Environmental Engineering Reference
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modeled values compared favorably with measured values. Duration and recovery
intervals between CPY water concentrations that exceeded two different threshold
values derived from toxicity data of Giddings et al. (
2014
) were also computed.
Based on modeling with the half-life of 28 d, no exceedances were identified in the
focus watersheds in Georgia or Michigan. Using the half-life of 96 d, only three
exceedance events of 1-d duration each were identified in the Michigan focus-
watershed. Frequency of exceedance was greater in the focus watershed from
California. There were 10-35 exceedance events (depending on threshold level)
during the 30-yr simulation period, or an average of less than one per year. Moreover,
even in this worst-case focus-watershed, the median event exceedance duration was
1 d. The greater concentrations in Orestimba Creek are attributed to a higher fre-
quency of applications, a higher frequency of runoff events (due to irrigation tail-
water), and less stream-flow for dilution.
Several advantages and insights gained from the modeling of concentrations of
CPY in surface waters were developed from range of modeling assumptions and
field conditions simulated. Conservative assumptions were used in the modeling of
concentrations in the focus-watersheds. Studies on environmental-fate of CPY have
demonstrated a range of rates of degradation in soil, crops, and aquatic systems.
Values from these data were selected to represent appropriate degradation and loss
processes in the modeling. For example, the 90th centile confidence interval on the
mean half-life was selected to err on the side of caution. Conservative assumptions
were also used in configuring the Georgia and Michigan watersheds, in that all eli-
gible crop acreage in each watershed was represented as if it were pecan or cherries,
respectively; thereby, the soil properties and applications of CPY represented by the
use-pattern selected produced the greatest estimates of exposure-concentrations.
Model simulations for Orestimba Creek used reported applications of CPY, but field
specific management practices to mitigate runoff and drift were not represented in
the simulations. In addition, volatilization was not included in the California simula-
tions. CPY drift for all three watersheds was assumed to occur with all treated crops
having a proximity to water equal to the minimum setback requirements on the
product label. The setback was used to simulate drift reductions to water, but reduc-
tions in pesticide loadings in runoff were not assumed to occur in the setback area.
Opportunities to verify the focus-watershed model results were limited for this
study. No calibration of runoff water, soil erosion, or CPY properties was con-
ducted. However, it was possible to model several field-specific runoff studies
(Cryer and Dixon-White
1995
; McCall et al.
1984
; Poletika and Robb
1994
; Racke
1993
) by using the environmental fate properties employed in the modeling of the
focus watersheds. Predicted volumes of runoff water, sediment, and CPY concen-
trations in runoff water and in eroded sediment were within an order of magnitude
of the amounts measured in runoff studies, and they were neither consistently high
nor consistently low, suggestive of a lack of model prediction bias.
The analyses used to characterize runoff of CPY at the national level incorpo-
rated a number of generalizations. For this reason, these analyses were only used to
evaluate the relative potential for runoff of CPY as a guide in selecting the focus
watersheds. The most significant generalization was representing all simulations as
a generic crop (corn) with a unit application rate. Applications were set to occur
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