Document ID: EPA-HQ-OPP-2005-0495-0024
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2005-12-21T05:00Z

1
UNITED
STATES
ENVIRONMENTAL
PROTECTION
AGENCY
WASHINGTON,
D.
C.
20460
OFFICE
OF
PREVENTION,
PESTICIDES
AND
TOXIC
SUBSTANCES
May
11,
2005
PC
Codes:
128821,
128829
DP
Barcode:
316746
MEMORANDUM
SUBJECT:
Tier
I
Drinking
Water
Concentrations
(
Surface
and
Ground
Water)
for
the
Herbicide
Imazapyr/
Arsenal
(
CAS
#
81334­
34­
1)
Applied
to
Terrestrial
and
Aquatic
Environments.

TO:
Sherrie
Kincade
,
Risk
Assessor
RRB2,
Health
Effects
Division
(
7509C)

Charles
Smith,
Risk
Assessor
RRB2,
Health
Effects
Division
(
7509C)

FROM:
Lucy
Shanaman,
Chemist
Environmental
Risk
Branch
III
Environmental
Fate
and
Effects
Division
(
7507C)

THRU:
Daniel
Rieder,
Branch
Chief
Environmental
Risk
Branch
III
Environmental
Fate
and
Effects
Division
(
7507C)

CC:
Mika
Hunter,
Chemical
Review
Manager
Special
Review
and
Re­
Registration
Division
(
7508C)

This
memo
presents
Tier
1
Estimated
Drinking
Water
Concentrations
(
EDWCs)
for
the
non­
selective
herbicide,
imazapyr
(
2­[
4,5­
dihydro­
4­
methyl­
4(
1­
methylethyl)­
5­
oxo­
1Himidazol
2­
yl]­
3­
pyridinecarboxylic
acid),
for
the
labeled
uses:
corn,
forestry
uses,
aquatic
and
terrestrial
non­
cropland
uses
including
manufacturing
sites
and
right­
of­
ways,
pasture
and
rangeland,
residential
non­
food
use,
and
non­
residential
turf.
1Clear
Field
is
a
variety
of
corn
that
has
been
developed
to
be
resistant
to
the
herbicidal
effects
of
imazapyr.

2PCA
(
Percent
Cropped
Area)
is
only
applied
to
cultivates,
agricultural
uses.

3Ibid
2
Imazapyr
is
expected
to
have
high
mobility,
and
to
be
persistent
in
the
open
environment.
Imazapyr
is
stable
to
both
aerobic
and
anaerobic
degradation.
Imazapyr
is
also
stable
to
hydrolysis,
but
susceptible
to
photolytic
degradation
with
a
reported
half­
lives
of
2.5,
4.5
and
5.3
days.
There
were
no
toxic
residues
of
concern
identified
for
the
imazapyr
transformation
products,
and
none
were
considered
in
this
drinking
water
assessment.

Terrestrial
Applications
of
Imazapyr
EDWCs
have
been
based
on
a
single,
aerial
application
at
a
maximum
annual
application
rate
of
1.5
pounds
a.
i./
acre
applied
to
various
non­
cropped
uses.
In
general,
the
labeled
uses
for
different
products
containing
imazapyr
have
single
maximum
annual
application
rates
between
0.9
and
1.5
pounds
a.
i./
acre.
While
labels
for
registered
products
do
allows
application
with
backpack
sprayers,
which
would
minimize
the
treated
area,
in
many
cases
aerial
application
is
also
permissible.
Maximum
annual
application
rates
for
uses
of
imazapyr
on
Clear
Field1
corn
is
much
lower,
at
0.014
pounds
a.
i./
acre.

