4
DNAPLs: Technologies for Characterization, Remediation, and Containment

Chlorinated hydrocarbons are among the most common pollutants in groundwater and soils at Department of Energy (DOE) sites (Riley et al., 1992), as well as other contaminated sites across the United States (Pankow and Cherry, 1996). Other types of dense non-aqueous phase liquid (DNAPL) components, including polychlorinated biphenyls (PCBs), also may be present, but chlorinated solvents are by the far most prevalent (see Table 1-4). Therefore, this chapter focuses on technologies for remediation of chlorinated solvent DNAPLs, although many of these technologies are applicable to other types of DNAPLs as well.

The conventional strategies of excavating soil and pumping and treating contaminated groundwater are generally ineffective at restoration of DNAPL-contaminated sites (NRC, 1994; Pankow and Cherry, 1996). The innovative technologies discussed in this chapter have demonstrated potential for use in remediation of DNAPL-contaminated sites. However, data for these evaluations are limited because only a few well-documented pilot tests on DNAPL sites have been reported.

THE DNAPL PROBLEM

The chlorinated organic compounds that comprise the DNAPLs common at DOE sites have low solubilities in water (Table 4-1).1 As

1  

This discussion of the DNAPL problem and the distribution of DNAPLs is after Fountain (1998).



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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants 4 DNAPLs: Technologies for Characterization, Remediation, and Containment Chlorinated hydrocarbons are among the most common pollutants in groundwater and soils at Department of Energy (DOE) sites (Riley et al., 1992), as well as other contaminated sites across the United States (Pankow and Cherry, 1996). Other types of dense non-aqueous phase liquid (DNAPL) components, including polychlorinated biphenyls (PCBs), also may be present, but chlorinated solvents are by the far most prevalent (see Table 1-4). Therefore, this chapter focuses on technologies for remediation of chlorinated solvent DNAPLs, although many of these technologies are applicable to other types of DNAPLs as well. The conventional strategies of excavating soil and pumping and treating contaminated groundwater are generally ineffective at restoration of DNAPL-contaminated sites (NRC, 1994; Pankow and Cherry, 1996). The innovative technologies discussed in this chapter have demonstrated potential for use in remediation of DNAPL-contaminated sites. However, data for these evaluations are limited because only a few well-documented pilot tests on DNAPL sites have been reported. THE DNAPL PROBLEM The chlorinated organic compounds that comprise the DNAPLs common at DOE sites have low solubilities in water (Table 4-1).1 As 1   This discussion of the DNAPL problem and the distribution of DNAPLs is after Fountain (1998).

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 4-1 Properties of Select DNAPL Components Commonly Found at DOE Sites Compound Aqueous Solubility (mg/liter) Density (g/cm3) Vapor Pressure (mm Hg) Henry's Law Constant (atm m3/mol) Absolute Viscosity (cP) Tetrachloroethylene 150 1.6227 14 1.46 × 10-2 0.89 Trichloroethylene 1,100 1.4642 57.8 9.9 × 10-3 0.57 1,2-Dichloroethylene 6,260 1.2565 265 5.23 × 10-3 0.40 Trichloroethane 4,500 1.4397 19 9.09 × 10-4 0.12 1,2-Dichloroethane 5,500 1.235 64 9.10 × 10-4 0.80 Carbon tetrachloride 800 1.594 90 3.02 × 10-2 0.97 Chloroform 8,000 1.483 160 3.20 × 10-3 0.58 NOTE: Properties are at 20°C. SOURCES: Mueller et al., 1989; Mercer and Cohen, 1990; Montgomery, 1991. a result, when released in the subsurface they typically do not dissolve totally in the groundwater but remain largely as a separate, nonaqueous-phase liquid (NAPL). However, solubilities of these DNAPL components are much higher than drinking water standards, so they create a persistent source of groundwater contamination as they slowly dissolve. The chlorinated solvents that comprise the most common DNAPL components are denser than water (Table 4-1) and tend to sink beneath the water table. These characteristics pose a challenge to all conventional groundwater remediation technologies (NRC, 1994; Pankow and Cherry, 1996). In the vadose zone (the soil above the water table), DNAPL flows downward under the influence of gravity with relatively little spreading (Schwille, 1988; Pankow and Cherry, 1996). Capillary forces retain a small quantity in each pore (or fracture) through which the DNAPL flows (see Box 4-1). This fraction, which is not mobile under static conditions, is termed residual saturation. Contamination in soils above the water table therefore tends to be both laterally restricted and of relatively low saturation (saturation is defined as the fraction of the pore space filled with DNAPL). Where the water table is deep, however, as at several DOE sites, the total quantity of DNAPL retained in the vadose zone may be large. Below the water table the distribution of DNAPLs tends to be much more irregular. Entry of DNAPL into water-filled pores requires overcoming a displacement pressure resulting from capillary forces between DNAPL and water (Box 4-1). The required entry pressure increases with decreasing grain size of the solid media in the aquifer (see Table 4-2). The downward flow of DNAPL therefore

