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Introduction: Using Natural Processes in Groundwater Restoration

Managers of contaminated sites nationwide are increasingly relying on unenhanced natural processes, rather than solely on engineered technologies, to clean up groundwater and soil. This approach has been given various names, but the most common term now in use is “natural attenuation.” Natural attenuation currently is being used at tens of thousands of contaminated sites around the country, in place of or in conjunction with engineered remediation systems (EPA, 1997).

The first large-scale attempts to restore contaminated sites, initiated after the Love Canal incident and passage of the Superfund1 law in 1980, employed engineered systems to attempt to remove contamination from groundwater and soil. These efforts often involved the excavation of large volumes of soil for secured landfilling or incineration or the pumping of large volumes of groundwater to the surface for treatment (see Figure 1-1). By the early 1990s, however, studies revealed that these “brute force” approaches have many shortcomings. Excavation often destroys the native ecosystems and may expose workers and nearby residents to elevated levels of contaminants. The pump-and-treat approach often cannot remove all contamination from the site (NRC, 1994). Both approaches are expensive.

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Superfund, the common name for the Comprehensive Environmental Response, Compensation, and Liability Act, requires the cleanup of abandoned waste sites.



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Natural Attenuation for Groundwater Remediation 1 Introduction: Using Natural Processes in Groundwater Restoration Managers of contaminated sites nationwide are increasingly relying on unenhanced natural processes, rather than solely on engineered technologies, to clean up groundwater and soil. This approach has been given various names, but the most common term now in use is “natural attenuation.” Natural attenuation currently is being used at tens of thousands of contaminated sites around the country, in place of or in conjunction with engineered remediation systems (EPA, 1997). The first large-scale attempts to restore contaminated sites, initiated after the Love Canal incident and passage of the Superfund1 law in 1980, employed engineered systems to attempt to remove contamination from groundwater and soil. These efforts often involved the excavation of large volumes of soil for secured landfilling or incineration or the pumping of large volumes of groundwater to the surface for treatment (see Figure 1-1). By the early 1990s, however, studies revealed that these “brute force” approaches have many shortcomings. Excavation often destroys the native ecosystems and may expose workers and nearby residents to elevated levels of contaminants. The pump-and-treat approach often cannot remove all contamination from the site (NRC, 1994). Both approaches are expensive. 1   Superfund, the common name for the Comprehensive Environmental Response, Compensation, and Liability Act, requires the cleanup of abandoned waste sites.

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Natural Attenuation for Groundwater Remediation FIGURE 1-1 Conventional pump-and-treat system treating groundwater beneath a landfill. SOURCE: Mercer et al. (1990) as reprinted in NRC, 1994. Concurrently, researchers began to understand more fully how naturally occurring subsurface processes can transform contaminants to harmless forms (Chapelle, 1999; Rifai et al., 1995; Rifai, 1998). Most of this early work focused on the effect of natural processes on benzene, toluene, ethylbenzene, and xylene (BTEX), which are components of gasoline that are commonly found at leaking underground storage tank sites. In 1993, the National Research Council (NRC) issued a report—In Situ Bioremediation: When Does It Work?—concluding that biologically mediated natural processes could, in some instances, prevent the migration of contaminants without human intervention other than careful monitoring. The report called this approach “intrinsic bioremediation” and defined it as using “the innate capabilities of naturally occurring microbes to degrade contaminants without taking any engineering steps to enhance the process” (NRC, 1993). Around the same time as the NRC report endorsed intrinsic bioremediation, a broader approach—called natural attenuation—was gaining favor among some government regulators and owners of contaminated sites. Natural attenuation was not a new concept; it is referenced in the 1990 National Contingency Plan (EPA, 1990), the regulatory document that specifies policies for cleanup of Superfund sites. However, it

