4
Capabilities of Enhanced Pump-and-Treat and Alternative Technologies

Given the limited capabilities of conventional pump-and-treat systems and the large number of contaminated sites, a substantial market exists for innovative ground water cleanup technologies. However, use of innovative technologies has not been as extensive as might be expected, considering the potential size of the market. For example, while conventional pump-and-treat systems were selected for use at 73 percent of Superfund sites with ground water contamination through fiscal year 1992, at the remaining 27 percent of sites the most common "remedies" were not innovative technologies but nontreatment measures such as providing alternative water supplies, aquifer use restrictions, and wellhead treatment (Kelly, 1994; K. Lovelace, Environmental Protection Agency, unpublished data, 1992). Furthermore, technologies that treat ground water in place rather than extracting it were specified as remedies at fewer than 2 percent of Superfund sites (Kelly, 1994).

This chapter evaluates the capabilities of innovative subsurface cleanup technologies and reviews why application of these technologies has been limited. Included in addition to reviews of technologies that treat ground water below the water table are reviews of technologies that treat soils above the water table, because ground water cleanup cannot be achieved if contaminants from the overlying soil continue to migrate downward. While the Environmental Protection Agency (EPA) defines innovative technologies as those for which limited or no cost and effi-



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Alternatives for Ground Water Cleanup 4 Capabilities of Enhanced Pump-and-Treat and Alternative Technologies Given the limited capabilities of conventional pump-and-treat systems and the large number of contaminated sites, a substantial market exists for innovative ground water cleanup technologies. However, use of innovative technologies has not been as extensive as might be expected, considering the potential size of the market. For example, while conventional pump-and-treat systems were selected for use at 73 percent of Superfund sites with ground water contamination through fiscal year 1992, at the remaining 27 percent of sites the most common "remedies" were not innovative technologies but nontreatment measures such as providing alternative water supplies, aquifer use restrictions, and wellhead treatment (Kelly, 1994; K. Lovelace, Environmental Protection Agency, unpublished data, 1992). Furthermore, technologies that treat ground water in place rather than extracting it were specified as remedies at fewer than 2 percent of Superfund sites (Kelly, 1994). This chapter evaluates the capabilities of innovative subsurface cleanup technologies and reviews why application of these technologies has been limited. Included in addition to reviews of technologies that treat ground water below the water table are reviews of technologies that treat soils above the water table, because ground water cleanup cannot be achieved if contaminants from the overlying soil continue to migrate downward. While the Environmental Protection Agency (EPA) defines innovative technologies as those for which limited or no cost and effi-

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Alternatives for Ground Water Cleanup ciency data exist, this report groups all technologies other than conventional pump-and-treat systems under the heading "innovative." It is important to note that some of the technologies reviewed in this chapter are gaining wider use. However, the increasing use of newer technologies applies mainly to soil above the water table. The most striking example of this desired technical evolution is the increased use of soil vapor extraction systems, which have now become a leading cleanup technology for soil (Kovalick, 1993). For cleaning up petroleum hydrocarbons, in situ bioremediation is also becoming increasingly common. Despite the increasing use of these two technologies, application of innovative technologies for cleaning up ground water remains rare. In this chapter, the committee has divided innovative technologies into two categories: enhanced pump-and-treat systems and alternative technologies. Enhanced pump-and-treat systems all involve, to some extent, the pumping of fluids such as water, water solutions, or air and thus will face some of the same difficulties as conventional pump-and-treat systems; the advantage of these enhanced systems is their potential to significantly increase the rate at which contaminant mass can be removed from the subsurface. Alternative technologies do not involve continuous pumping. For each of these technologies, the importance of thorough site characterization and field tests prior to implementation and of process monitoring after implementation cannot be overstated. Because of the lack of performance data for most of the technologies reviewed here, the uncertainty associated with these methods is proportionately greater than the uncertainty associated with conventional pump-and-treat systems. Thorough characterization of the site's geologic and chemical characteristics, field tests of the remediation method, and continual monitoring of the full-scale system are all essential steps for minimizing uncertainties. For innovative technologies even more than for conventional pump-and-treat systems, an observational approach to remediation—in which the design of the system evolves as information on field performance is collected—is a requirement for effective cleanup. ENHANCED PUMP-AND TREAT SYSTEMS Conventional pump-and-treat systems extract relatively large volumes of water with relatively low contaminant concentrations. Because of geologic complexity and slow rates of contaminant desorption and dissolution, these systems must displace many pore volumes of aquifer water to flush out contaminants, as explained in Chapter 3. Conventional pump-and-treat systems thus are inherently inefficient for removing contaminants from the subsurface. Many technologies currently being

