3
Scientific Basis for Natural Attenuation

To evaluate whether natural attenuation can achieve legal standards for groundwater cleanup, the fate of contaminants in the groundwater environment has to be well understood. In what direction do the contaminants move? How far will they spread? Will they degrade to innocuous compounds? While similar questions need to be answered for any proposed remedy for contamination, providing clear answers is especially important for natural attenuation remedies because of the unique public concerns described in Chapter 2.

This chapter describes the common classes of groundwater contaminants, the characteristics of the subsurface environment, and the subsurface processes that can affect contaminants. For each contaminant class, it describes case examples of sites at which natural attenuation has been carefully studied. The chapter then summarizes what is and is not known about natural attenuation of the different contaminant classes and when it is likely to succeed.

Although assessing contaminant fate in the subsurface environment is complex, in many cases science and experience provide a foundation for judging when different combinations of contaminants are likely to degrade or transform in different hydrogeologic settings. In other cases, better scientific understanding is needed before making decisions about natural attenuation. In some cases, natural attenuation works well to minimize risks, because the contaminant’s fate is controlled by a process that destroys the contaminant before it moves far. Biodegradation is the most common example. In other cases, the contaminant can be permanently



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Natural Attenuation for Groundwater Remediation 3 Scientific Basis for Natural Attenuation To evaluate whether natural attenuation can achieve legal standards for groundwater cleanup, the fate of contaminants in the groundwater environment has to be well understood. In what direction do the contaminants move? How far will they spread? Will they degrade to innocuous compounds? While similar questions need to be answered for any proposed remedy for contamination, providing clear answers is especially important for natural attenuation remedies because of the unique public concerns described in Chapter 2. This chapter describes the common classes of groundwater contaminants, the characteristics of the subsurface environment, and the subsurface processes that can affect contaminants. For each contaminant class, it describes case examples of sites at which natural attenuation has been carefully studied. The chapter then summarizes what is and is not known about natural attenuation of the different contaminant classes and when it is likely to succeed. Although assessing contaminant fate in the subsurface environment is complex, in many cases science and experience provide a foundation for judging when different combinations of contaminants are likely to degrade or transform in different hydrogeologic settings. In other cases, better scientific understanding is needed before making decisions about natural attenuation. In some cases, natural attenuation works well to minimize risks, because the contaminant’s fate is controlled by a process that destroys the contaminant before it moves far. Biodegradation is the most common example. In other cases, the contaminant can be permanently

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Natural Attenuation for Groundwater Remediation immobilized. However, the fate of a contaminant never is controlled by one process alone. Often, several physical, biological, and chemical processes act simultaneously. Which processes are important depends on the contaminant and the hydrogeologic setting. CONTAMINANTS AND HYDROGEOLOGIC SETTINGS Modern society uses enormous quantities of organic and inorganic chemicals. Through accidental and purposeful releases, some of these chemicals enter the environment and become contaminants. Contaminants enter the subsurface in a variety of forms, including in solid materials, liquids, and vapors. Figure 3-1 illustrates many ways by which wastes can contaminate soil and groundwater. In general, this report focuses on groundwater contaminants released from “point sources,” such as waste pits, landfills, mine wastes, buried containers, and leaking storage tanks. Nonetheless, the principles also apply to contaminants released from nonpoint sources such as agricultural fields, farm animal lots, urban runoff, and polluted rainfall. Table 3-1 lists the common classes of groundwater contaminants and provides examples in each class, along with common industrial sources or applications. The table is organized by chemical classes, because the various natural attenuation processes tend to affect contaminants within each class in similar ways. Although the chemicals in Table 3-1 are listed as pure materials, they commonly occur as mixtures. Many products used in industry and commerce are mixtures. For example, gasoline contains hundreds of hydrocarbons, as well as a range of organic and inorganic additives (Rittmann et al., 1994). Solvents and other industrial feedstocks are not 100 percent pure but contain small amounts of other compounds. Landfills usually receive a wide range of chemicals that can leach into groundwater. Wastes generated from cleaning operations contain the cleaning agents, as well as chemicals removed during cleaning. Wastes from nuclear weapons manufacturing sites typically contain mixtures of radionuclides, metals, solvents, and organic chelating agents (DOE, 1990; Rittmann et al., 1994). Complex interactions among contaminants, natural environmental chemicals, and microorganisms—as well as the complex processes affecting contaminant and groundwater movement—typically make understanding the fate of such mixtures in the subsurface a challenge. When a contaminant dissolves into the groundwater, it creates a plume, which moves with the groundwater. Figure 3-2 illustrates a groundwater plume formed below a leaking tank and compares it to a visible plume from a smokestack. The contaminants in the plume always move in the same direction, although not necessarily at the same speed, as the

