1

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

September 16, 2015

PC Code: 128712

MEMORANDUM DP Barcode: 425572

SUBJECT: Carfentrazone-ethyl: Drinking Water Exposure Assessment of Section 3 Proposed 

Label Amendments for New Uses, Expansions from Representative Crops and Crop 

Group Conversions.

FROM: Richard Shamblen, Biologist

ERB2/EFED (7507P)

THRU: Brian Anderson, Chief

Elyssa Arnold, Risk Assessment Process Leader

James Lin, PhD, Environmental Engineer, Secondary Reviewer

ERB2/EFED (7507P)

TO: Driss Benmhend, Risk Reviewer Manager

Shaja Joyner, Product Manager PM Team 20

Fungicide Herbicide Branch

Registration Division (7505P)

Barbara Madden, Minor Use Team Leader

Minor Use and Emergency Response Branch

Registration Division (7505P)

Thomas Bloem, Dietary Assessor

Thurston Morton, Acting Branch Chief

Health Effects Division (7509P)

The Environmental Fate and Effects Division (EFED) conducted a drinking water 

exposure assessment for three proposed Section 3 label amendments for carfentrazone-ethyl.

These proposed amendments include: 1) new use on Artichoke, Asparagus, Mint and Teff; 2) 

expansion from representative crops to subgroups for Oil seed Group 20, Low Growing Berry 

Subgroup 13-07G, Small Fruit Vine Climbing Subgroup 13-07F; and, 3) crop group 

conversions for Bulb Group 3-07, Fruiting Vegetable Group 8-10, Citrus Fruit Group 10-10,

Pome Fruit Group 11-10, Stone Fruit Group 12-12, Caneberry subgroup 13-07A, Bushberry 

subgroup 13-07B and Tree Nut Group 14-12.

RICHARD 

SHAMBLEN

Digitally signed by RICHARD 

SHAMBLEN 

DN: c=US, o=U.S. Government, 

ou=USEPA, ou=Staff, cn=RICHARD 

SHAMBLEN, dnQualifier=0000051207 

Date: 2015.09.17 14:38:11 -04'00'

JAMES LIN Digitally signed by JAMES LIN 

DN: c=US, o=U.S. Government, 

ou=USEPA, ou=Staff, cn=JAMES 

LIN, dnQualifier=0000014809 

Date: 2015.09.17 14:41:37 -04'00'

2015.09.17 

14:59:33 

For -04'00'
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Table 1 summarizes results of the three models used to assess current and proposed use 

scenarios. Results of the drinking water exposure assessment indicate that the use of 

carfentrazone-ethyl on rice in California yields the highest total residue estimated drinking water 

concentrations (EDWC). The revised percent cropped area (PCA) of 100% adjusted peak and 

annual average exposure estimates are 224 μg/L and 86 μg/L, respectively. 

The current rice use scenario with the previous adjusted PCA value of 56% yields a peak 

and annual average exposure estimates are 126 μg/L and 48 μg/L, respectively (DP Barcode 

384371, 2011). Results of the provisional cranberry model v2.0 at the maximum single 

application rate of 0.4 lbs a.i./A yields a peak and annual average exposure estimates are 131

μg/L and 53 μg/L, respectively. These lower EDWCs can be directly attributed to the minimum 

cranberry bog water column depth of 0.305 m (12 inches), versus the 0.1 meters (4 inches) in the 

rice paddy model.

Groundwater EDWC for current use on Crop Group 13 crops at 0.4 lbs a.i./A resulted in 

highest daily (peak exposure) and post-breakthrough (annual mean exposure) concentrations of 

48.9 μg/L and 43.9 μg/L, respectively.

Table 1: Screening-Level Total Residue EDWCs for Current and Proposed Carfentrazoneethyl Uses.

