Sethoxydim Risk Assessment

scenarios are given in worksheets F10 for acute exposures as well as Worksheets F11a and F11b 
for longer-term exposures. 
For the acute exposures, the assumption is made that the vegetation is sprayed directly – i.e., the 
animal grazes on site – and that 100% of the diet is contaminated (Worksheet F10).  While 
appropriately conservative for acute exposures, neither of these assumptions are plausible for 
longer-term exposures.  Thus, for the longer-term exposure scenarios for the large mammal, two 
sub-scenarios are given.  The first is an on-site scenario that assumes that a 70 kg herbivore 
consumes short grass for a 90 day period after application of the chemical.  The contaminated 
vegetation accounts for 10 to 100% of the diet assuming that the animal would spend 10 to 100% 
of the grazing time at the application site.  Because the animal is assumed to be feeding at the 
application site, drift is set to unity - i.e., direct spray.  This scenario is detailed in Worksheet 
F11a.  The second sub-scenario is similar except the assumption is made that the animal is grazing 
at distances of 25 to 100 feet from the application site (lowing risk) but that the animal consumes 
100% of the diet from the contaminated area (increasing risk).  For this scenario, detailed in 
Worksheet F11b, AgDRIFT is used to estimate deposition on the off-site vegetation.  Drift 
estimates from AgDrift are summarized in Worksheet A06 and this model is discussed further in 
Section 4.2.3.2. 
The consumption of contaminated vegetation is also modeled for a large bird.  For these exposure 
scenarios, the consumption of range grass by a 4 kg herbivorous bird, like a Canada Goose, is 
modeled for both acute (Worksheet F12) and chronic exposures (Worksheets F13a and F13b). 
As with the large mammal, the two chronic exposure scenarios involve sub-scenarios for on-site 
as well as off-site exposure. 
For this component of the exposure assessment, the estimated amounts of pesticide residue in 
vegetation are based on the relationship between application rate and residue rates on different 
types of vegetation.  As summarized in worksheet A04, these residue rates are based on the 
re-analysis of the data from Hoerger and Kenaga (1972) conducted by Fletcher et al. (1994).  This 
is the same approach taken by U.S. EPA in their ecological risk assessment of sethoxydim 
(Bryceland et al.  1997). 
Similarly, the consumption of contaminated insects is modeled for a small (10g) bird.  No 
monitoring data have been encountered on the concentrations of sethoxydim in insects.  Following 
the approach used by Bryceland et al. (1997), the empirical relationships recommended by 
Fletcher et al. (1994) are used as surrogates as detailed in worksheet F14. 
In addition to the consumption of contaminated vegetation and insects, sethoxydim may reach 
ambient water and bioconcentrate in fish.  Thus, a separate exposure scenario is developed for the 
consumption of contaminated fish by a predatory bird in both acute (Worksheet F08) and chronic 
(Worksheet F09) exposures.  Because predatory birds usually consume more food per unit body 
weight than do predatory mammals (U.S. EPA/ORD 1993, pp. 3-4 to 3-6), separate exposure 
scenarios for the consumption of contaminated fish by predatory mammals are not developed. 
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4.2.3.  Terrestrial Plants.  In general, the primary hazard to non-target terrestrial plants 
associated with the application of most herbicides is unintended direct deposition or spray drift. 
In addition, herbicides may be transported off-site by percolation or runoff or by wind erosion of 
soil. 
4.2.3.1. Direct Spray – Unintended direct spray will result in an exposure level equivalent to the 
application rate.  For many types of herbicide applications - e.g., rights-of-way management, it is 
plausible that some non-target plants immediately adjacent to the application site could be sprayed 
directly.  This type of scenario is modeled in the human health risk assessment for the 
consumption of contaminated vegetation.  As with any effective herbicide, it is likely that any 
non-target vegetation sprayed directly with sethoxydim at or near the range of recommended 
application rates would be damaged. 
4.2.3.2. Off-Site Drift – Data regarding the drift of sethoxydim during ground applications were 
not found in the literature. Because off-site drift is more or less a physical process that depends on 
droplet size and meteorological conditions rather than the specific properties of the herbicide, 
estimates of off-site drift can be made based on data for other compounds. 
Off-site drift will be estimated using AGDRIFT (Teske et al. 2001).  AGDRIFT is a model 
developed as a joint effort by the EPA Office of Research and Development and the Spray Drift 
Task Force, a coalition of pesticide registrants.  AGDRIFT is based on the algorithms in FSCBG 
(Teske and Curbishley.  1990), a drift model previously used by USDA.  AGDRIFT represents a 
detailed evaluation of a very large number of field studies and is likely to provide plausible 
estimates of drift.  Further details of AGDRIFT, including the executable file, are available at 
http://www.agdrift.com/.
 For aerial applications, AGDRIFT permits very detailed modeling of 
drift based on the chemical and physical properties of the applied product, the configuration of the 
aircraft, as well as wind speed and temperature.  For ground applications, AGDRIFT provides 
estimates of drift based solely on distance downwind as well as the types of ground application: 
low boom spray, high boom spray, and orchard airblast.  Representative estimates based on 
AGDRIFT (Version 1.16) are given in Worksheet A06b). 
Estimates of drift for ground applications is given in Worksheet A06.  Sethoxydim will typically 
be applied by low boom ground spray and thus these estimates are used in the current risk 
assessment. 
Drift distance can be estimated from a consideration of Stoke’s law, which describes the viscous 
drag on a moving sphere.  According to Stoke’s law: 
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