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Alternatives for Ground Water Cleanup developed or tested are designed to enhance the efficiency of pump-and-treat systems. Some of these technologies reduce the ultimate burden on the pump-and-treat system by removing from the soil contaminants that would otherwise migrate to the ground water or by removing volatile contaminants from the soil and ground water. Other innovative technologies improve the efficiency of contaminant extraction by increasing the amount of contaminant removed with each volume of pumped water. Another group of innovative technologies pumps minimal amounts of fluids to stimulate treatment of contaminants in place, either biologically or chemically, rather than requiring contaminant extraction and surface treatment. All of these technologies have in common the requirement to pump fluids through the subsurface, meaning that to varying degrees the geologic and chemical conditions that impose limitations on conventional pump-and-treat systems also present problems for these innovations. The committee has divided enhancements to pump-and-treat systems into two categories: demonstrated technologies and technologies in development. Demonstrated Technologies The following technologies are all close to being accepted or are already accepted for site cleanups. They have been tested in laboratory-scale batch and column studies, in controlled field experiments, and in large-scale site trials. Data collection and analysis are comprehensive. Soil Vapor Extraction Description Soil vapor extraction (SVE) is one of the few innovative technologies that has gained wide use. The technology extracts organic contaminants (primarily from the unsaturated zone) by flushing with air. Air flow is induced by applying a vacuum at a sealed wellhead or with blowers. Where the air stream contacts contaminants—which may be present as a nonaqueous-phase liquid (NAPL), dissolved in water in the soil pores, or associated with the soil—mass transfer to the air can occur, with subsequent transport of the air and contaminants to the surface. An incidental effect of SVE is that by increasing the subsurface oxygen supply, it can promote biodegradation of contaminants by aerobic microbes, although standard SVE systems are not specifically designed for this purpose.1 As shown in Figure 4-1, an SVE system usually consists of one or more extraction wells, vacuum pumps or air blowers, and a treatment system for the extracted vapors. In some cases, the ground is covered with an impermeable cap to improve system performance by controlling

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Alternatives for Ground Water Cleanup FIGURE 4-1 Process diagram for soil vapor extraction. the direction of air flow and ensuring total capture of the extracted vapors. SVE also has been called soil venting, subsurface venting, in situ soil air stripping, and vacuum extraction. (For detailed descriptions of SVE systems, see Hutzler et al., 1989, and American Academy of Environmental Engineers, 1994.) Application SVE has proven effective for removing substantial quantifies of certain volatile organic contaminants from the unsaturated zone at a variety of sites in the United States and abroad. Numerous Records of Decision at Superfund sites have specified SVE as the technology of choice for unsaturated zone cleanup. The technique has also been extensively used for cleanups at gas stations and other sites where large quantities of volatile organic compounds have leaked from underground storage tanks. Although SVE also can remove contaminants from dewatered portions of the saturated zone, in which the water table has been purposely lowered through pumping, its use for this purpose has not been as extensive as for unsaturated zone treatment. Conceptually, contaminant removal efficiency for SVE systems depends on the physical state of the contaminant. When present as a NAPL, contaminants will transfer from the pure liquid phase to the air via evaporation. If contaminants are adsorbed on or in the soil, then transfer must occur within the soil to the air-soil interface, with subsequent trans-

