UNITED
STATES
ENVIRONMENTAL
PROTECTION
AGENCY
WASHINGTON,
D.
C.
20460
OFFICE
OF
PREVENTION,
PESTICIDES
AND
TOXIC
SUBSTANCES
MEMORANDUM
PC
Code:
121001
DPBarcode:
D312559
SUBJECT:
Sethoxydim
Drinking
Water
Assessment
(
Tier
1)
for
Reregistration
Eligibility
Decision
FROM:
William
P.
Eckel,
Ph.
D.
Environmental
Risk
Branch
II
Environmental
Fate
and
Effects
Division
(
7507C)

TO:
Amaris
Johnson
Special
Review
and
Reregistration
Division
(
750C)

THRU:
Tom
Bailey,
Branch
Chief
Environmental
Risk
Branch
II
/
EFED
(
7507C)

DATE:
February
7,
2005
This
memorandum
provides
EFED's
drinking
water
assessment
for
the
sethoxydim
reregistration
eligibility
decision
(
RED).
The
estimated
drinking
water
concentrations
(
EDWCs)
for
total
sethoxydim
residues
are
1.5
ppb
for
ground
water,
and
130
ppb
(
acute)
and
16
ppb
(
chronic)
for
surface
water.

Usage
Assessment
Sethoxydim
is
registered
for
use
on
a
large
number
of
crops.
40CFR180.412
(
revised
as
of
7/
1/
2004)
lists
tolerances
for
sethoxydim
in
99
commodities.
The
BEAD
Usage
Report
for
sethoxydim
(
7/
30/
2004)
gives
estimates
for
usage
on
86
different
crops.
The
ten
crops
with
the
highest
estimated
usage
are
soybeans
(
CBI
material
deleted
here
by
request
of
the
registrant),
sunflowers
(
CBI
material
deleted
here
by
request
of
the
registrant),
alfalfa
(
CBI
material
deleted
here
by
request
of
the
registrant),
dry
peas/
beans
(
CBI
material
deleted
here
by
request
of
the
registrant),
sugarbeets
(
CBI
material
deleted
here
by
request
of
the
registrant),
peanuts
(
CBI
material
deleted
here
by
request
of
the
registrant),
corn
(
CBI
material
deleted
here
by
request
of
the
registrant),
oranges
(
CBI
material
deleted
here
by
request
of
the
registrant),
potatoes
(
CBI
material
deleted
here
by
request
of
the
registrant),
and
cotton
(
CBI
material
deleted
here
by
request
of
the
registrant).
The
U.
S.
Geological
Survey
has
estimated
1997
usage,
as
depicted
in
Map
1.
The
USGS
estimates
of
annual
usage
are
higher
for
soybeans,
alfalfa,
cotton,
sugarbeets,
and
peanuts.
This
may
represent
a
decrease
in
overall
annual
usage.
2
Map
1:
1997
Usage
of
Sethoxydim
as
Estimated
by
U.
S.
Geological
Survey
(
http://
ca.
water.
usgs.
gov/
cgi­
bin/
pnsp/
pesticide_
use_
maps_
1997.
pl?
map=
W1910).
3
Use
Rate
and
Frequency
The
maximum
seasonal
use
rate
in
the
Use
Closure
Memorandum
(
8/
24/
2004)
is
1.875
lb
active
ingredient
(
a.
i.)/
acre
per
year
(
4
applications
of
2.5
pints
each
formulated
product
containing
1.5
lb
a.
i.
per
gallon).
The
maximum
single
use
rate
is
0.47
lb
a.
i./
acre.
For
purposes
of
this
assessment,
four
applications
of
0.47
lb
a.
i./
acre
spaced
at
14
days,
will
be
modeled
(
LUIS
report
1/
14/
2004).
Fourteen
days
is
the
shortest
application
interval
given
in
the
LUIS
report,
however,
intervals
are
not
specified
for
many
uses.