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Natural Attenuation for Groundwater Remediation Excavation of contaminated soil can destroy native ecosystems and expose workers and nearby residents to contaminants. SOURCE: John Wilson, U.S. Environmental Protection Agency. was rarely invoked as a mechanism for site cleanup until the early 1990s. Whereas intrinsic bioremediation refers strictly to microbial reactions that destroy or control contaminants before they travel far from their source area, natural attenuation is defined more broadly: it includes all types of naturally occurring physical, chemical, and biological processes that can reduce water-phase concentrations of contaminants. The Environmental Protection Agency (EPA) defines natural attenuation as including “biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants” (EPA, 1999). Several other organizations also have recently written definitions of natural attenuation (see Box 1-1). Cleanup strategies based on natural attenuation, instead of the more strictly defined intrinsic bioremediation, are the predominant choices being pursued today. Whether natural attenuation is an appropriate strategy for managing contaminated sites has become a politically controversial issue. Owners of contaminated sites have pushed for the broader acceptance of this approach because of the cost savings it can offer. Natural attenuation can in some cases reduce the price tag for site cleanup by millions of dollars.

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Natural Attenuation for Groundwater Remediation BOX 1-1 Definitions of Natural Attenuation A variety of organizations, including the following, have written definitions of natural attenuation: Environmental Protection Agency: The EPA released a final policy directive for use of “monitored natural attenuation” in 1999 (EPA, 1999). This directive defines monitored natural attenuation as the “reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The ‘natural attenuation processes’ that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants.” American Society for Testing and Materials (ASTM): ASTM in 1997 approved a document entitled Standard Guide for Remediation of Groundwater by Natural Attenuation at Petroleum Release Sites that outlines a process for evaluating the potential for natural attenuation at contaminated sites (ASTM, 1997). The document defines natural attenuation as the “reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes, such as biodegradation, dispersion, dilution, adsorption, and volatilization.” Air Force: The Air Force developed two guides for monitoring sites for natural attenuation. The first, published in 1995 (Wiedemeier et al., 1995), defines the process as resulting “from the integration of several subsurface attenuation mechanisms that are classified as either destructive or nondestructive. Biodegradation is the most important destructive attenuation mechanism. Nondestructive attenuation mechanisms include sorption, dispersion, dilution from recharge, and volatilization.” The second guide, published in 1997 (Wiedemeier et al., 1997), covers chlorinated solvents and uses the EPA’s definition of natural attenuation. Army: The Army’s Science Board in 1995 prepared a draft report that assesses the utility of natural attenuation for the remediation of contaminated Army sites (U.S. Army Science Board, Infrastructure and Environment Issue Group, 1995). The report defines natural attenuation as “the process by which contamination in groundwater, soils, and surface water is reduced over time … [via] natural processes such as advection, dispersion, diffusion, volatilization, abiotic and biotic transformation, sorption/desorption, ion exchange, complexation, and plant and animal uptake.”

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Natural Attenuation for Groundwater Remediation For example, at the French Limited site in Houston, natural attenuation reportedly saved $12 million, compared to a conventional pump-and-treat system (Powers and Rubin, 1996). Meanwhile, however, environmental advocates have charged that industry is misusing natural attenuation in order to save money at the expense of public health protection and is “walking away” from contaminated sites at which active cleanup is needed. The multiprocess definition of natural attenuation fuels this controversy because it includes mechanisms such as dilution and dispersion that are unacceptable ways to manage contamination in the view of many environmental advocates. This report examines public concerns about and the scientific basis for natural attenuation. It answers several important questions: Why are informed members of the public near contaminated sites often skeptical about this approach? What does science tell about when natural processes can work in cleaning up a site? When do natural processes fail? What criteria for success or failure should be applied? What guidelines are available for making these determinations and ensuring that the natural processes remain effective? Are the currently available guidelines adequate from a scientific and public policy perspective? This report was prepared by the Committee on Intrinsic Remediation, appointed by the National Research Council in 1997 to assess the capabilities and limitations of natural attenuation. The committee included members with expertise in all the scientific disciplines needed to understand natural subsurface processes, the effects of these processes on contaminants, and risks associated with leaving contaminants in place. Disciplines represented on the committee were environmental engineering, hydrogeology, soil science, environmental microbiology, geochemistry, and risk assessment. Members were drawn from academia, government laboratories, consulting firms, industry, and environmental groups to represent a balance of experience and political viewpoints. This report’s findings reflect the consensus of the full committee. This chapter provides an overview of the history of use of natural attenuation. Chapter 2 describes community concerns about natural attenuation and suggests ways to address them. Chapter 3 evaluates the scientific basis for natural attenuation: it presents what is known about the fate of different classes of contaminants in different geologic settings and what is and is not possible to achieve with natural processes. Chapter 4 discusses how to integrate understanding of the various processes that affect contaminant fate when evaluating whether natural attenuation might be useful as part of a site remediation strategy. Chapter 5 reviews the adequacy of existing protocols for evaluating natural attenuation.

