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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants 3 Metals and Radionuclides: Technologies for Characterization, Remediation, and Containment Many different types of inorganic contaminants are present in groundwater and soil at Department of Energy (DOE) facilities. Table 1-3 presents the results of several studies that ranked metal and radionuclide contaminants according to frequency of occurrence. In many instances, contaminants occur as mixtures of metals and radionuclides, organic complexing agents, and organic solvents. These lists of frequency of occurrence indicate the types of contaminants encountered but do not provide information about toxicity, risk, and cost-benefit of cleanup. DOE's Subsurface Contaminants Focus Area (SCFA) singled out several inorganic contaminants of concern in an informal ranking procedure based on prevalence in the weapons complex, mobility, and toxicity, as shown in Table 3-1. This chapter focuses on key contaminants from this list (U, Pu, 137Cs, 90Sr, Ra, Th, Tc, and Cr). The chapter does not review remediation techniques for 3H because this element is generally treated using containment and natural attenuation. The chapter also does not assess remediation technologies for mercury because it is not prevalent throughout the DOE complex; it is found at Oak Ridge, primarily in sediment and surface water, not in groundwater. FACTORS AFFECTING RISKS OF METAL AND RADIONUCLIDE CONTAMINATION Persistence is one of the key factors considered in assessing the risk associated with a chemical in the environment. Many organic
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 3-1 Inorganic Contaminants of Particular Concern at DOE Sites Element Mediuma Priorityb Site Tc GW High Portsmouth Cr(VI) GW Medium-High Hanford Soil Medium Hanford, Sandia National Laboratories, White Sands U GW Medium-High Fernald, Oak Ridge, Rocky Flats Soil Medium Rocky Flats, Fernald, Oak Ridge 137Cs Soil Medium-High Hanford, Savannah River Site 90Sr GW Medium-High Hanford Soil Medium Hanford Pu Soil Medium Mound, Rocky Flats, Nevada Test Site Ra Soil Medium Uranium mill tailings sites 3H GW Medium-Low Savannah River Site, Hanford, Brookhaven Hg GW Low Oak Ridge Soil Low Oak Ridge Th Soil Low Uranium mill tailings sites a GW groundwater b Priority based on prevalence in DOE complex, mobility, and toxicity (according to a survey by the SCFA). compounds biodegrade, reducing the potential for human and ecological exposure over the long term. Metals, on the other hand, are infinitely persistent. Radionuclides undergo natural radioactive decay that, for some compounds (such as tritium), may significantly reduce risks over relatively short time periods. However, for other radionuclides (including various isotopes of Tc, U, Pu, and Th), half-lives are very long, meaning that risks posed by the presence of these compounds will persist for a very long time. As shown in Table 3-2, half-lives for radionuclides vary quite significantly depending on the isotopes present. The potential for humans or sensitive ecosystems to be exposed to metals and long-lived radioactive materials is strongly affected by a number of factors that must be considered in assessing these contaminants. Some metals and radioactive contaminants have more than one oxidation state, which differ in mobility and toxicity (see Box 3-1). Like organic compounds, metals and radioactive contaminants can partition into organic matter present in soils. They also can be sorbed by other soil components, including cation exchange sites and metal oxides, and they can precipitate. Because of the mul-
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 3-2 Half-Lives of Radioactive Compounds Common at DOE Installations Radionuclide Isotope Half-Life (years) Tc 99 2.12 × 105 U 234 2.47 × 105 235 7.1 × 108 238 4.51 × 109 Cs 137 30 Sr 90 28 Pu 238 86 239 24.4 × 103 240 6.58 × 103 3H 12 Th 228 1.91 230 8.0 × 104 232 1.41 × 1010 SOURCE: Weast, 1980. tiple possible associations of a metal or radioactive contaminant with a soil, determination of the total contaminant concentration is unlikely to provide sufficient information to allow valid assessments of potential risk or amenability to remediation by specific processes. Figure 3-1 summarizes the types of species in which metals may be present in the environment. BOX 3-1 Oxidation States Many metal and radionuclide contaminants exist in the environment in multiple forms. For example, chromium is found as Cr(VI) (+6 oxidation state) under environmental conditions known as oxidizing conditions and as Cr(III) (+3 oxidation state) under reducing conditions. Oxidizing conditions generally prevail in the absence of biodegradable organic matter and in near-surface environments. Reducing conditions generally prevail when an excess of biodegradable organic matter is present and the oxygen supply is limited. The different oxidation states of metals and radionuclides may exhibit greatly different chemical behavior. For example, in simple, dilute, neutral aqueous solutions, the predominant form of Cr(VI) is the highly soluble, mobile oxyanion CrO42-, while the predominant form of Cr(III) is the highly insoluble solid Cr(OH)3. Metals in different oxidation states also have different risks. For example, Cr(VI) is highly toxic, whereas Cr(III) is relatively harmless to humans.
