UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 120301 

DP Barcode: 427241 

MEMORANDUM 

DATE: May 13, 2015 

SUBJECT: Thidiazuron: Drinking Water Exposure Assessment Memorandum for 

Preliminary Risk Assessment in Support ofThidiazuron Registration Review. 

TO: Ideliz Encarnacion, Risk Assessor 

Michael Metzger, Branch Chief 

Registration Action Branch 7 

Health Effects Division (7509P) 

FROM: 

THRU: 

Khue Nguyen, Chemical Review Manager 

Neil Anderson, Branch Chief 

Risk Management and Implementation Branch I 

Pesticide Reevaluation Division (7508P) 

lwona Maher, Chemist $hQ lfJ ( 'hir "t. II\~~) V-\3 ·'), I iJ _ / / 

William P. Eckel, Ph.D., Senior Science Advisor .-111~~ f V-{ ':> 

11 1 

~ Environmental Risk Branch VI 

Environmental Fate and Effects Division (7507P) 

MonicaWait, RAPL ~ W~ 5/\~/\5 

Environmental Risk Branch VI \J.J '- ')°--\ 1 -\) Mark Corbin, Branch Chief ~r , k _ 

Environmental Fate and Effects Division 7507P) 

This memorandum provides the human health drinking water exposure assessment for 

thidiazuron in support of registration review. Thidiazuron, a cotton defoliant, was first registered 

in the U.S. in 1982. Thidiazuron is used as a pre-harvest treatment (treatment applied before 

harvesting) on cotton as a defoliant to reduce the foliage and immature fruiting structures which 
contribute to the staining of harvested cotton. Applications of thiadiazuron also facilitate cotton 

harvesting and reduce the pieces of dry leaves and other inert matter in harvested cotton.

Human drinking water exposure to thidiazuron was previously analyzed in support of the 

Reregistration Eligibility Decision (RED) on Aug. 24, 2004 (D244574). The 2004 DWA was a 

Tier I assessment using the FIRST and SCI-GROW models. The estimated drinking water 

concentrations (EDWC) at that time were 3.5 ppb for acute surface water and 1.0 ppb for chronic 

surface water, including the parent thidiazuron and two photodegradates. The EDWC for 

groundwater was 0.066 ppb (acute and chronic) for parent only.

For this assessment, the half-lives were recalculated to follow the “NAFTA Guidance to 

Calculate Representative Half-life Values,” and the modeling input parameters follow current 

guidance. Three degradates were included in this analysis: photodiazuron, 1-cyano-3-phenyl 

urea, and thiadiazolurea.

Table 1 lists the recommended exposure estimates for use of thidiazuron on cotton (the only 

currently registered use). The groundwater exposures are higher than estimated surface water 

exposure and are therefore recommended for use in the HED risk assessment.

Table 1. Estimated Drinking Water Concentrations (EDWCs) for Thidiazuron, its Photodegradates (I and 

II) and Thidiazolurea ResiduesA

Drinking water source 

(model)

Use

(modeled rate)

1-in-10 year acute 

(μg/L)

1-in-10 year 

chronic (μg/L)

30- year average 

(μg/L)

Surface water

(SWCC)

Cotton (MS)

(0.31 lb ai/A) aerial 3.8 0.66 0.46

Ground water

(PRZM-GW)

Cotton (as FL citrus)

(0.31 lb ai/A) aerial

Max. Daily 

Conc(μg/L)

Post-breakthrough 

mean (μg/L)

Entire Simulation 

Mean (μg/L)

21.7 21.5 10.6 A Based on the total annual rate of 0.31 lbs ai/A/yr (2 apps, 5-d interval, 0. 21b a.i/A and 0.1 a.i/A, respectively). 

Surface water corrected for national maximum cotton PCA of 0.21. 

Usage

Thidiazuron is currently registered for use as a defoliant (herbicide) to aid the harvest of cotton. 

There are no other registered use sites. The maximum single application rate is 0.21 pounds of 

active ingredient per acre (lb ai/A). A second application is recommended under low 

temperature conditions when defoliation does not reach the expected level, but the total of the 

two applications must not exceed 0.31 lb ai/A. The interval between applications range from 5-7 

days in some of the labels, while the other labels provide no recommendations of the

reapplication interval. According to the BEAD Chemical Profile (1/26/15, S. Ratnayake et al.), 

the top states, in terms of the percentage of the total pounds a.i. applied and acres treated, are 

Georgia, Texas, Mississippi, and Arkansas. 

