1

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

June 25, 2014

PC Code: 121001

MEMORANDUM DP Barcode: 418281

SUBJECT: Sethoxydim: Drinking Water Exposure Assessment for IR-4 Petition on 

Fescue, Oilseeds, Fruits, and Vegetables

FROM: James Lin, Environmental Engineer

ERB2/EFED (7507P)

REVIEWED BY: Greg Orrick, Environmental Scientist

ERB2/EFED (7507P)

THROUGH: Brian Anderson, Chief

ERB2/EFED (7507P)

TO: Sydney Jackson, Risk Manager

Barbara Madden, Team Leader

RIMUERB/RD (7505P)

James Parker, Risk Manager

Neil Anderson, Branch Chief

RMIB1/PRD (7508P)

This assessment provides estimated drinking water concentrations (EDWC) of sethoxydim in 

surface water and in ground water in support of human health risk assessment for IR-4 petitions. 

Screening EDWCs (Table 1) of sethoxydim were generated with the Surface Water 

Concentration Calculator (SWCC) and the Provisional Cranberry Model for surface water and 

with the Pesticide Root Zone Model for Ground Water (PRZM-GW) for ground water. Modeled 

application rates represent the maximum use patterns of proposed end-use labels and current 

registered labels. Remaining model input parameters were chosen according to current guidance 

(USEPA, 2009)

1

. EDWCs reflect exposure to sethoxydim residues of concern in drinking water, 

which include the parent compound and eight degradates of concern (DP 246356). If the 

screening EDWCs listed in this memo result in dietary risk exceedances, contact James Lin (703-

308-9591) of Environmental Risk Branch II (7507P) to request a refined drinking water exposure 

assessment.

 

1 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 Pesticide Programs, Environmental Fate 

and Effects Division, Oct. 22, 2009. http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
2

Table 1. Maximum Screening Drinking Water Exposure Estimates for Proposed 

Sethoxydim Uses (Based on Sethoxydim Residues of Concern)*

Source (Model) Use (Maximum Rate) Peak Exposure 

(μg/L)

Annual Mean 

Exposure (μg/L)

Surface water (SWCC) 4 apps @ 0.47 lb ai/A 

w/ a 14-day interval

65.3 6.6

Ground water (PRZM-GW) 0.565 0.511

Surface water (Provisional 

Cranberry Model)

2 apps @ 0.47 lb ai/A w/ a 14-day 

interval 79.6 13.9

*Maximum values in bold.

The Environmental Fate and Effects Division (EFED) has produced several drinking 

water assessments for sethoxydim, such as the following reports:

1. D239881, D239882, D239884, D238951 - Drinking Water Assessment for Sethoxydim 

for the Current Uses and for Proposed IR-4 Uses on Leafy Vegetables (excluding 

Brassica), Root and Tuber Crops (excluding Radish), Artichoke, and Caneberries. 

2. D257560, D257310 - Drinking Water Assessment and Ecological Risk Summary for 

Sethoxydim for the Maximum Use Crop (Citrus) and the Proposed IR-4 Uses on 

Pistachio and Safflower.

3. D312559 - Sethoxydim Drinking Water Assessment (Tier 1) for Reregistration 

Eligibility Decision.

4. D313456, D314420 - Drinking Water Assessment for Sethoxydim: Proposed Uses on 

Buckwheat, Okra, Borage, Dill, Root Vegetables, and Turnip Greens.

Due to recent updated modeling approaches, EFED is required to use new tools: the 

Surface Water Concentration Calculator (SWCC) for surface water and the Pesticide Root Zone 

Model for Ground Water (PRZM-GW) for ground water. In addition to these two new modeling 

tools, this drinking water exposure assessment also uses the provisional cranberry modeling tool.

USE INFORMATION

Sethoxydim is a broad-spectrum, postemergence herbicide for selective control of annual 

and perennial grass weeds. It is belongs to the cyclohexenone-class herbicide. The end use 

product is labelled as Poast® herbicide, which is formulated as an emulsifiable concentrate (EC) 

containing 1.5 pounds of active ingredient per gallon. This IR-4 petition expands the proposed 

uses to include:

1. New uses, R190, on fescue, Bushberry Subgroup 13-07B, Low Growing Berry Subgroup 

13-07H, Small Fruit Vine Climbing Subgroup 13-07F, Rapeseed Subgroup 20A, 

Sunflower Subgroup 20B (except safflower), and Cottonseed Subgroup 20C.
3

2. Crop group/subgroup additions, R175, for Citrus Fruit Group 10-10, Pome Fruit Group 

11-10, Caneberry Subgroup 13-07A, Bulb Vegetable Group 3-07, and Fruiting Vegetable 

Group 8-10.

