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

salmonids. The 85.1 μg/L represents one of the higher values observed in the monitoring data
despite the low use rate (0.5 lbs a.i./acre) and no-spray buffer of almost 500 ft.
Table 59. Mean concentration (μg/L) of carbaryl in the Little Missouri River following rangeland
application of Sevin-4-Oil
Hours after application
1 2 4 8 12 24 96
1991 85.1 - 12.3 3.08 5.30 10.3 0.100
1993 12.0 12.6 3.84 4.01 - 4.51 5.14
-No Data
The CDPR database contained data from several studies conducted by CDPR in the 1990s
evaluating concentrations of rice chemicals in the Colusa Basin Drain, Butte Slough, and the
Sacramento River (referenced in their metadata as studies 17, 30, 34, and 40). Carbofuran, along
with molinate, malathion, methyl parathion, and thiobencarb, was one of the analytes in all years.
Some years, analytes also included other pesticides such as 2,4-D, triclopyr, and propanil
(Norberg-King, Durhan et al. 1991). Sampling was conducted in March-July. Carbofuran was
quantifiable in 100% of the samples for which it was an analyte. Concentrations across all years
ranged from 0.087 – 2.97 μg/L (Table 60). Another study located in open literature, conducted
in 1991-1992 (Crepeau and Kuivila 2000), measured peak concentrations in the Colusa River
Basin of 0.6-1.1 μg/L. This study also measured concentrations in the Sacramento River at
Sacramento and the Sacramento River at Rio Vista. Carbofuran concentrations in the river were
typically an order of magnitude lower than the concentrations in the Colusa Basin Drain. The
CDPR studies included acute biotoxicity studies (test organism not specified in metadata) and
generally found no significant toxicity. However, a study on water from the Colusa Basin Drain
using Ceriodaphnia dubia as the test organism in a Toxicity Indicator Evaluation (TIE)
procedure found that an extract of Colusa Basin Drain water caused toxicity in laboratory tests
(Norberg-King, Durhan et al. 1991) The procedure indicated that carbofuran and methyl
parathion accounted for the toxicity of the sample, although other chemicals such as molinate
and thiobencarb were present as well. The concentrations observed in surface water monitoring
are, as expected; much lower than those predicted for the flooded rice paddies themselves. It
should be noted that California requires rice growers to obtain discharge permits or get
conditional waivers before discharging water from treated rice fields. Obtaining the permits or
waivers typically requires holding periods following pesticide applications, which helps reduce
300