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Chapter 5: Two-Phase Gas Transport
5.5
SIMPLIFIED MODELS FOR SOILS
There are simplified one-dimensional models for gas transport in soils that are often
used. They use the effective diffusion coefficient discussed above as well as con-
vection in the liquid phase due to evaporation or leaching and diffusion through a
stagnant boundary layer at the soil surface. Because these models use simplifying
assumptions such as constant water flux, constant atmospheric boundary layer thick-
ness, negligible advection in the gas phase, etc., the models are generally used as
screening models to investigate different scenarios.
The original screening models were developed by Jury et al. and are called the
Behavioral Assessment Model (BAM) for the screening of various pesticides (Jury et
al., 1983a, 1984a, b, c) and the Buried Chemical Model (BCM).The BCM is simply an
extension of the BAM for buried organic chemicals (Jury et al., 1990). The BAM and
BCM have been used to screen the behavior of various pesticides and volatile organic
compounds by identifying the differences under typical environmental conditions.
The BAM and BCM have been used in a study of landmine chemical signatures as dis-
cussed by Phelan (Chapter 21 of this book). Numerous extensions of Jury’s approach
have been made to include vapor-solid absorption such as Shoemaker et al. (1990).
The BAM and BCM are closed form solutions that are very amenable to fast cal-
culations of the various effects. Note that the original equations in Jury et al. (1983a)
have some errors in them. The equations are correct in Jury et al. (1983b, 1990).
Density effects may also influence unsaturated gas transport in soil. For example,
evaporation of Volatile Organic Compounds (VOCs) in the subsurface, whose gas
phase is heavier than air, may lead to gas stratification in the soil. This situation is
discussed by Falta et al. (1989).

5.6 EXPERIMENTAL DATA
In contrast to the data presented in Section 2.4 for gas transport in porous media,
there are no similar comprehensive data available for unsaturated flow conditions.
As mentioned earlier, most investigations of flow and transport in the unsaturated
(vadose) zone are concerned with liquid flow, not gas flow.

ACKNOWLEDGMENTS

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed
Martin Company, for the United States Department of Energy’s National Nuclear
Security Administration under Contract DE-AC04–94AL85000.

NOMENCLATURE
b Klinkenberg coefficient
c concentration
D ordinary diffusion coefficient
D iK Knudsen diffusion coefficient