Sethoxydim Risk Assessment

4.4.3.  Aquatic Organisms.  The quantitative risk characterization for aquatic species is 
summarized in Table 4-4.  As discussed in previous sections of this risk assessment (sections 
4.1.3.1,  4.1.3.3, and 4.3.3.1), Poast is much more toxic to aquatic organisms than sethoxydim. 
For this reason, all of the toxicity values used in this risk assessment for aquatic species are based 
on exposures to Poast, the formulated product.  Thus, the toxicity of the adjuvants – i.e., 
petroleum solvent and polyoxyethylene nonylphenol emulsified – are considered in the 
characterization of risk.  Based on the hazard quotients summarized in Table 4-4, there is no 
indication that fish, aquatic invertebrates, or aquatic plants are likely to be exposed to 
concentrations of sethoxydim that will result in toxic effects, although the upper range of the 
hazard quotient for aquatic plants – i.e., 0.75) approaches a level of concern. 
However, there is a very substantial limitation to the current risk assessment.  As discussed in 
Section 4.3.3.1, no chronic toxicity studies on aquatic animals are available for either sethoxydim 
or Poast.  The hazard quotients given in Table 4-4 for chronic exposures are based on the ratio of 
the longer-term concentrations of sethoxydim in water to the acute toxicity benchmarks.  These 
ratios are provided only for comparison to the corresponding acute values and cannot be directly 
used to characterize longer-term risks to fish or aquatic invertebrates.  Nonetheless, the upper 
range of the longer-term hazard quotients range from 0.0036 to 0.038.  These are factors of about 
25 to 275 below a level of concern.  In other words, the chronic toxicity of sethoxydim would 
have to be greater than the acute toxicity by factors of 25 to 275 to reach a level of concern. 
Aquatic plants appear to be only somewhat more sensitive to Poast than aquatic animals and there 
is no indication that adverse effects on aquatic plants are plausible.  Unlike the case with aquatic 
animals, even short-term toxicity studies in aquatic plants use endpoints involving changes in 
population density.  Thus, both the short-term and longer-term hazard quotients given in Table 
4-4 can be legitimately used to characterize risk. 
4-21  

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Table 4-1: Summary of Exposure Scenarios for Terrestrial Animals. 
Dose (mg/kg/day) 
Worksheet 
Scenario 
Typical 
Lower 
Upper 
Acute/Accidental Exposures 
Direct spray 
small mammal, first-order absorption 
small animal, 100% absorption 
bee, 100% absorption 
Contaminated vegetation 
small mammal 
large mammal 
large bird 
Contaminated water 
small mammal, spill 
stream 
Contaminated insects 
small bird 
Contaminated fish 
predatory bird, spill 
Longer-term Exposures 
1.90e-01 
7.27e+00 
4.81e+01 
3.75e-01 
5.16e+00 
8.08e+00 
3.99e-01 
8.78e-03 
1.12e+01 
9.81e-01 
2.55e-02 
2.27e+00 
1.50e+01 
1.17e-01 
1.61e+00 
2.52e+00 
6.21e-02 
2.74e-04 
3.51e+00 
7.63e-02 
5.29e-01 
9.09e+00 
6.01e+01 
1.00e+00 
1.82e+01 
2.85e+01 
9.97e-01 
2.74e-02 
4.22e+01 
3.68e+00 
F01 
F02a 
F02b 
F03 
F10 
F12 
F05 
F06 
F14 
F08 
Contaminated vegetation 
small mammal, on site 
off-site 
large mammal, on site 
off-site 
large bird, on site 
off-site 
Contaminated water 
small mammal 
Contaminated fish 
predatory bird 
1.80e-03 
1.82e-05 
7.44e-02 
2.51e-03 
1.17e-01 
3.92e-03 
3.51e-05 
5.04e-04 
2.82e-04 
1.63e-06 
7.75e-03 
4.50e-04 
1.21e-02 
7.04e-04 
2.74e-07 
1.97e-06 
9.66e-03 
1.81e-04 
8.76e-01 
1.64e-02 
1.37e+00 
2.56e-02 
6.59e-05 
1.42e-03 
F04a 
F04b 
F11a 
F11b 
F13a 
F13b 
F07 
F09 
4-22  

