1 OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION **UNITED STA TES ENVIRONMENTA**  **L PROTECTION A GENCY** 

WA SHINGTON, D.C. 20460 June 2 5 , 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 L in, 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 (750 8 P) This assessment provides estimated drinking water concentrations (EDWC) of sethox yd im in surface water and in ground water in support of human health risk assessment for IR - 4 petition s . Screening EDWCs ( **Table 1** ) of sethox ydim were generated with the Surface Water Concentration Calculator ( SWCC ) and the Provisional Cranberry Model for surf

ace water and with the Pesticide Root Zone Model for Ground Water ( PRZM - GW ) for ground water. Modeled application rates represent the maximum use pattern s of proposed end - use labels and current registered labels . Remaining model input parameters were cho sen according to current guidance (USEPA , 2009 ) 1 . EDWCs reflect exposure to sethox yd im residues of concern in drinking water , which include the parent compound and eight degradates of concern ( D P 246356 ). If the screening EDWC s listed in this memo result in dietary risk exceedances, contact James L in (703 - 30 8 - 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.htm2

**Table 1. Maximum Screening Drinking Water Exposure Estimates for P roposed**  **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 L eafy 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 t ools : 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 pro visional cranberry model ing tool . **USE INFORMATION**  Sethoxydim is a broad - spectrum, postemergence herbicide for sel ectiv e control of annual and perennial grass weeds. Itis 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 exp ands the proposed uses to include: 1. New uses, R190, on fescue, Bushbe rry 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 purpose s , two use sites are considered for the drinking water assessment. The first one is based on the highest label u se rate among all uses. T he highest label use rate is for citrus fruit group . T h e 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 bas ed on cranberry use, which has a PHI(pre harvest interval) of 30 day s. 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 highl y s oluble 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 s ethox ydim and total residues were 7X, 25X, and 21X for edible, nonedible, an d whole fish, respectivel y. Depuration was fast, with a half - life of 3.6 day s. 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), s ethox yd im i s not expected to be volatile under field conditions and f

rom water. Based on batch equilibrium experiments, s ethox ydim and its transformation products 2 , M2 - SO 2 , M - SO, M - SO 2 , and M 2 - SO, were determined to be mobile to very mobile in sterile (autoclaved ) sand, sandy loam, sandy clay loam, silt loam, and clay loam soils. Freundl ich Kd values were <1.00 for s ethoxydim and its transformation products M - SO and M - SO 2 . The Freundlich Kd values for M2 - SO and M2 - SO 2 ranged from 0.06 to 9.12. The only unsterili zed soils were used on M2 - SO 2 , for the reason that M2 - SO 2 is thought to be the most stable among test compounds and it is the terminal degradate . The Freundlich Kd values of M2 - SO 2 with non s terilized soil are 0.12, 2.89, 0.78, 5.64, and 9.42, respectivel y for sand, sandy loam, sandy clay loam, silt loam, and clay

loam. **Degradation** Sethoxydim hydrolyzes at moderatel y rapid rates at low pH's, but is more stable at high pH's. The calculated half - lives are 8.7, 155, and 284 day s in pH 5, 7, and 9 solutions, respectivel y. The major observed h ydrol ys is transformation product is M2 - S or 6 - (2 - (ethylthio)propyl) - 4 - oxo - 2 - prop yl - 4,5,6,7 - tetrahydrobenzoxazole. In contr ast, evaluation of the data of s ethox ydim total residues shows that they remain stabl e at all three pHs tested. Sethoxydim and s ethox ydim total residues degrade photolyticall y in both water and soil. In pH 8.7 buffered wate r, the calculated photoly sis half - life of s ethox yd im is 5.23 day s, 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 - (eth yl thio) - prop yl ) - cyclohex - 1,3 - dione. In sandy loam soil irradiated with a xenon light source, the half - life of s ethox yd im is approximately 1 hour, and the major transformation product is M - SO or 2 - (1 - ethox yi minobutyl) - 5 - (2 - (ethylsulfinyl)prop yl ) - 3 - hydroxycyclohex - 2 - enone. Sethoxydim total residues photodegrade slower than parent s ethox ydim in soil and water. Using a linear regression anal ys is, EFED calculated a half - life of 19.8 day s for the photoly sis in water of s ethox yd im total residues and a half - life of 20 hours in soil. Under aerobic conditions, parent s ethox ydim 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 cl ay

loam soils. The major transformation product at 2 months was M - SO, and after 12 months the major product was CO 2 . Using aerobic clay loam soil:water and aerobic clay soil:water sy stems, it was determined that, under aerobic aquatic conditions, s ethox yd im transformed with a half

