Environmental Engineering Reference
In-Depth Information
to >90%, and an 80% reduction in total NAPL content was reported based
on pre-and postflushing PITTs.
Brooks (2000) described a third cosolvent flushing field demonstration
at the Dover National Test Site (DNTS) at Dover Air Force Base, DE. This
test was conducted in an isolated test cell into which a known volume of
PCE had been injected. The dimensions of the cell were approximately 5 by
3 by 12 m deep, with a clay layer forming the lower boundary of the cell.
DNAPL was injected into the formation at approximately 10.5 m below the
surface, creating a source zone of approximately 1.5 m thickness. A total of
41,700 l of 95% ethanol was used during this flushing demonstration. Upon
extraction from the test cell, the cosolvent solution was treated on site to
remove PCE and reinjected into the test cell. Because of this recycling system,
a total of 69,700 l of ethanol was displaced through the test cell. Based on
mass balance measurements, 71% of the resident PCE was removed by this
flushing process.
These results and others have indicated that PCE- and TCE-constituted
DNAPLs are amenable to extraction-based mass reductions. However,
although the potential for substantial mass removal has been demonstrated,
the debate over the relative and absolute benefit of partial source removal,
in terms of costs and risk reduction, is currently unresolved. It is clear that
even under the best of extraction scenarios, residual amounts of contami-
nants will remain at levels that could preclude meeting regulatory require-
ments for site closure.
5.1.3 Biotransformation of chlorinated solvents and subsurface
microbial ecology
Heterotrophic organisms (humans and most bacteria) oxidize organic com-
pounds to obtain energy. In this process, electrons or reducing equivalents
from the oxidizable organic compound (substrate/electron donor) are trans-
ferred to, and ultimately reduce, an electron acceptor. The electron acceptor
may be an organic or inorganic compound. During this electron transfer
process, usable energy is recovered through a complex series of oxida-
tion-reduction (redox) reactions by the formation of energy storage com-
pounds or electrochemical gradients. The oxidation of organic compounds
coupled to the reduction of molecular oxygen is termed aerobic heterotrophic
respiration. When the oxidation of the organic compounds is linked to other
electron acceptors (i.e., sulfate, nitrate, ferric iron, chloroethenes) by bacteria,
it is referred to as anaerobic respiration. The oxidation of the organic matter,
linked to oxygen reduction or other electron acceptors, to yield energy and
carbon compounds for microbial growth is termed oxidative catabolism.
Pristine (oxygenated) groundwater environments are electron donor lim-
ited. When readily degradable organic matter enters the subsurface in suf-
ficient quantities, it can produce a variety of chronologically and spatially
defined metabolic zones. These zones are dependent on the availability of
electron acceptors. As organic matter enters an oxygenated aquifer, whether