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants BOX 4-1 Factors Influencing the Movement of DNAPLs Underground When two fluids are present in one environment, the fluid having a greater affinity for a solid surface tends to spread along the surface; this fluid is termed the wetting phase. Typically, water is the wetting phase relative to both air and DNAPLs, whereas DNAPLs are wetting relative to air. Capillary forces tend to favor the entry of the wetting phase into small pores or fractures. In contrast, capillary forces will resist the entrance of the nonwetting fluid into pores filled with the wetting phase (Pankow and Cherry, 1996). In the saturated zone (the portion of the subsurface below the water table), capillary forces resist the entry of DNAPLs into water-filled pores: the required pressure (displacement pressure) for DNAPL entry increases with decreasing grain size. This has several results, including (1) a tendency for DNAPLs to spread horizontally above finer-grained layers and thus form thin horizontal layers or lenses; (2) a tendency for DNAPLs to follow preferential pathways and thus have a highly inhomogeneous distribution; and (3) the concentration of DNAPL in larger pores. In the vadose zone (above the water table), the presence of air in soil pores allows DNAPLs to move downward without overcoming a displacement pressure. Therefore, above the water table, DNAPLs tend not to spread laterally as readily as they do in the saturated zone. Capillary forces and contact angle also affect the retention of DNAPLs. Capillary forces retain a small fraction of DNAPLs in every pore. The amount retained, referred to as residual saturation, is higher in the saturated zone, where water keeps the DNAPLs away from pore walls, than in the vadose zone. may be interrupted each time the DNAPL encounters a layer with a smaller grain size than the overlying one, causing the DNAPL to flow laterally above the fine-grained layer. In such cases, a DNAPL lens may accumulate until it reaches a sufficient thickness of DNAPL Table 4-2 Examples of the Entry Pressure Required for Downward Migration of Trichloroethylene (TCE) in Different Media Medium Required Entry Pressure (cm of TCE) Clean sand (K = 1 × 10-2 cm/sec) 45 Silty sand (K = 1 × 10-4 cm/sec) 286 Clay (K = 1 × 10-7 cm/sec) 4,634 Fracture, 20-µm aperture 75 Fracture, 100-µm aperture 15 Fracture, 500-µm aperture 3 NOTE: Calculations based on TCE as the DNAPL, an interfacial tension of 34 dynes/cm, a wetting angle of 0, and a porosity of 0.35. Sand and fracture entry were calculated, respectively, from Pankow and Cherry, 1996, equations 3.17 and 11.3.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Figure 4-1 Typical distribution of a DNAPL in the subsurface. to overcome the required displacement pressure. The result is a series of horizontal DNAPL lenses connected by narrow, vertical pathways (see Figure 4-1). As in the vadose zone, a small amount of DNAPL is retained as residual saturation in every pore through which it flows. If the DNAPL encounters a layer that has a sufficiently high entry pressure, the DNAPL will be retained as a pool on the top of this layer. Thus, DNAPL is typically found in multiple horizontal lenses connected by sparse vertical pathways, with one or more pools above fine-grained layers. Most of the horizontal lenses and vertical pathways will be at or below residual saturation; only pools will have higher saturations. The distinction between residual saturation and pools is important, since only the DNAPL in pools is expected to be mobile. The flow of DNAPL and the resultant distribution are discussed in detail in Pankow and Cherry (1996). A plume of dissolved contaminants, known as the dissolved-phase plume, will form when groundwater contacts either residual saturation, DNAPL pools, or DNAPL lenses. The volume of soil that contains DNAPL at or above residual saturation is termed the source zone. Removal of the source zone is required for restoration, because, if the source zone remains, groundwater will continue to be

