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

includes an excellent treatment of human exposure. It was originally designed to
assess exposure to chemicals present in hazardous waste sites. The environmental
fate equations are formulated using fugacities. Maddalena et al. (1995) compared
the output of and ChemCAN and found similar results, indicating that both
models were using similar predictive equations.
The reader should consult the Trent website for current versions of the model
software described in this text.
8.3.1 Introduction
©2001 CRC Press LLC
8.3 AN AIR-WATER EXCHANGE MODEL
Air-water exchange process calculations are useful when estimating chemical
loss from treatment lagoons, ponds, and lakes; for estimating deposition rates of
atmospheric contaminants, and for interpreting observed air and water concentrations
to establish the direction and rate of transfer. The complexity of the several processes
and the widely varying physical chemical properties of chemicals of interest leads
to situations in which chemical behavior is not necessarily intuitively obvious. The
simple model derived here provides a rational method of estimating exchange characteristics
and exploring the sensitivity of the results to assumed values of the various
chemical and environmental parameters. Bidleman (1988) gives an excellent review
of atmospheric processes treated by the model.
An elegant application of the fugacity concept to elucidating chemical exchange
in the air-water system is that of Jantunen and Bidleman (1997). Samples of air and
surface water from the Bering and Chukchi Seas (between Alaska and Russia) were
analysed for a-hexachlorocyclohexane (a-HCH) over a multiyear period, and the
ratios of air to water fugacities were deduced using the Henry’s law constant for
seawater at the appropriate temperature. Initially, in the mid 1980s, this ratio was
greater than 1, indicating that a-HCH was being absorbed by the ocean. This is
consistent with the source of a-HCH being evaporation of technical lindane following
application in South East Asia, India, and China, with subsequent atmospheric
transport. Later, in the mid 1990s, after the use of technical lindane was greatly
reduced, the fugacity ratio became less than 1 (because of the drop in air fugacity),
and net volatilization of a-HCH started. Essentially, the ocean acted first as a
“sponge,” absorbing a-HCH, then it desorbed the a-HCH in response to changes
in the concentration in air. Interpretation of data using of the fugacity ratio illustrated
this clearly. It is an example to be followed in cases where there is doubt about the
direction of net transport of chemicals between air and water.
8.3.2 Process Description
The situation treated here, and the resulting model, are largely based on the
study of air- -water exchange by Mackay et al. (1986) as is depicted in Figure 8.1.
The water phase area and depth (and hence volume) are defined, it being assumed
that the water is well mixed. The water contains suspended particulate matter, to