Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 16
Contaminants in the Subsurface: Source Zone Assessment and Remediation 1 Introduction Concern about polluting water supplies began over 100 years ago when the industrial revolution resulted in noticeable changes in the quality of surface waters. Because groundwater was generally believed to be protected from such pollution, however, groundwater contamination did not become a major issue until the 1970s (NRC, 1994; Pankow and Cherry, 1995). 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). The same cannot be said of groundwater, for which cleanup efforts represent a significantly more difficult challenge. Attempts at large-scale groundwater cleanup began in earnest in the 1980s after passage of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Resource Conservation and Recovery Act (RCRA). Results of early remediation efforts seldom produced the expected reduction in contamination levels. Studies by the U. S. Environmental Protection Agency (EPA) (EPA, 1989, 1992) found that the commonly used pump-and-treat technologies rarely restored sites that had contaminated groundwater to background conditions. This was confirmed in a much more extensive 1994 National Research Council (NRC) study that explicitly reviewed 77 sites across the country where full-scale pump-and-treat was being used. The inherent difficulty of groundwater cleanup results directly from fundamental aspects of hydrogeology and chemistry. First, heterogeneities in the subsurface, sorption of contaminants onto solid organic matter, and contaminant diffusion into low-permeability zones combine to make pump-and-treat much
OCR for page 17
Contaminants in the Subsurface: Source Zone Assessment and Remediation less efficient than originally envisioned (Mackay and Cherry, 1989). Second, most common organic pollutants in the subsurface have low solubilities in water and tend to remain as either a separate organic phase liquid in the subsurface (nonaqueous phase liquid or NAPL) 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 non-aqueous phase liquids (DNAPLs). During the late 1980s, it was recognized that the presence of DNAPLs made a site particularly difficult to remediate (Feenstra and Cherry, 1988; Mackay and Cherry, 1989; Mercer and Cohen, 1990; NRC, 1994). Before it was understood that DNAPLs commonly exist in source areas, it was assumed that by removing a few pore volumes of contaminated groundwater, the majority of the total contamination could be extracted. At sites where they are present, separate phase or sorbed contaminants serve as a long-lived contamination source. That is, groundwater that flows through the volume of subsurface containing the contaminant—termed the source zone—will be contaminated by the small amount of contaminant that dissolves. This suggests that groundwater remediation to background levels will not be achieved unless the contaminant source is removed or physically isolated from flowing groundwater (NRC, 1994). Unfortunately, due to a lack of effective characterization tools and the tendency of DNAPLs to have a spatially limited but extremely heterogeneous distribution, it is very difficult to find contaminant sources within the subsurface. In addition, although numerous new technologies have been developed to remediate source zones, the difficulty in evaluating these technologies (due to the lack of data from pilot studies) makes prediction of their effectiveness for full-scale applications problematic. Several NRC reports extend the findings of the 1994 report on pump-and-treat systems to include more comprehensive analysis and encompass new remediation technologies (NRC, 1997, 1999a, 2003). These reports have noted the general paucity of data available for evaluating remediation technology performance (including technologies for DNAPL sites). These and many other recent studies (e.g., ITRC, 2000, 2002; SERDP, 2002; EPA, 2003) have demonstrated that restoration of sites with DNAPL contamination to pre-contamination levels is rare and may not be practically achievable. Indeed, there are no reported cases of large DNAPL sites where remediation has restored the site to drinking water standards. At this time, most DNAPL sites have pump-and-treat systems in place to contain the dissolved phase plumes and thus minimize risk to the public. At only a small fraction of these sites has remediation of the DNAPL source actually been attempted. Layered onto this issue of technical impracticability are the opinions of stakeholders, including those who live or work near contaminated sites, as well as the high cost associated with remediation efforts. There is often pressure from the public to remediate when pollution is found. This pressure to clean up sites is contrasted by the fact that remediation technologies for DNAPL sites are under-
OCR for page 18
