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

EECs in the BEs may underestimate exposure of some listed organisms and designated
critical habitat.
Although EPA characterized these exposure estimates as “worst case” in the BEs, it has
also acknowledged that measured concentrations in the environment sometimes exceed
PRZM-EXAMs EECs (EPA 2007). EPA has subsequently clarified that rather than
providing worst case estimates, PRZM-EXAMS estimates are protective for the vast
majority of applications and aquatic habitats (EPA 2007). NMFS agrees that the model is
designed to produce estimates of exposure that are protective for a large number of
aquatic habitats.
Recent formal consultation and reviews of EPA informal consultations by the Services
found that concentrations measured in surface water sometimes exceed peak
concentrations predicted with PRZM/EXAMS modeling (NMFS 2007; NMFS 2008;
USFWS 2008). Concentrations of carbaryl, carbofuran, and methomyl were generally
less than concentrations predicted in the BEs using PRZM-EXAMs although there were
examples where measured concentrations in surface water exceeded these estimates (e.g.,
(Hurlburt 1986; Creekman and Hurlburt 1987; Tufts 1989; Tufts 1990; Beyers, Farmer et
al. 1995). These findings demonstrate that the EECs generated using PRZM-EXAMS
can underestimate peak concentrations that actually occur in some aquatic habitats, and
therefore, peak exposure experienced by some individuals of listed species may be
underestimated.
Exposure values derived by EPA using PRZM-EXAMS may underestimate or
overestimate salmonid exposure to carbaryl, carbofuran, and methomyl. Two
assumptions are discussed below that show salmonids may be exposed to higher
concentrations than predicted with PRZM-EXAMS modeling:
Assumption 1: Model outputs are 90th percentile time­weighted averages. It is
important to recognize that the model predicts concentrations based on site-specific
assumptions (e.g., rainfall) and that environmental concentrations provided for the
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