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Natural Attenuation for Groundwater Remediation HISTORICAL ROLE OF NATURAL CLEANUP PROCESSES The use of natural processes to degrade waste products is not a new concept. In fact, such processes were the sole means of cleansing the earth of excess waste until the development of sewage and other waste treatment systems in the nineteenth century. That the earth has been able to convert detritus from animal and plant wastes into products that can be reused in natural cycles is a testament to the effectiveness of such processes for treating naturally occurring organic chemicals (Madsen, 1991, 1998). The growth of cities and their associated waste production and the development of large numbers of synthetic organic chemicals have challenged nature’s ability to process wastes (Madsen, 1991; Bonaventura and Johnson, 1997). Approximately 8 million synthetic and naturally occurring organic chemicals have been widely used in the nineteenth and twentieth centuries for industrial, military, agricultural, commercial, and domestic activities (Lenhard et al., 1995; Swoboda-Colberg, 1995; Wackett, 1996). A wide variety of inorganic chemicals also has been used. More than 72,000 chemicals are listed on the EPA’s Toxic Substances Control Act Chemical Inventory, and some 2,300 new chemicals are added to this inventory each year (Bonaventura and Johnson, 1997). Many of these chemicals are vital for a variety of industrial processes, including the production of fuels, solvents, household products, and pesticides. Nonetheless, the large volume of human-produced wastes and the increasing complexity and diversity of the chemical content of these wastes have overwhelmed the assimilative capacity of many ecosystems. For example, the EPA estimates that some 217,000 sites nationwide have soil, groundwater, or both that have been contaminated by purposeful disposal or accidental spillage of human-produced chemicals and waste products (EPA, 1997). Earlier estimates, prior to closure of a large number of leaking underground gasoline storage tanks, placed the number of such sites at approximately 300,000 to 400,000 (NRC, 1994). (See Chapter 3 for information about typical sources of groundwater and soil contaminants and the fate of these contaminants in the subsurface.) LIMITATIONS OF ENGINEERED REMEDIATION SYSTEMS Engineered systems have met with limited success in restoring contaminated soil and groundwater. For example, in a 1994 review of conventional pump-and-treat systems for groundwater restoration at 77 sites, the NRC concluded that the pump-and-treat systems had achieved restoration goals at just 8 of the sites and were highly unlikely to achieve existing goals at 42 of the sites (NRC, 1994). Attempts to engineer treat-

