Natural attenuation and enhanced bioremediation of organic contaminants in groundwater
Introduction
Microbial degradation of organic contaminants in groundwater can occur naturally, supported by available electron donors, electron acceptors and nutrients, or through human intervention using enhanced or engineered bioremediation technologies. Monitored natural attenuation, also referred to as intrinsic or passive remediation, has been defined in different ways by various governmental agencies and scientists depending on its end use [1, 2, 3]. A general definition of natural attenuation is the reduction in toxicity, mass and/or mobility of a contaminant without human intervention owing to both physical (e.g. dilution, sorption and precipitation) and biological processes (biodegradation) [4]. Smets and Pritchard [5] reviewed recent approaches and data providing insight into the microbiology of natural attenuation processes. The term ‘enhanced bioremediation’ encompasses a broad continuum of technologies that differ with respect to their inputs [6, 7]. These technologies may involve the addition of electron acceptors, electron donors or nutrients to stimulate naturally occurring microbial populations (biostimulation) or could introduce specific microorganisms aimed at enhancing the biodegradation of the target compound (bioaugmentation). Bioaugmentation can involve the ex situ stimulation of naturally occurring microbial populations that are reinjected into the contaminated site, the addition of wild-type strains or mixed cultures not native to the site that are capable of biodegrading or co-metabolizing the target compound, or the addition of genetically modified organisms.
Three lines of evidence are recommended to demonstrate bioremediation: reduction in contaminant mass or concentration with time; changes in hydrogeologic or geochemical data providing indirect evidence of transformation; and in situ or microcosm studies providing direct evidence of biodegradation [3, 5]. New results from field studies have increased the database for the first of these and novel geochemical approaches are improving our ability to measure indirect evidence of transformation. The development and application of new technologies (e.g. measuring metabolic and taxonomic biomarkers or isotopic fractionation) is also making in situ or microcosm studies increasingly attainable. Our review reports recent advances in elucidating the processes and organisms involved in remediation of groundwater aquifers polluted by organic contaminants, with primary attention given to chlorinated solvents, the fuel additive methyl tert-butyl ether (MTBE), and petroleum hydrocarbons. We also consider research coupling aquifer geochemistry and biodegradation, the use of stable isotopes and molecular tools to demonstrate biodegradation processes, and studies to measure the transport of inoculated microorganisms.
Section snippets
Chlorinated solvents
A recurring issue for chlorinated solvents is whether bioaugmentation or biostimulation is required for successful treatment [8] and whether transformation of these contaminants terminates with the production of undesirable metabolites such as vinyl chloride. At Kelly air force base, biostimulation with lactate and nutrients was followed by bioaugmentation with a mixed methanogenic culture containing Dehalococcoides (not detected at the site) [9•]. Dehalococcoides includes strains capable of
In situ analyses of biodegradation processes by push-pull tests
Push-pull tests, in which isotopes are injected into aquifers and then quickly retrieved and analyzed, can provide direct evidence for the activity of in situ biodegradation processes. For example, monitoring of metabolites formed after the injection of deuterated surrogates of aromatic compounds into groundwater provided the basis for estimating in situ biodegradation rates [33]. In another case, the reduction of the fluorinated analog trichlorofluoroethene was used to estimate the
Interactions with aquifer geochemistry
The importance of understanding the mineral and aqueous geochemistry of aquifers to a successful bioremediation strategy is continually underscored in the emerging literature. Analyses using multivariate statistics or artificial neural networks can be useful to link geochemistry with biodegradation or microbial community data [37, 38, 39]. For example, the frequency of dissimilatory sulfite reductase genes was related to uranium and sulfate concentrations at a site [39]. A critical review of
Use of isotopes to monitor bioremediation
Field methods using isotopes, particularly stable isotopes of carbon and hydrogen, either in fractionation approaches or as tracers, provide evidence of bioremediation processes and give new insights [43, 44]. Compound-specific isotope analysis (CSIA) directly measures biodegradation in situ by analyzing changes in the isotopic fractionation of the remaining pool of contaminant or emerging metabolites, over time. Recent developments include applications to new contaminants (e.g. MTBE),
Use of molecular tools to study microbial communities involved in natural attenuation and biostimulation
Nucleic acid based tools are powerful for relating biodegradation processes to specific microbial populations [60, 61]. These methods offer insight into bioremediation by providing evidence of biodegradation, by implicating specific organisms or populations in biotransformation events, and by quantifying environmentally relevant organisms. An important challenge is to obtain representative samples of microbial communities in situ. One approach is to combine push-pull tests with the analysis of
Transport and tracking of bacteria added to aquifers during bioaugmentation
New techniques have been developed to study the effect of cell properties on rates of attachment to particles and to track inoculated populations after injection into aquifers. Cell-surface characteristics that have been measured include hydrophobic interactions, electrostatic interactions, and electrophoretic mobility. In a study of the MTBE-degrading bacterium Hydrogenaphaga flava ENV735, an adhesion-deficient mutant was found to be non-flagellated, highly mobile, less hydrophobic and
Conclusions
Microbial processes responsible for the biodegradation of organic contaminants in groundwater are the driving forces behind natural attenuation and can be harnessed in enhanced bioremediation technologies such as biostimulation and bioaugmentation. Increasing demands by regulators to provide evidence for bioremediation opens up new applications for the rapidly emerging field of molecular microbial ecology. Technologies that target environmentally important functional genes or specific
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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