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Figure 3-1 Forms of occurrence of metal species in the environment. Source: Stumm and Morgan, 1981 (Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters, Copyright 1981 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.).
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Geochemical Characteristics of Metal and Radionuclide Contaminants: Effects on Treatment Options Because metal and radionuclide contaminants are generally non-degradable except by radioactive decay, treatment technologies must involve some form of mobilization or immobilization for removal or containment, respectively. Thus, solubility and the propensity for complexation in solution or sorption to surfaces are key properties to consider when evaluating treatment options. Table 3-3 summarizes these properties for the contaminants that are the focus of this study. Table 3-3 Speciation of Inorganic Contaminants A. Elements with Multiple Oxidation States Element Oxidizing Conditions Reducing Conditions Tc Tc(VII): TcO4-, high solubility, very weak adsorption Tc(IV): TcO2nH2O(s); low solubility Cr Cr(VI): CrO42-, HCrO4-, Cr2O72- depending on total Cr concentration and pH value; high solubility, weak adsorption Cr(III): Cr(OH)3(s); low solubility U U(VI): UO22+ high solubility, moderate sorption; highly soluble, weakly sorbing anionic U(VI) carbonate complexes may predominate in waters with high carbonate concentrations U(IV): UO2(s), low solubility Pu Pu(VI), PU(V), Pu(IV): Pu4+, PuO2+, PuO2+ complex, redox-active aqueous chemistry with moderate solubility and moderately sorbing species Pu(IV): PuO2(s), moderately low solubility B. Elements with Single Oxidation States Element Speciation in Water Cs Cs(I): Cs+, essentially no hydrolysis, moderate adsorption, no oxidation-reduction activity Sr Sr(II): Sr2+, essentially no hydrolysis, moderate to weak adsorption, no oxidation-reduction activity Ra Ra(II): Ra2+, essentially no hydrolysis, moderate to strong adsorption, no oxidation-reduction activity Th Th(IV): Th(OH)n(4-n)+, strong hydrolysis, moderate solubility, very strong adsorption
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants As shown in Table 3-3, Tc, Cr, U, and Pu exhibit multiple oxidation states, of which the reduced forms are quite insoluble in water. The oxidized forms Tc(VII) and Cr(VI) are both anions in water and generally sorb weakly to the negatively charged surfaces typically encountered in nature. The alkali and alkaline earth ions Cs+, Sr+2, and Ra+2 do not exhibit redox activity and are hard cations, which do not hydrolyze strongly, are not expected to sorb strongly to oxide surfaces, and are subject to competition in any complexation reaction by other alkali and alkaline earth ions present at much higher concentrations (e.g., Na+, Ca+2). Thorium(IV) does hydrolyze strongly in water and adsorbs rather strongly to oxide surfaces. Table 3-4 summarizes treatment technologies for different classes of inorganic contaminants, and Table 3-5 summarizes technologies for different media. Box 3-2 provides a glossary of remediation technology terms. These technology options are discussed in more detail later in the chapter. CHARACTERIZATION OF METAL AND RADIONUCLIDE CONTAMINATION Because speciation controls the environmental transport and risks of metals and radionuclides, it is as important to characterize as the total amount (or total concentration) of the contaminant. Traditionally, concentrations of metals and radionuclides have been determined by taking samples of groundwater from monitoring wells or soil from borings to the laboratory for analysis. A variety of techniques, from computer models, to spectroscopic and electrochemical analyses, to sequential extraction methods, are available to determine speciation of metals and radionuclides in samples in a laboratory. More recently, techniques have been developed for measuring metal and radionuclide concentrations in situ, without bringing samples to the laboratory. The advantages of in situ analysis include reduction in time and cost of site characterization as well as reduction of exposure of personnel to hazardous contaminants. The following brief descriptions of laboratory and in situ techniques for characterizing metals and radionuclides are intended only as an introduction to this complex subject; the references cited with the descriptions provide technical details on carrying out these analyses. Ex Situ Analysis for Speciation Speciation of metals and radionuclides based on analysis of laboratory samples can be determined computationally or experimentally.