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants contaminated, typically for very long periods. Some of the remediation technologies discussed in this chapter (summarized in Tables 4-3 and 4-4) are primarily for treating source zones, whereas others are for plumes of dissolved contaminants. CHARACTERIZATION OF DNAPL CONTAMINATION The presence of DNAPL makes site characterization both more critical and more difficult due to the irregular distribution of DNAPLs. Site characterization includes characterization of a site's hydrogeology and of contaminant distribution at the site.2 Because the distribution of DNAPLs is controlled by heterogeneities in the soil or rock and because the effectiveness of most technologies is also affected by heterogeneities, careful characterization of the hydrogeology of the site is essential. Technologies for hydrogeological characterization are not unique to DNAPL-contaminated sites and are thus not discussed in this report. Characterization of contaminant distribution includes defining the extent of both dissolved-phase contamination and the DNAPL source areas. Methods for defining dissolved-phase plumes are well established and are not discussed here. Determination of the limits of the DNAPL source zone is necessary both to ensure that all DNAPL is within the containment or treatment area and to minimize the volume to be treated (and hence cost). This section discusses emerging technologies for DNAPL source-zone characterization. Characterization plans for DNAPL-contaminated sites must consider the risks inherent in penetrating a DNAPL pool. The high density and low viscosity of chlorinated solvent DNAPLs create a significant risk of mobilization if a pool is penetrated by drilling during site characterization. A DNAPL pool perched on a fine-grained layer may drain down a drill hole to lower, previously clean layers. In general, it is inadvisable to drill through DNAPL (Pankow and Cherry, 1996) Direct-Push Technologies Push-in tools, including large cone penetrometers, smaller units (e.g., geoprobes), and hand-held samplers driven by hammers, provide an extremely useful method of analyzing sites without drilling wells, 2   For a detailed discussion of methods for characterizing DNAPL-contaminated sites, see Pankow and Cherry (1996).

