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

Measured Concentrations in Willapa Bay and Grays Harbor following Carbaryl Applications
Numerous monitoring studies have been conducted in coordination with applications of carbaryl
to control burrowing shrimp in commercial oyster beds in Washington State. The results of the
studies are quite variable, as are the study designs and objectives. Several investigations have
documented water column concentrations exceeding several mg/L (which is substantially higher
than the μg/L concentrations observed in freshwater aquatic habitats) following the first flood
tide post-application (Hurlburt 1986; Creekman and Hurlburt 1987; Tufts 1989; Tufts 1990).
Water column concentrations measured at Willapa Bay in 1984 detected a mean concentration of
10.6 mg/L upon initial flooding of the treated area (Hurlburt 1986). Concentrations were
monitored at varying depths for several hours following flooding. The concentrations decreased
rapidly but were detectable in the low μg/L range throughout sampling. In 1985 samples were
collected along transects from treated areas following the direction of the tide to monitor the
dispersal of carbaryl (Creekman and Hurlburt 1987). The data indicate that carbaryl is
transported off the treated area by the tide. Average concentrations measured above the treated
area ranged from 2.4 – 5.5 mg/L. The highest average concentration (7.9 mg/L) was collected
510 ft from the treated area. Average concentrations at the most distant sampling point (650 ft
from initial treated area) were 2.5 mg/L. In 1996, further study evaluated carbaryl movement off
the treated spray area (Tufts 1989). Samples were collected along a transect corresponding to the
direction of the incoming tide. The peak concentration measured was 27.8 mg/L. Over one
treated area, the concentration of carbaryl decreased from 13.2 mg/L to 9.3 mg/L as the water
depth increased from 1.5 to 10 inches. The concentration further decreased to 600 µg/L when
the water rose to 16 inches. The monitoring indicated concentrations of carbaryl decreased with
increasing distances along the transect, but concentrations greater than 1 mg/L were detected in
several instances at distances several hundred feet from treated plots. Carbaryl was found in the
water column at the detection limit (0.1 mg/L) as far as 1,725 ft from the treated area.
In 1987, surface water monitoring was conducted in Willapa Bay to determine “the dilution
pattern of carbaryl washed from sprayed oyster beds, and to determine carbaryl concentrations in
shallow pools and streams where marine fish might be found” (Tufts 1990). Carbaryl
concentrations in the mg/L range were frequently detected with the incoming tide. Peak
concentration for one sample site was 18.8 mg/L upon initial flooding. The concentrations
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