Environmental
Fate
Assessment
In
terrestrial
environments,
photodegradation,
aerobic
metabolism,
and
acid­
catalyzed
hydrolysis
appear
to
be
the
primary
routes
of
dissipation
for
Sethoxydim
and
for
Sethoxydim
total
residues.
The
calculated
soil
photolytic
half­
life
for
Sethoxydim
is
1
hour
and
20
hours
for
Sethoxydim
residues
(
parent
+
transformation
products),
indicating
rapid
transformation
of
Sethoxydim
and
its
residues
after
application.
Other
potential
routes
of
dissipation
are
microbialmediated
degradation
and
surface
water
runoff.
Under
aerobic
conditions,
Sethoxydim
degrades
in
less
than
1
day,
while
Sethoxydim
total
residues
have
half­
lives
which
range
from
7
to
30
days.

In
aquatic
environments,
the
major
route
of
dissipation
for
total
Sethoxydim
residues
is
aqueous
photolysis
(
t
1/
2=
19.8
days),
followed
by
microbially­
mediated
degradation.
In
aerobic
aqueous
studies,
half­
lives
are
on
the
order
of
about
one
month
(
32.9­
38.1
days).

Laboratory
studies
show
that
Sethoxydim
is
a
highly
soluble
compound
(
4700
mg/
L
in
water)
and
that
it
hydrolyzes
at
moderately
rapid
rates
at
low
pH,
but
is
more
stable
at
high
pH.
The
calculated
half­
lives
are
8.7,
155,
and
284
days
in
pH
5,
7,
and
9
solutions,
respectively.
The
major
observed
hydrolysis
transformation
product
is
M2­
S
or
6­(
2­(
ethylthio)
propyl)­
4­
oxo­
2­
propyl­
4,5,6,7­
tetrahydrobenzoxazole.
In
contrast,
evaluation
of
the
data
of
Sethoxydim
total
residues
shows
that
they
remain
stable
at
all
three
pH
tested.

Sethoxydim
and
Sethoxydim
total
residues
degrade
photolytically
in
both
water
and
soil.
In
pH
buffered
water,
the
calculated
half­
life
of
Sethoxydim
is
5.23
days,
and
the
major
transformation
product
is
M1­
S
or
2­(
1­
aminobutylidene)­
5­(
2­(
ethylthio)­
propyl)­
cyclohex­
1,3­
dione.
In
sandy
loam
soil
irradiated
with
a
xenon
light
source,
the
half­
life
of
Sethoxydim
is
approximately
1
hour,
and
the
major
transformation
product
is
M­
SO
or
2­(
1­
ethoxyiminobutyl)­
5­(
2­(
ethylsulfinyl)
propyl)­
3­
hydroxycyclohex­
2­
enone.
Sethoxydim
total
residues
photodegrade
slower
than
parent
Sethoxydim
in
soil
and
water.
Using
a
linear
regression
analysis,
EFED
calculated
a
half­
life
of
19.8
days
for
the
photolysis
in
water
of
Sethoxydim
total
residues
and
a
half­
life
of
20
hours
in
soil.

Under
aerobic
conditions,
parent
Sethoxydim
transformed
with
short
half­
lives
(

1
day)
both
in
soil
and
aquatic
environments.
It
degraded
with
a
half
life
of
less
than
one
day
in
sandy
loam
and
sandy
clay
loam
soils.
The
major
transformation
product
at
2
months
was
M­
SO,
and
after
12
months
the
major
product
was
CO
2.
Using
an
aerobic
clay
loam
soil:
water
and
an
aerobic
clay
soil:
water
systems,
it
was
determined
that,
under
aerobic
aquatic
conditions,
Sethoxydim
transformed
4
with
a
half­
life
of

1
day.
After
28
days,
the
major
transformation
products
were
CO
2,
M­
SO,
M2­
S,
and
M­
SO
2
or
2­(
1­
ethoxyiminobutyl)­
5(
2­(
ethylsulfonyl)
propyl)­
3­
hydroxycyclohex­
2­
enone.
In
contrast
to
parent
Sethoxydim,
Sethoxydim
total
residues
were
more
persistent.
The
observed
halflife
was
1
month
for
Sethoxydim
total
residues
in
the
aerobic
sandy
loam
study,
and
7
days
in
the
aerobic
sandy
clay
loam
study.
In
an
aerobic
clay
loam
soil:
water
system,
the
calculated
half­
life
for
Sethoxydim
total
residues
was
38.1
days,
while
in
an
aerobic
clay
soil:
water
system
the
half­
life
was
32.9
days.

