Sethoxydim Risk Assessment

where v is the velocity of fall (cm sec
-1
), D is the diameter of the sphere (cm), g is the force of 
gravity (980 cm sec
-2
), and n is the viscosity of air (1.9 
@
 10
-4
 g sec
-1
 cm
-1
 at 20°C) (Goldstein et al. 
1974). 
In typical backpack ground sprays, droplet sizes are greater than 100 
:
, and the distance from the 
spray nozzle to the ground is 3 feet or less.  In mechanical sprays, raindrop nozzles might be used. 
These nozzles generate droplets that are usually greater than 400 
:
, and the maximum distance 
above the ground is about 6 feet.  In both cases, the sprays are directed downward. 
Thus, the amount of time required for a 100 µ droplet to fall 3 feet (91.4 cm) is approximately 3.2 
seconds, 
91.4 ÷ (2.87 
@
 10
5
(0.01)
2
). 
The comparable time for a 400 µ droplet to fall 6 feet (182.8 cm) is approximately 0.4 seconds, 
182.8 ÷ (2.87 
@
 10
5
(0.04)
2
). 
For most applications, the wind velocity will be no more than 5 miles/hour, which is equivalent to 
approximately 7.5 feet/second (1 mile/hour = 1.467 feet/second).  Assuming a wind direction 
perpendicular to the line of application, 100 
:
 particles falling from 3 feet above the surface could 
drift as far as 23 feet (3 seconds 
@
 7.5 feet/second).  A raindrop or 400 
:
 particle applied at 6 feet 
above the surface could drift about 3 feet (0.4 seconds 
@
 7.5 feet/second). 
For backpack applications, wind speeds of up to 15 miles/hour are allowed in Forest Service 
programs.  At this wind speed, a 100 
:
 droplet can drift as far as 68 feet (3 seconds 
@
 15 
@
 1.5 
feet/second).  Smaller droplets will of course drift further, and the proportion of these particles in 
the spray as well as the wind speed will affect the proportion of the applied herbicide that drifts 
off-site. 
4.2.3.3. Runoff – Sethoxydim or any other herbicide may be transported to off-site soil by runoff 
or percolation.  Both runoff and percolation are considered in estimating contamination of 
ambient water.  For assessing off-site soil contamination, however, only runoff is considered.  The 
approach is reasonable because off-site runoff will contaminate the off-site soil surface and could 
impact non-target plants.  Percolation, on the other hand, represents the amount of the herbicide 
that is transported below the root zone and thus may impact water quality but should not affect 
off-site vegetation. 
Based on the results of the GLEAMS modeling (Section 3.2.3.4.2), the proportion of the applied 
sethoxydim was estimated for clay, loam, and sand at rainfall rates ranging from 5 inches to 250 
inches per year.  These results are summarized in Worksheet G04. 
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4.2.3.4. Wind Erosion – Wind erosion is a major transport mechanism for soil (e.g., Winegardner 
1996) and is associated with the environmental transport of herbicides (Buser 1990).  Although 
numerous models were developed for wind erosion (e.g., Strek and Spaan 1997, Strek and Stein 
1997), the quantitative aspects of soil erosion by wind are extremely complex and site specific. 
Field studies conducted on agricultural sites found that annual wind erosion may account for soil 
losses ranging from 2 to 6.5 metric tons/ha (Allen and Fryrear 1977).  The upper range reported 
by Allen and Fryrear (1977) is nearly the same as the rate of 2.2 tons/acre (5.4 tons/ha) recently 
reported by the USDA (1998).  The temporal sequence of soil loss (i.e., the amount lost after a 
specific storm event involving high winds) depends heavily on soil characteristics as well as 
meteorological and topographical conditions. 
This risk assessment uses average soil losses ranging from 1 to 10 tons/ha
A
year, with a typical 
value of 5 tons/ha
A
year.  The value of 5 tons/ha
A
year is equivalent to 500 g/m
2
 [1 ton=1000 kg and 
1 ha = 10,000 m
2
] or 0.05 g/cm
2
 [1m
2
=10,000 cm
2
].  Thus, using a soil bulk density of 1.5 g/cm
3 
(Knisel et al.  1992, p. 56), the depth of soil removed from the surface per year would be 0.033 
cm[(0.05 g/cm
2
)÷ (1.5 g/cm
3
)].  The average amount per day would be about 0.00007 cm/day 
[0.033 cm per year ÷ 365 days/year].  The upper range of the typical daily loss would thus be 
about 0.00009 cm/day. 
The amount of sethoxydim that might be transported by wind erosion depends on  several factors, 
including the application, the depth of incorporation into the soil, the persistence in the soil, the 
wind speed, and the topographical and surface conditions of the soil.  Under desirable conditions, 
like relatively deep (10 cm) soil incorporation, low wind speed, and surface conditions that inhibit 
wind erosion, it is likely that wind transport of sethoxydim would be neither substantial nor 
significant. 
Any number of undesirable exposure scenarios could be constructed.  As a reasonable ‘worst 
case’ scenario, it is assumed that sethoxydim is applied to arid soil, that it is incorporated into the 
top 1 cm of soil, that minimal rainfall occurs for a 2-month period, that the degradation and 
dispersion of sethoxydim in the soil is negligible over the 2-month period, and that local 
conditions favor a high rate of soil loss (i.e., smooth, sandy surface with high wind speeds) that is 
a factor at  the upper limit of the typical rate (i.e., 0.00009 cm/day).  Under those conditions, 
0.0054 [0.00009 cm/day × 60 days ÷ 1 cm] of the applied sethoxydim would be lost due to wind 
erosion.  This is virtually identical to the estimates of off-site contamination from low-boom 
applications at a distance of 100 feet from the application site and is greater than drift that would 
be expected 500 feet offsite (0.0015 for low-boom applications from Worksheet A06) by a factor 
3.6 [0.0054 ÷ 0.0015 = 3.6].  Thus, in areas where wind erosion of soil may occur, wind erosion 
could be a more important mode of offsite movement than drift during application. 
The deposition of the sethoxydim contaminated soil also will vary substantially with local 
conditions.  Under desirable conditions, the soil might be dispersed over a very large area and be 
of no toxicological consequence.  In some cases, however, local topographical conditions might 
favor the deposition and concentration of contaminated dust from a large treated area into a 
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