Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites (2013) / Chapter Skim
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6 Technology Development to Support Long-Term Management of Complex Sites
6 Technology Development to Support Long-Term Management of Complex Sites
Pages 219-260
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From page 219... ...
Thus, for these sites the management challenges include optimization of active remedies, reducing mass flux/mass discharge of contaminants from source areas such that natural attenuation may be effective, or ensuring that any active or passive engineered containment system will remain effective over the long term. This chapter discusses technological developments that can aid in addressing these management challenges -- in particular, providing the scientific and technical bases for transitioning from active remediation to more passive strategies where applicable.
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In NRC (2005) , hydrogeologic heterogeneity was conceptually captured by identifying five generic geologic environments ranging from nearly uniformly homogeneous, unconsolidated porous media (Type I)
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Back-Diffusion and Desorption For many complex sites that have been subject to partial or complete source removal, the transition to long-term management is largely controlled by volatilization into the vapor phase (if applicable) and transport into the aqueous phase plume, as these two phases are the primary media for both off-site contaminant migration and the biotic and abiotic transformation processes associated with natural attenuation.
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Understanding whether back-diffusion and desorption are occurring at a site is challenging because the relative importance of each process is highly dependent on the site-specific contamination history and the presence and distribution of low-permeability and/or strongly sorbing materials. And yet, current site characterization techniques typically do not fully delineate the structure of these materials, particularly when they are distributed over small spatial scales within the plume interior.
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From page 223... ...
Such paths of preferential groundwater flow often control the distribution of contaminant mass in both source areas and downgradient plumes, and must be properly considered in the design and implementation of containment and remediation strategies. Chapman et al.
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Transformation Capacity As discussed in Chapter 7, monitored natural attenuation (MNA) is the dominant process during long-term management at sites not relying on physical or hydraulic containment.
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The tools discussed later in this chapter represent important initial steps toward the development of such a protocol. Vapor Intrusion Issues As described in Chapter 5, the vapor intrusion pathway is increasingly considered at complex sites with DNAPL contamination.
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Furthermore, it is not always the case that the greatest indoor air impacts are found in buildings overlying the highest groundwater concentrations. Groundwater-related vapor intrusion has been documented in some buildings overlying dissolved chlorinated solvent groundwater concentrations as
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. As a case in point, approximately 3,000 residences overlie chlorinated solvent groundwater plumes originating from Hill Air Force Base, although monitoring has indicated that a very small percentage of the residences have indoor air impacts attributable to groundwater contamination.
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new tools that can be applied to better understand whether MNA is working. Improved Application of Conventional Monitoring Tools The deployment of conventional site characterization tools has evolved in a manner that has emphasized greater spatial resolution in regions where contamination is significant.
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As it is unlikely that complete removal of contaminant source material will be feasible for many complex sites, the transition to long-term management will depend not only on the amount of source mass removed, but on the rate at which mass is transferred between the source and plume compartments during the post-remediation period. One of the most promising recent developments in source zone management is the development of tools for measuring contaminant mass flux, either at localized monitoring points or as an integrated mass discharge across a control plane.
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further clarification of the range of uncertainty associated with mass flux and mass discharge measurements, (2) continued refinement of specific aspects of the various techniques, including a better definition of the necessary preliminary site characterization, and (3)
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One vision for future sensing systems is something like a household CO monitor with real-time data communication to a home computer, tablet, or PDA to increase occupants' awareness of their indoor air quality. Evaluating Monitored Natural Attenuation Chapter 7 discusses the possibility, at many complex sites, of a transition from active source zone remediation to more passive strategies such as MNA or natural attenuation without monitoring.
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Determining if site-specific bacteria are capable of degrading the target contaminants can be accomplished using genomic tools to detect and quantify gene copies, such as quantitative polymerase chain reaction (qPCR) and PCRdenaturing gradient gel electrophoresis (PCR-DGGE)
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. The presence/absence of genes has also been correlated to the rates of reductive dehalogenation of chlorinated ethenes at sites where natural attenuation (Lu et al., 2006; Burgmann et al., 2008)
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Isotope Analysis Another emerging tool for understanding the effectiveness of MNA is compound-specific isotope analysis (CSIA) , which is used to monitor the changes in stable isotope ratios of elements within molecules (e.g., 13C/12C, 2H/1H)
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Predicting Source Zone Mass Removal during Remediation Although active source removal typically occurs prior to the transition to long-term management, new developments in remedial technology and/ or site characterization might result in scenarios where additional source zone activity is undertaken. Modeling of source zone processes remains challenged by a number of technical constraints, including the need to represent second-order processes such as pore clogging by biofilms and/or precipitated reaction products, reactions with natural organic matter and other non-target compounds, gas production, and other changes in aquifer properties (e.g., Heiderscheidt et al., 2008; Glover et al., 2007)
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, and metabolite identification to study natural attenuation of BTEX, including the effects of ethanol. Amaral et al.
