Document ID: EPA-HQ-OPP-2005-0495-0022
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2005-12-21T05:00Z

­
M
­
1
­
Appendix
M.
AgDrift
Model
(
spray
drift
estimates)

Surface
water
modeling
using
PRZM/
EXAMS
assumes
5%
and
1%
drift
deposition
integrated
across
the
surface
of
a
pond
adjacent
to
a
treated
field
for
aerially
and
ground
applied
pesticides,
respectively.
A
comparison
of
these
default
values
can
be
made
with
those
from
the
first
screening­
level
spray
drift
predictions
from
the
AgDRIFT
model
(
Version
2.01).
The
following
table
presents
AgDRIFT
predictions
for
deposition
of
spray
drift
(
fraction
of
application
rate)
integrated
across
the
surface
of
a
standard
pond
which
is
immediately
adjacent
and
downwind
to
a
treated
field
and
which
has
a
208.7­
foot
downwind
width.
In
situations
where
the
Agency's
screening
models
suggest
that
spray
drift
is
a
significant
source
of
exposure
and
therefore
risk,
the
following
information
is
considered
in
the
risk
characterization
to
evaluate
the
confidence
of
risk
assessment
conclusions.

Application
Method
PRZM/
EXAMS
Drift
AgDRIFT
Model
Drift
Ground
Low
Boom
Height
1%
1.5
%
very
fine
to
fine
spray,
50th
percentile
2.7
%
very
fine
to
fine
spray,
90th
percentile
0.9
%
fine
to
medium/
coarse
spray,
50th
percentile
1.1
%
fine
to
medium/
coarse
spray,
90th
percentile
High
Boom
Height
1%
5.5
%
very
fine
to
fine
spray,
50th
percentile
6.2
%
very
fine
to
fine
spray,
90th
percentile
1.5
%
fine
to
medium/
coarse
spray,
50th
percentile
1.7
%
fine
to
medium/
coarse
spray,
90th
percentile
Application
Method
PRZM/
EXAMS
Drift
AgDRIFT
Model
Median
Drift,
90th
Percentile
Application
Conditions
(
Based
on
Best
Professional
Judgment)

Aerial
5%
6.9
%
coarse
to
very
coarse
spray
8.9
%
medium
to
coarse
spray
12.7%
fine
to
medium
spray
24.3%
very
fine
to
fine
spray
From
this
comparison,
the
baseline
assumptions
of
drift
currently
used
for
PRZM/
EXAMS
modeling
exceed
the
90th
percentile
of
drift
predictions
from
AgDRIFT
modeling
for
ground
applications
only
for
the
medium/
coarse
spray
from
low
boom
sprayer
(
50th
percentile
assumption).
Aerial
drift
assumptions
are
below
drift
levels
predicted
by
AgDRIFT
for
all
droplet
spectra
sprays
using
90th
percentile
application
conditions.
The
exact
extent
to
which
the
currently
used
aerial
drift
assumption
represents
more
frequently
encountered
application
conditions
is
not
presently
quantified.

The
extent
to
which
a
5%
versus
another
drift
assumption
alters
estimated
aqueous
­
M
­
2
­
concentration
estimates
depends
on
specific
use
scenarios
and
can
be
influenced
by
the
degree
to
which
runoff
contributes
to
the
overall
receiving
water
concentration.
For
example,
if
a
persistent
pesticide
with
low
affinity
for
soils
is
used
in
a
high
runoff
potential
use
area,
drift
may
be
only
a
minor
route
for
pesticide
loading
to
the
receiving
waters
and
the
magnitude
of
assumed
drift
may
have
a
limited
effect
on
the
concentration
estimate.
However,
for
non­
persistent
chemicals
with
high
affinity
for
soils
used
in
low
runoff
areas,
drift
may
be
the
dominant
route
of
pesticide
entering
receiving
waters,
and
the
particular
level
of
spray
drift
chosen
may
appreciably
influence
aqueous
pesticide
concentration
estimates.

It
should
be
noted
that
the
baseline
drift
assumptions
for
a
water
body
located
adjacent
to
a
treated
field
are
much
higher
than
upper
bound
values
for
water
bodies
located
at
greater
distances
from
the
treated
area.
The
table
below
shows
distances
from
the
treated
area
where
AgDRIFT
assumptions
for
aerial
drift
to
a
water
body
would
be
approximated
by
the
baseline
drift
assumption
of
5%.
Water
bodies
located
closer
to
the
treated
field
than
shown
below
would
be
predicted
to
have
drift
loadings
greater
than
the
5%
assumption.
The
greater
the
distance
from
the
treated
field
required
to
reach
5%
drift,
the
greater
the
likelihood
that
actual
water
bodies
could
receive
drift
levels
higher
than
the
baseline
5%
assumption.