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 3-4 Treatment Technology Options for Different Classes of Inorganic Contaminants Redox-Active Anions (Tc, Cr) Redox-Active Actinide Cations (U, Pu) Weakly Hydrolyzing Cations (Cs, Sr, Ra) Strongly Hydrolyzing Cations (Th) Solidification and Stabilization Pozzolanic agents — — A A Vitrification A A A A Chemical and Biological Reaction In situ redox manipulation Gaseous reductants ? ? NA NA Liquid reductants A A NA NA Permeable reactive barriers Fe0 A A NA NA Microbiological ? ? NA NA Sorption — — A A Bioremediation A A NA NA Biological reduction and adsorption (wetlands) A A NA NA Separation, Mobilization, and Extraction Electrokinetic systems A A ? ? Soil flushing and washing A A A A Phytoremediation (macrophytes) — — A — NOTE: A = applicable; ? = application still in an experimental stage and not yet proven; — = lack of information for a quantitative comparison; NA = not applicable.
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 3-5 Technology Types Applicable to Different Contaminated Media Context Solidification, Stabilization Containment Biological and Chemical Reactions Separation, Mobilization, and Extraction Surface soils, sediments, and sludge Pozzolanic agents Cement Vitrification Capping Phytoremediation Chemical reduction Solvent extraction Soil washing • Acids, bases, and chelating agents • Surfactants and cosolvents Unsaturated zone Pozzolanic agents Cement Vitrification Chemical reduction Microbial reduction Electrokinetic systems Soil flushing • Acids, bases, and chelating agents • Surfactants and cosolvents Saturated zone Grout walls Slurry walls Sheet pile walls Chemical reduction Permeable-reactive barriers • Fe0 • Microbiological • Enhanced sorption • Ion exchange Electrokinetic systems Soil flushing • Acids, bases, and chelating agents • Surfactants and cosolvents High-concentration source areas in the saturated zone Grout walls Slurry walls Sheet pile walls Pump-and-treat systems Chemical reduction Electrokinetic systems Soil flushing • Acids, bases, and chelating agents • Surfactants and cosolvents
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants BOX 3-2 Remediation Technologies for Inorganic Contaminants* Stabilization-Solidification and Containment Technologies In Situ Precipitation or Coprecipitation. A permeable reactive barrier (see definition below) that causes the precipitation of a solid (usually carbonate, hydroxide, or sulfide mineral) to maintain a toxic metal in an immobile form. Formation of solid phases is controlled primarily by pH, redox potential, and concentrations of other ions. Pozzolanic Agents. Cement-like materials that form chemical bonds between soil particles and can form chemical bonds with inorganic contaminants, decrease permeability, and prevent access to contaminants. The most common pozzolanic agents are portland cement, fly ash, ground blast furnace stag, and cement kiln dust. Vitrification. Melting of contaminated soil to form a glass matrix from the soil, either in place (in situ vitrification) or in a treatment unit. Nonvolatile metals and radioactive contaminants become part of the resulting glass block after cooling. Organic contaminants are either destroyed or volatilized by the extremely high temperatures. The method is generally expensive due to the large energy requirements. Biological Reaction Technologies Phytoremediation. Removal of contaminants from surface soil through plant uptake. Subsequent treatment of the plant biomass may be necessary. Chemical Reaction Technologies Enhanced Sorption. A passive-reactive barrier (see definition below) that creates zones that cause contaminant sorption, either microbiologically (biosorption) or chemically (through materials with surface complexation, ion exchange, or hydrophobic partitioning properties). In Situ Redox Manipulation. The injection of chemical reductants into the ground Experimental methods can be further subdivided into those that provide characteristics of the contaminant and those that provide information on specific chemical contaminants. Computational Methods Chemical equilibrium computer programs are useful for computing the distribution of species in samples for which total concentrations of metals and ligands (ions or molecules that can attach to metals) have been measured, provided appropriate stability constants are available (Nordstrom et al., 1979). Commonly used programs include MINTEQA2 (Allison et al., 1991) and MINEQL+ (Schecher
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants to create reducing conditions in an aquifer, which will then lead to reduction and immobilization of certain contaminants in groundwater. Permeable-Reactive Barriers. Permeable containment barriers that intercept contaminant plumes and remove contaminants from groundwater solution through chemical and/or biological reactions within the barrier. Zero-Valent Iron Barrier. A passive-reactive barrier (see definition above) that creates strongly reducing conditions, resulting in hydrogen generation. Dissolved chlorinated solvents (chlorinated ethanes, ethenes, and methanes) are chemically degraded at relatively rapid rates. Some metals form relatively insoluble solids at low redox potential and can be treated with this method. Separation, Mobilization, and Extraction Technologies Electrokinetics. The movement of water and/or solutes through a porous medium under the influence of an applied electric field. Electromigration is the migration of ionic species through a soil matrix. The process can function in both saturated and unsaturated environments. Electroosmosis is the movement of pore water through a fine-grained matrix. This technique has long been understood as a means to control water movement in fine-grained media and is currently being investigated to remove contaminants at waste sites. Soil Flushing. An in situ process that uses chemical amendments and fluid pumping to