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Natural Attenuation for Groundwater Remediation FIGURE 3-1 Activities that can lead to the contamination of groundwater. SOURCE: Fetter, 1999. Reprinted, with permission from American Geophysical Union (1999). © 1999 by the American Geophysical Union.

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Natural Attenuation for Groundwater Remediation TABLE 3-1 Categories of Subsurface Contaminants, Frequency of Occurrence, and Sources Chemical Class Example Compounds Occurrence Frequencya Examples of Industrial Sources or Applications Organic   Hydrocarbons   Low molecular weight BTEX, alkanes F Crude oil, refined fuels, dyestuffs, solvents High molecular weight Polycyclic aromatic hydrocarbons, nonvolatile aliphatic hydrocarbons C Creosote, coal tar, crude oil, dyestuffs, lubricating oils Oxygenated hydrocarbons   Low molecular weight Alcohols, ketones, esters, ethers, phenols, MTBE F Fuel oxygenates, solvents, paints, pesticides, adhesives, pharmaceuticals, fermentation products, detergents Halogenated aliphatics   Highly chlorinated Tetrachloroethene, trichloroethene, 1,1,1-trichloroethane, carbon tetrachloride F Dry cleaning fluids, degreasing solvents Less chlorinated 1,1-Dichloroethane, 1,2-dichloroethene, vinyl chloride, methylene chloride F Solvents, pesticides, landfills, biodegradation by-products, plastics Halogenated aromatics   Highly chlorinated Pentachlorophenol, PCBs, polychlorinated dioxins, polychlorinated dibenzofurans, chlorinated benzenes C Wood treatment, insulators, heat exchangers, by-products of chemical synthesis, combustion by-products Less chlorinated Chlorinated benzenes, PCBs C Solvents, pesticides Nitroaromatics TNT, RDX C Explosives

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Natural Attenuation for Groundwater Remediation Chemical Class Example Compounds Occurrence Frequencya Examples of Industrial Sources or Applications Inorganic   Metals Cr, Cu, Ni, Pb, Hg, Cd, Zn F Mining, gasoline additives, batteries, paints, fungicides Nonmetals As, Se F Mining, pesticides, irrigation drainage Oxyanions Nitrate, (per)chlorate, phosphate F Fertilizers, paper manufacturing, disinfectants, aerospace Radionuclides Tritium (3H), 238,239,240Pu, 235,238U, 99Tc, 60Co, 137Cs, 90Sr I Nuclear reactors, weaponry, medicine, food irradiation facilities NOTE: BTEX = benzene, toluene, ethylbenzene, and xylene; MTBE = methyl tert-butyl ether; PCBs = polychlorinated biphenyls; TNT = trinitrotoluene; RDX = royal Dutch explosive (1,3,5-trinitrohexahydro-s-triazine). a F = very frequent; C = common; I = infrequent. groundwater. The core of the plume normally has the highest concentrations of the dissolved contaminant, while the fringes have lower concentrations. Just as a visible plume from a smokestack eventually disappears, a groundwater plume also can become nondetectable due to various subsurface processes, explained later in this chapter. REMOVAL OF CONTAMINANT SOURCES At most contaminated sites, the bulk of the contaminant mass is in what remediation professionals call “source zones.” Examples of source zones include landfills, buried tanks that contain residual chemicals, deposits of tars, and mine tailings piles. These types of sources sometimes can be easily located (especially if they are visible like landfills and tailings piles), and complete or partial removal or containment may be possible. However, other common types of sources often are extremely difficult to locate and remove or contain. One example of a source in this category is chemicals that have sorbed to soil particles but have the potential to later dissolve into groundwater that contacts the soil. Another, extremely important example is the class of organic contaminants known as “nonaqueous-phase liquids” (NAPLs). There are two types of NAPLs: those that are more dense than water (dense nonaqueous-phase liquids, or DNAPLs), and those that are less dense than water (light nonaqueous-

