| Eliminating
Vapors in Your Home Caused by Seepage into Your Basement of Dissolved Phase
Contamination: containing products such as gasoline, fuel oil, MTBE, benzene,
toluene, ethylbenzene, xylene, TCE, and PCE |
It has
been previously noted that it is possible to split a plume into two separate
plumes. In many subsurface cleanups, but not all, there is value in splitting
a plume into two portions. When vapors result from the off gassing
of contaminated ground water seeping into your basement, shouldn't the objective
of this type cleanup be to ensure that only clean ground water flows under
your home? This clean ground water will eventually flush much of
the contamination from below your home. Splitting the plume to redirect
the natural ground-water flow regime and thereby, induce clean ground water
to flush contamination from below your home will be illustrated. A previous
figure has illustrated vapor generation in a home from the off gassing
of seepage of contaminated ground water (such as dissolved product) into
the basement when the water table is above the basement slab.
 |
The first figure on this
page depicts an extensive plume of contaminated ground water engulfing this
residence with the water table higher than the basement slab so that seepage
of contaminated water into the basement occurs. The plume of contamination
that is dissolved in the water table is being transported with the ground
water as it moves from a point of higher hydraulic head to a lower hydraulic
head (generally put, but not quite correct - ground water and the product
dissolved in it are moving from points on the water table that have a higher
elevation to those points of lower elevation - similar to the ground water
rolling downhill). Notice that the plume of contaminated ground water is
not that extensive across the transverse-lateral extent (width and depth)
of the plume. Just outside the perimeter defining the transverse-lateral
extent of the plume is where clean ground water can be found. The objective
of splitting the plume into two portions is to influence, by the pumping
of ground water, these outer flowpaths that are free of contamination to
flow under your home flushing any remaining contamination off in a downgradient
direction.
 |
The objective of hydraulic
control is to influence clean ground water to flow under your home. Do I
need to add: that this should be accomplished in a timely manner. Generally,
this definition does not need much time to accomplish. In the second figure,
the recovery well is installed sufficiently upgradient of the house so that
the downgradient extent of the capture zone, known as the stagnation point,
is located at the upgradient side of the house. The stagnation point should
be located sufficiently close to and upgradient of the house so that the
clean ground water from just outside the sidegradient extent and below the
vertical extent of the plume is induced to flow the shortest distance to
the upgradient side of the house through those soils contaminated by dissolved
product. Once the soils on the upgradient side and just under the
upgradient side of the house are flushed of their contaminant load, the
clean ground water, which originated from just beyond the sidegradient extent
of the plume, can now flush the contaminant load in the more distant, downgradient
soils under the basement as will be depicted next.
The second figure depicts early capture of the plume just after the plume
splitting is effectuated. The next figure depicts a later stage of the plume
splitting when the objective of inducing clean ground water to flow beneath
your home is achieved. The objective of inducing clean ground water to flow
beneath your home is to rid those soils and waters underlying your home
of contamination. Since we are addressing
 |
remediation of dissolved
phase seeps into your basement, please remember that those soils surrounding
your home were contaminated by contact with dissolved phase contamination,
and not by contact with a separate phase which might include such products
as gasoline, MTBE, TCE, PCE, or some other toxic substance or mixture. In
other words, this discussion applies to soils contaminated by contacting
ground water that has first been contaminated by the dissolution of product
into the water and not by product directly contacting the soil. Inducing
clean ground water to flow beneath your home may not be the best remedy
if the soils proximate to your house have been contaminated by contact with
separate phase, unless these soils have been exhausted of their contaminant
load (mass) given sufficient time or some other remedial action. Dissolved
product contaminated soils (I call them solusols
- for solute contaminated soils) can be flushed with clean water to remediate
them. It may take eight or nine flushes with clean ground water to exhaust
these soils of the contamination load (mass). Separate phase contaminated
soils (I call them produsols
- for product contaminated soils) can require in excess of 400 flushes of
the pore volume to exhaust contaminants from them. Splitting a plume into
two parts can be accomplished relatively quickly when the transverse-lateral
extent of the plume is limited; the permeability of the soil to transmit
water is adequate; and, the soils surrounding your home are comtaminated
by dissolved product. When these conditions are met, splitting the plume
and flushing the plumelet beyond your basement can be measured in timeframes
of months, and not years, when the recovery well is properly installed to
locate the stagnation point of the capture zone just upgradient of the your
home.
Unfortunately, in practice, many recovery wells have not been located
in similar fashion to that depicted in the second and the third figures.
Mislocation of recovery wells will not abate the vapor problem and possibly
make it worse. |
