1

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

PC Code: 108201

DP Barcode: 423330

Date: April 20, 2015

MEMORANDUM

SUBJECT: Drinking Water Exposure Assessment in Support of the New Use Registration 

of Diflubenzuron Formulated Product on Alfalfa, Carrot, Cottonseeds

Subgroup 20C, Peach Subgroup 12-12B, Plum Subgroup 12-12-C 

Peppers/Eggplant Subgroup 8-10B and Conversion for Tree Nuts Group 14-

12.

TO: Carmen Rodia, Risk Manager Reviewer

Barbara Madden, Risk Manager

Deborah McCall, Branch Chief

Registration Division (7505P)

Thurston Morton, Chemist

Elissa Reaves, Branch Chief

Risk Assessment Branch IV

Health Effects Division (7509P)

FROM: Faruque Khan, Ph.D., Senior Fate Scientist

Environmental Risk Branch I

Environmental Fate and Effects Division (7507P)

APPROVED

BY: Sujatha Sankula, Ph.D., Branch Chief, 

Environmental Risk Branch I

Environmental Fate and Effects Division (7507P)

This assessment provides estimated drinking water concentrations (EDWCs) of diflubenzuron

and its major metabolite, 4-chlorophenyl urea (CPU) in surface water and groundwater in 

support of human health risk assessment on the Interregional Research Project Number 4 (IR4)

proposed new uses on alfalfa, carrot, cottonseed Subgroup 20C, peach Subgroup 12-12B, Plum 
2

Subgroup 12-12-C, peppers/eggplant Subgroup 8-10B, and conversion of tree Nut Group 14-12.

Proposed application rate information is provided in Table 1. 

Table 1. Proposed New Use Application Rates for Diflubenzuron

Use Single max 

app rate

(lb ai/A)

Max yearly 

app rate

(lb ai/A)

Min interval 

between 

applications

(days)

Additional Notes

Alfalfa/Alfalfa grown 

for seed

0.0325 0.0975 14 For use West of the Mississippi

Carrot 0.125 0.25 7 Not for use in CA

Cottonseed Subgroup 

20C

0.125 0.375 5 ---

Peach Subgroup 12-

12B

Plum Subgroup 12-12B

0.25 0.50 14 ---

Peppers/Eggplant 

Subgroup 8-10B

0.125 0.375 7 ---

The Metabolism Assessment Review Committee (MARC) of the Health Effects Division (HED) 

suggested to include CPU as a residue of concern for drinking water (USEPA, 2001, DP

D272976) and recommended to assess the concentrations of CPU and diflubenzuron in drinking 

water sources independently. Therefore, this drinking water assessment was performed for both 

diflubenzuron and CPU in support of the proposed Section 3 new uses. Foliar spray applications

(aerial, air-blast and ground spray applications) are being proposed for these crops. 

The EDWCs of diflubenzuron and CPU were generated using the annual maximum application 

rates of 0.098 to 1.00 lb a.i./A with the Surface Water Concentration Calculator model (SWCC) 

for surface water and the Pesticide Root Zone Model-Groundwater (PRZM-GW) model for 

ground water. Conversely, the application rates for CPU were adjusted based on maximum 

formation of CPU in the submitted environment fate studies and molecular ratio of CPU and 

diflubenzuron. Since additional environmental fate data became available and the 

implementation of SWCC and PRZM-GM models, the previously recommended EDWCs were 

reevaluated for citrus crop, for which the highest water concentrations were reported earlier. The 

maximum application rate of 0.312 lbs a.i./A X 3 applications/year for citrus as well as 0.25 lbs 

a.i./A X 4 applications/year for tree nut were also modeled with the SWCC and PRZM-GW 

models. Model input parameters were chosen according to current guidance (USEPA, 2009 and 

USEPA, 2013a). For surface water sources of drinking water, the recommended peak (acute) 

concentration of 4.61 μg/L, annual mean (non-cancer chronic) concentration of 0.41 μg/L and 

the 30 year annual average concentration (cancer chronic) of 0.38 μg/L based on LV application 

of diflubenzuron for citrus use. For CPU, the recommended surface water concentrations are

6.83 μg/L for acute, 0.89 μg/L for non-cancer chronic and 0.51 μg/L for cancer chronic also 

based on aeial application representative of citrus use.

For groundwater sources of drinking water, the maximum acute and chronic estimated concentrations of 

CPU in shallow groundwater from PRZM-GW model for applications to citrus were 8.62 μg/L and 8.02 

μg/L, respectively. However, the concentrations of diflubenzuron in groundwater were negligible.

Recommended surface water and groundwater EDWCs for human health are listed in Table 2.
3

Table 2. Drinking Water Exposure Estimates for Diflubenzuron and its Major 

Degradate, CPU for Use on Various Crops.

