2

Drinking Water Exposure Assessment for the Registration Review 

of Paclobutrazol on Ornamentals Grown in Outdoor/Indoor 

Nurseries, Turf (commercial, ornamental, and golf courses), and 

Seed Treatment of Tomato, Pepper, and Cucurbit 

Paclobutrazol 

CAS Number 76738-62-0

CAS Name - (αR,βR)-rel-β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol

Prepared by:

Faruque Khan, Ph.D. Senior Scientist

Environmental Risk Branch I

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch I

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Reviewed by:

Stephen Wente, Ph.D. Biologist

Christopher Koper, M.S., Chemist

Environmental Risk Branch I

Environmental Fate and Effects Division
3

1 EXECUTIVE SUMMARY

This document reports the drinking water exposure assessment for paclobutrazol to 

support the human health risk assessment for the registration review. Paclobutrazol is a plant 

growth regulator that is currently registered for application to turf grass and a number of 

ornamentals (trees, shrubs, bulbs). It is also registered for use as seed treatment for tomatoes, 

peppers, and cucurbits. Registered application methods for paclobutrazol are foliar spray

(greenhouse and outdoors), soil drench (greenhouse and outdoors), soil injection, tree injection, 

bulb soak, plug/liner soak, and seed treatment. Revised Tier 1 surface water and groundwater 

modeling were conducted for the highest application rate of 6.1 lbs/A for nursery production of 

container-grown ornamentals through soil drench method (US EPA, 2011). Two major 

environmental degradates, CGA 149907 (maximum of 40.4%) and NOA457654 (maximum of 

25.1%) were identified in the environmental fate and terrestrial field dissipation studies, and 

were included into the drinking water exposure assessment. However, there is no available 

recommendation to include these major metabolites from the Residues of Concern 

Knowledgebase Sub-committees (ROCKS) of the Health Effects Division (HED). The upperbound estimated drinking water concentrations (EDWCs) for total toxic residue (TTRpaclobutrazol plus two major degradates (CGA 149907 and NOA 457654) as well as for 

paclobutrazol (parent only) are provided in Table 1 for the human health risk assessment.

Table 1. Estimated Maximum Drinking Water Environmental Concentrations (EDWCs) of 

Paclobutrazol for Surface Water and Ground Water

Drinking Water 

Source (model)

Use

(modeled rate)a Application 

Method

Acute

(μg/L)

Chronic

(μg/L)

Parent only (paclobutrazol)

Surface Water

(FIRST)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 153.3b 49.0 b

Groundwater

(SCIGROW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 26.9 26.9 b

Groundwater

(PRZM-GW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 29.1 6.7

Total Toxic Residue (Paclobutrazol plus CGA 149907 and NOA 457654)

Surface Water

(FIRST)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 255.3 b 166.2 b

Groundwater

(SCIGROW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 25.0 25.0

Groundwater

(PRZM-GW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 945.0 b 622.0 b
4

Table 1. Estimated Maximum Drinking Water Environmental Concentrations (EDWCs) of 

Paclobutrazol for Surface Water and Ground Water

Drinking Water 

Source (model)

Use

(modeled rate)a Application 

Method

Acute

(μg/L)

Chronic

(μg/L)

a Example of application rate calculation: 6.1 lbs/A- (6.1lbs a.i./A*(1.0-0.59) = 2.50 lbs a.i./A

 Where

 6.1 lbs a.i./A is application rate for drench application for 8-inch container grown ornamentals (US EPA, 2011)

 0.59 is the minimum fraction of retention of parent (paclobutrazol) in column leaching study (MRID 

40685004)

b Bolded values are recommended for human health concerns

Tier I FIRST model concentrations represent untreated surface water concentrations. The 

SCIGROW and PRZM-GW concentrations represent groundwater concentration that might be 

expected in shallow unconfined aquifers under sandy soils. For TTR, EFED recommends acute 

surface water EDWC of 255.3 μg/L and a chronic EDWC of 166.2 μg/L for use in the human 

health risk assessment. For parent, EFED recommends acute surface water EDWCs of 153.3 

μg/L and a chronic EDWC of 49.0 μg/L for use in the human health risk assessment. The 

recommended EWDCs for groundwater are 945 μg/L for acute and 622 μg/L for chronic and 

29.1 μg/L for acute and 26.9 μg/L for chronic for TTR and parent respectively. These 

recommendations are based on the use of paclobutrazol on nursery at an annual application rate 

of 6.1 lbs a.i./A/season. The simulated concentrations provided in this assessment are upperbound estimates of concentrations for drinking water.

