McKay, Donald. "Front matter" Multimedia Environmental Models ...

Unlike the Level II calculation, it is now necessary to specify the emissions into
each compartment separately. Different mass distributions, concentrations, and residence
times result if 100 mol/h is emitted to air, water, or soil; thus, “mode of
entry” is an important determinant of environmental fate and persistence.
Having obtained the fugacities, all process rates can be deduced as Df, and a
steady-state mass balance should emerge in which the total inputs to each medium
equal the outputs. The amounts and concentrations can be calculated.
An overall residence time can be calculated as the sum of the amounts present
divided by the total input (or output) rate. A reaction residence time can be calculated
as the amount divided by the total reaction rate, and a corresponding advection
residence time can also be deduced. Doubling emissions simply doubles fugacities,
masses, and concentrations, but the residence times are unchanged.
An important property of this model is its linear additivity. This is also called
the principle of superposition. Because all the equations are linear, the fugacity in,
for example, water, deduced as a result of emissions to air, water, and soil, is simply
the sum of the fugacities in water deduced from each emission separately. It is thus
possible to attribute the fugacity to sources, e.g., 50% is from emission to water,
30% is from emission to soil, and 20% from emission to air. The masses and fluxes
are also linearly additive.
Figure 7.11 is a schematic representation of the results corresponding to the
computed output in Figure 7.12. This is a comprehensive multimedia picture of
chemical emission, advection, reaction, intermedia transport, and residence time or
persistence. The important processes are now clear, and it is possible to focus on
them when seeking more accurate rate data. Figure 7.11 contains information about
21 processes, some of which, such as air-water transfer, consist of several contributing
processes. The human mind is incapable of making sense of the vast quantity
of physical chemical and environmental data without the aid of a conceptual tool
such as a Level III program.
It is possible to add more compartments and to subdivide the existing compartments.
It may be advantageous to add vegetation as a separate compartment. The
atmosphere or water column could be segmented vertically. The soil can be treated
as several layers. If information is available to justify these changes, they can be
implemented, albeit at the expense of greater algebraic complexity. If the number
of compartments becomes large and highly connected, it is preferable to solve the
equations by matrix algebra.
Computer programs are provided on the Internet, as discussed in Chapter 8, that
undertake the Level III calculation of the multimedia fate of a specified chemical.
The user must provide physical chemical (partitioning) properties, reaction halflives,
and sufficient information to deduce intermedia transport D values. Assembly
of an entire Level III model for a chemical is a fairly demanding task, since there
are numerous areas, flows, mass transfer coefficients, and diffusivities to be estimated.
To assist in this task, Table 7.2 gives suggested order-of-magnitude values
for the various parameters. Such values are included as defaults in some programs,
but they can be modified as desired.
The user is encouraged to conduct Level III calculations for chemicals of interest,
or those specified in Chapter 3. It is instructive to prepare a mass balance diagram,
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