feature
Evaluating
Natural ATTENUATION for Groundwater Cleanup
The National Research Council has issued the first comprehensive assessment of w h e n natural attenuation works. J A C Q U E L I N E A. M A C D O N A L D
uring the 1990s, natural attenuation grew from a laboratory research phenomenon to a commonly used approach for the cleanup of contaminated groundwater. For those responsible for cleaning up a contaminated site, using natural attenuation involves filing a formal regulatory application to allow natural biological, chemical, and physical processes to treat groundwater contaminants, sometimes in conjunction with a constructed treatment system, but often without one. According to EPA data, natural attenuation use in the Superfund program climbed during the 1990s from application at 6% to more than 25% of groundwater contamination sites (1). Natural attenuation use increased even more in the Underground Storage Tank (UST) Program (see Figure 1, (2)). By 1997, natural attenuation had become the leading remedy for groundwater contamination from leaking underground storage tanks.
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"Interest in using natural attenuation has skyrocketed," says Ken Lovelace of EPA's Office of Emergency and Remedial Response. Opinions about whether natural attenuation is an appropriate strategy for managing groundwater contamination are highly polarized. Most environmental advocates charge that natural attenuation is little more than an excuse for industry to avoid the high costs of building cleanup systems. Linda Greer, a scientist at the Natural Resources Defense Council (NRDC), calls natural attenuation a "low-cost, ineffective approach that delays the inevitable need to clean up the site." (Greer allows that natural attenuation can be effective for a very limited set of highly biodegradable contaminants.) Industry representatives counter that natural attenuation should be used more broadly because of increasing scientific evidence that concentrations of contaminants in groundwater can decrease without human interven© 2000 American Chemical Society
tion. Bruce Bauman of the American Petroleum Institute says that natural attenuation is "always going to be a component of corrective action strategies for sites." Even if active treatment is required initially, he says, "once that proceeds to the point of diminishing returns, natural attenuation can be used to complete the cleanup." Is natural attenuation oversold or underused? How can sites with contaminated groundwater be evaluated to determine whether natural attenuation really will protect the public from contamination or whether engineered cleanup systems will be needed? According to a National Research Council (NRC) report scheduled for final publication this summer, "Natural attenuation is an established remedy for only a few types of contaminants . . . and . . . should be accepted as a formal remedy for contamination only when the processes are documented to be working and are sustainable" (3). The report is a consensus
document by a committee of volunteer experts from diverse backgrounds, including academia, government environmental laboratories, environmental consulting firms, industries, and environmental groups. The report provides the first comprehensive synthesis of current knowledge about natural attenuation for the full range of contaminants that can contaminate groundwater. Bruce Rittmann of Northwestern University, who chaired the NRC natural attenuation committee, says, "The report delivers a two-part message. On the one hand, natural attenuation is a valid concept and may be a remedy for a range of groundwater contaminants . . . under the right circumstances. On the other hand, natural attenuation should be selected as a remedy only when the mechanisms responsible for destroying or immobilizing the contaminant are scientifically recognized, documented to be working now at the site, and sustained for as long as the contamAUGUST 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 4 7
A
FIGURE 1
Natural attenuation use in the Underground Storage Tank Program According to a survey of state program managers, as of 1997, a variety of methods have been used to clean up substantial quantities of groundwater contaminants from leaking underground storage tanks.
contributed to the slow development of more effective, less costly treatment systems. According to Walter Kovalick of EPA's Technology Innovation Office, "Monitored natural attenuation has arrived on the scene... in the midst of a flat remediation marketplace. It is a factor amongst others in slowing the development of other technologies." NRDC's Greer predicts that at a number of sites, those responsible for the cleanup "will be back to the drawing board" searching for "more expensive remedies necessitated by contaminant migration" if natural attenuation fails to work as predicted. Kovalick suggests, "There is a possibility that natural attenuation could suffer bioremediation's early fate: Too many promises have been made, and expectations are too high."
Varied scientific basis
Source: Reference (2).
ination source is present." Says Rittmann, "Natural attenuation's 'protective technology' is knowledge. This means that natural attenuation is a valid remedy only when we have confidence in the causeand-effect relationship between loss of contaminant and the mechanism responsible for the loss."