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TECHNOLOGY DEVELOPMENT 237 FIGURE 6-2 Concurrent carbon and hydrogen isotope ratios of benzene measure at various sampling depths (B through E) together with isotope patterns for aerobic and anaerobic benzene degradation calculated from published enrichment factors for carbon and hydrogen as well as the isotope signature of the contaminant source using the Rayleigh equation.
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From page 238... ...
Of particular relevance to long-term management, a considerable body of work has addressed the simulation of natural attenuation processes for both petroleum and halogenated organic contaminants, including sequential parent/daughter reactions and multiple electron acceptor/donors. In particular, the modular code structure employed by MT3DMS has facilitated a number of extensions that address reaction scenarios typical of those found at complex sites, including SEAM3D (Waddill and Widdowson, 2003)
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. Modeling Natural Attenuation The decision to switch from an aggressive remediation strategy to MNA or natural attenuation without monitoring requires an estimate of post-remediation plume development, including (1)
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Recently, the plane source concept has been extended to include the time-dependent mass flux from a DNAPL source zone based on a power function approach, as implemented in the REMCHLOR software. Screening models such as REMCHLOR represents a significant step forward in the practical application of the 14-compartment model, but its implementation emphasizes mass transfer from the source area to the transmissive plume.
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This section provides a snapshot of several emerging technologies, none of which, with exception of nanotechnologies, have received extensive field testing. The preponderance of research on nanoparticles used in groundwater remediation has focused on nanoscale zero-valent iron (nZVI)
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, with pilot-scale tests ongoing.2 An alternative to traditional in situ chemical oxidation (ISCO) is the encapsulation of reactive agents in a permeable reactive ISCO barrier.
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None of these in situ electrode methods, however, has been tested at field sites, and both cost and scaling issues may be important. RESEARCH FUNDING As the focus at complex sites shifts from active remediation to longterm management, the development and effective deployment of appropriate concepts and tools may require a redirection of research efforts.
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Geological Survey (USGS) , occasionally fund projects that have relevance to groundwater remediation (e.g., the USGS Water Resources Institutes)
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EPA has funded research on groundwater remediation through a variety of mechanisms, including its Science to Achieve Results (STAR) basic research program, several agency research laboratories, and externally funded Hazardous Substance Research Centers (which have been discontinued)
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Existing protocols for assessing monitored natural attenuation and other remediation technologies should be expanded to integrate compound-specific isotope analysis and molecular biological methods with more conventional biogeochemical characterization and groundwater dating methods. The development of molecular and isotopic diagnostic tools has significantly enhanced the ability to evaluate the performance of degradation technologies and monitored natural attenuation at complex sites.
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• Tools to Assess Vapor Intrusion. Further research and development should identify, test, and demonstrate tools and paradigms that are practicable for assessing the significance of vapor intrusion, especially for multi-building sites and preferably through short term diagnostic tests.
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From page 248... ...
Currently, a national strategy for technology development to support longterm management of complex sites is lacking. It is not clear that the pertinent federal agencies will be capable of providing the funding and other support for the fundamental research and development that is necessary to meet the challenges facing complex sites.
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2008. Comparative assessments of benzene, toluene, and xylene natural attenuation by quantitative polymerase chain reaction analysis of a catabolic gene, signature metabolites, and compound-specific isotope analysis.
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2008. Changes in contaminant mass discharge from DNAPL source mass depletion: Evaluation at two field sites.
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2002. Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance)
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2012b. EPA's Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile Organic Compounds and Residential Buildings.
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2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water.
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2007. Vapor Intrusion Pathway: A Practical Guideline.
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2005. Reference Handbook for Site-Specific As sessment of Subsurface Vapor Intrusion to Indoor Air.
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2006. Final NYSDOH CEH BEEI Soil Vapor Intrusion Guidance.
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Environmental Science & Technology 35(2)
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Environmental Science & Technology 43(10)
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TECHNOLOGY DEVELOPMENT 259 Wilkins, M
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