Contaminants in the Subsurface: Source Zone Assessment and Remediation standably expensive, such that there have been relatively few large-scale remediation attempts. Whether it is worth the expense to undertake remediation at DNAPL sites depends on the objectives of the remediation project, on what can be achieved (which is often unknown), and on the competing needs of other critical sites. Because the cost of remediating the nation’s contaminated groundwater has been estimated to range from a few to several hundred billion dollars (NRC, 1999b), 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. It addresses what can be achieved given the fact that DNAPL is present at many sites (Villaume, 1985; Feenstra and Cherry, 1988; Mercer and Cohen, 1990; Pankow and Cherry, 1995) and given the findings of prior studies that remediation of DNAPL sites may not provide complete restoration (see NRC, 1994, 1999a; numerous case studies in Chapter 5). Certain technologies capable of significant source remediation are being increasingly utilized by large responsible parties, like the U.S. military. Just what can be accomplished by these more aggressive technologies, in terms of the percentage of total contaminant mass removed, risk reduction, and other metrics, is uncertain. The study was initiated at the request of the U.S. Army Environmental Center, which coordinates the Army’s efforts to restore thousands of contaminated sites at installations across the country. Although chlorinated solvent DNAPLs are the primary focus of the report, chemical explosives are also considered in depth because of the Army’s large potential liability in subsurface sites contaminated by explosives. THE STATUS OF CLEANUP IN THE UNITED STATES During the past two decades of cleaning up hazardous waste sites in the United States, there has been an evolution of activities, from the initial stages of the CERCLA or Superfund process to later remediation stages. Thus, a large percentage of sites have moved from initial characterization and investigation—activities embodied in the remedial investigation and feasibility study (RI/FS)—to remedy selection, remedy implementation, and, in some cases, site closure. The remedies chosen at hazardous waste sites across the country have also evolved from an initial emphasis on source treatment (reflecting the preference of the National Contingency Plan to treat so-called principal threats) to containment measures. In large part, this change in emphasis reflects the technical difficulty of cleaning up many of the more complex and recalcitrant hazardous waste sites as well as the limited resources available for cleanup. In the early 1980s, the limitations of remediation technology were unclear to Congress. In 1986, CERCLA was amended to provide a preference for attaining drinking water standards in groundwater, such that the number of remedies relying on treatment dramatically increased. Since that time, however, it has become widely known that at many
OCR for page 19
Contaminants in the Subsurface: Source Zone Assessment and Remediation contaminated sites it is not feasible to reduce groundwater concentrations to drinking water standards with pump-and-treat technology in a reasonable timeframe (e.g., decades) (NRC, 1994). Several government agencies have estimated the long-term costs of continuing to operate pump-and-treat systems, despite their ineffectiveness, with projected annual costs in the hundreds of millions of dollars and life cycle costs in the billions of dollars1 (e.g., DoD, 1998). In response to the rising costs of contaminated site cleanups and the growing recognition of the limitations of technology, federal and state regulatory agencies issued a number of explicit policies that led to the acceptance of more containment. For example, EPA released guidance in 1996 to select pump and treat as a presumptive remedy for DNAPL sites, reflecting the continuing debate at the time on whether it would be technically feasible to clean up these sites (EPA, 1996a). Although treatment as a source area remedy at Superfund sites increased from 14 percent to 30 percent during the 1982–1986 period to a peak of 73 percent in 1992, it has decreased ever since. Monitored natural attenuation (MNA) alone or in conjunction with other remedial actions increased from 0 percent in 1982 to between 28 percent and 48 percent in the 1998–2001 period (EPA, 2004). EPA’s 1990 Superfund remedy rules state that even though permanent remedies are preferred, EPA expects to use treatment to address the principal threats posed by a site wherever “practicable,” and engineering controls, such as containment, for sites that pose a relatively low long-term threat (EPA, 1991). Despite these trends toward containment and MNA, remedial actions and monitoring activities at many sites regulated under CERCLA and RCRA (which encompass almost all military sites) cannot legally be terminated unless the chemicals remaining at the site are reduced to levels that allow unrestricted use of the property. At the vast majority of sites, this goal corresponds to groundwater contaminant concentrations that are equal to or less than drinking water maximum contaminant levels (MCLs) within the source zone or at some specified location in the plume. NRC (1994) estimated that given such criteria, cleanup times will extend from a few years to thousands of years, with the actual treatment time being highly uncertain. Because it is a primary goal for the military to achieve site closeout at as many sites as possible within the next 10–15 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. As of FY2002, 58 percent of all Superfund source remediation actions used ex situ technologies (EPA, 2004), and trends at military 1 Life cycle cost estimated by assuming that the average life cycle cost for a pump-and-treat site is $9.8 million and that 10 percent of the 3,000 DoD sites have or will have full-scale pump-and-treat systems (Quinton et al., 1997).
