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Water and immiscible fluids interact during transport through pore spaces,
distributing themselves according to the properties of the liquids and of the solid
and gas phases. Above the retention capacity, subsurface pore geometry permits
the flow of nonwetting fluids, leaving behind clusters of water-immiscible liquids
that are disconnected from the main body of organic liquid. These clusters are
sometimes called blobs or ganglia, with the trapped immiscible fluid being
referred to as the residual organic liquid saturation. Figure 5.7 illustrates the
retention of such a liquid in a partially saturated porous medium. Thus, trapped
immiscible liquid can remain in the vadose zone for an indefinite time, serving as a
source of contamination that decreases in magnitude as a result of processes such
as volatilization into the gas phase, dissolution in the leaching water, or chemically
or biologically induced decomposition. The degree of porous medium saturation
by an immiscible liquid can be expressed in terms of the utilization of pore space
by the liquid and air phases (van Dam 1967 ; Schwille 1984 ) or as the organic
liquid content in volume units.
If the organic liquid saturation is measured as the volume of organic liquid per
unit void volume, measured over a representative volume of the porous medium,
then S o , the fraction of pore space occupied by the organic liquid is
S o ¼ V organic liquid = V voids ;
ð 5 : 15 Þ
where the subscript o indicates the organic liquid. The residual saturation at which
the organic liquid becomes discontinuous and immobile is then
S or ¼ V discontinuous organic liquid = V voids ;
ð 5 : 16 Þ
where the subscript or indicates residual organic liquid. In the saturated zone, the
water saturation (S w ) is given by S w = 1.0 - S o .
The extent of trapping is determined primarily by the physical properties of the
vadose zone. If the organic liquids are characterized by a low vapor pressure and a
low solubility in water, they remain trapped in the partially saturated zone. In this
particular case, the porous medium behaves like an inert material and the behavior
of the organic liquids depends only on their own properties, with no interaction
between the liquid and the solid phases.
5.7.3 Particle Deposition and Trapping
Retention of suspended particles in porous media occurs by straining (trapping),
physicochemical filtration (deposition), and detachment. Depending on the size of
the suspended particle, a number of mechanisms may be responsible for physi-
cochemical filtration: (1) gravitational sedimentation, where the gravitational
forces acting on the particle cause it to settle onto a sediment grain (collector), (2)
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