The
models,
FIRST
and
SCI­
GROW
were
used
in
estimating
EDWCs
derived
from
surface
water
and
groundwater
sources,
respectively.
Terrestrial,
non­
cropped,
non­
forest
uses
were
chosen
to
represent
the
labeled
uses
because
they
have
the
highest
annual
use
rate
for
imazapyr.
For
surface
water,
the
maximum
estimated
peak
acute
concentrations
(
only
corn
adjusted
for
national
percent
cropped
area2)
are
146,
88,
and
0.626
:
g/
L
from
use
of
imazapyr
on
for
terrestrial,
non­
cropped,
non­
forest
areas,
at
maximum
annual
application
rates
of
1.5,
0.9,
and
0.014
lbs.
a.
i./
acre,
respectively.
The
maximum
estimated
annual
mean
cancer
and
noncancer
chronic
exposure
concentrations
(
only
corn
adjusted
for
national
percent
cropped
area3)
are
79,
47,
and
0.337
:
g/
L
from
use
of
imazapyr
on
non­
cropped
areas,
at
maximum
annual
application
rates
of
1.5,
0.9,
and
0.014
pounds
a.
i./
acre,
respectively.
Results
are
also
presented
below
in
Table
1.

The
Tier
1
SCI­
GROW
generated
EDWC
values
derived
for
groundwater
are
36,
22,
and
0.34
:
g/
L
(
ppb)
for
maximum
annual
application
rates
of
1.5,
0.9,
and
0.014
pounds
a.
i./
acre,
respectively.
These
value
are
recommended
for
use
for
both
acute
and
chronic
exposures.
Results
are
also
presented
below
in
Table
2.
4Trademark
3
Direct
Application
of
Imazapyr
to
Surface
Water
Estimations
of
EDWC
resulting
from
direct
application
to
water
have
not
changed
substantially
from
the
April
2003
EFED
drinking
water
assessment
in
support
of
new
uses
for
aquatic
weeds.
However,
the
current
label
(
8/
25/
03)
for
HABITAT4
herbicide
indicates
that
a
one
half
mile
setback
from
drinking
water
intakes
is
required.
EFED
does
not
currently
have
an
approved
policy
for
calculating
EDWCs
in
water
bodies
where
pesticides
are
applied
with
a
set
back
distance
from
drinking
water
intakes.
Therefore,
the
half
mile
setback
distance
was
not
considered
in
April
2003
assessment,
and
is
not
considered
here.
As
a
result,
the
assessment
presented
below
is
more
conservative
than
one
where
a
setback
distance
had
been
considered.

By
simple
calculation,
and
assuming
no
attenuating
factors
such
as
foliar
interception,
direct
application
at
the
maximum
rate
of
1.5
pounds
a.
i./
acre
to
the
entire
surface
area
of
the
Index
Reservoir
yields
a
peak
concentration
of
61
:
g/
L
(
ppb)
,
which
is
lower
than
the
acute
peak
result
from
FIRST
of
146
ppb
from
terrestrial
applications
at
the
1.5
pounds
a.
i./
acre.
Thus,
the
current
EFED
scenario
for
runoff
and
spray
drift
into
the
Index
Reservoir
from
terrestrial
uses
is
more
conservative
than
direct
application
to
the
entire
Index
Reservoir.
This
result
is
consistent
with
the
relative
sizes
of
the
treated
areas
as
follows:
The
real
drinking
water
reservoir
simulated
(
Shipman
City
Lake,
located
in
Shipman,
Illinois)
is
relatively
small
(
13
acres
in
area
and
9.0
feet
or
2.74
meters
deep;
total
water
volume
of
approximately
144,000
cubic
meters),
but
has
a
relatively
large
watershed
or
runoff
area
of
427
acres,
of
which
87%
is
presumed
treated.
Thus,
with
the
transport
and
kinetic
processes
simulated
in
the
model
and
a
ratio
of
runoff
area
to
volume
of
water
in
the
Index
Reservoir
of
approximately
12,
a
greater
mass
of
imazapyr
enters
the
reservoir
from
the
427­
acre
runoff
and
spray
drift
area
than
that
from
direct
application
to
the
much
smaller
surface
area
of
the
reservoir.
Treated
in
their
entirety,
only
water
bodies
4.0
feet
deep
(
1.23
m)
or
shallower
could
have
concentrations
of
imazapyr
equal
to
or
greater
than
those
estimated
from
the
terrestrial
scenario,
FIRST.
Results
are
also
presented
below
in
Table
1.
5PCA
(
Regional
Percent
Cropped
Area)
not
applicable
6Ibid
7PCA
(
Regional
Percent
Cropped
Area)
=
0.46
8Application
to
entire
water
body
4
Table
1:
Estimated
Tier
I
(
FIRST)
Surface
Water
Concentrations
of
Imazapyr
for
Terrestrial
Uses,
and
for
Direct
Application
to
Water:

