Overview of In Situ Bioremediation
Bioremediation is the use of microorganisms (e.g., bacteria, fungi), plants (termed phytoremediation), or biological enzymes to achieve treatment of hazardous waste. Treatment can target a variety of media (wastewater, groundwater, soil/sludge, gas) with several possible objectives (e.g., mineralization of organic compounds, immobilzation of contaminants). In situ bioremediation (ISB) is the application of bioremediation in the subsurface – as compared to ex situ bioremediation, which applies to media readily accessible aboveground (e.g., in treatment cells/soil piles or bioreactors). In situ bioremediation may be applied in the unsaturated/vadoze zone (e.g., bioventing) or in saturated soils and groundwater. Given that we are interested in reactive transport modelling with RT3D, we focus here on bioremediation of contaminants in groundwater.
In situ bioremedation technology was originally developed as a less costly, more effective alternative to the standard pump-and-treat methods used to clean up aquifers and soils contaminated with organic chemicals (e.g., fuel hydrocarbons, chlorinated solvents), but has since expanded in breadth to address explosives, inorganics (e.g., nitrates), and toxic metals (e.g., chromium). 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.
One way to categorize ISB is by the type of metabolism involved. The two high-level 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).
Another way to categorize ISB is by the degree of human intervention. Accelerated in situ bioremediation is at one end of the scale, where there is a high degree of human intervention. In accelerated ISB, 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, which is 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 as the electron acceptor, while anaerobic ISB generally requires the addition of an electron donor (e.g., lactate, benzoate) and potentially 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.
At the other end of the scale, Monitored natural attenuation (MNA) is a method of applying in situ bioremediation with essentially no human intervention. MNA is a multi-faceted approach, one component of which is the degradation/transformation of contaminants by indigenous microorganisms without human intervention (i.e., using existing bacteria and whatever nutrients that are already available in the system). Site characterization, evaluation of the MNA potential (which often includes reactive flow and transport modeling with software like RT3D), and long term monitoring comprise the activities required to implement MNA. Site characterization determines the extent of contamination, the properties of the aquifer, the geochemistry of the aquifer, and the nature of biological reactions in the aquifer. This characterization information can then be used to assess the potential for MNA to prevent contaminant mass from impacting receptors of concern. Analytical and/or numerical models can be used to estimate fate of the contaminants as one line of evidence to support (or reject) MNA. Long-term monitoring is used to assess the fate and transport of the contaminants compared against the predictions. The evaluation can undergo further iterations as more data are collected and the understanding of the system improves.
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 rate of activity 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.
More on MNA
The U.S. EPA OSWER Directive [U.S. EPA, 1997], describes monitored natural attenuation as follows:
The term "monitored natural attenuation", as used in this Directive, refers to the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remedial objectives within a time frame that is reasonable compared to that offered by other more active methods. The "natural attenuation processes" that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants.
Other terms associated with natural attenuation in the literature include "intrinsic remediation", "intrinsic bioremediation", "passive bioremediation", "natural recovery", and "natural assimilation". While some of these terms are synonymous with "natural attenuation," others refer strictly to biological processes, excluding chemical and physical processes.
Natural attenuation processes are typically occurring at all sites, but to varying degrees of effectiveness depending on the types and concentrations of contaminants present and the physical, chemical, and biological characteristics of the soil and groundwater. Natural attenuation processes may reduce the potential risk posed by site contaminants in three ways:
- The contaminant may be converted to a less toxic form through destructive processes such as biodegradation or abiotic transformations
- Potential exposure levels may be reduced by lowering of concentration levels (through destructive processes, or by dilution or dispersion)
- Contaminant mobility and bioavailability may be reduced by sorption to the soil or rock matrix
Advantages of In Situ Bioremediation
- It may be possible to completely transform organic contaminants to innocuous substances (e.g., carbon dioxide, water, ethane).
- Accelerated ISB can provide volumetric treatment, treating both dissolved and sorbed contaminant.
- The time required to treat subsurface pollution using accelerated in situ bioremediation can often be faster than pump-and-treat processes.
- In situ bioremediation often costs less than other remedial options.
- The areal zone of treatment using bioremediation can be larger than with other remedial technologies because the treatment moves with the plume and can reach areas that would otherwise be inaccessible.
- As an in situ (versus ex situ) technology, there is typically little secondary waste generated
- As an in situ (versus ex situ) technology, there is reduced potential for cross-media transfer of contaminants
- As an in situ (versus ex situ) technology, there is reduced risk of human exposure to contaminated media
- With MNA, there is less intrusion because few surface structures are required
- MNA can be used in conjunction with, or as a follow-up to, other (active) remedial measures
- MNA has lower overall remediation costs than those associated with active remediation
Limitations of In Situ Bioremediation
- Depending on the particular site, some contaminants may not be completely transformed to innocuous products.