2 PROBLEM FORMULATION

This is a Tier I drinking water assessment that uses modeling and available monitoring 

data to estimate the groundwater and surface water concentrations in drinking water sources 

(pre-treatment) resulting from pesticide use on sites that are vulnerable. This initial tier screens 

out chemicals with low potential risk and allows the Office of Pesticide Programs (OPP) to focus 

resources on more refined risk assessments for chemicals which potentially present more 

significant risks. This drinking water assessment reports potential exposure concentrations for 

the human health dietary risk assessment and provides a description of how those concentrations 

were determined. 

3 ANALYSIS

3.1 Use Characterization

Paclobutrazol, also known by the trade names Clipper, Bonzi, Paczol, and PP333, is a plant 

growth regulator that is currently registered for application to a number of food (seed treatments) 

and non-food crops (ornamentals and turf) to induce compact plant growth. Registered 

application methods for paclobutrazol are foliar spray, soil injection, soil drench, bulb soak, seed 

treatment, plug/liner soak, and tree injections. It is used to control the size of numerous plants 

including turf, trees in right-of-ways, ornamental trees and shrubs, and bulbs (flower trade). Ten 

different companies currently have registered products with paclobutrazol concentrations ranging 
5

from 0.018 to 50.0 percent. Currently registered uses and rates of paclobutrazol are shown in 

Table 2. 

Table 2. Current Registered Uses and Application Rates of Paclobutrazol

Crop

Maximum Application 

Rate1 Number of 

Applications

Application 

Method

Shortest 

Application 

Interval

(days)

Reg # 2

Cucurbit 6.5 μg/seed

0.00029 lbs/A

1 Seed 

Treatment

N/A 000100-01014

Herbaceous 

and 

ornamentals 

(greenhouse)

5.7 mg ai/ft2

Spray Application

0.20 lbs/A for 6’-8 pot

0.26 lbs/A for 18”x9” 

propagation trays

drench application

4.3 lbs/A for 6” pot

6.1 lbs/A for 8’ pot

NS Spray, Drench, 

Bulb Soak, 

Plug/Liner 

Soak

As needed 000100-00992

000100-00996

000100-01079

000400-00515

062097-00011

080697-00003

000228-679

62097-25

Pepper 16 μg/seed

0.00079 lbs/A

1 Seed 

Treatment

N/A 000100-01014

Tomato 12.5 μg/seed

0.00040 lbs/A

1 Seed 

Treatment

N/A 000100-01014

Trees (shade 

and 

ornamental)

0.009 lb ai per inch DBH

Tree injection: 0.36 lbs/A

Soil Injection: 1.9 lbs/A

Basel drench: 1.9 lbs/A

1 Soil Injection, 

Basal Drench, 

Tree Injection

3 years or as 

needed

000100-01037

000100-00991

000100-00995

000100-01014

067690-00022

000228-680

74779-3

74779-7

80697-4

81943-31

62097-24

Turf 0.75 lb ai/A 3 Spray, 

Broadcast

14 009198-00196

009198-00199

009198-00201

009198-00205

009198-00215

000100-01014

067690-00022

000538-00201

000538-00199

000228-680

80697-4

81943-31

NS=not specified on label

N/A Not Applicable

1 Italic rates are provided by the Biological and Economic Analysis Division (US EPA, 2011)

2 Application rates represent maximum application rate for a least one of the EPA Reg# listed. Other labels may have slightly 

different application rates.
6

3.2 Environmental Fate and Transport Characterization

Paclobutrazol is a racemic mixture of the (2R, 3R)- and (2S, 3S)-enantiomers but the 

submitted physiochemical and fate properties of each enantiomer were not investigated in the 

submitted studies. Therefore, it is assumed that each enantiomer of paclobutrazol has similar 

physiochemical and fate properties based on submitted studies. Paclobutrazol has a low 

solubility (25.9 mg/L) in water. The vapor pressure (1.28 x 10-9mm Hg @25.0°C) and Henry's 

Law Constant (8.09 x 10-10 atm m3

/mol) indicate that the compound is relatively non-volatile 

under field conditions. The log Kow of 3.2 indicates the potential for this chemical to 

bioaccumulate in aquatic organisms. 