Use
Scenario
Acute
Exposure
from
One
in
Ten
Year
Peak
Values
in
:
g/
L
(
ppb)
Cancer
and
Non­
Cancer
Chronic
Exposure
from
One
in
Ten
Year
Annual
Mean
in
:
g/
L
(
ppb)

Non­
Cropped
Uses
(
high
application
rate)
1465
79
Non­
Cropped
Uses
(
low
application
rate)
886
47
Clear
Field
Corn
0.6267
0.337
Direct
Application
to
Water
(
Index
Reservoir)
618
(
½
mile
setback
not
considered)
­­­­­

Table
2:
Estimated
Tier
I
(
SCI­
GROW)
Groundwater
Screening
Concentration
Values
of
Imazapyr
for
Terrestrial
Uses:

Use
Scenario
Groundwater
Screening
Concentrations
in
:
g/
L
(
ppb)

Non­
Cropped
Uses
(
high
maximum
application
rate)
36
Non­
Cropper
Uses
(
low
maximum
application
rate)
22
Clear
Field
Corn
0.34
Models
Used
The
Tier
1
drinking
water
concentrations
derived
from
surface
water
for
the
proposed
cropped
uses
were
calculated
using
FIRST
version
1.0
(
dated
August
1,
2001).
The
FIRST
(
FQPA
Index
Reservoir
Screening
Tool)
model
is
typically
used
by
EFED
to
simulate
the
transport
of
the
pesticide
off
the
field
and
to
estimate
pesticides
concentrations
in
surface
waters.
FIRST
is
employed
to
evaluate
runoff
and
spray
drift
loading
to
a
receiving
surface
water
body,
and
to
estimate
pesticide
concentrations
after
the
residues
reach
surface
water,
by
taking
into
5
account
the
different
dissipation
mechanism
in
the
aqueous
and
sediment
phases.
While
EFED
recognizes
that
it
is
unlikely
that
an
entire
watershed
will
be
planted
with
turf
or
ornamental
plants,
we
have
no
policy
or
method
for
assessing
a
PCA
for
these
uses.
In
the
absence
of
data,
EFED
makes
the
conservative
assumption
that
the
entire
watershed
is
treated.
Model
background
information
is
found
in
Appendix
III.

Tier
1
drinking
water
concentrations
derived
from
groundwater
for
the
proposed
uses
were
calculated
using
the
model,
SCIGROW
(
version
2.3)
dated
July
29,
2003.
Input
parameters
used
to
model
imazapyr
are
presented
in
Tables
2
and
3.
EFED
model
input
guidance
was
followed.
(
http://
www.
epa.
gov/
oppefed1/
models/
water/
input_
guidance2_
28_
02.
htm).

Table
3.
Modeled/
Calculation
Annual
Application
Rates
for
Imazapyr:

Labeled
Use
Maximum
Application
Rate
(
lb
a.
i.
/
acre)
Number
of
Applications
Reference
Non­
Cropped
Uses
1.5
1
Label
for
Arsenal
Railroad
Herbicide
(
22.6%
a.
i.)
8/
27/
93
Non­
Cropped
Uses
0.9
1
Label
for
Chopper
Herbicide
(
22.6%
a.
i.)
9/
18/
01
Clear
Field
Corn
0.014
1
Label
for
Lightning
D
Hebicide
(
4.0%
a.
i.)
12/
19/
02
Direct
Application
to
Water
1.5
1
Label
for
Habitat
Herbicide
(
22.6%
a.
i.)
8/
25/
03
Table
4:
FIRST
Input
Parameters
for
Imazapyr:

Parameters
Values
&
Units
Sources
Molecular
Weight
261
amu
2003
Science
Chapter
for
Aquatic
Uses
of
Imazapyr
Vapor
Pressure
25oC
<
10­
7
mm
Hg
(<
1.3
x
10­
5
Pa)
at
60
°
C
(
method
limit)
2003
Science
Chapter
for
New
Aquatic
Uses
of
Imazapyr
Water
Solubility
@
20oC
11.1
x
103
mg/
L
25
°
C,
2003
Science
Chapter
for
New
Aquatic
Uses
Hydrolysis
Half­
Life
(
pH
7)
stable
MRID
00132359
Aerobic
Soil
Metabolism
Half­
life
stable
MRID
00131619
Aerobic
Aquatic
Metabolism
(
total
sediment/
water
system)
stable
MRID
40003712
Aqueous
Photolysis
5.3
days
MRID
00131617
(
maximum
value)

Soil
Water
Partition
Coefficient
­
Kd
0.639
Lowest
Non­
Sand
Kd
(
silt
loam
soil)
MRID
45119705
Pesticide
is
Wetted­
In
no
Product
Label
6
Table
5:
Imazapyr
Environmental
Fate
Input
Parameters
for
SCI­
GROW:

Parameters
Values
&
Units
Sources
Non­
Cropped
Uses
(
high
range
of
rates)
1.5
lb.
a.
i
/
acre
Label
for
Arsenal
Railroad
Herbicide
(
22.6%
a.
i.)
8/
27/
93
Non­
Cropped
Uses
(
low
range
of
rates)
0.9
lb.
a.
i
/
acre
Label
for
Chopper
Herbicide
(
22.6%
a.
i.)
9/
18/
01
Clear
Field
Corn
0.014
lb.
a.
i
/
acre
Label
for
Lightning
D
Hebicide
(
4.0%
a.
i.)
12/
19/
02
Organic
Carbon
Partition
Coefficient
99.8
mg/
g
o.
c.
Lowest
Non­
Sand
Koc
(
silt
loam)
MRID
45119705
Aerobic
Soil
Metabolism
Half­
life
Stable
MRID
41002301
7
Appendix
I
FIRST
Output
Data
for
Imazapyr
RUN
No.
2
FOR
imazapyr
ON
terrestria
*
INPUT
VALUES
*
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
RATE
(#/
AC)
No.
APPS
&
SOIL
SOLUBIL
APPL
TYPE
%
CROPPED
INCORP
ONE(
MULT)
INTERVAL
Kd
(
PPM
)
(%
DRIFT)
AREA
(
IN)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
1.500(
1.500)
1
1
.611100.0
AERIAL(
16.0)
100.0
.0
FIELD
AND
RESERVOIR
HALFLIFE
VALUES
(
DAYS)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
METABOLIC
DAYS
UNTIL
HYDROLYSIS
PHOTOLYSIS
METABOLIC
COMBINED
(
FIELD)
RAIN/
RUNOFF
(
RESERVOIR)
(
RES.­
EFF)
(
RESER.)
(
RESER.)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.00
2
N/
A
5.30­
657.20
.00
657.20
UNTREATED
WATER
CONC
(
MICROGRAMS/
LITER
(
PPB))
Ver
1.0
AUG
1,
2001
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
PEAK
DAY
(
ACUTE)
ANNUAL
AVERAGE
(
CHRONIC)
CONCENTRATION
CONCENTRATION
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
146.443
79.129
8
RUN
No.
4
FOR
imazapyr
ON
terrestria
*
INPUT
VALUES
*
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
RATE
(#/
AC)
No.
APPS
&
SOIL
SOLUBIL
APPL
TYPE
%
CROPPED
INCORP
ONE(
MULT)
INTERVAL
Kd
(
PPM
)
(%
DRIFT)
AREA
(
IN)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.900(
.900)
1
1
.611100.0
AERIAL(
16.0)
100.0
.0
FIELD
AND
RESERVOIR
HALFLIFE
VALUES
(
DAYS)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
METABOLIC
DAYS
UNTIL
HYDROLYSIS
PHOTOLYSIS
METABOLIC
COMBINED
(
FIELD)
RAIN/
RUNOFF
(
RESERVOIR)
(
RES.­
EFF)
(
RESER.)
(
RESER.)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.00
2
N/
A
5.30­
657.20
.00
657.20
UNTREATED
WATER
CONC
(
MICROGRAMS/
LITER
(
PPB))
Ver
1.0
AUG
1,
2001
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
PEAK
DAY
(
ACUTE)
ANNUAL
AVERAGE
(
CHRONIC)
CONCENTRATION
CONCENTRATION
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
87.548
47.059
RUN
No.
3
FOR
imazapyr
ON
corn
*
INPUT
VALUES
*
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
RATE
(#/
AC)
No.
APPS
&
SOIL
SOLUBIL
APPL
TYPE
%
CROPPED
INCORP
ONE(
MULT)
INTERVAL
Kd
(
PPM
)
(%
DRIFT)
AREA
(
IN)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.014(
.014)
1
1
.611100.0
AERIAL(
16.0)
46.0
.0
FIELD
AND
RESERVOIR
HALFLIFE
VALUES
(
DAYS)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
METABOLIC
DAYS
UNTIL
HYDROLYSIS
PHOTOLYSIS
METABOLIC
COMBINED
(
FIELD)
RAIN/
RUNOFF
(
RESERVOIR)
(
RES.­
EFF)
(
RESER.)
(
RESER.)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.00
2
N/
A
5.30­
657.20
.00
657.20
UNTREATED
WATER
CONC
(
NANOGRAMS/
LITER
(
PPTr))
Ver
1.0
AUG
1,
2001
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
PEAK
DAY
(
ACUTE)
ANNUAL
AVERAGE
(
CHRONIC)
CONCENTRATION
CONCENTRATION
9
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
626.458
336.730
Appendix
II
SCIGROW
Output
Data
for
Imazapyr
SCIGROW
VERSION
2.3
ENVIRONMENTAL
FATE
AND
EFFECTS
DIVISION
OFFICE
OF
PESTICIDE
PROGRAMS
U.
S.
ENVIRONMENTAL
PROTECTION
AGENCY
SCREENING
MODEL
FOR
AQUATIC
PESTICIDE
EXPOSURE
SciGrow
version
2.3
chemical:
imazapyr
time
is
5/
5/
2005
16:
15:
46
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
Application
Number
of
Total
Use
Koc
Soil
Aerobic
rate
(
lb/
acre)
applications
(
lb/
acre/
yr)
(
ml/
g)
metabolism
(
days)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
1.500
1.0
1.500
9.98E+
01
999.0
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
groundwater
screening
cond
(
ppb)
=
3.59E+
01
************************************************************************