- life of <1 day . After 28 day s, the major transformation products were CO 2 , M - SO, M2 - S, and M - SO 2 . In contrast to parent sethox ydim, s ethox ydim total residues were more persistent. The obse rved half - life was 1 month for s ethox yd im 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 sy stem, the calculated half - life for s ethox yd im total residues was 38.1 day s, while in an aerobic clay

soil:wate r sy stem the half - life was 32.9 day s. Unde r anaerobic conditions, parent s ethox ydim is even more persistent than under aerobic conditions. It transformed with half - lives of 11 to >60 day s under anaerobic soil conditions and 25 - 39 days in anaerobic aquati c 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 hydroly sis study , was only a minor transformation product in the anaerobic aquatic metabolism study. S ethox ydim total residues we re more persistent than parent s ethox yd im in the anaerobic studies . A half - life of 91.6 day s was observed in the anaerobic soil metabolism study , while half - lives of 132 - 187 days were observed in the anaerobic aquatic metab olism study . **Residues of Concern** The proposed degradation scheme of the parent and eight degradates has been discussed in an earlier memo (D P 312559). The Metabolism Assessment and Review Committee (MARC) of the Health Effects Division ( D P 246356 ) state s 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 - SO 2 , M1 - S, M1 - SO, M1 - SO 2 , M2 - S, M2 - SO, and M2 - SO 2 . The proposed scheme of the transformation of sethox yd im and the eight metabolites is presented in Attachment 1 . **Drinking Water Exposure A**  **ssessment** Drinking water exposure to the residues of concern is estimated using available chemical properties and enviro nmental fate data. A total residues of concern (TRC) approach was used because degradation studies were not performed on the specific degradate s of concern. **Table 2** 5

lists the environmental fate exposure modeling inputs of the sethox yd im TRC. 3 These par ameters 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 - SO 2 was used to represent the residues of concern because this compound was thought to be the most prominent terminal degradate. F or the batch equilibrium study (MRID 41475212), the parent and 4 degradates (M - SO, M - SO 2 , M2 - SO , and M2 - SO 2 ) 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 - SO 2 ) wa s thought to be most stable , so additionally it was also test ed using no n - sterile soil. The results indicate that M2 - SO 2 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 T**  **otal R**  **esidues of C**  **oncern Parameter**  **Value Source**  Molecular Weight (g/mol) 327.48 C 17 H 29NO 3 S Water Solubility (mg/L) 4700 EFED one - liner Vapor Pressure (torr) 1.6 x 10 - 7 EFED one - liner Hydrolysis half

- life (day) (25 o C) Stable (pH 5 , 7, 9) MRID 41475207 Aqueous photolysis half

- life (day) (25 o C) 19.8 (total residue) MRID 4 1475208 Aerobic soil metabolism half - life (day) (25 o C) 54 (upper 90 th percentile on two values of 30 days and 7 days for total residues) MRID 4 1475210 Aerobic aquatic metabolism half - life (day) (25 o C) 44 ( upper 90 th percentile on two values of 38.1 days and 32.9 days for total residue s ) MRID 42165604 Anaerobic aquatic metabolism half - life (day) (25 °C) 244 (upper 90 th percentile on two values of 132 days and 187 days for total residues) MRID 41475211 Ad sorption coefficients (K d) (25 o C) * 3.77 ( Mean of five values: 0.12, 0.78, 2.89, 5.64, and 9.42) MRID 4 147 5212 *T he only batch equilibrium test with non - sterile soil s, conducted on M2 - SO 2 (the terminal degradate) . **Drinking Water from Surface Water Source**  **s** The Surface Water Concentration Calculator (SWCC v 1.10 6 ) wa s 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 . B oth scenarios we re assessed to obtain EDWCs. **Table 3** lists the application related information and assumptions. Since s ethox yd im 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.htm6

