1. 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?

  2. 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?

  3. 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?

  4. 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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