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

The total rates of transfer are thus
©2001 CRC Press LLC
water to air f WD V mol/h
air to water f A(D V + D D + D R + D Q) = f AD T mol/h
The total amounts of chemical in each phase may be calculated as V AZ TAf A and
V WZ TWf W, where the subscript T refers to the total or bulk phase. The rate constants
(h –1 ) and half-times (h) for transfer from each phase are respectively
from air D T/(V AZ TA) h –1 and 0.693V AZ TA/D T h
from water D V/(V WZ TW) h –1 and 0.693V WZ TW/D V h
These quantities are useful as indicators of the rapidity with which chemical can be
cleared from one phase to the other, thus enabling the significance of these exchange
processes to be assessed relative to other processes such as reaction. Inspection of
the individual D values shows which processes are most important.
It is noteworthy that a steady-state (i.e., no net transfer) condition may apply in
which the air and water fugacities are unequal, i.e., a nonequilibrium, steady-state
condition applies. The steady-state condition will apply when
f WD V = f AD T
The steady-state water fugacity and concentration with respect to the air, and the
steady-state air fugacity and concentration with respect to the water, can thus be
calculated to give an impression of the extent to which the actual concentration
departs from the steady-state values, as distinct from the equilibrium (equifugacity)
value. It is noteworthy that, because D T exceeds D V, the fugacity in water will tend
to exceed the fugacity in air; however, this will be affected by removal processes in
water.
8.3.3 Model
The AirWater model is available from the website as Windows software and in
the older DOS-based BASIC format. In both cases, the calculations can be viewed
by the user, and sufficient comments are included to enable the logic to be followed.
A sample chemical and set of air and water properties are included as an example
for the user. Given the concentrations in the air and water, the steady-state fluxes
are calculated.
8.4.1 Introduction
8.4 A SURFACE SOIL MODEL
Chemicals are frequently encountered in surface soils as a result of deliberate
application of agrochemicals and sewage sludge, and by inadvertent spillage and