SciGrow
version
2.3
chemical:
imazapyr
time
is
5/
5/
2005
16:
17:
16
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
Application
Number
of
Total
Use
Koc
Soil
Aerobic
rate
(
lb/
acre)
applications
(
lb/
acre/
yr)
(
ml/
g)
metabolism
(
days)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
0.900
1.0
0.900
9.98E+
01
999.0
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
groundwater
screening
cond
(
ppb)
=
2.15E+
01
************************************************************************
10
SciGrow
version
2.3
chemical:
imazapyr
time
is
5/
5/
2005
16:
18:
8
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
Application
Number
of
Total
Use
Koc
Soil
Aerobic
rate
(
lb/
acre)
applications
(
lb/
acre/
yr)
(
ml/
g)
metabolism
(
days)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
0.014
1.0
0.014
9.98E+
01
999.0
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
groundwater
screening
cond
(
ppb)
=
3.35E­
01
************************************************************************
11
Appendix
III
Background
Information
on
FIRST
For
human
health
exposure
assessments
undertaken
for
FQPA,
EFED
has
attempted
to
develop
a
consistent,
parallel
modeling
system
to
replace
that
developed
for
ecological
risk
assessment.
In
recognition
of
the
fact
that
very
few
people
drink
water
derived
from
small
farm
ponds
or
from
small,
upland
streams,
EFED
has
developed
an
"
index
agricultural
watersheddrinking
water
reservoir"
or
"
Index
Reservoir"
scenario
to
replace
the
"
standard
field­
pond"
scenario.
In
addition,
it
is
recognized
that
most
watersheds
large
enough
to
support
a
community
drinking
water
system
(
CWS)
are
not
entirely
planted
in
only
one
crop.
The
modeling
system,
therefore,
has
develop
a
method
to
consider
a
maximum
percent
cropped
area
(
PCA)
factor
to
account
for
this
fact.

