Environmental Engineering Reference
In-Depth Information
subsurface (Figure 5.2d). If this is achieved, complete site restoration is
possible.
Mixing of the cosolvent and the contaminant for both dissolution and
removal of the separate phase DNAPL, and the subsequent biodegradation
of the remaining contaminant, is the most important criterion in the design
of the injection/extraction system for the SERB technology. Delivery of the
electron donor to the subsurface has been a problem for various
in situ
treatment technologies and is usually limited by transport considerations.
By conducting the cosolvent flush, the concurrent exposure of the electron
donor (cosolvent) to the electron acceptor (chlorinated solvent) is facilitated
by the injection/extraction system and the cosolvency effect. Thus, subse-
quent biodegradation of the chlorinated solvent becomes a much more viable
remediation strategy.
For application of the SERB technology at the demonstration site, the
reductive dechlorination of PCE was the bioremediation process that was to
be stimulated. The pathways for the biotransformation of chloroethenes
shown in Figure 5.1 demonstrate why PCE and TCE are extremely recalci-
trant contaminants. There is no evidence for oxidative mechanisms for deg-
radation of these compounds; thus, the reduction dechlorination pathway
is the only known mechanism for breakdown to nontoxic constituents. There
is evidence for both oxidative and reductive pathways for dechlorination of
the other daughter products.
The reductive dechlorination process is a stepwise removal of a chlorine
atom. Production of cis-DCE rather than the other isomers is an indication
that it is a product from the dechlorination of TCE. Monitoring of all daughter
products of PCE gives an indication of the extent of dechlorination occurring
in the system. The reductive dechlorination of chlorinated ethenes can be
conducted by both halorespiration (catabolic) and cometabolic mechanisms.
Different microbial populations are responsible for each of these processes.
Because of the complexity and heterogeneity of the subsurface environment,
it is hypothesized that there can be several competing mechanisms and organ-
isms active at the same time but in different portions of the subsurface. This
is displayed in Figure 5.3, where a consortium of microorganisms is shown to
be responsible for different oxidation-reduction reactions. Various electron
donors and electron acceptors are present in the subsurface and are utilized
and degraded by these reactions. Evaluation of the concentrations of electron
donors and electron acceptors in the site groundwater can be an indication of
the types of reactions that may be occurring. Because most monitoring wells
are screened over a large interval and water will move more quickly through
the layers with a higher hydraulic conductivity, this type of analysis gives a
general view of the screened interval.
5.2 Objectives
The overall objective of this research, as for the entire Flask to Field (FTF)
program, was to design, develop, and implement new biologically based