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Natural Attenuation for Groundwater Remediation FIGURE 3-2 Comparison of a plume of dissolved contaminant in groundwater (bottom) with the visible plume from a smokestack (top). In both cases, the contaminants move in the same direction as the air or groundwater.

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Natural Attenuation for Groundwater Remediation phase liquids, or LNAPLs). When released to the ground, these types of fluids move through the subsurface in a pattern that varies significantly from that of the water flow, because NAPLs have different physical properties than water. As shown in Figure 3-3, LNAPLs can accumulate near the water table; DNAPLs can penetrate the water table and form pools along geologic layers; and both types of NAPLs can become entrapped in soil pores. These NAPL accumulations contaminate groundwater that flows by them as they dissolve slowly at concentrations sufficient to pose a public health risk. Common LNAPLs include fuels (gasoline, kerosene, and jet fuel), and common DNAPLs include industrial solvents (trichloroethene, tetrachloroethene, and carbon tetrachloride). Once they have migrated into the subsurface, NAPLs are often difficult or impossible to locate in their entirety. Normally, the total mass of a contaminant within source zones is very large compared to the mass dissolved in the plume. Therefore, the source usually persists for a very long time. For example, the rate at which contaminants dissolve from a typical NAPL pool is so slow that many decades to centuries often are needed to dissolve the NAPL completely (NRC, 1994). Given the persistent nature of contaminant sources, removing them would seem like a practical way to speed natural attenuation of the contaminant plume. In many cases, environmental regulators require source removal or containment as part of a natural attenuation remedy. Although requiring source control or removal is good policy for many sites, expert opinions conflict on whether source removal is advisable when using natural attenuation as a remedy, even when such removal is technically feasible. Goals of source removal would be the following: remove as much contaminant mass as practical, in the hope of reducing the longevity and perhaps concentration of the contaminant plume; and avoid any changes that would reduce the effectiveness of natural attenuation. In theory, if one can delineate the source essentially completely and succeed in removing most of the mass, then a significant benefit may be achieved. Later, this chapter presents a case study of a polycyclic aromatic hydrocarbon (PAH) plume in which it appears that, after removal of the source, the plume itself attenuated rapidly. However encouraging this example might be, this kind of success may not always be realized. More commonly, the source cannot be delineated completely and/or cannot be removed to any significant degree even if located perfectly. Hence, source

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Natural Attenuation for Groundwater Remediation FIGURE 3-3 Examples of sources of groundwater contamination. In (a), the amount of LNAPL released is not large enough to reach the water table in pure form, but components of the LNAPL dissolve in infiltrating water and create the plume of contamination shown beneath the water table. The situation portrayed in (b) is analogous to the scenario in (a), except the contaminant is a DNAPL. As shown in (c), when an LNAPL release is large enough, free-phase LNAPL will pool near the water table. In contrast, as shown in (d), a DNAPL will migrate beneath the water table, because the DNAPL is more dense than water. In all cases, the undissolved LNAPLs and DNAPLs serve as a long-term source feeding the development of contaminant plumes in groundwater. removal options may be rejected because none are anticipated to remove enough of the source mass to warrant the expense and risks of the removal effort. In some cases, source removal efforts may directly and adversely affect natural attenuation. Technical guidelines on natural attenuation developed for the Navy present a summary of interactions between

