Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 96

microorganisms. Their function is to augment the indigenous microbial population such
that effective degradative capability can be obtained. If need be, biostimulation can also
be added to the bioaugmentation to further increase the likelihood of effective degrada-
tive capability. We need to be conscious of the risks that arise when unknown results
are obtained from interactions between the genetically engineered microorganisms and
the various chemicals in the contaminated ground. The use of microorganisms grown in
uncharacterized consortia, which include bacteria, fungi, and viruses can produce toxic
metabolites (Strauss, 1991). In addition, the interaction of chemicals with microorganisms
may result in mutations in the microorganisms themselves and/or microbial adaptations.
3.4.1.7 Enhanced Natural Attenuation
We show in Figure 3.11 a direct application of ENA as an in situ remediation process.
Enhanced treatment of a region (spatial and vertical) of the site down-gradient from the
contaminated site permits the ENA to function as planned. The treated region is called
the in situ reactive region (IRR) or treatment zone , and can be used in conjunction with other
treatment procedures. Figure 3.11 illustrates the use of the IRR as a treatment procedure
for the contaminant plume in the region in front of the PRB. Treatment procedures using
treatment wells or boreholes and associated technology include
• Geochemical procedures such as pH and Eh manipulation
• Soil improvement techniques such as introduction of inorganic and organic ligands,
introduction of electron acceptors and donors
• Various other biostimulation procedures and bioaugmentation
The choice of any of these, or a combination of these methods of augmentation, will
depend on the type, distribution, and concentration of contaminants in the contaminated
site, and also on the results obtained from microcosm and treatability studies.
3.4.1.8 In Situ Reactive Regions—Treatment Zones
We have seen from Figure 3.11 an example of the use of a treatment zone, known also as
an IRR, i.e., the region immediately in front of the PRB. The purpose of an IRR is to pro-
vide not only pretreatment or preconditioning in support of another treatment procedure,
but also as a posttreatment process for sites previously remediated by other technological
procedures. In the drawing shown in Figure 3.11, we show the IRR used in support of the
PRB treatment procedure. Other treatment procedures can also be used in place of the
PRB. The presence of heavy metals in combination with organic chemicals in the contami-
nant plume is not an uncommon occurrence. One could, for example, envisage using IRR
as a treatment procedure in combination with a subsequent procedure designed to ix or
remove the metals.
In application of IRR as a posttreatment process, one is looking toward the IRR as the inal
cap for some kind of design or technological process for remediation of a contaminated site.
This is generally part of a multiple-treatment process—as opposed to the use of IRR in a
pretreatment or preconditioning process. A good example of this is the use of pump-treat
procedure as the irst phase of the remediation program, followed by the IRR as a posttreat-
ment process where the treated contaminant plume will receive its inal cleanup. The efi-
ciency of cleanup using pump-and-treat methods rapidly decreases as greater contaminant