Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 112

Although the monitoring data on CPY provide relevant insight in quantifying the
range of concentrations in surface waters, few monitoring programs have sampled
at a frequency sufficient to quantify the time-series pattern of exposure. Therefore,
numerical simulations were used to characterize concentrations of CPY in water
and sediment for three representative high exposure environments in the U.S. The
fate of CPY in the environment is dependent on a number of dissipation and degra-
dation processes. In terms of surface waters, fate in soils is a major driver of the
potential for runoff into surface waters and results from a number of dissipation
studies in the laboratory were characterized. Aerobic degradation of CPY exhibits
bi-phasic behavior in some soils; initial rates of degradation are greater than overall
rates by factors of up to threefold. Along with fate in water, these data were
considered in selecting parameters for the modeling concentrations in surface
waters. An assessment of vulnerability to runoff was conducted to characterize the
potential for CPY to be transported beyond a treated field in runoff water and eroded
sediment across the conterminous U.S. A sensitivity analysis was performed on use
practices of CPY to determine conditions that resulted in the highest potential run-
off of CPY to aquatic systems to narrow the application practices and geographical
areas of the country for selecting watersheds for detailed modeling. The selected
focus-watersheds were Dry Creek in Georgia (production of pecans), Cedar Creek
in Michigan (cherries), and Orestimba Creek in California (intensive agricultural
uses). These watersheds provided realistic but reasonable worst-case predictions of
concentrations of CPY in water and sediment.
Estimated concentrations of CPY in water for the three watersheds were in gen-
eral agreement with ambient monitoring data from 2002 to 2010 in the datasets from
US Geological Survey (USGS), California Department of Pesticide Regulation
(CDPR), and Washington State Department of Ecology (WDOE). Maximum daily
concentrations predicted for the watershed in California, Georgia, and Michigan
were 3.2, 0.041, and 0.073 μg L −1 , respectively, with the 28-d aerobic soil metabo-
lism half-life and 4.5, 0.042, and 0.122 μg L −1 , respectively, with the 96-d soil half-
life. These estimated values compared favorably with maximum concentrations
measured in surface water, which ranged from 0.33 to 3.96 μg L −1 . For sediments,
the maximum daily concentrations predicted for the watersheds in California,
Georgia, and Michigan were 11.2, 0.077, and 0.058 μg kg −1 , respectively, with the
28-d half-life and 22.8, 0.080, and 0.087 μg kg −1 , respectively, with the 96-d soil
half-life. CYP was detected in 12 samples (10%) out of 123 sample analyses that
existed in the USGS, CDPR, and WDOE databases. The concentrations reported in
these detections were from <2.0, up to 19 μg kg −1 , with the exception of one value
reported at 58.6 μg kg −1 . Again, the modeled values compared favorably with these
measured values. Duration and recovery intervals between toxicity threshold con-
centrations of 0.1 and 1.0 μg L −1 were also computed. Based on modeling with the
half-life of 28 d, no exceedance events were identified in the focus watersheds in
Georgia or Michigan. Using the half-life of 96 d, only three events of 1-d duration
only were identified in the Michigan focus-watershed. Frequency of exceedance was
greater in the California focus watershed, though the median duration was only 1-d.