Source Residue Peak Exposure 

(μg/L)

Annual Mean 

Exposure (μg/L)

30-year Average 

Exposure (μg/L)

Surface water1

Diflubenzuron2 4.61 0.41 0.38

CPU2 6.83 0.89 0.51

Groundwater1

Diflubenzuron 0 --- 0

CPU 8.62 --- 8.02

1 EDWCs generated using SWCC and PRZM-GW models for aerial LV application of 0.312 lbs a.i./A X 3 

applications for citrus use 

2 Estimated in FL Citrus scenario

Environmental Fate and Transport Properties

The physicochemical properties suggest that diflubenzuron has a low solubility in water (0.8 

mg/L). The vapor pressure (9.0 × 10-10 mm Hg) and Henry's Law Constant (1.19 x 10-11atmm3

/mol) indicate that diflubenzuron is a non-volatile chemical. Based on registrant-submitted 

data, diflubenzuron appears to be relatively non-persistent and slightly mobile under normal use 

conditions. Diflubenzuron is relatively stable to hydrolysis and photolysis. The major route of 

dissipation appears to be biotic processes (half-life of approximately ≤ 14 days for aerobic soil 

metabolism). Two major degradates, a maximum of 48 % of 4-chlorophenylurea (CPU or pCPU) and a maximum of 31% of 2,6-diflubenzoic acid (DFBA) were identified in the submitted 

environmental fate studies for diflubenzuron. However, there is insufficient data on the major 

degradates of diflubenzuron to adequately assess their persistence and mobility. Detailed 

environmental fate properties can be obtained from the Registration Review Problem 

Formulation document (USEPA 2012a).

Residues of Concern

Two major degradates, a maximum of 48 % of 4-chlorophenylurea (CPU or p-CPU) and a 

maximum of 31% of 2,6-diflubenzoic acid (DFBA) were identified in the submitted 

environmental fate studies for diflubenzuron. The Residues of Concern Knowledgebase 

Subcommittee (ROCKS) of the Health Effects Division (HED) suggested to include CPU as a 

residue of concern for drinking water (USEPA, 2001, DP D272976) and recommended to assess 

the concentrations of CPU and diflubenzuron in drinking water sources independently.

Drinking Water Exposure Modeling Approach and Input Parameters

Models

The EDWCs were generated using EFED’s standard suite of models. Most surface water 

EDWCs and groundwater EDWCs were determined in previous assessments using older models.

In this assessment, SWCC and PRZM-GW (currently approved models) were used to estimate 

surface water and groundwater EDWCs. Since additional environmental fate data became 

available, the previously recommended EDWCs were reevaluated for citrus scenario. 
4

Information concerning the model can be found on the EPA Water Models web-page1(USEPA, 

2015). The maximum application rate of 0.312 lbs a.i./A X 3 application/year for citrus was also 

modeled with the SWCC (Surface Water Concentration Calculator) and PRZM-GW (Pesticide 

Root Zone Model-Groundwater) models. The highest predicted EDWCs for diflubenzuron 

scenarios were used in estimating EDWCs for CPU for surface water and groundwater. 

Surface Water

The Surface Water Concentration Calculator (SWCC v 1.106) model was used to generate 

EDWCs for the Tier II aquatic exposure assessment. The SWCC is a graphical user interface that 

runs the Pesticide Root Zone Model (PRZM, v 5, November 15, 2006) and the Variable Volume 

Water Body Model (VVWM, 3/6/2014) (USEPA, 2006b). Simulations are run for multiple 

(usually 30) years and the EDWCs represent peak values that are expected once every ten years 

based on the thirty years of daily values generated during the simulation. The default Percent 

Crop Area (PCA) of 1.0 was used for the proposed uses because diflubenzuron can be used in 

both agricultural and non-agricultural settings (USEPA 2014).

Groundwater 

Tier I groundwater EDWCs for diflubenzuron and CPU, resulting from its use on various crops 

estimated using the PRZM-GW model (USEPA, 2012b), with the GW-GUI (Graphical User 

Interface, version 1.0, August 31, 2012b). PRZM-GW is a one-dimensional, finite-difference 

model that estimates the concentrations of pesticides in groundwater. It accounts for pesticide 

fate in the crop root zone by simulating pesticide transport and degradation through the soil 

profile after a pesticide is applied to an agricultural field. PRZM-GW permits the assessment of 

multiple years of pesticide application (up to 100 years) on a single site. PRZM-GW output 

values represent pesticide concentrations in a vulnerable groundwater supply that is located 

directly beneath a rural agricultural field following many years of pesticide application. PCAs 

are not applicable for the PRZM-GW model.

Input Parameters

There is no environmental fate data available for CPU at this time. The aerobic soil metabolism

and aerobic aquatic input values for CPU used in SWCC and PRZM-GW were estimated from 

diflubenzuron submitted environmental fate studies. Assumed the highest fraction of CPU 

forming in the diflubenzuron studies for “Day 0” then adjusted the subsequent sampling days for 

CPU accordingly. The NAFTA degradation Kinetics tool, PestDF v0.8.13 tool2 was used in 

estimating representative degradation half-lives for aerobic soil and aerobic aquatic metabolisms. 

The calculation can be found in Appendix A. The EPISUITE 4.1 was also used in estimating 

selected physicochemical properties of CPU for model inputs. 

Summaries of the model input parameter values for diflubenzuron and CPU used in SWCC are 

presented in Tables 3 and 4 respectively. Summaries for PRZM-GW input parameter values for 

diflubenzuron and CPU used are presented in Table 5. Input parameters were selected in 

 

1 http://www.epa.gov/oppefed1/models/water /index.htm

2 http://www.epa.gov/oppefed1/ecorisk_ders/degradation_kinetics/NAFTA_Degradation_Kinetics.htm
5

accordance with EFED’s “Guidance for Selecting Input Parameters in Modeling the 

Environmental Fate and Transport of Pesticides,” Version 2.1 (USEPA, 2009) and “Guidance 

for Selecting Input Parameters for Modeling Pesticide Concentrations in Groundwater Using the 

Pesticide Root Zone Model,” Version 1 (USEPA 2013a). 