2
Thidiazuron is formulated as wettable powder in water soluble bags, suspension and emulsifiable 

concentrates, or water dispersible granules and applied as a pre-harvest application via ground or 

aerial application equipment. This product is not to be applied using any type of irrigation 

system.

Fate Summary

In soil, thidiazuron is persistent, as evidenced by laboratory and field half-lives and rotational 

crop intervals on the order of one year. It has intermediate soil sorption coefficients (sorption 

coefficients ranged from 4.4 to 19 Freundlich units). Such persistence and intermediate mobility 

would allow some year to year accumulation over time and, therefore, more opportunity for 

favorable conditions for runoff from application sites to occur. For example, with a one year soil 

half-life and without redistribution or dispersal from the site of application, theoretical build-up 

in soil would asymptotically approach twice that of annually applied amounts. Of course, given 

time and variable climatic conditions, redistribution or partitioning of thidiazuron into other 

environmental compartments does occur (via runoff, leaching, plant uptake, etc.); therefore, 

maximum build-up in surface soil averages less than the theoretical factor of two. 

When thidiazuron reaches surface water, photolysis is expected to be the major route of 

transformation; other degradative processes are essentially negligible by comparison. Aqueous 

photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of 

the photodegradates is a structural isomer of parent, whereas the other has a substantially altered 

chemical structure.

Based on its solubility and vapor pressure (Henry's law), and other laboratory evidence, 

thidiazuron is non-volatile. Based on its relatively low n-octanol/water partitioning coefficient, 

thidiazuron should not bioconcentrate.

No foliar dissipation data are currently available, and since thidiazuron is used to defoliate cotton

prior to harvest, dislodgement from or transformation in/on leaves may be an important fate

pathway. The default foliar half-life of 35 days was assumed. 

Thidiazuron may degrade to thiadiazolurea (also known as AE F132345 or 1,2,3-thidiazol-5-

ylurea) and unidentified simple phenyl compounds via hydrolysis of the urea bridge. These 

compounds in turn could degrade to unidentified minor compounds which are incorporated into 

the organic material and mineralized to CO2. However, there was insufficient data to support 

these contentions, and production of additional metabolites through AE F132345 appears to be a 

minor route of transformation.

The structural isomer is AE F132347 or photoproduct I (see Appendix 1 for chemical 

identification). It has a maximum experimental limit of production of 77% of parent equivalents 

at pH 5, 28% at pH 7, and 17% at pH 9. Photoproduct I was stable to further photolysis under 

3
experimental study conditions. The second photodegradate is AE C421200 or photoproduct II 

(see Appendix 1 for chemical identification). Photoproduct II is substantially different from 

parent thidiazuron, and has a complementary asymptotic limit of production to photoproduct I of 

23% of parent at pH 5, 72% at pH 7, and 83% at pH 9. This product was also stable to further 

photolysis under study conditions. Thus, at different environmental pHs, parent thidiazuron is 

always quantitatively converted in complementary proportions into the two photodegradates.

A selection of thidiazuron’s environmental fate and transport properties are summarized in Table 

2 below.

Table 2. Physical Chemical and Environmental Fate Properties of Thidiazuron and Two 

Photoproducts.

Parameter Value Source

Selected Physical/Chemical Parameters

Chemical Structure

Empirical Formula C9 H8 N4 O S D294544

IUAPC Name 1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea D294544

CAS Name N-phenyl-N’-1,2,3-thiadiazol-5-ylurea D294544

CAS No. 51707-55-2 (330-54-1 also found) D294544

PC Code No. 120301 D294544

SMILES string: O=C(Nc1ccccc1)Nc1cnns1 D294544

Molecular Weight 220.25 g/mole Product Chemistry

Physical state TGAI: amorphous solid; PAI: crystalline 

solid

MRID 45934001

Vapor pressure 3.5 x l0-9 Pa @ 20 oC MRID 94246001

Aqueous solubility (25°C) PAI: water = 31.0 mg/L (pH = 7.0) MRID 41364906

Dissociation constants (pKa ) PAI: pKa = 8.86 25 oC MRID 41364901

pH TGAI: 5.21 (20 0C); PAI: 6.50 MRID 45934001

Henry’s Law Constant 2.5 x 10-l6 atm m3/mol calculated

Octanol-to-water partition 

coefficient at 25 oC

PAI: K o/w = 50.0 (25 0C, pH = 7.3)

shake flask method

MRID 41364906

Persistence

Hydrolysis half-life (pH 5, 7, 9) Stable MRID 42069203

Aqueous photolysis half-life Stable due to formation of stable 

photodegradates

MRID: 44537108

Aerobic soil metabolism halflife (range in days)349 days (90th%ile upper bound on mean) MRID 41950101; 46119601

