Contaminants in the Subsurface Source Zone Assessment and Remediation (2005) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-15
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Where they are present, separate phase or sorbed contaminants serve as a long-lived contamination source to groundwater. The technical difficulties involved in characterizing and remediating source zones and the potential costs are so significant that there have been no reported cases of large DNAPL (dense nonaqueous phase liquid)
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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? SOURCE ZONES As a preliminary step, the NRC committee formed to conduct the study created a definition of "source" that would capture the essence of a source as a reservoir of contamination while making a distinction between the source zone and the plume of contaminated groundwater.
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Five hydrogeologic settings, described in Chapter 2, are broadly representative of the common conditions of concern: · Type I granular media with low heterogeneity and moderate to high permeability · Type II granular media with low heterogeneity and low permeability · Type III granular media with moderate to high heterogeneity · Type IV fractured media with low matrix porosity · Type V fractured media with high matrix porosity These settings differ in their permeability, heterogeneity, and porosity-parameters that control how contaminants are stored and released from source zones under natural and engineered conditions. For example, fractured media sites characterized by high matrix porosity (Type V)
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For both chlorinated solvent and explosives source zones, contaminant plumes develop downgradient of the source material in cases where the contaminants are soluble in water and are resistant to natural biodegradation. In general, groundwater plumes tend to have larger spatial extents and to be more continuous in nature in comparison to contaminant mass distributions within source zones.
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Decisions regarding the objectives of remediation and the remediation technologies selected will have a strong impact on the source zone characterization strategy and vice versa. Although it is impossible to prescribe a specific step-by-step source zone characterization process because of differing conditions from site to site, there are four broad categories of information that are critical to characterizing all source zones: 1.
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In general, successful source remediation requires information on the nature of the source material, on the site hydrogeology, on the source zone distribution, and on the site biogeochemistry. However, the level of characterization effort required and the tools used at any given site are dependent on site conditions, on the cleanup objectives chosen, and on the technology chosen to achieve those objectives.
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Physical objectives are discussed first, including mass removal, concentration reduction, mass flux reduction, reduction of source migration potential, plume size reduction, and changes in toxicity or mobility of residuals. Objectives relating to risk reduction, cost minimization, and scheduling are also discussed, many of which have been institutionalized within regulatory, risk assessment, and economic frameworks for site cleanup.
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SOURCE REMEDIATION TECHNOLOGY OPTIONS Chapter 5 presents those technologies that have surfaced as leading candidates for source zone remediation, including a description of each technology, a discussion of the technology's strengths and weaknesses, and special considerations for each technology. The discussion of chlorinated solvents focuses on contamination of the saturated zone, as this medium presents the greatest difficulties in terms of site cleanup.
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The table assesses the types of contaminants for which each technology is suitable and then qualitatively evaluates each technology's relative potential for mass removal, local aqueous concentration reduction, mass flux reduction, source migration, and changes in toxicity. This evaluation is presented for each of the five hydrogeologic settings described in Chapter 2.
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Chemical oxidation and enhanced bioremediation are more sensitive to heterogeneities than are thermal methods, and air sparging is the most sensitive to heterogeneity because there are no mitigating factors preventing the preferential flow of low-viscosity air and the bypassing of the target DNAPL. Heterogeneities are more likely to affect a technology's ability to achieve mass removal and local aqueous concentration reductions compared to mass flux from the source zone.
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Development of treatment technologies for explosives source zones is in its infancy because the characterization of explosive source materials and of their interactions with geologic media lags far behind the knowledge base that exists for DNAPLs. Before one can understand the utility or performance characteristics of treatment technologies for explosives contamination, one should understand the chemical and physical nature of the explosives source zones.
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The design and implementation of a successful source remediation project involve the iterative characterization of the source zone, development of remediation objectives, and evaluation of technologies -- a process that is sufficiently complex to warrant a formal protocol. Chapter 6 describes the elements of a protocol to assist project managers in designing, implementing, and assessing the effects of source remediation.
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A protocol specific to source zones is needed to aid stakeholders in optimizing the benefits derived from investments in remediating source zones. The key attributes that need to be addressed are pursuing actions that effect intended changes, understanding the extent to which objectives are attainable, and being able to measure progress toward desired
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With respect to the future use of source removal as a cleanup strategy, an important conclusion that can be made from reviewing source zone remediation attempts to date is that the data are inadequate to determine how effective most technologies will be in anything except the simpler hydrogeologic settings. Furthermore, it is unlikely that available source remediation technologies will work in the most hydrogeologically complex settings such as karst.
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From page 15... ...
EXECUTIVE SUMMARY 15 settings, to warrant further investigation to determine their long-term effects on water quality, especially if objectives such as mass flux reduction become more prevalent. Thus, future work should attempt to determine the full range of conditions under which these technologies can be successfully applied, and to better understand how mass removal via these technologies affects water quality.
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