Table 3. SWCC Input Parameters for Diflubenzuron Modeling

Parameter Value Source Comments

Crops 

Citrus

Scenario

CACitrus STD

FLCitrusSTD

All available 

standard crop 

scenarios of SWCC 

were used except 

for alfalfa. 

Alfalfa CAalfalfa WirrigOP

MNalfalfaOP

TXalfalfaOP

Regional Restriction west of 

Mississippi River. No standard 

scenario available for alfalfa. 

Surrogate scenarios from west of 

Mississippi River were used for 

alfalfa

Carrot FLCarrotSTD

Cottonseed Subgroup 

20C

CAcotton_WirrigSTD

MScottonSTD

NCCottonSTD

Peach and Plum 

Subgroup (12-12B)

CAFruitsWirrigSTD

GAPeachSTD

Peppers/Eggplant 

Subgroup 8-10B

Tree Nut Subgroup 14-

12

FLPeppersSTD

CAalmond_WirrgSTD

GAPecanSTD

Crops

Max single application 

rate lb a.i./A x Max # of 

application

Dimilin® 2L

(EPA Reg # 400-

461)

Citrus 0.312 lbs a.i./A X 3 Low- and high-volume (LV and 

HV) application applicable to citrus

Alfalfa

0.0325 lbs a.i./A x 3

---

---

Carrot 0.125 lbs a.i./A x 2 ---

Cottonseed subgroup 

20C

0.125 lbs a.i./A x 3 ---

Peach and Plum 

subgroup (12-12B)

0.25 lbs a.i./A x 2 ---
6

Table 3. SWCC Input Parameters for Diflubenzuron Modeling

Parameter Value Source Comments

Peppers/Eggplant 

subgroup 8-10B

0.125 lbs a.i./A x 3 ---

Tree Nut Subgroup 14-

12

0.25 lbs a.i./A x 4 ---

Crop

Scenario: Application 

date and minimum 

interval between 

applications (days)

Citrus

CA citrus: Dec 10 (7)

FL Citrus: Dec 10 (7)

Dimilin® 2L

(EPA Reg # 400-

461) and SWCC 

Scenario

Based on pre-harvest 

recommendation

Alfalfa

CAalfalfa: Feb 02 (30)

MNalfalfaOP:May 25(30)

TXalfalfaOP: Apr 15 (30)

Dimilin® 2L, 

SWCC Scenario 

and USDA, 2010

Based on three harvests of alfalfa 

per year. Harvest dates for the 

states (USDA 2010) and maturation 

of 30 days for successive harvest

Carrot

FL Carrot: Jan 08 (7)

Dimilin® 2L and 

SWCC Crop 

Scenario

Based on pre-harvest 

recommendation 

Cottonseed subgroup 

20C

CA Cotton: July 01(5)

MS Cotton: July 01(5)

NC Cotton: August 15(5)

Dimilin® 2L Assumed 1

st application at fully 

bloomed (USDA, 2006)

USDA Cotton crop profile

Peach Subgroup (12-

12B)

CA Fruit: July 01 (14)

GA Peach: Aug 01 (14)

Dimilin® 2L and 

SWCC Scenario Based on pre-harvest 

recommendation

Pepper/Eggplant

FL Peppers Nov 10 (7)

Tree Nuts

CA Almond: July 26 (7)

GA Pecan: Aug 06 (7)

Depth of Incorporation 

(inches) 0 Dimilin® 2L

For foliar application according to 

Input parameter guidance (USEPA, 

2009)

Method of Application

and 

crops

Aerial

Alfalfa, citrus, cottonseed, 

pepper/eggplant

Ground

Alfalfa, carrot, cottonseed, 

pepper/eggplant 

Air-blast spray

Citrus peach, plum and 

tree nut

Dimilin® 2L ---
7

Table 3. SWCC Input Parameters for Diflubenzuron Modeling

Parameter Value Source Comments

Application 

Efficiency1,2

Aerial: 0.95 (Default)

Aerial LV: 0.69 (Est.))

Ground: 0.99 (Default)

Input parameter 

guidance (USEPA, 

2009)

For LV, application efficiency was 

estimated from AgDRIFT model 

output.

Spray Drift Fraction Citrus scenario 

Aerial (default) 0.04

Aerial (LV) 0.21

Air-blast: 0.01 

AgDRIFT 

modeling

Based on LV and DV0.5 = 90 um 

and 150 ft buffer. 

Based on 25 ft buffer for ground 

application. 