DP Barcode D289980

Anaerobic aquatic metabolism 

half-life 

289 days MRID 42666101and 

44505132

Aerobic aquatic metabolism 

half-life (total system)

697 days (2 x soil metabolism) MRID: 45196011

Mobility

4
Table 2. Physical Chemical and Environmental Fate Properties of Thidiazuron and Two 

Photoproducts.

Parameter Value Source

Adsorption/desorption Kf/ads Kf/oc/ads 1/n r2

Sand: German Std. Soil # 2.1

4.36 908 0.721 0.997

Loamy sand: German Std. Soil #2.2

16.2 786 0.771 0.999

Sandy loam: German Std. Soil #2.3

7.33 780 0.719 0.999

Sandy clay loam: UK, Schering #171

18.8 494 0.817 1.000

Koc model is appropriate (r2 = 0.88)

Min Kf, Kfoc (n=4): 4.36, 494

Max Kf, Kfoc (n=4): 18.8, 908 

Median Kfd, Kfoc: 11.8, 783

Average Kfd, Kfoc: 11.7+6.9, 742+176

MRID 41364909

Surface Water Assessment

The Surface water Concentration Calculator (SWCC; version 1.106) was used to calculate 

EDWCs for the three standard cotton scenarios (CA, NC, MS). The application parameters are 

given in Table 3, the input parameter values are given in Table 4, and the results in Table 5.

Table 3. SWCC Management Practices Input Parameters for Thidiazuron

Application Information Value (unit) Source

Application Regime

Application Rate in kg ai/ha 

(lb ai/A)

0.235 kg a.i./ha (0.21) 1st

0.112 kg a.i./ha (0.1) 2nd Manufacturer Label

Number of Applications 2 Same as above

Application Interval 5 days Same as above

Application Date(s)

CAcotton_WirrigSTD:28 Oct &2 Nov

MScottonSTD: 12 & 17 Sep

NCcottonSTD: 29 Sep & 4 Oct

Selected following the label 

instructions and date of harvest.

Practices for All Modeled Programs

Application Method Aerial (spray) = foliar (2) Manufacturer Labels

Application Efficiency 95% Input Parameter Guidelines 

(USEPA, 2009)

Spray Drift 13.5% 

Default value; fine to medium 

droplet; Offsite Deposition 

Guidance (USEPA, 2013)

Scenarios Run CAcotton_WirrigSTD, MScottonSTD, and NCcottonSTD:

5
Table 4. SWCC Input Parameters (Physico-chemical and Environmental Fate) Used in Modeling 

Aquatic Exposure for Thidiazuron (including its Photodegradates (I and II) and Thidiazolurea).

Fate Property/Physicochemical ParameterValue Comment Source

Kfoc (ml/g) 742 Mean of four values ranging from 

494 to 908 ml/g for the parent.

MRID 41364909

Water Column Metabolism 

Half LifeA (days) @ 20 °C

697 Double aerobic soil metabolism halflife input.n/a

Benthic Metabolism Half 

Life (days) @ 20 °C

289 The parent is essentially stable. MRIDs 42666101

and 46119602

Aqueous Photolysis Half 

LifeB (days) 

0 Parent photolysis rapidly to form 

Photodegradate I and II which are 

stable to photolysis.

MRID 41188201; 

41364910; 

43075202; and 

44436901

Hydrolysis Half Life (days) 0 pH 5, 7, 9 MRID 42069203

Aerobic Soil Metabolism 

Half LifeC (days) @ 20 °C 349

Upper 90th percentile confidence 

bound on the mean (316) of four 

values (268, 354, 342, and 301, 

days).

MRIDs: 46119601

and 41950101

Molecular MWT (g/mole) 220.25 Value for parent Product Chemistry

Vapor Pressure (torr) 2.6 xl0-11 Value for parent 

Aqueous Solubility at 25oC 

(mg/L)

31.0 Value for parent MRID 41364906

A These input values reflect recalculated values based on the latest NFTA guidance B Stable pertains to the 

parent and its Photodegratates I and II. The half-lives were estimated for parent and its degradates 

thidiazolurea and Photodegradate I. 