New
Tier
1
modeling
uses
the
FIRST
program
in
place
of
the
GENEEC
program.
The
`
FIRST'
program
is
designed
to
mimic
a
more
complex
simulation
using
the
linked
PRZM3
and
EXAMS
2.97.7
models
but
requires
less
time
and
effort
to
complete.
Note:
Neither
of
these
tiers
considers
the
potential
impact
of
water
treatment
processes
on
removal
of
pesticide
from
the
water
that
eventually
reaches
the
consumer.

Index
Drinking
Water
Reservoir.

In
July,
1998,
OPP
presented
to
the
FIFRA
SAP
an
"
index"
reservoir
scenario
to
replace
the
"
field
pond"
scenario
used
at
that
time
in
its
screening
models
to
estimate
pesticide
concentrations
in
drinking
water
derived
from
surface
water.
The
concept
behind
use
of
a
model
of
an
"
index"
reservoir
to
screen
pesticides
is
that
the
chosen
reservoir
 
and
its
associated
characteristics
 
has
become
the
standard
set
of
conditions
by
which
EPA
would
judge
the
potential
of
a
pesticide
to
contaminate
drinking
water
derived
from
surface
water.
The
"
index"
reservoir
was
selected
from
a
group
of
reservoirs
that
provide
drinking
water
to
communities
throughout
the
country.
EPA
has
picked
a
particular
reservoir
with
characteristics
associated
with
a
higher
potential
for
pesticide
contamination
of
surface
water
and
use
those
real
world
characteristics
in
its
mathematical
screening
model.
Because
the
"
index"
reservoir
models
real
world
characteristics,
it
is
likely
to
produce
more
realistic
estimates
of
pesticide
concentrations
in
surface
water.
However,
because
the
"
index"
reservoir
has
characteristics
that
are
associated
with
a
higher
potential
for
pesticide
contamination
of
surface
water,
the
model
is
likely
to
be
protective
of
other
drinking
water
sources
which
are
less
vulnerable
to
contamination.

The
actual
reservoir
simulated
is
a
small
drinking
water
reservoir
located
in
Shipman,
Illinios.
Shipman
City
Lake
is
13
acres
in
area,
9
feet
deep,
and
has
a
watershed
area
of
427
acres.
Its
ratio
of
drainage
area
(
area
in
square
meters)
to
capacity
(
volume
of
water
in
the
lake,
in
cubic
meters)
is
approximately
12.
As
a
comparison,
the
"
field
pond"
currently
used
has
a
ratio
of
5.
Shipman
City
Lake
is
one
of
a
number
of
mid­
western
reservoirs
that
tend
to
be
small
and
shallow
12
with
small
watersheds,
and
frequently
have
Safe
Drinking
Water
Act
(
SDWA)
compliance
problems
with
atrazine,
a
herbicide
widely
used
on
corn
grown
in
these
watersheds.
The
Index
Drinking
Water
Reservoir
characteristics
have
been
incorporated
into
the
FIRST
model
and
are
implemented
in
conjunction
with
percent
cropped
area
adjustment
(
FIFRA
SAP,
1999).
While
estimates
of
pesticide
concentrations
in
drinking
water
based
on
a
Midwestern
Index
Drinking
Water
Reservoir
may
not
be
representative
of
residue
levels
in
drinking
water
sources
in
other
parts
of
the
country,
the
scenario
provides
an
effective
screening
tool
to
determine
the
need
for
more
extensive
refinements.
The
modeling
scenarios
currently
account
for
region­
specific
rainfall,
soil,
and
hydrologic/
runoff
factors.
Steps
to
develop
scenarios
for
regional
reservoirs
for
advanced
tiers
of
modeling
have
been
hampered
by
the
lack
of
monitoring
data
outside
of
the
Midwest
that
is
of
sufficient
quality
and
extent
to
develop
scenarios
for
additional
reservoirs.

