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Natural Attenuation for Groundwater Remediation (2000)

Chapter: 5 Protocols for Documenting Natural Attenuation

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Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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5
Protocols for Documenting Natural Attenuation

As interest in using natural attenuation to manage contaminated sites has surged, an increasing number of protocols have been developed to guide evaluations of the potential for natural attenuation to occur. This chapter reviews 14 such protocols, listed in Box 5-1.

For this review, the term “protocol” is defined very broadly to include any policy statement, state regulation, or technical document on how decision making and implementation of natural attenuation should be carried out. As defined here, a protocol is an outline of a strategy and methodology to be followed. It is an assessment and planning tool. A protocol is not necessarily a “how-to” manual, although some existing protocol documents (such as those prepared by the U.S. Air Force) have extensive appendixes that provide considerable information on field sampling techniques, analytical methods, and data interpretation. Standardizing the steps in data gathering, analysis, and decision making—using a process such as that outlined in Chapter 4—is the most important use of protocols.

Based on current activity, protocol documents for natural attenuation will continue to increase in number in the near future. Reviewing all existing protocols would be infeasible because new protocols continue to be promulgated at the rate of several per year. The protocols reviewed in this chapter represent the range of those available, from protocols prepared by federal and state agencies to those developed by private companies and industrial associations.

This chapter first defines a set of attributes that are important ele-

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

ments of natural attenuation protocols. It then discusses how well the existing protocols conform to these attributes and compares the various protocols and their intended uses. The chapter also discusses decision-making tools used in the protocols. The final sections discuss several critical topics—including monitoring of sites, training of those who implement the protocols, and involvement of the public in decision making—that existing protocols generally do not address adequately.

CRITERIA FOR A GOOD PROTOCOL

For this review, the Committee on Intrinsic Remediation developed a list of important subject areas and subtopics that natural attenuation protocols should address. This list is the outcome of extended committee deliberations following presentations by developers of natural attenuation protocols, users of the protocols, and local community organizations and environmental advocacy groups. Box 5-2 summarizes the topics, which are discussed briefly in turn below in no implied order of precedence.

Community Concerns

Community Involvement

As described in Chapter 2, early community involvement is especially important in order to gain public acceptance and confidence in decisions regarding natural attenuation. Without adequate community participation, natural attenuation may be viewed as a less aggressive and less costly remediation alternative that offers advantages to responsible parties without fully protecting human health and the environment. Further, natural attenuation decisions may affect community property values. For these reasons, a comprehensive natural attenuation protocol needs to identify critical decision points at which community involvement is necessary. Chapter 2 provides guidance on how community groups can be involved effectively. In some cases, no nearby community per se may exist, so flexibility on this issue is warranted.

Institutional Controls

Natural attenuation processes may operate for many years, during which time land reuse may have to be restricted. A comprehensive protocol has to describe the criteria for deciding whether institutional controls are necessary and how to ensure the long-term viability of these controls. Natural attenuation may differ little from other long-term remedies in

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 5-1
Natural Attenuation Protocols Reviewed

Federal Agencies

Environmental Protection Agency

  • “Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites,” Final OSWER Directive (OSWER Directive Number 9200. 4-17P), April 21, 1999, EPA Office of Solid Waste and Emergency Response.

  • “Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water,” Todd H. Wiedemeier, Matthew A. Swanson, David E. Moutoux, E. Kinzie Gordon, John T. Wilson, Barbara H. Wilson, Donald H. Kampbell, Patrick E. Haas, Ross N. Miller, Jerry E. Hansen, and Francis H. Chapelle, EPA/600/R-98/128, September 1998, EPA Office of Research and Development.

  • “Draft Region 4 Suggested Practices for Evaluation of a Site for Natural Attenuation (Biological Degradation) of Chlorinated Solvents,” Version 3.0, November 1997, EPA Region 4.

Department of Energy

  • “Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites,” Patrick V. Brady, Brian P. Spalding, Kenneth M. Krupka, Robert D. Waters, Pengchu Zhang, David J. Borns, and Warren D. Brady, Draft, August 30, 1998, Sandia National Laboratory.

Air Force

  • “Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination in Groundwater,” Todd Wiedemeier, John T. Wilson, Donald H. Kampbell, Ross N. Miller, and Jerry E. Hanson, Volume I and Volume II, November 11, 1995, Air Force Center for Environmental Excellence, Technology Transfer Division, Brooks AFB.

  • “Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater,” Todd H. Wiedemeier, Matthew A. Swanson, David E. Moutoux, E. Kinzie Gordon, John T. Wilson, Barbara H. Wilson, Donald H. Kampbell, Jerry E. Hansen, Patrick Haas, and Francis H. Chapelle, Draft—Revision 2, July 1997, Air Force Center for Environmental Excellence, Technology Transfer Division, Brooks Air Force Base, San Antonio, Tex.

Navy

  • “Technical Guidelines for Evaluating Monitored Natural Attenuation at Naval and Marine Corps Facilities,” Todd H. Wiedemeier and Francis H. Chapelle, Draft—Revision 2, March 1998.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

State Agencies

Minnesota Pollution Control Agency

  • “Draft Guidelines—Natural Attenuation of Chlorinated Solvents in Ground Water,” Working Draft, December 12, 1997, Minnesota Pollution Control Agency, Site Response Section.

New Jersey

  • “New Jersey Administrative Code 7:26E—Technical Requirements for Site Remediation, and Classification Exception Areas: Final Guidance 4-17-1995.”

Corporations

Chevron

  • “Protocol for Monitoring Intrinsic Bioremediation in Groundwater,” Tim Buscheck and Kirk O’Reilly, March 1995, Chevron Research and Technology Company, Health, Environment, and Safety Group.

  • “Protocol for Monitoring Natural Attenuation of Chlorinated Solvents in Groundwater,” Tim Buscheck and Kirk O’Reilly, February 1997, Chevron Research and Technology Company, Health, Environment, and Safety Group,.

Professional and Industry Associations

American Society for Testing and Materials

  • “Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites,” Draft, February 4, 1997.

American Petroleum Institute

  • “Methods for Measuring Indicators of Intrinsic Bioremediation: Guidance Manual,” American Petroleum Institute, Health and Environmental Sciences Department, Publication Number 4658, November 1997, API Publishing Services, Washington, D.C.

Public/Private Consortium

Remediation Technologies Development Forum

  • “Natural Attenuation of Chlorinated Solvents in Groundwater: Principles and Practices,” Industrial Members of the Bioremediation of Chlorinated Solvents Consortium of the Remediation Technologies Development Forum, Version 3.0, August 1997.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 5-2
Attributes for Assessing the Adequacy of Natural Attenuation Protocols

Community Concerns

  • Community involvement: The protocol should specify points in the evaluation process where community input is especially important. Effective means to involve community groups are presented in Chapter 2.

  • Institutional controls and long-term monitoring: The protocol should describe criteria for determining when institutional controls are needed and how the viability of these controls can be ensured.

  • Contingency plans: The protocol should address decision making on contingency plans. Criteria for reevaluating the effectiveness of natural attenuation must be defined, along with whether contingency treatment systems should be pre-positioned.

Scientific and Technical Issues

  • Establish cause and effect: The protocol should explain the scientific underpinnings and the evidence used to assess the relationship between what is observed and what is expected. Guidelines for establishing cause and effect are provided in Chapter 4.

  • Site condition assessment: The protocol should describe the level of data required to assess different types of sites and characterize different types of contaminant sources.

  • Sustainability: The protocol should address methods for the long-term viability of natural attenuation.

  • Peer review: The protocol should be peer reviewed by individuals representing various disciplines and viewpoints other than those of the organization that wrote it.

Implementation

  • Usability and user qualifications: The protocol should provide sufficiently detailed explanations so that users can follow it. The qualifications and training of implementers should be discussed.

requiring institutional controls until cleanup standards are achieved. Nevertheless, ensuring the adequacy of institutional controls is especially important for natural attenuation remedies, because this approach lacks continuously operated and supervised cleanup systems. Having clear institutional controls also helps to assure the affected community that natural attenuation is more than a walk-away solution.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

A comprehensive protocol should provide guidance on the long-term viability of institutional controls to prevent exposure to the contamination while natural attenuation takes place. SOURCE: Courtesy of Center for Health, Environment, and Justice.

Contingency Plans

A comprehensive protocol has to provide guidance on contingency plans in the event that natural attenuation does not perform as predicted. It describes the method for determining whether natural attenuation is adequate or is failing. It provides the decision-making tools necessary for triggering action plans and methods for deciding whether contingencies have to be pre-selected, pre-designed, or pre-positioned for deployment.

Scientific and Technical Issues

Cause-and-Effect Determination

A comprehensive protocol has to explain clearly what scientific evidence is needed to establish that specific natural attenuation processes are responsible for observed decreases in contaminant concentrations. It should describe the class of contaminants it addresses and the general

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

hydrogeologic setting in which natural attenuation is applicable for these contaminants. It must provide guidance on the intended use of field data to assess the relationship between what is observed and what is expected and to explain criteria for judging whether such information is adequate for evaluating critical processes. Concepts for establishing cause and effect are most advanced for fuel hydrocarbons and chlorinated solvents; thus, this issue is particularly important for natural attenuation of other classes of contaminants. The protocol’s guidelines for establishing cause and effect should follow the process recommended in Chapter 4.

Site Condition Assessment

A comprehensive protocol has to provide guidance on how much information is needed to understand important hydrogeologic factors affecting contaminant transport and fate at the site. It should provide criteria for delineating the sources of contamination and deciding whether they have to be removed or contained with physical or hydraulic engineered systems.

Sustainability

The long-term viability of natural attenuation has to be understood because a considerable period of time may be required to achieve cleanup goals. A comprehensive protocol should provide criteria for assessing the capacity of the natural attenuation processes at work at the site, whether the processes are likely to be constant, and the length of time required to reach cleanup goals. It also has to provide strategies for determining the rates of the processes, whether these are variable over space and time, and whether they are sufficient to prevent migration of contaminants to undesired locations. It must address complicating factors, such as site heterogeneity, that may influence reactions; the potential adverse effects of contaminant mixtures; the possible formation of harmful transformation by-products that may be more mobile or more toxic than the parent compound; variability with climate changes; and the potential adverse effects of other proposed remediation activities (such as source control measures) that may remove or add substances important for specific natural attenuation processes. It would provide guidance on the use of error analysis or confidence assessment for understanding the effects of important hydrogeologic factors and physicochemical and microbial phenomena.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
Peer Review

Peer review is important for ensuring scientific credibility and completeness of natural attenuation protocols, and all protocols should be independently peer reviewed prior to publication. Because a successful natural attenuation evaluation embraces various engineering and scientific disciplines, as well as public policy and management, the peer review process likewise has to involve participation from various disciplines. Protocols should explain the peer review process, including discussion of how concerns raised during the review were addressed. As defined here, peer review is not intended to ensure conformance with all applicable regulations. However, it should be conducted by persons not directly affiliated with the organization that developed the protocol.

Implementation Issues

Usability and User Qualifications

A comprehensive protocol has to be easily understood and implemented by individuals responsible for managing operations in the field, as well as by regulators. Detail must be sufficient to allow the protocol to serve as an effective tool for assessing natural attenuation implementation. The comprehensive protocol should describe the qualifications (such as disciplinary expertise, practical field experience, and specific training) needed to carry out the various analyses it recommends.

OVERVIEW OF PROTOCOLS

The protocols listed in Box 5-1 are at different stages of completeness, ranging from final published documents to drafts that may be revised in the future. Some of these protocols attempt to be comprehensive, with extensive appendixes, whereas others are much briefer, relying on previously published information. Some recent protocols are less voluminous than earlier ones, due in part to reliance on previously published information. For example, the Navy protocol, which combines discussion of hydrocarbon fuels and chlorinated solvents, is much shorter than either of the two Air Force protocols, which preceded the Navy protocol and treated these contaminants separately.

In reviewing the documents listed in Box 5-1, the committee considered the following points:

  • background and motivation,

  • intended audience,

  • scope,

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • development process,

  • organization and content, and

  • conformance with protocol attributes listed in Box 5-2.

The individual protocols are summarized briefly below; then the similarities, differences, and applicability of the different protocols are compared.

Federal Agency Protocols

Environmental Protection Agency Policy on Monitored Natural Attenuation

The EPA document Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites is a revision of a draft published in 1997. The directive is intended to “clarify EPA’s policy regarding the use of monitored natural attenuation for the cleanup of contaminated soil and groundwater in the Superfund, RCRA Corrective Action, and Underground Storage Tank programs.” These programs are administered by the EPA’s Office of Solid Waste and Emergency Response (OSWER). The EPA’s position, as stated in the policy directive, is that monitored natural attenuation, when used as a remediation strategy, must attain remedial objectives within a time frame that is reasonable compared to other, more active methods. The EPA prefers natural attenuation processes that degrade contaminants and expects that monitored natural attenuation will be most appropriate for sites having low potential for plume generation and migration. Nonetheless, the natural attenuation processes recognized in the directive include dispersion, dilution, and volatilization, as well as sorption and chemical or biological stabilization, transformation, or destruction of contaminants. According to the directive, a remedy that includes the introduction of an enhancement of any type is no longer considered natural attenuation.

The directive is intended as a policy document and, as such, provides very little technical guidance. The document comments briefly on three contaminant classes—petroleum-related contaminants, chlorinated solvents, and inorganics—but recognizes that natural attenuation processes are best understood for the benzene, toluene, ethylbenzene, and xylene (BTEX) components of petroleum fuels. The directive is intended to promote consistency in the way monitored natural attenuation remedies are proposed, evaluated, and approved, but there is no explanation of the means by which this consistency is ensured. It states several times EPA’s position that complete reliance on monitored natural attenuation is appropriate in only limited circumstances and that EPA expects source control measures almost always to be necessary. It presumes that monitored

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

natural attenuation will require more substantiation than other remedies, including more detailed site characterization data and performance monitoring than needed to support active remediation.

The EPA directive indicates that three types of characterization data provide evidence of natural attenuation: (1) historical data demonstrating decreasing contaminant mass or concentration, (2) hydrogeologic or geochemical data demonstrating indirectly the attenuation process, and (3) data from field or microcosm studies (conducted in or with actual contaminated media) that show the occurrence of a particular attenuation process. It indicates that the first line of evidence may be adequate if the overseeing regulatory authority determines that these data are of sufficient quality and duration to support a decision to select monitored natural attenuation as the remedy. Otherwise, the second line of evidence should be provided. Where both the first and second lines of evidence are inadequate or inconclusive, the third line of evidence also may be necessary.

According to the directive, criteria triggers that signal unacceptable performance include unpredicted increases in contaminant concentrations, indications of new or renewed release, sentry or sentinel wells showing contaminants not decreasing at a sufficiently rapid rate to meet remediation objectives, and changes in land or groundwater use. The directive says that contingency remedies generally should be included as part of a monitored natural attenuation remedy for selections that are based primarily on predictive analyses rather than on documented trends of decreasing contaminant concentrations. The EPA defines contingency as a backup technology or modification of the selected technology to be used if needed.

The directive cautions strongly against reliance on non-EPA documents that may provide technical information, despite the fact that it recognizes that little EPA guidance exists “concerning appropriate implementation of monitored natural attenuation remedies.” Non-EPA guidances are not “officially endorsed by EPA, the EPA does not necessarily agree with all their conclusions, and all parties involved should clearly understand that such guidances do not in any way replace current EPA … guidances or policies addressing the remedy selection process,” according to the directive. EPA may change this position with experience, as the agency acknowledges that non-EPA documents may provide useful technical information to site managers.

EPA Technical Protocol

In September 1998, the EPA Office of Research and Development released a protocol entitled Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. The intended audience for

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

the protocol includes project managers, contractors, consultants, scientists, and regulatory personnel. The document focuses on biological processes occurring in groundwater contaminated with mixtures of fuels and chlorinated aliphatic hydrocarbons. It relies on methods developed by the Air Force Center of Environmental Excellence for assessing natural attenuation at sites contaminated with fuel hydrocarbons and chlorinated solvents. In fact, all of the authors of the EPA protocol were coauthors of one or both of the Air Force documents. The protocol was a joint effort involving the Bioremediation Research Team at the EPA’s laboratory in Ada, Oklahoma; the Air Force Center for Environmental Excellence; the U.S. Geological Survey (USGS); and Parsons Engineering Science, Inc. For these reason, the EPA protocol is similar to the Air Force chlorinated solvent protocols (discussed later in this chapter).

The protocol includes a screening process for assessing contaminant biodegradation potential that employs a scoring system based on analytical parameters adopted from the Air Force protocol. The scoring system is designed to recognize geochemical environments where reductive dechlorination (see Chapter 3) is plausible. The protocol states that the scoring system is applicable to various chlorinated compounds and is weighted toward chemical indicators of a reducing environment and the production of associated daughter products. The scoring system compares data from the contaminant source area, within the plume, downgradient from the plume, and upgradient and lateral locations not affected by the plume. If the score is sufficiently high (15 or more points) in the zones of contamination, then the investigation continues with determination of groundwater flow and solute transport parameters. The protocol recommends using the analytical model BIOSCREEN to assess whether natural attenuation processes will be capable of meeting site-specific remediation objectives downgradient from the source. It suggests using the numerical model BIOPLUME III to estimate whether site contaminants are attenuating at a rate fast enough to restore the plume to appropriate cleanup levels.

The protocol provides case study examples illustrating the use of total chloride or chlorine as a tracer and mass-balance concepts to estimate the biodegradation rate. It does not provide guidance on long-term monitoring. The protocol underwent external and internal peer and administrative review by the EPA and the Air Force.

EPA Region 4 Protocol

EPA Region 4 in 1997 developed a document entitled Draft Region 4 Suggested Practices for Evaluation of a Site for Natural Attenuation (Biological Degradation) of Chlorinated Solvents that describes suggested practices for

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

evaluating natural attenuation of chlorinated solvents. Like the more recent EPA protocol described above, it relies on information contained in the Air Force protocol on this subject. The EPA Region 4 protocol, however, is much briefer, 42 pages. It focuses primarily on natural attenuation of perchloroethylene (PCE) and trichloroethene (TCE), although it could apply to other chlorinated solvents or mixtures of chlorinated solvents and fuel hydrocarbons.

Although the protocol repeats the EPA OSWER definition of natural attenuation, it emphasizes natural attenuation through biological degradation processes. The protocol presents the hydrogeological, geochemical, and physiological concepts in the context of a nine-step process, employing the same scoring system for preliminary screening as the Air Force protocol. It discusses the sustainability of natural attenuation in terms of the supply of electron donors versus electron acceptors. It briefly mentions institutional controls. Discussion of the possible failure of natural attenuation and subsequent reliance on engineered remediation strategies is explicit in the decision tree presented in the document.

When creating this document, EPA Region 4 explicitly recognized that it may be superseded: “The hope within the EPA nationwide is that a guidance document will be put forth from EPA’s Office of Research and Development that will also complement or supersede this document. At this time a date for this guidance is not known.” Presumably, the 1998 EPA technical protocol for natural attenuation of chlorinated solvents is a step toward that eventuality.

Department of Energy (DOE) Guidance for Monitored Natural Attenuation

The DOE document entitled Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites outlines a site-screening procedure for assessing the importance of natural attenuation at DOE sites. The general audience is DOE site managers. The document considers metals, radioactive materials, and organic chemicals. Most of it is devoted to discussion, justification, and use of “MNAtoolbox” (monitored natural attenuation toolbox). This is a software package developed by Sandia National Laboratories to guide site managers through the screening phase. The document does not discuss the peer review for this software package.

The guidance proposes a concept termed “natural attenuation factor” (NAF), which it discusses at some length. The NAF is calculated by adding together four variables representing dilution, sorption, irreversible uptake, and degradation. This concept is difficult to grasp; the DOE document itself states that the NAF does “not imply a specific physical meaning or reactive-transport implication. [The NAF] has as its sole

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

objective the interpretive summation of the various mass removal mechanisms.” The NAF concept is obscure, and additional discussion of the relationship of the NAF to classic retardation or attenuation concepts would be helpful. The guidance document relies on certain approaches and selected literature to justify procedures to compute the NAF, but it steps beyond the basis of scientific understanding and vastly oversimplifies various processes. For example, irreversible sorption, which is one of the processes represented in the NAF, is not understood for either organic or inorganic contaminants; much more scientific research is needed before this process can be quantified. In the end, the NAF is expressed as a score ranging from 0 to 100 [score = 100 ( NAF/(1 + 0.01NAF)], where a score greater than 50 is proposed to favor monitored natural attenuation. The toolbox is easy to use, and a version is available on the Internet, but the committee is concerned about whether the results are accurate enough to be meaningful.

The authors of this document undertook an overwhelming task. Contaminated DOE sites are complex from every scientific and technical perspective. While the DOE document represents a needed initial attempt to address a complicated subject, it does not adequately recognize the complexity of contamination on DOE lands, and the use of the NAF calculations is not justified from a scientific perspective.

The DOE protocol is the only document that addresses inorganic contaminants. Although the protocol mentions the various complicating factors that may affect the sorption and sequestration of inorganics in the subsurface, it nonetheless suggests using default values for sorption coefficients and fraction of contaminant metal taken up irreversibly. It indicates that these default values are probably no more accurate than plus or minus 30 percent, but this uncertainty range is not substantiated and in fact may be very inaccurate. The document mentions that site-specific measurements of the affinity of contaminants for specific soils are preferred, and this point needs more emphasis. Previously measured sorption values should be recommended for use only as reference indicating a possible range that may be encountered. Further, whether irreversible uptake of inorganic compounds can be measured by sequential extraction or isotopic pulsing, as suggested in this protocol, is a matter requiring further scientific deliberations.

Because the DOE protocol is the only one that covers inorganics, as well as the concept of irreversible sorption for both inorganics and organics, and because it is readily accessible on the Internet, the committee’s concerns about this protocol are particularly important. The protocol as is could be misused and could produce misleading results. The protocol suffers from oversimplifications resulting from (1) the fact that the long-term processes responsible for the immobilization and mobilization of

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

inorganic elements and radionuclides in groundwater are not well understood or studied, and (2) the fact that each inorganic element and radionuclide exhibits subtleties in geochemical behavior that are not easily generalized. For the time being, decisions concerning natural attenuation at DOE sites should be based on firm scientific and engineering studies and expert judgment, and this protocol should be reviewed and revised before it is used as a decision-making tool.

Air Force Technical Protocol for Fuel Contaminants

The U.S. Air Force Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination in Groundwater, released in 1995, was among the first such protocols. It was developed through a collaborative effort involving the Air Force, EPA, and Parsons Engineering Science, Inc. One of the reasons the Air Force sponsored development of the protocol was to instruct Air Force remediation site managers. Because Air Force personnel transfer frequently and change responsibilities, detailed instructions on evaluating and managing natural attenuation sites were needed. The protocol is not intended for use with plumes comprising mixtures of contaminants (e.g., fuels with chlorinated solvents or metals). It also does not discuss fuel additives such as methyl tert-butyl ether (MTBE). From a technical standpoint, this is among the most thorough and scientifically sound of the available protocols. The protocol and its appendixes describe in detail how to define the extent of the nonaqueous phase, determine groundwater chemistry, estimate aquifer parameters, and confirm biological activity. It provides methods for estimating the role of dilution, sorption, and dispersion in the apparent reduction in contaminant mass. Methods recommended in the protocol to demonstrate that biological activity is occurring include field dehydrogenase assays, microcosm studies, and the presence of volatile fatty acids. The protocol describes how to present and analyze field data to estimate the role of natural attenuation and how to calculate mass balances of electron donors and acceptors, using modeling to support the mass-balance calculations and perform sensitivity and uncertainty analyses. A method for calculating the intrinsic capacity (capacity of the aquifer to sustain natural attenuation) is described in detail using conservative approaches. Modeling is also used for sensitivity and uncertainty analyses.

Air Force Technical Protocol for Chlorinated Solvents

The Air Force Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater is intended for use where ground-

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

water is contaminated with either chlorinated solvents or mixtures of chlorinated solvents and fuel hydrocarbons. The protocol applies primarily to PCE and TCE and extends the concepts developed in the fuel protocol to the case of chlorinated solvents. The document is extensive, with six appendixes that provide detailed methodologies and case studies. Like the fuel protocol, the document was developed in recognition of the fact that Air Force personnel transfer frequently. It was developed jointly by the Air Force, EPA, USGS, and Parsons Engineering, Inc.

The protocol discusses three types of contaminant plumes in detail: type 1, in which the primary substrate is anthropogenic organic carbon (e.g., BTEX or landfill leachate), which drives reductive dechlorination; type 2, in which concentrations of biologically available native organic carbon are relatively high, which generally results in slower reductive dehalogenation rates than type 1 plumes; and type 3, in which native or anthropogenic organic carbon is inadequate and dissolved oxygen concentrations are greater than 1 mg/liter, which results in conditions under which reductive dechlorination will not occur. The protocol provides a scoring system intended to give the user an early indication of the potential for PCE or TCE natural attenuation. This scoring system has been adopted by others, including the EPA and the State of Minnesota.

Navy Technical Guidelines

The Navy document entitled Technical Guidelines for Evaluating Monitored Natural Attenuation addresses petroleum hydrocarbons and chlorinated solvents, whether singly or mixed. This document draws on the Air Force protocols for fuel contaminants and chlorinated solvents and the EPA’s definition of natural attenuation, with biodegradation mechanisms the principal focus. The document was prepared by several Naval Facilities Engineering Commands, the USGS, and Parsons Engineering Science, Inc.

The Navy guide is much shorter than the Air Force protocols (42 pages plus two case-study appendixes, one for jet fuel and one for chlorinated ethylenes) and relies on the extensive references in the Air Force protocols. The document is intended to provide general guidance, and it states that “ultimately the accurate assessment of natural attenuation will rely as much on the sound professional judgment of the practitioner responsible for the assessment as on the guidelines being followed.” Thus, the protocol, unlike the Air Force documents, is not a complete how-to manual. As the title suggests, professionals and their expertise are expected to contribute to the Navy’s natural attenuation practices.

The Navy’s technical guidelines include a logic diagram for deducing the final electron acceptor hierarchy, a brief six-step approach to evaluate

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

the efficiency of BTEX natural attenuation, and a brief eight-step method for evaluating the efficiency of natural attenuation of chlorinated solvents. The guidelines require two lines of evidence to confirm that natural attenuation is occurring: (1) plume stabilization and/or historical loss of contaminant and (2) groundwater chemistry data showing depletion of electron acceptors and donors, increasing metabolic by-product concentrations, decreasing parent compound concentrations, and increasing daughter product concentrations. A three-page table, unique to this protocol, explains the possible interactions among active remediation technologies and natural attenuation.

State Agency Protocols

Minnesota Draft Guidelines for Chlorinated Solvents

The Minnesota document entitled Draft Guidelines: Natural Attenuation of Chlorinated Solvents in Groundwater was developed for use by regulators in Minnesota’s state-level programs for cleanup of contaminated sites. Although the document is brief (24 pages), it captures essential technical points regarding natural attenuation of chlorinated solvents through the use of checklists. While it is too compressed for the nonexpert to use, a person familiar with the literature would find the protocol easy to follow. The protocol draws on the preliminary screening model put forward in the Air Force protocol for chlorinated solvents. It outlines procedures for comparing the estimated rate of biodegradation with the estimated rate of contaminant migration using the nonproprietary BIOSCREEN model to rapidly assess the potential for natural attenuation as a promising remedy. The nonproprietary BIOPLUME III model is recommended for simulating aerobic and anaerobic contaminant degradation processes by modeling the sequential use of electron acceptors.

New Jersey Natural Attenuation Policy

New Jersey has a policy on natural attenuation in a section of the state administrative code entitled “Technical Requirements for Site Remediation.” This section covers investigation or remediation of any contaminated site in New Jersey. The portion directly applying to natural attenuation is only about two pages long, but it cross-references sections in the technical requirements. The style is that of a terse legal guide. The regulations do not address specific contaminant types, meaning that the same standards apply to fuels and chlorinated solvents. In general, the technical regulations explain what data must be supplied, but provide little guidance on why such data are to be collected or how such data should be gathered or interpreted.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

One unique feature of the New Jersey policy is that if natural attenuation is accepted, then a “classification exception area” (CEA) is established that explains and maps the affected area, identifies the contaminants, and estimates the longevity of the CEA.1 In essence, the CEA is the regulatory tool used to permit remediation by natural attenuation, but it also provides an institutional mechanism for keeping track of natural attenuation sites.

Declining contaminant concentrations, not a steady-state plume, are required for a CEA to remain in effect. Eight consecutive quarterly water samples are required before establishing a CEA. Free-product nonaqueous-phase liquid (NAPL) must be removed or contained if removal is not practical. Implementation of institutional controls and periodic CEA reviews are laid out in the CEA guidance. The institutional controls include notification of authorities and property owners about the impacts of contamination and projected future use of groundwater assuming a 25-year planning horizon.

Corporate Protocols: Chevron

Chevron developed natural attenuation protocols to guide its staff on site analysis. Chevron has two protocols: one for fuels (entitled Protocol for Monitoring Intrinsic Bioremediation in Groundwater) and one for chlorinated solvents (entitled Protocol for Monitoring Natural Attenuation of Chlorinated Solvents in Groundwater). The Chevron protocols draw from other efforts, especially Air Force protocols. They are narrow in scope, detailing the company’s preferences regarding sampling and analytical procedures. The focus is on indicator parameters (e.g., dissolved oxygen) not on contaminants. Both protocols emphasize the importance of representative measurements of geochemical indicator parameters. They present case studies that illustrate the utility of geochemical indicators and contaminant concentrations for assessing the occurrence of intrinsic bioremediation. The documents are easy to read and use.

Professional and Industry Association Protocols

American Society for Testing and Materials (ASTM) Standard Guide

The ASTM Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites is intended to serve as a template for

1  

New Jersey requires that CEAs be established wherever groundwater is contaminated.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

others to use in developing more specific guidance. It was developed in response to a recognized need by private industry and government regulatory agencies for a consistent approach to evaluating and using natural attenuation as a remediation option. Details on site sampling, analytical methods, and data evaluation are provided in appendixes, with reliance on reference to ASTM or other publications.

Although the ASTM guide is intended to be consistent with EPA guidance, the two protocols differ in important ways. Key areas of difference include the following:

  • the range of contaminants addressed (ASTM’s document covers only petroleum hydrocarbons, while EPA’s guidance is intended to apply to all contaminants);

  • whether or not site characterization data have to be more detailed than those for active remediation, as stated by the EPA;

  • whether or not natural attenuation is more likely appropriate if the plume is not expanding, as described by the EPA; and

  • whether or not source removal or control is mandatory, as emphasized by the EPA.

The guide outlines in broad terms a mass-balance approach, which is explained mainly in terms of contaminant source strength, for estimating plume stability and shrinkage via a first-order decay rate, but it does not provide much technical detail. One appendix contains a discussion of various methods of evaluating field data, including examples of order-of-magnitude types of calculations using analytical models that rely on typical values of various parameters. The guide is relatively easy to understand and concise, with details in appendixes. The standard was distributed informally for external comments. It was then formally voted on and approved by ASTM members.

American Petroleum Institute (API) Manual

The API guidance manual entitled Methods for Measuring Indicators of Intrinsic Bioremediation focuses on sampling and analytical methods for geochemical parameters, such as dissolved oxygen, nitrate, sulfate, and oxidation-reduction potential. It was prepared under contract by CH2M-Hill through the auspices of the API’s Health and Environmental Sciences Department. The document states clearly that it is not intended as guidance for broader issues related to assessment of intrinsic bioremediation; the manual states that these broader issues are addressed in other documents such as those prepared by the Air Force, ASTM, Mobil, and Chevron. The API was concerned about lack of specific guidance on appropriate

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

sampling and analytical procedures to ensure that measurements generate quality data. This was a concern because the extent to which natural attenuation is ultimately embraced depends to a large degree on the valid characterization of site conditions. Thus, the API guidance manual is narrowly focused on sampling and analysis.

The document is easy to read and likely easy to use in practice. Information is provided on method selection, method implementation, and data interpretation. The manual was peer reviewed by industry members of the API Biodegradation Processes Research Group and two individuals from the EPA’s R.S. Kerr Environmental Research Laboratory.

Public-Private Partnership Protocol

The document Natural Attenuation of Chlorinated Solvents in Groundwater: Principles and Practices was prepared by the industrial members of the Bioremediation of Chlorinated Solvents Consortium of the Remediation Technologies Development Forum (RTDF), a public-private partnership organized by EPA to advance the development of innovative remediation methods. It was written to provide information to the public in nonscientific terms, but it also includes a detailed description of how to make decisions about natural attenuation. This document is unlike the other protocols or guides reviewed in that it is written in question-and-answer format. For example, one question is, How often is natural attenuation effective? The document is organized into sections, including one on technical challenges associated with sites contaminated with chlorinated solvents and the types of chlorinated solvent attenuation processes known to occur, one on how natural attenuation studies are generally conducted, and one on how a stepwise process and flow chart generally are used to implement natural attenuation at chlorinated solvent sites. The document discusses the three lines of evidence that form the basis for current protocol and guidance documents: (1) loss of contaminants at the field scale, (2) the presence and distribution of geochemical and biochemical indicators, and (3) direct microbiological evidence. It includes a table that explains, for various data types, data used in providing these lines of evidence. The complex interplay among patterns of chlorinated solvent biodegradation and behavior of electron acceptors is described schematically in two figures, one for anaerobic systems and one for aerobic or anaerobic systems. The document appears to accomplish its stated purpose of guiding the more general reader on “how to think about natural attenuation based on science.”

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

ADEQUACY OF PROTOCOLS

Table 5-1 compares the various protocol documents in terms of conformance with the desired protocol attributes outlined in Box 5-2. This qualitative comparison shows whether the particular topic is discussed in the protocol, merely mentioned, or not discussed or not applicable. The protocols vary widely in the breadth of topics covered and the level of detail provided. Seven of the documents are based on peer-reviewed literature and examples of field studies; these seven documents are

  1. Air Force protocol for natural attenuation of fuel contamination,

  2. Air Force protocol for natural attenuation of chlorinated solvents,

  3. Navy guidelines for natural attenuation of petroleum hydrocarbons and chlorinated solvents,

  4. Minnesota guidelines for natural attenuation of chlorinated solvents,

  5. ASTM standard guide for natural attenuation at petroleum release sites,

  6. EPA technical protocol for natural attenuation of chlorinated solvents, and

  7. EPA Region 4 suggested practices for natural attenuation of chlorinated solvents

The remaining protocol documents serve more limited purposes, such as providing guidance on sampling and analytical methods, clarifying legal and policy requirements, attempting to standardize evaluation methods, and educating the public. In short, not all of the protocols were intended to be comprehensive. Even the comprehensive protocols were not intended to cover broadly all issues related to natural attenuation as outlined in Box 5-2. Nonetheless, comparing the existing documents against the attributes listed in Box 5-2 shows where additional national guidance is needed.

With the exception of the EPA directive and the DOE guidance, a common characteristic of all the protocols is their focus on fuel hydrocarbons and/or chlorinated solvents. This limited focus reflects the volume of empirical evidence and scientific data supporting natural attenuation of these compounds under certain conditions. The EPA directive and DOE guidance are the only documents that address any other class of contaminants (specifically, inorganics), but these documents have significant limitations. The information in the EPA directive is general, with no detailed discussion or examples, whether for organic or inorganic contaminants. The limited amount of detail provided is consistent with the stated objective of the EPA directive, which is to clarify EPA’s position on natural attenuation. The DOE approach must be viewed with caution

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

TABLE 5-1 Natural Attenuation Policy Statements, Regulations, and Technical Protocols Reviewed

 

Community Concerns

Type of Document

Community Involvement

Institutional Controls, Long-Term Monitoring

Contingency Plans

Policy Documents

 

EPA (1999)

X

X

XX

Regulationsa

 

Minnesota (chlor. sol. 1997)

X

New Jersey (1995)

XX

XX

Technical Protocols

 

Chevron (chlor. sol. 1997)

RTDF (chlor. sol. 1997)

X

X

Air Force (chlor. sol. 1997)

X

EPA Region 4 (chlor. sol. 1997)

X

X

EPA ORD (chlor. sol. 1998)

Navy (fuels 1998)

X

Air Force fuels (1995)

X

X

Chevron fuels (1995)

ASTM (fuels 1997)

X

XX

API (fuels 1997)

DOE (inorganic and organic contaminants, 1998)

NOTE: X = mentioned; XX = discussed;—= not discussed or not applicable. Chlor. sol. = chlorinated solvents are primary focus of the document; ORD = Office of Research and Development; RTDF = Remediation Technologies Development Forum.

a The parts of the state regulations that dealt only with natural attenuation were reviewed.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Scientific and Technical Issues

Implementation Issues

Cause and Effect

Site Condition Assessment

Sustainability

Scope

Science-Based Underpinnings

Evidence

Geological and Hydrological Setting

Source Characterization

Intrinsic Capacity

Complicating Factors

Robustness

Peer Review

Qualifications, Training

Usability

X

X

X

X

X

X

X

X

XX

X

X

XX

X

XX

XX

XX

XX

XX

XX

XX

X

X

XX

XX

XX

XX

XX

XX

X

XX

XX

XX

XX

XX

XX

X

X

X

XX

XX

XX

XX

XX

XX

X

X

XX

X

XX

XX

XX

XX

XX

X

XX

X

XX

XX

XX

XX

XX

XX

XX

X

XX

XX

XX

XX

XX

X

XX

XX

XX

X

X

X

XX

XX

XX

XX

XX

X

XX

X

X

X

X

XX

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

until reviewed and tested, because it ignores many factors that influence the fate of inorganics in the subsurface. In general, the review of existing protocols makes clear that guidance on natural attenuation of contaminants other than fuel hydrocarbons and chlorinated solvents is lacking. Inorganic contaminants are not covered in sufficient detail, and other organic contaminants—particularly persistent hydrophobic organic compounds such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs), energetics, explosives, and pesticides—are not discussed. This is a major weakness in the current state of the art for natural attenuation protocols.

The following discussion assesses the adequacy of existing protocols with respect to the attributes listed in Box 5-2 and Table 5-1.

Community Concerns

Community Involvement

None of the protocol documents provides guidance on steps in the evaluation process where public input is important, although the EPA directive and the New Jersey and RTDF documents mention community participation in passing. The EPA directive notes that the process for remedial selection “should include opportunities for public involvement that serve to both educate interested parties and to solicit feedback concerning the decision making process,” but it does not provide details.

The New Jersey protocol is one of the few documents that specifically addresses community notification, albeit not solely for natural attenuation. As noted above, the New Jersey protocol links the acceptance of natural attenuation to the establishment of a classification exception area. The New Jersey Department of Environmental Protection (NJDEP) Classification Exception Areas: Final Guidance includes a two-page discussion of public notice requirements. Natural attenuation is only one of the potential reasons for establishing a classification exception area. NJDEP requires responsible parties to notify in writing the appropriate municipal authorities, public health agencies, and in some cases, individual property owners. The notification must explain the type and extent of groundwater contamination, the proposed remedial action, the duration of the proposed exception area, and those uses of the aquifer that will be suspended in the affected area for the term of the exception. Notification requirements depend on whether or not the proposed area is in a “groundwater use area,” which is determined by an analysis of the present and projected future uses of groundwater assuming a 25-year planning horizon. If the proposed exception area is in a groundwater use area, then all affected off-site property owners and local officials must be notified. The property

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

owners notified are those with property under which the contaminant plume may flow and on which wells either already exist or might be installed in the future. If the proposed area is not in a groundwater use area, only local authorities must be notified, even when the contamination has migrated off-site. Regardless of groundwater use, a 30-day comment period is common, after which the state establishes institutional controls via well restrictions. Classification exception areas may not be permanent, except where background levels of contaminants exceed the state’s standards.

In some cases the protocols reviewed here may not address public involvement because the developers assumed that other guidelines exist for public participation and communication. The regulatory programs governing contaminated sites have policies for community involvement. In addition, all of the armed services have strong policies providing for public participation (via advisory boards, public comment periods, and technical assistance programs). Nonetheless, the existing natural attenuation protocols should specify more clearly—for example, through the use of flow charts—the points in the evaluation process at which public input is important.

Institutional Controls and Long-Term Monitoring

The documents reviewed provide little or no discussion of the need for institutional controls and how these should be coordinated with long-term monitoring programs. Several sections of the EPA guidance document mention institutional controls, but the guidance does not discuss this topic in detail. Institutional controls are important for natural attenuation remedies, because many years may be required to achieve remedial objectives. Although a number of states have guidelines comparable to New Jersey’s classification exception area guidance, the guidelines vary by state. Often, they are intended to classify the state’s groundwaters by use and quality for decision making about environmental protection and restoration. Whether this process is suitable for natural attenuation is unclear. Government agencies may have trouble with record keeping and monitoring in support of institutional controls. Probably the worst example is the infamous Love Canal: the site was capped and deeded for use as an open space or park, but then was sold for housing lots—despite the deed restrictions—to raise funds for construction of a school.

The existing guidelines are also extremely variable in their recommendations for monitoring well placement, duration of monitoring, terminology for monitoring phases, and terminology for monitoring wells. Given this variability, assessing whether existing guidelines for long-term monitoring of natural attenuation are sufficiently protective of public

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

health is difficult. For example, one community group reported to the committee an example of a Florida town where the monitoring strategy for natural attenuation is based on periodically testing the town’s water supply well for contaminants from the plume (Ruhl, 1998).

Existing protocols provide insufficient guidance on how to coordinate monitoring results with the program of institutional controls that may be necessary to protect the community from exposure to contamination. The community will want to know whether natural attenuation is continuing to work and public health is being protected in the meantime. The purpose of long-term monitoring is to demonstrate that the important natural attenuation processes are sustained over time and that the plume is not expanding. The behavior of large contaminant plumes in the subsurface over decades is still a topic of active research. Especially in cases where residual NAPL acts as a continuous source of contamination that may eventually exhaust the natural attenuation capacity of the aquifer, monitoring is critical to ensure that attenuation processes continue to operate. Protocols should provide better guidelines on long-term monitoring.

Some protocols (e.g., Air Force, EPA chlorinated solvents, Navy) recommend placing sentinel wells within the plume and at its fringe to provide an ongoing indication of whether the plume is expanding or the geochemical processes are changing. However, other protocols (e.g., ASTM) require long-term monitoring only at point-of-compliance wells located at property boundaries or between the affected groundwater and potential sensitive receptors. Although these wells may be located in the projected path of the plume, many years may pass before the front of an expanding plume reaches them and triggers contingency measures. Problems can arise if a plume is allowed to grow substantially and active cleanup measures are delayed while natural attenuation is tried and subsequently found to be inadequate. The larger size of the plume makes the ultimate cleanup more expensive and longer in duration. Moreover, in some cases the delay may result in loss of available funds for contingency measures (Kelly, 1998). Long-term monitoring plans should also use sentinel wells to monitor plume behavior before the plume reaches receptors. The location and number of wells, as well as the frequency of sampling, may have to change with time, and these variables should be reflected in the protocols.

Contingency Plans

The EPA policy directive and the ASTM standard guide provide limited guidance on the specification of contingency plans in case natural attenuation does not perform as expected, but the other protocols men-

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

tion contingencies only in passing or do not mention them at all. The EPA directive recommends that contingency remedies be included as part of natural attenuation where the decision is based primarily on predictive analyses rather than on documented trends of decreasing contaminant concentrations. The ASTM guide states that if at any point during the long-term monitoring program, data indicate that natural attenuation is not adequate to achieve remedial goals, a contingency plan should be implemented. This plan could include consideration of changes in remedial approach, including additional source removal, containment measures, more rigorous institutional controls, and augmentation of natural attenuation with other remedial actions.

Scope

The scope of the available documents is explained clearly in each. Consideration of scope is essential to ensuring that the protocols are used for their intended purposes only.

Scientific and Technical Issues

Cause and Effect

The Air Force, Navy, ASTM, EPA Office of Research and Development, EPA Region 4, and RTDF protocols explain clearly that cause and effect must be documented when deciding whether to use natural attenuation. They describe methods that should be used to establish which processes are responsible for observed decreases in contaminant concentrations. In general, these documents carefully review field collection and sample analyses that are to be performed and the use of such data in confirming mechanisms for concentration decreases. The presentation format and information in these documents are readily understandable to a reader trained in the physical sciences or engineering aspects of contaminant transport and transformation in the subsurface, and the RTDF is accessible to broader audiences.

Although several of the protocols provide detailed guidance on establishing cause and effect, the body of protocols as a whole nonetheless treats issues of documentation in an uneven manner. In particular, the level of detail needed to characterize a contaminant plume is highly variable. In part, this variability occurs because the individual protocols address different types of contaminants. For example, a more thorough investigation is generally outlined in protocols for chlorinated solvents than for petroleum fuels, because the degradation pathways for solvents are more complex than those for fuels.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Ultimately, site closure decisions depend on the professional judgment of individual regulators. Investigators and regulators sometimes employ and advocate minimalist criteria or rules of thumb to make quick decisions on natural attenuation without using detailed technical protocols to show cause and effect (Arulanantham, 1998). However, such rules should not be substituted for experienced professional assessments based on a conceptual model and understanding of cause and effect that are consistent with all of the data, as described in Chapter 4. In particular, it is important to avoid creating a climate in which regulators feel pressured to apply simple rules. In general, providing guidelines for documenting cause and effect is one of the most critical roles of a natural attenuation protocol.

Site Condition Assessment

The protocols in general provide detailed guidelines on characterizing site hydrogeology. However, except for the Air Force protocols (and the related EPA protocol for solvents), the protocols give surprisingly little attention to characterizing contaminant sources and using source characterization data to determine whether source removal or control is necessary. Source control issues must be considered in evaluating the ability of natural attenuation to meet remedial objectives, because the total mass of contaminants within source zones is often very large compared to the mass of dissolved contaminants, and the source may persist for a very long time. The EPA policy directive notes several times the importance of source characterization in evaluating the feasibility of natural attenuation, but it does not provide details. The Air Force protocol provides the most complete guidance on this issue. It describes, in general terms, the need to determine the subsurface distribution of NAPL and how NAPL composition may be used to calculate contaminant partitioning from NAPL to water, referring readers to other sources for methods of calculation. It also recommends evaluation of source removal effects on natural attenuation, focusing on the effects of reduction in the target contaminant concentration at the source. Nonetheless, the other protocols provide insufficient guidance on source characterization and control.

Sustainability

A few of the protocols provide guidance on sustainability, which refers to the long-term viability of natural attenuation. For natural attenuation to be sustainable, the aquifer must have sufficient intrinsic capacity (e.g., sufficient supply of required electron acceptors and/or donors), complicating factors must not unduly interfere with natural attenuation,

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

and the natural attenuation processes must be robust to changes in environmental conditions.

The Air Force protocol on fuel contamination discusses the relationship between relative amounts of different fuel components present in comparison to the natural supply of various electron acceptors, which may provide a conservative estimate of the mass of fuel that can be degraded. Although other protocols reference this quantitative approach, most instead provide a qualitative approach. For example, the Navy protocol recommends looking for patterns in electron-donor and acceptor concentrations. The ASTM protocol recognizes that a quantitative approach has been used, but it cautions that the linkage between contaminant mass balance and consumption of electron acceptors is only qualitative because of the possible environmental sources and sinks for electron acceptors and metabolites. In general, the modeling assessments outlined in the ASTM protocol are minimalist approaches, invoking simple models with maximum flow and slowest degradation. The EPA chlorinated solvents protocol encourages the use of BIOSCREEN and BIOPLUME III to determine whether natural attenuation processes may meet site-specific remediation objectives and whether site contaminants are attenuating fast enough to restore the plume to appropriate cleanup levels.

Several of the protocols mention complicating factors, but only the Navy guidelines address this topic in detail. Even the Navy protocol’s discussion of complicating factors is limited: it addresses only interactions between natural attenuation and engineered remediation systems. The Navy guidelines explain that although engineered remediation systems may be effective in removing contaminant mass from groundwater, some engineered systems may adversely affect natural attenuation.

Robustness is addressed in the EPA chlorinated solvents, Air Force, and ASTM protocols through the use of models to perform sensitivity analysis by varying the input parameters. Low uncertainty implies robust predictions. Hence, robustness is not highlighted as a separate topic in these protocols. Rather robustness and the factors contributing to it are implicit in the modeling efforts, and long-term, continued performance must be verified by monitoring.

Although many of the existing protocols discuss aspects of sustainability, better guidance is needed. Especially important is additional guidance on how to evaluate potential complicating factors and explicit guidance on how to determine the sensitivity of the attenuation processes to environmental changes. In general, protocols tend to focus on evaluation of present-day behavior, not on how well the process will be working in the future.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
Peer Review

The ASTM standard guide is the only technical protocol that describes an explicit peer review process. ASTM solicited comments on a voluntary basis from interested outside parties. The guide was then reviewed and voted on by an ASTM subcommittee and committee, proceeding eventually to review by the full society. The document also was sent to underground storage tank programs in all 50 states and 10 EPA regions. Regardless, the formal ASTM review process involved only ASTM members, which is not the same as peer review by parties not directly affiliated with the authors of the protocol or their organizations.

The RDTF principles and practices document was reviewed by the industrial members of the RTDF, as well as by state regulators. Although the reviews are not specifically documented, the Air Force and Navy protocols and the EPA’s guidance document underwent wide in-house review, and the Air Force chlorinated solvents protocol underwent some external review. Preparation of the Air Force and Navy protocols, as well as the EPA guidance, drew extensively on peer-reviewed literature and involved groups from both the public and the private sectors, including some of the most knowledgeable natural attenuation experts in the country. Nonetheless, aside from the ASTM standard guide, the peer review process is not well documented for the natural attenuation documents in Box 5-1.

Although the peer review process may be adequate in certain cases, further substantiation of the process is needed. All protocols would benefit from external peer review to ensure that they are based on correct principles and sound methodologies and that they adequately address public concerns and environmental protection issues. Peer review is especially important in protocols designed for contaminants for which field experience is limited and the underlying science is not well understood. Inorganics, explosives, and persistent hydrocarbons (such as PAHs and PCBs) are examples of such compounds. The lack of sufficient external review of the underlying technical assumptions and methodologies employed in the DOE protocol is an especially significant problem. As with scientific peer review, the protocol peer review process should be conducted by individuals not closely associated with the author’s organization or branch. The process by which the protocol was reviewed, the comments received, and how those comments were addressed should be explained in the protocol.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Implementation

Usability

All of the protocols, except for the EPA policy directive and the New Jersey requirements, are relatively straightforward and easy for the knowledgeable individual to follow. The EPA directive provides only broad guidance on acceptance of natural attenuation, while the New Jersey technical requirements are too brief for practical use as a stand-alone protocol. However, as mentioned previously, one problem with some protocols is that they rely too heavily on easy-to-compute scores that mask the role of judgment and may produce erroneous conclusions due to questionable inputs and assumptions. The release of publicly available software, particularly software not subject to peer review (such as, DOE’s monitored natural attenuation toolbox), may instill false confidence, leading to misapplication of natural attenuation by those without adequate training. Although protocols should be easy for trained specialists to read and follow, they should not oversimplify the very complex processes that occur in the subsurface.

Training and User Qualifications

None of the protocols provide explicit guidelines on the training or background needed by implementers. Although some of the documents reference the need for trained individuals, they do not explain the type of training necessary.

ADEQUACY OF DECISION-MAKING TOOLS

Some of the protocols use decision-making tools such as flow charts, check lists, scoring systems, or interactive softwares. Table 5-2 lists tools used in the protocols reviewed in this chapter. Most of the tools were created by a small group of authors who work together. Thus, the decision tools for fuels and chlorinated solvents take quite similar approaches. All such tools have very limited ability to adequately recognize the differences between sites based on size, composition, or geological complexity.

Although a good decision-making tool can help to organize thinking about a site and could be useful for those not familiar with natural attenuation, well-informed users will probably not adhere rigidly to these tools any more than they would to highly specific protocols. For such individuals, it is not apparent how important these tools are, given all the existing regulations, guidances, and protocols. Further, inappropriate or inflexible use of these tools could make decision processes rigid rather

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

TABLE 5-2 List of Decision-Making Tools Included in Protocols

Type

Author

Application

Flow charts

Air Force

Fuel hydrocarbons

 

Air Force

Chlorinated solvents

 

Navy

Fuel hydrocarbons

 

Minnesota

Chlorinated solvents

 

EPA Region 4

Chlorinated solvents

 

EPA Office of Research and Development

Chlorinated solvents

 

RTDF

Chlorinated solvents

Check lists

RTDF

Chlorinated solvents

 

Minnesota

Chlorinated solvents

Scoring systems

Air Force

Chlorinated solvents

 

EPA Region 4

Chlorinated solvents

 

EPA Office of Research and Development

Chlorinated solvents

 

DOE

Fuel hydrocarbons, chlorinated solvents, inorganics

Interactive software

DOE

Fuel hydrocarbons, chlorinated solvents, inorganics

than fluid. The tools may also make the user inappropriately confident in the decision. In cases where the decision tools are not appropriate for a site being considered, these tools may simply empower individuals to impede the process of identifying the best remediation method.

Flow Charts

As shown in Table 5-2, a few of the protocols use flow charts, which can be very useful for organizing data needs and the decision process. For example, both the EPA Office of Research and Development and the EPA Region 4 protocols present flow charts showing the steps involved in a natural attenuation demonstration and the important regulatory decision points. The available flow charts deal with deciding whether to employ natural attenuation, to use natural attenuation in combination with another technology, or to use another technology without any contribution from natural attenuation. Existing policy directives, especially the EPA’s, would benefit from greater use of flow charts.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Check Lists

The only two available check lists are from the Minnesota and RTDF chlorinated solvent protocols. These check lists attempt to list all of the potential information that could usefully apply to a natural attenuation site. The Minnesota check list is considerably more complete and is broken down into investigative phases. The RTDF list attempts to convey how information needs correlate with the complexity of a site. Although rigid use of check lists would impede the decision-making process, they are very useful for maintaining consistency, ensuring thoroughness, and serving as a reminder of what sort of information may have to be gathered. Protocols could benefit from increased use of check lists, as long as they make clear that some information is mandatory and some is optional and as long as the lists are used flexibly.

Scoring Systems

At this time, only one scoring system—the system presented in the Air Force and EPA protocols for chlorinated ethylenes—is widely used for natural attenuation. This system is clearly described as being useful only for screening and only for sites contaminated with chlorinated ethenes. The scoring system assigns positive or negative numeric scores to various geochemical indicators, contaminant mixtures, and key biodegradation products. A high positive numeric score is intended to mean that a site should be evaluated further for natural attenuation, not that attenuation is proven.

Unfortunately, this scoring system is being widely adopted for uses that the authors never intended. For example, many states are using it to evaluate natural attenuation for all types of chlorinated solvents. Tables of natural attenuation scores are showing up in remedial investigation reports at Superfund sites. Maps and cross sections showing natural attenuation scores are being included in final reports as a key line of evidence. Some regulators are accepting this inappropriate use of scoring.

Limitations in the present scoring system that are not widely understood include the following:

  • The method applies only to chlorinated ethenes.

  • The scores emphasize reducing environments more than dehalogenation reactions.

  • A reduced geochemical environment does not guarantee that natural attenuation will occur, because geochemical environments can be very reduced without reductive dehalogenation of chlorocarbons occurring (for example, if dehalogenating bacteria are not present).

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • The scoring system includes items that are of current research interest (for example, hydrogen concentration), but that may have limited practical impact on making remediation decisions.

  • The system identifies interactions between contaminants only for electron donors.

Nyer et al. (1998) discuss the use of the Air Force scoring system for a case study at a former aerospace manufacturing facility in Irving, Texas, that is contaminated with TCE and trichloroethane. They found that site screening using the Air Force scoring method indicated limited potential for biodegradation. However, further analysis suggested that the groundwater sampling data were indicative of geochemical characteristics within the larger pore spaces of the sand and gravel alluvium and not of clayey soils where groundwater flow was much slower and reductive dehalogenation was believed to be occurring. The authors caution against using the scoring system as a primary method of substantiating natural attenuation and suggest that many sites will require assessments beyond those specified in the Air Force protocol.

As discussed previously, the DOE technical guidance for monitored natural attenuation proposes using a score known as the NAF that is estimated from the sum of four factors representing different processes that affect the contaminants. The NAF is expressed on a scale of 1 to 100, with a score of 50 or more indicative of natural attenuation. The calculation is facilitated by use of interactive software that can be accessed remotely through the Internet. The methodology, the engineering and scientific underpinnings, and validation have not been subjected to comprehensive, independent peer review. To date, the NAF has not been widely used, and the committee is concerned about whether the NAF is meaningful.

Because scoring systems are susceptible to misuse and because approaches to natural attenuation have been advanced in recent years, the committee recommends the abandonment of scoring systems in screening sites for natural attenuation. Instead, the committee recommends site-specific conceptual models and footprints as described in Chapter 4.

ADEQUACY OF TRAINING

Decisions regarding natural attenuation require a considerable amount of expert judgment. Thus, adequate training of protocol users is essential to ensure that the protocols are implemented properly. The training needed to implement natural attenuation protocols differs depending on an individual’s role. Regulators, responsible parties, remediation consultants, and community-based organizations that participate in decision making have different training needs.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Responsible Parties

Responsible parties need to be able to evaluate the work of remediation consultants and make remediation decisions based on consultants’ work. Thus, such individuals should have sufficient technical background and experience to actively manage their consultants and negotiate responsibly with environmental regulators. Responsible parties who rely solely on attorneys or external project managers to handle their remediation programs do so at the risk of receiving poor-quality work. Responsible party representatives should always have a technical degree and understand quality evaluation of field data and consultant reports, fundamentals of risk evaluation, and the necessity for community involvement. In some organizations, especially the military, environmental managers may be “short-timers” with little environmental background. A number of good short courses on natural attenuation are available from the EPA, the Interstate Technology Regulatory Cooperation (ITRC) Work Group, and the National Ground Water Association. Responsible party representatives without experience with natural attenuation (but who do have other qualifications) should take at least one such course.

Consultants

Remediation consulting firms employ two tiers of personnel on most projects: senior technical leaders and field personnel.

  1. Senior technical leaders: A consulting firm should have a lead natural attenuation expert with considerable understanding of natural attenuation science. These experts should specialize in natural attenuation to the degree possible, know the methods applied to natural attenuation studies, recognize the typical pitfalls of these studies, and be able to identify which natural attenuation protocols to use and apply. Consulting leaders develop professional judgment about natural attenuation based on fundamental understanding of the scientific principles combined with experience from a number of sites. These leaders need the technical skills to understand unique contaminants and situations outside the scope of protocols and communications skills to effectively transmit their conclusions. Experience suggests that broad training with grounding in environmental science and engineering fundamentals is considerably more useful than more focused training, such as a specific bioremediation curriculum.

  2. Field sampling personnel: High-quality field data are essential for understanding the geochemical and biochemical environment in the subsurface to determine which natural attenuation processes are possible at

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

sites. Field personnel who are trained to recognize and correct problems as they occur and adapt to changeable conditions in the field are a major asset. The necessary skills may be obtained via experience, although some schools offer associate’s degrees in environmental science that can be helpful. Specific key skills include the Occupational Health and Safety Administration 1910.120 hazardous materials training, well purging, and groundwater collection techniques; basics of chemical analysis; field instrument calibration and field lab techniques; sample shipping requirements; and first aid.

State and Federal Regulators

State and federal regulators are charged with evaluating the merits of the various remediation proposals they receive and making responsible judgments on whether natural attenuation proposals have sufficient technical justification. Regulators must be able to understand a wide variety of technical and policy information. Groundwater regulators should have technical degrees; a graduate degree is preferred to ensure an appropriate level of understanding. Desirable disciplinary skills include environmental engineering, geology, chemistry, and biology. Natural attenuation training for regulators should include the fundamentals of relevant environmental regulations, development and use of conceptual models, use of mathematical tools and models to estimate contaminant movement and degradation, fundamentals of risk evaluation, and methods for working with stakeholders. Training also should include education in natural attenuation protocols and how natural attenuation remedies compare to other potential remedies with respect to risk and cost. Training of regulators can be aided by short courses and mentoring under more experienced personnel. For example, ITRC offers a short course for state regulators (see Box 5-3). Some regulatory agencies (for example, in Oregon) employ technology specialists who are available to help regulators with complex situations.

Community-Based Organizations

As explained in Chapter 2, members of communities affected by contaminated sites where natural attenuation is proposed as a remedy should be included in the decision-making process as early as possible and have the resources necessary to participate in this process. Community members may desire technical training to help them understand the natural attenuation proposal, and they should have the opportunity to receive this training.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 5-3
ITRC Natural Attenuation Training for State Regulators

The Interstate Technology Regulatory Cooperation Work Group, which consists of about 26 state agencies working on guidance documents on various remediation techniques, offers a two-day training course aimed specifically at the needs of state groundwater regulators. The goals of the course are to provide current scientific information concerning natural attenuation of chlorinated solvents and to provide participants with tools for evaluating proposals for natural attenuation.

The first day covers basics of environmental biodegradation and provides detailed examples showing how real-world sites are evaluated. Day 2 emphasizes how to interpret information from real sites. The class divides into small groups and evaluates the technical suitability of natural attenuation on two computer-simulated sites. State regulators lead two free-form discussions on regulatory concerns about natural attenuation, before and after the practical exercises. An EPA representative explains the agency’s recent natural attenuation directive.

Although the course was developed for regulators, it is also open to consultants and industry. Preference in registration is given to regulators. Consultants and industry employees pay a modest registration fee. These fees are used to pay the travel and living expenses of state and federal regulators, public stakeholder representatives, and course instructors. The ITRC operates the course on a nonprofit basis.

There are several ways to provide training opportunities for community members. Possibilities include making information readily and frequently available; providing funds directly to a community organization to hire an expert; providing opportunities for community leaders to enroll in a training course offered by a neutral, nonprofit organization; holding workshops or seminars in the community to explain technical issues; and holding a regional conference that brings together scientists and community representatives. Information and training should be provided by individuals who are independent of the responsible party and the regulating agency. For training directed specifically at a local community, training topics should be selected in collaboration with community leaders, and training should be held at times that are convenient for the community.

The goal of training community leaders and members is to help them understand the technical complexities of natural attenuation, including its strengths and weaknesses, its effectiveness in addressing the specific contaminants, its suitability at a particular site, and the short- and long-term risks. Training topics might include a basic introduction to contami-

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

nant behavior in the environment, including chemical, physical, and microbial processes; a review of strengths and weaknesses of natural attenuation; a description of methods for measuring natural attenuation at a site; a review of how site conditions and contaminants affect natural attenuation; and a discussion of ways to verify the effectiveness of natural attenuation.

ADEQUACY OF POLICIES CONCERNING USE OF PROTOCOLS

Despite the limited EPA guidance on implementation of monitored natural attenuation remedies, EPA cautions strongly against reliance on non-EPA documents. Although it acknowledges that such documents may provide useful technical information, EPA does not officially endorse non-EPA protocols. Nonetheless, for very good reasons, users and agencies have proceeded with the development of various protocols. Some of these protocols have been prepared with the direct involvement of EPA research personnel. Yet, how these non-EPA protocols can be used in satisfying the regulatory requirements of groundwater and soil cleanup programs administered by EPA is unclear. Adding to the confusion, each state also administers its own groundwater cleanup programs for sites not regulated by EPA, and state requirements can vary widely. In some cases multiple agencies may have jurisdiction over a site, and the goals, criteria, constraints, and process of selecting remedies can be a mixture of federal, state, and local laws and policies. Natural attenuation remedies, like engineered remedies, must be approved by the site regulator, but regulatory acceptance of existing protocols to document natural attenuation varies highly from site to site. No standard regulatory approval process is available.

Legal requirements for cleanup using natural attenuation (or any other process) vary significantly depending on the regulatory program under which the site is being restored. Therefore, the application of natural attenuation protocols varies, as well. Site owners wishing to use natural attenuation must demonstrate to regulators, through use of an appropriate protocol or other means, that natural attenuation will achieve whichever remediation requirements apply to the site.

Although drinking water standards historically have been chosen as groundwater remediation goals at Superfund and Resource Conservation and Recovery Act (RCRA) corrective action sites, goals can vary depending on what decisions regulators make about future land use (NRC, 1997). If an aquifer is not used as a domestic water supply, there can be a substantial (e.g., hundredfold or more) difference between a federal drinking water standard and a risk-based concentration limit. Because this difference can be large, the applicability of natural attenuation at a site may

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

depend on whether a risk-based concentration limit or a drinking water standard has to be achieved.

Remediation requirements at sites regulated under state programs differ markedly, as well. Some state Superfund programs require restoration to background levels of contaminants, while others set higher concentration goals (NRC, 1994). Nondegradation policies for groundwater can be interpreted as discouraging, if not precluding, the selection of natural attenuation. If, however, the policy is interpreted as allowing a reasonable time to restore the aquifer, natural attenuation may be acceptable, depending on the local determination of what constitutes a reasonable time frame and the strength of the expectation that natural attenuation can restore the aquifer to local standards within the allowed time frame. For the cleanup of underground storage tanks, most of which are delegated to the states, remediation goals also vary considerably. Many states have adopted a risk-based approach to setting site-specific cleanup levels for leaking underground storage tanks; other states use state-specific groundwater standards to define remediation goals.

Like remediation requirements, the level of detail of data required to demonstrate that natural attenuation can achieve remediation goals—and whether natural attenuation can be accepted at all—varies with the regulatory program and with the individual regulator. For example, at Superfund and RCRA sites, EPA’s monitored natural attenuation policy generally requires applications for use of natural attenuation to demonstrate that (1) contaminant concentrations are decreasing and (2) hydrogeologic and geochemical conditions are sufficient to support natural attenuation at rates that will achieve cleanup goals in a reasonable time (EPA, 1999). In some cases, EPA, at the discretion of individual site regulators, also may require microcosm or field studies showing that natural attenuation potential is realized under actual site conditions. In contrast, at gas stations with leaking underground storage tanks, some state regulators require only proof that contaminant concentrations are decreasing over time (Arulanantham, 1998). Further complicating matters, the framework and structure of regulatory management of natural attenuation sites are in a rapid state of flux. In the case of sites contaminated with petroleum hydrocarbons, there has been a radical shift in some states over the past two years from a position of requiring a demonstration that natural attenuation is appropriate to expecting site owners to demonstrate why it is not adequate. The State of Wisconsin, for example, will no longer provide funds to reimburse owners of leaking underground storage tanks for cleanups that involve engineered remedies.

In sum, the goals that natural attenuation must achieve and the regulatory requirements for documenting natural attenuation are highly vari-

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

able from site to site. How natural attenuation protocols are to be used in the context of existing regulatory programs requires further clarification.

CONCLUSIONS

More than a dozen documents providing guidance on whether natural attenuation is an appropriate remedy for managing contaminated sites have been issued within the past few years. As proposals to use natural attenuation continue to multiply in number, more protocols likely will be developed.

With the exception of the DOE protocol, the available natural attenuation protocols address only organic contaminants and only two classes of such: fuel hydrocarbons and chlorinated solvents. A large body of empirical evidence and scientific and engineering studies in recent years has been developed to support understanding of natural attenuation of these classes of organic contaminants under certain conditions. However, natural attenuation of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, explosives, and other classes of persistent organic contaminants is not addressed in any protocol. Further, while the DOE protocol proposes a method for assessing natural attenuation processes for inorganics, such processes are not well understood, posing concerns about whether results generated with the DOE protocol are accurate enough to be meaningful.

The Committee on Intrinsic Remediation reviewed 14 of the available natural attenuation protocols in detail. These protocols were developed by a range of organizations, from federal and state agencies, to private companies, to industry associations. At the time of preparation of this report, they represented most of the available documents providing guidance on decisions related to natural attenuation. The committee compared these protocols against a list describing the characteristics of a comprehensive protocol, which would cover three broad subject areas:

  1. Community concerns: A comprehensive protocol would indicate key points for receiving community input. It would also include plans for maintaining institutional controls to restrict use of the site until cleanup goals are achieved, monitoring the site, and implementing contingency measures when natural attenuation fails to perform as expected.

  2. Scientific and technical issues: A comprehensive protocol would describe how to document which natural attenuation processes are responsible for observed decreases in contaminant concentration; how to assess the site for contaminant source, hydrogeologic, and geochemical characteristics that affect natural attenuation; and how to assess the sustainability

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

of natural attenuation over the long term. It should be independently peer reviewed.

  1. Implementation issues: A comprehensive protocol is easy to follow and would describe qualifications necessary to implement the protocol.

None of the protocols meets all of the characteristics defined by the committee. To some extent, this gap reflects the purposes for which these protocols were developed. Some are detailed technical guides; others are intended to help ensure consistency in site evaluation within a particular organization (such as a private corporation or a branch of the military); others are intended to guide policy. Nonetheless, key limitations in the existing body of protocols must be addressed.

In general, the existing protocols are silent on when and how to involve the public in site decisions and when and how to implement institutional controls. In the few instances where these matters are mentioned, the discussion is typically brief, almost in passing. Discussion of when and how to implement contingency plans in case natural attenuation does not work is inadequate in many of the protocols. The protocols also provide insufficient guidance on when engineered methods to remove or contain sources of contamination benefit natural attenuation and when they interfere with it. Guidance on how to conduct long-term monitoring to ensure that natural attenuation is continuing at an adequate rate is also inadequate. All of the protocols are silent, as well, on the issue of type and level of training or experience needed to implement the protocol. For the most part, the existing protocols have not been subjected to independent peer review.

An additional limitation of some of the protocols relates to uncertainties in “scoring systems” used to reach conclusions about whether a site is a candidate for treatment by natural attenuation. Protocols with such scoring systems yield numeric values for the site in question, and if this value is above a certain level, the site is judged an eligible candidate for natural attenuation. Typically, such scores imply more confidence in the decision than is justified by field experience and literature to date.

A final problem with the existing body of protocols is the lack of sufficient guidance on which protocols are appropriate for use in various regulatory programs. None of the existing protocols developed by organizations other than the EPA is officially recognized by the agency, yet even EPA recognizes that a number of these might prove very useful in assessing sites that it regulates. Although EPA does not officially recognize any protocols other than those developed by the agency, a number of regulators at the state level advocate that for some sites, no protocol is needed to judge whether natural attenuation is occurring and that such determinations can be based on trends in contaminant concentration

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

alone. A process is needed to ensure consistent, logical application of professional judgment at all sites where natural attenuation is being considered. As the federal agency with responsibility for addressing environmental contamination, the EPA has to take charge of developing a consistent evaluation process.

In sum, the existing body of natural attenuation protocols is limited in several important areas. Where and how the existing protocols can be used to meet regulatory requirements for documenting site cleanup—and whether such protocols are required at all—is also unclear.

RECOMMENDATIONS

  • The EPA should lead an effort to develop national consensus guidelines for protocols on natural attenuation. As soon as possible, EPA should undertake an effort to work with other federal agencies, professional organizations, industry groups, and community environmental organizations to assess natural attenuation protocols and how they can be used in existing regulatory programs (including Superfund, the RCRA corrective action program, and the leaking underground storage tank program). Ideally, these guidelines should address in detail the attributes listed across the top of Table 5-1. The guidelines should be updated regularly to include new knowledge and should allow flexibility for regional geologic differences and variations in policies by state or region. The guidelines should give special attention to community involvement, source removal, long-term monitoring, contingency plans, sustainability of natural attenuation, and training for protocol users.

  • The national consensus guidelines and all future natural attenuation protocols should be peer reviewed. The peer review should be conducted by independent experts who are not affiliated with the authoring organization.

  • The national consensus guidelines and future protocols should eliminate the use of “scoring systems” for making decisions on natural attenuation. The evaluation methods outlined in Chapter 4 of this report, using conceptual models and footprints of natural attenuation, should replace scoring systems. 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. For this reason, scoring systems, including DOE’s monitored natural attenuation toolbox and scorecard, should not be used.

  • Developers of natural attenuation protocols should write easy-to-understand documents to explain the protocol to nontechnical audiences. Such documents should be made available to interested members of communities near contaminated sites.

Suggested Citation:"5 Protocols for Documenting Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • The EPA, other federal and state agencies, and organizations responsible for contaminated sites should provide additional training on natural attenuation concepts for interested regulators, site owners, remediation consultants, and community and environmental groups. The training should be provided by neutral organizations. The cost of attendance should be subsidized for regulators and community group members.

REFERENCES

Arulanantham, R. 1998. Presentation to the Committee on Intrinsic Remediation, Third Meeting, Woods Hole, Mass., June 1-2.


EPA (Environmental Protection Agency). 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. Directive No. 9200.U-17P. Washington, D.C.: EPA, Office of Solid Waste and Emergency Response.


Kelly, M. 1998. Presentation to the Committee on Intrinsic Remediation, Second Meeting, Irvine, Calif., March 12-13.


NRC (National Research Council). 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.

Nyer, E., P. Mayfield, and J. Hughes. 1998. Beyond the AFCEE Protocol for Natural Attenuation. Ground Water Monitoring Review Summer (1998):70-77.


Ruhl, S. 1998. Presentation to the Committee on Intrinsic Remediation, Second Meeting, Irvine, Calif., March 12-13.

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In the past decade, officials responsible for clean-up of contaminated groundwater have increasingly turned to natural attenuation-essentially allowing naturally occurring processes to reduce the toxic potential of contaminants-versus engineered solutions. This saves both money and headaches. To the people in surrounding communities, though, it can appear that clean-up officials are simply walking away from contaminated sites.

When is natural attenuation the appropriate approach to a clean-up? This book presents the consensus of a diverse committee, informed by the views of researchers, regulators, and community activists. The committee reviews the likely effectiveness of natural attenuation with different classes of contaminants-and describes how to evaluate the "footprints" of natural attenuation at a site to determine whether natural processes will provide adequate clean-up. Included are recommendations for regulatory change.

The committee emphasizes the importance of the public's belief and attitudes toward remediation and provides guidance on involving community stakeholders throughout the clean-up process.

The book explores how contamination occurs, explaining concepts and terms, and includes case studies from the Hanford nuclear site, military bases, as well as other sites. It provides historical background and important data on clean-up processes and goes on to offer critical reviews of 14 published protocols for evaluating natural attenuation.

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