Table 5. PCA CorrectedA Estimated Drinking Water Concentrations (EDWCs) for Thidiazuron including

Photodegradates (I and II) and Thidiazolurea Residues. B,C

Drinking Water 

Source

(model/modeled rate)

Use Site

Peak 1-in-10 

Year Value (μg/L)

1-in-10 Year 

Chronic (μg/L)

30- Year Average

(μg/L)

Surface Water

(SWCC; 1st application 

0.21 and 2nd 0.1 lb ai/A; 

5 days apart) aerial

CAcotton_WirrigSTD 1.2 0.51 0.38

MScottonSTD 3.8 0.66 0.46

NCcottonSTD 2.4 0.90 0.65

A National crop-specific PCA adjustment factor for cotton (0.21) was applied to EDWCs when running SWCC. B Modeled with updated input parameters according to current EFED guidance on half-life calculation, modeling 

input parameters, and spray drift fraction. C Based on the total annual rate of 0.31 lbs ai/A/yr (2 apps, 5-d interval, 0. 21b a.i/A and 0.1 a.i/A, respectively). 

6
Groundwater Assessment

Chemical input parameters (Table 6) for thidiazuron were selected according to the PRZM-GW 

Input Parameter Guidance (USEPA, 2012). As per the product label, only foliar (aerial) 

application was modeled. Table 7 gives the PRZM-GW model results.

Table 6. PRZM-GW v 1.07 Input Parameters for Thidiazuron (including its Photodegradates (I and II) 

and Thidiazolurea Residues).

Input Parameter Value Justification Source

Application Rate 0.235 kg a.i./ha

0.112 kg a.i./ha,

Preharvest application via aerial spray

(foliar). Manufacturer label

Application dates 

NC Cotton 

29 Sep & 4 Oct

FL citrus

15 Oct & 20 Oct

DMV corn

01 Oct & 06 Oct

Initial application date was selected 

based on growing season data Manufacturer label

Hydrolysis Half-life 0 pH 5, 7, 9 MRID 42069203

Aerobic Soil Metabolism

Half-life 349 d

Upper 90th percentile confidence bound 

on the mean (316) of four values (268, 

354, 342, and 301, days).

MRIDs: 46119601

and 41950101

Organic Carbon Partition 

Coefficient (KfOC) 742 L/kg Represents the mean KFOC value (n=4) MRID 41364909

Table 7. Estimated Drinking Water Concentrations from Ground Water sources for Thidiazuron including its 

Photodegradates (I and II) and Thidiazolurea Residues.

A, B

Drinking water

source (model)

Modeled 

Scenario

Max. Daily Conc. 

(μg/L)

Post-breakthrough 

Mean (μg/L)

Entire Simulation

mean (μg/L)

Average Breakthrough 

Time (days)

Ground Water

(PRZM-GW)

DMV corn 13.7 12.0 2.9 9807

FL citrus 21.7 21.5 10.6 8184

NC cotton 16.6 16.0 5.7 8818

A Modeled with updated input parameters according to current EFED guidance on half-life calculation, modeling 

input parameters, and spray drift fraction. B Based on the total annual rate of 0.31 lbs ai/A/yr (2 apps, 5-d interval, 

0. 21 lb a.i/A and 0.1 lb a.i/A, respectively). 

Uncertainties

In the aerobic soil metabolism study (MRID 46119601), the study was terminated at 160-168 

days post-treatment. At that time, 43.2% of the applied thidiazuron remained undegraded in the 

sandy loam soil, 68.8% remained in the loamy sand soil, and 73.4% in the silt loam soil. 

However, for compounds that, because of their longevity, cannot meet halving-time criteria in 

less than one year, then a study lasting only one year is considered sufficient by the EPA. As a 

7
result of the experiments being terminated prematurely, half-lives for thidiazuron are essentially 

extrapolations beyond the final sampling interval. The accuracy of the half-lives is, therefore, 

based on the assumption that the pattern of transformation observes first-order kinetics 

throughout the process. 