National
Percent
Cropped
Area
(
PCA)
Adjustments:

The
PCA
is
a
generic
adjustment
which
represents
the
maximum
percent
of
any
watershed
that
is
planted
to
the
crop
or
crops
being
modeled
and,
thus,
may
potentially
be
treated
with
the
pesticide
in
question.
PCA
factors
are
generated
from
Geographic
Information
System
(
GIS)
overlays
of
cropping
area
and
watershed
delineations
and
are
applied
to
estimates
of
Index
Reservoir
surface
water
pesticide
concentration
values
from
the
FIRST
model.
The
output
generated
by
this
model
is
multiplied
by
the
maximum
decimal
fraction
of
cropped
area
in
any
watershed
generated
for
the
crop
or
crops
of
interest.
To
be
effective
as
an
adjustment
to
screening
model
estimates,
the
PCA
should
result
in
estimated
concentrations
that
are
closer
to,
but
not
less
than,
actual
pesticide
concentrations
in
vulnerable
(
prone
to
pesticide­
laden
runoff)
surface
water
sources.
While
it
moves
away
from
assuming
that
the
entire
watershed
would
be
treated
at
the
same
time,
the
PCA
is
still
expected
to
be
a
screen
because
it
represents
the
highest
percentage
of
crop
cover
of
any
large
watershed
in
the
U.
S.
and
it
assumes
that
the
entire
crop
is
being
treated.
Current
PCA
values
and
a
description
of
how
the
values
are
derived
and
applied
can
be
found
in
US
EPA
OPP
(
2000a).
The
PCA
adjustment
is
only
applicable
to
pesticides
applied
to
agricultural
crops.
The
PCA
is
not
to
be
applied
to
groundwater
modeling
assessments.

As
the
"
Index
Reservoir'
replaced
the
"
standard
field­
pond",
the
FIRST
model
replaced
GENEEC
as
the
surface
water
screen
for
screening
level
model
for
FQPA
pesticide
exposure
assessments.
Like
the
more
complex
and
resource­
intensive
PRZM
and
EXAMS
models
which
it
mimics,
FIRST
estimates
pesticide
concentrations
in
a
vulnerable
index
reservoir.
As
with
GENEEC,
FIRST
considers
adsorption
of
the
pesticide
to
soil
or
sediment,
incorporation
of
the
pesticide
at
application,
direct
deposition
of
spray
drift
into
the
water
body,
and
degradation
of
the
pesticide
in
soil
before
runoff
and
within
the
water
body.
Simulation
results
are
adjusted
to
account
for
the
percentage
of
the
watershed
in
the
crop
being
assessed.
FIRST
is
expected
to
be
exceeded
by
measured
pesticide
concentrations
in
drinking
water
only
very
rarely
due
to
the
conservative
nature
of
the
model
assumptions.
It
represents
a
small
drinking
water
reservoir
surrounded
by
a
runoff­
prone
watershed,
uses
maximum
pesticide
application
rates
and
assumes
13
that
no
buffer
exists
between
the
reservoir
and
the
treated
fields.
The
simulation
assumes
runoff
from
a
single,
large
rainfall
event.

Background
Information
on
SCI­
GROW
(
version
2.3;
July
29,
2003)

SCI­
GROW
is
a
Tier
I,
regression­
based
model
that
provides
a
groundwater
screening
exposure
value
to
be
used
in
determining
the
potential
risk
to
human
health
from
drinking
water
contaminated
with
the
pesticide.
Since
the
SCI­
GROW
concentrations
are
likely
to
be
approached
in
only
very
small
percentage
of
drinking
water
sources
(
i.
e.
highly
vulnerable
aquifers),
it
is
not
appropriate
to
use
SCI­
GROW
for
national
or
regional
exposure
estimates.

SCI­
GROW
estimates
likely
groundwater
concentrations
if
the
pesticide
is
used
at
the
maximum
allowable
rate
in
areas
where
groundwater
is
exceptionally
vulnerable
to
contamination.
In
most
cases,
a
large
majority
of
the
use
area
will
have
groundwater
that
is
less
vulnerable
to
contamination
than
the
areas
used
to
derive
the
SCI­
GROW
estimate.