| Basics of In Situ Bioremediation (ISB) |
In situ bioremediation (ISB) is the use of microorganisms to degrade contaminants in place with the goal of obtaining harmless chemicals as end products. Most often in situ bioremediation is applied to the degradation of contaminants in saturated soils and groundwater, although bioremediation in the unsaturated zone can occur. The technology was developed as a less costly, more effective alternative to the standard pump-and-treat methods used to clean up aquifers and soils contaminated with chlorinated solvents, fuel hydrocarbons, explosives, nitrates, and toxic metals. ISB has the potential to provide advantages such as complete destruction of the contaminant(s), lower risk to site workers, and lower equipment/operating costs. Battelle has applied their Chlorinated Solvent Bioremediation Design Service at many sites, focusing primarily on chlorinated solvent contamination.
ISB can be categorized by metabolism or by the degree of human intervention. At a high level, the two categories of metabolism are aerobic and anaerobic. The target metabolism for an ISB system will depend on the contaminants of concern. Some contaminants (e.g., fuel hydrocarbons) are degraded via an aerobic pathway, some anaerobically (e.g., carbon tetrachloride), and some contaminants can be biodegraded under either aerobic or anaerobic conditions (e.g., trichloroethene).
Accelerated in situ bioremediation is where substrate or nutrients are added to an aquifer to stimulate the growth of a target consortium of bacteria. Usually the target bacteria are indigenous, however enriched cultures of bacteria (from other sites) that are highly efficient at degrading a particular contaminant can be introduced into the aquifer (termed bioaugmentation). Accelerated ISB is used where it is desired to increase the rate of contaminant biotransformation, which may be limited by lack of required nutrients, electron donor, or electron acceptor. The type of amendment required depends on the target metabolism for the contaminant of interest. Aerobic ISB may only require the addition of oxygen, while anaerobic ISB often requires the addition of both an electron donor (e.g., lactate, benzoate) as well as an electron acceptor (e.g., nitrate, sulfate). Chlorinated solvents, in particular, often require the addition of a carbon substrate to stimulate reductive dechlorination. The goal of accelerated ISB is to increase the biomass throughout the contaminated volume of aquifer, thereby achieving effective biodegradation of dissolved and sorbed contaminant.
Monitored natural attenuation (intrinsic bioremediation) is the other method of applying in situ bioremediation. One component of natural attenuation is the use of indigenous microorganisms to degrade the contaminants of concern without human intervention (such as supplementing the available nutrients). Site characterization, reactive flow and transport modeling, and long term monitoring comprise the activities required to implement natural attenuation. The site characterization determines the extent of contamination and the properties of the aquifer. This characterization information can then be used in a reactive transport model to predict the fate of the contaminants and whether the contaminants will affect the receptors of concern. Long-term monitoring is used to assess the fate and transport of the contaminants compared against the predictions. The reactive transport model can then be refined to obtain better predictions.
Whether accelerated ISB or natural attenuation is used at a particular site will depend upon the aquifer properties, chemical concentrations, goals of the remediation project, and the economics of each option. The rate of contaminant degradation is typically slower in a natural attenuation scenario than for active bioremediation because the concentration of bacteria is much greater in accelerated bioremediation and the biodegradation rate is proportional to the amount of biomass. Thus, natural attenuation typically takes longer to complete. Accelerated ISB usually provides a faster solution, but has a much greater investment in materials, equipment, and labor.
Chlorinated hydrocarbons can undergo biotransformation via three different mechanisms: use of the chlorinated compound as an electron acceptor, use of the chlorinated compound as an electron donor, or by cometabolism (fortuitous reaction providing no benefit to the microorganisms) One or more of these mechanisms may be active at a given site.
A chlorinated hydrocarbon may be used as an electron acceptor. Use of chlorinated compounds as electron acceptors has been demonstrated under nitrate- and iron-reducing conditions, but the most rapid biodegradation rates, affecting the widest range of chlorinated aliphatic hydrocarbons, occur under sulfate-reducing and methanogenic conditions. This mode of biotransformation requires an appropriate source of carbon (electron donor) for microbial growth and reductive dehalogenation to occur. Electron donor carbon may come from natural organic matter, anthropogenic sources (e.g., fuel hydrocarbon co-contamination), or intentional introduction of organic carbon into the aquifer (i.e., in accelerated in situ bioremediation).
In this situation, the CS is used as the primary substrate (electron donor) and the microorganism obtains energy and organic carbon from the CS. This may occur under aerobic and some anaerobic conditions. Lesser oxidized chlorinated compounds (e.g., vinyl chloride, DCE, or 1,2-dichloroethane) are more likely to be amenable to this mode of biotransformation. Note that fuel hydrocarbons are biodegraded under this mode of operation because they can be used as an organic carbon source.
When a chlorinated aliphatic hydrocarbon is biodegraded via cometabolism, the degradation is catalyzed by an enzyme or cofactor that is fortuitously produced by the organisms for other purposes. The microbe receives no known benefit from the degradation of the chlorinated compound. The biotransformation of the CS may actually be harmful/inhibitory to the microorganisms. Cometabolism is best documented in aerobic environments, although it potentially could occur under anaerobic conditions.