Environmental Engineering Reference
In-Depth Information
granular and 529 μg L
−1
for spray applications. Given the similarity of the pore
water and puddle water values, only the highest value, 571 μg L
−1
was selected for
use in the risk assessment. With a daily intake of 47 μL
−1
, the predicted 95th centile
of the maximum daily dose was 0.027 μg bee
−1
d
−1
. The corresponding value for
honey bees collecting 1,250 μL of water d
−1
for the hive is 0.71 μg bee
−1
d
−1
. These
estimates include peak values after storm events and are much greater than the
equilibrium-based values in the Tier-1 Rice model. Exposure to these values is pos-
sible but depends on a combination of probabilities, limited to only a few use sce-
narios. In many use scenarios that were run in the PRZM/EXAMS model, the
median predicted puddle concentrations were zero due to the large time interval
between application and isolated heavy storm events during the 30-yr simulation
interval.
Dew and wet foliage
. The USEPA recommends a conservative (protective) equi-
librium partition model based on pesticide
K
OC
and plant carbon content to estimate
pesticide concentrations in dew (USEPA
2012
) (Equation 2).
C
Kf
()
plant t
(2)
C
=
()
dewt
´
oc
oc
Where:
C
dew
(
t
)
is the concentration of dissolved pesticide in dew (mg L
−1
);
C
plant
(
t
)
is the concentration of pesticide on and in plant leaves (mg kg
−1
(fresh weight)) at
time
t
and was set at 240 mg CPY kg
−1
foliage, corresponding to T-REX concentra-
tions on short grass; and
f
OC
is the fraction of organic carbon in leaves, set at 0.04
(4% of fresh wt) based on estimates of carbon in plants (Donahue et al.
1983
) and
water content (Raven et al.
1992
). As with the puddle model, this is an equilibrium
equation and the concentration does not increase as the water dries on the surface.
Partition into rainwater that remains on foliage after a rainfall is expected to be simi-
lar without runoff, or less if runoff occurs and reduces the amount of residue left on
the leaf surface. Using the mean
K
OC
of 8,216 (Solomon et al.
2014
),
C
dew
(
0
)
for a
spray application at 1.12 kg CPY ha
−1
is 730 μg L
−1
. If a bee consumed 100% of its
daily drinking water from contaminated dew and has an intake of 47 μL d
−1
, this
model predicts a point estimate dose of 0.034 μg CPY bee
−1
d
−1
. If the intake was
250 μL d
−1
, the dose would be 0.18 μg CPY bee
−1
d
−1
.
A second estimate of exposure for dew and/or wet foliage was obtained using the
LiquiPARAM model, which gives both a mean and an estimate of variability (Moore
et al.
2014
). Using the same data as USEPA (
2005
), with the
K
OC
for CPY and the
f
OC
value of 0.40 derived for alfalfa, clover, bluegrass, corn stalk, and small grain
straw this model predicts a worst-case mean CPY dew concentration (at 09:00 h,
immediately after application) of 102 μg CPY L
−1
and a 95th centile concentration
of 210 μg CPY L
−1
. If a bee consumed 100% of its daily drinking water from
contaminated dew and has an intake 47 μL d
−1
, this model predicts mean and 95th
centile daily doses of 0.0048 and 0.0099 μg CPY bee
−1
, respectively. The corre-
sponding values for collection of 1,250 μL d
−1
are 0.03 and 0.05 μg CPY bee
−1
.
A variety of uses of CPY involve application by mixing the product in irrigation
water or chemigation. These applications result in wet foliage with water that
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