In this first figure a
product, such as, gasoline, TCE, or PCE is moving somewhat laterally, but
overall downward, through soils above the water table. Product coating of
soil is indicated by the light brown layer surrounding a soil particle.
Layering of water is indicated by the light blue layer surrounding a soil
particle. Even in a hot, desert soil there will be layers of water surrounding
the individual soil particles. Most times, these water layers that surround
an individual soil particle will coalesce with other nearby water layers
to completely fill some small run of a soil pore. Obviously, the smaller
is the size of the soil-pore diameter, the more likely that water will occlude
that run of the soil pore. Occlusion of the soil pores with water limits
product movement through soils.
This second figure illustrates a latter stage of product
contamination of the soil. It is apparent from this
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figure that not all of
the soil becomes saturated with product, as a separate phase. The surface
tension of the water trapped by capillary forces in the smaller pore sizes
prevents the movement of free product into many of the soil pores. Free
product tends to migrate through the larger soil-pore sizes. The product
will coat the walls of the soil pores and eventually coat the surface of
nearby soil particles. Product layers may coalesce with other product layers
and occlude short runs of soil pores just as does water. These discrete,
disjunct runs of product trapped in the capillary pores are often referred
to as ganglia of product. Product contaminated soil is much more difficult
to remedy than are solusols (dissolved product contaminated soil).
The third figure illustrates that point in time when only clean ground water
enters the system. Since only clean water is entering the soil pore system,
then the flushing of contamination from the soil is now
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possible. As clean water,
indicated by the light blue arrows, flows through the system two things
can happen. Product as a separate phase may be mobilized and driven by the
total head of the soil solution out of the system. Another, possibility is
that the product will dissolve into the water and be transported out of
the system as a dissolved phase.
The product can be mixture of many different chemicals such as gasoline.
There may also be a single compound in a product. TCE (trichloroethylene)
is one such sole chemical in a product. The MTBE in many gasolines will
dissolve into the soil solution or ground water more readily than will benzene
or especially any of the xylene isomers. Therefore, MTBE is more easily flushed
from the soil
than is benzene and the xylene isomers. Product may remain trapped as ganglia
in the soil even after hundreds of flushes with clean water. Unlike produsols,
it is much easier to exhaust soils of contamination if it is only contaminated
by dissolved phase. |
