...
What
...
are
...
the
...
scenarios
...
for
...
the
...
present
...
and
...
future
...
situation
...
In
...
order
...
to
...
carry
...
out
...
a
...
risk-based
...
approach
...
for
...
the
...
megasite
...
of
...
concern,
...
it
...
is
...
necessary
...
to
...
assess
...
the
...
current
...
and
...
future
...
risks
...
to
...
receptors,
...
and
...
possible
...
effect
...
of
...
risk
...
reduction
...
measures.
...
A
...
representation
...
of
...
the
...
sources,
...
pathways
...
and
...
receptors
...
related
...
to
...
the
...
groundwater
...
system
...
at
...
the
...
megasite,
...
as
...
well
...
as
...
prediction
...
of
...
the
...
concentrations
...
and
...
mass
...
fluxes
...
of
...
prevailing
...
contaminants
...
at
...
specified
...
locations
...
over
...
time,
...
requires
...
development
...
of
...
a
...
quantitative
...
(mathematical)
...
model.
...
Such
...
a
...
model
...
allows
...
for
...
appropriate
...
description
...
of
...
groundwater
...
flow
...
and
...
transport
...
of
...
contaminants
...
in
...
groundwater
...
(see
...
figure
...
below).
...
A
...
stepwise
...
approach
...
to
...
modelling
...
is
...
recommended.
...
To
...
go
...
from
...
a
...
comparatively
...
simple
...
model
...
to
...
more
...
complex
...
models
...
requires
...
more
...
effort
...
and
...
data.
...
The
...
model
...
simulations
...
may
...
be
...
carried
...
out
...
in
...
various
...
contexts
...
and
...
with
...
different
...
objectives,
...
in
...
particular:
...
- to
...
- quantitatively
...
- summarize
...
- the
...
- state
...
- of
...
- knowledge
...
- about
...
- the
...
- megasite
...
- groundwater
...
- system
...
- including
...
- fate
...
- and
...
- transport
...
- of
...
- contaminants,
...
- to
...
- assess
...
- existing
...
- risks
...
- and
...
- to
...
- predict
...
- future
...
- development,
...
- to
...
- predict
...
- the
...
- effects
...
- of
...
- different
...
- scenarios
...
- of
...
- mitigating
...
- measures
...
As
...
a
...
result,
...
the
...
decision-support
...
tool
...
is
...
provided
...
for
...
the
...
selection
...
of
...
alternative
...
risk
...
reduction
...
measures
...
or
...
strategies.
...
This section provides guidance for carrying out contaminant fate and transport modelling at megasites, including interpretation of results and validation. The relevant facts and aspects of modelling are presented in a summarized form, focusing on issues that particularly need to be considered when dealing with megasites. The general presentation of modelling herein is a synthesis of a modelling protocol.
Output
The output of this step is detailed determination and characterization of the contaminants pathways from diverse sources to the receptors within the defined clusters, by means of the fate and transport modelling tools.