Drinking Water 

Source 

(Model)

Maximum Single 

Application Rate

(lbs a.i./A)

Current or 

Proposed/ 

Use

Peak

Exposure

ȝJ/

Annual Mean 

Exposure 

ȝJ/

Surface water

(Tier I Rice Model)

0.188, 0.10

(14-d interval)

Current/

Rice 126 48

Surface water

(Tier I Rice Model)

0.188, 0.10

(14-d interval)

Revised/

Rice 224 86

Surface water

(Provisional 

Cranberry Model)

0.4

Proposed/

Cranberry in Crop 

Group 13-07G

131 53

Ground water

(PRZM-GW) 0.4 Current/ Crop Group 13 48.9 43.9

Label Uncertainties

A large source of uncertainty arises from current and proposed label language. For 

example, the use of carfentrazone-ethyl on rice in California yielded the highest EDWCs. The 

maximum annual application rate is 0.3 lbs a.i./A; however, the label limits the maximum 

application rates in California are 0.188 lbs a.i./A for early post-seeding applications to 

submerged weeds and 0.10 lbs a.i./A for applications to emerged weeds above the water surface. 

Pursuant to the label use directions, this assessment assesses only the direct application of 

carfentrazone-ethyl to a flooded rice paddy. The current label enables pre-plant burn-down 

programs for rice, but does not specify, nor limit, the application rate. Thus this assessment does 

not account for the pre-plant, pre-flood application of up to 0.012 lbs a.i./A, or 4% of the total 

annual application rate.
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1. Background

EFED completed a preliminary problem formulation and conducted two drinking water 

exposure assessments for carfentrazone-ethyl in 2011 (DP Barcode 382665, 384371 and 

389087). Both drinking water exposure assessments included the residues of concern and total 

toxic residues (TTR) of carfentrazone-ethyl (F8426 parent plus its chloropropionic acid, its 

cinnamic acid, its propionic acid, its benzoic acid, and its 3-hydoxymethyl benzoic acid). EFED 

relied upon these documents for information and data to conduct this drinking water exposure 

assessment.

For surface water systems, EFED identified the use of carfentrazone-ethyl on cranberry 

and rice crops as likely to produce conservative (i.e. higher) screening-level Tier I drinking water 

exposure estimates. The previous drinking water assessment (DP Barcode 384371, 2011)

revealed that carfentrazone-ethyl applied to intentionally flooded rice fields in California resulted 

the highest estimated drinking water concentrations (EDWCs). Therefore, EFED used the Tier 1

Rice model to assess carfentrazone-ethyl use on flooded rice in California. Under the proposed 

label expansion from representative crops to include subgroups, the Low Growing Berry 

Subgroup 13-07G includes the crop cranberry. EFED used the provisional cranberry model to 

assess the proposed cranberry use site. Lastly, to assess screening-level Tier I groundwater 

exposure, EFED used the PRZM-GW model. The Low Growing Berry Subgroup 13-07 includes 

the highest annual application rate of 0.4 lbs a.i./A.

Unless otherwise noted, all model input parameterization values selected in the 2011 

drinking water exposure assessments were used for this assessment. Only changes to EFED 

guidance affected differences in models used and parameter input values.

2. Exposure Analysis

EFED conducted an exposure analysis for surface and groundwater drinking water 

systems using the Tier 1 Rice, provisional Cranberry and PRZM-GW models.

2.1 Surface Water: Tier 1 Rice Model

The previous drinking water assessment for rice (DP Barcode 384371, 2011) evaluated 

the use of carfentrazone-ethyl on rice production in California and Southern States. The 

California scenario yielded the highest estimated drinking water concentrations (EDWC) at 126

μg/L and 48 μg/L for peak and annual mean exposures, respectively. Since then, however, EFED 

has modified the percent cropped area (PCA) adjustment factors. A PCA for rice is not reported 

and, thus, has increased from 56% to 100% (Bohaty et al., 2014). For these reasons, the

California rice production scenario was re-assessed and compiled in the Tier 1 Rice model.

2.1.1 Model Inputs

Table 2 summarizes input parameters for the Tier I Rice Model, which were selected

from the previous drinking water assessments (DP Barcodes 384371, 2001; and, 389087, 2011). 

The maximum application rates in California are 0.188 lbs a.i./A for early post-seeding 
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applications to submerged weeds and 0.10 lbs a.i./A for applications to emerged weeds above the 

water surface. These rates were modeled with a 14-day reapplication interval although the 

minimum reapplication interval is not specified on the labels. The soil-water partition coefficient 

(Kd) input (0.89 L/kg) reflects the mean value for carfentrazone acid in five soils. The aerobic 

aquatic metabolism half-life input (106 d) is the upper 90% confidence bound on the mean of 

two half-lives for residues of concern (64 and 84 days). Derived model input values are fully 

discussed in the previous drinking water assessment (DP Barcode 384371, 2011).

