BIOAUGMENTATION FOR GROUNDWATER REMEDIATION: AN OVERVIEW - Bioaugmentation for Groundwater Remediation - page 4

The majority of the inocula were for treating fuel hydrocarbons and/or polycyclic aromatic
hydrocarbons (PAHs), but roughly 10% claimed the ability to treat halogenated aliphatic
compounds (Major and Cox, 1992 ). Bioaugmentation cultures for hydrocarbon degradation
were tested in several well-monitored studies, including controlled field trials following the
Exxon Valdez oil spill of 1989. In most cases, bioaugmentation inocula had little effect on the
rate or extent of removal of fuel hydrocarbons (Tagger et al., 1983 ; Lee and Levy, 1987 ; Venosa
et al., 1996 ). Numerous studies demonstrated that populations of oil-degrading bacteria in soil
and water increase in the presence of oil (Lee and Levy, 1987 ; Button et al., 1992 ; Atlas, 1993 ;
Prince, 1993 ), and results from field trials of bioaugmentation were generally no better than
biostimulation alone (Atlas and Bartha, 1972 ; Swannell and Head, 1994 ).
In many cases, the effectiveness of commercial bioaugmentation cultures has been
difficult to assess. Complete biodegradation pathways often were not understood or docu-
mented, and few controlled field trials were performed. Many doubted the ability of the added
microbes to thrive, or even survive, long enough to degrade the contaminants (Goldstein et al.,
1985 ). In addition, drastic changes in the ecosystem (e.g., aerobic to anaerobic) also slowed the
microbial community transition and adaptation to the targeted pollutant(s). The prevailing
ecological theory was that the microbial strains present at a site were those that were best
suited to their niche, so the natural communities would remain stable even when subjected to
moderate levels of biotic or abiotic stress (Suflita et al., 1989 ). Furthermore, the general
consensus in the early 1990s was that the genetic potential to degrade most if not all
contaminants already existed in the environment and could be expressed by manipulation of
environmental conditions.
1.2.2 Recent Developments: Bioaugmentation with Dehalococcoides
for Reductive Dehalogenation of Chlorinated Ethenes
Due to these early disappointments, developments in the area of bioaugmentation were met
with skepticism, and there was relatively little research interest until the chlorinated ethene
pollution problem was recognized in the late 1990s. The bioremediation of chlorinated ethenes
often had been unsuccessful using conventional bioremediation techniques. Few indigenous
organisms were capable of complete degradation, with long lag times and incomplete treatment
(e.g., the “ cis -1,2-dichloroethene [ cis -DCE] stall”) being typical. Reductive dechlorination of
perchloroethene (PCE; also termed perchloroethylene or tetrachloroethylene) and trichlor-
oethene (TCE) was recognized as early as 1983 (Bouwer and McCarty, 1983 ). The observation
that highly chlorinated compounds were degraded under anaerobic conditions (Vogel et al.,
1987 ; Mohn and Tiedje, 1990 ) led to an increase in the stimulation of anaerobic conditions in situ
for the degradation of these compounds, although the identity of the responsible organisms was
not known. Research demonstrated that each subsequent reductive dechlorination step was
slower than the preceding one, often resulting in the accumulation of vinyl chloride (VC),
with VC being a carcinogenic gas more hazardous than the more chlorinated compounds.
As a result, researchers temporarily abandoned the idea of anaerobic biodegradation of
PCE and TCE, and for several years, research focused on the use of aerobic cometabolic
biodegradation of these compounds (Fogel et al., 1986 ; Little et al., 1988 ; Oldenhuis et al., 1989 ).
However, cometabolic biodegradation proved difficult to implement successfully. In general,
the ineffective treatment of chlorinated compounds was due, in some cases, to the time needed
for growth of the competent microorganisms to sufficient numbers (Morse et al., 1998 ; Ellis
et al., 2000 ). In other cases, competent microorganisms did not exist at the cleanup sites, and
this is where bioaugmentation normally proves its advantage.