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Extracting Liquid Contamination from Soils and Groundwater
“Phase” in chemistry means, a discrete homogeneous part of a material system that is mechanically separable from the rest.
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Example:
In the material system of ice and water, the ice is a phase separable
from the water; and the water is a phase separable from the ice.
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Example: In the material system of the oil and the water shown, we see two phases separable from each other. |
A
“material system” is a sample of matter in which substances in
different phases are in equilibrium. In the above illustrations we see
different phases (oil and water, or ice and water) which are in states
of equilibrium with each other.
NAPL
is an acronym of “Non-Aqueous Phase Liquid.” meaning, liquid that
does not become a solution in water but rather floats on it, or sinks
in it. (Aqua in Latin means Water). In the above examples, the oil is a NAPL. The iceberg is not a NAPL because it is solid.
LNAPL
is an acronym of “Light Non-Aqueous Phase Liquid.” It means a NAPL that
is lighter than water, and therefore will float. The oil phase in the
above example is an LNAPL. Here are some LNAPLs that we meet in
the contaminated soil and groundwater cases:
Upon reaching groundwater, LNAPLs stay at the water table zone.
LNAPLs
possess low levels of solubility; only a small amount will dissolve
into the water. When the amount present at the water surface is greater
than the amount that can dissolve into the water, we get a free phase
product lying on the water.
Groundwater table is seldom static, moving up and down seasonally.
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As the water table fluctuates, the free phase smears up and down into the soil particles, creating a “smear zone.”
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In
the LNAPL case illustrated above, we first act to remove the body; that
is the free phase product that saturates the soil at and under the
point of discharge and “pancakes” on the water table underneath.
An efficient way to remove the LNAPL is by high vacuum extraction. Here is how it works:
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We
lower the water table. In the diagram you see a well installed
into the saturated zone. The bottom of the well is slotted
(screened) to allow the water in. Inside we have a “stinger”
tube. The stinger is perforated at the bottom.
We
apply high vacuum to the stinger. This create a strong suction at the
tip which gets the water in. As the water flows in, its level in the
soil surrounding the well drops in a shape of a cone. We’ve created a
“cone of depression” in the saturated zone.
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Through
the perforations in the stinger a vacuum (thus suction) is now induced
through the exposed soil inside the depression. This pulls in a flow of
air which evaporates the free phase liquid in the soil and carries it
to the stinger and out. The evaporation works very well in the case of
gasoline which is highly volatile. In the case of diesel that is less
evaporative, the free phase diesel gets pulled in as liquid with the
water and in the in the air stream; the operation produces good results
but overall is less efficient than in the case of gasoline.
The
free phase floating “pancake” (see earlier illustration) is pumped in
through the tip of the stinger with the extracted water.
The
picture below shows a similar system but with two stingers in the well:
one for lowering the water table and one for inducing vacuum to pull in
the product that is smeared in the saturated zone.
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