used to control post - emergence annual and perennial grass weeds , the first application date wa s assume d 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 1 5 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 man y use site s of sethox ydim, 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** . T he modeling results of the Florida citrus scenario provide d the higher EDWCs of 6 5. 3 μg/Land 6.6 μg/L to represent the 1 - in - 10 - ye ar peak and mean exposure value s , respectivel y. **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 modification s allow estimation of screening - level peak and annual mean EDWC s of sethox yd im residues of concern that may occur in untreated surface water used as drinking wate r 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 memoran dum. Dec. 20, 2013. 5 Jones, R. D., K. Costello, J. Hetrick, J. Lin, R. Parker, N. Thurman, C. Peck, G. Orrick. 2010 a . Development and Use of Percent Cropped Area Adjustment Factors in Drinking Water Exposure Assessments. U.S. Environmental Protection Ag ency , Office of Pesticide Programs. September 9, 2010. Online at: http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html7

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 b y the Cape Cod Cranberry

Growers Association 6

. 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, US EPA, 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, whi ch grow up to eight i nches high . Therefore, a twelve - inch deep flood is a reasonable refinement to model assumptions. The label use of sethox ydim on cranberries at the maximum proposed application rate (0.47 lbs a.i./A) may

occur no more than twice times per ye ar, based o n the annual application rate limit (0.94 lbs a.i./A). The minimum reapplication interval is 14 days. There is a pre harvest interval (PHI) of 30 day s requirement . Cranberry

bogs are not t ypically flooded until the night before harvesting. Therefore, f or the parent compound, the propos ed single application rate (0.47 lbs a.i./A) was aged on dry soil with the Provisional Cranberry Model for two durations: 4 4 and 3 0 day s. Peak and annual mean EDWCs were calculated beginning three day s after flooding to a ccount 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**  . B ecause PCA values were not available for cranberry uses, they

were not consider ed f

or 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. L abel Information Application intervals (d) 14 Minimum interval Label Information Adsorption coefficients (K d) (25 o C)* 3.77 Mean of five values: 0.12, 0.78, 2.89, 5.64, and 9.42 MRID 4 147 5212 Aerobic Soil Metabolism Half - life (days) 54 Upper 90 th percentile on two values of 30 days and 7 days for total residues) MRID 4 1475210 Aerobic aquatic metabolism half - life (days) 44 Upper 90 th 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 sethox ydim application dates calculated to be 30 day s (30 day s PHI) and 44 day s before flooding, the provisional cranberry model yields the EDWCs of 7 9. 6 μg/L and 13 .9 μg/L, respectivel y for peak value and annual average value . **D rinking Water**  **from Ground Water Source s**  PRZM - GW (v 1.07 ) is used to calculate EDWCs from ground water sources. The chemical specific input parameters including h ydroly sis 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 h ighest 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/Lto represent the acute and chronic exposure value s , respect ivel y. **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 con centrations.9

**Attachment 1**  **. M**  **olecular Structures of S**  **ethoxydim and E**  **ight M**  **etabolites** The following s cheme represents the relationship between parent sethox ydi m (M) and its eight identified degradates. There are three groups of degradates: a group formed from the parent b y oxidation of the sulfur atom (M group), the “amines” or M1 group , and the tetrah ydrobenzoxazoles, or M2 group . The first M1 group daughter (M1 - S) is formed by cleavage of the ethoxyi mino group to an amino group, either b y h ydrol ys is or aqueous photoly sis. 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 su lfone ( - SO2), to round out the total of eight degradates. This reaction dominates in aerobic soil metabolism and soil photoly sis experiments, especially to form the M SO and M - SO 2 degradates.

M Group M1 Group M2 Group M  **M1 -**  **S**  **M2 -**  **S**    **M -**  **SO**  **M1 -**  **SO**  M2 - SO    **M -**  **SO 2**   M1 - SO 2  M2 - SO 2 The eight degradates include 2 daughter products, which are formed from the parent b y a single reaction ( **M1 -**  **S and M -**  **SO** ), 3 granddaughter products that are two reactions removed from the parent (**M**

**- SO** **2 , M2**  **- S, and M1**  **-**

**SO** ), 2 great - granddaughters that are three reactions removed (

M1 - SO 2 and M2 - SO ), and one great - gr eat - granddaughter that is four reactions removed (M2 - SO 2 ). It is important to note that M1 - SO, M2 - SO, M1 - SO 2 , and M2 - SO 2 may be formed b y more than one pathway, and that the sulfur oxidation may be reversible under reducing conditions.