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 4-3 Treatment Technology Options for DNAPL Source-Zone Remediation Technology Mechanism(s) Objective Potential Application Limitations Steam Volatilization, mobilization Removal Any aquifer unit with adequate permeability Must be able to control steam flow and to heat units to appropriate temperatures. Heterogeneities may increase treatment time and produce tailing. Must consider implications of DNAPL mobilization. Surfactants Dissolution, mobilization Removal Any unit with adequate permeability Must be able to establish hydraulic control. Heterogeneities may increase treatment time and produce tailing. Must consider implications of DNAPL mobilization. Solvents Dissolution, mobilization Removal Any unit with adequate permeability Must be able to establish hydraulic control. Heterogeneities may increase treatment time and produce tailing. Must consider implications of DNAPL mobilization. In situ oxidation Chemical reaction Destruction Any unit with adequate permeability that does not react excessively with reagent Must be able to deliver adequate reagent to source zone. Heterogeneities may increase treatment time and produce tailing. Reaction with other compounds in aquifer may reduce effectiveness.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Technology Mechanism(s) Objective Potential Application Limitations In situ vitrification Thermal decomposition Destruction Any site with appropriate depth and groundwater conditions Soil must produce appropriate melt. Groundwater volume must not be excessive. Vapor emission must be controlled. Depth limited to about 10 m (30 ft). Soil vapor extraction Volatilization Removal Units with low soil-water content, volatile contaminants, and adequate permeability Must be able to induce adequate air flow through entire source zone. Heterogeneities and high water contents may limit effectiveness. Air sparging Volatilization Removal Saturated units with volatile contaminants and adequate permeability Must be able to induce air flow through entire source zone. Heterogeneities may limit effectiveness. Contaminants must be volatile at groundwater temperatures. Electrical heating Volatilization Removal Typically, low-permeability units with adequate moisture to provide conductivity Volatilized contaminants must be removed by another technology (typically soil vapor extraction or steam). Permeability must be sufficient for vapor flow.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 4-4 Technologies Primarily for Treatment of Contaminants Dissolved from DNAPLs Technology Mechanism(s) Objective Potential Application Limitations In-well stripping Volatilization Removal Saturated units with adequate permeability Treatment zone depends on groundwater flow pattern established. Bioremediation Biologically mediated chemical reaction Destruction Electron acceptor, donor, and nutrients required; specific conditions depend on biodegradation pathways. Only aqueous phase can be treated Reactive barriers Chemical reaction Dechlorination Dissolved phase must be treatable, and water must be chemically compatible with barrier. Treats only aqueous phase. Wall must intercept entire plume. Specific limitations depend on barrier type. Electrokinetic systems Electrically induced mobilization Mobilization, coupled with another technology for destruction or removal Low-permeablity units Applications to DNAPL not well established. Must be coupled with technology for destruction or removal of contamination. Physical barriers Containment Containment Any unit in which barriers can be physically emplaced Limitations vary with barrier type. If not keyed into impermeable unit at base, system will be open, and contamination will not be contained.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants which is costly and invasive. Push-in tools can recover or in some cases analyze core samples of soil and samples of water taken at multiple depths during insertion. Core samples provide direct evidence of DNAPL saturation, while water samples can determine the vertical extent of contamination. A thorough characterization requires many cores, each analyzed at small vertical intervals, because of the low probability of a given core intersecting the inhomogeneously distributed DNAPL. Many new types of push-in tools that allow direct detection of contamination, without bringing soil or water samples to the surface for analysis, have been developed in recent years. One of the most successful efforts, the site characterization and analysis penetrometer system (SCAPS) program, is discussed in detail in Chapter 5. Sensors for direct detection of contamination include the following: Laser-induced fluorescence sensors. Laser-induced fluorescence sensors use a laser to cause organic components to fluoresce in the subsurface. These tools have been highly successful in hydrocarbon detection. Although hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), generally fluoresce, most chlorinated solvents do not. Therefore this technique is of limited applicability for the detection of chlorinated solvent DNAPLs. Thermal desorption volatile organic compound (VOC) sampler. The SCAPS thermal desorption VOC sampler collects a small soil sample at a desired depth. The soil is heated, and volatile components are extracted and carried to the surface by a carrier gas. VOCs are analyzed on the surface by mass spectrometry. Multiple samples may be taken during a given push. This sampler provides essentially the same data as discrete-depth sampling of a conventional core for volatile compounds. Hydrosparge VOC sensing system. This system is mounted on a hydropunch direct-push device and pushed to the desired depth. A water sample is obtained and sparged using helium carrier gas. The gas is routed to the surface, where the sample is analyzed with an ion trap mass spectrometer. Video imaging system (GeoVIS). This system illuminates soil in contact with a sapphire window and images it with a miniature color camera. Both light nonaqueous-phase liquid (LNAPL) and DNAPL detection have been reported (Lieberman and Knowles, 1998; Lieberman et al., 1998). The NAPL phases are visible as discrete globules. Although data at this time are insufficient to establish the sensitivity of the method, the range of conditions in which it is effective, and the lower limit of NAPL saturation that can be reliably detected, the technology may offer promise for rapid source-zone delineation.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Neutron Probes Differences in the way air and water scatter and attenuate neutrons can be used to evaluate the amount of moisture in the soil, as is done with neutron soil moisture probes. Neutron logging tools based on the difference in neutron attenuation between water and hydro-carbons are widely used to determine the saturation of oil in reservoirs. In a similar manner, the difference in attenuation of neutrons between DNAPL and water can be used to estimate DNAPL saturation. This technique is based upon the much larger neutron capture cross section of chlorine compared to water. Neutron logging tools can be used to identify intervals with DNAPL within a few centimeters of a well (Newmark et al., 1997; Daly et al., 1998). Seismic Methods Seismic reflection, refraction, and acoustical tomography have been used in an attempt to locate DNAPLs. Although seismic methods have been widely used for petroleum exploration and site characterization and have been highly successful in defining subsurface geology, their success in detecting DNAPLs has been limited. Generally, the differences in response due to geologic heterogeneities are greater than those due to the presence of DNAPLs. At this time, there are no well-documented successes in locating DNAPL pools using seismic methods, although research is continuing.3 Ground Penetrating Radar (GPR) GPR has a proven capability to define shallow stratigraphy. It provides a detailed image of layering in the soil to a depth of from approximately a meter (a few feet) to 10 or so meters (a few tens of feet). It cannot penetrate clay layers and so is useful for defining confining layers. Where DNAPL is known to exist, GPR may have the potential to monitor its movement. However, with no prior data at a site, recognition of DNAPL cannot generally be accomplished, because the lithologic variations within a unit are generally greater than the variations caused by thin DNAPL pools (Grumman and Daniels, 1995; Pankow and Cherry, 1996; Young and Sun, 1996). 3   For a review of recent developments in shallow seismic methods, see Steeples (1998).