REFERENCES

USEPA (2005). Environmental Fate and Effects Division Revised Risk Assessment for the 

Reregistration Eligibility Decision of Thidiazuron for Use on Cotton. OCSPP/OPP/EFED

September 15, 2005

DP Barcode: D319339

USEPA (2015). Standard Operating Procedure for Using the NAFTA Guidance to Calculate 

Representative Half-life Values and Characterizing Pesticide Degradation. OCSPP/OPP/EFED, 

March 23, 2015

8
Appendix A

Thidiazuron Degradates

Thidiazolurea (AE F132345; ZK 85290)

IUPAC name: 1,2,3-thiadiazol-5-ylurea.

CAS name: Not reported.

CAS No: Not reported.

AE F132347; ZK 79173; ZK 80178; photoproduct I; photothidiazuron (peak 3 in 

MRID 41188201)

IUPAC name: 1-phenyl-3-(1,2,5-thiadiazol-3yl)urea

CAS name: N-phenyl-N′-1,2,5-thiadiazol-3ylurea

CAS No: Not reported.

9
AE C421200; photoproduct II (peak 1 in MRID 41188201)

1-cyano-3-phenylurea

ZK 44483; BTS 13225

IUPAC name: phenylurea

CAS name: phenylurea

CAS No: not reported

10
AE F147706; ZK 80062

IUPAC name: 1-(3-hydroxyphenyl)-3-(1,2,3-thiadiazol-5-yl)urea

CAS name: N-(3-hydroxyphenyl)-N’-1,2,3-thiadiazol-5-ylurea

CAS No: not reported

Other names: 1-m-hydroxyphenyl-3-(1,2,3-thiadiazol-5-yl)-urea

N-m-hydroxyphenyl-N’-(1,2,3-thiadiazol-5-yl)urea

11
Modeling output files from SWCC and PRZM-GW.

Summary of Water Modeling of Thidiazuron and the USEPA Standard 

Reservoir

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

standard reservoir with the MScottonSTD 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.

This model estimates that about 3.2% of thidiazuron applied to the field eventually reaches the water 

body. The main mechanism of transport from the field to the water body is by runoff (83.3% of the total 

transport), followed by spray drift (12.7%) and erosion (4.02%).

In the water body, pesticide dissipates with an effective water column half-life of 45.2 days. (This value 

does not include dissipation by transport to the benthic region; it includes only processes that result in 

removal of pesticide from the complete system.) The main source of dissipation in the water column is 

washout (effective average half-life = 48.2 days) followed by metabolism (725 days) and volatilization 

(1.285241E+09 days).

In the benthic region, pesticide dissipates very slowly (300.6 days). The main source of dissipation in the 

benthic region is metabolism (effective average half-life = 300.6 days). The vast majority of the pesticide 

in the benthic region (98.77%) is sorbed to sediment rather than in the pore water.

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

Peak (1-in-10 yr) 18.0

4-day Avg (1-in-10 yr) 17.1

21-day Avg (1-in-10 yr) 14.1

60-day Avg (1-in-10 yr) 10.6

365-day Avg (1-in-10 yr) 3.16

Entire Simulation Mean 2.21

Table 2. Summary of Model Inputs for thidiazuron.

Scenario MScottonSTD

Cropped Area Fraction 1.0

Koc (ml/g) 742

Water Half-Life (days) @ 20 °C 697

Benthic Half-Life (days) @ 20 °C 289

12
Photolysis Half-Life (days) @ 40 °Lat 0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 20 °C 349

Foliar Half-Life (days) 35

Molecular Wt 220.25

Vapor Pressure (torr) 2.62E-11

Solubility (mg/l) 31

Table 3. Application Schedule for thidiazuron.

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

09/12 Foliar 0.235 0.95 0.135

09/17 Foliar 0.112 0.95 0.135

Figure 1. Yearly Peak Concentrations

13
Groundwater Analysis for thidiazuron 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 thidiazuron 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 thidiazuron and the Florida Citrus - FL Central Ridge, Polk County -

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

Peak Concentration (ppb) 21.7

Post-Breakthrough Mean 

Concentration (ppb)

 21.5

Entire Simulation Mean 

Concentration (ppb)

 10.6

Average Breakthrough Time 

(days)

8183.798

Throughputs 1.339231

Table 2. Chemical Properties for Groundwater Modeling of thidiazuron.

Koc (ml/g) 742

Surface Soil Half Life (days) 349

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 thidiazuron. This application scheme was applied every 

year of the simulation.

Application Date

(Month/Day)

Application Method Application Rate

(kg/ha)

10/15 Above canopy application 0.235

14
10/20 Above canopy application 0.112

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

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

15