OCR for page 20
Contaminants in the Subsurface: Source Zone Assessment and Remediation facilities are expected to be the same. Of the 42 percent of Superfund sites where in situ source remediation technologies were used, over half utilized soil vapor extraction, with the remainder being composed primarily of solidification/ stabilization, bioremediation, and soil flushing. 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). Of the 69 thermal projects in the EPA database, 49 were completed in the last five years or are ongoing (www.cluin.org/products/thermal); similar upward trends in usage were observed for in situ chemical oxidation (www.cluin.org/products/chemox). Use of these more aggressive source remediation technologies at Superfund sites has increased substantially in the past six years despite an overall trend toward less private investment in innovative technologies during the 1990s (NRC, 1997). ARMY CLEANUP CHALLENGES AND THE ARMY’S REQUEST FOR THE STUDY The goals of the Army’s environmental restoration program are to “protect human health and the environment, to clean up contaminated sites as quickly as resources permit, and to expedite cleanup to facilitate disposal of excess Army properties for local reuse” (Department of the Army, 1997). In addition, the program aims to optimize risk reduction per dollar spent (Haines, 2002). Activities within the Army’s Installation Restoration Program mirror the trends discussed above for the nation in general, in that the majority of sites are now in the latter stages of cleanup. As of September 30, 2003, the Army had identified 10,367 sites at active bases and 1,899 sites at closing bases (DoD, 2003). For both type of bases, about 88 percent of the identified sites have reached “remedy-in-place/response complete,” which is a military milestone in the cleanup process that indicates the end of remedy construction or completion of cleanup activities. These numbers do not include sites contaminated with unexploded ordnance (UXO), discarded military munitions, or munitions constituents, of which there are 177 sites located at 26 closing bases and 819 sites located at 166 active bases. The Army estimates that the remaining cumulative cost to reach remedy-in-place/response complete in today’s dollars is $3.1 billion at active bases (DoD, 2003) and is $439 million at closing bases (not including UXO cleanup). It is expected that funding for the Active Installation Restoration Program will hold steady at around $400 million for FY2005 and FY2006. Like other branches of the military and large private responsible parties, the Army is responsible for hazardous waste sites that reflect a broad range of activities over the last century. Perhaps the most distinct characteristic of these
OCR for page 21
Contaminants in the Subsurface: Source Zone Assessment and Remediation facilities is the wide range of contaminant types—often present as mixtures of unknown composition and with no clear indication of how they were disposed. As summarized in Appendix A and other documents (NRC, 1999b), petroleum hydrocarbons are the most frequently reported organic compounds at Army and other military facilities, due to the high prevalence of large-scale transportation and industrial activities that utilize fuel. Petroleum hydrocarbons include components of gasoline [benzene, toluene, ethylbenzene, and xylene (BTEX) and oxygenates such as methyltertbutylether (MTBE)] as well as other fuels. 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), trichloroethene (TCE), and trichloroethane (TCA) and their degradation products vinyl chloride, dichloroethene (DCE), and dichloroethane (DCA)—all of which can be present in DNAPLs in the subsurface. Chlorinated organic solvents were widely used for cleaning and degreasing military equipment and were typically disposed of at the land surface or in drums. Within the Department of Defense (DoD), there are approximately 3,000 individual sites that require cleanup of chlorinated solvents (Stroo, 2003). The EPA has estimated that approximately 5,000 DoD, Department of Energy (DOE), and Superfund sites are contaminated with chlorinated solvents (EPA, 1996b), although DNAPL may not exist at all of these sites. Additionally, there are an estimated 20,000 solvent-contaminated drycleaner sites in the United States (Jurgens and Linn, 2004). Other frequently reported hard-to-treat organic compounds at military sites are polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), creosote, and coal tar. Mixtures of PCBs (the most common were Aroclor 1254 and 1260) were used as dielectric fluids in electrical transformers and capacitors before their use was restricted. PAHs are components of petroleum products, whereas creosote and coal tar, which were commonly used to treat wood, are mixtures of hundreds of compounds that include phenols, naphthalene and other PAHs, and nitrogen-heterocyclic compounds. Pesticides and herbicides are also frequently reported at military sites, as are heavy metals (particularly lead), paints, perchlorate, bis (2-ethylhexyl) phthalate, and nitrates. Of the military services, the Army has the largest number of sites affected by chemical explosives. The chemical explosives 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are reported at military sites where the contaminants were manufactured or at depots where they were disposed of. The Army has 42 installations that contain 230 sites with chemical explosives as contaminants (Haines, 2003), although the number of explosives sites that require source remediation may increase significantly when source zones become more fully characterized.
