Density-enhanced remediation of non-aqueous phase liquid contamination of subsurface environments

A method for remedying contamination of a subsurface environment by a non-aqueous phase liquid that is denser than water (DNAPL). The subsurface environment has a resident aqueous phase and a DNAPL phase. A dense aqueous solution that has a density greater than a density of the DNAPL phase is introduced to the subsurface environment. The dense aqueous solution displaces the resident aqueous phase and causes the DNAPL phase to rise above the greater density aqueous solution. The DNAPL phase is then recovered, which substantially remedies the contamination of the subsurface environment. An alternative method provides for introducing a dense solution layer that has a density greater than the density of the DNAPL phase beneath the subsurface region contaminated with the DNAPL phase. Next, the water table is lowered so as to create an unsaturated porous medium above the dense solution layer and to downwardly mobilize the DNAPL phase to the top of the dense solution layer. The DNAPL phase is then collected from the top of the dense solution layer.

TECHNICAL FIELD
 The present invention relates generally to methods for remedying subsurface
 soil contamination. More particularly, the present invention relates to a
 method for remedying the contamination of a subsurface environment by
 dense non-aqueous phase liquids (DNAPLs).
 The publications and other materials used herein to illuminate the
 background of the invention, and in particular cases, to provide
 additional details respecting the practice, are incorporated herein by
 reference, and for convenience, are referenced by author and date in the
 following text, and respectively grouped in the appended list of
 references.

Table of Abbreviations
 CF cosolvent flushing
 DERD density-enhanced remediation of DNAPLs
 DNAPL dense non-aqueous phase liquids
 IBD In situ biodegradation
 IFT interfacial tension
 ISGS in situ gas stripping
 LNAPL less dense non-aqueous phase liquids
 NAPL non-aqueous phase liquids
 PAT pump-and-treat
 PCE perchloroethylene
 RW reactive walls
 IS steam injection
 SF surfactant flushing
 1,1,1-TCA 1,1,1-trichloroethane
 1,1,2-TCA 1,1,2-trichloroethane
 TCE trichloroethylene
 VE vapor extraction
 BACKGROUND ART
 Contamination of subsurface environments by fluids that are immiscible with
 water has occurred routinely in the United States and industrialized
 countries around the world over the last 40 years. Mercer and Cohen
 (1990). Such fluids are often termed non-aqueous phase liquids (NAPLs) in
 general, or LNAPLs when less dense or DNAPLs when more dense than the
 groundwater present in the subsurface. Typical LNAPLs are petroleum
 products (e.g., gasoline, diesel fuel, jet fuels, heating oils), and
 typical DNAPLs are chlorinated solvents (e.g., trichloroethylene,
 tetrachloroethylene, dichloroethanes). Schwille (1988); Bartow and
 Davenport (1990).
 Species present in such NAPL phases can solubilize to the aqueous phase,
 volatilize to the gas phase, or sorb to the solid phase present in the
 subsurface. Environmental concerns result when such species are linked to
 human or ecological health concerns and become present in sufficient
 quantities in a mobile aqueous or gas phase. Such health concerns are
 associated with common constituents of NAPLs found routinely in the
 environment, such as trichloroethylene and benzene.
 Once released into the subsurface, NAPLs migrate and typically reach a
 stable, immobile state within a relatively short time scale (hours to
 days) after the source is removed. Immobile NAPLs can remain in the
 subsurface over time scales that can range from months to decades or
 longer under natural conditions, because they are comprised of species
 that are only sparingly soluble in water. Miller, Poirier-McNeill et al.
 (1990). As a result, NAPLs are considered a long-term source of
 groundwater contamination. Mayer and Miller (1996).
 DNAPLs tend to be an even more significant problem than LNAPLs because of
 the following characteristics:
 (1) they were routinely used in industrial practices, spilled, and
 intentionally disposed of in the subsurface in the United States, starting
 in the 1960's and continuing for two decades;
 (2) they often migrate larger distances than LNAPLs;
 (3) they often penetrate the water table;
 (4) they can form pools contained by low-permeability materials;
 (5) they are often comprised of species that tend to degrade slowly in many
 systems;
 (6) they are extremely difficult to locate and remove; and
 (7) they contain species that are typically regulated at low concentrations
 in drinking water (e.g., 5 .mu.g/L).
 Because of the above characteristics, remediation of subsurface
 contamination resulting from DNAPLs is a frequently encountered problem,
 which has proven to be extremely difficult. Methods that have been used to
 remove such contamination include: pump-and-treat, cosolvent flushing,
 surfactant flushing, steam injection, and in situ biodegradation. To
 varying extents, all of these strategies have been implemented at the
 laboratory and pilot or full field scale. Each of these methods results in
 the removal of some solute mass from a system contaminated with a DNAPL,
 but the rate at which the removal occurs and the expense involved with
 this standard set of methods leaves the problem of DNAPL remediation
 unsolved. The advantages and disadvantages of the common set of DNAPL
 removal and containment strategies are described in detail as follows.
 Pump-and-Treat
 Pump-and-treat (PAT) is perhaps the most common method of subsurface
 restoration. The method consists of installing a well in a region
 contaminated by a DNAPL and pumping it, which results in the induction of
 a flow of groundwater in all directions toward the extraction well for
 some local volume in the vicinity of the well called the capture zone. de
 Marsily (1986); Domenico and Schwartz (1990). As the groundwater passes
 through a capture zone that is contaminated with DNAPL, a portion of the
 DNAPL will dissolve into the groundwater and be transported with it. If a
 PAT scheme is continued for a sufficient period of time, all of the DNAPL
 present in the capture zone will be solubilized and exit the system
 through the pumping well. Mayer and Miller (1996). In theory, placement of
 a sufficient number of adequately designed pumping wells and appropriate
 operation for a sufficient period of time will result in the removal of
 all DNAPL contaminants from a contaminated system.
 Once these contaminants are brought to the surface from the wells,
 treatment of the contaminated waste stream is accomplished and the water
 is either injected back into the subsurface or disposed of using other
 means. The actual treatment processes used depends upon the
 characteristics of the waste stream, treatment objectives, discharge
 stream quality constraints, and other factors.
 The PAT method is appealing because of its simplicity. However, experience
 has shown that this approach is not very efficient, where efficiency is
 defined as the mass of DNAPL removal per volume of fluids removed. Mackay
 and Cherry (1989). This lack of efficiency results from the upper bound on
 the mass removal rate that results from the solubility of the DNAPL in the
 water phase, and because the water removed from the pumping wells may be
 far removed from the solubility limit of the DNAPL in the fluid phase. The
 solubility of the DNAPL forms the upper bound on the mass removal rate
 because capillary forces that trap a DNAPL are typically not overcome by
 the viscous forces induced by pumping; thus, mobilization of free-phase
 DNAPL typically does not occur in PAT operations. Further, if mobilization
 does occur, viscous forces must be greater than the sum of both capillary
 and gravity forces if free phase capture by a pumping well is to occur.
 Because efficiency is low for PAT methods and cleanup standards for DNAPLs
 are often stringent, PAT remediation is ineffective as a means of
 remediating DNAPL-contaminated groundwater over time scales of even years.
 For this reason, PAT methods have been classified in some instances as a
 contaminant containment strategy, rather than as a restoration method.
 PAT methods are inefficient not only because they do not mobilize the DNAPL
 into a capture well, but also because DNAPL distribution patterns in
 typical heterogeneous subsurface systems results in the formation of
 regions of high saturations of DNAPLs called pools. These pools in turn
 block the pore space of porous media, deflecting groundwater flow around
 the DNAPL zone. Since contaminant removal occurs only through mass
 transfer from the DNAPL to the aqueous phase, the surface area that exists
 between the groundwater and DNAPL, which is relatively small, results in a
 significant mass transfer limitation for most systems, which is manifest
 as concentrations of dissolved contaminants that are far removed from
 equilibrium values.
