Environmental Engineering Reference
In-Depth Information
long-term trend models, six sites had a sufficient number of uncensored concentra-
tions of CPY for analysis of flow-adjusted concentrations. Three of the six sites had
significant downward trends, one had a significant upward trend (Zollner Creek,
OR; a small agricultural site), and the other two sites had no significant trends.
Downward trends in concentrations of CPY at the small urban and California agri-
cultural sites appear to reflect the phase-out of CPY and/or reduction in its use.
Upward trends in the Yakima River, WA and Zollner Creek, OR might be due to
increases in use from an increase in the planted acreage of certain crops (e.g., corn)
and/or restrictions on the use of other organophosphate insecticides.
Overall, these three USGS analyses of the Martin and Eberle (
2009
) data gener-
ally indicate trends of decreasing pesticide concentrations, including CPY, in corn
belt streams and rivers (Sullivan et al.
2009
), urban streams in four regions of the
U.S. (Ryberg et al.
2010
), and in small urban and agricultural sites in California and
the Pacific Northwest (Johnson et al.
2011
).
3
Modeling of Chlorpyrifos in Aquatic Environments
Collectively, the CPY monitoring data provided useful and relevant insight towards
quantifying the range of concentrations expected in surface waters. However, rela-
tively few monitoring programs have sampled at a frequency sufficient to quantify
the time-series pattern of exposure. Therefore, numerical simulations were used to
characterize CPY concentrations in water and sediment for three representative high
exposure environments in the United States. The environments were selected by
parallel examination of use intensity across the U.S., susceptibility of CPY to runoff
with respect to soil and weather variability across the U.S., and a sensitivity analysis
of CPY runoff potential for various patterns of use. From the analyses, three geo-
graphical regions, each defined as several contiguous counties, were identified as
having greater potential exposure to CPY. These regions were in central California,
southwestern Georgia, and the Leelanau peninsula of Michigan. A small watershed,
defined as having a 3rd order stream outlet, was selected from each region based on
having a high density of cropland eligible for receiving CPY applications according
to registered uses. Models were configured for each watershed and simulations con-
ducted for up to 30 yr of consecutive CPY use using historical weather records for
each region modeled. Daily mean concentrations of CPY in water and sediment
from runoff, erosion, and drift sources were predicted at the watershed outlets.
3.1
Selection and Justification of the Models
Several models were used, the number of which depended on the level of detail
needed for the specific phase of the assessment. Models were selected for their abil-
ity to represent the key fate and transport processes of CPY. Based on use practices
Search WWH ::
Custom Search