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Costanza-Robinson and Brusseau
minimal. Mendoza and Frind (1990) demonstrate that in dilute (e.g., low vapor pres-
sure) vadose zone systems, diffusion from the soil to the atmosphere results in removal
of 95% and 69% of the contaminant mass from systems having gas-phase permeabil-
2
ities of 1.0 × 10 −11 and 1.0 × 10 −10 m , respectively. At the higher permeabilities
other transport mechanisms, such as density-driven advection, contribute more sig-
nificantly to gas transport. Lupo (1989) also demonstrates the importance of diffusion
in the transport of aromatic contaminants in a simulated landfill scenario. Diffusion
of chlorobenzene and benzene is calculated to be most critical in coarser soils and
under conditions of lower groundwater recharge. Similarly, Baehr (1987) reports the
important role diffusion plays in the transport of hydrocarbons in the vadose zone.

7.5 SUMMARY
Gas-phase dispersion is caused by mechanical mixing and diffusion processes. The
magnitude of diffusion is inversely proportional to compound molecular weight,
porous media bulk density, and soil-water content and directly proportional to tem-
perature. Under natural-gradient conditions, diffusion will likely be the dominant
transport mechanism. Mechanical mixing is likely to be dominant only under condi-
tions of induced gas advection (e.g., miscible displacement experiments and soil vapor
extraction systems) or relatively extreme changes in barometric pressure. The mag-
nitude of dispersion depends on the degree of heterogeneity of the physical system.
Experimental investigation of gas-phase dispersion has focused almost exclusively on
laboratory-scale systems. Understanding of gas-phase dispersion is important for con-
taminant transport applications, as well as atmosphere-soil gas exchange processes.

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