Chapter 2
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The Evolution of DNAPL Remediation Practice Richard E. Jackson INTERA Inc., 9111A Research Boulevard, Austin, TX 78758
The remediation of dense non-aqueous phase liquids (DNAPLs) and their dissolved components has proven to be an expensive and incomplete chapter in environmental history. In the 1980s, it was assumed by many that ground-water extraction would completely restore aquifers to their precontaminated conditions. By the early 1990s it was apparent that this condition would not be achieved and that pump-and-treat remediation was principally a means of containing dissolved-phase contamination without complete site restoration. This failure prompted experimentation with technologies from different engineering fields, including enhanced oil recovery and waste-water treatment, as potential remediation technologies. Independent performance assessment of all technologies indicates that good site characterization is essential to their successful implementation and that vendor claims of success must be carefully scrutinized. Recent field testing indicates that the benefits derivablefromeach technology can best be obtained by its use as a unit operation linked to other unit operations in the form of an in-situ "treatment train".
© 2003 American Chemical Society Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction The contamination of water-supply wells and suburban neighborhoods by DNAPLs became a national issue in the USA in the late 1970s. Two events caused this sudden awareness of a very much larger problem. The first was the discovery of DNAPL beneath the Love Canal neighborhood of Niagara Falls, New York in 1976 and then in the bedrock beneath nearby chemical plants, causing seepage of contamination into the Niagara River and Lake Ontario. The second was the discovery in 1978-9 of the contamination of municipal wells by dissolved trichloroethylene (TCE), perchloroethylene (PCE) and other chlorinated solvents on Long Island (New York), in the industrial towns around Boston, and in the urbanized alluvial basins of California. These events produced a number of responses to the crisis posed to public health by the vaporization and dissolution of these DNAPLs. The most profound was the Federal legislation that sought to provide society with the means with which to remediate the problems of contaminated neighbourhoods and wells. This involved the passage of Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the establishment of the Superfund in 1980, its amendment in 1986 (SARA), and of the Hazardous and Solid Waste Amendments to the Resource Conservation and Recovery Act of (RCRA)in 1984. While Love Canal spawned legislation, the irony of the discovery of TCE contamination of ground water across the USA was that it was accidental. TCE was discovered during the testing of municipal water supplies for the presence of trihalomethanes that was required under the 1978 amendment to the Federal Safe Drinking Water Act (SDWA). With the introduction of the gas chromatograph/mass spectrometer into analytical practice and the adoption of the purge-and-trap device for extracting volatile organic chemicals (VOCs) from ground-water samples in the mid 1970s, the stage was set to reliably detect dissolved TCE and other VOCs (i). The events and the legislation that followed generated the need to enlist and train environmental scientists and engineers who could characterize the hazardous-waste sites contaminated with DNAPLs and could then commission effective remedial operations to protect public health. It is no understatement to say that these demands of site characterization and remediation fell on a technical community that was not only few in numbers but was completely unprepared for the tasks implicit in their execution. They worked without a guiding technical paradigm relating the migration, trapping and dissolution of DNAPLs to their distribution in the subsurface and their detection, quantification and spatial characterization. As a consequence, the engineers and scientists who practiced DNAPL remediation made very limited progress in restoring sites to meet regulatory guidelines without resorting to active and continuing
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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containment. Just as a century before when the germ theory of disease opened the door to vast improvements in public health, so progress in DNAPL remediation was dependent upon the emergence of a paradigm that would explain the migration and fate of DNAPL in the subsurface and would guide research and practice. This commentary on DNAPL remediation considers the scientific basis of our understanding of DNAPL distribution in the field and the various technologies that have been developed to address the problem. It is in no way a complete survey of the field. Rather it considers the progress made and the failures encountered since Love Canal brought the issue to the public's attention.
The Schwille Paradigm The term 'paradigm' has become oversimplified in popular use in the years since it was used by T.S. Kuhn in 1962 to describe scientific development (2). In Kuhn's own words (2, page 10): "By choosing it, I mean to suggest that some accepted examples of scientific practice - examples which include law, theory, application, and instrumentation together - provide models from which spring particular coherent traditions of scientific research." Thus, M.K.Hubbert's analytical theory of ground-water flow in 1940 (3) provided a conceptual model based on the principle of conservation of mass and the laws of thermodynamics from which the modern paradigm of ground-water flow developed (4). Friedrich Schwille and his colleagues at the Federal Institute of Hydrology in Koblenz, West Germany developed the conceptual model of the migration and fate of DNAPL as shown in Figure 1. Their 1984 report (5) on laboratory experiments of the migration and fate of dense chlorinated solvents established the principal features of the paradigm, including rules and experiments that environmental engineers and scientists would subsequently follow. They showed the patterns of migration and trapping of solvents in sands of various particle size and the retention of this DNAPL by these same sands above and below the water table. Furthermore, they demonstrated the solubilization of the DNAPL to form dissolved-phase plumes, and the importance of the volume of solvent spilled and the geometrical -arrangement of zones of differing permeability in determining the spatial distribution of DNAPL in the subsurface. They also demonstrated the importance in the subsurface of the volatilization of chlorinated solvents and the migration of the vapor to depth. A synopsis of this report appeared in English in a 1984 conference proceedings that was little noticed in North America (6). Schwille first presented his results in North America in June 1985 at the 2 Canadian-American Hydrogeology Conference at Banff, Alberta (7) and, in 1988, the English translation of the 1984 report (8) was published. nd
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
Figure 1. Migration of DNAPL in a sand aquifer and the development of DNAPL and dissolved contamination zones. Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
25 Increasingly explicit descriptions of the Schwille paradigm appeared in two reviews published in Environmental Science and Technology in 1985 and 1989. The first (9) acknowledged Schwille's finding that chlorinated degreasing solvents are retained in significant quantities in the subsurface and act as longterm subsurface sources of contamination. The second review (10) described the Schwille paradigm in much greater detail and how it negatively affected attempts to restore contaminated ground water to pristine conditions by pump-and-treat methods. The general acceptance of the paradigm in North America was indicated by presentation of the conceptual model (i.e., Figure 1) in the first (1990) edition of Domenico and Schwartz (11). A more complete account of the migration, trapping, volatilization and dissolution of DNAPL was published in 1996 by the Solvents-in-Groundwater Consortium based at the University of Waterloo, Ontario, Canada (12). Thus, Schwille's work provided the rules by which the technical community could begin to develop appropriate subsurface characterization and remediation approaches. He warned (8, p. 112) that solvents might remain at the base of aquifers "for time periods ranging between several months to decades." It was this consideration that lead Mackay and Cherry (10), in 1989, to state that "the primary challenge in groundwater cleanup is to remove the organics masses that serve, in effect, as subsurface sources and cause the plumes to grow and persist, rather than simply to remove the dissolved contamination that defines the plume."
The Development of Technologies for DNAPL Remediation Although Mackay and Cherry (10) recommended DNAPL removal, the difficulty of that task was apparentfromthe outset. Wilson and Conrad (13) had already shown the impracticability of water-flooding to remove sufficient residual NAPL from alluvium to achieve regulatory goals. As Mackay and Cherry noted prophetically, "very little success has been achieved in even locating the subsurface NAPL sources, let alone removing them." Therefore, increasingly during the 1990s, DNAPL removal came to be seen as an impractical and futile goal by many in industry and the Solvents-in-Groundwater Consortium, which was supported by industrial funding. Alternative approaches were developed, including containment walls borrowed from geotechnical engineering and in-situ chemical oxidation (ISCO) and bioremediation, which were modified forms of waste-water treatment methods. Four general classes of DNAPL site remediation technology may be identified (see Table 1): containment, dissolved-phase destruction of VOCs, DNAPL removal from the saturated zone, and soil-vapor extraction to remove DNAPL from the vadosezone. The first two classes address the dissolved-phase contamination that is