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Natural Attenuation for Groundwater Remediation ment systems in situ have shown more promise but still are not proven for the wide range of contaminants and geologic settings of concern. In its most recent assessment of subsurface cleanup technologies, the NRC concluded, “Although considerable effort has been invested in ground water and soil cleanup, the technologies available for these cleanups are relatively rudimentary” (NRC, 1997). Increased understanding of the limitations of engineered systems for groundwater and soil restoration has coincided with increased recognition that, in some cases, groundwater and soil restoration will occur naturally. As a consequence, natural attenuation now is often considered a tool for supplementing or even replacing engineered treatment systems. In some cases, natural attenuation serves as a polishing step to manage the contamination remaining after engineered systems have removed the bulk of contamination. INCREASING RELIANCE ON NATURAL ATTENUATION Since the early 1990s, use of natural attenuation as a formally documented remedy has become increasingly common in all of the U.S. regulatory programs for the cleanup of contaminated sites. At some sites, natural attenuation is used as a stand-alone remedy, while at others it is used as a component of an overall remediation scheme. Box 1-2 provides brief explanations of regulatory programs governing site cleanup. Natural attenuation is being used or considered for use to varying degrees in each of these programs. Natural attenuation is used most commonly at sites regulated by the Underground Storage Tank Program, which applies to contamination emanating from leaks in underground chemical storage tanks. Most of these tanks contain gasoline or other fuels, which are relatively easily degraded by subsurface microorganisms. As Figure 1-2 shows, natural attenuation is the leading remedy—used at more than 15,000 sites—for groundwater contaminated by leaking underground storage tanks. It is the fourth most common remedy—used at more than 6,000 sites—for cleanup of soil contaminated by leaking underground storage tanks (see Figure 1-3). Few of the underground storage tank sites at which natural attenuation is a formal remediation strategy have been evaluated in detail to determine the long-term effectiveness of the natural processes in controlling contamination. Most of these sites are gas stations with limited funds for monitoring and regulatory oversight. The most comprehensive study to date was a 1995 report by Lawrence Livermore National Laboratory. The report evaluated plumes of groundwater contamination from 271 leaking underground fuel tanks in California. It concluded that 90 per-

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Natural Attenuation for Groundwater Remediation BOX 1-2 Regulatory Programs Requiring Cleanup of Contaminated Sites Contaminated sites for which natural attenuation is proposed as a remedy may be managed under one or more of a variety of programs. The cleanup requirements of these programs and their implementation from state to state can be highly variable (NRC, 1997). The main programs are as follows: Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA): CERCLA, also known as “Superfund” for the fund it established to pay for cleanup of abandoned sites where no responsible party can be identified, requires cleanup of natural resources at closed or abandoned industrial facilities. Congress enacted CERCLA in 1980 in response to the Love Canal incident, in which residents of a Niagara Falls, New York, community had to be relocated and the community school closed due to seepage of contamination from a closed industrial waste dump. Resource Conservation and Recovery Act (RCRA) Corrective Action Program: This program, authorized by Congress under the RCRA Hazardous and Solid Waste Amendments of 1984, requires cleanup of all environmental media at active facilities that treat, store, dispose of, or recycle solid and hazardous waste. It is essentially equivalent to the CERCLA cleanup program, except that it governs active rather than closed facilities. RCRA Underground Storage Tank (UST) Program: The UST program, authorized under the RCRA Hazardous and Solid Waste Amendments of 1984, requires cleanup of contamination resulting from leaks and spills from USTs. Uranium Mill Tailings Remediation Control Act (UMTRCA): This law, enacted in 1978 and subsequently amended, requires cleanup of contamination at 24 former uranium ore processing sites and is managed by the Department of Energy. State Superfund Programs: All 50 states have programs modeled after CERCLA that govern the restoration of contaminated sites not included in the federal CERCLA program. State Voluntary Cleanup and Brownfields Programs: The majority of states have established voluntary site cleanup programs to encourage private parties to address contamination problems without using state resources for oversight and enforcement. An increasing number of states are also establishing brownfields programs, which provide incentives to clean up industrial properties to levels that are suitable for industrial reuse.

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Natural Attenuation for Groundwater Remediation FIGURE 1-2 Methods used to clean up groundwater contamination from leaking underground storage tanks as of 1997, according to a survey of state program managers. SOURCE: Tulis et al., 1998. Reprinted, with permission, from Soil and Groundwater Cleanup (1998). © 1998 by Soil and Groundwater Cleanup. FIGURE 1-3 Methods used to clean up soil contamination from leaking underground storage tanks as of 1997, according to a survey of state program managers. SOURCE: Tulis et al., 1998. Reprinted, with permission, from Soil and Groundwater Cleanup (1998). © 1998 by Soil and Groundwater Cleanup.

