3
Source Zone Characterization

One of the goals of this study was to explain the importance of characterization to the effectiveness of source remediation, including a discussion of tools or methods used to delineate sources of organics contamination in the subsurface. The environs of a hazardous waste site described in Chapter 2—that is, the hydrogeologic environment and the distribution of contaminants—are revealed through site characterization. Site characterization is a continuous, dynamic process of building and revising a site conceptual model that captures relevant aspects of a hazardous waste site, including the source zone. The site conceptual model represents current understanding of the site in terms of the relevant subsurface materials and processes, serves as the basis for more sophisticated site characterization, and will ultimately support the evaluation of various remedial alternatives. Because of the inherent scarcity of available data at field sites, the site conceptual model can only provide an approximation to the real world. Indeed, at the early stages of site conceptual model development, it is possible that several realizations will be tenable. However, as more monitoring and other data become available, the various plausible site conceptual models should gradually converge into a single picture encompassing all salient fluid flow and material transport and transformation processes. Site conceptual models are continually refined, possibly using computer models, to address site-specific complexities involving spatial and temporal variations in flow, transport, and transformation processes.

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 for characterizing all source zones:



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Contaminants in the Subsurface: Source Zone Assessment and Remediation 3 Source Zone Characterization One of the goals of this study was to explain the importance of characterization to the effectiveness of source remediation, including a discussion of tools or methods used to delineate sources of organics contamination in the subsurface. The environs of a hazardous waste site described in Chapter 2—that is, the hydrogeologic environment and the distribution of contaminants—are revealed through site characterization. Site characterization is a continuous, dynamic process of building and revising a site conceptual model that captures relevant aspects of a hazardous waste site, including the source zone. The site conceptual model represents current understanding of the site in terms of the relevant subsurface materials and processes, serves as the basis for more sophisticated site characterization, and will ultimately support the evaluation of various remedial alternatives. Because of the inherent scarcity of available data at field sites, the site conceptual model can only provide an approximation to the real world. Indeed, at the early stages of site conceptual model development, it is possible that several realizations will be tenable. However, as more monitoring and other data become available, the various plausible site conceptual models should gradually converge into a single picture encompassing all salient fluid flow and material transport and transformation processes. Site conceptual models are continually refined, possibly using computer models, to address site-specific complexities involving spatial and temporal variations in flow, transport, and transformation processes. 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 for characterizing all source zones:

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Understanding source presence and nature. What are the components of the source, whether a DNAPL or explosive material, and what is the expected behavior of the individual components based on known information? Characterizing hydrogeology. What are the lithology of the subsurface and groundwater flow characteristics as they pertain to the source zone? Are there multiple aquifers at the site, and how are they connected? What are the properties and connectedness of the low permeability layers or zones? Can the flow system be described at the specific site and at a larger scale? Can the groundwater velocity and direction (and the spatial and temporal variation in both) be measured? Determining source zone geometry, distribution, migration, and dissolution rate. Where is the source with respect to lithology? Is it present as pooled DNAPL, distributed as residual saturation, or both? Is it crystalline explosive material, or is it sorbed? What is the current vertical and lateral extent of the source material, and what is the potential for future migration based on the hydrogeologic characteristics of the site? How fast is the source dissolving? Understanding the biogeochemistry. What roles do transport and transformation processes play in attenuating the source zone and the downgradient