Based
on
batch
equilibrium
experiments,
Sethoxydim
and
its
transformation
products,
M2­
SO
2,
M­
SO,
M­
SO
2,
and
M2­
SO,
were
determined
to
be
mobile
to
very
mobile
in
sterile
(
autoclaved)
sand,
sandy
loam,
sandy
clay
loam,
silt
loam,
and
clay
loam
soils.
Freundlich
K
ads
values
were
<
1.00
for
Sethoxydim
and
its
transformation
products
M­
SO
and
M­
SO
2.
The
Freundlich
K
ads
values
for
M2­
SO
and
M2­
SO
2
ranged
from
0.06
to
9.12.

Because
of
its
low
vapor
pressure
(
1.6x10­
7
mm
Hg)
and
Henry's
Law
Constant
(
1.47x1011
atm­
m3/
mol),
Sethoxydim
is
not
expected
to
be
highly
volatile.

Residues
of
Interest
in
Drinking
Water
The
Metabolism
Assessment
and
Review
Committee
of
the
Health
Effects
Division
met
of
May
14,
1998
to
consider
which
residues
of
sethoxydim
to
include
in
the
drinking
water
assessment.
The
memorandum
of
this
meeting
(
5/
27/
1998)
states
that
all
degradates
are
to
be
included.

Ground
Water
The
following
input
parameters
were
used
for
the
SCIGROW
model,
version
2.3,
to
estimate
drinking
water
concentrations
from
ground
water
sources.
Four
applications
of
0.47
lb
a.
i./
acre;
Organic
Carbon
partition
coefficient
(
Koc)
of
2.72
mL/
g
(
MRID
41475212)
and
an
average
aerobic
soil
metabolism
half­
life
of
18.5
days
for
total
residues
(
MRID
41475210).

The
estimated
drinking
water
concentration
is
1.5
ppb
for
ground
water
sources.

Surface
Water
For
Tier
1
drinking
water
assessments,
EFED
uses
the
surface
water
model
FIRST,
or
FQPA
Index
Reservoir
Screening
Tool.
FIRST
simulates
a
PRZM­
EXAMS
run
for
a
small
drinking
water
reservoir
in
Shipman
City,
Illinois.
The
reservoir
is
13
acres
in
area,
9
feet
deep,
and
has
a
watershed
area
of
427
acres,
and
was
selected
because
it
had
Safe
Drinking
Water
Act
compliance
problems
with
herbicides.

The
following
data
were
used
as
inputs
to
the
FIRST
model
for
aerial
application
of
sethoxydim
at
the
maximum
labeled
use
rate
and
minimum
application
intervals.
5
Model
Input
Variable
Input
Value
Comments
Application
Rate
(
maximum)
0.47
lb
a.
i./
acre
(
2.5
pints)
LUIS
report
1/
14/
2004
Maximum
Number
of
Applications
4
(
10
pints
max.
per
season)
LUIS
report
1/
14/
2004
Application
Interval
14
days
(
minimum)
LUIS
report
1/
14/
2004
Organic
Carbon
Partition
Coefficient
(
Kd)
0.03
(
lowest
non­
sand
Kd)
Source:
MRID
41475212
Aerobic
Soil
Metabolism
Half­
life
54
days
(
Upper
90th
%
ile
on
two
values
for
total
residues)
Source:
MRID
41475210
Pesticide
wetted­
in?
No
Source:
label
7969­
58,
9/
24/
99
Spray
Drift
16
%
Standard
for
Aerial
Application
Solubility
(
mg/
L)
4700
Source:
EFED
one­
liner
database
Aerobic
Aquatic
Half­
life
44
days
(
Upper
90th
%
ile
on
two
values
for
total
residues)
Source:
MRID
42165604
Photolysis
Half­
life
19.8
days
(
total
residues)
Source:
MRID
41475208
Percent
Cropped
Area
87%
default
The
FIRST
model
gave
the
following
results
for
untreated
drinking
water
concentrations
from
surface
water
sources,
to
two
significant
figures.