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Natural Attenuation for Groundwater Remediation cent of the plumes were stable or were declining in size as a result of natural attenuation (Rice et al., 1995). Following release of the Lawrence Livermore report, the closure of underground storage tank cleanup sites in California surged as regulators signed off on remedies allowing treatment to occur by natural attenuation (see Figure 1-4). More recently, discovery of methyl tert-butyl ether (MTBE), a fuel oxygenate used to decrease air pollution from automobiles, in plumes of contamination from underground storage tanks has slowed regulatory approval of natural attenuation remedies for these sites, especially in California. MTBE is resistant to biodegradation and currently appears unlikely to be transformed via natural attenuation (see Chapter 3). During the 1990s, use of natural attenuation increased in the Superfund program, which regulates cleanup of abandoned industrial sites. In 1991, natural attenuation was used—by itself or in conjunction with an engineered system—for treatment of contaminated groundwater at approximately 6 percent of Superfund sites. By 1996 (the most recent year for which data are available), this figure had increased to 20 percent or more (see Figure 1-5). Data from 1997, which were not complete at the time this report was prepared and therefore are not included on Figure 1-5, again show an upward trend in use of monitored natural attenuation, with natural attenuation slated for use at 23 new sites at least (as compared to 18 sites in 1996). Natural attenuation is also being used in the cleanup of active hazardous waste treatment, storage, and disposal facilities regulated under the Resource Conservation and Recovery Act (RCRA) Corrective Action Program, although quantitative data on remedies used in the RCRA program are not available from EPA. Because of the increased number of proposals to the EPA to use natural attenuation in Superfund and RCRA cleanups, EPA in 1999 released a directive clarifying its policy on natural attenuation (see Chapter 5) (EPA, 1999). Increasingly, federal agencies, especially the Air Force, are using or considering natural attenuation for the remediation of contaminated soil and groundwater at their facilities. For example, the Air Force estimates that it can use natural attenuation to restore 80 percent of its 1,500 jet-fuel-contaminated sites (Powers and Rubin, 1996). The Army plans to use natural attenuation for at least 84 of its contaminated sites, according to data compiled from the Army’s restoration data base as of August 4, 1998 (I. May, U.S. Army, personal communication, August 1998). Natural attenuation also has been increasingly applied in state soil and groundwater cleanup programs. As of early 1998, four state legislatures had passed statutes referring to natural attenuation, and one additional state was planning such a statute. Regulations for the use of natural attenuation were in place in 11 states, 1 state was planning to issue such regulations, 17 states had developed guidance documents for the use of

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Natural Attenuation for Groundwater Remediation FIGURE 1-4 Closure of leaking underground storage tank cleanup projects in the state of California, January 1, 1991, through January 1, 1998. SOURCE: James Giannopoulos, California Water Resources Control Board, Sacramento, California.

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Natural Attenuation for Groundwater Remediation FIGURE 1-5 Use of natural attenuation in the cleanup of contaminated groundwater at Superfund sites, 1985-1996, shown as a percentage of the total number of remedies for contaminated groundwater selected in the indicated year. Although there was a slight downturn in the use of monitored natural attenuation in 1996, data from 1997 show an upward trend. However, 1997 data were not complete as of the publication of this report and therefore could not be included on this chart. NOTE: RODs = records of decision (regulatory documents specifying remedies for Superfund sites). SOURCE: Ken Lovelace, U.S. Environmental Protection Agency, Washington, D.C. natural attenuation, and 14 states were considering or developing such guidance (Martinson, 1998). INCREASING VARIETY OF CONTAMINANTS CONSIDERED FOR NATURAL ATTENUATION Scientists have demonstrated convincingly that natural processes can substantially destroy some contaminants, primarily from gasoline and other fuels, but this level of scientific confidence is not as high for most other common groundwater contaminants. A strong scientific foundation for understanding how petroleum hydrocarbons biodegrade in the environment was available before empirical studies, such as the Livermore report, documented the effectiveness of natural attenuation of

