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sand unit. This strong advective sweeping would tend to quickly remove any volatile
contaminants from the coarse sand zones (as would natural processes like barometric
pumping). However contaminants trapped in the fine sand would not be subject to
much gas flushing, and the rate of contaminant removal would be limited by the rate
of gas diffusion out of the fine sand, into the coarse sand. This conceptual model has
been validated by the experimental and analytical work by Ho and Udell (1991,1992)
(see Section 2.6).

22.3 AIR SPARGING
22.3.1 Introduction to Air Sparging

Air sparging refers to the injection of a noncondensible gas (usually air) below the
water table for the purpose of evaporating NAPLs and volatilizing dissolved VOCs.
Air sparging wells are almost always combined with SVE wells, and they typically
inject at a rate of about one-fifth to one-tenth of the SVE flow rate to insure capture
of the injected gas. Air sparging is inexpensive and easy to implement at a site but
the efficiency in the field seems highly variable, and sometimes disappointing. The
apparent inefficiency of some sparging operations probably results from the nature of
the gas flow below the water table, which can lead to poor contact with contaminants
and poor mass transfer into the gas phase.

22.3.2 Nature of Noncondensible Gas Flow Below Water Table,
Bubbles Versus Channels, Capillary Barriers, Unstable Flows
The flow of noncondensible gases below the water table is completely different from
gas flow in the vadose zone. Although Darcy’s law still applies to the gas flow, below
the water table the gas movement is dominated by gravitational and capillary effects,
and it is almost always unstable.
Some early studies of mass transfer during air sparging assumed that the injected
gas moved upwards through the water by gravity as discrete bubbles. However,
experimental studies have shown that this is only true in coarse gravels (Ji et al.,
1993). In typical systems, the pore geometry is such that a nonwetting gas phase
bubble would be trapped by capillary forces. These capillary forces are only exceeded
when the vertical length of the gas bubble becomes very large, such as when the gas
formsachannel. Figure22.4showsaschematicdiagramofatrappedgasbubblebelow
the water table. In order for this gas to flow upwards as a bubble, the gravitational
buoyancy force must exceed the capillary force needed to squeeze through the pore
throat. Following Hunt et al. (1988), a force balance may be used to analyze the
conditions under which a gas bubble could migrate. Referring to the figure, the
bubble has a vertical length of l v , and the radius of curvature at the top of the bubble,
r 1 is approximately equal to the pore throat radius. The radius of curvature at the
bottom of the bubble, r 2 is approximately equal to the pore body radius. At static