For screening purposes, two use sites are considered for the drinking water assessment. 

The first one is based on the highest label use rate among all uses. The highest label use rate is 

for citrus fruit group. The maximum single application rate is 2.5 pt/A (0.47 lb ai/A). The 

maximum seasonal rate is 10.0 pt/A (1.88 lb ai/A) with a 14-day retreatment interval. The 

second one is based on cranberry use, which has a PHI (pre harvest interval) of 30 days. The 

single application rate is same as the citrus rate. The maximum seasonal rate is only 50% of the 

citrus use rate. 

FATE AND TRANSPORT CHARACTERIZATION

Transport and Mobility

Sethoxydim is a highly soluble compound (4700 mg/L in pH 7 water) with a low 

octanol/water partition coefficient (Kow is 45.1). The calculated bioconcentration factors (BCF)

for sethoxydim and total residues were 7X, 25X, and 21X for edible, nonedible, and whole fish, 

respectively. Depuration was fast, with a half-life of 3.6 days. For these reasons, 

bioaccumulation is not likely to occur. Also, due to its low vapor pressure (1.6x10-7 mm Hg) and 

Henry's Law Constant (1.47x10-11 atm-m3/mol), sethoxydim is not expected to be volatile under 

field conditions and from water. 

Based on batch equilibrium experiments, sethoxydim and its transformation products2, 

M2-SO2, M-SO, M-SO2, and M2-SO, were determined to be mobile to very mobile in sterile 

(autoclaved) sand, sandy loam, sandy clay loam, silt loam, and clay loam soils. Freundlich Kd

values were <1.00 for sethoxydim and its transformation products M-SO and M-SO2. The 

Freundlich Kd values for M2-SO and M2-SO2 ranged from 0.06 to 9.12. The only unsterilized 

soils were used on M2-SO2, for the reason that M2-SO2 is thought to be the most stable among 

test compounds and it is the terminal degradate. The Freundlich Kd values of M2-SO2 with nonsterilized soil are 0.12, 2.89, 0.78, 5.64, and 9.42, respectively for sand, sandy loam, sandy clay

loam, silt loam, and clay loam.

 Degradation

Sethoxydim hydrolyzes at moderately rapid rates at low pH's, but is more stable at high 

pH's. The calculated half-lives are 8.7, 155, and 284 days in pH 5, 7, and 9 solutions, 

respectively. The major observed hydrolysis transformation product is M2-S or 6-(2-

(ethylthio)propyl)-4-oxo-2-propyl-4,5,6,7-tetrahydrobenzoxazole. In contrast, evaluation of the 

data of sethoxydim total residues shows that they remain stable at all three pHs tested.

Sethoxydim and sethoxydim total residues degrade photolytically in both water and soil. 

In pH 8.7 buffered water, the calculated photolysis half-life of sethoxydim is 5.23 days, and the 

 

2 The molecular structures of sethoxydim and eight metabolites are presented in Attachment 1. 
4

major transformation product is M1-S or 2-(1-aminobutylidene)-5-(2-(ethylthio)-propyl)-

cyclohex-1,3-dione. In sandy loam soil irradiated with a xenon light source, the half-life of 

sethoxydim is approximately 1 hour, and the major transformation product is M-SO or 2-(1-

ethoxyiminobutyl)-5-(2-(ethylsulfinyl)propyl)-3-hydroxycyclohex-2-enone. Sethoxydim total 

residues photodegrade slower than parent sethoxydim in soil and water. Using a linear 

regression analysis, EFED calculated a half-life of 19.8 days for the photolysis in water of 

sethoxydim total residues and a half-life of 20 hours in soil.

Under aerobic conditions, parent sethoxydim transformed with short half-lives (<1 day) 

both in soil and aquatic environments. It degraded with a half-life of less than one day in sandy 

loam and sandy clay loam soils. The major transformation product at 2 months was M-SO, and 

after 12 months the major product was CO2. Using aerobic clay loam soil:water and aerobic clay

soil:water systems, it was determined that, under aerobic aquatic conditions, sethoxydim 

transformed with a half-life of <1 day. After 28 days, the major transformation products were 

CO2, M-SO, M2-S, and M-SO2. In contrast to parent sethoxydim, sethoxydim total residues 

were more persistent. The observed half-life was 1 month for sethoxydim total residues in the 

aerobic sandy loam study, and 7 days in the aerobic sandy clay loam study. In an aerobic clay

loam soil:water system, the calculated half-life for sethoxydim total residues was 38.1 days, 

while in an aerobic clay soil:water system the half-life was 32.9 days.

