UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION 

PREVENTION 

PC Code: 125851 

DP Barcode: 420998 

DATE: August 11, 

2014 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

TO: 

Isoxaben: Drinking Water Assessment for Registration Review 

Andrew Shelby, Physical Scientist ~ ~ ~ '6 - 1 1- 1 '-{ 

Environmental Fate and Effects Divison/ERB6 (7507P) {/ 

MarkCorbin,BranchChief ~~ e~ · k <-

Environmental Fate and Effects Divison!ERB6 (7507P) /'\I \ YDennis McNeilly, Risk Assessor, 

Christina Swartz, Branch Chief 

Health Effects Division/RAB2 (7509P) 

Christina Scheltema, Chemical Review Manager 

Pesticide Re-Evaluation Division (7508P) 

The Environmental Fate and Effects Division (EFED) ofthe Office of Pesticide Programs (OPP) 

has completed a Drinking Water Assessment for the registration review of isoxaben. The 

chemical is registered for use on turf grass, ornamentals, groundcover/perennials, non-cropland, 

Christmas tree/conifer plantations, hardwood nurseries, fruit trees, nut trees, vineyards, and 

Eucalyptus. The maximum single application rate for the broadcast use is 1.02 lbs a.i./ A and the 

maximum annual application rate is 4.03 lbs a.i./ A. The assessment considers a parent only 

approach but also provides for characterization the major degradates as parent in a total toxic 

residue (TTR) screening level approach assuming stability of toxic residues to biotic and abiotic 

degradation. Major degradates include hydroxy isoxaben, dimethoxy benzamide, 

methoxyphenyl pyrimidinol, and AEM hexenoyl. Degradates are characterized in detail in the 

drinking water degradates identification memo (D374526). Hydroxy isoxaben, methoxyphenyl 

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pyrimidinol, AEM hexenoyl isoxaben, and dimethoxy benzamide are included with parent 

isoxaben as residues of concern as identified by HED (0375245; see attached). Unidentified 

degradates from the aqueous photolysis study were not noted as residues of concern but their 

inclusion as residues of concern is characterized. 

Due to the persistence and moderate mobility of isoxaben, groundwater estimated drinking water 

concentrations (EDWCs) were higher than surface water EDWCs. Based on modeling results, 

the maximum estimated groundwater exposure concentrations for total toxic residues of isoxaben 

and the recommended concentrations for use in HED's dietary assessment are: 

1,010 Jlg/L for the peak concentration 

909 Jlg/L for the post-breakthrough average 

The maximum estimated 1-in-1 0 year peak surface water exposure concentration for TTR of 

isoxaben is 89.6 J..Lg/L derived from the Surface Water Concentration Calculator (SWCC v1.106). 

Fate and Transport Characterization 

The most significant degradation pathway for isoxaben differs depending on surface water or 

groundwater exposure. Due to isoxaben's moderate mobility (average Koc = 354 Llmg o.c.), 

both surface water and groundwater exposure are possible depending on soil permeability. In the 

case of groundwater exposure, anaerobic metabolism is the dominant form of dissipation with an 

anaerobic soil metabolism half-life of three days. In the case of surface water exposure, aqueous 

photolysis is the dominant form of dissipation with a half-life of six days. Isoxaben is stable to 

hydrolysis and persistent to aerobic metabolism with aerobic soil metabolism half-lives ranging 

from 150 to 891 days and aerobic aquatic half-lives ranging from 435 days to stable. Isoxaben is 

non-volatile with a Henry's Law Constant of 1.26* 1 o-9 atm-m3

/mol. 

Monitoring Data 

Neither USGS National Water-Quality Assessment Program (NA WQA) or the Water Quality 

Portal (WQP) include any monitoring data for isoxaben as of June 2, 2014. 

