OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

March 31, 2015

PC Code: 121011

MEMORANDUM DP Barcode: 425767

SUBJECT: Clethodim: Drinking Water Exposure Assessment for Proposed New Uses and 

Crop Group Conversions regarding Stevia, Pome Fruit Group 11-10, Stone Fruit 

Group 12-12, Bulb Onion Subgroup 3-07A, Low-growing Berry Subgroup 13-

07G, Rapeseed Subgroup 20A, Sunflower Subgroup 20B, Cotton Seed Subgroup 

20C, and Fruiting Vegetable Group 8-10

FROM: Greg Orrick, Environmental Scientist

ERB2/EFED (7507P)

THROUGH: Brian Anderson, Branch Chief

ERB2/EFED (7507P)

TO: Meheret Negussie, Risk Assessor

Christine Olinger, Branch Chief

RAB3/HED (7509P)

Sydney Jackson, Risk Manager

Barbara Madden, Team Leader

MUERB/RD (7505P)

Aswarthy Balan, Risk Manager

Shaja Joyner, Team Leader

FHB/RD (7505P)

This Environmental Fate and Effects Division (EFED) memorandum is a drinking water 

exposure assessment (DWA) of proposed labeled uses of clethodim (listed in Table 2). In light 

of the following three bullets, this DWA recommends use of the estimated drinking water 

concentrations (EDWC) of the previous DWA (USEPA, 2013), which are 1,430 μg/L (acute) 

and 1,150 μg/L (chronic) (also listed in Table 1).

• The previous DWA assessed a maximum annual application rate of 4.5 pounds of active 

ingredient per acre (lbs a.i./A) using Tier I screening models.

• Representative uses have been assessed previously for most of the proposed crop groups.

• All of the proposed uses are limited to a seasonal application rate of 0.5 or less lbs a.i./A, 

which is less than the maximum assessed annual application rate of 4.5 lbs a.i./A.

1
Table 1. Screening-Level ROC EDWCs for Clethodim Uses A

Source (Tier: Model) Use Site

(Max. Annual App. Rate)

Acute 

EDWC 

(μg/L)

Chronic 

EDWC 

(μg/L)

Surface water (Tier I: FIRST) Non-cropped (4.5 lbs a.i./A) 330 137

Ground water (Tier I: PRZM-GW) Non-cropped (4.5 lbs a.i./A) 1,430 1,150

Ground water (Tier I: SCI-GROW) Non-cropped (4.5 lbs a.i./A) ≤27 A Maximum values in bold.

The clethodim residues of concern (ROC) include clethodim and degradates with 5-(2-

ethylthiopropyl)-5-hydroxycyclohexene-3-one or 5-(2-ethylthiopropyl)cyclohexene-3-one 

moieties. The maximum use pattern for clethodim is for non-cropped areas, fallow ground, and 

alfalfa. Table 2 lists the proposed uses and crop group conversions and their maximum use 

patterns.

Table 2. Proposed Use Patterns of Clethodim A

Proposed Use Pattern

Similar 

Current Use 

Pattern

Single 

App. Rate

(lbs a.i./A)

Reapp. 

Interval 

(days)

Max. No. 

of App. 

per Crop

Seasonal 

App. Rate 

(lbs a.i./A)

App. 

Method

Additional 

Restrictions

Stevia Herbs 0.125 14 NS 0.50 NS –

Pome fruit group 11-10 (None) 0.125 14 NS 0.50 NS –

Stone fruit group 12-12 Peach 0.125 14 NS 0.50 NS –

Low-growing berry 

subgroup 13-07G Strawberry 0.125 14 NS 0.50 NS –

Rapeseed subgroup 20A Rapeseed/ 

canola 0.094 None NS 0.094 NS

Do not apply after crop 

has begun bolting. Not 

for use in CA.

Sunflower subgroup 20B Sunflower 0.25 NS B NS 0.50 NS –

Cotton subgroup 20C Cotton 0.25 NS B NS 0.50 NS –

Bulb onion subgroup 3-

07A C

Onion, garlic, 

shallot 0.125 14 NS 0.50 NS –

Fruiting vegetable group 

8-10

Fruiting 

vegetables 0.125 14 NS 0.50 NS –

NS = “not stated”

A Listed use patterns represent those of the maximum use pattern and do not represent all labeled use patterns for 

that use.

B Reapplication interval on 12.6% a.i. label is 14 days, on 26.4% a.i. label is not stated.

C Bulb onion subgroup 3-07A appears on 12.6% a.i. label, not on 26.4% a.i. label.

USEPA. 2013. Orrick, G. Clethodim: Drinking Water Exposure Assessment for Registration 

Review. DP barcode 412704. U.S. Environmental Protection Agency, Office of Chemical 

Safety and Pollution Prevention, Environmental Fate and Effects Division. Memorandum 

to the Health Effects Division and Pesticide Re-evaluation Division. Jun. 24, 2013.

2