Spray
Category
Water
Body
Distance
from
Treated
Field
to
Reach
5%
Surface
Integrated
Drift
in
AgDRIFT
Model
(
ft)

Coarse
to
very
coarse
spray
13.12
Medium
to
coarse
spray
39.4
Fine
to
medium
spray
105
Very
fine
to
fine
spray
643
This
comparison
suggests
that
the
OPP
assumption
of
5%
aerial
drift
would
reasonably
represent
high­
end
estimates
of
drift
for
most
water
bodies
when
medium
to
very
coarse
sprays
are
used
because
a
few
water
bodies
are
usually
found
within
40
feet
of
treatment
areas.
However,
for
very
fine
to
medium
spray
uses,
the
confidence
that
the
5%
drift
assumption
adequately
characterizes
drift
to
water
bodies
is
diminished
because
a
higher
number
of
water
bodies
can
be
assumed
to
be
located
within
650
feet
from
treated
fields.
It
should
be
noted
that
quantitative
probabilities
of
water
body
locations
from
treated
fields
are
likely
to
be
crop
and
regionally
specific.

The
Agency
includes
a
discussion
of
the
impacts
of
chemical
use­
specific
estimates
of
drift
as
computed
by
AgDRIFT
in
the
risk
characterization
and
evaluates
the
extent
to
which
alternative
drift
estimates
may
impact
overall
risk
conclusions.
Pesticide
application
conditions
indicated
by
the
product
labels
or
agronomic
practices
associated
with
a
specific
crop
or
target
pest
are
evaluated
with
respect
to
their
associated
droplet
spectra.
These
expected
spectra
are
compared
with
the
AgDRIFT
model
predictions
of
drift
to
determine
if
drift
assumptions
employed
in
EEC
modeling
are
likely
to
be
over­
or
underestimates.
The
degree
to
which
drift
is
over
or
underestimated
is
considered
when
establishing
bounds
around
EEC
predictions
and
the
extent
to
which
these
bounds
lead
to
RQs
that
exceed
listed
species
LOCs,
or
not,
is
presented.
­
M
­
3
­
A.
Terrestrial
Assessment:
Aerial
Applications
I.
Based
on
an
application
rate
of
1.5
lb
ae/
A
applied
by
air.
­
M
­
4
­
II.
Based
on
an
application
rate
of
0.9
lb
ae/
A
applied
by
air.
­
M
­
5
­
III.
Based
on
an
application
rate
of
0.5
lb
ae/
A
applied
by
air
­
M
­
6
­
IV.
Based
on
an
application
rate
of
0.014
lb
ae/
A
applied
by
air.
­
M
­
7
­
B.
Terrestrial
Assessment:
Ground
Applications.

I.
Based
on
an
application
rate
of
1.5
lb
ae/
A
applied
by
ground
­
M
­
8
­
II.
Based
on
an
application
rate
of
0.9
lb
ae/
A
applied
by
ground.

III.
Based
on
an
application
rate
of
0.5
lb
ae/
A
applied
by
ground.
­
M
­
9
­
­
M
­
10
­
IV.
Based
on
an
application
rate
of
0.014
lb
ae/
A
applied
by
ground.
­
M
­
11
­
C.
Aquatic
Assessment:
Aerial
applications
to
EPA
standard
pond
from
spray
drift
due
to
agricultural
use.

I.
Based
on
an
application
rate
of
1.5
lb
ae/
A,
applied
by
air.
­
M
­
12
­
D.
Aquatic
Assessment:
Aerial
applications
to
EPA
standard
wetland
from
spray
drift
due
to
agricultural
use.

I.
Based
on
an
application
rate
of
1.5
lb
ae/
A,
applied
by
air.
­
M
­
13
­
E.
Aquatic
Assessment:
Ground
applications
to
EPA
standard
pond
from
spray
drift
due
to
agricultural
use.

I.
Based
on
an
application
rate
of
1.5
lb
ae/
A,
applied
by
ground.
­
M
­
14
­
F.
Aquatic
Assessment:
Ground
applications
to
EPA
standard
wetland
from
spray
drift
due
to
agricultural
use.

I.
Based
on
an
application
rate
of
1.5
lb
ae/
A,
applied
by
ground.