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Alternatives for Ground Water Cleanup Vapor extraction facility that treats highly concentrated gasoline vapors from a large free-product plume. Courtesy of Peter Gerbasi, Roux Associates, Inc., Islandia, New York. fer to the air stream. Finally, if the contaminants are dissolved in water in the soil pores, mass transfer must occur through the water-air interface. The rate of extraction thus depends in part on the efficiency of each of these molecular-scale mass transfer processes. Based on this conceptual model, it is apparent that the efficiency of SVE depends strongly on contaminant and soil properties. Contaminant properties include vapor pressure, Henry's Law constant, hydrophobicity (usually quantified with the octanol-water partition coefficient), and diffusion characteristics. Soil properties include stratigraphy (for example, size distribution, permeability, and porosity), organic carbon content, mineralogy, and moisture. The design of the SVE system also influences contaminant extraction efficiency. Principal design variables include the number of extraction wells, the rate of air flow (level of vacuum applied or rate of air injection), and the depth and length of the screened zone. Especially important to consider is the vapor flow path relative to the contaminant location. If the air stream bypasses zones of low permeability, the slow process of diffusion will dominate, making contaminant removal extremely slow. As a rough rule of thumb for feasibility assessment, SVE is likely to

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Alternatives for Ground Water Cleanup be successful if the contaminant's boiling point is less than 150°C or if its vapor pressure (evaluated at the subsurface temperature) is greater than about 5 × 10-4 atm (Hutzler et al., 1989; Johnson et al., 1990). Also essential for SVE is a soil permeability sufficient to allow adequate air flow. Typically, if the soil's permeability to air is less than 1 darcy (10-16 m2), flow rates may be too low to achieve successful removal in reasonable time frames. Because of the complex interrelationships among all the factors that influence SVE, the effectiveness of SVE should be evaluated carefully on a site-by-site basis. Reviews of SVE systems in the United States show successful recovery of volatile organic compounds from the subsurface down to a depth of over 60 meters (see, for example, Hutzler et al., 1989; Buscheck and Peargin, 1991). Most of these systems addressed contaminant removal from the unsaturated zone rather than from dewatered portions of the saturated zone. Air flow rates ranged from 0.3 up to approximately 100 standard cubic meters per minute, with applied vacuums ranging from 0.0067 arm (5 mm of Hg) to 0.3 atm (230 mm of Hg). Unfortunately, the efficiency of the systems in terms of contaminant recovery was not reported. Typically, concentrations of volatile compounds in the extracted air stream decreased rapidly with time and approached asymptotic values similar to those seen in ground water pump-and-treat systems. Average removal rates for sites reviewed by Hutzler exhibited a narrow range: from 0.005 to 0.01 kg of contaminant per m3 of air extracted. Due to the apparent lack of detailed field investigations, a detailed assessment of SVE performance under controlled field conditions is needed. Limitations Flushing the subsurface with air, either injected or induced, is subject to the same limitations as flushing with water. The air stream is unlikely to flush zones of low permeability, which can contain significant quantities of contamination. In addition, SVE must overcome mass transfer limitations that inhibit the desorption of strongly adsorbed contaminants or contaminants that have penetrated the microstructure of the aquifer materials. Thus, all of the factors that inhibit release of contaminants during traditional pumping and treating also limit the performance of air flushing systems. Advantages A major advantage of air flushing versus water flushing is the higher fluid flow rates possible with air—provided that the soil permeability allows sufficient volumetric flow rates. Large numbers of pore volumes of air can be flushed through the subsurface in a short time, which permits recovery of a significant mass of released contaminants. Whether this increased flushing is sufficient to remove contaminants to acceptable levels is highly site specific.

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Alternatives for Ground Water Cleanup SVE appears very promising for enhancing contaminant removal from dewatered sections of the saturated zone, although the degree to which this technology can remove contaminant sources has yet to be thoroughly evaluated. It is probable that SVE will be more successful at sites with light NAPLs (LNAPLs) than at sites with dense NAPLs (DNAPLs) because LNAPLs tend to remain above the water table, where they are more accessible, whereas DNAPLs tend to sink. In Situ Bioremediation—Hydrocarbons Description In situ bioremediation systems stimulate subsurface microorganisms, primarily bacteria, to biodegrade contaminants. When given the proper stimuli, microorganisms can transform the contaminants to innocuous mineral end products, such as carbon dioxide and water. As explained in Chapter 2, the necessary stimuli for microbial growth in aquifers are oxygen or other electron acceptors (such as nitrate or sulfate) and nutrients (such as nitrogen and phosphorus). Typical in situ bioremediation systems therefore perfuse electron acceptors and nutrients through the contaminated region, as shown in Figure 4-2. In situ bioremediation near the land surface can be achieved by using infiltration galleries that allow water amended with nutrients and electron acceptors to percolate through the soil. When contamination is deeper, in situ bioremediation systems inject the amended water through FIGURE 4-2 Process diagram for in situ bioremediation.