plume? How will possible remediation strategies affect the geochemical environment (e.g., by releasing other toxic substances, or by adding or removing substances upon which microbial activity and contaminant degradation depend)? There may be an overall work plan directing that the source characterization activities described above be conducted in a particular order. However, each of the activities is related to the others, and a good deal of iteration between the general categories is not only desirable but critical to the process. Furthermore, iteration between source zone characterization and other site conceptual model building blocks should be employed to constantly reassess site understanding and integrate new data from all facets of the characterization. This chapter addresses several aspects of source zone characterization, beginning by examining some potential ramifications of inadequate source zone characterization. A subsequent section discusses the four primary categories of information important to source zone characterization and outlines a broad array of characterization methods and tools. General methods for site characterization have been described elsewhere (ITRC, 2003; EPA, 2003a; Thiboutot et al., 2003) and will not be detailed here. Specific source characterization methods for explosives are not well developed and are also not addressed in detail in this chapter. The chapter closes by discussing (1) the importance of source zone characterization to determining cleanup objectives, (2) scale issues, and (3) coping with uncertainty during the process. A recurring theme in this chapter is that source zone characterization should be carried out in a manner that best informs the entire source remediation process. Decisions regarding the objectives of remediation and the remediation tech-

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Contaminants in the Subsurface: Source Zone Assessment and Remediation nologies selected will have a strong impact on the source zone characterization strategy and vice versa. These subjects are addressed in Chapters 4 and 5, respectively, and the reader is encouraged to keep the interrelationship between these three key topics in mind. KEY PARAMETERS OF SOURCE ZONE CHARACTERIZATION AND THE TOOLS TO MEASURE THEM The four categories of information important for source zone characterization are (1) the nature and presence of the source material, whether it be a DNAPL or chemical explosives, (2) the hydrogeologic setting, (3) source zone delineation, including geometry, distribution, migration, and dissolution rate in the subsurface, and (4) the biogeochemical environment of the site. These categories of information, and the tools necessary to measure certain parameters, are discussed in detail below. Because of the variation and complexity of the subsurface environment and the various human activities performed at different sites, no two DNAPL or explosives-contaminated sites are the same. Therefore, there is not a standard suite of tools that can be prescribed for source characterization. Each site must be characterized in a manner that addresses its particular set of constraints and challenges. Before the necessary source zone characterization tools are chosen, it is important that the capabilities and limitations of the tools and the uncertainty of the data generated be generally understood. Many tools are appropriate for both source zone and general site characterization and can provide useful information that spans several of the four categories listed above. The impact of cost, regulator acceptance, and other nontechnical factors should also be considered in decisions on appropriate characterization tools. For example, drilling and core analysis to assess DNAPL distribution and saturation is an inexpensive method that is accepted by the regulatory community. Partitioning interwell tracer tests (PITT), on the other hand, have been less widely used (primarily for cost reasons), even though they have advantages over drilling and coring in terms of determining the volume of residual DNAPL. Costs and regulatory requirements pertaining to the handling and disposal of investigation-derived wastes can be high for both DNAPLs and explosives. Safety issues will also vary depending on the source material involved. When performing field characterization of suspected explosives source areas, field teams must be vigilant because of the risk of detonation (EPA, 1993). For example, soils contaminated with ~12 percent to 15 percent TNT or RDX could propagate a detonation after initiation by flame and shock (Kristoff et al., 1987). For this reason, the Army considers explosives in soil at greater than 10 percent to constitute a detonation risk. Thus, geophysical methods are often used to safely get information on site hydrogeology before drilling is commenced on production and training ranges (see Thiboutot et al., 2003).