Drinking Water Exposure Modeling 

Though uses in orchards, vineyards and on Eucalyptus allow fewer applications and/or have a 

lower maximum application rate, scenarios representing these uses also represent uses with 

maximum annual applications and the maximum application rate. Therefore, all surface water 

scenarios were modeled using maximum application rate (1.02 lbs a.i./A) and maximum number 

of reapplications ( 4 per year). Scenarios selected that represent orchards, vineyards and 

Eucalyptus are also representative of Christmas tree/conifer plantations, hardwood nursery, 

ornamentals, and non-cropland which have uses at the maximum application rate and number of 

reapplications. All available SWCC scenarios that are representative of orchards, vineyards, 

nurseries, or turf were modeled. This criterion includes 18 scenarios that are geographically 

distributed across the coterminous U.S. All six currently available groundwater scenarios were 

modeled with maximum application rate and number of reapplications. The currently available 

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groundwater scenarios are representative of vulnerable groundwater conditions which have the 

potential to be present for all isoxaben uses. 

Table 1: Isoxaben Summary Use 

Use Application Max. Max Max Min. Comment 

method Single # App App 

app rate apps rate per interval 

(lb per year (lb (days) 

a.i./A) year a.i/A) 

Turfgrass Ground 1.02 4* 4.03 56-60 Inconsistent restrictions on 

use in AZ, NY, and on 

chemigation 

Lawns Ground 1.0 4* 3.2 60* 

Ornamentals (landscape, Ground 1.02 4* 4.03 56-60* AZ restrictions for 

container, field) commercial use 

Groundcovers/perennials Ground 1.02 4* 4.03 60 AZ restriction. • Not for 

commercial use. 

Non-cropland Ground 1.02 4* 4.03 60 AZ restriction. • Not for 

commercial use. 

Non-bearing fruit & nut Ground 1.0 3* 3.0 60* AZ restriction 

trees & non-bearing 

vineyards 

Christmas tree/conifer Ground 1.02 4* 4.03 60 Not for commercial use in 

plantations, hardwood AZ 

nursery 

Fruit and nut trees and Ground 1.02 2 2.0 30* Not for use in NY 

vineyards 

Eucalyptus Ground 0.75 2 1.0 30 62719-612 only: Not for use 

in NY; do not apply within 

200 ft of susceptible nontarget plants 

*Not specified or inconsistent across all product labels 

Parameters for the derivation of surface water concentrations and the groundwater concentration 

for all uses are as follows: 

T a e bl 2 . SWCC an d PRZM -GW In apu t P arame t er va I ues t: or I sox a b en 

PARAMETER (units) VALUE (S) SOURCE COMMENT 

Application Rate (kg a.i./ha) 1.14 Product 1.02 lbs a.i./A; Maximum application rate for 

labels turfgrass, ornamentals, groundcover/perenials, noncropland, Christmas tree/conifer plantations, 

hardwood nursery, fruit and nut trees and vineyards. 

Number of Applications 4 Maximum number of applications for turfgrass, 

ornamentals, groundcover/perenials, non-cropland, 

Christmas tree/conifer plantations, and hardwood 

nursery 

3 
PARAMETER (units) VALUE(S) SOURCE COMMENT 

Interval between Applications 56 Minimum application interval for use patterns with 4 

(days) applications per year (turfgrass and ornamentals) 

Molecular weight (g/mol) 332.39 

Henry's Law Constant (atm- 1.26* to·9 Calculated (V APR/760)/(SOL/MWT), 

m3/mol) where 

V APR is vapor pressure in torrs, 

MWT is the molecular weight in g moi·1, 

and SOL is the solubility in water in mg L"1

• 

Vapor Pressure (torr) 4.1 * 10·

9 IUPAC MRID 153106 reports a valuce of <3.9* to·

7 torr 

Footprint 

Solubility in Water @ 20 °C, 1.42 MRID 

(mg/L or ppm) 47140001 

Soil Partition Coefficient 354 MRID Average of280, 190, 310, 420, and 570 mLig o.c. 1 

(Koc)(mUg) 41106303 

CAM (Chemical Application l Broadcast, applied to bareground and unincorporated. 

Method) 

Depth of Incorporation 0 

(inches) 

Application efficiency 0.99 Ground Application 

(decimal) 

Spray drift fraction (decimal) 0.061 Ground Application 

Percent Cropped Area 1.0 l 00% cropped area conservatively assumed due to 

(decimal) non-agricultural use patterns. 

Application date (day/month) Product 4 applications with 56 day retreatment intervals. 