Technological limitations The rush to use natural attenuation is driven largely by the high costs of constructing and operating engineered groundwater cleanup systems. The average cost of cleaning up a privately owned Superfund site is more than $25 million (4). A significant portion of this cost goes to the construction of treatment systems. For example, at one Houston site, the companies paying for cleanup reportedly saved $12 million by using natural attenuation instead of an engineered treatment system (5). Another factor responsible for the increased reliance on natural attenuation is the limited effectiveness of groundwater cleanup technologies. In its most recent comprehensive review of these technologies, NRC concluded, "Although considerable effort has been invested in groundwater and soil cleanup, the technologies available for these cleanups are relatively rudimentary" (6). Earlier studies by a number of organizations, including EPA and NRC, had indicated that conventional technologies were not reaching prescribed regulatory cleanup standards at many sites with contaminated groundwater. For example, in a 1994 review, NRC concluded that conventional technologies had restored contaminated groundwater to regulatory standards at 8 of 77 sites evaluated in previous studies (7). The increased use of natural attenuation in response to the high costs and technical limitations of engineered cleanup systems has served as a doubleedged sword. On the one hand, it may have saved money and reduced the backlog of contaminated sites in regulatory programs. On the other hand, it may have 3 4 8 A • AUGUST 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
NRC appointed a natural attenuation committee in late 1997 because of concerns expressed by some members of the National Academy of Engineering—an elected body of the nation's most accomplished engineers—about the controversies surrounding natural attenuation. A central part of the committee's task was to assess current scientific understanding of the fate of different contaminant classes in the subsurface absent human intervention. The committee based its assessment on a review of current scientific and technical literature, field reports, and protocols for assessing natural attenuation; presentations to the committee by other expert scientists, environmental regulators, environmental advocates, industry representatives, and consultants; and the expertise of its members. On the basis of its research, the committee rated the likelihood that natural attenuation will succeed as a remediation strategy as being "high", "moderate", or "low" for different types of contaminants (see Table 1). The ratings provide a qualitative indication of the probability that any given site will have the right conditions for natural attenuation of the particular contaminant. The committee defined high likelihood of success as meaning "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." A moderate rating means "natural attenuation can be expected to be protective at about half of the sites." A low rating generally means "natural attenuation is expected to be protective at less than 25% of contaminated sites." According to the report, "a low rating can also result from a poor level of scientific understanding." Contaminants that are poorly understood are rated low because natural attenuation is difficult to evaluate absent an understanding of the processes involved. As Table 1 shows, 3 types of contaminants are highly likely to be treated successfully with natural attenuation; 10 are moderately likely to be treated successfully; and 20 have a low likelihood of successful treatment. Table 1 also indicates the dominant natural processes—biological degradation, chemical degradation, or physical immobilization—that are the basis for natural attenuation, when it occurs. It also indicates how much scientists currently know about how each
TABLE 1
Likelihood of success of natural attenuation The NRC committee rated as "high", "moderate", or " l o w " the likelihood that natural attenuation will succeed as a remediation strategy for different types of contaminants. Also indicated are the dominant natural attenuation processes and the degree to which these processes are understood for each contaminant.