Under anaerobic conditions, parent sethoxydim is even more persistent than under 

aerobic conditions. It transformed with half-lives of 11 to >60 days under anaerobic soil 

conditions and 25-39 days in anaerobic aquatic metabolism studies. M-SO was the major 

transformation product in both studies. It was observed that M2-S, which was a major 

transformation product in the hydrolysis study, was only a minor transformation product in the 

anaerobic aquatic metabolism study. Sethoxydim total residues were more persistent than parent 

sethoxydim in the anaerobic studies. A half-life of 91.6 days was observed in the anaerobic soil 

metabolism study, while half-lives of 132-187 days were observed in the anaerobic aquatic 

metabolism study.

Residues of Concern

The proposed degradation scheme of the parent and eight degradates has been discussed 

in an earlier memo (DP 312559). The Metabolism Assessment and Review Committee (MARC)

of the Health Effects Division (DP 246356) states that all degradates are to be included. Because 

of the short metabolism half-lives of parent sethoxydim (< 1 day), the EDWCs represent total 

residues, including sethoxydim, M-SO, M-SO2, M1-S, M1-SO, M1-SO2, M2-S, M2-SO, and 

M2-SO2. The proposed scheme of the transformation of sethoxydim and the eight metabolites is 

presented in Attachment 1.

Drinking Water Exposure Assessment

Drinking water exposure to the residues of concern is estimated using available chemical 

properties and environmental fate data. A total residues of concern (TRC) approach was used 

because degradation studies were not performed on the specific degradates of concern. Table 2
5

lists the environmental fate exposure modeling inputs of the sethoxydim TRC.3 These 

parameters reflect regression of the total residues at each interval of the degradation studies. 

Chemical properties of the parent compound were used to represent those of the residues of 

concern due to the lack of data on the degradates. The mean soil adsorption coefficient for M2-

SO2 was used to represent the residues of concern because this compound was thought to be the 

most prominent terminal degradate. For the batch equilibrium study (MRID 41475212), the 

parent and 4 degradates (M-SO, M-SO2, M2-SO, and M2-SO2) were studied separately. Due to 

the possible short half-life values of the parent and degradates, the batch equilibrium experiments 

were tested using sterile (autoclaved) soils. The terminal degradate (M2-SO2) was thought to be 

most stable, so additionally it was also tested using non-sterile soil. The results indicate that M2-

SO2 is very mobile in sand, sandy loam, sandy clay loam, and silt loam and to be mobile in clay

loam. Freundlich Kd values were 0.12 for the sand soil, 0.78 for the sandy clay loam soil, 2.89 

for the sandy loam soil, 5.64 for the silt loam soil, and 9.42 for the clay loam soil. Adsorption 

increased as the CEC values of the soils increased.

Table 2. Chemical Model Inputs for Sethoxydim Total Residues of Concern

Parameter Value Source

Molecular Weight (g/mol) 327.48 C17H29NO3S

Water Solubility (mg/L) 4700 EFED one-liner

Vapor Pressure (torr) 1.6 x 10-7 EFED one-liner

Hydrolysis half-life (day) (25 oC) Stable (pH 5, 7, 9) MRID 41475207

Aqueous photolysis half-life (day)

(25 oC)

19.8 (total residue) MRID 41475208

Aerobic soil metabolism half-life (day)

(25 oC)

54 (upper 90th percentile on two values of 30 

days and 7 days for total residues)

MRID 41475210

Aerobic aquatic metabolism half-life 

(day) (25 oC)

44 (upper 90th percentile on two values of 

38.1 days and 32.9 days for total residues)

MRID 42165604

Anaerobic aquatic metabolism half-life 

(day) (25 °C)

244 (upper 90th percentile on two values of 

132 days and 187 days for total residues)

MRID 41475211

Adsorption coefficients (Kd) (25 oC)* 3.77 (Mean of five values: 0.12, 0.78, 2.89, 

5.64, and 9.42) 

MRID 41475212

*The only batch equilibrium test with non-sterile soils, conducted on M2-SO2 (the terminal degradate).

Drinking Water from Surface Water Sources

The Surface Water Concentration Calculator (SWCC v1.106) was used to calculate 

EDWCs from surface water sources. There are two citrus modeling scenarios in the SWCC 

database. They are California citrus and Florida citrus. Both scenarios were assessed to obtain 

EDWCs. Table 3 lists the application related information and assumptions. Since sethoxydim is 

 

3

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 Pesticide Programs, Environmental Fate 

and Effects Division, Oct. 22, 2009. http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
6

used to control post-emergence annual and perennial grass weeds, the first application date was 

assumed during the spring time. 