OCR for page 22
Contaminants in the Subsurface: Source Zone Assessment and Remediation This diversity of compounds reflects the wide array of activities typical at Army and other military installations. Activities include providing services, materials, and equipment to support military operations, designing and manufacturing weapons systems, and painting (which tends to release heavy metals and solvents). Military installations are characterized by industrial landfills, waste disposal pits, aboveground and underground storage tanks, and spill sites. 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. This was further defined to encompass those organic compounds that can potentially exist in the subsurface as DNAPLs (primarily solvents) and those that can form pure solid phases (chemical explosives). Table 1-1 summarizes the number of Army installations at which these key recalcitrant organic chemicals are found (with details provided in Appendix A, Table A-3). It should be noted that in the remainder of this report these compounds—as well as NAPL and DNAPL—are referred to exclusively by their abbreviations. TABLE 1-1 Prevalence of Organic Contaminants of Concern at Army Installationsa Chlorinated Solvents Explosives Contaminant Prevalence PCE TCE cis-1,2-DCEb 1,2-DCA TCAc DNT TNT HMX RDX Total number of installations with contaminant 51 74 32 24 35 26 30 19 14 Percentage of all installations 37% 54% 23% 17% 25% 19% 22% 14% 10% NOTE: An installation many contain many individual hazardous waste sites. aNumber of Base Realignment and Closure Act (BRAC) installations – 23; Number of active installations – 115 bDoes not include other DCE isomers. cIncludes 1,1,1-TCA and 1,1,2-TCA SOURCE: Compiled by Laurie Haines, Army Environmental Center.
OCR for page 23
Contaminants in the Subsurface: Source Zone Assessment and Remediation Given the technical difficulties inherent in source area cleanup and the potentially high costs associated with investigating and remediating such sites, the Army (like other branches of the military—see NRC, 2003) is concerned about its long-term management and cost responsibilities and its ability to reach site closure throughout the Installation Restoration Program. 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) the ability of the technology to achieve substantial mass removal and (2) the relationship between removal of contaminant mass from these sites and its long-term impact on groundwater contamination. There was also considerable uncertainty among Army managers about whether the costs of these efforts were commensurate with the risk reduction achieved. In addition, it was found that remedial project managers (RPMs) at the sites sometimes failed to form contingency plans and exit strategies in the event of remedy failure, which led to numerous iterations of aggressive source remediation efforts. To counteract these trends, a new approach to these complex, high-cost sites was recommended by the Army Environmental Center, which included (1) protecting receptors directly (with alternate water supplies, well-head treatment, etc.), (2) considering the need for a technical impracticability waiver early in the project, and (3) documenting the cost of the remedy as well as its risk reduction benefits. According to the Army Environmental Center, this approach has been met with considerable resistance, primarily from those stakeholders (e.g., regulators and the public) who desire mass removal and thus prefer more aggressive source remediation strategies and view technical impracticability as an excuse for no action. On a more practical level, however, there is a lack of scientific expertise and tools to do the recommended analyses. Thus, the Army Environmental Center requested the NRC’s input on several technical issues to help determine the usefulness and applicability of source remediation as a cleanup strategy. Several key questions served to guide the work of the committee. It should be noted that the term “source removal” (which appears below in the committee’s charge) is replaced by the term “source remediation” for the remainder of this report (as discussed at the end of this chapter). What is a meaningful definition of a “source” for the purpose of this study? How important is the source delineation step to the effectiveness of mass removal as a cleanup strategy? What tools or methods are available to delineate sources of organics contamination in complex sites? How should the uncertainty of these characterizations be quantified, in terms of both total mass and mass distribution? What are the data and analytical requirements for determining the effectiveness of various source removal strategies, and how do these requirements
OCR for page 24