Generic Default 

Aerial: 0.135

Ground: 0.066 Spray Drift 

Guidance (USEPA, 

2013b)

Based on AgDRIFT Default 

Scenarios 

Air-blast: 0.025 (Orchard)

Based on orchard scenario due to 

active growing season for 

peach/plum and nut trees

Molecular Mass (g/mol) 311 MRID 485969-01 Product chemistry data

Vapor Pressure (Torr) 9.0 x 10-10 MRID 485969-01 Product chemistry data

Henry’s Law Constant 

(atm m3/mol) 1.19 x 10-11 Estimated EPISUITE 4.1

Solubility in Water 

(mg/L) 0.08 MRID 485969-01 Product chemistry data

Hydrolysis t1/2 at pH 7.0 

(days)

119 MRID 014355 --

Aquatic Photolysis t1/2

(days)

80 MRID 40816301

MRID 41087802

--

Aerobic Soil 

Metabolism t1/2 (days)

5.1

MRID 41722801

MRID 46888703

MRID 46888708

90th percentile upper confidence 

bound on the mean half-life of 

diflubenzuron

Aerobic Aquatic 

Metabolism t1/2 (days)

25.2

MRID 44895001

MRID 46888707

90th percentile upper confidence 

bound on the mean half-life of total 

toxic residue of diflubenzuron

Anaerobic Aquatic 

Metabolism t1/2 (days)

102 (34 x 3)

3 MRID 41837601 Input parameter guidance (USEPA, 

2009)

Soil Partitioning 

Coefficient (Koc; ml/goc)

4862.1

MRID 00157842

MRID 46895401

Average Koc from 7 soils 
8

Table 3. SWCC Input Parameters for Diflubenzuron Modeling

Parameter Value Source Comments

1 Low volume application is restricted in California. FL Citrus STD was used for LV application. 

2 Spray drift fraction was estimated using AgDRIFT 2.1.1 model – Tier I Air-blast scenario (citrus, aerosol to fine 

droplet) and the Guidance on Development and Use of the Index Reservoir in Drinking Water Exposure 

Assessments. http://www.epa.gov/oppefed1/models/water/index_reservoir_dwa.html#memo

3 Due to reported half-life for a single anaerobic aquatic metabolism study, the input half-life was multiplied by 3 

according to Guidance for Selecting Input Parameters in Modeling for Environmental Fate and Transport of 

Pesticides. Version 2.1 October 22, 2009.

Table 4. SWCC Input Parameters for CPU Modeling

Parameter Value Source Comments

Crops

Max single application 

rate lb a.i./A x Max # of 

application

Estimated from 

label rate

Maximum CPU formation of 48% 

in aerobic aquatic system (MRID 

46888707 was used in application 

rate calculation1

Citrus 0.081 X 3 (Aerial Default)

Cotton 0.033 X 3 (Aerial Default)

Method of Application Ground application Assumed Contribution from degradation

Application Date Same as Table 2

Depth of Incorporation 

(inches) 0 Assumed Contribution from degradation

Application Efficiency Ground: 1.0 Assumed Contribution from degradation

Spray Drift Fraction 0 Assumed Contribution from degradation

Molecular Mass (g/mol) 177.6

TOXNET

Estimated value 

http://chem.sis.nlm.nih.gov/chemid

plus/rn/140-38-5#physical

Vapor Pressure (Torr) 1.65 x 10-04

Henry’s Law Constant 

(atm m3/mol) 1.49 x 10-10

Solubility in Water 

(mg/L) 1770

Hydrolysis t1/2 at pH 7.0 

(days)

0 Assumed No data

Aquatic Photolysis t1/2

(days)

0 Assumed No data

Aerobic Soil 

Metabolism t1/2 (days)

65.4

MRID 41722801

MRID 46888703

MRID 46888708

90th percentile upper confidence 

bound on the mean half-life derived 

from diflubenzuron study.

MRID 41722801 is the same study 

as MRID 46888708.
9

Table 4. SWCC Input Parameters for CPU Modeling

Parameter Value Source Comments

Aerobic Aquatic 

Metabolism t1/2 (days)

41.9

MRID 44895001

MRID 46888707

90th percentile upper confidence 

bound on the mean half-life derived 

from diflubenzuron study

Anaerobic Aquatic 

Metabolism t1/2 (days)

0 Assumed No data

Soil Partitioning 

Coefficient (Koc; ml/goc)

246 Van Dijk, N.R.M. 

1992

Cited from previous assessment 

(USEPA, 2002; D279732+)

1 Diflubenzuron application rate (0.312 lb/A) x [(0.48, the maximum conversion rate from the aerobic aquatic 

degradation of diflubenzuron to CPU in laboratory study) x ([177.6/311]), the molecular weight ratio of CPU to 

diflubenzuron) x (Application Efficiency, 0.95 for default).

Table 5. PRZM-GWInput Parameters1for Diflubenzuron and its Major Degradate, CPU 

Variable Name Data Value

Data Source

MRID(s) Comment

Diflubenzuron

Application Method Aerial and ground 

Dimilin®2L

EPA Reg# 400-461

---

Maximum Single 

Application Rate (lbs. 

a.i./acre) [kg/ha] X 

Maximum # of application

Citrus:0.312 [0.349] x 3 

Carrot: 0.125 [0.14] x 2

Tree Nuts: 0.25 [0.28] x 4

Since the highest 

application rate for 

citrus has never been 

modeled, it was also 

included in modeling 

Crop Scenario

Carrot

 Delmarva sand

 FL Jacksonville

 WI Sand

Citrus

 FL Central ridge

Tree Nuts

 GA Coastal

 NC Coastal

Specific scenario for FL 

carrot. In addition 

potential sites to grow 

carrot 

Citrus site specific

Specific scenario for 

GA Coastal. In addition 

potential sites to grow 

tree nuts

Application Interval (days)

7 (Citrus and tree nuts)

7 (Carrot)

7(Tree Nuts)

No specification for tree 

nuts. 