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Table 4-2: Summary of toxicity values used in ecological risk assessment 
Animal 
Type 
Value 
Units 
Section 
Mammals 
Acute 
180 
mg/kg bw 
4.3.2.1. 
Chronic 
9 
mg/kg bw/day 
4.3.2.1. 
Birds 
Acute 
500 
mg/kg bw 
4.3.2.2. 
Chronic 
10 
mg/kg bw/day 
4.3.2.2. 
Terrestrial 
invertebrates 
Acute 
107 
mg/kg bw 
4.3.2.3. 
Terrestrial 
vegetation, 
sensitive 
Drift 
Pre-emergence 
0.006
0.059
 lb/acre 
lb/acre 
4.3.2.4. 
4.3.2.4. 
Terrestrial 
vegetation, tolerant 
Drift 
Pre-emergence 
0.03
0.235
 lb/acre 
lb/acre 
4.3.2.4. 
4.3.2.4. 
Fish 
Acute 
1.2 
mg/L 
4.3.3.1. 
Aquatic 
invertebrates 
Acute 
2.6 
mg/L 
4.3.3.1. 
Aquatic plants 
Acute 
0.25 
mg/L 
4.3.3.2. 
4-23  

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Table 4-3: Summary of quantitative risk characterization for terrestrial animals
1 
Hazard Quotient
2 
Scenario 
Typical 
Lower 
Upper 
Acute/Accidental Exposures 
Direct spray 
small mammal, first-order absorption 
1.1e-03 
1.4e-04 
2.9e-03 
small animal, 100% absorption 
4.0e-02 
1.3e-02 
5.1e-02 
bee, 100% absorption 
4.5e-01 
1.4e-01 
5.6e-01 
Contaminated vegetation 
small mammal 
2.1e-03 
6.5e-04 
5.6e-03 
large mammal 
2.9e-02 
9.0e-03 
1.0e-01 
large bird 
1.6e-02 
5.0e-03 
5.7e-02 
Contaminated water 
small mammal, spill 
2.2e-03 
3.4e-04 
5.5e-03 
small mammal, stream 
4.9e-05 
1.5e-06 
1.5e-04 
Contaminated insects 
small bird 
2.2e-02 
7.0e-03 
8.4e-02 
Contaminated fish 
predatory bird, spill 
2.0e-03 
1.5e-04 
7.4e-03 
Longer-term Exposures 
Contaminated vegetation 
small mammal, on site 
2.0e-04 
3.1e-05 
1.1e-03 
off-site 
2.0e-06 
1.8e-07 
2.0e-05 
large mammal, on site 
8.3e-03 
8.6e-04 
9.7e-02 
off-site 
2.8e-04 
5.0e-05 
1.8e-03 
large bird, on site 
1.2e-02 
1.2e-03 
1.4e-01 
off-site 
3.9e-04 
7.0e-05 
2.6e-03 
Contaminated water 
small mammal 
3.9e-06 
3.1e-08 
7.3e-06 
Contaminated fish 
predatory bird 
5.0e-05 
2.0e-07 
1.4e-04 
Toxicity Indices 
3 
Acute toxicity value for mammal - NOAEL 
180 
mg/kg 
Chronic toxicity value for mammal - NOAEL 
9 
mg/kg/day 
Acute toxicity value for bird - NOAEL 
500 
mg/kg 
Chronic toxicity value for birds 
10 
mg/kg/day 
Toxicity value for bee -NOAEL 
107 
mg/kg 
1
 See Worksheet G01 (Table 4-1 in text) for summary of exposure assessment. 
2 
Estimated dose ÷ toxicity index 
3
 See Section 4.3. for a discussion of the dose-response assessments 
4-24  

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