Standard percent cropped area (PCA) values, represent of the extent of watershed in

which agricultural crops are grown, were used to modify estimated drinking water concentrations 

(EDWC). The previous drinking water exposure assessment for use on rice (DP Barcode 384371,

2011), a regional PCA for California of 56% was used to adjust the EDWC. However, in 2014, a 

PCA for rice is not reported and, thus, has increased from 56% to 100% (Bohaty et al., 2014).

Table 2: Tier I Rice Model Input Parameters for Residues of Concern. 

Input Parameter

(units) Value Source

Application Rates 

(lbs a.i./A)

0.188

0.10

Existing and Proposed 

labels

Soil-water Partition Coefficient (Kd)

(L/kg) 0.89 DP Barcode 384371389087

Aerobic Aquatic Metabolism Half-life 

(days) 106 DP Barcode 384371389087

Percent Cropped Area

(%) 100 USEPA, 2014

2.1.2: Modeling Results

Total residue exposure estimates from the Tier I Rice Model v2.0 are presented below in 

Table 3. These estimates represent peak and annual average exposure concentrations in the rice 

paddy tail water following the proposed 23-day holding period after the second application. The 

table compares the differences between the results of the previous drinking water assessment for 

rice and, results of the updated PCAs for rice (100%). The label directions for use on rice in 

California result in the highest estimated exposure. The peak estimated drinking water 

concentration (EDWC) 23 days after the second application is 224 μg/L. The annual mean 

EDWC is 86 μg/L.
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Table 3: Total Residue Estimated Drinking Water Concentrations in Surface Water from 

Split Applications of Carfentrazone-ethyl on Rice.

Regional Basin:

Reference State

Application 

Rate 

(lbs a.i./A)

Percent 

Cropped 

Area 

(%)

Peak EDWC 

(ppb)

Annual Mean EDWC 

(ppb)

18: California 0.188, 0.10

(14-d interval) 56 126 48

18: California 0.188, 0.10

(14-d interval) 100 224 86

Abbreviations: EDWC = estimated drinking water concentration

2.2 Surface Water: Provisional Cranberry Model v2.0

EFED used the Provisional Cranberry Model v2.0 because the registrant is seeking a 

label expansion from representative crops to subgroups for, among others, the Low Growing 

Berry Subgroup 13-07G, which includes cranberry.

2.2.1 Model Inputs

Input parameters (listed in Table 4) for the Provisional Cranberry Model v2.0 were 

selected from the previous drinking water assessments (DP Barcodes 384371, 2011; and, 

389087, 2011). The soil-water partition coefficient (Kd) input (0.89 L/kg) reflects the mean value 

for carfentrazone acid in five soils. The aerobic aquatic metabolism half-life input (106 d) is the 

upper 90% confidence bound on the mean of two half-lives for residues of concern (64 and 84 

days). The aerobic soil metabolism half-life input (193 d) is the upper 90% confidence bound on 

the mean of four half-lives for residues of concern (149, 203, 184 and 156 days).Because the 

label does not identify application timing intervals at rates lower than the maximum seasonal 

rate, EFED modeled the maximum proposed annual application rate as the maximum single

application rate of 0.4 lbs a.i./A.

Table 4: Provisional Cranberry Model v2.0 Input Parameters for Residues of Concern.

Input Parameter Value Source

Application Rates

(lbs a.i./A) 0.4 Proposed labels

Soil-water Partition Coefficient (Kd)

(L/kg) 0.89

DP Barcode 

384371

389087

Aerobic Aquatic Metabolism Half-life 

(days) 106

DP Barcode 

384371

389087

Aerobic Soil Metabolism Half-life

(days) 193

DP Barcode 

38437

389087
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2.2.2 Modeling Results

Total residue drinking water exposure estimates from the Provisional Cranberry Model

v2.0 are summarized below in Table 5. These estimates represent peak and annual average 

exposure estimates in bog tail water based on 0, 3 and 21-day holding periods after application.

Table 5: Total Residue Estimated Drinking Water Exposure (EDWC) in Surface Water 

from Single Application of Carfentrazone-ethyl on Cranberry.