Chemical class
Dominant
attenuation processes
Current level of understanding
Likelihood of success given current level of understanding
Organic Hydrocarbons BTEX Gasoline, fuel oil Nonvolatile aliphatic compounds PAHs Creosote
Biotransformation Biotransformation Biotransformation, immobilization Biotransformation, immobilization Biotransformation, immobilization
High Moderate Moderate Moderate Moderate
High Moderate Low Low Low
O x y g e n a t e d hydrocarbons Low-molecular-weight alcohols, ketones,
Biotransformation
High
High
esters MTBE
Biotransformation
Moderate
Low
H a l o g e n a t e d aliphatics Tetrachloroethylene, TCE, carbon tetrachlo-
Biotransformation
Moderate
Low
ride TCA Methylene chloride Vinyl chloride Dichloroethylene
Biotransformation, abiotic transformation Biotransformation Biotransformation Biotransformation
Moderate High Moderate Moderate
Low High Low Low
Biotransformation, immobilization
Moderate
Low
Biotransformation Biotransformation
Moderate Moderate
Low Moderate
Biotransformation, abiotic transformation, immobilization
Moderate
Low
Halogenated aromatics Highly chlorinated PCBs, tetrachlorodibenzofuran, pentachlorophenol, multichlorinated benzenes Less chlorinated PCBs, dioxins Monochlorobenzene Nitroaromatics TNT, RDX
Inorganic Metals Ni Cu, Zn Cd Pb Cr
Immobilization Immobilization Immobilization Immobilization Biotransformation, immobilization
Moderate Moderate Moderate Moderate Moderate
Hg
Biotransformation, immobilization
Moderate
Moderate Moderate Low Moderate Low to moderate Low
Nonmetals As Se
Biotransformation, immobilization Biotransformation, immobilization
Moderate Moderate
Low Low
Oxyanions Nitrate Perchlorate
Biotransformation Biotransformation
High Moderate
Moderate Low
Radionuclides Co 137 Cs 3 H ^Sr 99 Tc 238.239.24opu 235.238y
Immobilization Immobilization Decay Immobilization Biotransformation, immobilization Immobilization Biotransformation, immobilization
Moderate Moderate High High Low Moderate Moderate
Moderate Moderate Moderate Moderate Low Low Low
60
Note: BTEX • benzene, toluene, ethylbenzene, and xylene; MTBE • methyl iert-butyl ether; TCE = trichloroethylene; TCA = trichloroethane; PCBs = polychlorinated biphenyls; and PAHs = polycylic aromatic hydrocarbons.
AUGUST 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 4 9 A
TABLE 2
Natural attenuation footprints from National Research Council case studies Footprints of natural attenuation reactions at a dozen field sites that the committee reviewed as case studies.
Case study
Contaminant(s)
Contaminants controlled?
Footprints
Traverse City
BTEX
Yes
Depletion of 0 2 ; formation of CH 4 and Fe 2+
Vandenberg Air Force Base
MTBE
No
Insignificant decrease in 0 2 and S0 4 2 ~ concentration; extension of MTBE plume far beyond BTEX plume
Borden Air Force Base
Five chlorinated solvents
Partially
Detection of metabolites of solvent degradation
St. Joseph
TCE
Partially
Formation of CH 4 ; detection of degradation byproducts (vinyl chloride and ethene)
Edwards Air Force Base
TCE
No
Documentation of high N0 3 ~ and S 0 4 2 " concentration; demonstration that TCE moves with water
Dover Air Force Base
TCE, TCA
Yes
Formation of degradation byproducts (c/s-1,2-DCE, 1-1 -DCA, vinyl chloride, and ethene); CH 4 and H2S formation; increase in C r concentration
Hudson River
PCBs
Partially
Detection of breakdown products; detection of unique transient metabolites; observation of microbial metabolic adaptation
South Glens Falls
PAHs
Yes
Depletion of 0 2 ; detection of unique metabolic byproducts; detection of genes for degrading PAHs in site microorganisms; rapid PAH degradation in soils taken from site
Pinal Creek Basin
Metals, acid
Yes, but may not be sustainable
Observation of carbonate dissolution, leading to pH increase coincident with metal precipitation; observation of M n oxide precipitates in stream sediments
Hanford 216-B-5
Radionuclides
Yes
Sorbed radionuclides observed in site samples
Anonymous field site
BTEX
Yes
Loss of 0 2 , N G y , and S 0 4 2 " ; formation of Fe 2+ and CH 4 ; increase in inorganic carbon concentration; increase in alkalinity
Bemidji
Petroleum hydrocarbons
Partially
Loss of 0 2 ; formation of Fe 2 *, M n 2 * , and CH 4 ; formation of intermediate metabolites; observation of selective degradation of petroleum hydrocarbons relative to more stable chemicals
Note: BTEX = benzene, toluene, ethylbenzene, and xylene; MTBE · methyl (erf-butyl ether; TCE = trichloroethylene; TCA = trichloroethane; PCBs = polychlorinated biphenyls; PAHs = polycylic aromatic hydrocarbons; DCE = dichloroethene; and OCA = dichloroethane.
contaminant fares in natural environments. According to the report, a high level of understanding "means there is good scientific understanding of the process involved, and field evidence confirms attenuation processes can protect human health and the environment" A moderate level of understanding "means that studies confirm that the dominant attenuation process occurs, but the process is not well understood scientifically." Low understanding "means scientific understanding is inadequate to judge if and when the dominant process will occur and whether it will be protective." The report describes in detail how the various processes work, what site conditions are required, and what additional knowledge is needed.