Contaminants in the Subsurface: Source Zone Assessment and Remediation change for different organic contaminant types or hydrogeologic environments? Effectiveness would consider the metrics of groundwater restoration, plume shrinkage and containment, mass removed, risk reduction, and life cycle site management costs. What tools or techniques exist today, and what tools would need to be developed in the future, to help predict the likely benefits of source removal? What would be the most important elements of a well-designed protocol to assist project managers in the field to assess the effects of source removal? What can be concluded about the ability of source removal efforts to bring about substantial water quality benefits and to meet various cleanup goals? (For example, when can these efforts remove enough of the source to then rely on monitored natural attenuation?) What have been the results of source removal activities at Army and other facilities to date? 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. However, some brief comments are warranted here. Most NAPLs include several different chemical compounds due to a combination of mixing before release, reaction with aquifer and soil solids, and partial biodegradation of specific components. A DNAPL consisting of more than one compound (the general case) is a multicomponent DNAPL. A distinction should thus be made between a component of the DNAPL and the DNAPL itself. Chlorinated solvents are the most common DNAPL components, particularly PCE, TCE, 1,1,1-TCA, 1,2-DCA, cis-1,2-DCE, methylene chloride, and chloroform. These compounds vary from slightly soluble to moderately soluble in water, causing plumes of contaminated groundwater that migrate away from the source material. When released to the subsurface, DNAPLs flow downward through the vadose zone, typically traveling vertically with little spreading. When the water table is reached, capillary forces tend to produce horizontal spreading. In both vertical and horizontal flow, DNAPLs tend to be restricted to pathways of maximum permeability. As a result of this process, DNAPLs generally follow a very narrow, highly irregular path resulting in a source zone that contains narrow vertical pathways connected to thin, laterally extensive horizontal layers (see Figure 1-1). The limited and extremely heterogeneous distribution of DNAPLs makes both detection and remediation difficult. Given the many forces affecting
OCR for page 25
Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 1-1 Typical distribution of DNAPL. Gray areas show residual saturation of DNAPL, while black areas are pools of DNAPL (see Chapter 2 for further explanation). SOURCE: Adapted from NRC (1999a). DNAPL distribution discussed above, it is not surprising that only a small fraction of the subsurface volume at a contaminated site actually contains DNAPLs. Explosives are a class of chemicals that can undergo rapid oxidation—a process called detonation—which releases a tremendous amount of energy. Explosives are divided into organic and inorganic chemical classes, though the organic explosive class has caused the greatest environmental risk. 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.
OCR for page 26
Contaminants in the Subsurface: Source Zone Assessment and Remediation Explosive compounds exhibit low solubility in water. Nonetheless, surface deposits of solid phase explosives can dissolve into percolating rainwater and can present a long-term contaminant source that threatens groundwater quality. A more common scenario for groundwater contamination by explosives is discharge of wastewater containing these compounds to the environment. At sites impacted by such discharges, the bulk of the explosives contaminant mass is usually present as sorbed and aqueous phase contamination. However, when effluents with high aqueous concentrations are discharged into a significantly colder environment, the explosive compounds may precipitate out of solution and form a separate solid phase in the soil system. These surface deposits are often removed by excavation to practical depths, leaving deeper source areas to be treated in situ. DEFINING THE SOURCE ZONE In order to evaluate aggressive source remediation as a cleanup strategy and differentiate it from other activities, the term “source” must be defined. While seemingly simple, the term “source” can be defined from several perspectives. Implicit in all of the perspectives below, including the one endorsed by the committee, is that sites at which DNAPLs or explosives were released typically have a zone in which the mass of contamination is originally concentrated. This volume, termed the source zone, serves as the source for the development of a dissolved phase plume. As long as the source remains, a dissolved phase plume will continue to develop; hence, removal (or isolation) of the source zone is required to halt creation of the dissolved phase plume. One approach to defining “source” is to consider the phases in which the contamination may exist. An uncontaminated soil system contains a solid phase, an aqueous phase, and a vapor phase. 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. Volatilized contaminants were considered not to be sources because the percentage of total contaminant mass in the gas phase per unit volume of subsurface is generally insignificant compared to other phases. The challenge, then, was to determine to what extent sorbed or dissolved contaminants are considered to be sources. Two definitions, in particular, shed light on this argument. The EPA’s regulatory definition of “source material” (EPA, 1991) is: Material that includes or contains hazardous substances, pollutants or contaminants that act as a reservoir for migration of contamination to ground water, to surface water, to air, or acts as a source for direct exposure.