Hydrolysis, DT50 (Days) 119 MRID 014355 ---

Aerobic Soil Metabolism 

DT50 (Days) 5.1 days

MRIDs.: 

41722801

4688703

4688708

The 90% of the upper 

confidence limit (UCL) of 

the mean metabolism halflife.
10

Table 5. PRZM-GWInput Parameters1for Diflubenzuron and its Major Degradate, CPU 

Variable Name Data Value

Data Source

MRID(s) Comment

Koc (mL/goc) 4862.1

MRIDs:

00157842 and 

46895401

Average Koc from 7 

soils

CPU

Application Method Ground ---

Assumed for 

degradation product

Maximum Single 

Application Rate X 

Maximum # of application

(lbs. a.i./acre) [kg/ha]

0.086[ 0.096] x 3 (Citrus)

0.034[0.038] x 2 (Carrot)

0.069[0.077] x 4 (Tree nut)

Estimated2---

Crop Scenarios and 

Application Interval (days) Same as diflubenzuron

Dimilin®2L

EPA Reg# 400-461 ---

Hydrolysis, DT50 (Days) 0 ---

No study related to 

CPU

Aerobic Soil Metabolism 

DT50 (Days) 65.4

MRID

41722801

4688703

4688708

The 90% of the upper 

confidence limit (UCL) of 

the mean metabolism halflife.

Koc (mL/goc) 246 Van Dijk, N.R.M. 

(1992)

Cited from previous 

assessment (USEPA, 

2002;D279732+)

1 EFED input parameter guidance: http://www.epa.gov/oppefed1/models/water/przm_gw/wqtt_przm_gw_input_guidance.htm

2 Diflubenzuron application rate (0.312 lb/A) x [(0.48, the maximum conversion rate from the aerobic aquatic 

degradation of diflubenzuron to CPU in laboratory study) x ([177.6/311]), the molecular weight ratio of CPU to 

diflubenzuron)

Modeling Results

Estimated diflubenzuron and CPU concentrations in surface water used for drinking water are 

summarized in Table 6. Values in bold are recommended for use in the human health risk assessment. 

For surface water, the recommended peak (acute) concentration of 5.18 μg/L, annual mean (noncancer chronic) concentration of 0.63 μg/L and the 30 year annual average concentration (cancer 

chronic) of 0.49 μg/L based on LV application of diflubenzuron applications representative of 

cotton. The default Percent Crop Area (PCA) of 1.0 was used for the proposed uses because 

diflubenzuron can be used in both agricultural and non-agricultural settings (USEPA 2014). 

For CPU, the recommended surface water concentrations are 6.83 μg/L for acute, the 0.89 μg/L

for non-cancer chronic and 0.55 μg/L for cancer chronic also based on LV applications 

representative of citrus and cotton. Sample outputs with the highest EDWCs from the SWCC model 

runs is provided in Appendix B. The Percent Crop Area is not applicable for groundwater. 
11

Table 6. EDWCs of Diflubenzuron and its Major Degradate, CPU for Surface Water

Crop Crop Scenario Application 

Method

Peak (Acute) 

Exposure

Annual Mean 

(chronic)

Exposure 

30-year Mean 

(cancer chronic) 

Exposure

-----------------------(μg/L)-----------------------

Citrus1

CA Citrus1 Aerial (Default) 0.95 0.14 0.12

Air-blast 0.41 0.04 0.03

FL Citrus1 Aerial (Default) 1.52 0.12 0.09

Aerial LV 4.61 0.41 0.38

Air-blast 1.2 0.07 0.04

Alfalfa

CA Alfalfa Aerial 0.23 0.04 0.04

Ground 0.11 0.02 0.02

MN Alfalfa Aerial 0.23 0.05 0.05

Ground 0.11 0.02 0.02

TX Alfalfa Aerial 0.61 0.05 0.04

Ground 0.58 0.04 0.03

Carrot FL Carrot Ground 1.38 0.08 0.04

Cotton CA Cotton Aerial (Default) 1.52 0.15 00.14

Ground 0.86 0.10 0.09

MS Cotton Aerial (Default) 3.37 0.26 0.19

Ground 2.81 0.22 0.15

NC Cotton Aerial (Default) 4.22 0.51 0.33

Ground 4.11 0.47 0.27

Peach 

Subgroup 

12-12B

CA Fruit Air-blast 0.42 0.03 0.03

GA Peach 0.77 0.05 0.04

Peppers 

/Eggplant

Subgroup 

8-10B

FL Peppers Aerial 2.36 0.16 0.12

Ground

2.02 0.12 0.08

Tree Nut 

Subgroup 

14-12

CA Almond Air-blast 0.52 0.08 0.07

GA Pecan 2.91 0.22 0.14

CPU3

Aerial LV

5.00 (R)4 0.64 (R) 0.28(R)

FL Citrus 1.83 (D) 4 0.25 (D) 0.23 (D)

6.83 (T) 4 0.89 (T) 0.51 (T)

Cotton MS Cotton Aerial (Default) 3.96 0.39 0.16

NC Cotton 3.66 0.79 0.46 

1 For citrus scenarios EDWCs were generated using 150 ft and 25 ft buffer as prescribed in the diflubenzuron label 

for aerial and air-blast applications respectively

2 Bolded values are the highest EDWCs observed among various scenarios 
12

Table 6. EDWCs of Diflubenzuron and its Major Degradate, CPU for Surface Water

Crop Crop Scenario Application 

Method

Peak (Acute) 

Exposure

Annual Mean 

(chronic)

Exposure 

30-year Mean 

(cancer chronic) 

Exposure

-----------------------(μg/L)-----------------------

3 CPU EDWCs were estimated for selected high concentration of diflubenzuron scenarios (i.e. FL Citrus, MS Cotton 

and NC cotton) observed during SWCC modeling

4 CPU EDWCs generated from Runoff (R) and drift (D) for citrus UL application. The summation of EDWCs from 

runoff and drift is designated as Total (T) CPU.

