BIOAUGMENTATION FOR GROUNDWATER REMEDIATION: AN OVERVIEW - Bioaugmentation for Groundwater Remediation - page 27

of bioaugmentation with these compounds is varied (Singh et al., 2006 ). For example atrazine
(2-chloro-4-[ethylamine]-6-[isopropylamine]-s-triazine) was introduced as an herbicide in the
late 1960s. Repeated inoculation of the soil with atrazine-degrading organisms removed 72% of
the atrazine under field conditions after 11 weeks (Newcombe and Crowley, 1999 ).
Another herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), was the subject of a successful
field-scale gene bioaugmentation study in which bacteria carrying a 2,4-D degrading plasmid
pJP4 were able to transfer the plasmid to indigenous organisms that successfully expressed the
proteins, with transconjugants representing about 10% of the culturable population (Newby
et al., 2000 ). Similar plasmid transfer in some gene-bioaugmented soils has resulted in
successful 2,4-D degradation (Pepper et al., 2002 ).
Hexachlorocyclohexane (HCH, whose gamma isomer is commonly known as lindane), a
now-banned, highly-chlorinated insecticide (a gamma-aminobutyric acid [GABA] inhibitor), is
still found in high residual concentrations in areas where it was produced or used. HCH and
related chlorinated pesticides are resistant to biodegradation, and often have very low risk-
based cleanup levels because they are biomagnified. In one field-scale pilot test in India, a
single-species bioaugmentation inoculum was used to successfully remediate a site contami-
nated with HCH (Raina et al., 2008 ). The investigators used local products to grow and store the
inoculum, thus reducing cost and increasing the feasibility of bioaugmentation in economically
stressed regions.
1.7.5 Metals
Metals, particularly heavy metals, sometimes accumulate in areas due to industrial activity.
These metals, such as cadmium, mercury, lead, zinc, chromium and nickel, can either be
transformed to a less toxic version of the metal or accumulated and sequestered to reduce
bioavailability or facilitate removal. Microorganisms can reduce and precipitate metals such as
hexavalent chromium (Cr[VI]) and radionuclides such as uranium that are less soluble in reduced
forms. The technology has been successfully demonstrated in field-scale testing, and several
bacterial cultures have been isolated and cultured during field testing (Vrionis et al., 2005 ).
In situ bioremediation is likely to be an important technology for treating several metals
and radionuclides in soils and groundwater, but so far bioaugmentation has not proven
necessary or beneficial (Hazen and Tabak, 2005 ; Wu et al., 2006 ). The sequestration process
also can be aided by plants or in biofilms (Singh et al., 2006 ). Bioaugmentation can be
performed to increase plant growth and thus plant uptake and sequestration (Zaidi et al.,
2006 ; Lebeau et al., 2008 ). Rhizoremediation also can be a successful, plant-dependent
bioaugmentation strategy (Kuiper et al., 2004 ). Depending on the metal and on the soil,
microorganisms can increase metal bioavailability (although sometimes they also do the
opposite) by changing soil pH or by secreting compounds like biosurfactants and siderophores
that increase metal solubility and potential mobility.
1.7.6 Mixed Pollutants
Contaminated sites often contain more than one pollutant, and such mixtures can complicate
the remediation strategy considerably. The orchestration of such a site cleanup can involve more
than one remediation strategy and, if the strategy is bioaugmentation, more than one round or
type of inoculation with different strains. One notable example is the inhibition of reductive
dechlorination of TCE in the presence of TCA, a common cocontaminant (Duhamel et al., 2002 ).
Inhibitory pollutants should be removed prior to bioaugmentation for other target compounds.
For example, at sites contaminated with mixtures that include heavy metals, the metals often can