OCR for page 27
Contaminants in the Subsurface: Source Zone Assessment and Remediation The term “reservoir” suggests a large supply of contamination, although this is not further clarified. The phrase “for migration of contamination to ground water, to surface water, to air” implies that contamination can move from its point of origin. EPA (1991) goes on to state that “contaminated groundwater generally is not considered to be a source material.” EPA examples of source materials and nonsource materials are shown in Table 1-2. Although contaminated groundwater is explicitly excluded from the EPA definition, sorption always results in contamination of solid phases that are contacted by contaminated water. In fact, the amount of sorbed contaminant may greatly exceed the amount of contaminant in solution. The extent of this association depends upon both the contaminant and the soil characteristics. Is this newly contaminated soil now considered source material? 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. For this reason, the committee explored alternate definitions. One definition that includes such a distinction was made by a recent EPA expert panel (EPA, 2003) concentrating on DNAPL sites, which chose to define the term “source zone” as: the groundwater region in which DNAPL is present as a separate phase, either as randomly distributed sub-zones at residual saturations or “pools” of accumulation above confining units…. This includes the volume of the aquifer that has had contact with free-phase DNAPL at one time. The EPA panel excluded vadose zone issues from its definition and emphasized DNAPLs (chlorinated solvents, solvent/hydrocarbon mixtures, and coal tars/ creosotes) rather than light nonaqueous phase liquids (LNAPLs). The EPA panel TABLE 1-2 EPA Examples of Source and Nonsource Materials Source Materials Nonsource Materials • Drummed wastes • Groundwater • Contaminated soil and debris • Surface water • Pools” of dense nonaqueous phase liquids • Residuals resulting from treatment of site (NAPLs) submerged beneath groundwater materials or in fractured bedrock • NAPLs floating on groundwater • Contaminated sludges and sediments SOURCE: EPA (1991).
OCR for page 28
Contaminants in the Subsurface: Source Zone Assessment and Remediation definition provides a clear distinction between the region containing the dissolved phase plume (has not had contact with DNAPL) and the source zone (has had contact with DNAPL). The committee combined and modified the prior definitions in order to develop a definition that captures the essence of a source as a reservoir of contamination, while making a distinction between the source and the plume, and that encompasses pure solid sources: A source zone is a saturated or unsaturated subsurface zone containing hazardous substances, pollutants or contaminants that acts as a reservoir that sustains a contaminant plume in groundwater, surface water, or air, or acts as a source for direct exposure. This volume is or has been in contact with separate phase contaminant (NAPL or solid). Source zone mass can include sorbed and aqueous-phase contaminants as well as contamination that exists as a solid or NAPL. Recognition of a source zone may be accomplished either from direct observation of the separate phase contaminant (NAPL or solid) or from inference. Because of equilibrium partitioning theory, certain soil phase and aqueous phase concentrations of contaminants imply that a separate pure solid or liquid phase exists in the subsurface, even if one cannot be discovered. Thus, for example, in subsurface areas where the groundwater contaminant concentrations are at or near the temperature-dependent aqueous solubility limit, the presence of separate phase material can be inferred. A similar approach may be used to recognize a source zone that once contained DNAPL, but no longer does. For example, the most commonly cited example of such a source zone is the case in which DNAPL has been depleted due to matrix diffusion. That is, if DNAPL is trapped on top of a clay layer, or within a fracture in a clay unit, there will be a large concentration gradient established between the saturated water immediately in contact with the DNAPL and the water in the matrix. This will lead to diffusion of the contaminant into the matrix and subsequent sorption to the matrix solids. This relatively immobile contaminant mass is significant because if the source zone is treated by a flushing or chemical treatment that removes the NAPL (e.g., surfactants or in situ oxidation), the contamination in the matrix will remain largely unaffected, and at a later date it will diffuse back into the more permeable zones and recontaminate the groundwater. 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), others do not. Several approaches involve contaminant mass
OCR for page 29
Contaminants in the Subsurface: Source Zone Assessment and Remediation destruction (chemical oxidation, reduction, or biodegradation), or they combine removal and destruction (e.g., steam treatment). Others such as containment or immobilization do not involve any contaminant mass removal. In almost all cases, removal of the source, whether through physical removal or reaction, will not be complete. Thus, in this report the term “source remediation” is used, defined as any approach to reduce the problem associated with source zones. 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). In order to make informed statements about site characterization and the various technologies of interest to the Army, the committee analyzed the results of source remediation activities completed to date at dozens of Army and other facilities. The 11 Army sites for which the committee heard presentations and reviewed extensive reports are listed in Table 1-3. These sites are contaminated with either chemical explosives or chlorinated solvents, and they span a range of hydrogeologic conditions. Numerous source remediation activities at non-Army sites were also reviewed, including almost 100 technology-specific cases cited in Chapter 5 and five additional sites discussed in the EPA expert panel report (EPA, 2003). Throughout the report, case studies of Army and other sites where source characterization and source remediation have occurred are presented. A central theme to this report is the importance of understanding the relationship between the hydrogeologic setting of the site, the different objectives for remediation, and the effectiveness of source treatment technologies. These three independent variables were judged by the committee to be central to decision making at hazardous waste sites, and their multidimensional relationship is illustrated by the cube shown in Figure 1-2. (Other factors that are not included in Figure 1-2, but which play a role in source remediation, include the type of DNAPL and the magnitude of its release.) Envisioning source remediation as a multidimensional problem that links the appropriate source technology with the hydrogeologic setting at a given site and the cleanup objective is critical to maximizing the potential for success. The concepts embodied in this diagram are used throughout this report to illustrate the importance of the relationship of these independent variables to cleanup success. To elaborate on the physical setting, Chapter 2 presents a more comprehensive picture of DNAPL and explosives sites than is presented in this chapter, including information about DNAPL architecture and contaminant characteristics.