Paclobutrazol is stable to hydrolysis and photolysis. Degradation rates of aerobic soil 

metabolism were revised based on the NAFTA Guidance for evaluating degradation kinetics in 

environmental media (U.S. EPA, 2012a). Under aerobic conditions, paclobutrazol appears to be 

generally quite persistent in acidic soils, with half-lives less than 471 days. However in soils with 

neutral or higher pH, estimated half-lives were as short as 19 days. In anaerobic aquatic 

metabolism studies, paclobutrazol variably partitioned to the sediment and degraded with a halflife of more than one year in flood loam and silt loam soils. Since, the extrapolated half-lives 

were beyond the duration of experiment, paclobutrazol was considered stable in the anaerobic 

aquatic system. Paclobutrazol residues were persistent and mobile in terrestrial field dissipation 

studies in California, West Virginia, Florida, North Carolina, Illinois, and Mississippi Half-lives 

of paclobutrazol residues ranged from 450-950 days for orchard soils in California, West 

Virginia, and Florida, and 175 days to 252 days in agricultural soils in Mississippi, North 

Carolina, and Illinois. Two major environmental degradates, ketone analog of parent (CGA 

149907, ≤ 40.0%) and 1H-1,2,4-triazol-3-ol (NOA457654, ≤ 25.1%) were identified in the 

environmental fate and terrestrial field dissipation studies.

The potential mobility of this compound is quite variable; ranging from highly mobile to 

moderately mobile in soil according to the mobility classification of the Food and Agriculture 

Organization (FAO, 2000). Soil column leaching experiments involving aged (9 week) residues 

of methine-labeled paclobutrazol determined that the compound was relatively immobile in 30 

cm columns of sand, sandy loam, loamy sand except in clay loam soils, where it was leached 

with 66 cm rain over 9 weeks. In contrast, aged residue studies of triazole-labeled paclobutrazol 

determined the compound to be of low mobility in 30 cm columns of sand and sandy loam soils, 

and mobile in loamy sand and clay loam soils. In all cases, the majority (58.6-90.7 percent of the 

applied) of aged residue did not leach out of the upper 10 cm of the treated soil columns. Table 3

summarizes the physiochemical and environmental fate and transport properties of 

paclobutrazol. 

Table 3. Physical, Chemical and Environmental Fate properties of Paclobutrazol

Parameter Value Reference Comments

Chemical Paclobutrazol MRID 456625-01

Chemical Class Triazole

CAS No. 76738-62-0

Empirical Formula C15H20ClN3O MRID 456625-01
7

Table 3. Physical, Chemical and Environmental Fate properties of Paclobutrazol

Parameter Value Reference Comments

IUPAC (2RS,3RS)-1-(4-

chlorophenyl)-4,4-dimethyl2-(1H-1,2,4-triazol-1-

yl)pentan-3-ol

MRID 456625-01

CAS Name (αR,βR)-rel-β-[(4-

chlorophenyl)methyl]-α-

(1,1-dimethylethyl)-1H1,2,4-triazole-1-ethanol

MRID 456625-01

Structure MRID 456625-01

Selected Physical/Chemical Parameters

Molecular mass 293.8 g/mol MRID 456625-01

Solubility (20.0°C) 25.9 mg/L (water)

2.8 g /L (n-Octanol)

79.8 g/L (methanol)

5.22 g/L (xylene)

MRID 456625-05

MRID 456625-03

Vapor pressure (25.0°C) 1.7 x 10-7 Pa

(1.28 x 10-9 mm Hg)

MRID 456625-05

Henry’s law constant (25°C) 8.2 x 10-7 Pa M3 mol-1

(8.09 x 10-12 atm M3 mol-1)

MRID 456625-05

Octanol-water partition coefficient 

(LogKOW) (25°C)

3.18 MRID 456625-05

Persistence

Hydrolysis

pH 4: stable

pH 7: stable

pH 9: stable

MRID 00132698

Aqueous Photolysis (t 1⁄2 ) Stable MRID 00132699

Soil Photolysis (t 1⁄2 ) Stable MRID 40685002 After 33 days 

paclobutrazol decrease 

to 86.4 % of applied 

radioactivity

Aerobic Soil Metabolism (t 1⁄2 ) @ 25°C1 Sandy clay loam,2

443.4 days f or triazole 

label

[950.4 days f or triazole 

label] 3

470.9 days for methane 

label 

1020.4 days for methane 

label 3

MRID 40685003 Major metabolite

(CGA 149907 (≈18.7%)
8

Table 3. Physical, Chemical and Environmental Fate properties of Paclobutrazol

Parameter Value Reference Comments

Sandy clay loam

200.1 days 

[277.9 days] 3

MRID 47338504 Supplemental 

No major metabolites 

detected above 10%.