Table 3. Application Information Related to Sethoxydim Use on Citrus Scenarios

Model Input Variable Input Value Comments

1

st Application Date (2 scenarios) April 1 or April 15 In the spring time

Application Rate (maximum) 0.47 lb a.i./acre (2.5 pints) Label Information

Maximum Number of Applications 4 (10 pints max. per season) Label Information

Application Interval 14 days (minimum) Label Information

Percent Cropped Area 91% National default for 

all agricultural land

Application method Ground Label Information

App. Efficiency / Spray Drift 0.99 / 0.066 (for ground) USEPA, 2013 4

Standard percent cropped areas (PCA) are used as conservative default estimates of the 

extent of watershed on which agricultural crops of unknown specific PCA are grown.

5

 

Considering the many use sites of sethoxydim, the PCA value of 0.91 was used to represent the 

national default PCA for all agricultural land. The PCA-adjusted SWCC results are tabulated in 

Table 4. The modeling results of the Florida citrus scenario provided the higher EDWCs of 65.3

μg/L and 6.6 μg/L to represent the 1-in-10-year peak and mean exposure values, respectively.

Table 4. Sethoxydim Total Residue of Concerns EDWCs from Surface Water Sources 

Use PRZM 

Scenario

Initial 

App. 

Date

1-in-10-year 

Peak Exposure 

(μg/L)

1-in-10-year 

Annual Mean 

Exposure (μg/L)

30-year Mean 

Exposure (μg/L)

Citrus Group 10

CA Citrus Apr. 1 3.89 0.77 3.38

CA Citrus Apr. 15 3.90 0.75 3.55

FL Citrus Apr. 1 65.3 6.6 0.71

FL Citrus Apr. 15 56.6 6.3 0.68

The Provisional Cranberry Model is a provisional refinement to the Tier I Rice Model 

(v1.0, May 8, 2007). Refinements include the addition of simple degradation processes in dry 

and flooded conditions and a water depth of twelve inches, rather than the water depth of four 

inches used in the rice model. These modifications allow estimation of screening-level peak and 

annual mean EDWCs of sethoxydim residues of concern that may occur in untreated surface 

water used as drinking water following use on cranberries.

 

4 USEPA. 2013. Guidance on Modeling Offsite Deposition of Pesticides via Spray Drift for Ecological and Drinking 

Water Assessments. U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and 

Effects Division. Internal memorandum. Dec. 20, 2013.

5

Jones, R. D., K. Costello, J. Hetrick, J. Lin, R. Parker, N. Thurman, C. Peck, G. Orrick. 2010a. Development and 

Use of Percent Cropped Area Adjustment Factors in Drinking Water Exposure Assessments. U.S. Environmental 

Protection Agency, Office of Pesticide Programs. September 9, 2010. Online at: 

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html
7

The Provisional Cranberry Model is based on the same assumptions as the Tier I Rice 

Model, with the addition of degradation processes in dry and flood conditions and a water depth 

of twelve inches in the flooded cranberry bog. Degradation of sethoxydim residues of concern is 

assumed to occur predominantly via aerobic microbial metabolism, whether on dry cranberry

bog soil or in bog flood water.

The cranberry bog water depth of twelve inches is a maximum depth recommended by 

the Cape Cod Cranberry Growers Association6. Previous assessments of mesotrione uses that 

employed the Provisional Cranberry Model assumed a water depth of eighteen inches (DP 

barcode 339984, USEPA, 2004; DP barcode 325840, USEPA, 2006a), which is within the range 

of water depths traditionally used. However, modern water conservation pressures are causing 

cranberry growers to reduce the flood depth to fewer inches above the vertical cranberry

branches, which grow up to eight inches high. Therefore, a twelve-inch deep flood is a 

reasonable refinement to model assumptions. 

The label use of sethoxydim on cranberries at the maximum proposed application rate 

(0.47 lbs a.i./A) may occur no more than twice times per year, based on the annual application 

rate limit (0.94 lbs a.i./A). The minimum reapplication interval is 14 days. There is a preharvest interval (PHI) of 30 days requirement.

Cranberry bogs are not typically flooded until the night before harvesting. Therefore, for 

the parent compound, the proposed single application rate (0.47 lbs a.i./A) was aged on dry soil 

with the Provisional Cranberry Model for two durations: 44 and 30 days. Peak and annual mean 

EDWCs were calculated beginning three days after flooding to account for transport time to a 

down-gradient drinking water intake following release of bog tailwater not long after flooding. 

