously, the highest modeled EDWC for surface water 

drinking water resulted from uses on rangeland, with a peak value of 32.9 μg/L (ppb), annual 

mean value of 3.28 μg/L (ppb), and a 36-year overall mean of 1.76 μg/L (ppb). Previous 

drinking water assessments for fluroxypyr were based on the OPP/EFED linked Tier 2 PRZMEXAMS surface water models. See http://www.epa.gov/oppefed1/models/water/#przm and 

http://www.epa.gov/oppefed1/models/water/#exams for information on these two models. 

Percent Cropped Area (PCA) corrections of 87% were applied for these crops. Also see: 

http://www.epa.gov/pesticides/science/efed/models/water/archives/pca_adjustment_dwa_9sep20

10.pdf for information on use of PCA values for drinking water exposure modeling. 

Environmental Fate. Once fluroxypyr MHE is applied to the environment, it is expected to 

degrade to its herbicidal active ingredient, fluroxypyr acid. The fate of fluroxypyr-MHE in the 

environment is complicated by uncertainty in the formation rate and distribution of the initial 

degradate fluroxypyr acid following spray application. Fluroxypyr-MHE application rates are 

established based upon acid equivalents (ae) pursuant to fluroxypyr acid as the active ingredient 

(ai). However, the solubility of fluroxypyr-MHE is much lower (~0.1 mg/L) than fluroxypyr acid 

(~8,000 mg/L); according to mixing/application instructions, the tank must be agitated before 

and during applications to allow undissolved material to remain in suspension. In neutral or low 

pH aqueous conditions, fluroxypyr-MHE is sufficiently stable such that spray distributed on a 

target zone will initially contain mostly the fluroxypyr-MHE form. Under field conditions it is 

likely that most of the MHE will convert to the active ingredient (fluroxypyr acid) in the course 

of a few days. In the EFED models, it is assumed that the initial spray contains all the active 

ingredient, and that degradation rates for the active ingredient commence immediately after 

spraying. However, there may be initial on-site exposure to the MHE after spraying, with 

degradation occurring first from the MHE to acid form, then to other forms. Actual exposure 

could then potentially be longer than assessed. Conversely, this effect may also attenuate the 

impact of the active ingredient, as it will be introduced onto the site more gradually (according to 

degradation rates). In high-pH conditions, though, degradation of both MHE and acid forms are 

expected to occur more rapidly. For the purpose of EFED assessments, the most conservative 

fate and effects input parameters are used where available; in this instance, it is more 
3 

conservative to use the values for fluroxypyr acid. Furthermore, in the case of potential drinking 

water exposure, fate parameters for fluroxypyr acid were selected also because this compound is 

both more soluble in water and more likely to be transported in an aquatic medium. Physical/ 

chemical properties of fluroxypyr-MHE and fluroxypyr acid are listed in Tables 1 & 2. Chemical 

structures of fluroxypyr-MHE and its degradates are provided in Appendix A5. 

Table 1. General Environmental Fate & Physico-chemical Properties of Fluroxypyr-MHE

CAS Number 8140637-3 

Molecular Formula C15H21Cl2FN2O3

Parameter Value Source

Molecular Weight 367 g/mol OPP/Pesticide Fact 

Sheet/1998

Melting Point 56-57 o

C 

log Kow 3.788 OPP/Pesticide Fact 

Sheet/1998

Water Solubility 0.9 mg/L @ pH 7 OPP/Pesticide Fact 

Sheet/1998

Vapor Pressure (25o C) 1.07 X 10-7 mm Hg OPP/Pesticide Fact 

Sheet/1998

Henry’s Law Constant 5.75 X 10-7 atm m3/mol Calculated

Hydrolysis Half-life (25o C) (days) 

pH 5 

 pH 7 

pH 9 

pH 8.3 (Natural Water-Dark) 

 pH 8.3 (Natural Water-Lighted) 

stable 

454 days 

3.2 days 

20 days 

63 days (photo-inhibition) 