Focus on degradation and transformation In its review, the committee focused on processes that either transform contaminants to less harmful forms or immobilize them in place. This focus was narrower than EPA's current formal regulatory definition of natural attenuation. In 1999, EPA released a formal policy on natural attenuation, entided "Use of Monitored Natural Attenuation at Superfund, RCRA [Resource Conservation and Recovery Act] Correc3 5 0 A • AUGUST 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
tive Action, and Underground Storage Tank Sites." This policy defines natural attenuation as the "reliance on natural attenuation processes . . . to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods" (8). EPA's policy says the relevant processes include "biodégradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants." The NRC committee chose a narrower focus for its review for two reasons. First, transformation (including biological and chemical degradation) and immobilization (including sorption and stabilization) are more difficult to understand and have a more limited knowledge base than dilution and dispersion processes. Second, the committee believes a narrower definition of natural attenuation could reduce the doubts that community members often have about natural attenuation. The committee heard testimony from a number of community environmental advocates who would not accept natural attenuation as a remediation strategy because of the inclusion of dilution and dispersion in the formal reg-
ulatory definition of this phenomenon. "The multiprocess definition of natural attenuation fuels [the] controversy because it includes mechanisms such as dilution and dispersion that are unacceptable ways to manage contamination in the view of many environmental advocates," according to the report. The committee could not agree on whether dilution and dispersion are acceptable strategies for managing contamination. Nonetheless, the committee agreed to focus on degradation and immobilization processes because of the scientific complexity of these processes and because of the concerns expressed by community advocates.
Site-specific evaluations The report emphasizes that every site where natural attenuation is proposed as a remedy needs to be evaluated individually. Even when a site is contaminated with a chemical having characteristics that make it highly likely to degrade or transform naturally according to Table 1, natural attenuation may not occur or may be incomplete if geochemical conditions are not optimal. Conversely, natural attenuation can occur in special circumstances even for recalcitrant contaminants. "[I]t is important to keep in mind that natural attenuation processes are always site-specific: They depend on the hydrogeology and biogeochemistry of the site in question," the report finds. At some sites in the past, environmental regulators have approved the use of natural attenuation as a remedy based on documentation that contaminant concentrations in monitoring wells have declined over time without human intervention. The report stresses that this type of evidence is not sufficient for documenting natural attenuation. "Documenting that the contaminant concentration has become very low or undetectable in groundwater samples is an important piece of evidence that natural attenuation is working... [but] is not sufficient to show that natural attenuation is protecting human health and the environment," the report cautions. Contaminants can bypass monitoring wells, concentrations can decrease near the monitoring well but not in other locations, or the contaminant can transform to a harmful byproduct. "For these reasons, environmental regulators and others should not rely on simple rules of thumb (such as maximum contaminant concentration data or trends in these data over a relatively short time) in evaluating the potential success of natural attenuation," the report notes. The committee developed a three-part recommended process for evaluating the occurrence of natural attenuation at contaminated sites. The process requires (1) providing a conceptual model of the site, (2) looking for "footprints" of expected natural attenuation processes, and (3) monitoring the site. Every site needs at least a simple model showing groundwater flow, contaminant locations and concentrations, and possible natural attenuation reactions. Footprints—a term the committee adopted for its report—are changes in water chemistry left by the attenuation reactions. For example, biodégradation of toluene by bacteria that use oxygen consumes oxygen from the groundwater and adds inorganic carbon as the toluene is converted to
TABLE 3
Effort required to document natural attenuation The approximate level of effort required to document natural attenuation for different broad categories of sites is indicated relative to site hydrogeology and the nature of contaminants present.