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Alternatives for Ground Water Cleanup wells. As shown in Figure 4-2, some in situ bioremediation systems use extraction and injection wells in combination to control the flow of electron acceptors and nutrients and to hydraulically isolate the contaminated area. The most common electron acceptor for the full-scale in situ bioremediation systems used today is oxygen, although in the future other electron acceptors (such as nitrate) may become more common. In situ bioremediation systems typically supply oxygen by bubbling air or pure oxygen into the injection water or by dosing the water with hydrogen peroxide. Alternately, they may supply oxygen by injecting air directly into the ground water, with nutrients added through injection wells or infiltration galleries. Application In situ bioremediation has been well established as a successful method for treating soil and ground water contaminated with certain types of hydrocarbons, primarily petroleum products and derivatives. In situ bioremediation was first successfully demonstrated for cleaning up subsurface petroleum hydrocarbons at a Sun Oil pipeline leak in Ambler, Pennsylvania, in 1972 (Lee and Ward, 1985). Since then, the technique has been used to clean up subsurface spills of refinery wastes, crude oil, and fuels. It has also been used to treat other easily biodegraded organic contaminants such as phenols, cresols, acetone, and cellulosic wastes. Although in situ bioremediation of other types of or-garlic contaminants, such as chlorinated solvents, is possible, the technology has not yet been demonstrated for these other applications. Before an in situ bioremediation project is initiated, a specific microbial enhancement feasibility study and a general hydrogeologic site investigation are essential. The microbial study will help determine the types and amounts of substances required to stimulate optimum contaminant degradation. Site-specific geology and geochemistry also should be considered in project design. These parameters affect nutrient and electron acceptor availability, which may be hindered by sorption to the soils or reactions with naturally occurring subsurface chemicals. Limitations Like conventional pump-and-treat systems, in situ bioremediation systems are limited by geologic heterogeneities such as low-permeability zones, except the problem is reversed. For pump-and-treat systems, geologic heterogeneities limit the ability to extract contaminants, whereas for in situ bioremediation, geologic heterogeneities interfere with the ability to inject the necessary electron acceptors and nutrients. Adequate concentrations of electron acceptors and nutrients must be available to the bacteria throughout the contaminated zone to stimulate

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Alternatives for Ground Water Cleanup growth; delivery of these growth-stimulating materials to zones of low permeability is difficult. Mass transfer limitations that slow the dissolution of sorbed or NAPL contaminants and create problems for conventional pump-and-treat systems also interfere with in situ bioremediation. Microorganisms with the metabolic capability to degrade a contaminant will not do so if the contaminant is unavailable to the cell because it is contained in a NAPL or sorbed to subsurface particles. Slow dissolution from NAPLs and slow desorption from soils decrease the biodegradation rate, thereby increasing the cleanup time and the amount of chemicals that must be added to sustain microbial activity. Since there is currently no scientific consensus on what factors affect bioavailability or how bioavailability ultimately affects bioremediation, contaminant bioavailability must be considered on both a site- and a compound-specific basis. Toxicity of contaminants to the microorganisms may also limit in situ bioremediation. Many contaminants are toxic to bacteria at high concentrations. For example, concentrations within a NAPL pool are likely to be toxic and restrict bioremediation to the periphery of the NAPL zone. Fortunately, the soluble concentrations of hydrocarbons normally observed at field sites are well below the toxic range. Another limitation of in situ bioremediation is the requirement for a minimum contaminant concentration to maintain the microbial population and to induce the enzymes necessary for degradation. The existence of such a concentration threshold means that, theoretically, there is a minimum concentration below which no further bioremediation will occur. This minimum may exceed cleanup goals, particularly for heavier hydrocarbons. In studies of hydrocarbon biodegradation, minimum concentrations have ranged from 1 µg/liter to 1 mg/liter. An additional limitation is the difficulty of delivering sufficient oxygen to the microorganisms because of oxygen's low water solubility. Injecting air directly into the ground water, rather than applying it in dissolved form in the nutrient-amended water, is one approach used to improve oxygen delivery. Advantages In situ bioremediation has four unique advantages over conventional pump-and-treat systems. First, while pump-and-treat systems extract contaminants to the surface for disposal or treatment elsewhere, in situ bioremediation treats contaminants in place and can convert them to innocuous products (such as carbon dioxide and water). As a result, in situ bioremediation reduces the requirement for surface treatment and disposal of the recovered water and minimizes the contaminant exposure hazard. Second, pumping requirements are likely to be lower for in situ bioremediation than for conventional pump-and-treat systems. The wa-