Sandy loam

16.7 days4

[33.2 days] 3,4

Major metabolites

NOA 457654(25.1%)

CGA 149907(13.3%)

Sandy loam

19 days3

[24.0 days] 3,4

Major metabolites

NOA 457654 (20.5%)

CGA 149907(26.3%)

Anaerobic Soil Metabolism No data

Anaerobic Aquatic Metabolism (t 1⁄2 ) Stable MRID 40685003 After a year of 

incubation, 

paclobutrazol decrease 

to ≤79% of applied 

radioactivity 

Major transformation 

product:

(CGA 149907 (≈16.8%)

Aerobic Aquatic Metabolism

No data --

Mobility

Soil Partition Coefficient Adsorption Kd (mL/g)

9.5 (Clay loam)

10.2 (Sandy Clay loam)

7.6 (Silty clay loam)

17.3 (Silt loam)

5

4.1 (sandy loam)

2.4 (loamy sand)

5.8 (Silty clay)

1.4 (Sand)

MRID 4188113 There was no significant 

correlation (r2 = 0.12) 

between Kd and % 

organic carbon. 

Soil Partition Coefficient Desorption Kdes (mL/g)6

15.6 (Clay loam)

18.1 (Sandy Clay loam)

14.0 (Silty clay loam)

34.0 (Silt loam)

5

5.1 (sandy loam)

10.4 (loamy sand)

2.7 (Silty clay)

6.1 (Sand)

MRID 4188113
9

Table 3. Physical, Chemical and Environmental Fate properties of Paclobutrazol

Parameter Value Reference Comments

Adsorption Kd (mL/g)

1.77(Sandy loam) 5

0.8 (Sandy loam)

2.66 (Silty clay loam)

1.8 (Clay loam)

Desorption Kdes (mL/g)6

10.9 (Sandy loam) 5

3.81 (Sandy loam)

14.2 (Silty clay loam)

11.7 (Clay loam)

MRID 47338503 Supplemental

NOA 457654

Adsorption Kd (mL/g)

Borstel Loamy Sand- 0.09

Gartenaker Loam-0.25

Pappelacker Sandy loam0.19

18 Acres Sandy loam-0.30

MRID 47338502 NOA 457654 is a major 

degrradate of 

paclobutrazol

Supplemental

Leaching Aged methine 14C- labeled 

paclobutrazol residues were 

relatively immobile

Aged trizole 14C-labeled 

paclobutrazol residues were 

mobile in loamy sand and 

clay loam soils and low 

mobility in sands and sandy 

loam soils 

MRID 40685004 Major transformation 

product 

CGA 149907 (≈12.6%)

Field Dissipation

Terrestrial Field Dissipation (t 1⁄2 )

450 to 973 days

(CA, WV, FL)

MRID 

40685006-

406850012

Supplemental

Major transformation 

product

CGA 149907

88 days (CA)

133 days (NY) MRID 47338505

Supplemental

Major transformation 

product - CGA 149907 

(40%)

1 DT50 value were recalculated using the NAFTA Guidance for evaluating and calculating degradation kinetics in 

environmental media (U.S. EPA, 2012) and the estimated DT50s were adjusted to 25°C using Arrhenius equation. 

Calculations can be found in Appendix A. 2

Calculated half-lives are based on 14C labeled of triazole and methane of paclobutrazol 3

Italic half-lives are calculated based on paclobutrazol and its major degradates (CGA 149907 and NOA 457654). 

Calculation can be found in Appendix A.

4

High Soil pH (≥ 7.0) may have contributed faster degradation rates for aerobic soil metabolism 5 Japanese soils, excluded during exposure assessment

6 Reported values are based on second desorption step
10

3.3 Drinking Water Exposure Modeling

Tier 1 modeling was used to estimate the drinking water exposure for use in the dietary 

risk assessment of paclobutrazol. The following is a description of the models used, the selection 

of the model input parameters, and a characterization of the output from these simulations.

