Natural Attenuation for Groundwater Remediation (2000) / Chapter Skim
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3 Scientific Basis for Natural Attenuation
3 Scientific Basis for Natural Attenuation
Pages 65-149
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From page 65... ...
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.
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From page 66... ...
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.
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From page 68... ...
68 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION TABLE 3-1 Categories of Subsurface Contaminants, Frequency of Occurrence, and Sources Chemical Class Occurrence Examples of Industrial Sources or Applications Example Compounds Frequencya Organic Hydrocarbons Low molecular BTEX, alkanes F weight Crude oil, refined fuels, dyestuffs, solvents High molecular Polycyclic aromatic C Creosote, coal tar, weight hydrocarbons, crude oil, dyestuffs, nonvolatile aliphatic lubricating oils hydrocarbons Oxygenated hydrocarbons Low molecular Alcohols, ketones F weight esters, ethers, phenols, MTBE Halogenated aliphatics Fuel oxygenates, solvents, paints, pesticides, adhesives, pharmaceuticals, fermentation products, detergents Highly Tetrachloroethene, F Dry cleaning fluids, chlorinated trichloroethene, degreasing solvents 1,1,1 -trichloroethane, carbon tetrachloride Less chlorinated 1,1-Dichloroethane, F Solvents, pesticides, 1,2-dichloroethene, landfills, vinyl chloride, biodegradation methylene chloride by-products, plastics Halogenated aromatics Highly Pentachlorophenol, C Wood treatment, chlorinated PCBs, polychlorinated insulators, heat dioxins, exchangers, polychlorinated by-products of dibenzofurans, chemical synthesis, chlorinated benzenes combustion by-products Less chlorinated Chlorinated benzenes, C Solvents, pesticides PCBs Nitroaromatics TNT, RDX C Explosives continues
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From page 69... ...
lust 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.
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From page 70... ...
70 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION Wind Direction Smokestack Leaking Tank Leaked Contaminant Groundwater Flow Direction Groundwater Table $~~ FIGURE 3-2 Comparison of a plume of dissolved contaminant in groundwater (bottom) with the visible plume from a smokestack (top)
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From page 71... ...
Goals of source removal would be the following: 1. remove as much contaminant mass as practical, in the hope of reducing the longevity and perhaps concentration of the contaminant plume; and 2.
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From page 72... ...
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.
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From page 73... ...
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.
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From page 74... ...
enhance attenuation reduce interfere with electron reactions time for anaerobic acceptor attenuation degradation delivery reactions Free-product Decrease None Reduce Remove recovery source mass source electron donor for reductive dehalogenation; introduce oxygen continues
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From page 75... ...
SCIENTIFIC BASIS FOR NATURAL ATTENUATION TABLE 3-2 Continued 75 Natural Attenuation of Petroleum Hydrocarbons Natural Attenuation of Chlorinated Solvents Other Remediation Possible Possible Possible Possible Activities Benefits Detriments Benefits Detriments In-well Remove Interfere with Remove Interfere with stripping and mass; anaerobic mass; anaerobic recirculation enhance degradation enhance degradation aerobic aerobic de gradation de gradation Soil vapor Remove Interfere with Remove Interfere with extraction mass; anaerobic mass; anaerobic enhance degradation enhance degradation; aerobic aerobic remobilize degradation degradation DNAPL A. . r spargmg Remove mass; enhance aerobic degradation Interfere with anaerobic degradation Remove mass; enhance aerobic degradation Stop anaerobic degradation; remobilize DNAPL Bioslurping Control None Enhance Interfere with source; aerobic anaerobic enhance degradation degradation aerobic Passive O2 addition degradation Enhance aerobic degradation Not applicable Enhance aerobic degradation Interfere with aerobic degradation Carbon sources Not Not Stimulate Result in addition applicable applicable aerobic incomplete cometabolism utilization of or anaerobic carbon source; dechlorination form byproducts Cosolvent or Remove Cause spreading Remove Spread surfacant mass of contaminant; mass contaminant; flooding result in result in incomplete incomplete removal of removal of cosolvent or cosolvent or surfactant surfacant; result in removal of electron donors continues
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From page 76... ...
retained by mineral or organic matter in soil or the underlying vadose zone, (2) intercepted by plant roots, or (3)
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Because of the diversity of flow paths and biogeochemical reactions, groundwater composition varies considerably from one location to another. Nonetheless, some generalizations can be made.
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. MOVEMENT OF CONTAMINANTS IN THE SUBSURFACE Whether or not chemical or microbial reactions transform a contaminant, the contaminant always is subject to transport processes meaning that physical processes cause it to move.
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2. Hydraulic conductivity is the ability of porous rocks or sediments to transmit fluids and is measured from field tests or samples.
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Like hydraulic conductivity, it depends on the type of geologic material present, and it can be determined from field tests or samples. The equation for describing the rate of groundwater flow from one location to another is known as Darcy's equation: VD = KH ~: (3-1)
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From page 81... ...
The transfer of an organic compound from a NAPL source to the surrounding water increases the contaminant concentration in groundwater. The rate of transfer varies depending on the type of NAPL.
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. Some biogeochemical reactions can transform a contaminant into a benign form or immobilize it permanently.
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, which can be thought of as a cellular fuel. Like all living organisms, microorganisms generate ATE by catalyzing redox reactions: they transfer electrons from electron-rich chemicals to electron-poor chemicals.
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The amount of energy yielded varies depending on the electron donor and electron acceptor used. Collectively, microorganisms can use a wide range of electron donors, including both organic and inorganic chemicals.
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Microorganisms capture the electrons in an electron carrier, shown here as reduced nicotinamide adenine dinucleotide (NADH2~. The energy generated by redox reactions during respiration is captured in high-energy phosphate bonds of adenosine triphosphate (ATP)
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From page 86... ...
When oxygen is depleted in an advancing contaminant plume, anaerobic conditions can develop and lead to the formation of as many as five different downgradient zones, each with a different terminal electron acceptor (Anderson and Lovley, 1997~. In these zones, BTEX degradation processes are slower and less reliable than when oxygen is present.
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From page 90... ...
go o ¢ En lo o :.
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From page 91... ...
were transformed in less than 10 days. Under sulfatereducing conditions, toluene, m-xylene, and o-xylene were completely TABLE 3-4 Reliability of BTEX Biodegradation When Various Terminal Electron Acceptors are Present Terminal Benzene Toluene Ethylbenzene Xylenes Electron Acceptor Lab Field Lab Field Lab Field Lab Field Oxygen R R R R R R R R Nitrate IU IU R R R R R R Manganese NU NU NU NU NU NU NU NU Sulfate IU IU RS RS RS RS RS RS Iron RS RS RS RS RS RS RS RS Carbon dioxide IU IU LS LS LS LS LS LS NOTE: IU = investigated and found unreliable (meaning that while biodegradation may occur under the most favorable laboratory conditions, it is highly unlikely to occur in the field)
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From page 92... ...
. Polycyclic Aromatic Hydrocarbons In contrast to BTEX, PAHs biodegrade very slowly.
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A primary mechanism by which this transformation can occur is "reductive dechlorination," in which one C1- ion is released as the molecule accepts two electrons
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From page 95... ...
Figure 3-7 shows this sequential transformation process.2 Biodegradable organic materials must be present as electron donors for reductive dechlorination of chlorinated aliphatic hydrocarbons to occur. In addition, the transformation requires consortia of many microorganisms, as shown in Figure 3-8.
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From page 96... ...
The bottom step shows that PCE must compete for electrons with sulfate, iron, and carbon dioxide, meaning that a large amount of organic electron donors may be needed to supply enough electrons. NOTE: CDCE = cis-dichloroethene.
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From page 97... ...
Reductive dechlorination of the contaminants competes with other electron acceptors for the electrons from hydrogen and acetate (Smatlak et al., 1996; Yang and McCarty, 1998~. When reductive dechlorination is not highly successful in this competition, it gains only a small share of the available electrons.
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From page 98... ...
water emerges from the anaerobic environment of a plume into an aerobic stream or lake, oxygen may cause aerobic cometabolism of chlorinated aliphatics to occur. One exception to the general rule that chlorinated aliphatic hydrocarbons require special environmental conditions for biodegradation to occur is methylene chloride, known as dichloromethane.
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From page 99... ...
As the reductive dehalogenation process removes halogen atoms from the benzene ring, the molecules become more susceptible to biodegradation by aerobic microbes. When environmental conditions are right, natural attenuation may be able to control halogenated aromatic compounds, but these conditions generally are uncommon.
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From page 100... ...
The presence in the environment of congeners containing fewer chlorine atoms than the parent compounds is evidence that reductive dehalogenation reactions occur in nature. Lightly chlorinated PCBs (those containing one to four chlorine atoms)
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From page 101... ...
Microbial Transformation of Inorganic Contaminants Many research reports have documented that microorganisms can transform inorganic contaminants (Babu et al., 1992; Banaszak et al., 1999; Brierley, 1990; Chapatwala et al., 1995; Hinchee et al., 1995; Kalin et al., 1991; Lenhard et al., 1995; Lovley, 1993; McHale and McHale, 1994; Saouter et al., 1995; Summers, 1992; Thompson-Eagle and Frankenberger, 1992; Videla and Characklis, 1992; Whitlock, 1990~. However, unlike organic compounds, which microbes can convert completely to CO2, H2O, and other innocuous products, most inorganic contaminants can be changed only to forms with different mobilities.
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From page 102... ...
, and the use of Cr(VI) as a respiratory electron acceptor.
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From page 103... ...
) as a terminal electron acceptor.
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From page 104... ...
can serve as a final electron acceptor for anaerobic microorganisms, resulting in production of selenide and/or elemental Se. Methylation of the various selenium compounds is a detoxification mechanism that mobilizes Se away from microbial cells, but methylselenium is mobile and highly toxic to mammals.
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From page 105... ...
can serve as a terminal electron acceptor for anaerobic microorganisms (Lovley, 1995~. In this process, the organisms convert highly soluble U(VI)
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From page 106... ...
TRANSFORMATION BY CHEMICAL REACTIONS A variety of geochemical reactions can influence the potential success of natural attenuation in controlling contamination. Types of reactions include acid-base, redox, precipitation and dissolution, chemical sorption, hydrolysis, radioactive decay, and aqueous complexation.
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From page 107... ...
can reduce halogenated solvents, including TCE, by reductive dechlorination. Sulfides (S2-, HS-, and H2S produced from sulfate reduction)
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From page 108... ...
Soluble organic compounds from plant decay and/or microbial activity can participate in a wide range of redox reactions with groundwater contaminants and naturally occurring metals. The most important of these in groundwater are humic substances (soluble remains of decomposed plants and organisms)
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From page 109... ...
. Although complexation does not remove contaminant mass from the groundwater system, it is important in determining the natural attenuation of dissolved metals in groundwater for three reasons.
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, and Cu(II) are likely to be fully complexed by organic matter or OH-; and Ni(II)
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From page 111... ...
, and the concentration of competing species, especially H+ and exchanging cations. Further, sorption reactions sometimes take place in two steps: a rapid surface adsorption reaction followed by slow absorption.
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From page 112... ...
Under conditions likely to be found in groundwater, 1,1,1-trichloroethane (1,1,1-TCA) is the only major chlorinated solvent that can be chemically hydrolyzed within the one- to two-decade time span of general interest in site remediation.
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From page 113... ...
, and chemical reactions (acid-base, redox, precipitation, dissolution, complexation, sorption, hydrolysis, and radioactive decay)
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Biodegradation or chemical transformation of a contaminant produces or consumes other materials, and these compounds serve as footprints. Examples of footprints from reactions that destroy or immobilize contaminants include the following: · the products of reductive dechlorination of TCE and PCE, such as VC, ethene, and C1-; · the depletion of electron acceptors, such as oxygen, nitrate, and sulfate, or the formation of reduced end products, such as methane and Fe(II)
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From page 115... ...
Coast Guard Air Station at Traverse City, Michigan. The NAPL source and the resulting plume of dissolved contaminants went undetected until 1980, when BTEX contamination was discovered in drinking water wells downgradient from the release (see Figure 3-11~.
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From page 116... ...
Coast Guard Air Station at Traverse City, Michigan, prior to installation in 1985 of a pump-and-treat system to control the contaminant source. The top diagram shows the migration of the plume off Coast Guard property and through industrial and residential areas before it discharges to Traverse Bay.
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From page 117... ...
Once the contaminant was controlled, natural attenuation successfully reduced dissolved BTEX concentrations to below harmful levels. Vandenberg Air Force Base: Persistent MTBE in a Fuel Spill Leaking fuel-storage facilities at a General Services Administration gas station at Vandenberg Air Force Base in California created a plume of petroleum hydrocarbons and MTBE.
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, and BTEX contamination arising from gasoline leaks at a former service station at Site 60, Vandenberg Air Force Base, California. extends approximately 520 m (1,700 It)
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Borden Air Forces Base: Partial Bioclegraclation of ChIorinatec! Solvents Approximately 12 m3 of a contaminant solution were injected into a shallow sand aquifer at the Canadian Air Forces base in Borden, Ontario, for the purpose of studying the fate of chlorinated solvents in the subsurface in a controlled setting.
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From page 120... ...
Concentrations of cis-DCE, VC, and ethene are high at the site, providing an indicator that TCE is biodegrading. A large amount of organic matter leaching from a disposal lagoon is driving the biodegradation of TCE by reductive dechlorination (McCarty and Wilson, 1992~.
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SCIENTIFIC BASIS FOR NATURAL ATTENUATION 121 The factory from which the ICE plume arose at the St. Joseph, Michigan, site.
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Railroad I 1 ~ 5~50 ~ \\\\ . ~100_ \~\c 200~ · 1~ ~ ~ `` ~ T 1 i ;~1 IJ FIGURE 3-14 Concentrations of chlorinated aliphatic hydrocarbons (as represented by chemical oxygen demand)
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From page 123... ...
The absence of primary substrates to create reducing conditions and drive dechlorination of TCE explains why the TCE plume moves with the groundwater and without attenuation by biodegradation. Dover Air Force Base: Natural Attenuation by Sequential Bioclegraclation Reactions Groundwater at a portion of Dover Air Force Base, Delaware, known as Area 6 West is contaminated with TCE and 1,1,1-TCA, which were
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From page 124... ...
These data suggest that the methanogenic conditions normally accompanying reductive dehalogenation The X-15 rocket plane, which TCE was used to clean at Edwards Air Force Base. Disposal of TCE in the nearby desert created a large plume of groundwater contamination.
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SCIENTIFIC BASIS FOR NATURAL ATTENUATION 125 Aerial view of Edwards Air Force Base. SOURCE: Courtesy of Edwards Air Force Base, California.
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From page 126... ...
Joseph site, extensive groundwater analyses have been unable to identify the source of the electron donor that is driving reductive dechlorination of the solvents. Concentrations of organic carbon, which might serve as an electron donor, are unusually low, and only small amounts of BTEX (another potential donor)
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From page 127... ...
Sediment cores removed from the Hudson River show that the composition of the PCBs has changed over the years from highly chlorinated mixtures to lightly chlorinated ones due to natural biodegradation reactions (Brown et al., 1987~. Laboratory tests with microorganisms from the contaminated sediments confirmed that these organisms can remove chlorine atoms from PCBs via the reductive dechlorination process (Bedard and Quensen, 1995; Quensen et al., 1988~.
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From page 128... ...
Thus, the observed footprints of PCB dechlorination, although encouraging, are insufficient in themselves to ensure that natural attenuation will be sufficient to decrease contaminant concentrations to meet regulatory standards. South Glens Falls, New York: Natural Attenuation of PAHs Following Source Removal The Electric Power Research Institute and Niagara Mohawk Power Company collaborated to assess how removing the contaminant source at a coal tar disposal site would affect natural attenuation of dissolved contaminants from coal tar remaining in groundwater (Taylor et al., 1996~.
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From page 129... ...
By the later time, much of the region between the source removal area and transect B contained less than 10 ,ug/liter naphthalene, which is below the New York State Department of Environmental Conservation drinking water standard. Phenanthrene another dissolved constituent from coal tar has a similar fate.
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From page 130... ...
NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION Pre-Source Removal - June 1990 A Source area\ of November 1994 /; B C N \o// C D o D E ° Sample Location F A A Transect >2.0 ~1.0-2.0 1_ 0.5-1.0 - 0.5 1 1 0.01 - 0.1 FIGURE 3-16 Change in maximum concentration of naphthalene in a groundwater plume following coal tar source removal. SOURCE: Taylor et al., 1996.
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From page 131... ...
Pina! Creek Basin: Multiple Natural Attenuation Processes Affect Metals Acidic drainage from former mine sites is a common source of metal contamination in groundwater.
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From page 132... ...
Geological Survey.
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From page 133... ...
, but father downgradient the pH increases to 5-6 as carbonate minerals neutralize the acidity. The low-pH region corresponds to the region with high concentrations of dissolved metals.
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From page 134... ...
One particular well at Hanford, the 216-B-5 reverse well, used for disposal of medium-level radioactive wastes from 1945 to 1947, is representative of the effects of past disposal practices on groundwater quality in the nuclear weapons production complex, now controlled by the Department of Energy. The well was 92 m deep 2 m below the water table and the lower 18 m were perforated to distribute waste solutions into the surrounding sediments.
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From page 135... ...
Over the years, a small, stationary contaminant plume developed. The plume consists of radionuclides with longer half-lives (90Sr, Arcs, and 239, 240Pu)
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From page 137... ...
A moderate level of understanding indicates studies confirm that the dominant attenuation process occurs. Low understanding indicates that the attenuation processes may have been observed, but the level of scientific understanding of the processes involved is insufficient to judge whether natural attenuation can achieve regulatory standards for protection of public health and the environment.
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From page 138... ...
38 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION TABLE 3-6 Likelihood of Success of Natural Attenuation Likelihood of Dominant Current Success Given Attenuation Level of Current Level of Chemical Class Processes Understandinga Understandingb Organic Hydrocarbons BTEX Biotransformation High High Gasoline, fuel oil Biotransformation Moderate Moderate Nonvolatile aliphatic Biotransformation, Moderate Low compounds immobilization Polycyclic aromatic Biotransformation, Moderate Low hydrocarbons immobilization Creosote Biotransformation, Moderate Low immobilization Oxygenated hydrocarbons Low-molecular-weight Biotransformation High High alcohols, ketones, esters MTBE Biotransformation Moderate Low Halogenated aliphatics Tetrachloroethene, Biotransformation Moderate Low trichloroethene, carbon tetrachloride Trichloroethane Biotransformation, Moderate Low abiotic transformation Methylene chloride Biotransformation High High Vinyl chloride Biotransformation Moderate Low Dichloroethene Biotransformation Moderate Low Halogenated aromatics Highly chlorinated PCBs, tetrachloro- Biotransformation, Moderate Low dibenzofuran, immobilization pentachloro phenol, multi chlorinated benzenes Less chlorinated PCBs, dioxins Biotransformation Moderate Monochlorobenzene Biotransformation Moderate Nitroaromatics TNT, RDX Low Moderate Biotransformation, Moderate abiotic transformation, immobilization Low continues
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From page 139... ...
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.
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From page 140... ...
· 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.
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From page 141... ...
1995. Microbial reductive dechlorination of polychlorinated biphenyls.
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From page 142... ...
1997. Anaerobic degradation of polycyclic aromatic hydrocarbons and alkanes in petroleum-contaminated marine harbor sediments.
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From page 143... ...
1999. Comparison of eight innovative site characterization tools used to investigate an MTBE plume at Site 60, Vandenberg Air Force Base, California.
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From page 144... ...
Applied and Environmental Microbiology 65:47884792. Harkins, S
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From page 145... ...
Applied and Environmental Microbiology 56:1858-1864. Lovley, D
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From page 146... ...
Applied and Environmental Microbiology 64:646-650 Malmqvist, A., T Welander, and L
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From page 147... ...
1988. Reductive dechlorination of polychlorinated biphenyls by anaerobic microorganisms from sediments.
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From page 148... ...
1997a. In situ transformation of halogenated aliphatic compounds under anaerobic conditions.
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From page 149... ...
Applied Environmental Microbiology 5:200-202. Wackett, L
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