Chemical(
s)
modeled
Peak
day
(
acute)
Annual
Average
(
chronic)

Total
Residues
130
ppb
16
ppb
6
Characterization
of
EDWCs
As
Tier
1
modeling
estimates,
the
EDWCs
given
in
this
assessment
are
expected
to
be
at
the
high
end
of
residue
concentrations
actually
found
in
raw
drinking
water
sources.
Protective
estimates
of
input
parameters
(
aerial
application
to
maximize
spray
drift,
maximum
use
rate,
minimum
application
interval,
lowest
non­
sand
Koc
or
Kd)
were
used.
The
FIRST
model
is
based
on
a
high
runoff
scenario
(
Mississippi
cotton)
with
a
storm
two
days
after
application;
few
if
any
Tier
2
EDWCs
are
expected
to
exceed
FIRST
model
outputs.

The
SCIGROW
output
represents
concentrations
at
the
top
of
the
water
table
immediately
beneath
the
treated
field.
Concentrations
in
private
water
wells
near
the
treated
field
will
may
be
closer
to
SCIGROW
outputs
than
municipal
wells
that
draw
in
ground
water
from
larger
areas.

A
default
percent
cropped
area
factor
(
PCA)
of
87%
was
used
for
the
surface
water
assessment.
This
means
that
it
was
assumed
that
sethoxydim­
treated
crops
were
grown
on
87%
of
the
acres
of
land
in
the
modeled
watershed.
This
is
the
highest
PCA
for
any
watershed
in
the
continental
U.
S.,
and
is
used
as
a
default
value
for
minor
crops.
Lower
PCA
values
could
have
been
applied
for
the
major
crops
on
which
sethoxydim
is
used
(
soybeans,
cotton,
and
corn).
However,
because
there
is
such
a
great
variety
of
uses,
it
is
likely
that
there
is
usage
on
major
and
minor
crops
in
the
same
watershed,
so
the
crop­
specific
PCA
may
not
be
applicable.
EDWCs
are
proportional
to
the
PCA
used.

The
EDWCs
given
here
represent
concentrations
in
raw
drinking
water
sources,
and
do
not
account
for
possible
water
treatment
effects.
Because
of
the
short
metabolism
half­
lives
of
parent
sethoxydim,
the
EDWCs
represent
total
residues,
including
M­
SO,
M­
SO2,
M1­
S,
M1­
SO,
M1­
SO2,
M2­
S,
M2­
SO,
and
M2­
SO2.

Scheme
I
represents
the
relationship
between
parent
sethoxydim
(
M)
and
its
eight
identified
degradates.
There
are
three
groups
of
degradates:
a
group
formed
from
the
parent
by
oxidation
of
the
sulfur
atom
(
M
group),
the
"
amines"
or
M1
group,
and
the
tetrahydrobenzoxazoles,
or
M2
group.
The
first
M1
group
daughter
(
M1­
S)
is
formed
by
cleavage
of
the
ethoxyimino
group
to
an
amino
group,
either
by
hydrolysis
or
aqueous
photolysis.
The
first
member
of
the
M2
group
(
M2­
S)
is
then
formed
from
M1­
S
by
ring
closure
to
produce
a
tetrahydrobenzoxazole
ring.