3.3.1 Models

Tier 1 EDWCs for surface water were generated using FIRST, dated August 5, 2001. 

FIRST is a screening model designed by the Environmental Fate and Effects Division (EFED, 

2010a) of the Office of Pesticide Programs to estimate the concentrations found in drinking 

water from surface water sources for use in human health risk assessment. As such, it provides 

upper bound values on the concentrations that might be found in drinking water due to the use of 

a pesticide. FIRST is a single-event model (one runoff event), but can account for spray drift 

from multiple applications. FIRST is hardwired to represent the Index Reservoir, a standard 

water body used by the Office of Pesticide Programs to assess drinking water exposure (Office 

of Pesticide Programs, 2010b). It is based on a real reservoir, Shipman City Lake in Illinois, that 

is known to be vulnerable to pesticide contamination. The single runoff event moves a maximum 

of 8% of the applied pesticide into the reservoir. This amount can be reduced due to degradation 

on the field and the effects of binding to soil in the field. FIRST also uses a Percent Cropped 

Area (PCA) factor to adjust for the area within the watershed that is planted to the modeled crop. 

The national default PCA of 1.0 and 0.87 was applied for turf and nursery assessment,

respectively.

Tier 1 EDWCs for groundwater were generated with SCIGROW 2.3, dated July 29, 2003 

(2010a). SCIGROW is a regression model used as a screening tool for ground water used as 

drinking water. SCIGROW was developed by regressing the results of Prospective Ground 

Water studies against the Relative Index of Leaching Potential (RILP). The RILP is function of 

aerobic soil metabolism and the soil-water partition coefficient. The output of SCIGROW 

represents the concentrations that might be expected in shallow unconfined aquifers under sandy 

soils.

In addition, Tier 1 groundwater EDWCs for paclobutrazol and TTR, resulting from use 

for nursery were also derived using the PRZM-GW model (Pesticide Root Zone Model for 

Groundwater, version 1.0, August 31, 2012), with the GW-GUI (Graphical User Interface, 

version 1.0, August 31, 2012). 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. Six standard scenarios, each 

representing a different region known to be vulnerable to groundwater contaminations, are 

available for use with PRZM-GW for risk assessment purposes. In PRZM-GW simulations, each 

of these standard scenarios was used. 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. Descriptions of all executable 
11

versions of current models used in drinking water exposure can be obtained from URL link 

(http://www.epa.gov/oppefed1/models/water/).

3.3.2 Modeling Approach and Input Parameters

Tables 4, 5 and 6 summarize the model input parameter values used in FIRST, SCIGROW and PRZM-GW, respectively. These parameters were selected in accordance with 

EFED’s input parameter guidance (US EPA, 2010c and US EPA, 2012c). Half-lives (rate 

constants) listed in Table 3 were used for exposure modeling. The average soil-water partitioning 

coefficient was used in FIRST. For TTR, half-lives were estimated by accounting the percent of 

applied parent (paclobutrazol), CGA 149907 and NOA 457654 present on each sampling period 

of fate studies (Appendix A). The lowest Koc value was used in SCI-GROW because there was a 

greater than 3-fold variability in the available data. 

Table 4. FIRST (v 1.0) Input Parameter Values for Paclobutrazol and Total Toxic Residues 

(TTR) of Paclobutrazol

Parameter Value Source Comments

Application 

Rate 

(lb a.i./A)

2.50 lb a.i/A (based on 

59% retention of parent in 

soil column)1

0.55 lbs/A (based on 91% 

retention of parent in soil 

column

2.0 lbs/A for turf2

Memo from the Biological and 

Economic Analysis Division 

(BEAD) (US EPA, 2011)

Maximum application rate was 

estimated based on information 

provided by BEAD (US EPA 2011)

and adjusted with the retention of 

paclobutrazol in a soil column 

leaching study (MRID 40685004)

Number of 

Applications

1 (nursery)

3 (turf)

Product Label

Soil 

Partitioning 

Coefficient (Kd

ml/g)

4.5 (Parent)

0.16 (NOA)

MRIDs 4188113 and 47338503 Mean value 

Aerobic Soil 

Metabolism t1/2

(d)

791 (TTR)

381 (Parent)

MRIDs 40685003 and

47338504

90th percentile upper confidence 

bound on the mean.