- If biotransformation halts at an intermediate compound, the intermediate may be more toxic and/or mobile than the parent compound.
- Some contaminants cannot be biodegraded (i.e., they are recalcitrant).
- When inappropriately applied, injection wells may become clogged from profuse microbial growth resulting from the addition of nutrients, electron donor, and/or electron acceptor.
- Accelerated In situ bioremediation relies on appropriate distribution of amendments and thus, may be difficult to implement completely in low-permeability or heterogeneous aquifers.
- Heavy metals and toxic concentrations of organic compounds may inhibit activity of indigenous microorganisms.
- In situ bioremediation usually requires an acclimatized population of microorganisms, which may not develop for recent spills or for recalcitrant compounds.
- With MNA, longer time frames may be required to achieve remediation objectives, compared to active remediation
- With MNA, site characterization/monitoring may be more complex and costly; long-term monitoring and periodic re-evaluation of the remedy effectiveness will generally be necessary
- With MNA, institutional controls may be necessary to ensure long term protectiveness
- With MNA, hydrologic and/or geochemical conditions may change over time and could result in renewed mobility of previously stabilized contaminants, adversely impacting remedial effectiveness
- With MNA, more extensive education and outreach efforts may be required to gain public acceptance of the remedy
Biotransformation of Chlorinated Hydrocarbons
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.
Electron Acceptor Reactions
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).
Electron Donor Reactions
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.
Note:The terms "reductive dehalogenation," "direct dechlorination," and "primary substrate" have been used elsewhere in an inconsistant manner, leading to potential confusion. The electron donor/acceptor terminology provides an explicit description. In general, reductive dechlorination is the process whereby a chlorine atom is removed from the chlorinated compound and is replaced with a hydrogen atom. Direct dechlorination is usually associated with the chlorinated hydrocarbon acting as the electron donor. The primary substrate usually also refers to the electron donor.
References / Further Reading
- 2002 Technical Protocol for Using Soluble Carbohydrates to Enhance Reductive Dechlorination of Chlorinated Aliphatic Hydrocarbons Air Force Center for Environmental Excellence, Brooks City-Base, Texas PDF File
- 2004 Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents Air Force Center for Environmental Excellence, Brooks City-Base, Texas PDF File
- 1992 Ground Water Issue: Fundamentals of Ground-Water Modeling EPA/540/S-92/005 U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C PDF File
- 1994 Natural Bioremediation of Hydrocarbon-Contaminated Ground Water Handbook of Bioremediation R.D. Norris, R.E. Hinchee, R. Brown, P.L. McCarty, L. Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J. Bouwer, R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward eds Lewis Publishers, Boca Raton, Florida pp. 177-199
- 1994 Bioremediation of Chlorinated Solvents Using Alternate Electron Acceptors Handbook of Bioremediation R.D. Norris, R.E. Hinchee, R. Brown, P.L. McCarty, L. Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J. Bouwer, R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward eds Lewis Publishers, Boca Raton, Florida pp. 149-175
- 1993 In Situ Bioremediation: When Does it Work? National Academy Press, Washington, D.C Website
- 2000 Natural Attenuation for Groundwater Remediation National Academy Press, Washington, D.C Website
- 1991 Bioremediation of Chlorinated Solvents Using Alternate Electron Acceptors EPA/540/4-90/054 U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C
- 1992 Ground Water Issue: In-Situ Bioremediation of Contaminated Ground Water EPA/540/S-92/003 U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C PDF File
- 1997a Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites OSWER Directive 9200.4-17 U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C PDF File
- 1997b Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water EPA/540/R-97/504 U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C PDF File
- 2001 A Citizen's Guide to Bioremediation EPA/542/F-01/001 U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C PDF File
- 2006 In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated Sites EPA/625/R-06/015 U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, Cincinnati, Ohio PDF File
- Publications on Remediation Technologies U.S. Environmental Protection Agency, Technology Innovation Office, Washington, D.C Website
- Bioremediation of Chlorinated Solvents Website U.S. Environmental Protection Agency, Technology Innovation Program, Washington, D.C Website
- 1994 Natual Bioremediation of Chlorinated Solvents Handbook of Bioremediation R.D. Norris, R.E. Hinchee, R. Brown, P.L. McCarty, L. Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J. Bouwer, R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward eds Lewis Publishers, Boca Raton, Florida pp. 201-225
- 1998 Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water EPA/600/R-98/128 U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C PDF File