Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 109

maxima represent approximately 17,700 sample analyses (911 locations) during
2002-2010 that were collected following the ban on residential uses and other label
changes of CPY ca. 2001. However, detections are relatively infrequent.
Characterization of surface water CPY concentrations in this 9-yr period from
regional data sets that focused on use areas indicated that only 13-17% of collected
samples contained detectable CPY concentrations. The 95th centile concentrations
from these programs ranged from <0.01 to 0.3 μg L −1 across years.
Similarly, CPYO was not detected frequently or in large concentrations. The
databases of the USGS, CDPR, and WDOE contained reports of only 25 detections
of CPYO from 10,375 analyses (0.24% of samples) in surface water between 1999
and 2012. The concentrations reported in the 16 detections reported in the USGS
databases (9,123) were all below the LOQ and neither the CDPR nor the WDOE
databases contained any reports of detections of CPYO in surface waters.
One of the inherent limitations of monitoring data is that it may not be wholly
representative of all locations. Because of logistical and resource issues, not all
locations can be sampled with a frequency and duration that fully characterizes
peaks in exposures, especially those of short duration. Thus, modeling was used to
provide estimates of concentrations in surface waters across the country. From
knowledge of rates of and frequency of application, environmental settings were
identified as being more susceptible to runoff and CPY drift into surface water.
These included areas of higher rainfall and heavier soils and those receiving the
greatest single application rate or shortest intervals between multiple applications as
identified from the sensitivity analysis of use-patterns.
Because it was not possible to model all locations in detail and all scenarios of
CPY use in the USA, the analyses of sensitivity were used to identify three focus
watersheds—Dry Creek in Georgia, Cedar Creek in Michigan, and Orestimba Creek
in California. These watersheds were intended to provide realistic but reasonable
worst-case predictions of concentrations of CPY in runoff water and sediment. The
soil-water hydrologic model SWAT was combined with PRZM to predict these con-
centrations using a 30-y climatic record (1961-1990 for Dry Creek and Cedar Creek
and 2000-2008 for Orestimba Creek) and local cropping and soils information. Two
half-lives for aerobic soil metabolism of CPY in soil, 28 and 96 d, were selected for
the purposes of modeling.
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 of
the USGS, CDPR, and WDOE. Maximum daily concentrations predicted for the
California, Georgia, and Michigan watersheds were 3.2, 0.041, and 0.073 μg L −1 ,
respectively, with the 28-d aerobic soil metabolism half-life and 4.5, 0.042, and
0.122 μg L −1 , respectively, with the 96-d soil half-life. These 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
California, Georgia, and Michigan watersheds 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. Twelve detections out of 123 analyses were contained in
the USGS, CDPR, and WDOE databases with concentrations reported from <2.0,
up to 19 μg kg −1 with the exception of one value reported at 58.6 μg kg −1 . Again, the