Environmental Engineering Reference
In-Depth Information
mean half-life calculated from the 18 studies of accumulation of CPY (SI Table
A3). If a range of values was listed, the largest value was used in the calculation for
a study (SI Table A3). If the half-life value given was expressed as being less than a
given value, that value was used as a conservative estimate in the calculation. The
upper 90th centile confidence bound on the mean half-life was calculated (
1
),
(USEPA
2009
) and used in the model.
(
)
t
×
s
90
,
n
(1)
t
=− +
t
input
12
/
n
12
/
where,
t
input
= half-life input value (
time
)
t
1
/
2
= mean of sample half-lives (
time
),
s = sample standard deviation (
time
), n = number of half-live values available (−),
and
t
90
,
n
−
1
= one-sided Student's t value at α = 0.1.
A foliar wash-off fraction of 0.1 cm
−1
rainfall for CPY was used in the PRZM
model (Carousel et al.
2005
). This value was further supported by the results of
dislodgeable foliar residue studies and foliar wash-off measurements (Poletika and
Robb
1994
; Racke
1993
). The review by Racke (
1993
) indicates that the proportion
of applied CPY that can be washed off foliage decreases with time after application,
not unexpectedly, given the large values of K
OC
and Log K
OW
for CPY. Because
dislodgeable residues diminish within hours after application and sampling times
likely differ between studies, reported results have high variability. Relevant values
of the wash-off fraction could not be derived from many of the older studies, since
this would require assumptions about both the proportion of dislodgeable residue
released by rain and the amount of rainfall required to release it. For example, Hurto
and Prinster (cited in Racke
1993
), reported dislodgeable residues on turfgrass of
0.03 μg cm
−2
, and total residues of 0.53 to 0.68 μg cm
−2
within the first 1-2 h after
application, giving a wash-off fraction of 0.044 to 0.057. Since not all dislodgeable
residues as measured in the laboratory (released by immersion in detergent and
water) are washed off in rain, this is likely to be an overly conservative estimate.
Although no information existed on the amount of rain needed to achieve such a
reduction, it is likely to be substantial.
The best direct measure of foliar CPY wash-off was obtained from a field study
on cotton (Poletika and Robb
1994
). The results of this study (Fig.
4
) show that the
foliar wash-off fraction of CPY is strongly time-dependent, and has a maximum
value of ~0.08 approximately 2 h after application. Other results that were based on
irrigation of the treated area immediately after application were considered to be
unrealistic. Under actual use conditions, CPY products are not to be applied when
rainfall is anticipated within 2 h after application, since this could diminish the effi-
cacy against foliar pests (except where the product is intentionally watered in to
provide efficacy against pest in soil or thatch).
Finally, uptake of CPY by plants from the soil was considered negligible and this
parameter was set to 0.0 in the model. CPY is not systemic and tends not to enter
plants or move within the plant vascular system (Racke
1993
).
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