Page
1
of
4
UNITED
STATES
ENVIRONMENTAL
PROTECTION
AGENCY
WASHINGTON,
D.
C.
20460
OFFICE
OF
PREVENTION,
PESTICIDES
AND
TOXIC
SUBSTANCES
January
13,
2004
MEMORANDUM
TO:
The
Public
Docket
SUBJECT:
Guidance
for
Developing
and
Performing
Quality
Control
of
Water
Modeling
Standard
Scenarios
and
Standard
Scenario
Metadata
Files:
Response
to
Comments
[
OPP­
2002­
0182]

On
August
21,
2002,
EPA
requested
comments
on
the
following
documents,
which
were
published
in
the
Federal
Register
(
67
Federal
Register
54206,
FRL­
7193­
5):

1.
PRZM
Field
and
Orchard
Crop
Metadata.
This
document
provides
cropspecific
parameters
(
i.
e.,
specific
values
and
references)
used
in
field
and
orchard
crop
pesticide
exposure
scenarios.
2.
Standard
Procedures
for
Conducting
Quality
Control
and
Quality
Assurance
for
Pesticide
Root
Zone
Model
(
PRZM)
Field
and
Orchard
Crop
Scenarios.
This
document
defines
a
set
of
steps
(
i.
e.,
methods
f
for
selecting
or
estimating
specific
scenario
values
and
references)
to
develop
crop
scenariospecific
models
and
to
establish
a
sound
basis
for
quality
assurance.

These
documents
were
intended
to
provide
a
transparent
description
of
each
environmental
modeling
scenario
and
the
procedure
used
to
create
them.
During
the
public
comment
period,
which
ended
on
October
21,
2002,
EPA
received
comments
from
one
group:
CropLife
America
(
CLA).

In
responding
to
CLA s
comments,
EPA
grouped
similar
comments
related
to
the
two
documents
together.
Comments
not
specific
to
these
two
documents
were
also
listed
and
discussed
in
cases
where
they
were
relevant.

CropLife
Comments
1
and
2:
CLA
provided
perspectives
on
how
surface
water
scenarios
should
be
designed,
specifying
seven
steps
to
be
followed.
They
stated
that
the
current
documentation
does
not
adequately
describe
how
scenarios
were
developed
Page
2
of
4
(
including
how
the
relative
vulnerability
of
the
individual
scenario
is
determined)
and
used
in
risk
assessments.
Without
this
information,
they
found
it
difficult
to
comment
on
their
relevance.
They
also
concluded
that
 
there
is
no
apparent
logic
for
[
the
selection
of
scenarios]
or
certainty
about
their
relative
vulnerability....
existing
scenarios
(
many
of
which
have
been
calibrated)
should
continue
to
be
used
and
new
scenarios
should
be
placed
on
hold. 
CLA
also
suggested
a
mechanism
for
stakeholder
input
for
scenario
development.

EPA
Response:
The
Agency
agrees
that
transparency
in
scenario
identification
and
development
is
important.
We
believe
that
the
release
of
the
documents
referenced
above
is
an
important
step
in
the
right
direction.
At
the
same
time,
though,
we
recognize
that
more
comprehensive
guidance
is
needed
concerning
how
scenarios
are
selected
and
developed
and
in
assuring
that
adequate
quality
assurance/
quality
control
(
QA/
QC)
is
conducted.
Also,
part
of
OPP s
ongoing
efforts
for
process
improvement,
we
have
started
developing
new
tools,
using
Geographic
Information
System
(
GIS)
mapping
tools
technology,
to
identify
and
select
scenarios.

Although
the
Agency
believes
that
the
selection
of
sites
for
these
scenarios
is
logical
and
the
scope
is
well
defined,
it
will
expand
the
description
of
the
scope
to
make
the
guidance
more
understandable.
The
steps
used
in
site
selection
and
scenario
development
generally
followed
the
steps
outlined
in
CLA s
comments
with
the
exception
of
defining
and
verifying
the
relative
vulnerability
of
the
scenarios.
EPA
has
not
yet
established
a
method
to
determine
the
relative
vulnerability
of
scenarios,
but
recognizes
the
need
to
include
this
step
in
both
the
scenario
development
and
the
scenario
QA/
QC
process
that
follows.

In
its
FR
Notice,
the
Agency
requested
comments
on
two
distinct
groups
of
scenarios
that
were
developed
at
different
times
for
different
purposes.
Scenarios
in
the
first
group
were
developed
as
"
standard
scenarios"
for
tier
II
national­
level
assessments
in
which
a
single,
high­
vulnerability
scenario
represented
each
crop.
EPA
began
using
national,
multi­
year
PRZM/
EXAMS
cropping
scenarios
in
1992.
The
stated
goal
for
these
scenarios
was
to
locate
crop/
soil/
weather
combinations
that
would
produce
exposure
concentration
values
higher
than
90
percent
of
all
of
the
other
scenarios
across
the
country
for
that
crop.
The
90th
percentile
relative
vulnerability
of
these
scenarios
was
based
on
the
best
professional
judgment
of
the
scenario
developer.

The
scenarios
in
the
second
group
were
developed
in
the
fall
of
2001
for
a
cumulative
drinking
water
risk
assessment
for
organo­
phosphate
(
OP)
pesticides
to
be
used
in
an
aggregate
human
health
risk
assessment
under
the
Food
Quality
Protection
Act
(
FQPA).
For
this
assessment,
EPA
developed
approximately
25
regional
cropping
scenarios
that
represented
high
OP
usage
areas.
These
scenarios
supplemented
the
first
group
of
standard
national
scenarios
described
above.