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Electrical Resistivity DNAPLs are generally nonconductive, so methods based on conductivity or resistivity have the potential to distinguish water-filled from DNAPL-filled pores. Results to date have shown considerable success in monitoring the change in DNAPL saturation during remediation. Direct detection of DNAPL, without previous background measurements, does not generally appear to be possible (Santamarina and Fam, 1997; Daly et al., 1998; Lien and Enfield, 1998). Partitioning Tracers The partitioning of an organic compound between DNAPL and water is largely a function of the solubility of the compound in water. Methanol and isopropanol, which are miscible in water, partition nearly totally into the aqueous phase. On the other hand, less soluble alcohols (pentanol, heptanol, and all heavier alcohols) have limited aqueous solubilities and hence partition into DNAPL. If a solution of conservative tracers (e.g., bromide or isopropanol) and less soluble tracers (heavier alcohols) is pumped through a contaminated zone, partitioning of the less soluble compounds into the DNAPL will slow the rate of transport of the heavier alcohols exactly as sorption slows the transport of hydrophobic organic contaminants. The fraction of pore space occupied by DNAPLs can be derived from known partition coefficients and measured breakthrough curves in a well-to-well pump test (just as the retardation factor of a compound can be converted to the fraction of organic carbon in soil if the partition coefficient between soil and water is known). This technology has been demonstrated in numerous field tests (Jin et al., 1995; Brown et al., in review). Partitioning tracers can provide a quantitative measure of the fraction of pore space occupied by DNAPLs between an injection well and an extraction well. The test averages DNAPL saturation over the entire interval. Interpretation in nonhomogeneous units requires an accurate flow model. Tracer tests require a very large number of high-quality analyses; if DNAPL saturation is low, very precise data are needed over an extended time period. This technology has proven invaluable at several dozen sites for determining the quantity of DNAPL present and for evaluating the performance of remediation technologies in field trials through tests conducted before and after treatment. Since cleanup goals are seldom based on the amount of DNAPL left in place, but rather are