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Alternatives for Ground Water Cleanup ter circulation requirements for delivering growth-stimulating materials to the subsurface are much lower than the requirements for attempting to flush out contaminants with a pump-and-treat system. Third, in situ bioremediation may be faster than conventional methods. Bioremediation at the periphery of a NAPL pool or on surfaces where contaminants are sorbed decreases the contaminant concentration near these remaining sources, increasing the dissolution rate. In addition, microbially mediated chemical transformations are generally faster than the same reactions in the absence of microorganisms. Fourth, certain microorganisms are able to move toward regions of higher contaminant concentration through a process known as chemotaxis, helping to expand the zone of biodegradation and eventually achieve complete treatment (Bosma et al., 1988) Bioventing Description Bioventing is in situ bioremediation of the unsaturated zone. Like soil vapor extraction, bioventing involves inducing air movement through the unsaturated soil. However, the main purpose of bioventing is not to extract volatile contaminants but to enhance aerobic biodegradation of contaminants by supplying oxygen to soil microbes. Air flow requirements are therefore much lower for bioventing systems than for soil vapor extraction systems. Inorganic nutrients also may be added, if necessary. As shown in Figure 4-3, the components of a bioventing system resemble those of a soil vapor extraction system, with the addition of a mechanism for nutrient delivery. Bioventing systems use air recovery wells either alone or with air injection wells. Since they are designed to promote biodegradation rather than physical removal of vapors, air recovery wells are located at the periphery of the contaminated area, and air flow rates are kept at the minimum rate required to deliver oxygen. Application Bioventing is used primarily for petroleum hydrocarbons and some chlorinated solvents. The technology is particularly useful in cases where excavation of the site is impractical, such as under buildings, where underground utilities are present, or where the contaminated soils are deep. Because bioventing requires air flow, it is more easily applied to permeable soils such as sand than to clays. Soil moisture levels are also an important parameter. Although biodegradation rates improve with high moisture levels, high soil moisture inhibits air movement. Limitations Where the natural nutrient supply is insufficient, nutrient addition can be problematic, especially at capped sites with low-perme-

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Alternatives for Ground Water Cleanup FIGURE 4-3 Process diagram for bioventing. ability soils. Bioventing systems add nutrients in aqueous solution. The added liquid affects soil moisture content and, consequently, may inhibit air movement. The change in soil moisture can also affect the load-bearing capacity of the soil—an important consideration when treating soil under or near a building. In addition, flushing nutrients through the soil may transport contaminants from the unsaturated into the saturated zone. Researchers have investigated the possibility of using gaseous ammonia as a nitrogen source to eliminate these problems, but this method has not been very successful. To avoid causing unintended contaminant migration, before nutrients are added, the air circulation system can be operated for a period of time to biodegrade and/or physically remove the most mobile contaminants. Another limitation of bioventing systems is that they may cause air quality problems if large quantities of volatile contaminants are vented to the atmosphere. Off-gas treatment may be necessary to meet local regulatory discharge limits. Bioventing systems are also limited by the same factors mentioned for SVE. Significant masses of contaminants may remain in zones of low

