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

mean. For chemical between depths of 1 and 10 cm, a log mean depth of 3.9 cm is
more appropriate than the arithmetic mean of 5.5 cm. It may be useful to consider
layers of soil separately, e.g., 2 to 4 cm, 4 to 6 cm, etc., and calculate separate
volatilization rates for each. Chemical present at greater depths will thus volatilize
more slowly, leaving the remaining chemical more susceptible to other removal
processes. It is acceptable to specify a mean Y of, say, 10 cm to examine the fate
of chemical in the 2 cm depth region from 9 to 11 cm. This depth issue is irrelevant
to reaction or leaching, but it must be appreciated that, if the soil is treated as separate
layers, the leaching rate is applicable to the total soil, not to each layer independently.
The total rate of chemical removal is then f D T, where the total D value is:
©2001 CRC Press LLC
D T = D R + D L + D V
the individual rates being f D R, f D L, and f D V. The overall rate constant k O is thus
D T/V TZ T, where V TZ T is the sum of the V iZ i products, and the overall half-life t O is
0.693/k O hours. The half-life t i attributable to each process individually is
0.693V TZ T/D i, thus,
1/t O = 1/t R + 1/t L + 1/t V
It is illuminating to calculate the rates of each process, the percentages, and the
individual half-lives. Obviously, the shorter half-lives dominate. The situation being
simulated is essentially the first-order decay of chemical in the soil by three simultaneous
processes, thus the amount remaining from an initial amount M (mol) at
any time t (h) will be
M exp(–D Tt/V TZ T) = M exp(–k Ot) mols
This relatively simple calculation can be used to assess the potential for volatilization
or for groundwater contamination.
Implicit in this calculation is the assumption that the chemical concentration in
the air, and in the entering leaching water, is zero. If this is not the case, an appropriate
correction must be included. In principle, it is possible to estimate atmospheric
deposition rates as was done in the air-water example and couple these processes
to the soil fate processes in a more comprehensive air-soil exchange model.
It may prove desirable to segment the soil into multiple layers, especially if
evaporation or input from the atmosphere is important. Models of this type have
been reported by Cousins et al. (1999) for PCBs in soils.
8.4.3 Model
The Soil model is available from the website in both Windows software and in
the older DOS-based BASIC format, similarly to the AirWater model. The SoilFug
model is also available and can be used to explore the effects of varying precipitation
on soil runoff.