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Natural Attenuation for Groundwater Remediation these compounds. This scientific underpinning—which is not available for most other contaminants—helped gain support from the regulatory and scientific communities for using natural attenuation in the cleanup of petroleum hydrocarbons. For some types of contaminants, natural transformation processes can increase toxicity. For example, natural processes can convert trichloroethene (TCE) to vinyl chloride, which is more hazardous than its parent compound. Under some conditions, natural processes can dissolve metals, increasing the hazard they pose. When natural attenuation processes increase risks in these types of cases, they cannot be considered a remedy for contamination. Despite the limited scientific understanding and potential risks, natural attenuation is being approved more and more frequently for the remediation of a variety of other types of contaminants. For example, regulators have approved the use of natural attenuation for the treatment of a plume of trinitrotoluene, an explosive, at the Sierra Army Depot in Herlong, California (Ground Water Newsletter, 1996). Natural attenuation has also been approved for treating vinyl chloride, which is a potent carcinogen once thought to be extremely recalcitrant to natural degradation, at a chemical manufacturing site (Leetham and Larson, 1997). The Department of Energy (DOE) has proposed using natural attenuation as one of several strategies for cleaning up uranium and other groundwater contaminants at uranium mill tailings sites being cleaned up under the Uranium Mill Tailings Remediation Control Act (DOE, 1996). The degree to which natural attenuation can control risks from these types of contaminants under different environmental conditions is an active research topic among environmental scientists and engineers. Yet, natural attenuation is being approved as a formal remedy, despite the limitations in scientific understanding. INCREASING NUMBER OF NATURAL ATTENUATION PROTOCOLS An important question is whether the procedures used to evaluate the effectiveness of natural attenuation at the increasing variety of sites for which it is being considered are adequately rigorous, especially given the limitations of scientific understanding. Several organizations have issued technical protocols for evaluating when natural attenuation is appropriate for contaminant management, but these protocols vary in approach, level of detail, and completeness. The 1993 NRC report included the first set of general guidelines for evaluating sites for intrinsic bioremediation (natural attenuation involving only biological reactions). These guidelines, developed by a panel of

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Natural Attenuation for Groundwater Remediation expert scientists, specified that documenting intrinsic bioremediation requires three types of evidence: (1) documented loss of contaminants from the site; (2) laboratory assays or technical literature showing that microorganisms from site samples have the potential to transform the contaminants under the expected site conditions; and (3) one or more pieces of information showing that the biodegradation potential is actually realized in the field. In its report, the NRC emphasized that all three types of evidence, not just one or two, should be provided. The report said, “No one piece of evidence can unambigiously prove that microorganisms have cleaned up a site. Therefore, the … [NRC] recommends an evaluation strategy that builds a consistent, logical case for bioremediation based on converging lines of independent evidence.” Since the 1993 NRC report, the Air Force, American Society for Testing and Materials, EPA, American Petroleum Institute, Chevron, several states, and various other organizations have issued guidelines for evaluating sites for natural attenuation. Chapter 5 reviews these guidelines in detail. Many use the NRC 1993 report as a starting point. However, some require a less detailed evaluation, and some indicate that documenting loss of contaminants from the site—without proof of the mechanism involved—is sufficient proof of natural attenuation under certain circumstances. INCREASING PUBLIC CONCERNS As the use of natural attenuation has increased, some members of the public in areas affected by groundwater and soil contamination have voiced increasing dissatisfaction with this approach. Citizens’ groups often perceive natural attenuation as a “do-nothing” approach. In a special meeting with the Committee on Intrinsic Remediation, representatives of several community groups contended that legal loopholes, the desire of site owners to save on remediation costs, and pressure on regulators to close sites quickly have led to the use of natural attenuation at sites where this strategy is not sufficient to protect public health and the environment. According to citizen group representatives, a key concern of communities affected by contaminated sites is that natural attenuation is being used to justify the dilution of contaminants and the transfer of contaminants from one environmental medium to another. Community leader Diane Heminway of Citizens’ Environmental Coalition in Medina, New York, observed, At both the state and federal levels, it has been the goal to remove as many sites as possible from the Superfund list. Sometimes this is done with piecemeal cleanups; sometimes it is done with legal loopholes; and sometimes it is simply done with an eraser. Given this reality, it seems