Maximum

Application Rate

(lbs a.i./A)

Holding Period

(days)

Peak EDWC

(ppb)

Annual Mean EDWC 

(ppb)

0.4 0 130.7 53.2

0.4 3 128.1 52.2

0.4 21 113.9 46.4

2.3 Ground Water: PRZM-GW

EFED uses the Pesticide Root Zone Model for Ground Water (PRZM-GW) to conduct

screening-level (Tier 1) and refined (Tier II) drinking water exposure assessments for 

groundwater systems (Brady, 2012).

2.3.1 Model Inputs

Input parameters for the PRZM-GW model are summarized in Table 3.4 (USEPA, 

2012). From the proposed label amendments, the maximum total seasonal (annual) application 

rate of 0.4 lbs a.i./A is proposed for Caneberry (Subgroup 13-07A), Bushberry (Subgroup 13-

07B), Fruit, small vine climbing, except fuzzy kiwi fruit (Subgroup 13-07F), and Berry low 

growing (Subgroup 13-07G). Similar to the cranberry model, since the label does not identify 

application timing intervals at rates lower than the maximum seasonal rate, EFED modeled the 

maximum proposed annual application rate as the maximum single application rate of 0.4 lbs 

a.i./A. Application method is ground applied without incorporation.

From the previous drinking water exposure assessments (DP Barcodes 384371, 2011;

and, 389087, 2011), the following summarize the pesticide degradation properties. Hydrolysis of 

the carfentrazone acid, a residue of concern, showed no evidence of degradation at the three 

tested pH levels (DP Barcode 384371). The maximum hydrolysis half-life value, or in this case 

“stable” with a value of 0, was selected (USEPA, 2012).

The aerobic soil metabolism half-life input (193 d) is the upper 90% confidence bound on 

the mean of four half-lives for residues of concern (149, 203, 184 and 156 days) (DP Barcode 

389087, 2011). Batch equilibrium data for the predominant and most mobile residue, 

carfentrazone acid, were used to represent the residues of concern (MRID 43189233). 

Partitioning to soil was not correlated with the soil’s fraction of organic matter (DP Barcode 

382665, 2011). The soil-water partition coefficient (Kd) input (0.89 L/kg) reflects the mean value 

for carfentrazone acid in five soils (DP Barcodes 384371, 2011; and, 389087, 2011).
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Table 6: PRZM-GW Model Input Parameters.

Input Parameter Value Source

Application Rates

(lbs a.i./A) 0.4 Proposed labels

Hydrolysis

(days) 0

DP Barcode

384371

389087

Aerobic Soil Metabolism half-life

(days) 193

DP Barcode 

384371

389087

Soil-water Partition Coefficient (Kd)

(L/kg) 0.89

DP Barcode 

384371

389087

Application Method 2

(Above canopy application) Proposed labels

Incorporation Depth

(cm) 0 Proposed labels

2.3.2 Modeling Results

Total residue exposure estimates from the PRZM-GW model follow below in Table 3.5.

These estimates represent highest daily and post breakthrough average exposure estimates in 

ground water. The Wisconsin sands corn scenario yielded the highest daily, average postbreakthrough time and post breakthrough average.

Table 7: Total Residue Drinking Water Exposure Estimates in Ground Water from Single 

Maximum Annual Applications of Carfentrazone-ethyl on Rice.

Scenario Highest Daily

(ppb)

Average PostBreakthrough Time

(days)

Post Breakthrough 

Average 

(ppb)

DELMARVA_STD.SCN 25.4 8092.9 20.1

FLCITRUS_STD.SCN 48.5 4522.1 43.7

FLPOTATO_STD.SCN 20.5 3807.8 18.7

GACOASTAL_STD.SCN 15.7 6775 13.3

NCCOASTAL_STD.SCN 22 12139.3 20

WISANDS_STD.SCN 48.9 17015 43.9
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3. Uncertainties

The exposure values in this assessment are conservative for a number of reasons. The 

actual fraction of drinking water basins cropped to rice is likely to be less than 100% of the 

watershed area. Also, there are a number of factors inherent in the modeling that can affect the 

accuracy of these analyses including: the availability and quality of the model input data; the use 

of maximum labeled practices that may be greater than actual agricultural practices; and the 

neglect, due in part to lack of data, of dissipation processes such as dilution, flow, and adsorption 

to downstream sediment that may attenuate dissolved concentrations in surface water. Further,

the metabolism half-lives used to model exposure estimates reflect high-end confidence bounds 

based on limited data.

