Abstract
The U.S. Geological Survey monitored atmospheric
deposition of pesticides in the Central Valley of California
during two studies in 2001 and 2002–04. The 2001 study
sampled wet deposition (rain) and storm-drain runoff in the
Modesto, California, area during the orchard dormant-spray
season to examine the contribution of pesticide concentrations
to storm runoff from rainfall. In the 2002–04 study, the
number and extent of collection sites in the Central Valley
were increased to determine the areal distribution of organophosphate
insecticides and other pesticides, and also five more
sample types were collected. These were dry deposition, bulk
deposition, and three sample types collected from a soil box:
aqueous phase in runoff, suspended sediment in runoff, and
surficial-soil samples. This report provides concentration data
and describes methods and quality assurance of sample collection
and laboratory analysis for pesticide compounds in all
samples collected from 16 sites. Each sample was analyzed
for 41 currently used pesticides and 23 pesticide degradates,
including oxygen analogs (oxons) of 9 organophosphate
insecticides. Analytical results are presented by sample type
and study period.
The median concentrations of both chloryprifos and
diazinon sampled at four urban (0.067 micrograms per liter
[μg/L] and 0.515 μg/L, respectively) and four agricultural
sites (0.079 μg/L and 0.583 μg/L, respectively) during a
January
2001 storm event in and around Modesto, Calif.,
were nearly identical, indicating that the overall atmospheric
burden in the region appeared to be fairly similar during the
sampling event. Comparisons of median concentrations in the
rainfall to those in the McHenry storm-drain runoff showed
that, for some compounds, rainfall contributed a substantial
percentage of the concentration in the runoff; for other
compounds, the concentrations in rainfall were much greater
than in the runoff. For example, diazinon concentrations in
rainfall were about 70 percent of the diazinon concentration
in the runoff, whereas the chlorpyrifos concentration in
the rain was 1.8 times greater than in the runoff. The more
water-soluble pesticides—carbaryl, metolachlor, napropamide,
and simazine—followed the same pattern as diazinon and had
lower concentrations in rain compared to runoff. Similar to
chlorpyrifos,compounds with low water solubilities and higher
soil-organic carbon partition coefficients, including dacthal,
pendimethalin, and trifluralin, were found to have higher
concentrations in rain than in runoff water and were presumed
to partition to the suspended sediments and organic matter on
the ground.
During the 2002–04 study period, the herbicide dacthal
had the highest detection frequencies for all sample types
collected from the Central Valley sites (67–100 percent). The
most frequently detected compounds in the wet-deposition
samples were dacthal, diazinon, chlorpyrifos, and simazine
(greater than 90 percent). The median wet-deposition amounts
for these compounds were 0.044 micrograms per square meter
per day (μg/m2/day), 0.209 μg/m2/day, 0.079 μg/m2/day, and
0.172 μg/m2/day, respectively. For the dry-deposition samples,
detection frequencies were greater than 73 percent for the
compounds dacthal, metolachor, and chlorpyrifos, and median
deposition amounts were an order of magnitude less than for
wet deposition. The differences between wet deposition and
dry deposition appeared to be closely related to the Henry’s
Law (H) constant of each compound, although the mass
deposited by dry deposition takes place over a much longer
time frame.
Pesticides detected in rainfall usually were detected in the
aqueous phase of the soil-box runoff water, and the runoff concentrations
were generally similar to those in the rainfall. For
compounds detected in the aqueous phase and suspended-sediment
samples of soil-box runoff, concentrations of pesticides
in the aqueous phase generally were detected in low concentrations
and had few corresponding detections in the suspended-
sediment samples. Dacthal, diazinon, chlorpyrifos, and
simazine were the most frequently detected pesticides (greater
than 83 percent) in the aqueous-phase samples, with median
concentrations of 0.010 μg/L, 0.045 μg/L, 0.016 μg/L, and
0.077 μg/L, respectively. Simazine was the most frequently
detected compound in the suspended-sediment samples
(69 percent), with a median concentration of 0.232 μg/L.
Results for compounds detected in the surficial-soil
samples collected throughout the study period showed that
there was an increase in concentration for some compounds,
indicating atmospheric deposition of these compounds onto
the soil-box surface. In the San Joaquin Valley, the compounds
chlorpyrifos, dacthal, and iprodione were detected at
higher concentrations (between 1.4 and 2 times greater) than
were found in the background samples collected from the
San Joaquin Valley soil-box sites. In the Sacramento Valley,
the compounds chlorpyrifos, dacthal, iprodione, parathionmethyl,
and its oxygen analog, paraoxon-methyl, were
detected in samples collected during the study period in low
concentrations, but were not detected in the background concentration
of the Sacramento Valley soil mix.