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Natural Attenuation for Groundwater Remediation frighteningly possible that intrinsic remediation [natural attenuation] will offer polluters an easy, cheap approach and increase the number of sites where remedies have been selected, however inappropriate [these remedies] might be (Heminway, 1998). FOCUS OF THIS REPORT The scientific analyses presented in this report focus primarily on processes that transform contaminants to less harmful forms or immobilize them in place, rather than on physical processes that dilute and disperse the contaminants. This focus on transformation and immobilization processes is not intended as a value judgment about the validity of using dilution as a means to reduce contaminant concentrations in the environment. The members of the Committee on Intrinsic Remediation held conflicting opinions on whether reducing contaminant concentrations via dilution is, philosophically, an acceptable strategy. Likewise, the group did not agree on whether dilution processes should be included in the definition of natural attenuation. Nonetheless, the committee agreed to focus on transformation and immobilization processes for two reasons. First, these are the most difficult processes to understand from a scientific perspective. Increased understanding of transformation and immobilization processes is essential to increased understanding of natural attenuation because dilution and dispersion processes are already comparatively well understood. Second, at many contaminated sites, documenting the occurrence of transformation and immobilization processes is likely to be essential to community acceptance and successful use of natural attenuation. SUMMARY Natural attenuation is replacing or augmenting engineered remediation systems at an increasing number of sites with contaminated groundwater and soil. Regulators are increasingly approving this approach for treatment of a wide variety of groundwater contaminants. Yet, the public remains skeptical of natural attenuation in many cases. Furthermore, it is not clear that scientific understanding is sufficient to allow the broader use of this approach—beyond BTEX sites—while ensuring adequate protection of public health and the environment. The purpose of this report is to increase the understanding of processes involved in natural attenuation so that natural attenuation is applied only where it will adequately protect public health and the environment.

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Natural Attenuation for Groundwater Remediation REFERENCES American Society for Testing and Materials (ASTM). 1997. Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites: February 4, 1997 Draft. Philadelphia: ASTM. Baedecker, M. J., I. M. Cozzarelli, R. P. Eganhouse, D. I. Siegel, and P. C. Bennett. 1993. Crude oil in a shallow sand and gravel aquifer: III. Biochemical reactions and mass balance modeling in anoxic groundwater. Applied Geochemistry 8:569-586. Bonaventura, C., and F. M. Johnson. 1997. Healthy environments for healthy people: Bioremediation today and tomorrow. Environmental Health Perspectives 105 (supplement 1): 5-20. Chapelle, F. H. 1999. Bioremediation of petroleum hydrocarbon-contaminated groundwater: The perspectives of history and hydrology. Ground Water 37(1):122-132. Department of Energy (DOE). 1996. Final Programmatic Environmental Impact Statement for the Uranium Mill Tailings Remedial Action Ground Water Project. DOE/E15-0198. Springfield, Va.: National Technical Information Service. Environmental Protection Agency (EPA). 1990. Preamble to the national oil and hazardous substances pollution contingency plan, final rule. Federal Register 55(46): 8666-8732. EPA. 1994. National Water Quality Inventory, 1994 Report to Congress. EPA841-R-95-005. Washington, D.C.: U.S. Government Printing Office. EPA. 1997. Cleaning Up the Nation’s Waste Sites: Markets and Technology Trends. 1996 Edition. EPA 542-R-96-005. Washington, D.C.: EPA, Office of Solid Waste and Emergency Response. EPA. 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. Directive number 9200.4-17P. Washington, D.C.: EPA, Office of Solid Waste and Emergency Response. Ground Water Newsletter. 1996. Natural processes to degrade Army depot contaminants. Groundwater Newsletter 25(18):1. Heminway, D. 1998. Comments presented to Committee on Intrinsic Remediation, second meeting, National Academy of Sciences’ Beckman Center, Irvine, Calif., March 12. Washington, D.C.: National Research Council, Water Science and Technology Board. Leetham, J. T., and J. R. Larson. 1997. Intrinsic bioremediation of vinyl chloride in groundwater at an industrial site. In Situ and On-Site Bioremediation, Vol. 3. Columbus, Ohio: Battelle. Lenhard, R. J., R. S. Skeen, and T. M. Brouns. 1995. Contaminants at U.S. DOE sites and their susceptibility to bioremediation. Pp. 157-172 in Skipper, H. D., and R. F. Turco (eds.) Bioremediation Science and Applications. SSSA, Special Publication Number 43. Madison, Wis. Madsen, E. L. 1991. Determining in situ biodegradation: Facts and challenges. Environmental Science and Technology 25(10):1663-1673. Madsen, E. L. 1998. Epistemology of environmental microbiology. Environmental Science and Technology 32:429-439. Martinson, M. 1998. 1998 national RNA survey. Underground Tank Technology Update 12(3):14-16. Mercer, J. W., D. C. Skipp, and D. Giffin. 1990. Basics of Pump-and-Treat Ground-Water Remediation Technology. EPA/600/8-90/003. Ada. Okla.: EPA. NRC (National Research Council). 1993. In Situ Bioremediation: When Does It Work? Washington, D.C.: National Academy Press. NRC. 1994. Alternatives for Ground Water Cleanup. Washington, D.C.: National Academy Press. NRC. 1997. Innovations in Ground Water and Soil Cleanup: From Concept to Commercialization. Washington, D.C.: National Academy Press.