OCR for page 30
Contaminants in the Subsurface: Source Zone Assessment and Remediation TABLE 1-3 Army Sites Where Source Characterization and Source Remediation Were Reviewed by the Committee Installation and Site Technology Scale Contaminants Anniston In situ chemical oxidation—Fenton’s Pilot, then Full TCE + others Letterkenny OU3 In situ chemical oxidation—Fenton’s Pilot DCE, TCE, VC, PCE Watervliet In situ chemical oxidation—KMnO4 Pilot TCE, PCE Letterkenny OU11 In situ chemical oxidation—peroxone Pilot TCE + others Pueblo In situ chemical oxidation—Fenton’s Pilot TNT, TNB, RDX Milan In situ chemical oxidation—Fenton’s Pilot TNT, RDX, HMX Letterkenny OU10 Enhanced bioremediation Pilot TCE, TCA, DCE, DCA, VC Badger Enhanced bioremediation Pilot, then Full DNTs Redstone No remediation planned yet NA TCE, TCA, perchlorate Ft. Lewis Thermal treatment planned NA TCE Volunteer Monitored natural attenuation planned NA DNT, TNT Chapter 3 then discusses the role of source characterization, and it stresses the importance of understanding the nature of the source zone prior to making decisions about cleanup strategies. Without adequate characterization of the size, nature, and distribution of contamination, source cleanup is unlikely to succeed. The various objectives of source zone remediation are outlined in Chapter 4, with a focus on making sure project managers choose metrics that are appropriate for measuring whether their stated objectives are met. 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. It is here that conclusions are made about the ability of source remediation to bring about changes in water quality. Finally Chapter 6 presents a decision-making framework for source remediation based on Figure 1-2, as well as the committee’s conclusions about the future of source remediation as
OCR for page 31
Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 1-2 The success of source remediation is envisioned to depend on the physical setting, the chosen cleanup goal, and the selected remedy. 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. Rather, it is inherently iterative, requiring continual feedback from each stage of study. Each aspect, from the conceptual model to the objectives selected, the degree and methods of characterization, the remediation project design, and the basis for performance assessment, must be continually refined and reevaluated as understanding of the site develops. The report is intended to inform decision makers within the Army, the rest of the military, and many other government agencies and the private sector about
OCR for page 32
Contaminants in the Subsurface: Source Zone Assessment and Remediation potential options for their sites contaminated with DNAPLs and chemical explosives. The scientific information contained herein should help in the prioritization of cleanup efforts, which is clearly essential given the conflicting forces of reconciling the public’s desire for aggressive remediation, the apparent inability of current technologies to achieve aquifer restoration, and the high cost of remediation. REFERENCES Department of the Army. 1997. Environmental Protection and Enhancement. Army Regulation 200-1. http://www.usapa.army.mil/pdffiles/r200-1.pdf. Department of Defense (DoD). 1998. Evaluation of DoD waste site groundwater pump-and-treat operations. Report No. 98-090, Project No. 6CB-0057. Washington, DC: Office of the Inspector General. DoD. 2003. Defense Environmental Restoration Program Annual Report to Congress—Fiscal Year 2003. Washington, DC: DoD. http://184.108.40.206/DERPARC_FY03/do/report. Environmental Protection Agency (EPA). 