 Vapor Extraction
 Vapor extraction (VE) is a frequently used method of remediation in which a
 well (or wells) are installed into the unsaturated zone and a vacuum is
 applied to the well(s). Walter (1994); U.S. Army Corps of Engineers
 (1995). Much like PAT, a pressure gradient is established in a local area
 around the well, which results in gas-phase flow toward the well. If the
 contaminant of interest is present in the gas phase, then removal of the
 contaminant occurs as the gas phase is removed from the system. Because
 much less energy is required to extract a volume of gas from the
 subsurface compared to an equivalent volume of water, VE has the potential
 to be more efficient than PAT methods. Ross and Lu (1994); Travis and
 Macinnis (1992).
 In order to realize the potential efficiency of VE methods, a contaminant
 must be present in the gas phase. If the contaminant originally enters the
 subsurface as a NAPL, then volatilization to the gas phase must occur for
 all species of interest, thus VE is only a potentially viable technology
 for relatively volatile species. She, Sleep etal. (1995). In addition,
 volatilization must occur at a sufficiently rapid rate such that the
 gas-phase concentrations are sustained at a relatively high level compared
 to their vapor pressure. If this does not occur, then mass removal rates
 are said to be mass transfer limited. VE systems are often mass transfer
 limited, removing contaminants with a concentration that is much less than
 the vapor pressure of the target species.
 A more important problem with VE exists for DNAPL contamination that exists
 in the saturated zone: a contaminant must undergo a series of mass
 transfer steps, first from the DNAPL phase to the water phase and second
 from the water phase to the gas phase. For contaminants to be removed from
 the system, they must volatilize from the top of the water table into the
 gas phase and be removed in the VE system. The dissolution of the DNAPL
 into the aqueous phase and transport to interface of the unsaturated zone
 requires a series of physical and chemical transport steps that in sum are
 usually much slower that volatilization of NAPL in the unsaturated zone
 alone. Additionally VE does not aggressively control the movement of a
 DNAPL in the saturated zone, where the majority of such contaminants are
 likely to reside. For this reason, PAT and VE are often used together--a
 procedure that typically increases the rate at which contaminants are
 removed from the subsurface compared to the use of either method alone.
 However, this overall rate of removal is still sufficiently slow as a
 result of mass transfer limitations that VE is not an efficient means of
 DNAPL remediation for most sites. Mccann, Boersma et al. (1994); Larkin
 and Hemingway (1991).
 An effort to reduce the mass transfer and transport limitations of VE has
 resulted in a related process known as in situ gas stripping (ISGS). lSGS
 is similar to VE except that a gas phase is injected into the saturated
 zone. Marley (1991). As the gas phase is transported vertically under
 gravity forces through the saturated zone and exits into the unsaturated
 zone, the hope is that an increased rate of DNAPL removal from the
 saturated zone results compared to VE. While ISGS can often be more
 effective than VE or VE and PAT at removing DNAPL contaminants from the
 saturated zone, an injected gas phase often follows preferential pathways
 under gravity-dominated forces and directly influences a volume within a
 relatively small distance of the gas-phase injection well.
 In Situ Biodegradation
 In situ biodegradation (IBD) is a process that encourages the establishment
 and growth of microorganisms that will transform contaminants originally
 present in a DNAPL into products that pose less of risk than the original
 contaminant. The reactants and products, microorganisms required, and
 relative rates at which such transformations occur depend upon the
 contaminant of concern, the electron acceptor conditions, and a variety of
 other conditions. Domenico and Schwartz (1990), Dupont (1993). Determining
 specific pathways, rates, and conditions that encourage such
 transformations for a wide variety of contaminants is an active area of
 research.
 In cases in which contaminants are transformed quickly into inoculous
 products, IBD can be an attractive process. However, many DNAPLs are
 comprised of species for which rates of transformation have been found to
 be relatively slow, especially for native microbial populations and redox
 conditions. For example, contaminants such as trichloroethylene (TCE) and
 perchloroethylene (PCE) have proven relatively resistant to rapid
 microbial transformations. Tsien, Brusseau et al. 1989; Semprini, Roberts
 et al. (1990). An additional, but familiar, limitation occurs with IBD
 processes: mass transfer and transport limitations. In order for
 transformations to occur, the contaminant must be dissolved into the
 aqueous phase and come into contact with an effective microbial population
 in the presence of needed nutrients and appropriate electron acceptors.
 Zacharias, Lang et al. (1995). For these reasons, IBD has not, in general,
 proven to be an effective means of remediating DNAPL-contaminated
 subsurface systems.
 Cosolvent Flushing
 Cosolvent flushing (CF) is one of a group of remediation methods that are
 often collectively termed enhanced remediation methods, which together
 form a set of approaches that are more aggressive and typically more
 effective than the conventional methods described above. The fundamental
 notion involved with enhanced remediation methods is to take aggressive
 actions that markedly affect the mass removal rates of NAPL contaminated
 regions, either through increasing rates of mass transfer, Brandes and
 Farley (1993), or through mobilization, Larson, Davis et al. 1982, Roeder,
 Brame et al. (1996), of previously trapped NAPL residual contaminants.
 CF is implemented by flushing a contaminated region with a mixture of water
 and alcohol(s). Vancho (1994); Imhoff, Gleyzer et al. (1995b). The
 flushing solution is designed such that the NAPL contaminant is much more
 soluble in the cosolvent solution than it is in an aqueous solution alone,
 increasing the potential rate of removal. If present as a sufficient
 fraction of the flushing phase, cosolvent systems can completely
 solubilize a quantity of NAPL that they come in contact with--reducing a
 two-fluid-phase immiscible contamination problem to a single-fluid-phase
 miscible displacement problem. Such methods have worked relatively well in
 laboratory experiments, and these methods have been tried in the field as
 well. Rao, Annable et al. (1997).
 Two potentially significant drawbacks exist for CF approaches: cost and the
 heterogeneous distribution of NAPL in the subsurface. Alcohols are
 expensive in the quantities typically needed for in situ remediation of
 DNAPL contamination. This cost and the need to use relatively high
 fractions of alcohol for many DNAPL-alcohol combinations of potential use
 presents a considerable economical challenge, which for most cases will
 result in the need to recapture and reuse the alcohol solutions. This
 recapturing process will require the use of some sort of separation
 facility to separate contaminants, water, and alcohols into their
 component parts. The large volumes of alcohols needed and the complexity
 and expense of their recovery poses significant limitations to the
 widespread use of CF.
 In addition, as previously described, DNAPL contamination of subsurface
 systems often results in complex patterns of DNAPL distributions,
 including pooling of DNAPLs on low permeability materials. These pools
 attain length scales and distributions that result in accumulation in
 coarse-grained materials and pore openings that impede the flow of water,
 or a cosolvent mixture, through the DNAPL contaminated region. Under such
 circumstances, the flushing solution will largely bypass the
 DNAPL-contaminated zone and mass removal will occur through dissolution
 along the edges of the DNAPL pool. This is another sort of mass transfer
 limitation which can significantly increase the number of pore volumes of
 flushing needed to clean up a contaminated region compared to the case
 with an equivalent volume of DNAPL that is uniformly distributed
 throughout a region. As the number of pore volumes of flushing needed to
 meet a remediation target increases, the time and expense involved with CF
 remediation increases as well. The economics of CF are an important
 limitation and constraint involved with the method.
 Surfactant Flushing
 Surfactant flushing (SF) is another method in the class of enhanced
 remediation methods, which is similar in many respects to CF, including
 similar advantages and limitations. Similar to cosolvents, surfactants can
 significantly increase the solubility of a DNAPL in an aqueous solution
 and, under certain conditions, can result in the formation of a single,
 stable fluid phase that contains a mixture of water, surfactant, and
 DNAPL. Sharma and Shah (1989); Pope and Wade (1995); Shiau, Sabatini et
 al. (1996). Surfactants can significantly reduce the interfacial tension
 (IFT) that exists between a DNAPL and an aqueous phase, often by orders of
 magnitude under appropriate conditions. Reductions in IFT reduce capillary
 forces and encourage the mobilization of DNAPLs that were previously held
 immobile in a state of residual saturation. Pennell, Pope et al. (1996).
 When such mobilization occurs, rapid and efficient removal of DNAPLs can
 result--often removing the majority of DNAPL residual within a few volumes
 under controlled laboratory conditions. Willson, Hall et al. (1998).
 While surfactant methods show promise, Okuda, Mcbride et al. (1996), a
 number of important challenges arise, which have not been routinely
 overcome as yet. Pennell, Abriola et al. (1996). First, surfactants are
 expensive, so it is important to have a process that is efficient in terms
 of the number of pore volumes that must be flushed. It is also usually
 necessary for economic reasons to implement a separations strategy to
 capture and reuse the surfactant solution, which adds complexity and
 expense to the use of the process. Lipe, Sabatini et al. (1996). Second,
 reductions in IFT can mobilize a DNAPL, but if the viscous forces are not
 sufficient to overcome gravity forces, the DNAPL may be transported
 vertically, potentially spreading the contamination to previously
 uncontaminated portions of a subsurface system. Longino and Kueper (1995);
 Mason and Kueper (1996). Third, phase behavior for surfactant solutions is
 affected by a variety of chemical properties, including surfactant
 fraction in solution, pH, and ionic strength, and controlling the set of
 these factors to affect the desired behavior in the subsurface can be a
 challenge. Fourth, heterogeneous distributions of DNAPLs, especially
 pools, can lead to flow bypassing and mass transfer limitations that can
 greatly increase the number of pore volumes of a flushing solution needed
 to reach a given DNAPL removal fraction.
 Steam Injection
 Steam injection (IS) is an enhanced remediation process that relies upon
 the injection of steam into the subsurface to convert the DNAPL present to
 a gas phase, which may be easily removed from the system. Forsyth (1994).
 This process can be effective at removing entrapped DNAPL, but several
 challenges remain. Sittler, Swinford et al. (1992). The process is
 typically implemented such that the steam must contact DNAPL, which
 suggest that water and DNAPL in the system are volatilized. As a minimum,
 the system temperature must be brought to and maintained at the boiling
 point of the DNAPL until the DNAPL gas has been removed from the system.
 Since subsurface systems tend toward thermodynamic equilibrium, this
 implies that the water and solid materials present are also heated to a
 similar minimum level--either the boiling point of DNAPL or water. The
 energy required to accomplish this can be substantial, especially for
 cases in which the DNAPL has penetrated deeply into the saturated zone. A
 further complication is that when the DNAPL is heated a variety of
 physiochemical properties are altered, including surface tension, density,
 viscosity, and diffusion coefficients. For regions of large DNAPL
 saturation, such changes ahead of a steam front can mobilize a DNAPL by
 changing the balance of forces and allow the DNAPL to penetrate deeper
 regions of an aquifer.
 Reactive Walls
 Reactive walls (RW) are a technology that has evolved rapidly over the last
 few years. This technology is implemented by placing a barrier in the
 subsurface that directs the groundwater flow through specific regions in
 the construct. These regions are comprised of an appropriate material that
 facilitates the transformation of contaminants that are present in the
 groundwater phase. Groundwater then exits the wall free of the target
 contaminant and not a threat to potential down-gradient users of the
 resource. Kaplan, Cantrell et al. (1996). The material that such walls are
 constructed from varies depending upon the application. Zero-valent iron
 is a popular choice for sites that are contaminated with DNAPLS such as
 TCE and PCE. In some cases, RW are used in a region to establish and
 promote biological growth and transformation of the target contaminants of
 interest.
 There are some drawbacks to RW, the most obvious being that the DNAPL
 contaminated region is not affected by the method, only the contaminant in
 the dissolved phase. Because such walls are constructed typically under
 natural gradient conditions, their period of operation will be affected by
 the same set of mass transfer limitations as the PAT method, but over a
 longer period of time since DNAPL removal rates will be mass transfer and
 transport limited. Thus, RW for DNAPL contamination must be operative over
 periods of time similar to or longer than PAT methods, which can range to
 times over a century. In addition, RW can be expensive to construct and
 difficult to maintain, especially when the potentially large depths of
 DNAPL penetration are encountered and the long periods of maintenance
 needed are considered.
 Because each of these approaches has significant limitations, recent
 efforts have centered on containment or treatment of the groundwater that
 is contaminated by the DNAPL. However, such efforts are deemed
 unsatisfactory in that they treat a symptom of the subsurface
 contamination problem, as opposed to the problem itself.
 What is needed is a method to effectively, quickly, and economically
 remediate subsurface systems that are contaminated with DNAPLs. Such a
 method is currently unavailable in the art.
 DISCLOSURE OF THE INVENTION
 Disclosed herein is a method for remedying contamination of a subsurface
 environment by a non-aqueous phase liquid that is denser than water
 (DNAPL). The subsurface environment comprises a resident aqueous phase and
 a DNAPL phase, and the method includes the steps of introducing to the
 subsurface environment a dense aqueous solution that has a density greater
 than the density of the DNAPL phase present in the subsurface environment;
 displacing the resident aqueous phase with the dense aqueous solution,
 thereby causing the DNAPL phase to rise above the greater density aqueous
 solution; and recovering the DNAPL phase and thereby substantially
 remedying contamination of the subsurface environment.
 Also disclosed herein is an alternative method for remedying contamination
 of a subsurface region by a non-aqueous phase liquid (DNAPL) that is
 denser than water. The subsurface region comprises a resident aqueous
 phase and a DNAPL phase, and the method includes the steps of introducing
 to the subsurface region a dense solution layer (DSL) that has a density
 greater than the density of the DNAPL phase beneath the surface region
 contaminated with the DNAPL phase; dewatering downwardly to the top of the
 dense solution layer (DSL) so as to create an unsaturated porous medium
 above the dense solution layer (DSL) and to downwardly mobilize the DNAPL
 phase to the top of the dense solution layer (DSL); and collecting the
 DNAPL phase from the top of the dense solution layer (DSL) and thereby
 substantially remedying contamination of the subsurface region.
 Accordingly, it is an object of the present invention to provide a method
 for remedying contamination of a subsurface environment by a DNAPL which
 provides for effective and economical cleanup of a contamination site.
 It is another object of the present invention to provide a method for
 remedying contamination of a subsurface environment by a DNAPL which does
 not promote downward migration of DNAPLs.
 It is yet another object of the present invention to provide a method for
 remedying contamination of a subsurface environment by a DNAPL which works
 in presence of DNAPL pools.
 It is a further object of the present invention to provide a method for
 remedying contamination of a subsurface environment by a DNAPL which
 provides for rapid cleanup times.
 It is still a further object of the present invention to provide a method
 for remedying contamination of a subsurface environment by a DNAPL which
 utilizes a non-toxic and inexpensive flushing solution.
 Some of the objects of the invention having been stated hereinabove, other
 objects will become evident as the description proceeds, when taken in
 connection with the accompanying drawings as best described hereinbelow.
 The file of this patent contains at least one photograph executed in color.
 Copies of this patent with color photograph(s) will be provided by the
 Patent and Trademark Office upon request and payment of necessary fee.

BEST MODE FOR CARRYING OUT THE INVENTION
 Contamination of subsurface environments by non-aqueous phase liquids that
 are denser than water (DNAPLs), such as chlorinated solvents, is an
 important environmental problem. Efforts to date to remediate such
 contamination have been only partially successful, because of the physics
 that controls the movement and trapping of such contaminants below the
 water table. In accordance with the present invention a novel approach for
 remediation of such systems that relies upon reversal of the density
 gradient between the aqueous phase and the DNAPL is provided. This is
 accomplished by increasing the density of the aqueous phase by introducing
 a dense aqueous solution, such as a salt solution, to the subsurface
 environment and displacing the resident aqueous-phase solution within the
 subsurface environment with the dense aqueous solution. This density
 gradient reversal effectively floats the DNAPLs to the top of the water
 table, where they can be easily recovered.
 To overcome trapping in fine-grained materials, surfactant solutions can be
 added. Optionally, the surfactant solution can be introduced to the
 subsurface environment as a first or initial step of the method of the
 present invention. Alternatively, the surfactant solution can be
 introduced to the subsurface environment after the introduction of a first
 volume of the dense aqueous solution in accordance with the method of the
 present invention. After density-enhanced displacement, the remaining
 small amount of DNAPL present can be readily removed using conventional
 enhanced-remediation methods, such as cosolvent or surfactant flushing,
 with a relatively small volume of flushing solution. Density-enhanced
 remediation of DNAPLs approach is shown herein to be an effective and
 efficient approach for restoration of contaminated subsurface
 environments.
 The method of the present invention particularly addresses the problems
 observed in prior art methods with respect to NAPL entrapment. To
 elaborate on these problems, NAPL entrapment results in a static
 distribution of an organic liquid in the subsurface. The static state
 results from the release of DNAPLs into typical subsurface environments.
 Once present in the subsurface, NAPLs follow complex patterns of flow,
 which are influenced by the porous medium and fluid properties manifest as
 gravity, viscous, and capillary forces. Since LNAPLs are less dense than
 water, they reside in the unsaturated zone and generally above the water
 table. DNAPLs tend to move vertically under gravitational forces and, if
 present in sufficient quantities to exceed the capillary entry pressure of
 the saturated zone, they will penetrate the water table. Schwille (1988).
 Continuing with the problems associated with NAPL entrapment, capillary
 forces trap some fraction of a NAPL in the pore space, leaving behind a
 residual saturation present as a phase that may be continuous only over a
 single pore space or small set of connected pores. The distribution of
 residual NAPL is influenced by the morphology of the pore space, the
 wettability of the solid phase with respect to the fluids occupying the
 pore space, the density and interfacial tension of the NAPL phase, the
 velocity of the aqueous phase, and the boundary conditions associated with
 the aqueous phase and NAPL release. Pennell, Pope et al. (1996).
 The distribution of DNAPL in the subsurface plays a crucial role in
 determining the difficulty associated with removing it from the
 subsurface. NAPLs present in the unsaturated zone above the water table
 are often the intermediate wetting phase, with the gas phase occupying the
 larger pore sizes and water occupying the smallest pore sizes. Below the
 saturated zone interface, DNAPLs are usually assumed to be the non-wetting
 phase and tend to occupy the largest pore spaces.
 Consider a two-fluid-phase system consisting of an aqueous phase and a
 DNAPL phase that is non-wetting (i.e. non-aqueous), and a solid phase. In
 a homogeneous porous media, the pore size distribution is uniform in
 space. Since DNAPL residual saturation is often expressed as a function of
 funicular DNAPL saturation, homogeneous regions exposed to similar maximum
 concentrations of funicular DNAPL saturation would be expected to have
 similar levels of residual DNAPL saturation. However, natural systems are
 heterogeneous by nature, many markedly so. In heterogeneous systems, the
 largest pore sizes are distributed non-uniformly in the subsurface, which
 in turn leads to a non-uniform distribution of DNAPL residual. In
 heterogeneous finite entry pressure media, stable DNAPL pools of
 relatively high saturation can form in coarse-grained regions of the
 system. These pools have length scales that are consistent with the length
 scales of the porous media, which may be from centimeters to meters. Pools
 also form in depressions in hydraulic barriers, such as clay layers, that
 form the bottom boundary of a permeable hydraulic unit. Michalski, Metlitz
 et al. (1995). Because pools can have high DNAPL saturations, pooled
 regions are typically relatively impermeable to the flow of the wetting
 phase, which leads to mass transfer limitations for any method requiring
 phase change. Because of this, NAPL pools are the dominant feature
 limiting restoration of a DNAPL contaminated porous media. Whelan,
 Voudrias et al. (1994); Pearce, Voudrias et al. (1994).
 As detailed above, efforts to remediate DNAPL contaminated subsurface
 systems have generally failed for a number of reasons:
 (1) DNAPLs tend to move below the saturated zone and are difficult to
 locate;
 (2) DNAPLs form pools that can lead to mass transfer limitations and
 downward mobilization;
 (3) existing methods require many pore volumes of flushing, with the actual
 number increasing as a function of the heterogeneity of the system
 increases; and
 (4) existing enhanced methods of subsurface remediation are expensive.
 To overcome these limitations with existing methods, in accordance with the
 present invention a novel method of DNAPL remediation of saturated porous
 media is provided: density-enhanced remediation of DNAPLs (DERD). The
 basic notion behind DERD is straightforward: the wetting phase density is
 modified so that trapped DNAPLs become significantly less dense than the
 aqueous phase. This leads to gravity forces acting upward on the DNAPL
 phase and motivates the upward migration of DNAPL toward the water table.
 If the wetting (i.e. aqueous) characteristics of the aqueous phase are not
 modified, the tendency will be for the DNAPL to migrate through the larger
 pore openings, precisely the class of pore sizes favored during the
 downward migration phase that contaminated the media.
 Because DERD is a mobilization approach, mass transfer limitations are
 avoided. Also, unlike other enhanced remediation methods, mobilization
 caused by DERD must be in an upward direction--avoiding the potential
 problem of contaminating deeper, previously uncontaminated portions of an
 aquifer during a restoration effort.
 The density modifications of the aqueous phase that are needed can be
 easily and economically accomplished using a variety of means, such as the
 addition of salt or sugar solutions in either single or multi-component
 form to a create a dense aqueous solution. Other mixtures that form
 wetting phases that are denser than the target DNAPLs are possible as
 well, but the exact choice of the solution is secondary and the optimal
 choice will depend upon local geochemistry conditions, the DNAPL of
 concern, and economic considerations related to the cost of the compounds
 added to the wetting phase. As an example of densities that are readily
 achievable, Table 1 lists the properties of some simple, common,
 single-component salt solutions. These properties can be compared to
 properties of some common DNAPLs that are listed in Table 2. This shows
 that a wide variety of choices exist to achieve densities of an aqueous
 solution that are significantly greater than typical DNAPLs and which will
 accomplish the objective of DERD.
 TABLE 1
 Densities of Single-Component Salt Solutions
 temp. g/100 g density
 Name formula (deg. C) sat. soln. g/cm.sup.3
 ammonium iodide NH.sub.4 I 25 64.5 1.646
 ammonium nitrate NH.sub.4 NO.sub.3 25 68.3 1.320
 barium iodide BaI.sub.2 --7.5H.sub.2 O 25 68.8 2.277
 barium bromide BaBr.sub.2 20 51.0 1.710
 barium chlorate Ba(ClO.sub.3).sub.2 25 28.5 1.294
 barium chloride BaCl.sub.2 20 26.3 1.27
 barium perchlorate Ba(ClO.sub.4).sub.2 25 75.3 1.936
 calcium bromide CaBr.sub.2 20 58.8 1.82
 calcium chloride CaCl.sub.2 --6H.sub.2 O 25 46.1 1.47
 calcium iodide CaI.sub.2 20 67.6 2.125
 lithium bromate LiBrO.sub.3 18 60.4 1.830
 magnesium MgBr.sub.2 --6H.sub.2 O 18 50.1 1.655
 bromide
 magnesium iodide MgI.sub.2 --8H.sub.2 O 18 59.7 1.909
 potassium chloride KCl 25 26.5 1.178
 potassium citrate KC.sub.6 H.sub.5 O.sub.7 25 60.9 1.514
 potassium iodide KI 25 59.8 1.721
 potassium formate KCHO.sub.2 18 76.8 1.571
 sodium bisulfate NaHSO.sub.4 --H.sub.2 O 25 59.0 1.47
 sodium bromide NaBr--2H.sub.2 O 25 48.6 1.542
 sodium chlorate NaClO.sub.3 25 51.7 1.440
 sodium chloride NaCl 25 26.5 1.198
 sodium hydroxide NaOH 25 50.4 1.51
 sodium iodide NaI 25 64.8 1.919
 sodium perchlorate NaClO.sub.4 25 67.8 1.683
 sodium tungstate NaWO.sub.4 --10H.sub.2 O 18 42.0 1.573
 sucrose C.sub.12 H.sub.22 O.sub.11 25 67.89 1.340
 TABLE 2
 Densities of Common DNAPLS
 DNAPL Density
 carbon tetrachloride 1.59
 o-dichlorobenzene 1.31
 m-dichlorobenzene 1.29
 1,1-dichloroethane 1.17
 1,2-dichloroethane 1.26
 1,1,1-trichloroethane (1,1,1-TCA) 1.35
 1,1,2-trichloroethane (1,1,2-TCA) 1.44
 1,1-dichloroethylene 1.22
 trichloroethylene (TCE) 1.46
 tetrachloroethylene (PCE) 1.63
 Standard Application
 As an example, DERD can be implemented in the following fashion:
 (1) locate the approximate region of the DNAPL contamination;
 (2) bound the region with wells that will be used for injection of a dense
 wetting-phase solution;
 (3) install a shallow pumping well in the center of the region which
 penetrates the upper portion of the saturated zone;
 (4) begin pumping a rate sufficient to induce a gradient that captures the
 flow from the target region--running the effluent through a suitable
 treatment system;
 (5) inject a dense wetting-phase solution at the set of bounding wells at a
 depth that bounds the vertical extent of the contamination, which will
 result in the displacement of the native water with the injected solution;
 (6) maintain the pumping and injection pattern until free-phase DNAPL is no
 longer collected from the pumping well;
 (7) discontinue injection of the dense wetting-phase solution and maintain
 pumping to capture the residual injected solution and remove it from the
 subsurface; and
 (8) remove the remaining residual DNAPL by conventional methods, such as
 flushing through the installed wells with a surfactant or cosolvent
 solution.
 It is important to note that many modifications of this scheme are possible
 within the general concept of density-enhanced displacement. Aspects that
 might vary include: the method and pattern used to deliver the dense
 wetting-phase solution, the method and pattern used to withdraw the
 flushing and resident fluids (e.g., horizontal or vertical wells, drains),
 the rates of injection and withdrawal, the precise properties of the
 injected fluid, and the choice and implementation of schemes to remove the
 remaining residual left behind after the gravity enhanced displacement
 process phase. It is also envisioned that vapor extraction might be useful
 during a DERD process for some applications.
 Thus, in application, it is contemplated that the DERD method of the
 present invention can assume a large number of forms, with subsurface
 conditions and other factors affecting sound engineering design at any
 given location. This is the usual case for any remediation method applied
 in the field. Exemplary techniques for applying remediation techniques in
 the field can be found in U.S. Pat. Nos. 5,753,122 (IBD process) and
 5,449,251 (SI process), the entire disclosure of which are herein
 incorporated by reference.
 Heterogeneous Conditions
 When subsurface conditions are moderately to highly heterogeneous,
 modifications to the basic DERD scheme outlined above may prove useful.
 Under such conditions, trapping of DNAPL can occur by contact with
 relatively fine-grained, and small pore size, materials overlying a region
 of entrapped DNAPL. That is, as the dense wetting-phase solution contacts
 the trapped and pooled DNAPL, gravity forces will act to cause it to move
 upward. However, if these forces are insufficient to move the DNAPL
 through a fine-grain region, because of capillary considerations, the
 DNAPL can remain trapped. To overcome such situations, the basic DERD
 scheme outlined above can be modified by following a dense-wetting phase
 solution injection step with the injection of surfactant solution followed
 in turn by a second dense wetting phase solution injection step. The
 surfactant will lower the IFT of the DNAPL, allowing it to move into and
 through finer grained materials, but it will also be brought to the
 surface by gravity forces from the dense wetting-phase solution that
 resides below it. Residual DNAPL left in fine-grained regions after the
 surfactant and second dense wetting-phase solutions are flushed from the
 system can be removed by flushing with a conventional surfactant or
 cosolvent solution. As before, various modifications of this basic scheme
 are possible.
 The following Examples have been included to illustrate preferred modes of
 the invention. Certain aspects of the following Examples are described in
 terms of techniques and procedures found or contemplated by the present
 inventor to work well in the practice of the invention. These Examples are
 exemplified through the use of standard laboratory practices of the
 inventor. In light of the present disclosure and the general level of
 skill in the art, those of skill will appreciate that the following
 Examples are intended to be exemplary only and that numerous changes,
 modifications and alterations can be employed without departing from the
 spirit and scope of the invention.
 EXAMPLES
 The DERD method outlined above has been investigated in a set of laboratory
 experiments with encouraging results. Briefly, these experiments emplaced
 a dyed TCE solution into a heterogeneous porous media under saturated
 conditions in a thin, essentially two-dimensional glass cell packed with
 glass beads. Visual observations of the DNAPL distribution were possible
 at all phases of the investigation. Following emplacement, the aqueous
 phase was displaced with a dense salt solution by injecting in an up-flow
 mode. Rapid and substantial displacement of the trapped and pooled DNAPL
 occurred, with a substantial amount of the DNAPL readily captured at the
 surface of the cell. This displacement occurred after a single pore volume
 of the salt solution was added and captured an estimated about 65% of the
 trapped DNAPL from this highly heterogeneous system. The remaining portion
 could have been easily removed using a combination of a surfactant
 followed by a dense salt solution flush to remove vertically trapped
 regions of high saturation, and a phase of conventional surfactant or CF.
 This Example pertains to the use of a dense brine flushing solution to
 remove pooled DNAPLS from the subsurface. Experimental parameters
 including the dimensionality, media, packing geometry, and fluids were
 chosen to represent conditions similar to those commonly encountered in
 the field. The results clearly demonstrate the basic removal mechanism of
 the method of the present invention. Further, the results indicate that,
 with some minor modifications within the skill of the art, similar
 density--enhanced flushing strategies are feasible for many real-world
 DNAPL remediation scenarios.
 Experimental Setup
 A bench-scale experimental cell was constructed to qualitatively test the
 density-enhanced flushing method of the present invention. The dimensions
 of the cell and properties of the media and chemicals used in this
 experiment are detailed below.
 To represent field conditions involving DNAPL pools, an essentially
 two-dimensional flow cell was chosen rather than common but essentially
 one-dimensional laboratory columns. The construction and geometry of the
 cell are described in Table 3 and are essentially the same as that
 described in Thyrum (1994) and in Imhoff, Thyrum et al. (1996), as it is
 the same frame with some slight modifications.
 TABLE 3
 Details of the Experimental Flow Cell
 Parameter Value
 Construction:
 frame aluminum with stainless steel and brass fittings
 windows plate glass
 gasket neoprene rubber
 screens stainless steel
 Internal dimensions:
 sand width 20.0 .+-. 0.2 cm
 sand height 15.0 .+-. 0.2 cm
 sand thickness 2.0 .+-. 0.5 mm
 reservoirs 0.5 .times. 0.5 .times. 7.75 in
 Porous media:
 coarse Quackenbush Company, "Q-bead", 0.8-mm glass
 beads
 fine Cataphote MILG-9954A size 4, $&gt;0.25$-mm glass
 beads
 very fine Cataphote MIL-G-9954A size 10, 0.125 &lt; d.sub.50 &lt;
 0.150 mm glass beads
 The cell was packed with three different kinds of glass beads. The first
 was a very fine glass bead (size 10) that was used only along the sides of
 the cell within one-half inch of the gasket. It was placed there to keep
 free-phase DNAPL from contacting and destroying (dissolving) the gasket
 material. The fine (size 4) glass beads were used to form the bulk of the
 domain and they correspond to a relatively fine sand.
 The third glass bead (nominally 0.8-mm diameter) was used to create the
 coarse inclusion. The average size of these beads corresponds to a coarse
 sand. It was used as it has an appreciably lower entry pressure than the
 finer beads and, when used with them, could be expected to readily form
 stable TCE pools on the order of 15-cm in height. This is clearly
 sufficient for the size of the domain chosen.
 At the top and bottom of the sand domain, a stainless steel screen was used
 in conjunction with stainless steel wool. The screen acted as a porous
 barrier that would allow fluids to easily pass through while retaining
 even the finest of the glass beads. The stainless steel wool was used as a
 porous support for the relatively flimsy screen material. A stainless
 steel needle was inserted through the top screen as a means of injecting
 DNAPL into the domain.
 The DNAPL and the flushing solutions that were used are described in Table
 4. All water used in the experiment was de-ionized and then de-aired (DDI)
 using a vacuum pump. The trichloroethylene (TCE) used as a DNAPL was an
 analytical grade reagent and was subsequently dyed with less than 0.01% by
 weight Oil-Red-O (ORO) to give it a red color and makes it much more
 visible in FIGS. 1 and 2 attached hereto. The ORO dye has little or no
 effect on the physical properties of the TCE and will not partition into
 the aqueous phase.
 TABLE 4
 Fluid Properties
 p @ 20.degree. C. .mu. @ 20.degree. C.
 Fluid (g/cc) (cp)
 trichloroethylene (TCE) 1.4642 0.57
 DDI water 1.0 1.0
 NaI solution (58% by weight NaI) 1.7518 2.095
 NaI/Aerosol AY-OT mixture 1.6675 -ND-
 Experimental Procedure
 The following is a description of the sequence of events. Time-lines that
 correspond to still photographs that were taken during the experiment, as
 presented in FIGS. 1 and 2, are provided in Tables 5 and 6. The times
 recorded in FIGS. 1 and 2 and in Tables 5 and 6 are "wall-times" in
 hr:min:sec relative to the time at which DNAPL was first injected into the
 domain.
 After creating the packing, air in the cell was displaced with carbon
 dioxide. The cell was then flushed with DDI water in an up-flow fashion to
 displace and/or dissolve all of the carbon dioxide gas within the cell.
 This procedure resulted in a domain completely saturated with DDI water.
 The cell was then flushed with more than ten pore volumes of DDI water to
 clean (rinse) it and ensure that all gas bubbles were gone.
 A syringe was used to slowly inject the DNAPL (TCE with ORO) into the
 domain. When a pool of desired dimensions had formed, the TCE injection
 ceased. The TCE was allowed to reach a static fluid configuration by
 letting it sit overnight.
 The next morning, the Nal solution was pumped into the domain in an up-flow
 configuration. Since the Nal solution was much more dense than water (see
 Table 4), it was a stable displacement scenario and the experiment clearly
 demonstrated a sharp and level front between the DDI water and the
 advancing flushing front. At the front, a significant fraction of the
 total TCE mass collected and floated out of the sand domain. This is
 clearly shown in the still images presented in FIGS. 1 and 2.
 When the flush was finished, the TCE removed from the sand was extracted
 from the upper reservoir of the flow cell using a second syringe. The cell
 was once again flushed--this time with a down-flow current of DDI water.
 The DDI water was used to remove the Nal solution from the sand and to see
 what would happen to the TCE that remained. The TCE left within the domain
 (especially in the lower left-hand comer, where the most TCE remained) did
 shift downwards slightly during the DDI water flush which indicated that a
 small TCE pool was still present.
 TABLE 5
 Sequence of Events - Simulation of Contamination
 Date/Time Description
 Day 1-Day 3 The "Thyrum" cell was packed with glass
 beads. The pattern was chosen as it was a
 "clearly heterogeneous" packing. The rear
 glass plate cracked after packing (during
 final tightening of the bolts--gasket
 compression). To keep the cell from
 leaking, the cracked rear glass plate was
 covered by an acrylic (Plexiglass (tm)) insert
 and a seal was made using Vaspar (a
 putty-like wax). The cracks are evident
 during the experiments but they are
 sufficiently tight that the DNAPL is not able
 to enter or pass through them. The cell was
 flushed with CO.sub.2 gas and then many pore
 volumes (&gt;10) of DDI H.sub.2 O to dissolve away
 all gas bubbles and wash the cell and glass
 beads.
 Day 5 DNAPL was injected. Initially, the cell is
 saturated with DDI H.sub.2 O and the DNAPL is
 injected, using a syringe, through the SS
 needle at the top of the domain.
 Day 5 00:00:00 DNAPL (TCE with ORO) injection starts.
 00:00:11 .+-.2.5 mL cumulative DNAPL injected.
 00:00:14 .+-.3.5 mL cumulative DNAPL injected.
 00:00:15.5 .+-.4.35 mL cumulative DNAPL injected.
 00:00:17 .+-.5.6 mL cumulative DNAPL injected.
 00:00:19 .+-.6.45 mL cumulative DNAPL injected.
 00:00:21 .+-.7.35 mL cumulative DNAPL injected.
 00:00:22 DNAPL injection complete: .+-.7.35 mL total.
 The DNAPL clearly formed a single pool of
 high (&gt;50%) saturation with some residual
 DNAPL trapped near the point of injection.
 TABLE 6
 Sequence of Events - Remediation of Contamination
 Date/Time Description
 Day 6 DNAPL is allowed to reach static conditions
 overnight. Cell leaked slightly more than
 10 mL of water over night. However, no air
 entered the domain due to the
 constant-head water reservoirs connected
 at both ends.
 Day 7 22:15 Brine solution is injected into the domain
 from the bottom using a constant-head
 marriot. Brine solution first fills the lower
 reservoir (8.0 by 0.5 by 0.5 in) and the
 difference in densities is clearly visible due
 to diffraction.
 22:22 Brine begins to enter the glass beads from
 the bottom and is visible as it darkens the
 domain--NaI solution is clear yellow in color.
 22:24 Brine front has moved half-way through the
 domain.
 22:26 Brine reaching top of top of glass beads and
 TCE flotation is clearly evident.
 22:27 All the DNAPL that is removed by flotation
 is, at this point, removed from the glass
 beads.
 22:28-22:34 NaI solution fills the upper reservoir (8.0 by
 0.5 by 0.5 in).
 22:50-23:54 DNAPL is removed from the domain using
 a syringe. Unfortunately, a mass balance
 was not possible due to evaporative and
 other losses during the DNAPL extraction
 procedure.
 23:56 Start of reverse flush of DDI water to
 remove brine solution.
 24:02 Roughly half of the brine solution has been
 flushed from the domain.
 24:15 Essentially all of the brine solution has been
 flushed away. Figure 2 shows DNAPL
 entrapment caused by the geometry of the
 packing.
 From observation and image analysis of the bench-scale flow experiment
 performed during the period described in Tables 5 and 6 above, the
 following results are noted.
 (1) Density-enhanced approaches can remove a significant fraction of the
 DNAPL present in pools. Image analysis showed that roughly 65% of the TCE
 was removed in this experiment.
 (2) Density-enhanced approaches can accomplish rapid removal--only one pore
 volume of flushing solution is required.
 (3) While capillary trapping may limit the effectiveness of this method,
 the addition of a surfactant could drastically reduce trapping and result
 in a much higher percent removal.
 DNAPL contamination of subsurface systems is an important problem that
 remains largely unsolved. Existing methods to remediate subsurface systems
 suffer from a variety of limitations, which in most cases are amplified by
 heterogenous conditions that exist in virtually all natural systems. A new
 method, density-enhanced remediation of DNAPLs (DERD), is introduced as an
 alternative to existing technologies. A standard approach and some typical
 modifications to the DERD approach are outlined in sufficient detail to
 allow the implementation of the method at the field scale by one skilled
 in the art. DERD is an effective and economical method for restoration of
 DNAPL-contaminated subsurface systems, and some of the relative advantages
 over prior art alternative techniques are set forth in Table 7 below.
 TABLE 7
 Overview of Cleanup Strategies - Strengths and Weaknesses
 PAT
 (1) not likely to promote downward migration of DNAPLs
 (2) high operating and maintenance costs
 (3) very slow--essentially perpetual treatment required
 CF
 (1) potentially rapid cleanup of some sites
 (2) may cause further contamination by downward migration
 (3) expensive flushing solution
 (4) potentially toxic flushing solution
 SF
 (1) potentially rapid cleanup of some sites
 (2) may cause further contamination by downward migration
 SI
 (1) potentially rapid cleanup of some sites
 (2) unsuited to some (especially deep) spill scenarios
 (3) expensive process (large energy input)
 RW
 (1) low operating and maintenance costs
 (2) unsuited to some (especially deep) spill scenarios
 (3) very slow--essentially perpetual treatment required
 DERD
 (1) potential for rapid cleanup of all sites
 (2) not likely to promote downward migration of DNAPLs
 (3) works in presence of DNAPL pools
 (4) fast cleanup times
 (5) non-toxic flushing solution
 Alternative Embodiment of Invention
 As discussed previously, restoration of environmental systems contaminated
 with hazardous wastes is a costly problem, averaging about $9 million per
 site in 1996 with an expected $750 billion in restoration remaining to be
 accomplished over the next 30 years in the United States alone. Oppelt
 (1999). Contamination of subsurface environments by non-aqueous phase
 liquids that are denser than water (DNAPLs), such as chlorinated solvents,
 is an important subset of this overall restoration problem because such
 contaminants are believed to be a significant risk to human health when
 present at relatively low concentrations in the aqueous phase. Remediation
 of DNAPL-contaminated systems is among the most difficult problems in the
 broad field of environmental sciences, because DNAPLs are long-lived in
 the subsurface environment and extremely difficult to remove--so difficult
 that some researchers have opined that the removal of DNAPL sources is
 essentially a hopeless problem.
 Vigorous research efforts over the last two decades have continued in
 pursuit of an effective strategy. These efforts have included flushing a
 contaminated system with water, water-alcohol mixtures, water-surfactant
 mixtures, steam, and a gas phase. Significant efforts have also been
 expended to understand and promote biological transformations of DNAPLs,
 such as the chlorinated solvents tetrachloroethylene and
 trichloroethylene. Despite all of these efforts, an efficient and safe
 approach for removing DNAPLs; that is, one requiring relatively few pore
 volumes of an inexpensive flushing solution and presenting limited risk
 for spreading the contamination to previously uncontaminated portions of
 the system, has yet to be advanced and applied except as described and
 claimed herein.
 The second approach proposed by applicant's invention is as follows:
 1. a dense solution layer (DSL) is established underlying a region
 contaminated with a DNAPL;
 2. the DNAPL-contaminated region is dewatered down to the top of the DSL,
 creating an unsaturated porous medium above the DSL, which is termed the
 primary phase of treatment;
 3. DNAPL that is mobilized downward is collected from the top of the DSL;
 4. if needed, one or more pulses of a flushing solution, such as a
 surfactant or cosolvent, is flushed through the unsaturated zone and the
 flushing solution and any mobilized DNAPL is collected from the top of the
 DSL, which is termed the secondary phase of treatment; and
 5. a tertiary phase of treatment can optionally be applied to remove any
 remaining DNAPL from the unsaturated zone, which might, for example,
 consist of vapor extraction.
 The rationale with this approach is that the establishment of a DSL will
 provide a vertical barrier to restrict the movement of DNAPL into
 uncontaminated regions of the system. Upon dewatering, gravity forces
 acting on the DNAPL will be increased, resulting in a significant fraction
 of DNAPL migration in the vertical direction but only down to the DSL
 where it can be collected. Pulsing the unsaturated zone with a flushing
 solution, such as a surfactant, provides a means to significantly reduce
 the interfacial tension between the NAPL and aqueous phase, further
 mobilizing DNAPL down to the DSL where it can be collected. Because of
 interfacial tension reductions, such secondary treatment pulses can also
 result in the removal of water from the pore space, which has advantages
 for certain tertiary treatment phases. These pulses could be effective
 even when much less than a pore volume in magnitude. Under some situations
 where difficult conditions exist or a very high level of removal is
 desired, a tertiary treatment phase can be applied. For sufficiently
 volatile contaminants, vapor extraction can be an appealing approach.
 Many different solutions are possible to form the DSL including a wide
 range of salts, formate solutions, acetate solutions, and other fluid
 phases that have a density greater than that of the DNAPL. The actual
 solution used in any application will depend mostly upon the DNAPL
 characteristics, local geochemical conditions, and supplier costs. The
 selection of this solution is considered a typical aspect of site
 engineering, which is a part of any subsurface remediation method.
 An important variant of this approach that will be effective under certain
 hydrogeolocial conditions is to dewater the unsaturated zone, but not
 establish a DSL. Instead, a natural (e.g., silt, clay, or other low
 permeability material) or engineered barrier (e.g., slurry wall) could
 form an impermeable lower boundary and serve to confine the vertical
 migration of the DNAPL. Under such conditions, no DSL would be needed.
 Instead, the DNAPL could be collected via a layer of similar thickness to
 the DSL but consisting of a surfactant or cosolvent solution. The key
 concepts of the technology of mobilizing the DNAPL vertically by creating
 unsaturated conditions, pulsing a flushing solution through the
 unsaturated zone to encourage additional removal, collecting the DNAPL
 through a confined lower barrier and mobile fluid phase, and perhaps
 performing vapor extraction as a tertiary phase all would remain the same
 under this approach.
 Similar comments apply to the specification of the flushing solution used
 for the secondary treatment phase. Surfactants and cosolvents, such as
 water-alcohol mixtures, form two broad classes of approaches with many
 possible solutions existing from within these classes. Other flushing
 solutions and enhancements, such as thermal modification and steam
 injection, are possible. The key feature is that the secondary treatment
 is applied to the unsaturated media in such a way that primarily vertical
 flow occurs and the intent is to remove DNAPL trapped in the pore space
 and deliver it to the top of the DSL.
 The above approach will also work as well for less-dense than water
 non-aqueous phase liquids (LNAPLs), such as petroleum products. For this
 case, the density of the aqueous phase may be sufficiently high to impede
 the vertical migration of the NAPL phase, so the establishment of a DSL
 may not be necessary for some cases. Secondary and tertiary phases would
 apply for the case of LNAPLs in a manner analogous to the application for
 DNAPLs.
 EXPERIMENTAL INVESTIGATION
 Experimental Setup
 To investigate the practicality of this approach, a bench-scale
 experimentation cell was assembled to quantitatively test the new idea.
 The dimensions of the cell and properties of the media and chemicals used
 in this experiment are detailed below.
 To represent field conditions involving DNAPL pools, an essentially
 two-dimensional flow cell was chosen rather than a common but essentially
 one-dimensional laboratory column. The construction and geometry of the
 cell is described in Table 8 and is very similar to that described by
 Thyrum (1994) and Thyrum et al. (1996) as it is the same frame with some
 slight modifications.
 The cell was packed with three different sands. The fines sand (F-52) was
 used only along the sides of the domain to act as a capillary barrier to
 hinder the DNAPL (TCE) from contacting the gasket material. This was done
 to prevent the TCE from swelling, rupturing, or otherwise compromising the
 gasket between the glass and aluminum frame.
 The medium (C778) and coarse (Accusand 20/30) sands were packed in a
 layered configuration. The roughly horizontal layers were chosen to mimic
 conditions in many sandy stratified-drift aquifers. These continuous
 layers can act as capillary barriers that promote the formation of DNAPL
 pools. Given the differences in pore sizes and the properties of the DNAPL
 (TCE), these two sands can result in a maximum pool height of
 approximately 12 cm.
 At the top and bottom of the sand domain, a stainless steel screen was used
 in conjunction with stainless steel wool. The screen and wool acted as a
 porous barrier that allowed fluids to easily pass through the cell while
 retaining even the finest of the sands. The stainless steel wool was used
 as porous support for the relatively flimsy screen material.
 TABLE 8
 Details of the Experimental Flow Cell
 Parameter Value
 Construction:
 frame aluminum with stainless steel and brass fitting
 windows plate glass
 gasket neoprene rubber
 screens stainless steel
 Internal Dimensions:
 sand width 20.0 .+-. 0.2 cm
 sand height 15.0 .+-. 0.2 cm
 sand thickness 2.0 .+-. .5 mm
 reservoirs 1.27 .times. 1.27 .times. 19.7 cm
 Porous Media:
 coarse Accusand 20/30, 0.713 .+-. 0.023 mm
 medium Sand, C778, 0.355 mm
 fine Sand, F-52, 0.255 mm
 Two needles were inserted into the domain. The lower needle protruded up
 through the screen and was placed there to act as an extraction point
 during the desaturation process. The upper needle was inserted in a
 similar fashion and was used for the injection of the DNAPL.
 The DNAPL and the flushing solutions that were used are described in Table
 9. All water used in the experiment was de-ionized and then de-aired (DDI)
 using a vacuum pump. The trichloroethylene (TCE) used as a DNAPL was an
 analytical grade reagent and was subsequently dyed with less than 0.01% by
 weight Oil-Red-O (ORO) to give it a red color that was more visible to the
 camera. Past experience has shown that the ORO dye has little or no effect
 on the physical properties of the TCE and will not partition into the
 aqueous phase.
 TABLE 9
 Fluid Properties
 p @ 20.degree. C. .mu. @ 20.degree. C.
 Fluid (g/cc) (cp)
 trichloroethylene (TCE) 1.4642 0.57
 DDI water 1.0 1.0
 NaI solution (60% by weight NaI) 1.7957 2.10
 Experimental Procedure
 After creating the packing, air in the cell was displaced with carbon
 dioxide. The cell was then flushed with DDI water in an up-flow fashion to
 displace and/or dissolve all of the carbon dioxide gas within the cell.
 This procedure resulted in a domain completely saturated with DDI water.
 The cell was then flushed with more than ten pore volumes (&gt;2 L) of DDI
 water to rinse the domain and ensure that all gas bubbles were dissolved.
 The upper syringe was used to slowly inject the DNAPL (TCE with ORO) into
 the domain. When a total of 13.5 g had been transferred, the TCE injection
 ceased. The TCE was allowed to reach a static fluid configuration by
 letting it redistribute for a few hours.
 Next, the Nal solution was pumped into the domain in an up-flow
 configuration. Since the Nal solution was more dense than water (see Table
 9), it was a stable displacement and the experiment clearly demonstrated a
 sharp and level front between the DDI water and the advancing flushing
 front.
 Following the Nal injection, a syringe pump was used to extract fluid from
 the lower needle. This extraction was done at a rate of 1.0 ml/min and
 continued even when the DNAPL and air entered the syringe. When the
 initial desaturation had reached a point where mostly air was withdrawn,
 some surfactant solution was added. This surfactant was a 1% solution of a
 1:1 by weight mixture of sodium diamyl-and sodium dioctyl-sulfosuccinates
 (SDSS). Properties of this surfactant mixture were detailed by Willson et
 al. (1999). When the surfactant addition was cased and the rate of fluid
 extraction had become very slow (mostly gas extraction was occurring), the
 pump was turned off and the experiment ceased.
 FIGS. 3-8 shows a sequence of images from this experiment, where the
 darkest media has the largest grain size. The images describe: a DDI
 water-saturated condition was established (FIG. 3); TCE was added from the
 top and allowed to reach a stable configuration (FIG. 4); a thin brine
 layer was established on the bottom of the domain (FIG. 5); the upper
 portion of the cell was dewatered (FIG. 6); a 0.4 pore volume pulse of
 surfactant was added at the top of the cell (FIG. 7); and the surfactant
 mobilized both DNAPL and water towards the extraction needle (FIG. 8). The
 results from this experiment clearly show that a large fraction (&gt;90%) of
 the TCE from this heterogenous system was removed. Further, the addition
 of the surfactant resulted in water removal as well, leaving the small
 remaining fraction of TCE in a state of relatively large surface area
 volume and low water saturation, which is an ideal condition for removal
 of the remaining fraction of this volatile contaminant via gas-phase
 extraction.
 Results
 From observations of the bench-scale flow experiment applicant reports the
 following results:
 Density-enhanced approaches can remove a significant fraction of the DNAPL
 present in pools. Image analysis showed that over 90% of the TCE was
 removed in this experiment.
 Density-enhanced approaches can accomplish rapid removal.
 The addition of a surfactant can clearly improve DNAPL removal efficiency.
 The dense brine solution acted as an effective lower barrier, preventing
 the migration of the DNAPL out of the sand domain.
 Summary
 Remediation of DNAPLs and LNAPLs is an important environmental concern, for
 which effective and economical solution strategies have previously not
 been known. Applicant has discovered a class of three-fluid-phase
 approaches, including several variants, for mobilizing and capturing both
 DNAPL and LNAPL contaminants, that have been shown to be highly effective
 and economical.
 References
 Ahlfeld, D., Dahmani, A., Hoag, G., Farrel, M., and Ji, W. (1994),
 Proceeding of the Petroleum Hydrocarbons and Organic Chemicals in Ground
 Water: Prevention, Detection, and Remediation Conference, 175-190.
 Bartow, G. and Davenport, C. (1995), Ground Water Monitoring and
 Remediation, 15(2):140-146.
 Bass, D. H. and Brown, R. A. (1995), Proceeding of the 1995 Petroleum
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 It will be understood that various details of the invention may be changed
 without departing from the scope of the invention. Furthermore, the
 foregoing description is for the purpose of illustration only, and not for
 the purpose of limitation--the invention being defined by the claims.