March 1 Labels 

April26 

June 23 

August 20 

Hydrolysis Half-life (days) Stable MRID 

250449 

Aqueous Photolysis Half-life Parent 6.0 MRIO Parent input used in the calculation ofEDWCs as 

@pH 7 (days) TTR 93.5 47140003 requested by HED (0375245). TTR t 112 includes 

unidentified degradate and is provided for 

characterization. 

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PARAMETER (units) VALOB(S) SOURCE COMMENT 

Aerobic Aquatic Metabolism Parent 22.0 MRIO 63 (TTR) and21.5 (parent) days in Water-sand; SFO 

Half-life (days) TTR282 46393202 171 (TTR) and 21 (parent)Stable in Water-sandy 

loam; N/A 

90th percentile mean of values used as input1 

Anaerobic Aquatic Parent 17.4 Anaerobic soil input = single t 112 x 31 

Metabolism Half-life (days) TTR 1608 Anaerobic aquatic input = anaerobic soil input x 21 

Aerobic Soil Metabolism Parent 443 MRID 274 (TTR) and 150 (parent)days in Sandy Loam; 

Half-life (days) TTR 723 47 140004 DFOP; isoxazole label 

264 (TTR) and 199(parent) days in Sandy Loam; 

SFO; phenyl label 

340 (TTR) and 207 (parent)days in Silt Loam; SFO; 

phenyl label 

MRID 360(TTR) and 280 (parent)days in Silt Loam; SFO; 49090101 phenyl label 

l123(TTR) and 891 (parent) days in Sand; SFO; 

phenyl label 

185 days in Sandy clay loam; DFOP; combined label 

90th percentile mean of values used as input1 

Anaerobic Soil Metabolism Parent 2.9 MRID Sandy clay loam; SFO; total system 

Half-life (days) TTR268 46393201 

1 " Input Parameter Guidance" refers to Guidance for Selectmg Input Parameters m Modehng the Envtronmental Fate 

and Transport of Pestici<les; Version 2.1, October 22, 2009. 

Surface water and groundwater EDWCs are presented below with highest representative EDWCs 

highlighted in bold. All relevant surface water scenarios that are relatable to isoxaben uses were 

selected and parameterized at maximum application rates and with the maximum number of 

reapplications. Inclusion of aqueous photolysis degradates in the total toxic residue approach 

results in surface water EDWCs 1% to 17% higher than those reported below. 

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Table 3: EDWCs for Total Toxic Residues of Isoxaben in Surface Water. 

Crop Scenario 1-in-1 0 year 1-in-1 0 year 30-year annual 

(application date) Acute annual average (ppb) 

concentration average (ppb) 

(ppb) 

CA Almond 24.1 16.2 14.6 

CA Citrus 13.9 10.7 9.7 

CA Fruit 20.4 16.2 12.6 

CA Grapes 18.8 13.9 10.9 

CA Nursery 28.3 19.9 11 

FL Citrus 89.6 32.2 21.6 

FL Nursery 60.4 19.3 13.5 

FL Turf 24.1 15 11.3 

MI Nursery 54.3 30.7 24.7 

NC Apple 69.5 28.1 17 

NJ Nursery 59.2 28 20.1 

NY Grape 77.1 43.6 33.6 

OR Apple 35.8 21.5 16.3 

OR Nursery 26.5 18.6 16.1 

OR Xmas Tree 21.7 14.7 12.8 

PA Aj:>ple 59.7 31.7 21.9 

PA Turf 39.8 24.1 16.9 

TN Nursery 60.6 22.4 15.3 

All currently available groundwater scenarios were run in a screening level approach to assess 

for vulnerable groundwater conditions. Groundwater EDWCs are substantially higher than 

surface water EDWCs due to isoxaben's moderate mobility and high persistence. 

Table 4: EDWCs for Total Toxic Residues of Isoxaben in Groundwater 

Scenario Peak (ppb) Post Breakthrough Average 

(ppb) 

Delmarva 739 673 

Florida Citrus 805 721 

Florida Potato 126 125 

Georgia Coastal 301 259 

North Carolina Coastal 971 867 

Wisconsin Sands 1010 909 

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