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

decreased to 0.2 mg/L when covered by 18 inches of water. Concentrations of carbaryl were
variable among sites. For example, at one of the sampling sites carbaryl was not detected until
the water was 7 inches deep. A peak concentration of 8 mg/L was detected at this site when the
water depth was 11 inches. This sample station was located 300 ft from a treated area.
Maximum carbaryl concentrations detected at other sites were 17.4, 7.8, and 4 mg/L with
samples collected at depths of 6, 4, and 13 inches from the bottom. Carbaryl’s primary toxic
degradate, 1-naphthol, was detected at concentrations as high as 1.4 mg/L. Average
concentrations of carbaryl detected in tide pools and small streams ranged from 3.6 to 11.2 mg/L.
The 11.2 mg/L detection was associated with application of 5 lbs carbaryl/acre. An average
concentration of 7.8 mg/L in tide pools and streams was associated with an application rate of 4
lbs carbaryl/acre, approximately half the maximum rate allowed. Similarly, a more recent study
found a peak of 820 μg/L in the water column 50 ft from the application site following a typical
treatment application at the maximum rate of 8 lbs/acre (n=3) (Weisskopf and Felsot 1998).
Several studies demonstrate that carbaryl dissipates fairly rapidly from over the treatment site
due to degradation, metabolism, dilution, and off-site transport (Hurlburt 1986; Creekman and
Hurlburt 1987; Tufts 1989; Tufts 1990; Weisskopf and Felsot 1998). In 2006 and 2007, samples
collected at the mouth of channels adjacent to treated oyster beds in Willapa Bay had maximum
concentrations of 29.1 µg/L after 6 h (high tide), 38 µg/L after 12 h (low tide), and 21.1 µg/L
after 24 h (low tide) (Major, Grue et al. 2005).
The NPDES permit for Willapa Bay and Grays Harbor requires annual monitoring of water
column concentrations in treated areas. It specifies an acute effluent limit of 3 µg/L and a
chronic limit of 0.06 µg/L. However, those data are of highly questionable value because the
monitoring plan specifies that monitoring is suspended for the first 48 h following application to
assess the acute effluent, and further suspends monitoring for 30 days from the last application to
assess the chronic limit. We expect that most of the carbaryl will be degraded and transported to
other locations by the time carbaryl monitoring is initiated.
Carbaryl sprayed on mud flats can be transported substantial distances at concentrations that may
have ecological impacts. Researchers found that close to 100% of Dungeness crabs were killed
376