1989. Evaluation of Ground-Water Extraction Remedies: Volumes 1 and 2. Washington, D.C.: EPA Office of Emergency and Remedial Response. EPA. 1991. A Guide to Principal Threat and Low Level Threat Wastes. Publication 9380.3-06FS. Washington, DC: EPA Office of Solid Waste and Emergency Response. EPA. 1992. Evaluation of Ground-Water Extraction Remedies: Phase II, Volume I—Summary Report. Publication 9355.4-05. Washington, DC: EPA Office of Emergency and Remedial Response. EPA. 1996a. Presumptive Response Strategy and Ex Situ Treatment Technologies for Contaminated Ground Water at CERCLA Sites. EPA 540-R-96-023. Washington, DC: EPA Office of Solid Waste and Emergency Response. EPA. 1996b. A Citizen’s Guide to Treatment Walls. EPA 542-F-96-016. Washington, DC: EPA. EPA. 2000. U.S. EPA National Water Quality Inventory 2000 Report. Washington, DC: EPA Office of Water. EPA. 2003. The DNAPL Remediation Challenge: Is There a Case for Source Depletion? EPA 600/ R-03/143. Washington, DC: EPA Office of Research and Development. EPA. 2004. Treatment technologies for site cleanup: annual status report (11th edition). EPA 542-R-03-009. Washington, DC: EPA Office of Solid Waste and Emergency Response. Feenstra, S., and J. A. Cherry. 1988. Subsurface contamination by dense non-aqueous phase liquid (DNAPL) chemicals. In: Proceedings of the International Groundwater Symposium, International Association of Hydrogeologists, May 1–4, Halifax, Nova Scotia. Haines, L. 2002. Army Environmental Center. Presentation to the NRC Committee on Source Removal of Contaminants in the Subsurface. August 22, 2002. Haines, L. 2003. Army Environmental Center. Presentation to the Committee on Source Removal of Contaminants in the Subsurface. April 14, 2003. Jurgens, B., and W. Linn. 2004. Drycleaner Site Assessment & Remediation—A Technology Snapshot (2003). State Coalition for the Remediation of Drycleaners. http://drycleancoalition.org/download/2003surveypaper.pdf Interstate Technology Regulatory Council (ITRC). 2000. Dense Non-Aqueous Phase Liquids (DNAPLs): Review of Emerging Characterization and Remediation Technologies Technology Overview. Washington, DC: Interstate Technology and Regulatory Cooperation Work Group. ITRC. 2002. DNAPL Source Reduction: Facing the Challenge. Regulatory Overview. Washington, DC: Interstate Technology and Regulatory Council. MacKay, D., and J. A. Cherry. 1989. Groundwater Contamination: Pump and Treat Remediation. Environ. Sci. Technol. 23:630–636.
OCR for page 33
Contaminants in the Subsurface: Source Zone Assessment and Remediation Mercer, J. W., and R. M. Cohen. 1990. A Review of Immiscible Fluids in the Subsurface: properties, models, characterization and remediation. J. Contam. Hydrol. 6:107–163 National Research Council (NRC). 1994. Alternatives for Ground Water Cleanup. Washington, DC: National Academies Press. NRC. 1997. Innovations in Ground Water and Soil Cleanup: From Concept to Commercialization. Washington, DC: National Academy Press. NRC. 1999a. Groundwater and Soil Cleanup: Improving Management of Persistent Contaminants. Washington, DC: National Academy Press. NRC. 1999b. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: National Academy Press. NRC. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: National Academies Press. Pankow, J. F., and J. A. Cherry. 1995. Dense Chlorinated Solvents and other DNAPLs in Groundwater. Waterloo, Ontario: Waterloo Press. 522 p. Quinton, G. E., R. J. Buchanon, D. E. Ellis, and S. H. Shoemaker. 1997. A Method to Compare Groundwater Cleanup Technologies. Remediation 8:7–16. Strategic Environmental Research and Development Program (SERDP). 2002. SERDP/ESTCP Expert Panel Workshop on Research and Development Needs for Cleanup of Chlorinated Solvent Sites. Washington, DC: SERDP/ESTCP. Stroo, H. 2003. Retec. Presentation to the Committee on Source Removal of Contaminants in the Subsurface. January 30, 2003. Villaume, J. F. 1985. Investigations at sites contaminated with DNAPLs. Ground Water Monitoring Review 5(2):60–74.
Representative terms from entire chapter: