Natural Attenuation for Groundwater Remediation (2000) / Chapter Skim
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Executive Summary
Executive Summary
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In part, it is also a result of the high costs of engineered cleanup systems. Use of unenhanced natural processes as part of a site remediation strategy is called "natural attenuation." Some processes that occur during natural attenuation can transform contaminants to less harmful forms or immobilize them to reduce risks.
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Members of communities near contaminated sites often believe that natural attenuation is a "donothing" approach. They believe that relying on natural attenuation relieves those responsible for the contamination from the financial burden of site remediation without adequately protecting public health and the environment.
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Community leaders interviewed as part of this study expressed special concern that natural attenuation allows responsible parties to save on cleanup costs while exposing the community to undue risks. Community leaders believed that in many cases, natural attenuation leads to reductions in contaminant concentrations primarily because the contaminants are diluted or transferred to another environmental medium, where they may continue to pose risks.
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build an effective working relationship with the community. RECOMMENDATIONS: INVOLVING THE PUBLIC · At sites where natural attenuation is proposed as a formal remedy for groundwater contamination and where the contamination affects a community, environmental agencies and responsible parties should provide the community with clear evidence indicating which natural attenuation processes are responsible for the loss of contaminants.
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Strategies for encouraging public involvement include providing information regularly, holding meetings at times and locations that are convenient to the community, establishing rules for community participation at all meetings, using culturally sensitive materials, and where appropriate, translating materials for non-Englishspeaking communities. · The Environmental Protection Agency, state environmental agencies, and responsible parties should ensure that interested community groups can obtain independent technical advice about natural attenuation and other potential remedies.
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This table should serve only as a general guide; every site will have to be assessed individually because of the wide variation in conditions at individual sites. Judgments in the table are based on the current level of understanding of the dominant attenuation processes and the probability that sites will have the specific conditions necessary for effective natural
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For example, because BTEX attenuation processes are well understood, sites such as gas stations with BTEX contaminants will not require the same level of analysis as sites with contaminants that degrade less readily or are less well understood. · To achieve remediation objectives, natural attenuation may have to continue for many years or decades.
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8 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION TABLE ES-1 Likelihood of Success of Natural Attenuation Likelihood of Dominant Current Success Given Attenuation Level of Current Level of Chemical Class Processes Understandinga Understandingb Organic Hydrocarbons BTEX Biotransformation High High Gasoline, fuel oil Biotransformation Moderate Moderate Nonvolatile aliphatic Biotransformation, Moderate Low compounds immobilization Polycyclic aromatic Biotransformation, Moderate Low hydrocarbons immobilization Creosote Biotransformation, Moderate Low immobilization Oxygenated hydrocarbons Low-molecular-weight Biotransformation High High alcohols, ketones, esters MTBE Biotransformation Moderate Low Halogenated aliphatics Tetrachloroethene, Biotransformation Moderate Low trichloroethene, carbon tetrachloride Trichloroethane Biotransformation, Moderate Low abiotic transformation Methylene chloride Biotransformation High High Vinyl chloride Biotransformation Moderate Low Dichloroethene Biotransformation Moderate Low Halogenated aromatics Highly chlorinated PCBs, tetrachlorodibenzofuran, pentachlorophenol, multichlorinated benzenes Less chlorinated PCBs, dioxins Monochlorobenzene Biotransformation, Moderate Low immobilization Biotransformation Moderate Biotransformation Moderate Nitroaromatics TNT, RDX Biotransformation, Moderate Low abiotic transformation, immobilization Low Moderate Inorganic Metals Ni Immobilization Moderate Moderate Cu. Zn Immobilization Moderate Moderate
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BTEX = benzene, toluene, ethylbenzene, and xylene; MTBE = methyl tert-butyl ether; PCBs = polychlorinated biphenyls; TNT = trinitrotoluene; RDX = royal Dutch explosive. a Levels of understanding: "High" means that there is good scientific understanding of the process involved, and field evidence confirms attenuation processes can protect human health and the environment; "moderate" means that studies confirm the dominant attenuation process occurs but the process is not well understood scientifically; "low" means that scientific understanding is inadequate to judge if and when the dominant process will occur and whether it will be protective.
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However, in some cases, it can cut off natural attenuation entirely, if the source is serving as critical fuel for attenuation processes. APPROACHES FOR EVALUATING NATURAL ATTENUATION Documenting that the contaminant concentration has become very low or undetectable in groundwater samples is an important piece of evidence that natural attenuation is working.
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Table ES-2 shows the level of analysis required for different site conditions. RECOMMENDATIONS: EVALUATING NATURAL ATTENUATION · At every site where natural attenuation is being considered as a formal remedy for groundwater contamination, responsible parties should use footprints of natural attenuation processes to document which mechanisms are responsible for observed decreases in contaminant concentration.
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The conceptual model should show the groundwater flow, contaminant source, plume, and reactions and chemical species relating to natural attenuation at the site. The model should be tested and revised as new data are gathered, especially at complex sites.
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However, the natural attenuation of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, explosives, and other classes of persistent organic contaminants is not addressed in any protocol. Furthermore, although the DOE document proposes a method for assessing natural attenuation processes for inorganic contaminants, such processes are extremely complex, and the DOE document does not adequately reflect this complexity.
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2. Scientific and technical issues: The protocol should describe how to document which natural attenuation processes are responsible for observed decreases in contaminant concentrations, how to assess the site for contaminant source and hydrogeologic characteristics that affect natural attenuation, and how to assess the sustainability of natural attenuation over the long term.
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An additional problem is lack of sufficient guidance on which protocols are appropriate for use in various regulatory programs. EPA does not officially endorse any protocols other than those developed by the agency, and the specific information that individual EPA regulators require to document natural attenuation can vary substantially.
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X XX XX X X X X X X XX X X X NOTE: XX = discussed; X =mentioned; = not discussed or not applicable. ASTM = American Society for Testing and Materials; API =American Petroleum Institute; chlorinated solvents = chlorinated solvents are primary focus of the document; DOE = Department of Energy; EPA = Environmental Protection Agency; 0RD = Office of Research and Development; RTDF = Remediation Technologies Development Forum.
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EXECUTIVE SUMMARY and 17 Scientific and Technical Issues Implementation Issues Cause and Effect Site Condition Assessment Sustainability o .= .'4 a, u ~ < L)
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The cost of attendance should be subsidized for regulators and community group members. In summary, natural attenuation processes that degrade or transform contaminants can work well in controlling risks from groundwater contamination when the right combination of contaminants and environmental conditions exists.
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For these other contaminants, natural attenuation may work in some cases under very specific site conditions. For all contaminants, natural attenuation will work best when the geologic system is simple enough for the natural attenuation processes to be effectively monitored.
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