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Natural Attenuation for Groundwater Remediation various active remediation technologies, some of which are used in source removal efforts, and natural attenuation. Table 3-2 is adapted and condensed, with some revisions, from the Navy’s summary. As evident in Table 3-2, active technologies that introduce oxygen to the subsurface could have negative effects on the biodegradation of petroleum hydrocarbon or chlorinated solvent plumes. Source control methods that could introduce oxygen include excavation, pumping and treating of groundwater, free-product recovery, in-well stripping, soil vapor extraction, air sparging, bioslurping, cosolvent or surfactant flushing, and thermal treatment. (See NRC, 1997, 1999, for descriptions of these engineered remediation methods.) An additional potential problem is that removal of the source of one type of contaminant may adversely affect natural attenuation of another type and thus result in minimal or no overall benefit. A good example is the removal of a petroleum hydrocarbon source zone that was serving as a nutrition source for microbes involved in degrading a chlorinated solvent plume. (Details of this type of process are discussed later in this chapter.) Such an action could slow down or completely shut off natural attenuation of the chlorinated solvent. When natural attenuation is the primary remediation mechanism, source removal has to be undertaken with caution. When negative effects on natural attenuation are not anticipated, and where it is feasible and reasonably efficient, source removal is advisable. However, other than for fuel hydrocarbon NAPLs, removing sufficient contaminant mass to justify the effort can be extremely difficult. Furthermore, source removal efforts may interfere with the present or future efficiency of natural attenuation. For these reasons, source removal may often be unjustified or even undesirable. In such cases, natural attenuation, if effective can serve as a long-term source management method, but the attenuation reactions will have to be sustainable for a long period of time. Hydrogeologic Settings Water and contaminants do not flow freely in the subsurface as they would in a river, but instead must travel through the circuitous pore spaces of subsurface materials. In the upper portion of the subsurface, which is known as the “vadose” or “unsaturated” zone, the pore spaces are only partly filled with water (see Figure 3-4).1 Below the vadose zone 1   For the most part, the vadose zone is unsaturated and contains air, but shallow regions within the vadose zone can be transiently saturated, which occurs below stream beds and in perched zones above confining strata.

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Natural Attenuation for Groundwater Remediation TABLE 3-2 Potential Effects of Other Remediation Activities on Natural Attenuation Other Remediation Activities Natural Attenuation of Petroleum Hydrocarbons Natural Attenuation of Chlorinated Solvents Possible Benefits Possible Detriments Possible Benefits Possible Detriments Excavation and backfilling Remove mass; enhance oxygen input Alter flow field; enhance spreading Mass removal Interfere with anaerobic degradation; alter flow field; enhance DNAPL spreading Capping Reduce contaminant flux to groundwater Enhance spreading of vapors; reduce oxygen input Enhance anaerobic degradation Enhance spreading of vapors; reduce fermentative creation of substrates; reduce oxygen input for vinyl chloride biodegradation Pump and treat (for plume capture) Contain plume Reduce time available for attenuation reactions Contain plume Reduce time for natural attenuation; introduce oxygen into plume and source area Pump and treat (for mass removal) Control source; enhance electron acceptor delivery Reduce time available for attenuation reactions Control source; reduce time for attenuation reactions Introduce oxygen; interfere with anaerobic degradation Free-product recovery Decrease source mass None Reduce source Remove electron donor for reductive dehalogenation; introduce oxygen

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Natural Attenuation for Groundwater Remediation Other Remediation Activities Natural Attenuation of Petroleum Hydrocarbons Natural Attenuation of Chlorinated Solvents Possible Benefits Possible Detriments Possible Benefits Possible Detriments In-well stripping and recirculation Remove mass; enhance aerobic degradation Interfere with anaerobic degradation Remove mass; enhance aerobic degradation Interfere with anaerobic degradation Soil vapor extraction Remove mass; enhance aerobic degradation Interfere with anaerobic degradation Remove mass; enhance aerobic degradation Interfere with anaerobic degradation; remobilize DNAPL Air sparging Remove mass; enhance aerobic degradation Interfere with anaerobic degradation Remove mass; enhance aerobic degradation Stop anaerobic degradation; remobilize DNAPL Bioslurping Control source; enhance aerobic degradation None Enhance aerobic degradation Interfere with anaerobic degradation Passive O2 addition Enhance aerobic degradation Not applicable Enhance aerobic degradation Interfere with aerobic degradation Carbon sources addition Not applicable Not applicable Stimulate aerobic cometabolism or anaerobic dechlorination Result in incomplete utilization of carbon source; form byproducts Cosolvent or surfacant flooding Remove mass Cause spreading of contaminant; result in incomplete removal of cosolvent or surfactant Remove mass Spread contaminant; result in incomplete removal of cosolvent or surfacant; result in removal of electron donors