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Table 3-1 summarizes various characterization methods and tools and their applicability for addressing the four categories relevant to source zone characterization. More detailed information about each tool is presented in Table 3-2, including a brief description of the tool and what it measures, the general application of the tool, and the general limitations of the tool for source zone characterization. A variety of methods and tools are presented here including noninvasive characterization approaches ranging from collecting historical information to certain geophysical techniques, invasive sampling tools, methods for laboratory analysis, and tools that represent a combination of the above. The tools found in Tables 3-1 and 3-2 are not equivalent, as some are approaches to removing contaminant samples from the subsurface, some measure specific chemicals either in situ or following sample extraction, some perform both functions, and some do neither. Furthermore, some of the tool categories are much broader than others, and some may overlap slightly. The tables are meant to be inclusive and provide a broad overview of the array of tools and methods often used in source zone characterization. There are a number of references that provide additional information on the applications and limitations of these techniques. For example, Cohen et al. (1993) and ITRC (2002, 2003) provide details on many of these techniques. NRC (2003) TABLE 3-1 Various Characterization Methods and Their Potential for Providing Source Zone Information Method/Tool Source Material Hydrogeology Source Zone Delineation Biogeochemistry Historical Data Maybe Maybe Maybe No Regional Geology No Yes No Maybe Geophysical Tools No Yes No No Direct Push Maybe Yes Yes Yes Core Analysis Maybe Yes Maybe Yes Downhole Methods Maybe Yes No No Piezometers No Yes No No Pump Tests No Yes No No Groundwater Analysis Maybe No Maybe Yes Solid (Matrix) Characterization No No No Yes Microbial Analyses No No No Yes Soil Vapor Analysis Maybe No Maybe No DNAPL Analysis Yes No No No Partitioning Tracer Tests No Maybe Yes No Ribbon NAPL Samplers Yes No Yes No Dyes Maybe No Maybe No NOTE: The term “maybe” indicates that in some situations the method/tool could provide information relevant to the category.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation TABLE 3-2 Summary of Various Methods and Tools and Their Application to Source Zone Characterization Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Historical Data Information about types and amounts of chemicals used and practices for chemical handling and disposal. Provides understanding of DNAPL composition and source location. Subsurface solvent migration unknown. Chemical composition changes with time. Regional Geology Information about fractures, sink-holes, springs, and discharge points. Used for site conceptual model and determining hydrogeologic setting. Site-specific details difficult to infer from this information. Geophysical Methods Methods include:       a) Seismic refraction and reflection. Seismographs measure the subsurface transmission of sound from a point source. Provides 3-D stratigraphic map. Useful for defining geologic heterogeneities. Not specific for DNAPL detection.   b) Electrical resistivity measures bulk electrical resistance during transmission of current between subsurface and ground surface electrodes. Used to determine site stratigraphy, water table depth, buried waste, and conductive contaminant plumes. Not applicable for DNAPL detection.   c) Electrical conductivity measures bulk electrical conductance by recording changes in the magnitude of electromagnetic currents induced in the ground. Used for determining lateral stratigraphic variations and the presence of conductive contaminants, buried wastes, and utilities. Not applicable for DNAPL detection.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Geophysical Methods (Continued) d) Ground-penetrating radar measures changes in dielectric properties of materials by transmitting high-frequency electromagnetic waves and continuously monitoring their reflection from interfaces between materials with different dielectric properties. Used to determine site stratigraphy, and the location of buried wastes and utilities. Cannot penetrate clay layers. Not specific for DNAPL.   e) Magnetic techniques measure perturbations to the earth’s magnetic field caused by buried ferrous metal objects. Used for finding steel drums at landfill sites. Limited to ferrous metal detection. Direct Push Direct push techniques are used both for retrieving subsurface samples and for performing in situ analyses of physical and chemical parameters. Two major techniques include cone penetrometer (CPT) and rotary hammer methods. They are similar in their principles of operation but differ in scale and in some of their applications. CPT systems, which are used mainly for in situ measurements, make use of sensors that measure soil and sediment resistance. CPT is often used in conjunction with aqueous phase (drive point) sampling and probes [e.g., laser-induced fluorescence, neutron probe, membrane interface probe (MIP)]. Used for gaining information about the physical properties of soils, stratigraphy, depth to the water table, pore pressure, and hydraulic conductivity. Extracted aqueous phase samples may be analyzed quantitatively ex situ. MIP provides semiquantitative subsurface aqueous volatile organic compound (VOC) concentration data, while laser-induced fluorescence detects fluorescing compounds. Direct push techniques are generally quicker and more mobile than traditional drill rigs, and there is no drilling waste. However, they are not applicable in bedrock, boulders, and tight clays. They are limited to unconsolidated aquifers and to depths of less than 100 ft (30 m). They require calibration with borehole data for accurate interpretation of stratigraphy. Chlorinated solvents do not fluoresce.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Core Retrieval and Analysis A variety of drilling techniques (rotosonic technologies, flight augers, hollowstem augers, rotary drilling, and cable tool drilling) coupled with different sampling tubes (hollow stem or piston tubes) can be used to collect cores from unconsolidated or consolidated media. Provides direct information regarding porous media, geology, and stratigraphy. The samples can be tested for contaminants or other biogeochemical species. Provides a point measurement of spatially variable parameters. Collection methods may alter the physical–chemical properties of the core. Expensive at radioactive sites. Downhole Methods a) Downhole video (e.g., GeoVIS) illuminates soil in contact with a sapphire window and images it with a miniature color camera. Provides visual imaging of borehole. NAPL possibly visible as Conditions for effectiveness not well defined.   b) Downhole flow metering impeller or thermal flowmeters measure groundwater inflow rate. Identifies zones of preferential flow. Calibration with other flow metering techniques necessary to ensure accuracy.   c) Caliper logging tool follows borehole wall and measures hole diameter. Identifies cavities or fractures in borehole. Provides only point measurements.   d) Specific conductance probe determines fluid conductivity with depth. Can identify inflow zones and contamination zones. Limited to contaminants that change fluid conductivity (i.e., not DNAPL).   e) Natural gamma logging measures emissions from isotopes preferentially sorbed in clay and shale layers. Reveals presence of shale or clay layers.     f) Gamma-gamma log measures media response to gamma radiation. discrete globules. Provides information about formation density.  

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Downhole Methods (Continued) g) Neutron logging measures media response to neutron radiation. Measures moisture content and porosity.     h) Electrical resistance or conductance devices measure these properties of formation fluids and media. Enables identification of lithology, stratigraphy, or high ionic strength-contaminated water. Typically used in conjunction with core analysis or other borehole data. Piezometers Primarily used to determine pressure head spatially and temporally on the site. Used for potentiometric mapping to understand groundwater flow. Screen length is important. May not provide the detail needed within the source zone. Because head distribution changes over time, sampling can be required over an extended time. Pump Tests Pumping groundwater and then monitoring the drawdown cone and rebound can be used to estimate permeability, hydraulic conductivity, the radius of influence, and flow boundaries. Standard tracer tests (e.g., bromide or iodide) are frequently used during pumping to confirm flow models and optimize flow. This information is necessary for site conceptual model development and remedial activities. Provides a spatially averaged estimate. Not specific for locating preferential paths or highly permeable zones. Groundwater Analysis Discrete water samples can be collected with various pumps, bailers, or samplers and then analyzed for different contaminants and groundwater constituents of interest. Multilevel sampling allows water sampling at various depths within a single well. Helps delineate source areas on site and document preremediation conditions in order to later evaluate whether remedial objectives have been met. Good understanding of groundwater flow, biogeochemistry, and DNAPL composition is needed for proper interpretation.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Solid (Matrix) Characterization Includes analysis of organic matter content, percent clay and clay type, silt content, mineral composition, and wetting behavior. Improves understanding of the source zone and the impact of the subsurface environment on remedial actions (e.g., oxidation). Difficult to quantitatively relate bulk soil measurements to contaminant behavior. Microbial Analyses Microbial community composition and functional potential can be measured for extracted subsurface samples using a combination of molecular techniques and tools based upon conventional culturing and microcosm approaches. Identifies organisms and genes that are present within the subsurface community to evaluate potential activity and quantify functional activity associated with the active microbial population. Difficult to extrapolate laboratory activity measurements and rates to in situ field activity. Soil Vapor Analysis Soil probes or passive soil-gas collectors are used to withdraw soil gas. A variety of analytical techniques are used to measure the actual contaminants (e.g., GC-MS). May be used for indicating “hot spots” of contamination. Provides point measurements. Understanding of partitioning and NAPL composition is necessary for interpretation. Reflects DNAPL distribution in the vadose zone only. DNAPL Analysis A variety of analytical techniques are used to determine DNAPL chemical composition (e.g., GC-MS) and chemical-physical properties such as viscosity (e.g., viscometer), interfacial tension (e.g., pendant drop method), and density (e.g., densitometer). Used to better interpret groundwater and vapor sample measurements and to enhance site conceptual model (SCM) and modeling efforts. DNAPL samples are difficult to obtain and may be variable across the site and with time.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Method/Tool Tool Description and What Is Measured Application/ Relevance to Source Zones Limitations Partitioning Tracer Tests Hydrophobic chemicals such as higher-weight alcohols (partitioning tracer) are injected through a contaminated zone with a conservative tracer. The reactive tracers partition into DNAPL and experience a delay in breakthrough as compared to the conservative tracer. The retardation and partition coefficients are used to determine NAPL saturation. Provides in situ estimates of DNAPL saturation. Can provide information on DNAPL distribution when coupled with multilevel sampling. Expensive. Limited to media with sufficiently high permeability. Ribbon NAPL Samplers Material is placed on a core or in bore holes that reacts with NAPL. Flexible Liner Underground Technologies Everting (FLUTe) is an example. Provides continuous record of DNAPL distribution in borehole. Only indicates presence (not amount) of DNAPL. Not proven to be responsive in all cases; thus, negative results are not conclusive. Time in borehole may be important. Hydrophobic Dyes (such as Sudan IV) Hydrophobic dye shake test for detecting DNAPL in soil samples. Onsite screening tool for locating DNAPL. Can only indicate presence (not amount) of DNAPL. provides information on various sensors and analytical techniques and their appropriate applications. An expert panel report to the U.S. Environmental Protection Agency (EPA) on DNAPL source depletion presents a summary of characterization tools (EPA, 2003a), Kram et al. (2001, 2002) provide a comparison of various analytical techniques with cone penetrometers, and Griffin and Watson (2002) provide a comparison of field techniques to confirm DNAPLs. A large amount of information on sampling technology can be obtained from the Department of Energy’s (DOE) Environmental Management Science Program, EPA’s Technology Innovation Office (http://fate.clu-in.org), and EPA’s Environmental

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Technology Verification program (http://www.epa.gov/etv). Sampling in fractured rocks is discussed in Shapiro (2002). Thiboutot et al. (2003) provides extensive information on characterization of explosives soil sites, primarily on military training and testing ranges, including the risks of detonation and appropriate sampling strategies and chemical analysis methods. Source Presence and Nature Before extensive source zone characterization methods are undertaken, an effort should be made to first determine the nature of the source. Determining the composition of the DNAPL or explosive material is useful for a variety of site management activities. Knowing the components of the source and being able to predict the expected behavior of the individual components based on known information is important for performing risk assessment and surmising appropriate remedial actions for the site. For DNAPLs, the physical–chemical properties such as solubility, density, specific gravity, viscosity, interfacial tension, wettability, contact angle, and the tendency to partition between sediment and water should be determined if possible (see Cohen et al., 1993, for analytical methods relevant to characterizing DNAPLs). The concentrations analyzed in sediment and water can be related to health-based standards, and estimates of the human and ecosystem exposure to the contaminants can be predicted. This information is necessary to guide subsequent phases of site characterization. Kram et al. (2001) provide an excellent summary of field techniques and information for determining DNAPL source material information based both on direct and indirect evidence. Direct detection of a DNAPL source can be accomplished via various analyses of soil, rock, or water cores and samples. These range from such simple techniques as visual observation (such as with downhole video) and soil shake tests with hydrophobic dyes, to measurements of UV fluorescence in situ or within extracted samples or cores, or to ribbon NAPL samplers used either ex situ to test extracted cores or in situ within boreholes. These various techniques are generally used to determine the presence or absence of DNAPL and not necessarily total mass or chemical composition. Understanding the presence and nature of the source material can be a challenge at sites where a DNAPL or solid phase explosive sample cannot be isolated from the source zone. In such cases, indirect methods such as measuring high aqueous or vapor contaminant concentrations relative to saturated aqueous or vapor concentrations, or measuring high contaminant concentrations in soil cores, are used for inferring the presence of a separate phase (see Box 3-1 for an example relevant to chemical explosives). For example, aqueous concentrations in excess of 1 percent of DNAPL solubility (Mackay et al., 1991; Cohen et al., 1993) or soil concentrations greater than about 10,000 mg/kg (EPA, 1992) are generally considered to be indicative of DNAPL presence. Caution should be taken when using this technique to infer DNAPL presence because of the highly

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Contaminants in the Subsurface: Source Zone Assessment and Remediation BOX 3-4 Camp Lejeune Case Study MCB Camp Lejeune Site 88 is an example of a source depletion action that was accomplished after an adequate amount of source zone characterization was completed (ESTCP, 2001; NFESC, 2001, 2002). The potential PCE source zone area was initially identified at this former dry cleaning facility using chemical usage history and conventional site characterization techniques, such as borehole drilling and groundwater monitoring. Extensive and dense physical sampling efforts in the form of cone penetrometer and soil borings were first undertaken to map the soil lithology in terms of the permeability contrasts and porosity distributions that offer clues as to spatial DNAPL distribution. At the same time, chemical analysis of core samples was used, along with measured organic carbon content, to develop a more direct line of evidence regarding the distribution. As certainty about the DNAPL distribution above and along the clay unit began to increase, source remediation was presented as a potential objective, and wells were installed in and around the source zone. Using these wells, more specific characterization strategies, such as pump tests and partitioning interwell tracer tests (PITT), were undertaken to estimate the integrity of the clay unit as a capillary barrier and to estimate the volume of DNAPL in the source zone. By the end of the pre-removal characterization activity, three injection, six extraction and two hydraulic control wells were situated within the roughly 10-m by 10-m source zone area, which had already been densely probed by means of cone penetrometry and core sampling. This level of characterization was necessary to evaluate the feasibility of the surfactant-enhanced aquifer remediation (SEAR) source remediation technology and prepare for its application. Evaluation of the SEAR technology at Camp Lejeune involved extensive additional characterization efforts. Pre- and post-SEAR PITTs were used to estimate the amount of DNAPL volume remaining in the source zone, and 60 post-SEAR confirmational core samples were collected from within the small source zone and analyzed. The results from these characterization efforts suggested that while 92 percent to 95 percent of the DNAPL was removed from the permeable portions of the source zone, only 72 percent was removed overall. However, post-SEAR characterization and analyses suggested that the residual DNAPL was as unavailable to dissolution as it had been to the surfactant flushing. Subsequent modeling and flux reduction also indicate this 72 percent mass removal was accompanied by a greater than 90 percent reduction in the dissolution flux emanating from the source zone (Jayanti and Pope, 2004). Follow-up groundwater monitoring efforts aimed at confirming the sufficiency of this reduction in terms of plume size are ongoing. It is important to emphasize that, with the exception of the PITT, conventional characterization procedures were employed to characterize the source zone at Camp Lejeune.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 3-3 Illustration of potential repercussions of inadequate source zone characterization in the case of a poorly defined, discontinuous clay layer. Case 1 represents a prolonged pump-and-treat effort that will contain the source but not deplete the source in the short term. In Case 2, a chemical flushing technology is applied prematurely and results in the migration of a portion of the DNAPL to the lower aquifer. In Case 3, adequate source zone characterization leads to an accurate evaluation of the extent of the source and to more successful execution of the chemical flushing technology, substantial source strength reduction, and subsequent monitored natural attenuation.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Illustrative Case 1: Minimal Source Zone Characterization. Despite the uncertainty associated with the existence and nature of DNAPL at the site, the horizontal extremities of the plume seem reasonably well defined. Thus, one alternative is to forego any additional source zone characterization and implement a conventional pump-and-treat strategy. Personnel working on the site have experience in the hydraulic capture and treatment of the extracted water. The decision makers are wary of attempting source zone remediation, fearing that it will fail in the face of the uncertainties about the underlying clay layer, source size, and source strength. Thus, source zone remediation is not attempted and a conservative pump-and-treat plan is adopted. Given their prior decision to halt site characterization, the decision makers have selected a reasonable remedial plan. Negative consequences might have resulted if source zone remediation actions had been undertaken with the inadequate characterization data. First, the costs incurred by this would likely produce limited benefits. Second, additional costs may arise if the source remediation attempt results in transferring portions of the source deeper or into less accessible regions of the subsurface. However, because the source material is not efficiently reduced by the pump-and-treat remedy, the prognosis for this scenario is that there will be a long-term operation and maintenance horizon with costs continuing indefinitely. Illustrative Case 2: Insufficient Source Zone Characterization. In Case 2, the decision makers determine that the potential benefits of source zone remediation (e.g., plume size reduction to within a compliance boundary and reduced time to site closure) make additional source zone characterization worth pursuing. Thus, the decision makers choose to allocate additional resources toward source zone characterization, although their technical staff is relatively less experienced in this aspect of site assessment. A firm time limit of one year is set to ensure that the source zone characterization process does not delay remedial actions for too long. Additional core samples are collected, and monitoring wells are installed in and around the suspected source area. At the end of the one-year period, the corresponding data improve the