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants tion. One of the most extensively studied sites is a Superfund site in St. Joseph, Michigan, that is contaminated with TCE at concentrations as high as 100 mg/liter (McCarty and Wilson, 1992; Kitanidis et al., 1993; Haston et al., 1994; Wilson et al., 1994). At this site, Wilson et al. (1994) found nearly a 24-fold decrease in concentrations of chlorinated organic compounds across the site and attributed this decrease to reductive dehalogenation. Although the reductive dehalogenation was extensive, this study did not demonstrate complete transformation of the contaminants to harmless end products. The transformation products cis-DCE, vinyl chloride, and ethene were still present at the site. At another well-studied site, Edwards Air Force Base in California, detailed studies indicated that no biological transformation of TCE has occurred in 40 years (McCarty et al., 1998). Studies at this site have shown that the rate at which the TCE plume has grown is consistent with what would occur in the absence of biodegradation. Limitations Natural attenuation of chlorinated solvents is a slow process and thus will not be an appropriate strategy for sites at which relatively rapid cleanup of contaminants is required. Estimating the length of time required for transformation of the contaminants is often not possible due to the complexity of the microbial processes involved. In addition, the biological reactions responsible for attenuation of chlorinated solvents generally require the presence of other organic compounds to serve as electron donors or primary substrates; biodegradation will not occur in the absence of these other substances. Another limitation is that some transformation products that result during natural attenuation, such as vinyl chloride, are more harmful than the original contaminants and may accumulate at the site. Monitoring of sites for natural attenuation can be costly. Advantages The primary advantage of this method is that it can eliminate the need for an engineered solution that may disrupt the site, or it can reduce the size of the area requiring treatment with an engineered system. It also can be less costly than engineered methods, depending on the amount of site analysis and monitoring required.

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants COMMON LIMITATIONS OF DNAPL REMEDIATION TECHNOLOGIES Regardless of the technology used, hydrologic and geochemical conditions will impose limitations on performance. Geological heterogeneities are the most significant cause of limitation of remediation technologies. The severity of problems produced by heterogeneities can often be predicted based on a thorough site assessment. Variation in hydraulic conductivity within the contaminated zones results in two types of problems: (1) regular lithologic variation produces channeling of flow, and (2) interunit heterogeneity results in unequal access to the unit. Where some layers have higher conductivity than others, as is typical of layered sedimentary aquifer formations, flow will preferentially occur in the higher-conductivity units. Pumping any fluids, whether vapor or liquid, will require a longer time in the lower-conductivity units, resulting in much larger than necessary volumes being pumped through the high-permeability units. Variations within a given unit, such as horizontal grain size variation, cause some areas to receive less flow than others within the same horizon. As a result, some zones will receive little or no treatment if a fluid is pumped in or out of the zone. In some cases, this uneven treatment is acceptable if natural attenuation rates are sufficient to control contaminants in less permeable zones following treatment of the permeable zones. The slow movement of groundwater in less permeable soils also can result in reagents being spent before they penetrate into the formation. Failure of reagents to penetrate the contaminated area is especially a problem for chemical oxidants that can oxidize naturally occurring organics and for Fenton's reagent, which can decompose to oxygen and water. In these cases, most of the reagent is consumed unproductively. Other less obvious restrictions may result from the site lithology. For instance, sites at which a permeable, water-bearing interval is located immediately beneath a low-permeability unsaturated zone can be problematic. Remediation technologies that involve the injection of a gas phase, (e.g., air sparging, steam injection) or that can generate a gas phase (e.g., Fenton's reagent) are limited because the gas phase is difficult to collect for treatment. Karst hydrogeology presents unique challenges. Groundwater movement in karst terrain occurs mostly through relatively large channels. Horizontal movement can be quite rapid. Reagents introduced in the source area, to the extent this can be defined, may be able to intercept the path followed by the contaminants; however, the reagent