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Natural Attenuation for Groundwater Remediation Chemical Class Dominant Attenuation Processes Current Level of Understandinga Likelihood of Success Given Current Level of Understandingb Inorganic   Metals   Ni Immobilization Moderate Moderate Cu, Zn Immobilization Moderate Moderate Cd Immobilization Moderate Low Pb Immobilization Moderate Moderate Cr Biotransformation, immobilization Moderate Low to moderate Hg Biotransformation, immobilization Moderate Low Nonmetals   As Biotransformation, immobilization Moderate Low Se Biotransformation, immobilization Moderate Low Oxyanions   Nitrate Biotransformation High Low Perchlorate Biotransformation Moderate Low Radionuclides   60Co Immobilization Moderate Moderate 137Cs Immobilization Moderate Moderate 3H Decay High Moderate 90Sr Immobilization High Moderate 99Tc Biotransformation, immobilization Low Low 238,239,240Pu Immobilization Moderate Low 235,238U Biotransformation, immobilization Moderate Low NOTE: Knowledge changes rapidly in the environmental sciences. Some contaminants not rated as having high natural attenuation potential could achieve this status in the future, but this table represents the best understanding of natural attenuation potential at this time. a Levels of understanding: “high” means there is good scientific understanding of the processes involved, and field evidence confirms attenuation processes can protect human health and the environment. “Moderate” means studies confirm that the dominant attenuation process occurs, but the process is not well understood scientifically. “Low” means scientific understanding is inadequate to judge if and when the dominant process will occur and whether it will meet regulatory standards. b “Likelihood of success” relates to the probability that at any given site, natural attenuation of a given contaminant is likely to protect human health and the environment. “High” means scientific knowledge and field evidence are sufficient to expect that natural attenuation will protect human health and the environment at more than 75% of contaminated sites. “Moderate” means natural attenuation can be expected to meet regulatory standards at about half of the sites. “Low” means natural attenuation is expected to be protective at less than 25% of contaminated sites. A “low” rating can also result from a poor level of scientific understanding.

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Natural Attenuation for Groundwater Remediation evaluating natural attenuation potential, but each site must be evaluated individually to determine whether natural attenuation is sufficiently protective of human health and the environment. Chapter 4 describes site evaluation strategies in detail. CONCLUSIONS Natural attenuation is well established as a remediation approach for only a few types of contaminants, primarily BTEX. For most other contaminant classes, it is not as likely to succeed or not well established. In some cases, the likelihood of success is low because natural attenuation depends on special environmental conditions that may not be present at the site. In other cases, scientific understanding is too limited to judge the potential of natural attenuation as a remedial alternative. Also, at some sites, the possible production of toxic intermediate compounds raises too many regulatory or public concerns about the long-term acceptability of the process. Natural attenuation should never be considered a default or presumptive remedy. Although natural attenuation can be a technically valid means for protecting human health and the environment, its effectiveness must be documented at every site (even those contaminated with BTEX) overseen by environmental regulators. At sites where natural attenuation is shown to be effective, long-term monitoring will be necessary to ensure that key attenuation processes continue to control contamination. To achieve remediation objectives, natural attenuation may have to continue for many years or decades, over which time environmental conditions and natural attenuation processes may change. Natural attenuation of some compounds can form hazardous by-products that in some cases can persist in the environment. Evidence of transformation of a contaminant does not necessarily ensure detoxification. Natural attenuation processes cannot destroy metals but in some cases can immobilize them. The passage of time can enhance or reverse immobilization reactions, depending on the type of reaction, the contaminant, and environmental conditions. In some cases, removing contaminant sources can speed natural attenuation, but in other cases it can interfere with natural attenuation. Removing sources can reduce the mass of contamination that has to be treated by natural processes. However, in some cases it can cut off natural attenuation entirely, if the source is serving as critical fuel for attenuation processes.