areal resolution of the source zone, but the vertical extent remains highly uncertain. The decision makers assume that a clay layer identified in the source area is sufficiently continuous to provide a capillary barrier, and the decision is made to design and execute an aggressive chemical flushing technology aimed at mobilizing and extracting the DNAPL (see Chapter 5 for technologies). In this case, the decision makers have allowed a predetermined time limit to control the quality of the source zone characterization effort. The fact that the clay unit in question is actually not continuous in the area of the source means that the likelihood of success is low. In the end, the costs incurred by the source remediation action are wasted because the mobilization technology will likely drive DNAPL through the discontinuity and deeper into the subsurface.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Illustrative Case 3: Adequate Source Zone Characterization. The rationale of the decision makers in Case 3 is similar to that in Case 2 with one exception: in Case 3 they recognize the importance of allowing the source zone characterization to continue until uncertainty regarding the feasibility of partial source depletion is reduced to a manageable level. Outside expertise in the areas of source zone characterization and remediation has been retained to assist the site personnel. As in the previous case, additional core sampling is undertaken, and monitoring wells are installed in and around the suspected source area. At the end of the one-year period, the vertical extent of the source remains highly uncertain, and additional time and resources are allocated to the install and monitor multilevel monitoring wells to better resolve the vertical concentration gradient. At the end of the second year, the source zone and underlying clay unit have been resolved with enough certainty to correctly identify a less aggressive DNAPL solubilization strategy. The decision makers elect to design and execute a chemical flushing technology to reduce the source strength, but they orientate flushing and recovery hydraulics to guard against DNAPL migration through the discontinuity in the clay layer. In this case the decision makers have expended more time and money during source characterization relative to Cases 1 and 2, respectively. In doing so, they have maximized the likelihood of successful source depletion. The chemical flushing plan is designed, executed, and evaluated, requiring an additional two years. However, according to pre- and postremediation dissolution flux estimates, 95 percent of the original source strength has been depleted. Modeling efforts predict a reduction in source strength, and monitored natural attenuation becomes an acceptable alternative for all stakeholders. Four years have passed since the decision to aggressively characterize the source zone, but operations and maintenance costs for the site are now low compared to those for the previous two cases. (It is not clear, however, whether life cycle costs, which take into account all relevant expenditures over the life of a hazardous waste site, are lower for the third scenario. See Chapter 4 for an in-depth discussion of life cycle analysis.) * * * Although these case studies are hypothetical, they serve to illustrate common themes in source zone characterization. For many reasons, responsible parties may be disinclined to commit the considerable funds required for comprehensive source zone characterization. Source identification and delineation are considered technically challenging and expensive, and they may reveal that the contamination problem is more extensive than previously thought, leading to even larger costs. Detailed source zone characterization can be unappealing to the responsible party since it may pave the way to source remediation regarded as complicated, costly, and perhaps of questionable effectiveness. Indeed, it is worthwhile to note

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Contaminants in the Subsurface: Source Zone Assessment and Remediation that overzealous source zone characterization such as excessive source zone drilling without proper precautions can remobilize DNAPL or create worker safety issues related to chemical exposure. However, a nonexistent or halfhearted characterization attempt may unnecessarily lead to containment efforts with no foreseeable end (Case 1) or compromise the effectiveness of any source remediation effort (see Case 2). Avoiding these undesirable consequences requires that the decision process leading to a specific source assessment and mitigation plan be dynamic and iterative. That is, on the basis of preliminary information, funds are allocated for characterization and remediation. As new information is obtained, the goals of the remediation campaign must be revisited and are likely to change. Once it becomes clear that source remediation is a potentially promising approach, one must evaluate, on the basis of the