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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generated by ground-water flow through the DNAPL zone; the third addresses the removal of the DNAPL zone itself; and the fourth removes contamination by promoting volatilization. Table 1: Glasses andExamples of DNAPL Remediation Technologies Example Technoloiges Class "Pump-and-Treat"; Barrier Walls Containment ISCO; bioremediation; PRBs Dissolved-Phase Destruction Saturated-Zone DNAPL Water, steam, surfactant or alcohol Removal flooding Vadose-Zone DNAPL Removal Soil vapor extraction
Containment of Dissolved-Phase Contamination Mackay and Cherry (10) pointed out that the pump-and-treat approach to remediation "is best thought of as a management tool to prevent, by hydraulic manipulation of the aquifer, continuation of contaminant migration". This statement implied that methods of operations research would be useful, in particular those that would derive optimal solutions for containing and efficiently extracting contaminated ground water. Despite the elegance and utility of optimization algorithms, optimal hydraulic capture is seldom attempted at DNAPL sites. In fact, the example DNAPL site used by Gorelick and colleagues in their text on optimal capture and containment (14) was cornmissioned with extraction wells that were placed in a manner that the hydrogeologist responsible felt would ensure maximum ground-water recovery, irrespective of engineering efficiency or the costs of water treatment or the precise axis of the narrow contaminant plume that was unknown to him at the time of well construction. Similarly, the capture zones at other DNAPL sites are most often developed without reference to pumping tests for anisotropic porous media. Such tests are needed to determine minimum extraction rates to achieve capture of the contaminant plume (75). In short, although the mechanisms of DNAPL dissolution and plume generation are well understood (16) and the tools for efficient hydraulic containment are available (14,15), the application of those tools is frequently neglected and containment even less frequently confirmed following several years of pump-and-treat operation. It is seldom made explicit how important site characterization is to containment system design - whether hydraulic control or barrier walls, which are borrowed from geoteefuiical engineering practice (17). The performance of these walls depends not only on good construction practice but also on good site characterization. At Hill Air Force Base in Utah, the failure of a soil-bentonite
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
27 slurry wall to envelope the DNAPL zone - leaving about 3000 gallons of DNAPL outside the wall - has been presented publicly (18) as part of an effort at Hill to improve remediation engineering performance. The Hill AFB example shows very clearly the perils of designing and installing a barrier wall without first properly characterizing the DNAPL zone. A similar requirement is necessary with hydraulic control by ground-water extraction in order to ensure that the. wellfield installed to contain the aqueousphase contamination actually fully contains the contaminants, even in the event of failure of one of the extraction wells.
Destruction of Dissolved-Phase Contamination We may consider three technologies that involve the in-situ destruction of dissolved components of DNAPLs: permeable reactive barriers (PRBs), ISCO, and anaerobic bioremediation. As the name implies, PRB technology combines in-situ destruction with containment to create an entirely new technology, whereas ISCO and bioremediation are borrowed technologies that have involved adapting methods of waste-water treatment engineering. Warner and Sorel (19) provide a superb review of the development of PRB technology and the scientific basis for its function in this volume. They point out the critical dependence of the chemical component of the technology - the substrate or "treatment matrix" that acts on ground water to produce the hydrogen for dechlorination - upon the hydraulic control system that is designed to channel the contamination through the PRB. Given what has been said above about the failure of hydraulic containment systems to contain dissolved-phase contamination, it can be hardly surprising to learn from Warner and Sorel that those PRBs that have failed to function as designed have generally failed for hydraulic control reasons, rather than because the treatment matrix failed. The applicability of PRBs to shallow environments and narrow plumes is well established, although high dissolved concentrations, deep DNAPL contamination, and wide plumes may prove uneconomic for PRB application (20) . ISCO relies on electron transfer to oxidize the dissolved contaminant, as with KMn0 (22) and other chemical oxidants. Consequently, ISCO requires that the redox reactions occur in an electrolyte such as ground water that has a typical electrical conductivity of 10'MO" Siemen-meters (S-m), as opposed to non-electrolytes such as TCE (8x10" S-m) or PCE (6x10" S-m). Thus DNAPL dissolution must occur prior to oxidation. As Farquhar and colleagues (21) showed in controlled experiments at Borden, Ontario, permanganate oxidation of 8 liters of PCE DNAPL released into Borden sand required some ten months to destroy just 62% of the DNAPL. The slow rate of destruction 4
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Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
28 indicates that these methods should be reserved for systems containing little or no DNAPL, i.e., DNAPL saturations < 0.1% or
