In Situ Bioremediation When Does it Work? (1993) / Chapter Skim
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4 Evaluating In Situ Bioremediation
4 Evaluating In Situ Bioremediation
Pages 63-90
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From page 63... ...
Without evidence of microbial involvement, there is no way to verify that the contaminant did not simply volatilize, migrate off site, sorb to subsurface solids, or change form via abiotic chemical reactions. This chapter discusses a strategy for evaluating the effectiveness of in situ bioremediation projects, based on showing that microbes were responsible for declining contaminant concentrations.
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From page 64... ...
Thus, regulators and buyers of bioremediation services can use the strategy to evaluate whether a proposed or ongoing bioremediation project is sound. The first type of evidence in the strategy showing decreasing contaminant concentrations-comes from standard sampling of the ground water and soil over time as cleanup progresses.
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From page 65... ...
Measurements of Field Samples A number of techniques for documenting in situ bioremediation involve removing samples of soil and water from the site and bringing them to the lab for chemical or microbiological analysis. Many of these techniques require comparing conditions at the site once bioremediation is under way with site conditions under baseline circumstances, when bioremediation is not occurring.
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From page 66... ...
Third, they identified microbial breakdown products from the solvents in aquifer samples. Fourth, they used models to show that theoretical estimates of biodegradation rates could account for contaminant loss in the field.
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From page 67... ...
When contaminant biodegradation rates are low, such as when contaminant levels are low or biodegradable components are inaccessible, increases in the number of bacteria may not be great enough to detect above background levels, given the error in sampling and measurement techniques. Thus, the absence of a large increase in bacterial numbers does not necessarily mean that bioremediation is unsuccessful.
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From page 68... ...
Major increases in the number of bacteria in the ground water usually correlate to large increases in the total number of bacteria in the subsurface. The main advantages of ground water samples are that they can be taken repeatedly from the same location and that they are relatively inexpensive.
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From page 69... ...
Probing also can be used to determine whether the gene for a particular biodegradation reaction is present. The drawbacks of probing are that it requires considerable prior knowledge of the cells' genetic sequences, it is only semiquantitative in its current state, and it requires specialized equipment and knowledge.
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From page 70... ...
Therefore, samples correlating contaminant loss with growth in the protozoan population can provide further evidence of active bioremediation. The protozoan population can be counted using a statistical MEN technique similar to that used for bacteria.
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From page 71... ...
in the ground water below. An engineered bioremediation system was installed at the site in 1989.
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From page 72... ...
Comparing the microcosm-generated biodegradation rates under a variety of conditions can provide valuable information concerning whether environmental conditions in the field are conducive for high degradation rates. The careful control and monitoring possible in microcosms make rate determinations much less ambiguous than rates measured in the field.
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From page 73... ...
Gas chromatography is the method of choice for determining gaseous CO2 concentrations; inorganic carbon analysis is appropriate for water samples.
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From page 74... ...
Thus, if the inorganic carbon taken from site samples has a i3C/~2C ratio much lower than the ratio for carbon from mineral sources, it is likely that the carbon originates from contaminant biodegradation. The second type of application exploits isotope fractionation, in which microbial metabolism usually creates inorganic carbon that is enriched in :2C, while the remaining organic contaminant source becomes enriched in i3C.
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From page 75... ...
Electron Acceptor Concentration In the process of transforming contaminants, bacteria consume electron acceptors, usually O2, NO3-, or SO42-, as explained in Chapter 2. A depletion in the electron acceptor concentration that occurs simultaneously with contaminant loss is further evidence that bioremediation is occurring.
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From page 76... ...
environment, loss of electron acceptors other than oxygen (for example, nitrate and sulfate) , and consumption of electron donors responsible for the loss of the electron acceptors.
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From page 77... ...
In an effort to demonstrate the practical implications of these studies, the General Electric Corporation sponsored a 10-week in situ biodegradation field test. Researchers anchored six large adjacent cylinders in shallow areas of the Hudson River where the sediments are contaminated with lightly chlorinated PCBs.
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From page 78... ...
Stimulating Bacteria Within Subsites One type of field experiment involves adding materials that stimulate biodegradation to subsites within the contaminated area. Addition of stimulants such as electron acceptors, electron donors, and nutrients should speed biodegradation but not abiotic contaminant removal processes.
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From page 79... ...
Immediately after stopping the flow of sparged gases, an oxygen probe is lowered into ground water wells to measure the rate of oxygen consumption. To distinguish oxygen used by contaminantdegrading microbes from oxygen used by ordinary microbial activity, background oxygen uptake rates should be measured in adjacent uncontaminated wells.
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From page 80... ...
As discussed under "Measurements of Field Samples," some contaminants contain mixtures of degradable and nonbiodegradable compounds that move through the subsurface in similar ways. When the concentration of degradable compounds drops faster than the concentration of conservative tracers, the difference can be attributed to microbial activity in the field.
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From page 81... ...
Saturated flow models start by describing where and how fast the water flows through the saturated zone (the region below the water table)
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From page 82... ...
Multiphase flow models also can describe the flow of dense nonaqueousphase liquids such as chlorinated solvents, which move in a distinct mass separate from the ground water. The phenomena controlling multiphase flow are not as well understood and are much more difficult to represent mathematically than are those for water flow in the saturated zone because they involve complex interactions among solids, water, air, and nonaqueous phases.
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From page 83... ...
Biological reaction rate models represent how quickly the microorganisms transform contaminants. They are useful for evaluating bioremediation systems because the rate at which the microbes work is the key factor influencing how much time the cleanup will take.
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From page 84... ...
Because these conditions are not true in many other situations, biodegradation rate modeling may require different and more sophisticated approaches. Except when the biodegradation or geochemical models are very simple, coupling them with flow models requires more than an extension of the existing contaminant transport models used for conservative tracers.
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From page 85... ...
The approach recognizes that biodegradation rate models often have greater uncertainty that do models of abiotic processes. The uncertainty can be caused by poor understanding of the biochemical reactions, difficulty estimating parameters, and inadequate site characterization.
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From page 86... ...
The field measurements of both the contaminant concentrations and the dissolved oxygen levels taken since 1987 closely match the model's predictions. In addition, the biodegradation rate predicted by the model closely matches the rate measured in the field.
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From page 87... ...
One approach is to use the model to answer the question, "Does our best representation of the biodegradation rates, when combined with the simultaneously occurring abiotic rates, support the conclusion that biological reactions are responsible for observed changes in contaminant levels or other relevant observations? " If the answer is "yes," modeling provides a much greater measure of confidence that observations supporting biodegradation are not artifacts.
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From page 88... ...
LIMITATIONS INHERENT IN EVALUATING IN SITU BIOREMEDIATION Because the subsurface is complex and incompletely accessible, knowledge of the fate of ground water contaminants always will be limited. This situation is intensified for in situ remediation technologies of any type, because frequently the amount, location, and type of contamination are unknown.
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From page 89... ...
This localization makes it difficult to "find" the organisms when only a few samples can be taken. Microbial numbers, biodegradation rate estimates, or biodegradation potentials can vary tremendously, depending on whether the sample was from a location of high microbial activity or from a nearby location with low activity.
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From page 90... ...
Investigators cannot fully understand the details of whether and how bioremediation is occurring at a site. The goal in evaluating in situ bioremediation is to assess whether the weight of evidence from tests such as those described above documents a convincing case for successful bioremediation.
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