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Alternatives for Ground Water Cleanup to establish the reliability of the technology, to provide data for preparing design manuals, and to identify the inherent limitations for users. These field tests should evaluate performance for a range of chemical contaminants and contaminant mixtures (such as petroleum hydrocarbons, chlorinated solvents, polychlorinated biphenyls, and metals) and site conditions (such as shallow and deep aquifers and homogeneous and heterogeneous geology). They should be designed to allow a mass balance analysis comparing the amount of contamination present in the subsurface with the amount removed. Data from the field tests should undergo scientific peer review. In addition to field tests, focused research is needed to answer more fundamental questions about the technologies. The committee believes that focused research could lead to especially promising advances for engineered in situ bioremediation, intrinsic bioremediation, soil vapor extraction, air sparging, containment methods, and inorganic contaminant treatment methods. Engineered And Intrinsic In Situ Bioremediation Engineered and intrinsic in situ bioremediation are promising cleanup methods because they treat contaminants in place instead of requiring extraction, can convert contaminants to innocuous products, and minimize or eliminate pumping requirements. However, the microbial processes underlying bioremediation and how to optimize these processes are still not fully understood. Especially important is research to address the following questions: Under what chemical conditions are contaminants susceptible to biodegradation? What genetic characteristics and biochemical mechanisms control microbial degradation of particular types of contaminants? What are the ecological relationships between the various microbial communities involved in biodegrading contaminants? How can subsurface microbial populations be selected and manipulated to carry out specific biotransformations? In addition to advanced understanding of microbial processes, improvements are needed in the engineering systems used to promote bioremediation. The key engineering challenges are delivering adequate growth-stimulating materials to the microorganisms and ensuring adequate contact between the organisms and the contaminants. Research addressing the following questions would advance the design of bioremediation systems:

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Alternatives for Ground Water Cleanup How can systems for delivering oxygen and other growth-stimulating materials be optimized to provide sufficient quantities of the necessary materials, especially in low-permeability and heterogeneous soils? How can these delivery systems be improved to ensure mixing throughout the contaminated zone and prevent excessive microbial growth near the injection point? What is the efficiency of methods such as surfactant and solvent flushing in enhancing contact between the microorganisms and the contaminants? What is the ultimate fate in the subsurface of surfactants and solvents designed to promote microbial contact with contaminants? How can protocols for monitoring and evaluating the progress of bioremediation be improved? Soil Vapor Extraction and Air Sparging Soil vapor extraction and air sparging have the potential to rapidly remove large quantities of volatile organic contaminants from shallow zones, as documented in this chapter. However, accurately predicting mass removal rates and operating times for these systems is difficult because of limited process-level understanding. Research addressing the following questions would advance the state of the art for soil vapor extraction and air sparging: What is the relationship between air velocity and attainment of local equilibrium? What is the importance of diffusion-limited processes, such as movement of contaminants from low-permeability zones and dead-end pores, in the performance of vapor extraction and air sparging systems? What are the implications of assuming laminar flow versus assuming turbulent flow in design calculations? What is the response of multicomponent contaminant mixtures, such as gasoline, to removal by volatilization? How does the extraction efficiency decrease over time with loss of the more volatile fractions? How can the optimal well spacing be determined? What are the appropriate models for describing two-phase hydro-dynamic conditions generated by air flow in the saturated zone? How does the elevated air supply influence chemical and biological reactions? What is the most cost-effective balance between designing air delivery systems to promote contaminant volatilization and designing them to promote contaminant biodegradation?

