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

Figure 7.1 Illustration of nonequilibrium behavior in an air-water system. In the lower diagram,
the rate of reaction in air is constrained by the rate of evaporation.
material that has decided, for reasons unrelated to the presence of the chemical, to
make this journey. Examples include advective flows in air, water, or particulate
phases, as discussed in Chapter 6; deposition of chemical in rainfall or sorbed to
aerosols from the atmosphere to soil or water; and sedimentation of chemical in
association with particles that fall from the water column to the bottom sediments.
These are usually one-way processes. The rate of chemical transfer is simply
the product of the concentration C mol/m 3 of chemical in the moving medium, and
the flowrate of that medium, G, m 3 /h. We can thus treat all these processes as
advection and calculate the D value and rate as follows:
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N = GC = GZf = Df mol/h
The usual problem is to measure or estimate G and the corresponding Z value
or partition coefficient. We examine these rates in more detail later, when we focus
on individual intermedia transfer processes.
7.2.2 Diffusive Processes
The second group of processes are diffusive in nature. If we have water containing
1 mol/m 3 of benzene and add some octanol to it as a second phase, the benzene will