mobilize and recover contaminants (see also cosolvent flushing and surfactant flushing). Soil Washing. An ex situ process in which contaminated soils are segregated and then washed with a water-based solution. Generally, soil fines have a high concentration of contaminants, while coarse materials may be sufficiently clean that contaminant concentrations are below action levels, allowing coarse materials to be disposed of separately. Once fines are separated from coarse soils, the fines may be disposed of directly or extracted. * Some technologies may also be applicable to organic contaminants. and McAvoy, 1992). Systems in which the principal ligands are inorganic and that contain relatively uncomplicated solid surfaces are most amenable to modeling with computations. Considerable research has been conducted to describe the binding of metal ions to oxide surfaces. Applicable surface complexation models that can be used to describe this binding based on electrical double-layer models are discussed by Dzombak and Morel (1990) and Stumm (1992). The description of metal complexation with natural organic matter (NOM; for example, humic substances) is much more complicated. NOM is an unresolvable mixture of a very large number of compounds varying in their properties, including their ability to bind
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants metal ions. Several approaches have been proposed for modeling the way in which metals form complexes with NOM and humic substances. These include gaussian distribution models (Perdue and Lytle, 1983) and multiple discrete site models (Fish et al., 1986). Tipping (1994) has presented a model, using multiple classes of organic matter reaction sites, that is able to predict relatively accurately metal cation and proton binding to naturally occurring organic matter. Because of the heterogeneity of soils, metals can be associated with many types of surfaces in a single soil. Recently, Radovanovic and Koelmans (1998) presented a model to predict the binding of a series of cationic metals to suspended particles in natural waters as a function of the characteristics of the aqueous and solid phases. Because similar properties control the speciation and partitioning of metals between soil and pore water, this or similar models could be applied to soil samples. Experimental Methods Perhaps the most fundamental physical means for determining the speciation of metals is physical separation of the dissolved and particulate phases. A number of procedures are available for this separation (Bufflap and Allen, 1995). The separation processes are subject to significant error, particularly as a result of incomplete separation of particulate and dissolved phases. Analysis of chemical species is possible by chromatographic, spectroscopic, and electrochemical methods. The separation and quantitation of ethylenediaminetetraacetic acid (EDTA) and other complexes of metals has been achieved by ion chromatography (Hajós et al., 1996) and by capillary electrophoresis (Buergisser and Stone, 1997). Several electrochemical methods are widely used for the analysis of trace metals in natural waters and in soil solution. Among these are selective ion electrodes, anodic stripping voltammetry, and cathodic stripping voltammetry (Florence, 1989; Van den Berg, 1984). All are capable of determining submicrogram-per-liter concentrations of metals and can be used in titrations to determine the concentration of available binding sites for a metal and the strength of the complexation reaction. Sequential extraction procedures, most commonly that of Tessier et al. (1979), frequently are used to correlate the presence of metal species in samples with observed effects, including toxicity, bioavailability (availability for uptake by living organisms), and mobility. These procedures use increasingly strong extractants to release trace metals
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants important phytoremediation methods for treating contaminants in groundwater and soil (although phytovolatilization could be used to remove Hg). Phytostabilization is a special form of containment technology, whereas phytoextraction can be used to remove metals and radionuclides from the subsurface; thus, this discussion focuses on phytoextraction. Physical and Chemical Principles Many plants exude chelating agents from the roots to complex essential micronutrients and make them available to the plant. Phytoextraction causes plants to take up contaminant metals in the same way they extract micronutrients. For this method to be effective, some mechanism must exist for the plant to select a contaminant over other similar but more abundant metals (e.g., selection of Ra or Sr rather than Ca). Application Phytoextraction is most applicable to large areas of surface soils with low to moderate levels of contamination. Plants that accumulate at least 0.1 percent by weight of Co, Cu, Cr, Pb, or Ni or 1 percent by weight of Mn or Zn are defined as hyperaccumulators and are suitable for phytoextraction. Although many plants have been tested, plants of the genera Brassica, Thlaspi, Cardaminopsis, and Alyssum appear to be the most promising (Ahmann, 1997, after Kumar et al., 1995). For metals that are bound extremely tightly to soils, the addition of chelating agents, such as EDTA promotes their accumulation in the plant. In such cases, care must be exercised that adding the chelating agent does not cause leaching of the contaminant from the surface soil zone. Ultimately the plants are harvested, dried, and combusted or composted to reduce their mass prior to disposal. Multiple harvests are generally required to achieve cleanup goals. Performance Commercialization of phytoextraction and other phytoremediation methods has been relatively slow but appears to be occurring. Table 3-13 lists several field demonstrations of phytoextraction for the treatment of metals and radionuclides.