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Alternatives for Ground Water Cleanup Containment Technologies Construction difficulties and questions about long-term reliability have limited the application of containment technologies in the past. Research addressing the following questions would improve the ability to design effective containment systems: How can methods for detecting defects in containment systems be improved? How can the bottoms of vertical walls be effectively sealed? What is the long-term reliability of different materials used for containment? How significant is diffusive transport of contaminants across barriers over long time scales? Treatment Methods For Inorganic Compounds Most of the enhancements and alternative technologies reviewed in this chapter are for treating organic contaminants. However, as described in Chapter 1, metal contaminants such as lead and chromium are present at hundreds of Superfund sites as well as at the many other types of waste sites. One example of a metal removal technique currently being researched is electro-osmotic purging (Acar, 1992), in which electrodes are inserted in the soil to enhance the diffusion of metals and facilitate their extraction from low-permeability soils. Other suggested methods for treating metals involve using chemicals or microbes to either dissolve the metals and improve their recovery or immobilize the metals for long-term containment. In general, however, the techniques for removing metals are much less developed than those for removing organic compounds. Therefore, research is needed to develop existing metal recovery methods and to explore possible new techniques. EDUCATIONAL NEEDS As discussed in this chapter, an important barrier to the use of innovative technologies is lack of technical expertise on the many possible innovative cleanup methods. Advancing use of these technologies will require improved education, especially of the people in direct decisionmaking positions. The committee recommends three types of educational programs: Formal interdisciplinary programs: The nation's formal academic educational programs need to be updated and the interdisciplinary op-

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Alternatives for Ground Water Cleanup portunities within these programs expanded to train future generations of technical personnel. Technical training courses: Training courses are needed to improve the knowledge of existing technical personnel. Opportunities for representatives of industry, researchers, regulators, consultants, and contractors to exchange ideas and experiences: Opportunities are needed to discuss successes and failures, barriers to using more efficient treatment methods, and steps that could be taken to increase the diversity of available technologies. CONCLUSIONS Based on technical reports about innovative subsurface cleanup technologies and an assessment of the application of these technologies at contaminated sites, the committee reached the following conclusions: Enhancements to conventional pump-and-treat systems can significantly increase the mass of contaminants removed from the subsurface and the rate at which they are removed. Pulsed pumping can improve cleanup cost efficiency by allowing contaminants to desorb and diffuse into mobile fluid zones, increasing the mass of contaminant extracted with each volume of pumped water. In situ bioremediation and in situ chemical treatment can improve cleanup efficiency by promoting contaminant destruction in place, eliminating or minimizing the need to extract the contaminants. Soil vapor extraction, air sparging, and horizontal wells can improve cleanup efficiency by removing volatile contaminants via air, a more effective transport medium than water. Soil flushing and in situ thermal technologies can improve cleanup efficiency by enhancing contaminant removal from soil above the water table, preventing contaminant migration into the ground water. Although enhancements can substantially increase the efficiency of pump-and-treat systems, they are subject to similar limitations as conventional systems because they involve pumping fluids such as water, water solutions, and/or air. Geologic complexities can interfere with delivery of the fluids to zones where contaminants may be lodged. Contaminant sorption to solid materials and the presence of NAPLs can limit the availability of contaminants for capture or treatment by the circulating fluids. Sorption and NAPLs can also limit contaminant contact with microorganisms, which is required for in situ bioremediation. In situ nonpumping alternatives to pump-and-treat systems may reduce cleanup costs, but they have important limitations. Nonpumping alternatives reduce costs by eliminating the need to circulate fluids through the subsurface, which can have a high energy cost. Nonpump-