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants will be chasing the contaminants and may not mix sufficiently with the contaminated groundwater to produce the intended reaction. Fractured rock poses similar challenges. Flow often occurs through a few conductive fractures. Reaching the entire source area is impossible if contaminants are located in dead-end fractures, and even defining the flow pattern is difficult. Geochemistry varies among sites and is often affected by the release of contaminants. The groundwater pH, mineral content of soils and groundwater, and presence of nutrients beneficial to microbial processes affect many processes. For example, the release of petroleum hydrocarbons and/or oxygenated solvents, such as methyl ethyl ketone, results in the depletion of oxygen through biodegradation by the indigenous microorganisms. Following consumption of dissolved oxygen, microbial processes can result in lowered oxidation-reduction potentials and increased concentrations of reduced iron (both dissolved and on the surface of minerals), reduced manganese concentrations, and reduced sulfur species (sulfide, etc.) concentrations. Remediation processes that involve reduction, such as reduction of hexavalent chromium or reductive dechlorination of chlorinated solvents, benefit from these geochemical conditions. Conversely, these conditions will create problems for remedial technologies that introduce oxidants, resulting in excess consumption of oxidants to oxidize reduced iron, manganese, and sulfide and to increase the oxidation-reduction potential. The challenges presented by specific hydrogeologic conditions are likely to affect all remediation technologies but are likely to be less problematic for some technologies than for others. When evaluating and selecting site remedies, it is thus necessary to investigate, understand, and consider site-specific hydrogeology and geochemistry. In some cases there will be no easy answers, and remediation, if possible at all, will be more costly and require a longer time. CONCLUSIONS Several technologies have shown the ability to rapidly remove mass from DNAPL source zones. Other technologies have demonstrated the ability to clean up contaminants that have dissolved from these source zones. Following are brief summaries of the demonstrated capabilities of the technologies reviewed in this chapter: Soil vapor extraction is effective for mass removal of volatile compounds in homogeneous, permeable soils and, with the addition of thermal processes, can be extended to semivolatile compounds. Removal of

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants DNAPLs requires sufficient flow through the entire source zone, which may be difficult to achieve. Steam can remediate DNAPLs in permeable soil in both the saturated and the unsaturated zones. It may be combined with electrical heating when fine-grained layers are present. Successful application to DNAPL remediation requires adequate permeability and control of DNAPL mobility. Heterogeneities may limit efficiency. Surfactants have demonstrated the ability to remove DNAPLs nearly completely from permeable units under saturated conditions. DNAPL remediation requires adequate permeability and consideration of DNAPL mobility. Heterogeneities may reduce efficiency. Cosolvents have shown similar potential as surfactants for rapid removal of LNAPLs and should, in principle, be equally effective with DNAPLs. DNAPL remediation requires adequate permeability and consideration of DNAPL mobility. Performance may be limited by heterogeneities. In situ oxidation has proven effective for the destruction of specific chlorinated DNAPL compounds in permeable, relatively homogeneous soils. Its application to DNAPLs requires adequate permeability and delivery of sufficient reagent to the source zone. The volume of DNAPL that may be efficiently treated may be limited by mass transfer considerations. Electrical heating and electrokinetics have shown potential for remediation of DNAPLs in low-permeability units. Both must be accompanied by some form of contaminant retrieval and destruction system. Currently, data are inadequate to determine the effectiveness of electrokinetics for remediating DNAPL source zones. Biodegradation of both chlorinated compounds and PAHs has been demonstrated. Degradation apparently takes place primarily in the dissolved phase, so bioremediation is not a direct DNAPL source-zone treatment method. Degradation of DNAPL source zones may require an extended time. In situ vitrification has demonstrated the ability to vitrify soil and produce temperatures that should lead to the destruction or mobilization of DNAPL compounds. Data on its applicability to DNAPL sites are insufficient to provide a meaningful evaluation at this time. Reactive barrier walls have shown great promise for the treatment of chlorinated solvent dissolved-phase plumes. They do not directly address the DNAPL source zone. Barrier walls, with or without reactive components, may, however, contain DNAPL source zones. Although a range of technologies is emerging to help clean up DNAPL-contaminated sites, the number of carefully controlled field