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Table 3-13 Field Demonstrations of Phytoextraction of Metals and Radionuclides Location Application Plants Contaminants Performance Trenton, New Jersey Demonstration on 60 × 90 m plot at brownfield site Brassica juncea (Indian mustard) Pb Pb was removed to below action level in one season. Pennsylvania Mine waste site Thlaspi caerulescens Zn, Cd Rapid contaminant uptake was observed, but the soil was difficult to decontaminate. Findlay, Ohio Contaminated soil demonstration under the SITE program NA Pb, Cr, Ni, Zn, Cd NA NOTE: NA = published information not available from the indicated sources. SOURCES: Schnoor, 1997; Chappell, 1998.
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants Limitations Phytoextraction is applicable only to the rooting zone of soils Although the goal of phytoextraction is mobilization of contaminants from soils to plant biomass for subsequent recovery or disposal, care must be exercised to prevent mobilization into the biosphere (for example, from animals eating the plants) where recovery is no longer possible. Furthermore, care must be exercised that natural or synthetic chelating agents associated with the uptake of contaminants do not mobilize the contaminant into groundwater. The plants must be able to grow vigorously at the site. Disposal of plant biomass can be a problem when plants are contaminated with heavy metals and radionuclides. Advantages Phytoextraction is a low-cost method to remove contaminants from large areas of the surface zone of contaminated soils. Conclusions Treatment technologies for cleaning up metal and radionuclide contaminants in groundwater and soil act either by immobilizing contaminants in place to prevent transport to humans and sensitive ecosystems or by mobilizing contaminants for extraction and treatment at the surface. Because metals and radionuclides, unlike organic contaminants, are nondegradable except by radioactive decay and because the risk posed by these compounds is highly sensitive to geochemical conditions, managing these contaminants is very different from managing the types of organic contaminants discussed in Chapter 4. Few well-established technologies are available for treating metals and radionuclides in the subsurface, but a number of new technologies are being developed. Available technologies for treating metals and radionuclides are summarized in Tables 3-4 and 3-5 and include the following: Impermeable barriers are among the least expensive and most widely used methods for preventing the spread of metal and radionuclide contaminants in groundwater. Vertical barriers are well developed and widely available; methods are being developed for the installation of horizontal barriers beneath existing waste. In situ vitrification for immobilization of metal and radionuclide contaminants is an emerging technology that is particularly suitable for sites with high concentrations of long-lived radioisotopes within 6 to 9 m
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants of the soil surface (depending on water table depth and soil moisture). However, it is among the most expensive treatment technologies. Solidification and stabilization are mature technologies for use ex situ but are considered emerging technologies for use in situ. Ensuring sufficient mixing is difficult when this technology is used in situ. Improved mixing methods are being tested. The longevity of the solidified or stabilized material is another concern that must be addressed before these methods can be considered established technologies. Permeable reactive barriers are among the most promising and rapidly developing emerging treatment technologies for metal and radionuclide contaminants. A variety of reactive media has been tested for a variety of contaminants, including organics. Because the technology is relatively new, the longevity of the barrier is a major uncertainty. In situ redox manipulation is an emerging method that is appropriate at both shallow depths and depths at which trenches are impractical. It is an excellent technology for elements (e.g., Cr) that can be reduced to solids that are resistant to reoxidation by ambient oxygen, but it is less suitable for elements (e.g., Tc) that are susceptible to reoxidation. Bioremediation is in the early stages of development for the treatment of metals and radionuclides. If better developed, it could be a relatively low-cost alternative and could be used to treat mixtures of organic