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

©2001 CRC Press LLC
N = D T(f P – f A) = 2.4 ¥ 10 –6 mol/h = 6.4 ¥ 10 –4 g/h
The intermediate fugacities and concentrations are 3.6 ¥ 10 –5 Pa (3800 ng/m 3 )
and 11 ¥ 10 –5 Pa (11500 ng/m 3 ). The time for evaporation of 1 g of PCB will be 65
days. The amount of PCB in an air volume of 500 m 3 would be on the order of
0.004 g, a small fraction of the small amount of PCB spilled.
The significant conclusion is that, despite appreciable ventilation at an ACH of
0.5 h –1 , the indoor air concentrations are over 1000 times those outdoors. Fortunately,
the indoor fugacity is still very much lower than the pool fugacity. Similar behavior
applies to other solvents, pesticides, and chemicals that may be used and released
indoors. Although the amounts spilled or released are small, the restricted advective
dilution results in concentrations that are much higher than are normally encountered
outdoors. In many cases, this phenomenon is suspected to be the cause of the “sick
building” problem in which residents complain repeatedly about headaches, nausea,
and tiredness. The cure is to eliminate the source or increase the ventilation rate.
Fugacity calculations can contribute to understanding such problems.
8.12 UPTAKE BY PLANTS
Uptake of chemicals by plants is one of the most important but still poorly
understood processes. It is important because plants are at the base of food chains.
Thus, a chemical such as a dioxin absorbed by grass can be transported to the cow,
then to dairy and meat products, and thus to humans. Plants can be valuable monitors
of the presence of chemicals in the environment, but they are only of quantitative
value if the plant-environment partitioning phenomena are fully understood. Plants
may also affect the overall environmental fate of a substance by removing it from
the atmosphere or absorbing it from soils. An attractive “phyto-remediation” option
is to use plants to reduce concentrations in contaminated sites.
Ironically, although plants are much simpler organisms than animals on the scale
of biological evolution, they are more difficult to model. A major difficulty is that
plants grow quickly relative to animals, and their circulatory system is not as efficient
as those of animals. There are flows in xylem and phloem channels, but these are
not as well characterized as blood flows. Foliage, which is the primary contact area
with the atmosphere, is very complex and variable from species to species. It consists
of an external often waxy cuticle, but with access to the interior by stomata that are
designed to ensure entry of CO 2. The root, which is in contact with soil, presents a
complex barrier to uptake of chemicals. It is not clear how the wood or bark of trees
should be treated. Much of the mass of a tree is inaccessible to contaminants. Often,
the consumed material is fruit, nuts, or seeds that form rapidly, but processes of
chemical transport to them from foliage, roots, and stem are not yet well understood.
These and other issues have been discussed in the texts by Nobel (1991) and
Trapp and MacFarlane (1995). McLachlan (1999) has set out a framework for
assessing uptake of chemicals by grasses from the atmosphere. Severinson and Jager
(1998) have discussed the need for including plants in multimedia models. Actual
fugacity or related models of uptake have been developed by Trapp et al. (1990),