UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

 WASHINGTON, D.C. 20460 

 OFFICE OF 

 PREVENTION, PESTICIDES AND 

 TOXIC SUBSTANCES

 DP Barcode: 362366/362365 

 PC Code: 121011 

 October 1, 2009 

MEMORANDUM 

SUBJECT: Drinking water assessment for Clethodim on Caneberry, Bushberry, Peach, 

and Artichoke. 

TO: Paula Deschamp 

 Registration Action Branch 3 

 Health and Effects Division 

 and 

 

Jim Stone 

 Herbicide Branch 

 Registration Division 

FROM: Tiffany Mason, Environmental Engineer 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division 

THRU: William P. Eckel, PhD, Senior Scientist 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division 

Jean Holmes, PhD, RAPL 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division 

 Tom Bailey, PhD, Branch Chief 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division 

CONCLUSIONS

This drinking water assessment is for Clethodim and its sulfoxide and sulfone degradates. 

Clethodim is used as a post-emergence herbicide against a wide range of annual and perennial 

grasses (monocotyledon plant species) in a wide variety of broad leaf crops including soybeans, 
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cotton, flax, peanuts, sunflowers, sugar beets, potatoes, alfalfa and vegetables. The proposed 

new uses of caneberry, bushberry, peach and artichoke, have a maximum use rate per season, as 

specified on the label to be four applications, fourteen days apart, at the rate of 0.12 lb. a.i./acre. 

Based on the proposed application rates and the environmental fate properties of clethodim, 

surface and ground water contamination may occur from clethodim as well as its sulfoxide and 

sulfone degradates. The risk of water contamination is primarily associated with clethodim 

sulfone and clethodim sulfoxide rather than parent clethodim based on greater persistence and 

mobility for the degradates. Thus, this assessment is based on total toxic residues rather than 

parent only. 

 

EFED recommends that total toxic residues estimated environmental concentrations (EEC’s) 

generated by the FIRST model and listed above be used instead of EEC’s for parent clethodim. 

For drinking water derived from surface water, the acute concentrations to be used for drinking 

water are 38.9 μg/l for the use on flax, leafy greens, sesame, safflower, herbs, hops, asparagus, 

legumes (DW assessment November 30, 2006). However, the chronic (annual mean) values for 

clethodim in surface water used as drinking water is 13.0 μg/l (Table 1) for the use on caneberry, 

bushberry, peach, and artichoke. 

For drinking water derived from ground water, the SCI-GROW model estimates total toxic 

clethodim concentrations of 9.8 μg/l for the use on caneberry, bushberry, peach, and artichoke 

(Table 1). These SCI-GROW values should be used for acute, chronic, and cancer risk 

assessment on each of their respective crops. 

Based on FIRST and SCI-GROW modeling, the drinking surface and groundwater Expected 

Environmental Concentrations (EEC) of Clethodim and its sulfoxide and sulfone degradates for 

the use on caneberry, bushberry, peach, and artichoke are shown in Table 1. 

Table 1. Tier I EEC's for Clethodim and its sulfoxide and sulfone degradates for the use on 

caneberry, bushberry, peach, and artichoke using FIRST and SCI-GROW models. 

Compound Peak EEC (ug/L) Annual Mean EEC (ug/L) 

Total Toxic Clethodim (parent + 

sulfoxide + sulfone); (four apps 

of 0.12 lbs. ai/A at 14-day 

interval) 

 37.3 13.0 

SCI-GROW EEC (ug/L) 

9.8 

Environmental Fate Summary
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Surface and ground water contamination may occur from the sulfoxide and sulfone degradates of 

clethodim, as well as from parent clethodim. However, the risk of water contamination is 

primarily associated with clethodim sulfone and clethodim sulfoxide rather than parent 

clethodim based on greater persistence and mobility for the degradates. 

The only significant routes of dissipation of clethodim are microbial degradation in soil and 

movement by leaching or runoff. Parent clethodim is moderately persistent to hydrolysis at pH 5 

with half-lives of 26-42 days and stable at pH 7 and 9 with half-lives of greater than 300 days. 

Even though the acceptable aqueous photolysis study shows half-lives of 2.2 days at pH 5, 7.1 

days at pH 7, and 11.8 days at pH 9, this may not be an important route of dissipation because of 

suspended sediment and shading. Photolysis is only an important route of dissipation in shallow, 

well-mixed surface water with no shading. The half-lives in aerobic soil are 86.4 to 97.6 days 

for total toxic residues (parent + sulfoxide + sulfone). The sulfoxide and sulfone metabolites are 

more persistent than parent clethodim and are formed in significant quantities in soil. All 

residues of clethodim (parent and metabolites) are very mobile in soil with five out of six soil 

desorption coefficients (Kd ) less than one. The field dissipation studies show that parent 

clethodim was only found at levels at or near the quantitation limit of 0.02 ppm, which is 

consistent with the rapid degradation in soil. Clethodim sulfoxide had an apparent half-life of 

2.5 to 3.7 days, indicating that movement from the treated field may have been an important 

route of dissipation. 

Water Resources Summary

A. Monitoring 

 The EFED currently has no monitoring data on the concentrations of clethodim or its 

major degradates in surface or groundwater. 

 B. Surface Water 

The Tier I screening models FIRST Version 1.1.1 (FQPA Index Reservoir Screening Tool) with 

Percent Crop Area adjustment factor was used to determine drinking water concentrations of 

Clethodim and its sulfoxide and sulfone degradates derived from surface water sources. 

Parent clethodim may move from the treated field to surface water or ground water through runoff or leaching which occurs shortly after application (e.g. rainfall). Also, the sulfoxide and 

sulfone degradates may migrate by runoff or leaching for longer periods of time because they are 

more persistent. All residues of clethodim (parent and degradates) are very mobile in soil. Tier 

1 drinking water concentrations derived from surface water for total toxic residues (parent + 

sulfoxide + sulfone) were estimated using Version 1.1.1 of FIRST model. Input values for the 

FIRST model are reported in Table 2. 

Table 2. FIRST Input Parameters for Clethodim and its sulfoxide and sulfone degradates use on 
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caneberry, bushberry, peach, and artichoke. 

Parameter Input value Data Source/Rationale* 

Application Rate (lbs ai/A) 0.12 

 

IR-4 labels 

Maximum No. of 

Applications/season 

4 IR-4 labels 

Application Interval (days) 

14 IR-4 labels 

Percent Cropped Area (as 

decimal) 

0.87 Default 

Incorporation depth (inch) 0 Broadcast application 

Koc 5 

Lowest Koc from 5 soils (MRID 40974523), 

range of 5-271 for parent. 

Aerobic Soil Metabolic Halflife (days) 

Total Toxic residue 92 

Average half-lives for total toxic residue. 

Half-lives 86.4 and 97.6. (MRID’s 40974521 

and 41376801). 

Is the pesticide wetted-in? No 

Method of application B Ground 

Solubility (ppm) 5400 Joint meeting of the FAO panel of experts on 

pesticide residues in food and the 

environment and the WHO expert group on 

pesticide residues (Italy, 1994). 

Aerobic aquatic t1/2 (days) 184 2 x (aerobic soil value) 

Photolysis Half-life (days) 7.1 MRID 41030134 

* Parameters were selected in accordance with the Proposed Interim Guidance for Input Values 

document, dated September 22, 2009. 
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 C. Ground Water 

SCI-GROW model (Screening Concentration in Ground Water Program (SCI-GROW) 

VERSION 2.3.0 was used to predict the maximum chronic and acute concentration of parent 

Clethodim and its sulfoxide and sulfone degradates derived from shallow ground water. 

Parent clethodim is mobile, but has a short metabolic half-life in soil under aerobic conditions. 

Therefore, parent compound should not be a ground water concern in most environments. While 

EFED expects the parent to be transformed to sulfoxide or sulfoxone products fairly quickly by 

soil metabolism (t1⁄2 = 1 - 3 days), it may be more persistent since it is leached below the more 

biologically active topsoil. In such instances (i.e., leaching rainfall shortly after application) 

parent clethodim concentrations may be higher than estimated. In the event that parent 

clethodim did reach ground water, the available routes of disappearance would be dilution, some 

metabolism to persistent degradates, and slows hydrolysis with the rate depending on the pH of 

the ground water. Table 3 below summarizes the results of the screening level groundwater 

model, SCIGROW 2.3.0, for the total toxic clethodim (parent + sulfoxide+sulfone). 

Table 3. SCIGROW input parameters used to estimate total toxic clethodim concentrations for 

the use on caneberry, bushberry, peach, artichoke. 

Parameter Input value Data Source/Rationale* 

Koc (l/kg)

5 

Lowest Koc from 5 soils (MRID 

40974523), range of 5-271 for 

parent. 

Application Rate (lbs ai/A) 0.12 

 

IR-4 Labels 

Maximum No. of Applications 4 IR-4 Labels 

Aerobic soil metabolism half-life 

(days 

total toxic residue. 

92 

Average half-lives for total toxic 

residue. Half-lives 86.4 and 97.6. 

(MRID’s 40974521 and 

41376801). 

APPENDIX I 
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FIRST modeling input and output for the use of clethodim on caneberry, 

bushberry, peach, and artichoke 

 

SCIGROW 

VERSION 2.3 
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ENVIRONMENTAL FATE AND EFFECTS DIVISION 

OFFICE OF PESTICIDE PROGRAMS 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

SCREENING MODEL 

FOR AQUATIC PESTICIDE EXPOSURE