and inorganic contaminants. Electrokinetics may be advantageous for extracting metals and radionuclides from media with very low hydraulic conductivity. The consequences of generating large amounts of acid and base must be considered. Extensive field tests of electrokinetics for the remediation of metal and radionuclide contamination have yet to be conducted in the United States. Soil washing has been well developed for ex situ treatment of contaminated, coarse-grained, near-surface soils but requires excavation of the soil prior to treatment. Soil flushing can potentially flush metals and radionuclides from soil in situ. Although used in the mining industry, this technology has not seen widespread application in the remediation of metals and radionuclides. Phytoremediation is primarily advantageous for extracting metals from large areas of contaminated surface soils. Its advantages are low cost and ease of implementation. Potential disadvantages are the excessive mobilization of metals from chelators that often must be added and the need to dispose of plant biomass. In general, only containment and ex situ technologies are well developed for treating metal and radionuclide contaminants. Additional development work is needed to increase the range of options
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Ground Water & Soil Cleanup: Improving Management of Persistent Contaminants for treating metals and radionuclides in situ and for extracting them for ex situ treatment. REFERENCES Acar, Y. B., and A. N. Alshawabkeh. 1993. Principles of electrokinetic remediation. Environmental Science and Technology 27:2638–2647. 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. Adriano, D. C. 1986. Trace Elements in the Terrestrial Environment. New York: Springer-Verlag Ahmann, D. 1997. Bioremedation of metal-contaminated soil. Society for Industrial Microbiology News 47:218–233. Allen, H. E., and P-H. Chen. 1993. Remediation of metal contaminated soil by EDTA incorporating electrochemical recovery of metal and EDTA. Environmental Progress 12:284–293. Allen, H. E., and Y. Yin. 1998. Combining chemistry and biology to derive soil quality criteria for pollutants. Presented at Sixth World Congress of Soil Science, Montpellier, France, August 25. Allison, J. D., D. S. Brown, and K. J. Novo-Gradac. 1991. MINTEQA2/PRODEFA2, A geochemical assessment model for environmental systems: Version 3.0 users manual. EPA/600/3-91/021. Washington, D.C.: U.S. Environmental Protection Agency. Amonette, J. E., J. Szecsody, J. C. Templeton, Y. A. Gorby, and J. S. Fruchter. 1994. Abiotic reduction of aquifer materials by dithionite: A promising in situ remediation technology. In In-Situ Remediation: Scientific Basis for Current and Future Technologies, G. W. Gee and N. R. Wing, Eds. Richland, Wash.: Battelle Press. Ballard, J. H., and M. J. Cullinane, Jr. 1997. Tri-Service Site Characterization and Analysis Penetrometer System (SCAPS) Technology Verification and Transition. Vicksburg, Miss.: U.S. Army Corps of Engineers Waterways Experiment Station. Betts, K. S. 1998. Novel barrier remediates chlorinated solvents. Environmental Science and Technology 3(2):495A. Borns, D. J. 1997. Geomembranes with incorporated optical fiber sensors for geotechnical and environmental applications. Pp. 1067–1073 in Proceedings of the International Containment Technology Conference, St. Petersburg, Fla., February 9–12. Browne, C. L., Y. M. Wong, and D. R. Buhler. 1984. A predictive model for the accumulation of cadmium by container-grown plants. Journal of Environmental Quality 13:184. Buergisser, C. S., and A. T. Stone. 1997. Determination of EDTA, NTA, and other amino carboxylic acids and their Co(II) and Co(III) complexes by capillary electrophoresis. Environmental Science and Technology. 31:2656–2664. Bufflap, S. E., and H. E. Allen. 1995. Sediment pore water collection methods: A review. Water Research 29:165–177. Chappell, J. 1998. Phytoremediation of TCE in ground water using populus. Washington, D.C.: EPA. Cifuentes, F. R., W. C. Lindemann, and L. L. Barton. 1996. Chromium sorption and reduction in soil with implications to bioremediation. Soil Science 161:233–241. Daily, W., and A. Ramirez. 1997. A new geophysical method for monitoring emplacement of subsurface barriers, Pp. 1053–1059 in Proceedings of the International Containment Technology Conference, St. Petersburg, Fla., February 9–12.
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Representative terms from entire chapter: