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
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
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