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 Wat e r Exposure Assessment Memorandum for Preliminary Risk Assessment in Support ofThid i azuron 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 Ne il Anderson

, Branch Chief Risk Management and Implementation Branch I Pesticide Reevaluation Division (7508P)

lwona Maher

, Chemist 

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William P. Eckel

, Ph.D

., Senior Science Advisor

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# ~ Environmental Risk Branch VI Environmental Fate and Effects Division (7507P)

MonicaWait

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Environmental Risk Branch VI 

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- \) Mark Corbin

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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 re gistered in t h e U

. S. in 1982. Thidiazuron is u sed as a pre-harvest treatment (treatment applied before harvesting) on cotton as a defoliant to reduce the fol i age 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.

F or this assessment, t he half

- lives were recalculated to follow t he “ 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 highe r 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 Residues A 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

T hidiazuron is currently registered for use as a defoliant (herbicide) to aid the harvest of cotton. There a re no other registered use site s

. The maximum single application rate is 0.2 1 pounds of active ingredient per acre ( l b ai/A ) . A second ap plication is recommended under low temperature conditions when defoliation does n ot reach th e expected level, but the total of the two applica tions must not exceed 0.3 1 lb ai/A. The in terval between applications range from 5

- 7 days in some of the labels, while the other la bels provide n o recommendations of the reapplication interval

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

, t he top states, in terms of the percentage of the total pounds a.i. applied and acre s treate d , are Georgia, T exas, 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 applicat ion 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 persis tence and intermediate mobility would allow some year to year accumulation over time and, the refore, more opportunity for favorable conditions for runoff from application sites to occur. Fo r example, with a one year soil half

- life and without redistribution or dispersal from the site of ap plication, theoretical build

- up in soil would asymptoticall y approach twice that of annually applied amounts. Of cour se, given time and variable climatic conditions, redistribution or partiti oning 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 t heoretical factor of two.

When thidiazuron reaches surface water, photolysis is ex pected to be the major route of transformation; other degradative processes are ess entially ne gligible 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 ot her has a substantially altered chemical struct ure.

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 transformatio n in/ on leaves may be an important fate pathway. The default foliar half

- life of 35 days was assumed.

T hidiazuron may degrade to thiadiazolurea (also known as AE F132345 or 1,2,3

- thi d iazol

- 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 CO 2 

### . 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 stud y 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 photoprod uct 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 proporti ons 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 Environment al Fate Properties of Thidiazuron and Two P hotoproducts

. Parameter Value Source Selected Physical/Chemical Parameters Chemical Structure

Empirical Formula C 9 H 8 N 4 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 o C 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 o C MRID 41364901 pH TGAI: 5.21 ( 20 0 C ); PAI: 6.50 MRID 45934001 Henry’s Law Constant 2.5 x 10

- l6 atm m 3 /mol calculated O ctanol

- to

- w ater partition coefficient at 25 o C PAI: K o/w = 50.0 (25 0 C, pH = 7.3) shake flask method MRID 413649 0 6

Persistence Hydrolysis half

- life (pH 5, 7, 9) Stable MRID 420692 03 Aqueous photolysis half

- life Stable due to formation of stable photodegradates MRID: 44537108

Aerobic soil metabolism half

life (range in days) 349 days (90 th %ile upper bound on mean) MRID 41950101 ; 46119601 DP Barcode D289980 Anaerobic aquatic metabolism half

- life 289 days MRID 42666101 and 44505132 Aerobic aquatic metabolism half

- life (total system) 697 days (2 x soil metabolism) MRID: 45196011

Mobility

4Table 2. Physical Chemical and Environment al Fate Properties of Thidiazuron and Two P hotoproducts

. Parameter Value Source Adsorption/desorption Kf/ads Kf/oc/ads 1/n r 2 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 (r 2 = 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 ) 1 st 0.112 kg a.i./ha (0.1 ) 2 nd 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 labe l instructions and date of harvest

.

Practices for All Modeled Programs Application Method Aerial (spray) = foliar (2) Manufacturer Label s 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:

5Table 4 . SWCC Input Parameters (Physico

- chemical and Environmental Fate) Used in Modeling Aquatic Exposure for Thidiazuron (including its Photodegradates (I and II) and Thidiazo lurea )

. Fate Property/Physico

chemical Parameter Value Comment Source

K foc (ml/g) 742 Mean of four values ranging from 494 to 908 ml/g for the parent. MRID 41364909

Water Column Metabolism Half Life A (days) @ 2 0 °C 697 Double aerobic soil metabolism half

life input

. n/a

Benthic Metabolism Half Life (days) @ 20 °C 289 The parent is e ssentially stable

. MRID s 42666 1 01 and 46119602 Aqueous Photolysis Half Lif e B (days) 0 Parent photolysis rapid ly to form Photode gradate 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 Life C (days ) @ 20 °C 349 Upper 90th percentile confidence bound on the mean (316) of four values (268, 354, 342, and 301, days). MRID s : 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 25 o C (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 Corrected A E stimated Drinking Water Concentrations (EDWCs) f or Th idiazuron including Photodegradates ( I and II) and Thidiazolurea Residues. B ,C Drinking Water S ource (model/modeled rate) Use Site Peak 1

- in

- 10 Year Value (μg/L) 1

- in

- 10 Year Chronic (μg/L) 3 0

- Year A verage (μg/L)

Surface W ater (SWCC; 1 st application 0.21 and 2 nd 0.1 lb ai/A ; 5 days apart ) a erial 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 param eters, and spray drift fracti on

. 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, o nly 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 0.1 12 kg a.i./ha, Preharvest application via aerial spray (foliar)

. Manufacturer label

Application dates NC Cotton 2 9 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 90 th percentile confidence bound on the mean ( 316 ) of four values ( 268, 354, 342 , and 301, days )

. MRID s : 46119601 and 41950101

Organic Carbon Partition Coefficient (K f OC ) 742 L/kg Represents the mean K FOC value (n=4 ) MRID 41364909

Table 7

. Estimated Drink ing 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 param eters, 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 l b a.i/A and 0.1 lb a.i/A, respectively).



### Uncertainties

In the aerobic soil metabolism study (MRID 46119601 )

, t he study was terminated at 160

- 168 days post

- t reatment . At th at 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 ass umption 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

. OCSP P /OPP/EFED September 15, 2005 DP Barcode: D 319339

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

- life Values and Chara cterizing Pesticide Degradation. OCSPP/ OPP/EFED, M arch 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

- hy droxyphenyl

- 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 T hi di azuron and the USEPA Standard Reservoir

Estimated Environmental Concentrations for thi di azuron 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 Co ncentration 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 thi di azuron applied to the field eventually reaches the water body. The main mechanism of transport f rom 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 di ssipation 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 meta bolism (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 thi di azuron.

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 thi di azuron.

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

12Photolysis 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 thi di azuron.

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 mod el 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

1410/20 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