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

Carbaryl can also be applied at 8 lbs a.i./acre in estuarine areas in Washington state to kill ghost
shrimp and mud shrimp in commercial oyster beds. Application of 8 lbs of carbaryl to a static
water body of 10 cm deep would result in an initial average concentration of approximately 9
mg/L (8,968 μg/L). This estimate does not consider tidal influences. The 24(c) label for the use
of carbaryl on oyster beds does not specify when applications are to be made with relation to
incoming or outgoing tides. The label does specify applicators must obtain an NPDES permit for
this use. A current NPDES permit specifies that applications to these tidal areas are made when
the beds are uncovered by the outgoing tide. Peak concentrations in the water column are
expected as the treated area is re-flooded by the incoming tide. These concentrations are
expected to decrease as the volume of water over the treated area increases. The area
contaminated by the carbaryl application will increase beyond the treatment site, and water
column concentrations will simultaneously decrease due to transport and dilution from the
incoming tide. This use is discussed in more detail in the Risk Characterization portion of the
Opinion (see Field studies in ESA-listed salmonid habitat).
Table 51. Average initial concentration of any a.i. in surface water resulting from a direct
overspray of aquatic habitat.
Application Rate Water Depth A.i. Concentration in Surface
Water
(lbs a.i. / acre) (meters) (μg/L)
0.25 2 14
0.5 2 28
1 2 56
3 2 168
10 2 560
0.25 1 28
0.5 1 56
1 1 112
3 1 336
10 1 1121
0.25 0.5 56
0.5 0.5 112
1 0.5 224
3 0.5 673
10 0.5 2242
0.25 0.3 93
0.5 0.3 187
1 0.3 374
3 0.3 1121
10 0.3 3736
287