Likelihood of success of natural attenuation of the contaminant of concern Site hydrogeology Simple flow, uniform geochemistry, and low concentrations Simple flow, small-scale physical or chemical heterogeneity, and medium-high concentrations Strongly transient flow,
High (e.g., BTEX) 1
Moderate (e.g., Pb)
Low (e.g., MTBE, TCE)
2
2
2
2
3
2
3
3
large-scale physical or chemical heterogeneity, or high concentrations Note: Level of effort refers to number and frequency of samples taken, parameters analyzed in site samples, and type of data analysis. 1 = low effort; 2 = moderate effort; and 3 = high effort. BTEX = benzene, toluene, ethylbenzene, and xylene; MTBE = methyl fert-butyl ether; and TCE · trichloroethylene. 'Likelihood of success refers to judgments in Table 1.
carbon dioxide. Many such changes in water chemistry can be measured to provide evidence that the contaminant is transforming, via known possible reactions. Once such reactions are postulated, monitoring is necessary to show that the attenuation processes continue to occur. The report emphasizes that the types of data gathered to evaluate whether natural attenuation is occurring need to vary considerably depending on the types of contaminants present, the chemistry of the groundwater, and the geologic characteristics of the site. Different contaminants can produce very different footprints upon degradation or transformation. Further, most contaminants can degrade or transform by a number of different mechanisms, depending on site conditions, and often many different mechanisms act in concert. For example, the initial stages of benzene biodégradation often consume all of the oxygen in the groundwater, so later stages proceed by different biotransformation pathways. As a consequence, the search for footprints of natural attenuation must consider the unique conditions of the site (see Table 2). The amount of data collected also will vary with site type. For example, a UST site with one class of contaminant, such as petroleum hydrocarbons, will not require nearly the amount of analysis as required at a site with multiple sources and complex mixtures of contaminants. Likewise, a site in a relatively simple geologic setting, such as a sandy aquifer, will not require the extent of data required for sites in more complex settings, such as rock with complex fracture networks (see Table 3). AUGUST 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 5 1 A
Policy changes
TABLE 4
Natural attenuation protocols reviewed in the NRC study The National Resource Council report reviewed 11 protocols by national organizations, 1 protocol from Minnesota, and 1 protocol from New Jersey. Year published
Contaminants addressed
Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination in Groundwater
1995
Fuels
New Jersey Department of Environmental Protection
New Jersey Administrative Code 7:26E—Technical Requirements for Site Remediation and Classification Exception Areas
1995
General
Chevron
Protocol for Monitoring Intrinsic Bioremediation in Groundwater
1995
Fuels
EPA, Region 4
Draft Region 4 Suggested Practices for Evaluation of a Site for Natural Attenuation (Biological Degradation) of Chlorinated Solvents
1997
Chlorinated solvents
Air Force
Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater
1997
Chlorinated solvents
Minnesota Pollution Control Agency
Draft Guidelines: Natural Attenuation of Chlorinated Solvents in Ground Water
1997
Chlorinated solvents
Chevron
Protocol for Monitoring Natural Attenuation of Chlorinated Solvents in Groundwater
1997
Chlorinated solvents
American Society for Testing and Materials
Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites
1997
Fuels
American Petroleum Institute
Methods for Measuring 1997 Indicators of Intrinsic Bioremediation: Guidance Manual
Fuels
EPA, Office of Research and Development
Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water
1998
Chlorinated solvents
Department of Energy
Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites
1998
General
Navy
Technical Guidelines for Evaluating Monitored Natural Attenuation at Naval and Marine Corps Facilities
1998
Fuels
EPA
Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites
1999
General
Publisher
Title
Air Force
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The report identifies important areas where current policies for cleanup of contaminated sites— promulgated before formal reliance on natural attenuation was common—need to be updated to account for the special concerns that natural attenuation raises. First, the report says, programs for community involvement in deciding on remedies for groundwater contamination need to be revised: "Requirements for public participation need not be any different at sites using natural attenuation remedies than at other sites, but existing public participation programs are inadequate to address the special concerns about natural attenuation." The report points out that community members near contaminated sites often perceive natural attenuation as a "do-nothing" approach. This view is exacerbated when interested members of the affected public are involved late in the decision making, after regulators and those responsible for the cleanup have identified a list of potential remedies for the contamination. At this stage, community