In Situ Bioremediation When Does it Work? (1993) / Chapter Skim
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Executive Summary
Executive Summary
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While soil cleanup efforts have come closer to meeting regulatory goals, the technologies typically used to decontaminate soils often increase the exposure to contaminants for cleanup crews and nearby residents. The limitations of conventional ground water cleanup technologies and the hazards of conventional soil treatment methods-along with the high costs of both have spurred investigations into alternative cleanup technologies, including in situ bioremediation.
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The microorganisms act against the contaminants only when they have access to a variety of materials compounds to help them generate energy and nutrients to build more cells. In a few cases the natural conditions at the contaminated site provide all the essential materials in large enough quantities that bioremediation can occur without human intervention a process called intrinsic bioremediation More often, bioremediation requires the construction of engineered systems to supply microbestimulating materials-a process called engineered bioremediation Engineered bioremediation relies on accelerating the desired biodegradation reactions by encouraging the growth of more organisms, as well as by optimizing the environment in which the organisms must carry out the detoxification reactions.
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The types of site conditions that favor bioremediation differ for intrinsic and engineered bioremediation. For intrinsic bioremediation, the key site characteristics are consistent ground water flow throughout the seasons; the presence of minerals that can prevent pH changes; and high concentrations of either oxygen, nitrate, sulfate, or ferric iron.
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Intrinsic Bioremediation Intrinsic bioremediation is an option when the naturally occurring rate of contaminant biodegradation is faster than the rate of contaminant migration. These relative rates depend on the type and concentration of contaminant, the microbial community, and the subsurface hydrogeochemical conditions.
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Bioremediation may also be combined with a vapor recovery system to extract volatile contaminants from soils. Finally, it is possible to follow engineered bioremediation, which cleans up most of the contamination, with intrinsic bioremediation, which may be used for final polishing and contaminant containment.
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Because none of these three techniques alone can show with complete certainty that biodegradation is the primary cause of declining contaminant concentrations, the most effective strategy for demonstrating bioremediation usually combines several techniques. Measurements of Field Samples The following techniques for documenting in situ bioremediation involve analyzing the chemical and microbiological properties of soil and ground water samples from the contaminated site: .
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An increase in the ratio of compounds that are difficult to degrade to those that are easily degraded indicates that bioremediation may be occurring. Experiments Run in the Field The following methods for evaluating whether microorganisms are actively degrading the contaminant involve conducting experiments in the field: · Stimulating bacteria within subsites.
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The first strategy, useful when biodegradation is the main phenomenon controlling the contaminant's fate, is to model the abiotic processes to determine how much contaminant loss they account for. Bioremediation is indicated when the concentrations of contaminant actually found in field sites are significantly lower than would be expected from predictions based on abiotic processes (such as dilution, transport, and volatilization)
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Limitations Inherent in Evaluating In Situ Bioremediation Although microorganisms grown in the laboratory can destroy most organic contaminants, the physical realities of the subsurfacethe low fluid flow rates, physical heterogeneities, unknown amounts and locations of contaminants, and the contaminants' unavailability to the microorganisms make in situ bioremediation a technological challenge that carries inherent uncertainties. Three strategies can help minimize these uncertainties: (1)
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Examples of relevant site measurements include distribution of hydraulic conductivities, contaminant concentrations associated with solid or other nonaqueous phases, native biodegradation potential, and abundance of different microbial populations. · Improved models.
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The combination of the intricacies of microbial processes and the physical challenge of monitoring both microorganisms and contaminants in the subsurface makes bioremediation difficult to understand, and it makes some regulators and clients hesitant to trust bioremediation as an appropriate cleanup strategy. The inherent complexity involved in performing bioremediation in situ means that special attention must be given to evaluating the success of a project.
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