Wetted in? No --- ---

Depth of 

Incorporation 

(inches)

0 --- ---

Method of 

Application

Drench

Ground Paclobutrazol labels ---

Spray Drift 

Fractions

0 (drench)

6.4 (ground foliar)

Paclobutrazol labels Model input guidance 

(US EPA, 2010)

Percent 

Cropped Area 

0.87

1.0

Nursery

Turf

National Default

No PCA established for turf
12

Table 4. FIRST (v 1.0) Input Parameter Values for Paclobutrazol and Total Toxic Residues 

(TTR) of Paclobutrazol

Parameter Value Source Comments

Solubility in 

Water (mg/L)

25.9 (Parent)

336000 (TTR)

MRID 456625-05

EPISUITE

Estimated value

Aerobic 

Aquatic 

Metabolism t1/2

(days)

1594 (TTR

762 (Parent)

2X of Aerobic soil metabolism 

half-life

Model input guidance 

(US EPA, 2010)

Hydrolysis t1/2 0 MRID 00132698 Stable to hydrolysis

Aquatic 

Photolysis t1/2

(days)

0 MRID 0013299 Stable to aquatic photolysis

1 Example of application rate calculation: 6.1 lbs/A- (6.1lbs/A*(1.0-0.59)

 Where

 6.1 lbs/A is application rate for drench application for 8-inch container grown ornamentals (US EPA, 2011)

 0.59 is the minimum fraction of retention of parent (paclobutrazol) in column leaching study (MRID 40685004) 2 Maximum single application rate of 0.67 lbs/A was used instead of 0.75 lbs/A to account maximum seasonal rate of 

2.0 lbs/A due to Tier I model limitation. Model accepts only uniform rate for multiple applications. 

Table 5. SCIGROW 2.3 Input Parameter Values for Paclobutrazol and Total Toxic 

Residues (TTR) of Paclobutrazol

Parameter Value Source Comments

Maximum 

Application Rate (lbs 

a.i./A)1

2.50 

Maximum application rate were 

estimated based on information 

provided by BEAD (US EPA 2011 

and adjusted with the retention of 

chemicals in a soil column leaching 

study (MRID 40685004)

Maximum Number 

of Applications per 

Year

1 (Nursery) Product Label

#s of application were based on 

information provided in the 

submitted labels for vegetables, 

grape, hops, and ornamentals and 

flower bulbs

Aerobic Soil 

Metabolism Half-life 

(days)

277.9 (Total Residue)

200.1 (Parent)

MRIDs 40685003 

and 47338504 median value 

Organic Carbon 

Partition Coefficient 

(Koc, L/Kg)2

40.4 (Parent)

0.16 (NOA)

MRIDs 4188113 

and 47338503

MRID 47338501

Lowest value; > 10X variability in 

data 

Average of four values 1 See calculation in Table 4 2 There is no significant correlation between Kd and organic carbon. Since, model does not have an option to use 

Kd , the lowest value of Koc was used in the model
13

Table 6. PRZM-GW (v1.0) Input Parameter Values for Paclobutrazol and Total Toxic 

Residues (TTR) of Paclobutrazol

Parameters1 Value Data Source Comments

Application Method

Ground

Table 4 Same as input for FIRST Model

Maximum Application 

Rate (lbs a.i./A)

 

2.50 Table 4 Same as input for FIRST Model

Application 

Frequency 1 Table 4 Same as input for FIRST Model

Hydrolysis (DT50) d 0 (Parent and 

TTR) Table 4 Same as input for FIRST Model

Aerobic soil 

metabolism (DT50) d

381 (Parent)

791 (TTR) Table 4 Same as input for FIRST Model

Koc

40.4 (Parent)

0.16 L/Kg (NOA)

Table 4 Same as input for FIRST Model

1 EFED input parameter guidance: 

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

Modeling Results

Revised modeling results are depicted in Table 6. Tier I FIRST model concentrations 

represent untreated surface water concentrations. The SCIGROW and PRZM-GW concentrations 

represent groundwater concentration that might be expected in shallow unconfined aquifers 

under sandy soils. For TTR, EFED recommends acute surface water EDWC of 255.3 μg/L and a 

chronic EDWC of 166.2 μg/L for use in the human health risk assessment. For parent, EFED 

recommends acute surface water EDWC of 153.3 μg/L and a chronic EDWC of 49.0 μg/L for 

use in the human health risk assessment. The recommended EWDCs for groundwater are 945 