MRID 40244539 

Aqueous Photolysis Half-life (days) 550 days 

197 days (buffered) 

MRID 44080342 

Aerobic Soil Metabolism Half-life (days) 

Total: (Fluroxypyr-MHE + Fluroxypyr) 

silt loam: 12.26 days 

sandy loam: 23.43 days 

loam: 11.65 days 

clay: 7.44 days 

MRID 42137317 

Anaerobic Aquatic Metabolism Half-life (days) (Under Review) MRID 44080344 

Aerobic Aquatic Metabolism Half-life (days) (Under Review) MRID 44080345 

Soil Photolysis Half-life (days) 152.7 days MRID 44080343 

Adsorption Kd / Koc 

 Kads Koc 

loam 190 6,200 

sandy loam 95 43,000 

silt loam 260 12,000 

clay 210 17,000 

MRID 42137319 
4 

Table 2. General Environmental Fate and Physico-chemical Properties of Fluroxypyr Acid 

CAS Number 69377-81-7 

Molecular Formula C7H5Cl2FN2O3

Parameter Value Source

Molecular Weight 255.03 g/mol OPP/Pesticide Fact 

Sheet/1998

Melting Point 57.5 0C OPP/Pesticide Fact 

Sheet/1998

log Kow 0.04 @ pH 7 IUPAC Pesticide Properties 

Database

Water Solubility 7950 mg/L OPP/Pesticide Fact 

Sheet/1998

Vapor Pressure (25o C) 9.5x10-11 mm Hg MERCK 

Henry’s Law Constant 4.01 x10-15 ( atm m3/mol ) Calculated

Hydrolysis Half-life (25o C) (days) 

pH 5 

pH 7 

pH 9 

Stable 

stable 

stable

MRID 40244539 

Aqueous Photolysis Half-life (Natural Water) 

(days) 

429 days (buffered) MRID 44080342 

Aerobic Soil Metabolism Half-life (days) 

Total: (Fluroxypyr-MHE + Fluroxypyr) 

silt loam: 12.26 days 

sandy loam: 23.43 days 

loam: 11.65 days 

clay: 7.44 days 

MRID 42137317 

Anaerobic Aquatic Metabolism Half-life (days) (Under Review) MRID 44080344 

Aerobic Aquatic Metabolism Half-life (days) (Under Review) MRID 44080345 

Soil Photolysis Half-life (days) 152.7 days MRID 44080343 

Adsorption Kd / Koc 

 Kads Koc 

loam 1.9 62 

sandy loam 0.11 51 

silt loam 1.7 78 

clay 1.0 81 

MRID 42137319 
5 

Drinking Water Exposure Modeling. Fluroxypyr-MHE is formulated as an emulsifiable 

concentrate that is mixed with water and applied as ground or aerial spray. GF-2764 is used for 

post-emergent broadleaf weed suppression. Applications to rice may be made after the paddy has 

been flooded. 

Estimation of Exposure Concentrations for Surface Water in Rices Paddies (Tier 1 Rice Model) 

The proposed new use label for application to rice permits a single maximum application of 0.34 

lb ai per acre per year and allows for two applications with a maximum total of 0.568 lb ai per 

acre per year with a minimum retreatment interval (RTI) of ten days. The label also requires a 

pre-harvest interval (PHI) of 60 days of harvest. This assessment reflects a high-exposure 

scenario by assuming the maximum application amount of 0.568 lb ai/A (0.64 kg/ha) is used per 

year. 

Tier 1 Rice Model (v1) When using the Tier 1 rice model, EFED assumes pesticide application 

to a static rice paddy with no inflow from rainfall or outflow from seepage, overflow or 

evaporation. The only process simulated by the model is partitioning of the applied chemical 

between the 10-centimeter deep water column and the 1-centimeter deep sediment layer of the 

paddy. The partitioning is based on the pesticide’s Kd. Degradation in the paddy is not calculated 

by the model so the pesticide is assumed to remain at the initial concentration in the water 

indefinitely. Results of the tier 1 rice modeling are presented in Table 3. Acute and chronic 

values are equal because the model does not simulate degradation of the pesticide in the paddy. 

