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

concentrations are likely to be large and thus easier to determine accurately. When
monitoring for PCBs in lakes, it is thus common to sample sediment or fish rather
than water, since the expected concentrations in water are very low. Likewise, those
concerned with PCB behavior in the atmosphere may measure the PCBs on aerosols
or in rainfall containing aerosols, since concentrations are higher than in the air.
In general, when assessing the likely environmental behavior of a new chemical,
it is useful to calculate the various Z values and from them identify the larger ones,
since it is likely that the high Z compartments are the most important. It is no
coincidence that solutes such as halogenated hydrocarbons, about which there is
great public concern, have high Z values in humans!
It should be borne in mind that, when calculating the environmental behavior of
a solute, Z values are needed only for the phases of concern. For example, if no
atmospheric partitioning is considered, it is not necessary to know the air-water
partition coefficient or H. An arbitrary value of H can be used to define Z for water
and other phases, because H cancels. Intuitively, it is obvious that H, or vapor
pressure, play no role in influencing water-fish-sediment equilibria.
In summary, in this chapter we have introduced the concept of equilibrium
existing between phases and have shown that this concept is essentially dictated by
the laws of thermodynamics. Fortunately, we do not need to use or even understand
the thermodynamic equations on which equilibrium relationships are based. However,
it is useful to use these relationships for purposes such as correlation of partition
coefficients. It transpires that there are two approaches that can be used to conduct
equilibrium calculations. First is to develop and use empirical correlations for partition
coefficients. Using these coefficients, it is possible to calculate the partitioning
of the chemical in a multimedia environment.
The second approach, which we prefer, is to use an equilibrium criterion such
as fugacity or, in the case of involatile chemicals, an aquivalent concentration. The
criterion can be related to concentration for each chemical and for each medium
using a proportionality constant or Z value. The Z value can be calculated from
fundamental equations or from partition coefficients. We have established recipes
for the various Z values in these media using information on the nature of the media
and the physical chemical properties of the substance of interest. This enables us to
undertake simple multimedia partitioning calculations.
©2001 CRC Press LLC
5.7 LEVEL I CALCULATIONS
Calculation of the equilibrium Level I distribution of a chemical is simple, but
it can be tedious. It is ideal for implementation on a computer. The obvious steps are
1. Definition of the environment, i.e., volumes and compositions
2. Input of relevant physical chemical properties
3. Calculation of Z values for each medium (see Table 5.1)
4. Input of chemical amount
5. Calculation of fugacity, and hence concentrations, amounts, and percent distribution