μg/L for acute and 622 μg/L for chronic and 29.1 μg/L for acute and 26.9 μg/L for chronic for 

TTR and parent respectively. These recommendations are based on the use of paclobutrazol on 

nursery at an annual application rate of 6.1 lbs a.i./A/season. The simulated concentrations 

provided in this assessment are upper-bound estimates of concentrations for drinking water. If 

dietary risks require refinement, higher-tiered, crop-specific and location-specific models and 

modeling scenarios can be used. Sample outputs of Tier I FIRST and SCIGROW as well as 

Results and sample output PRZM-GW are provided in Appendix B. 
14

Table 6. Estimated Maximum Drinking Water Environmental Concentrations (EDWCs) of 

Paclobutrazol for Surface Water and Ground Water

Drinking Water 

Source (model)

Use

(modeled rate)a Application 

Method

Acute

(μg/L)

Chronic

(μg/L)

Parent only (paclobutrazol)

Surface Water

(FIRST)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 153.3 49.0

Groundwater

(SCIGROW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 26.9 26.9

Groundwater

(PRZM-GW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 29.1 6.7

Total Toxic Residue (Paclobutrazol plus CGA 149907 and NOA 457654)

Surface Water

(FIRST)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 255.3 166.2

Groundwater

(SCIGROW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 25.0 25.0

Groundwater

(PRZM-GW)

Nursery:

(1app. X 2.50 lbs ai/acre) Drench 945.0 622.0

a Example of application rate calculation: 6.1 lbs/A- (6.1lbs/A*(1.0-0.59) 

 Where

 6.1 lbs/A is application rate for drench application for 8-inch container grown ornamentals (US EPA, 2011)

 0.59 is the minimum fraction of retention of parent (paclobutrazol) in column leaching study (MRID 

40685004)

3.4 Monitoring Data

There were no national-scale monitoring data available for this assessment. 

3.5 Drinking Water Treatment

There is no available information on the drinking water treatment effects on 

Paclobutrazol.

3.6 Uncertainties

In general, the EDWCs provided in this assessment are upper bound estimates of 

paclobutrazol concentrations in untreated drinking water. A major source of uncertainty is the 

leaching of paclobutrazol from indoor drench application of potted nursery. It is assumed that 

treated pots would be placed outdoors immediately after drench application throughout the 

watershed of the index reservoir. If the treated pots remain indoors and/or the applied 

paclobutrazol is taken up by the potted plants, the residual paclobutrazol would be less than the 

modeled application rate and consequently exposure in surface water and groundwater would be 

less than the reported values. Additionally, exposure assessments were performed based on one 

seasonal application rate for drench method. Indoor potted plants can be grown several times 
15

within a year. If this chemical is applied to multiple batches of potted plants and placed outdoors

during the growing season, exposure would be higher than the reported values in Table 6. In 

addition, exposure would be higher or lower based on residence time of treated potted plants 

remain outdoors before planting or distributing to retailers. 
16

REFERENCES

Food and Agriculture Organization (FAO) of the United Nations. 2000. FAO Pesticide Disposal 

Series. Assessing Soil Contamination: A Reference Manual. Appendix 2. Parameters of 

Pesticides that Influence Processes in the Soil. Editorial Group, FAO Information 

Division: Rome. http://www.fao.org/DOCREP/003/X2570E/X2570E00.htm

US EPA. 2011. Per-Acre Application Rates for Ornamental Trees in Urban Forests, Seed

Treatment and Container-Grown Ornamental Uses of Pac1obutrazol. Biological and Economic 

Analysis Division. November, 2011

US EPA. 2010a. Water Models. U.S. Environmental Protection Agency, Pesticides: Science 

and Policy. Online at: http://www.epa.gov/oppefed1/models/water/

US EPA. 2010b. Development and Use of the Index Reservoir in Drinking Water Exposure 

Assessments. U.S. Environmental Protection Agency, Office of Prevention, Pesticides 

and Toxic Substances, Office of Pesticide Programs, Environmental Fate and Effects 

Division, April 15, 2010. Online at: 

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

USEPA. 2010c. 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. 