Table 3. Summary of Tier 1 Rice Model EDWC for Use on Rice 

Assessment Type Acute EDWC Chronic EDWC 

Tier 1 Rice Model (conc in paddy) 540 μg/L (ppb) 540 μg/L (ppb) 

 
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Appendix A. Model Inputs and Outputs. 

Table A1 is a summary of the input values used in the Tier 1 Rice Model. 

Table A1. Summary of EFED Tier 1 Rice Model Input Values for Fluroxypyr

Environmental Fate 

Parameter 

Value Justification Source 

Adsorption Coefficient 

(Kads) (mL/g ) 

1.0 Lowest Non-sand Kd MRID 42137319 

Appendix 2. Physical Values of the Tier I Rice Model v1.0. 

Table A2. Physical Values of the Tier I Rice Model v1.0. 

Parameter Symbol Value Reference 

Water column depth dw 0.10 m USDA (1998, 2002) 

Sediment depth dsed 0.01 m Calibration described in Section III C of this 

document. 

Organic content of 

sediment 

foc 0.01 Justification provided in Section III B of this 

document. 

Sediment bulk density ρb 1300 

kg/m3

Brady (1984), Hillel (1982) 

Grain density ρg 2650 

kg/m3

Density of quartz, typical assumption; 

Brady(1984), Hillel (1982) 

Sediment porosity θsed 0.509 θsed = 1- (ρb/ρg) 

Most of the assumptions used in this model help ensure that the outputs are protective of most 

environments associated with rice agriculture. The following is a summary of the Tier 1 EECs 

resulting from application of fluroxypyr on rice. The model assumptions include the following: 

1. Movement of pesticide on suspended sediment is not considered. 

2. Degradation does not occur. 

3. Volatilization and other dissipation processes are not considered. 

4. Partitioning to sediment is instantaneous. 

5. Water is available for human or wildlife exposure instantaneously. 

6. Water column depth is 10 cm. 

7. Sediment depth is 1 cm. 

8. All pore space is saturated with water. 

9. Organic carbon fraction is 0.01. 

10. Bulk density is 1300 kg/m3

. 

11. Grain density is 2650 kg/m3

. 
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Appendix 3. SCI-GROW Model Input Values 

Table A3: Summary of Input Values Used in SCI-GROW Tier 1 Groundwater Model. 

Table A3. Summary of EFED SCI-GROW Model Input Values for Fluroxypyr

Environmental Fate Parameter Value Justification Source 

Organic Carbon Normalized Adsorption 

Coefficient (Koc) 

68 Mean of 4 Values 

(Table 2) MRID 42137319 

Aerobic Soil Metabolism Half-life (days) 

Total: (Fluroxypyr-MHE + Fluroxypyr) 

13.7 Mean of 4 Values 

(Table 2) MRID 42137317 

Appendix 4. SCI-GROW Model Output File. 

 SCIGROW 

 VERSION 2.3 

ENVIRONMENTAL FATE AND EFFECTS DIVISION 

 OFFICE OF PESTICIDE PROGRAMS 

 U.S. ENVIRONMENTAL PROTECTION AGENCY 

 SCREENING MODEL 

 FOR AQUATIC PESTICIDE EXPOSURE 

 

 SciGrow version 2.3 

 chemical:Fluroxypyr 

 time is 8/15/2012 14:45:32 

 ------------------------------------------------------------------------------------------ 

 Application Number of Total Use Koc Soil Aerobic 

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days) 

 ------------------------------------------------------------------------------------------- 

 0.568 1.0 0.568 6.80E+01 13.7 

 ------------------------------------------------------------------------------------------- 

 groundwater screening cond (ppb) = 5.47E-02 

 ************************************************************************ 
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Appendix 5. Structure of Fluroxypyr and Related Degradation Products