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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants tests is insufficient to establish the ultimate cleanup level attainable for each technology. Each technology discussed in this report is based on well-established chemical, biological, and physical principles. Performance limitations are thus more likely to be a function of the hydrogeologic conditions of the site than of the processes themselves. Since an accurate characterization of the occurrence of DNAPLs is essential for the design of a remediation system and an accurate knowledge of geological heterogeneities is vital for evaluating the hydrogeological limits on remediation, thorough site characterization is required for DNAPL sites. Once site assessment has provided the means to evaluate the applicability of the technologies discussed in this report and the probable limitations of remediation, these technologies can be compared to baseline technologies such as excavation and pump-and-treat systems. REFERENCES AATDF (Advanced Applied Technology Demonstration Facility). 1997. Technology Practices Manual for Surfactants and Cosolvents. Houston, Tex.: AATDF, Rice University. Abriola, L. M., K. D. Pennell, G. A. Pope, T. J. Dekker, and D. J. Luning-Prak. 1995. Impact of surfactant flushing on the solubilization and mobilization of dense nonaqueous-phase liquids. ACS Symposium 594:10–23. Acar, Y. B., A. N. Alshawabkeh, and R. J. Gale. 1993. Fundamentals of extracting species from soils by electrokinetics. Waste Management 13:141–151. Acar, Y. B., R. J. Gale, A. N. Alshawabkeh, R. E. Marks, S. Puppala, M. Bricka, and R. Parker. 1995. Electrokinetic remediation: Basics and technology status. Journal of Hazardous Materials 40:117–137. Aines, R. 1997. Results from Visalia: Rapid thermal cleanup of dense nonaqueous-phase liquids. Presentation to the National Research Council Committee on Technologies for Cleanup of Subsurface Contaminants in the DOE Weapons Complex, Third Meeting, Livermore, Calif., December 15–17. Baran, J. R. J., G. A. Pope, W. H. Wade, V. Weerasoorlya, and A. Yapa. 1994. Microemulsion formation with chlorinated hydrocarbons of differing polarity . Environmental Science and Technology 287(7):1361–1365. Bass, D. H., and R. A. Brown. 1996. Air sparging case study database update. In Proceedings of the 1st International Symposium on In Situ Air Sparging for Site Remediation, Las Vegas, October 24–25. Potomac, Md.: INET. Beeman, R. E., J. E. Howell, S. H. Shoemaker, E. A. Salazar, and J. R. Butram. 1994. A field evaluation of in situ microbial reductive dehalogenation by the biotransformation of chlorinated ethylenes. Pp. 14–27 in Bioremediation of Chlorinated and Polycyclic Aromatic Hydrocarbon Compounds, R.E. Hinchee, A. Leeson, L. Semprini, and S.K. Ong, Eds. Boca Raton, Fla.: Lewis Publishers. Brockman, F. J., W. Payne, D. J. Workman, A. Soong, S. Manley, and T. C. Hazen. 1995. Effect of gaseous nitrogen and phosphorus injection on in situ bioremediation of a trichloroethylene-contaminated site. Journal of Hazardous Materials 41:287–298. Brown, C. L., M. Delshad, V. Dwarakanath, R. E. Jackson, J. T. Londergan, H. W. Meinardus, D. C. McKinney, T. Oolman, G. A. Pope, and W. H. Wade. In review. A successful demonstration of surfactant flooding of an alluvial aquifer contaminated with DNAPL. Submitted as a research communication to Environmental Science and Technology.

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