hydrogeochemical conditions and the types of the contaminants, what methods of remediation are appropriate and what levels of characterization they require. While this iterative process may appear time-consuming in the face of stakeholders’ desires to expedite cleanup, a rush to judgment on the nature and extent of the contamination can worsen site conditions and result in contamination inadvertently remaining behind. The public is more likely to respond positively to an honest acknowledge of the limitations on and uncertainties in the data than to perceived certainty that is later revealed as incorrect. CONCLUSIONS AND RECOMMENDATIONS The committee’s review of dozens of case studies of source characterization and remediation (see Chapter 1) suggests that at many DNAPL or explosives-contaminated sites there is inadequate site characterization to support the remediation strategies that were employed and/or to evaluate the actual results of the trial in terms of improvement in water quality, the fraction of the total mass removed, or other appropriate success metrics. In several cases the data were not adequate to determine if there even was a source. This is most likely due to pressure to show progress and meet deadlines, to financial constraints, or to unclear objectives. Despite these shortcomings, the Army has made substantial progress at improving its source characterization activities, particularly with respect to confirming that DNAPL is present. The Army case studies reviewed by the committee also suggest that the development of site conceptual models is evolving rapidly in parallel with improved Army characterization efforts. At Fort Lewis, for example, a number of nontraditional characterization technologies were used including CPT-LiF, MIP, GPR, resistivity, dye studies, and multilevel wells. Other sites using innovative technologies included Watervliet, where down-hole geophysics were used to characterize the fractured rock, and Redstone Arsenal, where a major effort was mounted to characterize flow in a complex karst terrain. These efforts are impressive, given that karst, epikarst, and fractured bedrock settings

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Contaminants in the Subsurface: Source Zone Assessment and Remediation are resistant to detailed source characterization using existing technologies. The Army’s realization of the difficulty of site characterization in these hydrogeologic settings has led it to consider applying for technical impracticability waivers (e.g., at Anniston and Letterkenny). Such waivers are appropriate if the objectives of remediation are clearly defined and if sufficient data are obtained to show that the objectives cannot be met by any feasible approach. The following recommendations regarding source characterization are made. Source characterization should be performed iteratively throughout the cleanup process to identify remedial objectives, metrics for success, and remediation techniques. All sites require some amount of source characterization to support the development and refinement of a site conceptual model. 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. An evaluation of the uncertainties associated with the conceptualization of the source strength and location, with the hydrogeologic characteristics of the subsurface, and with the analytical data from sampling is essential for determining the likelihood of achieving success. This is often accomplished through the use of statistical, inverse, and stochastic inverse methods. Unfortunately, quantitative uncertainty analysis is rarely practiced at hazardous waste sites. Obtaining a better handle on uncertainty via increased source characterization would allow eventual remediation to be more precise. It is likely that at most sites, there is not an optimum combination of resources and effort expended on source characterization and thus uncertainty reduction vs. remedial action. REFERENCES American Petroleum Institute (API). 2003. Groundwater remediation strategies tool. Publication Number 4730. Washington, DC: API Publishing Services. American Society of Civil Engineers (ASCE). 2003. Long-term groundwater monitoring: the state of the art. Prepared by the Task Committee on the State of the Art in Long-Term Groundwater Monitoring Design. Reston, VA: American Society of Civil Engineers. American Society for Testing and Materials (ASTM). 2003. Standard guide for developing conceptual site models for contaminated sites. Document No. ASTM E1689-95(2003)e1. West Conshohocken, PA: ASTM International. Annable, M. D., P. S. C. Rao, K. Hatfield, W. D. Graham, and C. G. Enfield. 1998. Partitioning tracers for measuring residual NAPLs for field-scale test results. J. Env. Eng 124:498–503. Bair, S., C. M. Safreed, and E. A. Stasny. 1991. A Monte Carlo-based approach for determining travel time-related capture zones of wells using convex hulls as confidence regions. Ground Water 29 (6):849–855.

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