Estimated diflubenzuron and CPU concentrations in groundwater used for drinking water are 

summarized in Table 6. Values in bold are recommended for use in the human health risk assessment.

For groundwater sources of drinking water, the maximum acute and chronic estimated concentrations of 

CPU in shallow groundwater from PRZM-GW model for applications to citrus are 8.61 μg/L and 8.02 

μg/L, respectively. However, the concentrations of diflubenzuron in groundwater were negligible (Table 

7). Sample outputs with the highest EDWCs from the PRZM-GW model runs is provided in Appendix B.

Table 7. Results from PRZM-GW Modeling Based on Diflubenzuron Uses on Citrus and 

Proposed New Uses

Crop

(Application Rate) Scenario

Highest

Daily

Value (μg/L)

Average (μg/L)

Average Simulation 

Breakthrough Time 

(days)

Citrus1

(0.312 lbs a.i./A x 3 

applications)

Other Crops

(0.25 lbs a.i./A x 3 

applications)

Diflubenzuron

FL-Central Ridge 0 --

Incomplete3

FL-Jacksonville 0 0

GA Coastal 0 0

NC Coastal cotton 3.04E-18 --

DELMARVA Sandy 6.59E-20

WI Sand 0 0

CPU

FL-Central Ridge 8.61 8.02 3191

FL-Jacksonville 0.01 0.00 5942

GA Coastal 2.07 1.94 5291

NC Coastal cotton 4.26 3.81 4228

DELMARVA Sandy 1.07 0.94 3970

WI Sand 3.85 3.47 5665

1 PRZM-GW modeled the highest diflubenzuron application rate of 0.312 lbs a.i/A for diflubenzuron uses using 

FL Central ridge scenario known for growing citrus crops.

2 Bolded concentrations are the highest EDWCs determined for groundwater

3

Incomplete with 30 and 100-years simulation
13

Monitoring Data

Monitoring data for diflubenzuron are available from the United States Geological Survey 

(USGS) National Water-Quality Assessment (NAWQA) Program Data Warehouse3, searched on 

April 06, 2015. No diflubenzuron was detected in 11 surface water samples from Texas. 

However, the study design of NAWQA is not targeted to account for all diflubenzuron use areas;

timing of application and other factors which may more accurately represent spatially and 

temporally dependent variables influencing runoff vulnerability. No groundwater and sediment 

data are available at this time.

Monitoring data for surface water, groundwater, and sediment from the California Department of 

Pesticide Regulation (CDPR)4 were also searched on April 06, 2015. No diflubenzuron was 

detected in 66 surface water samples from California. No groundwater and sediment data are 

available at this time. No monitoring data related to CPU is available at this time.

 

3

(http://infotrek.er.usgs.gov/nawqa_queries/jsp/swmaster.jsp

4 http://www.cdpr.ca.gov/docs/emon/ehap.htm
14

References

USEPA, 2001. Health Effects Division (HED) Metabolism Assessment Review Committee 

(MARC) Meetings of 2/20/01 & 5/8/01. Diflubenzuron. Residues of Concern for Cancer 

Risk Assessment. Chemical 108201. Barcode D272976. Case 239100. Submission 

S590172.

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 Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, 

Environmental Fate and Effects Division, October 22, 2009. 

US EPA. 2012a. Registration Review: Problem Formulation for the Environmental Fate, 

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

Diflubenzuron. Environmental Fate and Effects Division, Office of Chemical Safety and 

Pollution Prevention. U. S. Environmental Protection Agency (D398815).

US EPA. 2012b. Memorandum: Approval of PRZM-GW for use of Drinking Water Exposure 

Assessment. Environmental Fate and Effects Division, Office of Chemical Safety and 

Pollution Prevention. U. S. Environmental Protection Agency.

USEPA, 2013a. Guidance for Selecting Input Parameters for Modeling Pesticide Concentrations 

in Groundwater Using the Pesticide Root Zone Model. Version 1. October 15, 2012. 

Environmental Fate and Effects Division. Office of Pesticide Programs. United States 

Environmental Protection Agency. Available at 

http://www.epa.gov/oppefed1/models/water/przm_gw/wqtt_przm_gw_input_guidance.ht

m

USEPA, 2013b. Guidance on modeling off-site deposition of pesticides via spray drift for 

ecological and drinking water assessments. Environmental Fate and Effects Division, 

Office of Chemical Safety and Pollution Prevention. U. S. Environmental Protection 

Agency.

USEPA. 2014. Guidance on Development and Use of Community Water System Drinking Water 

IntakePercent Cropped Area Adjustment Factors for Use in Drinking Water Exposure 

Assessments:2014 Update. Environmental Fate and Effects Division, Office of Chemical 

Safety and Pollution Prevention, United States Environmental Protection Agency.