Model input parameters are listed in Table 5. Because PCA values were not available for 

cranberry uses, they were not considered for exposure estimates in this cranberry use.

Table 5. Provisional Cranberry Model input parameters for total residues of sethoxydim.

Input Parameter Value Comments Source

Application Rate

(lbs a.i./A) 0.47 Maximum single application rate. Label Information

Applications per Year 2

Maximum number of applications per 

year at the maximum proposed single 

application rate.

Label Information

Application intervals (d) 14 Minimum interval Label Information

Adsorption coefficients 

(Kd) (25 oC)* 3.77 Mean of five values: 0.12, 0.78, 2.89, 5.64, and 9.42 

MRID 41475212

Aerobic Soil Metabolism 

Half-life (days) 54 Upper 90th percentile on two values of 30 days and 7 days for total residues)

MRID 41475210

Aerobic aquatic 

metabolism

half-life (days)

44

Upper 90th percentile on two values of 

38.1 days and 32.9 days for total 

residues

MRID 42165604

 

6 Cape Cod Cranberry Growers Association, 2001. “Cranberry Water Use – An information fact sheet”.
8

Based on the inputs from Table 5 and the sethoxydim application dates calculated to be 

30 days (30 days PHI) and 44 days before flooding, the provisional cranberry model yields the 

EDWCs of 79.6 μg/L and 13.9 μg/L, respectively for peak value and annual average value.

Drinking Water from Ground Water Sources

PRZM-GW (v1.07) is used to calculate EDWCs from ground water sources. The 

chemical specific input parameters including hydrolysis half-life, aerobic soil metabolism halflife, and adsorption coefficient (Kd) are taken from Table 2 and application related inputs are 

taken from Table 3. All six available scenarios were modeled as surrogates for turf scenarios, 

with the Florida citrus scenario producing the highest EDWCs as shown in Table 6. The 

modeling results of Florida citrus scenario provide the higher EDWCs of 0.565 μg/L and 0.511 

μg/L to represent the acute and chronic exposure values, respectively.

Table 6. PRZM-GW Output of EDWCs for Sethoxydim Total Residue of Concerns

Modeled 

Scenario

Max. Daily Conc. 

(μg/L)

Mean Breakthrough 

Time (years)

Post-breakthrough 

Mean (μg/L)

DMV corn 0.0444 86.12 0.0394

FL citrus 0.565 45.96 0.511

FL potato 0.0656 35.28 0.0585

GA peanut 0.104 67.69 0.0923

NC cotton 0.00225 Incomplete* Incomplete

WI corn 0.00384 Incomplete Incomplete

*Incomplete indicates no breakthrough during the modeling simulation

Figure 1. PRZM-GW EDWCs over 100 years for the Florida citrus scenario

The overall estimated drinking water concentrations from surface water sources and 

ground water sources are summarized in Table 1. The surface water concentrations are about 

two order of magnitudes higher than the ground water concentrations.
9

Attachment 1. Molecular Structures of Sethoxydim and Eight Metabolites

The following scheme represents the relationship between parent sethoxydim (M) and its 

eight identified degradates. There are three groups of degradates: a group formed from the 

parent by oxidation of the sulfur atom (M group), the “amines” or M1 group, and the 

tetrahydrobenzoxazoles, or M2 group. The first M1 group daughter (M1-S) is formed by

cleavage of the ethoxyimino group to an amino group, either by hydrolysis or aqueous 

photolysis. The first member of the M2 group (M2-S) is then formed from M1-S by ring closure 

to produce a tetrahydrobenzoxazole ring. Each of the three groups of degradates also has a 

sulfoxide (-SO) and a sulfone (-SO2), to round out the total of eight degradates. This reaction 

dominates in aerobic soil metabolism and soil photolysis experiments, especially to form the MSO and M-SO2 degradates.

M Group M1 Group M2 Group

M  M1-S  M2-S

  

M-SO  M1-SO  M2-SO

  

M-SO2  M1-SO2  M2-SO2

The eight degradates include 2 daughter products, which are formed from the parent by a 

single reaction (M1-S and M-SO), 3 granddaughter products that are two reactions removed 

from the parent (M-SO2, M2-S, and M1-SO), 2 great-granddaughters that are three reactions 

removed (M1-SO2 and M2-SO), and one great-great-granddaughter that is four reactions 

removed (M2-SO2). It is important to note that M1-SO, M2-SO, M1-SO2, and M2-SO2 may be 

formed by more than one pathway, and that the sulfur oxidation may be reversible under 

reducing conditions.