Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

estimate do not represent the highest aquatic concentrations predicted given the
assumptions. Rather, the exposure estimates provided in the BEs are time-weighted
average concentrations for one day (i.e., peak), 21 d, and 60 d. Although EPA refers to
the 1 d-averages as peak concentrations, they do not represent the maximum
concentration predicted. Rather, they represent the average concentration over a 24-hour
(h) period. Additionally, the concentrations reported represent the 90 th percentile of the
estimates derived using PRZM-EXAMS (Lin 1998). Although NMFS agrees this is a
relatively protective approach for evaluating exposure in most aquatic habitats, it does
not represent the possible “worst case” exposure.
Assumption 2: Model inputs used the highest use rates and greatest number of
applications. NMFS compiled information on the maximum use rates permitted (single
and seasonal), number of applications allowed, and minimum application intervals
required through EPA product labeling (Tables 1-3).
Several of the PRZM-EXAMS scenarios did not match up well with current use
restrictions and did not account for maximum permitted application rates. For example,
PRZM-EXAMS scenarios for carbaryl evaluated a maximum single application rate of
3.5 lb a.i./acre versus higher single application rates authorized by EPA for oyster beds,
home lawn, golf course, and sod farms (8-9.1 lb a.i./acre), nut trees (5 lbs a.i./acre), and
citrus (7.5 – 12 lbs a.i./acre). Some of the simulations included multiple applications that
exceed single application rates. For example, there were simulations for 4 applications of
5 lbs a.i./acre in citrus and 2 applications of 7 lbs a.i./acre in peaches. However, these
scenarios do not cover the maximum rate of carbaryl allowed per applications (12 lbs
a.i./acre in California citrus) or annually (24 lbs a.i./acre/year in apple, pear, crabapple,
and others). The maximum single application rate for methomyl is 0.9 lbs a.i./acre. One
scenario assumed 3 applications of 1.8 lbs a.i./acre (California peaches). This scenario
may overestimate aquatic concentrations for single applications at comparable use sites.
However, considering multiple applications, the maximum application rate of methomyl
evaluated in the BE was 9 lbs a.i./acre (lettuce) versus allowable application rates of 10-
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