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

potential ingredients in tank mixtures were not addressed in EPA’s BEs and remain a significant
source of uncertainty.
Environmental Mixtures
As described in the Approach to the Assessment, we analyze the status of listed species, in
conjuction with the Environmental Baseline in evaluating the likelihood that action stressors will
reduce the viability of populations of listed salmonids. This involves considering interactions
between the stressors of the action and the Environmental Baseline. For example, we consider
that listed salmonids may be exposed to the wide array of chemical stressors that occur in the
various marine, estuarine, and freshwater habitats they occupy throughout their life cycle.
Exposure to multiple pesticide ingredients is most likely in freshwater habitats and nearshore
environments adjacent to areas where pesticides are used. As of 1997, about 900 a.i.s were
registered in the U.S. for use in more than 20,000 different pesticide products (Aspelin and
Grube 1999). Typically 10 to 20 new a.i.s are registered each year (Aspelin and Grube 1999). In
a typical year in the U.S., pesticides are applied at a rate of approximately five billion lbs of a.i.
per year (Kiely, Donaldson et al. 2004). Pesticide contamination in the nation’s freshwater
habitats is ubiquitous and pesticides usually occur in the environment as mixtures (Gilliom,
Barbash et al. 2006). “More than 90% of the time, water from streams with agricultural, urban,
or mixed-land-use watersheds had detections of two or more pesticides or degradates, and about
20% of the time they had detections of 10 or more (Gilliom, Barbash et al. 2006).” The
likelihood of exposure to multiple pesticides throughout a listed salmonids’ lifetime is great
considering their migration routes and habitats occupied for spawning and rearing. In a threeyear
monitoring study conducted by the Washington DOE, pesticide mixtures were found to be
common in both urban and agricultural watersheds (Burke, Anderson et al. 2006). An average of
three pesticides was found in each sample collected on urban sampling sites with as many as nine
pesticides found in a single sample. Agricultural sites averaged three to five pesticides per
sample with as many as 14 pesticides being detected in a single sample (Burke, Anderson et al.
2006). Mixtures of chemicals that share a common mode or mechanism of action are of
particular concern. Six to 11 million lbs of cholinesterase-inhibiting insecticides are used
annually in California (CDPR 2007). One a.i., thiodicarb, degrades into methomyl. Potential
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