Each
of
the
three
groups
of
degradates
also
has
a
sulfoxide
(­
SO)
and
a
sulfone
(­
SO2),
to
round
out
the
total
of
eight
degradates.
This
reaction
dominates
in
aerobic
soil
metabolism
and
soil
photolysis
experiments,
especially
to
form
the
M­
SO
and
M­
SO2
degradates.
7
Scheme
I
M

M1­
S

M2­
S



M­
SO

M1­
SO

M2­
SO



M­
SO2

M1­
SO2

M2­
SO2
The
eight
degradates
include
2
daughter
products,
which
are
formed
from
the
parent
by
a
single
reaction
(
M1­
S
and
M­
SO),
3
granddaughter
products
that
are
two
reactions
removed
from
the
parent
(
M­
SO2,
M2­
S,
and
M1­
SO),
2
great­
granddaughters
that
are
three
reactions
removed
(
M1­
SO2
and
M2­
SO),
and
one
great­
great­
granddaughter
that
is
four
reactions
removed
(
M2­
SO2).

It
is
important
to
note
that
M1­
SO,
M2­
SO,
M1­
SO2,
and
M2­
SO2
may
be
formed
by
more
than
one
pathway,
and
that
the
sulfur
oxidation
may
be
reversible
under
reducing
conditions,
so
that
the
kinetics
of
the
formation
is
very
complicated,
and
probably
cannot
be
elucidated
with
any
confidence
from
the
current
fate
data.

Monitoring
Data
Little
if
any
monitoring
data
is
available
for
sethoxydim
to
determine
its
concentration
in
drinking
water
sources.
EFED
is
relying
on
modeling
estimates
to
quantify
possible
drinking
water
exposure.

The
USGS
Pesticide
National
Synthesis
Project
(
http://
ca.
water.
usgs.
gov/
pnsp/
)
did
not
include
sethoxydim
(
nor
any
of
its
degradates)
in
its
list
of
pesticides
for
laboratory
analysis
of
ground
water
and
surface
water
samples.

The
Pesticides
in
Ground
Water
Database
report
(
EPA
734­
12­
92­
001,
Sept.
1992)
states
that
sethoxydim
was
analyzed
in
65
Missouri
wells
in
1986,
and
was
not
detected.
8
Appendix.
FIRST
and
SCIGROW
model
outputs.

RUN
No.
1
FOR
sethoxydim
ON
any
*
INPUT
VALUES
*
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
RATE
(#/
AC)
No.
APPS
&
SOIL
SOLUBIL
APPL
TYPE
%
CROPPED
INCORP
ONE(
MULT)
INTERVAL
Kd
(
PPM
)
(%
DRIFT)
AREA
(
IN)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
.470(
1.465)
4
14
.0
4700.0
AERIAL(
16.0)
87.0
.0
FIELD
AND
RESERVOIR
HALFLIFE
VALUES
(
DAYS)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
METABOLIC
DAYS
UNTIL
HYDROLYSIS
PHOTOLYSIS
METABOLIC
COMBINED
(
FIELD)
RAIN/
RUNOFF
(
RESERVOIR)
(
RES.­
EFF)
(
RESER.)
(
RESER.)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
54.00
2
N/
A
19.80­
2455.20
44.00
43.23
UNTREATED
WATER
CONC
(
MICROGRAMS/
LITER
(
PPB))
Ver
1.0
AUG
1,
2001
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
PEAK
DAY
(
ACUTE)
ANNUAL
AVERAGE
(
CHRONIC)
CONCENTRATION
CONCENTRATION
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
130.708
16.156
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:
sethoxydim
time
is
1/
31/
2005
12:
31:
44
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
Application
Number
of
Total
Use
Koc
Soil
Aerobic
rate
(
lb/
acre)
applications
(
lb/
acre/
yr)
(
ml/
g)
metabolism
(
days)
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
0.470
4.0
1.880
2.72E+
00
18.5
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
groundwater
screening
cond
(
ppb)
=
1.51E+
00
************************************************************************