Van Dijk, N.R.M. 1992. Adsorption test with 4-chlorophenylurea with four soil types. 

SOLVAY DUPHAR B.V., Environmental Research department, Graveland, The 

Netherlands (preliminary EFED review).

USDA, 2006. Fruits and Tree Nuts: Blooming, Harvesting, and Marketing Dates. USDA, 

National Agricultural Statistics Service, Agriculture Handbook No. 729

http://www.nass.usda.gov
15

USDA, 2010. Field Crops Usual Planting and Harvesting Dates. USDA, National Agricultural 

Statistics Service, Agriculture Handbook No. 628. 

http://www.nass.usda.gov
16

APPENDIX A

Note: DT50 cannot be calculated for Clay loam soils from French and UK. Not enough data points 

to regress. 

MRID 41722801_Soil Metabolism Rate_SL_CPU

DAT % of Applied

0

0

0

 3

 3

 3

 7

 7

 7

 14

 14

 14

31.2

30.2

30.9

30.4

29.8

31.9

27.9

27.7

 28.

26.3

24.5

24.3

0 2 4 6 8 1 0 1 2 1 4

0

5

1 0

1 5

2 0

2 5

3 0

Time Concentration

SFO

DFOP

IORE

-4

-2

0

2

4

-4

-2

0

2

4

-4

-2

0

2

4

0 2 4 6 8 1 01 21 4

DT50 DT90

2 C0

44.1 146 1.5 31.3

44.1 146 Inf 31.3 f 0.867

36869418033631588520406844480028204484828842402 36869418033631629084862048200842064608868428624 8.5 28.6 N

MRID 46888703_Soil Metabolism Rate_SL_CPU

DAT % of Applied

0

 11

 32

 46

19.2

15.9

7.50

4.50

0 1 0 2 0 3 0 4 0

0

5

1 0

1 5

Time Concentration

SFO

DFOP

IORE

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 0 2 0 3 0 4 0

DT50 DT90

2 C0 Parameters

SFO 23.6 78.3 6.2 20 k 0.0294

DFOP 23.6 78.3 Inf 20 f 0.577 k0 0.0294 k1 0.0294

IORE 26.7 56.3 4.3 19.5 N 0.323 k 0.155

SC

SSFO

Slow t1 2

tR IORE

5.95

3.22

23.6

16.9
17
18

Appendix B

Sample Outputs:

Summary of Water Modeling of Diflubenzuron and the USEPA Standard Reservoir

Estimated Environmental Concentrations for Diflubenzuron are presented in Table 1 for the 

USEPA standard reservoir with the FLcitrusSTD field scenario. A graphical presentation of the 

year-to-year peaks is presented in Figure 1. These values were generated with the Surface Water 

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

Table 1. Estimated Environmental Concentrations (ppb) for Diflubenzuron.

Peak (1-in-10 yr) 4.61

4-day Avg (1-in-10 yr) 3.83

21-day Avg (1-in-10 yr) 2.75

60-day Avg (1-in-10 yr) 1.61

365-day Avg (1-in-10 yr) 0.410

Entire Simulation Mean 0.376

Table 2. Summary of Model Inputs for Diflubenzuron.

Scenario FLcitrusSTD

Cropped Area Fraction 1.0

Koc (ml/g) 4862.1

Water Half-Life (days) @ 25 °C 25.2

Benthic Half-Life (days) @ 25 °C 102

Photolysis Half-Life (days) @ 40 °Lat 80

Hydrolysis Half-Life (days) 119

Soil Half-Life (days) @ 25 °C 5.1

Foliar Half-Life (days)

Molecular Wt 311

Vapor Pressure (torr) 9.0E-10

Solubility (mg/l) 0.08

Table 3. Application Schedule for Diflubenzuron.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

12/10 Foliar 0.35 0.69 0.21

12/17 Foliar 0.35 0.69 0.21

12/24 Foliar 0.35 0.69 0.21
19

Figure 1. Yearly Peak Concentrations

Summary of Water Modeling of CPU and the USEPA Standard Reservoir

Simulated Concentration from Runoff Contribution only

Estimated Environmental Concentrations for CPU are presented in Table 1 for the USEPA 

standard reservoir with the FLcitrusSTD field scenario. A graphical presentation of the year-toyear peaks is presented in Figure 1. These values were generated with the Surface Water 

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

Table 1. Estimated Environmental Concentrations (ppb) for CPU.

Peak (1-in-10 yr) 5.00

4-day Avg (1-in-10 yr) 4.73

21-day Avg (1-in-10 yr) 3.81

60-day Avg (1-in-10 yr) 2.69

365-day Avg (1-in-10 yr) 0.639

Entire Simulation Mean 0.284

Table 2. Summary of Model Inputs for CPU.