The
Agency
believes
that
it
used
a
logical,
three­
step
process
to
design
the
scenarios
in
each
of
these
two
groups.
Briefly,
the
first
step
was
to
locate
the
area
of
expected
maximum
pesticide
usage.
For
the
standard
scenarios,
the
first
step
was
accomplished
by
using
the
area
of
maximum
national
cropping
intensity
for
the
scenario
Page
3
of
4
under
development.
For
the
cumulative
scenarios,
EPA
identified
the
areas
of
highest
OP
usage.
In
the
second
step,
high
usage
areas
(
located
in
step
one),
located
in
areas
with
the
highest
rainfall/
runoff
history,
were
identified
for
both
types
of
scenarios.
The
third
step,
which
was
common
to
both
types
of
scenarios,
involved
selecting
a
soil
series
with
the
appropriate
hydrologic
group
to
maximize
both
runoff
and
soil
erosion.

The
two
reference
documents,
which
were
provided
for
public
comment,
describe
data
and
procedures
for
selecting
scenario
parameters
.
This
selection
process
is
an
important
topic
that
must
be
addressed
to
ensure
that
scenarios
are
reproducible
and
consistent.
The
second
topic
that
must
be
addressed
is
a
description
of
QA/
QC
processes
for
ensuring
that
parameter
selection
follows
the
guidance
and
parameters
are
adequately
documented.
This
topic
is
described
briefly
in
the
guidance,
but
will
need
to
be
expanded
to
include
more
detailed
information.
Thirdly,
the
description
of
how
a
scenario
fits
into
the
current
conceptual
model
represented
at
a
specific
assessment
tier
(
problem
formulation)
is
another
important
topic
which
will
need
a
more
detailed
description.

The
Agency
encourages
public
participation
on
issues
such
as
site
selection
for
scenario
development
as
reflected
in
the
request
for
comments
on
the
scenario
development
document
published
in
the
Federal
Register
Notice
and
on
scenarios
existing
at
that
time.
We
are
aware
that
available
documentation
of
these
procedures
is
incomplete.
The
Agency
will
initiate
a
discussion
specifically
to
address
all
of
the
issues
raised
in
these
comments
on
site
selection
for
development
of
surface
water
modeling
scenarios
for
simulation
of
pesticide
aquatic
exposure.
The
Agency
plans
to
engage
both
internal
and
external
stakeholders
in
further
discussion
concerning
the
process
of
scenario
development.

CropLife
Comments
3
and
5:
CropLife
provided
a
separate
document
with
detailed
comments
on
each
scenario.
They
also
provided
a
Microsoft
Access
database
that
can
be
used
to
compare
parameters
between
scenarios
and
to
automate
cross­
checking
between
similar
parameters
in
different
scenarios.
CropLife
was
critical
of
the
apparent
level
of
QA/
QC
of
the
individual
scenarios
and
documentation
provided.

EPA
Response:
CropLife s
detailed
comments
and
database
provide
valuable
feedback
on
scenario
inputs
and
parameters,
which
will
be
useful
in
current
and
future
QA/
QC
efforts.
EPA
is
currently
engaged
in
the
following
activities
to
address
these
specific
comments:

1)
Scenario­
by­
scenario
checking
of
all
points
listed
in
the
comments,
revision
of
the
scenarios,
and
a
separate
response
to
comments
document
addressing
the
specific
points
raised.

2)
Separation
of
the
cumulative
scenarios
that
were
developed
specifically
for
the
organophosphate
cumulative/
aggregate
FQPA
assessment
from
those
generally
used
as
national,
standard
scenarios
as
described
above.
Page
4
of
4
3)
More
rigorous
parameter
selection
criteria
and
a
more
comprehensive
reporting
on
the
parameter
values
selected
not
only
within
scenarios
but
between
scenarios.

CropLife
Comment
4:
CropLife
provided
results
of
a
modeling
exercise
for
a
single
chemical
across
all
scenarios.
They
highlighted
the
variability
in
output
and
related
that
to
the
90
th
percentile
concentration
estimates.
They
also
described
differences
between
 
old 
and
 
new
scenarios, 
and
noted
that
multiple
scenarios
(
used
for
the
same
crop
in
EPA s
risk
assessments),
have
not
been
adequately
described.

EPA
Response:
Although
the
guidance
document
quantifies
a
specific
percentile
of
vulnerability,
which
a
scenario
is
expected
to
represent,
a
scenario
is
expected
to
represent
a
site
which
simulates
upper­
end
(
but
not
the
extreme)
of
potential
exposure
for
people
relying
on
surface
water
for
their
drinking
water.
The
Agency
recognizes
that
relative
vulnerability
needs
to
be
addressed
in
a
more
rigorous
manner
and
that
the
documentation
should
be
strengthened
and
revised
to
clarify
and
expand
on
this
topic.
Future
documentation
should
also
include
an
explanation
of
how
results
of
multiple
scenarios
are
used
in
estimating
exposure.

CropLife
Comment
6:
Additional
Comments
and
Concerns:

CropLife
made
several
additional
statements
concerning
the
need
to
ensure
compatibility
with
recommended
models
and
model
shells
and
to
ensure
that
procedures
exist
for
scenario
version
control
and
verification.
They
also
recommended
maintaining
standard
scenarios
(
unmodified),
creating
additional
scenarios
to
deal
with
special
cases,
standardizing
documentation
of
scenario
parameters
(
Good
Modeling
Practices),
developing
internal
systems
to
check
newly
drafted
scenarios
for
 
obvious 
errors,
and
ensuring
that
the
scenarios
fully
comply
with
the
OMB
data
quality
guidelines.

EPA
Response:
EPA
agrees
that
these
issues
are
important
and
is
currently
developing
additional
guidance
for
scenarios
to
improve
tools
and
processes
and
to
engage
the
public
in
a
process
that
should
address
these
issues.