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Natural Attenuation for Groundwater Remediation Powers, M. B., and D. K. Rubin. 1996. Doing what comes naturally—Nothing. Engineering News-Record (October 14):26-28. Rice, D. W., R. D. Grose, J. C. Michaelsen, B. P. Dooher, D. H. MacQueen, S. J. Cullen, W, E. Kastenberg, L. G. Everett, and M. A. Marino. 1995. California Leaking Underground Fuel Tank (LUFT) Historical Case Analyses. UCRL-AR-122207. Livermore, Calif.: Lawrence Livermore National Laboratory. Rifai, H. 1998. One hundred years of natural attenuation. Bioremediation Journal 2(3&4):217-219. Rifai, H., R. Borden, J. Wilson, and C. H. Ward. 1995. Intrinsic bioattenuation for subsurface restoration. Pp. 1-30 in Hinchee, R. E., J. Wilson, and D. Downey (eds.) Intrinsic Bioremediation. Columbus. Ohio: Battelle Memorial. Institute. Swoboda-Colberg, N. G. 1995. Chemical contamination of the environment: Sources, types, and fate of synthetic organic chemicals. Pp. 27-74 in Young, L. Y., and C. E. Cerniglia (eds.) Microbial Transformation and Degradation of Toxic Organic Chemicals. New York: Wiley-Liss. Tulis. D., P. F. Prevost, and P. Kostecki. 1998. Study points to new trends in use of alternative technologies at LUST sites. Soil and Groundwater Cleanup (July):12-17. U.S. Army Science Board, Infrastructure and Environment Issue Group. 1995. Remediation of Contaminated Army Sites: Utility of Natural Attenuation. Draft Report. Washington, D.C.: Department of the Army. Wackett, L. P. 1996. Co-metabolism: Is the emperor wearing any clothes? Current Opinions in Biotechnology 7:321-325. Wiedemeier, T., J. T. Wilson, D. H. Kampbell, R. N. Miller, and J. E. Hansen. 1995. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater, Vols. 1 and 2. San Antonio, Tex.: Air Force Center for Environmental Excellence, Brooks Air Force Base. Wiedemeier, T. H., M. A. Swanson, D. E. Moutoux, E. K. Gordon, J. T. Wilson, B. H. Wilson, J. H. Kampbell, J. E. Hansen, P. Haas, and F. H. Chapelle. 1997. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater. San Antonio, Tex.: Air Force Center for Environmental Excellence, Brooks Air Force Base.