Scenario FLcitrusSTD

Cropped Area Fraction 1.0
20

Koc (ml/g) 246

Water Half-Life (days) @ 25 °C 41.9

Benthic Half-Life (days) @ 25 °C 0

Photolysis Half-Life (days) @ 40 °Lat 0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 25 °C 65.4

Foliar Half-Life (days)

Molecular Wt 177.6

Vapor Pressure (torr) 1.65E-04

Solubility (mg/l) 1770

Table 3. Application Schedule for CPU.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

12/10 Ground 0.066 1 0

12/17 Ground 0.066 1 0

12/24 Ground 0.066 1 0

Figure 1. Yearly Peak Concentrations

Simulated concentrations from Drift Contribution only

Estimated Environmental Concentrations for CPU are presented in Table 1 for the USEPA 

standard reservoir with the FLcitrusSTD field scenario. A graphical presentation of the year-toyear peaks is presented in Figure 1. These values were generated with the Surface Water 

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.
21

Table 1. Estimated Environmental Concentrations (ppb) for CPU.

Peak (1-in-10 yr) 1.83

4-day Avg (1-in-10 yr) 1.73

21-day Avg (1-in-10 yr) 1.42

60-day Avg (1-in-10 yr) 1.04

365-day Avg (1-in-10 yr) 0.248

Entire Simulation Mean 0.232

Table 2. Summary of Model Inputs for CPU.

Scenario FLcitrusSTD

Cropped Area Fraction 1.0

Koc (ml/g) 246

Water Half-Life (days) @ 25 °C 41.9

Benthic Half-Life (days) @ 25 °C 0

Photolysis Half-Life (days) @ 40 °Lat 0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 25 °C 65.4

Foliar Half-Life (days)

Molecular Wt 177.6

Vapor Pressure (torr) 1.65E-04

Solubility (mg/l) 1770

Table 3. Application Schedule for CPU.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

12/10 Ground 0.020 0 1

12/17 Ground 0.020 0 1

12/24 Ground 0.020 0 1

Figure 1. Yearly Peak Concentrations
22

Groundwater Analysis for Diflubenzuron and the Florida Citrus - FL Central Ridge, Polk 

County - (Tampa) Met File (12842.Dvf) - Astatula Sand, Hrgb A Scenario

Estimated groundwater concentrations and breakthrough times for Diflubenzuron are presented 

in Table 1 for the Florida Citrus - FL Central Ridge, Polk County - (Tampa) Met File 

(12842.dvf) - Astatula sand, hrgb A groundwater scenario. A graphical presentation of the daily 

concentrations in the aquifer is presented in Figure 1. These values were generated with the 

PRZM-GW (Version 1.07). Critical input values for the model are summarized in Tables 2 and 

3.

Table 1. Groundwater Results for Diflubenzuron and the Florida Citrus - FL Central 

Ridge, Polk County - (Tampa) Met File (12842.dvf) - Astatula sand, hrgb A Scenario.

Peak Concentration (ppb) 1.6E-12

Post-Breakthrough Mean 

Concentration (ppb)

Incomplete

Entire Simulation Mean 

Concentration (ppb)

2.74E-13

Average Breakthrough Time 

(days)

Incomplete

Throughputs 0.2207278

Table 2. Chemical Properties for Groundwater Modeling of Diflubenzuron.

Koc (ml/g) 4862.1

Surface Soil Half Life (days) 5.2

Hydrolysis Half Life (days) 116
23

Diffusion Coefficint Air (cm2/day) 0.0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 3. Pesticide application scheme used for Diflubenzuron. This application scheme was applied every 

year of the simulation.

Application Date

(Month/Day)

Application Method Application Rate

(kg/ha)

12/10 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.349

12/17 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.349

12/24 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.349

Figure 1. Aquifer Breakthrough Curve for Diflubenzuron and the Florida Citrus - FL 

Central Ridge, Polk County - (Tampa) Met File (12842.Dvf) - Astatula Sand, Hrgb A 

Scenario

Groundwater Analysis for CPU and the Florida Citrus - FL Central Ridge, Polk County -

(Tampa) Met File (12842.Dvf) - Astatula Sand, Hrgb A Scenario

Estimated groundwater concentrations and breakthrough times for CPU are presented in Table 1 

for the Florida Citrus - FL Central Ridge, Polk County - (Tampa) Met File (12842.dvf) - Astatula 

sand, hrgb A groundwater scenario. A graphical presentation of the daily concentrations in the 

aquifer is presented in Figure 1. These values were generated with the PRZM-GW (Version 

1.07). Critical input values for the model are summarized in Tables 2 and 3.
24

Table 1. Groundwater Results for CPU and the Florida Citrus - FL Central Ridge, Polk 

County - (Tampa) Met File (12842.dvf) - Astatula sand, hrgb A Scenario.

Peak Concentration (ppb) 8.61

Post-Breakthrough Mean 

Concentration (ppb)

 8.02

Entire Simulation Mean 

Concentration (ppb)

 6.25

Average Breakthrough Time 

(days)

3191.4

Throughputs 3.434229

Table 2. Chemical Properties for Groundwater Modeling of CPU.

Koc (ml/g) 246

Surface Soil Half Life (days) 65.4

Hydrolysis Half Life (days) 0

Diffusion Coefficint Air (cm2/day) 0.0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 3. Pesticide application scheme used for CPU. This application scheme was applied every year of the 

simulation.

Application Date

(Month/Day)

Application Method Application Rate

(kg/ha)

12/10 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.096

12/17 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.096

12/24 Ground application with mass distribution 

increasing proportionally with depth to 4 

cm

0.096

Figure 1. Aquifer Breakthrough Curve for CPU and the Florida Citrus - FL Central Ridge, 

Polk County - (Tampa) Met File (12842.Dvf) - Astatula Sand, Hrgb A Scenario
25