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

US EPA. 2012a. Memorandum: NAFTA Guidance for Evaluating and Calculating Degradation 

Kinetics in Environmental Media. Environmental Fate and Effects Division, Office of 

Chemical Safety and Pollution Prevention. U. S. Environmental Protection Agency.

US EPA. 2012b. Development and Use of Percent Cropped Area and Percent Turf Area 

Adjustment Factors in Drinking Water Exposure Assessments: 2012 Update 

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html
17

APPENDIX A

Degradation Kinetic Estimates of Paclobutrazol

DT50 = 20.9 months = 627 days @ 20°C

DT50 = 443.4 days @ 25°C

Estimated DT50 value was adjusted to temperature of 25°C using Arrhenius equation below

http://www.epa.gov/oppefed/models/water/input_parameters_guidance.html
18

DT50 = 22.2 months = 666 days @ 20°C

DT50 = 470.9 days @ 25°C

DT50 = 283 days @ 20°C

DT50 = 200.1 days @ 25°C
19

DT50 = 23.6 days @ 20°C

DT50 = 16.7 days @ 25°C

DT50 = 26.8 days @ 20°C

DT50 = 19.0 days @ 25°C
20

Degradation Kinetic Estimates of total Toxic Residue of Paclobutrazol @ Temp 20°C

DT50 = 44.8 months = 1344 days @ 20°C

DT50 = 950.4 days @ 25°C

DT50 = 48.1 months = 1433 days @ 20°C

DT50 = 1020.4 days @ 25°C

DT50 = 950.4 days @ 25°C
21

DT50 = 393 days @ 20°C

DT50 = 277.9 days @ 25°C

DT50 = 47.1 days @ 20°C

DT50 = 33.2 days @ 25°C
22

DT50 = 34 days @ 20°C

DT50 = 24.0 days @ 25°C
23

APPENDIX B

Surface Water

Drench Application

 RUN No. 1 FOR Paclobutrazol ON Nurnery * INPUT VALUES * 

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

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) AREA (IN)

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

 2.500( 2.500) 1 1 4.5 25.9 GRANUL( 0.0) 100.0 0.0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

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

 381.00 2 0.00 0.00- 0.00 762.00 762.00

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.1 MAR 26, 2008

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

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

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

 153.399 49.020

RUN No. 1 FOR Paclobutazol_TTR ON Nursery * INPUT VALUES * 

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

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) AREA (IN)

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

 2.500( 2.500) 1 1 0.2******* GRANUL( 0.0) 100.0 0.0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

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

 797.10 2 0.00 0.00- 0.00 ****** 1595.20

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.1 MAR 26, 2008

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

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

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

 255.337 166.243
24

 

Ground WaterSciGrow version 2.3

chemical:Paclobutrazol

time is 2/10/2014 10:27:26

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

Application Number of Total Use Koc Soil Aerobic

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

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

2.500 1.0 2.500 4.04E+01 200.1

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

groundwater screening cond (ppb) = 2.69E+01

************************************************************************

chemical:Paclobutrazol_TTR

time is 2/10/2014 10:28: 5

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

Application Number of Total Use Koc Soil Aerobic

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

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

2.500 1.0 2.500 6.06E+01 277.9

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

groundwater screening cond (ppb) = 2.52E+01

********************************************************
25

PRZM-GW

Table B-1. PRZM-GW Generated Estimated Drinking Water Cconcentrations of 

Paclobutrazol and it’s Total Toxic Residue

Application Rate and 

Frequency Scenario1

Highest

Daily

Value (μg/L)

Post 

Breakthrough 

Average (μg/L)

Average Simulation 

Breakthrough Time 

(days)

2.50 lbs a.i./A x 

1applications

Parent

FL Citrus 29.1 6.69 274

FL Potato 15.2 4.07 338

GA peanut 0.01 0.001 1064

NC Cotton 0.03 0.003 1089

Delmarva sweet corn 0.25 0.03 252

WI corn 9.33E-11 6.88E-12 938

Total Toxic Residue of Paclobutrazol

FL Citrus 635 458 238

FL potato 447 328 246

GA peanut 312 246 993

NC cotton 615 437 1054

Delmarva sweet corn 632 426 217

WI corn 945 622 868 1 These scenarios are regional vulnerable sites for groundwater evaluation. 

Sample output of PRZM-GW