BIOAUGMENTATION FOR GROUNDWATER REMEDIATION: AN OVERVIEW - Bioaugmentation for Groundwater Remediation - page 22

introduction of foreign material also introduces unknowns into the system, possibly resulting in
undesired effects like toxic intermediates and clogging. Degradation of the pollutant might
itself lead to secondary water quality impacts, such as taste or odor issues. The selection
process used to develop the bioaugmentation strain might select bacteria with undesirable
properties, such as enhanced antibiotic resistance (Davison, 2005 ). Introduction of a strain that
can grow to large population numbers would almost certainly alter the microbial community
structure (Coppotelli et al., 2008 ). Foreign genes could enter the gene pool and be horizontally
transferred to the indigenous strains. Existing models are simply not sophisticated enough to
predict these effects.
However, there are certain undesirable side-effects that are foreseeable and preventable.
For example, in certain cases, injection of bacteria leads to clogging of the subsurface due to
uncontrolled growth (Vogel, 1996 ). In such cases, the choice of a slow-growing degrader may be
favored over a fast-growing degrader that would quickly use up nutrients in a system and lead
to clogging (Cases and de Lorenzo, 2005 ). The use of potentially pathogenic strains also should
be avoided (Singer et al., 2005 ).

1.6 BIOAUGMENTATION TO REMEDIATE CHLORINATED
COMPOUNDS
The primary focus of this volume is on bioaugmentation to remediate chlorinated solvent
pollution. Chlorinated compounds are particularly difficult to degrade due to the presence of
the halide, which often makes these compounds more recalcitrant to biodegradation than
unsubstituted hydrocarbons. Halides can be bulky and often obstruct enzymes from reaching
their target bonds, and they are electrophilic (like oxygen) and thus render oxidizing enzymes
less useful. The most widely used chlorinated compounds were often chlorinated solvents,
including PCE and trichloroethane (TCA), carbon tetrachloride (CT) and chlorinated aromatic
compounds like chlorobenzene. Chlorinated solvents were heavily used as cleaning agents and
to synthesize other chemicals. Due to use, spillage from tanks or pipes and improper disposal of
these agents, chlorinated solvent contamination is widespread.
Besides being recalcitrant, these low solubility chlorinated solvents often sink through soil
and aquifers to form DNAPL pools at the bottom of aquifers (Figure 1.8 ). These DNAPLs
present hard-to-remediate source zones of contamination due to the pure product nature of
DNAPLs and the difficulty of reaching them. As groundwater flows through these DNAPLs, it
spreads soluble phase contamination to an even larger area.
The magnitude of this problem is reflected in part by the quantity of literature on the
subject and the number of government and industry-sponsored research publications.
The Interstate Technology & Regulatory Council (ITRC) has published a guide to evaluating
and implementing in situ bioremediation strategies, including bioaugmentation, at sites

contaminated with chlorinated ethenes (ITRC, 2008 ).

1.6.1 Chlorinated Aliphatic Hydrocarbons (CAHs):
Dehalococcoides and the Chloroethenes

Chlorinated ethenes are the most prevalent groundwater contaminants and pose difficult
remediation challenges, so this contamination is a major environmental concern and a sizeable
commercial opportunity. As discussed earlier, chlorinated ethenes can be degraded to different
degrees both aerobically and anaerobically. Under anaerobic conditions, PCE can be trans-
formed by reduction past the toxic VC intermediate to the non-toxic gas ethene (Freedman