Sethoxydim Risk Assessment

environments, the actual lower limit would approach zero.  The resulting estimates of peak 
concentrations of sethoxydim in a small stream based on the application rates that might be used 
by the Forest Service are given in Worksheet B06. 
3.2.3.4.2.  LONGER-TERM EXPOSURE -- The scenario for chronic exposure to sethoxydim 
from contaminated water is detailed in worksheet D07.  This scenario assumes that an adult (70 
kg male) consumes contaminated ambient water for a lifetime.  As with the above stream 
scenario, there are no monitoring studies available on sethoxydim that permit an assessment of 
concentrations in ambient water associated with applications of sethoxydim.  Consequently, for 
this component of the exposure assessment, estimates of levels in a small pond are based on 
GLEAMS modeling as detailed in the previous section.  The specific methods used to calculate 
the concentration of sethoxydim in a small pond based on the GLEAMS output are detailed in 
Section 5.4 of Attachment 2. 
The results of the GLEAMS modeling for the pond is summarized in Table 3-5 and the specific 
estimates of concentrations of sethoxydim in ambient water that are used in this risk assessment 
are summarized in Worksheet B06.  As with the corresponding values for a small stream, these 
estimates are expressed as the water contamination rates (WCR) in units of mg/L per lb/acre. 
The typical WCR is taken as 0.0008 mg/L.  This is about the average concentration that could be 
expected at rainfall rates of about 100 inches per year from loam – i.e., 0.81 µg/L in Table 3-4. 
The upper limit is taken as 0.0012 mg/L, approximately the longer-term average concentration 
from loam soils at rainfall rates of 250 inches per year – i.e., 1.23 µg/L in Table 3-4.  The lower 
limit of the WCR is taken as 0.00002 mg/L, the average concentration from loam soil at an annual 
rainfall rate of 10 inches per year – i.e., 0.02 µg/L in Table 3-4. 
Using these water contamination rates, the expected concentrations of sethoxydim in ambient 
water range from about 0.0000019 to 0.00045 mg/L with a central value of 0.00024 mg/L.  These 
values are used in all of the worksheets involving long-term exposures to contaminated water 
(e.g., Worksheet D07). 
3.2.3.5. Oral Exposure from Contaminated Fish  -- Many chemicals may be concentrated or 
partitioned from water into the tissues of animals or plants in the water.  This process is referred 
to as bioconcentration.  Generally, bioconcentration is measured as the ratio of the concentration 
in the organism to the concentration in the water.  For example, if the concentration in the 
organism is 5 mg/kg and the concentration in the water is 1 mg/L, the bioconcentration factor 
(BCF) is 5 L/kg [5 mg/kg ÷ 1 mg/L].  As with most absorption processes, bioconcentration 
depends initially on the duration of exposure but eventually reaches steady state.  Details 
regarding the relationship of bioconcentration factor to standard pharmacokinetic principles are 
provided in Calabrese and Baldwin (1993). 
Several studies are available on the bioconcentration of sethoxydim (Appendix 3).  In catfish, 
bioconcentration factors in whole fish and edible tissue have been measured at 0.75 and 0.71, 
respectively (BASF 1982).  In other words, the concentration in the fish tissue was less than the 
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concentration in water.  Substantially higher bioconcentration factors have been measured in 
bluegill sunfish.  In an early study (Vilkas and Kuc 1981a), bioconcentration factors of 6.98 in 
whole body and 2.87 in edible tissue were measured.  In a more recent study (McKenna and Patel 
1991), somewhat higher bioconcentration factors are reported for bluegills: 21 in whole body and 
7 in edible tissue. 
For this risk assessment, the higher values from McKenna and Patel (1991) are used.  For the 
human health risk assessment, the BCF of 7 in edible tissue is used for the chronic risk 
assessment.  For the acute risk assessment, the BCF is adjusted for the expected bioconcentration 
after 1 day.  As summarized in Appendix 3, the elimination half-life of sethoxydim residue in fish 
was 3.6 days, corresponding to an elimination coefficient of 0.19 days
-1
 [ln(2)÷3.6 days].  Thus, 
the proportion to steady-state after one day would be 0.173 [1-e
-0.19/day ×1day
] and the estimated 
one-day bioconcentration factor is 1.211 [0.173×7].  For the acute risk assessment, this BCF is 
rounded to 1.2 as summarized in Worksheet B02. 
For both the acute and longer-term exposure scenarios involving the consumption of 
contaminated fish, the water concentrations of sethoxydim used are identical to the concentrations 
used in the contaminated water scenarios (see Section 3.2.3.4).  Because of the available and 
well-documented information and substantial differences in the amount of caught fish consumed 
by the general public and native American subsistence populations (U.S. EPA/ORD 1996), 
separate exposure estimates are made for these two groups (Worksheets D08a and D08b).  The 
chronic exposure scenarios (Worksheet D09a and D09b) are constructed in a similar way, except 
that estimates of sethoxydim concentrations in ambient water are based on GLEAMS modeling as 
discussed in Section 3.2.3.4. 
For the acute scenarios, the consumption of contaminated fish is based on the maximum amount 
of fish that an individual might consume in a single day.  For the chronic scenarios, the 
consumption of contaminated fish is based on the average amount of fish that an individual might 
consume in a single day.  These values and the documentation for these values are given in 
Worksheet A03. 
3.2.3.6. Oral Exposure from Contaminated Vegetation -- Under normal circumstances and in 
most types of applications, it is extremely unlikely that humans will consume vegetation 
contaminated with sethoxydim.  Any number of accidental scenarios could be developed involving 
either spraying of crops, gardens, or edible wild vegetation.  Again, in most instances and 
particularly for longer-term scenarios, treated vegetation would probably show signs of damage 
from exposure to sethoxydim (Section 4.3.2.4), thereby reducing the likelihood of consumption 
that would lead to significant levels of human exposure. 
Notwithstanding that assertion, it is conceivable that individuals could consume contaminated 
vegetation that is accidentally sprayed.  One of the more plausible scenarios involves the 
consumption of contaminated berries after the accidental spray of an area in which wild berries 
grow.  The two accidental exposure scenarios developed for this exposure assessment include one 
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