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Natural Attenuation for Groundwater Remediation REFERENCES Abramowicz, D. A. 1990. Aerobic and anaerobic biodegradation of PCBs: A review. Critical Reviews in Biotechnology 10(3):241-249. Anderson, J. E., and P. L. McCarty. 1997. Transformation yields of chlorinated ethenes by a methanotrophic mixed culture expressing particulate methane monooxygenase. Applied and Environmental Microbiology 63(2):687-693. Anderson, R. T., and D. R. Lovley. 1997. Ecology and biogeochemistry of in situ groundwater bioremediation. Advances in Microbial Ecology 15:289-350. Atlas, R. M., and R. Bartha. 1997. Microbial Ecology: Fundamentals and Applications, 4th Ed. Menlo Park, Calif.: Benjamin Cummings Publishing Co. Babu, G. R. V., J. H Wolfram, and K. D. Chapatwala. 1992. Conversion of sodium cyanide to carbon dioxide and ammonia by immobilized cells of Pseudomonas putida. Journal of Industrial Microbiology 9:235-238. Baedecker, M. J., D. I. Siegel, P. Bennett, and I. M. Cozzarelli. 1989. The fate and effects of crude oil in a shallow aquifer. I. The distribution of chemical species and geochemical facies. Pp. 13-20 in Millar, G. E., and S. E. Rabone (eds.) U.S. Geological Survey Water Resources Division Report 88-4220. Reston, Va.: U.S. Geological Survey. Baedecker, M. J., J. M. Cozzarelli, D. I. Siegel, P. C. Bennett, and R. P. Eganhouse. 1993. Crude oil in a shallow sand and gravel aquifer. 3. Biogeochemical reactions and mass balance modeling in anoxic ground water. Applied Geochemistry 8:569-586. Banaszak, J. E., D. T. Reed, and B. E. Rittmann. 1999. Subsurface interactions of actinide species and microorganisms: Implications on bioremediation of actinide-organic mixtures. Journal of Radioanalytical and Nuclear Chemistry 241:385-435. Barkay, T., and B. H. Olson. 1986. Phenotypic and genotypic adaptation of aerobic heterotrophic sediment bacterial communities to mercury stress. Applied and Environmental Microbiology 52:403-406. Barker, J. F., G. C. Patrick, and D. Major. 1987. Natural attenuation of aromatic hydrocarbons in a shallow sand aquifer. Ground Water Monitoring Review 7:64-71. Barkovski, A. L., and P. Adriaens. 1996. Microbial dechlorination of historically present and freshly spiked chlorinated dioxins and diversity of dioxin-dechlorinating populations. Applied and Environmental Microbiology 62:4556-4562. Bedard, D. L., and J. F. Quensen III. 1995. Microbial reductive dechlorination of polychlorinated biphenyls. Pp. 127-216 in Young, L.Y., and C. E. Cerniglia (eds.) Microbial Transformation and Degradation of Toxic Organic Chemicals. New York: Wiley-Liss, Inc. Beller, H. R., D. Grbic-Galic, and M. Reinhard. 1992a. Microbial degradation of toluene under sulfate-reducing conditions and the influence of iron on the process. Applied and Environmental Microbiology 58:786-793. Beller, H. R., M. Reinhard, and D. Grbic-Galic. 1992b. Metabolic by-products of anaerobic toluene degradation by sulfate-reducing enrichment cultures. Applied and Environmental Microbiology 58: 3192-3195. Binks, P. R., S. Nicklin, and N. C. Bruce. 1995. Degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Stenotrophomonas maltophilia PB1. Applied and Environmental Microbiology 61:1318-1322. Borden, R. C., R. A. Daniel, L. E. LeBrun, and C. W. Davis. 1997. Intrinsic biodegradation of MTBE and BTEX in a gasoline-contaminated aquifer. Water Resources Research 33:1105-1115. Bouwer, E. J., B. E. Rittmann, and P. L. McCarty. 1981. Anaerobic degradation of halogenated 1- and 2-carbon organic compounds. Environmental Science and Technology 15(5):596-599.

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