members are more likely to be suspicious of candidate remedies, especially natural attenuation, because they were not involved in the earlier evaluations and will not understand the factors considered. The public needs to be involved in reviewing data from the beginning, so that the community will understand why natural attenuation may be a remedy worth considering. "Federal and state environmental regulations and guidelines for cleaning up contaminated sites affecting communities should be changed to allow community involvement as soon as the presence of contamination is confirmed," the report recommends. Second, the report says, EPAs policies on use of protocols for evaluating sites for natural attenuation need to be re-evaluated. Natural attenuation assessment protocols have proliferated along with formal applications to use this approach. In 1993, no such protocols were available. In that year, NRC issued a report, In Situ Bioremediation: When Does It Work?, with a recommended strategy for evaluating the potential for biological attenuation, which the report termed "intrinsic bioremediation", to occur. Since then, at least a dozen national protocols for natural attenuation have been published, and many states have published such documents, as well. The new NRC report reviews 13 of the available protocols (see Table 4). The report gives favorable reviews to many of the protocols but nonetheless concludes that important gaps in the body of protocols need to be filled. To fill these gaps, the report recommends that EPA "lead an effort to develop national-consensus guidelines for protocols on natural attenuation." The report notes that guidelines on how the existing protocols can be used in regulatory programs are lacking. Further, it says, the existing body of protocols generally provides detailed guidance only for fuels and chlorinated solvents; comprehensive technical guides for evaluating other classes of contaminants are lacking. The protocols also provide insufficient direction in several other areas: when and how to involve the public, when and how to implement
contingency plans in case natural attenuation fails, when contaminant sources need to be removed, how to conduct long-term monitoring, and what level of training users need to implement the protocol. The national-consensus guidelines need to address these deficiencies, the report says. An additional weakness of some protocols is the use of "scoring systems" that rate the likelihood that natural attenuation will occur based on site data. "Scoring systems are generally too simple to represent the complex processes involved and often are used erroneously in judging the suitability of a site for natural attenuation," the report notes. The national-consensus guidelines should phase out the use of scoring systems, the report recommends, and should replace scoring systems with the type of three-part evaluation process (using a conceptual model, footprints, and monitoring) that the committee developed.
Report impact The NRC report is likely to prompt more research and a review of current natural attenuation policies. A subgroup of EPA's Science Advisory Board (SAB) is writing a plan for the agency's natural attenuation research. The NRC report will be one of the major pieces of input in developing that plan, according to Domenico Grasso of Smith College, chair of the SAB effort. According to Lovelace, EPA might revise its natural attenuation policy based on the report. "Recommendations in the report might cause us to revisit our policy," Lovelace says. NRC committee chair Rittmann predicts, "Full application of [the recommendations in
the report] will gready improve the confidence levels of all those involved widi a natural-attenuation decision: responsible parties, regulators, and the affected communities." The report may help to forge a consensus among the disparate viewpoints about whether and when allowing nature to take its course is an acceptable way to manage groundwater contamination.
References (1) Lovelace, K. U.S. EPA. Personal communication, 1999. (2) Tulis, D.; Prévost, R E; Kostecki, R Soil Groundwater Cleanup 1998, 7 (July), 12-17. (3) National Research Council. Natural Attenuation for Groundwater Remediation; National Academy Press: Washington, DC, 2000. (4) Congressional Budget Office. The Total Costs of Cleaning Up Nonfederal Superfund Sites; U.S. Government Printing Office: Washington, DC, 1994. (5) Powers, M. B.; Rubin, D. K. Eng. News-Rec. 1996, Oct. 14, 26-28. (6) National Research Council. Innovations in Ground Water and Soil Cleanup: From Concept to Commercialization; National Academy Press: Washington, DC, 1997. (7) National Research Council. Alternatives for Ground Water Cleanup; National Academy Press: Washington, DC, 1994. (8) EPA. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites, Directive Number 9200.4-17P; EPA, Office of Solid Waste and Emergency Response, U.S. Government Printing Office: Washington, DC, 1999.
Jacqueline A. MacDonald is an engineer at the RAND Corporation and is RAND's liaison to the White House Office of Science and Technology Policy. Before joining RAND, MacDonald helped to launch NRC's study of natural attenuation, and she served as study director for the project.
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