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Alternatives for Ground Water Cleanup ing approaches can contain the contamination, route the dissolved contamination through a stationary reactive medium, or use the capability of indigenous microbes to degrade the contamination without human intervention. However, the long-term integrity of containment systems is unproven, stationary barriers for treating contamination have been tested only in small-scale laboratory experiments, and very little operating history exists to judge the effectiveness of intrinsic biodegradation. In addition, members of the public may be unwilling to accept these approaches, which they may perceive as equivalent to no action or as resulting in incomplete site remediation. No known technology can overcome all of the limitations to ground water cleanup. For innovative cleanup technologies as for conventional pump-and-treat systems, the geologic conditions at the site and the chemical nature of the contaminants can prevent restoration of ground water to health-based standards. Nevertheless, the use of innovative technologies should be encouraged even if the technologies cannot currently reach health-based cleanup goals, because new technologies may outperform conventional systems and because wider use of the technologies may lead to discoveries that improve their performance. For enhanced pump-and-treat systems and nonpumping alternatives, thorough site characterization, pilot testing, and monitoring are critical to effective performance. An observational approach to remediation, in which the treatment system is adjusted as new information becomes available, is even more important for innovative technologies than for conventional pump-and-treat systems because of the greater uncertainties associated with the innovations. A combination or sequence of remedial technologies is likely to be more effective—and necessary—at most sites. The majority of waste sites contain mixtures of contaminants with varying physical and chemical properties. A single remedial process is typically effective at removing only a subset of the compounds in a waste mixture. Treatment trains that couple remedial techniques may be necessary to treat the different types of contaminants that may be present at the site. Despite the potential for innovations to increase the efficiency and reduce the costs of ground water cleanup, significant barriers have obstructed the development and application of these innovations . A combination of technical, institutional, and economic barriers discourages those involved in cleanups from assuming the risks associated with using technologies that lack proven track records. Options need to be explored for developing mechanisms that allow risk sharing when innovative technologies are used, so that neither the site owner nor the technology developer must assume the full burden of risk if the technology fails.

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Alternatives for Ground Water Cleanup Federal agencies supporting innovative technology development should assess the effectiveness of current technology development efforts. Cooperative research and development agreements among government agencies, the DOE's research at its national laboratories, and the EPA's efforts under the SITE program, through the TIO, and through the hazardous substance research centers are important steps in expanding use and development of innovative technologies. However, whether these programs are sufficient to overcome the many barriers to technology development is unknown. Given the size and cost of the ground water cleanup problem, the importance of ensuring that adequate resources are directed toward technology development cannot be overstated. NOTE 1.   Technologies that circulate air specifically to promote biodegradation are known as bioventing systems and are described later in this chapter. REFERENCES Abdul, A. S., and T. L. Gibson. 1991. Laboratory studies of surfactant enhanced washing of polychlorinated biphenyl from sandy material. Environ. Sci. Technol. 25(4):665-671. Abdul, A. S., T. L. Gibson, and D. N. Rai. 1990. Selection of surfactants for the removal of petroleum products from shallow sandy aquifers. Ground Water 28(6):920-926. Acar, Y. D. 1992. Electrokinetic cleanups. Civil Eng. 62(10):58-60. American Academy of Environmental Engineers. 1994. Vacuum Vapor Extraction. Annapolis, Md.: American Academy of Environmental Engineers. Angell, K. G. 1992. In situ remedial methods: air sparging. Natl. Environ. J. 2(1):20-23. Baehr, A. L., G. E. Hoag, and M. C. Marley. 1989. Removing volatile contaminants from the unsaturated zone by inducing advective air-phase transport. J. Contam. Hydrol. 4(1):2-6. Borden, R. C., C. Gomez, and M. Becker. In press. Natural bioremediation of a gasoline spill. In Proceedings of the Second International Symposium on In Situ and On-Site Bioremediation, San Diego, April 5-8, 1994. Chelsea, Mich.: Lewis Publishers. Bosma, T. N. P., J. L. Schnoor, G. Schram, and A. J. B. Zehnder. 1988. Simulation model for biotransformation of xenobiotics and chemotaxis in soil columns. J. Contam. Hydrol. 2:225-236. Bouwer, E. J., and P. L. McCarty. 1983. Transformations of halogenated organic compounds under denitrification conditions. Appl. Environ. Microbiol. 45(4):1295-1299. Bouwer, E. J., and J. P. Wright. 1988. Transformations of trace halogenated aliphatics in anoxic biofilm columns. J. Contain. Hydrol. 2:155-160. Brown, R. 1992. Air Sparging: A Primer for Application and Design. Trenton, N.J.: Groundwater Technology, Inc. Buscheck, T., and R. Peargin. 1991. Summary of a nationwide vapor extraction system performance study. In Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground Water. Dublin, Ohio: National Ground Water Association. Chiang, C. Y., I. P. Salanitro, E. Y. Chai, J. D. Colthart, and C. L. Klein. 1989. Aerobic biodegradation of benzene, toluene, and xylene in a sandy aquifer: data analysis and computer modeling. Ground Water 27(6):823-834.

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