Contaminants in the Subsurface Source Zone Assessment and Remediation (2005) / Chapter Skim
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1 Introduction
1 Introduction
Pages 16-33
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. Strict regulation of point-source discharges, including the development of more effective sewage treatment plants, has led to substantial improvements in water quality for some surface waters in the United States (EPA, 2000)
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as in the case of chlorinated solvents or a separate solid phase as where chemical explosives have precipitated in the subsurface. Organic liquids that are denser than water are referred to as dense nonaqueous phase liquids (DNAPLs)
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, giving priority to sites where remediation efforts can make the most impact is essential. Unlike previous NRC reports, this report focuses on active remediation of source zones and the effect of that remediation on a number of factors including groundwater quality.
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Because it is a primary goal for the military to achieve site closeout at as many sites as possible within the next 1015 years, there have been renewed efforts to reduce the time required for remedy operation and monitoring by attempting to remove a significant portion of contaminant mass at many hazardous waste sites with more aggressive source remediation technologies. Source remediation can involve ex situ and in situ technologies, both conventional and innovative.
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However, several additional innovative in situ technologies, which are the focus of this report, have recently demonstrated potential for effecting at least partial depletion of the source. Although comprehensive data on most of these innovative technologies are not available, the EPA has compiled information on in situ chemical oxidation and in situ thermal treatment (which includes steam injection, electrical resistance heating, conductive heating, radio-frequency heating, and hot air injection)
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Because many petroleum hydrocarbons are amenable to natural degradation processes, they are less likely to present long-term contamination problems that might eventually necessitate aggressive source remediation. The greater concern at military facilities is with recalcitrant organic compounds such as the chlorinated solvents perchloroethene (PCE)
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From page 22... ...
In addition, they are also burdened with typical domestic waste streams, such as from municipal solid waste landfills, wastewater treatment plants, hospitals, laundries, golf courses, and underground storage tanks for automobile and truck fuels. Despite the breadth of contamination problems discussed above, the Army Environmental Center's request to the NRC was specifically focused on contamination by recalcitrant organic compounds.
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During 2001, the Army Environmental Center oversaw independent technical reviews at seven of its facilities where DNAPLs are present in hydrogeologically complex locations. It was observed that certain aggressive source remediation technologies were being pursued at these sites with little understanding of (1)
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From page 24... ...
More generally, what can be said about the future use of source removal as a cleanup strategy and the specific technologies investigated during the study? CHARACTERISTICS AND DISTRIBUTION OF DNAPLS AND CHEMICAL EXPLOSIVES Chapter 2 of this report describes in detail the physical properties of DNAPLs and chemical explosives that affect their distribution and persistence in the subsurface.
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Most chemical explosives have melting points well above near-surface soil temperatures, and thus exist as solids at environmentally relevant temperatures. Unlike DNAPLs, when explosives are deposited on the ground surface as solids, they do not migrate into the subsurface.
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Any contamination that is present can exist as a separate liquid or pure solid phase, it can be dissolved in the aqueous phase, it can be associated in some fashion with the soil solid phase, or it can be volatilized. For the purposes of this report, the committee agreed that separate solid or liquid phase contaminants were indeed sources.
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This seems unlikely, and it is probable that EPA intended for "contaminated soil" to mean only in-place contaminated soil and debris in the same context as drummed waste. However, nothing in the EPA definition allows one to make a clear distinction between source zones and the solids impacted by dissolved phase plumes.
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The presence of this source zone can be inferred from rebounding aqueous concentrations after treatment or recognized from the high concentrations within the matrix. The charge to this committee included the term "source removal," but this term was abandoned in the course of writing this report because while many technologies involve contaminant mass removal (e.g., excavation and surfactant/ cosolvent flooding)
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REPORT ROADMAP Among potentially responsible parties, scientists and engineers, regulatory agencies, and other stakeholders, rapidly growing interest in using more aggressive source remediation technologies has generated numerous questions and uncertainties that this report attempts to address. The strengths and weaknesses of source zone remediation as a strategy for hazardous waste remediation are discussed, focusing on recalcitrant organic contaminants (e.g., solvents, other DNAPLs, explosives)
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From page 30... ...
In many instances of source remediation observed by the committee, objectives for cleanup are not adequately defined in advance, leading to misunderstandings regarding the expected outcomes. Chapter 5 discusses the specific technologies that are commonly associated with aggressive source remediation, including the conditions under which they are optimal, their limitations, and their effectiveness for meeting the objectives outlined in Chapter 4.
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From page 31... ...
a cleanup strategy at hazardous waste sites containing DNAPLs and chemical explosives. It is important to note, as discussed in detail in Chapter 3, that the process of recognizing, characterizing, and remediating a site is not linear, in that it does not follow the successive stages listed in these chapters, flowing directly from initial studies to final remediation.
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2003. Army Environmental Center.
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From page 33... ...
1995. Dense Chlorinated Solvents and other DNAPLs in Ground water.
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