A large source of uncertainty arises from current and proposed label language. More 

specifically for rice, holding periods on the labels do not appear to restrict releases of paddy 

water following all applications. For example, current and proposed labels contain statements 

that holding periods apply in California “when applications are made to flooded fields.” If 

applications are made prior to flooding (i.e. pre-plant, pre-flood), it appears that fields may be 

subsequently flushed with immediate release, which may result in substantially higher exposure 

than is estimated. Also, label statements to “hold the floodwater static for at least five days after 

application” appear to allow for release of paddy water prior to the end of the holding period. 

Label language that disallows the release of any paddy water for a period of time following any 

application would reduce uncertainty in these assessments.

The use of carfentrazone-ethyl on rice in California yielded the highest EDWCs. The 

maximum annual application rate is 0.3 lbs a.i./A; however, the label limits the maximum 

application rates in California are 0.188 lbs a.i./A for early post-seeding applications to 

submerged weeds and 0.10 lbs a.i./A for applications to emerged weeds above the water surface. 

Pursuant to the label use directions, this assessment assesses only the direct application of 

carfentrazone-ethyl to a flooded rice paddy. The current label enables pre-plant burn-down 

programs for rice, but does not specify, nor limit, the application rate. Thus this assessment does 

not account for the pre-plant, pre-flood application of up to 0.012 lbs a.i./A, or 4% of the total 

annual application rate.

Lastly, the variance between modeled and actual depths of rice paddy water between 

application and release of the water is a source of uncertainty. Proposed and current labels 

stipulate that “floodwater must be 3 to 6 inches deep” for post-seeding applications to submerged 

weeds. The Tier I Rice Model is parameterized with a 4-inch water depth. If actual flood depths 

are higher, exposure may be lower than estimated. If actual flood depths are less than 4 inches 

(such as 3 inches), than exposure may be substantially (26-27%) higher than estimated.
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4. Literature Citations

Bohaty, R.F.H., Carleton, J.N. Crk, T., Echeverria, M., Ruhman, M., Thawley, M. Thurman, N., 

Villanueva, P., White, K. 2014. Development and Use of Percent Cropped Area

Adjustment Factors in Drinking Water Exposure Assessments. U.S. Environmental 

Protection Agency, Office of Pesticide Programs. September 9, 2014.

USEPA. 2009. Guidance for Selecting Input Parameters in Modeling the Environmental Fate 

and Transport of Pesticides. Version 2.1. U.S. Environmental Protection Agency, Office 

of Pesticide Programs, Environmental Fate and Effects Division, Oct. 22, 2009. Online 

at: http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm

Orrick, Greg. 2011. Carfentrazone-ethyl: Assessment of Proposed Label Amendments for Use 

on Rice. DP barcode 384371. U.S. Environmental Protection Agency, Office of Chemical 

Safety and Pollution Prevention, Environmental Fate and Effects Division. February 9, 

2011

Orrick, Greg. 2011. Carfentrazone-ethyl: Drinking Water Exposure Assessment for Proposed

Use on Crop Group 18 (Non-grass Animal Feed) Crops. DP barcode 389087. U.S. 

Environmental Protection Agency, Office of Chemical Safety and Pollution Prevention, 

Environmental Fate and Effects Division. Memorandum to the Health Effects Division 

and Registration Division.

USEPA. 2011. Registration Review: Preliminary Problem Formulation for Environmental Fate, 

Ecological Risk, Endangered Species, and Drinking Water Exposure Assessments for 

Carfentrazone-ethyl. DP barcode 382665. U.S. Environmental Protection Agency, Office 

of Chemical Safety and Pollution Prevention, Environmental Fate and Effects Division. 

Memorandum to the Pesticide Re-evaluation Division.

USEPA. 2012. Guidance for Selecting Input Parameters for Modeling Pesticide Concentrations 

in Groundwater Using the Pesticide Root Zone Model Version 1.0. U.S. Environmental 

Protection Agency, Office of Chemical Safety and Pollution Prevention, Environmental 

Fate and Effects Division. Health Canada, Pesticide Management Regulatory Agency, 

Environmental Assessment Directorate. October 15, 2012